HF Backscatter From High-Latitude, Ionospheric Barium Releases


  • Howard F. Bates


Of the six barium releases made in Alaska in March 1969, two gave good amplitude data from the 4- to 64-MHz step-frequency radars installed near Kodiak and Homer, Alaska. The effective cross section for isotropic scatter exceeded 1000 km2, whereas the projected area of the cloud was less than 100 km2; this indicates that the scatter was directive, not isotropic. The echoes were found to have an aspect sensitivity of 10 to 20 decibels per degree of off-perpendicularity from the magnetic field. The backscatter cross section apparently had a maximum in the 15-to 30-MHz range; this indicates that transverse irregularity sizes of 5 to 10 meters were dominant n the clouds. At early times the length of the irregularities was of the order of a kilometer. Pronounced optical striation was accompanied by a decrease in aspect sensitivity; no other correlation was found between the optical and radar data. This indicates that the changes affecting the radar cross section were invisible. When all the available radio data are considered, the cloud at early times acts as if it were an over-dense ball (not necessarily spherical) embedded in a field-aligned cloud that is thin perpendicular to the direction of ion drift and that contains weak-scattering field-aligned irregularities. After striation the cloud acts as if it were a group of weak-scattering, field-aligned irregularities. The radar behavior of the clouds appears to be strongly dependent on ionospheric conditions.