Iku SHINOHARA

Abstract Ground‐based very low frequency transmitters emit signals that primarily propagate within the Earth–ionosphere waveguide, and some of their energy can propagate into the magnetosphere. Ionospheric observations from the DEMETER satellite reveal a distinct concentric rings pattern of the wave power distribution of the North West Cape transmitter on the transverse (longitude–L‐shell) plane. Using data from the Van Allen Probes and Arase/energization and Radiation in Geospace satellites, we find that the concentric rings pattern is still visible but becomes less distinct. The pattern shifts outward, and becomes more elliptic as the wave propagates from the southern ionosphere to the Northern Hemisphere. To investigate the cause of this evolution, we performed ray tracing simulations under three conditions: ducted propagation, non‐ducted propagation with vertical initial wave normal angles and non‐ducted propagation with spread initial wave normal angles. The results show that non‐ducted propagation with spread wave normal angles best explains the observed evolution of wave power distribution pattern during the propagation.

Abstract 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.

Abstract Inverted‐V ion structures in energy‐time spectrograms are typically associated with quasi‐static potential structures and have generally been observed as unidirectional signatures in previous studies. Based on observations from the Arase satellite, we report an event featuring counter‐streaming inverted‐V ion structures that occurred on 16 February 2021. The inverted‐V ions parallel and anti‐parallel to the magnetic field are observed with a time difference of ∼5‐min, likely because they originate from the quasi‐static structures in the southern and northern hemispheres, which may have slightly different spatial locations along the satellite trajectory. This spatial difference between the two structures is also suggested by a time difference in the electron flux depletion observed in the parallel and anti‐parallel directions. Auroral images from multiple satellites further support the existence of quasi‐static structures in both the northern and southern hemispheres. In addition, the parallel inverted‐V ions exhibit a wider pitch angle distribution than that of the anti‐parallel ions, possibly due to pitch angle scattering of about 5° as they crossed the magnetic equator from the southern hemisphere. These results contribute to a better understanding of the spatial configuration and dynamics of auroral acceleration processes.

Abstract 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.

Abstract The afternoon detached auroral arc is an important phenomenon in the subauroral region, reflecting coupling processes between the Earth's magnetosphere and ionosphere. Previous studies have not identified fine‐scale structures in such arcs, leaving the dynamics underlying their formation poorly understood. Here we report an afternoon detached auroral arc event on 13 September 2017 during the recovery phase of a storm. For the first time, the sawtooth‐like undulations were observed along the equatorward boundary of the afternoon detached arc in the Lyman‐Birge‐Hopfield Long (LBHL) wavelength band of Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager (DMSP/SSUSI). This auroral structure is accompanied by >10 keV ion precipitation and by tens to hundreds of eV electron precipitation at higher latitudes. Detailed analyses based on coordinated observations from the Arase satellite indicate that the structure is associated with a plasmaspheric plume, with surface waves occurring along its boundary. Joint observations from ground‐based magnetometer stations indicate that magnetic pulsations in the Pc1‐2 band were also distinctly detected. We suggest that surface waves perturb the cold plasma density within the plume, thereby modulating Electromagnetic Ion Cyclotron (EMIC) waves. The modulated EMIC waves resonate with energetic ions, producing precipitation that contributes to the formation of the sawtooth‐like undulations in afternoon detached auroral arc.