[Forum] Tom: Coronal hole streams & Aurora

[Cary Oler]

Hi Tom. Sorry I missed your question. I'll try to answer it now. I'll quote
it below, since I had to dig to find it.

Tom stated:
"Couple years ago in the last days of solar max years, you said that during
declining years of the solar cycle the coronal holes will become a force (in
aurora watching) and it happened. For me the year 2003 was especially the
year the strong coronal hole streams, when repeatedly coronal holes produced
at least 1 evening of strong aurora and up to 4 little less good aurora
nights in a row after  that. 2003 was the first year for me when i saw good
aurora from a coronal, but if i remember right, Lyndon Anderson and some
other peoples from high middle lat's and middle lat's in North America saw
few good shows from CH streams even on earlier years. Last year predicting
the nights when some aurora should be visible was pretty easy, just look
from "tables" when big coronal hole streams was supposed to arrive and how
long it lasted, it was pretty much like that. YET...the reason for this post
was not to the tell what happened last year, but to ask where did strong CH
streams go??? This is the first autumn in my aurora watching career...in 7-8
years, when i have not seen ANY aurora worth mentioning or worth
photographing. Ok, i may be bit unpatient here, have had some bad luck with
the clouds and i am aware that some peoples, like Stephane Levesque, in
maybe similar mag lat's as mine are seeing aurora in north america about all
the time, but...was the era of the truly strong coronal holes just 1 year
long or should it still continue??? Is it probable that a big coronal holes
will get better situated and that there will be strong ones in coming few
years?"

We will continue to see the influence of coronal holes through the solar
minimum. The occurrence of "truly strong coronal holes" is something that
isn't strictly very predictable. Coronal hole effects at the Earth (during
the peak of coronal hole effects) vary from cycle to cycle. There is a very
broad period where geomagnetic activity typically peaks in a solar cycle,
and we are now well into the decline from that peak (for a graphic
depiction, see: http://www.sec.noaa.gov/ftpdir/weekly/Ap.gif. What we will
tend to see through the remainder of this solar cycle and around the minimum
years (2006-2007) will be an increase in the size of the polar coronal
holes. You will also continue to see occasional excursions of the polar
coronal holes toward lower latitudes - "fingers" that will influence
geomagnetic and auroral activity at predictable recurrent intervals. Since
the Earth's orbit carries it above and below the solar equator, there may be
an increased tendency for enhanced levels of activity near the times of the
season where the Earth's orbit is furthest north or south of the solar
equator (bringing it closest to the equatorward boundary of those polar
coronal holes).

"Ok, one more thing....I know the phrase, that the best time to look for
aurora is around magnetic midnight and so on. Know the facts and all behind
that, have done this long enough to know how the earth turns under the oval.
Just wondering  why clearly big majority of all the substorms here seem to
occur between 2 and 4 am local time here? Same thing more than half of the
nights, you wait till 24 or  01, go to sleep and see the possible aurora
would have happened around 3am. Usually around 3 am...Why? This is no
coincidence, it has been happening over the years so often that it really
isn't possible anymore. While on the contrary, if there isn't any auroral
storm, those peaks almost never happen around 21 in the evening. Or is this
little similar as with meteorites, that the earth is swimming against the
stream in morning and swimming along the stream in the evening? Or is it so
that the aurora is just simply gathering strenght during our night time, so
that it would look as good as possible during North American night.... "

Yeah, sure... (-; that's it!  Actually, the time of peak activity is best
correlated with the simultaneous hooting of all the owls living in higher
latitudes.
(-;

But seriously, extensive studies indicate that substorms happen most
frequently in the hours around magnetic midnight, which for your area (if I
remember where you live) is around 21:30 UTC, which should be just a bit
after your local midnight.

I suspect your observations are suffering from bias in one or more of the
following manners. Your observing career is 7 or 8 years. That places the
start of your career around the rising phase of the solar cycle. During the
rising and maximum phase of the cycle, geomagnetic and auroral activity is
dominated by the impact of coronal mass ejections. Thus, your ability to
observe activity will be largely dependent upon the impact time and the
orientation of the IMF at the time darkness falls in your area. Your memory
of the strongest auroral activity events is likely also dominated by those
auroral storms that were associated with the most vigorous coronal mass
ejections. As a result, your observations may be more closely related to the
time of arrival of favorably oriented IMF fields in your region rather than
the statistical occurrence of substorm maxima. It may also be related to the
statistical frequency of clouds. Clouds often dissipate the most in the
hours after midnight (I don't know if this is true for your region though).
Thus, you might have a greater number of nights where the skies are clear
enough to observe auroral activity in the hours after 3 am.

If you could get above the obscuring influence of the clouds and if you
could observe the aurora at wavelengths less influenced by sunlight, you
would see that substorms do typically occur most frequently around local
magnetic midnight. Thus, the effects you are observing are almost certainly
the result of undetected observational bias.