Occasionally, our sun belches massive plumes of plasma from its corona that stream Earthward. When a fast coronal mass ejection (CME) arrives, it compresses Earth’s magnetosphere and can reconfigure the planet’s magnetic field lines. The reconfiguration enhances electric currents and energizes charged particles. Those currents and particles heat the tenuous atmosphere—hundreds of kilometers in altitude—which then expands outward. Low-orbiting satellites should thus experience more drag. But measurements of their orbital decay reveal that the drag from CMEs isn’t always as great as expected. Researchers led by Delores Knipp (University of Colorado Boulder) now explain why. Under some circumstances, the same CME that heats the upper atmosphere also triggers chemical reactions that quickly cool it. Charged particles with energies greater than about 10 keV split molecular nitrogen, and the free N atoms react with oxygen to produce nitric oxide. The NO molecules, often created in a vibrationally excited state, spontaneously radiate...

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