The formation of auroral beads, wavy onset arcs, is often discussed in terms of substorm initiation. The present study addresses several aspects of their development and propagation by examining ...ground all‐sky images for representative events for each aspect. The results are summarized as follows: (a) the expansion and propagation of auroral beads do not depend on whether they form east or west of a preceding equatorward flow; (b) Auroral beads propagate either eastward or westward when the westward auroral electrojet intensifies apparently not preceded by the approach of an equatorward flow in neighboring sectors; (c) A nearby arc does not seem to be affected even if auroral beads form only a few tenths of degree apart in latitude; (d) two wavy arcs occasionally form next to each other and propagate in different directions; (e) the evolution of beading arcs is not always conjugate in two hemispheres. Results (a) and (b) suggest that meso‐ or large‐scale ionospheric convection as inferred from ground magnetic disturbances does not regulate the development and propagation of auroral beads. Results (c) and (d) imply that the development of auroral beads can be attributed to a small‐scale process, possibly a convection flow, at a latitudinal scale of a few tenths of degree or less. Such a small‐scale process may not be conjugate in two hemispheres, which potentially explains Result (e). These results do not necessarily mean that the cause of auroral beads is ionospheric, but suggest that the magnetosphere‐ionosphere coupling plays an important role in their development.
Key Points
The propagation direction of auroral beads is not regulated by meso‐ or large‐scale convection as inferred from ground magnetic disturbances
Some events show an adjacent arc apparently unaffected by the bead formation, and other events show two beading arcs propagating oppositely
Evolution of auroral beads is not always conjugate between two hemispheres suggesting the contribution of an ionospheric or M‐I process
A high‐resolution global magnetohydrodynamic simulation is conducted with the Lyon‐Fedder‐Mobarry (LFM) model for idealized solar wind conditions. Within the simulation results high‐speed flows are ...seen throughout the magnetotail when the interplanetary magnetic field (IMF) is southward. Case study analysis of these flows shows that they have an enhancement in BZ and a decrease in density preceding a peak in the flow velocity. A careful examination of the structure within the magnetotail shows that these features are driven by bursts of magnetic reconnection. In addition to the case study, a superposed epoch analysis of flows occurring during a 90 min interval of southward IMF yields statistical properties that are in qualitative agreement with observational analysis of bursty bulk flows (BBFs). For the idealized simulation conditions, the most significant differences with the observational results are a broader velocity profile in time, which becomes narrower away from the center of the current sheet, and a larger density drop after flow passage. The peak BZ amplitude is larger than in observations and precedes the peak in the flow velocity. We conclude that the LFM simulations are reproducing the statistical features of BBFs and that they are driven by spatially and temporally localized reconnection events within the simulation domain.
Key Points
High‐resolution LFM simulations contain BBFs
BBFs have statistical properties similar to observations
BBFs are generated by reconnection within simulation
Explosive magnetotail activity has long been understood in the context of its auroral manifestations. While global models have been used to interpret and understand many magnetospheric processes, the ...temporal and spatial scales of some auroral forms have been inaccessible to global modeling creating a gulf between observational and theoretical studies of these phenomena. We present here an important step toward bridging this gulf using a newly developed global magnetosphere‐ionosphere model with resolution capturing
≲ 30 km azimuthal scales in the auroral zone. In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, we find the self‐consistent formation and destabilization of localized magnetic field minima in the near‐Earth magnetotail. We demonstrate that this destabilization is due to ballooning‐interchange instability which drives earthward entropy bubbles with embedded magnetic fronts. Finally, we show that these bubbles create localized field‐aligned current structures that manifest in the ionosphere with properties matching observed auroral beads.
Plain Language Summary
The aurora has long been used as a window onto the magnetosphere. However, auroral observations are inherently limited in trying to reconstruct global magnetospheric dynamics from the “magnetic shadow” they cast on Earth. For this reason modeling has been used in tandem with observations to better contextualize and understand the data. Substorms, the violent reconfiguration of the magnetotail and one of the most dynamic magnetospheric phenomena, have been known to be preceded by the formation of bead‐like structures in the aurora. The processes responsible for auroral beading and their causal versus correlative role with substorm onset have remained an enduring mystery. The vast disparity between the spatial scales of auroral beads and those of the global magnetosphere has greatly complicated the use of modeling in unraveling this mystery. We show here for the first time a demonstration of the self‐consistent formation of a magnetospheric configuration that becomes unstable during the period preceding the substorm onset and that this instability manifests in the ionosphere with similar morphology to auroral beads. The global context of the model shows that the magnetospheric processes responsible for beading are not necessarily causal to onset but a consequence of the slow magnetotail reconfiguration that precedes onset.
