Substorm Recovery Phase Research Articles

Fields and flows at GEOTAIL during a moderate substorm

The behavior of the tail field and plasma during a moderate substorm with onset at ∼1120 UT on Dec. 13, 1994 is examined using data from the ISTP spacecraft GEOTAIL and WIND, synchronous orbit and the ground. Five substorms were observed on this day while GEOTAIL was located near the center of the tail at Xgsm = −46 R e. In each substorm the field and plasma variations were similar to those observed during substorms by spacecraft closer to the earth. A southward turning of the IMF caused accumulation of lobe flux, development of a more tail-like field, and eventually an expansion phase and its consequences. The ∼1120 UT onset immediately followed a northward turning which ended a lengthy interval in which the IMF was alternately northward and southward. Shortly after the onset a flux rope passed GEOTAIL with a delay consistent with its formation at 20–30 R e several minutes earlier than a Pi 2 burst began at midnight. Immediately after the onset the lobe field decreased and the plasma sheet disappeared. During the substorm recovery phase the plasma sheet reappeared with plasma moving earthward. The plasma data show that the tailward flow is a combination of convecting and streaming plasma. All of the substorms exhibited multiple onsets. The main onset of each can be determined by a combination of negative bay onsets, Pi 2 bursts, synchronous field-aligned currents, dispersionless particle injection, and midlatitude positive bays. In one event the flux rope at 46 R e arrived before dispersionless injection at synchronous orbit suggesting that reconnection in the tail begins at or before major onsets. In fact most of the major onsets were preceded by pseudo breakups early in the growth phase, and weak tailward flows carrying a weak vertical field fluctuating about zero. These observations suggest that reconnection begins in the middle tail early in the growth phase and that it is substantially intensified or begins again at another location at the expansion onset.

High‐latitude Hall and Pedersen conductances during substorm activity in the SUNDIAL‐ATLAS campaign

Simultaneous high time resolution observations of the Hall and Pedersen conductances, ΣH and ΣP, respectively, the ionospheric electric field and the ground magnetic field during a magnetospheric substorm are reported. The measurements discussed here were taken during the SUNDIAL/ATLAS 1 campaign of March 24 to April 2, 1992. The European Incoherent Scatter (EISCAT) UHF special programme SP‐UK‐ATLAS, which operated on March 27, 1992, provided continuous measurements of the electron density and the ion vector velocity from which ΣH and ΣP and the ionospheric electric field were calculated. During the substorm growth phase, ΣP and ΣH were less than 10 S and the ratio, R = ΣH/ΣP, was less than 1. Although both ΣH and ΣP increased at the onset of the expansion phase, R remained close to 1. This ratio provides information on the mean energy of the precipitating particles responsible for the enhanced conductances. A ratio of 1 implies a mean energy of the particles of 2.56 keV. Two distinct expansion phases were identified, the second of which included a number of intensifications. Both ΣH and ΣP increased toward the end of the second expansion phase with peak values of 71 S and 34 S, respectively. The ratio R also increased to values exceeding 2, equivalent to mean energies of more than 5.78 keV. The largest value of R was 3.25, which occurred during the substorm recovery phase and is equivalent to mean energies of more than 10 keV. The increase in mean energy as the substorm progresses may be interpreted in terms of changes in the acceleration processes in the magnetosphere. The maximum zonal (east‐west) current during this interval was 2.20 A m−1 and occurred toward the end of the second substorm expansion phase. At a number of intensifications, reversals or enhancements in the zonal current were observed, for which the electric field was responsible in most cases. While the peak value of the Pedersen conductance of 40 S is similar to recent published results, the Hall conductance peak of 75 S is less than recent published measurements. Furthermore, the westward current during the initial part of the expansion phase is dominated by the electric field, in contrast to a previously published model which suggested that this current would be conductivity‐dominated. It is suggested that further spatial structure of the current is necessary to explain these observations.

