DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

aurora australis
Earth’s magnetic field. They enter the Earth’s
upper atmosphere where the field lines intersect
the atmosphere, i.e., in the polar regions. There
they excite atoms in the high thin atmosphere at
altitudes of 95 to 300 km. The red and green
colors are predominantly produced by excitations
of oxygen and nitrogen. The polar lights
are typically seen within 5000 km of the poles,
but during times of intense solar activity (which
increases the electron population), they can become
visible at midlatitudes as well. Any body
thatpossessesbothamagneticfieldandanatmosphere
can produce aurorae. Aurorae are commonly
seen not only on Earth but also the Jovian
planets of Jupiter and Saturn.
aurora australis Southern light, aurora in
the southern hemisphere. See aurora.
aurora borealis Northern light, aurora in the
northern hemisphere. See aurora.
auroral cavity A region on magnetic field
lines which guides the aurora, typically within
10,000 km or so of Earth, where abnormally low
ion densities are observed at times of strong aurora,
presumably caused by it.
auroral electrojet A powerful electric current,
flowing in the auroral oval in the ionospheric
E-layer, along two branches that meet
near midnight. The branches are known as the
eastward and westward auroral electrojets, respectively,
and the region in which they meet,
around 2200 magnetic local time, is the Harang
discontinuity.
The electrojets are believed to be Hall currents
in the ionospheric E-layer and to be a
secondary effect of the currents linking Birkeland
currents of region 1 with those of region 2.
Because of Fukushima’s theorem, the magnetic
disturbance due to the Birkeland current sheets
on the ground is very weak, and the main signature
of their circuit — which can be quite strong
— comes from the electrojets. The usual way
of estimating the current flowing in that circuit
— which is a major signature of substorms —
is therefore by means of the AE, AL, and AU
indices which gauge the strength of the electrojets.
© 2001 by CRC Press LLC
auroral oval Circular region several degrees
wide around the geomagnetic pole at a geomagnetic
latitude of about ±70 ◦ , its center shifted by
about 200 km towards the nightside; the region
in which aurora is observed at any instant, covering
the region of the diffuse aurora, which is also
where the discrete aurora can be seen. The auroral
oval can be seen in satellite images in UV
as a closed circle. From Earth, in visible light,
in the auroral oval aurora can be seen nearly
each night, during polar night for a full 24 hours.
Shapes and structure of the aurora vary with local
time: with a rather diffuse auroral brightening
between local noon and midnight, quiet arcs
during the evening hours up to around 21 local
time, followed by homogeneous or rayed
bands or draperies, which after about 3 local
time, are complemented by patches at the southern
rim of the auroral oval. These patches, together
with short arcs, dominate the appearance
of the aurora during the morning hours. The
size of the auroral oval varies greatly; it grows
during magnetic storms and may sometimes extend
well beyond the region where aurora is ordinarily
seen (auroral zone). At magnetically
quiet times the oval shrinks and may assume a
non-circular “horsecollar” shape, narrower near
noon. Physically, the auroral oval is related to
upward flowing Birkeland currents coupling the
ionosphere and magnetosphere. See Birkeland
current.
auroral zone The region where auroras are
ordinarily seen, centered at the magnetic pole
and extending between magnetic latitudes 66 ◦
and 71 ◦ . The auroral zone is generally derived
from ground observations of discrete aurora, but
it also approximates the statistical average of the
auroral oval, averaged over many nights.
autumnal equinox The epoch at the end of
Northern hemisphere summer on which the sun
islocatedattheintersectionofthecelestialequator
and the ecliptic; on this day, about September
21, the night and day are of equal length
throughout the Earth. The date of autumnal
equinox is the beginning of the Southern hemispherespring.
Autumnalequinoxalsoreferstoa
direction of the celestial sphere: 12 h RA, 0 ◦ declination,
antipodal to the direction of the vernal
equinox. See vernal equinox. After autumnal