Showing papers on "Substorm published in 1992"

Current disruptions in the near-Earth neutral sheet region

Abstract: Current disruption events observed by the Charge Composition Explorer during 1985 and 1986 are examined. Occurrence of current disruption was accompanied by large magnetic field turbulence and frequently with reversal in the sign of the field component normal to the neutral sheet. Current disruptions in the near-earth region are found to be typically shortlived (about 1-5 min), and their onsets coincide well with the ground onsets of substorm expansion or intensification in the local time sector of the footpoint of the spacecraft. These events are found almost exclusively close to the field reversal plane of the neutral sheet (within about 0.5 RE). Prior to current disruption the field strength can be reduced to as low as one seventh of the dipole field value and can recover to nearly the dipole value after disruption. The temporal evolution of particle pressure in the near-earth neutral sheet during the onset of current disruption indicates that the current buildup during the substorm growth phase is associated with enhancement in the particle pressure at the neutral sheet.

Substorm intensifications and field line resonances in the nightside magnetosphere

Abstract: Magnetometer and HF radar data often indicate the presence of magnetohydrodynamic, field line resonances in the nightside magnetosphere. These resonances have frequencies of about 1.3, 1.9, 2.6, and 3.4 mHz and are due to cavity modes or waveguide modes which form between the magnetopause and turning points on dipolelike magnetic shells. Energy from these cavity modes tunnels to the field line resonances which are seen in the F region by the HF radar and on the ground by the magnetometers. The presence of these field line resonances gives us an excellent diagnostic tool for determining the position of the mechanism leading to the energetic electrons and field-aligned currents associated with substorm intensifications and auroral brightening. Using data from the Canadian CANOPUS array of magnetometers, meridian scanning photometers, riometers, and bistatic auroral radars and data from the Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay in Canada, we have identified a number of intervals in which substorm intensifications occurred during times when field line resonances existed in the region of the magnetosphere where the intensification occurred. In the events that we have analyzed in detail, the ionospheric signatures of the substorm intensification began equatorward (earthward) of existing field line resonances. These observations give very strong evidence indicating that at least one component of the substorm mechanism must be active very close to the Earth, probably on dipolelike field lines in regions with trapped and quasi-trapped energetic particles. Furthermore, the auroral intensifications started near the position of one of the equatorward resonances, indicating that the field line resonances may play a role in triggering or producing the substorm intensifications. One possible scenario is mode conversion to kinetic Alfven waves in the resonance.

Proton aurora and substorm intensifications

Abstract: Ground based measurements from the CANOPUS array of meridian scanning photometers and precipitating ion and electron data from the DMSP F9 satellite show that the electron arc which brightens to initiate substorm intensifications is formed within a region of intense proton precipitation that is well equatorward (approximately four to six degrees) of the nightside open-closed field line boundary. The precipitating protons are from a population that is energized via earthward convection from the magnetotail into the dipolar region of the magnetosphere and may play an important role in the formation of the electron arcs leading to substorm intensifications on dipole-like field lines.

Radial expansion of the tail current disruption during substorms: A new approach to the substorm onset region

Abstract: A new method is used to examine the radial expansion of the tail current disruption and the substorm onset region. The expansion of the disruption region is specified by examining the time sequence (phase relationship) between the north-south component and the sun-earth component. This method is tested by applying it to the March 6, 1979, event. The phase relationship indicates that the current disruption started on the earthward side of the spacecraft, and expanded tailward past the spacecraft. The method was used for 13 events selected from the ISEE magnetometer data. The results indicate that the current disruption usually starts in the near-earth magnetotail and often within 15 RE from the earth.

Transverse ion acceleration by localized lower hybrid waves in the topside auroral ionosphere

Abstract: Up to now, observations had been unable to show conclusively a one-to-one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O(+) source region below 2000 km. In this paper we demonstrate that intense (100-300 mV/m) lower hybrid waves are responsible for transversely accelerating H(+) and O(+) ions to characteristic energies of up to 6 eV. This wave-particle interaction takes place in thin filamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at latitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse.

Particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase

Abstract: The particle scattering and current sheet stability features in the geomagnetic tail during the phase of substorm growth were investigated using Tsyganenko's (1989) magnetic field model. In a study of four substorm events which were observed both in the high-altitude nightside tail and in the auroral ionosphere, the model magnetic field was adjusted to each case so as to represent the global field development during the growth phase of the substorms. The model results suggest that the auroral brightenings are connected with processes taking place in the near-earth region inside about 15 earth radii. The results also suggest that there is a connection between the chaotization of the electrons and the auroral brightenings at substorm onset.

