Substorm Expansion Phase Research Articles

Thermospheric density perturbations in response to substorms

AbstractWe use 5 years (2001–2005) of CHAMP (Challenging Minisatellite Payload) satellite data to study average spatial and temporal mass density perturbations caused by magnetospheric substorms in the thermosphere. Using statistics from 2306 substorms to construct superposed epoch time series, we find that the largest average increase in mass density of about 6% occurs about 90 min after substorm expansion phase onset about 3 h of magnetic local time east of the onset region. Averaged over the entire polar auroral region, a mass density increase of about 4% is observed. Using a simple model to estimate the mass density increase at the satellite altitude, we find that an energy deposition rate of 30 GW applied for half an hour predominantly at an altitude of 110 km is able to produce mass density enhancements of the same magnitude. When taking into account previous work that has shown that 80% of the total energy input is due to Joule heating, i.e., enhanced electric fields, whereas 20% is due to precipitation of mainly electrons, our results suggest that the average substorm deposits about 6 GW in the polar thermosphere through particle precipitation. Our result is in good agreement with simultaneous measurements of the NOAA Polar‐orbiting Operational Environmental Satellite (POES) Hemispheric Power Index; however, it is about 1 order of magnitude less than reported previously.

Coordinated ionospheric observations indicating coupling between preonset flow bursts and waves that lead to substorm onset

AbstractA critical, long‐standing problem in substorm research is identification of the sequence of events leading to substorm expansion phase onset. Recent Time History of Events and Macroscale Interactions during Substorms (THEMIS) all‐sky imager (ASI) array observations have shown a repeatable preonset sequence, which is initiated by a poleward boundary intensification (PBI) and is followed by auroral streamers moving equatorward (earthward flow in the plasma sheet) and then by substorm onset. On the other hand, substorm onset is also preceded by azimuthally propagating waves, indicating a possible importance of wave instability for triggering substorm onset. However, it has been difficult to identify the link between fast flows and waves. We have found an isolated substorm event that was well instrumented with the Poker Flat incoherent scatter radar (PFISR), THEMIS white‐light ASI, and multispectral ASI, where the auroral onset occurred within the PFISR and ASI fields of view. This substorm onset was preceded by a PBI, and ionospheric flows propagated equatorward from the polar cap, crossed the PBI, and reached the growth phase arc. This sequence provides evidence that flows from open magnetic field lines propagate across the open‐closed boundary and reach the near‐Earth plasma sheet prior to the onset. Quasi‐stable oscillations in auroral luminosity and ionospheric density are found along the growth phase arc. These preonset auroral waves amplified abruptly at the onset time, soon after the equatorward flows reached the onset region. This sequence suggests a coupling process where preexisting stable waves in the near‐Earth plasma sheet interact with flows from farther downtail and then evolve to onset instability.

M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

Abstract. We study substorms from two perspectives, i.e., magnetosphere–ionosphere coupling across the auroral oval at dusk and at midnight magnetic local times. By this approach we monitor the activations/expansions of basic elements of the substorm current system (Bostrøm type I centered at midnight and Bostrøm type II maximizing at dawn and dusk) during the evolution of the substorm activity. Emphasis is placed on the R1 and R2 types of field-aligned current (FAC) coupling across the Harang reversal at dusk. We distinguish between two distinct activity levels in the substorm expansion phase, i.e., an initial transient phase and a persistent phase. These activities/phases are discussed in relation to polar cap convection which is continuously monitored by the polar cap north (PCN) index. The substorm activity we selected occurred during a long interval of continuously strong solar wind forcing at the interplanetary coronal mass ejection passage on 18 August 2003. The advantage of our scientific approach lies in the combination of (i) continuous ground observations of the ionospheric signatures within wide latitude ranges across the auroral oval at dusk and midnight by meridian chain magnetometer data, (ii) "snapshot" satellite (DMSP F13) observations of FAC/precipitation/ion drift profiles, and (iii) observations of current disruption/near-Earth magnetic field dipolarizations at geostationary altitude. Under the prevailing fortunate circumstances we are able to discriminate between the roles of the dayside and nightside sources of polar cap convection. For the nightside source we distinguish between the roles of inductive and potential electric fields in the two substages of the substorm expansion phase. According to our estimates the observed dipolarization rate (δ Bz/δt) and the inferred large spatial scales (in radial and azimuthal dimensions) of the dipolarization process in these strong substorm expansions may lead to 50–100 kV enhancements of the cross-polar-cap potential due to inductive electric field coupling.

