The quiet Sun's silent storms
Posted: September 18, 2009
We shouldn’t be misled into thinking that a lack of sunspots means a quiet Sun, say researchers from the USA who have found that our planet was bombarded with high speed streams of solar energy throughout 2008.The Sun is slowly beginning to wake up from solar minimum – this giant prominence was seen leaping in to the solar corona by the joint NASA/ESA Solar and Heliospheric Observatory (SOHO) on 5 September 2009. Image: SOHO (NASA and ESA).
The Sun has a roughly eleven-year cycle of activity, peaking at solar maximum when there are lots of sunspots, flares and coronal mass ejections. Then activity drops to a minimum, which we have seen in recent years, before the cycle begins again. However, over the past two or three years we have experienced an unusually long solar minimum, with barely a sunspot to speak of. However, a team of solar physicists led by Sarah Gibson of the National Center for Atmospheric Research in Boulder, Colorado and colleagues at the University of Michigan focused instead on fast moving streams in the solar wind that drag the Sun’s magnetic field out into space. These streams should have by and large disappeared during solar minimum, but throughout 2008 they were still going strong, playing havoc in Earth’s upper atmosphere. Gibson’s team found that the streams were three times stronger than they had been during the previous solar minimum in 1996.
“The solar wind can hit Earth like a fire hose even when there are virtually no sunspots, and the fact that Earth can continue to ring with solar energy has implications for satellites and sensitive technological systems,“ says Gibson. The streams energise Earth’s outermost radiation belts, which can fry the electronics on satellites used for communications, weather monitoring or GPS navigation, as well as posing a threat to the health of astronauts on the International Space Station or the space shuttle. On Earth, we often see the effects as beautiful aurorae. The energy imparted by the streams can even heat Earth’s upper atmosphere, causing it to expand and exert more drag on orbiting satellites, which then slow down and drop to potentially dangerous lower altitudes.
The reason for these fast streams, according to Gibson’s team, are holes in the Sun’s corona that have lingered near the Sun’s equator during the wait for the new solar cycle. The streams blow out through these gaping gaps, and can buffet Earth for seven to ten days in some cases. The data behind these results were accrued during two major international projects, the Whole Sun Month back in 1996 and the Whole Heliosphere Interval in spring 2008. During the Whole Heliosphere Interval, Earth was in the path of the streams for 55 percent of the time, compared to 31 percent in 1996. Interestingly, the streams have dropped off during 2009, almost two years after the predicted solar minimum.
“The intensity of the magnetic activity at Earth in this extremely quiet solar minimum surprised us all,” says co-researcher Janet Kozyra of the University of Michigan. “The new observations from last year are changing our understanding of how quiet solar intervals affect the Earth and how and why this might change from cycle to cycle.”
The research is published in the Journal of Geophysical Research – Space Physics.
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