Sudden Ionospheric Disturbances Supplement

A very strong gamma ray burst, GRB, was detected by the Italian GRB satellite, Beppo SAX on 22 February at 0723.5 UT. It was described as being the strongest GRB Beppo SAX had ever detected. When I saw this notice on the AAVSO's GRB network as a GCN Circular I immediately emailed five of our SID observers to see if any of them recorded anything unusual at that time. All USA observers were in darkness at 0723 UT and none of them reported seeing anything unusual in their nighttime traces that might be the GRB. A few days later I did receive a daytime chart from Danie Overbeek and Domenic Toldo, A-52, that showed a very clear sudden ionospheric disturbance, SID, at that time on all three multiplexed signals that they record, 24 kHz, NAA, in Cutler, Maine, USA; 16 kHz, GBR in Rugby, England, UK and 19'8 kHz, NWC, at Northwest Cape, West Australia. This seemed like a definite recording of an SID at the time of GRB010222 so I sent it to Arne Hendon at the US Naval observatory in Flagstaff, Arizona, USA. Arne posted it on the GRB network where it came to the attention of professional astronomers interested in GRBs who asked many questions about the SID. You can see this Rustrak strip-chart recording at ftp://ftp.aavso.org/grb/sid010222.jpg I was able to present enough data to convince them it was an SID caused by the GRB. The original chart as sent by A-52 is shown below.

There is one thing about the chart that is curious and raised some doubts. The chart shows the SID taking ~6 minutes to rise to maximum and yet the GRB lasted only 30 seconds. A later GCN Circular announced that The Chandra x-ray satellite was detecting the GRB as a strong x-ray source 19 hours after the burst. This suggests that the SID may have been actually a recording of an x-ray transient that accompanied the optical transient that AASO variable star observers attempt to monitor with CCD equipped telescopes to plot its light curve.

The daytime ionization of the D-layer of the ionosphere is maintained by solar ultraviolet. The recombination rate there is very high and responds very quickly to any additional ionizing radiation. X-rays from solar flares disturb the delicate balance between ionization and recombination quite suddenly. These sudden ionospheric disturbances change the propagation characteristics of the D-layer for very low frequency, VLF, radio propagation. Many nations use powerful VLF radio transmitters to communicate with their submerged submarines taking advantage of the laws of optics which allow electromagnetic radiation to be scattered into a conducting medium (salt water) for a small fraction of a wavelength. The propagation paths of these VLF transmitters are very sensitive to SIDs which enhance the signal strength of their signal at the receiving end of the propagation path. AAVSO observers monitor the signal strength of VLF transmitters to detect sudden enhancements of the signal, SES, to detect solar flares. The VLF propagation paths are surprisingly sensitive detectors of solar flares making it possible detect small C-1 C-2 flares and sometimes the weaker B-7 and up flares as SESs. Such good sensitivity is comparable to that of the NASA x-ray satellites. Looking at the SES recording made by A-52 in South Africa of the GRB, a rough guess might be that the x-radiation from GRB010222 was about the same as that from perhaps a C-5 solar flare. An SES chart recording on the next page was made using the propagation path of Italian VLF station ICV as the sensor.

AAVSO Director, Dr. Janet Mattei, has invited Solar Division SID observers to subscribe to the GRB alerts and become part of the GRB discussion group. To find a form to fill out that will get you on the alert mailing list log into http://www.aavso.org/grb/filterdatabase.shtml and fill out the form. If you choose "all" in the right places you will receive all GCN Circulars as emails. This way you will know for yourself when each new GRB is discovered by the Satellites and you can check your SID chart to see if you have anything interesting at the given time. Log into http://www.aavso.org/grb/join.shtml To join the GRB discussion group. Good luck and enjoy.

A new observer joins our SID monitoring group this month. He is Roberto Battaiola, A-96, in Italy. He has been trying to get his receiver on the air and record by computer for some time. Now at last he has everything working and has recorded some SIDs as SESs. These are shown in his chart below that he sent as a DAT file. The file is plotted using Piclogger software that duplicates the format of Rustrak strip-chart recordings. Anyone who would like to use this plotting program can get it free from Al McWilliams who wrote the program. Write to Al at his email address which is --- amcwill417@email.msn.com --- The signal Roberto monitors in his chart recording below is ICV on the island of Sardinia off the coast of Italy in the Mediterranean Sea. ICV transmits on a frequency of 20.27 kHz and puts out a good signal that is also monitored by long-time SID observer Jim Ellerbe, A-63, in Spain. Jim has been very successful for many years monitoring the ICV signal to detect solar flares. The A-96 chart shows the sunrise and sunset patterns at the ends of the chart. Three SESs are recorded between 1000 and 1200 UT. Offsets in the trace may be due to interference but could also be changes in signal strength at the ICV transmitter.

I received SES recordings made on 22 February from A-96 and also A-84 in Switzerland. The GRB occurred during the sunrise pattern on these charts and I do not see the GRB signature. SIDs are hard to find in sunrise patterns. Sunrise patterns in South Africa on the A-52 chart are well before the GRB occurred so its signature as an SID is easy to find. The strange interference marked on the South African chart is caused by VLF signals superimposed on the 50 Hz power lines. Their purpose is to turn off hot-water heaters to conserve power during peak loads.

It is very important to run charts with time set accurately if they are to be used to search for gamma ray bursts. All charts should be run on Universal Time with the dates also in Universal Time. I like to publish charts in the Solar Bulletin that show the whole civil day with its sunrise and sunset patterns. The SIDs are easier to understand if they can be compared to the whole day with sunrise and sunset patterns. This is especially true for those not so familiar with how SIDs are detected as SESs.

CHH

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• Editor's Notes
• Sudden Ionospheric Disturbance Report