TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1394
SUBJECT: XRF 020427: Sudden Ionospheric Disturbance (SID)
DATE:    02/05/24 19:16:01 GMT
FROM:    Peter Woods at UAH/MSFC  <peter.woods@msfc.nasa.gov>

G.J. Fishman, P.M.. Woods, C. Hossfield and L. Anderson report that the
X-ray rich event of April 27, 2002 was detected as a Sudden Ionospheric
Disturbance (SID) by Len Anderson in South Perth Australia, indicating
that XRF 020427 (GCN 1383), produced a prodigious amount of ionizing
x-ray flux.   The disturbance is clearly seen, lasting about one minute,
in a low-resolution all-day strip chart recording from a Very Low
Frequency (VLF) radio receiver near Perth.  Although a quantitative
measure of the flux of this event could not be determined by this
method, the SID magnitude (and thus the ionizing flux) is comparable to
that observed from the Aug. 27, 1998 super-flare from SGR 1900+14 (Inan,
et al., Geophys. Res. Lett., v.26, p.3357, 1999).  A large flux of
x-rays below ~15 keV was deduced from that flare (op. cit.).  One
intense gamma-ray burst, GRB830801, was also observed to produce a SID
but at a much weaker level (Fishman and Inan, Nature v.331, p.418,
1988).  The strong ionizing present in this burst confirms that this is
a long, x-ray rich event of unknown origin, as suggested by in't Zand et
al. (GCN 1383).

Note: SIDs have been used to study solar flares since the 1950s.  In
this case, the SID was observed as a major change in the propagation of
a continuous VLF radio transmission at 19.8 kHz from Northwest Cape,
Australia.  The VLF radio receiver trace can be seen at:

http://gammaray.nsstc.nasa.gov/~woods/xrf020427.html

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