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Magnetic Portals Connect Sun and Earth

Oct. 30, 2008: During the time it takes you to read this article,something will happen high overhead that until recently manyscientists didn't believe in. A magnetic portal will open, linking Earth

to the sun 93 million miles away. Tons of high-energy particles mayflow through the opening before it closes again, around the time youreach the end of the page.

"It's called a flux transfer event or 'FTE,'" says space physicist DavidSibeck of the Goddard Space Flight Center. "Ten years ago I waspretty sure they didn't exist, but now the evidence isincontrovertible."

Indeed, today Sibeck is telling an international assembly of spacephysicists at the 2008 Plasma Workshop in Huntsville, Alabama, thatFTEs are not just common, but possibly twice as common as

anyone had ever imagined.

Right: An artist's concept of Earth's magnetic field connecting to the sun's--a.k.a.a "flux transfer event"--with a spacecraft on hand to measure particles and fields.

Researchers have long known that the Earth and sun must be connected.Earth's magnetosphere (the magnetic bubble that surrounds our planet) is filledwith particles from the sun that arrive via the solar wind and penetrate theplanet's magnetic defenses. They enter by following magnetic field lines that canbe traced from terra firma all the way back to the sun's atmosphere.

"We used to think the connection was permanentand that solar wind could trickle into the near-Earth environment anytime the

wind was active," says Sibeck. "We were wrong. The connections are notsteady at all. They are often brief, bursty and very dynamic."

Several speakers at the Workshop have outlined how FTEs form: On thedayside of Earth (the side closest to the sun), Earth's magnetic field pressesagainst the sun's magnetic field. Approximately every eight minutes, the twofields briefly merge or "reconnect," forming a portal through which particles

can flow. The portal takes the form of a magnetic cylinder about as wide as Earth. The European SpaceAgency's fleet of four Cluster spacecraft and NASA's five THEMIS probes have flown through andsurrounded these cylinders, measuring their dimensions and sensing the particles that shoot through."They're real," says Sibeck.

Now that Cluster and THEMIS have directly sampled FTEs, theorists can use those measurements to

simulate FTEs in their computers and predict how they might behave. Space physicist Jimmy Raeder ofthe University of New Hampshire presented one such simulation at the Workshop. He told his colleaguesthat the cylindrical portals tend to form above Earth's equator and then roll over Earth's winter pole. InDecember, FTEs roll over the north pole; in July they roll over the south pole.

Right: A "magnetic portal" or FTE mapped in cross-section by NASA's fleet of THEMIS spacecraft.

Sibeck believes this is happening twice as often as previously thought. "I think there are two varieties ofFTEs: active and passive." Active FTEs are magnetic cylinders that allow particles to flow through rathereasily; they are important conduits of energy for Earth's magnetosphere. Passive FTEs are magnetic

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cylinders that offer more resistance; their internal structure does not admit such an easy flow of particlesand fields. (For experts: Active FTEs form at equatorial latitudes when the IMF tips south; passive FTEsform at higher latitudes when the IMF tips north.) Sibeck has calculated the properties of passive FTEsand he is encouraging his colleagues to hunt for signs of them in data from THEMIS and Cluster. "PassiveFTEs may not be very important, but until we know more about them we can't be sure."

There are many unanswered questions: Why do the portals form every 8 minutes? How do magnetic fieldsinside the cylinder twist and coil? "We're doing some heavy thinking about this at the Workshop," saysSibeck.

Meanwhile, high above your head, a new portal is opening, connecting your planet to the sun.

Author: Dr. Tony Phillips

Below: In a presentation at the 2008 Plasma Workshop, Robert Fear of the University of Leicester, UK,presented some alternatives for the magnetic topology of FTEs. Possibilities include ropes (left column),cylinders (middle column), or bubbles (right column): abstract.

mailto:james.a.phillips@earthlink.nethttp://science.nasa.gov/headlines/y2008/images/ftes/fear.rtfmailto:james.a.phillips@earthlink.nethttp://science.nasa.gov/headlines/y2008/images/ftes/fear.rtf

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Flux transfer events: Bursty reconnection at the Earthsmagnetopause

R. C. Fear (1)S. E. Milan (1)

A. N. Fazakerley (2)C. J. Owen (2)

r.fear@ion.le.ac.uk(1) Radio & Space Plasma Physics Group, University of Leicester, Leicester, LE1 7RH, United

Kingdom(2) Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking,

Surrey, RH5 6NT, United Kingdom

A flux transfer event (FTE) is a burst of reconnection at the Earths magnetopause (the boundarybetween terrestrial magnetic field lines and the interplanetary magnetic field). FTEs can beobserved either via their in situ signatures (in the magnetic field and plasma distributions) or by

the effect they have on the ionosphere (pulsed flows and poleward-moving auroral and radarfeatures). Early work showed that FTE occurrence at the dayside magnetopause is correlatedwith intervals of southward-directed interplanetary magnetic field (Rijnbeek et al., 1984; Berchemand Russell, 1984). In this talk, we present an overview of recent work that we have carried outon FTEs using in situ observations from the European Space Agencys four-spacecraft Clustermission, supported by ionospheric observations from the SuperDARN radar network. Weexamine three main topics. First, we discuss the occurrence of FTEs when the interplanetarymagnetic field is northward. Under such conditions, magnetopause reconnection may occur athigh latitudes, but the net force exerted on the FTE structures may drag them equatorward,where they can be observed in the post-terminator region (Fear et al., 2005). Second, weconsider the more general matter of FTE motion in a range of interplanetary magneticconditions. We compare the velocities of FTE structures (deduced from multi-spacecraft

observations) with a simple model of open field line motion developed by Cooling et al. (2001),and find that the model explains the observed motion of open field lines across themagnetopause reasonably well (Fear et al., 2007). Finally, we present some observations ofFTEs made by the Cluster spacecraft at their largest separation of the entire mission (~10,000km), along with supporting ionospheric observations, which show that FTE structures at themagnetopause can exist with a variety of azimuthal extents, either extending further azimuthallythan they do poleward, or existing as more spatially localized features (Fear et al., 2008a,b).

mailto:r.fear@ion.le.ac.ukmailto:r.fear@ion.le.ac.uk