Showing papers on "Substorm published in 1984"

Structure of the magnetotail at 220 RE and its response to geomagnetic activity

Abstract: Using plasma electron and magnetic field measurements from ISEE 3, 220 RE from earth, we find that the magnetotail at that distance is a coherent structure that evidently waves about through distances comparable to its own lateral scale size. For about one-third of the time it was inside the magnetotail, ISEE 3 was in the plasma sheet. During quiet times the plasma sheet is apparently quite thin, but in response to geomagnetic activity it expands, becoming filled with hot plasma flowing tailward at speeds sometimes exceeding 1000 km/sec, and forces the magnetotail cross-section itself to expand. The plasma sheet’s expansion is delayed typically by about 30 minutes from the onset of the associated geomagnetic activity (often a clearly identified isolated substorm). The magnetic field in the newly-expanded plasma sheet usually exhibits a few-minute steep northward excursion followed by a more prolonged (and often steep) southward excursion. We believe these to be the signatures of arrival of a plasmoid formed and released near the earth at the onset of the corresponding geomagnetic activity. The discreteness of these plasma releases through the magnetotail and their close association with onsets of geomagnetic activity at earth suggest that they are consequences of spontaneous release, probably by magnetic reconnection, of energy and plasma earlier stored in the magnetotail.

Substorm associated traveling compression regions in the distant tail: Isee‐3 Geotail observations

Abstract: While in the lobes of the distant magnetotail, ISEE-3 encountered regions of compressed magnetic field at a rate of several per day. The duration of these events was 5 to 20 minutes and they were observed 10 to 30 minutes following the onset of substorm activity near the earth. During each event, the lobe magnetic field tilted first northward and then southward with the inflection point near the time of peak field strength. Following the compression events, the lobe field weakened and retained a southward component for 20 to 40 minutes. It is suggested that these traveling compression regions are the lobe signatures of plasmoids moving rapidly down the tail in the plasma sheet. Comparison of ISEE-3 compression event times with substorm onset times yielded propagation speeds of 350 to 750 km/s.

Longitudinal structure in pi-2 pulsations and the substorm current wedge

Abstract: The characteristics of mid-latitude Pi 2 pulsations are studied using a subauroral east-west chain of magnetometers which spans over 4 hours local magnetic time. The mid-latitude magnetic bays associated with the Pi 2 pulsations are used to define a longitudinal coordinate system based on the substorm current wedge model. The wave properties, frequency, polarization, and east-west phase variations are examined in this coordinate system. This allows us to compare the wave characteristics at longitudes between the meridians of the two field-aligned currents of the substorm current wedge with the characteristics both east and west of these meridians. In an attempt to minimize the influence of preexisting current systems on our estimate of the substorm current location we use only Pi 2 pulsations which follow a magnetically quiet interval and which start simultaneously with a magnetic bay. We find that the longitudinal pattern of the horizontal polarization ellipse azimuth found earlier extends beyond the meridians of the field-aligned currents. The sense of wave horizontal polarization is predominantly counterclockwise at all longitudes, though the incidence of linear and clockwise polarization increases with distance from the field-aligned current meridians. Estimates of signal phase differences between station pairs, if interpreted as azimuthal phase propagation, show that westward propagation dominates west of and within the field-aligned current meridians but the eastward propagation dominates east of the current system. This latter observation fits a recent model introduced to explain the longitudinal variation in polarization azimuth. We also looked for variations in the signal frequency among stations and found some variation in about half the events studied but could find no systematic behavior.

