RADAR AURORAL ECHO HEIGHTS AS SEEN BY A 398 MHZ PHASED ARRAY RADAR OPERATED AT HOMER, ALASKA.

摘要

Backscatter data collected with a 398 MHz phased-array radar operated at Homer, Alaska (59.72 deg N, 151.53 deg W) have been analyzed for information on the height of radar auroral echoing. Altitude was resolved through the variation of backscattered power with antenna beam elevation angle. The mean height of backscattering could be determined with an accuracy of 1-2 km over small ((TURN)20 x 20 km('2)) areas and short ((TURN)1 min) periods. In this thesis, the results are presented in the form of maps of the spatial distribution of echo height.; The data base encompassed approximately 40 hrs of observation carried out in 1973, 1976, and 1978. Echo activity most often spanned a 10-20 km range of height centered on 100-110 km. The echoing region was always sharply bounded from below at 96-98 km. The upper altitude limit of echo activity was 115-120 km. The height range in events of the post-midnight sector was 97-110 km vs. 97-118 km in events of the pre-midnight sector. Echoing was restricted to directions nearly perpendicular to the geomagnetic field, giving rise to systematic spatial and temporal variations of height. Magnetic aspect control of echo height was weaker in events of discrete radar aurora.; The height range of the Homer echo activity is shown to be consistent with the onset of primary two-stream plasma instability within auroral electrojet current. The modulation of height within the altitude limits of echo activity by the magnetic aspect geometry is attributed to strong directional confinement of plasma wave growth. It is suggested that the spatial and temporal variabilities of radar auroral altitude derive from structure within auroral ionization and variability of auroral electrojet current.