The sun’s rotation produces changes in its magnetic field, which flips completely every 11 years or so, triggering a phase of intense activity. Solar flares—huge eruptions from the surface of the sun lasting minutes or hours—emit intense bursts of particles and high levels of electromagnetic radiation. The release of energy during solar flares heats the chromosphere, causing almost full ionization of the atomic hydrogen present in the region.
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Source: Phys.org