Substorm Recovery Phase Research Articles

A proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution

Multisatellite data are used to examine the temporal relationship between Subauroral Ion Drifts (SAID) and the phases of an auroral substorm. Utilizing images of auroral luminosities taken by the Dynamics Explorer 1 (DE 1) spacecraft and observations of particle injection at geosynchronous orbit, we identify the time of expansive phase onset and estimate the time at which recovery begins. Noting the times at which SAID are observed simultaneously by the DE 2 spacecraft, we find that SAID typically occur well after substorm onset (more than 30 min), during the substorm recovery phase. Substantial westward ion drifts and field‐aligned currents are observed well equatorward of the auroral oval during the expansion phase of a substorm, but the drifts lack the narrow spike signature associated with SAID. Prior to substorm onset and after substorm recovery, field‐aligned currents are absent equatorward of the auroral oval and the ionosphere is very nearly corotating. A phenomenological model of SAID production is proposed that qualitatively agrees with the observed ionospheric signatures and substorm temporal relationship. In this model, substorm‐generated, subauroral field‐aligned currents close via Pedersen currents with the outward flowing, region 1 currents at higher latitudes. These Pedersen currents flow in the region of low conductivity equatorward of the auroral oval and are associated with relatively large, poleward directed electric fields. The frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of ion‐atom interchange between O+ and N2. Subsequent fast recombination of NO+ with electrons further reduces the subauroral F region conductivities with a corresponding increase in the electric field and the frictional heating. This heating leads to thermal expansion, substantial field‐aligned plasma flow, and very large depletions in the F peak concentration, thus additionally reducing the height‐integrated Pedersen conductivity.

Is there a near-Earth neutral line?

High correlations between southward turnings of the IMF, increases of open flux in the tail lobes, and magnetic activity confirm dayside magnetic reconnection plays a fundamental role in magnetospheric substorms. The absence of a time delay between expansion phase onset and the decrease in tail lobe flux suggest that nightside reconnection is involved in the expansion. At 20–30 R e, the disappearance of the center of the plasma sheet and the appearance of tailward flows threaded with southward fields strongly suggest reconnection occurs earthward of this distance. Recent AMPTE/CCE and DMSP observations suggest that current disruption begins near synchronous orbit on closed field lines and expands tailward. Auroral images from the VIKING spacecraft support this. Reports of a lack of correlation between plasma and field changes at the ISEE spacecraft and onsets has led some researchers to reject the hypothesis that reconnection is the cause of the expansion. To counter these reports we present magnetic field and plasma data from substorms observed on an outbound pass by ISEE-2. We show that the tail field responds in the manner predicted by the near-earth neutral line (NENL) model, growing in strength and tilting earthward in the growth phase, and decreasing and tilting upward in the expansion phase. Near the earth the plasma sheet gradually thins in the growth phase while further away it suddenly thins at expansion onset. In the expansion phase the plasma sheet rapidly recovers close to the earth, but its recovery is delayed until the substorm recovery phase further from the earth. We explain many of the recent reports of disagreement between the NENL model and observations by a combination of factors. These include: Large changes in the location of a spacecraft relative to the neutral sheet as the size of the magnetosphere changes with dynamic pressure, or as the central meridian and latitude of substorm onset change due to inherent variability; Differences between the outbound and inbound portions of the ISEE orbit; Change in shape of successive orbits in GSM coordinates. Most important is multiple substorm onsets. Successive intensifications cause quite different effects at a spacecraft as the disturbances moves relative to the spacecraft. We conclude that rather than one x-line, there are probably multiple, localized pairs of x- and o-type lines formed in the near-earth plasma sheet during the substorm expansion.

Characteristics of Suprathermal Electron Bursts Observed by the DMSP-F6/F7 Satellites in the Diffuse Aurora Region.

The particle data of the DMSP-F6 and -F7 satellites show the presence of suprathermal electron bursts in the diffuse aurora region. These bursts are characterized by intense (∼108-109 (cm2sr sec)-1), low energy (∼100-500 eV), and localized (∼20-30 km at satellite altitude h ≅ 830 km) electron precipitation. Simultaneous magnetic field data from the DMSP-F7 satellite show that these bursts are often, but not always, accompanied with intense (few μAm-2), small-scale (only few tens of km at satellite altitude) upward field-aligned currents. The bursts are observed from 19 h to 11 h MLT through midnight during the maximum to recovery phase of substorm. These bursts are less likely to be observed during geomagnetic storms than during times of isolated substorms. These results suggest that suprathermal electron bursts are generated by localized heating of ionospheric electrons which flow out from the ionosphere toward the magnetosphere during substorm.

