JGR:Space physics

Different from power line harmonic radiation (PLHR) events at high harmonics (∼kHz) in the ionosphere and inner magnetosphere, the wave dynamics of power line emission (PLE) (the fundamental frequency 50/60 Hz or PLHR at low harmonics) can be significantly affected by various ion species. In order to investigate the evolution of the wave properties of PLE from power lines to satellite altitudes in a dipole field, a numerical model is developed to perform full-wave simulations, in which the lithosphere and atmosphere are characterized by electrical conductivity and the ionosphere (inner magnetosphere) is treated as collisional (collisionless) cold plasma consisting of electron, H+, He+, O+, and NO+. Our simulation results show that the spatial distribution and wave properties of PLE are determined by the magnetic latitudes of power lines and plasma densities. PLE from power lines at middle and high magnetic latitudes (|MLAT| > 40°) can propagate to high L shells as whistler waves; PLE from power lines at |MLAT| < 30° usually propagate at low L shells below local He+ cyclotron frequency as left-handedly polarized or right-handedly He+ band electromagnetic ion cyclotron (EMIC) waves. The amplitude of PLE is usually stronger with smaller electron density in the space plasma medium. With power lines at |MLAT| < 30°, the coupling efficiency between different right-handedly polarized EMIC wave modes of PLE decreases significantly with electron density. Wave properties of PLE including Poynting vector direction, wave normal angle and wave polarization obtained from our simulation results are consistent with some of the recent observations using Van Allen Probes.

The magnetosphere-ionosphere-thermosphere system is externally driven by the energy input from the solar wind. A part of the solar wind energy deposited in the magnetosphere during geomagnetically active periods dissipates into the thermosphere. Previous studies have reported temperature perturbations in the lower thermosphere during geomagnetic storms. The present study aims to assess the climatological spatial pattern of the lower thermospheric response to geomagnetic activity at high latitudes based on 21 years of temperature measurements by the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument onboard the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite and their comparison with the recently developed half-hourly geomagnetic activity index Hp30. The temperature response to geomagnetic activity, evaluated at different seasons and altitudes, is better organized in magnetic coordinates than in geographic coordinates. At 110 km, the temperature increases with Hp30 at all magnetic local times, but with a prominent dusk-dawn asymmetry in the magnitude. That is, the temperature variation per unit Hp30 is larger in the dusk sector than in the dawn sector. At 106 km, the response in the dawn sector is further reduced or even negative. These results provide observational evidence to support earlier theoretical predictions; according to which, both storm-induced vertical wind and Joule heating contribute to the temperature increase in the dusk sector, while in the dawn sector, the vertical wind acts to cool the air and thus counteracts Joule heating.

No abstract is available for this article.

The relationship between auroral, ground, and plasma sheet signatures in the late growth phase is crucial for understanding the sequence of events during a substorm expansion phase onset. Here we show conjugate ground-auroral-satellite observations of a substorm that occurred on 18 September 2021, between 04:45 and 05:00 UT, where four auroral activations were detected in the all-sky imagers. An initial activation showed the brightening of an equatorward arc within the cutoff of the 630 nm emissions, indicating activity on closed field lines well inside the open-closed field line boundary (OCFLB). During a second activation, auroral beads were observed on a brightening arc, equatorward and within the OCFLB, followed by the transformation from small-scale to large-scale vortices. The tail current sheet was highly disturbed during the auroral vortex evolution, including pressure and magnetic disturbances, an apparent broadening of a previously thin current sheet, and a breakdown of the frozen-in condition. Our observations clearly show late growth phase dynamics, including arc brightenings, the formation of auroral beads, and auroral vortex development, can occur well in advance of fast Earthward flows in the tail. Indeed, it is only during that later activity that auroral breakup and strong Earthward flows, which we associate with magnetic reconnection further down the tail, are observed together with strong magnetic bays on the ground. The sequence of events is consistent with an inside-to-outside model at substorm expansion phase onset, most likely via a shear-flow ballooning instability in the transition region from dipole to tail-like fields in the near-Earth plasma sheet.

