On the cause of substorm expansion onset and the processes driving the substorm expansion phase

Magnetospheric substorms can be a consequence of an enhanced magnetospheric convection driven by the solar wind. The ionosphere responds gradually to an enhanced magnetospheric convection during the growth phase of a substorm, leading to the substorm expansion onset. The growth phase and the expansion onset are powered by the directly-driven process tapping energy from the solar wind. After the expansion onset, a substorm intensifies explosively during the expansion phase which is powered jointly by the directly-driven process and the unloading process. The unloading process is responsible for the explosive release of the stored energy in the magnetotail. Substorm currents associated with the unloading process is found to be ~ 1.6–2 times the substorm currents associated with the directly-driven process based on a substorm event on 7 March 1979.     

Convecting towards the catastrophe of substorm onset

A nonlinear model of substorm evolution is constructed which self-consistently unifies the directly driven and the loading-unloading systems. Using only the principle that the magnetosphere minimizes its potential energy within the constraints imposed by external conditions, substorms are described with reference to trajectories on a cusp catastrophe surface. The model predicts the conditions required for substorm onset and the amount of energy explosively released at the beginning of the expansive phase.     

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread-F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s⁻¹, and with a median velocity of 720 m s⁻¹. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.     

Energy input and spectral variations of precipitated particles during three nights of the Energy Budget Campaign

Observations were carried out during the Energy Budget Campaign with the aim of measuring keV particle precipitation in the ionosphere over a region covering the rocket range. These measurements are important as particle precipitation constitutes one of the major energy inputs into the ionosphere during an auroral substorm. Estimates of electron precipitation have been made in three different regions of the energy spectrum. Two optical emissions were measured that are proportional to electron precipitation of a few hundred eV and a few keV, respectively, and cosmic noise absorption related to electrons of a few tens of keV. The relative variation between the signals may be interpreted as changes of the energy spectrum of the particles. Measurements from the ground have been compared with satellite measurements to obtain the relation between the two methods. The ground-based observations, which are continuous in time and cover a large area, may be used to separate the spatial and temporal variations and to give the large scale substorm frame of reference for the rocket data. The results may also be used in the interpretation of other measurements on board rockets where a knowledge of the precipitated particles is required.     

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.     

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.     

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.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Recent advances in magnetospheric substorm research

More than two decades of magnetospheric exploration have led to a reasonably clear morphological picture of geomagnetic substorms, which is often summarized in terms of the near-Earth neutral line (NENL) model of substorms. Although this qualitative theory is quite comprehensive and explains a great many observations, it is hard pressed to explain both recent observations of consistently earthward flow within 19 R_E and also the prompt onset of magnetic turbulence at 8 R_E at the time of substorm onset. Other theories have recently been proposed which tend to be more quantitative, but which explain a more limited number of substorm observations. The challenge seems to be to understand the essential physics of these various quantitative theories and integrate them into a larger structure such as is provided by the near-Earth neutral line model.     

A servo-model of the night-time F2-layer above St. Santin

A modified form of the ionospheric servo-model is used to describe the night-time F2-layer above St. Santin. Data taken by the incoherent scatter radar on nine nights in 1974–1977 were used to determine the height profiles of electron density, electron and ion temperature and electric field. The servo-model was then used to compute the theoretical variation through the night of the height of the F2 peak and the field-aligned plasma velocity, using gas concentrations and horizontal pressure gradients derived from the MSIS79 atmospheric model. On magnetically quiet nights these calculated values agreed closely with the observations. On disturbed nights, however, the calculations and observations began to diverge an hour or so after the onset of a substorm. The divergence could be explained by an enhanced southward wind.     

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.     

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.     

The auroral distribution and its mapping according to substorm phase

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.     

