A new study based on data from European Space Agency’s Cluster mission shows that it is easier for the solar wind to penetrate Earth’s magnetic environment, the magnetosphere, than had previously been thought. Scientists from NASA's Goddard Space Flight Center in Greenbelt, Md. have, for the first time, directly observed the presence of certain waves in the solar wind—called Kelvin-Helmholtz waves that can help transfer energy into near-Earth space—under circumstances when previous theories predicted they were not expected.
The recent paper, published on Aug 29, 2012, in the Journal of Geophysical Research shows that the presence of these waves help the incoming charged particles of the solar wind breach the magnetopause—the outer region of the magnetic "shield" ahttp://cms.nasa.gov/support/cms/templates/authoring/detail_page_template.html#4round our planet. As a result, the boundary of Earth’s magnetic bubble behaves less like a continuous barrier and more like a sieve allowing entry to the continuous onslaught of energetic electrons and protons.
"The complex environment near Earth varies continuously, but it is always filled with strong and complex magnetic fields. Variations in the pressure of the solar wind and in the orientation of the magnetic field can lead to changes in how the magnetosphere responds to the solar wind," says Melvyn Goldstein, a geospace scientist at Goddard and an author on the paper. "And understanding how the solar wind impacts these changes by transferring material, momentum and energy across the magnetopause, is one of the most important questions in magnetospheric physics in general and space weather effects in particular."
This latest discovery was made possible by the unique configuration of the four identical Cluster spacecraft, which fly in a closely controlled formation through near-Earth space. As they sweep from the magnetosphere into interplanetary space and back again, the flotilla provides unique three-dimensional insights on the processes that connect the sun to Earth.
Previous discoveries derived from Cluster measurements have shown that the magnetopause is commonly subject to Kelvin-Helmholtz waves. These waves have a distinctive shape that is quite familiar: they look like large amplitude ocean waves that are whipped up by strong winds. Such waves generate turbulence as they crest and break. In the case of the solar wind, the waves are made of huge swirls of electrified gas called plasma, up to 25,000 miles across, which develop along the outer edge of the magnetosphere. Moving plasma, and therefore the Kelvin-Helmholtz waves, trap magnetic fields along with them, which turn out to be crucial in trying to determine how the solar wind can enter the magnetosphere. As the magnetic field becomes wrapped up in the Kelvin-Helmholtz waves, oppositely directed fields can "reconnect", allowing plasma to move from the solar wind into the magnetosphere.
"The space weather community pays considerable attention to Kelvin-Helmholtz waves," says Kyoung-Joo Hwang, a research scientist at Goddard and the University of Maryland Baltimore County and lead author of the paper. "Because they have global influence on Earth's magnetic system and are important for understanding Earth’s response to changes on the sun."