JGR:Space physics

The east–west component of the geomagnetic field (Y-component) at the Trivandrum (TRD) station, an equatorial site, is used to investigate the characteristics of Inter-Hemispheric Field-Aligned Currents (IHFACs) in the Indian sector during solar cycles 21 and 22. The observations reveal an annual variation of IHFAC in the dawn/noon sector with positive/negative peaks around northern summer. Furthermore, it is noted that IHFAC amplitudes are modulated by solar activity, superimposed with seasonal effects, with consistently higher amplitudes during the northern summer months. The dusk-time IHFAC also shows a pattern modulated by seasons and solar cycles, with negative peaks around northern summer only during the low solar active years. The seasonal pattern of IHFAC shows a similar trend from April to September, where dawn/noon IHFAC is south/north directed, and for the remaining months, both are northward directed. The dusk-side IHFAC is mostly south-directed irrespective of the season. Moreover, it is demonstrated that the seasonal pattern of dawn and noon-time IHFAC amplitudes show a strong to moderate correlation with amplitudes of Sq asymmetries between two hemispheres, but dusk-time IHFAC does not follow this trend. The IHFAC directions at TRD follow the Fukushima model for May to September during the dawn-noon sector for 70%–95% of the days and less than 50% of the days for remaining months. The noon-dusk sector shows Fukushima model-type polarity for ∼60% of days in June–July and less than 50% for remaining months. The global analysis of IHFAC, using monthly averages of hourly values from eight observatories spanning Asia, Africa, and America, reveals spatial differences in amplitude and directions. The directions of noon-time IHFAC in the Indian-East Asian and East-African sectors follow similar trends, and the amplitude variations of noon-time IHFAC can be explained by DE-3 tidal effects. The IHFAC from the South American and West African sectors show intense southward and northward currents compared to other sites, indicating the influence of the South Atlantic Anomaly (SAA).

We present the first observations of the dayside coronal oxygen emission in far ultraviolet (FUV) measured by the Emirates Mars Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM). The high sensitivity of EMUS is providing an opportunity to observe the tenuous oxygen corona in FUV, which is otherwise difficult to observe. Oxygen resonance fluorescence emission at 130.4 nm provides a measurement of the upper atmospheric and exospheric oxygen. More than 500 oxygen corona profiles are constructed using the long–exposure time cross–exospheric mode (OS4) of EMUS observations. These profiles range from ∼200 km altitude up to several Mars radii (>6 RM) across all seasons and for two Mars years. Our analysis shows that OI 130.4 nm is highly correlated with solar irradiance (solar photoionizing and 130.4 nm illuminating irradiances) as well as changes in the Sun–Mars distance. The prominent short term periodicity in oxygen corona brightness is consistent with the solar rotation period (quasi–27–day). A comparison between the perihelion seasons of Mars Year (MY) 36 and MY 37 shows interannual variability with enhanced emission intensities during MY 37, due to the rise of Solar Cycle 25. These observations show a highly variable oxygen corona, which has significant implications on constraining the photochemical escape of atomic oxygen from Mars.

The magnetotail current sheet's (CSs) spatial configuration and stability control the onset of magnetic reconnection - the driving process for magnetospheric substorms. The near-Earth CS has been thoroughly investigated by numerous missions, whereas the midtail CS has not been adequately explored. This is especially the case for the long-term variation of its configuration in response to the solar wind. We present a statistical analysis of 1261 magnetotail CS crossings by the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission orbiting the moon (X ∼ −60 R E ), collected during the entirety of Solar Cycle 24. We demonstrate that the magnetotail CS typically remains extremely thin, with a characteristic thickness comparable to the thermal ion gyroradius, even at large distances from Earth's dipole. We also find that a substantial fraction (∼one quarter) of the observed CSs have a partially force-free magnetic field configuration, with a significant contribution of the magnetic field shear component to the pressure balance. Further, we quantify the impact of the changing solar wind driving conditions on the properties of the midtail around the lunar orbit. During active solar wind driving conditions, we observe an increase in the occurrence rate of thin CSs, whereas quiet solar wind driving conditions seem to favor the formation of partially force-free CSs.

