Theoretical models of polar-cap convection under the influence of a northward interplanetary magnetic field

The unexpected patterns of high-latitude auroral luminosity and ionospheric convection that are observed when the interplanetary magnetic field (IMF) has a northward orientation have inspired a variety of theoretical interpretations. The existing models, all referring to steady-state conditions, can be classified according to the topology of the polar magnetic field lines and of the polar-cap convection streamlines. The classes of model include: (1) a closed magnetosphere model, (2) a conventional open model with a distorted, but topologically unchanged, polar-cap boundary, (3) a conventional open model with distorted, but topologically unchanged, polar-cap convection cells, (4) a modified open model with ‘lobe convection cells’ contained wholly on open magnetic-field lines, and (5) a modified open model with a bifurcated polar cap. The third and fourth types require significant regions of sunward flow on open polar-cap field lines, a concept that presents serious theoretical difficulties. The other three types appear equally viable from a theoretical point of view, and the comparison against observations is an ongoing enterprise. Outstanding theoretical questions include (a) how do observed structures in the polar ionosphere map along magnetic field lines into the magnetosphere?, (b) what is the mechanism that drives the observed sunward convection at highest latitudes on the day side?, and (c) what role does time dependence play in the observed phenomena?     

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.     

Electric field generation at the magnetospheric boundary for northward IMF

We discuss three different processes which generate electric fields at the magnetopause during northward interplanetary magnetic field (IMF) conditions. These are (1) Petschek-type magnetic field reconnection, (2) magnetic field diffusion, and (3) viscous-like interaction resulting from the Kelvin-Helmholtz instability. For northward IMF all three processes lead to the formation of a boundary layer on closed magnetic field lines adjacent to the magnetospheric boundary. The thickness of the boundary layer depend on Petschek's parameter in the first case, the magnetic Reynolds number in the second case, and an effective Reynolds number in the third case. In each case coupling between the boundary layer and the ionosphere occurs via field-aligned currents. These field-aligned currents result from the penetration into the polar ionosphere of the electric field generated at the magnetospheric boundary. These currents are closed by a transverse current in the boundary layer and the associated Lorentz force causes a decrease of the kinetic energy of the solar wind plasma inside the boundary layer. As a result of this velocity decrease the thickness of the boundary layer increases on both flanks of the magnetosphere near the equatorial plane. The convergence of the boundary layer on the dawn and dusk sides leads to antisunward plasma flow in the magnetospheric tail.     

Field-perpendicular and field-aligned plasma flows observed by EISCAT during a prolonged period of northward IMF

The effect of a prolonged period of strongly northward Interplanetary Magnetic Field (IMF) on the high-latitude F-region is studied using data from the EISCAT Common Programme Zero mode of operation on 11–12 August 1982. The analysis of the raw autocorrelation functions is kept to the directly derived parameters N_e, T_e, T_i and velocity, and limits are defined for the errors introduced by assumptions about ion composition and by changes in the transmitted power and system constant. Simple data-cleaning criteria are employed to eliminate problems due to coherent signals and large background noise levels. The observed variations in plasma densities, temperatures and velocities are interpreted in terms of supporting data from ISEE-3 and local riometers and magnetometers. Both field-aligned and field-perpendicular plasma flows at Tromsø showed effects of the northward IMF: convection was slow and irregular and field-aligned flow profiles were characteristic of steady-state polar wind outflow with flux of order 10¹² m⁻² s⁻¹. This period followed a strongly southward IMF which had triggered a substorm. The substorm gave enhanced convection, with a swing to equatorward flow and large (5 × 10¹² m⁻² s⁻¹), steady-state field-aligned fluxes, leading to the possibility of O⁺ escape into the magnetosphere. The apparent influence of the IMF over both field-perpendicular and field-aligned flows is explained in terms of the cross-cap potential difference and the location of the auroral oval.     

A unified view on convection and field-aligned current patterns in the polar cap

During the last two decades measurements of polar cap ionospheric electric fields and currents, field-aligned currents, and global auroral forms have been made from ground-based and space-based platforms. An attempt is made to unify these observations into a large-scale view of polar phenomena. In this view, plasma convection patterns and the corresponding electrodynamics in the polar region can consistently be ordered by the orientation of the interplanetary magnetic field (IMF). The different patterns of the electric potential and of field-aligned currents depend on where the main interaction between the terrestrial and interplanetary fields occurs, on the morning or evening side of the central polar cap, or on the dayside portion of the ‘closed’ cusp region, or on the nightside portion of the ‘open’ cusp region. One of the essential elements of this unified view is that it is possible to account for various convection patterns ranging from the four-cell pattern (during periods of strong northward IMF and B_y ~ 0), to the three-cell pattern (B_z > 0 and |B_y| 2> 0), to the conventional two-cell pattern (B_z < 0) with its possible deformation into a convection throat near the dayside cusp (during southward IMF). We also discuss the way in which the complicated field-aligned current systems can consistently be accounted for in terms of these convection patterns.     

