In the IPS Solar Geophysical Summary you will find a list of the
geomagnetic storms occurring each month. A geomagnetic storm is a
time during which
the direction and the strength of the magnetic field of the
earth is observed to be varying. Whilst these changes are of small amplitude,
not able to be seen on an ordinary magnetic compass, they are
important to many systems such as magnetic surveys
for minerals, and the operation of pipe lines and power lines. They are
also important because geomagnetic disturbances usually
correspond with a disturbed ionosphere and difficult HF
communications.
Geomagnetic disturbances are not equally likely in every month of the
year. The graph illustrates this fact for geomagnetic disturbances
where the daily planetary Ap index is greater than a value of
25 (a disturbance which might occur on average 5 times per month). The data
in the graph come from the years 1932 to 1994 inclusive.
Geomagnetic disturbances tend to occur most frequently in the equinox months of
March/April and September/October. They are least common during the
solstice months of December/January and June/July. For disturbances
of this size there are approximately twice the number of disturbances
near the equinoxes as near the solstices.
If we consider much larger disturbances
then we find an even greater concentration near the equinoxes. So,
if your work is affected by geomagnetic disturbances, then it is wise
to avoid the equinox months if you can!!
Educational