Low-cost instrumentation combined with volunteering and citizen science educational initiatives allowed the deployment of L-band scintillation monitors to remote sense areas that are geomagnetically conjugated and located at low-to-mid latitudes in the American sector (Quebradillas in Puerto Rico and Santa Maria in Brazil). On 10 and 11 October, 2023, both monitors detected severe scintillations, some reaching dip latitudes beyond 26°N. The observations show conjugacy in the spatio-temporal evolution of the scintillation-causing irregularities. With the aid of collocated all-sky airglow imager observations, it was shown that the observed scintillation event was caused by extreme equatorial plasma bubbles (EPBs) reaching geomagnetic apex altitudes exceeding 2,200 km. The observations suggest that geomagnetic conjugate large-scale structures produced conditions for the development of intermediate scale (few 100 s of meters) in both hemispheres, leading to scintillation at conjugate locations. Finally, unlike previous reports, it is shown that the extreme EPBs-driven scintillation reported here developed under geomagnetically quiet conditions.

Collisionless shock waves are efficient ion accelerators. Previous numerical and observational studies have shown that quasi-parallel (Q ‖) shocks are more effective than quasi-perpendicular (Q ⊥) shocks at generating energetic ions under steady upstream conditions. Here, we use a local, 2D, hybrid particle-in-cell model to investigate how ion acceleration at super-critical Q ⊥ shocks is modulated when tangential discontinuities (TDs) with large magnetic shear are present in the upstream plasma. We show that such TDs can significantly increase the ion acceleration efficiency of 2D Q ⊥ shocks, up to a level comparable to Q ‖ shocks. Using data from the hybrid model and test particle simulations, we show that the enhanced energization is related to the magnetic field change associated with the discontinuity. When shock-reflected ions cross the TD during their upstream gyromotion, the sharp field change causes the ions to propagate further upstream, and gain additional energy from the convection electric field associated with the upstream plasma flow. Our findings illustrate that the presence of upstream discontinuities can lead to bursts of energetic ions, even when they do not trigger the formation of foreshock transients. These results emphasize the importance of time-variable upstream conditions when considering ion energization at shocks.

Space-based observations of the signatures associated with STEVE show how this phenomenon might be closely related to an extreme version of a SAID channel. Measurements show high velocities (>4 km/s), high temperatures (>4,000 K), and very large current density drivers (up to 1 μA/m2). This phenomena happens in a small range of latitudes, less than a degree, but with a large longitudinal span. In this study, we utilize the GEMINI model to simulate an extreme SAID/STEVE. We assume a FAC density coming from the magnetosphere as the main driver, allowing all other parameters to adjust accordingly. We have two main objectives with this work: show how an extreme SAID can have velocity values comparable or larger than the ones measured under STEVE, and to display the limitations and missing physics that arise due to the extreme values of temperature and velocity. Changes had to be made to GEMINI due to the extreme conditions, particularly some neutral-collision frequencies. The importance of the temperature threshold at which some collision frequencies go outside their respective bounds, as well as significance of the energies that would cause inelastic collisions and impact ionization are displayed and discussed. We illustrate complex structures and behaviors, emphasizing the importance of 3D simulations in capturing these phenomena. Longitudinal structure is emphasized, as the channel develops differently depending on MLT. However, these simulations should be viewed as approximations due to the limited observations available to constrain the model inputs and the assumptions made to achieve sensible results.

