25,286 research outputs found
On the origin of two X-class flares in active region NOAA 12673 - Shear flows and head-on collision of new and pre-existing flux
Flare-prolific active region NOAA 12673 produced consecutive X2.2 and X9.3
flares on 06/09/2017. To scrutinize the morphological, magnetic, and horizontal
flow properties associated with these flares, a 7-hour time-series was used
consisting of continuum images, line-of-sight/vector magnetograms, and 1600
{\AA} UV images. These data were acquired with the SDO HMI and AIA. The
white-light flare emission differed for both flares, while the X2.2 flare
displayed localized, confined flare kernels, the X9.3 flare exhibited a
two-ribbon structure. In contrast, the excess UV emission exhibited a similar
structure for both flares, but with larger areal extent for the X9.3 flare.
These two flares represented a scenario, where the first confined flare acted
as precursor, setting up the stage for the more extended flare. Difference maps
for continuum and magnetograms revealed locations of significant changes, i.e.,
penumbral decay and umbral strengthening. The curved magnetic polarity
inversion line in the {\delta}-spot was the fulcrum of most changes. Horizontal
proper motions were computed using the DAVE4VM. Persistent flow features
included (1) strong shear flows along the polarity inversion line, where the
negative, parasitic polarity tried to bypass the majority, positive-polarity
part of the {\delta}-spot in the north, (2) a group of positive-polarity spots,
which moved around the {\delta}-spot in the south, moving away from the
{\delta}-spot with significant horizontal flow speeds, and (3) intense moat
flows partially surrounding the penumbra of several sunspots, which became
weaker in regions with penumbral decay. The enhanced flare activity has its
origin in the head-on collision of newly emerging flux with an already existing
regular, {\alpha}-spot.Comment: 7 pages, 6 Figures, Accepted to be published in Astronomy and
Astrophysic
Evaluating local correlation tracking using CO5BOLD simulations of solar granulation
Flows on the solar surface are linked to solar activity, and LCT is one of
the standard techniques for capturing the dynamics of these processes by
cross-correlating solar images. However, the link between contrast variations
in successive images to the underlying plasma motions has to be quantitatively
confirmed. Radiation hydrodynamics simulations of solar granulation
(e.g.,CO5BOLD) provide access to both the wavelength-integrated, emergent
continuum intensity and the 3D velocity field at various heights in the solar
atmosphere. Thus, applying LCT to continuum images yields horizontal proper
motions, which are then compared to the velocity field of the simulated
(non-magnetic) granulation. In this study, we evaluate the performance of an
LCT algorithm previously developed for bulk-processing Hinode G-band images,
establish it as a quantitative tool for measuring horizontal proper motions,
and clearly work out the limitations of LCT or similar techniques designed to
track optical flows. Horizontal flow maps and frequency distributions of the
flow speed were computed for a variety of LCT input parameters including the
spatial resolution, the width of the sampling window, the time cadence of
successive images, and the averaging time used to determine persistent flow
properties. Smoothed velocity fields from the hydrodynamics simulation at three
atmospheric layers (log tau=-1,0,and +1) served as a point of reference for the
LCT results. LCT recovers many of the granulation properties, e.g.,the shape of
the flow speed distributions, the relationship between mean flow speed and
averaging time, and also--with significant smoothing of the simulated velocity
field--morphological features of the flow and divergence maps. However, the
horizontal proper motions are grossly underestimated by as much as a factor of
three. The LCT flows match best the flows deeper in the atmosphere at log
tau=+1.Comment: 11 pages, 16 figures, accepted for publication in Astronomy and
Astrophysic
Computation of Kolmogorov's Constant in Magnetohydrodynamic Turbulence
In this paper we calculate Kolmogorov's constant for magnetohydrodynamic
turbulence to one loop order in perturbation theory using the direct
interaction approximation technique of Kraichnan. We have computed the
constants for various , i.e., fluid to magnetic energy ratios
when the normalized cross helicity is zero. We find that increases from
1.47 to 4.12 as we go from fully fluid case to a situation when , then it decreases to 3.55 in a fully magnetic limit .
When , we find that .Comment: Latex, 10 pages, no figures, To appear in Euro. Phys. Lett., 199
Incompressible Turbulence as Nonlocal Field Theory
It is well known that incompressible turbulence is nonlocal in real space
because sound speed is infinite in incompressible fluids. The equation in
Fourier space indicates that it is nonlocal in Fourier space as well. Contrast
this with Burgers equation which is local in real space. Note that the sound
speed in Burgers equation is zero. In our presentation we will contrast these
two equations using nonlocal field theory. Energy spectrum and renormalized
parameters will be discussed.Comment: 7 pages; Talk presented in Conference on "Perspectives in Nonlinear
Dynamics (PNLD 2004)" held in Chennai, 200
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