Identifying Alfvénic Modulation of Quasi‐Static Potential Structures Above Aurora by Optical Methods

Abstract

Much observational evidence exists for the presence of quasi‐electrostatic potential drops above aurorae and their role in acceleration of auroral electrons. However, the temporal evolution of these potential structures is less well studied. We present an optical‐only method for investigating counter‐flowing auroral forms, hypothesized to be due to a U‐shaped potential structure in the auroral acceleration region. The method estimates electric field vectors perpendicular to the background magnetic field based on: (a) optical flow vectors obtained from sequences of high spatial and temporal resolution auroral images; and (b) the spatial gradient of two‐dimensional characteristic precipitation energy maps, obtained from the brightness ratio of the OI 777.4 nm emission to N2 1PG emissions (measured at 673.0 nm). Results from the two methods are compared, with emphasis on temporal evolution. We find that they are generally consistent, thereby supporting the U‐shaped potential hypothesis, although there are notable discrepancies. Toy modeling shows that these discrepancies could be attributed to electron acceleration by dispersive Alfvén waves, via one‐bounce reflection off the wave electric field.