Keisuke Hosokawa

On June 1, 2025, an intense coronal mass ejection (CME) arrived at Earth and caused a large geomagnetic storm. Immediately after the arrival of the CME structure including an intense southward-directed interplanetary magnetic field (IMF), the sporadic E layer (EsL) observed over Japan suddenly disappeared or weakened. In this study, we examined the abrupt EsL disappearance/weakening based on ionosonde and very-high-frequency radio wave observations. The observations showed that Es across Japan vanished at approximately 06 UT on June 1, 2025, and reappeared 1–3 h later, during a period when the IMF was predominantly southward. Variations in the F region height suggested that the storm-time electric fields penetrate the mid-latitude ionosphere and cause the EsL disappearance/weakening. A numerical simulation was conducted and supported the suggestion. The simulation generally reproduced the disruption/weakening and subsequent reappearance of the EsL over Japan, demonstrating that the storm-time electric fields associated with intense CME can drive the strong upward ion motion and disrupt the EsL above ~110 km. After the storm-time electric fields weaken, wind shear recreates the EsL. We provide further evidence that storm-time electric fields can significantly alter the EsL dynamics at mid latitudes during geomagnetic storms as well as wind shear and clarify its underlying mechanism.

Auroral emissions modulated at geomagnetic pulsation periods signal magnetospheric wave–particle interactions, yet how pulsations synchronize chorus waves, electron scattering, precipitation, and aurora has not been fully understood. Here we analyze a well-conjugated event observed by the Arase satellite, the EISCAT radar, and auroral imaging, combining high-time-, energy-, and pitch-angle-resolution satellite measurements with high-energy-resolution ionospheric precipitation measurements. These observations reveal synchronization among chorus waves, electron pitch-angle scattering, precipitating electron fluxes, and diffuse aurora at the pulsation period, linking chorus-driven scattering to periodic precipitation. We further suggest that geomagnetic pulsations regulate chorus waves not only by modulating source electrons but also by facilitating chorus-wave propagation. The westward propagation of the modulated diffuse aurora was consistent with that of the geomagnetic pulsations, indicating that the spatial propagation of precipitation reflected the geomagnetic pulsation structure. These results show that geomagnetic pulsations can organize chorus-driven precipitation in both time and space, revealing multiscale magnetosphere–ionosphere coupling.

Pulsating aurora (PsA) is one of the common diffuse auroras typically generated by ∼10 keV electron precipitation. Precipitating electrons are scattered via wave‐particle interactions with whistler‐mode chorus waves in the magnetosphere. Previous studies have observationally demonstrated that, during PsAs, highly energetic electrons with energies ranging from a few hundred keV to a few MeV simultaneously penetrate into lower altitudes and can cause ozone depletion in the mesosphere and upper stratosphere. Recent attention has focused on the importance of high‐latitude propagation of chorus waves through magnetospheric density ducts for the precipitation of such highly energetic electrons. Our previous study showed that patchy structures of PsAs reflect the cross section of ducts. However, no statistical study has examined the relationship between ducted propagation of chorus waves and PsAs using ground‐satellite conjugate observations. Thus, we perform a statistical analysis to evaluate this relationship using data from the ground‐based optical network in Scandinavia and the Arase satellite. The results show that when chorus waves propagate to higher latitudes while maintaining their electromagnetic properties, PsAs exhibit patchy structures with more than 90% probability, and occurrence increases from midnight to the morning sector. In addition, the background electron density irregularities indicative of ducts were also observed in four events during high‐latitude propagation of electromagnetic chorus waves. For these events, the equatorial projection sizes of PsA patches ranged from several hundred to several thousand kilometers, comparable to the scale required to confine chorus waves. These observations suggest the horizontal scale of ducts at the magnetic equator.