The average ionospheric electrodynamics for the different substorm phases

The average patterns of the electrostatic potential, current vectors, and Joule heating in the polar ionosphere, as well as the associated field‐aligned currents, are determined for a quiet time, the growth phase, the expansion phase, the peak epoch, and the recovery phase of substorms. For this purpose, the Kamide‐Richmond‐Matsushita magnetogram‐inversion algorithm is applied to a data set (for March 17, 18, and 19, 1978) from the six meridian magnetometer chains (the total number of magnetometer stations being 71) which were operated during the period of the International Magnetospheric Study (IMS). This is the first attempt at obtaining, on the basis of individual substorms, the average pattern of substorm quantities in the polar ionosphere for the different epochs. The main results are as follows: (1) The substorm‐time current patterns over the entire polar region consist of two components. The first one is related to the two‐cell convection pattern, and the second one is the westward electrojet in the dark sector which is related to the wedge current. (2) Time variations of the two components for the four substorm epochs are shown to be considerably different. (3) The dependence of these differences on the ionospheric electric field and the conductivities (Hall and Pedersen) is identified. (4) It is shown that the large‐scale two‐cell pattern in the electric potential is dominant during the growth phase of substorms. (5) The expansion phase is characterized by the appearance of a strong westward electrojet, which is added to the two‐cell pattern. (6) The large‐scale potential pattern becomes complicated during the recovery phase of substorms, but the two‐cell pattern appears to be relatively dominant again during their late recovery as the wedge current subsides. These and many other earlier results are consistent with the present ones, which are more quantitatively and comprehensively demonstrated in our global study. Thus the two components are tentatively identified as the directly driven and the unloading components, respectively, although there is some indication that both components are actually coupled in the ionosphere. In the present paper we show that the directly driven component is present throughout the lifetime of substorms, becoming a dominant feature during the recovery phase of substorms as the unloading component wanes. The fact that the two components exist and that their time variations are different indicates that different processes are in progress even for the same value, say, −500 nT, of AL and that we must be cautious in using a single electrojet index, such as AL, in ordering satellite data.

Anisotropy Reversals in the Distant Magnetotail and Their Association with Magnetospheric Substorms

We present a series of tailward-to-Earthward particle anisotropy reversals observed by the GEOTAIL spacecraft in the distant magnetotail plasma sheet at a distance of | x |≈ -90RE during the 2.5-day period of 11/01/94 1200 UT to 13/01/94 2400 UT. The reversals were detected in the first order anisotropy of energetic protons and the first moment of the low energy plasma distribution function. The X-component of the cross-field flow exhibits the clearest indicator of a particle anisotropy reversal. A comprehensive examination of ground magnetometer and geosynchronous, satellite data reveals that each distant tall anisotropy reversal occurred during the late expansion or recovery phase of a magnetospheric substorm. Out of 14 substorms during which adequate monitoring of the geomagnetic activity and the plasma sheet was possible all but possibly one exhibited a tailward-to-Earthward reversal in the cross field flow ∼1-2 hours after substorm onset. The median time delay between substorm onset and the beginning of the subsequent Earthward convective flow was 94 min. Some uncertainty is associated with this time delay due to the commonly observed exits of the spacecraft to the lobe/mantle. The lower limit on the time delay based on the time of exit of the spacecraft to the lobe/mantle is 61 min. The implications of the particle anisotropy reversals for substorm dynamics are considered.

Mapping of the auroral oval and individual arcs during substorms

Magnetospheric mappings of the auroral oval during the course of a substorm are presented for an event where conjugate auroral imaging was available (CDAW 9 event C on May 3, 1986). The mappings have been computed using time‐evolving magnetic field models developed for the substorm growth and recovery phases. We show that even though the models do not contain field‐aligned currents, conjugacy between images from the two hemispheres is good. Furthermore, the high time resolution Viking images show an accurate comparison with the east‐west spreading of the substorm current wedge observed by four spacecraft in the near‐Earth tail. We also show that the double oval configuration present during the substorm recovery phase maps mostly within the lunar distance. The void (weak luminosity) regions between the poleward and equatorward arc systems are connected to the region 20–30 RE down the tail. To complement the global view, mappings of individual growth phase auroral arcs were undertaken. These mappings did not result in a coherent pattern, suggesting that these arcs may be formed by low‐altitude acceleration region processes rather than plasma sheet instabilities.