The recovery phase of substorms

A special SPA: Magnetospheric Physics session dedicated to the recovery phase of substorms is being planned for the AGU 1992 Fall Meeting. Substorms have provided an important focus for magnetospheric research for nearly 3 decades, and continue to do so today. Of the three phases (growth, expansive, and recovery) that constitute a substorm, the growth and expansive phases have received the most attention, and a fairly good understanding of them has developed.In contrast, the recovery phase has received scant attention. Perhaps that is because it lacks the dramatic auroral and geomagnetic effects that mark the expansive phase at Earth and the similarly impressive particle and magnetic field effects that mark the growth phase to expansive phase transition at geosynchronous orbit. The recovery phase was originally defined as the period when auroras, having advanced far toward the pole during the expansive phase, become less intense and retreat back toward the auroral oval. Wavy structures—omega bands—and pulsating intensities often characterize the recovery phase auroras. It has become customary, also, to define the recovery phase as that period when the Auroral Electrojet (AE) index is subsiding from the peak value it reached during the substorm. These two definitions, probably not altogether compatible, may identify somewhat different periods of a substorm. There is no obvious feature in geosynchronous observations that has been specifically identified with the recovery phase.

Response of equatorial ionosphere to the transit of interplanetary magnetic cloud of January 13–15, 1967. Transient disturbance in F region

Quarter-hourly ionograms of Kodaikanal (10°4′N, 77°29′E, dip 3°N) are analysed to evaluate the response of equatorial ionosphere to the severe geomagnetic storm (| Dst|max, 176 nT) associated with the transit of an interplanetary magnetic cloud at Earth during January 13–15, 1967. It is shown that during the early stage of the storm initial phase on January 13, which corresponded to the local dusk-midnight sector, the entire bottomside F region over Kodaikanal experienced a sudden and spectacular downward drift for a short-duration of ∼2 h (18:45 – 20:30 L.T.). The decrease in height of F layer peak ( hm) by 223 km over the interval 18:45–20:30 L.T. implied a gross apparent downward drift of ∼35.4 m s −1. The height disturbance occured during the recovery phase of an auroral substorm (bay disturbance) that developed at the commencement of the geomagnetic storm. It also displayed an apparent temporal association with a prominent northward swing in IMF Bz and decrease in polar cap potential drop, φ, estimated from IMF parameters. The anomalous and rapid post-sunset descent of F layer was accompanied by an increase in electron density at and below the F layer peak. The evident perturbation in F region height near the dip Equator is interpreted as the signature of a transient westward electric field disturbance generated by the decrease in magnetospheric convection (decrease in polar cap potential) during the recovery phase of the substorm. The values of vertical downward drift derived from the time rate of change of h′F and corrected for chemical loss effects indicated the maximum amplitude of the westward electric fields to be ∼1.9 mV m −1. The increase in Nm is shown to be the outcome of the ionization convergence rate induced by the abnormally large downward drift exceeding the chemical loss rate ( βNe) at altitudes ⩾250 km.

Electromagnetic ion cyclotron waves observed near the oxygen cyclotron frequency by ISEE 1 and 2

Pc 2 electromagnetic ion cyclotron waves at 0.1 Hz, near the oxygen cyclotron frequency, have been observed by ISEE 1 and 2 between L = 7.6 and 5.8 on an inbound near‐equatorial pass in the dusk sector. The waves occurred in a thick plasmapause of width ≈1.5 RE and penetrated ≈1 RE into the plasmasphere. Wave onset was accompanied by significant increases in the thermal (0–100 eV) He+ and the warm (0.1–16 keV/e) O+ and He+ heavy ion populations. The most intense waves (8 nT) were observed in the outer plasmasphere where convection drift velocities (E × B)/B² were largest and the Alfven velocity was a minimum. Wave polarization is predominantly left‐handed with propagation almost parallel to the ambient magnetic field, and the spectral slot and polarization reversal predicted by cold plasma propagation theory are identified in the wave data. Poynting fluxes calculated during the first 15 min of the event show wave energy propagation directions both parallel and antiparallel to the field. Computations of the experimental wave spectra during the passage through the plasmapause show that the spectral slots relate to local plasma parameters, possibly suggesting an ion cyclotron wave growth source near the spacecraft. A regular wave packet structure seen over the first 30 min of the event may be attributed to the modulation of this energy source by the Pc 5 waves seen at the same time. Overall, the results are considered an example of an electromagnetic ion cyclotron wave‐particle interaction occurring during the outer plasmasphere refilling process at the time of the substorm recovery phase.