This study highlights the need to re‐evaluate energy transfer in the upper thermosphere at high latitudes through analysis of 9‐year measurements of neutral wind ( u ) from a Fabry‐Perot interferometer and ion velocity ( v ) from a Dynasonde in Tromsø, Norway. This study introduced a ratio R (thermospheric energy conversion ratio) of the Joule heating rate, j · E ′ , to the electromagnetic energy transfer rate, j · E , that is, j · E ′ / j · E , where j and E are the current density and the electric field, respectively, and E ′ = E + u × B . Evaluating the energy transfer process using the ratio R does not require ionospheric conductivity information, which is an advantage of this method. This study found that the sign of u · v is associated with the energy transfer process at F‐region altitudes. In the dusk‐midnight sector, we found 0 < R < 1 under moderately active conditions, indicating that electromagnetic energy is converted to thermospheric thermal energy through Joule heating and kinetic energy via wind acceleration by the Lorentz force. R < 0 occurred under quiet conditions throughout the night and at post‐midnight across geomagnetic activity levels, indicating the Lorentz force performs negative work on the thermosphere. R can exceed 1 under quiet dusk‐to‐dawn and moderately active post‐midnight, with all instances of u · v < 0 being classified as R > 1. During evening and morning with u · v < 0, sunward flow dominates ionospheric plasma convection, causing ion drag to oppose the day‐night pressure gradient. However, ion drag cannot overcome the pressure gradient due to low geomagnetic activity, resulting in opposing thermospheric wind and ionospheric plasma convection directions.

Pulsating aurorae are prominent auroral emissions in the polar regions, typically occurring in the morning hours during the recovery phase of auroral substorms. These aurorae usually consist of round‐shaped patches of emission, with luminosity that pulsates at intervals ranging from less than a second to several tens of seconds. Here, we present, for the first time, a unique case of a pulsating aurora that expanded radially outward in all the directions and repeatedly formed a ring‐shaped structure. The speed of expansion, which was at least several tens of kilometers per second at ionospheric altitudes, cannot be attributed to the horizontal convective motion of plasma in the ionosphere. In the magnetosphere, corresponding to the expanding ring‐shaped aurora, the Arase satellite detected successive enhancements of natural electromagnetic waves known as a “chorus.” These chorus waves scatter energetic magnetospheric electrons into the ionosphere, resulting in pulsating diffuse aurorae. Notably, the satellite observed systematic delay in the timing of chorus detections, which suggests that a similar circularly expanding feature existed in space. These simultaneous observations of expanding features in both the ionosphere and the magnetosphere demonstrate that the temporal evolution of the shape of a pulsating aurora manifests the spatiotemporal evolution of the source of plasma waves in space.

On 23 November 2022, during exceptionally quiet geomagnetic conditions, a distinct ultra‐low frequency (ULF) wave in the Pc5 band (∼2.4 mHz) was observed in the duskside sub‐auroral region by two Canadian SuperDARN (Super Dual Auroral Radar Network) radars. The wave exhibited a periodic Doppler velocity signature resembling a “caterpillar,” characterized by anti‐sunward propagation and an azimuthal wave number of ∼12. Ground‐based magnetometer data revealed latitudinal variations in wave amplitude and phase consistent with field line resonance, peaking at ∼66° magnetic latitude. The Arase satellite, whose ionospheric footprint traversed the region of the caterpillar ULF wave, detected toroidal oscillations in electric and magnetic fields at magnetically conjugate locations in the inner magnetosphere, with high coherence and phase consistency with ground‐based observations. Arase observed the electric field variation preceding the magnetic field variation by ∼45°, which is unusual for a pure standing mode and suggests a departure from a simple standing‐wave interpretation. Instead, the observations can be interpreted as mixed standing and propagating characteristics along the field line. Simultaneously, the clear anti‐sunward azimuthal phase propagation and sustained wave activity suggest a driving mechanism by a large‐scale surface perturbation at the magnetospheric flank. These properties are consistent with excitation by the Kelvin‐Helmholtz instability. A minor enhancement in solar wind dynamic pressure was observed, potentially contributing to the wave onset. This study highlights SuperDARN's capability to detect ULF waves during quiet intervals and demonstrates the utility of multi‐instrumental conjugate observations in revealing the spatiotemporal variations of ULF waves and their generation mechanisms.

