Negative Bay Research Articles

Particle and field signatures during pseudobreakup and major expansion onset

The temporal and spatial scales of the onset of two types of substorm events are investigated. These substorms were cases where the expansion onset had precursor localized auroral activation without significant negative bay enhancement, that is, “pseudobreakup”. High‐resolution energetic particle and magnetic field data at synchronous orbit are used for the analysis together with auroral and magnetic field data simultaneously taken from ground‐based instrumentation. The auroral structure following the pseudobreakup significantly resembled the major expansion aurora, except in its spatial scale. Typical magnetospheric onset signatures such as tail current diversion, dipolarization, and injection were observed associated with some of the pseudobreakups. The major expansion, on the other hand, consisted of a number of rather localized injections and expansions, each of which had timescales of 2‐8 min, a comparable timescale to that of pseudobreakups. This study shows that there does not appear to be any phenomenological differences between pseudobreakups and major expansion onsets. The major difference between pseudobreakups and major expansion onsets would be the number of occurrences, as well as the intensity and the scale size of the magnetospheric source.

Experimental evidence for the formation and entry of patches into the polar cap

A mechanism for the formation of polar cap patches is proposed based on data collected by the Sondrestrom incoherent scatter radar, magnetometers located on the west and east coasts of Greenland and Digisondes, digital ionospheric sounders, operated at Qaanaaq and Sondrestrom. This instrumentation has served to identify the formation of patchlike structures and follow their temporal evolution and entry into the polar cap. Prior to the onset of the event, the background ionospheric plasma was moving in a poleward direction and formed part of a tongue of ionization directed toward and into the polar cap. The event starts with the appearance of a fast plasma jet containing eastward directed velocities in excess of 2 km s−1. This plasma jet consists of a channel extending 300 km in width, where the F region ion temperature reaches values in excess of 4000 K and the E layer Ti is enhanced above 5000 K. The existence of a region containing high electron temperatures associated with soft precipitation (likely the cusp) and located adjacent and equatorward of the fast plasma jet is very suggestive of the jet being the result of a tension force acting upon recently reconnected field lines. This view is supported by the fact that the fast plasma jet was seen in the prenoon sector under IMF By negative conditions. The elevated Ti values inside the plasma jet are exactly collocated with depleted F region densities. We suggest that the recombination loss of O+ is increased by a factor >10 due to the dependence of the O+ + N2 reaction on the ion temperature. The eroding action of the fast plasma jet proceeds until the poleward moving tongue of ionization is divided into regions containing high and low densities. Magnetic field perturbations associated with the fast plasma jet were observed by the different magnetometer stations. The large negative bays recorded at most of the sites have been used to trace the poleward motion of the jet. At the time that the fast plasma jet reaches Qaanaaq, the Digisonde there measured low ƒ0F2 values. A few minutes before and after this minimum, high ƒ0F2 values were observed. The ratio of the enhanced ƒ0F2 values on either side of the minimum, to the minimum ƒ0F2 is about 2 (or a density ratio of 4). This is the commonly accepted signature of a polar cap patch. The series of events leading to the formation of the patchlike density structure has been designated a density breakoff event.

A multisatellite study of a pseudo‐substorm onset in the near‐Earth magnetotail

This paper reports the multisatellite and ground observations of two pseudo‐substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The distinguishing features of the 0747 UT event are as follows. (1) The substorm‐associated tail reconfiguration started in a very localized region in the near‐Earth magnetotail. (2) The magnitude of the current disruption decreased markedly as the disruption region expanded tailward. (3) On the ground the onset of a very small negative bay (∼ 40 nT) was observed simultaneously with the onset of the current disruption, but over a much wider local time sector than the near‐Earth tail reconfiguration. Positive bay onsets at mid‐latitudes also had a longitudinally wide distribution. From these features we infer that in the present event the current disruption took place filamentarily near AMPTE/CCE at ∼8.8 RE. It is also inferred that pseudo‐substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption, and that the spatial scale of the onset region in the magnetosphere is not a major difference between the two. The present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption. It is also suggested that the current disruption is basically an internal process of the magnetosphere.