A model for the propagation of the westward traveling surge

Abstract: A unified model is developed for the propagation of the westward traveling surge (WTS) that can explain the diversity in the observed surge characteristics. We start with the Inhester-Baumjohann model for the surge region, which implicitly includes both the Hall and Pedersen currents. It is found that precipitating electrons at the conductivity gradient modify the gradient, causing it to propagate as a wave front. The velocity of propagation is directly dependent on the ionization efficiency of the precipitating electrons and therefore increases dramatically when they become more energetic during substorm onsets. For example, we predict that when the incident electron energy changes from 1 keV to 10 keV the surge velocity should increase from 2 km/s to 34 km/s. The direction of the surge motion depends on the presence of polarization charges on the poleward surge boundary. This is related to the efficiency with which the poleward ionospheric currents are closed off into the magnetosphere by the field-aligned currents. Inclusion of the electron-ion recombination rate modifies the surge propagation velocity and leads to explicit expressions for the conductivity profile. Sufficient precipitation current is required to overcome electron-ion recombination in order for the surge to expand. When the precipitating current is less than this threshold the WTS retreats. Therefore, the model describes the ionospheric response to both the expansion and recovery phases of the magnetic substorm.

GEOS 2 identification of rapidly moving current structures in the equatorial outer magnetosphere during substorms

Abstract: The ULF magnetic and the dc electric antennas operating onboard GEOS have been used to study the origin and characteristics of short irregular pulsations (SIP's). Strong SIP's are always observed during substorm “onsets”, which are characterized by an abrupt change of the GEOS dc magnetic field from a taillike configuration to a more dipolar one and which is known to be associated with rapid poleward displacement of aurorals forms in the vicinity of the GEOS field line. By applying to the three-component ULF signal a complex processing it is possible to demonstrate that most of SIP's are in fact the magnetic signatures of localized current structures passing by the spacecraft at a high velocity. Very intense spikes in the electric field (E ≃ 3–25mV/m) are observed in connection with the passing over of such structures at the satellite location, but these E field spikes are generally observed 10–20 s earlier than the SIP's. On the average the E field direction is earthward with a smaller component towards dusk. Provided that the duration of the magnetic signature of the signal (2τ) is short enough (less than 2 s) it is possible to show that they correspond to field aligned current tubes (FACT's) passing by the spacecraft. When 2τ > 2 s, the signature of the SIP is still consistent with that of a field-aligned current tube, but this cannot be assessed without ambiguity from the ULF experiment alone. Nevertheless, the direction of the velocity of the moving structure can still be deduced from the magnetic signature. For structures detected during substorm onsets this direction is consistent (within 40°) with the drift velocity direction as determined by E × B. This situation occurs for 28 events out of a total of 42, which were analyzed in detail. For these events, and assuming that the amplitude of the velocity is given by E × B/B², it is possible to compute the characteristic parameters of the structure: current density J, radius R (by assuming a cyclindrical structure for simplification), and velocity v. The parameters that have thus been obtained are 6.10−9 < J < 3.10−7 A/m², 20 < R < 900 km, 15 < v < 170 km/s, with the following average values: 8×10−8 A/m², 215 km, and 70 km/s. When transposed to ionospheric altitudes these values are consistent with those found for localized field-aligned current structures by ground or low orbiting spacecraft observations. The origin and nature of these localized current structures are discussed. A simple theory based on the MHD stability of a field-aligned current structure is proposed to explain why the current density can hardly exceed in the equatorial plane a critical value that is of the order of 10−7 A/m².

Can substorm expansive phase effects and low frequency Pc magnetic pulsations be attributed to the same source mechanism

Abstract: Geomagnetic pulsations and magnetospheric substorms are often treated as separate topics in the literature. These phenomena do, however, share common properties including spatial localization and characteristic propagation velocities. In this paper we discuss the possibility that both phenomena originate from Kelvin-Helmholtz instabilities at the interface between the low latitude boundary layer/boundary layer plasma sheet and the central plasma sheet. The variations in horizontal scale size and azimuthal velocities found in ground-based observations of Pc 4,5, Ps 6 and westward travelling surges can be explained by variations in the shear flow and width of the regions where the instability occurs.

