Premidnight Sector Research Articles

The correlation of AKR waves with precipitating electrons as determined by plasma wave and X‐ray image data from the POLAR spacecraft

Simultaneous measurements are presented of the intensities of AKR waves in the 60 kHz to 800 kHz range and the local time distributions of the fluxes of electrons precipitating into the atmosphere using wave and bremsstrahlung x‐ray data acquired on the POLAR spacecraft. The images of 2 to 12 keV x‐ray emission measured with the PIXIE spectrometer were used to obtain the positions and intensities of the electron precipitation regions. Electric field wave measurements of the AKR were obtained with the Plasma Wave Instrument (PWI). Data are presented from a satellite pass on 19 April 1996 in which there were several short term enhancements in the intensities of both waves and x‐rays. The AKR electric field strengths were correlated with x‐rays emitted over a six hour local time range in the pre‐midnight sector with a correlation coefficient of 0.51. At the times of some of the rapid increases in the x‐ray fluxes the AKR intensities exhibited similar sharp onsets.

Thermospheric meridional winds in the vicinity of the auroral zone: observations and modelling

Optical Fabry-Perot interferometer measurements of the Doppler shift of the 557.7 nm OI emission were used to obtain the meridional thermospheric wind velocities at heights of the E-layer in the pre-midnight sector of the ionosphere at Loparskaya (68 °N, 33 °E geographic). They distinctly reveal a decrease in the mean meridional wind velocity when passing across the auroral zone from the pole to the equator, and also the near-counterphase character of the wind velocity fluctuations on both sides of the precipitation zone. The observed behaviour of the meridional wind has been reproduced in the modelling calculations using the global ionosphere-thermosphere model and interpreted in terms of the ion-neutral coupling due to the momentum interchange and generation of atmospheric internal gravity waves.

Synoptic auroral distribution: A survey using Polar ultraviolet imagery

The global distribution of the ultraviolet auroral emission was investigated for the period between April and July 1996 using over 17,000 imagery acquired by the ultraviolet imager (UVI) on board the Polar satellite. Average brightness of the N2 Lyman‐Birge‐Hopfield (LBH) auroral emissions at 1700 Å, which is approximately proportional to the total energy flux of precipitating electrons, was calculated with dayglow subtracted. The results of this investigation indicate that there exist two distinctive auroral emission regions, one in the premidnight sector of the auroral oval and one in the postnoon sector of the auroral oval. The maximum occurrence of nightside aurorae is found to be centered at 2230 magnetic local time (MLT) and 68° magnetic latitude (MLAT) while the dayside aurorae maximize at both 1500 MLT and 75° MLT and 1000 MLT and 75° MLAT, with the later one much weaker. This statistical auroral distribution is quite similar to previously reported distribution of discrete aurorae, suggesting that at this wavelength and at the sensitivity of the UVI detector, discrete aurorae contribute a major portion of the total emissions. The seasonal distribution of the nightside LBH auroral emissions is found to be consistent with previously reported particle result, namely nightside discrete auroral activities are more common in the dark hemisphere (winter) than in the sunlit hemisphere (summer). However, on the dayside part of auroral oval, auroral emissions are brighter in summer than in spring. The dayside auroral emissions, in particular the 1500 MLT bright spots, are also found to be correlated with the maximum region 1 upward field‐aligned currents which are most intense in summer because of a higher ionospheric conductivity produced by photoionization in the dayside region. These results point up the controlling role played by ionospheric conductivity and further illustrate how dayside and nightside aurorae behave in fundamentally different ways.

