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Planetary auroral emissions reveal the configuration of magnetospheric field‐aligned current systems. In this study, Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations of Saturn's pre‐equinox infrared H3+ aurorae were analysed to show (a) rotational modulation of the auroral intensity in both hemispheres and (b) a significant local time dependence of the emitted intensity. The emission intensity is modulated by the ‘planetary period’ rotation of auroral current systems in each hemisphere. The northern auroral intensity also displays a lesser anti‐phase dependence on the southern rotating current system, indicating that part of the southern current system closes in the northern hemisphere. The southern hemisphere aurorae were most intense in the post‐dawn sector, in agreement with some past measurements of auroral field‐aligned currents, UV aurora and SKR emitted power. A corresponding investigation of the northern hemisphere auroral intensity reveals a broader dawn‐noon enhancement, possibly due to the interaction of the southern rotating current system with that of the north. The auroral intensity was reduced around dusk and post‐midnight in both hemispheres. These observations can be explained by the interaction of a rotating field‐aligned current system in each hemisphere with one fixed in local time, which is related to the solar wind interaction with magnetospheric field lines. Key Points Rotational modulation of Saturn's H3+ auroral intensity is detected A strong local time asymmetry in intensity is observed Trends attributed to interaction of rotating and solar wind‐driven currents

In this study we present ultraviolet and infrared auroral data from 26 July 1998, and we show the presence of transient auroral polar spots observed throughout the postdusk to predawn local time sector. The polar dawn spots, which are transient polar features observed in the dawn sector poleward of the main emission, were previously associated with the inward moving flow resulting from tail reconnection. In the present study we suggest that nightside spots, which are polar features observed close to the midnight sector, are related to inward moving flow, like the polar dawn spots. We base our conclusions on the near‐simultaneous set of Hubble Space Telescope (HST) and Galileo observations of 26 July 1998, during which HST observed a nightside spot magnetically mapped close to the location of an inward moving flow detected by Galileo on the same day. We derive the emitted power from magnetic field measurements along the observed plasma flow bubble, and we show that it matches the emitted power inferred from HST. Additionally, this study reports for the first time a bright polar spot in the infrared, which could be a possible signature of tail reconnection. The spot appears within an interval of 30 min from the ultraviolet, poleward of the main emission on the ionosphere and in the postdusk sector planetward of the tail reconnection x line on the equatorial plane. Finally, the present work demonstrates that ionospheric signatures of flow bursts released during tail reconnection are instantaneously detected over a wide local time sector.

Since the discovery of the large‐scale field‐aligned currents it is widely acknowledged that gaps exist between the Region 1 (R1) and Region 2 (R2) currents in which the current values are relatively small as compared to neighboring regions. Assuming that the field‐aligned currents are generated by plasma pressure gradients, we analyzed data collected by the THEMIS satellites between 2007 and 2011 to identify regions with very low plasma pressure gradients (pressure plateaus), which could be responsible for the appearance of these gaps. It was found that the pressure profiles with low radial gradients are typically located between 8 and 10 Radii around the Earth. Projections of pressure plateau regions onto ionospheric altitudes, for both individual events and on a statistical basis, coincide with the locations of gaps between Iijima and Potemra field‐aligned currents. The role played by identified pressure plateaus in shaping the pattern of large‐scale field‐aligned currents is discussed. Plain Language Summary Field‐aligned currents that flow between the magnetosphere and ionosphere along magnetic field lines play a crucial role in magnetosphere‐ionosphere interactions. Despite being discovered at the start of the space era, their origins are still subject to debate. In our current study, we have established that the observed gaps between the upward and downward currents correspond to the plateau regions of constant plasma pressure, as obtained using data from the THEMIS mission. This finding provides strong evidence in favor of the generation of field‐aligned currents by plasma pressure gradients in the transition region between the dipole and tail‐like geomagnetic field. This is a critical piece of information for understanding the global dynamics of Earth's magnetosphere. Key Points During quiet geomagnetic conditions, the regions of plasma pressure plateau are observed in the inner magnetosphere These regions are located at geocentric distances of 8–10 RE forming a ring surrounding the Earth The localization of the plasma pressure plateau region and gaps between the Region 1 and 2 currents should be located at the same distance

On 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation. Plain Language Summary The solar wind applies pressure on the Earth's magnetic field, distorting it from a dipole into its compressed dayside and stretched tail configuration. However, this typical structure can be disrupted by eruptive solar events such as Coronal Mass Ejections (CMEs), which may cause the solar wind's pressure to drop low enough that it is no longer able to push the magnetosphere back to form a single unified tail. When this occurs, the tail splits into two separate structures, called Alfvén Wings. While this configuration is rare at Earth, it is common from interactions of the outer planets' magnetosphere's with their moons, where Alfvén Wing configurations have been studied and modeled. However, because the observing spacecraft lacked the necessary instrumentation, we have not yet directly observed the Alfvén Wing current structures. On 24 April 2023, a CME event led to the creation of an Alfvén Wing formation in the Earth's magnetosphere. We observed this event using the Magnetospheric Multiscale spacecraft, which enabled us to make the first direct observations of Alfvén Wing current structures. These currents were found to be mainly parallel to the local magnetic field, in contrast to typical magnetopause currents. Key Points On 24 April 2023, the Magnetospheric Multiscale (MMS) spacecraft observed an Alfvén Wing formation along the dawn‐flank of Earth's magnetosphere MMS's observations represent the first in situ measurements of Alfvén Wing current structures The current structures are found to be primarily anti‐field‐aligned, electron‐driven, and filamentary

