Sprites in low‐frequency radio noise

Abstract

Low‐frequency radio noise is the electromagnetic background radiation which is compared here to the luminosity of 39 sprites recorded with a low‐light video camera. It is found that the sprite luminosities coincide with ∼10–30 ms long sudden enhancements of the electromagnetic background radiation ∼6–8 μV m−1Hz−1/2(∼6–9 dB) with a relative maximum near ∼125 kHz as measured with a wideband (∼1–400 kHz) digital radio receiver. The sprites cluster in 10 groups of 2–5 consecutive sprites which are paralleled by up to ∼1 s long slowly varying enhancements of the electromagnetic background radiation ∼4–5 μV m−1Hz−1/2(∼2–4 dB). The observed electric field strengths place an upper bound on the low‐frequency radiation from the electron multiplication associated with the exponential growth and branching sprite streamers predicted by Qin et al. [2012a]. This upper bound corresponds to a maximum of ∼300–5000 sprite streamers at ∼40 km height above thunderclouds. Some part of the observed electromagnetic background radiation might result from the superposition of low‐frequency radiation emanating from the quick succession of numerous horizontal lightning strokes and/or stepped leaders inside thunderclouds which would constitute a fundamentally novel quasi‐static discharge process inside thunderclouds radiating slowly varying low frequency radio noise.