Journal of Geophysical Research: Space Physics最新文献

Past kinetic simulations and spacecraft observations have shown that traveling foreshocks (TFs) are bounded by either foreshock compressional boundaries (FCBs) or foreshock bubbles (FBs). Here we present four TFs with a different kind of structure appearing at one of their edges. Two of them, observed by the Cluster mission, are bounded by a hot flow anomaly (HFA). In one case, the HFA was observed only by the spacecraft closest to the bow shock, while the other three probes observed an FCB. In addition, two other TFs were observed by the MMS spacecraft to be delimited by a structure that we call HFA-like FCB. In the spacecraft data, these structures present signatures similar to those of HFAs: dips in magnetic field magnitude and solar wind density, decelerated and deflected plasma flow and increased temperature. However, a detailed inspection of these events reveals the absence of heating of the SW beam. Instead, the beam almost disappears inside these events and the plasma moments are strongly influenced by the suprathermal particles. We suggest that HFA-like FCBs are related to the evolution and structure of the directional discontinuities of the interplanetary magnetic field whose thickness is larger than the gyroradius of suprathermal ions. We also show that individual TFs may appear together with several different types of transient upstream mesoscale structures, which brings up a question about their combined effect on regions downstream of the bow shock.

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.

Charge sensitive amplifiers (CSA) form the first stage of most detector readout circuits. The operating range of the readout circuits depends on the saturation limits of the Charge sensitive amplifiers, thereby limiting the range of the detector system as well. A number of methods have been introduced to extend the dynamic range of the readout circuits. The Boston Extended Range Amplitude integrated circuit, BEAR 1, uses two switchable capacitors, which when added in parallel to the detector, shares the deposited charge and prevents the circuit from saturating. This article describes the design, simulation, fabrication and experimental results of a new version of this integrated circuit (IC), called BEAR 2. BEAR 2 has new features, mainly adjustable thresholds and variable energy ranges using different switchable capacitor values. Experimental results indicate that BEAR 2 can measure input energies from 6 MeV to 4.7 GeV with a resolution dE/E of about 7.1$��\%�$ with switchable capacitors of 1 and 20 nF. At 11.4 mW and with analog to digital converter clock speeds as high as 5 MHz, BEAR 2 is a low power, high speed, single channel readout circuit that can be used with detector systems like energetic particle telescopes. This has applications for a number of areas including high energy radiation belt physics, cosmic rays as well as Solar Energetic Particle (SEP) physics.

Enhanced radiation at aviation altitudes is a concern for flight crew and passengers. During space weather events, solar flares and coronal mass ejection (CME) driven shocks are sources of energetic particles that can reach Earth's near-space environment and interact with its magnetic field and atmosphere. Although Earth's magnetic field and atmosphere offer some protection, at high aviation altitudes and particularly near the poles, this shielding effect is weaker leading to increasing radiation exposure and related health risks. In this study, we use data from the Automated Radiation Measurements for Aerospace Safety (ARMAS) instrument onboard a commercial United Airlines flight from San Francisco to Paris that deviated its flight path to mitigate the risk of increased radiation doses during the extreme geomagnetic storm in May 2024. This allows investigation of how the crew and passengers may have experienced enhanced radiation onboard the aircraft. For comparison, we estimate radiation exposure for an alternative flight from San Francisco to Paris around the same time that did not deviate from its planned path. The results show that during the 10 May 2024 geomagnetic storm, ARMAS measured sporadic high absorbed dose rates onboard the deviated flight. However, exposure could have been significantly higher (up to three times higher) if the airline had not deviated to lower latitudes, highlighting the need for precautionary measures during space weather events. Additionally, it is shown that precipitating electrons from the Van Allen radiation belts may significantly contribute to radiation levels at flight altitudes during enhanced geomagnetic activity.

We investigate the empirical relationship between the spacecraft potential (V_s) measured by the Electric Field Double Probes, and the electron density (N_e) measured by the Fast Plasma Instrument on the MMS spacecraft. We derive their relationship during fast-mode intervals when the Active Spacecraft Potential Control Devices are off. Then we apply this relationship to slow-mode intervals during the perigee passes where V_s can be less than +2 V and N_e can exceed 1,000 cm⁻³. Because such a parameter range is never observed by MMS during fast-mode intervals, we define this part of the relationship using simultaneous observations from the Van Allen mission where N_e is measured up to 3,000 cm⁻³ while V_s is less than +2 V. We compare the empirical relationship to the predictions by an orbital motion limited theory. This suggests how to model the photoelectron current above +5 V and the collection of ambient electrons for moderate Debye lengths. We apply the empirical relationship (i.e., theoretical curves are not used here) to several consecutive plasmapause crossings by MMS during two magnetic storms. The erosion rates of the plasmasphere were ∼60 cm⁻³/day at 7 MLT and ∼30 cm⁻³/day at 14 MLT. In the duskside, the erosion rate decreases with L due to increasing flux tube volumes, being about 10–20 cm⁻³/day at L ∼ 7. The refilling rates are similar to the erosion rates, but in the dawnside, the refilling is delayed due to a slow expansion of the plasmapause to higher L.

