TITLE: GCN CIRCULAR NUMBER: 32744 SUBJECT: GRB221009A: Detection as sudden ionospheric disturbances (SID) DATE: 22/10/13 21:42:27 GMT FROM: Doug Welch at McMaster University P.W. Schnoor (Kiel Longwave Monitor, Germany), P. Nicholson (Todmorden, UK), D.L. Welch (McMaster University, Canada) A sudden disturbance of the Earth's ionosphere (SID) was observed by the Kiel Longwave Monitor (Germany) and a VLF-Monitor at Todmorden (near Manchester, UK) coincident with the detection of GRB221009A (SWIFT, #32635). This SID was seen as a sudden increase or decrease in the signal strengths from radio transmitters below 100 kHz (19.6 to 63.9 kHz; VLF/LF) received at Kiel and Todmorden. These naval transmitting stations are located at France, Germany, Iceland, Israel, Italy, Japan (Okinawa), United Kingdom and United States. Note: This is not a radio detection of GRB221009A; this disturbance was caused by the prompt X-rays and/or gamma-rays from GRB221009A ionizing the upper atmosphere and modifying the radio propagation properties of the waveguide between ground and ionosphere. According to the SWIFT-BAT refined analysis (RA, Dec = 288.254, 19.809 deg, GCN #32688) GRB221009A was above local horizon at both receiving sites (2022-10-09, 13:16:59 UT). Kiel:      az = 101.8, el = 32.9 deg Todmorden: az = 94.6,  el = 28.3 deg Plots of the SID detection are available at the following URLs: Kiel https://www.qsl.net/df3lp/grb221009/KLM_grb221009a_magnitudes.png Todmorden http://abelian.org/vlf/grb221009a-DHO.png http://abelian.org/vlf/grb221009a-NAA.png http://abelian.org/vlf/grb221009a-NSY.png The Kiel Longwave Monitor consists of a set of very low frequency radio receivers attached to a crossed pair of loop antennas (16m^2 each), monitoring the radio spectrum from 1.0 to 96.0 kHz at 50Hz-steps. The VLF-monitor at Todmorden consists of a set of very low frequency radio receivers attached to a crossed pair of loop antennas (20m^2 each) and a vertical antenna, monitoring the radio spectrum below 100 kHz.