478 research outputs found
The Role of fast magnetosonic waves in the release and conversion via reconnection of energy stored by a current sheet
Using a simple two-dimensional, zero-beta model, we explore the manner by
which reconnection at a current sheet releases and dissipates free magnetic
energy. We find that only a small fraction (3%-11% depending on current sheet
size) of the energy is stored close enough to the current sheet to be
dissipated abruptly by the reconnection process. The remaining energy, stored
in the larger-scale field, is converted to kinetic energy in a fast
magnetosonic disturbance propagating away from the reconnection site, carrying
the initial current and generating reconnection-associated flows (inflow and
outflow). Some of this reflects from the lower boundary (the photosphere) and
refracts back to the X-point reconnection site. Most of this inward wave energy
is reflected back again, and continues to bounce between X-point and
photosphere until it is gradually dissipated, over many transits. This phase of
the energy dissipation process is thus global and lasts far longer than the
initial purely local phase. In the process a significant fraction of the energy
(25%-60%) remains as undissipated fast magnetosonic waves propagating away from
the reconnection site, primarily upward. This flare-generated wave is initiated
by unbalanced Lorentz forces in the reconnection-disrupted current sheet,
rather than by dissipation-generated pressure, as some previous models have
assumed. Depending on the orientation of the initial current sheet the wave
front is either a rarefaction, with backward directed flow, or a compression,
with forward directed flow
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.
Heating of Flare Loops With Observationally Constrained Heating Functions
We analyze high cadence high resolution observations of a C3.2 flare obtained
by AIA/SDO on August 1, 2010. The flare is a long duration event with soft
X-ray and EUV radiation lasting for over four hours. Analysis suggests that
magnetic reconnection and formation of new loops continue for more than two
hours. Furthermore, the UV 1600\AA\ observations show that each of the
individual pixels at the feet of flare loops is brightened instantaneously with
a timescale of a few minutes, and decays over a much longer timescale of more
than 30 minutes. We use these spatially resolved UV light curves during the
rise phase to construct empirical heating functions for individual flare loops,
and model heating of coronal plasmas in these loops. The total coronal
radiation of these flare loops are compared with soft X-ray and EUV radiation
fluxes measured by GOES and AIA. This study presents a method to
observationally infer heating functions in numerous flare loops that are formed
and heated sequentially by reconnection throughout the flare, and provides a
very useful constraint to coronal heating models.Comment: This paper is revise
Direct Measurements of Magnetic Twist in the Solar Corona
In the present work we study evolution of magnetic helicity in the solar
corona. We compare the rate of change of a quantity related to the magnetic
helicity in the corona to the flux of magnetic helicity through the photosphere
and find that the two rates are similar. This gives observational evidence that
helicity flux across the photosphere is indeed what drives helicity changes in
solar corona during emergence.
For the purposes of estimating coronal helicity we neither assume a strictly
linear force-free field, nor attempt to construct a non-linear force-free
field. For each coronal loop evident in Extreme Ultraviolet (EUV) we find a
best-matching line of a linear force-free field and allow the twist parameter
alpha to be different for each line. This method was introduced and its
applicability was discussed in Malanushenko et. al. (2009).
The object of the study is emerging and rapidly rotating AR 9004 over about
80 hours. As a proxy for coronal helicity we use the quantity
averaged over many reconstructed lines of magnetic field. We argue that it is
approximately proportional to "flux-normalized" helicity H/Phi^2, where H is
helicity and Phi is total enclosed magnetic flux of the active region. The time
rate of change of such quantity in the corona is found to be about 0.021
rad/hr, which is compatible with the estimates for the same region obtained
using other methods Longcope et. al. (2007), who estimated the flux of
normalized helicity of about 0.016 rad/hr
- …