A drift-diffusion model is used to simulate the low-altitude electron distribution, accounting for azimuthal drift, pitch angle diffusion, and atmospheric backscattering effects during a rapid electron dropout event on 21st August 2013, at L = 4.5. Additional external loss effects are introduced during times when the low-altitude electron distribution cannot be reproduced by diffusion alone. The model utilizes low-altitude electron count rate data from five POES/MetOp satellites to quantify pitch angle diffusion rates. Low-altitude data provides critical constraint on the model because it includes the drift loss cone region where the electron distribution in longitude is highly dependent on the balance between azimuthal drift and pitch angle diffusion. Furthermore, a newly derived angular response function for the detectors onboard POES/MetOp is employed to accurately incorporate the bounce loss cone measurements, which have been previously contaminated by electrons from outside the nominal field-of-view. While constrained by low-altitude data, the model also shows reasonable agreement with high-altitude data. Pitch angle diffusion rates during the event are quantified and are faster at lower energies. Precipitation is determined to account for all of the total loss observed for 450 keV electrons, 88% for 600 keV and 38% for 900 keV. Predictions made in the MeV range are deemed unreliable as the integral energy channels E3 and P6 fail to provide the necessary constraint at relativistic energies.

Mercury is the smallest and innermost planet of our solar system and has a dipole-dominated internal magnetic field that is relatively weak, very axisymmetric and significantly offset toward north. Through the interaction with the solar wind, a magnetosphere is created. Compared to the magnetosphere of Earth, Mercury's magnetosphere is smaller and more dynamic. To understand the magnetospheric structures and processes we use in situ MESSENGER data to develop further a semi-empiric model of the magnetospheric magnetic field, which can explain the observations and help to improve the mission planning for the BepiColombo mission en-route to Mercury. We present this semi-empiric KTH22-model, a modular model to calculate the magnetic field inside the Hermean magnetosphere. Korth et al. (2015, https://doi.org/10.1002/2015JA021022, 2017, https://doi.org/10.1002/2017gl074699) published a model, which is the basis for the KTH22-model. In this new version, the representation of the neutral sheet current magnetic field is more realistic, because it is now based on observations rather than ad-hoc assumptions. Furthermore, a new module is added to depict the eastward ring shaped current magnetic field. These enhancements offer the possibility to improve the main field determination. In addition, analyzing the magnetic field residuals allows us to investigate the field-aligned currents and their possible dependencies on external drivers. We see increasing currents under more disturbed conditions inside the magnetosphere, but no clear dependence on the z-component of the interplanetary magnetic field nor on the magnetosheath plasma β.

This study investigates the diurnal variation of topside ionospheric plasma density using the Ionospheric Connection Explorer (ICON) observations from May to July in 2020 and 2021. The total ion density exhibits daytime double-maxima (DDM) patterns, also known as “bite-out” at magnetic latitudes from 10°S to 20°N and longitudes of 180°–276°E, but a single peak in other longitudes. The total ion density between 180° and 276°E reaches its first peak around 12 LT (Local Time) in both hemispheres and the second peak at 14–15 LT in the Northern Hemisphere, gradually shifting to around 16–17 LT in the Southern Hemisphere. The formation of the first peak is mainly influenced by vertical plasma drift, while the second peak is associated with both neutral wind and vertical plasma drift. Furthermore, this study provides direct observational evidence for the influence of neutral winds on the longitude and latitude differences of the topside DDM. Stronger southward magnetic meridional winds and vertical plasma drifts are observed at approximately 11–15 LT in the longitude sector with DDM compared to other longitudes, leading to a valley of ion density in the Southern Hemisphere around 15 LT. In the Northern Hemisphere, ion density continues to accumulate to form a second peak at 15 LT.

