This study presents initial results of the ionospheric scintillation in the F layer using the S4 index derived from the radio occultation experiment (RO‐S4) on FORMOSAT‐7/COSMIC‐2 (F7/C2), and a global map of equatorial plasma bubbles (EPBs) is constructed by using the RO-S4. The F7/C2 RO‐S4 during August 2019 to April 2020 show clear scintillation distributions around American and the Atlantic Ocean longitudes and compared with Jicamarca range‐time‐intensity (RTI) maps of the 50 MHz radar. Result shows that the occurrence of intense RO‐S4 in the range 0.125–0.5 is co‐located with the bottomside of the spread‐F patterns. The increase of RO‐S4 at the upward phase of bottom‐side oscillations is consistent with the theory that EPB seeding by the large‐scale wave. The locations and occurrences of the RO‐S4 greater than 0.5 are consistent with airglows depletions from the NASA GOLD mission. Climatology analyses show that monthly occurrences of RO‐S4 > 0.5 agree well with the monthly EPB occurrences in the GOLD 135.6 nm image, and show a similar longitudinal distribution to in‐situ measurements. The results suggest that the RO‐S4 intensities can be utilized to identify EPBs of specific scales.

Non-migrating atmospheric tides are evident throughout Earth’s upper atmosphere. Features corresponding to these non-migrating tides are also found in observations of the ionosphere, and especially in the location and strength of the equatorial ionization anomaly. It is widely accepted that the primary mechanism responsible for imprinting this signature of the atmospheric waves on the ionosphere involves the E-region dynamo. However, both theory and modeling suggest that other mechanisms should also contribute to the longitudinal modulation of the equatorial ionization anomaly. Providing observational confirmation of these various mechanisms was one of the primary goals for NASA’s Ionospheric Connection Explorer (ICON) mission. Utilizing data from multiple ICON instruments at once, we provide observational constraints on the effectiveness of various mechanisms by which non-migrating tides can produce longitudinal variations in the equatorial ionization anomaly.

The neutral wind dynamo plays an important role in generating low-latitude ionospheric variability and is associated with creating favorable conditions for space weather effects. The neutral wind in the lower thermosphere is highly variable due to its influence from lower atmospheric upward propagating waves and tides. In addition, observational and modelling studies have indicated large variability of the plasma drift on time scales from days to seasons associated with the wind dynamo at low and middle latitudes. The relationship of the ionospheric drift variability to the neutral wind variations is still not fully understood. The Ionospheric Connection explorer (ICON) mission is designed to focus on the low to middle latitude region and measures key parameters, such as the plasma drift and density and neutral temperatures and winds, to examine the vertical coupling of the lower to upper atmosphere. In this presentation, we will focus on the ICON observations and compare to Whole Atmosphere Community Climate Model-Extended (WACCM-X) simulations to examine the daytime low latitude ion drift and neutral wind variations. We investigate the variation of ion drift and neutral wind on different time scales. We compare WACCM-X simulations with the observations to shed light on what variability can be captured by this whole atmosphere model. We conclude with discussing possible reasons for disagreement.

Atmospheric tides play important roles in the dynamics of the upper atmosphere. The sources of atmospheric tides can be from convection in the troposphere, solar heating throughout the atmosphere, or generated by nonlinear wave-tide interactions locally. The Michelson Interferometer for Global High-resolution Thermosphereic Imaging (MIGHTI) onboard the Ionospheric Connection Explorer (ICON) is designed to measure the neutral winds and temperatures in the thermosphere. During day of year 85 to 105 in 2020, in the fixed local time frame, zonal wavenumber 3 and 4 are observed in the neutral wind from 95 km to 300 km. This event provides the opportunity to study the vertical evolution of the zonal wave structure, and the response in the ionosphere.  In this study, we focus on the vertical variation of the wave structure in neutral wind.  The possible atmospheric non-migrating tides propagating from lower to higher altitudes will be discussed based on theory and comparison of MIGHTI measurements with several TIE-GCM runs.

This study examines the ionospheric response during a minor magnetic storm under deep solar minimum conditions using the Global Ionospheric Specification (GIS) electron density constructed by using the radio occultation (RO) slant total electron content (TEC) measurements of the FORMOSAT-7/COSMIC-2 (F7/C2) constellation and ground-based global navigation satellite system (GNSS) TEC. The equatorial ionization anomaly (EIA) crest density increased by about ~300%, with a localized region of more intense enhancements over the European sector. These enhancements are validated with global ionosphere map (GIM) TEC data product and ground based GNSS observations. The vertically resolved electron density structures reconstructed by the GIS help us to understand the physical processes giving rise to such unexpectedly intense ionosphere responses during the storm.  The altitude distribution and poleward shift of the EIA crests indicate that prompt penetration electric fields (PPEF) play an important role in producing the observed positive storm responses, with the storm-induced equatorward circulations likely contributing to an accumulation of plasma that is competing against recombination losses. In addition, storm-time thermosphere composition changes, which appear to be more effective under deep solar minimum conditions, might also play a crucial role in producing these large low-latitude enhancements during a relatively minor storm.

The FORMOSAT‐7/COSMIC‐2 (F7/C2) satellite mission was launched on 25 June 2019 with six low‐Earth‐orbit satellites and can provide thousands of daily radio occultation (RO) soundings in the low‐latitude and midlatitude regions. This study shows the preliminary results of space weather data products based on F7/C2 RO sounding: global ionospheric specification (GIS) electron density and Ne‐aided Abel and Abel electron density profiles. GIS is the ionospheric data assimilation product based on the Gauss‐Markov Kalman filter, assimilating the ground‐based Global Positioning System and space‐based F7/C2 RO slant total electron content, providing continuous global three‐dimensional electron density distribution. The Ne‐aided Abel inversion implements four‐dimensional climatological electron density constructed from previous RO observations, which has the advantage of providing altitudinal information on the horizontal gradient to reduce the retrieval error due to the spherical symmetry assumption of the Abel inversion. The comparisons show that climatological structures are consistent with each other above 300 km altitude. Both the Abel electron density profiles and GIS detect electron density variations during a minor geomagnetic storm that occurred within the study period. Moreover, GIS is further capable of reconstructing the variation of equatorial ionization anomaly crests. Detailed validations of all the three products are carried out using manually scaled digisonde NmF2 (hmF2), the results show that both GIS and Ne‐aided Abel are reliable products in studying ionosphere climatology, with the additional advantage of GIS for space weather research and day‐to‐day variations.

