Natural and man‐made emissions at 1.0–5.6 MHz measured between 10 and 18 RE

A survey has been undertaken of the 1.0‐ to 5.6‐MHz electric field spectrum measured with the Active Magnetosphere Particle Tracer Explorers/Ion Release Module spacecraft at distances of 15–18 R E (100,000–120,000 km) from the Earth. Data from one full year of operation were considered in order to obtain complete local time coverage at the ecliptic plane. The data were further binned according to the presence or absence of intense auroral kilometric radiation (AKR) emissions at lower frequencies, in order to resolve emissions processes or interference sources that might be related to the level of AKR. For the times of intense AKR, the principal features found in this survey are (1) the occurrence statistics of the intense AKR with local time generally confirm previous AKR surveys using, for example, the ISEE satellite; and (2) the only 1‐ to 5.6‐MHz emissions unique to the times of intense AKR are the occasional occurrences of signals at the first harmonic of the AKR. The latter observation seems likely to be natural rather than artificial, and it is interesting because so few examples of harmonic AKR have previously been published that the effect has been controversial. For the times of weak or absent AKR, the results of this survey may be summarized as follows: for times of roughly 0800–1600 LT (dayside), a nearly constant level was recorded which at 1–3.2 MHz was equal to the galactic background level, and at 3.2–5.6 MHz was above the galactic backgound level and probably was due to spacecraft‐generated noise; at other local times, the same nearly constant level was sometimes recorded, but frequently bursts of noise at 3–5.6 MHz were observed. These bursts seem more likely to represent ionospheric breakthrough of man‐made radiation rather than natural emissions. A comparison with previous satellite observations implies that the terrestrial background level in this frequency range may have increased considerably between 1970 and 1985.