Don't Get Fooled by Phony Sharks

One of the ways we recognize some solar flares on our radiotelescope charts is by the characteristic "shark fin" signature. The signal rises rapidly and drops slowly over a matter of several seconds. I happened to be listening to the audio when the following shark fin began to grow on the chart...

What I heard was a broad slowly drifting signal that was drifting in frequency. The signal had definite audio structure which made it identifiable as from a human-made source. As it drifted through the frequency on which the receiver was set the shark fin pattern emerged. What is going on here?  The receiver being used on this day was a single sideband receiver for amateur radio. There was no choice for any IF bandwidth except SSB. SSB filters typically have a sharp cutoff on one side and taper more gently on the other side of their bandpass.  As the signal drifted through the filter bandpass, the shape of the filter was revealed on the strip chart.

The chart above was taken on the same morning from the WCCRO radio observatory online data stream using Radio-SkyPipe software. This is a real shark, in other words it is somewhat typical of the signature of a solar flare. The signal was confirmed to of flare origin by simultaneous reception thousands of miles away. One indicator that is often seen with real solar flares is the small bump in the rising edge of the signature. This bump has been referred to as the precursor bump. Its origins are a bit mysterious. The precursor bump can also be seen in VLF observations of Xray solar flares by indirect means.