We provide observational evidence that the stability of the stratospheric Polar vortex (PV) is a significant driver of sub-seasonal variability in the thermosphere during geomagnetically quiet times when the PV is anomalously strong or weak. We find strong positive correlations between the Northern Annular Mode (NAM) index and subseasonal (10–90 days) Global Observations of the Limb and Disk (GOLD) O/N2 perturbations at low to mid-northern latitudes, with a largest value of +0.55 at ∼30.0°N when anomalously strong or weak (NAM >2.5 or < −2.1) vortex times are considered. Strong agreement for O/N2 variability and O/N2-NAM correlations is found between GOLD observations and the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) simulations, which is then used to delineate the global distribution of O/N2-NAM correlations. We find negative correlations between subseasonal variability in WACCM-X O/N2 and NAM at high northern and southern latitudes (as large as −0.54 at ∼60.0°S during anomalous vortex times). These correlations suggest that PV driven upwelling at low latitudes is accompanied by corresponding downwelling at high latitudes in the lower thermosphere (∼80–120 km), which is confirmed using calculations of residual mean meridional circulation from WACCM-X.

Ionospheric heavy ions in the distant tail of the Earth's magnetosphere at lunar distances are observed using the ARTEMIS mission. These heavy ions are originally produced in the terrestrial ionosphere. Using the ElectroStatic Analyzers (ESA) onboard the two probes orbiting the Moon, these heavy ions are observed as cold populations with distinct energies higher than the baseline energy of protons, with the energy-per-charge values for the heavy populations highly correlated with the proton energies. We conducted a full solar cycle survey of these heavy ion observations, including the flux, location, and drift energy, as well as the correlations with the solar wind and geomagnetic indices. The likelihood of finding these heavy ions in the preferred regions of observation called “loaded” quadrants of the terrestrial magnetotail is ∼90%, regardless of the z orientation of the IMF. We characterize the ratio of the heavy ion energy to the proton energy, as well as the velocity ratio of these two populations, for events from 2010 to mid-2023. This study shows that the “common velocity” assumption for the proton and heavy ion particles, as suggested in previous work through the velocity filter effect, is not necessarily valid in this case. Challenges in the identification of the mass of the heavy ions due to the ESA's lack of ion composition discrimination are addressed. It is proposed that at the lunar distances the heavy ion population mainly consists of atomic oxygen ions (O+) with velocities ∼25% more than the velocity of the co-located proton population.

Calculating the magnetic flux transfer across the open-closed boundary (OCB) per unit time and distance—the reconnection electric field—is an important means of remotely monitoring magnetospheric dynamics. Ground-based measurements of plasma convection velocities together with velocities of the OCB are commonly used to infer reconnection rates. However, this approach is limited by spatial coverage and often lacks robust uncertainty quantification. In this paper, we assimilate Super Dual Auroral Radar Network convection measurements, ground magnetometer data, and estimates of the conductance derived from the Imager for Magnetopause-to-Aurora Global Exploration satellite imagers, using the Local mapping of polar ionospheric electrodynamics (Lompe) framework over a region in North America. We present a new method to assess various contributions to uncertainties in the derived reconnection electric fields, including a novel approach to estimate uncertainties in conductance from global auroral imaging. Our method is demonstrated on a substorm event with an associated pseudobreakup during a period of favorable observational coverage. In this case study, the uncertainties in the reconnection electric field are ∼5–10 mV/m at the peak of substorm expansion, roughly 15% of the peak reconnection electric field. We find that the main contributor to the reconnection electric field estimates after substorm onset is the OCB motion, whereas during the pseudobreakup the main contributor is ionospheric plasma convection.

An unseasonal equatorial plasma bubble (EPB) event occurred in the East/Southeast Asian sector during the geomagnetic storm on 1 December 2023, causing strong amplitude scintillations from equatorial to middle latitudes. Based on the observations from multiple instruments over a large latitudinal and longitudinal region, the spatial features of the super EPB were investigated. The EPB developed vertically at a fast rising speed ∼470 m/s over the magnetic equator and extended to a very high middle latitude more than 40°N, despite that the storm intensity was not very strong with the minimum SYM-H index −132 nT. In the zonal direction, the super EPB covered over a specific region ∼95–140°E, where the local sunset roughly coincided with southward turning of interplanetary magnetic field (IMF) Bz component. Before the onset of the super EPB, significant upward plasma drift up to ∼110 m/s was observed over the magnetic equator, which could amplify the growth rate of Rayleigh-Taylor instability and lead to the generation of the super EPB. The significant drift was likely caused by eastward penetration electric field (PEF) due to sharp southward turning of IMF Bz. The local time of storm onset and duration of IMF Bz southward turning during the storm main phase may partly determine the onset region and zonal coverage of the EPB.