Simultaneous observation of the poleward expansion of substorm electrojet activity and the tailward expansion of current sheet disruption in the near‐Earth magnetotail

In this paper we present observations of a substorm that occurred on June 7, 1985. The data consist of energetic ion and magnetic field data from AMPTE/CCE, magnetometer data from the ground stations of the EISCAT magnetometer cross, STARE radar data, and Pi 1 data from Sodankylä. CCE was in the Finnish local time sector, and the model field line threading CCE came down near the center of the EISCAT magnetometer cross. Furthermore, CCE was located near the neutral sheet and observed the disruption of the cross‐tail current in situ. At 2209 UT, Sodankylä registered a Pi 1 burst and the EISCAT stations recorded the onset of a negative bay. Simultaneously, CCE observed the onset of current sheet disruption and ion energization. The energetic ions observed during this initial burst had gyrocenters earthward of CCE. An intensification of activity occurred at 2212 UT, and it was observed essentially simultaneously on the ground and in space. It consisted of an intensification of Pi 1 activity, an intensification of the westward electrojet poleward of the initial breakup latitude (as determined from magnetometer and radar data), and magnetic turbulence and a burst of energetic ions at CCE in the magnetotail. Significantly, this second burst was composed only of ions with gyrocenters tailward of the satellite, and the magnetic turbulence was considerably weaker than that observed during the initial current disruption. This strongly suggests that the current disruption region had moved tailward of CCE. The event provides a direct observational link between the tailward expansion of active regions in the near‐Earth magnetotail and the poleward expansion of ionospheric electrojet activity during substorms.

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

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.

Seasonal and diurnal variations of the latitude of the westward auroral electrojet in the nightside polar cap

Simultaneous measurements of magnetic field variations at the nominally conjugate sites of South Pole station, Antarctica, and Iqaluit, Canada, have been used to study latitude differences in the location of corresponding westward electrojets in the nightside polar cap. Substorm‐related auroral events were selected from 1986 data with the criterion that the magnitude of the negative bay exceed 100 nT in at least one hemisphere and that the electrojet, oriented mainly in the magnetic east–west direction, pass overhead of this station. The resulting data base comprised 60 events distributed seasonally as follows: austral summer (17); fall (6); winter (18); spring (19). The interhemispheric latitude difference in the locations of the electrojets was obtained by examining the ratio of the vertical and horizontal component magnetic variations. Seasonal and diurnal variations of the latitude of westward electrojets were evident during solstice periods. In particular, it is noted that the substorm westward electrojet flows at higher latitudes in the winter hemisphere than in the summer hemisphere, after 2000 MLT. The latitude difference between the two hemispheres maximizes at ∼4° near local midnight. The latitude difference becomes smaller away from midnight and reverses for local times prior to ∼2000 MLT. The observed seasonal and diurnal variations are generally consistent with predictions of the effect of the magnetic dipole tilt on the location of last closed field lines. However, because on the nightside the field lines originating from South Pole station and Iqaluit reach deep into the magnetotail, they are likely to be influenced by the plasma sheet. Thus the implications of an asymmetric distribution of the plasma sheet with respect to the neutral sheet have also been considered. Such a postulated distribution can account for the seasonal variations while conserving magnetic flux and maintaining pressure balance across the tail.

