Your students can better understand their SID data if they understand the processes going on in the ionosphere. How and why do VLF signals bounce off the ionosphere, and thus provide communication "around" the Earth? Why are the daytime and nighttime SID signals different? Why would one be lower than the other? How does the Sun normally influence the ionosphere? What happens to the ionosphere during a solar flare? During a lightning storm? During a gamma ray burst?

Sometimes the SID monitors will pick up a solar flare by showing a decrease in signal strength, rather than an increase. This occasionally happens and represents a complex situation that students might not understand. The signal received is a sum of several different "modes" bouncing back and forth between the Earth and ionosphere. The modes effectively travel differently -- some are more horizontal, others are more zigzagged.

At any location, the signal is a simple sum of all these modes, but because those modes have different phases, they may well destructively interfere. This particular solar flare likely caused the ratios between the different modes to change in such a way that they interfered with each other at that specific location to a greater degree, and hence the signal strength appeared to drop. Had they taken the same measurement several miles away, they may well have gotten the opposite result.

For lightning-induced electron precipitation events, we also sometimes see positive and sometimes negative perturbations. The complicated mode structure of the signal is the reason. The result can change if we change the location of the receiver (or the transmitter, or the perturbation, for that matter). To help your students visualize destructive and constructive interference, see this Powerpoint presentation on Waves and Vibrations

For projects, they might look into the following questions:

• Compare x-ray flux (photon flow) and ionization levels, using the GOES satellite and their SID data. Is there constructive (additive) or destructive (subtractive) interference from the sky wave?
• What could be ionizing the atmosphere at night to explain the variability in your data?
• Can you verify that the SID nighttime data will not correspond to the x-ray flux? Do you understand why?
• Is there a "moon bounce" effect for a strong flare? If so, does it have to be a full moon and an x-class flare?
• It would be a nice research project to put the size and direction (i.e. signal strength increase or decreased) of flare signals on a map. You might want to have separate maps for sites monitoring different stations. What could you deduce from this?

To get started, your students might like to read The Effect of Solar Flares on the VLF Radio Waves Transmitted in the Ionosphere (Word document)