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R. A. Treumann

Publications and source records attributed to R. A. Treumann.

At least 73 records · Page 4Linked to original sources
arXiv2011

Electron-cylotron maser radiation from electron holes: Upward current region

Electron holes are suggested to be an important source for generation of electron-cyclotron maser radiation. We demonstrate that electron holes generated in a ring-horseshoe distribution in the auroral-kilometric radiation source region have the capacity to emit band-limited radiation. The radiation is calculated in the proper frame of a circular model hole and shown to be strictly perpendicular in this frame. Its bandwidth under auroral conditions is of the order of $\sim1$ kHz, which is a reasonable value. {It is also shown that much of the drift of fine structure in the radiation can be interpreted as Doppler shift. Estimates based on data are in good agreement with theory. Growth and absorption rates have been obtained for the emitted radiation. However, the growth rate of a single hole obtained under conservative conditions is small, too small for reproducing the observed fine structure flux. Trapping of radiation inside the hole for the hole's lifetime helps amplifying the radiation additionally but introduces other problems. This entire set of questions is discussed at length and compared to radiation from the global horseshoe distribution.} The interior of the hole produces a weak absorption at slightly higher frequency than emission. The absorptivity is roughly two orders of magnitude below the growth rate of the radiation thus being weak even when the emission and absorption bands overlap. Transforming to the stationary observer's frame it is found that the radiation becomes oblique against the magnetic field. For approaching holes the radiated frequencies may even exceed the local electron cyclotron frequency.

physics.space-ph↗
arXiv2011

Relativistic transformation of phase-space distributions

We investigate the transformation of the distribution function in the relativistic case, a problem of interest in plasma when particles with high (relativistic) velocities come into play as for instance in radiation belt physics, in the electron-cyclotron maser radiation theory, in the vicinity of high-Mach number shocks where particles are accelerated to high speeds, and generally in solar and astrophysical plasmas. We show that the phase-space volume element is a Lorentz constant and construct the general particle distribution function from first principles. Application to thermal equilibrium lets us derive a modified version of the isotropic relativistic thermal distribution, the modified Jüttner distribution corrected for the Lorentz-invariant phase-space volume element. Finally, we discuss the relativistic modification of a number of plasma parameters. Keywords: Relativistic invariance of the one-particle distribution function, Jüttner distribution, relativistic plasma parameters

physics.space-ph↗
arXiv2011

Fundamentals of collisionless shocks for astrophysical application, 2. Relativistic shocks

We review recent progress on collisionless relativistic shocks. Kinetic instability theory is briefed including its predictions and limitations. The main focus is on numerical experiments in (i) pair and (ii) electron-nucleon plasmas. The main results are: (i) confirmation of shock evolution in non-magnetised relativistic plasma in 3D due to either the lepton-Weibel instability or the ion-Weibel instability; (ii) sensitive dependence on upstream magnetisation ; (iii) the sensitive dependence of particle dynamics on the upstream magnetic inclination angle $\thetabn$, where particles of $\thetabn>34^\circ$ cannot escape upstream, leading to the distinction between `sub-luminal' and `super-luminal' shocks; (iv) particles in ultra-relativistic shocks can hardly overturn the shock and escape to upstream; they may oscillate around the shock ramp for a long time, so to speak `surfing it' and thereby becoming accelerated by a kind of SDA; (v) these particles form a power law tail on the downstream distribution; their limitations are pointed out; (vi) recently developed methods permit the calculation of the radiation spectra emitted by the downstream high-energy particles; (vii) the Weibel-generated downstream magnetic fields form large amplitude vortices which could be advected by the downstream flow to large distances from the shock and possibly contribute to an extended strong field region; (viii) if cosmic rays are included, Bell-like modes can generate upstream magnetic turbulence at short and, by diffusive re-coupling, also long wavelengths in nearly parallel magnetic field shocks; (ix) advection of such large-amplitude waves should cause periodic reformation of the quasi-parallel shock and eject large amplitude magnetic field vortices downstream where they contribute to turbulence and to maintaining an extended region of large magnetic fields.

