The proverbial calm before the storm may be portentous and dramatic, but in space physics, the calm after the storm is important, too. That’s particularly true for a class of solar storms caused by gusts in the solar wind called high-speed streams (HSSs).
Scientists have spent decades trying to forecast HSS arrivals and for good reason—they trigger geomagnetic storms that can produce aurora, disrupt communications, and damage satellites. But until recently, scientists have paid little attention to when the storms leave and how Earth’s magnetic field responds.
Now, in a new study, Denton and Borovsky have given the trailing edge of these HSSs the attention they deserve, analyzing what happens when the fast wind departs and the ambient wind returns.
These HSSs originate in the Sun’s corona, where wisps of plasma swirl and loop back to the surface, trapped by the star’s magnetic field. Sometimes the corona gets patchy, and “holes” appear that allow plasma to fly off into space faster than the surrounding solar wind—buffeting Earth’s magnetic field when it arrives. If the hole persists, Earth may encounter this stream multiple times because of the Sun’s rotation—it rotates once every 27 days or so—allowing plasma to spray from coronal holes like a garden sprinkler and hit Earth as HSS.
To examine the departures of HSSs, the team analyzed 43 such events recorded in public data sets from spacecraft, including satellites operated by the National Oceanic and Atmospheric Administration, NASA, and the Los Alamos National Laboratory.
They found that in each event, the trailing edge appears in the data as a characteristic bend: The solar wind speed levels off and sometimes shifts direction slightly. For typical events, the solar wind speed would be more than 550 kilometers per second for roughly 6 days as Earth bore the brunt of the high-speed stream and then would fall to 300–400 kilometers per second for 2 or 3 days as the trailing edge washed over.
The team also found corresponding changes in Earth’s magnetic field, relaxing as the stream passes: Although the field is compressed and gains strength at the onset as the leading edge of HSS strikes, 1 or 2 days before the trailing edge passes over, the magnetic field fluctuations go quiet, and its strength dips slightly.
All of this indicates that the trailing edge is like the trough of a passing wave: The solar wind eases, and Earth’s magnetic field goes quiet. But one thing the team found that doesn’t fit with this picture is that during this period, Earth’s outer radiation belts become more active. The number of high-energy electrons in the belts, which can damage satellites, increases, whereas solar and magnetic field activity dies down, peaking as the trailing edge washes over. This is counterintuitive and an important finding. Until now, radiation belts were almost always considered to be at their most dangerous during storms, not during the calm period afterward.
The team also found that the trailing edges occur with a predictable 27-day interval, just like the leading edges. That is crucial and reassuring: These events don’t happen randomly but can, indeed, be predicted. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1002/2016JA023592, 2017)
—Mark Zastrow, Freelance Writer