45 research outputs found
"Magneto-elastic" waves in an anisotropic magnetised plasma
The linear waves that propagate in a two fluid magnetised plasma allowing for
a non-gyrotropic perturbed ion pressure tensor are investigated. For
perpendicular propagation and perturbed fluid velocity a low frequency
(magnetosonic) and a high frequency (ion Bernstein) branch are identified and
discussed. For both branches a comparison is made with the results of a
truncated Vlasov treatment. For the low frequency branch we show that a
consistent expansion procedure allows us to recover the correct expression of
the Finite Larmor Radius corrections to the magnetosonic dispersion relation.Comment: 16 pages, 9 figure
Coupling between whistler waves and slow-mode solitary waves
The interplay between electron-scale and ion-scale phenomena is of general
interest for both laboratory and space plasma physics. In this paper we
investigate the linear coupling between whistler waves and slow magnetosonic
solitons through two-fluid numerical simulations. Whistler waves can be trapped
in the presence of inhomogeneous external fields such as a density hump or hole
where they can propagate for times much longer than their characteristic time
scale, as shown by laboratory experiments and space measurements. Space
measurements have detected whistler waves also in correspondence to magnetic
holes, i.e., to density humps with magnetic field minima extending on
ion-scales. This raises the interesting question of how ion-scale structures
can couple to whistler waves. Slow magnetosonic solitons share some of the main
features of a magnetic hole. Using the ducting properties of an inhomogeneous
plasma as a guide, we present a numerical study of whistler waves that are
trapped and transported inside propagating slow magnetosonic solitons.Comment: Submitted to Phys. of Plasma
"Ideal" tearing and the transition to fast reconnection in the weakly collisional MHD and EMHD regimes
This paper discusses the transition to fast growth of the tearing instability
in thin current sheets in the collisionless limit where electron inertia drives
the reconnection process. It has been previously suggested that in resistive
MHD there is a natural maximum aspect ratio (ratio of sheet length and breadth
to thickness) which may be reached for current sheets with a macroscopic length
L, the limit being provided by the fact that the tearing mode growth time
becomes of the same order as the Alfv\`en time calculated on the macroscopic
scale (Pucci and Velli (2014)). For current sheets with a smaller aspect ratio
than critical the normalized growth rate tends to zero with increasing
Lundquist number S, while for current sheets with an aspect ratio greater than
critical the growth rate diverges with S. Here we carry out a similar analysis
but with electron inertia as the term violating magnetic flux conservation:
previously found scalings of critical current sheet aspect ratios with the
Lundquist number are generalized to include the dependence on the ratio
where de is the electron skin depth, and it is shown that there are
limiting scalings which, as in the resistive case, result in reconnecting modes
growing on ideal time scales. Finite Larmor Radius effects are then included
and the rescaling argument at the basis of "ideal" reconnection is proposed to
explain secondary fast reconnection regimes naturally appearing in numerical
simulations of current sheet evolution.Comment: 15 pages, 3 Figures, 1 Tabl
Dynamics of ion-scale coherent magnetic structures and coupling with whistler waves during substorms
A new model of the self-consistent coupling between low frequency, ion-scale coherent magnetic structures and high frequency whistler waves is proposed in order to interpret space data gathered by Cluster satellites during substorm events, in the night sector of the Earthâs magnetosphere. The coupling provides a mechanism to spatially confine and transport whistler waves by means of a highly oblique, propagating nonlinear carrier wave. The present study relies on a combination of data analysis of original in situ measurements, theoretical modeling and numerical investigation. During substorms, the magnetosphere undergoes strong magnetic and electric field fluctuations ranging from low frequencies, of the order or less than the typical ion-time scales, to higher frequencies, of the order or higher than the typical electron time-scales. To understand basic plasma physical processes which characterize the magnetosphere dynamics during substorms an analysis of whether, and by which mechanism, waves occurring at these different time scales are coupled, is of fundamental interest. Low frequency magnetic structures are commonly detected in environments such as the magnetosheath and the solar wind, as well as in the dusk magnetosphere, possibly correlated with higher frequency whistler waves. In this Thesis it is shown that similar magnetic structures, correlated with whistler waves, are observed in the magnetospheric plasma sheet during substorms. The interesting question arises as to how the inhomogeneity associated with such magnetic structures affects the propagation of higher frequency waves. The Cluster mission, thanks to its four satellites in tetrahedron configuration and high temporal resolution measurements, provides a unique opportunity on the one hand to explore the spatial structure of stationary and propagating perturbations observed at low frequencies and on the other hand to study dynamics occurring at higher temporal scales, via whistler mode waves.