How the solar wind influences the magnetospheres of the outer planets is a fundamentally important question, but is difficult to answer in the absence of consistent, simultaneous monitoring of the upstream solar wind and the large-scale dynamics internal to the magnetosphere. To compensate for the relative lack of in-situ solar wind data, propagation models are often used to estimate the ambient solar wind conditions at the outer planets for comparison to remote observations or in-situ measurements. This introduces another complication: the propagation of near-Earth solar wind measurements introduces difficult-to-assess uncertainties. Here, we present the Multi-Model Ensemble System for the outer Heliosphere (MMESH) to begin to address these issues, along with the resultant multi-model ensemble (MME) of the solar wind conditions near Jupiter. MMESH accepts as input any number of solar wind models together with contemporaneous in-situ spacecraft data. From these, the system characterizes typical uncertainties in model timing, quantifies how these uncertainties vary under different conditions, attempts to correct for systematic biases in the input model timing, and composes a MME with uncertainties from the results. For the Juno-era (04/07/2016–04/07/2023) MME hindcast for Jupiter presented here, three solar wind propagation models were compared to in-situ measurements from the near-Jupiter spacecraft Ulysses and Juno spanning diverse geometries and phases of the solar cycle across >14,000 hr of data covering 2.5 decades. The MME gives the most-probable near-Jupiter solar wind conditions for times within the tested epoch, outperforming the input models and returning quantified estimates of uncertainty.

The Emirates Mars Ultraviolet Spectrometer (EMUS), aboard the Emirates Mars Mission (EMM), has been conducting observations of ultraviolet emissions within the Martian exosphere. Taking advantage of the distinctive orbit of the EMM around Mars, EMUS utilizes a dedicated strafe observation strategy to scan the illuminated Martian exosphere at tangential altitudes ranging from 130 to over 20,000 km. To distinguish between emissions of Martian origin and those from the interplanetary background, EMUS conducts specialized background observations by looking away from the planet. This approach has allowed us to investigate the radial and seasonal variations in Martian coronal emission features at H Lyman-α, β and γ wavelengths. Our analysis supports the previous studies indicating that Martian exospheric hydrogen Lyman emission brightness attains its highest levels around the southern summer solstice and reaches its lowest levels when Mars is near aphelion. Additionally, a secondary peak emission at all altitudes is observed after perihelion during Martian Year (MY) 36, which can be attributed to a Class C dust storm. Our study establishes a strong correlation between solar flux and coronal brightness for these emissions, highlighting the impact of solar activity on the visibility of Martian corona. In addition, we have examined interannual variability and found that emission intensities in MY 37 surpassed those in MY 36, primarily due to increased solar activity. These observations help to understand potential seasonal patterns of exospheric hydrogen, which is driven by underlying mechanisms in the lower atmosphere and solar activity, eventually suggesting an impact on water loss in the Martian atmosphere.

Quasi 16-day waves (Q16DWs) are a prominent and recurrent phenomenon in the middle atmosphere, typically observed over winter mid and high latitudes. This study investigates the intense Q16DW event during the 2018–2019 Northern Hemisphere (NH) winter, and explores its propagation in the middle atmosphere and its notable influence on the E-region ionosphere. Long-term geopotential height estimates of Aura Microwave Limb Sounder (MLS) reveal that the wave activity under consideration exhibited the largest amplitudes in the mesosphere for past 16 years. An analysis of wind data obtained from medium frequency (MF) and meteor radars, as well as from Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis, reveals the presence of a westward-propagating Q16DW with zonal wavenumber 1 exhibiting notable asymmetry about the equator, with the majority of the wave activity being confined to the NH. The prominently large amplitudes and vertical wavelengths of the wave suggest potential for the wave propagation to extend deep into the E-region ionosphere. Swarm satellite observations reveal concurrent ∼16-day oscillations in the eastward component of the geomagnetic field at low latitudes. These oscillations can be attributed to the periodic variations in interhemispheric field-aligned currents (IHFACs). The ∼16-day oscillations in the IHFACs are likely a consequence of asymmetric wind-dynamo action, which is directly or indirectly associated with the Q16DW. These findings suggest that planetary waves originating in the middle atmosphere can cause interhemispheric coupling in the ionosphere.

