1,069 research outputs found
Features of ion acoustic waves in collisional plasmas
The effects of friction on the ion acoustic (IA) wave in fully and partially
ionized plasmas are studied. In a quasi-neutral electron-ion plasma the
friction between the two species cancels out exactly and the wave propagates
without any damping. If the Poisson equation is used instead of the
quasi-neutrality, however, the IA wave is damped and the damping is dispersive.
In a partially ionized plasma, the collisions with the neutrals modify the IA
wave beyond recognition. For a low density of neutrals the mode is damped. Upon
increasing the neutral density, the mode becomes first evanescent and then
reappears for a still larger number of neutrals. A similar behavior is obtained
by varying the mode wave-length. The explanation for this behavior is given. In
an inhomogeneous plasma placed in an external magnetic field, and for
magnetized electrons and un-magnetized ions, the IA mode propagates in any
direction and in this case the collisions make it growing on the account of the
energy stored in the density gradient. The growth rate is angle dependent. A
comparison with the collision-less kinetic density gradient driven IA
instability is also given.Comment: The following article has been accepted by Physics of Plasmas. After
it is published, it will be found at http://pop.aip.org
Solar nanoflares and other smaller energy release events as growing drift waves
Rapid energy releases (RERs) in the solar corona extend over many orders of
magnitude, the largest (flares) releasing an energy of J or more.
Other events, with a typical energy that is a billion times less, are called
nanoflares. A basic difference between flares and nanoflares is that flares
need a larger magnetic field and thus occur only in active regions, while
nanoflares can appear everywhere. The origin of such RERs is usually attributed
to magnetic reconnection that takes place at altitudes just above the
transition region. Here we show that nanoflares and smaller similar RERs can be
explained within the drift wave theory as a natural stage in the kinetic growth
of the drift wave. In this scenario, a growing mode with a sufficiently large
amplitude leads to stochastic heating that can provide an energy release of
over J
Observational evidence favors a resistive wave heating mechanism for coronal loops over a viscous phenomenon
Context. How coronal loops are heated to their observed temperatures is the subject of a long standing debate.
Aims. Observational evidence exists that the heating in coronal loops mainly occurs near the loop footpoints. In this article, analytically and numerically obtained heating profiles produced by resonantly damped waves are compared to the observationally estimated profiles.
Methods. To do that, the predicted heating profiles are fitted with an exponential heating function, which was also used to fit the observations. The results of both fits, the estimated heating scale heights, are compared to determine the viability of resonant absorption as a heating mechanism for coronal loops.
Results. Two results are obtained. It is shown that any wave heating mechanism (i.e. not just resonant absorption) should be dominated by a resistive (and not a viscous) phenomenon in order to accomodate the constraint of footpoint heating. Additionally it is demonstrated that the analytically and numerically estimated heating scale heights for the resonant absorption damping mechanism
fit the observations very well
Numerical simulations of a flux rope ejection
Coronal mass ejections (CMEs) are the most violent phenomena observed on the Sun. One of the most successful models to explain CMEs is the flux rope ejection model, where a magnetic flux rope is expelled from the solar corona after a long phase along which the flux rope stays in equilibrium while magnetic energy is being accumulated. However, still many questions are outstanding on the detailed mechanism of the ejection and observations continuously provide new data to interpret and put in the context. Currently, extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) are providing new insights into the early phase of CME evolution. In particular, observations show the ejection of magnetic flux ropes from the solar corona and how they evolve into CMEs. However, these observations are difficult to interpret in terms of basic physical mechanisms and quantities, thus, we need to compare equivalent quantities to test and improve our models. In our work, we intend to bridge the gap between models and observations with our model of flux rope ejection where we consistently describe the full life span of a flux rope from its formation to ejection. This is done by coupling the global non-linear force-free model (GNLFFF) built to describe the slow low- β formation phase, with a full MHD simulation run with the software MPI-AMRVAC, suitable to describe the fast MHD evolution of the flux rope ejection that happens in a heterogeneous β regime. We also explore the parameter space to identify the conditions upon which the ejection is favoured (gravity stratification and magnetic field intensity) and we produce synthesised AIA observations (171 Å and 211 Å). To carry this out, we run 3D MHD simulation in spherical coordinates where we include the role of thermal conduction and radiative losses, both of which are important for determining the temperature distribution of the solar corona during a CME. Our model of flux rope ejection is successful in realistically describing the entire life span of a flux rope and we also set some conditions for the backgroud solar corona to favour the escape of the flux rope, so that it turns into a CME. Furthermore, our MHD simulation reproduces many of the features found in the AIA observations.Publisher PDFPeer reviewe
Perpendicular electron collisions in drift and acoustic wave instabilities
Perpendicular electron dynamics and the associated collisions are discussed
in relation to the collisional drift wave instability. In addition, the limit
of small parallel wave numbers of this instability is studied and it is shown
to yield a reduced wave frequency. It is also shown that in this case the
growth rate in fact {\em decreases} for smaller parallel wave numbers, instead
of growing proportional to . As a result, the growth rate appears to
be angle dependent and to reach a maximum for some specific direction of
propagation. The explanation for this strange behavior is given. A similar
analysis is performed for acoustic perturbations in plasmas with unmagnetized
ions and magnetized electrons, in the presence of a density gradient.Comment: 7 figure
Nonlinear three wave interaction in pair plasmas
It is shown that nonlinear three-wave interaction, described by
vector-product type nonlinearities, in pair plasmas implies much more
restrictive conditions for a double energy transfer, as compared to
electron-ion plasmas
Is the Weibel instability enhanced by the suprathermal populations, or not?
The kinetic instabilities of the Weibel-type are presently invoked in a large
variety of astrophysical scenarios because anisotropic plasma structures are
ubiquitous in space. The Weibel instability is driven by a temperature
anisotropy which is commonly modeled by a bi-axis distribution function, such
as a bi-Maxwellian or a generalized bi-Kappa. Previous studies have been
limited to a bi-Kappa distribution and found a suppression of this instability
in the presence of suprathermal tails. In the present paper it is shown that
the Weibel growth rate is rather more sensitive to the shape of the anisotropic
distribution function. In order to illustrate the distinguishing properties of
this instability a \emph{product-bi-Kappa distribution} is introduced, with the
advantage that this distribution function enables the use of different values
of the spectral index in the two directions, . The growth rates and the instability threshold are derived and
contrasted with those for a simple bi-Kappa and a bi-Maxwellian. Thus, while
the maximum growth rates reached at the saturation are found to be higher, the
threshold is drastically reduced making the anisotropic product-bi-Kappa (with
small kappas) highly susceptible to the Weibel instability. This effect could
also rise questions on the temperature or the temperature anisotropy that seems
to be not an exclusive source of free energy for this instability, and
definition of these notions for such Kappa distributions must probably be
reconsidered
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