What determines the intensity of magnetospheric substorms?

One of the central issues in substorm research is what determines the substorm intensity. Through an introduction on what constitutes a magnetospheric substorm, we discuss several parameters which are available to measure the substorm intensity. In terms of ionospheric quantities, we have the auroral electroject indices, the total current in the westward auroral electrojet, the area of bright aurora, the maximum poleward advance of the auroral bulge, and the duration of auroral substorm activities. In terms of magnetospheric quantities, we have the innermost location of the substorm injection boundary and the amount of current reduction in the cross-tail current within the substorm current wedge. A measure reflecting substorm activities in both the ionosphere and the magnetosphere is the total substorm energy dissipation but its drawback lies in the difficulty of assessing it accurately if the energy loss due to plasmoids is to be included. We also discuss the predictability of substorm intensity, which leads us to the issue of whether a substorm is a directly-driven or an unloading process. The recent success in predicting the auroral electrojet index from solar wind parameters with a cross-correlation of ~ 0.9 suggests that substorm activities over a long time scale are primarily directly-driven while those over a short time scale are governed by impulsive unloading processes. This understanding allows us to reconcile the apparently conflicting dual nature of magnetospheric substorms.     

Low latitude signatures of substorm activity

Although magnetospheric substorms have their most easily detectable observable consequences in the high latitude auroral oval, most of the early studies of the phenomenon took place using data from low latitude observing sites. Many of these early studies concentrated on magnetic storms, yielding observations and interpretations of the ring current and sudden storm commencements which are valid to this day. Over the years, ground-based observations have been concentrated in the high latitude regions under and immediately adjacent to the auroral oval and studies of storm and substorm phenomena using low latitude data have been few in number. Despite the intensity of research activity in recent years, the physics of the substorm process still remains a matter of controversy. The STEP period represents an excellent opportunity for substorm physicists to solve some of the outstanding problems they still face. Space scientists monitoring the geospace environment at observing sites well equatorward of the auroral oval have an excellent opportunity to contribute to the ultimate solution of the substorm problem during the STEP interval, however they will have to focus on specific observational manifestations of substorms if they are to optimize their probability of success. In this paper I shall try to point out some areas where successful observations and interpretation of substorm phenomena at low latitudes would be extremely useful in helping the substorm community resolve their outstanding scientific problems.     

Magnetospheric tail structure: concepts,problems and storm-time development of the auroral oval

This paper reviews current knowledge on links between the Earth's magnetic tail and the auroral oval, and identifies some problems remaining. It considers electrons as tracers of the geomagnetic field, boundaries between different regions, plasma flows, and pressure balance conditions. The auroral arc is considered as a standing discontinuity in the flow of central plasma sheet (CPS) plasma field-aligned current systems are also proposed. The plasma instability responsible for the breakup phase of an auroral substorm is also discussed.     

Dynamics of auroral electrojets and energetics of substorms

The dynamics of westward auroral electrojets in the course of magnetospheric substorms is studied according to the data of a meridional chain of magnetometers. It is shown that, during active phases of the substorm, the westward electrojet becomes inhomogeneous and some current filaments appear in it; some of them drift polewards, some other shift equatorwards.A method is proposed to estimate both the potential and curl parts of the magnetospheric electric field, the value of the electromagnetic energy entering the plasma sheet in the magnetotail, and the rate of Joule heating in the ionosphere based on data on the dynamics of the auroral electrojets.     

Identification of the magnetospheric cusp and cleft using Pc1–2 ULF pulsations

Geomagnetic pulsations in the 0.1–2.5 Hz (Pc1–2) range recorded over 12 quiet summer days at six Antarctic stations between −62.3 and −80.6° invariant latitude were examined in order to map the spatial and temporal distribution of spectral characteristics. Ionospheric particle signatures associated with the magnetospheric cusp and boundary layer were deduced for three of these days using ground riometer, magnetometer and ionosonde measurements, and in-situ ionospheric particle data. Comparison with the magnetic pulsation data shows that specific Pc1–2 emissions are associated with these regions. Within the cusp, intense unstructured ULF noise in the 0.15−0.4 Hz range is observed. Less intense waves of this type are seen near the cusp location on mantle and plasma sheet boundary layer flux tubes. These emissions are quite distinct from the discrete, structured and narrowband emissions seen equatorward of the cusp. Whereas past discussions of cusp and cleft identification have usually focused on optical or satellite data, we conclude that ground-based observations of Pc1–2 pulsations can provide a more convenient, although less precise, monitor of high latitude features.     