Premidnight plasmaspheric “plumes”

To explain observations of brief intervals of cold, dense plasma by geosynchronous satellites in the midnight sector prior to or during substorm onset, it has recently been proposed that dense plasmaspheric plasma is drawn out to geosynchronous orbit in the premidnight region by inductive electric fields during the growth phase of a geomagnetic substorm. We present here the results of a statistical study of such intervals observed with the Los Alamos magnetospheric plasma analyzer (MPA) on geosynchronous satellite 1989‐046 between March and December 1993. We find that these premidnight cold plasma intervals occur only after extended periods of low magnetospheric activity identified by Kp and the midnight boundary index (MBI). We also find that the satellite typically enters the cold plasma region from the trough region and exits it into the plasma sheet. Finally, while measurements of the flow velocity of the cold plasma are rendered uncertain by the asymmetric spacecraft charge or photoelectron sheath, such measurements show no evidence of the outward flow that would be expected from the extrusion hypothesis. Rather, there are some indications that these cold plasma regions flow sunward in the (corotation) satellite frame. These results suggest an alternative explanation for the premidnight cold plasma: corotation‐dominated transport of day side plasmaspheric structures into the premidnight sector. The implications of our observations for the extrusion hypothesis and for the alternative explanation are discussed.

Observations of a current pulse in the near‐Earth plasma sheet associated with a substorm onset

The onset of an isolated magnetospheric substorm occurred at about 0437 UT on 9 February 1995 while the Geotail spacecraft was located in the premidnight sector at a geocentric radial distance of about 30 RE. The onset occurred during a slow northward turning of the interplanetary magnetic field after a period of weakly southward fields as observed by the Wind spacecraft upstream from Earth's bow shock. For several minutes immediately following the onset of the substorm an intense field‐aligned current, with maximum current densities of 30 nA/m², was observed at the Geotail spacecraft. This is the first report of a directly detected current pulse coincident with onset. Unlike most of the magnetotail field‐aligned currents which are carried primarily by electrons, this impulsive current at onset is carried by both electrons and protons. It is possible that the onset current pulse is a signature of a “trigger” mechanism for the subsequent expansion phase of a substorm.

Characteristics of downward flowing ion energy dispersions observed in the low‐altitude central plasma sheet by Akebono and DMSP

We present characteristics of downward flowing ion (DFI) energy dispersions observed in the low‐altitude central plasma sheet by two polar‐orbiting satellites: Akebono and DMSP F8. In general, the typical energy decreases with decreasing latitude. Their main ion composition frequently consists of singly charged oxygen, suggesting that the energy dispersing ions are of ionospheric origin. We conclude that the energy‐dispersed signature represents a spatial structure, produced mainly by the global plasma convection driven by the E × B drift on the closed field lines. A probable source of these DFI energy dispersions is the upward flowing ion (UFI) beam accelerated by parallel electrostatic potential at low altitudes. Some case studies show that the energy ratios of the multiple (overlapped) energy dispersions and the pitch angle distributions have some important characteristics consistent with features expected from this model. Clear DFI energy dispersions are observed frequently in stable eastward convection in the postmidnight region, rather than in the premidnight sector. Although the occurrence frequency is different from the local time dependence of the UFIs as reported previously, this discrepancy is due probably to some differences of the convection patterns in the premidnight and postmidnight sectors. The clear energy dispersions might be smeared out by the distorted or complicated convection pattern, irregularities, and disturbances frequently seen in the premidnight sector.

Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude

Abstract. A statistical analysis of the power spectra of the geomagnetic field components H and D for periods ranging between 3 min and 1 h was conducted at a low-latitude observatory (L'Aquila, L=1.6) at the minimum and maximum of the solar cycle. For both components, during daytime intervals, we found evidence of power enhancements at frequencies predicted for global modes of the Earth\\'s magnetosphere and occasionally observed at auroral latitudes in the F-region drift velocities (approximately at 1.3, 1.9, 2.6, and 3.4 mHz). Nighttime observations reveal a relative low frequency H enhancement associated with the bay occurrence together with a peak in the H/D power ratio which sharply emerges at 1.2 mHz in the premidnight sector. The strong similarity between solar minimum and maximum suggests that these modes can be considered permanent magnetospheric features. A separate analysis on a two-month interval shows that the observed spectral characteristics are amplified by conditions of high-velocity solar wind.

Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude

A statistical analysis of the power spectra of the geomagnetic field components H and D for periods ranging between 3 min and 1 h was conducted at a low-latitude observatory (L'Aquila, L=1.6) at the minimum and maximum of the solar cycle. For both components, during daytime intervals, we found evidence of power enhancements at frequencies predicted for global modes of the Earth\'s magnetosphere and occasionally observed at auroral latitudes in the F-region drift velocities (approximately at 1.3, 1.9, 2.6, and 3.4 mHz). Nighttime observations reveal a relative low frequency H enhancement associated with the bay occurrence together with a peak in the H/D power ratio which sharply emerges at 1.2 mHz in the premidnight sector. The strong similarity between solar minimum and maximum suggests that these modes can be considered permanent magnetospheric features. A separate analysis on a two-month interval shows that the observed spectral characteristics are amplified by conditions of high-velocity solar wind.

Ground‐satellite correlation of low‐latitude Pi 2 pulsations: A quasi‐periodic field line oscillation in the magnetosphere

Magnetometer data at dip‐equator and at geosynchronous altitudes (GOES 5 and 6 satellites) and energetic particle data from geosynchronous spacecraft (1982‐019, 1984‐037, 1984‐129) are studied to investigate field line and particle signatures associated with ground Pi 2 onset. Pi 2 pulsations observed at the ground station are discussed as field disturbances that axe launched in the midnight sector of the magnetosphere in association with field reconfiguration at substorm onset. In the geosynchronous orbit, both quasi‐periodic field line oscillations and particle injections are often observed in the nightside sector in association with the onset of ground Pi 2 pulsations. Of 216 Pi 2 events selected in this report, 16% showed associated quasi‐periodic field line oscillations. The dominant periods of the oscillations in space are similar to but not equal to those of the corresponding ground Pi 2 pulsations. The oscillations in space are polarized primarily in the azimuthal direction with a dc offset that develops to form a field‐aligned current signature, namely, eastward (westward) directed in the premidnight (postmidnight) sector. Simulation of forced field line oscillations has been carried out by adding the sudden increase in plasma pressure in the equatorial midnight sector region that is expected to come from particle injection at substorm expansion onset. We discuss the relevance of the quasi‐periodic oscillations to plasma injections at geosynchronous altitudes with reference to the result of the model calculations.

Relationship of theoretical patch climatology to polar cap patch observations

During a southward orientation of the interplanetary magnetic field (IMF), patches are often observed moving antisunward across the polar cap. In saying “patches” we refer to structures in which the F region electron densities are enhanced relative to lower background levels; we do not in this paper consider patches which are observed optically (see J. J. Sojka et al., Ambiguity in identificiation of polar cap F region patches, submitted to the Journal of Atmospheric and Terrestrial Physics, 1995). The patches can be modeled by a process which involves the “chopping up” of the tongue of ionization (TOI) [Sojka et al., 1993a]. Various mechanisms for chopping the TOI have been suggested; our preferred method is to introduce temporal changes in the convection electric field pattern. In any case the present study is quite independent of any particular mechanism, so long as the TOI is considered to be the source of the patches. In this study we have used the Utah State University Time‐Dependent Ionospheric Model (TDIM) to model the TOI for various IMF By orientations. In our simulations the location of the TOI in the polar cap is mainly determined by the IMF By component, and hence the patch locations are also expected to be By dependent. This suggests that a polar ground‐based instrument may not see patches even when they are present in the polar ionosphere. This is because the typical field of view of a ground‐based instrument, such as an all‐sky camera, covers less than 10% of the polar region. The TDIM simulation results were used to predict the By dependence of patches that different ground‐based sites would observe. Eureka (Canada) at the magnetic pole is predicted not to observe patches for southward IMF conditions if the By component is strongly negative. Sondrestrom (Greenland) and NyAlesund (Svalbard), although at similar cusp latitudes, are expected to see quite different diurnal responses to patches. At Sondrestrom, patches are seen at noon in winter; both sites should see them in the premidnight sector. These model predictions are the “groundwork” for detailed patch observation‐model comparisons at all three sites.

