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In this short communication, we qualitatively analyze possible effects of the 10 October 2024 geomagnetic storm on accelerating the reentry of a Starlink satellite from very low-Earth orbit (VLEO). The storm took place near the maximum of solar cycle (SC) 25, which has shown to be more intense than SC24. Based on preliminary geomagnetic indices, the 10 October 2024, along with the 10 May 2024, were the most intense events since the well-known Halloween storms of October/November 2003. By looking at a preliminary version of the Dst index and altitudes along with velocities extracted from two-line element (TLE) data of the Starlink-1089 (SL-1089) satellite, we observe a possible connection between storm main phase onset and a sharp decay of SL-1089. The satellite was predicted to reenter on 22 October, but it reentered on 12 October, 10 days before schedule. The sharp altitude decay of SL-1089 revealed by TLE data coincides with the storm main phase onset. We compare the de-orbiting altitudes of another three satellites during different geomagnetic conditions and observe that the day difference between actual and predicted reentries increases for periods with higher geomagnetic activity. Therefore, we call for future research to establish the eventual causal relationship between storm occurrence and satellite orbital decay. As predicted by previous works, SC25 is already producing extreme geomagnetic storms with unprecedented satellite orbital drag effects and consequences for current megaconstellations in VLEO., Comment: 11 pages, 2 figures, 1 table

The impact of interplanetary (IP) shocks on the Earth's magnetosphere can greatly disturb the geomagnetic field and electric currents in the magnetosphere-ionosphere system. At high latitudes, the current systems most affected by the shocks are the auroral electrojet currents. These currents then generate ground geomagnetically induced currents (GICs) that couple with and are highly detrimental to ground artificial conductors including power transmission lines, oil/gas pipelines, railways, and submarine cables. Recent research has shown that the shock impact angle, the angle the shock normal vector performs with the Sun-Earth line, plays a major role in controlling the subsequent geomagnetic activity. More specifically, due to more symmetric magnetospheric compressions, nearly frontal shocks are usually more geoeffective than highly inclined shocks. In this study, we utilize a subset (332 events) of a shock list with more than 600 events to investigate, for the first time, shock impact angle effects on the subsequent GICs right after shock impact (compression effects) and several minutes after shock impact (substorm-like effects). We use GIC recordings from the Finnish natural gas pipeline performed near the M\"ants\"al\"a compression station in southern Finland. We find that GIC peaks (> 5 A) occurring after shock impacts are mostly caused by nearly frontal shocks and occur in the post-noon/dusk magnetic local time sector. These GIC peaks are presumably triggered by partial ring current intensifications in the dusk sector. On the other hand, more intense GIC peaks (> 20 A) generally occur several minutes after shock impacts and are located around the magnetic midnight terminator. These GIC peaks are most likely caused by intense energetic particle injections from the magnetotail which frequently occur during substorms., Comment: 29 pages, 9 figures

The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (d$B$/d$t$), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of d$B$/d$t$ variations in response to two similarly strong shocks, but with one being nearly frontal, and the other, highly inclined. Multi-instrument analyses by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Los Alamos National Laboratory spacecraft show that nightside substorm-time energetic particle injections are more intense and occur faster in the case of the nearly head-on impact. The same trend is observed in d$B$/d$t$ variations recorded by THEMIS ground magnetometers. THEMIS all-sky imager data show a fast and clear poleward auroral expansion in the first case, which does not clearly occur in the second case. Strong field-aligned currents computed with the spherical elementary current system (SECS) technique occur in both cases, but the current variations resulting from the inclined shock impact are weaker and slower compared to the nearly frontal case. SECS analyses also reveal that geographic areas with d$B$/d$t$ surpassing the thresholds 1.5 and 5 nT/s, usually linked to high-risk GICs, are larger and occur earlier due to the symmetric compression caused by the nearly head-on impact. These results, with profound space weather implications, suggest that shock impact angles affect the geospace driving conditions and the location and intensity of the subsequent d$B$/d$t$ variations during substorm activity., Comment: 44 pages, 18 figures, 3 tables

Satellites, crewed spacecraft and stations in low-Earth orbit (LEO) are very sensitive to atmospheric drag. A satellite's lifetime and orbital tracking become increasingly inaccurate or uncertain during magnetic storms. Given the planned increase of government and private satellite presence in LEO, the need for accurate density predictions for collision avoidance and lifetime optimization, particularly during extreme events, has become an urgent matter and requires comprehensive international collaboration. Additionally, long-term solar activity models and historical data suggest that solar activity will significantly increase in the following years and decades. In this article, we briefly summarize the main achievements in the research of thermosphere response to extreme magnetic storms occurring particularly after the launching of many satellites with state-of-the-art accelerometers from which high-accuracy density can be determined. We find that the performance of an empirical model with data assimilation is higher than its performance without data assimilation during all extreme storm phases. We discuss how forecasting models can be improved by looking into two directions: first, to the past, by adapting historical extreme storm datasets for density predictions, and second, to the future, by facilitating the assimilation of large-scale thermosphere data sets that will be collected in future events. Therefore, this topic is relevant to the scientific community, government agencies that operate satellites, and the private sector with assets operating in LEO., Comment: 14 pages, 2 figures

