841 research outputs found
The Acceleration Mechanism of Resistive MHD Jets Launched from Accretion Disks
We analyzed the results of non-linear resistive magnetohydrodynamical (MHD)
simulations of jet formation to study the acceleration mechanism of
axisymmetric, resistive MHD jets. The initial state is a constant angular
momentum, polytropic torus threaded by weak uniform vertical magnetic fields.
The time evolution of the torus is simulated by applying the CIP-MOCCT scheme
extended for resistive MHD equations. We carried out simulations up to 50
rotation period at the innermost radius of the disk created by accretion from
the torus. The acceleration forces and the characteristics of resistive jets
were studied by computing forces acting on Lagrangian test particles. Since the
angle between the rotation axis of the disk and magnetic field lines is smaller
in resistive models than in ideal MHD models, magnetocentrifugal acceleration
is smaller. The effective potential along a magnetic field line has maximum
around in resistive models, where is the radius where the
density of the initial torus is maximum. Jets are launched after the disk
material is lifted to this height by pressure gradient force. Even in this
case, the main acceleration force around the slow magnetosonic point is the
magnetocentrifugal force. The power of the resistive MHD jet is comparable to
the mechanical energy liberated in the disk by mass accretion. Joule heating is
not essential for the formation of jets.Comment: 15 pages, 15 figures, 1 table, accepted for publication in Ap
Signatures of Coronal Heating Mechanisms
Alfven waves created by sub-photospheric motions or by magnetic reconnection
in the low solar atmosphere seem good candidates for coronal heating. However,
the corona is also likely to be heated more directly by magnetic reconnection,
with dissipation taking place in current sheets. Distinguishing observationally
between these two heating mechanisms is an extremely difficult task. We perform
1.5-dimensional MHD simulations of a coronal loop subject to each type of
heating and derive observational quantities that may allow these to be
differentiated.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Three-dimensional MHD Simulations of Jets from Accretion Disks
We report the results of 3-dimensional magnetohydrodynamic (MHD) simulations
of a jet formation by the interaction between an accretion disk and a large
scale magnetic field. The disk is not treated as a boundary condition but is
solved self-consistently. To investigate the stability of MHD jet, the
accretion disk is perturbed with a non-axisymmetric sinusoidal or random
fluctuation of the rotational velocity. The dependences of the jet velocity
, mass outflow rate , and mass accretion rate
on the initial magnetic field strength in both non-axisymmetric cases are
similar to those in the axisymmetric case. That is, ,
and where is the
initial magnetic field strength. The former two relations are consistent with
the Michel's steady solution, , although
the jet and accretion do not reach the steady state. In both perturbation
cases, a non-axisymmetric structure with appears in the jet, where
means the azimuthal wave number. This structure can not be explained by
Kelvin-Helmholtz instability and seems to originate in the accretion disk.
Non-axisymmetric modes in the jet reach almost constant levels after about 1.5
orbital periods of the accretion disk, while all modes in the accretion disk
grow with oscillation. As for the angular momentum transport by Maxwell stress,
the vertical component, , in the wide range of initial magnetic field
strength.Comment: Accepted for publication in ApJ. The pdf file with high resolution
figures can be downloaded at
http://www.kusastro.kyoto-u.ac.jp/~hiromitu/3j050806.pd
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Measurements of reactive nitrogen produced by tropical thunderstorms during BIBLE-C
The Biomass Burning and Lightning Experiment phase C (BIBLE-C) aircraft mission was carried out near Darwin, Australia (12°S, 131°E) in December 2000. This was the first aircraft experiment designed to estimate lightning NO production rates in the tropics, where production is considered to be most intense. During the two flights (flights 10 and 13 made on December 9 and 11-12, respectively) enhancements of NOx (NO + NO2) up to 1000 and 1600 parts per trillion by volume (pptv, 10-s data) were observed at altitudes between 11.5 and 14 km. The Geostationary Meteorological Satellite (GMS) cloud (brightness temperature) data and ground-based lightning measurements by the Global Positioning and Tracking System (GPATS) indicate that there were intensive lightning events over the coast of the Gulf of Carpentaria, which took place upstream from our measurement area 10 to 14 h prior to the measurements. For these two flights, air in which NOx exceeded 100 pptv extended over 620 × 140 and 400 × 170 km2 (wind direction x perpendicular direction), respectively, suggesting a significant impact of lightning NO production on NOx levels in the tropics. We estimate the amount of NOx observed between 11.5 and 14 km produced by the thunderstorms to be 3.3 and 1.8 × 1025 NO molecules for flights 10 and 13, respectively. By using the GPATS lightning flash count data, column NO production rates are estimated to be 1.9-4.4 and 21-49 × 1025 NO molecules per single flash for these two flight data sets. In these estimations, it is assumed that the column NO production between 0 and 16 km is greater than the observed values between 11.5 and 14 km by a factor of 3.2, which is derived using results reported by Pickering et al. (1998). There are however large uncertainties in the GPATS lightning data in this study and care must be made when the production rates are referred. Uncertainties in these estimates are discussed. The impact on the ozone production rate is also described. Copyright 2007 by the American Geophysical Union
