JGR:Space physics

The ionospheres of Jupiter's icy moons have been observed by in situ plasma measurements and radio science. However, their spatial structures have not yet been fully characterized. To address this, we developed a new ray tracing method for modeling the radio occultation of the ionospheres using Jovian auroral radio sources. Applying our method to Jovian auroral radio observations with the Galileo spacecraft, we derived the electron density of the ionosphere of Ganymede and Callisto. For Ganymede's ionosphere, we found that the maximum electron density on the surface was 76.5–288.5 cm−3 in the open magnetic field line regions and 5.0–20.5 cm−3 in the closed magnetic field line region during the Galileo Ganymede 01 flyby. The difference in the electron density distribution was correlated with the accessibility of Jovian magnetospheric plasma to the atmosphere and surface of the moons. These results indicated that electron-impact ionization of the Ganymede exosphere and sputtering of the surface water ice were effective for the producing Ganymede's ionosphere. For Callisto's ionosphere, we found that the densities were approximately 350 and 12.5 cm−3 on the night side hemisphere during Callisto 09 and 30 flybys, respectively. These results combined with previous observations indicated that atmospheric production through sublimation controlled the ionospheric density of Callisto. This method is also applicable to upcoming Jovian radio observation data from the Jupiter Icy Moon Explorer, JUICE.

To ensure the robustness of both civilian and military infrastructure, it is important to protect electric power grids, smart grids, and other electrotechnologies from known and possibly as-of-yet unknown space weather hazards. Space weather can generate intense geoelectric fields at the surface of the Earth, as well as large voltage gradients across long distances of the Earth. These voltage gradients can lead to geomagnetically induced currents (GICs), which are known to produce hazards to electric power grids. The finite-difference time-domain (FDTD) method is a powerful and versatile method that has already been applied to the study of geoelectric fields. The advantages of FDTD over other methods are that it can account for more geometrical complexities and realistic time waveforms and that it directly solves for geoelectric fields. Snell's Law predicts that any electromagnetic waves incident on the ground should essentially propagate straight downwards into the low resistivity ground. For this reason, vertical FDTD grid resolutions of 1/3 of a skin depth were usually chosen, while the horizontal grid resolution was relaxed. We find, however, that there is another important consideration for choosing an FDTD grid resolution applied to real-world scenarios: localized field variations due to currents generated by ground features. It turns out the grid resolution requirements are much stricter when taking this physics into account.

Mercury possesses a miniature yet dynamic magnetosphere driven primarily by magnetic reconnection occurring regularly at the magnetopause and in the magnetotail. Using the newly developed Magnetohydrodynamics with Adaptively Embedded Particle-in-Cell (MHD-AEPIC) model coupled with planetary interior, we have performed a series of global simulations with a range of upstream conditions to study in detail the kinetic signatures, asymmetries, and flux transfer events (FTEs) associated with Mercury's dayside magnetopause reconnection. By treating both ions and electrons kinetically, the embedded PIC model reveals crescent-shaped phase-space distributions near reconnection sites, counter-streaming ion populations in the cusp region, and temperature anisotropies within FTEs. A novel metric and algorithm are developed to automatically identify reconnection X-lines in our 3D simulations. The spatial distribution of reconnection sites as modeled by the PIC code exhibits notable dawn-dusk asymmetries, likely due to such kinetic effects as X-line spreading and Hall effects. Across all simulations, simulated FTEs occur quasi-periodically every 4–9 s. The properties of simulated FTEs show clear dependencies on the upstream solar wind Alfvénic Mach number (MA) and the interplanetary magnetic field orientation, consistent with MESSENGER observations and previous Hall-MHD simulations. FTEs formed in our MHD-AEPIC model tend to carry a large amount of open flux, contributing ∼3%–36% of the total open flux generated at the dayside. Taken together, our MHD-AEPIC simulations provide new insights into the kinetic processes associated with Mercury's magnetopause reconnection that should prove useful for interpreting spacecraft observations, such as those from MESSENGER and BepiColombo.

