The agenda for today is to explore the latest applications of magnetic reconnection in space and astrophysical plasmas.

Camille will talk about:

A gyrofluid model to investigate collisionless reconnection with finite β_e effects

Magnetic reconnection is an ubiquitous phenomenon in magnetized plasmas and is responsible for explosive release of magnetic energy in astrophysical systems, such as solar flares and coronal mass ejections. This process causes a modification of the topology of the magnetic field which is accompanied by a transformation of magnetic energy into kinetic energy and heat. The study of reconnection for a finite β_e, where β_e is the ratio between the electron pressure and the magnetic pressure, can be relevant especially for plasmas with relatively large temperatures, such as in the Earth magnetosheath. Recently, for instance, studies of observations of the MMS space mission in the magnetotail have revealed electron-only reconnecting current sheet, where β_e values can be observed to be greater than 1. The gyrofluid modelling (a fluid-like description of plasmas valid for phenomena at frequencies much less than the cyclotron frequencies), can be effective for investigating β_e effects on reconnection. The analysis presented in this talk is based on a recently derived Hamiltonian gyrofluid model. The finite β_e value involves a magnetic perturbation along the guide field direction, the latter typically corresponding to the mean magnetic field in astrophysical plasmas, and electron finite Larmor radius effects.  By means of numerical simulations, we show that, for small β_e values, the equilibrium electron temperature is seen to enhance the linear growth rate of the reconnecting instability. Whereas, for high β_e values, we observe a stabilizing role when electron finite Larmor radius effects become more important. In the nonlinear phase, stall phases and faster than exponential growth rate are observed.

Hayk will talk about:

Energy dissipation and high-energy radiation from young pulsars

Some of the young pulsars exhibit rotation-modulated gamma-ray emission, captured by the Fermi observatory. The luminosity of this emission suggests that a significant fraction (0.1-10%) of the spin-down power is dissipated in the magnetosphere and reradiated as high-energy photons. This fraction is referred to as the gamma-ray efficiency of the pulsar. In this work we examine the reconnection process and its radiative signatures in detail using global 3D particle-in-cell simulations of pulsar magnetospheres with synchrotron cooling. We show that the fraction of the spin-down power dissipated in the magnetospheric current sheet is uniquely controlled at microphysical plasma scales and only depends on the pulsar inclination angle. We demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle near the current sheet: plasma magnetization parameter. Ultimately, the shape and the extent of the plasma distribution is imprinted in the observed high-energy emission. While the cutoff energy in gamma-rays is dictated by the synchrotron emission from the highest energy pairs, we show that the peak of the emission is also sensitive to the interplay between the efficiency of synchrotron cooling and the particle acceleration rate. We show that there are two separate parameter regimes applicable to young pulsars with low and high spin-down powers. In the former case the synchrotron cooling is dynamically weak, and the peak of the emission is close to the cutoff energy in 1-10 GeV range (e.g., Vela). In pulsars with higher spin-down power the cooling is dynamically important, resulting in broader spectral shapes which peak at lower energies (e.g., for Crab pulsar the peak lies in the MeV band). This picture naturally explains why pulsars with higher spin-down power have lower gamma-ray efficiency in the 0.1 to 100 GeV band of the Fermi satellite.