Journal of Geophysical Research: Space Physics最新文献

On 27 January 2017, Magnetospheric Multi-Scale observed a series of electron vortexes, which are driven by the electron Kelvin-Helmholtz (K-H) instability in the reconnection outflow at terrestrial magnetopause. We find the electron vorticity can reach above 200 s⁻¹ inside the vortexes, which is comparable to the strong vorticity events in the electron diffusion region. Using the First-Order Taylor Expansion for Velocity field method, we further reconstruct the 3D topology of two electron vortexes and find one vortex is converging while another vortex is diverging. Interestingly, enhancement of electron temperature was observed in the diverging vortex but not in the converging vortex, indicating electron dynamics is related to the vortex topology. Our study suggests that electron K-H vortexes formed by the intense electron shear flow in the reconnection outflow have different types of topologies and will help us better understand the reconnection picture at the terrestrial magnetopause.

We report recent findings for the magnetic field configurations of small-scale magnetic flux ropes (SFRs) broadly defined and identified by using the Grad-Shafranov-based techniques for in situ measurements via the Parker Solar Probe (PSP), Solar Orbiter (SolO), and two Helios spacecraft. Since the current sheets were found to occur at boundaries of SFRs and/or inside SFRs at 1 AU via the partial variance increment (PVI) and the Grad-Shafranov (GS) reconstruction technique by Pecora et al. (2019), https://doi.org/10.3847/2041-8213/ab32d9, we further examine such a co-existence in this study by assessing the maximum PVI indices within SFR intervals using the above four spacecraft observations throughout the inner heliosphere ($��⩽�$1 AU). Less than 15% of SFRs have maximum PVI indices exceeding a threshold value of 6 that is used to indicate a current sheet structure. Three representative events are selected to explain the most common situations. (a) Current sheets occur at SFR boundaries and near the center. Each could be a weak switchback feature in the time-series profile of the gradually bipolar magnetic field rotations. (b) An SFR configuration is confirmed by both the measurement of counterstreaming electrons and the GS reconstruction result, despite that a large PVI value occurs near the SFR center which is due to an arbitrary kink instead of a current sheet. (c) A current sheet is falsely identified as an SFR where a significant PVI value ($��\sim �$ 7) occurs near the center. In the end, we discuss the necessity of using multi-point spacecraft measurements to discern the structures associated with SFRs.

Specific polar cap auroras, such as 15MLT-PCA, linked to lobe reconnection due to the influence of the interplanetary magnetic field (IMF) B_y component, were only observed in the summer. Although the variance in ionospheric conductivity between winter and summer has been proposed as a potential explanation for this seasonal dependency, it has also been argued that the differences in lobe reconnection between the winter and summer hemispheres could be the cause. To address this debate, we examined two data periods with similar IMF conditions when the northern hemisphere was in summer and winter, respectively. Using DMSP/SSUSI and AMPERE observations, we detected clear 15MLT-PCA and associated field-aligned currents in the summer, but not in the winter. These observations were compared with global MHD simulations from OpenGGCM. Lobe reconnection signatures were identified for both winter and summer in the simulation results. However, a detailed analysis showed that the pattern of lobe reconnection in the winter hemisphere was different from that in the summer. Based on the combined observation and simulation results, we suggest that particular lobe reconnection in summer is critical for generating 15MLT-PCA, while the winter's reconnection may lead to transient or small-scale auroral responses that were not easily identified by DMSP/SSUSI observations as a 15MLT-PCA event.

We use the Magnetospheric Multiscale mission to study electron kinetic entropy across Earth's quasi-perpendicular bow shock. We perform a statistical study of how the change in electron entropy depends on the different plasma parameters associated with a collisionless shock crossing. The change in electron entropy exhibits strong correlations with upstream electron plasma beta, Alfvén Mach number, and electron thermal Mach number. We investigate the source of entropy generation by correlating the change in electron entropy across the shock to the measured electric and magnetic field wave power strengths for different frequency intervals within different regions in the shock transition layer. The electron entropy change is observed to be greater for higher electric field wave power within the shock ramp and shock foot for frequencies between the lower hybrid frequency and electron cyclotron frequency, suggesting electrostatic waves are important for electron kinetic entropy generation at Earth's quasi-perpendicular bow shock. Any eventual cross-shock potential contribution to the electron entropy generation has not been considered in this study.

