keV electron precipitation lFrank and Ackerson,. 1971). The precipitation is ...... Density Profiles in lonosphere and Exosphere, edited by Jon Fri- hagen, p. 358 ...
Radio Science,Volume 18, Number 6, Pages 1140-1150, November-December 1983
Alouette-ISIS radio wave studies of the cleft, the auroral zone, and the main trough and of their associated irregularities D. B. Muldrew Communications Research Centre, Department of Communications, Ottawa, Ontario, Canada K2H 852 (Received November 22,1982; accepted
April 18'
1983')
The principal highlatitude ionospheric features such as the cleft, the nighttime auroral zone' the main trough and their associated irregularities, as observed with the Alouette/ISIS topside sounders, plots as are reviewed. These features can be clearly distinguished in Alouette I electron density contour a function
oflatitude and height below
1000 km. Alouette 2 data recorded near apogee (3000 km) show
more the location of the light-ion (H* and He+) trough as a function of local time and show it to be location closely related to the plasmapause than is the main trough. An attempt is made to clarify the and terminology of troughlike structures. Regions of small-scale irregularities (tens of meters) are for the presassociated with itre high-latitude ionospheric features. These irregularities are responsible in the cleft found is spread intense most The ionograms. on topside F spread ence of aspect-sensitivi work region. Spiead F regions usually have a sharp equatorward boundary. Some recent unpublished be located tends to zone auroral the nighttime for boundary F spread that the indicates bithe author generated indirectly several degrees equatorward of the keV electron precipitation zone and is probably is sometimes region spread main trough In the equatorward. convected and piecipitaiion by this pi.r"nt urd other times absent. Some of the individual small-scale irregularities responsible for spread F can extend along magnetic-field lines for hundreds of kilometers'
1.
2. HIGH.LATITUDE
INTRODUCTION
FEATURES
By using topside ionograms, the electron density signatures of the principal high-latitude features can be determined from latitude-height plasma frequency (or electron density) contours extending from the satellite height to the height of the F ,layer peak [Nelms high-latitude trough, auroral cavity, slot, total-ion and W arren, 1965 ; N elms, 1966; N elms and Lotrough, light-ion trough, and no doubt others have ckwood, 1967 ; W arren, 1969 l'{ elms ond Chapman, been used. This paper will attempt to clarify this situl97O). Three examples are given in Figure 1. A chain ation. telemetry stations which were situated approxiof The relative locations and some characteristics of along the 75"W meridian just after the launch mately the main trough, the nighttime auroral zone, the cleft, 1 made it possible to obtain almost conAlouette of and the polar cap will also be reviewed by using data from the south to the north geocoverage tinuous recorded by the Alouette and ISIS topside sounders. poles. graphic The location and properties of the spread F boundFigure la lN elms, 19661 shows a well-defined cleft ary at the satellite height (which is presumably close centered on about 77" invariant latitude. signature to the scintillation bound aty in the region of the The electron 3 density (electron density in cm - is 1.24 trough), the nighttime auroral zone, and the dayside x 104 f k, where ,f, is the plasma frequency in MHz)
Some confusion exists regarding the relative locations of or differences among the troughlike features of the high-latitude ionosphere. Terms such as plasmapause, main trough, mid-latitude trough,
cleft will also be discussed.
topside-sounder review of the highlatitude ionosphere is given by I',i elms and Chapman
An earlier
[ 1 e70].
Copyright 1983 by the American Geophysical Union. Paper number 350612.
is approximately doubled at the center of the cleft region and shows an increase over about 4" of latitude. The Alouette 1 ionograms recorded in this region also show an increase in spre ad F at the satellite height over the range of the density increase. An example of spread F in the cleft region will be discussed below.
Poleward of the cleft, ripples
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