浅村 和史

Abstract Electron conics are a distinct type of electron distribution observed in Earth’s magnetosphere, characterized by enhanced fluxes of upgoing electrons at several-keV energies, particularly in the auroral acceleration region. This study analyzes high-altitude (27,000–32,000 km) observations made by the Arase satellite to investigate the characteristics of electron conics after passing through the heating region, employing the high angular resolution of the low-energy particle experiments—electron analyzer (LEPe) onboard the satellite. We analyzed eight electron conic events between 2017 and 2022 to estimate their source altitudes using mirror ratios and potential differences and by comparing pre- and post-heating data to investigate heating properties. Our results show that the source region of conics has an upper boundary at 9,000–14,000 km, with the peak flux originating from a central altitude of 3,000–7,000 km. This region spatially coincides with the source of auroral kilometric radiation (AKR): the central altitude of the source of conics corresponds to the lower boundary of the AKR source, suggesting that a longer residence time of particles within the AKR source region leads to stronger heating. The comparison of pre- and post-heating populations demonstrated that upgoing conic electrons exhibit higher temperatures and lower densities. The number flux remains conserved, indicating the energization of a magnetospheric population, whereas the energy flux is enhanced by up to a factor of four, significantly higher than that reported in previous studies. A test particle simulation, using observed plasma parameters and incorporating stochastic perpendicular heating, reproduces the main features of observed conics in terms of both energy and pitch angle. Our simulation shows that electron conics evolve into narrow, field-aligned beams at higher altitudes, suggesting that some of the anti-Earthward-flowing beams observed in the magnetotail may actually be unresolved conics. These findings contribute to the understanding of energy transport between the auroral acceleration region and the magnetotail and show the importance of high-angular-resolution instrumentation. Graphical Abstract

Abstract Using Arase satellite observations, this study provides a comprehensive statistical analysis of ions (H⁺, He⁺, O⁺) and electron contributions to the total ring current pressure during storms with two different drivers. The results demonstrate the effect of different solar wind drivers on the composition, energy distribution, and spatial characteristics of the ring current. Using 32 CIR‐ and 30 Interplanetary Coronal Mass Ejection (ICME)‐driven storms, we characterize the ring current pressure evolution during the prestorm, main, early‐recovery, and late‐recovery storm phases as a function of magnetic local time and L‐shell. In CIR‐driven storms, H⁺ ions are the dominant (∼70%) contributor to the total ring current pressure during main/early recovery phases and increasing to ∼80% during late recovery. In contrast, the O⁺ pressure (E = 20–50 keV) response is significantly stronger in ICME‐driven storms contributing ∼40% to the overall pressure during the main/early recovery phases and even dominate (∼53%) in certain MLT sectors. Additionally, ICME‐driven storms tend to have peak pressure at lower L‐shells (L ≈ 3–4), while CIR‐driven storms show pressure peaks at slightly higher L‐shells (L ≈ 4–5). Interestingly, electron pressure also plays a notable role in specific MLT sectors, contributing ∼18% (03–09 MLT) during the main phase of CIR‐driven storms and ∼11% (21–03 MLT) during ICME‐driven storms. The results highlight that the storm time electron pressure plays a crucial role in the ring current buildup. Another noteworthy feature of this study is that Arase's fine‐energy resolution and broad coverage enable a detailed investigation of energy‐dependent ring current dynamics.