Geology Reference
In-Depth Information
Figure 3.4
Typical 'quiet day' magnetic field variation at mid-latitudes.
The magnetic field within about 5
◦
of the magnetic equator is strongly
influenced by the
equatorial electrojet
, a band more than 1000 km wide of
high conductivity in the ionosphere. Diurnal variations in the affected regions
may be well in excess of 100 nT and may differ by 10 to 20 nT at points only
a few tens of kilometres apart. In polar regions, similarly severe short-period
fluctuations can be explained by the existence of
auroral electrojets
.Itis
therefore particularly important in these regions that background variations
are monitored continuously. Returning to a base station at intervals of one
or two hours may be quite insufficient.
3.2.4 Magnetic storms
Short-term auroral effects are special cases of the irregular disturbances
(Ds and Dst) known as
magnetic storms.
These are produced by sunspot
and solar flare activity and, despite the name, are not meteorological, often
occurring on clear, cloudless days. There is usually a sudden onset, during
which the field may change by hundreds of nanotesla in a few minutes,
followed by a slower, erratic return to normality. Time scales vary widely
but the effects can persist for hours and sometimes days. Micropulsations
are generally at their strongest in the days immediately following a storm,
when components with periods of a few tens of seconds can have amplitudes
of as much as 5 nT.
Ionospheric prediction services in many countries give advance warning
of the general probability of storms but not of their detailed patterns, and
the field changes in both time and space are too rapid for corrections to be
applied, unless high-frequency (10 or 20 Hz) base stations are used. Other-
wise, the only solution is to wait until the storm is over. Aeromagnetic data
are severely affected by quite small irregularities, and for contract purposes
technical magnetic storms
may be defined, sometimes as departures from
linearity in the diurnal curve of as little as 5 nT in an hour. Similar criteria