Pressure changes in the plasma sheet during substorm injections

Abstract: Data from the CHEM instrument on AMPTE CCE, data from the 3D plasma instrument and the SULEICA instrument on AMPTE IRM, and magnetometer data from both spacecraft are used to determine the particle pressure and total pressure as a function of radial distance in the plasma sheet for periods before and after the onset of substorm-associated ion enhancements over the range 7-19 RE. Events were chosen that occurred during times of increasing magnetospheric activity, as determined by an increasing AE index, in which a sudden increase, or 'injection', of energetic particle flux is observed. It is shown that the simultaneous appearance of energetic particles and changes in the magnetic field results naturally from pressure balance and does not necessarily indicate that the local changing field is accelerating the particles.

Modelling of the magnetic field of magnetospheric ring current as a function of interplanetary medium parameters

Abstract: The models are examined which are proposed elsewhere for describing the magnetic field dynamics in ring-currentDR during magnetic storms on the basis of the magnetospheric energy balance equation. The equation parameters, the functions of injectionF and decay τ, are assumed to depend on interplanetary medium parameters (F and τ during the storm main phase) and on ring-current intensity (τ during the recovery phase). The present-day models are shown to be able of describing theDR variations to within a good accuracy (the r.m.s. deviation 5 <δ < 15 nT, the correlation coefficient 0.85

Ionospheric convection and the substorm cycle

Abstract: During substorms, magnetic energy is stored and released by the geomagnetic tail in cycles of growth and expansion phases, respectively. Hence substorms are inherently non-steady phenomena. On the other hand, all numerical models (and most conceptual ones) of ionospheric convection produced to date have considered only steady-state situations. In this paper, we investigate the relationship of substorms to convection. In particular, it is shown that the steady-state convection pattern represents an average over several substorm cycles and does not apply on time scales shorter than the substorm cycle period of 1-2 hours. The flows driven by the growth and expansion phases of substorms are integral (indeed dominant) part of, as opposed to a transient addition to, the overall convection pattern.

Traveling magnetospheric convection twin-vortices : observations and theory

Abstract: Ground-based magnetometer arrays are an important tool to image dynamical features of the Earth's magnetosphere, and have more recently been used to study a peculiar type of geomagnetic field variation, impulsive magnetic events, occurring predominantly in the dayside magnetosphere. Magnetic field imaging has been able to show that such magnetic impulsive events are due to tailward-propagating magnetospheric convection twin-vortices with the vortex centers separated by 1000-2000 km and being located at high geomagnetic latitudes, on field lines probably mapping back to the dayside magnetopause. Several mechanisms have been suggested to generate such twin-vortex systems and are discussed in detail, among them magnetic reconnection and changes of the magnetopause pressure balance. Detailed analysis of a twin-vortex structure exhibits that they are associated with a pair of upward and downward flowing field-aligned current with current densities up to 10 −9 A/m 2 . That such large currents may eventually become unstable, and potentially lead to particle acceleration is supported by observations of dayside auroral activity associated with such twin-vortex systems. They may be looked at as a dayside equivalent of the nightside, substorm current wedge

Is Substorm Occurrence a Necessary Condition for a Magnetic Storm

Abstract: It is shown that “having substorms” is not a necessary condition for a magnetic storm. The main phase of magnetic storms develops because of sustained, southward interplanetary magnetic field (IMF), not because of frequent occurrence of intense substorms.

Distribution in magnetotail of O+ ions from cusp/cleft ionosphere: A possible substorm trigger

Abstract: The transport of O(+) ions from the cusp/cleft ionosphere to the magnetotail during highly disturbed times was determined by computing the guiding-center trajectories of the ions to a distance of 6 R(E) from the ionosphere and the full-motion trajectories at later times. Case histories were tallied in six planes perpendicular to the X(GSM) axis, three planes perpendicular to the Y(GSM) axis, and in the center plane of the tail. At various times relative to the enhancement of the convection electric field, the following ion properties were constructed from the case histories: number density, mean energy, energy and pitch angle distributions of the flux, and ion pressure components parallel and perpendicular to the magnetic field. It was found that, after about 1.7 hours, the ion flux in the near-earth magnetotail increased dramatically and the spectrum hardened, much as observed during periods just preceding substorms. This increase is attributed to (1) the increase in the O(+) outflux from the ionosphere, (2) the increased energization of the ions by the convection electric field, and (3) ion trapping, which generally occurs because the ion magnetic moments generally increase after the ions first cross the geomagnetotail center plane.

