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We have used THEMIS measurements to determine how the ion and electron temperatures and their ratio (Ti/Te) change spatially in the magnetosheath and plasma sheet and to identify the processes responsible for the variations. Magnetosheath Ti/Te varies from ∼4–12 with higher ratios observed during larger solar wind speed and at locations closer to the magnetopause. Ti/Te remains almost unchanged as particles flow downstream and cool adiabatically. Across the flank magnetopause from the magnetosheath to a plasma sheet that is cool with abundant cold plasma, temperature and specific entropy for ions and electrons increase significantly while Ti/Teremains similar, indicating that the magnetosheath ions and electrons are non‐adiabatically energized with similar proportion while entering the magnetosphere. Within the tail plasma sheet, Ti/Te varies from ∼6 to 10 when plasma is relatively cool to ∼2 to 5 when relatively warm. With this correlation, Ti/Te is higher closer to the flanks and during northward interplanetary magnetic field (IMF), while lower Ti/Te is more often seen during higher AE around midnight. The distinguishably lower Ti/Tefor warmer plasma in the near‐Earth plasma sheet is likely due to additional non‐adiabatic heating of electrons more than ions as particles move earthward and are adiabatically energized. As particles move into the near‐Earth magnetosphere, strengthening magnetic drift brings more hotter ions toward dusk and more hotter electrons toward dawn, resulting in a strong Ti/Tedawn‐dusk asymmetry with very high Ti/Te (∼15 to 100) near dusk and very low Ti/Te (∼1) near dawn. Key Points Temperature ratio in the magnetosheath is larger during higher solar wind speed Temperature ratio in the plasma sheet is larger when plasma is colder Strong dawn‐dusk ratio asymmetry in the near‐Earth magnetosphere

A new observational phenomenon, named Simultaneous Global Ionospheric Density Disturbance (SGD), is identified in GNSS total electron content (TEC) data during periods of three typical geospace disturbances: a Coronal Mass Ejection‐driven severe disturbance event, a high‐speed stream event, and a minor disturbance day with a maximum Kp of 4. SGDs occur frequently on dayside and dawn sectors, with a ∼1% TEC increase. Notably, SGDs can occur under minor solar‐geomagnetic disturbances. SGDs are likely caused by penetration electric fields (PEFs) of solar‐geomagnetic origin, as they are associated with Bz southward, increased auroral AL/AU, and solar wind pressure enhancements. These findings offer new insights into the nature of PEFs and their ionospheric impact while confirming some key earlier results obtained through alternative methods. Importantly, the accessibility of extensive GNSS networks, with at least 6,000 globally distributed receivers for ionospheric research, means that rich PEF information can be acquired, offering researchers numerous opportunities to investigate geospace electrodynamics. Plain Language Summary Electric fields of solar wind and geomagnetic disturbance origin can penetrate into the low latitude upper atmosphere, influencing the ionospheric dynamics and electron density variations. This study employs a new method that utilizes global and continuous GNSS total electron content (TEC) observations to investigate the electric field effects. The analysis focuses on three geospace disturbance events of different intensities and solar‐terrestrial conditions. The study identifies a novel phenomenon named Simultaneous Global Ionospheric Density Disturbance (SGD), primarily occurring on the sunlit portion of the Earth's ionosphere and also near dawn hours with 1% or larger amplitudes of the background TEC, or a few tenths of a TEC unit (1016 m3). The remarkable global extent of ionospheric responses to minor solar‐geomagnetic conditions is noteworthy. The solar wind magnetic field directed southward is highly correlated with most SGDs, lasting for up to 30 min. The findings present an effective approach for continuously monitoring electric field penetration and ionospheric impacts, leading to an improved understanding of space weather and its technological implications. Key Points Simultaneous global ionospheric disturbances (SGDs) are often observed even during minor solar and geomagnetic disturbances SGDs occur predominately on dayside and are related to penetration electric fields (PEFs) of solar wind and geomagnetic disturbance origin Global GNSS networks offer a novel and effective technique for continuous PEF monitoring, providing rich data sets for further study

A distinct class of aurora, called transpolar auroral arc (TPA) (in some cases called “theta” aurora), appears in the extremely highlatitude ionosphere of the Earth when interplanetary magnetic field (IMF) is northward. The formation and evolution of TPA offers clues about processes transferring energy and momentum from the solar wind to the magnetosphere and ionosphere during a northward IMF. However, their formation mechanisms remain poorly understood and controversial. We report a mechanism identified from multiple-instrument observations of unusually bright, multiple TPAs and simulations from a high-resolution three-dimensional (3D) global MagnetoHydroDynamics (MHD) model. The observations and simulations show an excellent agreement and reveal that these multiple TPAs are generated by precipitating energetic magnetospheric electrons within field-aligned current (FAC) sheets. These FAC sheets are generated by multipleflow shear sheets in both the magnetospheric boundary produced by Kelvin–Helmholtz instability between supersonic solar wind flow and magnetosphere plasma, and the plasma sheet generated by the interactions between the enhanced earthward plasma flows from the distant tail (less than −100 RE) and the enhanced tailward flows from the near tail (about −20 RE). The study offers insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it challenges existing paradigms of the dynamics of the Earth’s magnetosphere.

