Winter Anomaly of the Ionosphere Affecting 10m Radio Propagation

In response to my post on “Is the 10 Meter Band Dead?“, many friends on X responded mentioning that you can expect poor propagation in summer while the propagation will be better in Winter. Looking back, I could remember that it was in January that I was happy with 10m and after April, propagation became poor to non-existent for me. W1UE on Ham Radio Science Citizen Investigation (HamSCI) Google Groups told me that right now we are going through the “Summer Doldrums” and even with the high solar flux that exists, East/West propagation is poor. He also mentioned that several contacts have been made to Europe from United States of America during a recent contest, mostly by Sporadic E propagation. So that explains things a lot. Most of my 10m contacts earlier this year were to the West and some to the East. Couple of contacts to the North had very poor reports and there was no one to contact to the South of South India!

W6RZ told us that this is technically known as ‘Winter Anomaly’ (of the ionosphere). Winter anomaly of the ionosphere is noted in mid latitudes in the level of ionization of F2 layer of the ionosphere. Even though production of ions is higher in summer when the Sun shines directly over the Earth in this region, the loss of ions is also said to be higher. This results in net ionization level of F2 layer to be lower in summer than in winter. This has been called as the winter anomaly. It is mentioned that it is a feature of the Northern hemisphere than the Southern hemisphere. In polar regions, winter anomaly does not occur due to the prolonged darkness in winter. So I presume that I have got an answer for my dilemma why my favourite DX band earlier this year, that is 10m, is almost dead now. I am waiting for winter to see if it comes back to life again!

As I was finishing this post, another email by K8TE in HamSCI group explained things better. He quoted the explanation given by W3LPL for not seeing 10m propagation during ‘Summer Doldrums’. High velocity thermospheric winds at the F2 region blow from the high temperature dayside hemisphere to the low temperature nightside hemisphere reducing the volume of neutral atomic oxygen available for ionizing the F2 region during daytime hours. This lowers the Maximum Usable Frequency (MUF) for F2 propagation crossing the Northern hemisphere to less than 24 MHz through mid-September and less than 28 MHz through late September. That is an excellent to the point explanation, which I was waiting for!