Key Points
We present the first global magnetosphere simulation to reveal ballooning‐interchange instability of a narrow Bz minimum in the near‐Earth magnetotail
The instability is prominent during the substorm growth phase and generates earthward entropy bubbles with embedded magnetic fronts
The bubbles drive mesoscale ionospheric field‐aligned currents and auroral structures (beads) with properties matching to those observed
The equatorial magnetic field of the nightside magnetosphere is critical for understanding not only the configuration of the magnetotail but also its state and dynamics. The present study ...observationally addresses various aspects of the equatorial magnetic field, such as its spatial distribution, possible antisunward gradients, and extremely weak magnetic fields, with emphasis on the transition region between dipolar and stretched magnetic configurations. The results are summarized as follows: (1) the transition of the tail magnetic field from a near‐Earth dipolar configuration to a stretched one farther out takes place around −12 ≤ Xagsm ≤ −9 RE, although instantaneous configurations can vary significantly; (2) the average equatorial magnetic field in this transition region is noticeably weaker at solar minimum presumably reflecting weaker nightside magnetospheric currents closer to Earth; (3) the statistical comparison of equatorial magnetic fields measured simultaneously at two locations indicates that the gradient of the equatorial magnetic field is directed predominantly earthward, and it is suggested that apparent tailward gradients observed can be very often attributed to other factors such as structures in the Y direction and local fluctuations; (4) however, the gradient can be transiently directed tailward in association with the dipolarization of local magnetic field; (5) extremely weak (≤ 2 nT) magnetic fields are occasionally observed in the transition region during the substorm growth phase and during prolonged quiet intervals, but the association with steady magnetospheric convection, which was suggested before, cannot be confirmed possibly because of its rare occurrence.
Key Points
The tail magnetic field changes from a stretched to a dipolar configuration in the transition region around X = 9–12 RE
The X gradient of equatorial BZ is directed predominantly earthward in the transition region, but it can be tailward during dipolarization
Equatorial BZ can become extremely weak (<2 nT) in the transition region during the substorm growth phase and prolonged quiet intervals
Magnetospheric substorms represent key explosive processes in the interaction of the Earth's magnetosphere with the solar wind, and their understanding and modeling are critical for space weather ...forecasting. During substorms, the magnetic field on the nightside is first stretched in the antisunward direction and then it rapidly contracts earthward bringing hot plasmas from the distant space regions into the inner magnetosphere, where they contribute to geomagnetic storms and Joule dissipation in the polar ionosphere, causing impressive splashes of aurora. Here we show for the first time that mining millions of spaceborne magnetometer data records from multiple missions allows one to reconstruct the global 3‐D picture of these stretching and dipolarization processes. Stretching results in the formation of a thin (less than the Earth's radius) and strong current sheet, which is diverted into the ionosphere during dipolarization. In the meantime, the dipolarization signal propagates further into the inner magnetosphere resulting in the accumulation of a longer lived current there, giving rise to a protogeomagnetic storm. The global 3‐D structure of the corresponding substorm currents including the substorm current wedge is reconstructed from data.
Plain Language Summary
Using several millions of historical magnetometer records and data mining techniques, we form virtual spacecraft constellations of tens of thousands of spacecraft to reconstruct the global shape of the terrestrial magnetosphere at the moments of its most dramatic reconfigurations responsible for major space weather disturbances.
