Subauroral red arcs and associated ionospheric phenomena

Abstract

This paper describes the phenomenon of the midlatitude red arc of September 29, 1967, through observations of the properties of the ionospheric plasma. The ion and electron temperatures, ion composition and density, and suprathermal electron flux during this period are measured by retarding potential analyzers near 900 km from Ogo 4 and near 2000 km from Explorer 31. These parameters show the following changes in the region L=2.3 to 3.0 during the red arc period, as compared with their values during normal periods: (1) Electron and ion temperatures increase to above 4000°K from a normal value of 2000°K at 900 km, while at 2000 km electron temperature increases to above 5000°K from a normal value of 2500°K. (2) At 900 km the ratio of O+/(H+ + He+) changes from 1 to 5, while the total density remains approximately the same. (3) At 2000 km the ion density decreases by a factor of 10 with the composition remaining all H+. (4) There is no significant increase in the flux of 5- to 10-ev electrons. The relative importance of electric field heating, magnetospheric conduction, and the changes in the neutral composition in the lower atmosphere are examined in the light of these observations. It is concluded that the subauroral red arc is caused by a combination of thermal conduction of energy from the magnetosphere and changes in the neutral compositions in the lower atmosphere.