109 research outputs found
Relating magnetic reconnection to coronal heating
It is clear that the solar corona is being heated and that coronal magnetic
fields undergo reconnection all the time. Here we attempt to show that these
two facts are in fact related - i.e. coronal reconnection generates heat. This
attempt must address the fact that topological change of field lines does not
automatically generate heat. We present one case of flux emergence where we
have measured the rate of coronal magnetic reconnection and the rate of energy
dissipation in the corona. The ratio of these two, , is a current
comparable to the amount of current expected to flow along the boundary
separating the emerged flux from the pre-existing flux overlying it. We can
generalize this relation to the overall corona in quiet Sun or in active
regions. Doing so yields estimates for the contribution to corona heating from
magnetic reconnection. These estimated rates are comparable to the amount
required to maintain the corona at its observed temperature.Comment: To appear in Phil. Trans. Royal Soc.
Calculating energy storage due to topological changes in emerging active region NOAA AR 11112
The Minimum Current Corona (MCC) model provides a way to estimate stored
coronal energy using the number of field lines connecting regions of positive
and negative photospheric flux. This information is quantified by the net flux
connecting pairs of opposing regions in a connectivity matrix. Changes in the
coronal magnetic field, due to processes such as magnetic reconnection,
manifest themselves as changes in the connectivity matrix. However, the
connectivity matrix will also change when flux sources emerge or submerge
through the photosphere, as often happens in active regions. We have developed
an algorithm to estimate the changes in flux due to emergence and submergence
of magnetic flux sources. These estimated changes must be accounted for in
order to quantify storage and release of magnetic energy in the corona. To
perform this calculation over extended periods of time, we must additionally
have a consistently labeled connectivity matrix over the entire observational
time span. We have therefore developed an automated tracking algorithm to
generate a consistent connectivity matrix as the photospheric source regions
evolve over time. We have applied this method to NOAA Active Region 11112,
which underwent a GOES M2.9 class flare around 19:00 on Oct.16th, 2010, and
calculated a lower bound on the free magnetic energy buildup of ~8.25 x 10^30
ergs over 3 days.Comment: 36 pages, 14 figures. Published in 2012 ApJ, 749, 64. Published
version available at http://stacks.iop.org/0004-637X/749/64 Animation
available at http://solar.physics.montana.edu/tarrl/data/AR11112.mp
A Model-based Technique for Ad Hoc Correction of Instrumental Polarization in Solar Spectropolarimetry
We present a new approach for correcting instrumental polarization by
modeling the non-depolarizing effects of a complex series of optical elements
to determine physically realizable Mueller matrices. Provided that the Mueller
matrix of the optical system can be decomposed into a general elliptical
diattenuator and general elliptical retarder, it is possible to model the
cross-talk between both the polarized and unpolarized states of the Stokes
vector and then use the acquired science observations to determine the best-fit
free parameters. Here, we implement a minimization for solar
spectropolarimetric measurements containing photospheric spectral lines
sensitive to the Zeeman effect using physical constraints provided by polarized
line and continuum formation. This model-based approach is able to provide an
accurate correction even in the presence of large amounts of polarization
cross-talk and conserves the physically meaningful magnitude of the Stokes
vector, a significant improvement over previous ad hoc techniques.Comment: 16 pages, 4 figures, Accepted for publication in Ap
Identification and Characterization of Novel Compounds Blocking Shiga Toxin Expression in Escherichia coli O157:H7
Infections caused by Shiga toxin-producing E. coli strains constitute a health problem, as they are problematic to treat. Shiga toxin (Stx) production is a key virulence factor associated with the pathogenicity of enterohaemorrhagic E. coli (EHEC) and can result in the development of haemolytic uremic syndrome in infected patients. The genes encoding Stx are located on temperate lysogenic phages integrated into the bacterial chromosome and expression of the toxin is generally coupled to phage induction through the SOS response. We aimed to find new compounds capable of blocking expression of Stx type 2 (Stx2) as this subtype of Stx is more strongly associated with human disease. High-throughput screening of a small-molecule library identified a lead compound that reduced Stx2 expression in a dose-dependent manner. We show that the optimised compound interferes with the SOS response by directly affecting the activity and oligomerisation of RecA, thus limiting phage activation and Stx2 expression. Our work suggests that RecA is highly susceptible to inhibition and that targeting this protein is a viable approach to limiting production of Stx2 by EHEC. This type of approach has the potential to limit production and transfer of other phage induced and transduced determinants
A Hydrodynamic Model of Alfvénic Wave Heating in a Coronal Loop and Its Chromospheric Footpoints
Alfv\'enic waves have been proposed as an important energy transport
mechanism in coronal loops, capable of delivering energy to both the corona and
chromosphere and giving rise to many observed features, of flaring and
quiescent regions. In previous work, we established that resistive dissipation
of waves (ambipolar diffusion) can drive strong chromospheric heating and
evaporation, capable of producing flaring signatures. However, that model was
based on a simplified assumption that the waves propagate instantly to the
chromosphere, an assumption which the current work removes. Via a ray tracing
method, we have implemented traveling waves in a field-aligned hydrodynamic
simulation that dissipate locally as they propagate along the field line. We
compare this method to and validate against the magnetohydrodynamics code
Lare3D. We then examine the importance of travel times to the dynamics of the
loop evolution, finding that (1) the ionization level of the plasma plays a
critical role in determining the location and rate at which waves dissipate;
(2) long duration waves effectively bore a hole into the chromosphere, allowing
subsequent waves to penetrate deeper than previously expected, unlike an
electron beam whose energy deposition rises in height as evaporation reduces
the mean-free paths of the electrons; (3) the dissipation of these waves drives
a pressure front that propagates to deeper depths, unlike energy deposition by
an electron beam.Comment: Accepted to Ap
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