F.3 Additional information on short-term variation

The profile given in Table 6‑4 starts from medium activity levels. Then an x-ray / gamma-ray flare occurs. On the sun-surface this goes together with the ejection of highly energetic charged particles. The flare can push up the F10.7 index near Earth almost instantaneously. The a_p is still at its normal value for medium activity between 7 and 15. The F10.7 then fades within about 9 hours to pre-flare values. After a little less than a day the fastest solar wind particles arrive at Earth, disturb the magnetic field, and thus the a_p index increases. The disturbance lasts only about 6 hours. Thereafter it settles at its pre-event levels between 7-15 hours. The F10.7 stays at its average value.

The profile covers 1 day. It can be recurring as a disturbance event can easily last about a week. During such an event, flares occur repeatedly within the same active region on the sun. Extreme values of F10.7 and a_p do not usually occur simultaneously. Such a situation only occurs at the Earth if a second flare, essentially unrelated to the first, occurred on the sun at the same time that the charged particles from the first flare arrived at Earth. That situation is possible but happens rarely and seems not to have occurred within the historical data set.

The activity profile given in Table 6‑4 describes a “worst case” event but is neither unrealistic nor overly conservative. The x-rays (high F10.7) can lead to enhanced intensities and heating at equatorial and low latitude regions where the solar radiation intensity is highest. The high a_p values lead mainly to an increase of atmospheric density in the high latitude and polar regions where the effect of dissipation of energy and momentum from the magnetosphere is largest.

The short term variation profile in Table 6‑4 addresses a_p only. Short term variations of the geomagnetic field have larger effects on the atmosphere than short term variations of the solar activity. The atmospheric effects of solar activity variations on time scales of hours are still uncertain. Large short term variations of a_p can occur during all phases of a solar cycle.