In Earth’s low atmosphere, hurricanes are destructive due to their great size, strong spiral winds with shears, and intense rain/precipitation. However, disturbances resembling hurricanes have not been detected in Earth’s upper atmosphere. Here, we report a long-lasting space hurricane in the polar ionosphere and magnetosphere during low solar and otherwise low geomagnetic activity. This hurricane shows strong circular horizontal plasma flow with shears, a nearly zero-flow center, and a coincident cyclone-shaped aurora caused by strong electron precipitation associated with intense upward magnetic field-aligned currents. Near the center, precipitating electrons were substantially accelerated to ~10 keV. The hurricane imparted large energy and momentum deposition into the ionosphere despite otherwise extremely quiet conditions. The observations and simulations reveal that the space hurricane is generated by steady high-latitude lobe magnetic reconnection and current continuity during a several hour period of northward interplanetary magnetic field and very low solar wind density and speed. Hurricanes in the Earth’s low atmosphere are known, but not detected in the upper atmosphere earlier. Here, the authors show a long-lasting hurricane in the polar ionosphere and magnetosphere with large energy and momentum deposition despite otherwise extremely quiet conditions.

Bright auroral emissions during geomagnetic storms provide a good opportunity for testing the proposal that substorm onset is frequently triggered by plasma sheet flow bursts that are manifested in the ionosphere as auroral streamers. We have used the broad coverage of the ionospheric mapping of the plasma sheet offered by the high-resolution THEMIS all-sky-imagers (ASIs) and chose the main phases of 9 coronal mass ejection (CME) related and 9 high-speed stream (HSS)-related geomagnetic storms, and identified substorm auroral onsets defined as brightening followed by poleward expansion. We found a detectable streamer heading to near the substorm onset location for all 60 onsets that we identified and were observed well by the ASIs. This indicates that substorm onsets are very often triggered by the intrusion of plasma with lower entropy than the surrounding plasma to the onset region, with the caveat that the ASIs do not give a direct measure of the intruding plasma. The majority of the triggering streamers are “tilted streamers,” which extend eastward as their eastern tip tilts equatorward to near the substorm onset location. Fourteen of the 60 cases were identified as “Harang streamers,” where the streamer discernibly turns toward the west poleward of reaching to near the onset latitude, indicating flow around the Harang reversal. Using the ASI observations, we observed substantially less substorm onsets for CME storms than for HSS storms, a result in disagreement with a recent finding of approximately equal substorm occurrences. We suggest that this difference is a result of strong non-substorm streamers that give substorm-like signatures in ground magnetic field observations but are not substorms based on their auroral signature. Our results from CME storms with steady, strong southward IMF are not consistent with the ~ 2–4 h repetition of substorms that has been suggested for moderate to strong southward IMF conditions. Instead, our results indicate substantially lower substorm occurrence during such steady driving conditions. Our results also show the much more frequent occurrence of substorms during HSS period, which is likely due to the highly fluctuating IMF.

The Earth’s magnetosphere is the region of space where plasma behavior is dominated by the geomagnetic field. It has a long tail typically extending hundreds of Earth radii ( R E ) with plentiful open magnetic fluxes threading the magnetopause associated with magnetic reconnection and momentum transfer from the solar wind. The open-flux is greatly reduced when the interplanetary magnetic field points northward, but the extent of the magnetotail remains unknown. Here we report direct observations of an almost complete disappearance of the open-flux polar cap characterized by merging poleward edges of a conjugate horse-collar aurora (HCA) in both hemispheres’ polar ionosphere. The conjugate HCA is generated by particle precipitation due to Kelvin-Helmholtz instability in the dawn and dusk cold dense plasma sheets (CDPS). These CDPS are consist of solar wind plasma captured by a continuous dual-lobe magnetic reconnections, which is further squeezed into the central magnetotail, resulting in a short “calabash-shaped” magnetotail.

