Observations Suggesting Particle Precipitation at Latitudes Below 40°N

Abstract

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 34ø15'N, 75ø28'W geographic (or 45ø40'N, 352ø20'E geomagnetic coordinates), rapid simultaneous phase and amplitude fluctuations are frequently observed at night. Statistically these events occur more frequently near the equinox (seaso.nally) 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 30øN. 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 ø 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 -a sec -x 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.

VLF propagation disturbances and electron precipitation at mid-latitudes

“…In the two examples presented here, evidence for the enhanced precipitation of energetic electrons at mid-latitudes is obtained from measurements of substorm-related bremsstrahlung X rays and 30-MHz riometer absorption associated with VLF emission activity near L --4. These data support previous suggestions [Belrose, 1968;Reder and Westerlund, 1970; Doherty, 1971] that these mid-latitude VLF propagation disturbances are due to the precipitation of energetic electrons (E • 40 kev).…”

“…In contrast to the prolonged ionospheric events characteristic of geomagnetic storm conditions we discuss in this paper a different class of nighttime ionospheric disturbance also observed as phase or amplitude anomalies on midlatitude LF and VLF transmissions [Belrose, 1968;Reder and Westerlund, 1970;Doherty, 1971; R. N. Bracewell, personal communication, 1972]. These commonly occurring events may last up to 4 hours and are associated with moderate magnetic activity at mid-latitudes (_•100 7).…”

Recurrent VLF amplitude fluctuations: Detection using a filter and correspondence with geomagnetic disturbances

“…The sensitivity of these stations to the disturbances depends on the various radio propagation paths involved, the more northerly paths (all below 58øN dipole latitude) generally showing more of the activity. Doherty [1971] has reported examples of some LF (100 kHz) radio propagation disturbances over lower latitude paths which have the character of rapid amplitude and phase fluctuations and which have power spectra which peak at periods of a few minutes; i.e., the periods are similar to what we report here. We have observed signal level changes with rather long periods, some of the order of hours, in some of the nighttime recordings.…”

A coordinated study of energetic electron precipitation andDregion electron concentrations over Ottawa during disturbed conditions

“…In contrast to the long-lived ionospheric effects described above there have also been observations of short-lived (approximately a few hours duration) D region disturbances at middle latitudes. These effects are detected, e.g., on LF and VLF transmission [Belrose, 1968;Reder and Westerlund, 1970;Doherty, 1971;Potemra and Rosenberg, 1973]. Other studies [e.g., Manson and Merry, 1970;Manson, 1972;Potemra andZmuda, 1970, 1972;Antonova et al, 1972;Jones, 1972;Sears, 1972; Thorne, 1972a, b] have also been concerned with aspects of the mid-latitude D region and its anomalous ionization sources.…”