Vortex streets driven by sheared flow and applications to black aurora

Black aurora often exhibits the signature of vortex structures. In this paper, it is proposed that pseudo‐three‐dimensional drift‐like electrostatic modes can play a very important role in those phenomena. Employing a two‐fluid model, we derive a set of nonlinear equations governing the dynamics of long wavelength inertial convective cells and coupled drift‐acoustic waves in the presence of geomagnetic field‐aligned plasma flows. It is shown that free energy stored in the latter can cause purely growing and oscillatory instabilities in the auroral ionosphere. The threshold conditions as well as expressions for the linear growth rates are presented. Furthermore, it is found that possible stationary solutions of the nonlinear equations can be represented in the form of vortex structures. It is, therefore, likely that the nonlinear low‐frequency electrostatic modes, as described here, may possibly account for the coherent vortex structures within black aurora, as they are often observed during the late recovery phase of a substorm.

Magnetic field changes at the neutral sheet associated with substorm expansion onset: A model prediction and observations

Using a simple numerical model of the magnetotail current system, substorm‐associated changes in the large‐scale distribution of the neutral sheet current and the corresponding magnetic field at the neutral sheet are examined. In the current model it is assumed that the radial gradient of the neutral sheet current at the late expansion phase of substorms is steeper than that just prior to the expansion onset. It is shown that our current model can successfully reproduce the observed characteristics of the so‐called magnetic field dipolarization in the near‐Earth magnetotail. Comparing various satellite observations with what our current model predicts, the location of the substorm initiation and the propagation of the magnetic field dipolarization is discussed. It is discussed that there are at least three distinct types of the radial distribution of the neutral sheet current, depending on magnetospheric conditions: quiet times, just prior to substorm expansion onset, and near the end of the expansion phase (or the beginning of the recovery phase) of substorms.

Energetic particle dropouts observed in the morning sector by the geostationary satellite GEOS‐2

Energetic particle dropouts were observed in the local morning sector with the particle spectrometer S321 onboard the European geostationary satellite GEOS‐2. The dropouts usually last 10 to 30 minutes. They differ from those commonly observed in the midnight sector during growth phases of substorms and also from magnetopause crossings. They occur during the recovery phase and are well inside the magnetosphere. During the dropouts the total magnetic field increases. Ground magnetic recordings and AE indices show that the dropouts are related to the substorm recovery phase during multiple substorm events. Ground magnetic recordings also reveal that in the majority of the cases a new substorm activation is seen in the midnight sector simultaneously with the recovery phase signatures in the morning sector. The dropouts are most likely associated with locally thinning of the morning side plasma sheet.

Hybrid state of the tail magnetic configuration during steady convection events

Previous observations have shown that during periods of steady magnetospheric convection (SMC) a large amount of magnetic flux crosses the plasma sheet (corresponding to ∼10° wide auroral oval at the nightside) and that the magnetic configuration in the midtail is relaxed (the current sheet is thick and contains enhanced BZ). These signatures are typical for the substorm recovery phase. Using near‐geostationary magnetic field data, magnetic field modeling, and a novel diagnostic technique (isotropic boundary algorithm), we show that in the near‐Earth tail the magnetic configuration is very stretched during the SMC events. This stretching is caused by an intense, thin westward current. Because of the strongly depressed BZ, there is a large radial gradient in the near‐tail magnetic field. These signatures have been previously associated only with the substorm growth phase. Our results indicate that during the SMC periods the magnetic configuration is very peculiar, with co‐existing thin near‐Earth current sheet and thick midtail plasma sheet. The deep local minimum of the equatorial Bz that develops at R ∼ 12 RE is consistent with steady, adiabatic, Earthward convection in the midtail. These results impose constraints on the existing substorm theories, and call for an explanation of how such a stressed configuration can persist for such a long time without tail current disruptions that occur at the end of a substorm growth phase.