From the global simulation, we reproduce the solar wind-magnetosphere-ionosphere (S-M-I) interaction under the northward interplanetary magnetic field (IMF) with negative B y. Reconnection structures, the plasma sheet, and lobes are formed in magnetospheric convection, while lobe/round-merging/reciprocal/nightside cells appear in the ionosphere. Associated with the S-M interaction, northern open field is generated at the evening open-closed (O/C) boundary, due to successive cusp and interchange reconnections (in round-merging cell) or by Dungey-type reconnection (in nightside cell). Corresponding interchange and Dungey-type reconnections occur at the southern null. Dungey-type reconnection at the same time generates southern open field on the outer-most magnetopause. Open field injected into the northern polar cap/void/lobe constructs the open field part of the round-merging and nightside cells. After open-field convection in the lobe, reclosures occur by again successive cusp and interchange reconnections on the dayside separator, or separator reconnection on the nightside separator. Former closed field line proceeds toward the evening O/C boundary through the dayside closed-field convection in the round-merging cell, while latter closed field line through the nightside closed-field convection in the nightside cell. Shear that causes the large-scale sun-aligned arc is generated by the process of injecting open magnetic field into the void and the conjugate of process of connecting return flux from the plasma sheet to the nightside cell in counter hemisphere.

We use multiple instruments data to investigate the behavior of the equatorial and low-latitude ionosphere during the geomagnetically active and quiet period of November 1–6, 2021. In this context, total electron content (TEC) data obtained from the Global Positioning System (GPS) receivers in the equatorial and low-latitude regions of Asia, Africa, and America are used to assess variations in plasma density during the storm. The storm-time ionization levels were found to vary significantly in the crests of the Equatorial Ionization Anomaly (EIA) region over the 3 longitudes. The Rate of Change TEC Index (ROTI) derived from GPS receiver measurements, is used to study the equatorial/low-latitude ionospheric plasma irregularities at various longitudes under geomagnetically quiet and disturbed conditions. Observations showed longitudinal variations in the ionospheric irregularities under both quiet and disturbed conditions. Some days exhibit a decrease in the strength of the midnight plasma irregularities toward the East, that is, the irregularities are more pronounced in West America, less common in East America, and almost non-existent in Africa and Asia. Our investigations show this storm prevented the occurrence of plasma irregularities at the equatorial/low-latitude region in the American sector during the night following the main phase. In general, no significant storm effects were observed at the target locations in Africa and Asia. The existence of westward Prompt Penetration Electric Field (PPEF) and the Equatorial Electrojet (EEJ) during the main phase, from midnight to noon, is clearly related with the constriction of plasma diffusion and the consequent suppression of plasma irregularities. Thus, the longitudinal dependence for the generation of midnight plasma irregularities during this storm is mainly influenced by local time occurrence of maximum ring current, and the ionospheric electric fields.

The present study investigates wavenumber-4 (wave-4) structure in the longitude variation of zonal and meridional winds observed by the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer (ICON) satellite. The amplitude of the wave-4 pattern in meridional wind displays semi-annual variation with equinoctial maxima whereas its seasonal variation in zonal wind shows maxima during August–October at the equatorial and low latitudes. The wave-4 longitude variation maximizes at lower thermospheric heights (below 130 km) in zonal and meridional winds. It is considered primarily driven by the non-migrating eastward propagating diurnal tide with zonal wavenumber-3 (DE3) in the zonal wind. However, the amplitude of DE3 tide in the meridional wind does not show any enhancement during September–October. The seasonal variations of the wave-4 amplitude and the DE3 tide are not similar in the zonal and meridional winds. The migrating ter-diurnal tide (TW3) exhibits significant amplitudes during March–April and September–November in the meridional wind. In addition, the latitude variation of non-migrating TE1 tide shows maximum amplitude during September–October. These results suggest that the non-linear interaction between the TW3 and TE1 tides can serve as a potential source for the wave-4 longitude variation in the meridional wind at lower thermospheric altitudes.