Ionospheric signature of the tail neutral line during the growth phase of a substorm

An isolated substorm occurred on October 21, 1981 at the end of a large geomagnetic storm. Observations of particles and fields were made during the presumed growth phase of this substorm by the DE 2 satellite. The ionospheric convection at the time indicates a pattern commonly observed during the maximum of substorms with convection velocities averaging ∼500 m/s in an eastward channel of flow in the premidnight sector. Despite the convection signature, geomagnetic conditions were quiet with the AL index >−50 nT and very weak field‐aligned current. We will present an in‐depth study of the nightside gap region within the channel of eastward flow. We will discuss the possible effects of a neutral wind flywheel on the convection. Also, we will present a detailed analysis of the electrodynamic structures within the nightside gap which indicates a magnetospheric source. Possible ionospheric signatures of the tail neutral line are discussed.

Substorm correlated absorption on a 3200 km trans-auroral HF propagation path

Abstract. A high-frequency transmitter located at Clyde River, NWT, Canada, and a receiver located near Boston, USA, provide a 3200 km trans-auroral, near-meridional propagation path over which the propagation characteristics have been measured. Out of the fourteen frequencies in the HF band sampled every hour for the duration of the experimental campaign (16 January–8 February 1989), the signal level measurements of 6.800 MHz transmissions were selected in order to determine the extent and occurrence of auroral absorption. The median level of auroral absorption along the path is found to increase with geomagnetic activity, quantified by the index Kp, with the increase being greater in the post-midnight sector than in the pre-midnight sector. This asymmetric behaviour is attributed to the precipitation of high energy electrons into the midnight and morning sector auroral D region. The measured diurnal variation in the median level of absorption is consistent with previous models describing the extent and magnitude of auroral absorption and electron precipitation. Individual substorms, identified from geosynchronous satellite data, are found to cause short-lived absorption events in the HF signal level of ~30 dB at 6.800 MHz. The occurrence of substorm correlated auroral absorption events is confined to the midnight and morning sectors, consistent with the location of the electron precipitation. The magnitude of absorption is related to the magnetotail stress during the substorm growth phase and the magnetotail relaxation during the substorm expansion phase onset. The absorption magnitude and the occurrence of substorms during the period of the campaign increase at times of high Kp , leading to an increase in median auroral absorption during disturbed periods.

Substorm correlated absorption on a 3200 km trans-auroral HF propagation path

<strong class="journal-contentHeaderColor">Abstract.</strong> A high-frequency transmitter located at Clyde River, NWT, Canada, and a receiver located near Boston, USA, provide a 3200 km trans-auroral, near-meridional propagation path over which the propagation characteristics have been measured. Out of the fourteen frequencies in the HF band sampled every hour for the duration of the experimental campaign (16 January–8 February 1989), the signal level measurements of 6.800 MHz transmissions were selected in order to determine the extent and occurrence of auroral absorption. The median level of auroral absorption along the path is found to increase with geomagnetic activity, quantified by the index <i>K_p</i>, with the increase being greater in the post-midnight sector than in the pre-midnight sector. This asymmetric behaviour is attributed to the precipitation of high energy electrons into the midnight and morning sector auroral D region. The measured diurnal variation in the median level of absorption is consistent with previous models describing the extent and magnitude of auroral absorption and electron precipitation. Individual substorms, identified from geosynchronous satellite data, are found to cause short-lived absorption events in the HF signal level of ~30 dB at 6.800 MHz. The occurrence of substorm correlated auroral absorption events is confined to the midnight and morning sectors, consistent with the location of the electron precipitation. The magnitude of absorption is related to the magnetotail stress during the substorm growth phase and the magnetotail relaxation during the substorm expansion phase onset. The absorption magnitude and the occurrence of substorms during the period of the campaign increase at times of high <i>K_p </i>, leading to an increase in median auroral absorption during disturbed periods.