Electrodynamic properties of auroral surges

The incoherent scatter radar technique provides an excellent means to study the ionization and electric fields associated with auroral precipitation events. One of the most intense and dynamic auroral events is the so‐called surge or breakup aurora that accompanies auroral substorms. For our purposes we define a surge as a transient intensification of auroral precipitation that occurs simultaneously with a pronounced negative bay in the ground magnetometer data. We present data obtained during five such events in 1980 and 1981. Measurements made by the Chatanika radar are combined with simultaneous data obtained from ground‐based magnetometers and all‐sky cameras and space‐based instruments on the Dynamics Explorer satellites. The combined data sets yield information about the auroral forms, electric fields, and currents that are associated with surges. Prior to the surge, auroral forms move equatorward, develop ray structure, and intensify. The surge is identified by an apparent poleward motion of the aurora producing aurorally associated ionization that extends over several hundred kilometers in latitude. The presurge auroral forms are embedded in a region of northward electric field. The auroral forms that comprise the surge span a region within which the meridional electric field is small and at times southward. A westward electric field is often but not always present within the surge. The behavior of the westward electric field is significantly different from the north‐south field, in that sharp spatial gradients are absent even in very disturbed conditions. Although the westward Hall currents are mostly responsible for the negative bays that accompany the surge, at times the westward Pedersen current sustained by the westward electric field can be important. Sudden variations in the H component of the ground magnetogram can be caused by motions of the aurora or by temporal variations in the fields or conductivities. We present a model that simulates the observed changes in electric field and precipitation that accompany surges. The perturbation in the electric field produced by the surge is simulated by adding negative potential in regions of intense precipitation.

Comment on “ionospheric signature of plasma sheet thinning prior to a substorm” by J.R. Dudeney and A.S. Rodger

In a recent paper, Dudeney and Rodger (1988) have presented a study of a substorm expansive phase which they use to attempt to distinguish between the near-Earth neutral line model for substorms (cf. Hones, 1984) and the boundary layer model for substorms proposed by Rostoker and Eastman (1987). One of the features of the event studied by Dudeney and Rodger is a Pi2 pulsation which, the authors contend, is not accompanied by a substorm expansive phase. The purpose of this Comment is two-fold. In the first place I wish to demonstrate that the evidence that the Pi2 pulsation alluded to by the authors is not accompanied by a substorm expansive phase is far from compelling. In the second instance, reasons for rejecting the boundary layer model as an explanation for the event under consideration are shown to be without foundation. which is connected to the magnetosphere by fieldaligned currents at its eastern and western edges (viz. a current wedge). In this case, it is clear that to the equatorward side of the ionospheric electrojet, the magnetic signature is a negative Z-component bay. A 10 nT Z-component bay is quite consistent with the development of a westward electrojet of peak strength 150 nT whose center is 10” poleward of the observing station (e.g. Halley). (Note that Fig. 1 shows a westward electrojet for the Northern Hemisphere, so that the corresponding signature for a negative Z-component bay would be a positive Z-component bay at the Southern Hemisphere station of Halley.) Based on these considerations, I would suggest that the authors have no strong reason to believe that their 00: 50 U.T. pulsation is not accompanied by an expansive phase onset or intensification.

Observations of low‐frequency radio emissions in the Earth's magnetosphere

A study is made of electromagnetic radiation in the Earth's magnetosphere in the frequency range between 10 kHz and 80 kHz using data from the University of Minnesota Plasma Wave Experiment aboard the IMP 6 satellite. Two types of radio emissions are investigated. First is the nonthermal continuum radiation, and it is found that discrete enhancements above ambient levels are correlated with enhancements of the magnetic substorm index AE and appear to follow the onset of the negative bay feature of the AU index by about 20 min or so. The directions of these discrete source regions of continuum radiation are measured as a function of time, and movement of the source region in a dusk‐to‐dawn direction is directly observed. This drift motion is used to measure the energy of the generating electrons by a time‐of‐flight method, and a range between 10 keV and 50 keV is found in agreement with previous studies. A second type of radiation is also observed which correlates with auroral kilometric radiation (AKR) on a time scale of ∼1 min. This radiation lies between 10 and 60 kHz with a spectral peak near 30 kHz and is found to have a source direction very near that of the coincident AKR. The lower frequency of the spectral peak, in conjunction with the analysis of the spin‐modulated wave data, suggests a source location at a higher elevation than the higher‐frequency AKR indicating a source altitude of roughly 3 Earth radii.

