Lightning as an embryonic source of VLF hiss

Abstract

Data from the DE 1 satellite show that lightning‐generated whistlers often trigger hiss emissions that endure for up to 10‐ to 20‐s periods. The data consist of the measured electric and magnetic fields in the frequency range of 1.5 kHz to 6.0 kHz, during 22 DE 1 passes during the period December 28, 1986, to January 18, 1987. The 22 passes were nearly identical in terms of their projections on a magnetic meridional plane, and they covered L shells of 3.4 to 5.1 and geomagnetic latitude of 20°N to 40°S in the afternoon (∼1400–1500 MLT) sector. The geographic longitudes of the 22 passes were within ±50° of 80° W, and the geomagnetic activity during the period covered was relatively quiet (ΣKp<20 for most of the days). The whistler‐triggered hiss emissions were observed on 16 of the passes, and they generally exhibited the following characteristics: (1) emission spectra were wide band (1–2 kHz) and rather structureless, (2) well‐defined and sustained fading patterns were observed at twice the spin frequency over 10‐ to 20‐s periods, (3) the spin fading characteristics of the triggered hiss bursts were similar to those reported for background plasmaspheric hiss (Sonwalkar and Inan, 1988), indicating a large wave normal angle with respect to the ambient magnetic field. Our results indicate that lightning‐generated whistlers may be an important embryonic source for magnetospheric hiss and that whistlers and emissions triggered by them often constitute the dominant wave activity in the ∼1.5‐ to 6‐kHz range on L shells of 3.5 to 5 in the afternoon sector during geomagnetically quiet periods. Through cyclotron and Landau resonant scattering, it is likely that these lightning‐generated waves play a dominant role in the loss of ∼0.5‐ to 50‐keV electrons trapped on these field lines in the afternoon sector. Through anisotropic proton instability, these waves can also interact with ring current protons in the range of several tens of keV leading to a loss mechanism for ring current protons.