Space-time structure of the morning aurora inferred from coincident DMSP-F6, -F8, and Søndrestrøm incoherent scatter radar observations

On rare occasions, observations from the DMSP-F6 and -F8 spacecraft and the Søndrestrøm incoherent scatter radar coincide in space. Such coincidence offers a unique opportunity to study temporal vs spatial variations on a small scale. We discuss data from one of those occasions, with observations made in the dawn sector in the presence of moderate auroral precipitation during a magnetically quiet period. The DMSP satellites measured vertical electron and ion flux and cross-track plasma drift while the radar measured the ionospheric electron density distribution and line-of-sight plasma velocities. We combine these data sets to construct a two-dimensional map of a possible auroral pattern above Søndrestrøm. It is characterized by the following properties. No difference is seen between the gross precipitation patterns measured along the DMSP-F6 and -F8 trajectories (separated by 32 km in magnetic east-west direction and some 4 s in travel time in magnetic north-south direction), except that they are not exactly aligned with the L shells. However, F6 and F8 observed minor differences in the small-scale structures. More significant differences are found between small-scale features in the DMSP precipitation measurements and in radar observations of the E-region plasma density distribution. These measurements are separated by 74 km, equivalent to 2.4°, in magnetic longitude, and 0–40 s in time along the spacecraft trajectories (varying with magnetic latitude). Large-scale magnetospheric-ionospheric surfaces such as plasma flow reversal, poleward boundary of the keV ion and electron precipitation, and poleward boundary of E-region ionization, coincide. The combined data suggest that the plasma flow reversal delineates the polar cap boundary, that is, the boundary between precipitation characteristic for the plasma mantle and for the plasma sheet boundary layer.     

Boundary populations in the polar caps

The morphology of precipitating particles, measured at low altitude in the polar regions, varies systematically with the strength and direction of IMF B_z and with solar wind speed V_sw. We use particle data taken onboard the DMSP satellites to determine these variations. Both individual satellite passes during the storm/quieting period of 26 and 27 August 1990, and statistical maps compiled from a data base over 4.5 yr are presented. We focus attention on those magnetospheric populations that have magnetosheath characteristics, the boundary populations. We show that the precipitating ion boundary population, whose down-coming spectra can be fitted to streaming Maxwellians, expands from a region confined near the dayside cusp for southward IMF, to a thick, annular region, including the dayside cusp, for northward IMF. The expansion in local time is inhibited by increasing solar wind speed. Boundary electrons behave somewhat differently. They have easier access to the polar regions and their variations have shorter spatial/temporal scale lengths than the boundary ions. For strongly northward IMF, intense, agitated boundary electrons can be found over all or part of the polar cap. Broad regions (up to ~ 100 km) of strongly accelerated electrons (several keV) that produce visible arcs are embedded in this population. Two features of the ion boundary population help identify its source. (1) The spectra of the boundary ions expanding into the polar cap exhibit field-aligned streaming, which, downtail, is toward the Earth. (2) The region into which the boundary ions expand best maps magnetically to a dawn-dusk cut across the neutral sheet, rather than to the low-latitude boundary layer. Therefore, we conclude that the immediate source for boundary ions in the polar regions during northward IMF is the plasma sheet boundary layer. These ions reach tail lobe field lines by convection whose direction when mapped to the ionosphere is sunward. Significant change in the topology of the magnetospheric magnetic field, and, in particular, the closing of high-latitude field lines, is not required to explain the data.     

Interpretation of optical substorm onset observations

The ionospheric location of substorm onset is generally found to be at the most equatorward arc in the poleward portion of the diffuse aurora. The observation that most activity occurs in this region provides a reference from which the source region in the magnetotail may be assessed. This reference can be examined in two ways. First, magnetic field mappings of these onset locations to the equatorial plane suggest that the onset is associated with processes quite near the Earth. For example, for 14 cases the average GSM X value was found to be ≈ −7.8 R_E. However, this identification is based on a static magnetic field model and while these results are consistent with some earlier findings there is not sufficient confidence in this technique to discriminate between topological regions in the magnetotail. A second way to examine the ionospheric onset location is in relation to the open/closed field line boundary. It is evident from Viking satellite images that optical substorm expansions can occur well equatorward of the poleward extent of emissions, both during quiet and active periods. There is no reason to suspect that this poleward region of emissions is not on closed field lines and that the onset location is therefore unrelated to the open/closed field line boundary, a result consistent with some (but not all) near-Earth mechanisms but only under some conditions with the distant tail boundary layer theory.     

