Ingrid Sandahl

The auroral distribution and its mapping according to substorm phase

Abstract An attempt is made to reconcile two competing views as to where the auroral distribution maps from in the magnetosphere. The structure of the aurora is shown to have two distinctive parts which vary according to the magnetic activity. The low latitude portion of the structured distribution may be a near-Earth central plasma sheet phenomenon while the high latitude portion is linked more closely to boundary layer processes. During quiet times, the polar arcs may be the ionospheric signature of a source region in the deep tail low latitude boundary layer/cool plasma sheet. The structured portion of the ‘oval’ has a dominantly near-Earth nightside source and corresponds to an overlap region between isotropic 1–10 keV electrons and 0.1–1 keV structured electrons. The ionospheric local time sector between 13 and 18 MLT is the meeting point between the dayside boundary layer source region and this near-Earth nightside source. Late in the substorm expansion phase and/or start of the substorm recovery phase, the nightside magnetospheric boundaries (both the low latitude and Plasma Sheet Boundary Layers) begin to play an increasingly important role, resulting in an auroral distribution specific to the substorm recovery phase. These auroral observations provide a means of inferring important information concerning magnetospheric topology.

Electron populations above the nightside auroral oval during magnetic quiet times

Abstract In several studies of particle morphology above the nightside auroral oval, the electrons have been divided into two separate spatial regions, often called the BPS (from “boundary plasma sheet”) and the CPS (from “central plasma sheet”) (Winningham et al., 1975, J. geophys. Res.80, 3148). The names were derived from the source regions suggested by Winningham et al. In many cases this classification has worked well, but there are also many cases in which it has not. In this paper an alternative classification is proposed and explored by investigating the spatial distribution of electrons at altitudes between 2000 and 13,500 km, using particle spectrograms from the Viking satellite. A major difference between the newly proposed and the earlier classification is that spatial regions of populations may overlap in this new scheme. Electrons above the auroral oval could be divided into two populations. The first one is spatially unstructured and has a characteristic energy of a few kiloelectron volts. It is usually trapped in its equatorward part, while it is isotropic in its poleward part. The second one is spatially structured and normally has a characteristic energy of 100 eV or less. It is always present when there are signs of electron acceleration along magnetic field lines. The global distributions of both the structured and the unstructured electrons are ring-shaped. The two regions partially overlap, and the average latitude of the structured electrons is higher than the average latitude of the unstructured electrons. The majority of bright auroras appear in the region of overlap. The average poleward edge of the overlap region seems to coincide with the average poleward edge of region 1 field-aligned currents. We suggest that this boundary maps to the boundary between the central plasma sheet and the plasma sheet boundary layer. We also suggest that the sources for the region where only structured electrons are present are the low-latitude boundary layer and plasma sheet boundary layer. The conclusions concerning source regions are supported by mapping of the particle population regions into the equatorial plane of the magnetosphere using the Tsyganenko (1987, Planet. Space Sci. 35, 1347) magnetic field model. The average boundary between region 1 and region 2 field-aligned currents in the afternoon and evening is approximately at the average equatorward boundary of unstructured electrons. Through the midnight, morning and prenoon sectors it is at the average equatorward boundary of structured electrons.

Cusp and boundary layer observations by INTERBALL

Abstract Interball Tail Probe was launched on August 2, 1995 together with its subsatellite MAGION-4 into a highly elliptical orbit with apogee at 31 RE and inclination 63 degrees. During the course of one year all local times are visited. In this paper we will present initial results obtained from measurements in the cusp, mantle and eveningside plasma sheet in January, 1996. It is found that the cusp was well defined and persistent at altitudes of 4–10 RE. In one case both the main satellite and the subsatellite were in the cusp proper for two uninterrupted hours. We believe this to be the first ever multi-point satellite observation of the high-altitude cusp. The data indicate that the cusp was very stable with a wide entry area and that plasma entry took place at high latitudes rather than at the subsolar point. Pressure pulses, possibly due to Alfven waves were found. Sunward and antisunward moving plasma was measured simultaneously during a case of northward IMF but no convection was discovered. Plenty of plasma of cusp/magnetosheath type was also found mixed with plasma sheet plasma both equatorward of the cusp and in the eveningside plasma sheet.

Dispersive magnetosheath‐like ion injections in the evening sector on January 11, 1997

While the plasma cloud of the January 1997 CME event was passing the Earth, the Interball Auroral Probe (Interball-2) crossed the duskside auroral oval toward a contracted polar cap. The PROMICS-3 plasma instrument observed several consecutive dispersion ramps of magnetosheath-like protons over a wide range of magnetic local times (16–21 MLT). The last dispersion ramps were observed at later local times than previously reported, likely caused by the extreme conditions during a period when the magnetosphere was immersed in the dense plasma cloud. The Polar/VIS images of the oval show a very contracted polar cap. During the first clear dispersion event energetic oxygen ions were detected. They were also observed by the the particle detectors of Polar which was in close conjugation with Interball-2 right at this moment. We suggest that the dispersion ramps are signatures of ion injections formed by impulsive entry of magnetosheath ions through the dusk-side flank into the magnetosphere during a period of strongly northward IMF and contracted polar cap.

