Khan‐Hyuk Kim

Low‐energy ion data (E < 25 keV/q) acquired by the Kaguya spacecraft were used to study ions originating near the lunar surface when the Moon was in the terrestrial magnetotail lobe. We focus on three intervals during which Kaguya detected these ions at different altitudes: ∼100 km on 15 October 2008, ∼50 km on 9 April 2009, and a descent from ∼60 to ∼30 km on 9 May 2009. In each interval, the strongly enhanced fluxes of lunar‐origin ions exhibited a band structure spanning less than 100 eV/q to ∼1,000 eV/q, with pitch angles from 30° to 140°. To understand where and how cold lunar‐origin ions (<1 eV) are energized to the observed energy levels, we conducted test‐particle simulations. The simulations indicate that these ions can reach energies of ∼300 eV through multiple reflections from the lunar photoelectron sheath, and are further accelerated by the motional electric field, increasing their energies from several hundred eV to ∼1,000 eV, consistent with Kaguya observations.

On 2015 October 2, MMS spacecraft observed an electron micro‐injection event near the southern hemispheric high‐altitude cusp, coinciding with intense wave activity across several frequency bands. We investigated the MMS magnetic field and plasma during this event to explore cross‐scale coupling among the wave modes. Employing the Hilbert‐Huang transform, we perform an empirical mode decomposition to extract frequencies and amplitudes of the intrinsic mode functions (IMFs). In this analysis, we establish both linear and nonlinear relationships and examine the information transfer between the IMFs. Notably, the transfer entropy suggests that high frequency ion cyclotron waves may be driven by the mirror mode structures. Our case study effectively demonstrates the utility of the information theory based tools for studying cross‐scale wave coupling phenomena.

We investigate how the wave normal angle (WNA) and polarization of proton‐band electromagnetic ion cyclotron (EMIC) waves change as they travel from their source to Earth. This paper marks a significant milestone as the first full‐wave simulation of proton‐band EMIC waves reflecting from the ionosphere. Our findings show that the WNA can change rapidly during propagation, primarily due to plasma inhomogeneities, such as variations in the Alfvén speed. The wave polarization is strongly related to the WNA, consistent with theory. Newly generated EMIC waves near the equator propagate with a WNA of 0, then the WNA gradually shifts to 90 as they move toward Earth. In contrast, reflecting waves having 90 of WNA at Earth maintain a relatively larger WNA even near the magnetic equator. As a result, only the newly generated waves close to the source, where the magnetic latitude is less than approximately 20, show left‐handed polarization, while linear polarization remains dominant throughout the rest of the propagation.

To examine the temporal and spatial evolution of cold ions within the inner magnetosphere during geomagnetic storms, we conducted superposed epoch analyses on the electron density (N _e ) and the partial ion densities of H ⁺ , He ⁺ , and O ⁺ within the instrument frame energy range of 1–10 eV, using data obtained from the Van Allen Probes. The analyses were performed on a set of 14 large storms (SYM‐H < −100 nT) over a 5‐year period from 2013 to 2017. We found that the electron density and cold H ⁺ and He ⁺ densities outside the plasmasphere decrease as large storms develop. However, the cold O ⁺ density outside the plasmasphere remains largely unchanged during the storm's main and early recovery phases. Based on these observations, we suggest that the less‐dense cold population outside the plasmasphere during storm activities contains more cold O ⁺ ions.

In near-Earth space, the interplanetary magnetic field (IMF) is generally characterized by the Parker spiral configuration with a spiral angle of approximately 45° clockwise from the radial axis of the Sun. However, spacecraft observations reveal that IMF lines can exhibit non-Parker spiral configurations with either a nearly radial (outward–inward), tangential (westward–eastward), or normal (northward–southward) direction defined in radial–tangential–normal coordinates on even hourly timescales. Using the IMF and solar wind data measured by the Wind spacecraft for the period 1995–2023, we present distinctive features in the occurrence and various solar wind parameters between these non-Parker spiral IMF configuration types. Our study reveals that (1) the occurrence of non-Parker spiral IMF configuration types has the Hale cycle (∼22 yr magnetic dynamo cycle) variations with their own phases depending on the sign of the major IMF components, (2) the solar wind in the radial/tangential IMF structure is deflected eastward/westward, which corresponds to the direction opposite to/same as the rotation direction of the Sun on the plane perpendicular to the line of sight, and (3) these IMF configuration types can be characterized by magnetic field topology, which is open (for radial IMF and tangential IMF) or closed (for normal IMF). Our findings therefore provide insights into the formation mechanisms of these non-Parker spiral IMF configurations observed in near-Earth space. Moreover, our results suggest that spacecraft missions to various areas in interplanetary space are needed to better understand spatiotemporal evolutions of the solar-originated magnetic field structure in the heliosphere.