Inductive electric fields in the magnetotail and their relation to auroral and substorm phenomena

Abstract: The paper reviews the importance of inductive electric fields in explaining different magnetospheric and auroral phenomena during moderately and highly disturbed conditions. Quiet-time particle energization and temporal development of the tail structure during the substorm growth phase are explained by the presence of a large-scale electrostatic field directed from dawn to dusk over the magnetotail. Conservation of the first adiabatic invariant in the neutral sheet with a small value of the gradient in the magnetic field implies that the longitudinal energy increases at each crossing of the neutral sheet. At a certain moment, this may result in a rapid local growth of the current and in an instability that triggers the onset. During the growth phase energy is stored mainly in the magnetic field, since the energy density in the electric field is negligible compared to that of the magnetic field (ratio 1: 107). An analytical model is described in which the characteristic observations of a substorm onset are taken into account. One major feature is that the triggering is confined to a small local time sector. During moderate disturbances, the induction fields in the magnetotail are stronger by at least one order of magnitude than the average cross-tail field. Temporal development of the disturbed area results in X- and O-type neutral lines. Particles near to these neutral lines are energized to over 1 MeV energies within a few seconds, due to an effective combination of linear and betatron acceleration. The rotational property of the induction field promotes energization in a restricted area with dimensions equivalent to a few Earth's radii. The model also predicts the existence of highly localized cable-type field-aligned currents appearing on the eastern and western edges of the expanding auroral bulge. It is shown that the predictions agree with satellite observations and with the data obtained from the two-dimensional instrument networks operated in Northern Europe during the International Magnetospheric Study (IMS, 1976–79).

Energetic (>100 keV) O+ ions in the plasma sheet

Abstract: We present the first measurements of very energetic (112-157 keV) 0/sup +/ ions in the earth's magnetosphere. The observations were made with the UMd/MPE ULECA sensor on ISEE-1 on 5 March 1981 at geocentric distances approx.20 R/sub E/ in the earth's magnetotail. During this time period an Energetic Storm Particle event was observed by our nearly identical sensor on the ISEE-3 space-craft, located approx.250 R/sub E/ upstream of the earth's magnetosphere. The ISEE-1 sensor observed a similar temporal profile except for several sharp intensity enhancements, corresponding to substorm recoveries during which the plasma sheet engulfed the spacecraft. During these plasma sheet encounters we observe 0/sup +//H/sup +/ abundance ratios, at approx.130 keV, as large as 0.35. In between plasma sheet encounters with 0/sup +//H/sup +/ ratio at this energy is consistent with zero.

Pc5 pulsations and their possible source mechanisms: a case study

Abstract: During the interval 0400-1600 UT on August 31, 1978 strong pc5 activity was observed in the morning and afternoon sector of the magnetosphere. Using data from a world-wide network of ground-magnetometer stations and from the geostationary satellites GEOS 2, GOES 2 and 3 as well as the satellite pair ISEE 1 and 2 a case study was performed with special respect to the question of possible source mechanisms responsible for the observed pulsations. Like earlier workers we came up with the result that the Kelvin-Helmholtz instability at the magnetopause or in the low latitude boundary layer is a likely candidate. In particular we found a change in the azimuthal phase propagation direction from westwards on the morning side and eastward on the afternoon side. Also the sense of polarization and the azimuth of the ground magnetic disturbance in the horizontal plane changed across the meridian of the stagnation point as predicted by a Kelvin-Helmholtz instability source mechanism. A more detailed analysis was carried out for an isolated, large amplitude pc5 event at 1040 UT. At the same time magnetic field observations from the ISEE satellite pair indicate a flux transfer event (FTE) like disturbance at the magnetopause. Also indications of magnetopause boundary oscillations were found, and we feel that this FTE-like event constitutes a possible source for the observed impulsive pc5 event recorded on the ground and on GEOS 2. Furthermore, as the observed pc5 pulsations exhibit a wave-packet structure, we studied whether these wave packets coincided with substorms or substorm intensifications observed simultaneously in the nighttime magnetosphere. However, only a partial one-toone correlation was found.

Direct observations of passages of the distant neutral line (80‐140 RE) following substorm pnsets: ISEE‐3

Abstract: Specific events recorded by ISEE-3 passage through the earth magnetotail at 80-140 earth radii are discussed. The data were taken during March 20-28, 1983, when Kp ranged from +6 to -40, and included both plasma and magnetic field signatures. Magnetic field polar angle reversals from N-S flows were detected, along with alteration in the bulk plasma flow from tailward to stagnation. Tailward flow was associated with negative field values, while stagnation was mainly present with positive field values. The tail plasma at 100 radii exhibited changes shortly after a substorm event which featured particle ejection at 6.6 earth radii. The plasma sheet swept by the events could have a 25-50 radii length scale.

