Lynn M. Kistler

During geomagnetic storms, energetic ions are injected from the Earth's magnetotail along the plasma sheet boundary layer (PSBL). Owing to its off‐equatorial inclined orbit, the Arase satellite is in a prime position to observe these injections in the PSBL on the nightside at the apogee. The injections are observed as energy and pitch angle dispersed features in the Arase LEP‐i and MEP‐i data. While these injections have often been observed in H ⁺ , reports of O ⁺ dispersions are rare. During the main phase of a geomagnetic storm on 16 July 2017 we identify frequent strong earthward injections of O ⁺ . The distance to the injection location varied from about 10 to 40 Earth radii. Some successive injections showed an increasing trend in distance, indicating that the ion acceleration region moved down the magnetotail over time. The earthward flux of these ions was comparable to the flux of nightside auroral outflow during this time, indicating that these injections may be a significant contributor to the hot O ⁺ population in the storm‐time near‐earth plasma sheet. Additionally, we present statistics derived from 180 individual O ⁺ injections during storms in 2017–2020, confirming that the injection source is mainly located in 10–40 R _E with a median at 20 R _E .

The Sun continuously releases a stream of charged particles known as the solar wind, threaded by magnetic field lines that occasionally fold back on themselves in sharp reversals called magnetic switchbacks. These structures are frequently observed by spacecraft near the Sun, but their formation mechanism remains an open question. Using measurements from Solar Orbiter at 0.55 AU, we report heavy ion observations within a magnetic switchback that provide direct evidence of its solar source. Elevated charge-state ratios of oxygen and carbon, combined with a depletion of helium relative to protons, indicate that the switchback plasma originated from hot, closed coronal loop structures. A semi-empirical coronal model, constrained by electron temperature and density measurements from the Solar Dynamics Observatory, traces the ion signatures to either a coronal hole boundary or loops within the coronal hole, implicating the source mechanism as interchange reconnection, where closed loops reconnect with open magnetic field and eject plasma. The proton velocity and magnetic field fluctuations of the switchback are in accordance with an outward-propagating Alfvén wave. These findings suggest that interchange reconnection and wave or turbulence mechanisms jointly govern switchback origin and propagation, respectively. Most importantly, we show that switchbacks preserve measurable signatures, carrying information about the physical processes that heat the solar atmosphere and accelerate the solar wind.

The Heavy Ion Sensor in Solar Wind Analyser suite on Solar Orbiter has provided unprecedented high-resolution elemental and ionic signatures around the heliospheric current sheet (HCS) from a vantage point near the Sun. Its high cadence time-series observations are used to determine the magnetic connectivity and map these complex transitions, revealing embedded processes of the observed signatures around the HCS crossing at ∼0.3 au. Electron strahl dropouts observed before the HCS crossing are accompanied by distinct local drops in the oxygen–proton temperature ratio ( T _O / T _p ), while carbon and oxygen charge-state ratios remain nearly invariant, with at most slight reductions depending on the interval. In contrast, near the true sector boundary (TSB) the heavy-ion composition profiles remain nearly constant, despite marked changes in magnetic and strahl topology. These measurements show that small-scale HCS-related structures at 0.3 au can be systematically matched with their heavy-ion thermal and compositional signatures. A gradual decrease of the Fe/O ratio toward the TSB is observed, reaching a minimum of (4.54 ± 0.54) × 10 ⁻² at the sector boundary and recovering within a few hours, establishing a clear association between Fe/O depletion and the HCS. After the HCS crossing, T _O / T _p frequently exceeds the super-mass-proportional threshold and is positively correlated with O ⁷⁺ /O ⁶⁺ , while C ⁶⁺ /C ⁵⁺ shows a negative correlation, indicating distinct properties of fully striped carbon within the same sector. As these signatures undergo minimal distortion during radial propagation and evolution, the observations presented in this study offer critical insights into the plasma provenance and the underlying physical processes governing the HCS environment.

Super geomagnetic storms are characterized by extreme intensification of the ring current in near-Earth space. The origin of the ions that carry the ring current is key to understanding its development. In situ measurements of ring current ions by the Arase satellite demonstrate an unprecedented dominance of heavy ions originating from the Earth during the May 2024 super geomagnetic storm, despite the high solar wind density. The solar wind, another expected source of ions, contributes little to the energy density of the ring current. This observational evidence highlights the critical role of ion supply processes from the Earth and transport in the magnetosphere in developing the ring current for the super geomagnetic storm. Furthermore, the super-intense ring current penetrated close to the Earth, strongly deforming the local geomagnetic field and driving unusual outward transport of electrons, which led to the loss of radiation belt electrons from the near-Earth region.