Automated determination of auroral breakup during the substorm expansion phase using all‐sky imager data

AbstractThis paper describes a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers. Substorm onset is characterized by a brightening of the aurora that is followed by auroral poleward expansion and auroral breakup. This breakup can be identified by a sharp increase in the auroral intensity i(t) and the time derivative of auroral intensity i(t). Utilizing both i(t) and i'(t), we have developed an algorithm for identifying the time interval and spatial location of auroral breakup during the substorm expansion phase based solely on quantifiable characteristics of the optical auroral emissions. We compare the time interval determined by the algorithm to independently identified auroral onset times from three previously published studies. In each case the time interval determined by the algorithm is within error of the onset independently identified by the prior studies. We further show the utility of the algorithm by comparing the breakup intervals determined using the automated algorithm to an independent list of substorm onset times. We demonstrate that 50% of the breakup intervals characterized by the algorithm are within the uncertainty of the times identified in the list. The quantitative description and routine identification of an interval of auroral brightening during the substorm expansion phase provides a foundation for unbiased statistical analysis of the aurora and to probe the physics of the auroral substorm and identify the processes leading to auroral substorm onset.

Substorm onset process: Ignition of auroral acceleration and related substorm phases

AbstractThe substorm onset process was studied on the basis of the vertical evolution of auroral acceleration regions derived from auroral kilometric radiation (AKR) spectra and Pi pulsations on the ground. The field‐aligned auroral acceleration at substorm onset demonstrated two distinct phases. Low‐altitude acceleration (h~3000–5000 km), which accompanied auroral initial brightening, prebreakup Pi2, and direct current of ultralow frequency (DC‐ULF) pulsation were first activated and played an important role (precondition) in the subsequent substorm expansion phase onset. Prebreakup Pi2 is suggestive of the ballooning mode wave generation, and negative decrease in DC‐ULF suggests increasing field‐aligned current (FAC). We called this stage the substorm initial phase. A few minutes after this initial phase onset, high‐altitude acceleration, which accompanied auroral breakup and poleward expansion with breakup Pi1 and Pi2 pulsations, suddenly broke out in an altitude range from 8000 to 16,000 km. Thus, substorm expansion onset originated in the magnetosphere‐ionosphere (M‐I) coupling region, i.e., substorm ignition in the M‐I coupling region. Statistical investigations revealed that about 65% of earthward flow bursts observed in the plasma sheet were accompanied by enhanced low‐altitude AKR, suggesting that flow braking of bursts causes FAC and resulting low‐altitude field‐aligned acceleration in the M‐I coupling region. On the basis of these observations, we propose a substorm onset scenario in which FAC that originated from the braking of plasma flow bursts first enhances low‐altitude acceleration (substorm initial phase onset) and then the increasing FAC induces current‐driven instability in the M‐I coupling region, which leads to high‐altitude acceleration and resulting substorm expansion phase onset.

The detailed spatial structure of field‐aligned currents comprising the substorm current wedge

We present a comprehensive two‐dimensional view of the field‐aligned currents (FACs) during the late growth and expansion phases for three isolated substorms utilizing in situ observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and from ground‐based magnetometer and optical instrumentation from the Canadian Array for Realtime Investigations of Magnetic Activity and Time History of Events and Macroscale Interactions during Substorms ground‐based arrays. We demonstrate that the structure of FACs formed during the expansion phase and associated with the substorm current wedge is significantly more complex than a simple equivalent line current model comprising a downward FAC in the east and upward FAC in the west. This two‐dimensional view demonstrates that azimuthal bands of upward and downward FACs with periodic structuring in latitude form across midnight and can span up to 8 h of magnetic local time. However, when averaged over latitude, the overall longitudinal structure of the net FACs resembles the simpler equivalent line current description of the substorm current wedge (SCW). In addition, we demonstrate that the upward FAC elements of the structured SCW are spatially very well correlated with discrete aurora during the substorm expansion phase and that discrete changes in the FAC topology are observed in the late growth phase prior to auroral substorm expansion phase onset. These observations have important implications for determining how the magnetosphere and ionosphere couple during the late growth phase and expansion phase, as well as providing important constraints on the magnetospheric generator of the FACs comprising the SCW.

Where is the magnetic energy for the expansion phase of auroral substorms accumulated?

