Earth's magnetosphere behaves like a sieve24 October 2012

When Earths magnetic field and the interplanetarymagnetic field are aligned, for example in a northwardorientation as indicated by the white arrow in thisgraphic, KelvinHelmholtz waves are generated at low(equatorial) latitudes. Credit: AOES Medialab

ESA's quartet of satellites studying Earth'smagnetosphere, Cluster, has discovered that ourprotective magnetic bubble lets the solar wind inunder a wider range of conditions than previouslybelieved.

Earth's magnetic field is our planet's first line ofdefence against the bombardment of the solarwind. This stream of plasma is launched by theSun and travels across the Solar System, carryingits own magnetic field with it.

Depending on how the solar wind's interplanetarymagnetic field IMF is aligned with Earth'smagnetic field, different phenomena can arise inEarth's immediate environment.

One well-known process is magnetic reconnection,where magnetic field lines pointing in oppositedirections spontaneously break and reconnect withother nearby field lines. This redirects their plasmaload into the magnetosphere, opening the door to

the solar wind and allowing it to reach Earth.

Under certain circumstances this can drive 'spaceweather', generating spectacular aurorae,interrupting GPS signals and affecting terrestrialpower systems.

In 2006, Cluster made the surprising discovery thathuge, 40 000 km swirls of plasma along theboundary of the magnetosphere themagnetopause could allow the solar wind toenter, even when Earth's magnetic field and theIMF are aligned.

These swirls were found at low, equatorial latitudes,where the magnetic fields were most closelyaligned.

These giant vortices are driven by a process knownas the KelvinHelmholtz (KH) effect, which canoccur anywhere in nature when two adjacent flowsslip past each other at different speeds.

Examples include waves whipped up by windsliding across the surface of the ocean, or inatmospheric clouds.

Analysis of Cluster data has now found that KHwaves can also occur at a wider range ofmagnetopause locations and when the IMF isarranged in a number of other configurations,providing a mechanism for the continuous transportof the solar wind into Earth's magnetosphere.

"We found that when the interplanetary magneticfield is westward or eastward, magnetopauseboundary layers at higher latitude become mostsubject to KH instabilities, regions quite distant fromprevious observations of these waves," saysKyoung-Joo Hwang of NASA's Goddard SpaceFlight Center and lead author of the paperpublished in the Journal of Geophysical Research.

"In fact, it's very hard to imagine a situation where

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solar wind plasma could not leak into themagnetosphere, since it is not a perfect magneticbubble."

The findings confirm theoretical predictions and arereproduced by simulations presented by theauthors of the new study.

"The solar wind can enter the magnetosphere atdifferent locations and under different magneticfield conditions that we hadn't known about before,"says co-author Melvyn Goldstein, also fromGoddard Space Flight Center.

"That suggests there is a 'sieve-like' property of themagnetopause in allowing the solar wind tocontinuously flow into the magnetosphere."

The KH effect is also seen in the magnetospheresof Mercury and Saturn, and the new results suggestthat it may provide a possible continuous entrymechanism of solar wind into those planetarymagnetospheres, too.

"Cluster's observations of these boundary waveshave provided a great advance on ourunderstanding of solar wind magnetosphereinteractions, which are at the heart of spaceweather research," says Matt Taylor, ESA's Clusterproject scientist.

"In this case, the relatively small separation of thefour Cluster satellites as they passed through thehigh-latitude dayside magnetopause provided amicroscopic look at the processes ripping open themagnetopause and allowing particles from the Sundirect entry into the atmosphere."

More information: Journal of GeophysicalResearch 117, A08233, doi:10.1029/2011JA017256

Provided by European Space Agency

APA citation: Earth's magnetosphere behaves like a sieve (2012, October 24) retrieved 20 May 2015 from

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