In the Northern Hemisphere, we are in the summer doldrums with lower MUFs (maximum usable frequencies) during the day, but higher MUFs (than winter) in the evening and night. In the other hemisphere, this is reversed.
With more hours of daylight during the summer, wouldn’t the increased exposure to solar radiation cause greater ionization? The surprising answer is that, no, that is generally not the case. A look at many signal paths reveals that there are higher peaks during the winter daytime than during the summer daytime. However, during the summer night, those same paths may have higher MUFs than during the winter nights—the Summer Anomaly.
This seasonal variation is due to a change in the ionospheric chemistry with a shift towards more diatomic species and fewer monatomic ones (a species refers to the gas molecules which react to incoming solar energy). These molecules (diatomic species) are harder to ionize as they are more tightly bonded, leading to the lower levels of ionization, seen at <https://tinyurl.com/yxv99jsc>.
It was formerly believed that this anomaly was in part caused by temperature differences. This model held that during the Northern Hemisphere’s winter months the atmosphere is cold and therefore denser, and that because the Earth is closer to the Sun, more intense daytime ionization occurs; thus,
