U.S.A. National Committee Report, Fifteenth URSI General Assembly, Munich, September 1966: Commission 3, Ionospheric Radio; Progress in Ionospheric Radio

Abstract

Auroral absorption measured in the auroral zone with riometers is on the average more pronounced in the winter months for both hemispheres. Also, in winter it has a pronounced diurnal variation with a maximum a few hours before noon. The average absorption in winter is twice that in summer, but sunlight has no influence in its value [Leinbach and Basler, 1963; Basler, 1963]. Individual events at night are characterized by a quiet phase of relatively small absorption, a sharp increase of absorption associated with the breakup phase of the visible aurora, and a fast recovery with irregular fluctuation of absorption. Detailed correlation of areas of absorption and luminosity exists for such events [Ansari, 1964]. The structure is less than 200 km in the north‐south direction and 700 km in the east‐west direction [Parthasarathy and Berkey, 1965]. An event seen only after midnight consists of a relatively slow increase and recovery of extremely high absorption with no de~ailed correlation of light emission and absorption. It is associated with a harder electron spectrum [Ansari, 1964]. A peculiar type of absorption event frequently observed during midmorning usually increases very smoothly, reaches a high value of absorption, and is not accompanied by appreciable magnetic activity [Ansari, 1965]. Absorption associated either with negative bays or with breakup events shows no difference in intensity between day and night [Brown and Barcus, 1963; Brown, 1964]. There is a limited correlation between the high‐energy part of the electron spectrum, as measured by X‐rays in balloons, and the absorption [Barcus, 1965].