Empirical convection models for northward IMF

It is clear that polar cap convection during times of northward IMF is more structured and of lower mean speed than at times of southward IMF. This, coupled with the fact that the polar cap is smaller, means that empirical models are more difficult to construct with certainty. It is also clear that sunward flow deep in the polar cap is often observed, but its connection with the rest of the flow pattern is controversial.At present, empirical models are of three types: ‘statistical’ models wherein data from different days but with similar IMF conditions are averaged together; ‘pattern recognition’ models, which are built up by examining individually hundreds of passes to derive a ‘typical’ pattern which embodies features frequently observed; and ‘assimilative’ models, which use data of different types and from as many locations as possible, but all taken at the same time, in order to derive a snapshot (or series of snapshots) of the entire pattern.Each type of model has its own difficulties. Statistical models, by their very nature, smooth out flow features (e.g. the convection reversal, and the locus of sunward flow deep in the polar cap) which are not found at precisely the same invariant latitudes and magnetic local times on different days. Pattern recognition models are better at reproducing small-scale features, but the large-scale pattern can be a matter of interpretation. Assimilative models (such as AMIE) hold out the best hope for creating instantaneous, global convection patterns; however, the analysis technique tends to be most irregular (and least reliable) in the regions which are not well covered by in situ data. It appears that, at least at times, a four cell model with sunward flow at the highest and lowest latitudes, and antisunward flow in between, is consistent with the observations. At other times, the observations may be consistent with a two-cell convection pattern, but which includes significant meanders within the polar cap.     

Magnetopause plasma transients: signatures in the auroral ionosphere

The magnetopause and adjacent boundary layers of the Earth's magnetosphere play important roles in transferring momentum and energy from the solar wind to the magnetosphere-ionosphere system. The details of the different boundary processes, their ionospheric signatures and relative importance are not well known at present. Particle precipitation, field-aligned current, auroral emission, ionospheric ion drift and ground magnetic perturbations are among the low-altitude parameters that show signatures of various plasma processes in the LLBL and the magnetopause current layer. Magnetic merging events, Kelvin-Helmholtz waves, and pressure pulses excited by the variable solar wind/magnetosheath plasma are examples of boundary phenomena that may be coupled to the ionosphere via field-aligned currents. In this paper, attention is focussed on a specific category of auroral activity occurring in the cusp/cleft region predominantly during the southward directed interplanetary magnetic field (IMF). Co-ordinated observations from the ground and satellites in polar orbit have been used to study the temporal/spatial development of the events in relation to the background patterns of particle precipitation and ionospheric convection as well as the field-aligned current and ion drift characteristics of the individual events. The auroral phenomenon is characterized by a sequence of elongated forms moving laterally into the polar cap. Spatial scales of major events repeating every 5–10 min are ∼200 km (N-S) times 300–1000 km (E-W). Smaller scale auroral structures with more irregular occurrence rates are observed at times. The preliminary evidence suggests that the motion pattern is regulated by the IMF orientation, that is, the direction of longitudinal motion along the polar cap boundary is determined by the IMF B_Y polarity. The examples reported here occurred within 1000–1400 MLT, near the zero point potential line separating the morning and post-noon convection cells. During nonzero IMF B_Y the auroral structures are associated with channels of enhanced zonal ionospheric ion flow and Birkeland current sheets of opposite polarity, imbedded within the larger scale IMF B_Y-related cusp-mantle current system. These characteristics are discussed in relation to model predictions of ionospheric signatures of magnetopause plasma transients, with particular emphasis placed on impulsive magnetic merging events.     