Key Points
Substorm tail current sheet thinning and dipolarization are reproduced using novel data mining technique
Global 3‐D structure of substorm currents including the substorm current wedge is reconstructed from data
Substorms contribute to an accumulation of a longer‐lived thick current in the innermost part of the magnetosphere
The present study investigates dipolarization signatures in the inner magnetosphere using sharp geosynchronous dipolarizations as a reference. The results are summarized as follows: (1) The region of ...sharp and structured dipolarizations expands earthward while dipolarizations are sustained at geosynchronous orbit; (2) within 5 RE from Earth, dipolarization signatures are often smooth and gradual, resembling midlatitude positive bays, and they start simultaneously with substorm onsets; (3) off the equator (>0.5 RE), sharp dipolarizations often take place before geosynchronous dipolarizations. These results can be explained by a model current system with R1‐sense and R2‐sense current wedges (R1CW and R2CW) if (a) the R1CW, which is located outside, is more intense than the R2CW in total current, (b) the R1CW stays outside of geosynchronous orbit, and (c) the R2CW moves earthward. The model suggests that the region of sharp dipolarizations is confined between the two current wedges, and it expands earthward as the R2CW moves earthward (Result 1). Sufficiently earthward of the R2CW, the remote effect of the R1CW dominates that of the R2CW, and accordingly, magnetic disturbances resemble midlatitude positive bays (Result 2). Since the timing of sharp dipolarizations is determined by the passage of the R2CW, they take place earlier for outer flux tubes. Away from the magnetic equator, sharp dipolarizations can precede geosynchronous dipolarizations especially if the magnetic configuration is stretched (Result 3). Thus, this double‐current wedge model explains the variability of dipolarization signatures at different distances, and it may be regarded as a generalized substorm current wedge model.
Key Points
The region of sharp dipolarizations expands earthward in the inner magnetosphere while geosynchronous dipolarizations are sustained
Dipolarizations at r < 5 Re are often smooth and gradual resembling midlatitude positive bays, and start simultaneously with substorm onsets
In the inner magnetosphere, sharp dipolarizations start earlier off the equator, often before geosynchronous dipolarizations
The auroral intensification at the poleward boundary of the auroral oval is often considered to be the ionospheric manifestation of the distant reconnection. In the present study, however, we propose ...that the poleward boundary intensifications (PBIs) are initiated by ionospheric polarization due to fast polar cap flows, which are known to be well correlated with PBIs. The current continuity at the ionosphere can be described in two different ways, that is, the reflection of an Alfvén wave and the closure of Pedersen and Hall currents with field‐aligned currents (FACs). The required consistency between the two approaches sets a framework for modeling the ionospheric polarization, and we numerically test the aforementioned idea focusing on an induced upward FAC as indicative of PBIs. The results show that in case the polar cap flow channel approaches the auroral oval perpendicularly from poleward, (i) upward and downward FACs are induced at the poleward boundary to the west and east of the longitudinal center of the flow channel, respectively; (ii) those induced FACs extend much wider in longitude than the flow channel; (iii) the peak densities of those induced FACs are significantly larger than those of the incident FACs; (iv) those induced FACs are distributed almost symmetrically in longitude, indicating that the Pedersen polarization dominates the Hall polarization; and (v) if the polar cap flow inclined dawnward (duskward), an upward (downward) FAC is induced first. These results are consistent with the reported characteristics of PBIs, which are rather difficult to explain otherwise.
Key Points
The poleward boundary intensification of auroral emission can be an effect of ionospheric polarization associated with fast polar cap flows
Field‐aligned currents induced by polarization at the poleward boundary close primarily with ionospheric Pedersen currents
The spatial extent and timing of PBIs relative to the polar cap flows are well explained in terms of ionospheric polarization
We investigate the electrodynamic coupling of the nightside magnetosphere‐ionosphere system using the analogy of a current circuit. In our model circuit the generator drives a constant current, which ...flows through the magnetotail and ionosphere branches. The magnetotail branch has a capacitor C and resistor RT, whereas the ionospheric branch has an inductor L and resistor RI. Each element is physically described with local quantities and geometries. For RT ≪ RI the electric circuit is characterized by three time constants: τCR(=CRT),
τLC=LC, and τL/R(=L/RI). It is found that τCR is of the order of the ion gyroperiod in the plasma sheet, and τLC and τL/R correspond to the eigenperiod and decay time of the field line oscillation, respectively. Therefore, despite the variability of each circuit element, τCR ≪ τLC ≪ τL/R holds generally. It is found that under this condition the current circuit is characterized as overdamped, and its decay time constant is given by τL/R. RI is smaller, and therefore, τL/R is longer as the structure is more elongated in the direction of convection. This may explain why the auroral streamers, which are considered to be the ionospheric manifestation of fast flows in the plasma sheet, last significantly longer than the flows themselves. Another application is the Pi2 pulsations at the substorm onsets. If RT increases by a factor of τLC/τCR, the system indeed becomes underdamped, and the oscillation period is given by 2πτLC. It is suggested that the substorm initiation is a distinct process with a significant enhancement of tail resistivity in a localized area.
