Consider a collisionless, homogeneous plasma in which the electron velocity distribution is a bi-Maxwellian with T⊥<T∥, where the subscripts refer to directions relative to the background magnetic field B0. If this anisotropy is sufficiently large and the electron β∥ is sufficiently greater than one, linear dispersion theory predicts that a cyclotron resonant electron firehose instability is excited at propagation oblique to B0 with growth rates less than the electron cyclotron frequency |Ωe| and zero real frequency. This theory at constant maximum growth rate yields threshold conditions for this growing mode of the form 1−T⊥e/T∥e=Se′/β∥eαe′, where the two fitting parameters satisfy 1≲Se′≲2 and αe′≲1.0 over 2.0⩽β∥e⩽25.0. The first particle-in-cell computer simulations of the resonant electron firehose instability are described here. These simulations show that enhanced magnetic field fluctuations reach a maximum value of |δB|2/B02 which increases with β∥e. These enhanced fields scatter the electrons, reducing their anisotropy approximately to a linear theory threshold condition and yielding a dimensionless scattering rate which increases as β∥e increases. These results are consistent with the general principle that, for a given plasma species, scattering by enhanced fluctuations from anisotropy-driven electromagnetic instabilities acts to make the velocity distribution more nearly isotropic as the β∥ of that species increases.

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.[1] An Alfvén-cyclotron fluctuation of sufficiently short wavelength has a strong proton cyclotron resonance at propagation parallel to the background magnetic field B o in a homogeneous, collisionless electron-proton plasma. As k k , the wavevector component parallel to B o , decreases, the proton cyclotron wave-particle interaction becomes nonresonant, and the electron Landau resonance becomes effective at propagation oblique to B o . Here linear Vlasov theory is used to determine the dispersion and damping properties of Alfvén-cyclotron fluctuations associated with the transition from the proton cyclotron resonance regime to the electron Landau resonance regime. Also, a particle-incell plasma simulation is used to examine the electron response to the initial imposition of an Alfvén-cyclotron wave in the electron Landau resonance regime. The computation shows heating of the electrons in the direction parallel to B o and the formation of a beam in the direction of the parallel component of k.

The magnetic properties and magnetocaloric effect (MCE) in the ternary intermetallic compound ErMn2Si2 have been studied by magnetization and heat capacity measurements. A giant reversible MCE has been observed, accompanied by a second order magnetic phase transition from paramagnetic to ferromagnetic at ∼4.5 K. Under a field change of 5 T, the maximum value of magnetic entropy change (−ΔSMmax) is 25.2 J kg−1 K−1 with no thermal and field hysteresis loss, and the corresponding maximum value of adiabatic temperature change (ΔTadmax) is 12.9 K. Particularly, the values of −ΔSMmax and ΔTadmax reached 20.0 J kg−1 K−1 and 5.4 K for a low field change of 2 T, respectively. The present results indicate that the ErMn2Si2 compound is an attractive candidate for low temperature magnetic refrigeration.

Using a 2-1/2-dimensional particle-in-cell (PIC) code to simulate the relativistic expansion of a magnetized collisionless plasma into a vacuum, we report a new mechanism in which the magnetic energy is efficiently converted into the directed kinetic energy of a small fraction of surface particles. We study this mechanism for both electron-positron and electronion (m i /m e =100, m e is the electron rest mass) plasmas. For the electron-positron case the pairs can be accelerated to ultra-relativistic energies. For electron-ion plasmas most of the energy gain goes to the ions. An outstanding problem in astrophysics is the acceleration of high-energy particles. The challenge is to find natural mechanisms which can efficiently convert bulk energy, whether it is magnetic, bulk motion, thermal or gravitational energy, into the relativistic energy of a small number of nonthermal particles. Here we report the results of PIC simulations [1], which suggest a new mechanism for the energization of relativistic particles via magnetic expansion.