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 kra 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 4000øK from a normal value of 2000øK at 900 km, while at 2000 km electron temperature increases to above 5000øK from a normal value of 2500øK. (2) At 900 km the ratio of O*/(H* • He*) changes from I 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. Recent measurements of electron temperatures in the region of subauroral red arcs have given direct support to the concept that the arc is produced primarily by the thermal electron excitation of atomic oxygen [Norton and Findlay, 1969]. The mechanism exciting the arc is therefore related to the mechanism he.ating the ambient plasma. In recent years, a number of models for heating the electron gas have been proposed. These are (1) adc electric field [Megill et al., 1963]; (2) a precipitating flux of low-energy electrons [Dalgarno, 1964]; (3) the thermal conduction of energy from the magnetosphere in the ionosphere along the geomagnetic field lines [Cole, 1965, 1970]. Walker and Rees [1968] investigated the relative merits of the three processes in raising electron temperature to the point of exciting the 600-R red arc of June 21, 1961. They concluded that all satisfied the necessary criteria so far as ability to heat the ambient plasma was concerned.

Vertical lifting of ionization during geomagnetic storms from satellite measurements of ion composition.

“…The storm related changes in ion composition observed at low latitudes resemble those observed at mid latitudes as discussed by CHANDRA et al (1971) and MAIER et al (1975). The mid-latitude phenomena have been attributed to the changes in neutral composition and the increase in neutral and plasma temperatures.…”

Simultaneous observations of subauroral electron temperature enhancements and electromagnetic ion cyclotron waves

“…This paper investigates the role of electromagnetic ion cyclotron (EMIC) waves in thermal electron heating on field lines associated with subauroral electron temperature enhancements. Subauroral electron temperature (Te)enhancements produce stable auroral red (SAR) arcs wh6n the peak in T e exceed a certain threshold [Chandra et al, 1971]. The energy source of T e enhancements and the associated SAR arcs have been identified as ultimately coming from the ring current although the process of energy transfer from the ring current to thermal electrons is still under debate [Brace et al, 1967;Kozyra et al, 1986].…”

The light-ion trough, the main trough, and the plasmapause

“…, which according to Chandra et al, [1971] may be critically dependent upon chemical processes in the lower thermosphere, the relative behavior of the ion constituents must be carefully examined. Also, for studies involving the interaction of the cool thermal plasma with the hot energetic plasma trapped in the radiation belts, information on the LIT structure and pos-ition should be of particular concern.…”