Lightning‐induced electron precipitation events observed at L ∼ 2.4 as phase and amplitude perturbations on subionospheric VLF signals

Abstract

Lightning‐induced electron precipitation (LEP) events are studied using the Trimpi effect, in which the precipitation‐induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals. In 1983, the phase and amplitude of signals from the NPM transmitter in Hawaii (23.4 kHz) and the Omega transmitter in Argentina (12.9 kHz) were measured at Palmer, Antarctica (L ∼ 2.4), together with the magnetospheric whistler background. The long baseline and over‐sea great circle paths from these two sources make it possible for the observed perturbations to be interpreted using a single waveguide mode theory. Analytical expressions are used to relate the magnitude of the phase perturbations to differential changes in ionospheric reflection height along a segment of the propagation path. The predicted relationship between relative perturbation sizes on the two different signals is compared with measurements. From this information, the whistler‐induced flux levels are inferred to be in the 10−4 − 10−2 erg cm−2 s−1 range and the precipitation regions are inferred to be roughly “circular” in shape, rather than elongated along L shells. Measured amplitude changes tended to be small (∼ 0.5 dB) and negative, as expected from a single‐mode theory, but the ratios of simultaneous amplitude and phase perturbations were slightly larger than the theory predicts, probably due to the effects of an additional mode(s). An assessment of the relative detectability of amplitude versus phase perturbations favors phase perturbations by ∼ 10 dB, irrespective of the detection scheme used.

Massive disturbance of the daytime lower ionosphere by the giant γ‐ray flare from magnetar SGR 1806–20

“…Signals at PA received from other VLF transmitters NAA (24.0 kHz, Cutler, ME) and NLK (24.8 kHz, Jim Creek, WA) are similarly perturbed (not shown), but the better defined NPM signal is used from here on. The NPM-PA signal has been extensively studied for nighttime ionospheric disturbances [e.g., Lev-Tov et al, 1996;Inan et al, 1999], and is well suited due to its singlewaveguide-mode content, as an all-sea-based and midto-low latitude VLF path [Inan and Carpenter, 1987].…”

“…[2] VLF remote sensing is a sensitive technique to detect transient disturbances of the nighttime lower ionosphere ($40 to 90 km altitude), resulting from high energy auroral precipitation [e.g., Potemra and Rosenbert, 1973;Cummer et al, 1997], lightning-induced electron precipitation [e.g., Inan and Carpenter 1987], electromagnetic and quasielectrostatic coupling produced by lightning discharges (e.g., sprites and elves) Moore et al, 2003;Haldoupis et al, 2004;Cheng and Cummer, 2005], cosmic gamma-ray bursts (GRBs) [Fishman and Inan, 1988], and g-ray flares from a magnetar [Inan et al, 1999]. VLF detection of daytime ionospheric disturbances is less common, but include solar X-ray flares [Mitra, 1974].…”

Trimpi perturbations from large ionisation enhancement patches

“…Numerical simulations are important in order to be able to quantify the amplitude and phase perturbations expected for a given transmitter frequency and transmitter-receiver path when one or more LIEs lie along it or close to it. Early modelling of this scenario (Tolstoy, 1983;Inan et al, 1985;Tolstoy et al, 1982;Inan and Carpenter, 1987) was generally restricted to a 2D Earthionosphere waveguide model, in®nite and homogeneous in the direction transverse to the propagation path. Thus, the LIE was assumed to lie directly on the path and extend to in®nity both sides of it.…”

Study of Antarctic-South America Connectivity From Ionospheric Radio Soundings

“…They are attributed to a secondary ionization produced by precipitating electrons with > 50keV energy (helliwell et al 1973) from the radiation belts, which alter the earthionosphere waveguide mode structure, changing the conductivity of the D-region of the ionosphere. The VLF amplitude variations associated with electron precipitating events tend to be negative, as expected from single-mode theory (Inan & Carpenter 1987). The occurrence of VLF bursts is highly variable and presents a close association with geomagnetic storms, when the energetic electron population in the slot region of the radiation belts increases (Peter et al 2005).…”

“…The electromagnetic waves have been considered to be predominantly the whistler waves generated by lightning, and the associated VLF bursts called lightning - LEP (e.g. helliwell et al 1973, Inan et al 1978, Inan & Carpenter 1987. But, statistical analysis of these bursts had shown they occur mostly during the equinoxes (Fernandez et al 2003) in close association with the occurrence of geomagnetic storms (Peter & Inan 2004, Peter et al 2005, suggesting a connection with magnetospheric phenomena.…”