AbstractCharacteristics of the ionospheric current system (the directly driven (DD) / unloading (UL) currents) and the power ε during auroral substorms are examined semiquantitatively in terms of input‐output relationship. The study confirms (a) The magnetic energy for the expansion phase is accumulated with the rate of a little more than 1018 erg/s during the growth phase which lasts for about 1 h (because the ionosphere cannot dissipate the input power). (b) The global energy dissipation rate in the ionosphere (including the Joule heat production and the particle precipitation) is more than 3.3 × 1018 erg/s. (c) Thus, the total energy consumption by the ionosphere during the impulsive expansion phase (lasting for about 1 h) is 1.2 × 1022 erg or up to 1023 erg for medium substorms. (d) However, it is shown that even the total magnetic energy in the magnetotail at a distance between 10 RE and 20 RE (6.5 × 1021 ergs = 6.5 × 1014 J) cannot supply the energy for the expansion phase. This estimate is consistent with the most recent satellite observations (4.3 × 1013 J (= 4.3 × 1020 ergs)). Thus, the following conclusions may be applicable for medium intensity substorms: (i) The major part of the magnetic energy for the expansion phase must be accumulated within a distance of 10 RE and be released almost totally during the expansion phase (because the duration of the growth phase and the expansion phase is about the same). (ii) This suggests that there is an upper limit of the energy which the magnetosphere can accumulate even at distances less than 10 RE. Further, it is confirmed that: (a) The expansion phase is characterized by a single cell current system UL (not a twin cell current system DD which is driven by the earthward plasma convection from the magnetotail). (b) The UL current is independent of the DD current, requiring a different origin. (c) During the recovery phase, the UL current is absent and the DD current approximately follows ε (no accumulation of the energy), because the ionosphere is capable of dissipating the power even if the power is moderate). (d) Based on these results, as an example, one possible chain of processes within 10 RE is proposed as the release process of the accumulated magnetic energy by taking into account the fact that the frozen‐in field condition is observed to be broken down.

Tail reconnection region versus auroral activity inferred from conjugate ARTEMIS plasma sheet flow and auroral observations

AbstractPlasma sheet flow bursts have been suggested to correspond to different types of auroral activity, such as poleward boundary intensifications (PBIs), ensuing auroral streamers, and substorms. The flow‐aurora association leads to the important question of identifying the magnetotail source region for the flow bursts and how this region depends on magnetic activity. The present study uses the ARTEMIS spacecraft coordinated with conjugate ground‐based auroral imager observations to identify flow bursts beyond 45 RE downtail and corresponding auroral forms. We find that quiet‐time flows are directed dominantly earthward with a one‐to‐one correspondence with PBIs. Flow bursts during the substorm recovery phase and during steady magnetospheric convection (SMC) periods are also directed earthward, and these flows are associated with a series of PBIs/streamers lasting for tens of minutes with similar durations to that of the series of earthward flows. Presubstorm onset flows are also earthward and associated with PBIs/streamers. The earthward flows during those magnetic conditions suggest that the flow bursts, which lead to PBIs and streamers, originate from further downtail of ARTEMIS, possibly from the distant‐tail neutral line (DNL) or tailward‐retreated near‐Earth neutral line (NENL) rather than from the nominal NENL location in the midtail. We find that tailward flows are limited primarily to the substorm expansion phase. They continue throughout the period of auroral poleward expansion, indicating that the expansion‐phase flows originate from the NENL and that NENL activity is closely related to the auroral expansion of the substorm expansion phase.

Distinction between auroral substorm onset and traditional ground magnetic onset signatures

AbstractTo evaluate whether ground magnetic responses traditionally viewed as signatures of substorm onset may be related to postonset auroral streamers rather than to substorm onset seen in aurora, we have analyzed ground magnetic and all‐sky image responses for 14 substorm onsets selected solely on imager coverage. We find evidence supporting this possibility, the response being approximately coincident for midlatitude positive bay and Pi2 pulsation enhancements and for abrupt auroral zone H decreases at stations near a streamer. These are all signatures of current wedge formation, indicating that substorm current wedge formation may be more of a response to plasma sheet flow channels associated with postonset streamers than to the process leading to auroral onset and that the current wedge may develop via a series of distinctly narrower wedge‐like structures. We also find evidence that periods of streamers can occur more than once during a substorm expansion phase and can give magnetic signatures of multiple onsets even if there are no additional auroral onsets. Furthermore, the peak auroral zone H decreases are seen in association with streamers and at times varying from just a few minutes to well over a half hour after substorm auroral onset if there is a prolonged period of streamers. This indicates that peak substorm AE and AL values are likely related to features of postonset streamers and their locations relative to observing stations and are not a measure of the strength of the substorm auroral onset processes. We discuss possible additional implications on substorm expansion phase development.

