Aurora (astronomy) - Frequency of Occurrence

Frequency of Occurrence

Eurasia These NOAA maps of North America and Eurasia show the local midnight equatorward boundary of the aurora at different levels of geomagnetic activity; a Kp=3 corresponds to low levels of geomagnetic activity, while Kp=9 represents high levels

Auroras are occasionally seen in temperate latitudes, when a magnetic storm temporarily enlarges the auroral oval. Large magnetic storms are most common during the peak of the eleven-year sunspot cycle or during the three years after that peak. Within the auroral zone the likelihood of an aurora occurring depends mostly on the slant of interplanetary magnetic field (IMF) lines (the slant is known as Bz), however, being greater with southward slants.

Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoctes. It is not well understood why geomagnetic storms are tied to Earth's seasons while polar activity is not. But it is known that during spring and autumn, the interplanetary magnetic field and that of Earth link up. At the magnetopause, Earth's magnetic field points north. When Bz becomes large and negative (i.e., the IMF tilts south), it can partially cancel Earth's magnetic field at the point of contact. South-pointing Bzs open a door through which energy from the solar wind can reach Earth's inner magnetosphere.

The peaking of Bz during this time is a result of geometry. The IMF comes from the Sun and is carried outward with the solar wind. The rotation of the Sun causes the IMF to have a spiral shape called the Parker spiral. The southward (and northward) excursions of Bz are greatest during April and October, when Earth's magnetic dipole axis is most closely aligned with the Parker spiral.

Bz is not the only influence on geomagnetic activity, however, the Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. The solar wind blows more rapidly from the Sun's poles than from its equator, thus the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest – by about 50 km/s, on average – around 5 September and 5 March when Earth lies at its highest heliographic latitude.

Still, neither Bz nor the solar wind can fully explain the seasonal behavior of geomagnetic storms. Those factors together contribute only about one-third of the observed semiannual variations.

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