A Solar Flare Effect
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.
Material prepared by Richard Thompson