Cyclotron resonance precipitation of energetic electrons from the inner magnetosphere

Abstract

Cyclotron resonance between trapped energetic electrons and VLF waves produces pitch angle scattering which leads to electron precipitation in the inner magnetosphere. Previous experiments have shown that in the drift loss cone at L values from 1.5 to 1.8 the energy spectrum of the electrons above 50 keV is often dominated by a single narrow peak. The center energy of this peak varies with L in a manner characteristic of cyclotron resonance between the electrons and monochromatic VLF waves in the vicinity of the geomagnetic equator. The source of the waves is probably VLF communication or navigation transmitters. We report here the results of a study of 680 occurrences of these peaks detected by the low altitude polar orbiting satellite S81-1. The present data, from altitudes between 170 and 270 km, show the resonance peaks only in two restricted longitude zones centered at 110øE and 300øE; this result contrasts with the previous measurements at higher altitudes, which detected peaks over a wide range of longitudes. The majority of nighttime events were near transmitter locations where the cyclotron resonance condition was met by electrons with pitch angles near the edge of the loss cone. Detailed study of the events occurring near 110øE reveals a 27-day periodicity in the frequency of occurrence; peaks are most often observed during minima in solar/geomagnetic activity. The peaks observed at nighttime in both longitude zones have a possible seasonal variation, being most frequent when it is wintertime at the site of the VLF transmitters responsible for the precipitation.

Ground‐based transmitter signals observed from space: Ducted or nonducted?

“…In Table 1 we summarize the likely electron precipitation energies expected from each transmitter described in this study based on the L ‐shells of the peak wave power observed in this study, the transmitter frequency, and assuming parallel (0° wave normal angle) cyclotron resonance interactions at the geomagnetic equator on each of the field lines [ Datlowe and Imhof , 1990]. We are unable to estimate the energy range for nonducted NPM waves; however, we note here that the ducted electron precipitation energy would be ∼0.5 MeV.…”

“…We are unable to estimate the energy range for nonducted NPM waves; however, we note here that the ducted electron precipitation energy would be ∼0.5 MeV. NWC is shown to be the most effective transmitter in terms of the largest energy range of precipitation energies and is also well positioned in being west of the South Atlantic Anomaly which provides increased sensitivity of the loss of scattered electrons into the drift loss cone [ Datlowe and Imhof , 1990].…”

“…Previous theoretical studies into the impact of VLF transmitters on the radiation belt electron population [e.g., Abel and Thorne , 1998a] have generally relied on techniques based on magnetospherically ducted propagation alone [e.g., Inan et al , 1984]. Ducted waves experience equatorial gyroresonant interactions with electrons typically in a narrow energy range of only a few tens of keV, depending on transmitter frequency, location, and the wave propagation conditions described by magnetic field intensity, plasma number density, and wave propagation angle [ Datlowe and Imhof , 1990]. For example, for a ducted 20 kHz transmitter wave at L = 2.0 the equatorial resonant electron energy would be 50 keV.…”

“…However, several studies have shown that a combination of ducted and nonducted VLF waves from lightning‐generated whistlers and communication transmitters can efficiently precipitate radiation belt energetic electrons into the upper atmosphere through equatorial gyroresonance [ Kennel and Petschek , 1966], Landau and higher resonances [ Abel and Thorne , 1998b], and off‐equatorial gyroresonance [ Johnson et al , 1999; Lauben et al , 1999; Peter and Inan , 2004; Inan et al , 2007]. Datlowe and Imhof [1990] suggested that SEEP data showing extended L ‐shell ranges of equatorial cyclotron resonant electron precipitation from VLF transmitters had energies consistent with ducted wave normal angles, but they argued for nonducted wave propagation because of the lack of discrete striations in L ‐shell as would be expected by ducted interactions. Nonducted VLF waves propagate such that they could rapidly spread throughout large portions of the inner magnetosphere, resonantly interacting with a broad range of high‐energy electrons, with highest energies typically >100 keV [ Bortnik et al , 2006a].…”

Differences between transmitter precipitation peaks and storm injection peaks in low‐altitude energetic electron spectra

“…In 1982 the S81‐1 satellite observed a large number of occurrences (680) of VLF transmitter resonance peaks [ Datlowe and Imhof , 1990]. On the basis of the geographic distribution of events, the paper established clearly that the VLF transmitters NWC in Australia and NAA/NSS in the eastern United States are the sources of waves for nearly all of the peaks observed by that instrument.…”