The Low lAtitude long Range Ionospheric raDar (LARID), which consists of two high frequency (HF) radars looking toward the east and west of Hainan Island, respectively, has been developed and installed at Dongfang (19.2°N, 108.8°E, dip lat. 13.8°N), China. This paper describes the system design of LARID and its first observational results of equatorial plasma bubble (EPB) irregularities. The antenna array of LARID is composed of a west-looking array and an east-looking array. Each array consists of 20 log-periodic antennas for transmission and reception and 4 log-periodic antennas for interferometry, and has a beam steering capability in the azimuth angles of  ±24° due the boresight pointing east (or west). Observational results show that the LARID is capable of detecting backscatter echoes from EPB irregularities and the ground, with a distance of 4,000 km or more away from Hainan Island. Multiple EPB structures were continuously observed on 17 April 2023, with eastward drifts ranging between 70 and 130 m/s. Based on ray tracing simulations, the backscatter echoes of EPB irregularities were due to the 0.5-hop and 1.5-hop propagation modes. The distances between the successive EPB structures were estimated ranging between 500 and 900 km in longitude. The LARID observations, together with other instruments in the East and Southeast Asian sector, provided a clear picture of longitudinal variation of EPBs in 90–125°E. It is expected that the LARID will provide an important tool to study the generation and evolution of EPBs and their short-term prediction in East and Southeast Asia.

Space-weather conditions can often have a detrimental impact on satellite communications and limited experimental data has made it challenging to understand the complex processes that occur in the upper atmosphere. To overcome this challenge, we utilized a coordinated multi-instrumental dataset consisting of GNSS airglow remote sensing, ionosonde, magnetometer, and in-situ satellite data to investigate plasma depletions. We present a case study focused on the geomagnetic storm that occurred on 27 February 2014. During the storm, GNSS positioning errors exceeded undisturbed levels by at least 2 times, and ionospheric corrections reached amplitudes of up to ±20 m at the Rabat station. We identified 3 large depletions that were most likely generated by sudden vertical ionospheric drifts that began at approximately 17:00 UTC at sunset in Morocco and the southern regions of Spain. These drifts reached ∼500 m/s and lasted until 22:00 UTC. The observed depletions propagated to the northeast, as seen through ionosonde echoes and ground-based airglow images. Satellite limb-images revealed an ionospheric uplift of about 100 km due to the storm, consistent with ionosondes in Spain. The observed local anomalies may be influenced by variations in equatorial electric current flows, which are correlated with fluctuations in ground-based magnetometer data. These variations are likely a result of the effects of the inner radiation belt on the development of plasma bubbles in the African longitude sector. Sudden enhancements in upward E × B drift caused ionospheric uplift to higher altitudes, enhancing the “fountain effect” and shifting the Equatorial Ionospheric Anomaly crests to higher latitudes.

The abundant observations and research established a detailed category of the terrestrial ionospheric irregularities, which significantly advanced our understanding of how the Earth system's complicated physical and chemical process generates the intermediate-scale structures of the charged particles. Motivated by a future attempt at categorizing the Martian ionospheric irregularity, this study designs a method for naive classification of the plasma density depletion, enhancement, and oscillation based on the in situ measurements of the Martian ionosphere. The technique consists of several procedures: trend estimation, detrending and candidate extraction, and parameterization. The classification is achieved through a machine-learning-like process using some testing artificial density profiles. A preliminary credence test shows a good performance in separating the terrestrial low-latitude Equatorial Plasma bubble (depletion) and mid-latitude Median-scale Traveling Ionospheric Disturbance (oscillation). Another detection experiment of the Martian plasma depletion events (collected by Basuvaraj et al. (2022a, https://doi.org/10.1029/2022je007302)) showed a recall rate (i.e., true positive) of 38% but with a high precision of 67.8%. Therefore, we believe the proposed method could convincingly extract different Martian ionospheric irregularities and help uncover the climatological characteristics in the future.

An ultralow frequency (ULF) wave was simultaneously observed in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) radar at Hankasalmi, Finland and on the ground by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers with close proximity to the radar. The onset time of the wave event was around 03:00 magnetic local time. Fourier wave analysis of the event suggests a wave period of about 1,340 s with an equatorward latitudinal and eastward longitudinal wave phase propagation, and an effective azimuthal wave number of 17 ± 1, in the intermediate range of those observed in ULF waves. This wave has been interpreted as resulting from drifting electrons of energies of 13 ± 5 keV in a drift resonance condition linked to energetic particle populations during a magnetospheric substorm. The latitudinal phase characteristics of this wave experienced temporal evolution, believed to be caused by additional injected particle populations associated with the same substorm driving the wave, which resulted in an observed loss of HF backscatter. This observation of a unique type of temporal evolution in the phase propagation characteristics of ULF waves enhances current understanding about the structure, dynamics and source of these types of ULF waves.

