I’ve posted here several times stating that radio propagation has changed since the way it was when I was younger. I did not know and do not know fully what is responsible for that change, but clearly there has been change.
Now an article in Science Daily entitled, “Boundary Between Earth’s Upper Atmosphere And Space Has Moved To Extraordinarily Low Altitudes, NASA Instruments Document“, confirms that the Ionosphere has changed. It is lower than it used to be.
For the record, I believe the title of this article is scientifically horrid; there is no “boundary” between the Earth’s upper atmosphere and space. The atmosphere is a gas, and it gets progressively thinner as you move away from the planet. The definition of the boundary is arbitrary, at some “pressure” one can call that the boundary, but at what pressure you decide to do that is arbitrary.
There are many factors affecting the upper atmosphere and how “inflated” it is, but the largest is probably extreme UV from the Sun which is absorbed in the upper atmosphere heating and expanding it. The last solar cycle was the most intense on record, and then ended and a new one really has not started. There is indication that a magnetic reversal has happened, but so far there has only been a sparse very short lived sun spot every now and then, nothing like a normal solar cycle.
There are other factors as well. The Earth’s own magnetic field is weakening. How much of this relates to any internal dynamo action and how much relates to changes in the solar flux is hard to say. The interaction between the solar wind and internal sources is extremely complex.
The strength of the Earth’s magnetic field determines the latitude that cosmic rays and solar particles enter the atmosphere. When the field is strong, they enter in very concentrated zones near the poles, when the field is weaker these particles can enter farther from the poles. In the last 100 years, the average latitude that these particles enter the atmosphere has shifted towards the equator by about ten degrees.
The long term nature of this shift suggests more of an internal, change in the Earth’s dynamo, cause. One effect of this shift is to shift rainfall patterns to some degree because these high energy charged particles create ionized paths that serve as condensation points where raindrops can start to form.
Then there are changes in atmospheric chemistry. We’ve added additional carbon dioxide, methane, water vapor, halides, and other sulfur.
And then there is the intentional manipulation of the ionosphere with HAARP and similar projects around the globe.
So I’m not going to pretend to understand the relative impact of each factor, but clearly our atmosphere and ionosphere are changing.