The theta aurora

Abstract

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 observed over the poleward zones of the auroral oval. This latter region is currently thought to be magnetically mapped into the boundary layer of the plasma sheet in the magnetotail, namely, the boundary layer of high‐speed ion beams and field‐aligned currents. When the transpolar arc is dim or at its initial stages of brightening, there is a severe corresponding change in the character of luminosities, convection electric fields, fields, and plasmas over the polar cap region. The plasma convection is no longer the signature of a simple four‐cell convection pattern. Plasma convection is generally turbulent or sunward in the polar cap. Remarkably, the zone of hot H+, He++, and O+ ions, which is a unique signature of the presence of a transpolar arc, is still present for the two examples of dim or absent transpolar arc luminosities presented here. However, the ionosphere displays a complex system of polar arcs and glows over almost the entire polar cap region. Similarly, the occurrence of field‐aligned electron acceleration, broadband electrostatic noise, and field‐aligned currents is widespread within the polar cap region. Outside the zone of hot H+, He++, and O+ ions associated with the position of the transpolar arc the primary hot ions are H+ and He++, a composition that is indicative of direct access of these ions from the magnetosheath. The existence of the theta aurora implies a corresponding unique magnetospheric convection pattern and magnetospheric topology. The dynamics of the magnetosphere during these periods is currently an actively contested issue. The comprehensive, correlated observations presented here are intended to provide a foundation for future theoretical and interpretive efforts.