111 research outputs found

### CfA Plasma Talks

Notes from a series of 13 one hour (or more) lectures on Plasma Physics given
to Ramesh Narayan' research group at the Harvard-Smithsonian Center for
Astrophysics, between January and July 2012.
Lectures 1 to 5 cover various key Plasma Physics themes. Lectures 6 to 12
mainly go over the Review Paper on "Multidimensional electron beam-plasma
instabilities in the relativistic regime" [\emph{Physics of Plasmas}
\textbf{17}, 120501 (2010)]. Lectures 13 talks about the so-called Biermann
battery and its ability to generate magnetic fields from scratch.Comment: 58 pages, 21 figure

### Density jump as a function of magnetic field strength for perpendicular collisionless shocks with anisotropic upstream pressure

Shock waves are common in astrophysical environments. On many occasions, they
are collisionless, which means they occur in settings where the mean free path
is much larger than the dimensions of the system. For this very reason,
magnetohydrodynamic (MHD) is not equipped to deal with such shocks, be it
because it assumes binary collisions, hence temperature isotropy, when such
isotropy is not guaranteed in the absence of collisions. Here we solve a model
capable of dealing with perpendicular shocks with anisotropic upstream
pressure. The system of MHD conservation equations is closed assuming the
temperature normal to the flow is conserved at the crossing of the shock front.
In the strong shock sonic limit, the behavior of a perpendicular shock with
isotropic upstream is retrieved, regardless of the upstream anisotropy.
Generally speaking, a rich variety of behaviors is found, inaccessible to MHD,
depending on the upstream parameters. The present work can be viewed as the
companion paper of MNRAS 520, 6083-6090 (2023), where the case of a parallel
shock was treated. Differences and similarities with the present case are
discussed.Comment: 9 pages, 12 figures, to appear in MNRA

### Second law from the Noether current on null hypersurfaces

I study the balance law equation of surface charges in the presence of
background fields. The construction allows a unified description of Noether's
theorem for both global and local symmetries. From the balance law associated
with some of these symmetries, I will discuss generalizations of Wald's Noether
entropy formula and general entropy balance laws on null hypersurfaces based on
the null energy conditions, interpreted as an entropy creation term. The
entropy is generally the so-called improved Noether charge, a quantity that has
recently been investigated by many authors, associated to null future-pointing
diffeomorphisms. These local and dynamical definitions of entropy on the black
hole horizon differ from the Bekenstein-Hawking entropy through terms
proportional to the first derivative of the area along the null geodesics. Two
different definitions of the dynamical entropy are identified, deduced from
gravity symplectic potentials providing a suitable notion of gravitational flux
which vanish on non-expanding horizons. The first one is proposed as a
definition of the entropy for dynamical black holes by Wald and Zhang, and it
satisfies the physical process first law locally. The second one vanishes on
any cross section of Minkowski's light cone. I study general properties of its
balance law. In particular, I look at first order perturbations around a non
expanding horizon. Furthermore, I show that the dynamical entropy increases on
the event horizon formed by a spherical symmetric collapse between the two
stationary states of vanishing flux, i.e the initial flat light cone and the
final stationary black hole. I compare this process to a phase transition, in
which the symmetry group of the stationary black hole phase is enlarged by the
supertranslations.Comment: Accepted in Phys.Rev.D. Clarifications added, some parts have been
cleaned u

### A note on the physical process first law of black hole mechanics

I give a simple proof of the physical process first law of black hole
thermodynamics including charged black holes, in which all perturbations are
computed on the horizon.Comment: v2, 7 pages, 1 figure, minor modifications with respect to v

- â€¦