Signatures of the substorm recovery phase at high‐altitude spacecraft

The substorm recovery phase typically commences ∼30 min after the substorm expansion phase onset and covers a period of roughly 1 hour. Several signatures have previously been associated with the recovery phase such as plasma sheet expansion in the midtail and magnetic field return toward the quiet time configuration. However, the detailed temporal sequence during the recovery phase is still not very well established. A total of 66 events in February‐April 1979 have been investigated where ISEE 2 observed a plasma sheet expansion associated with fast earthward flows. Of these, 50 events were clearly associated with the substorm recovery phase as identified in ground magnetic records. For this data set, energetic electron (>30 keV) and proton (>145 keV) observations from two geostationary spacecraft were available in 41 cases. Of the 41 cases, 32 of the midtail plasma sheet recoveries were associated with distinctive ion or electron flux increases at geostationary orbit. These flux increases, often in both protons and electrons, were generally observed from the predusk to the postdawn sector. However, very few enhancements were found near local noon. The lack of large energy dispersion in the flux increases and the simultaneous occurrence of both electron and ion enhancements suggests that the particles do not drift from a more distant location (as in substorm expansion phase onsets) but are accelerated or redistributed locally. These events are suggested to be associated with a large‐scale reconfiguration of the near‐Earth tail as the neutral line retreats to large distances.

Magnetospheric field and current distributions during the substorm recovery phase

We have studied 11 substorm recovery phase events in which magnetic field and energetic particle data were available near the midnight sector from the GEOS 2 satellite. Comparison with the Tsyganenko magnetic field model shows that, after the expansion phase, BZ is large and decreases gradually toward the model value during the recovery phase, whereas deviations of BX and BY relative to the model values are small after the effects of the substorm current wedge have disappeared. We have modeled this sequence by using temporally evolving current systems implemented as additions to the Tsyganenko model. The tail current sheet thickness and the cross‐tail current intensity at different radial distances were varied using six free parameters in the model. The parameters were evaluated using a least squares fit for each of the 11 events separately. The results suggest that at the beginning of the recovery phase the current sheet was relatively thick close to the inner edge of the plasma sheet. Model fittings produced two different field configurations. In seven events the cross‐tail current was weak, and the field configuration was highly dipolar. In four events the near‐Earth current was weak, but stronger currents remained in the midtail region. In these latter events the field configuration at the beginning of the recovery phase included a region where BZ was negative. This negative BZ and the associated near‐Earth neutral line disappeared later as the current system developed toward the quiet time configuration. The magnetic field configuration, current distributions, and particle drift paths during the substorm recovery phase are examined and compared with those prevailing during the substorm growth phase.

Substorm time constants

We show that during magnetospheric substorms there is a clear tendency for the westward electrojet current to follow a characteristic curve that can be matched with an exponential equation. The substorm recovery phase has a characteristic timescale, or exponential time constant, that does not vary substantially from one substorm to the next. There is a slight trend toward shorter substorms as the peak magnitude of the electrojet current increases. This behavior can be explained by a simple model of the coupling between the currents and electric fields in the magnetotail and ionosphere. We derive an equation for the evolution of the substorm currents, including those in the magnetotail, using fundamental physical principles. There is good agreement between the measured and predicted time constants. This model is also consistent with other reported measurements of plasma flows and magnetic perturbations in the magnetotail.

Modeling ionospheric convection during a major geomagnetic storm on October 22‐23, 1981

Following the passage of an interplanetary shock at ∼0500 UT, a major geomagnetic storm developed on October 22‐23, 1981. Numerous auroral substorms occurred during this storm leading to an AE index greater than 1000 nT. We have used the expanding/contracting polar cap (ECPC) model (Moses et al., 1989) and data from the Dynamics Explorer 2 spacecraft to study the ionospheric electric fields for 12 consecutive traversals of the polar regions. The ECPC model can determine the voltage drops across the dayside merging and nightside reconnection gaps. We determined the relationship of the AL index (i.e., the intensity of the westward electrojet) to the nightside reconnection potential drop. An excellent linear correlation was found between the nightside reconnection gap voltage drop and the AL index. These results show that the solar wind strongly drives the magnetosphere‐ionosphere system throughout the geomagnetic storm. A substantial level of dayside merging seems to occur throughout the event. Nightside reconnection varies from satellite pass to satellite pass and within the substorm recovery phase. We find that tail reconnection is an important feature of the recovery phase of substorms.

