The solar wind fluctuates, but last December it surprised astronomers when it suddenly stopped completely for a couple of days. The anomaly was detected by MAVEN, a NASA orbiter studying the atmosphere of Mars, whose instruments recorded a drop in solar wind density by a factor of 100. Without the pressure of the solar wind, the Red Planet's magnetosphere and ionosphere swelled up to three times their normal size. "When we first saw the data, and how dramatic the drop in the solar wind was, it was almost unbelievable," said Jasper Halekas, lead author of the study. "We formed a working group to study the event, and we have found this time period to be rich with incredible findings."