A Comparison of the Occurrence of Very-Low-Latitude Pi 2 Pulsations with Magnetic-Field and Energetic-Particle Flux Variations (30-300 keV) at Geosynchronous Altitudes.

The occurrence of Pi 2 pulsations (20-150 s) at very-low-latitude stations is compared with the energetic-particle and magnetic-field variations at geosynchronous orbit during the PROMIS (Polar Region and Outer Magnetosphere International Study) period (March 10-June 16, 1986). Ground magnetometer data from Huancayo, Peru (dip equator, 75°W), and Kuju, Japan (L = 1.2, 135°E), are utilized to monitor the magnetic pulsations, while the particles (electron, 30-300 keV; proton, 145-560 keV) and magnetic fields in space are monitored by geosynchronous satellites S/C1982-019, S/C1984-129, S/C1984-037, GOES-5, and GOES-6. Of 175 Pi 2 events surveyed during the PROMIS period, 110 events are accompanied by electron flux enhancement, and 51 events have a concurrent onset of the field-aligned current (FAC). The FAC events correlated with ground Pi 2 pulsations indicate a local time change in polarities. If currents are assumed to flow along the field lines tailward of the geosynchronous orbit, the polarities are upward in the premidnight sector and downward in the postmidnight sector. We classified Pi 2 events in the nightside sector in several different ways. The results imply that if Pi 2 events in the present study can be assumed to be of substorm origin, then 47% of Pi 2 events occur in regions close to geosynchronous altitude.

A substorm observed by EISCAT and other ground-based instruments — evidence for near-Earth substorm initiation

We present observations of the development of a substorm in the ionosphere made by the EISCAT radar, optical and magnetic instruments. A typical growth phase started 30 min before the substorm onset with equatorward drifting arcs. The arc which crossed the EISCAT beam drifted equatorward with approximately the same velocity as the ambient plasma and it approached the most equatorward discrete arc. The most equatorward discrete arc (the breakup arc) stopped its drift at L = 5.3. Arcs about 1° of latitude poleward of the breakup arc continued their equatorward motion with no significant changes in intensity after the onset of the substorm, which was timed on the basis of the explosive intensification in the breakup arc. Similarly, no dramatic changes in electric fields or other plasma parameters measured by EISCAT were observed poleward of the breakup arc. The observations indicate that the instability that triggered the substorm onset was localized in the near-Earth magnetotail. A westward travelling surge (WTS) developed in the breakup arc and moved westward with a very high velocity, 13 ± 3 km s −1. Extremely high conductances were measured by EISCAT from the poleward boundary of the poleward expanding bulge with a maximum value of Σ H = 214 S obtained with a time resolution of 0.2 s. The westward electrojet (WEJ) was observed to be latitudinally very inhomogenous and concentrated near conductivity enhancements, especially close to the head of the WTS. The localization of the WEJ close to discrete arcs and consequent motions with the arcs gave the charasteristic spiky appearance of the magnetic X-component in the pre-midnight sector.

Radar observations of auroral zone flows during a multiple-onset substorm

Abstract. We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every ~12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the pre-existing extended current system driven by the large-scale flow. These filaments are ~1 h MLT wide (~600 km), and initially expand poleward to a width of ~300 km before contracting equatorward and coalescing with the pre-existing current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s–1 or less during the first few minutes, before surging equatorward at 0.5–1.0 km s–1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed ~100 mho, before decaying over a few minutes. This value compares with Hall conductivities of ~20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.