Interplanetary (IP) shocks are one of the dominant solar wind structures that can significantly impact the Geospace when impinge on the Earth's magnetosphere. IP shocks severely distort the magnetosphere and induce dramatic changes in the magnetospheric currents, often leading to large disturbances in the geomagnetic field. Sudden enhancements in the solar wind dynamic pressure (P Dyn) during IP shocks cause enhanced high-latitude convection electric fields which penetrate promptly to equatorial latitudes. In response, the equatorial electrojet (EEJ) current exhibits sharp changes of magnitudes primarily controlled by the change in P Dyn and the local time. In this paper, we further investigated the influence of shock impact angle on the EEJ response to a large number (306) of IP shocks that occurred during 2001–2021. The results consistently show that the EEJ exhibits a heightened response to the shocks that head-on impact the magnetosphere (frontal shocks) than those with inclined impact (inclined shocks). The greater EEJ response during the frontal shocks could be due to a more intensified high-latitude convection electric field resulting from the symmetric compression of the magnetosphere. Finally, an existing empirical relation involving P Dyn and local time is improved by including the effects of impact angle, which can quantitatively better predict the EEJ response to IP shocks.

This study focuses on the causes for the generation of equatorial plasma bubbles (EPBs) over the Indian subcontinent and their correlation with atmospheric-ionospheric disturbances resulting from the eruption of the Tonga volcano on 15 January 2022. Concurrent ionosonde observations obtained from Tirunelveli (8.67°N, 77.81°E) and Prayagraj (25.41°N, 81.93°E) show the presence of spread-F traces in ionograms. Notably, the EPBs are also accompanied by plasma blobs (PBs), with their pronounced occurrence during midnight at Prayagraj and Tirunelveli. Analysis of in situ electron density observations obtained from the Swarm B and C satellites reveals substantial plasma density depletions associated with EPBs. An intriguing observation is the intensification of Pre-Reversal Enhancement (PRE) immediately preceding the onset of spread-F at Tirunelveli due to enhanced eastward F region zonal winds by Tonga Volcano, as seen in the satellite observations. Furthermore, the isofrequency analysis from Tirunelveli shows the presence of gravity wave-like oscillations in the equatorial F-region over India. The investigation of Total Electron Content (TEC) obtained from a Pseudo Random Number (PRN)-14 over Indian longitudes suggests the presence of two dominant modes of Traveling Ionospheric Disturbances (TIDs) with speeds ∼452 m/s and ∼406 m/s having periods in the range of ∼65–75 min. These observations reaffirm that volcano triggered atmospheric/ionospheric disturbances can propagate long distances for several hours and can provide necessary seeding conditions for the generation of EPBs.