Effects of substrate differences on water availability for Arctic lichens during the snow-free summers in the High Arctic glacier foreland
AbstractWe used observational and experimental analyses to investigate the photosynthetic activity and water relationships of five lichen species attached to different substrates in a glacier foreland in the High Arctic, Ny-Ålesund, Svalbard (79°N) during the snow-free season in 2009 and 2010. After the rains ceased, lichens and their attached substrates quickly dried, whereas photosynthetic activity in the lichens decreased gradually. The in situ photosynthetic activity was estimated based on the relative electron transportation rate (rETR) in four fruticose lichens: Cetrariella delisei, Flavocetraria nivalis, Cladonia arbuscula ssp. mitis, and Cladonia pleurota. The rETR approached zero around noon, although the crustose lichen Ochrolechia frigida grown on biological soil crust (BSC) could acquire water from the BSC and retain its WC to perform positive photosynthesis. The light-rETR relationship curves of the five well-watered lichens were characterized into two types: shade-adapted with photoinhibition for the fruticose lichens, and light-adapted with no photoinhibition for O. frigida. The maximum rETR was expected to occur when they could acquire water from the surrounding air or from substrates during the desiccation period. Our results suggest that different species of Arctic lichens have different water availabilities due to their substrates and/or morphological characteristics, which affect their photosynthetic active periods during the summer
Controlling pre-tilt angles using rubbed PEDOTI PSS and DMOAP films for ITO-freeLC devices
In this report, we propose a method to induce a high pretilt angle by laminating DMOAP film on PEDOT / PSS layer
A novel bistable LCD with function of monostable operation
We propose a novel LCD mode named Bistable of Twisted Direction Switching (BTDS) LCD enables to
work not only a memory LCD with the bistability but also as a conventional LCD with the monostability
Structure of Magnetic Tower Jets in Stratified Atmospheres
Based on a new approach on modeling the magnetically dominated outflows from
AGNs (Li et al. 2006), we study the propagation of magnetic tower jets in
gravitationally stratified atmospheres (such as a galaxy cluster environment)
in large scales ( tens of kpc) by performing three-dimensional
magnetohydrodynamic (MHD) simulations. We present the detailed analysis of the
MHD waves, the cylindrical radial force balance, and the collimation of
magnetic tower jets. As magnetic energy is injected into a small central volume
over a finite amount of time, the magnetic fields expand down the background
density gradient, forming a collimated jet and an expanded ``lobe'' due to the
gradually decreasing background density and pressure. Both the jet and lobes
are magnetically dominated. In addition, the injection and expansion produce a
hydrodynamic shock wave that is moving ahead of and enclosing the magnetic
tower jet. This shock can eventually break the hydrostatic equilibrium in the
ambient medium and cause a global gravitational contraction. This contraction
produces a strong compression at the head of the magnetic tower front and helps
to collimate radially to produce a slender-shaped jet. At the outer edge of the
jet, the magnetic pressure is balanced by the background (modified) gas
pressure, without any significant contribution from the hoop stress. On the
other hand, along the central axis of the jet, hoop stress is the dominant
force in shaping the central collimation of the poloidal current. The system,
which possesses a highly wound helical magnetic configuration, never quite
reaches a force-free equilibrium state though the evolution becomes much slower
at late stages. The simulations were performed without any initial
perturbations so the overall structures of the jet remain mostly axisymmetric.Comment: 9 pages, 11 figures, 1 table, accepted for publication in Ap
Revision of Solar Spicule Classification
Solar spicules are the fundamental magnetic structures in the chromosphere
and considered to play a key role in channelling the chromosphere and corona.
Recently, it was suggested by De Pontieu et al. that there were two types of
spicules with very different dynamic properties, which were detected by space-
time plot technique in the Ca ii H line (3968 A) wavelength from Hinode/SOT
observations. 'Type I' spicule, with a 3-7 minute lifetime, undergoes a cycle
of upward and downward motion; in contrast, 'Type II' spicule fades away within
dozens of seconds, without descending phase. We are motivated by the fact that
for a spicule with complicated 3D motion, the space-time plot, which is made
through a slit on a fixed position, could not match the spicule behavior all
the time and might lose its real life story. By revisiting the same data sets,
we identify and trace 105 and 102 spicules in quiet sun (QS) and coronal hole
(CH), respectively, and obtain their statistical dynamic properties. First, we
have not found a single convincing example of 'Type II' spicules. Secondly,
more than 60% of the identified spicules in each region show a complete cycle,
i.e., majority spicules are 'Type I'. Thirdly, the lifetime of spicules in QS
and CH are 148 s and 112 s, respectively, but there is no fundamental lifetime
difference between the spicules in QS and CH reported earlier. Therefore, the
suggestion of coronal heating by 'Type II' spicules should be taken with
cautions. Subject headings: Sun: chromosphere Sun:transition region Sun:coronaComment: accepted by Ap
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