On the nature of substorm-related transient electric field disturbances in the equatorial ionosphere

Data derived from 5-min interval ionograms of Kodaikanal (10°14′N, 77°29′E, geomag. lat. 0.6°N) on the night of 29–30 August 1957 showed the presence of a marked perturbation in F-region height ( h′ F) in the midnight-to-morning period. The perturbation is characterized by a sudden decrease in h′ F (65 km in 1 hr, with h′ F reaching a low value of 200 km) followed by a prominent increase (200 km in 75 min) over a 3-h period. Changes in h′ F of essentially the same nature were also noticed at Calcutta (geomag. lat. 12.25°N) and Ahmedabad (geomag. lat. 14°N) in the same longitude sector simultaneous with those at Kodaikanal. Examination of the auroral electrojet (AE) index and magnetograms of high latitude stations widely separated in longitude revealed the prevalence of an isolated substorm of moderate strength at the time of the perturbations in equatorial F-region height. The decrease in h′ F is found to occur around the onset of the substorm and the subsequent increase during the substorm recovery phase. The observed F-region height disturbance is interpreted as the signature of a transient composite disturbance in the equatorial east-west (E-W) electric field caused by the prompt penetration of substormrelated perturbations in high latitude electric fields. The nature of the electric field disturbance is in good agreement with the recent modelling results which predict a westward peturbation in E-W field at equatorial latitudes in response to an increase in polar cap potential (around the onset of a substorm) and an eastward one with a decrease in polar cap potential (during the recovery phase) in the midnight-dawn sector.

Sondrestrom radar measurements of the reconnection electric field

Solar wind energy is transferred to and from the closed field line region of the magnetosphere by transport across the boundary between open and closed field lines (the separatrix). The rate of this transfer is measured by the reconnection electric field, which is tangential to the separatrix. Although it is not possible to measure directly the reconnection electric field in the magnetosphere, the electric field in the ionosphere can readily be used to calculate the magnetic reconnection rate. This paper describes a technique for using the Sondrestrom incoherent scatter radar in Greenland to estimate this rate from measurements of the plasma velocity in the separatrix reference frame. The ionospheric plasma drift and the separatrix location and velocity are determined from the radar observations, and the separatrix orientation is inferred from all‐sky images. This technique has been applied to obtain measurements of the reconnection electric field in the midnight sector, with 3‐ to 5‐min time resolution during the night of January 14–15, 1989. The reconnection electric field was found to be less than 15 mV/m during periods of local polar cap expansion, which corresponded to substorm recovery phases, and to be 30 to 40 mV/m during times of polar cap contraction, which corresponded to substorm expansive phases. During a short interval, the measurements showed the separatrix moving equatorward faster than the plasma, which implies that there was plasma transfer from closed to open field lines, rather than the usual nightside transfer from open to closed field lines.

Electrodynamic response of the middle atmosphere to auroral pulsations

On the nights of 21 and 28 October 1987, two Nike Orion payloads (NASA 31.066 and 31.067) were launched from Andøya, Norway, as part of the MAC/EPSILON campaign, to study the effect of auroral energetics on the middle atmosphere. Each payload carried detectors to measure relativistic electrons from 0.1 to 1.0MeV in 12 differential energy channels, and bremsstrahlung X-rays from >5 to >80keV in 5 integral channels. In addition, instrumentation to measure bulk ion properties and electric fields was also carried by these and/or near simultaneous flights. Flight 31.066 was launched during the recovery phase of a moderate magnetic substorm, during relatively stable auroral conditions. Flight 31.067 was launched during highly active post-break-up conditions during which Pc 5 pulsations (> 150s period) were in progress. The energetic radiation of the first event was composed almost entirely of relativistic electrons below 200 keV with negligible contributions from bremsstrahlung X-rays, while the radiation of the second event was dominated by much softer electrons ( < 100 kcV), which produced high X-ray fluxes that exceeded the cosmic ray background as an ionizing source down to altitudes below 30 km. Simultaneous conductivity measurements during both events show consistency with the ionizing radiations, with the pulsation event producing free electrons down to 55 km. far below their expected altitude range during night-time. These comparisons are discussed to evaluate the impact of such events on the middle atmosphere.

