The re‐formation of the polar cap during quiet times: The theta aurora

Abstract

The theta aurora, a form of auroral arc that stretches across the polar cap roughly parallel to the Sun‐Earth line, was first detected by an optical imager on the Dynamic Explorer 1 satellite [Frank et al., 1982]. But to date, its origin has not been satisfactorily explained. The new availability of nearly continuous measurements of the solar wind and the interplanetary magnetic field (IMF) by the WIND satellite whenever the POLAR satellite was in a position to observe the presence of these arcs (see Figure 1) has renewed interest in solving the mystery of the theta aurora's origin.The new data were examined with the notion that some feature of the solar wind's interaction with the magnetosphere must lead to the development of the theta arcs. Conclusions revealed that the theta aurora provides an unexpected way in which the polar cap can grow and reform during or after extended periods when the IMF has a northward component. This new understanding has strong roots in previous observational and theoretical work within the space plasma physics community.