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AURORA.DOC
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1992-02-18
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Since this question has been raised more than once in both public and
private discussions, perhaps an explanation is in order for those who are
not familiar with the behavior of auroral activity.
The auroral oval is, as the name implies, oval and not circular. It
represents a boundary between what is called the Polar Cap and the High
Latitude regions. This oval is a dynamic activity center. A great number
of physical processes occur in the oval which can affect atmospheric
heating, ionization, etc, which in turn can lead to significant changes in
radio signal propagation conditions, etc.
The auroral oval is centered approximately over the magnetic dipoles of the
earth (ex. the boreal and austral magnetic poles). The oval usually spans
only a few degrees of latitude during quiet periods. During storm periods,
the oval can expand polewards and equatorward covering an area as large as
15 degrees in latitude. It is therefore a fairly narrow band of luminosity.
The oval is more closely related to geomagnetic latitude than geographic
latitude. However, since most people have no idea what their geomagnetic
latitude is, the auroral activity warnings and watches use approximate
geographical equivalents. The main disadvantage of doing this is that the
stated geographical latitude is very dependent on location. It will be
fairly accurate over North America, somewhat questionable over Europe, and
very questionable over the USSR. It also will not apply to the southern
hemisphere regions. This should soon be corrected in future watches and
warnings that are issued.
The question of how far from the equator one must be to spot activity if
the auroral oval is at a given latitude is a good question, and one which
is not easily answered. The auroral oval resides at a height of about 100
kilometers (~ 60 miles) but can extend up to several hundred km's depending
on how energetic the activity becomes. The combination of changes in
height as well as changes in latitudinal width, equatorward position, and
luminosity all contribute to how far toward the equator aurorae may be seen.
But this is not all that must be considered. Your unobstructed view of the
northern-northeastern horizon must also be accounted, as must the phase of
the moon and the background brightness of the sky (from an ideal dark sky)
caused by light pollution from cities or nearby towns, as well as
atmospheric conditions describing the transparency of the horizon. So as
you can see, it is nearly impossible to say that aurorae will be visible
over specific latitudes, since observing conditions will vary widely from
one location to another. Additionally, aurorae undergo dramatic localized
changes in luminosity and position associated with what are called
"substorm" periods. Substorms are basically localized disturbances of the
geomagnetic field which can enhance auroral activity over the substorming
region.
Since the auroral oval is eccentric, there is a always a position on the
earth where the oval is closest to the equator. The auroral oval "shadows"
the sun. That is, its most poleward position is on the daylit side of the
earth, while its most equatorward position is near the midnight sector.
For this reason, you will notice (particularly over the higher latitudes
such as in areas of Canada) that the oval gradually slips further south until
around midnight, afterwhich it begins its trek back to the north. This is
why you are most likely to see aurorae near midnight, although this will
depend significantly on whether the oval is quiet or active as you move
closer toward the equator.
As far as the auroral activity watches and warnings go, the APPROXIMATE
geographical latitudinal boundaries for North America as we have defined them
follow below. Note that these boundaries are liable to change over time as
better estimates are made.
NORTHERN HEMISPHERE SOUTHERN HEMISPHERE
High latitudes >= 55 deg. N. | High latitudes >= 55 deg. S.
Middle latitudes >= 40 < 55 deg. N. | Middle latitudes >= 30 < 55 deg. S.
Low latitudes < 40 deg. N. | Low latitudes < 30 deg. S.
The differences in latitude of the northern and southern hemisphere
boundaries are required since the northern and southern auroral ovals are
not mirrors of each other. These boundary definitions are only valid for
North America (N. Hem.) and Australia/New Zealand (S. Hem.).
I hope this has helped some of you better understand the relationships
of the visibility of the auroral oval with geographic latitude.
Best regards.
-Cary Oler
--
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