INTERBALL magnetotail boundary case studies

Abstract We present two examples of INTERBALL-1 data near both the high and low-latitude tail magnetopause (MP) under disturbed conditions. For the high-latitude case, MAGION-4 data determine the scales of the MP current sheets which are in the order of 100–500 km for the main ones, 50–200 km for Flux Transfer Events (FTEs) and a few km for the fine structures and ULF turbulence. The MP speed was 15–30 km/s. The energetic protons in the magnetosheath (MSH) provide evidence of reconnection upstream of the spacecraft (S/C). The tailward flows grow for the northward MSH magnetic field when the reconnection site is believed to be shifted tailward of the cusp. The inner boundary layer (BL) after the disturbance consists of tailward and earthward flowing plasma of MSH origin and cold mantle plasma flowing tailward The earthward flow is evidence of reconnection tailward of the S/C, which is regarded as a specific feature of the disturbed conditions. Local production of a plasma-sheet-like plasma at high latitudes is argued based on the inner BL plasma characteristics. The following features are observed in both cases: (a) FTEs for both northward and southward MSH fields; (b) waves in the current sheet vicinities over ten mV/m and 15 nT peak-to-peak; (c) electron fluxes with scales down to a few km with extra heating especially parallel to the magnetic field; (d) outer turbulent boundary layers with a deflected magnetic field; (e) ions with time-energy dispersion-like features and deflected ion fluxes. In the downstream dawn region at the transition between the low-latitude boundary layer and the plasma sheet (LLBL/PS), multiple MP encounters are observed. In the LLBL parallel electron intensifications correlate with ULF magnetic fluctuations.

Measurements of O+ in the high latitude magnetosheath

Abstract The Prognoz-7 satellite was in operation for about 8 months and the data set includes about 50 orbits where the satellite passed through the magnetopause and the magnetosheath at high latitudes. Most of the data set of hot plasma (0.5–15 keV) measurements from the satellite was surveyed for this study. Particular attention was given to different ion species in the high latitude magnetosheath region. On many occasions the instrument detected O+ in the magnetosheath, which we believe to be of ionospheric origin. Several cases of large and persistent O+ fluxes at the same energy as the shocked solar wind plasma were detected, while on other occasions the O+ ions of higher energy during a shorter period of time could be seen. The data are interpreted as evidence that mechanisms other than magnetic field merging are also necessary to explain the cases of persistent O+ fluxes in the high latitude magnetosheath. The lack of correlation between IMF orientation and O+ in the magnetosheath suggests that merging rate is not the main limiting factor, but that the levels of available magnetospheric O+ controls the occurrence of detected O+.

The CUSP as a source of plasma for the magnetosphere

Abstract For the magnetosphere as a whole, the cusp, or even the dayside is relatively insignificant as a plasma source, but for the magnetosphere near the Earth the situation is different. Most important, quantitatively, is that the cusp acts as a gateway for magnetosheath plasma. Evidence is found in the data for different entry mechanisms; sub-solar point reconnection, merging in anti-parallel fields at different locations on the dayside magnetopause, and more direct entry at high latitudes through the turbulent boundary layer. Much remains to be understood regarding the relative importance of different source regions and mechanisms. Plasma also enters the cusp from the ionosphere at the bottom of the cusp. This region is very active, but due to its small size, the overall importance of the ionosphere as a cusp plasma source is not very big. It is, however, very interesting from both a plasma physics and a magnetospheric physics point of view. Via magnetospheric plasma circulation the cusp and the rest of the magnetosphere exchange particles. Interball data can be used to study many of the processes in both the high- and mid-altitude cusp and offer a solar cycle minimum complement to the Cluster solar cycle maximum data.

Recent cusp and cleft results from interball

Abstract The Interball project has given important contributions to our understanding of the morphology and the physical processes in the cusp and cleft. Interball Tail and Magion-4 have performed more extensive measurements in the high altitude cusp than any previous spacecraft. Interball has also been a part in the ISTP program and data have been used in many multipoint studies. In this paper recent cusp and cleft studies based entirely or partly on Interball data will be reviewed. Interball data show that processes at high latitudes are very important for plasma entry into the magnetosphere. A case study for southward IMF conditions agrees with a model in which the mantle is populated via entry along open high-latitude field lines. A statistical study of events dominated by IMF B y shows that merging in anti-parallel fields, rather than subsolar point reconnection, populates the mantle. Plasma entry also takes place through the turbulent boundary layer, TBL, a region of strong, Alfvenic ULF turbulence above the cusp and cleft. The TBL is almost always present. It extends tailward from the cusp and is proposed to be related to the magnetospheric sash. For the overall magnetosheath plasma entry into the magnetosphere the magnetotail boundary is probably more important than the cusp. The position of the cusp is controlled by the solar wind in a similar way as the low altitude cusp. The mid-altitude cusp was found to maintain its fine structure over periods of the order of one hour. A suprathermal proton population not previously described has been detected in the mid-altitude cusp.

Fine structure of aurora

Fine structure is present in most types of aurora, but much of it has previously not been possible to study properly because of instrument limitations. However, recent advances in optical instrumentation have provided considerable improvements in temporal and spatial resolution. Optical measurement systems are able to use a higher resolution than other types of ground-based instruments used in auroral studies. New results have been obtained regarding, for example, elemental structures in discrete auroras, generation of flickering aurora, generation of Alfvén waves in shear regions, dynamic rayed aurora, fine structure of diffuse auroras and fine structure of auroral curls. Outstanding questions are highlighted and recommendations for future research are given. The importance of a coordinated infrastructure for ionospheric research, simultaneous measurements on different scales, optical calibration facilities and the development of time-dependent high-resolution models is stressed.

The High- and Low-Latitude Boundary Layers in the Magnetotail

Interball has unique possibilities for boundary layer studies. In one year Interball-1 systematically scans all local times, crossing the magnetopause at high latitudes outbound (ZSE in the range 10-17 RE) and near the equatorial plane inbound. Most other spacecraft, which have been used for boundary layer studies in the tail, for example ISEE 1 and 2 and Geotail, have been equatorial.