To understand the processes responsible for the formation and structure of plasma sheet and ring current particles, we have used THEMIS and Geotail data to investigate statistically the distributions of ions and electrons from the midtail to the inner magnetosphere and compared them with results from the Rice convection model (RCM). The observed distributions show clear magnetic local time (MLT) asymmetries in the thermal energy and energy fluxes of plasma sheet particles but many more MLT symmetric ring current particles. Our RCM runs include both self‐consistent electric and magnetic fields and realistic MLT‐dependent outer particle sources. Starting with no initial particles, particles released from the RCM outer sources move along electric and magnetic drift paths and change energy adiabatically. Comparison of the observation with the simulation indicates that the particles along the open drift paths can account for the observed plasma sheet populations and that the observed significant MLT variations are a combined result of species‐ and energy‐dependent drift and location‐dependent source strength. The simulated energy and spatial distributions of the particles within closed drift paths are found to be consistent with the observed ring current particles. These ring current particles are originally plasma sheet particles which became trapped along closed paths due to temporal variations of drift paths. The good agreement in key features of the spatial distributions of thermal energy and energy fluxes between the RCM and observations clearly indicates that electric and magnetic drift transport and the associated energization play dominant roles in plasma sheet and ring current dynamics. Key Points We established global structures of particles from THEMIS‐Geotail observations We conducted simulations of particle transport using the Rice convection model We determined that adiabatic transport and energization are the primary process

The goal of this paper is to understand the formation of the Harang reversal and its association with the region 2 field‐aligned current (FAC) system, which couples the plasma sheet transport to the ionosphere. We have run simulations with the Rice convection model (RCM) using the Tsyganenko 96 magnetic field model and realistic plasma sheet particle boundary conditions on the basis of Geotail observations. Our results show that the existence of an overlap in magnetic local time (MLT) of the region 2 upward and downward FAC is necessary for the formation of the Harang reversal. In the overlap region the downward FAC, which is located at lower latitudes, is associated with low‐energy ions that penetrate closer to Earth toward the dawn side, while the upward FAC, which is located at higher latitudes, is associated with high‐energy ions. Under the same enhanced convection we compare the Harang reversal resulting from a hotter and more tenuous plasma sheet with the one resulting from a colder and denser plasma sheet. For the former case the shielding of the convection electric field is less efficient than for the latter case, allowing low‐energy protons to penetrate further earthward, resulting in a Harang reversal that extends to lower latitudes, expands wider in MLT, and is located further equatorward than the upward FAC peak and the conductivity peak. The return flows of the Harang reversal in the hot and tenuous case are located in a low conductivity region. This leads to an enhancement of these westward flows, resulting in subauroral polarization streams (SAPS).

In this study, we combine GPS vertical total electron content (VTEC) and other complementary instruments, such as the Poker Flat incoherent scatter radar and all‐sky imagers, to investigate the dynamics of the mid‐latitude trough during non‐storm time substorms for solar minimum condition and focus on Alaska region. We find that the poleward wall of the mid‐latitude trough shifts equatorward rapidly after substorm onset with a maximum speed reaching 4°–5° of geomagnetic latitude per hour. This equatorward motion results in narrowing and even disappearance of the mid‐latitude trough and is due to enhanced energetic electron precipitation. The mid‐latitude trough can reappear during the substorm recovery phase as auroral activity retreats poleward. This phenomenon has not been reported before probably because of limited field‐of‐view of previous instruments used in trough studies. Comparisons of the trough minimum location predicted by models that are based on global Kp and AE indices show good agreement before substorm activity reaches the peak and relatively poor agreement during the recovery phase. The observations suggest that a local index, such as the local AL index, may be a better index to use to parameterize the trough location at a given meridian than a global index. Key Points Multi‐instrument study of mid‐latitude trough dynamics during substorms Trough narrows/disappears during expansion and reappears during recovery phase Observed trough locations are compared with empirical model predictions

We have statistically analyzed Geotail data to investigate the processes that result in a plasma sheet that is denser under a prolonged northward than southward interplanetary magnetic field (IMF) period. The observations show that the change of number density with the IMF conditions is mainly due to the changes of particle number per unit magnetic flux (particle content N), with N increasing (decreasing) as the period of northward (southward) IMF extends. The changes are quicker in the first ∼2 to 4 h then substantially slow down. The Y profiles show that N is always lowest around midnight and becomes higher toward the flanks. The observed plasma velocities suggest that plasma sheet particles undergo earthward and flankward drift transport, as well as diffusive transport resulting from drift fluctuations. The diffusion coefficients associated with fluctuating drift are estimated to be ∼105 to 106 km2/s. We have simulated evolution of N resulting from drift and diffusive transport with particle sources along the flanks. The simulation results show that the observed temporal and Y variations of N under different IMF conditions can be accounted for by the competition between the particle increase owing to particles diffusing toward midnight from the flank sources and the particle decrease owing to particles drifting away from midnight. As the IMF turns northward (southward), it is mainly the strengthening (weakening) of diffusive transport owing to the increase (decrease) of the flank source that results in the increase (decrease) of N.