Understanding extreme space weather events is of paramount importance in efforts to protect technological systems in space and on the ground. Particularly in the thermosphere, the subsequent extreme magnetic storms can pose serious threats to low-Earth orbit (LEO) spacecraft by intensifying errors in orbit predictions. Extreme magnetic storms (minimum Dst $\leq$ --250 nT) are extremely rare: only 7 events occurred during the era of spacecraft with high-level accelerometers such as CHAMP (CHAllenge Mini-satellite Payload) and GRACE (Gravity Recovery And Climate experiment), and none with minimum Dst $\leq$ --500 nT, here termed magnetic superstorms. Therefore, current knowledge of thermospheric mass density response to superstorms is very limited. Thus, in order to advance this knowledge, four known magnetic superstorms in history, i.e., events occurring before CHAMP's and GRACE's commission times, with complete datasets, are used to empirically estimate density enhancements and subsequent orbital drag. The November 2003 magnetic storm (minimum Dst = --422 nT), the most extreme event observed by both satellites, is used as the benchmark event. Results show that, as expected, orbital degradation is more severe for the most intense storms. Additionally, results clearly point out that the time duration of the storm is strongly associated with storm-time orbital drag effects, being as important as or even more important than storm intensity itself. The most extreme storm-time decays during CHAMP/GRACE-like sample satellite orbits estimated for the March 1989 magnetic superstorm show that long-lasting superstorms can have highly detrimental consequences for the orbital dynamics of satellites in LEO., Comment: 25 pages, 7 figures, 3 tables

Being footprints of major magnetic storms and hence major solar eruptions, mid- to low-latitude aurorae have been one of the pathways to understand solar-terrestrial environments. However, it has been reported that aurorae are also occasionally observed at low latitudes under low or even quiet magnetic conditions. Such phenomena are known as "sporadic aurorae". We report on a historical event observed by a scientist of the Brazilian Empire in Rio de Janeiro on 15 February 1875. We analyze this event on the basis of its spectroscopic observations, along with its visual structure and coloration, to suggest this event was a possible case of sporadic aurorae. Given the absence of worldwide aurora observations on that day as a consequence of low magnetic activity recorded on the days preceding the observation, in addition to a detailed description, the event observed can most likely be classified as a sporadic aurora. We discuss the geographic and magnetic conditions of that event. Thus, we add a possible case of sporadic aurora in the South American sector., Comment: 17 pages, 3 figures

We investigate satellite orbital drag effects at low-Earth orbit (LEO) associated with thermosphere heating during magnetic storms caused by coronal mass ejections. CHAllenge Mini-satellite Payload (CHAMP) and Gravity Recovery And Climate Experiment (GRACE) neutral density data are used to compute orbital drag. Storm-to-quiet density comparisons are performed with background densities obtained by the Jacchia-Bowman 2008 (JB2008) empirical model. Our storms are grouped in different categories regarding their intensities as indicated by minimum values of the SYM-H index. We then perform superposed epoch analyses with storm main phase onset as zero epoch time. In general, we find that orbital drag effects are larger for CHAMP (lower altitudes) in comparison to GRACE (higher altitudes). Results show that storm-time drag effects manifest first at high latitudes, but for extreme storms particularly observed by GRACE stronger orbital drag effects occur during early main phase at low/equatorial latitudes, probably due to heating propagation from high latitudes. We find that storm-time orbital decay along the satellites' path generally increases with storm intensity, being stronger and faster for the most extreme events. For these events, orbital drag effects decrease faster probably due to elevated cooling effects caused by nitric oxide, which introduce modeled density uncertainties during storm recovery phase. Errors associated with total orbit decay introduced by JB2008 are generally the largest for the strongest storms, and increase during storm times, particular during recovery phases. We discuss the implication of these uncertainties for the prediction of collision between space objects at LEO during magnetic storms., Comment: 33 pages, 13 figures

We present the first quantitative calculations of thermospheric heating and cooling times for geomagnetic storms of different intensity, including extreme events. We utilize the neutral mass density database of the CHAllenging Mini‐satellite Payload and Gravity Recovery And Climate Experiment missions to produce thermospheric global system response to geomagnetic storms caused by coronal mass ejections via superposed epoch analysis during May 2001 to December 2015. Storm events are grouped in five different categories based on the minimum value of the SYM‐H index. We calculate the time from storm onset for the thermosphere to reach maximum intensification (heating time) and the time from onset for the thermosphere to recover (cooling time). We find that heating and cooling times decrease as storm intensity increases and the effect is more pronounced for the cooling times. For extreme storms, the thermospheric heating time is 9.5 hr, while the cooling time is 22 hr.

