Journal of Geophysical Research: Space Physics最新文献

Landau resonance by electromagnetic ion cyclotron (EMIC) waves is considered to be one of the potential mechanisms responsible for the heating and acceleration of electrons in the range of eV to keV. However, the sensitivity of this process to wave properties and plasma environment is still unclear. Here, we perform a detailed parametric study on Landau resonance scattering rates of electrons by H⁺ band EMIC waves using different wave properties and plasma conditions, including the peak wave frequency, bandwidth, peak wave normal angle, angular width, and plasma density. We find that the variations of peak frequency, bandwidth, peak wave normal angle, and ambient plasma density play an important role in changing both the resonance zone and the scattering rates, while the variations of angular width have a smaller effect. In addition, the minimum resonant energy of electrons interacting with H⁺ band EMIC waves tends to increase with decreasing plasma density. Our results are beneficial for improving the understanding of the electron Landau heating and acceleration caused by EMIC waves in the magnetosphere.

This study aims to investigate the possible influence of Alfvénic and compressive low-frequency fluctuations in a solar wind corotating high-speed stream on the geomagnetic activity from mid to high latitudes. We selected a stream in the declining phase of solar cycle 23, impinging on Earth's magnetosphere on 5–12 January 2008. To study solar wind fluctuations, we rotated velocity and magnetic field components into a reference frame aligned with the main magnetic field. This approach allowed us to define the fluctuation power along the direction aligned with the main magnetic field, associated with purely compressive fluctuations, and the power within the plane orthogonal to the main field, containing Alfvénic fluctuations. The Alfvénicity has been studied through the normalized cross helicity and residual energy, which describe the degree of correlation between kinetic and magnetic fluctuations and their energy balance. We used ground data from seven geomagnetic observatories from mid to high latitudes, removing well-known Sun-Earth-related periodicities during quiet periods. We analyzed the dependence of the average geomagnetic power on the down-dusk interplanetary electric field component $��E_y�$, by distinguishing the more and less Alfvénic populations. For the more Alfvénic population, we observe an increase in average geomagnetic power for positive $��E_y�$ values, corresponding to open magnetosphere conditions, particularly at auroral and sub-auroral latitudes. Additionally, we find that geomagnetic power fluctuations are coherent with those of the interplanetary medium; the coherence reaches higher values with a greater persistence for the orthogonal direction, associated with Alfven waves, with respect to the parallel one.

The Global-scale Observations of Limb and Disk (GOLD) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments were used to investigate the thermospheric composition and temperature responses to the geomagnetic storm on 23–24 April, 2023. Global-scale Observations of Limb and Disk observed a faster recovery of thermospheric column density ratio of O to N₂ (ΣO/N₂) in the southern hemisphere (SH) after the storm ended at 12 Universal time (UT) on 24 April. After 12 UT on 25 April, ΣO/N₂ had mostly recovered in both hemispheres. Global-scale Observations of Limb and Disk also observed an increase of middle thermospheric temperature (140–200 km) (Tdisk) on 24 April with a maximum of 340 K. Within 4–6 hr of the storm ending on 24 April, Tdisk enhancement persisted between 30°N and 60°N, 100°W and 30°W, while Tdisk lower than pre-storm quiet day (17 April) was observed between 45°W and 15°W, 40°S and 50°N. Tdisk recovered between 100°W and 45°W, 30°N and 55°S. On 25 April, Tdisk was lower than on 17 April across the entire GOLD Field-of-Regard (FOR) by ∼50–110 K. Additionally, solar irradiance decreased by 15%–20% from 17 to 25 April, indicating that the lower Tdisk on 25 April resulted from both storm and solar irradiance variations. Latitudinal variations of Tdisk and the SABER observed Nitric Oxide (NO) cooling rate revealed that NO cooling is crucial for the lower Tdisk in the northern hemisphere (NH) mid-high latitudes on 25 April. These results provide direct evidence of decreased thermospheric temperature during storm recovery phase than pre-storm quiet times.

Relatively little is known about the causes of fine-scale aurora with scale sizes of $��\sim �1�$ km and less. We analyze images from the Auroral Structure and Kinetics (ASK) instrument, a multi-monochromatic spectral imager located in the Norwegian High Arctic. We analyze five categories of fine-scale aurora. The categories described in this paper are: “drift arcs”, “drift arcs (dynamic)”, “chocolate sauce”, “chocolate sauce (turbulent)”, and “psychedelic”; these are categories based on those in the citizen science project, Aurora Zoo. The analysis comprises: magnetic local time (MLT) dependence, characteristic precipitation energies, occurrence relative to the auroral oval, and adapted local intermittency measure (LIM) analysis of radially averaged image 2D power spectra. LIM analysis allows us to estimate the widths of the smallest structures typically associated with each aurora type. We find that chocolate sauce (turbulent) and psychedelic aurora are associated with nightside processes. This is based on their tendency to be higher energy, to occur at late dusk/midnight MLTs, and to occur on the poleward boundary of the auroral oval. For non-psychedelic aurora types, we find the smallest characteristic widths ($��\sim �330�-�500�$ m) and distribution of energies (typically less than 10 keV) to be consistent with the dissipative non-linear inertial Alfvén wave acceleration mechanism described by Wu and Chao (2004a), https://doi.org/10.1029/2003ja010126. For psychedelic aurora, the smallest characteristic widths ($��\sim �200�$ m) and high energies (median 10.9 keV) make it unlikely that it is produced by the aforementioned mechanism, which predicts arc widths of $��\sim �0.3�-�4.6�$ km and energies less than 10 keV.