Nonadiabatic heating of the central plasma sheet at substorm onset

Abstract: Heating events in the plasma sheet boundary layer and central plasma sheet are found to occur at the onset of expansive phase activity. The main effect is a dramatic increase in plasma temperature, coincident with a partial dipolarization of the magnetic field. Fluxes of energetic particles increase without dispersion during these events which occur at all radial distances up to 23 RE, the apogee of the ISEE spacecraft. A major difference between these heating events and those observed at geosynchronous distances lies in the heating mechanism which is nonadiabatic beyond 10 RE but may be adiabatic closer to earth. The energy required to account for the increase in plasma thermal energy is comparable with that required for Joule heating of the ionosphere. The plasma sheet must be considered as a major sink in the energy balance of a substorm. Lobe magnetic pressures during these events are estimated. Change in lobe pressure are generally not correlated with onsets or intensifications of expansive phase activity.

ISEE 3 plasmoid and TCR observations during an extended interval of substorm activity

Abstract: On April 9-11, 1983, the ISEE 3 spacecraft was continuously located within the earth's magnetotail for more than 36 hours at downstream distances of X = -76 to -80 R(e). During this span of time, 12 major intervals of substorm activity were observed in the AL index with good ISEE 3 telemetry coverage for 11 of them. In addition, there were two small substorms outside of these intervals, both with complete observations in the distant tail. This unusual ISEE 3 data set provides a unique opportunity to test the predictions of the near-earth neutral line model. In particular, the hypothesis that energy stored in the tail lobes during the growth phase is later dissipated, in part, through the release of one or more plasmoids following expansion phase onset is examined. Clear growth phase enhancements in the lobe magnetic field intensity preceded the onsets of nine of the substorms. Plasmoids, or their lobe signatures, traveling compression regions (TCRs), were observed at ISEE 3 in association with all 11 of the major substorm intervals for which there were ISEE observations, as well as for the two small substorms. No plasmoids or TCRs were observed in the absence of substorm activity. If these ISEE 3 observations are representative, then the release of plasmoids down the tail may be a feature common to all substorms.

Thinning and expansion of the substorm plasma sheet

Abstract: More than 50,000 measurements with the AMPTE IRM satellite in the magnetotail are used for a superposed epoch analysis of the occurrence frequency of the near-Earth central plasma sheet, which is directly related to its thickness, around 39 major substorm onsets. For all radial distances between 10 and 19 RE, the central plasma sheet gets thinner during the 30–45 min preceding substorm onset, but starts to expand immediately after the onset. The central plasma sheet regains its initial thickness around the beginning of the recovery phase. These observations fit into the framework of the near-Earth neutral line model, if the neutral line is assumed to form beyond the IRM apogee.

Advances in magnetospheric storm and substorm research: 1989–1991

Abstract: Recent advances in magnetospheric storm and substorm research is reviewed, with emphasis on how the large southward fields and high velocities produced in the solar wind influence the magnetosphere and cause the enhanced transfer of energy, momentum, and mass to the magnetosphere. Overwhelming evidence indicates that the southward interplanetary magnetic field is the primary controlling factor in the generation of substorms. The immediate cause of the expansion phase onset is controversial, but the suddenness of the onset is suggestive of an instability that disrupts the cross-tail currents. Measurements increasingly suggest the region of 7-10 R sub E near midnight as the likely point of origin, but it is not clear that the long-popular tearing mode can go unstable this close to the earth, where it may be stabilized by a small northward field component.

Plasma and magnetic field parameters at substorm onsets derived from GEOS 2 observations

Abstract: Measurements of energetic particles and the geomagnetic field by the geostationary satellite GEOS 2 were used to investigate the plasma and magnetic field parameters at substorm onset. The distribution of energetic ions and electrons are found to be statistically isotropic at the nightside synchronous orbit. The number density and pressure of electrons Ne and Pe are much lower than Ni and Pi of ions. Electrons and ions increase substantially at substorm onset when compared with magnetically quiet days. Earthward pressure gradients of energetic ions with scale length of ∼10 ri (ri = ion gyroradius) are commonly observed during the substorm growth phase. The magnetic field lines are stretched out tailward with a curvature radius in the equatorial plane being considerably smaller than that in the dipole field. Plasma β values increase to the order of 1. All these parameters are found to be consistent with the development of the ballooning instability in connection with substorm onsets.