We observed a significant enhancement in 630.0 nm airglow intensity near midnight on 15–17 April, 2023, with maximum on 17 April at Devasthal (29.4°N, 79.7°E; Mlat ∼20.7°N). Simultaneous airglow measurements from Silchar (24.68°N, 92.76°E; Mlat ∼15.5°N) corroborated these observations on 15 and 16 April, thereby confirming the latitudinal and longitudinal extent of the phenomenon and suggesting its association with a well-known large-scale geophysical process, the midnight temperature maximum (MTM). The moderate enhancements occurred on 15–16 April, but 17 April showed a much stronger peak. Higher GPS-TEC levels contributed to increased post-sunset airglow intensity on these days, but TEC alone could not explain the midnight peak variations in the OI 630.0 nm intensity. Ionosonde data from a longitude chain (from dip latitude to the airglow observation locations) showed a descending F-layer (h′F and hmF2), with a stronger descent on 17 April, suggesting a role for ionospheric midnight descent in the airglow enhancement. The ionosonde-derived F-region winds showed a notable poleward flow on 17 April. Power spectral density (PSD) analysis revealed common wave periodicities (∼5–6 hr) in both the F-region winds and the mesosphere–lower thermosphere (MLT) regions. Furthermore, the wind associated with MLT region waves was observed to be in phase with the F-region wind during the strongest midnight peak on 17 April. Therefore, atmospheric waves are believed to have played a crucial role in the post-midnight descent of the ionosphere through wind reversal, amplifying the airglow peak on 17 April.

The soft X-ray imager (SXI) on the SMILE mission promises to revolutionize our understanding of the magnetopause by observing solar wind charge exchange emission from the magnetosheath on a global scale. The primary goal of this instrument is to infer the position and shape of the magnetopause from these images. One method involves devising a 3D X-ray emissions model through which an image can be simulated and adjusted to match a real image. The magnetopause position can be extracted from the fitted 3D model. Previous work by Wharton et al. (2025b), https://doi.org/10.1029/2025ja033837 showed this method was effective with noisy SXI images if solar wind data was used to initialize their Cusp and Magnetosheath Emissivity Model (CMEM1). This study develops CMEM2, a constrained and simplified model that does not require solar wind data for initialization but uses information in the image instead. It can also be fitted by a range of fitting algorithms, which we thoroughly compare. CMEM2 is shown to perform as accurately as CMEM1. We also test this method for the first time on realistic scenarios with changing solar wind conditions. We find the method struggles during median solar wind conditions when the magnetosheath is dim and largely out of the expected FOV of SMILE-SXI. However, during active conditions, the method tracks the movement of the magnetopause well and produces errors within the 0.5 $��R_E�$ requirement for the SMILE mission, providing that the initialization of CMEM2 is reasonably accurate.

On December 2023, the Juno spacecraft made a flyby of Io above the northern hemisphere at a closest approach (CA) altitude of ∼1,500 km (PJ57). The Juno/Waves and Radio-occultation measurements showed a surprising large electron density ∼28,000 cm⁻³ near closest approach. We run 2D numerical simulations of the plasma/atmosphere interaction to explore the causes of this high plasma density. Our numerical simulations are based on (a) A prescribed atmospheric composition and distribution of S, O, SO₂, and SO; (b) A MHD code to calculate the plasma flow into Io's atmosphere; (c) A multi-species physical chemistry code to compute the change in plasma properties (electron and ion densities, composition and temperature) during the plasma/atmosphere interaction; and (d) Ionization by field-aligned electron beams. We show that during the PJ57 flyby, the increased plasma density is strictly localized inside the Alfven Wing and is caused by ionization of the polar atmosphere by both the thermal electrons of the torus penetrating the Alfven Wing and field-aligned electron beams detected inside the wing. We explore several hypotheses leading to the very large electron density observed along PJ57: a dense upstream plasma impinging on the atmosphere; a dense polar atmosphere; and a change in its composition. We also assess the sensitivity of our results to the plasma flow speed inside the Alfven Wing. We show that the high electron density observed by Juno requires a significant atmospheric density above the pole, the composition of which is still undetermined.

Two extreme space weather events that occurred in May and October 2024 triggered severe modifications in the geospace environment leading to super geomagnetic storms of class G5 and G4, respectively. These two events that occurred in the increasing phase of the current 25th solar cycle during different seasons provided a unique opportunity to investigate the distinct nature of storm induced impacts on the ionospheric dynamics and electron density distribution, especially with regard to their seasonal dependence. The behavior of Equatorial Ionization Anomaly during these two superstorms has been explored in detail using Total Electron Content (TEC), digisonde, and satellite based measurements in the Indian low latitude sector. Ionospheric TEC variations during the recovery phase of the May storm revealed strong positive enhancement near the equator due to the suppressed fountain effect with a negative storm effect at the low latitudes. In the recovery phase of the October storm, a negative storm effect is seen over the low latitudes. The responses of the bottom side and top side of the ionosphere to the storm induced disturbances have shown distinct differences with respect to the magnitudes of deviations and settling times. The relative dominance of prompt penetration and disturbance dynamo electric fields and thermospheric composition changes on the observed modifications of ionospheric dynamics during these two storms are discussed. This study provides crucial information on the distinct impacts of two severe geomagnetic storms under varying seasonal background conditions over the equatorial and low latitude sectors.

We present a methodology for deriving the horizontal two-dimensional distribution of low-energy electron precipitation, specifically the possible lower bound of the differential energy flux of electron precipitation at 100 eV, from a 630-nm auroral image obtained with a ground-based all-sky imager. The electron energy flux required to reproduce the auroral intensity distribution can be obtained using the GLobal airglOW (GLOW) model for the magnetic field lines within the field of view of the imager. The distribution of the electron precipitation occurring in a roughly circular region with a diameter of approximately 1,400 km centered on the observation point can be determined every 10 s or so. This methodology is implemented using the data obtained during the early recovery phase of a substorm from an imager operating in Longyearbyen, Norway. A characteristic spatio-temporal variation of the electron precipitation in the polar cap boundary near midnight is revealed; the region of enhanced differential energy flux repeatedly expanded during the 20-min interval, reaching nearly 30,000 km² on three occasions when evaluated at an altitude of 250 km.