THE SOUTHERN AURORAE : PART 3
GENERAL FEATURES OF AURORAE
|c. ARCS WITH RAYS
|e. BANDS (RAY PARTS)
|g. DRAPERIES (RIBBONS)
(At Magnetic Zenith)
(a) G L O W S
Appearances and colours seen in auroral outbreaks vary
significantly in shape, intensity and duration. Most common aurora
will just appear as simple homogeneous coloured glows. These commonly
resembling a ‘false dawn’ towards the southern horizon. Many of these
types are either whitish-green or reddish in colour that vary in
intensity over time. Typically they will cover the sky as a small
curved arc towards or along the southern horizon. Sometimes these
glows can appear bright enough to cast colour and shadows across
ground. Such bright reddish glows are sometimes mistaken by the
general public as fires, and many calls are made to the local Fire
Brigade. In 1983, the aurora in Sydney did exactly that.
Faint glows are also sometimes seen, but are normally missed
unless they are actually being purposely sort. Frequently, these
glows vary in their intensity over period of several minutes to about
half-an-hour, though a number of peaks in brightness can occur over
the duration of the display.
Glows sometimes are precursors to more active aurora, that is
either further south or of increased activity. The glows themselves
may appear from bright concentrated light very close the horizon,
transforming into glows covering most of the southern horizon.
Sometimes contained with the glows are other phenomena, such as arcs
Observers under city lights or in the built-up of
the cities or surrounding suburbs are unable to distinguish between
aurorae from light pollution.
(b) A R C S
(or so-called Quite Arcs)
Sometimes the glows mentioned above appear as distinct arcs near
the southern horizon, whose widths may vary considerably. The
so-called homogenous arcs. show little variation in their
light intensity. However some appear as partial arcs, or as
double arcs, though these are rather rare. Some may have
pulsating arcs, that change in intensity over periods of
several seconds to about one minute. Change in light can be so
intense, that the arc appears and disappears from view — just
like turning on and off a light switch!
(c) A R C S and R A Y S
(or so-called Quite Arcs)
Some also have arcs and rays, making them looking like a crown.
(d) R A Y S
At odd times bands may not be visible but are replaced by rays
within their particular shapes. Often these rays are very bright,
joined sometimes along a continuous band or arc joining together the
bottom of the rays. This makes them look like a child’s drawing of the sun rising or setting. Some
rays can be illuminated by sunlight by aurorae that occur before
sunrise or after sunset. Even glows also sometimes contain rays.
(e) B A N D S
(or R A Y P A R T S)
Others auroral display instead of arcs can be bands of light with
very irregular boundaries. Such bands change their appearance
rapidly, moving like a woman’s hair
ribbon falling though the air. Often, the lower border of the band is
clearly sharp and distinct, while the upper boundary may fade into
the sky above it. Some bands seem very thin but others appear like a
huge hanging curtain.
(f) D R A P E R I E S
Unlikely seen from most latitude in mainland Australia. These are
huge curtains of light that are impossible to describe adequately.
These comprise of rays which come down to a very long band, which
sometimes is brighter and more luminous. Several may appear together,
and if near the magnetic zenith might appear as fanlike created by
the perspective. They can be thousands of kilometres long and tend to
favour an east to west direction.
(g) D R A P E R I E S (RIBBONS)
Many are just like looking at general household curtains, but
these are seen as if viewed from ground or below them. These kind of
features are in the minds of the general public the ideal or
quintessential example of all auroral forms. Most appear very
beautiful and intriguing to the eye, especially as the slowly move
like some curtain wavering in the gentle breeze, where they are seen
to continuously waver across a central line or curve in the sky.
Most are classed by the apparent height of the drapery (or arc),
being either faint, medium, strong or very strong.
All draperies are notably average about 105±10 in altitude,
whose bottom edges are brighter arcs with the upper edges being
much less defined. Most of these appear vertical or perpendicular
to the Earth’s surface, but this is
not necessarily always typical. They can appear as either loose or
tight curls and may undulate, and some evolve into several or many
bright vertical rays along the whole drapery. A cause for these
draperies are surmised as being caused by the magnetic field lines as
being in magnetic sheets coming down towards the surface.
(h) C O R O N A S
These are the most rare and spectacular of all the aurora types
that are visible to the naked-eye. Usually they disperse
multicoloured streamers which are viewed from some central point that
radiating from near zenith. Some are quite frightening in the way
they suddenly appear and dramatically brighten. Most coronas are
caused by aurorae the observe standing on one of the magnetic field
lines when auroral rays forms above them towards magnetic zenith.
Recording the Southern Aurorae
Detail on the phenomena and frequency of the aurorae is important
in the understanding of its nature. Frequently ships passing through
the Southern Ocean also report many aurorae. They can also be easily
recorded by photography or filming, and can make delightful images
showing significant changes in structure, colour and form. Exposures
are typically from seconds to tens of seconds, depending mostly on
their brightness — ranging from faint glows just detectable to
the eye to ones which cast bright shadows on the ground.
Call for Southern Aurorae Observational Reports
Any detailed reports of aurora seen in recent years
from mainland Australia I would like to hear of them that are
available please contact me. Any written observations or
recollections of this or past events would be greatly appreciated. At
present I receive the RASNZ’s “Aurora and Solar Section Circular”, and sometime would like to produce an
article regarding low latitude auroral activity. This is part of an
ongoing observation program and have been slowly collating material
for sometime now. If you have some comment or observation, I would
offer to place it here (if you like too!)
Last Update : 13th September 2014
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