Low-frequency noise observed in the distant magnetosphere with OGO 1

Abstract

Two new types of low‐frequency noise, designated ‘broadband’ and ‘highpass’ have been detected in the distant magnetosphere by the VLF/LF experiment on the OGO 1 satellite. Broadband noise extends over the entire range of observations from 0.2 to 100 kHz and the intensity decreases with increasing frequency. It occurs in bursts having durations of a few minutes or less. It shows no connection with any of the expected plasma cutoff or resonance frequencies and is believed to be a nonpropagating disturbance generated in the vicinity of the satellite. Highpass noise extends from a characteristic low‐frequency cutoff to above 100 kHz and occurs in bursts lasting tens of minutes. This cutoff has only been observed above 20 kHz. Above the cutoff the intensity shows little variation with frequency. Both types of noise are observed predominantly at L greater than 5 in or near the night hemisphere. The occurrence of both types of noise is highly correlated with the auroral electrojet index. Several times noise bursts began within 2 min of the onset of micropulsations in the polar region even though the satellite was near apogee (24 earth radii). The peak rms magnetic intensity in a highpass noise burst has a maximum of 8×10−5 γ Hz1/2 and averages 10−5 γ Hz−1/2. In free space these magnetic intensities correspond, respectively, to 10−12 and 3×10−14 watt m−2 Hz−1. The peak levels of both types of noise are 3 or more orders of magnitude stronger than emissions from extraterrestrial sources observed with interplanetary probes.

First results from the Cluster wideband plasma wave investigation

“…The first observations of this radiation were by Benediktov et al (1968) who detected a radio emission of unknown origin escaping from the Earth at frequencies of 725 and 1100 kHz that was correlated with the Kp planetary magnetic disturbance index. Dunckel et al (1970) also observed what they termed high-pass noise, primarily due to the low frequency part of the spectrum below about 100 kHz. A few years later, Gurnett (1974) showed that the radiation had its peak intensity in the frequency range from about 100 to 500 kHz, that it was very intense, approximately 10 9 Watts of total radiated power, and that it was closely correlated with the occurrence of discrete auroral arcs.…”

On the polarization and origin of auroral kilometric radiation

“…Although many spacecraft have made observations of these rad io emissions , including OGO 1 [ Dunckel et al, 1970] , IMP 6 [Brown, 1973;Gurnett , 1971d , Hawkeye 1 [ Kurth et al , 1 975 ] , IMP 8 [Gurnett, 1975] , and RAE 2 [Alexander and Kaiser, 1976] , up to the present time none have ;L taiued measurements at low-altitudes over the auroral zones wherẽ~~ ~~r ora l kilome tric radia tion is bel ieved to be ge nerated . Fortu-:~ately, because of changes in the orbit parameters after launch, the polar-orbiting Hawkeye 1 spacecraft , which was launched on June 3, 19714, has recently been able to prov~.de some limi ted coverage of this very interesting region.…”

Relationship between auroral substorms and the occurrence of terrestrial kilometric radiation

“…Several authors (Renediktov et al, 1968;Dunckel et al, 1970;Gurnett, 1974;Kaiser and Alexander, 1977) have shown that the intense magnetospheric radio emissions known as terrestrial kilometric radiation (TKR) or auroral kilometric radiation (AKR) are correlated both in occurrence and in intensity with magnetospheric substorms as indicated by the auroral electrojet index (AE). In the most detailed study to date, Voots et al (1977) showed that TKR can be a very reliable indicator of auroral disturbances since TKR power flux density and AE are well correlated on a coarse time scale (z 1 hr).…”