Multiple transpolar arcs appearing simultaneously in the polar cap have gained much interest in recent years. By analyzing Defense Meteorological Satellite Program Special Sensor Ultraviolet ...Spectrographic Imagers data, we report for the first time, that less than half of the multiple arc events occur simultaneously in both hemispheres. In 60% of the cases, multiple arcs appear in only one hemisphere. There is a clear difference in interplanetary magnetic field (IMF) conditions for those two groups. Conjugate multiple arcs appear on average during stronger northward IMF and smaller IMF clock angles than non‐conjugate multiple arcs. Only non‐conjugate multiple arcs show a dependence on IMF BX. They form in the northern (southern) hemisphere during negative (positive) BX. An IMF BX induced interhemispheric asymmetry in the magnetospheric field line topology might explain why multiple arcs appear sometimes in only one hemisphere.
Plain Language Summary
Occasionally, multiple auroral arcs form far poleward of the main auroral oval. These multiple transpolar arcs were so far believed to almost always be conjugate (appearing in both hemispheres simultaneously). We show for the first time that more than half of them appear in only one hemisphere. Conjugate multiple arcs appear when the magnetic field in the solar wind (IMF) is strongly northward. Non‐conjugate multiple arcs show a less strong dependency on northward IMF. Interestingly, we found a clear correlation between non‐conjugate multiple arc events and IMF BX. This is unexpected, as in general, polar auroral arcs do not show any clear dependence on BX. Non‐conjugate multiple arcs appear mainly in the southern hemisphere when the IMF points sunward, and in the northern hemisphere when it points tailward. As IMF BX is known to introduce an interhemispheric asymmetry in the field‐line topology close to the reconnection sites, this may affect the formation of multiple arcs differently in the two hemispheres, and thus might explain the non‐conjugacy of those events.
Key Points
Multiple transpolar arcs (TPAs) appear during stronger northward interplanetary magnetic field (IMF) than isolated TPAs
Hemisphere‐conjugate multiple TPAs appear for smaller IMF clock angles than non‐conjugate multiple arcs
Non‐conjugate multiple arcs show an IMF BX dependence with northern (southern) hemisphere arcs appearing for negative (positive) BX
We present the first high resolution global MHD with coupled inner magnetosphere simulation results of an observed theta aurora event. We use the Space Weather Modeling Framework in the Geospace ...configuration, which produces accurate field aligned current closure in the ionosphere that is integral to theta aurora formation. At the location of the observed theta aurora, the simulation produces a narrow channel of Joule heating along both open and closed field lines, and between a pair of oppositely directed field‐aligned current sheets in the ionosphere. We demonstrate that this Joule heating pattern that we identify as theta aurora maps to a reconnection region at the magnetotail flanks as well as in the distant magnetotail. The theta aurora maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail.
Plain Language Summary
The light of the aurora observed in the Earth's atmosphere is a signature of magnetic processes in outer space. The Sun produces hot magnetized plasma that blows through the solar system like a wind. The Earth's magnetic field shields us from the harmful impacts of the Sun's magnetized plasma wind. The interaction between the Earth's magnetic field and the Sun's plasma wind causes the ring of auroral light we see in the polar regions. Sometimes the ring of auroral light grows a bar of auroral light across its center (over the Earth's magnetic poles), transforming the ring into a structure like the Greek letter theta. We call this the theta aurora. It is unknown what physical processes in the Earth's magnetic field create the theta aurora. In this study we show the first realistic simulation that uses solar wind driver conditions of an observed theta aurora event, and demonstrate that we can successfully produce the theta aurora structure in the simulation. We find that the theta aurora's center bar of auroral light comes from multiple regions in the vast Earth's magnetotail. This new result helps us better understand the dynamics of the Earth's space environment and better protect ourselves against the Sun's hazardous plasma wind effects.