The intense electron-scale current structures (ECSs) with the current density J ≥ 30 nA/m2 are often observed in the Plasma Sheet (PS) during high-speed bulk flows. Using MMS observations we have analyzed 41 earthward and 37 tailward flow intervals and found 452 and 754 ECSs distributed over the PS region, respectively. Almost all ECSs are generated by high-speed electron beams. The duration of ECSs is ≤1 s, and many of them have a half-thickness L ≤ a few ρ e (ρ e is the gyroradius of thermal electrons). In such thin ECSs electrons become demagnetized and experience the dynamics like that observed in the electron diffusion region. Strong nonideal electric fields (E’) associated with violation of frozen-in condition for electrons are observed in the ECSs. This results in the intense energy conversion with J·E’ up to hundreds pW/m3. The major part of the dissipating energy is transferred to electron heating and acceleration. We suggest that the ECSs are manifestations of kinetic-scale turbulence driven by the high-speed ion bulk flows. The inductive electric fields generated by the growing magnetic fluctuations accelerate electron beams which, in turn, generate the ECSs. The ECSs thinning during their evolution, probably, stops for L ≤ a few ρ e . Further thinning leads to development of kinetic instability causing the current disruption and strong electric field generation. The last accelerates new electron beams which generate new ECSs in other locations. Thus, the life cycles of the ECSs contribute to energy cascade in turbulent plasma at electron kinetic scales.

In the Earth's magnetosheath (MSH), several processes contribute to energy dissipation and plasma heating, one of which is wave-particle interactions between whistler waves and electrons. However, the overall impact of whistlers on electron dynamics in the MSH remains to be quantified. We analyze 18 hr of burst-mode measurements from the Magnetospheric Multiscale (MMS) mission, including data from the unbiased magnetosheath campaign during February-March 2023. We present a statistical study of 34,409 whistler waves found using automatic detection. We compare wave occurrence in the different MSH geometries and find three times higher occurrence in the quasi-perpendicular MSH compared to the quasi-parallel case. We also study the wave properties and find that the waves propagate quasi-parallel to the background magnetic field, have a median frequency of 0.2 times the electron cyclotron frequency, median amplitude of 0.03–0.06 nT (30–60 pT), and median duration of a few tens of wave periods. The whistler waves are preferentially observed in local magnetic dips and density peaks and are not associated with an increased temperature anisotropy. Also, almost no whistlers are observed in regions with parallel electron plasma beta lower than 0.1. Importantly, when estimating pitch-angle diffusion times we find that the whistler waves cause significant pitch-angle scattering of electrons in the MSH.

A Southern Auroral Electrojet (SAE) index has been recently constructed using several Antarctica magnetometer stations. It has been compared for case studies with the standard Auroral Electrojet (AE) index, and a near-conjugate to the southern stations Northern Auroral Electrojet (NAE) index. We compare the three indices statistically as a function of the accompanying solar wind (SW) and Interplanetary Magnetic Field (IMF) conditions to further explore conjugacy issues. We use 274 days of common north/south data presence between December 2005 and August 2010. We calculate the cross-correlation coefficients and differences between all three pairs. We estimate the effect of the SW/IMF conditions on the index correlations and differences using three groups of data: (a) the entire data set, (b) two separate sets based on the presence or not of Southern Hemisphere stations within the 21-03 Magnetic Local Time (MLT) sector where substorms occur, and (c) separately for the four different seasons. We find that high north-south correlation coefficients are more common during strong SW/IMF driving, while the index differences are also higher, suggesting that the SAE index follows better the northern indices' trend, but has even lower values during active times. The UT study shows that the number of high AE/SAE correlations is slightly lower at all clock angles and dynamic pressure levels for the periods within 1454–1941 UT (when no southern station is within 21–03 MLT). Finally, the results show that the number of high correlations is greater during the northern spring than the winter period.

The Magnetospheric Multiscale (MMS) mission has probed Earth's magnetosphere, magnetosheath, and near-Earth solar wind for over 8 years. We utilize an unsupervised learning algorithm, Gaussian mixture model clustering, along with feature generation and simple post-cleaning methods to automatically classify 8 years of MMS dayside observations into four plasma regions (magnetosphere, magnetosheath, solar wind, and ion foreshock) at 1-min resolution. With these plasma regions distinguished, we have also identified boundary surfaces (e.g., magnetopause, bow shock). We validate our results on manually generated and rule based region labels described in the literature. We report overlap rates in our cluster determined magnetopauses and bow shocks against Scientist-in-the Loop (SITL) identified transitions and published databases. Our features are general and our model is extensible, potentially making it applicable to observational data from multiple other missions.