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-LoranView by DF6NM.


In the past, there have been a few examples of reported VLF/LF disturbances (SID's) being caused by intense gamma ray bursters.

1998 Aug 27: http://www-star.stanford.edu/~vlf/hail/hail.htm (Fig. 9),

2003 Mar 29: http://www.qsl.net/df3lp/projects/sid/,

However, I do not think the 100 kHz propagation anomaly I observed on Dec. 17th is in any way related to the gamma ray burst on the same day.

Method and description of my observation:

My LoranView system uses a simple direct conversion receiver with a 100.21 kHz LO, feeding the soundcard of a 233 MHz Pentium notebook computer. The audio is sampled at 22.05 kHz, converted to baseband. The software first synchronizes itself to UTC by the pulse arrval times of three chains, and then tries to keeps lock to the rising edge of Sylt pulses. For each of the 64 observed Loran stations, 24 samples (1.09 ms around the expected pulse arrival) are coherently added up for 5 minutes. The averaged impulse responses are then referenced to the phase of Sylt, and scaled by the reciprocal distance and inverse square root of listed TX power. Then they are stored in a bitmap using a quasilinear, saturating amplitude scale (~a/(1+a)) for brightness and phase for colour hue (see colourscale.png). The bitmaps can be converted back to complex voltage with reasonable precision over a range of about 60 dB. The last observation channel is set to 100000 cycles repetition, and monitors GPS 1-pps pulses inserted capacitively into the receiver input. Its is not really used for timing reference, just as a control to check consistency.

In the appendix, you can find the interesting part of the original bitmap for Dec. 17th, along with a stacked plot of the received powers (incoherently added over each impulse response) versus time. The bitmap was rotated right to have the 5 min/pix slow time axis left to right. The received pulses are then displayed at 44 µs/pix downward. The power plot has 30 dB boxes. There are three small dips (at 4:30, 13:45, 23:00) which are common to all traces and are artifacts due to loss of sync and automatic reaquisition.

Looking at the Ejde trace (9007m, fifth from the top), you can see the normal diurnal evolution of the path. At night, the main component is E-layer skywave, appearing with nearly constant phase in bright blue. After a transient period with advancing phase around sunrise, it disappears and is replaced by an earlier, yellow reflection which I attribute to the D-layer, with a strength and phase following the elevation of the sun.

The first peculiarity can be seen between 1:55 and about 3 hours, where the Ejde trace abruptly shifts upward and turns yellow. The power dropped by about 6 dB, with 10 dB dips at the beginning and end which are presumably the result of integrating across nearly-antiphase contributions. The similarity to the daytime pattern suggests a sudden and transient increase of ionisation in the D-region.

A second, inverse anomality happened at midday between 12:05 and 12:45, when the E reflection briefly reappeared, suggesting a transparent "hole" in the D-layer.

The effect is most pronounced on the Ejde signal, but correlated phase changes can be seen on Jan Mayen, Verlandet, Bo and Berlevag. Interestingly, Newfoundland / Labrador paths (top three traces) were not affected. This suggests a limited region of interaction, somewhere around the North Sea or southern Scandinavia.

Properties of the Gamma Ray Burster:

The GCN has a number of notes on GRB 061227 at http://gcn.gsfc.nasa.gov/other/061217.gcn3 (appended text file). The Swift burst alert telescope (BAT) was triggered by a short spike at 03:40:08 UT which lead to the alert (GCN # 5926). Several instruments were immediately turned to the source position (RA 10:41 DEC -21:07) to observe possible x-ray or optical afterglow.

Comparing the two events:

- Time of occurence:

The first LF anomality began at 1:55 UT (+- 5 minutes), about 105 minutes before BAT triggered at 03:40 UT. In my opinion, this definitely rules out any connection between the two events. GCN 5931 explicitly mentions that no pre- or post-trigger activity was detected. I have also rechecked that my display is indeed showing UT.

- Duration

The GRB produced a single burst with about 400 ms duration. Generally, most GRB's last only a few seconds, compared to tens of minutes for solar flares. My current five-minute averaging would not be able to detect anything shorter than a minute. After DF3LP's observation, there was a discussion in the AAVSO board whether the induced SID could have lasted longer than the irradiation, but apparently there were no indications for that.

- Location

The reported position of the GRB was about 20 degrees above my horizon, almost due south. It would have been visible up to a latitude of 69 N, including the supposed SID region. This still leaves us with the question why the GRB should have any special effect at low angle rather than from the zenith.

- Intensity:

The SID's in the past were all related to "monster GRB's" of unusual intensity, whereas 061217 was comparitively weak.

- Dual SID's:

Even if the GRB had matched the time of the first SID, we would still have no explenation for the second anomaly after 12:05.

I think that in principle Loran-C based observations are very good at detecting SID's, I can normally see all solar class-C x-ray bursts. The coherent and wideband character of the signals allows us to precisely measure the phase of different skywave components. The large number of transmitters distributed ocross the northen hemisphere gives an instant view of LF propagation. If several receiving stations would take simultaneous recordings, the interaction zone could be spatially defined or even imaged using tomographic principles. With a minimum practical integration time of perhaps one or two seconds, the detection of very strong GRB's seems quite feasible.

So what did I see on Dec 17th? I still have no idea. Could it have been just a natural irregularity in the ionospheric layers, or some sub-threshold aurora effect? I have speculated about artificial ionospheric heating, but EISCAT / Tromsoe would be too far north. Or has Rudi eventually been put out of work, and Santa's sleigh is now equipped with a new ion drive?

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