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.