The payload of the polar‐orbiting Ogo 6 spacecraft includes an experiment with four broadband receivers (0.02–15, 15–30, 92.5–107.5, and 280–295 kHz), two narrowband receivers at 200 and 540 kHz, and a broadband intensity detector. The receivers are connected to an electric dipole antenna and thus respond not only to electromagnetic but also to electrostatic waves, such as lower hybrid resonance (LHR) noise. LHR noise bands, observed below auroral latitudes in the range from a few to about 20 kHz are, in fact, among the most intense phenomena observed by the experiment. The broadband intensities of the signals often exceed the full‐scale reading of about 3 mv/m of the broadband detector. The intense signals are usually contained in a bandwidth of only a few kHz. Full‐scale readings are also exceeded by auroral hiss, but its bandwidth often extends from a lower cutoff near the LHR frequency to the highest frequency monitored by the experiment (540 kHz). The intensity and spectral shape of auroral hiss vary both within an event and from event to event, but a typical E field intensity is about 1 μv/m/Hz1/2 at audio frequencies, decreasing to 0.15 μv/m/Hz1/2 at 200 kHz and 0.025 μv/m/Hz1/2 at 540 kHz. For longitudinal propagation in the whistler mode with an index of refraction near unity, this would yield a flux density of about 6×10−17 w/m2/Hz at 200 kHz and 1.7×10−18 w/m2/Hz at 540 kHz, indicating a rapid decrease in the intensity of auroral hiss with increasing frequency. Auroral hiss has been observed with an intensity as high at 4.5 μv/m/Hz1/2 (≃5.5×10−14 w/m2/Hz) at 200 kHz. Under geomagnetically quiet conditions the center of the ‘auroral hiss zone’ extends from about 70° invariant at magnetic midnight, through 75° invariant at 0600 and 1800 MLT, to about 78° invariant at magnetic noon. The zone moves on the average about 5° toward the equator under disturbed conditions. In subpolar and polar latitudes very intense signals are observed in a narrow band near the low‐frequency cutoff of the experiment, suggesting the presence of dc electric fields. Intense VLF hiss has been observed near the equator with the dipole antenna nearly parallel to the geomagnetic field. These signals may result from some type of interaction between the spacecraft and the local plasma, but they could also be related to the recently detected fluxes of trapped low‐energy electrons at very low L values.
Our experiment on the polar-orbiting Ogo 6 spacecraft (perigee 400 km, apogee l100 km) y•elded real time analog data in several'broad band channels and essentially continuous tape-recorded data from two narrow band (200 Hz) receivers operating at 200 and 540 kHz. The results show that the worldwide distribution of signals at 200 and 540 kHz falls into a number of different categories: (1) naturally generated broad band (auroral) hiss at polar latitudes with typical 200-kHz intensities of around 0.1/•V m -• Hz -•/2, maximum intensities of up to several/•V m -• Hz -•/2, and generally lower intensities at 540 kHz; (2) nighttime mid-latitude enhancements of a few microvolts per meter, which probably result either from a superposition of signals from a number of 200-and 540-kHz stations or from interference from intense signals of much higher frequencies; (3) well-defined signal peaks associated with individual ground stations operating at 200 kHz (these upward-propagating signals appear to be guided along the earth's magnetic field and may reach an intensity of several hundred microvolts per meter); (4) striking signal enhancements in the conjugate region of a low-latitude 200-kHz station (Achkhabad), suggesting propagation in the whistler mode to the opposite hemisphere; (5) occasional signal enhancements at the magnetic equator, which are at present unexplained. The observations were made by Ogo 6, which was launched on June 5, 1969, into a polar orbit (82 ø inclination) having a perigee of about 400 km and an apogee of about 1100 km. The experiment had a broad band receiver that covered four 15-kHz bands between 20 Hz and 295 kHz and two narrow band (200 Hz) receivers at 200 and 540 kHz [Carden et al., 1969; Laaspere et al., 1971 ]. In some of our previous studies of Ogo 6 data [Laaspere et al., 1971; Laaspere and Johnson, 1973] we made extensive use of the broad band receiver records in identifying and studying the various phenomena occurring in the VLF and the LF bands (whistlers, LHR noise, broad band (auroral) hiss with LHR cutoff, signals from ground stations, etc.). The broad band data were obtained in real time during passes overNASA's telemetry receiving stations and thus had only a limited spatial coverage. The present study thus had to be based on observations made with the narrow band receivers, whose outputs were tape-recorded at the satellite on a global basis. In normal orbital operation the output of each narrow band receiver was sampled 6.9 times per second. The receivers were followed by low-pass filters, whose response was down 3 dB at 0.32 Hz.Our experiment was connected to an electric dipole antenna that was about 80 ft long tip to tip. (In presenting the results we will assume that the effective antenna length was 20 m.)We will also assume that the sheath effects caused the antenna impedance to be negligible in comparison with the input impedance of the preamplifier. (The preamplifier impedance can be approximated by a resistance of more than 20 Mft in parallel with a capacitance of about 30 p...
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