The propagation of kinetic Alfvén waves (assumed sourced from intermittent dayside reconnection) is investigated with a gyrofluid‐kinetic electron model compared with Cluster observations. These observations reveal electron distributions that are preferentially field‐aligned or field‐opposed, with signatures that are unidirectional or counterstreaming and skews that vary with the current sense. The simulations reproduce, with good fidelity, the observed local characteristics when the conditions match the observed local plasma conditions. The wave energy conversion is predominantly positive at mid‐altitudes, indicating a transfer of wave to electron energy. This conversion rate increases significantly at low‐altitudes (in the inertial Alfvén wave regime) and is accompanied by the formation of highly field‐aligned electron beams peaking at several hundred eV in energy, with a directionality that is opposite to the current sense. This low‐altitude energization results in the dissipation of the majority of the wave Poynting flux and would lead to substantial soft electron precipitation.

The duskside and dawnside subauroral polarization streams (SAPS) refer to high‐velocity westward and eastward plasma flows located equatorward of the auroral oval. While extensive research has focused on the duskside SAPS, the simultaneous evolution of both dawnside and duskside SAPS remains unreported. In this study, for the first time, we investigated the simultaneous evolution of duskside and dawnside SAPS using multiple Super Dual Auroral Radar Network radars during an intense storm. Observations indicate that the duskside SAPS exhibits a wider magnetic local time extension (∼7 MLT) and longer duration (∼1 hr) than the dawnside SAPS. Furthermore, the duskside SAPS resides within low‐density mid‐latitude troughs, whereas the dawnside SAPS is not located within the trough. The dawnside SAPS exhibits significantly higher electron density but comparable velocity to the duskside SAPS. These findings highlight the distinct evolution of dawnside and duskside SAPS, providing new insights into the electrodynamic processes of subauroral ionosphere and magnetosphere coupling.

The time delay from an interplanetary driver arriving at the magnetopause to the response in the ionosphere has never been quantified separately for different types of storm drivers. This study investigates the delay for storms driven by high‐speed streams and associated stream interaction regions (HSS/SIR), or by interplanetary coronal mass ejection sheaths and magnetic clouds (MC). The total field‐aligned current (FAC) and SME index lag the Newell coupling function (NCF) by 40 ± 10 min during storms driven by HSS/SIR and sheaths, and by 60 ± 10 min for MCs. The correlation coefficient between FAC and NCF reaches maximum value as NCF is averaged over the preceding 80 min for sheath, 90 min for HSS/SIR, and 140 min for MC storms. Plain Language Summary The Sun causes perturbations in the solar wind, which may drive geomagnetic storms associated with strong field‐aligned currents to the ionosphere. The solar wind drivers studied in this paper are high‐speed stream/stream interaction regions (HSS/SIR), and sheath and magnetic cloud (MC) interplanetary coronal mass ejections. The exact time from the arrival of the solar wind interplanetary driver at the magnetopause to the response in the ionospheric and field‐aligned currents (FAC) have not been quantified for different types of solar wind drivers. We study this time delay during geomagnetic storms and find that it is typically 40 min for HSS/SIR‐ and sheath‐driven storms, and 60 min for MC‐driven storms. Additionally, the total FAC best correlate with the solar wind averaged over the preceding 80 min for sheath, 90 min for HSS/SIR, and 140 min for MC‐driven storms. These results may help improve the accuracy of forecasting solar wind disturbances on the high‐latitude ionosphere. Key Points Correlation between Newell coupling function (NCF) and total field‐aligned current (FAC) is studied for different storm drivers Best correlation for sheath, high‐speed stream, and magnetic cloud storms is found by integrating NCF over 80, 90, and 140 min, respectively Sheath‐driven storms are associated with the highest values of total FAC and NCF

Within the fully integrated magnetosphere-ionosphere system, many electrodynamic processes interact with each other. We review recent advances in understanding three major meso-scale coupling processes within the system: the transient field-aligned currents (FACs), mid-latitude plasma convection, and auroral particle precipitation. (1) Transient FACs arise due to disturbances from either dayside or nightside magnetosphere. As the interplanetary shocks suddenly compress the dayside magnetosphere, short-lived FACs are induced at high latitudes with their polarity successively changing. Magnetotail dynamics, such as substorm injections, can also disturb the current structures, leading to the formation of substorm current wedges and ring current disruption. (2) The mid-latitude plasma convection is closely associated with electric fields in the system. Recent studies have unraveled some important features and mechanisms of subauroral fast flows. (3) Charged particles, while drifting around the Earth, often experience precipitating loss down to the upper atmosphere, enhancing the auroral conductivity. Recent studies have been devoted to developing more self-consistent geospace circulation models by including a better representation of the auroral conductance. It is expected that including these new advances in geospace circulation models could promisingly strengthen their forecasting capability in space weather applications. The remaining challenges especially in the global modeling of the circulation system are also discussed.

