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ANGEO

Annales Geophysicae

ANGEO

Ann. Geophys.

1432-0576

Copernicus Publications

Göttingen, Germany

10.5194/angeo-25-2175-2007

Low-frequency ionospheric sounding with Narrow Bipolar Event lightning radio emissions: regular variabilities and solar-X-ray responses

Jacobson

A. R.

¹ Holzworth

¹ Lay

¹ Heavner

² Smith

D. A.

Earth and Space Sciences, University of Washington, Seattle, WA, USA

Physics Dept., University of Alaska Southeast, Juneau, AK, USA

ISR Division, Los Alamos National Laboratory, Los Alamos, NM, USA

06 11 2007

25 10 2175 2184

2007

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We present refinements of a method of ionospheric D-region sounding that makes opportunistic use of powerful (10<sup>9</sup>–10<sup>11</sup> W) broadband lightning radio emissions in the low-frequency (LF; 30–300 kHz) band. Such emissions are from "Narrow Bipolar Event" (NBE) lightning, and they are characterized by a narrow (10-μs), simple emission waveform. These pulses can be used to perform time-delay reflectometry (or "sounding") of the D-region underside, at an effective LF radiated power exceeding by orders-of-magnitude that from man-made sounders. We use this opportunistic sounder to retrieve instantaneous LF ionospheric-reflection height whenever a suitable lightning radio pulse from a located NBE is recorded. We show how to correct for three sources of "regular" variability, namely solar zenith angle, radio-propagation range, and radio-propagation azimuth. The residual median magnitude of the noise in reflection height, after applying the regression corrections for the three regular variabilities, is on the order of 1 km. This noise level allows us to retrieve the D-region-reflector-height variation with solar X-ray flux density for intensity levels at and above an M-1 flare. The instantaneous time response is limited by the occurrence rate of NBEs, and the noise level in the height determination is typically in the range ±1 km.

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