In recent years, geomagnetically induced currents (GIC) in national power networks by space weather have been widely recognised as a serious threat to global socio-economic stability; with extreme geomagnetic storms likely to cause the widespread collapse of national power grids, both predicting and mitigating these currents is of major international importance. New Zealand is fortunate to have a significant amount of dense, high-quality GIC measurements from the national power-grid, which provides us with a unique opportunity to not only study the impact of GIC on the electricity network, but the potential to forecast the occurrence of GIC and protect the network from GIC-related damage through mitigation protocols. In this presentation we will outline the aims and objectives of our current project to investigate GIC from a New Zealand context. The primary scientific questions have been developed in consultation with the New Zealand energy industry, spanning both electrical power networks and gas distribution pipelines. Our actvities include the creation of a national magnetometer array and the execution of a dense magnetotelluric survey, both of which will feed into a state-of-the-art induction model allowing us to estimate the impact of an extreme solar storm. From this modelling, we hope to develop GIC prediction and mitigation techniques applicable not only to New Zealand, but to other mid- and low-latitude countries at risk of GIC.

Taking into account the recently discovered response of seismicity to artificial electromagnetic impacts on the Earth crust [1,2], a hypothesis is proposed on a possibility of earthquake triggering by severe space weather conditions that is currently under discussion [3]. One of the most significant manifestations of space weather on the Earth is geomagnetically-induced electric currents (GIC), excited in the surface layers of the Earth and conductors during sharp changes in the geomagnetic field. Practically unexplored are GIT in the conductive fault zones of the Earth crust, as well as their impact on deformation processes in the earthquake sources. It should be noted that, due to the saturation of crustal faults with highly mineralized fluids or graphitization of the fault, its electrical conductivity may exceed the conductivity of the host rocks by several orders of magnitude. The numerical estimations demonstrated that under strong disturbances of the geomagnetic field of ~ 10² nT during strong solar flares or geomagnetic storms the GIC density in a conductive fault can reach 10^-6 A/m² that is an order of magnitude higher than the current density generated in earthquake source by artificial pulsed power systems [1]. Thus, under certain conditions (the level of the stress-strain state of a fault, its conductivity and orientation), telluric currents (GIC) excited in faults by sharp variations of geomagnetic field can trigger earthquakes. The results obtained were confirmed by observation of sharp increase of global seismicity after the strong solar flare of X9.3 class on September 6, 2017 followed by strong magnetic storm (Kp>8). The reported study was funded by RFBR and NSFC, project number 21-55-53053. References: [1]. Zeigarnik V. et al (2018) Geoph Res Abstr 20: EGU2018-15436-1. [2]. Novikov V.A. et al (2017) Earthq. Sci. 30, 4, 167-172. [3]. Sorokin V.M. et al ( 2019) Earthq. Sci., 32, 1, 26-34.

Thanks to recent advances in radio interferometric instrumentation, we’ve entered a new era of solar radio observations—broadband dynamic imaging spectroscopy. In this talk, I will first introduce the history of solar radio observations based on either dynamic spectral measurements radiation integrated over the Sun or imaging at a few discrete frequencies, then review some recent progress based on dynamic imaging spectroscopy over a wide frequency range that has placed us in a strong position to make revolutionary breakthroughs in measuring the coronal magnetic fields. Future perspectives will also be briefly discussed.

Magnetic flux ropes are the centerpiece of solar eruptions. Direct measurements for the magnetic field of flux ropes are crucial for understanding the triggering and energy release processes, yet they remain heretofore elusive. Here we report microwave imaging spectroscopy observations of an M1.4-class solar flare occurred on 2017 September 6, using data obtained by the Expanded Owens Valley Solar Array. This flare event is associated with a failed eruption of a twisted filament observed in Hɑ by the Goode Solar Telescope at the Big Bear Solar Observatory. The filament, initially located along the magnetic polarity inversion line prior to the event, undergoes a failed eruption during the course of the flare. The upper portion of the erupting filament has a counterpart in microwaves, whose spectral properties indicate gyrosynchrotron radiation from flare-accelerated nonthermal electrons. Using spatially resolved microwave spectral analysis, we derive the magnetic field strength along the filament spine, which ranges from 600--1400 Gauss from its apex to the legs. The results agree well with the non-linear force-free magnetic model extrapolated from the pre-flare photospheric magnetogram. The multi-wavelength signatures of the event are consistent with the standard scenario of eruptive flares, except that the eruption failed to fully develop and escape as a coronal mass ejection. We conclude that the failed eruption is likely due to the strong strapping coronal magnetic field above the filament.

In the standard model of solar flares, a large-scale reconnection current sheet is postulated to be the central engine for powering the flare energy release and accelerating particles. However, where and how the energy release and particle acceleration occur remain unclear owing to the lack of measurements of the magnetic properties of the current sheet. Here we report the measurement of the spatially resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a "magnetic bottle". This spatial structure agrees with theoretical predictions and numerical modeling results. A strong reconnection electric field of about 4,000 V m^-1 is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons instead concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is probably the primary site for accelerating and confining the relativistic electrons.

The interaction between the magnetic field and an atom/ion breaks the symmetry of an atomic system allowing atomic states with the same magnetic quantum number and parity to mix and gives rise to a magnetic-field-induced transition (MIT) to appear in spectra. The intensities of these lines have a strong, to first order quadratic dependence on the external field strength and therefore can be used for magnetic field diagnostics. It was found that a pseudo-degeneracy of the 3p⁴3d ⁴D_7/2,5/2 occurred in Fe X, and an external magnetic field could induce a mixing of these two levels and thereby cause an MIT at 257.26 Å from the ⁴D_7/2 to the ²P^o_3/2 level. The pseudo-degeneracy of the two levels for Fe X causes this MIT to be sensitive to the relatively small magnetic fields expected in the solar corona. Recent studies have illustrated the potential of this method that allows the measurement of the coronal magnetic field strength utilizing spectra lines observed by the Hinode/EIS satellite. This new method can provide 2D and temporal resolution maps of the magnetic field from the solar disk to the limb, complementing the DKIST and CoMP ground-based observatories, and extending our reach to the disk for which the only measurements are done using radio measurements.