A study of a magnetospheric substorm recorded by the Viking auroral imager

A substorm, recorded at 80 second time resolution in images from the Viking auroral imager, has been studied using data from ground observations, from a geosynchronous satellite and from ISEE 1 in the magnetotail plasma sheet. Substorm onset, marked by negative bays at the ground, particle injection at synchronous orbit and plasma sheet dropout at ISEE 1, appeared to produce only local brightening of the night side oval. A rapid poleward expansion of the auroras twenty minutes later was accompanied by a major reduction of tail energy density and swelling of the inner plasma sheet. Recovery, 15 minutes later, of auroral zone bays and the plasma sheet was accompanied by fading of the equatorward auroras that left a bright poleward arc as the primary auroral emission.

EISCAT observations of large, fluctuating electric fields during a Pc4 pulsation event in the midnight sector

Large, irregular electric fields were observed in the midnight sector on 25 February 1984 with the European incoherent-scatter (EISCAT) radar facility. Utilizing its unique ability to determine the local vector electric fields from F-region ion velocity measurements (in contrast to F-region velocity measurements by other ground-based sensors), time-varying electric fields were detected during extremely quiet geomagnetic conditions. Peak electric field intensities approaching 45 mV m −1 were observed to commence under these conditions and to persist through a negative bay event. The electric fields were characterized by a near-linear polarization (northwest-southeast orientation) and an oscillation period that was correlated with the mean oscillation amplitude. We show that the characteristics of the time-varying electric field are similar to those of a relatively rare type of Pc4 pulsation event found in the midnight sector. We further argue that (1) the large pulsation electric fields measured in the ionosphere during quiet conditions were probably produced by hydromagnetic wave energy reflected from a poorly conducting ionosphere, (2) wave energy was probably provided by the westward component of the convection electric field and (3) variations in oscillation period were also associated with the westward electric field through changes in the length of geomagnetic field lines

Associations of geomagnetic activity with plasma sheet thinning and expansion: A statistical study

Associations of geomagnetic activity in the auroral zone with thinnings and expansions of the magnetotail plasma sheet are examined statistically in this paper. We first identified many plasma sheet thinnings and expansions in plasma and particle data from VELA satellites and from OGO 5 without reference to ground magnetic data. These events were grouped according to the location of the detecting satellite in the magnetotail. For each such group the times of thinning or expansion were then used as fiducial times in a superposed‐epoch analysis of the geomagnetic AL index values that were recorded in 8‐hour intervals centered on the event times. The results show that many plasma sheet thinnings and expansions are related to discrete negative bay structures that are the classical signature of substorms. Furthermore, they support earlier findings that plasma sheet thinning and expansion at the VELA orbit (r ≈ 18 RE) tend to be associated with the onset of the auroral zone negative bay and the beginning of its subsidence, respectively. Earthward of r ≈ 13–15 RE, plasma sheet expansion occurs near the time of the onset of the negative bay, again in agreement with earlier findings. A large fraction of plasma sheet expansions to half thicknesses of ≳ 6 RE at the VELA orbit are associated not with a baylike geomagnetic disturbance but with subsidence of a prolonged interval of disturbance. The study also shows that many plasma sheet expansions are related simply to generally enhanced geomagnetic activity showing no baylike or other distinctive features.

The fluctuation and fading of auroral arcs preceding auroral substorm onsets

Studies involving magnetometer and high time resolution photometer data from the Alaska Meridian Chain Network have been initiated to document the characteristics of optical emissions leading up to an auroral substorm. Fluctuations of the peak intensity of 5577Å [OI] and 4278Å [N2+] emissions are observed to occur with a period of 1 to 4 minutes prior to a 4 to 7 minute fade which leads into a substorm. These fluctuations usually occur for twenty minutes or more prior to break‐up and at times appear to be part of a cyclic sequence of similar events (i.e. fluctuations of other periods of amplitudes), which may indicate the initiation of magnetospheric and/or plasmaspheric activity prior to substorm onset. In the majority of cases a minimum of intensity takes place just before the abrupt brightening associated with the start of the substorm. When more than one arc is present the sky frequently does not fade as a whole, but some arcs will dim while others brighten. Aside from a slight decline in the magnitude of the H‐component of the geomagnetic field some 15 to 20 minutes prior to negative bays associated with the onset of the substorms, no characteristic magnetic activity has been correlated with the observed fluctuations.

