April activity discussion
In the week of April 3, we saw some classic SID events (see http://www.spacenv.com/~rice/vlf/prv-nmlThru20060409.png), specifically 3 Apr 2130 UT, 5 Apr 1530 UT, and a real beauty on 6 Apr 2030 UT.
There were also some bursts that I'm very curious about: 8 Apr 2000 UT through 9 Apr 0200 UT, 14 Apr 2200-2330 UT, and 17 Apr 1700-2000 UT. There were significant auroral disturbances on 9 and 14 April, but nothing particular on 17 April (poke around in http://www.sec.noaa.gov/ftpmenu/plots/kp.html to see geomagnetic activity levels.) So did anyone else see these bursts, or are they some kind of local interference? Could they be
whistler or chorus emissions? Our operating frequency is rather high for that sort of thing,
but I believe they have been observed up to 40 kHz in some cases. Guess we need an
AWESOME receiver to sort that out.
Nick Gross's comments on quiet time activities are very good. One thing I would add is recording weather events, particularly thunderstorms. Thunderstorms can produce spikes
in the data and may have other effects, even at considerable distances.
Regarding radio propagation, VLF signals are a little tricky. The wavelengths are so long
(tens of kilometers) that they get into a sort of waveguide situation between the earth
and ionosphere. But basically you are correct, the conditions we see in the signal are due to
things going on between the receiver and the transmitter. Rather than simply being reflected
between the transmitter and receiver like shortwave signals, VLF signals can also be soaked up by the lower ionosphere during the day (D region absorption) and may be bounced around in complicated ways around sunrise and sunset. Does Stanford have any VLF ray-tracing (or waveguide mode) software they would care to share with the effort? The stuff I have is for HF and above. Comparing computer predictions with observations would be extremely interesting during quiet conditions.
One activity I would suggest for students interested in the behavior of radio waves at lower frequencies is to keep track of AM broadcast stations with an ordinary AM radio. During the day, you will only hear local stations due to D region absorption. As the sun sets, you will start to hear more distant stations, and after dark the AM band is jam-packed with stations, often talking on top of each other. In the morning you can follow sunrise, as stations in the east fade out before local dawn...then all of the distant stations go away around local sunrise. Here in Utah, I can hear stations from California to Ohio, Mexico to Canada at night. There are all sorts of strange fading and fluttering effects that illustrate various basic physics principles. The
main drawback with listening to AM stations is the content isn't very interesting, especially
at night (unless you are into Art Bell...) Anyway, if you compare the behavior of distant AM
stations to your VLF signal, you will see similarities and differences that are due to the behavior of the ionosphere at the different frequencies.
There is also a rather dramatic change in behavior between winter and summer seasons. I'm
seeing it in the VLF data this month. You can see very definite seasons in our various data sets.
A given day may not be very exciting, but if you collect data for several months, all sorts of
patterns and trends begin to pop out. See if you can figure out the plots on:
Why do the radio station signals have a sort of sine wave variation during the year? Can you
see effects of the Halloween 2003 geomagnetic storm in the 2003 data?
When we collect enough data I'll plot the VLF signal in the same format...should look cool,
though the Tuesday shutdowns are going to mess up the natural variations a bit...