Multisatellite data are used to examine the temporal relationship between Subauroral Ion Drifts (SAID) and the phases of an auroral substorm. Utilizing images of auroral luminosities taken by the Dynamics Explorer 1 (DE 1) spacecraft and observations of particle injection at geosynchronous orbit, we identify the time of expansive phase onset and estimate the time at which recovery begins. Noting the times at which SAID are observed simultaneously by the DE 2 spacecraft, we find that SAID typically occur well after substorm onset (more than 30 min), during the substorm recovery phase. Substantial westward ion drifts and field‐aligned currents are observed well equatorward of the auroral oval during the expansion phase of a substorm, but the drifts lack the narrow spike signature associated with SAID. Prior to substorm onset and after substorm recovery, field‐aligned currents are absent equatorward of the auroral oval and the ionosphere is very nearly corotating. A phenomenological model of SAID production is proposed that qualitatively agrees with the observed ionospheric signatures and substorm temporal relationship. In this model, substorm‐generated, subauroral field‐aligned currents close via Pedersen currents with the outward flowing, region 1 currents at higher latitudes. These Pedersen currents flow in the region of low conductivity equatorward of the auroral oval and are associated with relatively large, poleward directed electric fields. The frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of ion‐atom interchange between O+ and N2. Subsequent fast recombination of NO+ with electrons further reduces the subauroral F region conductivities with a corresponding increase in the electric field and the frictional heating. This heating leads to thermal expansion, substantial field‐aligned plasma flow, and very large depletions in the F peak concentration, thus additionally reducing the height‐integrated Pedersen conductivity.

The ultraviolet photometer of the University of Iowa spin scan auroral imaging instrumentation on board the Dynamics Explorer 1 satellite has returned numerous images of the geocorona from altitudes of 570 km to 23,300 km. The geocoronal observations from 1981 through 1985 are compared to a spherically symmetric isothermal Chamberlain model of the exospheric density distribution. Model parameters are varied to obtain an acceptable fit. The radiative transfer equation is solved numerically. Stellar intensities are monitored for an independent calibration of the DE 1 instrument in flight. The solar Ly • flux is estimated through concurrent measurements made by the Solar Mesosphere Explorer satellite, supplemented by published values of ground-observable solar indices. Extraterrestrial background intensities are adopted from earlier OGO 5 high-altitude measurements. The optimum fit for 1981 imaging data utilizes a Chamberlain model of temperature T = 1050 K and exobase density n½ = 44,000 atoms cm-3. The exobase is taken as r e ---1.08 Rg (500 km altitude), and a critical radius for satellite atoms of r•s = 3.0 r e is adopted. This model continues to compare well with the DE 1 measurements over the entire 4-year period studied, even though the exobase conditions are expected to have changed appreciably during this interval of declining solar activity. It is concluded that the apparently constant hydrogen density and scale height observed by DE 1 are not directly indicative of the exobase conditions through the classical Chamberlain model but rather show the effects of charge exchange with thermal ions in the plasmasphere. A readily observable departure from spherical symmetry is the geotail, an enhancement in the atomic hydrogen column densities in the antisunward direction. atmosphere of the earth and the northern auroral oval are visible mainly in the emission lines of atomic oxygen (primarily 1304 •), which are transmitted with approximately equal sensitivities by the two filters represented on this plate. The sensitivity to Lyman • (1216 •) is lesser by a factor of • 12 for the filter used to obtain the image in the right-hand panel. These are consecutive images, each requiring 12 min to collect and telemeter. The spacecraft motion during this interval is revealed by the slight change in location of the back-ground star field. Emission rates represented by the color code for this and the following plates range from ~ 25 kR in white at the dayside limb down to ~ 2.5 kR in red; lesser values are black. The 6.8-hour DE 1 orbit, with an apogee altitude of 23,300 km and a perigee altitude of 570 km, is ideally suited to return images from a useful range of depths within the geocorona. The views in Plate 1, obtained on October 20, 1981, are from a near-apogee altitude of approximately 22,300 km above the north pole. In Plate 2 a sequence of six consecutive images collected on November 20, 1981, shows a rapidly changing aspect as the spacecraft climbs from 8300 km to 17,900 km in altitude. Three months later, the orbit...