Coronal magnetic field diagnostics plays a crucial role in advancing our understanding on the mechanisms of a variety of solar activities, including solar flares and coronal mass ejections (CME). Here we report results from the microwave spectral imaging observation from the Expanded Owens Valley Solar Array (EOVSA) of a C1.4 flare occurring on 2017 July 14^th. We obtained information on the evolving coronal magnetic field in the flare region using the microwave spectra fitting technique. We also examined the temporal and spatial correspondence between the magnetic field evolution and atmospheric dynamics observed by the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (SDO/AIA) and the Interface Region Imaging Spectrograph (IRIS). The analysis result suggests that the energy released during the decay of magnetic field directly contributes to the particle acceleration and plasma heating in the active region. Our study demonstrates that microwave spectral imaging observations are capable of determining the evolving coronal magnetic field during small-scale solar flares, thus shedding new light on the details of magnetic reconnection and particle acceleration in these eruptive events.

Magnetic field is the key to understand extreme activities, such as flares and coronal mass ejections, in the solar and stellar coronae. However, measurements of the coronal magnetic fields are limited. Recently, it was proposed that the intensity ratios of some EUV spectral lines from the Fe X ion can be used to measure the magnetic field in the solar corona based on the theory of magnetic field induced transitions (MIT). In order to test the validity of this method, we performed forward modeling with different 3D MHD models of the solar atmosphere. We first synthesized emissions of the Fe X lines from each model, then calculated a magnetic field based on the line ratios. Finally, we compared the magnetic field derived from the Fe X line pairs to that in each model to evaluate the accuracy of magnetic field measurements based on the MIT theory. We also compared the results using different atomic databases and different Fe X line pairs. Moreover, we also performed forward modeling with a series of 3D stellar MHD models that correspond to different levels of stellar activity, and discussed the possibility of magnetic field measurements of the stellar coronae based on the MIT theory.

The external magnetic field introduced as an additional Hamiltonian will cause additional mixing of some adjacent energy levels. This will produce some new transition channels, namely magnetic field induced transition. By observing the intensities of these spectral lines whose intensity is proportional to the square of the external magnetic field, it is expected to realize more comprehensive real-time detection of the coronal magnetic field. However, in order to achieve this method, we need to calibrate the relationship between the magnetic field and the actual spectral line intensity, while there were many theoretical calculations already, but we found some inconsistencies between the calculation and the astrophysical observation. In this work, we use the electron beam ion trap device which can simulate the coronal plasma environment to observe the spectral line intensity under controllable plasma parameters. We find that there is an obvious systematic deviation between the observation results and the theoretical expectation and with a same trend. So more extensive experiments for this deviation is arranged accordingly, which possibly achieve a more reliable and experimental evaluated method to measure the coronal magnetic field.

We examined wave perturbations of the geomagnetic field of the Pi2 and Pc3 type (periods of 1–2 min) recorded simultaneously by magnetometers at low latitude stations in the Indian sector and low-orbit SWARM satellites. At night, the Pi2 waves in the upper ionosphere and on Earth are almost identical in amplitude and in phase. These waves on the satellite are mainly manifested in the field-aligned and radial magnetic components. Night-time low-latitude Pi2 signals are supposedly produced by magnetospheric fast magnetosonic waves propagating through the opacity region to the Earth. The results of analytical estimates and numerical simulation with the model of interaction of MHD waves with the ionosphere – atmosphere – Earth system are consistent with the properties of Pi2 signals recorded in the upper ionosphere and on Earth.

In magnetohydrodynamics (MHD), magnetic reconnection has been traditionally discussed by Sweet--Parker and Petschek models. Recently, it was found that a laminar Sweet--Parker reconnection evolves to plasmoid-dominated turbulent reconnection in a large-scale system. The reconnection rate during the plasmoid-dominated stage is known to be 0.01, regardless of other parameters. Plasma beta in the inflow region is a key parameter in the reconnection system, and it is extremely low around reconnection sites in a solar corona. However, despite its importance, many aspects of the plasmoid-dominated reconnection in the low-beta regime remain unexplored, partly because of numerical difficulties.In this contribution, we explore basic properties of plasmoid-dominated reconnection in a low-beta background plasma, by means of resistive MHD simulations. We have found that the system becomes highly complex due to repeated formation of plasmoids and shocks. We have also found that the average reconnection rate increases in the beta < 1 regime, in contrast to popular results. We attribute this to compressible effects. Using a compressible reconnection theory, we have derived a scaling law to predict the rate. This was verified by extensive numerical survey. We will further present recent updates in our simulation code, OpenMHD. The latest version can be accelerated by NVIDIA GPUs. Reference:
S. Zenitani and T. Miyoshi, Astrophys. J. Lett., 894, L7 (2020)

To investigate the time history of spatial variation of global electron density in the ionosphere during geomagnetic storms, we conducted a super epoch analysis of interplanetary magnetic field (IMF), solar wind, geomagnetic indexes (AE and SYM-H), and global navigation satellite system (GNSS) – total electron content (TEC) data for 20 years (2000–2019). In this study, we analyzed the ratio of the TEC difference (rTEC) for 663 geomagnetic storms with the minimum SYM-H value of less than -40 nT. During the main phase, the rTEC enhancement appeared with a wide latitudinal range in the daytime (9–18 h, magnetic local time: MLT) mid-latitude (>30 degrees in geomagnetic latitude: GMLAT) in the northern and southern hemispheres. The enhanced rTEC region expanded to the midnight with time. Around the noon, the rTEC enhancement related to the storm-enhanced density (SED) plume was observed at high latitudes (60–70 degrees, GMLAT). In the low-latitude and equatorial region (<30 decrees, GMLAT), the clear rTEC enhancement occurred in the evening – morning sector after -6 h epoch time. During the recovery phase, the magnitude of the rTEC enhancement at mid-latitudes decreased rapidly within 4 h, and the rTEC value at high latitudes became negative in all the MLT range. The equatorial (<10 degrees, GMLAT) rTEC value increased significantly in the nighttime while that in the off equatorial region decreased slightly in a wide MLT range (8–24 h, MLT). These situations persisted at least for 16 h. The equatorial rTEC signatures imply that an over-shielding or disturbance dynamo field suppresses the equatorial fountain effect.

