Ground-based radio sounding of the Earth magnetosphere

Abstract

A sounding link at I-IF established between tightly synchronized ground-based terminals located at shell conjugate sites (Jupiter, Florida, and Ushuaia, Argentina) has obtained echoes with 62 millisec delay. The echoes can be interpreted as being due to propagation along an enhancement of electron density aligned with the L ----1.8 shell. The echoes are characterized by path losses 80 db worse than free space and were received at 7.46, 9.04, and 12.00 MI-Iz for 7.6% of the over-all observing time, during night hours. Typically their strength was 15 db below noise and they were recovered with an integration time of 1 hour (3600 pulse repetition periods).We report that a sounding link at HF established between shell conjugate ground-based terminals has obtained long-delay echoes that can be interpreted as being due to propagation guided along a field-aligned enhancement of electron density. Enhancements of this type were detected by the electron probe of the Ariel 1 satellite [Sayers et al., 1963]. Early observations of man-made long-delay 'cosmic echoes' were reported by Stormer [1928] and Van der Pol [1928]. These experimenters worked with radio links that operated at 9.55 MHz and observed echoes with delay up to 15 sec. Although the. mechanism that was active in these observations must have been different [Villard et al., 1969] from wave guidance along field-aligned structures, it is of interest to note that these early researchers interpreted the observed echoes as signals penetrating through the ionosphere and reflected by a toroidal surface formed around the earth by a stream of charged particles emitted from the sun [Alperr, 1963]. Similar observations were made at 6.7 and 9.5 MHz by Appleton [1934]. However, tests carried out at 13 and 20 MHz by Budden and Yates [1952] failed to provide evidence of longdelay echoes at these frequencies, and airborne listenings at 13.7 MHz by Gassman et al. [1965] were also inconclusive. Manczarski [1964] performed experiments with links opera-Copyright ¸ 1971 by the American Geophysical Union. ting between 6.075 and 17.180 MHz. He reported the reception of long-delay echoes for several per cent of the observation time at frequencies near the lower end of his tuning range and also saw some rare occurrence of these echoes near the upper end. Booker [1962] has shown that if magnetospheric wave guidance is involved in cosmic echo events the probability of their occurrence decreases sizeably above approximately 10 MHz. From experimentation conducted with HF radars, echoes with delays up to a few hundred milliseconds have been observed and interpreted in terms of wave guidance in the laminar structure of the magnetosphere [Obayashi, 1959; Gallet and Utlaut, 1961; DuCastel, 1963]. However, an interpretation in terms of transmission below the ionosphere has not been eliminated [Booker, 1962]. Rumi [1968], working with a 13.866-MHz radar, obtained lunar echoes with the antenna beam aimed in the direction of the geomagnetic field and concluded that the em waves were strongly guided i...

Ionospheric propagation of HF and VHF radio waves across the geomagnetic equator

“…On the other hand, the postmidnight irregularities, evidenced on both bottom side (frequency-spread) and top side (ducting) ionograms, may satisfy TEP guidance conditions at low magnetic latitudes (•20 ø) when terminals are magnetically conjugate. Ducting at higher latitudes is very rare and limited to frequencies in or below the low HF band [Grossi and Padula-Pintos, 1971].…”