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 apearing 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 lwer ratio of 5577-¿ intensity compared to 3914 ¿ 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 cylcotron 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.