Studies commonly assumed that variations in ionospheric conductance were insignificant and proposed that vorticities can be a reliable proxy or diagnostic for ionospheric field-aligned currents (FACs). We propose a complete method for measuring FACs using data from the Super Dual Auroral Radar Network radar and the Defense Meteorological Satellite Program. In our method, the FACs are determined by three terms. The first term is referred to as magnetospheric-origin FACs, while the second and third terms are known as ionospheric-origin FACs. This method incorporates height-integrated conductances based on observational data, thereby addressing the limitation of assuming uniform conductances. Different from previous works, we can calculate FACs at a low altitude of 250 km and obtain high-resolution measurements within observable areas. Another advantage of this method lies in its ability to directly calculate and analyze the impact of ionospheric vorticity and conductance on FACs. We apply this method to obtain FACs in the Northern Hemisphere from 2010 to 2016 and analyze the distributions of height-integrated conductances and total FACs. Our analysis reveals that the average FACs clearly exhibit the large-scale R1 and R2 FAC systems. We conduct statistical analysis on magnetospheric-origin FACs and ionospheric-origin FACs. Our findings show that within the auroral oval, ionospheric-origin FACs reach a comparable level to magnetospheric-origin FACs. However, ionospheric-origin FACs are significantly minor and almost negligible in other regions. This implies that height-integrated conductance gradients and vorticities play equally significant roles within the auroral oval, whereas vorticities dominate in other regions.

In this paper, the equatorial thermosphere anomaly (ETA) is investigated using accelerometer measurements to determine whether the feature is density-dominated, wind-dominated, or some combination of the two. An ascending-descending accelerometry (ADA) technique is introduced to address the density-wind ambiguity that appears when interpreting the ETA in atmospheric drag acceleration analyses. This technique separates ascending and descending acceleration measurements to determine if a wind's directionality influences the interpretation of the observed ETA feature. The ADA technique is applied to accelerometer measurements taken from the Challenging Minisatellite Payload mission and has revealed that the ETA is primarily density-dominated from 9:00 to 16:00 local time (LT) near 400 km altitude, with the acceleration perturbations behaving similarly between 2003 and 2004 across all seasons. This finding suggests that the perturbations in the acceleration due to in-track wind perturbations are small compared to the perturbations due to mass density, while indicating that the formation mechanisms across these local times are similar and persistent. The results also revealed that in the terminator region at 18:00 LT the acceleration perturbations deviate appreciably between ascending and descending passes, indicating different or multiple processes occurring at this local time compared to the 9:00–16:00 LT ascribed to the ETA. These results help constrain ETA formation theories to specific local times and thermospheric property responses without the use of supplemental wind measurements, while also indicating regions where in-track winds cannot always be neglected.

We study electromagnetic ion cyclotron (EMIC) waves based on observations from the ionosphere, magnetosphere, and ground during a geomagnetic storm recovery phase on 28 August 2018. In this case, multiple ducting EMIC waves in the ionosphere show higher frequencies in the post-midnight than those in the pre-midnight. Ionospheric EMIC wave frequency differences in magnetic local time (MLT) are consistent with MLT frequency differences in the equatorial magnetosphere, which are mainly caused by different background magnetic field at different L-shells. Moreover, we report the first observation of frequency range selections in ionospheric ducting EMIC waves and find that frequency selections depend on the magnetic field intensity in the main part of the ionospheric waveguide, with higher frequency corresponding to larger magnetic field. This study reveals the important role of background magnetic field in regulating ducting EMIC wave frequencies in the ionosphere.

Bursty reconnection models predict that flux transfer events (FTEs) moving along the magnetopause launch fast mode compressional waves into the magnetosheath that push the bow shock outward. By contrast, increases in the solar wind density striking the bow shock should push that boundary inward and launch fast mode compressional waves that propagate across the magnetosheath, drive waves on the magnetopause, and generate transient events in the outer magnetosphere. Multipoint ACE, Wind, THEMIS, and GOES-11/12 solar wind, bow shock, and magnetospheric observations on 14 October 2008 provide direct evidence for solar wind pressure pulses producing a large amplitude indentation with crater FTE-like properties on the magnetopause.

