Space Weather Monitors SID Users Manual - Stanford Solar Center ...

Space Weather Monitors SID Manual
the Earth, the ionosphere becomes suddenly more ionized, thus changing the density and
location of layers. Hence the term “Sudden Ionospheric Disturbance” to describe the
changes we are monitoring.
It is the free electrons in the ionosphere that have a strong influence on the propagation of
radio signals. Radio frequencies of very long wavelength (very low frequency or “VLF”)
“bounce” or reflect off these free electrons in the ionosphere thus, conveniently for us,
allowing radio communication over the horizon and around our curved Earth.
The strength of the received radio signal changes according to how much ionization has
occurred and at what level of the ionosphere the VLF wave “bounces” from.
Daytime Nighttime
The space weather monitor data graph below shows the changes to the signal strength
caused by sunrise, sunset, and 4 solar flares. (The solar flares are classified by strength,
from low to high as A, B, C, M, or X. These will be described later.)
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