Observations Suggesting Particle Precipitation at Latitudes Below 40°N

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Low‐frequency pulse transmissions are received over reciprocal paths after being reflected once from the ionospheric D region. The daytime reflection height is near 70 km and the nighttime height is near 90 km. Over an 1830‐km path between Nantucket, Massachusetts, and Jupiter Inlet, Florida, with a midpoint at 34°15′N, 75°28′W geographic (or 45°40′N, 352°20′E geomagnetic coordinates), rapid simultaneous phase and amplitude fluctuations are frequently observed at night. Statistically these events occur more frequently near the equinox (seasonally) and near dawn (diurnally). They seem to occur more frequently on the path indicated above than on higher latitude paths at quite different geomagnetic longitudes; the events have not appeared at all on paths at geomagnetic latitudes below 30°N. A possible explanation for the more frequent occurrence on the Nantucket‐Jupiter path is the fact that the midpoint lies slightly west of the magnetic conjugate to the South Atlantic anomaly. Detailed correlation of both phase and amplitude at both ends of reciprocal propagation paths tends to rule out gradients, offpath reflections, or instrumentation as causes of these events. Investigations of several different propagation paths suggest that these effects may be quite localized but that paths with similar geomagnetic latitudes can be similarly affected even when they are separated by more than 100° in geomagnetic longitude. LF ionospheric propagation is very sensitive to relatively small changes in electron densities in the D region. The events described above become less pronounced as the path length increases; this pattern suggests that they are quite localized and that the integrating effect of longer paths tends to mask the effects. Particle fluxes capable of producing electrons in the 10‐ to 100‐electron cm−3 sec−1 range near 80 km at night would be expected to produce variations such as those measured. These LF phase measurements over oblique paths may prove to be an excellent method for detecting leakage or dumping of charged particles from the radiation belts into the lower ionosphere.