3,193 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
A semi-analytical foreshock model for energetic storm particle events inside 1 AU
We have constructed a semi-analytical model of the energetic-ion foreshock of a CME-driven coronal/interplanetary shock wave responsible for the acceleration of large solar energetic particle (SEP) events. The model is based on the analytical model of diffusive shock acceleration of Bell (1978), appended with a temporal dependence of the cut-off momentum of the energetic particles accelerated at the shock, derived from the theory. Parameters of the model are re-calibrated using a fully time-dependent self-consistent simulation model of the coupled particle acceleration and Alfvén-wave generation upstream of the shock. Our results show that analytical estimates of the cut-off energy resulting from the simplified theory and frequently used in SEP modelling are overestimating the cut-off momentum at the shock by one order magnitude. We show also that the cut-off momentum observed remotely far upstream of the shock (e.g., at 1 AU) can be used to infer the properties of the foreshock and the resulting energetic storm particle (ESP) event, when the shock is still at small distances from the Sun, unaccessible to the in-situ observations. Our results can be used in ESP event modelling for future missions to the inner heliosphere, like the Solar Orbiter and Solar Probe Plus as well as in developing acceleration models for SEP events in the solar corona
Solar off-limb line widths: Alfven waves, ion-cyclotron waves, and preferential heating
Alfven waves and ion-cyclotron absorption of high-frequency waves are
frequently brought into models devoted to coronal heating and fast solar-wind
acceleration. Signatures of ion-cyclotron resonance have already been observed
in situ in the solar wind (HELIOS spacecrafts) and, recently, in the upper
corona (UVCS/SOHO remote-sensing results). We propose a method to constrain
both the Alfven wave amplitude and the preferential heating induced by
ion-cyclotron resonance, above a partially developed polar coronal hole
observed with the SUMER/SOHO spectrometer. The instrumental stray light
contribution is first substracted from the spectra. By supposing that the
non-thermal velocity is related to the Alfven wave amplitude, it is constrained
through a density diagnostic and the gradient of the width of the Mg X 625 A
line. The temperatures of several coronal ions, as functions of the distance
above the limb, are then determined by substracting the non-thermal component
to the observed line widths. The effect of stray light explains the apparent
decrease with height in the width of several spectral lines, this decrease
usually starting about 0.1-0.2 Rs above the limb. This result rules out any
direct evidence of damping of the Alfven waves, often suggested by other
authors. We also find that the ions with the smallest charge-to-mass ratios are
the hottest ones at a fixed altitude and that they are subject to a stronger
heating, as compared to the others, between 57" and 102" above the limb. This
constitutes a serious clue to ion-cyclotron preferential heating.Comment: 15 pages, 12 figures, 3 table
The Temperature Structure of Be Star Disks in the Small Magellanic Cloud
The temperature structure of Be star circumstellar disks at the sub-solar
metallicity appropriate to the Small Magellanic Cloud (SMC) is investigated. It
is found that for central stars of the same spectral type, Be star disks in the
SMC are systematically hotter by several thousand degrees compared to Milky Way
(MW) disks with the same density structure. For early spectral types (B0e -
B3e), this results in systematically smaller H{\alpha} equivalent widths for Be
stars in the SMC. The implication of this result on Be star frequency
comparisons between MW and SMC clusters is shown to be a 5 - 10% lowering of
the detection efficiency of Be stars in SMC clusters. These calculations are
also compared to the known H{\alpha} equivalent width distributions in the MW
and SMC. For the MW, reasonable agreement is found; however, for the SMC, the
match is not as good and systematically larger Be star disks may be required.Comment: 31 pages, 10 figures, Accepted for publication in the astrophysical
Journa
The universally growing mode in the solar atmosphere: coronal heating by drift waves
The heating of the plasma in the solar atmosphere is discussed within both
frameworks of fluid and kinetic drift wave theory. We show that the basic
ingredient necessary for the heating is the presence of density gradients in
the direction perpendicular to the magnetic field vector. Such density
gradients are a source of free energy for the excitation of drift waves. We use
only well established basic theory, verified experimentally in laboratory
plasmas. Two mechanisms of the energy exchange and heating are shown to take
place simultaneously: one due to the Landau effect in the direction parallel to
the magnetic field, and another one, stochastic heating, in the perpendicular
direction. The stochastic heating i) is due to the electrostatic nature of the
waves, ii) is more effective on ions than on electrons, iii) acts predominantly
in the perpendicular direction, iv) heats heavy ions more efficiently than
lighter ions, and v) may easily provide a drift wave heating rate that is
orders of magnitude above the value that is presently believed to be sufficient
for the coronal heating, i.e., J/(ms) for active
regions and J/(ms) for coronal holes. This heating
acts naturally through well known effects that are, however, beyond the current
standard models and theories.Comment: To appear in MNRA
Assessing the tilt of the solar magnetic field axis through Faraday rotation observations
Context: Faraday rotation measurements of extragalactic radio sources during
coronal occultation allow assessment of both the electron density distribution
and the three-dimensional magnetic field topology in the outer solar corona.
