New experimental evidence suggests that every ducted whistler component may precipitate bursts of radiation belt electrons into geomagnetically conjugate ionospheric regions. Strong spatial and temporal associations are seen between transient ionospheric disturbances observed in conjugate regions and ducted whistlers monitored at Palmer Station, Antarctica. The ionospheric disturbances were detected by their characteristic perturbing effects (“Trimpi events”) on subionospheric VLF, LF, and MF signals recorded at Palmer Station and at northern hemisphere sites. Of 74 such events examined on four different days, all were time‐associated with ducted whistlers. In no case was the arrival azimuth or dispersion of the associated whistlers inconsistent with the locations of the conjugate ionospheric disturbances, which were inferred from the configuration of perturbed signal paths. Other whistlers occurring independently of detected disturbances were found to be either weak or to have arrived from regions where a disturbance would not have been detected for lack of monitored signal paths. Signal perturbation onset behavior was consistent with multiple regions of precipitation induced by components of multipath whistlers and with theoretical predictions for ducted whistler‐induced precipitation. The results not only support the hypothesis that ducted whistlers are responsible for burst precipitation of energetic electrons but imply that such bursts may be induced by every ducted whistler component. Since radio energy from a lightning discharge can excite whistler ducts located 2500 km or more away from the flash, every ducted whistler observed at Palmer Station may indicate the presence of precipitation bursts associated one‐to‐one with excited whistler ducts distributed over a 5000‐km‐wide portion of the Earth's surface, and over its geomagnetic conjugate as well. The estimated effect of this precipitation on 70‐ to 200‐keV radiation belt electron populations for 2 < L < 3 is comparable to that predicted as a result of plasmaspheric hiss, indicating that ducted whistlers may contribute as significantly as hiss to radiation belt equilibrium at those electron energies.
Transient perturbations of subionospheric very low frequency (VLF) radiowave signals provide new evidence for lightning‐induced electron precipitation (LEP) events involving short (<1 s) bursts of >1 MeV electrons from the earth's inner radiation belt at L ≤1.8. The signal amplitude changes are attributed to increased absorption in the earth‐ionosphere waveguide and/or alterations of the waveguide mode structure due to localized secondary ionization enhancements produced in the nighttime lower ionosphere and the mesosphere by the precipitating electrons. The otherwise stably trapped electrons are believed to be scattered in pitch angle during cyclotron resonant interactions in the magnetosphere with the lightning‐generated whistler waves. That some precipitation bursts consist partly of MeV electrons is suggested by (i) confinement of the perturbed subionospheric signal path to low magnetic latitudes (L ≤1.8), for which corresponding electron energies for gyroresonance with typical whistler‐wave frequencies in the magnetosphere are >1 MeV, and (ii) the temporal signatures of the perturbation events, which often exhibit an unusually rapid initial recovery (time constant of τ <1 s) followed by further recovery at rates believed characteristic of less energetic events (τ ∼5–20 s). The latter is interpreted as a manifestation of the rapid variation with altitude of the effective loss rate for excess ionization over an exceptionally wide range of mesospheric altitudes (40–70 km) penetrated by the >1 MeV electrons.
Characteristic whistler‐associated amplitude perturbations of subionospheric VLF or LF signals (“Trimpi events”) observed within one second of each other at Palmer Station, Antarctica and at Arecibo, Puerto Rico suggest that ionospheric regions in both northern and southern hemispheres are disturbed together in association with individual lightning flashes. During a one hour period on March 21, 1989, the onsets of 44 out of 47 perturbations measured on a 21.4 kHz signal from Maryland to Arecibo occurred within l s of perturbation onsets measured on a 23.4 kHz signal from Hawaii to Palmer Station. Similar activity occurred before and after this period, and on the preceding and following days. The observations are consistent with the disturbance of geomagnetically conjugate ionospheric regions by multiple bounces between hemispheres of bursts of radiation belt electrons, scattered in pitch angle by whistlers in the magnetosphere. Analysis of patterns of perturbations with corresponding whistler and lightning information from this period suggests that there were at least two distinct ionospheric disturbances in each hemisphere.
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