The decay of suprathermal ion fluxes during the substorm recovery phase

During a magnetospheric substorm, the energetic particle flux in the plasma sheet increases by as much as 2 orders of magnitude. During the recovery phase the ion flux decreases. The decay rate is energy dependent, with the decay time constant decreasing for increasing energies. By comparing the decay rates of different species, we are able to test whether the decay rate is organized by total energy, energy per charge, velocity, or rigidity. Using the suprathermal energetic ion charge analyzer (SULEICA) instrument on Active Magnetospheric Particle Tracer Explorers Ion Release Module (AMPTE IRM), we have determined the decay rates for the species H+, He+, O+, and He++ for individual cases and by using a superposed epoch analysis. We find that the decay rate is organized by energy per charge. We have calculated the decay rates predicted by two different models: adiabatic cooling in a radially expanding plasma sheet, and particle drift loss, or “leakage,” from the plasma sheet. Both models predict that the decay rates will be organized by energy per charge, and in both cases the predicted decay rate is faster than that actually observed. This indicates that the production of energetic particles continues at a reduced rate in the the tail during the recovery phase, as the neutral line retreats down the tail.

Plasma sheet expansion at r = 15 ‐ 22 RE: A recovery phase or expansion phase phenomenon?

It has been widely accepted that at radial distances r ∼ 15‐20 RE the plasma sheet abruptly thins at the onset of the substorm expansion phase and later expands during the substorm recovery phase. This dynamical behavior is dramatically different from that observed at r ≲ 15 RE and is a fundamental component of substorm models. Recent results, however, have shown an expansion, rather than a thinning, of the plasma sheet at onset within the range r = 15‐22 RE. Here we reanalyze previously published data that led to the earlier conclusion. We find that it too shows an expansion of the plasma sheet at onset, and we find no evidence for a change in plasma sheet behavior at r ∼ 15 RE. We suggest that satellite location relative to the plasma sheet boundary, effects of multiple onsets, and previous selection of events that preceded extended quiet periods all contributed to the difference between our results and the earlier results. Our interpretation of these findings is that the substorm‐associated dynamics of the plasma sheet is very similar from near synchronous orbit to at least r = 22 RE. We suggest that the dynamics beyond 15 RE can be explained in terms of an extension into this region of a portion of the substorm current wedge, the response to the current wedge being very similar to that which has been well documented at lower r.

Statistical study of the late substorm recovery phase and quiet time plasma sheet based on ISEE 1 ∼30‐keV ion observations

We present a statistical survey of the near‐Earth plasma sheet (r = 7‐23 RE, distance from the neutral sheet center Zns = 0‐10 RE, LT = 2100‐0200) during (late) substorm recovery phase and quiet time based on ISEE 1 medium energy particle instrument observations of first‐order ∼30‐keV ion anisotropies, comprising ∼112 hours of the 1978 tail season. The data reveal predominantly isotropic conditions, with intervals of generally weakly anisotropic (<10 highest‐to‐lowest flux ratio) field‐aligned ion flows (FAIFs) and ion flux gradients. Both earthward and tailward streaming FAIFs are observed for Zns between 0 and 10 RE and are probably unassociated with the plasma sheet boundary layer. The occurrence rate of tailward FAIF (∼4.5%) is relatively insensitive to AE conditions. The occurrence rate of earthward FAIF increases during “active” conditions to as much as 12.4% and decreases to as low as 1.3% during “quieter” conditions. An increase in the occurrence rate of tailward FAIFs is observed with increasing Z (mapping along B to the more distant current sheet). We suggest that the tailward streaming flows may arise during stretching of especially distant closed field lines during extended recovery and quiet time. Ions will lose energy as they cross the tailward moving current sheet, just as they gain energy in crossing an earthward moving field reversal region. No evidence for a quiet time distant tail source is seen in the energetic (24 keV to 2 MeV) ion data during these late recovery phase and quiet time periods. A weak local time asymmetry in the ratio of occurrence rates for tailward and earthward FAIFs (T∶E) is observed, with earthward streaming FAIFs outnumbering tailward streaming FAIFs in the dusk sector (T∶E = 0.9‐1.0) and vice versa in the dawn sector (T∶E = 1.4‐ 2.1). The occurrence rate of dawn‐to‐dusk flux anisotropies, interpreted here as gradients indicating higher ion flux toward the plasma sheet equatorial plane, increases during active conditions and decreases during quieter conditions. North‐to‐south and south‐to‐north flux anisotropies, interpreted here as dawnward and duskward flux gradients (3‐5%), are independent of AE conditions.