Large total electron content (TEC) variations along the line from Mars to Earth have been demonstrated by analyzing the ground received Tianwen-1 differential one-way range (DOR) signals when corona mass ejections (CMEs) and co-rotating interaction regions (CIRs) pass through the signal path. Here, the TEC variations along the line from Mars to Earth are calculated from the Wang-Sheeley-Arge (WSA)-Enlil space weather forecast model. The results are compared with the Tianwen-1 line of sight (LOS) observations as well as the STEREO-A in situ solar wind density measurements. It is found that the TEC variations calculated from the WSA-Enlil model are usually much smaller in amplitude, especially in the cases of CIRs passing through the signal path. In addition, the timings of CMEs and CIRs passing through the DOR signal path in the WSA-Enlil model can be half a day earlier or later than the observed ones. In comparison with in situ solar wind measurements, the ground received Tianwen-1 DOR signals carry solar wind density information over a large spatial region along the signal path. Such remote sensing measurement can be valuable for constraining and improving global solar wind forecast models.

The dual frequency Global Navigation Satellite System (GNSS) observations could determine the total electron content (TEC) in the ionosphere. In this study, GNSS TEC was applied to detect traveling ionospheric disturbances (TIDs) after the eruption of Hunga Tonga–Hunga Ha’apai (HTHH) on 15 January 2022. The eruption caused two types of tsunamis, first is tsunami generated by atmospheric wave (meteo-tsunami) and second is caused by eruption induces water displacement or tsunami classic. At the same time with former tsunami, the atmospheric wave (shock and lamb waves) also caused TIDs at a speed of approximately ∼0.3 km/s. We found moderate correlation between this TIDs amplitude and the tsunami wave height model from tide gauge stations in New Zealand (0.64) and Australia (0.65). Further we attempted to reveal 3D structure of the TIDs in New Zealand, South Australia, and Philippines using 3D tomography. The tomography was set up > 1,170 blocks, as large as 1.0° (east–west)  × 1.0° (north–south) × 100 km (vertical), up to 600 km altitude over selected regions. Tomogram shows beautiful concentric directivity of the first TIDs generated by atmospheric wave (AW).

Ions in the F region ionosphere at 150–400 km altitude consist mainly of molecular NO+ and O2+ ${\mathrm{O}}_{2}^{+}$, and atomic O+. Incoherent scatter (IS) radars are sensitive to the molecular-to-atomic ion density ratio, but its effect to the observed incoherent scatter spectra is almost identical with that of the ion temperature. It is thus very difficult to fit both the ion temperature and the fraction of O+ ions to the observed spectra. In this paper, we introduce a novel combination of Bayesian filtering, smoothness priors, and chemistry modeling to solve for F1 region O+ ion fraction from EISCAT Svalbard IS radar (75.43° corrected geomagnetic latitude) data during the international polar year (IPY) 2007–2008. We find that the fraction of O+ ions in the F1 region ionosphere is controlled by ion temperature and electron production. The median value of the molecular-to-atomic ion transition altitude during IPY varies from 187 km at 16–17 MLT to 208 km at 04–05 MLT. The ion temperature has maxima at 05–06 MLT and 15–16 MLT, but the transition altitude does not follow the ion temperature, because photoionization lowers the transition altitude. A daytime transition altitude maximum is observed in winter, when lack of photoionization leads to very low daytime electron densities. Both ion temperature and the molecular-to-atomic ion transition altitude correlate with the Polar Cap North geomagnetic index. The annual medians of the fitted transition altitudes are 14–32 km lower than those predicted by the International Reference Ionosphere.

