Takanori Nishiyama

The upper thermosphere is important in the context of space weather, but continuous monitoring of the thermosphere at high latitudes remains limited despite its strong variations associated with geomagnetic activity. Orthohelium, He(2 ³ S), is found from the upper thermosphere to lower exosphere, and is a promising tracer of atmospheric dynamics. Here we report on unusual enhancements in He(2 ³ S) airglow during a severe geomagnetic storm on New Year's Day in 2025, associated with impulsive changes in the solar wind. Multi‐spectral optical observations show that the enhancement at the cusp coincided with electron and proton aurora and an increased density at an altitude of 850 km. The morphology of the He(2 ³ S) airglow closely matched that of electron aurora. The observed enhancement was attributed to electron precipitation and recombination of upwelling , while charge exchange of precipitating and ‐particles had a minor role in causing the airglow.

Iterative black auroral stripes were identified in auroral images obtained by the low‐altitude microsatellite Reimei. These steady and narrow black stripes are a remarkable feature of diffuse auroras generated by energetic electrons precipitating at higher latitudes than pulsating auroras. The Reimei data confirmed that the iterative black stripes were caused by the recurring depletion of energized plasma‐sheet electron fluxes within specific energy and pitch‐angle ranges, as previously reported. Another common feature of the events presented here is coincident electron energization accompanied by well‐developed inverted‐V components produced by upward (magnetic‐)field‐aligned quasi‐electrostatic fields. This feature is frequently observed in and near the higher‐latitude edge of the inner plasma sheet, where intensified fluxes of isotropic electrons are further energized beyond the inverted‐V electrons. Reimei also occasionally observed another type of electron flux depletion signature, similar to that causing the iterative black auroral stripes, in the plasma sheet populated by less energized electrons. In such cases, faint auroral forms without distinct black stripes and small inverted‐Vs were occasionally detected. Although plasma particle and wave data from the equatorial plasma sheet were not available, we hypothesize that ineffective pitch‐angle scattering near the loss cones of inner‐plasma‐sheet electrons could substantially deplete energetic electron fluxes precipitating into the loss cones, thereby producing black auroras. Spatially striped modulations of pitch‐angle scattering efficiency in the plasma sheet would likely induce alternating diffuse auroras and black stripes, consistent with the plasma‐sheet electron flux distributions observed at low altitudes by Reimei.

The ionospheric electric field is a key parameter for understanding ionospheric electrodynamics and the magnetosphere–ionosphere coupling. Several methods have been developed to reconstruct the two‐dimensional distribution of the ionospheric electric field from line‐of‐sight (LOS) ion velocities observed by incoherent scatter radars. Approaches tailored to the upcoming tri‐static incoherent scatter radar system, EISCAT_3D (E3D), have also been proposed. In this study, we propose a new method to reconstruct mesoscale flows using a Bayesian estimation. We applied maximum a posteriori estimation to reconstruct shear flow fields with maximum velocities of 1,000 m/s at 200 km altitude. The electric field was then estimated under the assumption of drift. The reconstructed shear flows showed root‐mean‐square errors (RMSEs) of 57–101 m/s in the ion velocity and 3–5 mV/m in the electric field. Typical mesoscale flow structures were reconstructed with errors of less than 100 m/s within about 150–200 km centered on the magnetic zenith. These results demonstrate the feasibility of resolving mesoscale ionospheric structures. Our method will be extended to reconstruct electric current structures in future studies.

This study focuses on ionosphere‐thermosphere coupling over North America during the 3–4 November 2021 strong geomagnetic storm (Kp 8−). We comprehensively analyze storm‐time ionospheric and thermospheric disturbances using data from ground‐based instruments at Poker Flat, Millstone Hill, and Urbana, as well as satellite observations and model data. Significant features of ionosphere‐thermosphere dynamics during the storm include (a) equatorward (−300 m/s) and poleward (50 m/s) surges at Millstone Hill (42.37 glat) and a poleward surge (75 m/s) at Urbana (40.13 glat) in the magnetic local time (MLT) post‐midnight sector, (b) ion velocities greater than −1,500 m/s at Poker Flat accompanied by a deep ionospheric trough moving to latitudes as low as 40 glat suggesting SAPS activity, (c) a widespread electron density depression in the morning hours due to an extended composition disturbance zone and (d) Large‐Scale Traveling Ionospheric Disturbances (LSTIDs) related to disturbances in equatorward winds in the post‐poleward wind interval. Results show that the equatorward and poleward surges were not accompanied by LSTIDs, which suggests that the extreme disturbances in meridional winds could have mainly been caused by SAPS forcing, due to ion drag and pressure gradients, rather than equatorward propagating Traveling Atmospheric Disturbances (TADs). Furthermore, the MLT location of Millstone Hill and Urbana indicates that during the disturbances both stations would have been located within the eastward backflow sector of the SAPS, which likely contributed to the complicated ion‐neutral dynamics.

