The Origin of the Aurora



Electric currents originating in such fashion apparently give auroral electrons their energy. The magnetospheric plasma has an abundance of electrons: some are magnetically trapped, some reside in the magnetotail, and some exists in the upwards extension of the ionosphere, which may extend (with diminishing density) some 25,000 km around the Earth.


The convergence of magnetic field lines near Earth creates a “mirror effect” which turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity.


Some O+ ions (“conics”) also seem accelerated in different ways by plasma processes associated with the aurora.


In addition, the aurora and associated currents produce a strong radio emission around 150 kHz known as auroral kilometric radiation (AKR, discovered in 1972).


These “parallel voltages” accelerate electrons to auroral energies and seem to be a major source of aurora.


Other processes are also involved in the aurora, and much remains to be learned. Auroral electrons created by large geomagnetic storms often seem to have energies below 1 keV, and are stopped higher up, near 200 km. Such low energies excite mainly the red line of oxygen, so that often such auroras are red. On the other hand, positive ions also reach the ionosphere at such time, with energies of 20-30 keV.

Frequency of occurrence



Large magnetic storms are most common during the peak of the 11-year sunspot cycle, or during the 3 years after that peak.


Geomagnetic storms that ignite auroras actually happen more often during the months around the equinoxes.During spring and autumn, the earth’s and the interplanetary magnetic field 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 Bz’s open a door through which energy from the solar wind can reach Earth’s inner magnetosphere.


However, Bz is not the only influence on geomagnetic activity. The Sun’s rotation axis is tilted 7 degrees with respect to the plane of Earth’s orbit. Because the solar wind blows more rapidly from the Sun’s poles than from its equator, 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 Sept. 5th and March 5th when Earth lies at its highest heliographic latitude.


Still, neither Bz nor the solar wind can fully explain the seasonal behaviour of geomagnetic storms. Those factors together contribute only about one-third of the observed semi-annual variation.