We present the observations of field‐aligned currents and the equatorial electrojet during the 23 March 2023 magnetic storm, focusing on the effect of the drastic decrease of the solar wind dynamic pressure occurred during the main phase. Our observations show that the negative pressure pulse had significant impact to the magnetosphere‐ionosphere system. It weakened large‐scale field‐aligned currents and paused the progression of the storm main phase for ∼3 hr. Due to the sudden decrease of the plasma convection after the negative pressure pulse, the low‐latitude ionosphere was over‐shielded and experienced a brief period of westward penetration electric field, which reversed the direction of the equatorial electrojet. The counter electrojet was observed both in space and on the ground. A transient, localized enhancement of downward field‐aligned current was observed near dawn, consistent with the mechanism for transmitting MHD disturbances from magnetosphere to the ionosphere after the negative pressure pulse. Plain Language Summary The solar wind is a continuous stream of charged particles blowing from the Sun. The Earth's magnetic field forms a protective shield around our planet, called the magnetosphere, which deflects most of the solar wind particles away from the Earth. Disturbances in the solar wind can interact with the magnetosphere and impact the Earth's upper atmosphere (ionosphere). The interaction creates electric fields forcing charged particles to move in the magnetosphere, which creates electric currents flowing along the magnetic field lines connecting to the high‐latitude ionosphere and drives the movement of charged particles there. The low‐latitude ionosphere is generally shielded from these electric fields. Sudden changes in the solar wind can break such balance, leading to the electric field penetration to low latitudes. We examined how the magnetosphere and ionosphere interacted during the 23 March 2023 geomagnetic storm, focusing on what happened when the solar wind dynamic pressure suddenly decreased. We found the pressure drop caused a sudden decrease of the high‐latitude electric field, resulting in a brief period of overshielding and the electric field in the equatorial ionosphere reversed its direction. This changed the direction of the equatorial electrojet, a major electric current in the ionosphere at the magnetic equator. Key Points Direct evidence of prompt penetration of electric field in the equatorial ionosphere caused by negative solar wind pressure pulse Transient counter electrojet caused by westward penetration electric field after the arrival of negative pressure pulse Significant decrease of global large‐scale field‐aligned currents (FACs) and transient enhancement of localized FAC in response to negative pressure pulse

The South Atlantic Anomaly (SAA) refers to a region where the strength of the magnetic field is notably weaker compared to a dipole field. While previous studies have primarily focused on its effects on the inner radiation belt, this study investigates its impact on the aurora system. By analyzing 2 years' worth of data obtained by the Fengyun‐3E/ACMag instrument, we discover that magnetic fluctuations within the auroral oval are significantly weaker in the longitude sector corresponding to the SAA, as compared to those outside this area. This characteristic remains permanent and independent of seasons and geomagnetic activities. Additional investigation using Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager (DMSP/SSUSI) observations reveals a similar phenomenon in the auroral intensity. Therefore, our results demonstrate that the SAA substantially weakens the aurora system, shedding new light on the effects of magnetic anomalies on planetary auroras and magnetosphere‐ionosphere‐thermosphere coupling. Plain Language Summary The South Atlantic Anomaly (SAA) is a unique location on Earth where the magnetic field is weaker than normal. This region has drawn a lot of attention because its weakened magnetic field brings the inner Van Allen radiation belt unusually close to the Earth's surface, which poses a threat to satellites passing through it. Here, we uncovered another interesting aspect of the SAA: its impact on the aurora system. To investigate this, we first examined 2 years' worth of data from the ACMag instruments on the Fengyun‐3E satellite, which orbits the Earth at an altitude of 836 km in a dawn‐dusk, Sun‐synchronous orbit. Our findings reveal that the magnetic fluctuations within the southern auroral oval are significantly weaker in the region that aligns with the SAA. This weakening effect is consistently present, regardless of the season or the level of geomagnetic activity. To reinforce our results, we also analyzed auroral intensity from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on the Defense Meteorological Satellite Program (DMSP) satellite, and it corroborated the same weakening trend in this data set. In conclusion, our observations demonstrate that the SAA has a substantial impact on weakening the aurora system. This discovery deepens our understanding of how magnetic anomalies can influence planetary auroras. Key Points The effects of the South Atlantic Anomaly (SAA) on the terrestrial aurora system are examined using multiple instruments Observations reveal a substantial weakening of auroral magnetic fluctuations and auroral intensity in the SAA longitude sector The results indicate considering magnetic anomalies like the SAA is essential for comprehensively understanding planetary aurora systems