48 research outputs found
The horizontal internetwork magnetic field: numerical simulations in comparison to observations with Hinode
Observations with the Hinode space observatory led to the discovery of
predominantly horizontal magnetic fields in the photosphere of the quiet
internetwork region. Here we investigate realistic numerical simulations of the
surface layers of the Sun with respect to horizontal magnetic fields and
compute the corresponding polarimetric response in the Fe I 630 nm line pair.
We find a local maximum in the mean strength of the horizontal field component
at a height of around 500 km in the photosphere, where it surpasses the
vertical component by a factor of 2.0 or 5.6, depending on the initial and
boundary conditions. From the synthesized Stokes profiles we derive a mean
horizontal field component that is, respectively, 1.6 and 4.3 times stronger
than the vertical component. This is a consequence of both the intrinsically
stronger flux density of, and the larger area occupied by the horizontal
fields. We find that convective overshooting expels horizontal fields to the
upper photosphere, making the Poynting flux positive in the photosphere, while
this quantity is negative in the convectively unstable layer below it.Comment: 4 pages, 3 figures, minor revisions, esp. concerning top boundary
cond., ApJL accepte
Excitation of Slow-Modes in Network Magnetic Elements Through Magnetic Pumping
From radiation magnetohydrodynamic simulations of the solar atmosphere we
find a new mechanism for the excitation of longitudinal slow modes within
magnetic flux concentrations. We find that the convective downdrafts in the
immediate surroundings of magnetic elements are responsible for the excitation
of slow modes. The coupling between the external downdraft and the plasma
motion internal to the flux concentration is mediated by the inertial forces of
the downdraft that act on the magnetic flux concentration. These forces, in
conjunction with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric and
chromospheric layers of the magnetic element. Subsequent to the transient
pumping phase, the atmosphere rebounds, causing a slow mode traveling along the
magnetic flux concentration in the upward direction. It develops into a shock
wave in chromospheric heights, possibly capable of producing some kind of
dynamic fibril. We propose an observational detection of this process.Comment: 5 pages, 4 figures, accepted for publication in ApJ Lette
Progress in Modeling Very Low Mass Stars, Brown Dwarfs, and Planetary Mass Objects
We review recent advancements in modeling the stellar to substellar
transition. The revised molecular opacities, solar oxygen abundances and cloud
models allow to reproduce the photometric and spectroscopic properties of this
transition to a degree never achieved before, but problems remain in the
important M-L transition characteristic of the effective temperature range of
characterizable exoplanets. We discuss of the validity of these classical
models. We also present new preliminary global Radiation HydroDynamical M
dwarfs simulations.Comment: Submitted to Mem. S. A. It. Supp
Magnetic Energy Spectra in Active Regions
Line-of-sight magnetograms for 217 active regions (ARs) of different flare
rate observed at the solar disk center from January 1997 until December 2006
are utilized to study the turbulence regime and its relationship to the flare
productivity. Data from {\it SOHO}/MDI instrument recorded in the high
resolution mode and data from the BBSO magnetograph were used. The turbulence
regime was probed via magnetic energy spectra and magnetic dissipation spectra.
We found steeper energy spectra for ARs of higher flare productivity. We also
report that both the power index, , of the energy spectrum, , and the total spectral energy are comparably
correlated with the flare index, , of an active region. The correlations are
found to be stronger than that found between the flare index and total unsigned
flux. The flare index for an AR can be estimated based on measurements of
and as , with and . We found that the regime of the fully-developed turbulence occurs in
decaying ARs and in emerging ARs (at the very early stage of emergence).
Well-developed ARs display under-developed turbulence with strong magnetic
dissipation at all scales.Comment: 14 pages, 4 figure
Morphology and Dynamics of the Low Solar Chromosphere
The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn
Solar Telescope of the NSO/SP is used to investigate the morphology and
dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere
below. The study addresses in particular the structure of magnetic elements
that extend into these layers. We choose different quiet Sun regions in and
outside coronal holes. In inter-network regions with no significant magnetic
flux contributions above the detection limit of IBIS, we find intensity
structures with the characteristics of a shock wave pattern. The magnetic flux
elements in the network are long lived and seem to resemble the spatially
extended counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight magnetic
field strength and explain some of the difficulties in deriving the magnetic
field vector from observations of the fluctosphere.Comment: accepted by ApJ, 16 pages, 8 figure
Illuminating shadows: introducing shadow interaction in spatial augmented reality
Computer Systems, Imagery and Medi