IMF effect on the polar cap contraction and expansion during a period of substorms

Abstract. The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

Temporal and spatial dynamics of the regions 1 and 2 Birkeland currents during substorms

We use current density data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to identify the location of maximum region 1 current at all magnetic local times (MLTs). We term this location the R1 oval. Comparing the R1 oval location with particle precipitation boundaries identified in DMSP data, we find that the R1 oval is located on average within 1° of particle signatures associated with the open/closed field line boundary (OCB) across dayside and nightside MLTs. We hence conclude that the R1 oval can be used as a proxy for the location of the OCB. Studying the amount of magnetic flux enclosed by the R1 oval during the substorm cycle, we find that the R1 oval flux is well organized by it: during the growth phase the R1 oval location moves equatorward as the amount of magnetic flux increases whereas after substorm expansion phase onset significant flux closure occurs as the R1 current location retreats to higher latitudes. For about 15 min after expansion phase onset, the amount of open magnetic flux continues to increase indicating that dayside reconnection dominates over nightside reconnection. In the current density data, we find evidence of the substorm current wedge and also show that the dayside R1 currents are stronger than their nightside counterpart during the substorm growth phase, whereas after expansion phase onset, the nightside R1 currents dominate. Our observations of the current distribution and OCB movement during the substorm cycle are in excellent agreement with the expanding/contracting polar cap paradigm.

Substorm onset and expansion phase intensification precursors seen in polar cap patches and arcs

Localized, transient polar cap flows impinging on the poleward boundary of the nightside auroral oval have been suggested to precede poleward boundary intensifications (PBIs), which are often followed by auroral streamers, some of which propagate equatorward and appear to lead to substorm auroral onset. While echo coverage often limits radar flow measurements, imaging of polar cap patches and arcs has the potential to monitor localized polar cap flows by tracing emission structures, previously associated with enhanced flows, over long distances. We use such imaging to examine if polar cap patches and arcs moving over large distances can be seen as possible precursors to presubstorm‐onset PBIs. We find evidence that such features, which are longitudinally narrow, propagate from the dayside polar region toward the nightside polar cap boundary prior to pre‐onset PBIs. This sequence indicates that transient activity in the dayside polar region may initiate polar cap patches and arcs that propagate across the polar cap and are followed by pre‐onset PBIs. Furthermore, we find evidence that expansion‐phase active aurora expanded poleward soon after additional polar cap patches and arcs reached close to the nightside polar cap boundary. The expansion phase auroral activity significantly weakened when polar cap patches/arcs disappeared. Our findings suggest that plasma transport originating from the dayside and reaching the nightside open‐closed boundary may trigger in plasma sheet flow bursts and play a crucial role in pre‐ and post‐onset auroral activity. Polar cap imaging offers the possibility for monitoring such localized, transient plasma transport over large distances.

Statistical properties of substorms during different storm and solar cycle phases

Abstract. Substorm properties during different storm phases have been studied using an automated recognition of substorm and storm phases in the auroral electrojet (AL) and ring current (Dst) index data from 1995–2009. The large number of events (about 500 storms and 15 000 substorms) provides statistically reliable distributions, average behaviour and long time series of simple parameters, such as durations and intensities. The phases of storms and substorms have been examined independently. Substorm phases have been further combined to single and multi-cycle events. The former consist of one growth, one expansion and one recovery phase, while the latter include multiple expansion and recovery phases after one growth phase. Our findings show that most substorms take place during non-storm times, and substorms during storm initial phases resemble isolated non-storm time substorms. Both during storm initial phases and non-storm times, the substorm growth phases may last longer than the other substorm phases. Substorm recovery phase is typically the longest phase but its duration also varies most. The longest substorm recovery phase duration was observed during multi-cycle substorms. The longest substorm expansion and storm main phases were found during the years close to the solar maximum. The shortest substorm events (the shortest phase durations) are the single-cycle substorms. The period of expansion onsets during multi-cycle substorms varied hugely for events with a small number of expansion phases. For events with a larger number of expansions, a clearer periodicity of about one hour (median value) was suggested.

