2011 SuperDARN Workshop
Auroral high frequency waves: Possible scattering targets?
Dartmouth College, USA
abstract. From the early yearss of space physics, rocket- and satellite-borne wave experiments probing auroral regions detected electrostatic waves between the plasma and upper hybrid frequencies. These high frequency waves are most prevalent at high altitudes, above about 600 km, but they occur down to or even below the F-peak. One consequence is two types of mode-conversion radiation observable at ground level: Auroral roar originates from upper hybrid waves at the matching condition between the upper hybrid frequency and cyclotron harmonics, most commonly near 275 km altitude; and auroral MF burst is of unknown origin but proposed to come either from Langmuir waves extending from the F-peak up to about 600 km or from electron sound waves. The upper hybrid waves, with wave vectors perpendicular to the magnetic field and wavelengths from a few meters to tens of meters, might be effective scatterers of superDARN frequencies, although a bistatic system may be needed to receive the scattered wave which would refract differently from the upgoing wave due to its significant (several MHz) frequency offset. However, experiments suggest that much of the time the upper hybrid waves form standing wave structures in pre-existing field-aligned density enhancements, in which case there may sometimes be no significant frequency offset of the scattered wave. In such cases it seems possible for SuperDARN to detect the scatter. The other high-frequency wave, MF burst, associated either with Langmuir waves or electron sound waves, either of which has parallel wave vectors, are targets for radars that look up the magnetic field line; they may be related, for example, to enhanced ion lines (NEIALs) detected with incoherent scatter radars pointed directly up the magnetic field, but they seem less likely targets for SuperDARN radars than do the upper hybrid waves. A search for scatter from field-aligned upper hybrid waves near 275-375 km altitude, possibly offset by about 3 MHz, could add significant contributions to knowledge of HF waves and open another type of regular SuperDARN diagnostic measurement.