Further investigation of the CDAW 7 substorm using geosynchronous particle data: Multiple injections and their implications

Abstract: A substorm which occurred on April 24, 1979 was the subject of the CDAW 7 workshop and several in-depth studies of ISEE and ground-based data. In this paper we continue the analysis by looking in detail at geosynchronous energetic particle data from the Los Alamos charged particle analyzer instruments. Observations show three distinct injection events during this substorm. We have used a drift shell tracing technique to determine the time and location on the geosynchronous drift shell of each injection region. We then compare the timing of events on the ground, in the tail and at geosynchronous orbit. The first injection is very closely associated with observations from ISEE and ground-based measurements which have been interpreted as the signatures of plasmoid formation in the near-Earth tail. The later two injections occurred while ISEE 1 and 2 were in the lobes following the exit of the plasmoid downtail. All three injections are calculated to have originated in the same local time sector in which the ISEE spacecraft were located and also to have overlapped one another in local time extent. These observations show that the spatial and temporal evolution of the substorm is more dynamic than previous studies have implied. We have considered possible interpretations of the more complete set of observations the context of the both the traditional near-Earth neutral line model and the inner magnetospheric current disruption model. Either model can provide an interpretation of the data but neither by itself, provides a completely satisfactory explanation. We suggest a new synthesis of these two models which provides a simple interpretation for all the observations. We propose that reconnection in the tail initiates the substorm but diverts much of the cross-tail current around the reconnection region enhancing the near-Earth cross-tail current. This creates an unstable situation and current disruptions dipolarize the field in three steps creating the three injections.

Transient composite electric field disturbances near dip equator associated with auroral substorms

Abstract: Ionosonde data of Kodaikanal (Geog.Long. 77° 29′E, dip 3.0°N) and Huancayo (Geog.Long. 75°18′W, dip 2.0°N) are used to show the simultaneous occurrence of a transient disturbance in F region height of composite polarity in day and night sectors near the dip equator during the auroral substorm activity on 20 August 1979. At Kodaikanal which is on the nightside at the time of the substorm activity, h′F first underwent an abrupt and rapid decrease (80km in 1 hr) followed by a much larger increase (120km in 1 hr). Perturbation in hpF2 of exactly opposite polarity was simultaneously seen at Huancayo which is on the dayside. The decrease in h′F at Kodaikanal (increase in hpF2 at Huancayo) occurred in association with an increase in polar cap potential drop, o (estimated from IMF parameters), and the subsequent increase (decrease at Huancayo) with a decrease in polar cap potential. The F-region height disturbance is interpreted as the manifestation of a global transient composite disturbance in equatorial zonal electric field caused by the prompt penetration of substorm-related high latitude electric fields into the equatorial ionosphere. The polarity pattern of the electric field disturbance is consistent with the global convection models which predict westward (eastward) electric fields at night (by day) near the geomagnetic equator in response to an increase in polar cap potential drop, and fields of opposite signs for a decrease in polar cap potential.

Formation of the stable auroral arc that intensifies at substorm onset

Abstract: In a companion paper, we present observational evidence that the stable, growth-phase auroral arc that intensifies at substorm expansion phase onset often forms on magnetic field lines that map to within approximately 1 to 2 R(sub e) of synchronous. The equatorial plasma pressure is 1 to 10 nPa in this region, which can give a cross-tail current greater than 0.1 A/m. In this paper, we propose that the arc is formed by a perpendicular magnetospheric-current divergence that results from a strong dawn-to-dusk directed pressure gradient in the vicinity of magnetic midnight. We estimate that the current divergence is sufficiently strong that a is greater than 1 kV field-aligned potential drop is required to maintain ionospheric-current continuity. We suggest that the azimuthal pressure gradient results from proton drifts in the vicinity of synchronous orbit that are directed nearly parallel to the cross-tail electric field.