Key Points
We show first global MHD simulation event study that successfully produces a Joule heating signature that we identify as theta aurora
Simulation associates transpolar arc signatures with reconnection at the magnetospheric flank and in the distant magnetotail
The transpolar arc maps to a cross‐tail current disruption and field‐aligned current source region in a highly twisted magnetotail
A large study of Kelvin‐Helmholtz (KH) waves at the magnetopause of Mercury covering 907 days of data from the MErcury Surface Space ENvironment GEochemistry Ranging spacecraft have resulted in 146 ...encounters of not only nonlinear KH waves but also linear surface waves, including the first observations of KH waves at the dawnside magnetopause. Most of the waves are in the nonlinear phase (90%) occur at the duskside magnetopause (93%), under northward magnetosheath magnetic field conditions (89%) and during greater magnetosheath Bz (23 nT) values than in general. The average period and amplitude is 30 ± 14 s and 14 ± 10 nT, respectively. Unlike duskside events, dawnside waves do not appear at the magnetopause flank (<6 magnetic local time). This is in agreement with previous observations and modeling results and possibly explained by finite Larmor radius effects and/or a lack of a large‐scale laminar flow at the dawnside magnetopause boundary.
Key Points
Observing Kelvin‐Helmholtz waves at the dawnside Mercury magnetopause
Confirming a dawn‐dusk asymmetry associated with the Kelvin‐Helmholtz at Mercury
Determine characteristics associated with Kelvin‐Helmholtz waves
We report two events of high‐latitude dayside aurora (HiLDA), a large‐scale aurora in the dayside polar cap, observed by the Defense Meteorological Satellite Program (DMSP) spacecraft in the northern ...and southern hemispheres, respectively. While HiLDA in the northern hemisphere was reported before under interplanetary magnetic field (IMF) positive By conditions, we show for the first time a HiLDA event in the southern hemisphere when the IMF negative By component was dominant. Our observations also show that HiLDA is highly dynamical: change in its forms, size, location, and development of fine structures during its long lifetime of hours. The co‐occurrence of HiLDA and the duskside oval‐aligned transpolar aurora (TPA) may be a common feature during IMF By dominant conditions. Both are associated with the high‐latitude reconnection and the cusp. Based on the linear Knight relation, we estimate the distribution of the electron density in the magnetospheric source region of HiLDA. These results indicate that HiLDA maps most probably to the high‐latitude lobe tailward of the cusp, where the electron density is down to 0.03−3 cm−3. The lobe electrons are accelerated by the field‐aligned potential drop (up to 10 kV) set up in the poleward part of upward Region 0 field‐aligned current (FAC). The total energy flux of HiLDA electrons can be up to 50 mW/m2, indicating HiLDA precipitation as a potential energy source that impacts the polar ionosphere‐thermosphere system.
Key Points
Defense Meteorological Satellite Program (DMSP) observations prove that high‐latitude dayside aurora (HiLDA) events appear in the southern hemisphere during predominantly negative interplanetary magnetic field (IMF) By
HiLDA is dynamical: change in its forms, size, and location with fine structures during its long lifetime of hours
Our estimates show that HiLDA maps to the high‐latitude lobe with electron density down to 0.03 cm−3
This flux transfer event (FTE) study is based on 984 FTEs originally identified by Wang et al. (2005, https://doi:10.1029/2005JA011150) in Cluster data. Due to Cluster's orbit, the FTE list ...exclusively contains events detected at the high‐latitude dayside magnetopause and low‐latitude flanks. The focus of this study is on FTE separation time. The results show that FTEs appearing in cascades are mainly located at the northern dusk and southern dawn magnetopause, while isolated FTEs are equally spread over the region covered by Cluster. This difference may be explained by the different interplanetary magnetic field (IMF) conditions during which the subsets occur. For isolated FTEs, average IMF By is close to zero. During such conditions, FTEs are expected to form at arbitrary longitudes along an equatorial merging line. After formation, they propagate northward and southward, causing an equal distribution at higher latitudes. In contrast, FTE cascades typically occur during weakly southward IMF with a negative By component. Their asymmetric distribution at higher latitudes is consistent with both the component and the antiparallel merging model for nonzero By. In both scenarios, newly formed FTEs are expected to move to the northern dusk and southern dawn regions, as observed. Many FTE cascades appearing during northward IMF are located close to the low‐latitude flanks, confirming previous reports. We discovered that such FTEs appear during large IMF values. Another new result is that 16% of all isolated FTEs appear during small IMF cone angles, suggesting that these may form as a result of magnetosheath jets impacting on the magnetopause.