Space plasmas display fluctuations and nonlinear behavior at a broad range of scales, being in most cases in a turbulent state. The majority of these plasmas are also considered to be heated, with dissipation of turbulence as a possible explanation. Despite of many studies and advances in research, many aspects of the turbulence, heating and their interaction with several space plasma phenomena (e.g., shocks, reconnection, instabilities, waves), remain to be fully understood and many questions are still open. Plasma irregularities and turbulence are believed common in the F-region ionosphere and because of their impact on the GNSS and the human activity in the polar regions, a detailed understanding is
required. This study provides a characterization of the turbulence developed inside the polar-cusp ionosphere, including features as intermittency, not extensively addressed so far. The electron density of ICI-2 and ICI-3 missions have been analyzed using advanced time-series analysis techniques and a standard diagnostics for intermittent turbulence. The following parameters have been obtained: the autocorrelation function, that gives useful information about the correlation scale of the field; the energy power spectra, which show the average spectral indexes ∼−1.7, not far from the Kolmogorov value observed at MHD scales, while a steeper power law is suggested below kinetic scales. In addition, the PDFs of the scale-dependent increments display a typical deviation from Gaussian that increase towards small scales due to intense field fluctuations, indication of the presence of intermittency and coherent structures. Finally, the kurtosis-scaling exponent reveals an efficient intermittency, usually related to the occurrence of structures.

A distinct class of aurora, called transpolar auroral arc (TPA) (in some cases called “theta” aurora), appears in the extremely high latitude ionosphere of the Earth when interplanetary magnetic field (IMF) is northward. The formation and evolution of TPA offers clues about processes transferring energy and momentum from the solar wind to the magnetosphere and ionosphere during a northward IMF. However, their formation mechanisms remain poorly understood and controversial. We report a new mechanism identified from multiple-instrument observations of unusually bright, multiple TPAs and simulations from a high-resolution three-dimensional global MagnetoHydroDynamics (MHD) model. The observations and simulations show an excellent agreement and reveal that these multiple TPAs are generated by precipitating energetic magnetospheric electrons within field-aligned current (FAC) sheets. These FAC sheets are generated by multiple flow shear sheets in both the magnetospheric boundary produced by Kelvin-Helmholtz instability between super-sonic solar wind flow and magnetosphere plasma, and the plasma sheet generated by the interactions between the enhanced earthward plasma flows from the distant tail (less than -100 R_E) and the enhanced tailward flows from the near tail (about -20 R_E). The study offers a new insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it challenges existing paradigms of the dynamics of the Earth’s magnetosphere.

Taking advantage of the high spatial-resolution and global coverage of DMSP/SSUSI observations, we investigated the critical interplanetary factors in controlling the cusp auroral emission by dividing the midday auroras into the gap (weak emission) and non-gap (intense emission) events. Although the cusp auroral intensity is essentially determined by a parameter related to the IMF direction, IMF magnitude (|B_y|), and solar wind speed (V), we found that the cusp aurora is statistically weak during the southward IMF but intense when the V and IMF |B_y| are greater. Further, we confirmed that even with V > 600km/s, the intense-aurora event still shows a minimum occurrence near the IMF |B_y| = 0. However, when the IMF |B_y| is greater, the V becomes less significant for the intense-aurora occurrence. These results demonstrate that the IMF |B_y| is critical in controlling the cusp auroral intensity, most likely by producing an electric field through V×B_y.

The subauroral polarization stream (SAPS) refers to persistent westward plasma flows driven by enhanced poleward electric fields in the subauroral ionosphere equatorward of the electron auroral precipitation zone. The presence of SAPS in the coupled ionosphere-magnetosphere plays an important role in controlling the formation and evolution of some large-scale features with significant space weather effects, such as enhanced ion vertical flows, main ionospheric trough, storm-enhanced density (SED), and sunward-convecting plasmaspheric drainage plumes. This study will discuss our recent findings of the SAPS-related ion-neutral coupling processes and associated ionosphere-thermosphere responses in the North American sector via both statistical and case analysis.

Helioseismic response to solar flares ("sunquakes") occurs due to localized force or/and momentum impacts observed during the flare impulsive phase in the lower atmosphere. Such impacts may be caused by precipitation of high-energy particles, downward shocks, or magnetic Lorentz force. However, the current theories of solar flares are unable to explain the origin of sunquakes. Our statistical analysis of M-X class flares observed by the Solar Dynamics Observatory during Solar Cycle 24 has shown that contrary to expectations, many relatively weak M-class flares produced strong sunquakes, while for some powerful X-class flares, helioseismic waves were not observed or were weak. The analysis also revealed that some active regions were characterized by the most efficient generation of sunquakes during the solar cycle. We found that the sunquake power correlates with maximal values of the X-ray flux derivative better than with the X-ray class. One could expect that significantly more powerful stellar flares produce strong asteroseismic responses. However, analysis of Kepler data failed to find starquake signals in global stellar oscillations.

Stellar flares are believed to share the same physical origin with solar flares. However, compared to their solar counterparts, our knowledge on the plasma dynamics during stellar flares and their connection to coronal mass ejections (CMEs) remains very limited. Using the time-resolved spectroscopic data from the CHANDRA X-ray Observatory, we have conducted a survey of X-ray stellar flares. To this end, we have established a flare catalogue for CHANDRA from its public data archive, in which 40 flare events were chosen because of the relatively complete coverage of different flare phases. Our investigation was focused on the Doppler shifts of several strong emission lines and the intensity variations in different spectral bands. Our preliminary results are the following: (1) in a couple of strong flare events, the high-temperature spectral lines reveal a transition from an obvious blueshift in the impulsive phase to a weak redshift in the decay phase, whereas the low-temperature lines mainly display persistent blueshift during the decay phase; (2) the intensity variations in some flare events show a possible “flaring-to-dimming” signature, possibly indicating the presence of an associated CME; (3) none of these X-ray flare events show a signature of late phase that is commonly observed in solar flares.  We attempt to give possible physical interpretations for these observational features and further discuss their implications for solar-stellar connection.

