Thousands of superheated plasma bubbles are bursting around the earth and no one knows what鈥檚 blowing them. They鈥檙e not dangerous, but they could provide a clue to why a puzzling interplanetary phenomenon called the bow shock exists.
The bubbles are forming around 100,000 kilometres above our heads on the day side of the planet, where the speeding solar wind 鈥� a constant stream of charged particles flowing out of the sun 鈥� suddenly slows down so that it can sweep around Earth鈥檚 magnetic field, much like water is deflected before the bow of a ship. This area is called the bow shock.
A flotilla of European and Chinese spacecraft from the Cluster and Double Star missions, which are studying the effects of the sun on the Earth鈥檚 environment, discovered the bubbles after detecting sudden changes in the magnetic field, temperature and density of the solar wind. 鈥淚 saw these glitches, and wondered what they could be,鈥� says George Parks of the Space Sciences Lab at the University of California, Berkeley. When his team analysed the data, they found that the plasma was full of holes even less dense than their surroundings.
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In each of Parks鈥� glitches a small region of the solar wind is suddenly heated to 10 million 掳C, inflates to 1000 kilometres in size and then collapses again, all in as little as 4 seconds (Physics of Plasmas, vol 15, p 050701).
There are probably many thousands of them going off at any one moment, and yet no one has spotted them before. Parks doesn鈥檛 yet know what blows the bubbles, but suspects it is something to do with energetic ions bouncing back off the bow shock and hitting the solar wind head on, heating it up.
鈥淭he bubbles could provide a clue to why a puzzling interplanetary phenomenon called the bow shock exists鈥�
The bubbles seem to get more tightly packed and more sharply defined closer to the bow shock, and Parks suggests that they might merge together to actually form it. If so, this could be a clue to the long-standing puzzle of why it exists at all. Because the particles that make up the solar wind rarely collide with each other, physicists have struggled to explain how they can all slow down together so suddenly.