The ionospheric electric field is a key parameter for understanding ionospheric electrodynamics and the magnetosphere–ionosphere coupling. Several methods have been developed to reconstruct the two-dimensional distribution of the ionospheric electric field from line-of-sight (LOS) ion velocities observed by incoherent scatter radars. Approaches tailored to the upcoming tri-static incoherent scatter radar system, EISCAT_3D (E3D), have also been proposed. However, they assumed a narrow field of view (FOV; 0.6 latitudinal and 0.9 longitudinal) or required long integration times (10 min). These conditions make it challenging to resolve mesoscale (~400 km) auroral variations. This study presents a feasibility analysis of reconstructing ion velocity and electric field structures in the ionospheric F-region with a wider FOV of 400 400 km and a shorter integration time of 54 s. We applied maximum a posteriori estimation to reconstruct shear flow fields with maximum velocities of 1000 m/s at 200 km altitude. The electric field was then estimated under the assumption of E × B drift. The reconstructed shear flows showed root mean square errors (RMSEs) of 57–101 m/s in ion velocity and 3–5 mV/m in the electric field. Typical mesoscale flow structures were reconstructed with errors of less than 100 m/s within about 150–200 km centred on the magnetic zenith. These results demonstrate the feasibility of resolving mesoscale ionospheric structures. Our method will be extended to reconstruct electric current structures in future studies.

We report the first observations of continuum emission at the poleward boundary of the dayside auroral oval. Spectral measurements of high-latitude continuum emissions resemble those of Strong Thermal Emission Velocity Enhancement (STEVE), with light characterized by colours such as white, pale pink, or mauve. The emission enhancement spans the entire visible wavelength range. However, unlike STEVE, the high-latitude dayside continuum emission events tightly follow the auroral particle precipitation, often forming field-aligned rays and other dynamic shapes. Some dayside emissions appeared as wide arcs or cloud-like structures within the red-emission-dominated dayside aurora. Our spectral measurements further suggest that the broadband continuum emission may extend into the near-infrared (NIR) regime. Similar to the STEVE emission, low-Earth-orbit measurements of plasma flow in the region of continuum emission show a strong horizontal cross-track velocity shear. Ground-based radar and optical observations provide evidence of both plasma and neutral heating, as well as upwelling, in connection to the continuum emissions. We conclude that the interplay between different heating mechanisms may be an important factor in generating high-latitude continuum emissions.

Electromagnetic ion cyclotron (EMIC) waves are recognized as one of the primary drivers of energetic electron precipitation (EEP) into the Earth's atmosphere. A problematic discrepancy has remained between the occurrence frequency of waves in magnetic local time (MLT) and the resultant precipitation observed by satellites. This study attempts to characterize the ionization profiles induced by EMIC‐wave‐driven EEP from ground‐based measurements. Our combined observational data sets, comprising imaging riometer, atmospheric radar, and magnetometer data obtained at Syowa Station from 2016 to 2019, enable us to find more than 850 events of EMIC‐driven EEP identified through concurrent ionization and EMIC wave activity. The MLT distribution of events peaks in the afternoon sector (13:00–14:00 MLT), consistent with the MLT distributions of waves previously reported. More than 60% of the events exhibit ionization above 60 km altitude caused by sub‐MeV EEP, which is attributed to non‐resonant scattering by EMIC waves.

This study presents the time variability of metastable orthohelium, , in the polar region associated with a geomagnetic storm. A data set consisting of continuous observations of airglow brightness at 1,083 nm for 17 nights was obtained from a short‐wavelength infrared imaging spectrograph (NIRAS‐2) at the Kjell Henriksen Observatory, Svalbard (78.1°N, 16.0°E). The observed airglow brightness clearly displayed responses to the geomagnetic storm on different time scales. It began to decrease sharply within an hour of sudden commencement of the storm, and it gradually decreased over the next few days before recovering slowly. This behavior in brightness agreed to helium density variations at a 500‐km altitude calculated by MSIS 2.1. The depletion of was mainly caused by Penning ionization due to upwelling from the lower atmosphere; this was consistent with the decreased O/ ratio in the MSIS 2.1 calculation and TIMED/GUVI measurements, and electron density depletion in the F region observed by the EISCAT Svalbard Radar (ESR). Additionally, sudden increases in airglow brightness were clearly associated with intermittent electron precipitations observed by the ESR. Therefore, direct impacts by precipitating electrons injected from space into the polar upper atmosphere can play significant roles in production of . The NIRAS‐2 measurements demonstrated that from the upper thermosphere to the exosphere changed drastically as a result if forcing both from the lower atmosphere and from space. measurements will improve our understanding of the thermosphere‐ionosphere coupling system and extend the coverage of space weather forecasting up to the exobase.

We report the first observations of continuum emission at the poleward boundary of the dayside auroral oval. Spectral measurements of high-latitude continuum emissions resemble those of STEVE, with light characterised by colours such as white, pale pink or mauve. The emission enhancement spans the entire visible wavelength range. However, unlike STEVE, the high-latitude dayside continuum emission events tightly follow the auroral particle precipitation often forming field-aligned rays and other dynamic shapes. Some dayside emissions appeared as wide arcs or cloud-like structures within the red-emission dominated dayside aurora. Our spectral measurements further suggest that the broad band continuum emission may extend into the near-infrared regime. Similar to the STEVE emission, low-Earth orbit measurements of plasma flow in the region of continuum emission show a strong horizontal cross-track velocity shear. Ground-based radar and optical observations provide evidence of both plasma and neutral heating, as well as upwelling, in connection to the continuum emissions. We conclude that the interplay of different heating mechanisms may be an important factor in generating high-latitude continuum emissions.