Active solar-type stars (G-type main-sequence stars) show some signatures of gigantic star spots and sometimes produce superflares. The observations of star-spots　emergence/decay can also lead to the understandings of the stellar superflares as well as the underlying stellar dynamo. However, the temporal evolution of star spots has been hardly measured, especially on solar-type stars. In this talk, we report the measurements of the temporal evolution of individual star-spot area on solar-type stars by using Kepler photometric data. We estimated it (i) by modeling of the rotational modulations of spotted solar-type stars (Namekata et al. 2019) and (ii) by modeling of the small stellar brightness variation due to the spots during exoplanet transits (Namekata et al. 2020a). We successfully obtained temporal evolution of individual star spots showing clear emergence/decay and derived the statistical values of the lifetimes and emergence/decay rates of star spots. As a result, we found that their lifetimes are ranging from 10 to 350 days when spot areas (A) are 0.1-2.3% of a solar hemisphere (SH). They are much shorter than those extrapolated from an empirical relation of sunspots while being consistent with other research on star spot lifetimes. The emerging/decay rates of star spots on solar-type stars are obtained for the first time in this study and they are derived to be typically 5×10²⁰ Mx/h with the area of 0.1-2.3% of SH. In contrast to lifetimes, it is found that the emergence/decay rates of star spots are mostly consistent with those expected from sunspots observations (Petrovay et al. 1997, Norton et al. 2017). This supports that the emergence/decay mechanisms of gigantic star spots (0.1-2.3% of SH) are probably the same as those of sunspots (< 0.5% of SH), which can constrain the stellar dynamo theory.

Flares are energetic explosions in the stellar atmosphere, and superflares are the flares having the energy 10-10⁶ times larger than that of the largest solar flares. It had been thought that superflares cannot occur on slowly-rotating stars like the Sun. Recently, many superflares on solar-type (G-type main-sequence) stars were found in the initial 500 days data obtained by the Kepler space telescope (e.g., Maehara et al. 2012). Notsu et al. (2019) conducted precise measurements of the stellar parameters and binarity check, using spectroscopic data and the Gaia-DR2 data. However, the number of Sun-like (slowly-rotating solar-type) superflare stars were very small.We report the latest statistical analyses of superflares on solar-type stars using all of the Kepler primary mission data and Gaia-DR2 catalog. We updated the flare detection method by using highpass filter to remove rotational variations caused by starspots. We also examined the sample biases on the frequency of superflares, taking into account gyrochronology and flare detection completeness. The sample size of solar-type stars and Sun-like stars are ~4 and ~12 times, respectively, compared with Notsu et al. (2019). As a result, we found 2341 superflares on 265 solar-type stars, and 26 superflares on 15 Sun-like stars. This enabled us to have a more well-established view on the statistical properties of superflares. The observed upper limit of the flare energy decreases as the rotation period increases in solar-type stars. The frequency of superflares decreases as the stellar rotation period increases. The maximum energy we found on Sun-like stars is 4x10³⁴ erg. Our analysis of Sun-like stars suggest that the Sun can cause superflares with energies of ~7x10³³ erg (~X700-class flares) and ~1x10³⁴ erg (~X1000-class flares) once every ~3,000 years and ~6,000 years, respectively (Okamoto et al. 2021, ApJ, 906, 72). 

Solar flares are sudden increase of brightness on the sun, interpreted as the result of magnetic reconnection occurring in the solar atmosphere. Sun-like stars also manifest similar phenomena in their atmosphere, though the order of magnitude is much larger. For comparing the stellar flares with the counterparts happening on the sun, we perform this study by examining the short cadence data (1-min time resolution) of stellar light curves obtained by the Kepler mission. We refer the concept of sun-like stars by the three criteria: a) the effective temperature is among the range of 5600 to 6000K；b) the surface gravity (log g) is greater than 4.0; c) it should be a single star. We sought out nearly 200 flare samples from the selected sun-like stars, excluding some samples with data-gap, overlapping flare signals, or very low signal-to-noise ratio. Then, we carried out statistical analyses on the characteristic times of the samples and evaluated some key parameters. The median duration of the flare rising phase is about six minutes and the median duration of the decay phase is about 24 minutes. This result basically agrees with the previous studies on solar flares. In addition, the morphology of the flare light-curve profiles can be classified to compact flares and long-duration flares, akin to that of the solar flares. This suggests that stellar flares and solar flares may follow the same physical process. Furthermore, minutes-scale flares must take place in a local region on the stars, just like the sun.

Starspots are apparent manifestations of stellar magnetic activities, such as stellar flares, on the stellar surface, and they can be ubiquitously observed on M, K, and G-type stars. Spots fluctuate the light curves by rotating in and out of the line of sight, and the periodicity and amplitude of the light curves provide information on the location and size of spots, their emergence and decay rates, and the stellar differential rotation. However, it is difficult to estimate such many parameters simultaneously by simple analyses since there are many spots on the surface. Therefore, we implement a computational code to decipher the surface information of stars from their light curves (Ikuta et al. 2020). The code enables to deduce many parameters by a parallel tempering and to calculate the model evidence for comparing the number of spots. First, to evaluate the performance, we revisit synthetic light curves emulating Kepler data. Light curves are produced with three spots and optimized by three, two, and four-spot models. We found that parameters can be unimodally deduced in three and two-spot models, deduced parameters of the three-spot model are consistent with the input values, and the three-spot model is the most preferable. Then, the number of spots could be determined for observational data. Second, we apply this code to the light curves of bright M-type flare stars AU Mic, EV Lac, and YZ CMi observed by TESS. These targets have been observed by spectroscopic monitoring of their flares, and they are suitable for study of the relation between spots and flares. As a result, the positions and sizes of the spots are uniquely deduced and almost consistent with studies by Doppler Imaging techniques. The phase of spots is suggested to be uncorrelated with the flares by a simple analysis (Ikuta et al. 2021, in preparation).

Low-mass stars on the main sequence are known to spin down over time, which is likely caused by the angular momentum loss by magnetized stellar wind (magnetic braking). Once the stellar intrinsic magnetic field is weakened by the spin-down, the efficiency of magnetic braking also becomes weaker. The stellar rotational evolution is described by this dynamo-wind feedback mechanism, and thus, knowing the properties of the stellar wind at different ages (or rotation rates) is crucial. Motivated by the above background, we investigate the rotation dependence of the stellar wind properties (velocity, mass-loss rate, angular-momentum-loss rate) using numerical simulation. The simulation is an extended version of the self-consistent solar wind model. In connecting the global magnetic field and stellar rotation rate, we utilized the recent Zeeman-Doppler Imaging results. We have obtained a scaling law of the angular momentum loss rate that accounts for the observed stellar rotational evolution. Meanwhile, the mass-loss rate is found to saturate with respect to fast rotation, contradictory to the conventional understanding. The saturation of the mass-loss rate is caused by the Alfven-wave energy loss in the chromosphere, and thus, the energy propagation in the chromosphere is never negligible in the context of the stellar-wind modeling.

