GCN Circular 32744

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
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 ) 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.