Aims: We simulate the three-dimensional structure of both the coronal magnetic
field and the electron density distribution in order to reproduce the excess
Faraday rotation measures (RMs) of the occulted radio sources observed during
solar activity minimum. In particular, we infer the tilt of the solar magnetic
axis with respect to the rotation axis. Methods: We compare the output of the
model with Very Large Array (VLA) radio polarimetric measurements of a sample
of extragalactic sources observed in May 1997. Information on the magnetic
field geometry can be retrieved by fine-tuning the set of model free parameters
that best describe the observations. Results: We find that predicted and
observed Faraday rotation measures are in excellent agreement, thus supporting
the model. Our best-fitting model yields a tilt angle
of the solar magnetic axis with respect to the solar rotation axis around
Carrington Rotation 1923. This result is consistent with analogous but
independent estimates computed from the expansion coefficients of the
photospheric field observed at the Wilcox Solar Observatory (WSO).Comment: 8 pages, 5 figures, accepted for publication in A&
Disk Loss and Disk Renewal Phases in Classical Be Stars I: Analysis of Long-Term Spectropolarimetric Data
(Abridged) Classical Be stars occasionally transition from having a gaseous
circumstellar disk (''Be phase'') to a state in which all observational
evidence for the presence of these disks disappears (''normal B-star phase'').
We present one of the most comprehensive spectropolarimetric views to date of
such a transition for two Be stars, pi Aquarii and 60 Cygni. 60 Cyg's disk loss
episode was characterized by a monotonic decrease in emission strength over a
time-scale of 1000 days, consistent with the viscous time-scale of the disk,
assuming alpha is 0.14. pi Aqr's disk loss was episodic in nature and occurred
over a time-scale of 2440 days. An observed time lag between the behavior of
the polarization and H-alpha in both stars indicates the disk clearing
proceeded in an ''inside-out'' manner. We determine the position angle of the
intrinsic polarization to be 166.7 +/- 0.1 degrees for pi Aqr and 107.7 +/- 0.4
degrees for 60 Cyg, and model the observed polarization during the quiescent
diskless phase of each star to determine the interstellar polarization along
the line of sight. Minor outbursts observed during the quiescent phase of each
star shared similar lifetimes as those previously reported for mu Cen,
suggesting that the outbursts represent the injection and subsequent viscous
dissipation of individual blobs of material into the inner circumstellar
environments of these stars. We also observe deviations from the mean intrinsic
polarization position angle during polarization outbursts in each star,
indicating deviations from axisymmetry. We propose that these deviations might
be indicative of the injection (and subsequent circularization) of new blobs
into the inner disk, either in the plane of the bulk of the disk material or in
a slightly inclined (non-coplanar) orbit.Comment: 30 pages, 14 figures; accepted in Ap
Probing the properties of Be star discs with spectroastrometry and NLTE radiative transfer modelling: beta CMi
While the presence of discs around classical Be stars is well established,
their origin is still uncertain. To understand what processes result in the
creation of these discs and how angular momentum is transported within them,
their physical properties must be constrained. This requires comparing high
spatial and spectral resolution data with detailed radiative transfer
modelling. We present a high spectral resolution, R~80,000, sub milli-arcsecond
precision, spectroastrometric study of the circumstellar disc around the Be
star beta CMi. The data are confronted with three-dimensional, NLTE radiative
transfer calculations to directly constrain the properties of the disc.
Furthermore, we compare the data to disc models featuring two velocity laws;
Keperian, the prediction of the viscous disc model, and angular momentum
conserving rotation. It is shown that the observations of beta CMi can only be
reproduced using Keplerian rotation. The agreement between the model and the
observed SED, polarisation and spectroastrometric signature of beta CMi
confirms that the discs around Be stars are well modelled as viscous decretion
discs.Comment: Accepted for publication in MNRA
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