4,772 research outputs found
The stability of coronal and prominence magnetic fields
The significance of the localized instabilities is not yet fully understood. The nonlinear coupling of these modes may give rise to an explosive instability, with the modes coupling to longer wavelengths. On the other hand, if the modes saturate early, than the main effect of the instability may be an enhancement of transport coefficients. Nonetheless, the equation provides a simple test for the stability of cylindrical magnetic fields
A numerical model of standard to blowout jets
We report on three-dimensional (3D) MHD simulations of the formation of jets produced during the emergence and eruption of solar magnetic fields. The interaction between an emerging and an ambient magnetic field in the solar atmosphere leads to (external) reconnection and the formation of "standard" jets with an inverse Y-shaped configuration. Eventually, low-atmosphere (internal) reconnection of sheared fieldlines in the emerging flux region produces an erupting magnetic flux rope and a reconnection jet underneath it. The erupting plasma blows out the ambient field and, moreover, it unwinds as it is ejected into the outer solar atmosphere. The fast emission of the cool material that erupts together with the hot outflows due to external/internal reconnection form a wider "blowout" jet. We show the transition from "standard" to "blowout" jets and report on their 3D structure. The physical plasma properties of the jets are consistent with observational studies.Peer reviewe
MHD mode conversion in a stratified atmosphere
Mode conversion in the region where the sound and Alfven speeds are equal is
a complex process, which has been studied both analytically and numerically,
and has been seen in observations. In order to further the understanding of
this process we set up a simple, one-dimensional model, and examine wave
propagation through this system using a combination of analytical and numerical
techniques. Simulations are carried out in a gravitationally stratified
atmosphere with a uniform, vertical magnetic field for both isothermal and
non-isothermal cases. For the non-isothermal case a temperature profile is
chosen to mimic the steep temperature gradient encountered at the transition
region. In all simulations, a slow wave is driven on the upper boundary, thus
propagating down from low-beta to high-beta plasma across the mode-conversion
region. In addition, a detailed analytical study is carried out where we
predict the amplitude and phase of the transmitted and converted components of
the incident wave as it passes through the mode-conversion region. A comparison
of these analytical predictions with the numerical results shows good
agreement, giving us confidence in both techniques. This knowledge may be used
to help determine wave types observed and give insight into which modes may be
involved in coronal heating.Comment: 7 pages, 5 figure
MHD Mode Conversion around a 2D Magnetic Null Point
Mode conversion occurs when a wave passes through a region where the sound
and Alfven speeds are equal. At this point there is a resonance, which allows
some of the incident wave to be converted into a different mode. We study this
phenomenon in the vicinity of a two-dimensional, coronal null point. As a wave
approaches the null it passes from low- to high-beta plasma, allowing
conversion to take place. We simulate this numerically by sending in a slow
magnetoacoustic wave from the upper boundary; as this passes through the
conversion layer a fast wave can clearly be seen propagating ahead. Numerical
simulations combined with an analytical WKB investigation allow us to determine
and track both the incident and converted waves throughout the domain.Comment: 4 pages, 2 figure
Sunspot rotation. I. A consequence of flux emergence
Context. Solar eruptions and high flare activity often accompany the rapid
rotation of sunspots. The study of sunspot rotation and the mechanisms driving
this motion are therefore key to our understanding of how the solar atmosphere
attains the conditions necessary for large energy release.
Aims. We aim to demonstrate and investigate the rotation of sunspots in a 3D
numerical experiment of the emergence of a magnetic flux tube as it rises
through the solar interior and emerges into the atmosphere. Furthermore, we
seek to show that the sub-photospheric twist stored in the interior is injected
into the solar atmosphere by means of a definitive rotation of the sunspots.
Methods. A numerical experiment is performed to solve the 3D resistive
magnetohydrodynamic (MHD) equations using a Lagrangian-Remap code. We track the
emergence of a toroidal flux tube as it rises through the solar interior and
emerges into the atmosphere investigating various quantities related to both
the magnetic field and plasma.
Results. Through detailed analysis of the numerical experiment, we find clear
evidence that the photospheric footprints or sunspots of the flux tube undergo
a rotation. Significant vertical vortical motions are found to develop within
the two polarity sources after the field emerges. These rotational motions are
found to leave the interior portion of the field untwisted and twist up the
atmospheric portion of the field. This is shown by our analysis of the relative
magnetic helicity as a significant portion of the interior helicity is
transported to the atmosphere. In addition, there is a substantial transport of
magnetic energy to the atmosphere. Rotation angles are also calculated by
tracing selected fieldlines; the fieldlines threading through the sunspot are
found to rotate through angles of up to 353 degrees over the course of the
experiment
The effect of the relative orientation between the coronal field and new emerging flux: I Global Properties
The emergence of magnetic flux from the convection zone into the corona is an
important process for the dynamical evolution of the coronal magnetic field. In
this paper we extend our previous numerical investigations, by looking at the
process of flux interaction as an initially twisted flux tube emerges into a
plane parallel, coronal magnetic field. Significant differences are found in
the dynamical appearance and evolution of the emergence process depending on
the relative orientation between the rising flux system and any preexisting
coronal field. When the flux systems are nearly anti-parallel, the experiments
show substantial reconnection and demonstrate clear signatures of a high
temperature plasma located in the high velocity outflow regions extending from
the reconnection region. However, the cases that have a more parallel
orientation of the flux systems show very limited reconnection and none of the
associated features. Despite the very different amount of reconnection between
the two flux systems, it is found that the emerging flux that is still
connected to the original tube, reaches the same height as a function of time.
As a compensation for the loss of tube flux, a clear difference is found in the
extent of the emerging loop in the direction perpendicular to the main axis of
the initial flux tube. Increasing amounts of magnetic reconnection decrease the
volume, which confines the remaining tube flux.Comment: 21 pages, 16 figures Accepted for Ap
The South Dakota cooperative land use effort: A state level remote sensing demonstration project
Remote sensing technology can satisfy or make significant contributions toward satisfying many of the information needs of governmental natural resource planners and policy makers. Recognizing this potential, the South Dakota State Planning Bureau and the EROS Data Center together formulated the framework for an ongoing Land Use and Natural Resource Inventory and Information System Program. Statewide land use/land cover information is generated from LANDSAT digital data and high altitude photography. Many applications of the system are anticipated as it evolves and data are added from more conventional sources. The conceptualization, design, and implementation of the program are discussed
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