Collisionless momentum transfer in space and astrophysical explosions

Abstract

The AMPTE (Active Magnetospheric Particle Tracer Explorers) mission provided in situ measurements of collisionless momentum and energy exchange between an artificial, photo-ionized barium plasma cloud and the streaming, magnetized hydrogen plasma of the solar wind [1][2][3] . One of its most significant findings was the unanticipated displacement of the barium ion 'comet head' (and an oppositely directed deflection of the streaming hydrogen ions) transverse to both the solar wind flow and the interplanetary magnetic field, defying the conventional expectation that the barium ions would simply move downwind , a collisionless momentum exchange mechanism believed to occur in various astrophysical and space-plasma environments 10,11 and to participate in cosmic magnetized collisionless shock formation [12][13][14] . Here we present the detection of Larmor coupling in a reproducible laboratory experiment that combines an explosive laser-produced plasma cloud with preformed, magnetized ambient plasma in a parameter regime relevant to the AMPTE barium releases. In our experiment, time-resolved Doppler spectroscopy reveals ambient ion acceleration transverse to both the laser-produced plasma flow and the background magnetic field. Utilizing a detailed numerical simulation, we demonstrate that the ambient ion velocity distribution corresponding to the measured Doppler-shifted spectrum is qualitatively and quantitatively consistent with Larmor coupling.The AMPTE barium release experiments aimed to better understand a ubiquitous category of astrophysical and space phenomena characterized by the rapid expansion or relative motion of a dense 'debris' plasma cloud through relatively tenuous, magnetized, ambient plasma. Examples include the expansion of stellar material through the surrounding interstellar medium in supernova remnants 10 , the formation of cometary plasma tails due to the solar wind 15 , the interaction of interplanetary coronal mass ejections with the Earth's magnetosphere 16 , and man-made ionospheric explosions 17 . In these rarified environments, the Coulomb collisional mean free paths exceed the observed interaction length scales by many orders of magnitude, signifying that the debris plasma exchanges momentum and energy with the ambient plasma via collisionless, collective, electromagnetic effects. In addition, the relative motion of the debris cloud produces electric polarization fields between the magnetically confined electrons and the relatively freestreaming ions, resulting in E × B drift electron currents that expel the magnetic field within the cloud volume (the diamagnetic cavity) and enhance it at the cloud edge (the magnetic compression) 18 . The general evolution in the reference frame of the magnetized ambient plasma is thus a deceleration of the debris cloud as it couples to the ambient plasma via collisionless processes and deforms the magnetic field.We here consider explosive astrophysical and space environments characterized by a perpendicular-to-B debris plasma flow speed V d that exceed...

Laboratory study of collisionless coupling between explosive debris plasma and magnetized ambient plasma

“…Emission spectroscopy, wavelength-filtered imaging, and a magnetic flux probe diagnose the debris-ambient interaction. The key result, first reported in a recent publication 33 by the same authors and considerably expounded upon in the present paper, is the direct observation of laminar collisionless coupling through Doppler shifts in a He 1þ ion spectral line, which indicates ambient ion motion in response to the laserproduced debris along a trajectory qualitatively consistent with the hybrid model laminar electric field. Specifically, directed ambient ion acceleration along the axis perpendicular to both the debris flow and the background magnetic field demonstrates that the transverse component of the laminar electric field (associated with Larmor coupling) contributes significantly.…”

A platform for high-repetition-rate laser experiments on the Large Plasma Device

“…Indeed, the experiments presented here were modeled after previous low-repetition experiments on the LAPD that explored perpendicular magnetized collisionless shocks [24,25] . In this setup, the laser plasma acts as a supersonic piston to sweep up and accelerate ambient ions until they form a shock [26] . While the higher-energy 6 Schaeffer et al…”

Collisionless coupling of a high-β expansion to an ambient, magnetized plasma. I. Rayleigh model and scaling

“…33 or the asymmetric response of the ambient plasma across V m of the passing MLPP as is the case in Ref. 35.…”

“…Many are conducted such that the majority of debris is directed unilaterally away from the target and across the background magnetic field. 5,6,[28][29][30][31][32][33][34][35] This configuration includes the presence of a polarization field, which arises from the center-of-mass motion of the expanding plasma. If the center-of-mass moves with velocity V m , then the polarization field has the approximate strength E pol ¼ ÀV m  B.…”