Distribution of auroral precipitation at midnight during a magnetic storm

On the night of November 4, 1986, a very complex precipitation pattern was observed by Viking in the magnetic midnight sector over Scandinavia and Svalbard. The pass took place during a magnetic storm, and during substorm recovery phase. Going from north to south, the satellite first encountered a plasma region of BPS‐type (name derived from boundary plasma sheet) and then a region of CPS type (derived from central plasma sheet). Then, however, a new region of BPS‐type was traversed. The quite intense, most equatorward aurora corresponded to a plasma region which was not of ordinary CPS type but contained sharp quasi‐monoenergetic peaks. The high‐latitude midnight sector was totally dominated by eastward convection. The Harang discontinuity had passed northern Scandinavia the first time as early as 17 to 20 MLT, more than three hours before the Viking pass. It is suggested that the particle precipitation pattern and the general shape of the aurora as observed by the Viking imager can be explained in a natural way by the convection pattern. The northernmost BPS‐ and CPS‐type regions originated in the morningside convection cell, while the more equatorward population of BPS type had drifted in from the eveningside. The interpretation is supported by ground‐based measurements by EISCAT and magnetometers.

Response of equatorial electric field to polarity of interplanetary magnetic field

The effect of the polarity of the interplanetary magnetic field, IMF, on the equatorial zonal electric field is assessed through a comparative study of the diurnal patterns of equatorial electrojet strength on days of sunward and antisunward IMF during vernal and autumnal equinoxes of high sunspot activity years (1957–1959 and 1979–1981). The results showed that, on average, the electrojet strength is reduced around noon (09–15 L.T.) on sunward IMF days when compared with antisunward days in the vernal equinox and the pattern is reversed in the autumnal equinox. This modulation in electrojet strength collaborates the trends noticed earlier in the behaviour of the equatorial ionization anomaly (Sastri, 1985b, Adv. Space Res. 5, 199), and strengthens the view that IMF polarity exerts a discernible influence on the equatorial electric field through its effect on the semiannual variation of geomagnetic activity. The average temporal profiles of the auroral electrojet index ( AE) showed not only the difference in the overall level of AE between sunward and antisunward IMF days and the opposite changes in it from vernal to autumnal equinox, but also a preferential occurrence of substorm activity apparently 19–24 h prior to local noon on sunward (antisunward) IMF days in the vernal (autumnal) equinox. A substorm recovery phase is generally seen in the forenoon period more or less independent of IMF polarity in the two equinoxes. These characteristic features in AE are conspicuously absent during the autumnal equinox of 1979 1981 when the IMF related changes in electrojet strength are not apparent. The results strongly suggest that the response of the noon time equatorial electric field to IMF polarity receives substantial contribution from westward electric field disturbances associated with the ionospheric “disturbance dynamo” mechanism.

Multipoint measurements from substorm onset to recovery: The relation between magnetic pulsations and plasma sheet thickening

Long duration waves in the Pi 2 frequency band (6.7 – 22 mHz) were observed by the Air Force Geophysics Laboratory (AFGL) Magnetometer Network during a set of substorms previously analyzed by Hones et al. For these events, Hones et al. found that on the order of 1 hour after the expansive phase onset of a substorm, there was a sudden appearance of riometer absorption observed at South Pole Station (magnetic latitude about -75°) and a reappearance of the plasma sheet as observed in 6-keV electrons by ISEE 1 and 2 satellites at a distance of 15 to 20 R e in the magnetotail. The wave activity observed at the AFGL stations during these events diminished significantly at approximately the time of the plasma sheet recovery or riometer increase. One explanation for the long duration of these waves is that following substorm onset they are continuously generated in a near-tail reconnection region and then decay when the neutral line moves tailward during the substorm recovery phase.

