What does the sun’s X-ray flux tell us?

The sun, seen as a large yellow sphere with small dark spots with labels.
Sun on May 1, 2022, with the most active regions labeled. When there are few active regions visible on the sun, you can expect the sun’s X-ray flux to be low. Image posted by Armando Caussade.

The sun’s X-ray flux

You often hear solar physicists speak of the sun’s X-ray flux for a given day as being high or low. What does that mean, and what’s it tell us? X-ray flux is a measure of how much X-ray light is coming from the sun over time. There’s always a background X-ray flux, which primarily comes from sunspots or active regions. So the more active regions on the sun, the higher the background X-ray level.

And that means X-ray flux is a good descriptor of the sun’s activity for the day.

So the X-ray flux is kind of like the level of water in the ocean. The level is low during low tide, or solar minimum. It rises with high tide, or solar maximum. Solar flares are like waves on the background ocean of X-ray flux from the sun.

How X-ray flux is measured

The background level for the sun’s X-ray fluxes is measured the same as for solar flares. Solar flares can be seen as spikes in the X-ray flux. When the sun is not very active – such as during solar minimum – the background X-ray flux sits at the A or B range. During more active times, the background rises to around the C level. See the chart below, which is from the Space Weather Services of the Australian Government Bureau of Meteorology.

3-column chart listing flare levels, wattage, and description for each level.
View larger. | A numerical description of the sun’s X-ray flux, via Richard Thompson/ Australian Government Bureau of Meteorology/ Space Weather Services.

Bottom line: You often hear solar physicists speak of the day’s X-ray fluxes from the sun as being high or low. What does that mean, and what’s it tell us? Explanation and chart here.

May 1, 2022

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