Direct measurements of the size and amplitude of irregularities in the topside ionosphere

Abstract

Data obtained from the Alouette 2 Langmuir probe experiment during a 3‐month period in the fall of 1966 have been examined for effects due to electron concentration irregularities (fine structure). Irregularities occur in patches hundreds of kilometers in horizontal extent, and individual irregularities have amplitudes between 5% (limit of resolution) and 70% and dimensions of less than 2 km along the satellite path. These irregularities appear more often at night than during the day, and they occur in a region that contains the auroral oval and extends to both higher and lower latitudes. Comparison of the probe data with Alouette 2 ionograms obtained simultaneously shows that spread F occurred at the height of the satellite whenever fine structure was observed. However, particularly at mid latitudes, spread F often occurs at the satellite even when the probe does not detect irregularities. When spread F occurs, but the probe does not observe fine structure, the irregularities are less than 5% in amplitude (i.e., below the probe’s resolution). Calculations using an irregularity model indicate that spread F is often caused by irregularities less than 0.5 km thick. Several irregularity production mechanisms that could account for the results are discussed.

A review of electrostatic wave measurements on auroral magnetic field lines.

“…Experimentally determining the dissipation scale length is important since computer models are limited by mesh constraints to relatively long wavelengths. Also imbedded in the magnetospheric flow are irregular plasma density patterns which have been detected by numerous low-altitude satellite probe experiments (DYSON, 1969;DYSON et al, 1974;SAGALYN et al, 1974). They are observed throughout the polar cap, auroral, and sub-auroral zones often with extremely sharp equatorward boundaries at all local times.…”

Ionospheric and magnetic observations at 1000 kilometers during the geomagnetic storm and aurora of May 25-26, 1967

“…The top strip represents typical nor-structured data, while the lower strip represents an extreme case of structure. These variations in collected current are thought to represent density variations along the satellite path (Dyson, 1969). The horizontal scale is time;…”

A whistler wave interaction with a density striation: a laboratory investigation of an auroral process

“…First, these waves are commonly found in almost all parts of the magnetosphere and ionosphere [Helliwell, 1965;Walker, 1976;Shawhan, 1979;Al'pert, 1980Al'pert, , 1983and Carpenter, 1983]. Second, at altitudes below approximately 5000 km, the high latitude ionospheric plasma is found to contain short scale-length, field-aligned irregularities or striations [Herman, 1966;Dyson, 1969;Clark and Raitt, 1976;Kelley and Mozer, 1972;Fejer and Kelley, 1980] at all accessible altitudes and all local times. The striations are almost always of lower density than the background and the density amplitude distribution peaks between 10% and 30% of the background level [Dyson, 1969].…”

“…Second, at altitudes below approximately 5000 km, the high latitude ionospheric plasma is found to contain short scale-length, field-aligned irregularities or striations [Herman, 1966;Dyson, 1969;Clark and Raitt, 1976;Kelley and Mozer, 1972;Fejer and Kelley, 1980] at all accessible altitudes and all local times. The striations are almost always of lower density than the background and the density amplitude distribution peaks between 10% and 30% of the background level [Dyson, 1969]. It is generally believed that the size of the striation parallel to the background magnetic field is very large compared to its extent across the field so that the shape is either cylinder-like or sheet-like.…”