“Early/slow” events: A new category of VLF perturbations observed in relation with sprites

Abstract

Analysis of subionospheric VLF transmissions, observed in relation with sprites, has led to the identification of a new category of VLF perturbations caused by the direct effects of tropospheric lightning on the overlying lower ionosphere. They constitute a large subset of the so‐called “early/fast” events where now the term “fast,” which implies rapid onset durations less than ∼20 ms, does not apply. In contrast with early/fast, the perturbations have a gradual growth and thus “slow” onset durations ranging from about 0.5 to 2.5 s; thus these events are labeled herein as “early/slow.” They are indicative of a new physical process at work which, following a sprite‐causative cloud‐to‐ground discharge, leads to a gradual buildup of conductivity changes in the lower ionosphere which must be responsible for the long onset durations of the observed perturbations. Analysis of broadband VLF sferic recordings, made with a two‐channel receiver near the sprite producing storms, shows that the growth phase of an early/slow event coincides with the occurrence of complex and dynamic lightning action. This is composed of a few sequential cloud‐to‐ground lightning strokes and clusters (bursts) of sferics which are attributable to intracloud lightning. We postulate that the long onset durations are due to secondary ionization buildup in the upper D region below the nighttime VLF reflection heights, caused mainly by the impact on sprite‐produced electrons of sequential electromagnetic pulses radiated upward from horizontal in‐cloud discharges.

ELF/VLF signatures of sprite-producing lightning discharges observed during the 2005 EuroSprite campaign

“…A mechanism involving a quasielectrostatic (QE) field generated above the cloud in the upper atmosphere soon after a +CG lightning discharge has been proposed for the excitation of sprites (Inan et al, 1991;Pasko et al, 1995). Moreover, the intra-cloud (IC) horizontal discharges are also believed to play a role in sprite initiation (Adachi et al, 2004;Ohkubo et al, 2005;Haldoupis et al, 2006;van der Velde et al, 2006) as theoretically proposed (Valdivia et al, 1997;Rycroft and Cho, 1998) but their role is not yet well understood.…”

“…Sprites have been investigated in the US since the early 1990's while extensive research in Europe began only in 2000 (Neubert et al, 2001). Although the thunderstorms over Southern Europe are generally less intense compared to the mesoscale convective systems (MCS) over the US high plains, hundreds of sprites have been imaged throughout the EuroSprite campaigns over France with cameras placed at the Pic du Midi Observatory in the French Pyré né es Haldoupis et al, 2006).…”

Underlying mechanisms of transient luminous events: a review

“…The experimental evidence provided by the Crete receiver together with Nançay broadband time series has shown that the early/slow VLF events can be accompanied by a few sequential CG lightning strokes and bursts of IC lightning (Neubert et al, 2008). It was hypothesized by Haldoupis et al (2006) that the observed ionization of the lower ionosphere builds up as a result of sprite-produced electrons accelerated by sequential electromagnetic pulses radiated upwards from horizontal IC discharges. One may speculate that the electron impact upon the ionosphere can result in the generation of secondary electron avalanche thereby ionizing the ionosphere.…”

Optical and VLF Imaging of Lightning-Ionosphere Interactions

“…Completion of an ion-chemistry model of the upper mesosphere/lower ionosphere [Inan et al, 2006;Lehtinen and Inan, 2007] has enabled research into the temporal evolution of transient ionization in the earth-ionosphere waveguide (Both LEP and Early VLF events). In agreement with previous results [e.g., Pasko and Inan 1994;Inan et al, 1988], the model predicts relaxation time scales which span several orders of magnitude, and are dependent on the altitude range in which the transient ionization is located.…”

“…High school students can view 1-s resolution VLF amplitude or phase data, recorded at their host school or at any other HAIL site, and explore ionospheric effects of recent events such as solar storms, galactic gamma ray bursts, and local thunderstorms. The various MATLAB-based analysis software developed by Stanford for the HAIL research project are being used by interested high school students at the schools that house our equipment, as well as by collaborating researchers from other institutions [e.g., Haldoupis et al, 2004Haldoupis et al, , 2006Mika et al, 2005Mika et al, , 2006.…”