GCN Circular 2176

P.W. Schnoor, D.L. Welch, G.J. Fishman and A. Price report, on behalf 
of the AAVSO GRB-SID Network, on the detection of GRB030329 as a sudden 
ionospheric disturbance (SID), observed by Peter Schnoor of Kiel, Germany.

A disturbance of the Earth's ionosphere was observed coincident with the
HETE detection of GRB030329. This SID was seen as an increase in the signal
strength from a Low Frequency (LF) radio beacon received in Kiel, transmitted 
as a time signal from station HBG (75 kHz) near Geneva, 920 km from the 
receiver. (Note: This is not a radio detection of GRB030329; this
disturbance was caused by the prompt X-rays and/or gamma-rays from GRB030329
ionizing the upper atmosphere and  modifying the radio propagation properties 
of the Earth's ionosphere.) Due to the sub-burst longitude and latitude and 
the geographical distribution of LF/VLF beacons and monitoring stations, this 
was the only recording (positive or negative) where GRB030329 illuminated the 
ionosphere along a signal path.

Several plots of the SID detection, including one with an overlay of 
the HETE X-Ray lightcurve are available at the URL
          http://www.qsl.net/df3lp/projects/sid/index.html
Additional details of the observation are also available at this site.

While this type of observation is not yet quantitative, future observations
of enough GRBs may allow a quantitative measurement to be made of the prompt, 
total ionizing flux (X-rays and gamma-rays) incident at the Earth over an 
extremely broad energy range. This measurement is not now attainable with any 
single spacecraft and will not be, until the launch of the NASA GLAST mission 
in 2006. 

Previously, at least three other transient, high-energy sources have 
produced detectable ionospheric disturbances, as measured with VLF
receivers: GRB830801 (Fishman and Inan, Nature v.331, p.418, 1988); XRF
020427 (GCN 1394
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 ), and the Aug. 27, 1998 super-flare from SGR 1900+14 (Inan,
et al., Geophys. Res. Lett., v.26, p.3357, 1999).

The AAVSO SID-GRB network is a worldwide network of observers monitoring 
VLF and LF beacons for SIDs of non-solar origin. The AAVSO Solar Committee 
has been monitoring and reporting solar-induced SIDs since the 1950's. This 
group intends to continue and expand this monitoring network.

This message may be cited.