Key Points
FTE distribution along the high‐latitude and flank magnetopause in the Cluster data set depends on FTE separation time
Isolated FTEs are equally distributed over the magnetopause region covered by Cluster and appear for small IMF By
FTEs with short separation time appear in the Cluster data set mainly in northern dusk and southern dawn during negative IMF By
In this work, the Polar UVI data set by Kullen et al. (2002) of 74 polar arcs is reinvestigated, focusing on bending arcs. Bending arcs are typically faint and form (depending on interplanetary ...magnetic field (IMF) By direction) on the dawnside or duskside oval with the tip of the arc splitting off the dayside oval. The tip subsequently moves into the polar cap in the antisunward direction, while the arc's nightside end remains attached to the oval, eventually becoming hook‐shaped. Our investigation shows that bending arcs appear on the opposite oval side from and farther sunward than most regular polar arcs. They form during By‐dominated IMF conditions: typically, the IMF clock angle increases from 60 to 90° about 20 min before the arc forms. Antisunward plasma flows from the oval into the polar cap just poleward of bending arcs are seen in Super Dual Auroral Radar Network data, indicating dayside reconnection. For regular polar arcs, recently reported characteristics are confirmed in contrast to bending arcs. This includes plasma flows along the nightside oval that originate close to the initial arc location and a significant delay in the correlation between IMF By and initial arc location. In our data set, the highest correlations are found with IMF By appearing at least 1–2 h before arc formation. In summary, bending arcs are distinctly different from regular arcs and cannot be explained by existing polar arc models. Instead, these results are consistent with the formation mechanism described in Carter et al. (2015), suggesting that bending arcs are caused by dayside reconnection.
Key Points
Bending arcs are polar arcs that split from the prenoon or postnoon oval and bend into the polar cap
Bending arcs form during dayside reconnection
The initial location of regular polar arcs is determined by IMF By conditions at least 1–2 h earlier
This study presents a re‐evaluation of the Kullen and Janhunen (2004, https://doi.org/10.5194/angeo-22-951-2004) global northward interplanetary magnetic field (IMF) simulation, using the Grand ...Unified Magnetosphere–Ionosphere Coupling Simulation version 4 (GUMICS‐4), a global MHD model. We investigate the dynamic coupling between northward IMF conditions and the Earth’s magnetotail and compare the results to observation‐based mechanisms for the formation of transpolar arcs. The results of this study reveal that under northward IMF conditions (and northward IMF initialization), a large closed field line region forms in the magnetotail, with similarities to transpolar arc structures observed from spacecraft data. This interpretation is supported by the simultaneous increase of closed flux measured in the magnetotail. However, the reconnection configuration differs in several respects from previously theorized magnetotail structures that have been inferred from both observations and simulations results and associated with transpolar arcs. We observe that dawn–dusk lobe regions form as a result of high‐latitude reconnection during the initialization stages, which later come into contact as the change in the IMF By component causes the magnetotail to twist. We conclude that in the GUMICS simulation, transpolar arc‐like structures are formed as a result of reconnection in the magnetotail, rather than high‐latitude reconnection or due to the mapping of the plasma sheet through a twisted magnetotail as interpreted from previous analysis of GUMICS simulations.
Plain Language Summary
When the magnetic field associated with the solar wind is directed “northward,” the Earth’s aurora can adopt a formation where a “bar” of emission crosses the otherwise dim region that lies poleward of the usual auroral emissions. These phenomena are called “theta auroras” (due to their resemblance to the Greek letter), or “transpolar arcs,” and have been observed from spacecraft observing the auroral regions. As spacecraft cannot directly observe the global configuration of magnetic field lines within planetary magnetospheres, simulations can be useful to gain insight into the possible structures that occur during different solar wind conditions. We use a global simulation to model the interaction between the solar wind and the Earth’s magnetosphere (the region of space around our planet) and see that a large‐scale field line structure can form during certain northward‐directed magnetic field conditions. When we trace these field lines to the ionospheric boundary, we find that they resemble the global structures that have been observed for auroral arcs that stretch across the polar cap (also named theta aurora or transpolar arcs). We also note some key differences in their formation process from observational results.
