TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2176 SUBJECT: GRB030329 observed as a sudden ionospheric disturbance (SID) DATE: 03/04/28 22:38:19 GMT FROM: Doug Welch at McMaster U,PhysAstro. 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), 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.