An unusually low density solar wind event was observed in December 2022 moving past both Earth and Mars. The source was traced back to a coronal hole and active region on the Sun's surface. The resulting solar wind lead to the development of a co-rotating interaction region (CIR) and trailing rarefaction region that lasted for multiple solar rotations. Within this structure, the solar wind conditions, including density, velocity, and magnetic field magnitude and orientation drastically changed. In this study we analyze the response of the Martian ionosphere using MAVEN data to these changing solar wind conditions. The low density solar wind region associated with the December event resulted in the expansion of the Martian ionospheric boundaries. We show that the ion composition boundary (ICB) is located at extreme altitudes that are beyond previously observed locations from the MAVEN mission between 2015 and 2018. Furthermore, the boundary between shocked solar wind and the Martian ionosphere identified using electron and ion data moved together on the dayside of the planet with the changing solar wind conditions. However, at the flank region these boundaries do not move together, and we show here that the decoupling of the two boundaries may be the result of a change in the interplanetary magnetic field azimuthal angle.

Fabry Perot interferometer (FPI) is an essential ground-based passive optical observation device to detect middle and upper atmospheric information. Three FPIs are located at Kunming (103.8°E, 25.6°N), Xinglong (117.4°E, 40.2°N), and Mohe (122.3°E, 53.5°N), China. The diurnal and annual variation of night wind at 87, 97, and 250 km are investigated from 2019 to 2021, compared to Horizontal Wind Model 14 (HWM14) to check the prediction accuracy for the local wind field. Our results are as follows: (a) At 87 km, the zonal winds are similar in Kunming and Xinglong, but the meridional winds are generally stronger in Kunming. The zonal and meridional winds in both locations are dominated by semidiurnal variations. (b) At 97 km, the duration of semidiurnal variation of zonal winds and diurnal variation of meridional winds in Kunming is long, which is opposite to that of Xinglong. (c) At 250 km, the wind speed increases with latitude for both zonal and meridional winds, which are both dominated by diurnal variations for all the three sites. Unlike 87 and 97 km, the entire wind field at 250 km is dominated by annual variation, except for a significant semiannual variation at midnight in Mohe. (d) Overall, the HWM14 predictions are stronger than the FPI measurements at peak wind speeds and similar at 87 and 97 km, which surpasses the performance at 250 km. Especially at 250 km, the model results are worse for the zonal winds of Kunming and the meridional winds of Mohe. These results will help to understand the wind field in the middle and upper atmosphere and improve the accuracy of the model.

In 1995, the Galileo spacecraft traversed the wake of Io at ∼900 km altitude. The instruments onboard detected intense bi-directional field-aligned electron beams (∼140 eV–150 keV), embedded in a dense, cold and slow plasma wake (N el ∼ 35,000 cm−3, T i  < 10 eV, V < 3 km/s). Similar electron beams were also detected along subsequent Galileo flybys. Using numerical simulations, we show that these electron beams are responsible for the formation of Io's dense plasma wake. We prescribe the composition of Io's atmosphere in S, O, SO and SO2, compute the atmospheric ionization by the beams with a parameterization adapted from study of auroral electrons at Earth, the plasma flow into Io's atmosphere with a Magneto-Hydro-Dynamic code, and the ion composition and temperature with a multi-species physical chemistry code. Results reveal contrasting chemistries between the upstream and wake regions, leading to different ion compositions. The upstream chemistry is driven by the torus thermal electrons at 5 eV with SO2 + becoming the dominant ion because of electron-impact ionization of the SO2 atmosphere. The wake chemistry is driven by the high-energy electrons in the beams with S+ and SO+ becoming the dominant ions produced by dissociative-ionization of SO2. We show that the wake ion composition is highly sensitive to the atmospheric composition. Juno, in its extended mission, will traverse Io's wake and determine its ion composition, which, compared with our numerical simulations will enable us to infer the detailed composition of the atmosphere.