Substorm recovery phase

The recovery phase of the magnetospheric substorm is studied numerically by means of a two-dimensional time-dependent nonlinear resistive MHD code. The initial configuration was chosen from the earlier numerical model in which the magnetospheric substorm was driven by the solar wind plasmas. In order to study the recovery phase, the entering solar wind energy flux was reduced when the magnetospheric substorm was in its expansive phase. The system was found to respond instantly to this change and the result showed many characteristic features related to the recovery phase including the tailward motion of thex-point of the reconnected magnetic field lines and the restoration of a tail-like configuration of the magnetic field. Thex-point moved at almost the same speed of the plasma flow in the upstream region, which was considerably smaller than the speed of the plasma jetting or the speed of the plasmoid. As the recovery phase progressed, the plasma jetting across thex-point was reduced very much in the Earthside region. Although the plasma flow was generally in the Earthward direction in the Earthside region of thex-point, the tailward flow was also found near thex-point. The current density was reduced near thex-point and the neutral sheet was broadened in the recovered region. The plasma sheet also became thick in this region. During the recovery of the substorm, the energy conversion rate, both in the form of plasma acceleration and the Joule heating, was reduced. These results on the recovery phase together with the earlier simulation result on the expansive phase indicate that driven reconnection can be a viable mechanism for the magnetospheric substorm including the recovery phase.

Correlative studies using the Viking imagery

Observations made with the UV Imager onboard the Viking satellite provide an ideal means whereby diverse data sets can be correlated together into the global context of magnetospheric processes. This is possible not only after the fact, but an important part of the overall mission was to modify observing goals in real-time and provide guidance in the decision making process for campaigns such as rocket launches. With the temporal and spatial resolution available auroral imaging is providing new insights into several processes monitored by other instrumentation. Substorm observations reveal the frequent occurrence on very short time scales (less than a minute) of extended arc brightenings during the early recovery phase of substorms. These brightenings are accompanied by the formation of multiple spirals due probably to instabilities operating along the field lines connecting the source region for the particles with the ionosphere. On the dayside by comparing the UV observations with Interplanetary Magnetic Field data it is found that during positive B z and away sector configurations, localized regions of emission can propagate either duskward or dawnward depending on the sign of B y. In the context of current merging/reconnection theories it seems plausible that these motions are the ionospheric signature of the drift of newly merged flux tubes.

Simultaneous observations of the near-earth and distant geomagnetic tail during a substorm by ISEE-1, ISEE-3 and geostationary spacecraft

The structure of the geomagnetic tail during a substorm is investigated by combining plasma, magnetic field and energetic particle data from the ISEE-3 spacecraft in the deep tail with similar near-Earth observations from ISEE-1 and geostationary spacecraft. The observations can be interpreted in terms of the neutral-line model of substorms and indicate the formation of a closed-loop field region (“plasmoid”) following substonn onset, which is ejected down the tail. The plasmoid is observed to have a double-loop field structure. This may be the result of a second substonn onset occurring ≈ 25 min after the first, producing a further near-Earth neutral line and closed field loop. During the substorm recovery phase, the substonn neutral line moves tailward to beyond 130 R E from Earth by some 3 h after substorm onset.

Convection patterns in the polar ionosphere for northward IMF inferred from ground-based magnetometer data.

Instantaneous, not statistical, convection patterns in the polar ionosphere are examined for periods of the northward interplanetary magnetic field (IMF) on the basis of the equivalent ionospheric current systems obtained from data of the IMS ground magnetometer. The equivalent current system is sufficient for this purpose, as long as one knows the relationship between the equivalent current system and the electrostatic potential distribution for such relatively quiet periods when no great spatial gradients in the ionospheric conductivity are expected to be present in the polar cap. In spite of the complexity of individual patterns, the following characteristic features are found. (1) During the late recovery phase of substorms, the clockwise current vortex in the postmidnight sector decays more rapidly than the counterclockwise one in the dusk sector. (2) As the IMF Bz component increases, a current system consisting of two current vortices with a sunward convection flow between them appears in the polar cap region. (3) As the IMF Bz component decreases, the clockwise current vortex in the afternoon sector decays rapidly while the counterclockwise vortex in the noon sector is shifted toward the afternoon sector. In the meantime, a new clockwise vortex emerges in the nightside and becomes enhanced.Thus the whole current system evolves gradually into the familiar two-cell type with an antisunward convection flow over the polar cap. In particular, it is important to note that all these changes occur before the IMF turns southward. (4) The effect of the IMF By dependence on the equivalent current system is also confirmed on our instantaneous data base during a prolonged Bz positive condition.

