The dynamics of low‐β plasma clouds as simulated by a three‐dimensional, electromagnetic particle code

Abstract

The dynamics of low‐β plasma clouds moving perpendicular to an ambient magnetic field in vacuum and in a background plasma is simulated by means of a three‐dimensional, electromagnetic, and relativistic particle simulation code. The simulations show the formation of the space charge sheaths at the sides of the cloud with the associated polarization electric field which facilitate the cross‐field propagation, as well as the sheaths at the front and rear end of the cloud caused by the larger ion Larmor radius, which allows ions to move ahead and lag behind the electrons as they gyrate. Results on the cloud dynamics and electromagnetic radiation include the following: (1) In a background plasma, electron and ion sheaths expand along the magnetic field at the same rate, whereas in vacuum the electron sheath expands much faster than the ion sheath. (2) Sheath electrons are accelerated up to relativistic energies. This result indicates that artificial plasma clouds released in the ionosphere or magnetosphere may generate optical emissions (aurora) as energetic sheath electrons scatter in the upper atmosphere. (3) The expansion of the electron sheaths is analogous to the ejection of high‐intensity electron beams from spacecraft: a stagnation point is reached where the expansion is halted, allowing only a fraction of the electrons to escape. These electrons may have energies exceeding the initial beam energy (sheath electrostatic potential energy). (4) Second‐order and higher‐order sheaths are formed which extend out into the ambient plasma. These sheaths are curved, the curvature increasing with the order of the sheath. (5) The formation of the sheaths and the polarization field reduces the forward momentum of the cloud. Furthermore, as the cloud moves across the field, sheath particles are peeled off and left behind, while fresh particles continue to drift into the sheaths in order to maintain these. This process requires continued energy supply which further decelerates the cloud. (6) The coherent component of the particle gyromotion (the particles are born with a coherent component at t=0) is damped in time as the particles establish a forward directed drift velocity. In addition, particles undergo orbital phase mixing associated with the establishment of the polarization electric field. For a dense cloud in vacuum the damping occurs very quickly, while for clouds in a background plasma the polarization fields are shorted, leaving the particles to gyrate as single particles. In this case, the coherent component is almost undamped. (7) The coherent particle gyrations generate electromagnetic radiation. The simulations indicate that artificial plasma clouds released in space will radiate close to the cloud electron or ion cyclotron/upper hybrid frequencies.

Transport and entry of plasma clouds/jets across transverse magnetic discontinuities: Three‐dimensional electromagnetic particle‐in‐cell simulations

“…Galvez et al, 1988;Livesey and Pritchett, 1989;Galvez and Borovsky, 1991;Cai and Buneman, 1992) particle-in-cell codes, but also electromagnetic threedimensional PIC simulations (e.g. Neubert et al, 1992). The computer experiments performed emphasized the self-polarization of plasma elements and their propagation across the transverse magnetic field, as described by Schmidt's model (Schmidt, 1960).…”

“…Table 2 compares the input parameters of particle-in-cell simulations performed to study the plasma motion across transverse magnetic fields and published in the literature in the last two decades: the two-dimensional electrostatic simulations of Livesey and Pritchett (1989) and Galvez and Borovsky (1991), the three-dimensional electromagnetic simulations of Neubert et al (1992) and the three-dimensional electrostatic simulations by Gunell et al Cluster and THEMIS spacecraft (Karlsson et al, 2012;Plaschke et al, 2013). The numerical simulations of Livesey and Pritchett (1989), Galvez and Borovsky (1991) and Neubert et al (1992) have been performed for a uniform magnetic field, while the geometry used in the study of Gunell et al (2009) is closely related to laboratory experiments with large Larmor radius plasma clouds entering a curved magnetic field (see also Hurtig et al, 2003). To our knowledge, the present study shows the first three-dimensional particle-in-cell simulations of a small Larmor radius plasma cloud transported across a non-uniform magnetic profile typical to a tangential discontinuity (as the magnetopause).…”

On a mechanism of highest-energy cosmic ray acceleration

“…The energy of accelerated particles is ¥ ZeU where Ze is the particle charge and U is one-half of the electrostatic potential difference across the plasma stream. This phenomenon of particle acceleration and the above estimate of particle energy has been verified in experiments (Lindberg 1978) and in computer simulations (Galvez & Borovsky 1991, Neubert et al 1992.…”

Plasmoid Impacts on Neutron Stars and Highest Energy Cosmic Rays

“…This phenomenon has been observed in numerical simulations [14][15][16] which showed that charge layers can accelerate particles to relativistic energies even for relatively slow (sub-Alfvénic) plasma flows; the accelerated particle energy E is ∼ qΦ 0 , where q is the particle charge. This estimate for E derives from the fact that the electrostatic field is dipole-like outside the plasma, giving rise to the potential drop on the order of Φ 0 along B [15].The process of particle acceleration is transient: The energetic particle outflow from boundary layers of the plasmoid gives rise to plasma current; the resulting force decelerates the plasmoid cross-field motion (see below).As an example, consider a plasmoid with h ∼ 10 km infalling at the the free-fall velocity onto the surface of a canonical neutron star of mass M * = 1.4M , radius R * = 10 km and surface magnetic field B * = 5 × 10 …”

“…This phenomenon has been observed in numerical simulations [14][15][16] which showed that charge layers can accelerate particles to relativistic energies even for relatively slow (sub-Alfvénic) plasma flows; the accelerated particle energy E is ∼ qΦ 0 , where q is the particle charge. This estimate for E derives from the fact that the electrostatic field is dipole-like outside the plasma, giving rise to the potential drop on the order of Φ 0 along B [15].…”