Topband: 160 Mid Winter Lull

[Donald Chester]

Over the years I have noticed that the better conditions on 160m often occur 
between early November and late December.  Propagation sometimes goes into 
the doldrums after Xmas and through the end of February, even if noise 
levels are low during this period.  More than once, I recall a weather front 
moving through sometime between Xmas and New Year's, and the the static 
stayed with us for the rest of the season.  We  may be enjoying right now 
the best conditions on the band that we'll see the entire season.


Someone brought to my attention a couple of web pages that may help explain 
the phenomenon.

"D Region Mid Winter Absorption Anomaly - A period of increased MF radio 
wave absorption at high and mid latitudes occurring in mid winter and is 
associated with sudden stratospheric warming and the Quasi Biennial 
Oscillation (QBO). You may notice that most of your good DX contacts are in 
the fall and spring. This is due to the D Region Mid Winter Absorption 
Anomaly. (See definition #17. Sporadic-D (Ds) Absorption & Wave Guiding). 
(See definition #19. Sudden Stratospheric Warming (STRATWARM ALERT)."
http://www.kn4lf.com/kn4lf8.htm


The relation between ionospheric winter anomalies and solar wind:

"There are two different winter anomalies. A small one that appears in 
connection with ionization at relatively low latitudes in the bottom of the 
D-region of the ionosphere. There, the electron densities in the winter 
happen to be less than should be expected. On the other hand, the classic 
winter anomaly is present when in the winter the upper D-region, again at 
relatively low latitudes, has more ionization than should be expected. Both 
these effects are due to the slant compression of the geomagnetic field 
produced by the solar wind in the winter season (which is, of course, the 
summer season when reference is made to events in the other hemisphere)."

http://www.ingv.it/~wwwannali/rumi443.htm




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