Optical measurements of a nightside poleward expanding aurora

Coordinated optical observations were performed from the poleward side of the midnight auroral oval. Height measurements of the auroral emissions at 4278, 5577 and 6300 Å, as well as their intensity ratios in the poleward expanded auroral substorm, have been carried out. The findings indicate a significantly softened electron spectrum compared with similar data from the equatorward part of this substorm. Typical values for the poleward expanded aurora are 300 eV and lower, while keV particles dominate the auroras at 10° lower latitudes. Emission altitudes and spectral characteristics are comparable to the transient burst emissions frequently observed from the same site in the post-noon sector, i.e. within the cusp.The 6300 Å atomic oxygen emission is used as a tracer of F-region wind and temperature. Interferometer observations show that there exists a prevailing crosspolar antisunward wind, increasing with geomagnetic activity to several hundred m s⁻¹. The temperature shows an increase of 150 K associated with high geomagnetic activity.     

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.     

Mechanisms and time-scales of the magnetospheric response to the interplanetary magnetic field changes during the 8 May 1986 substorm

On 8 May 1986, between 1113 and 1600 UT, an isolated magnetospheric substorm was observed, during which the AE-index exceeded 700 nT (CDAW 9E event). Three available sets of measurements (a) of the solar-wind parameters (IMP-8 satellite), (b) of the magnetotail energy flux (ISEE-1 spacecraft), and (c) of ground magnetic observatories, allowed us to make a detailed study of the overall magnetospheric response to changes of the interplanetary magnetic field (IMF) direction, during this event of weak solar-wind coupling.In order to study the mechanisms and time-delays of the magnetospheric response to the abrupt increase of the solar-wind energy input, we have evaluated the total magnetospheric energy output U_T following two different methods: (a) Akasofu's method, taking the ring current decay time τ_R constant, and (b) Vasyliunas' method where the values of u_t are independent of the solar-wind energy input as determined from the epsilon parameter. Both methods suggest that the driven system has been considerably developed during this substorm, while an unloading event has been superposed at the expansion onset.     

Variability of d-region electron densities at tromsø

The 2.75 MHz partial-reflection radar at Ramfjordmoen near Tromsø has been used for a study of D-region electron densities by the differential-absorption method on a number of days during 1978–79. Received signals are generally stratified in several layers, typically over 60–80 km. Strong stable echoes are seen down to 55 km during periods of enhanced riometer absorption. Inferred electron densities vary between ~ 100–1000 cm⁻³ at ~ 60–80 km and show well-defined features which persist for ~ 10–20 min. During periods of high absorption, enhanced electron densities (~ 600 cm⁻³) are observed below 65 km. During a Polar Cap Absorption event, the inferred electron densities at 60–70 km show a very stable profile. Possible sources of D-region ionization at high latitudes are briefly discussed     

D-region electron densities observed by incoherent-scatter radar during auroral-absorption spike events

Electron density profiles in the night-time auroral ionosphere were obtained with the incoherent-scatter radar at Chatanika, Alaska, during short duration precipitation events characterized by riometer data as spike events. The measurements show exceptionally large electron densities in the D-region during spike events, the electron density typically exceeding 10⁶ cm³ at 90 km altitude for a short time. The existence of a steep horizontal gradient, particularly on the poleward edge of the event, is inferred. The altitude and thickness of the absorbing layer are deduced. It is shown that 20–40 keV electrons make the greatest contribution to an absorption spike and that the spectrum of electrons producing such an event is probably softer than that producing a more slowly varying absorption peak. These absorption layers are too high for their altitudes to be measured by the technique of multi-frequency riometry.     

High-latitude D- and E-region investigations using imaging riometer observations

The imaging riometer technique has proved a valuable tool for investigations of a variety of ionspheric and magnetospheric disturbances. To illustrate the potential of the new technique, this presentation will discuss the observations at cusp latitudes (approx. 75° inv. lat.) of PCA events, substorms and poleward progressing absorption features for which imaging riometer observations have provided important new information. For the PCA studies an exceptionally nice data set for the sunrise/sunset asymmetry is presented. It is argued that the asymmetry is so modest that temperature effects offer a simple explanation. For the substorm generation an augmented current wedge model is suggested on the basis of imaging riometer observations. Finally, imaging riometer observations of IMF-dependent poleward progressing absorption events are presented. This type of disturbance is considered the convecting ionospheric footprint of the B_Y component of the interplanetary magnetic field. A typical example is examined.     

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.