With regard to this, I will describe the Cluster spacecraft detection of large amplitude whistler wave packets inside coherent ion-scale magnetic structures embedded in a fast plasma flow during the August 17th 2003 substorm event. In this period the Cluster satellites were located in the plasma sheet region and separated by a distance which is less than the magnetotail typical
ion-scale lengths, namely the ion gyroradius and the ion inertial length. The observed whistler emissions are correlated with magnetic field structures showing magnetic depletions associated with density humps. As a first step, the latter have been modeled as one dimensional nonlinear slow waves which spatially confine and transport whistlers, in the framework of a two-fluid
approximation. This schematic model is investigated through a theoretical and numerical study by means of a two-fluid code, and it is shown that the proposed model goes quite well with data interpretation. Its possible role in substorm dynamics is also discussed.
This new trapping mechanism, studied here by using a highly oblique slow magnetosonic soliton as a guide for whistler waves, is of more general interest beyond the specific context of the observations reported in this Thesis. Other nonlinear structures showing similar features, for example highly oblique nonlinear Alfvén waves or kinetic Alfvén waves in high beta plasmas, can in principle act as wave carriers. The model proposed provides an explanation for the recurrent detection of whistlers inside ion-scale magnetic structures which is alternative to usual models of stationary magnetic structures acting as channels. Moreover, the study described in this Thesis addresses more general questions of basic plasma physics, such as wave propagation in inhomogeneous plasmas and the interaction between wave modes at different temporal scales
Parametric decay and the origin of the low frequency Alfv\'enic spectrum of the solar wind
The fast solar wind shows a wide spectrum of transverse magnetic and velocity
field perturbations. These perturbations are strongly correlated in the sense
of Alfv\'en waves propagating mostly outward, from the Sun to the
interplanetary medium. They are likely to be fundamental to the acceleration
and the heating of the solar wind. However, the precise origin of the broadband
spectrum is to date unknown. Typical periods of chromospheric Alfv\'en waves
are limited to a few minutes, and any longer period perturbations should be
strongly reflected at the transition region. In this work, we show that minute
long Alfv\'enic fluctuations are unstable to the parametric instability.
Parametric instability enables an inverse energy cascade by exciting several
hours long periods Alfv\'enic fluctuations together with strong density
fluctuations (typically between 1 and ). These results may
improve our understanding of the origin of the solar wind turbulent spectrum
and will be tested by the Parker Solar Probe.Comment: 11 pages, 11 figures, to appear in the Astrophysical Journa
Dynamic evolution of current sheets, ideal tearing, plasmoid formation and generalized fractal reconnection scaling relations
Magnetic reconnection may be the fundamental process allowing energy stored
in magnetic fields to be released abruptly, solar flares and coronal mass
ejection (CME) being archetypal natural plasma examples. Magnetic reconnection
is much too slow a process to be efficient on the large scales, but accelerates
once small enough scales are formed in the system. For this reason, the fractal
reconnection scenario was introduced (Shibata and Tanuma 2001) to explain
explosive events in the solar atmosphere: it was based on the recursive
triggering and collapse via tearing instability of a current sheet originally
thinned during the rise of a filament in the solar corona. Here we compare the
different fractal reconnection scenarios that have been proposed, and derive
generalized scaling relations for the recursive triggering of fast, `ideal' -
i.e. Lundquist number independent - tearing in collapsing current sheet
configurations with arbitrary current profile shapes. An important result is
that the Sweet-Parker scaling with Lundquist number, if interpreted as the
aspect ratio of the singular layer in an ideally unstable sheet, is universal
and does not depend on the details of the current profile in the sheet. Such a
scaling however must not be interpreted in terms of stationary reconnection,
rather it defines a step in the accelerating sequence of events of the ideal
tearing mediated fractal cascade. We calculate scalings for the expected number
of plasmoids for such generic profiles and realistic Lundquist numbers.Comment: 11 pages, 2 figure