An index, denoted by AE, is derived as a measure of global electrojet activity. The basic data used are 2.5‐minute readings of the H trace in the standard magnetograms from seven auroral‐zone observatories. The readings are referenced to a level determined for each observatory from quiet intervals. All the data from the seven observatories are then plotted against UT, and two envelopes are drawn to embrace all the points. The index AE at any epoch is defined by the distance (or separation) between the upper and lower envelopes at that epoch. When viewed as functions of UT the upper and lower envelopes themselves show development and decay of positive and negative variations. It is found that a positive excursion usually accompanies a negative (larger) variation, confirming the well known feature of polar disturbance. The AE index for a 6‐day period, February 10–15, 1958, shows that polar disturbances statistically repeat with a time interval of about 4 hours, and that the average duration of their most active phase is a little more than 1 hour. It is pointed out that the average repetition time of 4 hours is comparable with that of the electron flux enhancements observed by Anderson et al. in the magnetosphere tail. It is suggested that polar disturbances are directly related to Anderson's ‘electron islands’ in the magnetosphere tail.

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...

The seasonal dependence of large‐scale Birkeland currents has been determined from the analysis of vector magnetic field data acquired by the TRIAD satellite in the northern hemisphere. Statistical characteristics of single sheet (i.e., net currents) and double sheet Birkeland currents were determined from 555 TRIAD passes during the summer, and 408 passes during the winter (more complicated multiple‐sheet current systems were not included in this study). The average Kp value for the summer events is 1.9 and for the winter events is 2.0. The principal results include the following: (1) The single sheet Birkeland currents are statistically observed more often than the double sheet currents in the dayside of the auroral zone during any season. The single sheet currents are also observed more often in the summer than in the winter (as much as 2 to 3 times as often depending upon the MLT sector). (2) The intensities of the single and double sheet Birkeland currents on the dayside, from approximately 1000 MLT to 1800 MLT, are larger during the summer (in comparison to winter) by a factor of about 2. (3) The intensities of the double sheet Birkeland currents in the nightside (the dominant system in this local time) do not show a significant difference from summer to winter. (4) The single and double sheet currents in the dayside (between 0600 and 1800 MLT) appear at higher latitudes (by about 1° to 3°) during the summer in comparison to the winter. These characteristics suggest that the Birkeland current intensities are controlled by the ionospheric conductivity in the polar region. The greater occurrence of single sheet Birkeland currents during the summertime supports the suggestion that these currents close via the polar cap when the conductivity there is sufficiently high to permit it. Since the intensities of Birkeland currents are larger during periods of greater ionospheric conductivity, an important source (but perhaps not the only source) of these currents must be a voltage generator in the magnetosphere, possibly related to the convective electric field.

Plasma, magnetic‐field and dc electric‐field observations from Dynamics Explorers 1 and 2 are used to investigate the morphology of solar‐wind ion injection, Birkeland currents, and plasma convection in the morning sector for both positive and negative interplanetary magnetic field (IMF) By components. The results of the study are used to construct a By‐dependent global convection model for southward IMF. A significant element of the model is the coexistence of three types of convection cells (“merging cells,” “viscous cells,” and “lobe cells”). This model can account for observations of a nearly stationary (in local time) convection “throat,” a sunward‐antisunward convection reversal zone at the polar‐cap boundary in both the morning and afternoon quadrants, the morphology of solar‐wind ion injection and transport in the mid‐altitude polar cusp, and the By‐dependent dawn‐dusk asymmetry of polar‐cap electron fluxes.