The Fokker-Planck diffusion equation is widely used for simulating the evolution of Earth's radiation belt electrons, which pose significant hazards to space-borne systems. To preserve the positivity of the numerical solution of the electron phase space density (PSD), several finely designed finite difference, Monte Carlo, spatiotemporal interpolation, and finite volume schemes have been developed. However, these schemes often suffer from either high implementation complexity or low execution efficiency. Here we propose an efficient, easy-to-implement, and positivity-preserving finite difference scheme, named the Semi-Implicit Logarithmic Linearization (SILL) scheme. The basic principle is to linearize the nonlinear equation of the natural logarithmic PSD. This scheme ensures accuracy and stability, even with large time steps, up to hundreds of seconds for typical radiation belt electron diffusion processes. Nonetheless, it exhibits oversensitivity to near-vanishing phase space densities, which necessitates reduced time steps when handling extremely large variations in orders of magnitude between neighboring grid points. We have publicly released the protype code of the SILL scheme, which could be useful for the radiation belt modeling community.

The characteristics of the westward ionospheric current flowing at the geomagnetic equator, known as Counter Electrojet (CEJ), are studied here using multi-satellite Swarm mission magnetic field data collected during January 2014 to March 2021. For the first time, the simultaneous measurements from multiple satellites are used to investigate the various features of the CEJ, including its longitudinal extent. The CEJ events occur in ∼19.8% of quiet-time electrojet profiles during the period of study. The local time, seasonal, and longitudinal variations of the CEJ amplitude and occurrence percentage during geomagnetically quiet days confirm many of the earlier reported observations. It is found that the CEJ is more prone to occur during morning and less frequent at noon, while the amplitude of CEJ peaks during noon hours. Longitudinal pattern of CEJ occurrence shows distinct four-peaks at ∼50°E, ∼150°E, ∼230°E and ∼310°E longitudes. Also, CEJ occurrence peaks from June to August. The CEJ occurrence and meteorite ablation rates are found to be moderately correlated on a monthly scale, and no one-to-one correlation found in the Indian region. For the first time, the longitudinal extent of CEJ is estimated using simultaneous observations from Swarm A and B. It is found that the CEJ has predominant longitudinal extent of 15°–25°, although it can also have larger longitudinal coverage more than or up to 165°, under the assumption of temporal and spatial stability of CEJ.

Spacecraft discharge time constants are calculated from measurements of electron differential flux before and during operation of an ionospheric sounding radar. Determining these time constants provides insight into how the operation of a sounding radar affects the surrounding plasma's interaction with the spacecraft. The analysis is enabled by the fixed-frequency operation mode of a sounding radar which enhances resonant interaction with the ambient plasma. This mode's effect on measured energy spectra of ion and electron fluxes is described. Measurements of electron fluxes disturbed by radar operation serve as input to a model of spacecraft discharge for calculating capacitive discharge time constants. A case study using electron fluxes measured at Mars yields discharge time constants in the range 0.6–0.8 ms and reveals that a residual potential around $��-�4�$ V remains on the spacecraft long after radar operation ceases. The minimum spacecraft potential cannot be determined with these data and model due to the narrow energy range of electrons in the ambient plasma.

It is traditionally assumed that upward currents correspond to electron precipitation, suggesting that electron precipitation should align with Region 2 (R2) Field-Aligned Currents (FACs) in the dawn sector and Region 1 (R1) FACs in the dusk sector. However, some previous studies have indicated systematic discrepancies between the locations of auroral electron precipitation and R1/R2 FACs, a topic that remains controversial and lacks an adequate explanation. This study aims to investigate and explain these differences. We conducted a detailed analysis of the relationship between FACs and auroral electron precipitation throughout the substorm phases, including the growth, expansion, and recovery phases. It is shown that (a) the region of large energy flux in auroral electron precipitation corresponds to the transition zone between R1 and R2 FACs, (b) auroral electron precipitation enhances height-integrated conductances, which combine with downward vorticities to be associated with upward magnetospheric-origin FACs, and (c) auroral electron precipitation also strengthens the gradient of height-integrated conductances and this enhanced gradient, together with drift velocities, contributes to the formation of downward ionospheric-origin FACs. The interplay between downward ionospheric-origin FACs and upward magnetospheric-origin FACs shifts the transition zone to poleward, resulting in the observed correlation between the region of large energy flux and the transition zone. This is similar to the mechanism of feedback instability. Specifically, the height-integrated conductance gradients due to auroral electron precipitation drive polarized currents, fed by downward and upward FACs. These FACs shift the transition zone poleward and enhance auroral electron precipitation energy flux, forming a closed feedback loop.