Large-scale solar eruptions are believed to have a magnetic flux rope as the core structure. However, it remains elusive as to how the flux rope builds up and what triggers its eruption. Recent observations found that a prominence erupted following multiple episodes of "flux feeding." During each episode, a chromospheric fibril rose and merged with the prominence lying above. In this Letter, we carried out 2.5-dimensional magnetohydrodynamic (MHD) numerical simulations to investigate whether the flux-feeding mechanism can explain such an eruption. The simulations demonstrate that the discrete emergence of small flux ropes can initiate eruptions by feeding axial flux into the preexistent flux rope until its total axial flux reaches a critical value. The onset of the eruption is dominated by an ideal MHD process. Our simulation results corroborate that the flux feeding is a viable mechanism to cause the eruption of solar magnetic flux ropes.

We report results from a high resolution global simulation study of the ionosphere/thermosphere system that self-consistently generates large-scale equatorial spread F (ESF) plasma bubbles. The coupled model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere codes WACCM-X and HIAMCM. An important feature of the study is that a high resolution grid is used globally with scale sizes 50 km. Several cases are studied for different seasons and solar activity. We show one case that is consistent with recent GOLD observations [Huba and Liu, 2020]. In addition to seasonal and solar activity variations in the onset of ESF, we also find a strong longitudinal and day-to-day variation. Importantly, the bubbles are initiated self-consistently by atmospheric gravity waves in the WACCM-X and HIAMCM models as opposed to artificial, numerical ionosphere perturbations. Huba, J.D. and H.-L. Liu, Global modeling of equatorial spread F  with SAMI3/WACCM-X, Geophys. Res. Lett.,  47, e2020GL088258. https://doi.org/10.1029/2020GL088258
2020.

The formation of equatorial irregularities is influenced by the electrodynamic environment of the equatorial ionosphere. Improvements in the specification and forecasting of the equatorial electrodynamics is thus crucial for improving current understanding of, and potentially predicting, the occurrence of equatorial irregularities. By capturing the day-to-day variability coming from the lower atmosphere, whole atmosphere data assimilation systems offer the potential for improved specification of the ionosphere electrodynamics. The Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCMX) combined with the Data Assimilation Research Testbed (DART) ensemble Kalman filter is used to demonstrate the impact of whole atmosphere data assimilation on the ionospheric electrodynamics. The impact of assimilating lower and middle atmosphere observations on capturing the day-to-day variability of the equatorial ionosphere will be presented. Results will also be presented demonstrating the impact of assimilating Ionosphere Connection Explorer (ICON) Michelson Interferometer for Global High-resolution Thermosphere Imaging (MIGHTI) neutral wind observations in WACCMX+DART. Assimilation of the ICON/MIGHTI observations improves the specification of the thermosphere neutral winds, which also leads to improvements in the equatorial electrodynamics in WACCMX+DART.   

The Naval Research Laboratory first-principles ionosphere model SAMI3/ESF is performed to study the interaction between the nighttime medium-scale traveling ionospheric disturbances (MSTIDs) and equatorial plasma bubbles (EPBs). The synthetic dynamo currents are imposed into the potential equation to induce polarization electric fields for generating the MSTIDs. Simulations demonstrate that the MSTIDs can inhibit the upward growth of EPBs; however, MSTIDs alone are insufficient to explain the disappearance of EPBs. We found that the meridional winds likely contribute to the disappearance of MSTIDs by reducing the background electron density and polarization electric fields within the EPBs. Then, the MSTIDs transport plasma to fill the EPB depletions up via E × B drifts. Both MSTIDs and meridional winds are necessary to comprehend the underlying mechanism of EPB disappearance. We also found that the zonal and vertical E × B drifts within the MSTIDs affect the morphology of EPBs, leading to a reverse-C shape structure.

Several studies for the F-region ionosphere associated with volcano eruptions based on GPS-total electron content (TEC) data have been reported so far (e.g., Heki, 2006). These studies reported that acoustic waves excited by volcano eruptions reach up to the F-region ionosphere and caused F-region perturbations. After eruption of the Kelud Volcano, Indonesia, in February 2014, acoustic resonance between the Earth’s surface and lower thermosphere was reported based on TEC data and the seismic wave data (Nakashima et al., 2015). However, little studies on the D-region ionosphere associated with volcano eruptions have been reported. In this study, we investigate the D-region ionospheric effects of 2016 eruptions of Mt. Aso, Japan, using intensity of low frequency (LF, 30-300 kHz) transmitter signals. The LF propagation paths used in this study were JJY (JJY60) - Sasaguri (SGR, Japan, Observatory of Kyushu University), JJY (JJY40) - SGR, and BPC (68.5 kHz) -SGR. Mt. Aso erupted at 16:46 UT on 7 October, 2016. The volcanic explosivity index (VEI) was 3 out of 8, and the eruption height was 11 km. The LF intensity on all three paths varied with frequency of 2-5 mHz based on wavelet spectra during 16:53-17:20 UT after the eruptions. We compared the perturbations with atmospheric pressure data obtained by the Kochi university of Technology Infrasound Sensor Network, and seismic waves in the NIED Full Range Seismograph Network of Japan (F-net) data. The atmospheric pressure and vertical velocity of the seismic waves had the similar frequencies of 3-6 mHz during 16:46-17:20 UT. These similar frequencies suggest that the perturbations would be caused by acoustic resonance between the Earth's surface and lower thermosphere, or by acoustic and atmospheric gravity waves generated by the volcanic eruptions. In the presentation, we will discuss the cause of the VLF/LF perturbations in more detail.