Drifting Holes in the Energetic Electron Flux at Geosynchronous Orbit Following Substorm Onset

Abstract: A new type of substorm-related particle flux change was discovered at geosynchronous orbit. It is manifested as a decrease in energetic electron flux (of initial duration about 1–2 min) starting near local magnetic midnight at a time of high-energy particle injection to geosynchronous distance. Unlike “dropouts,” which are observed only in the midnight local time sector, this flux signature drifted eastward showing the typical energy and LT dependent time patterns of electron drift. This phenomenon was seen only in electron fluxes above a certain threshold energy and was characterized by a moderate amplitude of flux variation (within a factor of 3). Below the threshold energy the normal substorm-associated increase in electron flux (drifting cloud of accelerated particles) was detected. The phenomenon in question, called a drifting electron hole, is explained as being a result of adiabatic redistribution of an already existing high-energy electron component recently described by Cayton et al. (1989). It is argued that in the case of a strong (inward directed) radial gradient of electron flux and of a hard energy spectrum the electron fluxes which are brought to a lower drift shell by the inward injection may still be smaller than the previously existing fluxes at this drift shell. This leads to an apparent loss of electrons in some localized portion of the drift shells (e.g., at geosynchronous orbit). Another explanation of the drifting electron hole based on rapid particle losses due to precipitation seems less probable.

Magnetospheric substorms and nighttime height changes of the F2 region at middle and low latitudes

Abstract: Using the published plots of 15-min interval values of h′F2, the minimum virtual height of the ionospheric F2 region, at five Japanese stations spread over 26°–45°N (15°–35°N geomagnetic) and 128°–142°E, it is shown that there are frequent rapid increases and less frequent decreases of h′F2 by 100–200 km in about 2 hours with near-simultaneous start and end at all five stations in the postmidnight hours and at three to five stations in the premidnight hours. In the months of September and December 1988 and March 1989, about 80 h′F2 perturbations of the above type are identified. The near simultaneity of the h′F2 disturbances at the five stations suggests that an plasma drift due to an east-west electric field is the cause of the h′F2 disturbances. This interpretation is validated by showing that there is a simultaneous presence of bay disturbances in auroral latitude magnetograms over a wide longitude range during almost all the h′F2 increases. It is shown that neutral wave disturbances propagating equatorward even with a speed of 750 m/s cannot account for the h′F2 disturbances, The study reveals two categories of h′F2 perturbations: (1) h′F2 increases starting near the beginning of the geomagnetic negative bays at auroral latitudes and (2) rapid h′F2 increases starting near the maximum intensity phase of the auroral bays and preceded by h′F2 decreases starting at the onset time of the bay disturbance or a little earlier. Using the vast amount of h′F2 data collected all over the globe, it is possible to quantify and characterize the substorm-related electric fields and their variabilities. Such characterization is very useful in understanding the magnetospheric and the magnetosphere-ionosphere interaction processes involved in the generation of magnetospheric substorms. It is also pointed out that the understanding of F2 region changes during magnetic storms may improve greatly if the large F2 region changes caused by electric fields on the substorm time scale are taken into account. The h′F2 disturbances reported in this study are similar in time scale and appearance to the h′F2 disturbances caused by long-period atmospheric waves; and this suggests the possibility of a mixup of the two effects and the consequent wrong identification on some occasions.

A solar flare model including the formation and destruction of the current sheet in the corona

Abstract: A numerical simulation method is used to show the possibility of forming a current sheet in the solar corona in an active region with four magnetic poles. The evolution of the quasi-stationary current sheet can lead to its transfer to an unsteady state. The MHD instability of this sheet causes its decay, accompanied by a set of events which characterizes the solar flare. The electrodynamical model of a solar flare includes a system of field-aligned currents typical of a magnetospheric substorm. Several events in substorms and solar flares are explained by the generation of field-aligned currents.

A case study of plasma structure in the dusk sector associated with enhanced magnetospheric convection

Abstract: In a case study from June 8–9, 1982, data from ground whistler stations Siple and Halley, Antarctica, located at L ∼4.3 and spaced by ∼2 hours in MLT, and from satellites DE 1 and GEOS 2, have provided confirming evidence that the bulge region of the magnetosphere can exhibit an abrupt westward “edge,” as reported earlier from whistlers. The present data and previous MHD modeling work suggest that this distinctive feature develops during periods of steady or declining substorm activity, when dense plasma previously carried sunward under the influence of enhanced convection activity begins to rotate with the Earth at angular velocities that decrease with increasing L value and becomes spirallike in form. For the first time, whistler data have been used to identify a narrow dense plasma feature, separated from the main plasmasphere and extending sunward into the late afternoon sector at L values near the outer observed limits of the main plasmasphere bulge. The westward edge of the main bulge, found by both whistler stations to be at ∼1800 MLT, appeared to be quasi-stationary in Sun-Earth coordinates during the prevailing conditions of gradually declining geomagnetic agitation. It is possible that outlying dense plasma features such as the one observed develop as part of the process leading to the occurrence of the more readily detectable abrupt westward edge of the bulge. It was not possible in this case to determine the extent to which the outlying feature was smoothly attached to or isolated from the main bulge region.