Key Points
The nightside M‐I coupling current system is characterized as overdamped based on the ordering of its three time constants
Auroral streamers have long decay times (L/R) because they are elongated along convection and their ionospheric resistance (R) is small
Pi2 onsets may be a manifestation of the transition of the M‐I system from overdamped to underdamped owing to enhanced tail resistance
Substorm‐type evolution of the Earth's magnetosphere is investigated by mining more than two decades (1995–2017) of spaceborne magnetometer data from multiple missions including the first two years ...(2016‐2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal magnetohydrodynamics (MHD) behavior: X‐lines, thin current sheets, and regions with the tailward gradient of the equatorial magnetic field Bz. X‐lines are found to form mainly beyond 20 RE, but for strong driving, with the solar wind electric field exceeding ∼5mV/m, they may come closer. For substorms with weaker driving, X‐lines may be preceded by redistribution of the magnetic flux in the tailward Bz gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 RE, comparable to the thermal ions gyroradius, and at the same time, as long as 15 RE. Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement microscale analysis of nonideal effects.
Plain Language Summary
The sun emits a steam of charged particles called the solar wind that flows past the Earth interacting with the planet's dipole magnetic field. This stretches the dipolar magnetic field away from the sun on the nightside of the planet storing energy in the stretched field. Once every few hours, this stretched configuration suddenly becomes more dipolar bringing particles and magnetic flux closer to the planet and powering aurora in the polar regions. During these processes, termed substorms, the gas of charged particles, protons, and electrons trapped by the dipole and known as plasma, behaves largely as a perfectly conducting fluid. However, only deviations from this ideal conducting plasma behavior can explain the substorm mechanisms. We mine two decades of spacecraft magnetometer data from multiple missions to form swarms of thousands of synthetic probes. They help reveal effects of nonideal plasma evolution during substorms, which cannot be captured by direct in situ observations because of their extreme paucity.
Key Points
X‐lines, including the 11 July 2017 reconnection event, are reconstructed at and beyond 20 RE, but for strong driving they can come closer
Current sheets in the growth phase may be as thin as 0.2 RE, and at the same time, as long as 15 RE, violating isotropic force balance
Bz humps form in the growth phase, and their reconfiguration may precede X‐line formation and substorm onset
The dawn‐dusk asymmetry of magnetic depression is a characteristic feature of the storm main phase. Recently Ohtani (2021, https://doi.org/10.1029/2021JA029643) reported that its magnitude is ...correlated with the dawnside westward auroral electrojet (AEJ) intensity, and suggested that the dawnside AEJ intensification is a fundamental process of the stormtime magnetosphere‐ionosphere coupling. In this study we observationally address the cause of the dawnside AEJ intensification in terms of four scenarios. That is, the dawnside AEJ intensifies because (a) the external driving of global convection strengthens, (b) solar wind compression enhances energetic electron precipitation, and therefore, ionospheric conductance, through wave‐particle interaction, (c) the substorm current wedge forms in the dawn sector, and (d) energetic electrons injected by nightside substorms drift dawnward, and the subsequent precipitation enhances ionospheric conductance. We find an event that fits each scenario, and therefore, none of these scenarios can be precluded. However, the result of a superposed epoch analysis shows that some causes are more prevalent than others. More specifically, (a) although the enhancement of external driving may precondition the dawnside AEJ intensification, it is rarely the direct cause; (b) external compression probably explains only a small fraction of the events; (c) prior to the dawnside AEJ intensification, the westward AEJ tends to intensify in the midnight sector along with mid‐latitude positive bays, which suggests that the substorm injection of energetic electrons is the most prevalent cause. This last result may also be explained by the dawnside expansion of the substorm current wedge, which, however, is arguably far less common.
Key Points
During storms the dawnside westward auroral electrojet (AEJ) often intensifies causing temporal (∼2 hr) large (>1,000 nT) ground H reductions
Enhancement of dawnside ionospheric conductance following substorm e‐injection is the most prevalent cause of such AEJ intensifications
Conductance enhancement following solar wind compression explains a small fraction of the dawnside AEJ intensification events
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