Previous studies have proposed that both thermospheric neutral wind and equatorial electrojet (EEJ) near sunset play important roles in the pre-reversal enhancement (PRE) mechanism. In this study, we examined the influences of the eastward neutral wind and the EEJ on the PRE strength using observations in the equatorial region of Southeast Asia. We employed the data of the zonal (east-west direction) neutral wind at an altitude of ~250 km (bottomside F region) at longitudes of 90°–130°E in the dusk sector from the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite. We utilized three ionosondes at Chumphon (dip lat.: 3.0°N) in Thailand, at Bac Lieu (dip lat.: 1.7°N) in Vietnam, and at Cebu (dip lat.: 3.0°N) in Philippines to derive the PRE strength. Two magnetometers at Phuket (dip lat.: 0.1°S) in Thailand and at Kototabang (dip lat.: 10.3°S) in Indonesia were used to estimate the EEJ strength. We collected the data from those observations during March–April and September–October in 2010–2013. We particularly accumulated the data on the days with the magnetically quiet conditions. We have found that the eastward neutral wind and EEJ are closely correlated with PRE with cross-correlation coefficients of 0.42 and 0.47, respectively. The relationship between the eastward neutral wind and the EEJ has a weaker cross-correlation coefficient (0.26). Our observations suggest that both eastward neutral wind and the EEJ near sunset are involved in the PRE mechanism, and both parameters could in a balanced manner control the PRE magnitude. Based on the relationship between the wind and the EEJ, these two parameters could be independent of each other. Thus, they could be also independent to control the PRE.

A sudden stratospheric warming (SSW) is a large-scale meteorological phenomenon, which is known to disturb the whole atmosphere. An SSW usually takes place in the Arctic region during winter months. In September 2019, a rare SSW occurred in the Antarctic region, providing a unique opportunity to study its impact on the middle and upper atmosphere. Geopotential height measurements by the Microwave Limb Sounder aboard NASA's Aura satellite reveal a burst of westward-propagating quasi-6-day wave (Q6DW) with zonal wavenumber 1 in the mesosphere and lower thermosphere following the SSW. At this time, ionospheric data from ESA's Swarm satellite constellation mission show prominent 6-day variations in the daytime equatorial electrojet intensity and low-latitude plasma densities. The whole atmosphere model GAIA reproduces salient features of the middle and upper atmosphere response to the SSW. GAIA results suggest that the observed ionospheric 6-day variations are not directly driven by the Q6DW but driven indirectly through tidal modulations by the Q6DW. An analysis of global total electron content data reveals signatures of secondary waves arising from the nonlinear interaction between the Q6DW and tides.

Free traveling Rossby wave normal modes (RNMs) are often investigated through large-scale space-time spectral analyses, which therefore is subject to observational availability, especially in the mesosphere. Ground-based mesospheric observations were broadly used to identify RNMs mostly according to the periods of RNMs without resolving their horizontal scales. The current study diagnoses zonal wave numbers of RNM-like oscillations occurring in mesospheric winds observed by two meteor radars at about 79°N. We explore four winters comprising the major stratospheric sudden warming events (SSWs) 2009, 2010, and 2013. Diagnosed are predominant oscillations at the periods of 10 and 16 days lasting mostly for three to five whole cycles. All dominant oscillations are associated with westward zonal wave number m=1, excepting one 16-day oscillation associated with m=2. We discuss the m=1 oscillations as transient RNMs and the m=2 oscillation as a secondary wave of nonlinear interaction between an RNM and a stationary Rossby wave. All the oscillations occur around onsets of the three SSWs, suggesting associations between RNMs and SSWs. For comparison, we also explore the wind collected by a similar network at 54°N during 2012–2016. Explored is a manifestation of 5-day wave, namely, an oscillation at 5–7 days with m=1), around the onset of SSW 2013, supporting the associations between RNMs and SSWs.

Sporadic E (Es) is a transient phenomenon where thin layers of enhanced electron density appear in the ionospheric E region (90-120 km altitude). Es can influence radio propagation, and its global characteristics have been of great interest to radio communications and navigations. The presence of neutral wind shear caused by atmospheric tides will lead ions to converge at E-region heights and form the so-called Es layers.
This research aims to examine the role of atmospheric solar and lunar tides in Es occurrence. For this purpose, radio occultation data of FORMOSAT-3/COSMIC, which provide complete global coverage for Ionospheric investigations, have been used. The results show both lunar and solar tidal signatures in Es occurrence. These tidal signatures are longitudinally dependent, which can result from non-migrating tides or modulation of migrating tidal signatures by zonally varying geomagnetic field. Moreover, GAIA (Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy) model data have been employed to evaluate the rate of vertical ion convergence due to solar tides

Solar and stellar flares are thought to be the rapid releases of magnetic energy through magnetic reconnection in the corona. Spectroscopic observations of stellar flares on M dwarfs have shown that blue-asymmetries (enhancement of the blue wing) in chromospheric lines (especially H-alpha) are often observed during flares. They are thought to be caused by upward motions of cool plasma (e.g., chromospheric evaporations, filament/prominence eruptions).Here we report the results from spectroscopic and photometric observations of the M-type flare star YZ CMi in the framework of the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER) collaborations during the Transiting Exoplanet Survey Satellite (TESS) observation period. We detected 4 H-alpha flares from the OISTER observations. One of them did not show clear brightening in the continuum; during this flare, the H-alpha line exhibited blue-asymmetry which has lasted for ~60 min. The line of sight velocity of the blue-shifted component is -80 -- -100 km/s.This suggests that there can be upward flows/motions of chromospheric cool plasma even without detectable brightening in the optical continuum. Under the assumption of that observed blue-asymmetry in H-alpha line was caused by a prominence eruption, the mass and kinetic energy of the upward-moving material are estimated to be 10¹⁶ -- 10¹⁸ g and 10^29.5 -- 10^31.5 erg, respectively. The estimated mass is comparable to expectations from the empirical relation between the X-rat flare energy and mass of solar coronal mass ejections (CMEs). In contrast, the estimated kinetic energy for the non-white-light flare on YZ CMi is roughly 2 orders of magnitude smaller than that expected from the relation between flare X-ray energy and kinetic energy for solar CMEs. This discrepancy could be understood by the difference in the velocity between CMEs and prominence eruptions (Maehara et al. 2021 PASJ, 73, 44).