Substorm‐associated radar auroral surges

Abstract: We report a recurrent convection signature observed in the E region ionosphere within ∼2 hours of the dusk meridian by the SABRE radar facility. In a typical event, the irregularity drift speed in the SABRE field of view is seen to increase from about 300 m s−1 to of the order of 1 km s−1 in the space of about 10 min. The speed subsequently remains at the enhanced level for 10 min or longer before declining as rapidly as its onset. The total event duration ranges between 30 min and 1 hour. As the irregularity drift speed increases the direction of the drift velocity changes, rotating poleward. At the same time, the radar backscatter power decreases. The onset of the drift speed enhancement crosses the SABRE field of view as a front moving from east to west. Detailed study of individual events indicates that the events are associated with increases in the |AL| index and with the injection of energetic particles into geosynchronous orbit. We thus suggest that the events are part of the magnetospheric response to the onset of a geomagnetic substorm. However, while each event appears to be associated with a substorm onset, not every substorm onset is associated with an event, at least not at SABRE. We estimate the speed at which the substorm-initiated ionospheric flow enhancement moves from the nightside to be 1–4 km s−1, a figure that is consistent with the rate at which the drift velocity front crosses the SABRE field of view. Although the front is associated with a rotation in the drift velocity, we see little evidence of strong vertical vorticity as the front passes. However, shears in the flow do develop subsequently which seem likely to correspond to field-aligned current. Although associated with substorm onset, we argue that these events are distinct from westward traveling surges and appear to differ from the midlatitude phenomenon known as subauroral ion drifts. We thus call this new-found signature a substorm-associated radar auroral surge or SARAS event.

Observations of a giant pulsation across an extended array of ground magnetometers and on auroral radar

Abstract: It has often been suggested that energetic protons are involved in the generation of giant pulsations (Pgs). A Pg event occurring in the early morning of 29 December 1987 has been observed by magnetometers in Scandinavia, the Faroe Islands and Iceland. The Pg has also been observed by the SABRE coherent radar system. The azimuthal drift velocity of the Pg disturbance region across the available longitudinal extent has been measured using the magnetometer data and the radar data have been used to estimate the ionospheric convection electric field. The energies of protons that would drift at this velocity in the region of the magnetosphere where the Pg occurred have been deduced to be ∼ 10–20 keV, depending on the pitch angle of the protons. Using azimuthal wavenumbers calculated on the EISCAT magnetometer cross and the SABRE radar for this event, the energies of protons that would satisfy the bounce resonance condition for this Pg have been calculated to be ∼ 5–18 keV. The Pg is thought to be driven by protons associated with a substorm particle injection that occurred on 28 December 1987. The energies of protons that would drift westward from this injection region and reach the Pg disturbance region have been calculated to be ∼ 11–24 keV. This study therefore presents good evidence to suggest that the bounce resonance interaction with protons is the mechanism responsible for the Pg and that these protons originated from a substorm particle injection.

Variations of the polar cap potential measured during magnetospheric substorms

Abstract: Measurements of the polar cap potential drop and size have been obtained during magnetospheric substorms. Using double-probe electric field measurements on the DE 2 satellite, 148 measurements have been obtained at random times preceding, during, and after 64 substorms. The polar cap potentials are graphed as a function of the difference between the time of the polar cap measurement and the time of the expansion onset of the corresponding substorm. The ratios of the auroral electrojet (AE) indices and the potential are also determined. The results show that on the average the polar cap potential starts to increase at 1.5 hours before onset, reaches a level of 70 kV in the 30-min period before onset, and starts to decline at 1.5 hours after onset. However, on a case-by-case basis there are substantial variations from the average, as polar cap potentials over 120 kV were measured as early as 1 hour before substorm onset and values as low as 40 kV were observed during the expansion phase. The size of the polar cap ranged from 23° to 38° invariant latitude at the time of onset, and had an average value of 31°. The AE/ΦPC ratio is nearly constant before and after substorms, but decreases slightly during the substorm growth phase and increases greatly during the expansion phase. This increase is most likely due to a higher conductivity and westward electric field within the electrojet during expansion, which causes AE to increase without a corresponding change in the polar cap potential.