Electrodynamic response of the middle atmosphere to auroral pulsations

On the nights of 21 and 28 October 1987, two Nike Orion payloads (NASA 31.066 and 31.067) were launched from Andøya, Norway, as part of the MAC/EPSILON campaign, to study the effect of auroral energetics on the middle atmosphere. Each payload carried detectors to measure relativistic electrons from 0.1 to 1.0MeV in 12 differential energy channels, and bremsstrahlung X-rays from >5 to >80keV in 5 integral channels. In addition, instrumentation to measure bulk ion properties and electric fields was also carried by these and/or near simultaneous flights. Flight 31.066 was launched during the recovery phase of a moderate magnetic substorm, during relatively stable auroral conditions. Flight 31.067 was launched during highly active post-break-up conditions during which Pc 5 pulsations (> 150s period) were in progress. The energetic radiation of the first event was composed almost entirely of relativistic electrons below 200 keV with negligible contributions from bremsstrahlung X-rays, while the radiation of the second event was dominated by much softer electrons ( < 100 kcV), which produced high X-ray fluxes that exceeded the cosmic ray background as an ionizing source down to altitudes below 30 km. Simultaneous conductivity measurements during both events show consistency with the ionizing radiations, with the pulsation event producing free electrons down to 55 km. far below their expected altitude range during night-time. These comparisons are discussed to evaluate the impact of such events on the middle atmosphere.     

How does magnetospheric convection relate to the expansion onset of substorms?

It is well recognized that magnetospheric and ionospheric convection play a key role in substorm development. Some characteristic implications of the relationship are reviewed and discussed. Southward turning of the IMF or a sudden magnetospheric compression and the associated effects in the magnetotail lead to enhanced earthward plasma flow and to a gradual growth of the ionospheric DP 2 current system. Ionospheric conductivities are enhanced due to increased (mainly Fermi accelerated) electron precipitation. Finally, after an extensive period of convection growth, plasmas in a confined region of the magnetotail become unstable leading to a substorm onset. Occasionally, the entire magnetosphere may experience continuous stable enhanced convection for several hours (up to 10) without clear signatures of magnetospheric substorm-type processes. Impulsive heated plasma beams are observed in the far magnetotail indicating that powerful acceleration processes are in operation. The DP 2 current system in the ionosphere shows a high constantly disturbed level lasting for several hours. The role of ionospheric Hall and Pedersen conductivities is discussed in detail. Three different time constants (ranging from 1 to 1000 min) are identified in the magnetospheric response to convection changes. It is concluded that changes in the tail configuration are needed to start a substorm. Also different types of precipitation mechanisms are active in connection with the various types of magnetospheric response mechanisms. Similarities are found in the wedge-type field-aligned current generation mechanisms during normal substorms and the prolonged stationary magnetospheric convection cases.     

The dynamics of the poleward edge of the trough deduced using data from a meridional chain of vertical-incidence sounding stations

Data from four ionospheric stations located along the 902E meridian in the range 55–702 of corrected geomagnetic latitude, were used to construct latitude-time electron density distributions in the F2-layer peak for 17 winter nights of 1982–1983. It is concluded that under stationary convection conditions the poleward edge of the trough during the nighttime displaces only 0.5-l°/h, that is, significantly less than obtained from existing analytical models of the ionospheric trough. When the stationarity is upset (due to the development of a substorm or abrupt changes of the north-south component of the IMF), the poleward edge of the trough is observed to displace abruptly equatorward. In the substorm expansion phase these displacements can amount to 4–5° in less than an hour. Such displacements of the poleward edge in the evening hours can characterize the dynamics of the inner edge of the plasma sheet.     

An alternative interpretation of auroral precipitation and luminosity observations from the DE,DMSP, AUREOL,and Viking satellites in terms of their mapping to the nightside magnetosphere

We present an interpretation, which differs from that commonly accepted, of several published case studies of the patterns of auroral electron precipitation into the high-latitude upper atmosphere in the near-midnight sector based on their mapping to the nightside magnetosphere. In our scheme bright discrete auroral structures of the oval and respective precipitation are considered to be on the field lines of the Central, or Main, Plasma Sheet at distances from 5–10 to 30–50 R_E, depending on activity. This auroral electron precipitation pattern was discussed in detail by Feldstein and Galperin [(1985) Rev. Geophys.23, 217] and Galperin and Feldstein [(1991) Auroral Physics, p. 207. Cambridge University Press. It is applied and shown to be consistent with the results of case studies based on selected transpolar passes of the DE, DMSP, AUREOL-3 and Viking satellites.A diagram summarising the polar precipitation regions and their mapping from the magnetospheric plasma domains is presented. It can be considered as a modification of the Lyons and Nishida (1988) scheme which characterizes the relationship between the gross magnetospheric structure and regions of nightside auroral precipitation. The modification takes into account non-adiabatic ion motions in the tail neutral sheet, so that the ion beams characteristic of the Boundary Plasma Sheet (BPS) originate on closed field lines of the distant Central Plasma Sheet (say, at distances more than ~30 R_E).     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

D-region signatures of substorm growth phase and onset observed by EISCAT

High resolution electron density measurements by EISCAT during the pre-onset phase and onset of an auroral absorption substorm are used to investigate the characteristics of electron precipitation during these substorm phases. The development of the pre-onset phase is the result of a uniform increase of electron fluxes with energies of a few tens of keV, with no particularly hard component. The absorption spike observed at substorm onset contains fine structure when investigated at 10 s resolution, indicating a rapid hardening of the precipitating spectrum at the onset.     

