Cluster Spacecraft Catch Crashing Waves inEarth's Magnetic Bubble12 August 2004

A bevy of satellites buzzing around in the Earth'smagnetosphere has found at least part of theanswer to a long-standing puzzle about thesource of the charged particles that feed theaurora.

The charged particles come from explosions on thesun and smash into the Earth's magnetic field,which repels the bulk of them. But many slipthrough, often via a physical process calledmagnetic reconnection, where the magnetic fieldtraveling with the particles breaks and reconnectswith the Earth's field, opening a window for theparticles to surge through. Once inside, theseexcited particles can spiral down toward the polesand create brilliant auroras when they hit theatmosphere.

But magnetic reconnection happens only when thesolar wind's magnetic field direction is 180 degreesopposite from that of the magnetic field of theEarth. When the two fields are aligned, there is noobvious physical process allowing entry of chargedparticles, at least at the leading edge of the Earth'smagnetosphere.

Three years ago, however, the four satellites of theCluster mission, operated by the European SpaceAgency (ESA), passed through the tail of theEarth's magnetic field, which stretches hundreds ofthousands of miles in the shadow of the Earth, andobserved a new process that could allow entry ofthe solar wind particles.

Picture: Three-dimensional computer simulation ofthe space waves or vortices that inject the solarwind plasma into the Earth's magnetic field. Green-blue areas represent the solar wind plasma, andred-orange areas represent plasma trapped inEarth's magnetic field. Earth's magnetospheredevelops ripples and folds like a flag in the wind asthe solar wind blows past. This turbulence createsrolling waves on the edges of the magnetospherethat engulf the solar wind into the magnetosphere(see orange wavelike structure in the cut-awayportion of the image). The blue, green and orangeswirl behind the wave is the vortex that mixes thesolar wind into the magnetosphere. The vorticesare huge structures, measuring more than 20,000miles across. (Kentaro Tanaka of Tokyo Institute ofTechnology)

The satellite data revealed eddies and vortices inEarth's magnetosphere. These waves are kickedup by the solar wind as it blows past themagnetosphere. If these vortices, called non-linearKelvin-Helmholtz waves, detach and spin off intoEarth's magnetosphere, they could carry chargedparticles from the solar wind inside - enough toexplain the hot, magnetically charged gas, orplasma, stored inside the tail of Earth's field.

"The Kelvin-Helmholtz instability has often beenignored as an important solar wind entry process,"said Tai Phan, a space physicist at the University ofCalifornia, Berkeley's Space Sciences Laboratoryand a co-author of the paper. "Thanks to its multi-

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spacecraft measurements, Cluster has now proventhe existence of these large-scale vortices thatcould lead to substantial entry of solar wind topopulate the Earth's magnetosphere."

Phan, lead-author Hiroshi Hasegawa of DartmouthCollege, New Hampshire, and their colleagues inJapan and Europe report their conclusions in theAug. 12 issue of Nature.

A solar wind of charged particles blows incessantlypast the Earth, compressing its magnetic field andpulling it into a teardrop shape pointing away fromthe sun. Periodic solar storms pump up the windand send more particles toward Earth, which createatmospheric disturbances - auroras, magneticstorms, and radiation belt storms - that can affectsatellites as well as radio communications. Thegoal of the Cluster mission and numerous otherEarth satellites is to understand how space weatheraffects the Earth environment.

One big question is how these charged particlespenetrate the protective magnetic field and fill upthe magnetic bubble around Earth, and whattriggers these particles to suddenly flame downonto the poles, creating colorful auroras. Magneticreconnection explains the entry of charged particleswhen the solar wind magnetic field is anti-parallel tothe Earth's field, but when the fields are parallel,they should present an impenetrable barrier to thisflow. Spacecraft measurements dating to 1987clearly show, however, that the magnetosphere isthree to five times fuller when the fields are alignedthan when they are not. So how is the solar windgetting in?

Part of the answer came on Nov. 20, 2001, whenthe Cluster flotilla was heading around from behindthe Earth and had just arrived at the dusk side ofthe planet, where the solar wind slides past theEarth's magnetosphere. There it began toencounter gigantic vortices of gas at themagnetopause, the outer edge of themagnetosphere.

"People have seen waves on the surface of themagnetosphere, but they couldn't tell if they were

small ripples or crashing, rolling waves," Phan said."You have to have big vortices to get the solar windinside."

"These vortices were really huge structures, aboutsix Earth radii across," said Hasegawa. The team'sresults place the size of the vortices at almost40,000 kilometers each.

These vortices are products of non-linear Kelvin-Helmholtz instability, which occur when twoadjacent flows travel past each other at differentspeeds and the friction between them kicks upeddies and vortices. Examples of such instabilitiesare the waves whipped up by the wind slippingacross the surface of the ocean. When a KHI-waverolls up into a vortex, it becomes known as a"Kelvin Cat's eye." The data collected by Clusterhave shown density variations of the electrified gasat the magnetopause precisely like those expectedwhen traveling through a Kelvin Cat's eye.

Scientists had postulated that, if these structureswere to form at the magnetopause, they might beable to pull large quantities of the solar wind insidethe magnetosphere as they collapse. Once thesolar wind particles are carried into the inner part ofthe magnetosphere, they can be excited strongly,allowing them to smash into the Earth's atmosphereand give rise to the auroras.

Cluster's discovery strengthens this scenario butdoes not show the precise mechanism by which thegas is transported into the Earth's magnetic bubble.Thus, scientists still do not know whether this is theonly process to fill up the magnetosphere when themagnetic fields are aligned. For thosemeasurements, Hasegawa said, scientists will haveto wait for a future generation of magnetosphericsatellites.

Phan, in fact, suspects there are other mechanismsthat allow entry of solar wind particles when thesolar wind and Earth's field's are parallel. Detectionlast year by UC Berkeley scientists of a protonaurora over the Earth's poles may indicate thatmagnetic reconnection around the Earth's polarregions, instead of at the bow of the

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magnetosphere, can allow particles in.

The Cluster satellites, built by ESA with significantparticipation from the National Aeronautics andSpace Administration, were launched in summer2000. The Cluster mission investigates three-dimensional structures throughout the Earth'smagnetosphere and solar wind. NASA supportsU.S.-based researchers associated with themission.

"These multi-point, high time-resolutionobservations open a new window intounderstanding the connection of the solar wind tothe Earth's magnetosphere," said William Peterson,NASA's geospace program scientist.

Coauthors, in addition to Hasegawa and Phan, areMasaki Fujimoto, Henri Rme, Andre Balogh,Malcolm W. Dunlop and graduate students C.Hashimoto and R. TanDokoro.

Source: UC Berkeley

APA citation: Cluster Spacecraft Catch Crashing Waves in Earth's Magnetic Bubble (2004, August 12)retrieved 20 May 2015 from http://phys.org/news/2004-08-cluster-spacecraft-earth-magnetic.html

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