Data from the FAST spacecraft are used to study the temporal progression of the energy inputs to the dayside cusp and the nightside aurora, including Poynting flux, electron number flux and amplitude of extremely low frequency (ELF) waves, during a storm driven by CME (Coronal Mass Ejections), and the resulting H+ and O+ outflows. The results show that (1) On the dayside, Poynting flux, ELF waves activity and soft electron precipitation are all enhanced during the initial and main phases of the storm, and decrease during the recovery phases. On the nightside, the Poynting flux increases during the initial and main phase, but the enhancements are smaller than on the dayside. The variations in the ELF wave activity and electron precipitation are similar before and during the storm. (2) The energy inputs are strongly correlated with the solar wind – magnetosphere coupling functions, dΦMP/dt and p1/2dΦMP/dt, especially in the dayside cusp region where the energy inputs and the ion outflows are localized. (3) The O+ and H+ ion outflow flux, fO+ and fH+, and the flux ratio fO+/fH+ all increase during the storm. Both the fluxes and the flux ratio reach their peaks on the initial phase and are enhanced during the main phase. Nightside auroral H+ and O+ outflows have lower outflow number fluxes than that in the dayside cusp region. These observations show how the solar wind changes characteristics of CME storms and results in strong sustained ion outflow during the initial and main phases.

Plasma populations within the solar system are distributed over a substantial range of densities, kinetic temperatures, and bulk velocities, from the solar wind to Earth's ionosphere. Measuring the full range of plasma conditions for some of these populations is difficult for current top‐hat electrostatic analyzers, let alone for multiple populations. A modification to the standard top‐hat electrostatic analyzer (ESA) design is developed and explored that extends the differential energy flux dynamic range of the instrument. The modification uses a secondary electrode placed on the outer portion of the analyzer optics to electrically control and vary the instrument's geometric factor (GF) by up to three orders of magnitude. The design space of the modification, such as its size and position within the analyzer channel, is investigated through ion optics simulations. As the GF is reduced, the energy and angular passbands of the analyzer narrow. The modification's impact on other instrument parameters and its advantages over other variable geometric factor systems (VGFS) are discussed. A prototype ESA with the presented modification was developed and tested. Laboratory results are consistent with the behavior of the modification from simulation predictions, verify the extension of the analyzer's dynamic range, and show the feasibility of the modification for future instrument designs.

We have recently undertaken a recalibration of data from the Time‐of‐flight Energy Angle Mass Spectrometer (TEAMS) on the Fast Auroral SnapshoT (FAST) satellite. The recalibration effort accounts for the known instrumental issues, including changing instrumental efficiencies, dead time correction, misclassification of ions by the electronics, spacecraft charging, and spacecraft motion. In determining the correction due to spacecraft motion, we derive the ion convection velocity, which is an additional data product. The corrected data are available as pitch angle data and as a 3‐D data product.

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.

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.

Data from the FAST spacecraft are used to study the temporal progression of the energy inputs to the dayside cusp and the nightside aurora, including Poynting flux, electron number flux and amplitude of extremely low frequency (ELF) waves, during a CME-driven storm, and the resulting H⁺ and O⁺ outflows. The results show that (1) On the dayside, Poynting flux, ELF waves activity and soft electron precipitation are all enhanced during the initial and main phases of the storm, and decrease during the recovery phases. On the nightside, the Poynting flux increases during the initial and main phase, but the enhancements are smaller than on the dayside. The variations in the ELF wave activity and electron precipitation are similar before and during the storm. (2) The energy inputs are strongly correlated with the solar wind – magnetosphere coupling functions, 𝑑Φ_MP/𝑑𝑡 and 𝑝^1/2 𝑑Φ_MP/𝑑𝑡, especially in the dayside cusp region where the energy inputs and the ion outflows are localized. (3) The O⁺ and H⁺ ion outflow flux, f_????+ and f_????+, and the flux ratio f_????+ /f_????+ all increase during the storm. Both the fluxes and the flux ratio reach their peaks on the initial phase and are enhanced during the main phase. Nightside auroral H⁺ and O⁺ outflows have lower outflow number fluxes than that in the dayside cusp region. These observations show how the solar wind changes characteristics of CME storms and results in strong sustained ion outflow during the initial and main phases.