The theta aurora is a remarkable configuration of auroral and polar cap luminosities for which a generally sun‐aligned transpolar arc extends contiguously from the dayside to nightside sectors of the auroral oval. Four individual occurrences of theta aurora over earth's northern hemisphere are examined in detail with the global auroral imaging instrumentation on board the high‐altitude, polar‐orbiting spacecraft DE 1. Simultaneous measurements of fields and plasmas with this high‐altitude spacecraft and its low‐altitude, polar‐orbiting companion, DE 2, are examined in order to establish an overview of auroral and polar cap phenomena associated with the appearance of the theta aurora. For these series of observations, two general states of the polar cap are found corresponding to (1) a bright, well‐developed transpolar arc and (2) a dim or absent transpolar arc. During periods of a relatively bright transpolar arc the plasma convection in the polar cap region associated with the transpolar arc is sunward. Elsewhere over the polar cap the convection is antisunward. The convection pattern over the auroral zones and polar cap is suggestive of the existence of four cells of plasma convection. Field‐aligned electron acceleration into the polar atmosphere and field‐aligned current sheets are present in the transpolar arc plasmas. This electron precipitation and these current sheets are relatively absent over the rest of the polar cap region. The transpolar arc plasmas exhibit similar densities and ion compositions relative to those plasmas observed simultaneously over the poleward zone of the auroral oval. The ion compositions include hot H+, He++, and O+ ions and thus are of both ionospheric and solar wind origins. Principal hot ions in the remainder of the polar cap region are H+ and He++, indicating access from the magnetosheath for these ions. Low‐energy electrons identified with a magnetosheath source are also present in this region. The dominant thermal ions in the polar cap region are O+ ions flowing upward from the ionosphere. These thermal ions are heated along magnetic flux tubes within the transpolar arc plasmas. Pairs of current sheets with oppositely directed current densities occur in the transpolar arc region and with magnitudes similar to those associated with the poleward zones of the auroral oval. The upward currents are carried by electrons accelerated by a field‐aligned potential. Funnel‐shaped auroral hiss and broadband electrostatic noise are associated with the presence of the transpolar arc plasmas. Energetic solar electrons are employed to show that the magnetic field lines threading both the transpolar arc and the poleward zone of the auroral oval are probably closed. In contrast, the accessibility of these electrons to the remainder of the polar cap indicates that these polar regions are characterized by a magnetic topology that is connected directly to field lines within the interplanetary medium. Thus the overall character of the transpolar arc region appears to be very similar to that obser...

The characteristics of the hrge-scale electrodynamic parameters, field-aligned currents (FACs), electric fields, and electron prec/pitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulgetype auroral emission region has been deduced from auroral images taken by the Dynamics Explorer I (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with litfie net current in the surge horn and the west of bulge. The downward net current/s distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are associated with narrow, intense antisunward convection, corresponding to an equatorward d/rected spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high-latitude boundary of the upward region I FAC. In the surge, the convection reversal is near the low-latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region I and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents. INTRODUCTION Auroral substorms are defined by a systematic sequence of auroral motions and magnetic disturbances [Akasofu, 1964]. Since field-aligned currents (FACs), ionospheric electric fields or convection, charged particle precipitation, and resulting ionospheric conductivity are closely...

Preliminary examination of the first global auroral images gained with the vacuum‐ultraviolet imaging photometer on board DE 1 reveals a remarkable spatial configuration of auroral luminosities. Frequently the northern auroral oval is bifurcated by a sun‐aligned arc extending from the midday auroral zone to the nighttime sector of the oval. Simultaneous plasma measurements with the low‐altitude DE‐2 spacecraft are used to show that the character of plasmas above the polar arc is similar to those found over the poleward zones of the auroral oval. The spatial distribution of auroral luminosities is suggestive of a two‐cell structure of current systems and convection electric fields over the earth's polar cap and a similar division of the corresponding lobe in the magnetotail.