Auroral zone thermospheric dynamics: 2. Individual nights

Ground‐based Fabry‐Perot spectrometer measurements of the O I 15867 K (630.0 nm; 1 K = 1 cm−1) emission line were obtained from College, Alaska, during the winters of 1981 to 1983. Averages of the temperatures and winds, derived from these measurements, show a general flow driven by ion drag due to magnetospheric convection. This hypothesis was supported by results from a global thermospheric circulation model. Individual nights, however, show differences from the average flow pattern. During the recovery phase of an auroral substorm, when the westward electrojet is over the observing site, the equatorward meridional wind decreases in magnitude. The zonal wind is generally westward in the evening and eastward in the morning. However, the crossover from westward to eastward flow is a function of the location of the substorm onset and the arrival of the westward electrojet over College. Zenith winds can be very large (200 m/s) and show no systematic pattern, perhaps due to a superposition of wave modes from the auroral zone source region. An example of a night with little auroral activity highlights the effects of magnetospheric convection and the auroral electrojet on the normal auroral zone wind pattern. Temperature measurements occasionally show large enhancements, usually associated with type‐A red aurora.

Some recent progress in substorm studies.

After reciting the present definition of a substorm as incorporating both the driven and the loading/unloading process, this review will be addressed to two topics where major advances were made during the last two years: the downtail loss of substorm energy via plasmoid release and observational and theoretical studies of the substorm onset triggering mechanism. It will be concluded by pointing out the need of further studies, both observational and theoretical, of the substorm recovery phase and the poleward leap phenomenon.

Ion cyclotron waves observed near the plasmapause

Pc2 electromagnetic ion cyclotron waves at 0.1 Hz, near the oxygen cyclotron frequency, have been observed by ISEE-1 and -2 between L = 7.6 − 5.8 on an inbound near equatorial pass in the dusk sector. The waves occurred in a thick plasmapause of width ⋍ 1 R e and penetrated ⋍ 1 R e into the plasmasphere. Wave onset was accompanied by significant increases in the thermal (0–100 eV) He + and the warm (0.1–16 keV/e) O + and He + heavy ion populations. Wave polarization is predominantly left-handed with propagation almost parallel to the ambient magnetic field, and the spectral slot and polarization reversal predicted by multicomponent cold plasma propagation theory are identified in the wave data. The results are considered an example of wave-particle interactions occurring during the outer plasmasphere refilling process at the time of the substorm recovery phase.

Suprathermal O+ and H+ ion behavior during the March 22, 1979 (CDAW 6), substorms

We present measurements of energetic (∼130 keV) O+ ions in the earth's magnetosphere during the two Coordinated Data Analysis Workshop 6 substorms on March 22, 1979. The behavior of thermal and suprathermal H+ (10–130 keV) and suprathermal He++ (30–130 keV/e) ions is also discussed. The observations were made with the University of Maryland/Max‐Planck‐Institut ultralow energy charge analyzer (ULECA) sensor on ISEE 1 at geocentric distances ∼16 to 7 RE in the earth's magnetotail. Approximately 15 min before the 1054 UT onset of the first substorm, energetic O+ ions are observed streaming tailward. H+ and He++ ions at all energies were generally streaming tailward from ∼1059 to 1115 UT, consistent with the presence of a near‐earth neutral line during this interval. From 1117 to 1124 UT the H+ and He++ ions were observed flowing earthward, suggesting that at ∼1117 UT the neutral line retreated tailward. A brief interval in the southern tail lobe, from ∼1124 to 1126 UT, was highlighted by an intense O+ beam streaming tailward; the O+/H+ ratio at 130 keV was 7±2. This suggests that high energy O+ ions may be accelerated directly out of the ionosphere. The recovery phase of the first substorm began a few minutes later and was characterized by large intensities of nearly isotropic suprathermal ions. The O+/H+ differential intensity ratio at 130 keV was quite large (∼1) during the recovery phase of both substorms.