The high‐frequency (HF) Doppler sounder array is a well‐established technique for monitoring vertical fluctuations in the ionospheric layer at specific altitudes. On 29 July 2025, at 23:24:50 UT, a devastative magnitude 8.8 earthquake occurred in the Far East of Russia. The phenomena of Doppler Frequency Shifts (DFSs) were observed by the HF Doppler sounder array in southwestern China at a considerable distance of approximately 5,400 km from the epicenter. Analytical results show that DFSs retrieved from arrays in China are not associated with primary (P) waves, but instead correspond well to the wave packet of secondary (S) and Rayleigh waves, with a delay of approximately 420 s. Combined Doppler data from China, Japan, and the Taiwan region further indicate that DFSs are strongly associated with the Rayleigh waves of the Kamchatka earthquake. However, signatures of these acoustic waves were neither identified in concurrent total electron content (TEC) from the ground‐based global navigation satellite system receivers nor magnetic data from magnetometers in the far‐field (southwestern China ∼5,400 km away). These results demonstrate that HF Doppler measurements provide a highly sensitive method for detecting seismo‐acoustic waves and studying lithosphere‐atmosphere‐ionosphere coupling, especially when other geophysical parameters show no discernible response.

This study investigates, for the first time, the effect of pulsating aurora on the Global Navigation Satellite System (GNSS) total electron content (TEC). TEC measurements from multiple ground-based GNSS receivers are compared with auroral intensities recorded by two all-sky cameras during a pulsating aurora event on 15 March 2018 in northern Scandinavia. At hour-long timescales, the GNSS-derived TEC exhibits clear correspondence with the overall auroral activity. At shorter timescales of a few minutes, the auroral emission brightness measured by the all-sky cameras highly correlates with the time derivative of the TEC, which is consistent with the local production/recombination balance in the ionosphere. In addition, the TEC is observed to decrease more slowly than the optical brightness within each pulsation, indicating that electron recombination does not occur instantaneously after the emission. These findings demonstrate that GNSS-derived TEC measurements can be used to detect and study key signatures of pulsating aurora, opening the possibility of using the extensive network of GNSS satellites and ground-based receivers to perform unprecedented large-scale studies of this type of aurora. Graphical Abstract

Auroral arcs in the high latitude are frequently observed during periods of northward interplanetary magnetic field (IMF). However, their generation mechanisms are not yet fully understood, due to the limited availability of simultaneous in-situ and optical auroral observations. In this talk, we present observations of recurrent tailward-propagating auroral arcs in the dawnside during a period of northward IMF. THEMIS satellites located near the dawnside magnetopause in the equatorial plane observed strong fluctuations in magnetic field and plasma parameters with periods of 5–7 minutes, suggesting the presence of surface waves or ULF waves. Subsequently, shorter-period fluctuations of 2-3 minutes were observed for an hour, which may indicate the development of Kelvin–Helmholtz instability (KHI). During these intervals, a ground-based all-sky imager detected recurrent auroral arcs detached from the poleward boundary of the auroral oval, with a period of 5–10 minutes, propagating predominantly tailward. DMSP observations revealed multiple auroral arcs on the dawnside, accompanied by electron precipitation associated with paired upward and downward field-aligned currents. Based on these coordinated space- and ground-based observations, we discuss several possible generation mechanisms for the observed auroral arcs, including surface waves, ULF waves, and KHI.

We present a methodology for deriving the horizontal two‐dimensional distribution of low‐energy electron precipitation, specifically the possible lower bound of the differential energy flux of electron precipitation at 100 eV, from a 630‐nm auroral image obtained with a ground‐based all‐sky imager. The electron energy flux required to reproduce the auroral intensity distribution can be obtained using the GLobal airglOW (GLOW) model for the magnetic field lines within the field of view of the imager. The distribution of the electron precipitation occurring in a roughly circular region with a diameter of approximately 1,400 km centered on the observation point can be determined every 10 s or so. This methodology is implemented using the data obtained during the early recovery phase of a substorm from an imager operating in Longyearbyen, Norway. A characteristic spatio‐temporal variation of the electron precipitation in the polar cap boundary near midnight is revealed; the region of enhanced differential energy flux repeatedly expanded during the 20‐min interval, reaching nearly 30,000 km ² on three occasions when evaluated at an altitude of 250 km.