Comparison of Viking onset locations with the predictions of the Thermal Catastrophe Model

In the thermal catastrophe model of substorms, onset is attributed to an explosive heating of the plasma sheet boundary layer (PSBL) due to the resonant absorption of Alfvèn waves. These waves are assumed to have their origins at the magnetopause where solar‐wind‐driven Kelvin‐Helmholtz surface waves launch magnetoacoustic waves through the lobes toward the PSBL which are then mode‐converted to kinetic Alfvèn waves in the PSBL. On the basis of this wave source mechanism, the thermal catastrophe model predicts the locations in the tail where catastrophe and hence onset is most likely to occur. In this paper, the model predictions are extended to include the effects of a realistic tail field (represented by the Tsyganenko 1987 long model) and are compared to observations of substorm onset as determined from Viking auroral images. The use of a realistic tail field causes a deflection of the catastrophe domain towards the dusk (dawn) flanks of the magnetosphere for gardenhose (orthogardenhose) interplanetary magnetic field (IMF) conditions, and it is also found that this region extends into the deep tail only for higher solar wind speeds, thereby confining catastrophe to the relatively near‐Earth portions of the tail for typical conditions. Most of the other results obtained with the inclusion of a realistic tail field produce trends similar to the original model predictions including the results that onset should get closer to midnight for increasing solar wind speed and that onset should occur in the premidnight (postmidnight) sector for gardenhose (orthogardenhose) IMF conditions. Although a weak consistency with measured onset locations is observed, many more events than are contained in the Viking database are required in order to provide a firm conclusion. However, the reasonably well established result that onset occurs very close to the Earth on closed field lines is inconsistent with the predictions of the model which places onset on PSBL field lines. Thus the thermal catastrophe model is not a likely mechanism for typical substorm onsets. Nevertheless, evidence is shown that the mechanism may still operate in the magnetosphere to produce a different type of auroral intensification.

Observations in the vicinity of substorm onset: Implications for the substorm process

Multi‐instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty‐six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307±115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity for 80 onsets was 65.9±3.5 CGMlat, 22.9±1.2 Mlt, whereas the average location of the AAF onsets was at 63.8±3.3 CGMlat, 22.9±1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low‐latitude wavelike onsets appear as precursors in the form of long‐period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region and the distance to the open‐closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low‐ and high‐altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross‐tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

Phase scintillation and irregularities in the northern polar ionosphere

Radio transmissions from Navy Navigation Satellite System (NNSS) satellites have been used to study phase scintillation and ionospheric irregularities at northern polar latitudes. The observations were made at Ny Alesund, a station in the vicinity of the daytime cusp and under the polar cap on the nightside. The analysis concentrates on data obtained over an extended period when more than 7000 satellite passes were recorded to provide estimates of the σф phase scintillation parameter for some 300,000 data intervals. The results show a marked difference in scintillation and irregularity occurrence with season. During the summer months the main feature is a maximum around magnetic noon, apparently associated with irregularities in the cusp region. In winter, irregularities are found at all times of day though there is a noon maximum that extends in a latitudinal belt into the afternoon and evening. The premidnight sector shows the highest occurrence levels in the polar cap. Investigation of signal fading rates indicates that doppler shifting resulting from irregularities convecting rapidly towards the station in the premidnight polar cap may contribute to the occurrence patterns. Under quiet geomagnetic conditions during summer the irregularities form a belt essentially in the morning sector, following the auroral oval. There is expansion to lower latitudes with increased geomagnetic activity and increased occurrence in the afternoon. In winter, irregularities can be seen at all hours even under quiet conditions, with the afternoon and evening showing greatest occurrence at times of geomagnetic disturbance. The pattern of scintillation occurrence with respect to the station indicates that the irregularities may be in the form of field‐aligned sheets of low axial ratio. In contrast to observations of L shell confined sheets at auroral latitudes there are indications from the current work that the irregularities are stretched in the antisunwards convective flow across the polar cap.

Radar observations of auroral zone flows during a multiple-onset substorm

We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every ~12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the pre-existing extended current system driven by the large-scale flow. These filaments are ~1 h MLT wide (~600 km), and initially expand poleward to a width of ~300 km before contracting equatorward and coalescing with the pre-existing current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s–1 or less during the first few minutes, before surging equatorward at 0.5–1.0 km s–1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed ~100 mho, before decaying over a few minutes. This value compares with Hall conductivities of ~20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.