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)
