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.