Fall = Auroral Season
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ABOVE: "Still frames from a digital movie show how solar wind gusts rattle Earth's magnetosphere and trigger auroras. Click to view the 750 kb Quicktime animation created by Digital Radiance, Inc. NASA
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02 October 2005, 03:10 UTC: The season for auroras. Autumn is almost synonymous with "aurora season" in the northern latitudes. It is strange how the Earth can effect solar activity. But, truth is stranger than fiction. According to NASA, there are twice as many geomagnetic storms in the fall and spring as there are during summer and winter.
Perhaps the only good thing to come from geomagnetic storms are pretty pictures of the sky with nature's greatest light show. At least that is true for those involved with communications, power distribution, and other fields effected by Earth's Magnetic Field.
It may be wrong to call a force of nature evil or good, so we will not use the term "evil ". But the solar wind, among other things, is a damaging wind that travels through space. Earth's magnetic field ( the magnetosphere) serves as a shield partially protecting Earth from the damaging effects of the solar wind. The area where the Earth's magnetic field (EMF) outer limit meets the magnetic field produced by the Sun (the Interplanetary Magnetic Field) is called the Magnetopause. It is the magnetosphere that keeps Earth's atmosphere in close to our planet and most of the solar wind's damaging effects out in space. These two magnetic fields meet at the magnetopause. Normally the Earth's magnetic field points northward at the magnetopause.
A Geomagnetic Storm is caused when a high speed solar wind stream with a strongly southward pointing Interplanetary Magnetic Field strikes the magnetopause. These conditions distort the Earth's magnetic field allowing more than the usual amount of the solar wind to penetrate the Earth's ionosphere. Some of the energy given off by ionized gas produced by the solar wind moving through Earth's ionosphere is in the form of visible light. The light is similar to the light produced by a very small meteor (dust to rice grain size) at it ionizes gas leaving a lighted tail of twelve mile or so as the meteor burns up.
The
interplanetary magnetic field direction is represented by " Bz". Bz can
point in a northward or southward direction. Its strength is measured on a
special scale. Normally Bz swings back and forth from a northward to
southward direction with changes in the solar wind, and other factors, such as
the normal oscillation of the Earth on its axis as viewed from the Sun .
The oscillation of the Earth on its axis (viewed from the Sun) speeds up and
slows down with Earth's seasons. At a point in its swing, the direction of flow
of the solar wind is parallel to the Earth's axis. Several forces
influence the direction of Bz. Solar storms and coronal holes can distort the
Earth's magnetic field at the magnetopause possibly causing a strongly southward
pointing Bz. These occurrences are not effected by Earth's seasons. But, added
to speeding up of Earth's oscillations on its axis during the spring
and fall can influence the likely-hood of several forces acting together
to produce a strongly southward pointing Bz according to the seasons of the
year. (The Webmaster's article on the significance of Bz in medium to long
range propagation forecasts is located on the Web Site's
Conjecture page.
ABOVE: In the early 1970's Russell and colleague R. L. McPherron recognized a connection between Bz and Earth's changing seasons. ILLUSTRATION: Geomagnetic activity from 1875 to 1927, from "Semiannual Variation of Geomagnetic Activity" by C.T. Russell and R.L. McPherron, JGR, 78(1), 92, 1973. See also a more recent analysis by NASA solar physicist David Hathaway.
The over-all effect of strongly southward pointing of Bz is a geomagnetic storm. During a geomagnetic storm, communications suffer in quality and individuals living in the higher latitudes are often rewarded by the aurora.