Morphology of spatial patterns in Pi 1-2 magnetic field pulsation activity - a review.

The source regions of Pi 1-2 activity in the night-morning sector consists of two regions or “spirals”. The night region consists of impulsive bursts, i. e., called Pi B events, that are correlated with auroral breakups, the Harang discontinuity, and lower frequency magnetic agitation. The morning region consists of more continuous Pi activity called Pi C events; these are associated to negative bays in the H component of the magnetic field and to pulsating aurora. A cleft and a dayside polar cap region of Pi 1-2 activity are also recognized. These regions form a domain with characteristics different from those for the night and morning region domain.

Global properties of the magnetosphere during a substorm growth phase: A case study

At approximately 0100 UT on December 29, 1976, a large injection of energetic (>30 keV) particles was observed by Los Alamos instrumentation onboard spacecraft 1976‐059 (35°W longitude) at geostationary orbit. This injection was closely associated with the onset of a major substorm (also at 0100 UT) identified by sharp negative bays in the H components of magnetic records at Leirvogur (22°W) and Narssarssuaq (45°W) and by the occurrence of a positive H component bay at 0100 UT in the mid‐latitude magnetogram record at M'Bour (17°W). This substorm expansion onset (and concomitant particle injection) was preceded (between 2330 and 0100 UT) by a pronounced ‘stretching’ of the magnetic field at synchronous orbit into a taillike configuration and by a development of highly cigarlike (field‐aligned) electron distributions at geostationary orbit that we have in the past identified with the substorm growth phase. Of principal importance in this case are two other auxiliary data sets. The first is a well‐timed set of DMSP auroral images taken during the course of the growth and expansion phases of the substorm. The images before and during the growth (cigar) phase, including one auroral zone crossing at ∼0050 UT, show quiet aurora with no observable substorm activity in the visible polar region. The second relevant data set is a broad set of riometer data from 13 separate stations in three general meridians (west coast Greenland, east coast Greenland, and northern Scandinavia) from magnetic latitudes of ∼65° to ∼90°. The riometer data also show clearly that there was no measurable substorm activity anywhere, either in longitude or latitude, as the magnetosphere developed its very stressed, growth‐phase configuration prior to substorm expansion onset. These results support the concept of a storage of energy (growth phase) prior to its rapid release at substorm onset. IMP 8 observations in the high southern magnetotail lobe at ∼35 RE geocentric radial distance provide strong corroborative evidence of this storage and subsequent release of magnetic energy.

In situ measurements of heating parameters in the auroral ionosphere

Four sounding rocket payloads were launched in early 1977 to measure heating parameters in the auroral oval. Geophysical conditions were different for the four flights: auroral arc substorm main phase diffuse aurora, and auroral arc with negative bay. The conductivity tensor and the heating rates of particle and Joule heating are determined. The heating rates range in the order of a few tens of mWm −2. These magnitudes accord with those determined with the aid of backscatter facilities and other sounding rocket observations.

High‐resolution altitude profiles of the auroral zone energy dissipation due to ionospheric currents

The first height‐resolved altitude profiles of the Joule heating rate obtained by using the Chatanika, Alaska, incoherent scatter radar are presented. The measured Joule heating rate typically maximizes at an altitude that is well above the altitude of the current density maximum. For a reasonable model neutral atmosphere, it is also found that the maximum Joule heating rate occurs above the altitude of the corresponding Pedersen conductivity maximum. For these data the eastward auroral electrojet was centered near 130 km, while the westward electrojet was centered 10 km lower in altitude during its most intense period. It is inferred that this large difference in height between the two electrojets and the corresponding altitude change in the Joule heating will have a significant effect on the source mechanisms for such phenomena as atmospheric gravity waves, infrasonic waves, and spread F irregularities. One should expect an asymmetry in the occurrence and structure of such events with respect to the Harang discontinuity. This asymmetry is noted for aurorally excited gravity waves. A tentative calibration of the total Joule dissipation in the neutral atmosphere relative to the magnetic field disturbance vector measured by a nearby ground‐based magnetometer has been performed. The heating efficiency was found to be 10 times larger in a positive bay than in a negative bay. The heating efficiency for the negative bay event is close to the value deduced by Cole (1971) on theoretical grounds.

