Ionospheric Electron Content during Solar Cycle 23

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Tuesday, March 13, 2018

Solomon & Qian show that solar minima are not all the same, and may have implications for understanding the Sun during the extended periods of very low activity known as “grand minima” that have sometimes occurred in the past, the best known of which was the Maunder Minimum during 1650–1700.

Global Total Electron Image
Comparison of measured to modeled global total electron content, showing daily mean values (dots) and 365-day smoothed results (lines). Black: data derived from the JPL-35 analysis by Emmert et al. (2017). Magenta: model results from the TIE-GCM, including extrapolation and assumed constant plasmasphere. Blue dotted line: gTEC=9, approximately the minimum of the 365-day smoothed values during 2008-2009, as a guide.

The Sun goes through activity cycles, best known with regard to the number of sunspots, with a cycle of about 11 years. At solar minimum, when sunspots mostly vanish, the extreme-ultraviolet region of the solar spectrum also diminishes. This, in turn, causes lower temperautre and density in the ionosphere and the thermosphere (the neutral atmosphere above 100 km), because that is where the extreme-ultraviolet radiation is absorbed. In the past, it seemed that most of these solar minimum periods were basically similar with regard to extreme-ultraviolet radiation, and other manifestations of solar activity. However, the solar minimum during 2008-2009 was longer and had fewer sunspots than its predecessor during 1996, or in fact than any minimum period in the past century. The thermosphere was also lower in density, and the ionosphere also appeared to be lower in density, but there were some contradictory measurements. Recent work by Emmert et al. (2017) re-analyzed the ionospheric measurements, and found that they are now in agreement with the thermospheric measurements. Meanwhile, a series of modeling studies attempted to explain the changes in the ionosphere and thermosphere as the result of a decrease in solar extreme-ultraviolet radiation, estimated to be around 10%. In this paper, we show that the model results, the thermosphere density observations, and the ionosphere density measurements, are now in good agreement. This demonstrates that solar minima are not all the same, and may have implications for understanding the Sun during the extended periods of very low activity known as “grand minima” that have sometimes occurred in the past, the best known of which was the Maunder Minimum during 1650–1700.

Publication Name: Journal of Geophysical Research Space Physics

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