Observations of the Earth's continuum radiation in the distant magnetotail with ISEE 3

The authors analyze observations of the Earth's nonthermal continuum emission made between 30 and 80 kHZ with the radio receiver on board ISEE 3 when the spacecraft was in the distant magnetotail and crossed the magnetopause many times, and when the magnetosheath plasma density varied over a wide range. They correct for quasi-thermal noise by calculating its magnitude from measured plasma parameters and subtracting it from the data. The intensity and the anisotropy of the nonthermal continuum radiation field are always found to be larger in the lobes than in the magnetosheath; i.e., the apparent source is less bright and its size is larger when observed from the magnetosheath. These properties sometimes extend to frequencies as high as 80 kHz. They interpret the radiation as arising from a source located at or near the plasmapause, much closer to Earth than the spacecraft distance {approx equal}200 R{sub E}. They suggest that the observed variations across the magnetopause of the intensity and anisotropy can be understood by taking into account the facts that the magnetotail is a plasma waveguide whose walls are sometimes rough and leaky and that there are density fluctuations in the magnetosheath. The efficiency with which the continuum powermore » at 30 kHz is collected in the near-Earth regions and ducted toward the distant tail is a strong, increasing function of the magnetosheath plasma frequency f{sub p,msh}. The percentage of that power which travels in the magnetosheath decreases with increasing f{sub p,msh}. There is no need for a radiation source in the deep magnetotail regions to account for the nearly isotropic radiation observed in some locations and at some frequencies. In the few cases in which they could derive an upper limit to the spectral density due to a local source, it was less than 10 to 20% of that of quasi-thermal noise at frequencies {approx gt} 1.7 {times} f{sub p,msh}.« less