77 research outputs found
Simulations of the Kelvin-Helmholtz instability driven by coronal mass ejections in the turbulent corona
Recent high resolution AIA/SDO images show evidence of the development of the
Kelvin-Helmholtz instability, as coronal mass ejections (CMEs) expand in the
ambient corona. A large-scale magnetic field mostly tangential to the interface
is inferred, both on the CME and on the background sides. However, the magnetic
field component along the shear flow is not strong enough to quench the
instability. There is also observational evidence that the ambient corona is in
a turbulent regime, and therefore the criteria for the development of the
instability are a-priori expected to differ from the laminar case. To study the
evolution of the Kelvin-Helmholtz instability with a turbulent background, we
perform three-dimensional simulations of the incompressible magnetohydrodynamic
equations. The instability is driven by a velocity profile tangential to the
CME-corona interface, which we simulate through a hyperbolic tangent profile.
The turbulent background is generated by the application of a stationary
stirring force. We compute the instability growth-rate for different values of
the turbulence intensity, and find that the role of turbulence is to attenuate
the growth. The fact that the Kelvin-Helmholtz instability is observed, sets an
upper limit to the correlation length of the coronal background turbulence
Using the Dipolar and Quadrupolar Moments to Improve Solar-Cycle Predictions Based on the Polar Magnetic Fields
The solar cycle and its associated magnetic activity are the main drivers
behind changes in the interplanetary environment and Earth's upper atmosphere
(commonly referred to as space weather and climate). In recent years there has
been an effort to develop accurate solar cycle predictions, leading to nearly a
hundred widely spread predictions for the amplitude of solar cycle 24. Here we
show that cycle predictions can be made more accurate if performed separately
for each hemisphere, taking advantage of information about both the dipolar and
quadrupolar moments of the solar magnetic field during minimum
Magnetic reconnection in incompressible fluids
We investigate the dynamical relaxation of a disturbed X-type magnetic neutral point in a periodic geometry, with an ignorable coordinate, for an incompressible fluid. We find that the properties of the current sheet cannot be understood in terms of steady state reconnection theory or more recent linear dynamical solutions. Accordingly we present a new scaling law for magnetic reconnection consistent with fast energy dissipation, i.e., the dissipation rate at current maximum is approximately independent of magnetic diffusivity (η). The flux annihilation rate, however, scales as η1/4, faster than the Sweet-Parker rate of η1/2 but asymptotically much slower than the dissipation rate. These results suggest a flux pile-up regime in which the bulk of the free magnetic energy is released as heat rather than as kinetic energy of mass motion. The implications of our results for reconnection in the solar atmosphere and interior are discussed
The Minimum of Solar Cycle 23: As Deep as It Could Be?
In this work we introduce a new way of binning sunspot group data with the
purpose of better understanding the impact of the solar cycle on sunspot
properties and how this defined the characteristics of the extended minimum of
cycle 23. Our approach assumes that the statistical properties of sunspots are
completely determined by the strength of the underlying large-scale field and
have no additional time dependencies. We use the amplitude of the cycle at any
given moment (something we refer to as activity level) as a proxy for the
strength of this deep-seated magnetic field.
We find that the sunspot size distribution is composed of two populations:
one population of groups and active regions and a second population of pores
and ephemeral regions. When fits are performed at periods of different activity
level, only the statistical properties of the former population, the active
regions, is found to vary.
Finally, we study the relative contribution of each component (small-scale
versus large-scale) to solar magnetism. We find that when hemispheres are
treated separately, almost every one of the past 12 solar minima reaches a
point where the main contribution to magnetism comes from the small-scale
component. However, due to asymmetries in cycle phase, this state is very
rarely reached by both hemispheres at the same time. From this we infer that
even though each hemisphere did reach the magnetic baseline, from a
heliospheric point of view the minimum of cycle 23 was not as deep as it could
have been
Temperature and Emission-Measure Profiles Along Long-Lived Solar Coronal Loops Observed with TRACE
We report an initial study of temperature and emission measure distributions
along four steady loops observed with the Transition Region and Coronal
Explorer (TRACE) at the limb of the Sun. The temperature diagnostic is the
filter ratio of the extreme-ultraviolet 171-angstrom and 195-angstrom
passbands. The emission measure diagnostic is the count rate in the
171-angstrom passband. We find essentially no temperature variation along the
loops. We compare the observed loop structure with theoretical isothermal and
nonisothermal static loop structure.Comment: 10 pages, 3 postscript figures (LaTeX, uses aaspp4.sty). Accepted by
ApJ Letter
Hinode Calibration for Precise Image Co-alignment between SOT and XRT (November 2006 -- April 2007)
To understand the physical mechanisms for activity and heating in the solar
atmosphere, the magnetic coupling from the photosphere to the corona is an
important piece of information from the Hinode observations, and therefore
precise positional alignment is required among the data acquired by different
telescopes. The Hinode spacecraft and its onboard telescopes were developed to
allow us to investigate magnetic coupling with co-alignment accuracy better
than 1 arcsec. Using the Mercury transit observed on 8 November 2006 and
co-alignment measurements regularly performed on a weekly basis, we have
determined the information necessary for precise image co-alignment and have
confirmed that co-alignment better than 1 arcsec can be realized between Solar
Optical Telescope (SOT) and X-Ray Telescope (XRT) with our baseline
co-alignment method. This paper presents results from the calibration for
precise co-alignment of CCD images from SOT and XRT.Comment: 8 pages, 9 figures, accepted for publication in PASJ (Hinode Special
issue
Temperature distribution of a non-flaring active region from simultaneous Hinode XRT and EIS observations
We analyze coordinated Hinode XRT and EIS observations of a non-flaring
active region to investigate the thermal properties of coronal plasma taking
advantage of the complementary diagnostics provided by the two instruments. In
particular we want to explore the presence of hot plasma in non-flaring
regions. Independent temperature analyses from the XRT multi-filter dataset,
and the EIS spectra, including the instrument entire wavelength range, provide
a cross-check of the different temperature diagnostics techniques applicable to
broad-band and spectral data respectively, and insights into cross-calibration
of the two instruments. The emission measure distribution, EM(T), we derive
from the two datasets have similar width and peak temperature, but show a
systematic shift of the absolute values, the EIS EM(T) being smaller than XRT
EM(T) by approximately a factor 2. We explore possible causes of this
discrepancy, and we discuss the influence of the assumptions for the plasma
element abundances. Specifically, we find that the disagreement between the
results from the two instruments is significantly mitigated by assuming
chemical composition closer to the solar photospheric composition rather than
the often adopted "coronal" composition (Feldman 1992). We find that the data
do not provide conclusive evidence on the high temperature (log T[K] >~ 6.5)
tail of the plasma temperature distribution, however, suggesting its presence
to a level in agreement with recent findings for other non-flaring regions.Comment: 14 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
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