Active solar-type stars sometimes show large `superflares' that may cause huge mass ejections. The possible stellar mass ejections can greatly affect the planetary environment and the stellar mass evolution. However, no observational indication of mass ejection has been reported especially for solar-type stars. We conducted spectroscopic monitoring observations of the active young solar-type star EK Draconis (EK Dra) by our new 3.8-m Seimei telescope, simultaneously with TESS satellite. Our time-resolved optical spectroscopic observation shows clear evidence for a stellar filament eruption associated with a superflare on the solar-type star (Namekata et al. submitted). After the superflare brighntenings with the radiated energy of 2.0×10³³ erg observed by TESS, a blue-shifted H-alpha absorption component with a velocity of -510 km s^-1 appeared. The velocity gradually decayed in 2 hour and the deceleration 0.34 km s^-2 was consistent with the surface gravity on EK Dra (0.30 ± 0.05 km s^-2), which means that the erupted mass is decelerated by stellar gravity. In order to compare it with solar observations, we obtained the-Sun-as-a-star H-alpha spectra of solar filament eruption/surge observed by the SMART telescope at Hida observatory. As a result of the comparison, we found that the temporal changes in the spectra of EK Dra greatly resemble that of solar mass ejections observed. Moreover, the ejected mass of 1.1×10¹⁸ g roughly corresponds to those predicted from the solar flare-energy/ejected-mass relation. These discoveries imply that a huge stellar filament eruption occurs possibly in the same way as solar ones. Our high-quality dataset can be helpful for future studies to estimate its impacts on the young-planet atmosphere around young solar-type stars as well as stellar mass/angular momentum evolution. 

The chromospheric activity level of a solar-type star can be indicated by the line-core emissions of the chromospheric spectral lines in the optical wavelength band, such as Ca II H&K, Hα, Ca II IRT, etc. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also named Guoshoujing Telescope) has the ability to observe optical spectra of thousands of celestial objects simultaneously through the 4000 fibers at its focal plane. After ten years of observation, the LAMOST sky survey has obtained more than 10 million stellar spectra. This large volume of stellar spectra by LAMOST sky survey provide a great opportunity for analyzing the overall properties of stellar chromospheric activity. A dedicated research project on the activities of solar-type stars based on the LAMOST sky survey was initiated at National Astronomical Observatories, Chinese Academy of Sciences (NAOC). In this presentation, we describe the data processing workflow of the project and give perspective on scientific yields.

We establish the largest eruptive/confined flare database to date and analyze 322 flares of \emph{GOES} class M1.0 and larger that occurred during 2010$-$2019, i.e., almost spanning the entire solar cycle 24. We find that the total unsigned magnetic flux ($\Phi$$_{AR}$) of active regions (ARs) is a key parameter in governing the eruptive character of large flares, with the proportion of eruptive flares exhibiting a strong anti-correlation with $\Phi$$_{AR}$. This means that an AR containing a large magnetic flux has a lower probability for the large flares it produces to be associated with a coronal mass ejection (CME). This finding is supported by the high positive correlation we obtained between the critical decay index height and $\Phi$$_{AR}$, implying that ARs with a larger $\Phi$$_{AR}$ have a stronger magnetic confinement. Moreover, the confined flares originating from ARs larger than 1.0$\times$$10^{23}$ Mx have several characteristics in common: stable filament, slipping magnetic reconnection and strongly sheared post-flare loops. Our findings reveal new relations between the magnetic flux of ARs and the occurrence of CMEs in association with large flares. These relations obtained here provide quantitative criteria for forecasting CMEs and adverse space weather, and have also important implications for ``superflares" on solar-type stars and stellar CMEs.

We study the light curves in multiple wavebands including white-light, Lyman-alpha, soft X-ray, and hard X-ray for a few tens of solar white-light flares. The shape of the white-light curves and the rise and decay times deduced from the light curves are investigated and compared with those of superflares on solar-type stars. We find that the white-light curve can show single and multiple peaks in both solar and stellar flares and that the white-light rise and decay times are correlated with the bolometric energy of flares. We also find that in solar flares, the Lyman-alpha rise and decay times have a relationship with the flare energy in Lyman-alpha, which are similar to the parameters in white-light. Based on the results of solar white-light flares as well as comparison with the stellar superflares, we can provide some information on energy release and properties of the white-light and Lyman-alpha emissions in solar and stellar flares.

M dwarf's atmosphere and wind is expected to be highly magnetized. The nonlinear propagation of Alfv\'en wave could play a key role in both heating the stellar atmosphere and driving the stellar wind. Along this Alfv\'en wave scenario, we carried out the one-dimensional compressive magnetohydrodynamic (MHD) simulation about the nonlinear propagation of Alfv\'en wave from the M dwarf's photosphere, chromosphere to the corona and interplanetary space. Based on the simulation results, we develop the semi-empirical method describing the solar and M dwarf's coronal temperature, stellar wind velocity, and wind's mass loss rate. We find that M dwarfs' coronae tend to be cooler than solar corona, and that M dwarfs' stellar winds would be characterized with faster velocity and much smaller mass loss rate compared to those of the solar wind.

Self-organized criticality (SOC) and turbulence are the two intrinsic physical processes during energy releases in a nonlinear dynamical system. Both of them can produce instabilities with power-law frequency distributions. The essential differences are that the SOC process predicts intermittent avalanches without interconnection, while the turbulent events exist memory-dependent correlation. In present study, we develop a new version of SOC cellular automaton (CA) model based on specific magnetic topology to simulate the flaring events. The statistical characteristics of the CA simulated flares are then compared with the ones of MHD simulation and real observation, that allows us to interpret the roles of SOC and MHD turbulence in solar and stellar eruptions.

It has been revealed that solar-like stars can produce massive "superflares". On the Sun, strong flares are almost always associated with active regions (ARs) that are large, complex, and rapidly evolving. While it is still difficult to spatially resolve the starspots and determine their structural complexity, one possible way to probe their evolutions and structures is to monitor the star in multiple wavelengths. In this study, we perform multi-wavelength irradiance monitoring of transiting solar ARs by using full-disk observation data from four satellites. As a result of this Sun-as-a-star spectral irradiance analysis, we find, for instance, that the near UV light curves show strong correlations with photospheric total magnetic flux and that there are time lags between the coronal and photospheric light curves while ARs are close to the limb, which together may enable one to discern how high bright coronal loops extend above stellar ARs. It is also found that the EUV light curves sensitive to transition-region temperatures are sometimes dimmed because the emission measure of 0.6–0.8 MK is reduced because the plasma is heated to higher temperatures over a wide area around the AR. These results demonstrate the potential of multi-wavelength photometry for obtaining the information of stellar ARs.