GCN Circular 2932

P.  Campbell, M. Hill, R. Howe, J.F. Kielkopf, N. Lewis, J.
Mandaville, A. McWilliams, W. Moos, D. Samouce, J. Winkler, G.J.
Fishman, A. Price, D.L. Welch, P. Schnoor, A. Clerkin, and D. Saum
report, on behalf of the AAVSO International High Energy Network
and SID Program, on the detection of the Dec. 27 outburst from SGR
1806-20 (GCN #2920
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 ; Borkowski, et. al) as a sudden ionospheric
disturbance (SID) in the Earth's atmosphere.

A disturbance of the Earth's ionosphere was observed coincident
with the INTEGRAL detection of a burst from SGR 1806-20. This SID
was seen as a change in the signal strength from Very Low Frequency
(VLF) radio transmitters being monitored by stations around the
globe. (Note: This is not a radio detection of SGR 1806-20; this
disturbance was caused by the prompt X-rays from SGR 1806-20
ionizing the upper atmosphere and modifying the radio propagation
properties of the Earth's ionosphere.)

The monitoring stations that report detections of the SID are:

Observer     Receiver Location     Transmitter and Location
Moos         Switzerland           FTA - St. Assie, France 
Hill         Massachusetts, USA    NAA - Cutler, ME, USA
Winkler      Texas, USA            NAA - Cutler, ME, USA
Kielkopf     Kentucky, USA         NAA - Cutler, ME, USA
Campbell     Alberta, CA           NLK - Jim Creek, WA, USA
Howe         Colorado, USA         NML - LaMoure, ND, USA
Mc. Williams Minnesota, USA        NML - LaMoure, ND, USA
Samouce      Montana, USA          NML - LaMoure, ND, USA
Kielkopf     Kentucky, USA         NPM - Lualualei, HI, USA (2nd receiver)
Mandaville   Arizona, USA          NPM - Lualualei, HI, USA
Lewis        California, USA       NPM - Lualualei, HI, USA
Winkler      Texas, USA            NPM - Lualualei, HI, USA (2nd receiver)

The observing method employs the monitoring of distant, powerful
VLF radio transmitters; this is a sensitive monitor of the state
of the lower ionosphere along the radio propagation path. Due to
the sub-burst longitude and latitude and the geographical
distribution of LF/VLF beacons and monitoring stations, this burst
was not detected by active monitoring stations in Germany,
Australia and Canada. However, one monitoring station in
Massachusetts, USA (separate from Hill) did not detect the SID
while being in a good location to do so.

Several plots of the SID detection are available at:
          
http://www.aavso.org/observing/programs/solar/sid-sgr1806.shtml

Additional details of the observations are also available at this
site, including the raw data of these and future observations as
they become available.

Previously, at least four other transient, high-energy sources have
produced detectable ionospheric disturbances, as measured with VLF
receivers:  GRB030329 (Price et. al, IBVS #5415, 2003); 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 is greatful for generous grants from the Curry Foundation
and NASA for the financial support for the High Energy Workshops
for Amateur Astronomers.