On the altitude at which some solar-flare-induced lonization is released

Abstract

During the occurrence of certain solar flares and subflares (about 20 to 25 per cent of those optically observed), the instantaneous frequency of a highly stable, CW, HF signal transmitted obliquely through the ionosphere, over distances of the order of 5000 km, is momentarily changed by a few cycles. The change consists of an increase in the frequency, followed by a decrease and, subsequently, a gradual return toward the original received frequency. The rapidly changing portion of the frequency variation lasts only a few minutes (usually less than 5). In addition, the frequency change varies inversely with the operating frequency and proportionally with the path length. Paths separated by many hundreds of kilometers are simultaneously affected. It is of interest that these pronounced frequency shifts invariably occur prior to the loss of signal characteristic of a short-wave fadeout, and hence could conceivably be used to warn of impending signal loss in modern HF communication systems affording continuous feedback of propagation conditions over the path. In certain cases, however, the flare-induced frequency shift is not followed by a fadeout. The azimuthal angle of arrival of the same signals that have frequency changes during solar flares and subflares also deviates in about one-third of the cases. The fact that a I-IF wave exhibits bearing deviations signifies that a horizontal gradient of ionization, capable of bending the wave, exists. Electron-density gradients are probably produced as a result of solar-flare-induced ionization, especially at times when the sun's energy falls at a grazing angle on the path. The angle usually deviated to the south (toward the subsolar point) of the great-circle path for the Puerto Rico-Palo Alto (PR-SU) path during the season at which these measurements were made. These observations of instantaneous frequency and angle-of-arrival changes during solar flares suggest that new ionization is introduced initially somewhere just above the E region, very probably in the height region 120 to 200 km, although even greater heights are possible. This initial ionization may or may not be followed by the generation of ionization in the absorbing D region. The time variation of height at which solar-flare-induced ionization is released suggests that the ionization-producing radiant energy is initially soft, and then 'hardens' as the flare progresses. released, Fall URSI-IRE Meeting, Austin, Texas, October 1961, TR 45, Radioscience Lab., Stanford Univ., December 4, 1961. Kanellakos, D. P., and O. G. Villard, Jr., Some relationships between frequency and azimuth angle of arrival of I-IF waves propagated over long distances, URSI-IRE meeting

Studies of solar flare effects and other ionospheric disturbances with a high frequency Doppler technique

“…An atmospheric model specifying electron collision frequency (v) as a function of height can then be used to locate the "slab" of ionization assumed responsible for both effects. Similar work has been carried out by Kanellakos, Chan, and Villard [1962]. These studies have helped to establish the fact that solar-flare effects are relatively frequent at heights above the D region of the ionosphere Davies [1962c] has also shown that the Doppler technique can he used for study of ionospheric drifts.…”

Systems for the detection and identification of nuclear explosions in the atmosphere and in space

“…These disturbances may be the result either of atmospheric waves [36] or of additional ionization [37], [38]. CW, H F transmissions over long paths, using one or more ionospheric reflections, may be used.…”

“…The nuclear-induced effects discussed in the last section may also be detected by H F sounders of various types [38]. Sounders which record the change of phase or the rate of change of phase on a fast time base are particularly useful.…”

Enhancement in electron and ion temperatures due to solar flares as measured by SROSS-C2 satellite

“…The low latitudes showed a larger increase in the TEC compared with the high latitude. Some events of solar flares have also been studied by Kanellakos et al (1962) to see the effect on the E-and F-region ionosphere and it has been found that the electron density was enhanced at these heights. Thome and Wagner (1971) effect of soar flares on electron density and found that the electron density is enhanced in the E-and F-regions of the ionosphere.…”