The graphs below show an event which is known as a Solar Flare Effect or
sometimes as a Magnetic Crochet. This example occurred on
Nov 04, 1997 during an X2 class flare at around 06 UT. The upper panel
shows the variation of the X-ray flux during the flare indicating
its very rapid rise starting at around 05:55 UT.
The peak of the flare occurred near 05:58 UT - a very short time
during which the X-ray flux increased by a factor of 100.
The lower panel shows the magnetic field recorded at Canberra at the same
time. As the flare started there is a sharp jump
in the magnetic field which
peaked at about the same time as the flare reached its
maximum; and as
the flare began to decline in strength the magnetic field
also decreased towards its pre-flare level. By 06:20 UT
the flare had ended with the X-ray flux
back to C class levels. The magnetic field has already returned to
its previous level by this time.
A magnetic crochet arises from the increased ionisation in the D and E
layers of the ionosphere caused by the massive increase in X-ray
radiation generated by the solar flare. This ionisation changes
the properties (especially the conductivity)
of these ionospheric layers allowing electric currents to flow more
easily. It is the magnetic effect
of these currents which produce the jump in the
earth's magnetic field. As the flare declines, the ionospheric layers
quickly return to their previous state, the electric currents in the
layers return to normal, and the change in the magnetic field
ends.
Magnetic crochets are quite rare because they are only
observed during large flares which rise to a peak very quickly. Also,
they are mostly observed in locations close to the sub-solar point (i.e.
the point on earth when the sun is overhead). In the case of the November
04 event, the sun at Canberra was well to the west. Similar
magnetic effects were observed from many stations in the sunlit hemisphere
at the time.
Educational