Key Points
Signatures consistent with magnetotail reconnection are observed in a simulation driven by northward interplanetary magnetic field conditions
The simulation results in the production of a closed magnetotail structure and polar cap arcs
The above structure occurs within a magnetotail that is highly twisted such that open lobe regions cross the equatorial plane
Fast earthward plasma flows are commonly observed in the magnetotail plasma sheet. These flows are often termed as bursty bulk flows because of their bursty nature, and they are considered to be ...generated by magnetic reconnection. Close to the neutral sheet (Bx ∼ 0), the fast flows are considered to be associated with an enhanced dawn‐to‐dusk electric field (Ey > 0), which together with the northward magnetic field component (Bz > 0) protrude the plasma earthward via enhanced E × B‐drift. Sometimes, reversals in the dawn‐dusk velocity component perpendicular to the magnetic field (V⊥y) are measured in association with Bx sign changes in the flows. This suggests that the electric field component in the north‐south direction (Ez) can play a role in determining the dawn‐dusk direction of the enhanced drift. We present data measured by the Magnetospheric Multiscale, which demonstrate that Ez can have a dictating role for V⊥y of fast flows. Furthermore, it is shown that the critical contribution of Ez is not limited only to V⊥y, but it can also dominantly determine the enhanced drift of the fast flows in the X direction (V⊥x). The latter can occur also near and at the neutral sheet, which adds an alternative configuration to the conventional picture of Ey and Bz being the main players in driving the earthward fast flows. The domination of Ez in the studied events appears with potential signatures of an influence of a nonzero dawn‐dusk component of the interplanetary magnetic field (IMF By) on the magnetotail.
Key Points
Magnetospheric Multiscale data for earthward fast flow events exhibiting reversals in V⊥y are investigated
Ez can have a dictating role in determining V⊥y in fast flows
The critical contribution of Ez is not only limited to V⊥y, but also earthward V⊥x can dominantly be determined by Ez
We statistically investigate convective earthward fast flows using data measured by the Magnetospheric Multiscale mission in the tail plasma sheet during 2017–2021. We focus on “frozen in” fast flows ...and investigate the importance of different electric field components in the Sun‐Earth (V⊥x) and dusk‐dawn (V⊥y) velocity components perpendicular to the magnetic field. We find that a majority of the fast flow events (52% of 429) have the north‐south electric field component (Ez) as the most relevant or dominating component whereas 26% are so‐called conventional type fast flows with Ey and Ex as the relevant components. The rest of the flow events, 22%, fall into the two ’mixed’ categories, of which almost all these fast flows, 20% of 429, have Ey and Ez important for V⊥x and V⊥y, respectively. There is no Y‐location preference for any type of the fast flows. The conventional fast flows are detected rather close to the neutral sheet whereas the other types can be measured farther away. Typical total speeds are highest in the mixed category. Typical perpendicular speeds are comparably high in the conventional and mixed categories. The slowest fast flows are measured in the Ez category. Most of the fast flow events are measured in the substorm recovery phase. Prevailing interplanetary magnetic field By conditions influence the V⊥y direction and the influence is most efficient for the Ez‐dominated fast flows.
Key Points
Most of the “frozen in” perpendicular bursty bulk flows (BBFs) detected in the 2017–2021 Magnetospheric Multiscale tail seasons have Ez as the most important E‐field component
Typically, the BBFs have fastest total speeds when Ey and Ez are important for V⊥x and V⊥y, respectively
The interplanetary magnetic field By influence on BBF plasma is most efficient when Ez is the most important E‐field component
Theoretical considerations, observations, and simulations have shown that the By component of the interplanetary magnetic field (IMF) may cause twisting of the magnetotail. However, the fundamental ...issues, the temporal and spatial responses of the magnetotail in the twisting process, are still unresolved. We report unique multipoint observations of the response of the magnetotail to the variations in IMF By on 1–2 January 2009. For the first time, estimates of the tail twisting response time at different (Time History of Events and Macroscale Interactions during Substorms, THEMIS) distances in the same event are inferred. Using cross‐correlation and timing analyses, we find that the tail twisting propagates from farther out toward the Earth and the response time increases significantly to the inner magnetosphere.
Key Points
Magnetotail twisting response time to IMF By variations
Tail twisting propagates from farther in the tail toward the Earth
The response time increases significantly at the inner magnetosphere
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