Answer:  A
magnetic storm is period of time during which the magnetic field displays
rapid temporal variation. The causes of magnetic storms are explained,
in general terms, in the Introduction
to Geomagnetism page given on the Geomagnetism website. Briefly, then,
magnetic storms have two basic causes. First of all, let us be reminded
that the Sun is always emitting a wind of charged particles that flows
outward into space away from the Sun itself. Occasionally the Sun emits
a strong surge of solar wind, something called a coronal mass ejection.
When this gust of solar wind impacts upon the outer part of the Earth’s
magnetic field, the magnetosphere, the field is disturbed and it undergoes
a complex oscillation. This causes the generation of associated electric
currents in the near-Earth space environment, which, in turn, generate
additional magnetic-field variations -- all of which constitute a 'magnetic
storm'. The second cause of magnetic storms is due to an occasional direct
linkage of the Sun’s magnetic field with that of the Earth’s.
This direct magnetic connection is not the normal state of affairs in
the space environment, but when it occurs, charged particles, traveling
along magnetic-field lines, can easily enter the magnetosphere, generate
currents, and cause the magnetic field to undergo time-dependent variation.
On occasion, the Sun emits a coronal mass ejection at a time when the
magnetic-field lines of the Earth and Sun are directly connected. Then
we can experience a truly large magnetic storm, which can be easily measured
by magnetic observatories on the Earth’s
surface.
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