Recent study has demonstrated that electron cyclotron harmonic (ECH) waves can be excited by a low energy electron beam. Such waves propagate at moderately oblique wave normal angles (∼70°). The potential effects of beam-driven ECH waves on electron dynamics in Earth's plasma sheet is not known. Using two-dimensional Darwin particle-in-cell simulations with initial electron distributions that represent typical plasma conditions in the plasma sheet, we explore the excitation and saturation of such beam-driven ECH waves. Both ECH and electron acoustic waves are excited in the simulation and propagate at oblique wave normal angles. Compared with the electron acoustic waves, ECH waves grow much faster and have more intense saturation amplitudes. Cold, stationary electrons are first accelerated by ECH waves through cyclotron resonance and then accelerated in the parallel direction by both the ECH and electron acoustic waves through Landau resonance. Beam electrons, on the other hand, are decelerated in the parallel direction and scattered to larger pitch angles. The relaxation of the electron beam and the continuous heating of the cold electrons contribute to ECH wave saturation and suppress the excitation of electron acoustic waves. When the ratio of plasma to electron cyclotron frequency ω pe /ω ce increases, the ECH wave amplitude increases while the electron acoustic wave amplitude decreases. Our work reveals the importance of ECH and electron acoustic waves in reshaping sub-thermal electron distributions and improves our understanding on the potential effects of wave-particle interactions in trapping ionospheric electron outflows and forming anisotropic (field-aligned) electron distributions in the plasma sheet.

We made observations of magnetic field variations in association with pulsating auroras with the magneto-impedance sensor magnetometer (MIM) carried by the Loss through Auroral Microburst Pulsations (LAMP) sounding rocket that was launched at 11:27:30 UT on 5 March 2022 from Poker Flat Research Range, Alaska. At an altitude of 200–250 km, MIM detected clear enhancements of the magnetic field by 15–25 nT in both the northward and westward components. From simultaneous observations with the ground all-sky camera, we found that the footprint of LAMP at the 100 km altitude was located near the center of a pulsating auroral patch. The auroral patch had a dimension of ∼90 km in latitude and ∼25 km in longitude, and its major axis was inclined toward northwest. These observations were compared with results of a simple model calculation, in which local electron precipitation into the thin-layer ionosphere causes an elliptical auroral patch. The conductivity within the patch is enhanced in the background electric field and as a result, the magnetic field variations are induced around the auroral patch. The model calculation results can explain the MIM observations if the electric field points toward southeast and one of the model parameters is adjusted. We conclude that the pulsating auroral patch in this event was associated with a one-pair field-aligned current that consists of downward (upward) currents at the poleward (equatorward) edge of the patch. This current structure is maintained even if the auroral patch is latitudinally elongated.

Magnetic field line curvature (FLC) scattering is an effective mechanism for collisionless particle scattering. In the terrestrial magnetosphere, the FLC scattering plays an essential role in shaping the outer boundary of protons radiation belt, the rapid decay of ring current, and the formation of proton isotropic boundary (IB). However, previous studies have yet to adequately investigate the influence of FLC scattering on charged particles in the Earth's dayside magnetosphere, particularly in the off-equatorial magnetic minima regions. This study employs T89 magnetic field model to investigate the impacts of FLC scattering on ring current protons in the dayside magnetosphere, with a specific focus on the off-equatorial minimum regions. We analyze the spatial distributions of single and dual magnetic minima regions, adiabatic parameter, and pitch angle diffusion coefficients due to FLC scattering as functions of Kp. The results show that the effects of FLC scattering are significant not only on the dusk and dawn sides but also in the off-equatorial minima regions on the noon. Additionally, we investigate the role of dipole tilt angle in the hemispheric asymmetry of FLC scattering effects. The dipole tilt angle controls the overall displacement of the dayside magnetosphere, resulting in different FLC scattering effects in the two hemispheres. Our study holds significance for understanding the FLC scattering effects in the off-equatorial region of Earth's dayside magnetosphere and for constructing a more accurate dynamic model of particles.

