3,772 research outputs found
Kinetic instability of drift-Alfven waves in solar corona and stochastic heating
The solar atmosphere is structured and inhomogeneous both horizontally and
vertically. The omnipresence of coronal magnetic loops implies gradients of the
equilibrium plasma quantities like the density, magnetic field and temperature.
These gradients are responsible for the excitation of drift waves that grow
both within the two-component fluid description (in the presence of collisions
and without it) and within the two-component kinetic descriptions (due to
purely kinetic effects). In the present work the effects of the density
gradient in the direction perpendicular to the magnetic field vector are
investigated within the kinetic theory, in both electrostatic and
electromagnetic regimes. The electromagnetic regime implies the coupling of the
gradient-driven drift wave with the Alfven wave. The growth rates for the two
cases are calculated and compared. It is found that, in general, the
electrostatic regime is characterized by stronger growth rates, as compared
with the electromagnetic perturbations. Also discussed is the stochastic
heating associated with the drift wave. The released amount of energy density
due to this heating should be more dependent on the magnitude of the background
magnetic field than on the coupling of the drift and Alfven waves. The
stochastic heating is expected to be much higher in regions with a stronger
magnetic field. On the whole, the energy release rate caused by the stochastic
heating can be several orders of magnitude above the value presently accepted
as necessary for a sustainable coronal heating.Comment: To appear in ApJ (2010
X-Ray Determination of the Variable Rate of Mass Accretion onto TW Hydrae
Diagnostics of electron temperature (T_e), electron density (n_e), and
hydrogen column density (N_H) from the Chandra High Energy Transmission Grating
spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a
classical accretion model, allow us to infer the accretion rate onto the star
directly from measurements of the accreting material. The new method introduces
the use of the absorption of Ne IX lines as a measure of the column density of
the intervening, accreting material. On average, the derived mass accretion
rate for TW Hya is 1.5 x 10^{-9} M_{\odot} yr^{-1}, for a stellar magnetic
field strength of 600 Gauss and a filling factor of 3.5%. Three individual
Chandra exposures show statistically significant differences in the Ne IX line
ratios, indicating changes in N_H, T_e, and n_e by factors of 0.28, 1.6, and
1.3, respectively. In exposures separated by 2.7 days, the observations
reported here suggest a five-fold reduction in the accretion rate. This
powerful new technique promises to substantially improve our understanding of
the accretion process in young stars
Improved Constraints on the Preferential Heating and Acceleration of Oxygen Ions in the Extended Solar Corona
We present a detailed analysis of oxygen ion velocity distributions in the
extended solar corona, based on observations made with the Ultraviolet
Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. Polar coronal holes at
solar minimum are known to exhibit broad line widths and unusual intensity
ratios of the O VI 1032, 1037 emission line doublet. The traditional
interpretation of these features has been that oxygen ions have a strong
temperature anisotropy, with the temperature perpendicular to the magnetic
field being much larger than the temperature parallel to the field. However,
recent work by Raouafi and Solanki suggested that it may be possible to model
the observations using an isotropic velocity distribution. In this paper we
analyze an expanded data set to show that the original interpretation of an
anisotropic distribution is the only one that is fully consistent with the
observations. It is necessary to search the full range of ion plasma parameters
to determine the values with the highest probability of agreement with the UVCS
data. The derived ion outflow speeds and perpendicular kinetic temperatures are
consistent with earlier results, and there continues to be strong evidence for
preferential ion heating and acceleration with respect to hydrogen. At
heliocentric heights above 2.1 solar radii, every UVCS data point is more
consistent with an anisotropic distribution than with an isotropic
distribution. At heights above 3 solar radii, the exact probability of isotropy
depends on the electron density chosen to simulate the line-of-sight
distribution of O VI emissivity. (abridged abstract)Comment: 19 pages (emulateapj style), 13 figures, ApJ, in press (v. 679; May
20, 2008
Electron and proton heating by solar wind turbulence
Previous formulations of heating and transport associated with strong
magnetohydrodynamic (MHD) turbulence are generalized to incorporate separate
internal energy equations for electrons and protons. Electron heat conduction
is included. Energy is supplied by turbulent heating that affects both
electrons and protons, and is exchanged between them via collisions. Comparison
to available Ulysses data shows that a reasonable accounting for the data is
provided when (i) the energy exchange timescale is very long and (ii) the
deposition of heat due to turbulence is divided, with 60% going to proton
heating and 40% into electron heating. Heat conduction, determined here by an
empirical fit, plays a major role in describing the electron data
A Kinetic Alfven wave cascade subject to collisionless damping cannot reach electron scales in the solar wind at 1 AU
(Abridged) Turbulence in the solar wind is believed to generate an energy
cascade that is supported primarily by Alfv\'en waves or Alfv\'enic
fluctuations at MHD scales and by kinetic Alfv\'en waves (KAWs) at kinetic
scales . Linear Landau damping of KAWs increases with
increasing wavenumber and at some point the damping becomes so strong that the
energy cascade is completely dissipated. A model of the energy cascade process
that includes the effects of linear collisionless damping of KAWs and the
associated compounding of this damping throughout the cascade process is used
to determine the wavenumber where the energy cascade terminates. It is found
that this wavenumber occurs approximately when ,
where and are, respectively, the real frequency and
damping rate of KAWs and the ratio is evaluated in the limit as
the propagation angle approaches 90 degrees relative to the direction of the
mean magnetic field.Comment: Submitted to Ap
A Deep Chandra X-ray Spectrum of the Accreting Young Star TW Hydrae
We present X-ray spectral analysis of the accreting young star TW Hydrae from
a 489 ks observation using the Chandra High Energy Transmission Grating. The
spectrum provides a rich set of diagnostics for electron temperature T_e,
electron density N_e, hydrogen column density N_H, relative elemental
abundances and velocities and reveals its source in 3 distinct regions of the
stellar atmosphere: the stellar corona, the accretion shock, and a very large
extended volume of warm postshock plasma. The presence of Mg XII, Si XIII, and
Si XIV emission lines in the spectrum requires coronal structures at ~10 MK.
