On the Formation of Cup-like Ion Beam Distributions in the Plasma Sheet Boundary Layer

Abstract

In the end of the 1970ies fast ion flows were discovered, jetting along the boundary of the plasma sheet (PSBL) in the Earth's magnetotail. Now, with the help of new technology devices on board of GALILEO and the GEOTAIL spacecraft, more and more exciting details of the three-dimensional beam velocity distributions are being discovered like, e.g., filaments and rings. In order to understand the origin of these structures and in order to use them for remote diagnostics of acceleration processes, we have derived a quasi-adiabatic theory of the non-linear dynamics of strong acceleration in two-dimensional current sheet fields. Together with the typical magnetotail velocity dispersion our approach reveals, in general, cup-like ion beam velocity distributions. In dependence on the accelerating current sheet electro-magnetic fields the cup-like ion beam velocity distributions may degenerate either to the well known lima-bean shape or to ring distributions. We illustrate our findings by test particle simulations of the beam velocity distribution functions and compare them with ring distributions, observed by GEOTAIL. IntroductionSpace plasma and energetic particles are able to carry information about their sources over long distancies. The reason is their large collisional mean free path, which easily can exceed the distance between source and observer. The investigation of anomalous "collisions" due to resonant interactions of particles with their oscillating collective fields have revealed in almost all situations that they do not influence the originally formed distribution functions considerably. As a result velocity dispersion remains the only one practically important factor influencing particle distributions after their initial formation. On the other hand traditional gyrotropic models have reached the limits of their explanatory power. Non-adiabatic effects have to be taken into account. The necessity to consider non-adiabatic particle dynamics arises also from the high-resolution measurements which are now becoming available. Higher temporary resolution and the observation of the three-dimensional distribution functions allow to deduce much more information from the observations. Improved tools have to be developed, therefore, which use advanced theoretical understanding of the formation of particle distributions. With this paper we try to contribute to this development. We intentionally focus on the formation of the velocity distribution of ion beams flowing along the plasma sheet boundary layer (PSBL) of the Earth's magnetotail. We think that they are due to strong acceleration in current sheet fields, varying in more than the cross-sheet direction.The paper is organized as follows. In Section 2 we review existing observations of PSBL ion beams. We begin with the main features (Subsection 2.1) and proceed with the shape of the beam-velocity distribution (Subsection 2.2), including current observations of ring structures. In the third section we review existing explanatory models. No one of them has predicted...

Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection

“…Ring structures in the outflows jets form cup-like distributions in the 3D velocity space as first shown for the ion-VDFs. 11,21,22 The temporal evolution of the EVDFs was studied in Ref. 13.…”

“…They are obtained also in symmetric 1D CS fields as it was shown by test particle calculations. 20,21,41 Before discussing the EVDFs formed by stronger guidefield reconnection, let us first clarify one terminology. As it was found earlier, in guide-field reconnection, the plasma density is different at different separatrices: [42][43][44][45][46][47][48][49] around two anti-symmetrically located separatrices, the plasma density is enhanced, while it drops near the other two separatrices, forming density cavities.…”

The effects of density inhomogeneities on the radio wave emission in electron beam plasmas

“…For ECME, a positive gradient in the EVDF is needed in the direction perpendicular to the magnetic field, , e.g. in loss-cone (Benáček & Karlický 2017), ring (Pritchett 1984; Lee, Omura & Lee 2011), horseshoe (Bingham & Cairns 2000; Melrose & Wheatland 2016), cup-like (Büchner & Kuska 1996) or shell-shaped distribution functions. The corresponding ‘inverted’ population in the velocity space led to the early authors calling this cyclotron-resonance-related mechanism a ‘maser’ mechanism.…”

Nongyrotropic particle distributions in space plasmas

“…These distributions were observed at pickup processes in space plasmas especially when the pickup is burst-like. Evidence for nongyrotropy was found at comets (Neubauer et al, 1993;Glassmeier and Neubauer, 1993;Coates et al, 1993;Cao et al, 1998), at collisionless shocks (Gosling et al, 1982;Anderson et al, 1985;Thomsen et al, 1985;Sckopke et al, 1990;Fuselier et al, 1990), in the plasma environment of the shuttle (Cairns, 1990), at tangential discontinuities in the solar wind (Astudillo et al, 1996), in the plasma sheet boundary layer (BuÈ chner and Kuska, 1996), and in the geomagnetotail (Ashour-Abdalla et al, 1996).…”