Multiple‐satellite studies of magnetospheric substorms: Distinction between polar magnetic substorms and convection‐driven negative bays

It is generally believed that long intervals of intense (500–1000 γ) negative auroral zone bays are caused by magnetospheric substorm occurrences at a high repetition rate. Since individual substorm phases may then overlap in time and occur within limited local time sectors, it may be difficult to identify their characteristic signatures. In this paper we have examined ground and multiple‐satellite recordings during an ∼5‐hour interval of persistent auroral zone activity in a systematic search for substorm expansion signatures. It appears that this enduring activity started as a rather weak substorm which thereafter developed into strong magnetospheric activity driven by a continuously southward interplanetary magnetic field (IMF). During this period there were no clear mid‐latitude positive bays, and pulsation recordings from different local time sectors showed only one instance of weak Pi 2‐like variations. The associated high‐latitude equivalent ionospheric current pattern is consistent with a convection system, and we refer to this activity as the convection bay. Following the plasma sheet and ground signatures of substorm recovery early in the convection bay, there was no definite indication of substorm expansions other than this weak magnetic pulsation in any recordings during the next ∼3½ hours until a clear substorm emerged. This substorm onset, which was associated with a not particularly strong auroral zone bay, was accompanied by the full complement of substorm expansion signatures. At this time a minimum plasma sheet thickness of only a few tenths of an earth radius was observed in the region where a neutral line appears to form during substorms. Prior to this onset, the plasma sheet was relatively thick, and it seems that the neutral line was located well tailward of the Vela orbit (r ∼ 18 RE). This event and other similar observations suggest that continuous auroral zone activity during southward IMF, such as a geomagnetic storm period, may be driven by a steady state magnetospheric convection. The long absence of clear substorm onset signatures may indicate that the nonsteady enhancements of convection associated with localized substorm expansions then make only small contributions to the total convection pattern. Similarly, the ground magnetic disturbances may then be caused mainly by Sqp currents and enhanced currents due to strong particle precipitation rather than by substorm‐associated current wedges. During this quasi‐steady state the tail reconnection rate appears nearly to balance the dayside merging rate, allowing most energy extracted from the solar wind to be released continuously rather than to be first stored in the tail. It seems that a large‐scale substorm expansion may be triggered if this convection mode is perturbed, for instance, by a significant reduction of the imposed convection electric field.

Characteristics of the association between the interplanetary magnetic field and substorms

The geomagnetic response to changes in the orientation of the interplanetary magnetic field (IMF) has been investigated for 18 IMF events. These events consisted of clear southward shifts of the IMF when the IMF Bz(GSM) component had been northward for more than 2 hours. It was found that when the IMF thus shifted southward and remained southward for at least 2 hours, a magnetospheric substorm always ensued. Several properties of this subsequent geomagnetic activity were determined to be associated with IMF parameters. The amplitude of auroral negative bays was confirmed to be a function of the southward IMF flux preceding the onsets. Auroral bay activity was also observed to cease abruptly coincident with permanent northward recoveries in the IMF. Finally, it was observed that many of the ground expansion onsets were associated with either IMF northward fluctuations or partial northward recoveries, which we interpret as indicative of the existence of a class of IMF-triggered substorms.

Correlation between modulation of the intensity of precipitating electrons in the auroral zone and a coincident Pi 2

Modulation of electrons precipitated at the time of a negative bay at Kiruna and the coincident Pi observed at Lerwick are compared. Both waveforms show a dominant spectral line at approximately 82 sec and a secondary peak at 48–50 sec. The correlation between the two is significant enough to imply that electron precipitation and field line oscillation are related in such a way that field line inflation (compression) corresponds to increase (decrease) in the flux of precipitating particles.