Ionospheric plasma convection in the southern hemisphere

The first ionospheric plasma convection maps ordered by the y- and z-components of the IMF using only data from the southern hemisphere are presented. These patterns are determined from line-of-sight velocity measurements of the Polar Anglo-American Conjugate Experiment (PACE) located at Halley, Antarctica, with the majority of the observations coming from 65°–75° magnetic latitude. For IMF Bz positive and negative conditions, the observed plasma motions are consistent with a standard two cell pattern. For the periods from dusk through midnight to dawn, flow speeds are at least twice as large for Bz negative component compared with Bz positive. The observations about noon are significantly different from each other. For Bz positive, little ordered plasma motion is observed. For Bz negative, there are large anti-sunward flows the orientation of which is ordered by IMF By. These By orientated flows are consistent with theoretical predictions, and are anti-symmetric to those reported from the northern hemisphere. The two most significant differences from previous observations are that the convection reversal in the late morning sector for By negative conditions occurs at about a 4° lower latitude than the Heppner and Maynard (1987) model. This may be due to a seasonal bias in the PACE dataset. Also, the separatrix between eastward and westward flow near midnight has a very different shape dependent upon the orientation of IMF By. For positive By conditions, the separatrix is observed at progressively lower latitudes at later local times, but for By negative conditions, the separatrix appears at increasingly higher latitudes at later times.     

Simulations of the behaviour of the polar thermosphere and ionosphere for northward IMF

The dynamics and structure of the polar thermosphere and ionosphere within the polar regions are strongly influenced by the magnetospheric electric field. The convection of ionospheric plasma imposed by this electric field generates a large-scale thermospheric circulation which tends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the magnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulations of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the UCL/Sheffield coupled thermosphere/ionosphere model has been used, including convection electric field models for a northward IMF configuration. During periods of persistent strong northward IMF B_z, regions of sunward thermospheric winds (up to 200 m s⁻¹) may occur deep within the polar cap, reversing the generally anti-sunward polar cap winds driven by low-latitude solar EUV heating and enhanced by geomagnetic forcing under all conditions of southward IMF B_z. The development of sunward polar cap winds requires persistent northward IMF and enhanced solar wind dynamic pressure for at least 2–4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward winds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B_y negative (positive) in the northern hemisphere (reverse in the southern hemisphere). The magnitude of sunward polar cap winds will be significantly modulated by UT and season, reflecting E-and F-region plasma densities. For example, in northern mid-winter, sunward polar cap winds will tend to be a factor of two stronger around 1800 UT, when the geomagnetic polar cusp is sunlit, then at 0600 UT, when the entire polar cap is in darkness.     

A survey of simultaneous observations of the high-latitude ionosphere and interplanetary magnetic field with EISCAT and AMPTE-UKS

This paper surveys the results of simultaneous observations by the EISCAT incoherent scatter radar and the AMPTE-UKS satellite, made during three periods in September and October 1984, when AMPTE-UKS was in the solar wind on the dayside of the Earth and the UK-POLAR EISCAT experiment was measuring ionospheric parameters at invariant latitudes 70.8–75.0°. A total of 42 h of EISCAT convection velocity data, with 2.5 min resolution, were obtained, together with 28 h of simultaneous 5 s resolution AMPTE-UKS observations of the solar wind and interplanetary magnetic field (IMF). The general features of the AMPTE-UKS data are described in Section 2 and those of the EISCAT data are described in Sections 3 and 4. The main subjects discussed are the form of the plasma convection patterns and their dependence on all three components of the IMF (Section 5), the ionospheric response to abrupt changes in the IMF (Section 6), in particular a sharp ‘southward turning’ of the IMF on 27 October 1984, and a crossing of an IMF sector boundary. Section 7 describes ‘short lived rapid flow burst’, which are believed to be related to flux transfer events at the magnetopause.     

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 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.     

Comparisons between EISCAT observations and model calculations of the high latitude ionosphere

Calculations using a numerical model of the convection dominated high latitude ionosphere are compared with observations made by EISCAT as part of the UK-POLAR Special Programme. The data used were for 24–25 October 1984, which was characterized by an unusually steady IMF, with Bz < 0 and By > 0; in the calculations it was assumed that a steady IMF implies steady convection conditions. Using the electric field models of Heppner and Maynard (1983) appropriate to By > 0 and precipitation data taken from Spiroet al. (1982), we calculated the velocities and electron densities appropriate to the EISCAT observations. Many of the general features of the velocity data were reproduced by the model. In particular, the phasing of the change from eastward to westward flow in the vicinity of the Harang discontinuity, flows near the dayside throat and a region of slow flow at higher latitudes near dusk were well reproduced. In the afternoon sector modelled velocity values were significantly less than those observed. Electron density calculations showed good agreement with EISCAT observations near the F-peak, but compared poorly with observations near 211 km. In both cases, the greatest disagreement occurred in the early part of the observations, where the convection pattern was poorly known and showed some evidence of long term temporal change. Possible causes for the disagreement between observations and calculations are discussed and shown to raise interesting and, as yet, unresolved questions concerning the interpretation of the data. For the data set used, the late afternoon dip in electron density observed near the F-peak and interpreted as the signature of the mid-latitude trough is well reproduced by the calculations. Calculations indicate that it does not arise from long residence times of plasma on the nightside, but is the signature of a gap between two major ionization sources, viz. photoionization and particle precipitation.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