A mechanism of formation of pulsating aurorae

We consider a mechanism of pulsating aurora formation relevant for the substorm recovery phase. It is based on the spikelike regime of whistler cyclotron instability in auroral ducts with enhanced plasma density, elongated with geomagnetic field lines. We present the rigorously derived self‐consistent quasi‐linear theory of flow cyclotron maser operation, which describes spikelike regimes of whistler wave generation and energetic particle precipitation in a duct. We show that pulsations are formed due to broadening of the resonance region in velocity space in the course of instability development. We have obtained theoretical estimations for temporal characteristics of pulsating auroral patches. Pulsating regime is realized when the energetic electron number density is below some critical value depending on background plasma density and particle source parameters. Pulsation “on” time is determined by the nonlinear instability dynamics whereas “off” time is nearly equal to the time of reaching the wave excitation threshold. We compare our analytical and numerical results with known experimental data and show that the model explains reasonably most observations concerning pulsation time pattern, electron precipitation fluxes, VLF wave spectrum dynamics, latitudinal and longitudinal dependencies of pulsation characteristics, etc. We have made also preliminary estimations of nonlinear particle trapping effects during the maximum of the on phase that can give rise to a fine temporal structure of pulsations.

Recovery phase of magnetospheric substorms and its association with morning‐sector aurora

In contrast to the extensively studied growth and expansion phases of magnetospheric substorms, the substorm recovery phase has not received much attention in the published literature. It has generally been considered as a period in which all disturbances caused by the previous two phases decay, the magnetosphere “recovers” to reach a quiet state again. Using mainly ground‐based data, we show that the “recovery phase” contains a number of features which are qualitatively different from the expansion phase, that is, not just a decay of previously excited forms of energy release. Typical recovery phase phenomena include intense electrojet activity in the morning sector, high‐energy particle precipitation, the development of large‐scale auroral vortex streets (so‐called Ω bands and associated magnetic Ps 6 pulsations), and very often new eastward expanding active auroral phenomena, not unlike evening‐sector expansion phase features but concentrated to the morning sector of the auroral oval. Such features must be associated with the substorm mechanism itself. We suggest that our observations during the substorm recovery phase are explained by the magnetospheric reorganization after the ejection of a plasmoid. We show that a more detailed investigation of the late substorm features can increase our understanding of the physical processes leading to the complete cycle of magnetospheric energy release.

The auroral distribution and its mapping according to substorm phase

An attempt is made to reconcile two competing views as to where the auroral distribution maps from in the magnetosphere. The structure of the aurora is shown to have two distinctive parts which vary according to the magnetic activity. The low latitude portion of the structured distribution may be a near-Earth central plasma sheet phenomenon while the high latitude portion is linked more closely to boundary layer processes. During quiet times, the polar arcs may be the ionospheric signature of a source region in the deep tail low latitude boundary layer/cool plasma sheet. The structured portion of the ‘oval’ has a dominantly near-Earth nightside source and corresponds to an overlap region between isotropic 1–10 keV electrons and 0.1–1 keV structured electrons. The ionospheric local time sector between 13 and 18 MLT is the meeting point between the dayside boundary layer source region and this near-Earth nightside source. Late in the substorm expansion phase and/or start of the substorm recovery phase, the nightside magnetospheric boundaries (both the low latitude and Plasma Sheet Boundary Layers) begin to play an increasingly important role, resulting in an auroral distribution specific to the substorm recovery phase. These auroral observations provide a means of inferring important information concerning magnetospheric topology.

VLF plasmaspheric emissions—‘wave’ signatures of inner magnetosphere boundaries

VLF quarter-gyrofrequency emissions have been observed in the plasmasphere by low-altitude (up to 2500 km) Intercosmos satellites. Their experimental characteristics indicate that the origin and the distribution function of resonant electrons and the type of instability leading to their production can differ during various phases of magnetospheric disturbances. Their close connection to the earthward edge of the lowest energy plasma sheet electron fluxes during active periods is shown. During recovery periods following magnetospheric substorms, they persist (together with other symptoms of a certain internal plasmaspheric structure—subauroral T e -peaks, SAR arcs, etc.) at roughly the same L-value within the new, more distant plasmapause. A new kind of instability is mentioned which could lead to their production during the recovery phase of magnetospheric substorms.