Auroral images of the N2 Lyman-Birge-Hopfield emissions from Polar Ultraviolet Imager (UVI) for 1.5 years are used to investigate the auroral characteristics related to the AU and AL indices that represent the directly driven and unloading processes in the solar wind-magnetosphere-ionosphere coupling, respectively. Findings include: (a) Growing AL mainly relates to the nightside aurora and tends to keep the general auroral morphology. (b) As AU increases, the aurora brightens and expands more globally, especially in the midnight to postmidnight sector. (c) The regional auroral power (AP), equatorward boundary, poleward boundary, and peak intensity change quasi-linearly with intensifying AU and AL. (d) The rates of change depend on the MLT and AU/AL levels. The same dataset has been used to construct the empirical Feature Tracking of Auroral Precipitation (FTA) model which specifies the global energy flux and mean energy determined by the AE index (FTA-AE). As an extension of FTA-AE, the relationships of the auroral emission with the AU&AL indices were derived to construct the FTA-AU&AL model which specifies a more consistent aurora during different magnetospheric driving modes compared to FTA-AE. Comparisons of AP from the Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) measurements and empirical auroral models show that the FTA-AE and -AU&AL models predicted larger AP than the Fuller-Rowell and Evans (1987) model and the OVATION-prime model did. As the activity level increased, all four models tended to underestimate the AP but the FTA APs increased relatively faster and were therefore more consistent with the data.

At Mars, the solar wind is usually decelerated and heated at the bow shock, then diverted around the planet by the induced magnetosphere. A recent study by Crismani et al. (2019, https://doi.org/10.1029/2018ja026251), however, presented evidence of near-pristine solar wind below the exobase of Mars (<200 km) during one Mars Atmosphere and Volatile EvolutioN (MAVEN) periapsis pass, implying the solar wind penetrated into the upper atmosphere with little modification. In this work, we search through 7 years of MAVEN Solar Wind Ion Analyzer (SWIA) periapsis observations to determine how often, and the conditions under which, these low-altitude solar wind events occur. We find 23 candidate events that contain signatures of the solar wind below 200 km. The events are much more common at low solar zenith angles and tend to occur over weak crustal field regions where the radial component of the field is oriented downward. The observations also point to significant interactions between the incoming solar wind and the neutral atmosphere, including evidence of solar wind alphas becoming singly ionized helium through collisions with atmospheric CO2. Finally, we find the events are four times more likely to be detected when the interplanetary magnetic field (IMF) is radial (cone angle <30°), suggesting the events are triggered when the IMF is nearly aligned with the solar wind flow.

We analyze data acquired by the Kaguya satellite on 14 October 2008 when the Moon was in the terrestrial magnetotail lobe to gain new insight into the energization of ions originating from the Moon. The Moon-originating ions were detected over a broad range of latitudes from −80° to 50° above the Moon's dayside at ∼100 km altitude. The fluxes of the Moon-originating ions were observed at energies from ∼50 to ∼1,000 eV. Additionally, these ions exhibited a wide distribution pitch angle spanning from ∼45 to 90°. The energy levels of ions originating from the Moon show rapid changes, either increasing or decreasing by a factor of ∼10 within 8 min without the solar zenith angle dependence. Such rapid energy changes were observed over the highland regions. These observations are discussed in light of possible acceleration mechanisms of Moon-originating ions, including temporal and spatial effects.

Geomagnetic storms are critical space weather phenomena resulting from the interaction between the solar wind and the Earth's magnetosphere. However, most studies focus on the main phase of magnetic storms, leaving the morphology of the recovery phase an open question. In this study, we analyze 82 intense magnetic storms with the minimum Dst index ≤ −100 nT between 1995 and 2018, finding that these storms can be classified into two distinct types: one-stage recovery storms that exhibit a single rapid exponential recovery and two-stage recovery storms that are characterized by a rapid exponential recovery in the early recovery phase and a slow linear recovery in the later recovery phase. We find that the two-stage recovery storms are dominant, accounting for approximately 60% of the events. Interestingly, the proportion of two-stage recovery storms peaks during solar minimum. The two-stage recovery storms tend to be accompanied by more Alfvén waves with long-duration and intense southward interplanetary magnetic fields. In addition, we find that the decay rate of the Dst index in the later recovery phase is correlated with the average B Z of the interplanetary magnetic field when the solar wind has a high degree of Alfvénicity. Overall, our results shed new light on the recovery phase morphology of intense magnetic storms and highlight the role of Alfvén waves in this process.