astro-ph.HE↗
arXiv2011

Downward auroral currents from the reconnection Hall-region

We present a simple (stationary) mechanism capable of generating the auroral downward field-aligned electric field {that is} needed for {accelerating the ionospheric electron component up into the magnetosphere and confining the ionospheric ions at low latitudes (as is required by observation of an ionospheric cavity in the downward auroral current region). The lifted ionospheric electrons carry the downward auroral current. Our model is based on the assumption of collisionless reconnection in the tail current sheet. It makes use of the dynamical difference between electrons and ions in the ion inertial region surrounding the reconnection {\sf X}-line which causes Hall currents to flow. We show that the spatial confinement of the Hall magnetic field and flux to the ion inertial region centred on the {\sf X}-point generates a spatially variable electromotive force which is positive near the outer inflow boundaries of the ion inertial region and negative in the central inflow region. Looked {at} from the ionosphere it functions like a localised meso-scale electric potential.} The positive electromotive force gives rise to upward electron flow from the ionosphere {during substorms (causing `black aurorae')}. A similar positive potential is identified on the earthward side of the fast reconnection outflow region which has the same effect, explaining the observation that auroral upward currents are flanked from both sides by narrow downward currents. keywords{Field-aligned auroral currents, parallel fields, Hall field in reconnection, substorms}

physics.space-ph↗
arXiv2011

A model of so-called `Zebra' emissions in type IV radio bursts

A simple mechanism for the generation of electromagnetic Zebra pattern emission is proposed. The mechanism is based on the generation of an ion-ring distribution in a magnetic mirror geometry in the presence of a properly directed field-aligned electric potential field. The ion-cyclotron maser then generates a number of electromagnetic ion-cyclotron harmonics which modulate the electron maser emission. The mechanism is capable of switching the emission on and off or amplifying it quasi-periodically which is a main feature of the observations.

physics.space-ph↗
arXiv2011

Collisionless magnetic reconnection: Flux quanta, field lines, `composite electrons' -- Is the quantum-Hall effect involved in its micro-scale physics?

Microscopically, collisionless reconnection in thin current sheets is argued to involve `composite electrons' in the ion inertial (Hall current) domain, a tiny fraction of electrons only. These `composite electrons' are confined to lower Landau levels $ε_L\ll T_e$ (energy much less than temperature). They demagnetise by absorbing magnetic flux quanta $Φ_0=h/e$, decouple from the magnetic field, transport the attached magnetic flux into the non-magnetic centre of the current layer, where they release the flux in the form of micro-scale magnetic vortices, becoming ordinary electrons. The newly born micro-scale magnetic vortices reconnect in their strictly anti-parallel sections when contacting other vortices, ultimately producing the meso-scale reconnection structure. We clarify the notions of magnetic field lines and field line radius, estimate the power released when two oppositely directed flux quanta annihilate, and calculate the number density and Landau-level filling-factor of `composite electrons' in the Hall domain. As side product we find that the magnetic diffusion coefficient in plasma also appears in quanta $D_0^m=eΦ_0/m_e=h/m_e$, yielding that the bulk perpendicular plasma resistivity is quantised, with quantum (lowest limit) $η_{\,0\perp}=μ_0 eΦ_0/m_e=μ_0h/m_e\sim 10^{-9}$ Ohm m. Keywords: Reconnection, thin current sheets, quantum Hall effect, quantised diffusivity, quantised plasma resistivity, composite electrons

physics.space-ph↗
arXiv2010

Magnetic guide field generation in thin collisionless current sheets

In thin ($Δ<$ few $λ_i$) collisionless current sheets in a space plasma like the magnetospheric tail or magnetopause current layer, magnetic fields can grow {from thermal fluctuation level by the action of the non-magnetic Weibel instability \citep{weibel1959}.}The instability is driven by the counter-streaming electron inflow from the `ion diffusion' (ion inertial Hall) region into the inner current (electron inertial) region from where the ambient magnetic fields are excluded when released by the inflowing electrons which become non-magnetic on scales smaller than the electron gyroradius and $<$ few $λ_e$. It is shown that under magnetospheric tail conditions it takes $\sim$ 20-40 e-folding times ($\sim$ 10-20 s) for the Weibel field to reach observable amplitudes $|{\bf b}_{\rm W}|\sim 1$ nT. In counter-streaming inflows these fields are predominantly of guide field type. This is of interest in magnetic guide field reconnection. Guide fields are known to possibly providing the conditions required for the onset of bursty reconnection \citep {drake2006,pritchett2005a,pritchett2006a,cassak2007}. In non-symmetric inflows the Weibel field might itself evolve a component normal to the current sheet which could also contribute to reconnection onset.

