next up previous
Next: Los Alamos Sferic Array Up: Introduction Previous: Introduction

Lightning LF/VLF Radio Emissions

The transient electrical activity of thunderstorms generates electromagnetic radiation events known as atmosferics, or sferics. A typical return stroke produces radiation with peak energy at $\sim$10 kHz. Radiation at these frequencies propagates through the earth-ionosphere waveguide and can be observed at large distances ($>$ 2000 km) from the source. The large current return stroke usually produces the strongest electric field transient and other lower-amplitude pulses in electric field change waveform can be attributed to other aspects of the discharge process (e.g. channel tortuosity and branching [Willett et al.(2000)], leader activity [Heavner et al.(2002)], or general intra-cloud activity).

In terms of LF/VLF electromagnetic radiation, intracloud lightning is generally not as well characterized as cloud-to-ground (CG) lightning. This is primarily because CG lightning is a much stronger radiator at these frequencies. Intracloud lightning can not, in general, be observed at distances as great as CG lightning. For example, one study of intracloud lightning rates used sensors with a range of detection of approximately 14 km [Mackerras et al.(1998)]. VHF systems (such as the New Mexico Tech Lightning Mapping Array or the Kennedy Space Center Lightning Detection and Ranging System) are sensitive to intracloud lightning within a range of approximately 150 km.

Intracloud lightning activity is of interest for several reasons. First, optical satellite observations may be more sensitive to intracloud lightning than to CG lightning [Light et al.(2001),Boccippio et al.(2000)]. LASA is a system which provides ground truth for satellite observations of intracloud lightning activity (specifically CIDs) over a large spatial region (1000 km in radius or more). Second, the possibility of determining thunderstorm convective state based on the ratio of intracloud and CG lightning activity [Williams et al.(1989),Boccippio et al.(2001)]. Finally, monitoring total lightning activity on a large scale until continuous satellite observations are available. The difficulty in large-scale, ground-based observations of intracloud lightning, and the fairly long range observation capabilities of LF/VLF systems has motivated the routine identification of intracloud lightning by LASA.


next up previous
Next: Los Alamos Sferic Array Up: Introduction Previous: Introduction
Matt Heavner 2003-04-02