Global ionospheric current distributions during substorms

Abstract: The growth and decay of global ionospheric currents during magnetospheric substorms on March 17, 18, and 19, 1978 are examined on the basis of magnetic records from the six IMS meridian chains of observatories and others (the total number being 71). The computer code developed by Kamide et al. (1981) and the conductivity model developed by Ahn et al. (1983) are used. Several substorms centered around 1000–1200 UT are chosen in this presentation, since the simultaneous all-sky and riometer records are essential in timing the substorm epochs. Several global features that are common to most substorms during the three-day interval include the following: (1) During a quiet period, currents are often present in the cusp and/or polar cap regions. The cusp current consists of a pair of east-west currents and the polar cap current consists of several vortices. (2) When the interplanetary magnetic field (IMF) Bz component is positive, but decreases in magnitude, a well-defined westward electrojet develops in the midnight sector. However, this development is not evident in the AE index. (3) A gradual, but distinct growth (often followed by a rapid and large increase) in the AE index is identified as the intensification of a weak substorm current system, which was mentioned in (2), accompanied by typical auroral substorm features, including riometer absorption. (4) The subsequent sharp increase of the AE index arises primarily from a deep intrusion of the westward electrojet into the pre-midnight sector and its equatorward shift. (5) The overall increase of the global current can be significantly different from what a sharp increase of the AE index indicates. (6) During the recovery phase, the intruded westward electrojet recedes towards the dawn sector. The electrojets recede also poleward.

Local time variation in the response of Pc 5 pulsations in the morning sector to substorm expansive phase onsets near midnight

Abstract: Since substorm expansive phase activity in the midnight sector and Pc 4,5 pulsation activity in the morning sector are both colocated with the auroral oval, it is logical to assume that sudden changes in plasma and field parameters in the oval would cause perturbations in the activity level of both types of activity. In an earlier study, Samson and Rostoker [1981] demonstrated that the frequency of Pc 4,5 pulsations on the dayside increased at the onset of a substorm expansive phase in the night sector. In that study, only pulsations recorded near noon were investigated for their frequency content. In this paper we consider the response of pulsation activity across the entire morning sector to substorm onsets near midnight. Using superposed epoch analyses we demonstrate that the conclusions of Samson and Rostoker [1981] are valid across the morning sector. However close to dawn, Pc 5 activity appears to be initiated at substorm onset rather than decreasing by shifting frequency toward the Pc 5 band, as appears to be the case closer to noon. We present a model of Pc 4,5 activity to explain this phenomenon in which the origin of the pulsations is proposed to be a Kelvin-Helmholtz instability at the interface between the low-latitude boundary layer and the central plasma sheet.

A multisatellite study of the plasma sheet dynamics at substorm onset

Abstract: A multi-satellite study has been conducted on the temporal relationship between energetic particle injections observed at the geostationary orbit onboard GEOS-2 and decreases of thermal plasma sheet particle fluxes (''plasma sheet thinnings'') observed in the more distant geomagnetic tail onboard the ISEE-1 and -2 space-craft. A case by case analysis as well as a statistical study of 100 events recorded from February to May 1979 and 1980 show that particle injection and particle flux decrease are detected to within less than 5 minutes of each other, for an average intersatellite distance of 15.7 R/sub E/. The observed spread in ..delta..t corresponds best to Alfven wave transit times. These particle phenomena are observed at the onset of the auroral zone magnet bays. The results reported here suggest that the dynamics of the inner and outer boundaries of the plasma sheet are closely related to each other and controlled by large scale processes that develop at substorm onset inside the magnetospheric tail and result in a cross-tail current disruption.