Auroral electron precipitation is often classified into three categories: diffuse, monoenergetic,and broadband. However, some precipitating electrons do not fit any of the three categories, which areassumed to be "diffuse" electrons for convenience. Many of these nonconforming electrons exhibit highfluxes at low energies. Most studies assume that the precipitating electrons originate from themagnetosphere. However, this primary precipitation is only the first step in the formation of the electronprecipitation. The primary precipitation creates secondary electrons and both electron populations bouncemany times between the conjugate points. Thus, the full electron precipitation consists of not only theprimary but the Multiply Reflected Primary and Secondary (MRPS) electrons. The MRPS electrons aremodeled with Superthermal Electron Transport model. The modeldata comparisons suggest that the lowenergy electrons in the nonconforming precipitating electrons can be attributed to MRPS electrons.

Many space plasmas (especially electrons generated in planetary ionospheres) exhibit fine-detailed structures that are challenging to fully resolve with the energy resolution of typical space plasma analyzers (10% → 20%). While analyzers with higher resolution have flown, generally this comes at the expense of sensitivity and temporal resolution. We present a new technique for measuring plasmas with extremely high energy resolution through the combination of a top-hat Electrostatic Analyzer (ESA) followed by an internally mounted Retarding Potential Analyzer (RPA). When high resolutions are not required, the RPA is grounded, and the instrument may operate as a typical general-purpose plasma analyzer using its ESA alone. We also describe how such an instrument may use its RPA to remotely vary the geometric factor (sensitivity) of a top hat analyzer, as was performed on the New Horizons Solar Wind at Pluto and MAVEN SupraThermal and Thermal Ion Composition instruments. Finally, we present results from laboratory testing of our prototype, showing that this technique may be used to construct an instrument with 1.6% energy resolution, constant over all energies and angles.

A community workshop was held in Greenbelt, Maryland, on 5-6 May 2016 to discussrecommendations for the future of ground magnetometer array research in space physics. The community reviewed findings contained in the 2016 Geospace Portfolio Review of the Geospace Section of the Division of Atmospheric and Geospace Science of the National Science Foundation and discussed the present state of ground magnetometer arrays and possible pathways for a more optimal, robust, and effective organization and scientific use of these ground arrays. This paper summarizes the report of that workshop to the National Science Foundation (Engebretson & Zesta, 2017) as well as conclusions from two follow-up meetings. It describes the current state of U.S.-funded ground magnetometer arrays and summarizes community recommendations for changes in both organizational and funding structures. It also outlines a variety of new and/or augmented regional and global data products and visualizations that can be facilitated by increased collaboration among arrays. Such products will enhance the value of ground-based magnetometer data to the community's effort for understanding of Earth's space environment and space weather effects.

This paper discusses the major pathways of electron distribution function formation in the region of diffuse aurora. The diffuse aurora accounts for about of 75% of the auroral energy precipitating into the upper atmosphere, and its origin has been the subject of much discussion. We show that an earthward stream of precipitating electrons initially injected from the Earth's plasma sheet via wave-particle interactions degrades in the atmosphere toward lower energies and produces secondary electrons via impact ionization of the neutral atmosphere. These electrons of magnetospheric origin are then reflected back into the magnetosphere along closed dipolar magnetic field lines, leading to a series of reflections and consequent magnetospheric interactions that greatly augment the initially precipitating flux at the upper ionospheric boundary (700-800 km). To date this, systematic magnetosphere-ionosphere coupling element has not been included in auroral research models, and, as we demonstrate in this article, has a dramatic effect (200-300%) on the formation of the precipitating fluxes that result in the diffuse aurora. It is shown that wave-particle interaction processes that drive precipitating fluxes in the region of diffuse aurora from the magnetospheric altitudes are only the first step in the formation of electron precipitation at ionospheric altitudes, and they cannot be separated from the atmospheric collisional machine that redistributes and transfers their energy inside the magnetosphere-ionosphere-atmosphere coupling system.

In this research, we used numerical simulation to study the effect of interhemispheric field-aligned currents (IHCs), going between two conjugate ionospheres in two hemispheres, on the equivalent ionospheric currents (EICs). We computed the maps of these EICs In two hemispheres during summer-winter conditions, when the effect of the IHCs is especially significant. The main results may be summarized as follows. In winter hemisphere, the IHCs may significantly exceed and be a substitute for the local R1 currents, and they may strongly affect the magnitude, location, and direction of the EICs In the nightside winter auroral ionosphere. While in summer polar cap the EICs tend to flow sunward, and in winter polar cap the EICs turn toward dawn due to the effect of the IHCs. The well-known reversal in the direction of the EICs in the vicinity of the midnight meridian, in winter hemisphere, is observed not at the polar caps boundary (as usually expected) but equatorward of this boundary in the region of the IHCs location. The IHCs in winter hemisphere may be, in fact, not only a substitute for the R1 currents but also the major source of the Westward Auroral Electrojet, observed In both hemispheres during substorm activity.

The objective of this work is to provide the center with a fully tested, flexible, low cost, miniaturized science magnetometer system applicable to small satellite programs, like Cubesats, and to rides of opportunity that do not lend themselves to the high integration costs a science magnetometer on a boom necessitates.

This presentation describes orbital drag effects on satellite orbits, selected satellite drag impacts, thermosphere effects, tracking errors, and observations of specific storm results.