Observations of polar cap flow channel and plasma sheet flow bursts during substorm expansion

AbstractWe present the first simultaneous observations of an enhanced polar cap flow impinging on the nightside polar cap boundary (PCB), two flow bursts in the plasma sheet and a conjugate ionospheric flow burst within the auroral oval. The ionospheric measurements on 3 September 2006 were made by the European Incoherent Scatter (EISCAT) radars and the magnetospheric measurements by the four Cluster spacecraft. In the end of a substorm growth phase, EISCAT measured a channel of enhanced equatorward plasma flow within the polar cap, which was about 1° wide in latitude and drifted slowly equatorward. During the substorm expansion phase, the PCB started to contract poleward. The interaction between the equatorward drifting polar cap flow channel and the poleward contracting PCB took 2–3 min. During this time, the F‐region electron temperature was elevated at the PCB, which is interpreted as a possible signature of an auroral poleward boundary intensification (PBI). After that, enhanced equatorward flows were measured inside the auroral oval by EISCAT. During this period, the Cluster satellites measured two fast earthward flow bursts in the plasma sheet, which were associated with dipolarizations of the magnetic field, depletions in plasma density, and return flows. We suggest that the second flow burst in the plasma sheet represents the same flow burst that is seen in the ionosphere by EISCAT and propose that the plasma sheet flow bursts were triggered by the enhanced flow structure on open polar cap field lines. The suggestion is in line with Lyons et al. (2011).

Effect of prompt penetration on the low latitude ASY indices

Onset of a geomagnetic substorm often intensifies the westward auroral electrojet, as well as produces asymmetric magnetic field at low-/mid-latitudes. Auroral electrojet and low latitude asymmetric indices are known to correlate well during substorms. These indices have been widely used to monitor the duration and strength of substorm activities. However, several processes, other than substorms, introduce local time asymmetry in magnetic field at low latitudes, which can substantially influence the ASY indices. Large number of substorms are observed in association with changes in the interplanetary magnetic field (IMF). It is known that sharp IMF Bz orientation changes result in penetration of interplanetary electric field (IEF) to lower latitudes, which affects the geomagnetic fields to different degree in different local times. In the present study, we demonstrate that sharp IMF Bz fluctuations during the expansion phases of substorms introduce additional asymmetry at low latitudes. The effect is clearly seen in ASYH, whereas ASYD remains almost unaltered.

Spectral characteristics of the plasma dispersionless injection during the storm recovery phase on 11 March 1998

A substorm dispersionless injection event observed during the storm recovery phase on 11 March 1998 at geosynchronous orbit is carefully studied. The event shows the notable characteristics that for energetic ions the flux enhancement ratio before and after injection increases and remains elevated with increasing energy, while for energetic electrons it tends to decrease with increasing energy. In order to explain the unique injection feature, the authors propose a possible mechanism that velocity space diffusion in common to electric acceleration adjusts the particle injection state. Spectral characteristics of four different phases (pregrowth phase, the growth phase, the substorm expansion phase, and the recovery phase) have been investigated. The differential fluxes of electrons from 50 keV to 1.5 Mev and ions from 50 keV to 1.2 MeV measured by Synchronous Orbit Plasma Analyzer (SOPA) instrument onboard LANL satellite 1991–080 are found to be best fitted with the three‐parameter kappa distribution function (f ∼ A0 · E[1 + E / (κE0)]−κ−1) by Levenberg‐Marquardt and Universal Global Optimization methods. The evolutions of the three parameters in the above kappa distribution in different substorm phases have been depicted for both electrons and ions. In each phase, E0 and κ show an approximately linear relationship κ(E0) = κ0 + ηE0. This linear relationship can be obtained by solving the velocity space diffusion equation with an initial superthermal kappa distribution. Ion and electron are found to have opposite trend of parameters κ0 and η in each phase, which indicates that the different species of particles exert different velocity space diffusion processes so that their flux enhancement ratios before and after injection are rather different. This implies that not only electric field acceleration, but also velocity space diffusion plays a very important role in the particle injection.

Reduction in field‐aligned currents preceding and local to auroral substorm onset

We examine the global field‐aligned current (FAC) topology associated with a clear substorm on the 16 February 2010. We show that for this particular substorm there is a clear and localised reduction in the FACs observed by AMPERE at least 6 minutes prior to auroral onset. A new auroral arc forms in the region of reduced FAC and on closed field lines which subsequently brightens and expands poleward, signifying the start of the substorm expansion phase. We argue that the change in FACs observed prior to onset is the result of a change in the magnetosphere‐ionosphere (M‐I) coupling in a region local to the subsequent auroral onset. Such a change implies an important role for M‐I coupling in destabilising the near‐Earth tail during magnetospheric substorms and perhaps more importantly in selecting the location in the ionosphere where auroral onset begins.

