[HCRA] More on the A and K indicies (more scientific)

[djs13@hotmail.com]

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To: <yccc@yccc.org>
Sent: Friday, December 06, 2002 3:14 AM
Subject: Re: [YCCC] A-Index (Long)


> I've received several questions, comments and requests for further
> explanation about some of things that I wrote in response to Ted W3TB's
> question. I'm going to try to answer these all in one fell swoop, so here
> goes...
>
> If you don't like the "science channel" kind of stuff, please hit delete
> now. :-)
>
> The aurora are permanent features of the Earth's polar regions and always
> have some effect on HF propagation for paths that pass near either pole.
>
> Sometimes the aurora becomes intense enough and the auroral oval expands
> far enough equatorward to: (a) more adversely affect HF propagation on a
> wider range of paths than usual and (b) become visible to those of us who
> live at mid-latitudes. Everyone remember CQWW CW last year (2001)?
>
> The aurora outlines a boundary line that results from the dynamic balance
> of the solar wind blowing past the earth's magnetic field. When the solar
> wind blows harder the aurora gets brighter and the auroral regions expand
> away from the poles towards the equator.
>
> The A & K indices measure, among other things, the degree to which the
> aurora is enhanced above its quiescent levels.
>
> Sometimes there are abrupt changes in the speed, direction or density
> of the solar wind. Such discontinuous changes are called "shocks" and they
> are typically associated with rather energetic events on the sun called
> coronal mass ejections (CMEs) and flares.
>
> When a solar wind shock passes by the earth we typically experience events
> that are called geomagnetic storms and sub-storms. Classical geomagnetic
> storms have to do with an increase of the "ring current" that flows
through
> the equatorial ionosphere. The most notable impact of a geomagnetic storm
> at HF is a lowering of the MUF on paths at middle and high latitudes.
>
> A geomagnetic storm will also often result in an increase in the MUF on
> paths that cross the (geomagnetic) equator. When the A/K indices begin to
> rise at the start of a geomagnetic storm is a good time to point your
> antenna south and check 6 meters for trans-equatorial propagation (TE or
> spread-F) towards Africa, South America and the South Pacific.
>
> Sub-storms are what one usually thinks of upon hearing the term
> "geomagnetic storm"; a sub-storm is the aurora ovals rapidly expanding and
> brightening. A sub-storm increases absorption and raises the LUF on paths
> passing though the higher latitudes.
>
> Bottom line: high A or K are bad news for HF propagation in general and
are
> particularly tough on polar paths.
>
> In addition to shocks and geomagnetic storms, there are times when the
> solar wind just seems to ratchet up a couple of notches and starts blowing
> at gale force. Once established, the high speed solar wind usually
persists
> for a while.
>
> These high velocity streams of solar wind originate in features on the sun
> called coronal holes. The higher pressure of the wind alters the balance
> point with the earth's magnetic field. As a result, the auroral oval
> expands and brightens. This is recorded as an increase in the A & K
indices.
>
> And this is what Ted W3TB noticed: an extended period of elevated, but not
> really storm level, A-indices.
>
> We get really high A/K indices with a geomagnetic storm; but storm
> conditions only last for one or a few days. With a coronal hole, we can
get
> stuck with elevated A/K indices, and the kind of mediocre HF conditions
> that go along with them, for several days up to a couple of weeks.
>
> Whether it's shocks or holes, the solar wind is most effective at driving
> the aurora and messing with HF propagation when the magnetic field carried
> along by the solar wind (which is really the outer reaches of the Sun's
> magnetic field) matches the direction (polarity) of earth's field. This is
> what's called having the interplanetary magnetic field (IMF) "southward."
> And that's also what has been going on of late.
>
> It turns out that CMEs and interplanetary shocks are a lot more common
> around solar maximum when there are lots of sunspots around. The kind of
> coronal holes that have an effect on the earth tend to get disrupted when
> there are lots of sunspots around. As a result holes are seen more
> frequently closer to solar minimum.
>
> I don't know exactly all of what Don K2KQ has planned for February, but if
> there's any interest in these sorts of things perhaps I could pull some
> kind of presentation together for that meeting.
>
> Of course space weather is a lot like terrestrial meteorology, no matter
> how much we might know about the weather, we're still stuck operating with
> whatever conditions we've got at the moment. ;-)
>
> And my recommendation for a space weather site that hasn't been mentioned
yet:
>
> NOAA Space Weather Now <http://www.sec.noaa.gov/SWN/index.html>
>
> 73 & KB,
> Mike K1MK
>
> Michael Keane, K1MK
> k1mk@alum.mit.edu
>
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