The electron density profiles produced from the ionospheric sounding system are traditionally estimated by the inversion procedure based on the image analysis of the observed ionograms. Jang Bogo Vertical Incidence Pulsed Ionospheric Radar (VIPIR) with Dynasonde (hereafter, JVD), however, uses the three-dimensional electron density inversion approach named “NeXtYZ” to produce ionospheric density, ion velocity, and tilt of the ionization in the bottomside ionosphere based on the list of detected radio echoes with their physical parameters. Sometimes, the resulting density profiles can be erroneous, not reflecting real ionosphere, probably due to severely disturbed ionosphere in the polar region. In this study, the automatic classification procedure of the estimated electron density profiles is developed to filter out unusable data for the 5-year period from 2017 to 2021. The ionograms are classified into four categories: ‘Unavailable’, ‘Sporadic E’, ‘Needs Reprocessing’, and ‘Available’. It is found that approximately 50% of ionograms are evaluated to be reasonable with proper electron density profiles and about 35% of them tend to be affected by sporadic-E like structures, blocking the F-region ionosphere to be observed. It should be noted that the sporadic-E like structures in the polar ionosphere seems to be main obstacles for the ionospheric sounding observation of the F-region ionosphere. Only less than 10% of ionograms are classified as a reprocessing type which needs to be reprocessed. Finally, no echoes are recognized by Dynasonde analysis for about 5% of ionograms. The reprocessing and/or unavailable types might be associated with auroral precipitations that disturbs the ionosphere in the polar region.

This study demonstrates the influence of electromagnetic ion cyclotron (EMIC) rising‐tone emissions on relativistic electrons in the inner magnetosphere using data obtained from the Van Allen Probes and Arase satellites. We find that the intense EMIC rising‐tone emissions occur during the increase in the solar wind pressures, creating favorable conditions for triggering nonlinear wave growth. The strong flux drop‐out of relativistic electrons in the parallel directions of the magnetic field, with energies of 0.2–4 MeV, was associated with the wave activity. We calculated the nonlinear triggering conditions and the minimum resonant energy of relativistic electrons interaction with EMIC waves, based on our observations. We conclude that EMIC rising‐tone emissions contribute not only to the rapid loss of MeV electrons through EMIC wave‐particle interactions while extending the resonance energy to a few MeV by broadening bandwidth via nonlinear wave growth but also to interactions with sub‐MeV electrons through the nonresonant effect.

This study investigated electron phase space densities (PSDs) in geostationary orbits using data from Korea's geostationary satellite, GK2A, as well as GOES‐16 and GOES‐17 satellites. The PSDs were computed from electron fluxes measured by each satellite during a geomagnetically quiet period as defined by stringent conditions on geomagnetic activity. The conjunction of the satellites over invariant coordinates enabled electron PSDs from pairs of satellites to be compared to assess the extent of deviation of the PSD ratios from the expected ratio of one, as inferred from the Liouville theorem. The results show that PSDs from the satellites are sufficiently consistent to allow the estimation of the radial gradient of the PSDs. Comparisons of radial gradients estimated in this manner show that positive radial gradients prevail during geomagnetically quiet periods, whereas both positive and negative gradients may occur at similar frequencies during active periods. This study provides statistical insights into the physical mechanisms responsible for the observed radial gradient profiles based on findings from a wide range of local times during both geomagnetically quiet and active periods.

During the May 2024 storm, the minimum Dst index was approximately −412 nT, marking the largest geomagnetic storm of the past decade. This event caused the inner edge of the ring current to penetrate deeply into the inner magnetosphere during the main phase of the storm. We present observations of high‐frequency electromagnetic ion cyclotron (HF EMIC) wave activity during this intense geomagnetic storm using data from the Arase satellite. Arase observations showed that HF EMIC waves with frequencies of 5–36 Hz at L ∼ 2, occurred mainly during the main and early‐recovery phases. The minimum resonance energy of energetic protons and relativistic electrons associated with HF EMIC waves suggests their potential to cause the loss of relativistic electrons in the low L‐shell region. Our observations provide new insights into the generation of EMIC waves and the dynamics of energetic particles at low L‐shells in the inner magnetosphere.