Talk: Northern Lights

Dave Rice <ric...@rs1.tcs.tulane.edu> wrote:
>
>ric...@isl.uit.no (Curt Rice) writes:
>>
>>jit...@erehwon.caltech.edu (Mike Jittlov) wrote:
>>
>>> Aurora borealis and its southern counterpart, aurora australis, are phenomena
>>> that occur only in the high latitudes, about 65 degrees North or South.
>>
>>> The glow is not terribly bright, in general.  Thus the glow is seen usually
>>> under conditions of low ambient light from other sources.  This means that
>>> the sightings are mostly in winter, because it is light all the time in the
>>> summer at high latitudes.
>>
>>> Most auroras are diffuse greenish-white clouds.  These are not so interesting
>>> because they resemble clouds lit by moonlight.
>>
>>This interesting report is reinforced with a great display on the topic
>>currently at Tromsoe Museum.  A couple of comments/questions:
>>
>>I live in Tromsoe during the school year and northern Minnesota during the
>>summers.  We routinely see northern lights both places.  No surprise in
>>Tromsoe, of course, but Minnesota doesn't fulfill either the latitude or
>>season requirements.
>>Also, the northern lights in these places are very different.  In Tromsoe
>>they're usually green and "vertical", so to say, but in Minnesota they
>>tend to be lots of colors, and more spread out through the sky, or if they
>>are a sort of stream there, then they often cover a greater portion of the
>>sky.
>>
>>And, as for the diffuse one's not being so interesting.... well, spend a
>>few weeks in the dark and I bet they'll be quite welcome!
>>
>I agree that all aurora are interesting, but the threshold of what gets
>the heart pumping must vary from person to person.
>
>Aurora occur in a circumpolar ring-shaped region centered
>on the magnetic pole.  The peak frequency of occurrance occurs over
>Fairbanks, Alaska, and, I presume, Tromsoe [Troms0?], because
>they are equidistant from the north magnetic pole.

   There is an incredible time-lapse sequence taken by a polar orbiting
satellite showing the circular nature of the aurora.  It is a rare bit of
video, but worth seeing.  I have never seen anythig like it.

>They are rarer further north and south of that
>region.  The more spectacular displays cover more area and can be seen
>over larger regions of the earth.  NASA (I don't know where or when)
>released a really high space photo of the northern hemispere which showed
>not only where the sun lit the earth, but also an aurora that filled the
>entire circumpolar region.  A more earthly photo is in
>National Geographic, November 1991, pp 98-99.  It's a time exposure
>as evidenced by the star tracks, but it is a good representation.
>The trees are probably black spruce.

   I have also seen a video of the aurora taken with a very special
camera capable of photographing the dimmest of light.  The images of the
aurora are quite amazing, especially to those who have never seen it in
person.

>Incidentally, aurora occur day and night.  This is known from measuring
>induced electric currents in the earth's surface and other means.
>It also seems that aurora borealis and aurora australis occur simultan-
>eously at either end of a magnetic field line.  This was predicted from
>symmetry, and confirmed directly by instrumentation.
>
>Why do the aurora appear different at different latitudes?  Since the
>particles that cause the air to glow generally follow the earth's magnetic
>field lines, the more northern views, in regions of great magnetic dip,
>would be caused by particles coming in more close to vertical than at
>lower lattitudes like Minnisota (~45 degrees N) where the particles would
>come in at grazing angles.  This may give "curtains" v. "streams."

   The particles that couse the aurora are highly charged spewage from
the sun.  Solar wind, traveling at a million miles an hour hits the
earth's atmosphere and gets cought by the magnetic field, where it travels
down to the poles.  The energy level of this event is tremendous.
Probably on the order of billions of watts per second.

   It's interesting to note that the solar wind wildly distorts the
earth's magnetic field, causing it to flatten on the side of the earth
facing the sun, and to form a tale on the other side.  It is what causes
comets to always have their tales facing away from the sun.

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