The paper presents the results of a numerical assessment of the contribution of the ionospheric D region to the total electron content during six powerful X-ray flares that occurred in September 2017. The calculation of the electron concentration in the lower ionosphere was carried out using a plasma-chemical model of the ionospheric D region. This model was verified using the data of ground-based radiophysical measurements in the VLF (very low frequency) range and data of the incoherent scattering radar. To calculate the ionization rate at the D region heights, we used real data on the radiation flux measured by the GOES and SDO satellites during the considered flares. The total electron content was estimated using GNSS data. As a result of the analysis, it was found that the contribution of the lower ionosphere to the TEC change varied from 7% to 23% for flares with different spectra. A functional dependency has been obtained that can be used to estimate the contribution of the D region to the TEC increment depending on the spectrum of the flare.

Microbursts are short duration intensifications in precipitating electron flux that are believed to be a significant contributor to electron losses in the magnetosphere. Microbursts have been observed in the form of bouncing electron packets, which offer a unique opportunity to study their properties and importance as a loss process. We present a collection of bouncing microbursts observed by the HILT instrument on SAMPEX from 1994 to 2004. We analyze the locations of the bouncing microbursts in L and MLT and find they align well with the properties of relativistic microbursts as a whole. We find that the majority of bouncing microbursts observed by SAMPEX have scale sizes of ∼30 km at the point of observation, or ∼300 km when mapped to the magnetic equator. The time separation between the peaks of these bouncing microbursts is usually either half a bounce period or a whole bounce period.

Using the High attitude Interferometer WIND observation balloon and Antarctic Jang Bogo station high latitude conjugate observations of the thermospheric winds we investigate the seasonal and hemispheric differences between the northern and southern hemispheres in June 2018. We found that the summer (northern) hemisphere dayside meridional winds have a double-hump feature, whereas in the winter (southern) hemisphere the dayside meridional winds have a single hump feature. We attribute that to stronger summer, perhaps, northern hemisphere cusp heating. We also compared the observation with NCAR Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) model. The TIEGCM reproduced the double-hump feature because of added cusp heating. The summer hemisphere has stronger anti-sunward winds. This is the first time we have very high latitude conjugate thermospheric wind observations.

Recent simulation studies using the RAM-SCB model showed that proton precipitation contributes significantly to the total energy flux deposited into the subauroral ionosphere thereby affecting the magnetosphere-ionosphere coupling. In this study, we use the BATS-R-US + RAM-SCB model to understand the evolution of ElectroMagnetic Ion Cyclotron (EMIC) waves in the inner magnetosphere, their correspondence to the proton precipitation into the subauroral ionosphere, and to assess the performance of the model in reproducing the EMIC wave-particle interactions. During the 27 May 2017 storm, Arase and RBSP-A satellites observed typical signatures of EMIC waves in the inner magnetosphere. Within this interval, Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA)/MetOp satellites observed significant proton precipitation in the dusk-midnight sector. Simulation results show that H- and He-band EMIC waves are excited within regions of strong temperature anisotropy near the plasmapause. The simulated growth rates of EMIC waves show a similar trend to that of the EMIC wave power observed by the Arase and RBSP-A satellites, suggesting that the model can reproduce the EMIC wave activity qualitatively. The simulated H-band waves in the dusk sector are stronger than He-band waves possibly due to the presence of excess protons in the boundary conditions obtained from the BATS-R-US code. The precipitating proton fluxes reproduced by the simulation with EMIC waves are found to agree reasonably well with the DMSP and NOAA/MetOp satellite observations. It is suggested that EMIC wave scattering of ring current ions can account for proton precipitation observed by the DMSP and MetOp satellites during the 27 May 2017 storm.

In recent studies, two-dimensional propagation model of fast magnetosonic (MS) waves has been proposed to interpret the satellite observations of MS waves knocking into a density boundary. Although the theoretical model is able to capture the main properties of the two-dimensional propagation of MS waves, quantitative description about the MS wave behaviors has not been given yet. Here, with the assumption of a parabolic function for the potential function near its minimum, we solve the wave equation only with a potential function to obtain the reflection coefficients. It is found that the wave equation with a potential function can describe the full reflection and full transmission of MS waves rather well. Furthermore, the first-order derivative term in the wave equation is utilized to modify the reflection coefficient when the minimum of the potential function is near zero. Our result is helpful for further understanding the two-dimensional propagation of MS waves.

No abstract is available for this article.