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Since solar flares are the largest explosions in the solar
system, it isn't surprising that they're often
associated with high-velocity flows. Blobs of ejecta are
frequently observed flying outwards from flares at
speeds of several hundred kilometers per second. In
1999, however, we discovered the unexpected phenomenon of
flows into the flare sites. Using the Soft
X-ray Telescope on the Yohkoh solar
observatory satellite, we detected features moving
downward into the tops of post-eruption arcade at
speeds of a few hundred kilometers per second. (See
the publication by McKenzie & Hudson,
1999; and also the press
release.)
We now believe that these flows represent the
retraction of magnetic flux tubes, shrinking to
relax magnetic tension after reconnection. My PhD student
Sabrina
Savage used Yohkoh, TRACE, and Hinode telescopes to
make quantitative measurements of
Supra-Arcade Downflows, in order to determine
the observational characteristics of magnetic
reconnection in flares. These observations formed the basis
of Sabrina's dissertation, and we are still learning much these downflows
about the conditions that control reconnection in current sheets.
The ability of eruptive flares to imprint a signature onto the solar wind and inner
heliosphere is well-documented. A component of this signature, hitherto unexplored,
is due to the turbulent flows in flare current sheets, turbulence which is driven by
the supra-arcade downflows. Our analysis has demonstrated that the plasma beta in
these sheet-like structures can reach values of order unity, so that gas pressure
forces and magnetic tensions have significant, and important, interplay in this
crucial region. The high spatial and temporal resolution made available by the most
recent solar telescopes (Hinode/XRT, SDO/AIA) facilitates tracking and measurement of
these turbulent flows, through a local correlation tracking technique adapted by us
for use in the corona. The velocity fields determined in this way show temporally and
spatially varying vorticity, and cascades to smaller length scales--classic hallmarks
of turbulence. With PhD students Roger Scott and Mike Freed, and with Prof. Dana Longcope and SAO's Dr.
Kathy Reeves, we are using the measurements of turbulence in the flare current sheets
to explore the conditions that initiate, accelerate, and prolong magnetic
reconnection, as well as the generation of Alfven waves that radiate into the inner
heliosphere.
In solar physics, we are now convinced that magnetic
fields are rearranged, and magnetic energy is released, via a
set of processes which we refer to as magnetic
reconnection. However, we're lacking some specifics: How
fast does reconnection occur? How much flux is included in a
given reconnection event? What triggers an event? By
combining observational measurements with analytical estimates
and computer modeling, we hope to continue finding answers
to some of these questions.
A handful of links for checking on current solar
activity. Each of these will open in a new browser window,
since they are outside of the MSU site.
Back to McKenzie's
home page
Back
to Solar Physics group
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