One of the best proxies to estimate the daytime ionospheric zonal electric field from the dynamo region is by probing the drift of the plasma irregularities in the ionospheric E-region by coherent radars, although there have not been many at HF frequencies. The present study is based on the observations of the anomalous variability in the zonal drift of 8.3 m scale size plasma irregularities in the ionospheric E-region during the daytime on 12 October 2021, and 28 November 2022, as observed by an 18 MHz HF radar at Thumba (8.5°N, 77°E, and dip lat. = 1.96°N). The wind perturbations from the lower atmosphere and the electric field penetrating from the solar wind, can both cause the perturbations in the E-region electric field. Based on a comparison with the predicted Prompt Penetration Equatorial Electric Field (PPEF) as ascribed by Manoj et al. (2008, https://doi.org/10.1029/2008ja013381), and the quiet-time dynamo electric field over the longitude of Thumba, we conclude that the perturbations in the electric field on these two events are caused by PPEF mapping to the equatorial region, solely driven by IMF B_Z oscillations. The former event is a result of a Coronal Mass Ejection, whereas the latter event was caused by a high-speed solar wind associated with coronal hole. The fluctuations in the strength of the Equatorial Electrojet (EEJ) calculated from the magnetic field observations from a pair of equatorial (Tirunelveli) and off-equatorial (Alibag) stations are also found to be well in agreement with the radar observations and modeled PPEF, thereby verifying the E-region observations.

Cassini flew past Jupiter in 2000/2001, and Juno has been orbiting the gas giant since mid-2016. Here we focus on the spectral properties of Jovian broadband kilometric radiation (bKOM), and we classified them according to their slope in the time–frequency spectrum and distinguish four categories with negative, positive, mixed, or no slope in frequency. These bKOM structures had mostly negative slopes during the inbound portion of the Cassini Jupiter flyby, whereas they were mostly positive for the outbound. The reason for this could be higher intensities of bKOM emissions from midnight to dawn local times. We interpret the semi-circles of the Jovian radio emission nicknamed “bullseyes” as negatively sloped bKOM connected with positively sloped bKOM, and we detected 40 bullseyes in Juno Waves data until the end of 2017. Cassini and Juno show similar distributions of bKOM with respect to the Jovian Central Meridian Longitude like the Voyagers and Ulysses. This means that bKOM has not changed over several decades and “rotates” with the system III period. Several features of the bKOM structures suggest emission cones with thick mantles and moderate beaming angles around $��50�°�$. We found an enhancement of the occurrence probability of southern bKOM for certain phases of Ganymede and Callisto.

The dynamics of giant planet magnetospheres is controlled by a complex interplay between their fast rotation, their interaction with the solar wind, and their diverse internal plasma and momentum sources. In the ionosphere, the Hall and Pedersen conductances are two key parameters that regulate the intensity of currents coupling the magnetosphere and the ionosphere, and the rate of angular momentum transfer and power carried by these currents. We perform a comparative study of Hall and Pedersen conductivities and conductances in the four giant planets of our Solar System - Jupiter, Saturn, Uranus and Neptune. We use a generic ionospheric model (restraining the studied ions to $��H_3^{+}�$, $��{\text{CH}}_5^{+}�$, and meteoric ions) to study the dependence of conductances on the structure and composition of these planets' upper atmospheres and on the main ionization sources (photoionization, ionization by precipitating electrons, and meteoroid ablation). After checking that our model reproduces the conclusions of Nakamura et al. (2022), https://doi.org/10.1029/2022ja030312 at Jupiter, that is, the contribution of meteoric ions to the height-integrated conductances is non-negligible, we show that this contribution could also be non-negligible at Saturn, Uranus and Neptune, compared with ionization processes caused by precipitating electrons of energies lower than a few keV (typical energies on these planets). However, because of their weaker magnetic field, the conductive layer of these planets is higher than the layer where meteoric ions are mainly produced, limiting their role in magnetosphere-ionosphere coupling.

Incoherent scatter radar measurements rely on the application of a priori parameters from empirical models to initialize the analysis of incoherent scatter spectra. Currently, there is a need to transform ionosphere models to enable reliable space weather predictions through data assimilation of observations. Very often the data assimilation relies on electron densities measured with incoherent scatter radars. Erroneous a priori parameters would lead to the assimilation of inaccurate and physically inconsistent data depending on the ionospheric model. It might therefore be beneficial to assimilate the entire radar spectrum and infer the plasma parameters from the assimilated spectrum by applying the a priori parameters as given by the model. To assess the potential assimilation of incoherent scatter spectra into models, we investigate synthetic EISCAT incoherent scatter spectra calculated from TIE-GCM results. At F1 region altitudes, the atomic-to-molecular ion ratio strongly affects the shape of the incoherent scatter spectrum. Since the vertical profiles of the atomic-to-molecular ion ratio are distinctly different in the EISCAT a priori model and TIE-GCM, the assimilation of single plasma parameters induces additional, unbalanced forces into the model. A similar problem arises in the E region due to different ion-neutral collision frequency profiles. These problems could be solved by assimilation of the entire incoherent scatter spectrum followed by an in-model evaluation of the plasma parameters. We demonstrate the effect of different a priori profiles on the spectral analysis and how the derived plasma parameters are changing when leveraging a more comprehensive approach of using forward modeling with TIE-GCM.