The double auroral oval in the dusk‐midnight sector: Formation, mapping and dynamics

The present study observationally examines the double auroral oval structure in the evening‐premidnight sector. The results are summarized as follows; (1) The poleward auroral branch can be attributed to accelerated auroral precipitation, occasionally accompanied by Alfvénic precipitation, and it is collocated with the upward R1 current; (2) The equatorward auroral branch, which is embedded in the downward R2 current, very often if not always consists of the diffuse precipitation of energetic electrons and extends mostly poleward of the b2i boundary collocated with energetic ion precipitation; (3) The associated ionospheric convection is consistent with the two‐cell pattern, and the poleward branch is located at the zonal flow shear. It is inferred that the equatorward branch is mapped to the geosynchronous region and outside, whereas the poleward branch is mapped farther down the tail to the source region of the R1 system. It is suggested that the intense ring current and injection of energetic electrons are necessary for the equatorward branch to be distinct, but the substorm expansion phase is not favorable for the formation of the emission gap. This explains why the double oval often takes place during magnetospheric storms and during the recovery phase of intense substorms. The equatorial magnetic field is expected to be strong in the mid‐tail region, which might be related to the auroral gap region. It is also found that the poleward branch occasionally intensifies in an extended MLT sector in a few minutes, which may be explained in terms of the sudden enhancement of the return (sunward) convection flow.

The correlation of ULF waves and auroral intensity before, during and after substorm expansion phase onset

We present case studies of the evolution of Ultra‐Low Frequency (ULF) magnetic wave amplitudes and auroral intensity through the late growth phase and the expansion phase of the substorm cycle. We present strong evidence that substorm‐related auroral enhancements are clearly and demonstrably linked to ULF wave amplitudes observed at the same location. In all three case studies presented, the correlation analysis shows that the ULF wave activity and auroral intensities are highly correlated at close to zero lag. We discuss four possible explanations that may be able to explain both the timing and the high correlations between these two phenomena, including a simple coincidence, an artifact of instrumental effects, the response of the ionosphere to magnetic waves and auroral particle precipitation, and finally, that ULF waves and auroral particle precipitation are physically linked at their source. We discount coincidence and instrumental effects since we present multiple events where instrumental effects have a negligible contribution, and we find that the ionospheric response to waves and precipitation can explain some, but not all, of the results contained within this paper. Specifically, the ionospheric response to substorm waves and auroral precipitation cannot explain the result backed up by previous studies that the onset of ULF wave activity and the onset of auroral particle precipitation occur at the same time and in the same location. This leaves the possibility that ULF waves and auroral particles are physically linked at their source. We therefore re‐emphasize the importance of ULF wave observations in fully understanding the mechanism or mechanisms responsible for rapid auroral brightenings.

Behavior of substorm auroral arcs and Pi2 waves: implication for the kinetic ballooning instability

Abstract. We present synoptic observations of the 21 December 2006 substorm event by the THEMIS ground-based All-Sky-Imagers, the ISUAL CCD Imager aboard the FORMOSAT-2 satellite, the geosynchronous satellites and the ground-based magnetometers, and discuss the implication of the observations. There are three subsequent arc breakups with time separation of <1 min during the substorm expansion phase. In particular, we investigated the mode number of the substorm arc bead-like structure and the concurrent behavior of the arc intensity, the westward electroject intensity, and the ground Pi2 pulsation amplitude. Prior to each arc breakup there was a clear azimuthally-spaced bright spot structure along the arc with high mode number (~140–180) and the arc intensity increased together with the westward electrojet and the ground Pi2 pulsation amplitude under the arc. The Pi1 perturbations observed under the arc appeared at or after the arc breakup started. This suggests that the Pi2 pulsation is related to the arc formation. The Pi2 pulsation may be caused by the kinetic ballooning instability (KBI) that is excited in the strong cross-tail current region. The longitudinal extent of the earthward expansion front of the substorm dipolarization region at the geosynchronous orbit is estimated from timings of the energetic proton and electron injections and is roughly located between ~19.50 MLT and ~23.00 MLT, which is consistent with the corresponding longitudinal extent of the auroral substorm activity.