Observations relating changes in thermospheric composition to depletions in topside ionization during the geomagnetic storm of September 1982

Abstract: In situ measurements taken over midlatitudes by the Dynamics Explorer 2 spacecraft during a geomagnetic substorm show that a direct correlation exists between depletions in the O/N2 density ratio and in the topside electron density, Ne. On the basis of this correlation, it is concluded that, for the storm of September 1982, losses of free electrons via oxygen and nitrogen chemistry near the F2 peak dominate over wind-induced plasma transport in determining the observed stormtime variations of topside Ne at higher latitudes. However, the appearance of an enhancement in topside Ne along 20-deg invariant latitude indicates that an equatorial wind did develop and was effective in increasing gas temperatures and in generating topside Ne enhancement at lower latitudes.

Simultaneous observation of the plasma sheet in the near Earth and distant magnetotail: ISEE-1 and ISEE-3

Abstract: Particle data have been acquired by the 1981-025 and 1982-019 spacecraft at geosynchronous orbit, as well as ISEE-1 in the near earth geomagnetic tail, and ISEE-3 in the distant geomagnetic tail. These observations are supplemented by ground-based magnetograms from near local midnight stations. Attention is given to a substorm recovery phase, and to observations of ion beams at the plasma sheet boundary in the near earth and distant tail, respectively, which are found to flow in opposite directions.

Ground-based observations of a very intense substorm-related pulsation event

Abstract: Les intenses micropulsations Pi 2 ont ete observees le 2 nov. 1982 au nord de la Scandinavie en meme temps que l'on a effectue des mesures magnetometriques (STARE/EISCAT). Les perturbations au sol sont reliees a un intense electrojet. Juste avant le debut de la perturbation on a localise une discontinuite de Harang. Les pulsations debutaient avec le sous-orage. On interprete les phenomenes comme etant des ondes d'Alfven generees dans des zones de derive differentielle du plasma et subissant des reflexions impaires

Association between energetic particle bursts and Birkeland currents in the geomagnetic tail

Abstract: IMP 8 observations in the geomagnetic tail of energetic particles at hundreds of keV and of the magnetic field are examined for a substorm period from 1700 to 1800 UT on Mar. 3, 1976. For this interval, it is found that bursts of energetic protons detected at a downstream distance of about 37 earth radii in the midnight tail region are accompanied by east-west magnetic field perturbations indicative of Birkeland current sheets. The magnitude of the magnetic field perturbations range from about 1 to 12 nT, corresponding to current densities of 1 to 9 mA/m when integrated over the current sheet thickness. These values are comparable to previous observations of Birkeland currents in the near-earth tail (X greater than -20 earth radii) and with the auroral-arc-associated Birkeland current densities at low altitudes scaled out to the tail region. The favorable comparison suggests that the closure of small-scale Birkeland current systems, at least in these instances, lies tailward of about 40 earth radii. The observed Birkeland current system and the magnitude of current densities are found to be consistent with ion tearing instability in the tail. The inferred wavelength of the tearing mode along the tail axis is about 2 to 17 earth radii. In the context of the tearing model, the presence of Birkeland currents and the occasional isotropic distribution of energetic electrons (of more than 0.22 MeV) suggest that the energetic particle events in this study occur on closed magnetic field lines, even though strong tailward streaming dominates the particle anisotropy in these events.

Stare observations of an eastward propagating Pc5 pulsation with large azimuthal wavenumber

Abstract: The results of observations by the Stare radar of an eastward propagating Pc5 pulsation with short azimuthal wavelength are presented. Previously observed high-m phenomena have been propagating westward. This event occurred in the predawn sector during a time of substorm activity. The azimuthal wave number is m = 30 ± 3. Calculations of the drift and bounce periods of energetic particles show that the most plausible particle resonances are with near-equatorial drifting electrons in the 5 to 20 keV range or with bounce resonant protons in the 0.03 - 1 keV range. If the drift mirror instability is important it must be with guiding center-drifting electrons or the wave velocity must match the diamagnetic drift velocity which is only possible if the pressure gradient is inwards.

Generation of intensity covariations of the oxygen green and red lines in the nightglow.

Abstract: On presente une etude comparative des variations d'intensite des raies de l'oxygene et de l'activite geomagnetique (indice AE, Pi2 et magnetogramme). On montre qu'une intensification de la raie rouge est associee a un sous-orage auroral