Lower temperature lines (e.g., from O VIII, Ne IX, and Mg XI) formed at 2.5 MK
appear more consistent with emission from an accretion shock. He-like Ne IX
line ratio diagnostics indicate that T_e = 2.50 +/- 0.25 MK and N_e = 3.0 +/-
0.2 x 10^(12) cm^(-3) in the shock. These values agree well with standard
magnetic accretion models. However, the Chandra observations significantly
diverge from current model predictions for the postshock plasma. This gas is
expected to cool radiatively, producing O VII as it flows into an increasingly
dense stellar atmosphere. Surprisingly, O VII indicates N_e = 5.7
^(+4.4}_(-1.2) x 10^(11) cm^(-3), five times lower than N_e in the accretion
shock itself, and ~7 times lower than the model prediction. We estimate that
the postshock region producing O VII has roughly 300 times larger volume, and
30 times more emitting mass than the shock itself. Apparently, the shocked
plasma heats the surrounding stellar atmosphere to soft X-ray emitting
temperatures and supplies this material to nearby large magnetic structures --
which may be closed magnetic loops or open magnetic field leading to mass
outflow. (Abridged)Comment: 13 pages (emulateapj style), 10 figures, ApJ, in pres
TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics
The nearest accreting T Tauri star, TW Hya was observed with spectroscopic
and photometric measurements simultaneous with a long se gmented exposure using
the CHANDRA satellite. Contemporaneous optical photometry from WASP-S indicates
a 4.74 day period was present during this time. Absence of a similar
periodicity in the H-alpha flux and the total X-ray flux points to a different
source of photometric variations. The H-alpha emission line appears
intrinsically broad and symmetric, and both the profile and its variability
suggest an origin in the post-shock cooling region. An accretion event,
signaled by soft X-rays, is traced spectroscopically for the first time through
the optical emission line profiles. After the accretion event, downflowing
turbulent material observed in the H-alpha and H-beta lines is followed by He I
(5876A) broadening. Optical veiling increases with a delay of about 2 hours
after the X-ray accretion event. The response of the stellar coronal emission
to an increase in the veiling follows about 2.4 hours later, giving direct
evidence that the stellar corona is heated in part by accretion. Subsequently,
the stellar wind becomes re-established. We suggest a model that incorporates
this sequential series of events: an accretion shock, a cooling downflow in a
supersonically turbulent region, followed by photospheric and later, coronal
heating. This model naturally explains the presence of broad optical and
ultraviolet lines, and affects the mass accretion rates determined from
emission line profiles.Comment: 61 pages; 22 figures; to appear in The Astrophysical Journa
Heavy MSSM Higgs Bosons at CMS: "LHC wedge" and Higgs-Mass Precision
The search for MSSM Higgs bosons will be an important goal at the LHC. In
order to analyze the search reach of the CMS experiment for the heavy neutral
MSSM Higgs bosons, we combine the latest results for the CMS experimental
sensitivities based on full simulation studies with state-of-the-art
theoretical predictions of MSSM Higgs-boson properties. The experimental
analyses are done assuming an integrated luminosity of 30 or 60 fb^-1. The
results are interpreted as 5 \si discovery contours in MSSM M_A-tan_beta
benchmark scenarios. Special emphasis is put on the variation of the Higgs
mixing parameter mu. While the variation of mu can shift the prospective
discovery reach (and correspondingly the ``LHC wedge'' region) by about Delta
tan_beta= 10, the discovery reach is rather stable with respect to the impact
of other supersymmetric parameters. Within the discovery region we analyze the
accuracy with which the masses of the heavy neutral Higgs bosons can be
determined. An accuracy of 1-4% should be achievable, depending on M_A and
tan_beta.Comment: Talk given by G.W. at EPS07 (Manchester, July 2007) and talk given by
S.H. at SUSY07 (Karlsruhe, July 2007). 4 pages, 2 figure
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