Thursday, September 26, 2013

'The Most Complete Observational Evidence for Solar Magnetic Reconnection To Date'

The plasma flow represents the motion relative to the existing (B)-field(say in a local solar field), and its "cutting" action on the field line leads to a change in magnetic flux and induced current.  The only way to avoid this unrealistic situation is for the plasma to be constrained to follow B-field lines rather than cross them. This is what is meant by field freezing.

The region of a large Solar flare, Note the magnetic field lines which follow the solar plasma - this is at about the level of the solar chromosphere.

Magnetic reconnection - the topic of this post- requires solar plasma to temporarily deviate from the infinitely conducting condition displayed in the very top image.  In this situation, when opposite polarity magnetic regions get pushed together, the changing magnetic flux generates a sheet of electric current (called a 'current sheet') that flows perpendicular to the plane of the page. Let this current be strong enough and the plasma's ability to conduct electricity is impeded- which allows the electric field in the plasma to build up and the magnetic field lines to reconnect, generating heat via joule dissipation. The newly connected field lines are then able to lower their energy by snapping back from the reconnection region, converting the magnetic energy into bulk kinetic energy and heat - what we observe in solar flares.

Image of solar corona taken at 211Å by the Atmospheric Imaging Assembly of the Solar Dynamics Observatory (SDO). Note carefully the lines - which are magnetic field lines - which have actual reality in the solar corona and are not merely artificial constructs.

      Three years ago NASA launched the Solar Dynamics Observatory, with great expectations. Much was expected on account of the Atmospheric Imaging Assembly (AIA)  equipped with the capability of obtaining images of the Sun with greater speed and resolution than ever before.
     Now that expectation has more than paid off, with the first detailed observation yet of magnetic reconnection in a solar flare. As one solar researcher, Eric Priest, has put it (Physics Today, September, p. 12): "This is exactly the type of event many of us have been waiting for, for years". 
       On paper- or in theoretical plasma physics classes - magnetic field lines are treated as theoretical constructs with no reality in and of themselves. But in the Sun's corona, they have  physical reality, as seen in the last image taken with the SDO's AIA.  Plasma in the corona is an excellent conductor or electricity because the ratio of kinetic energy density to magnetic energy density is less than one, in fact much less than one. In such a circumstance, the magnetic fields are said to be "force free" and the condition applies such that:  J x B = 0
     Here J is the current density and B denotes the magnetic field intensity.  This condition means that the magnetic field lines cannot move perpendicularly to the underlying solar plasma - which case is depicted in the top most graphic.  In that case, for a non-force free situation one would have:  grad p = J X B

      where grad p denotes the pressure gradient.  As opposed to this, in the corona, the magnetic field lines are entrained with the plasma and move in the same sense. As the Sun rotates and its plasma convects, the field lines will be affected dynamically by the rotation (think of Coriolis forces as on Earth) and become contorted into an ever more series of complicated loops whose topology (under ideal plasma conditions, i.e. J x B = 0) cannot change. But in force free conditions they can - as described in the text for the second image.
        Before going on to the comprehensive SDO discovery - observation, let's be clear what the AIA's limits and capabilities are: It basically uses four telescopes to obtain images of then in ten channels - 7 in the extreme ultra violet (EUV) ranging in wavelength from 94   to  335   -  two in the ultraviolet (UV) and one in the visible spectrum. It can collect one image of the Sun every 12 seconds with a spatial resolution of 0.6 arc seconds (for comparison, the full Moon's diameter is about a half degree or 1800 arcsecs.). It is also possible to collect images even more rapidly by restricting the field of view - say to just part of the Sun, maybe to observe a flare in real time - such as the second image shows.
        In the case of the solar flare of interest, and the subject here - Yang Su (University of Graz, Austria) and an international team of collaborators reported the most comprehensive observations of field reconnection ever for a flare that occurred on Aug. 17, 2011, and just after 4h 00m Universal time. While many flares have erupted in the 3 years since SDO's launch none have shown the explicit features of reconnection as well as the 8/17/11 flare. This also meant it didn't saturate the sensitive AIA detectors and obscure the flare region as other events have.
      Movies obtained of the entire flare - at the full 12 sec resolution - are even more impressive. The film shows hot solar plasma pinned to the magnetic loops flowing away from the reconnection region. One of the images extracted from the movie is shown here (credit Physics Today:)
Solar magnetic reconnection seen in detail (2013_07_25)
       One problem the researchers need to reckon with is reconciling the inherent 3D nature of the Aug. 17, 2011  flare  and the typical 2D models of magnetic reconnection. What we need therefore is a proper three dimensional model of reconnection as it would apply to the solar corona's events
     TO see other events occurring the same date;


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