Simulations of the seasonal variations of the thermosphere and ionosphere using a coupled,three-dimensional,global model,including variations of the interplanetary magnetic field

The University College London Thermospheric Model and the Sheffield University Ionospheric Convection Model have been integrated and improved to produce a self-consistent coupled global thermospheric/high latitude ionospheric model. The neutral thermospheric equations for wind velocity, composition, density and energy are solved, including their full interactions with the evolution of high latitude ion drift and plasma density, as these respond to convection, precipitation, solar photoionisation and changes of the thermosphere, particularly composition and wind velocity. Four 24 h Universal Time (UT) simulations have been performed. These correspond to positive and negative values of the IMF BY component at high solar activity, for a level of moderate geomagnetic activity, for each of the June and December solstices. In this paper we will describe the seasonal and IMF reponses of the coupled ionosphere/thermosphere system, as depicted by these simulations. In the winter polar region the diurnal migration of the polar convection pattern into and out of sunlight, together with ion transport, plays a major role in the plasma density structure at F-region altitudes. In the summer polar region an increase in the proportion of molecular to atomic species, created by the global seasonal thermospheric circulation and augmented by the geomagnetic forcing, controls the plasma densities at all Universal Times. The increased destruction of F-region ions in the summer polar region reduces the mean level of ionization to similar mean levels seen in winter, despite the increased level of solar insolation. In the upper thermosphere in winter for BY negative, a tongue of plasma is transported anti-sunward over the dusk side of the polar cap. To effect this transport, co-rotation and plasma convection work in the same sense. For IMF BY positive, plasma convection and co-rotation tend to oppose so that, despite similar cross-polar cap electric fields, a smaller polar cap plasma tongue is produced, distributed more centrally across the polar cap. In the summer polar cap, the enhanced plasma destruction due to enhancement of neutral molecular species and thus a changed ionospheric composition, causes F-region plasma minima at the same locations where the polar cap plasma maxima are produced in winter.     

Polar cap potential distributions during periods of positive IMF By and Bz

We compare the DE-2 electric field measurements used by Heppner and Maynard [(1987) J. geophys. Res.92, 4467] to illustrate strongly distorted, BC convection patterns for IMF B_z > 0 and large |B_y|, with simultaneous detections of particle spectra, plasma drifts and magnetic perturbations. Measured potentials >50 keV, driven by the solar wind speeds exceeding 500 km/s, are greater than published correlation analysis predictions by up to 27%. The potential distributions show only two extrema and thus support the basic conclusion that under these conditions the solar wind/IMF drives two- rather than fourcell convection patterns. However, several aspects of the distorted two-cell convection pattern must be revised. In addition to the strong east-west convection in the vicinity of the cusp, indicated by Heppner and Maynard, we also detect comparable components of sunward (equatorward) plasma flow. Combined equipotential and particle precipitation distributions indicate the presence of a lobe cell embedded within the larger, afternoon reconnection cell. Both types rotate in the same sense, with the lobe cell carrying 20–40% of the total afternoon cell potential. We detected no lobe cell within morning convection cell.     

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).     

The effect of electric field-induced vertical convection on the precipitation E-layer

The effects of perpendicular electric fields on the high-latitude nocturnal precipitation E-layer are studied in terms of model calculations. The deformations of the electron density profiles caused by vertical plasma movements are described assuming various incident electron energies and flux densities, as well as different field intensities and directions. Manifestations of the profile variations in vertical sounding observations and changes in the conductivity profiles are examined. The results show that field-induced vertical convection is important in plasma densities even higher than 10¹¹ m⁻³. Erroneous results may be obtained if the effect of the electric field is neglected when determining the incident electron spectrum from a measured ionospheric density profile.     

Differential Doppler measurements of the ionosphere during a solar eclipse

The effects of the solar eclipse of 26 February 1979 on the ionosphere were measured using differential Doppler techniques. Nayy navigation satellite passes were monitored at 12 sites located across the North American continent. These data yield a measurement of the vertical columnar electron content along a north-south line. Different sites monitoring the same pass provide simultaneous observations of ionospheric variations along different longitude lines. Two satellite passes occurred during or just after the eclipse. These data show a shoulderjust northward of the umbra region and a trough just behind the umbra containing large horizontal gradients. This sharp trough recovered quickly with a half-life of about 10 min.