VLF plasmaspheric emissions—‘wave’ signatures of inner magnetosphere boundaries

VLF quarter-gyrofrequency emissions have been observed in the plasmasphere by low-altitude (up to 2500 km) Intercosmos satellites. Their experimental characteristics indicate that the origin and the distribution function of resonant electrons and the type of instability leading to their production can differ during various phases of magnetospheric disturbances. Their close connection to the earthward edge of the lowest energy plasma sheet electron fluxes during active periods is shown. During recovery periods following magnetospheric substorms, they persist (together with other symptoms of a certain internal plasmaspheric structure—subauroral T_e-peaks, SAR arcs, etc.) at roughly the same L-value within the new, more distant plasmapause. A new kind of instability is mentioned which could lead to their production during the recovery phase of magnetospheric substorms.     

Kelvin-Helmholtz instability in a compressible plasma with a magnetic field and velocity shear

The development of the Kelvin-Helmholtz instability (KHI) in a compressible plasma containing a magnetic field is studied for a finite thick layer with a velocity shear in a linear two-dimensional MHD approximation. Approximate analytical expressions for the phase speed, growth rate and wavenumber of the fastest growing unstable mode are derived for the velocity profile across the layer with the velocity shear having a sudden onset, termed a sharp elbow. The Dispersion equation is obtained from boundary conditions on the sharp elbow where the logarithmic derivative of the total pressure (or normal component of velocity) plays the main role.The KHI, driven by velocity shear, is considered as a possible generator of plasma oscillations in magnetospheric regions such as at the boundary between the inner plasma sheet and the plasmasphere, the boundary between the plasma sheet and the tail lobes and the boundary between the plasma sheet and the magnetopause. Analytical expressions for the phase speed, growth rate, period and wavenumber of the growing unstable mode obtained with some simplifying assumptions leads to values similar to the results of numerical analyses.     

Model calculation and analysis of magnetosphere-ionosphere-thermosphere coupling

Model calculations are used to analyse the function of the magnetosphere-ionosphere electrodynamic coupling and that of the ionospheric tide-dynamo. The phenomena concerned are discussed.It is shown that for geomagnetic and ionospheric research in low-latitude and equatorial regions, the penetration and shielding of magnetospheric disturbances should be taken into account. On the other hand, the effect of the tidal dynamo on the ionosphere of the auroral region should not be ignored, especially on geomagnetically disturbed days when the conductivity is very high. The features and the relative importance of these two effects are different at different stages of a magnetic storm, and depend on latitude. There is an evident dawn-dusk asymmetry in these effects. The influence of weak geomagnetic disturbances on the low-latitude ionosphere should also be taken into account.     

Observations of high latitude ionospheric conductances

A brief historical review of the development of models of the ionospheric conductivities with special emphasis on high latitude regions and the auroral zone is presented. It is with great admiration that we must conclude that the physical understanding of the importance of the ionospheric conductances was well perceived by pioneers like Schuster and Birkeland a hundred years ago. Progress in the basic theoretical fundamentals was achieved in the late 1920s and 1930s. Realistic estimates were not derived until the first rocket probes measured the electron and ion content at different altitudes in the 1950s.Today we have a superior technique in resolving electron density profiles of high time and height resolution by incoherent scatter radars on the ground. The challenge that we are facing is to obtain global conductivity maps, especially at high latitudes, with a time and spatial resolution which match the details in auroral substorm phenomena. If that can be achieved, great progress in the understanding of detailed dynamical coupling in the ionosphere, magnetosphere, and thermosphere systems is expected. The imaging technique as demonstrated by the DE-satellite can be the tool which eventually materializes our desires for increased knowledge.     

Substorm activity precursors in the dayside magnetic perturbations

Magnetic data from a meridional chain of stations in Greenland and AL-indices of magnetic activity have been used to study the relationship between magnetic perturbations in the dayside cleft region and substorm activity in the night-time auroral zone. The analysis of 14 substorms, isolated and prolonged, has shown that intensification of westward currents in the postnoon sector of the cleft precedes or accompanies substorm development in the night-time auroral zone. Westward currents appear in the northern cleft as substorm precursors even under the adverse influence of the IMF positive B_y component. These currents trend to extend in the prenoon sector. To explain the relationship between the cleft currents and auroral electrojet the connection between neutral layer currents and noon Birkeland currents is proposed. This connection can be realized by means of the source region acting just inside the daytime magnetopause owing to stationary reconnection of geomagnetic field and IMF, the source region flowing downstream to the tail magnetopause.