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Two distinct types of auroras have been observed in the trough region (almost exclusively in the northern hemisphere) equatorward of the diffuse auroral boundary by the Isis 2 satellite scanning auroral photometer. The first appears as east‐west‐aligned oval‐shaped ‘patches,’ often appearing in groups and corotating with the earth just poleward of the plasmapause. The second consists of isolated east‐west ‘detached arcs’ which decrease in latitude and intensity to the east and show no evidence of corotation. The arcs occur at higher latitudes than the patches near the poleward edge of the F layer trough. Both patches and arcs are evening sector phenomena, although patches can be found in the morning. Both also show a longitude dependence, with the arcs centered mainly over the Alaska sector and the patches over Canada. Patches are more intense than arcs, while both tend to be less intense than the diffuse aurora poleward of them, although they have a lower ratio of 5577‐Å intensity compared to 3914 Å than the diffuse aurora, a finding suggestive of greater particle energies. Patches occur at times when Dst is increasing following a magnetic storm, while detached arcs occur about 10–12 hours after a peak in the AE index and at a time when the (statistically averaged) interplanetary field component Bz is positive following an extended period when it was negative. These dependencies, together with the particle observations reported in a companion paper, suggest that the sources for the trough region auroras are plasma sheet particles injected during magnetic storms or substorms and left behind by a poleward retreat of the auroral oval. The similar latitude‐local time dependencies of ELF emission regions detected by Chan and Holzer suggest cyclotron resonance scattering of trapped particles as the likely mechanism for the precipitation of these particles. Two rather distinct types of cold plasma regions are then required in order to account for the two types of optical features.

Isis 2 satellite measurements of electron and ion temperatures and densities and of airglow emissions have resolved certain features of the response of the night side ionosphere to the major magnetic storm of August 4–6, 1972, an event in which Kp reached 9o at one point. The near‐polar orbit of Isis 2 is circular at about 1400‐km altitude and at the time of the storm was oriented in the 0500–1700 LT plane. During the initial phases the plasmapause moved equatorward from L=4.5 to about 2.7, followed by a further compression to L=1.9–2.0 at the peak of the storm and a slow recovery during the ensuing days. The plasmapause was identified by an abrupt decline in H+ accompanied by a sharp peak in both Te and Ti, Te approaching 6000°K and Ti exceeding 3000°K in some passes. A pronounced trough developed within the plasmasphere during the days of greatest disturbance. Equatorial temperatures were also enhanced from normal values, Te exhibited great spatial variation there on some passes, and this variation was associated with the development of troughs of Ne at the dip equator. On the most disturbed day (August 5), when the plasmapause had fallen to L=2.3, a deep trough developed in the region L=1.3‐1.8, Ne falling below 1 × 10² cm−3, more than 2 orders of magnitude below the normal protonosphere concentration at this altitude. Top side sounder observations of radio breakthrough from the ground indicated that the trough extended through the F region as well. A red arc observed on this day at L=1.23 apparently was associated with an equatorial trough in Ne and enhancement in Te. Whether the arc and the troughs were caused by magnetospheric processes or by enhanced recombination within the F region is not clear.

Isis 2 observations of a variety of topside ionospheric ‘signatures’ of the plasmapause are compared with simultaneously acquired equatorial electron density profiles obtained by the whistler technique. The satellite data were acquired at ∼1400‐km altitude at dusk and dawn in the sunlit northern hemisphere summer ionosphere within ∼15° longitude of the VLF receiver. Results suggest that the dynamics of plasma coupling between the ionosphere and plasmasphere dominate the topside data and obscure the location of the equatorial plasmapause field line. The total density and light ion troughs begin 2°–10° equatorward of the field line through the equatorial plasmapause and are not clear plasmapause signatures. The invariant latitude of the region of steep spatial gradient in thermal plasma density, i.e., the plasmapause, appears to increase with altitude. Thus measurements of its position at different altitudes may give different results. Plasma sheet electrons, however, are observed on field lines just outside the equatorial plasmapause at both dawn and dusk. Their low‐latitude extent at 1400‐km altitude can be used as a signature of the equatorial plasmapause position.