Yesterday, the vernal equinox, the sun returned to the Northern Hemisphere at last. Nothing here or anywhere else in our corner of the galaxy would exist without the sun, yet a surprising number of solar mysteries persist. Much of heliophysics is focused on “space weather,” predicting what the sun will do. That’s because solar flares and coronal mass ejections spew charged particles and radiation into space, occasionally toward Earth. These seething bursts of energy can jeopardize telecommunications on the ground and in space, not to mention the lives of astronauts.