arXiv subjects

Yoshiya Kasahara

Publications and source records attributed to Yoshiya Kasahara.

5 recordsLinked to original sources

arXiv2026

First Observation of Multiple Very-Near-Earth Reconnection Events During a Single Storm Main Phase

For the first time, this paper presents three very-near-Earth reconnection (VNERX) events observed within the same 12-hour-long storm main phase. The THEMIS inner probes observed the hallmarks of three episodes of tailward retreating x-lines positioned between magnetic local time (MLT) 23-24 and radial distance 12-13 Earth radii (RE). The events occurred within a thin current sheet, < 1 RE thick. Simultaneously, dispersionless energetic particle injections above 10s of keV and magnetic field dipolarizations were observed near and earthward of geosynchronous altitude by the KOMPSAT and Arase satellites. Arase observed earthward flow bursts at or below geosynchronous altitude via ExB enhancements, suggesting VNERX ejecta proceed below geosynchronous orbit. These observations demonstrate that VNERX events, which predominantly occur during the storm main phase, can be frequent and essential for driving injections that can effectively power the ring current. However, they can be observed only at the pre-midnight sector, close to the neutral sheet.

physics.space-ph↗

arXiv2025

A Characteristic Signature of Magnetospheric Wave-Particle Interactions Found in the Turbulent E-region

Plasma waves in the magnetosphere scatter electrons, causing them to precipitate into Earth's atmosphere, imparting their temporal characteristics to diffuse auroras. In a case study of conjugate radar and satellite observations, we demonstrate a close and unprecedented association between enhanced electrostatic cyclotron harmonic wave activity in the magnetosphere and the appearance of meter-scale plasma turbulence a few seconds later in the lower ionosphere on nearby magnetic field lines. Such direct structuring of the ionosphere carries implications for our understanding of space weather.

physics.space-ph↗

arXiv2024

Variation of Whistler-Mode Wave Characteristics Along Magnetic Field Lines: Comparison of Near-Equatorial THEMIS and Middle-Latitude ERG Observations

The latitudinal distribution of whistler-mode wave intensity plays a crucial role in determining the efficiency and energy of electrons scattered by these waves in the outer radiation belt. Traditionally, this wave property has mostly been derived from statistical measurements of off-equatorial spacecraft, which collect intensity data at various latitudes under different geomagnetic conditions and at different times. In this study we examine a set of events captured by both the near-equatorial THEMIS spacecraft and the off-equatorial ERG spacecraft. Specifically, we compare the whistler-mode wave intensity from THEMIS and ERG measurements at the same MLT and time sectors. Similar wave spectrum characteristics confirm that THEMIS and ERG indeed observed the same wave activity. However, upon closer examination of the wave intensity variations, we identify two distinct categories of events: those that follow the statistically predicted variations in wave intensity along magnetic latitudes, and those that exhibit rapid wave intensity decay away from the equatorial plane. We analyze main characteristics of events from both categories and discuss possible implications of our analysis for radiation belt models.

physics.space-ph↗

arXiv2017

Statistical study on propagation characteristics of Omega signals (VLF) in magnetosphere detected by the Akebono satellite

This paper shows a statistical analysis of 10.2 kHz Omega broadcasts of an artificial signal broadcast from ground stations, propagated in the plasmasphere, and detected using an automatic detection method we developed. We study the propagation patterns of the Omega signals to understand the propagation characteristics that are strongly affected by plasmaspheric electron density and the ambient magnetic field. We show the unique propagation patterns of the Omega 10.2 kHz signal when it was broadcast from two high-middle-latitude stations. We use about eight years of data captured by the Poynting flux analyzer subsystem on board the Akebono satellite from October 1989 to September 1997. We demonstrate that the signals broadcast from almost the same latitude (in geomagnetic coordinates) propagated differently depending on the geographic latitude. We also study propagation characteristics as a function of local time, season, and solar activity. The Omega signal tended to propagate farther on the nightside than on the dayside and was more widely distributed during winter than during summer. When solar activity was at maximum, the Omega signal propagated at a lower intensity level. In contrast, when solar activity was at minimum, the Omega signal propagated at a higher intensity and farther from the transmitter station.

physics.space-ph↗

arXiv2016

Automatic Detection of Omega Signals Captured by the Poynting Flux Analyzer (PFX) on Board the Akebono Satellite

The Akebono satellite was launched in 1989 to observe the Earth's magnetosphere and plasmasphere. Omega was a navigation system with 8 ground stations transmitter and had transmission pattern that repeats every 10 s. From 1989 to 1997, the PFX on board the Akebono satellite received signals at 10.2 kHz from these stations. Huge amounts of PFX data became valuable for studying the propagation characteristics of VLF waves in the ionosphere and plasmasphere. In this study, we introduce a method for automatic detection of Omega signals from the PFX data in a systematic way, it involves identifying a transmission station, calculating the delay time, and estimating the signal intensity. We show the reliability of the automatic detection system where we able to detect the omega signal and confirmed its propagation to the opposite hemisphere along the Earth's magnetic field lines. For more than three years (39 months), we detected 43,734 and 111,049 signals in the magnetic and electric field, respectively, and demonstrated that the proposed method is powerful enough for the statistical analyses.

astro-ph.IM↗