physics.space-ph↗
arXiv2009

On the linear mirror mode threshold in high-$β$ space plasma

The linear threshold condition of the mirror mode including weak drift and finite Larmor radius effects is analysed and shown that it can be given to arbitrary precision in closed analytical form. Combined with observations of mirror modes in the magnetosheath it is shown that only magnetic `dips' are produced by the mirror instability. A semi-experimental (non-linear) threshold condition is derived from the data. The occasionally observed so-called `peaks' cannot be attributed to mirror modes. They result probably from nonlinear evolution of the compressive fast mode.

physics.space-ph↗
arXiv2009

Resistive scales for collisionless reconnection in space plasma

We derive restrictions on collisional reconnection under the conditions in near-Earth space. The arguments are based on the precise definition and estimates of the resistive scale $L_η$ at the ion inertial site in collisionless reconnection, i.e. in the so-called `ion diffusion region' which is the region of Hall-current flow.

physics.space-ph↗
arXiv2008

On the competition between the mirror and electromagnetic ion cyclotron modes

We give a simple argument for the exclusive existence of mirror and electromaghetic ion cyclotron modes in anisotropic high-$β$ plasmas. It is shown that, in addition to a large domain of coexistence of both modes, two domains exist in parameter space $(A,β_\perp)$ where solely either mirror modes or electromagnetic ion cyclotron modes can be excited. In the overlap region the modes with the larger growth rate should win. However nonlinear effects may modify such a conclusion.

physics.space-ph↗
arXiv2008

A note on the theory of transverse diffusion in shock particle acceleration

We investigate the role of the form of the spatial diffusion coefficient in shock acceleration of fast particles. Referring to non-classical diffusion and using the results of numerical (hybrid) simulations tailored for the downstream shock population in quasi-perpendicualr high-Mach number collisionless shocks to which we apply the theory, we demonstrate that the inferred diffusion coefficients are in excellent agreement with the requirements of the theory and its predictions. Diffusion in the collisionless regime turns out to be non-classical Gibbsian (Lévy flight), time-dependent though weak.

astro-ph↗
arXiv2008

Deformation of electron holes in phase space as prerequisite for narrow band maser emission: A qualitative discussion

A qualitative discussion is given of the role electron holes play in generating fine structure on the electron cyclotron maser radiation. It is argued that electron holes become deformed in phase space when interacting with an incomplete ring or horseshoe distribution which occurs in the presence of strong field aligned electric fields in the upward current region and in the presence of a loss cone. This interaction is based on momentum balance considerations. Deformed narrow electron holes cause steep velocity space gradients on the ring distribution that lead to intense but narrow band emission from their high speed sides and absorption at slightly higher frequency from their low speed sides. The twins of banded emission and absorption move in frequency space due to the average real space displacement of the deformed electron hole.

physics.space-ph↗
arXiv2008

Auroral evidence for multiple reconnection in the magnetospheric tail plasma sheet

We present auroral evidence for multiple and, most probably, small scale reconnection in the near Earth magnetospheric plasma sheet current layer during auroral activity. Hall currents as the source of upward and downward field-aligned currents require generation of the corresponding electron fluxes. The auroral spatial ordering in a multiple sequence of these fluxes requires the assumption of the existence of several -- and possibly -- even many tailward reconnection sites.

physics.space-ph↗
arXiv2008

On deformation of electron holes in phase space

This Letter shows that for particularly shaped background particle distributions momentum exchange between phase space holes and the distribution causes acceleration of the holes along the magnetic field. In the particular case of a non-symmetric ring distribution (ring with loss cone) this acceleration is nonuniform in phase space being weaker at larger perpendicular velocities thus causing deformation of the hole in phase space.

physics.space-ph↗
arXiv2008

Planetary Bow Shocks

Our present knowledge of the properties of the various planetary bow shocks is briefly reviewed. We do not follow the astronomical ordering of the planets. We rather distinguish between magnetised and unmagnetised planets which groups Mercury and Earth with the outer giant planets of the solar system, Mars and Moon in a separate group lacking magnetic fields and dense atmospheres, and Venus together with the comets as the atmospheric celestial objects exposed to the solar wind. Asteroids would, in this classification, fall into the group together with the Moon and should behave similarly though being much smaller. Extrasolar planets are not considered as we have only remote information about their behaviour. The presentation is brief in the sense that our in situ knowledge is rather sporadic yet, depending on just a countable number of bow shock crossings from which just some basic conclusions can be drawn about size, stationarity, shape and nature of the respective shock. The only bow shock of which we have sufficient information to deal in sufficient depth with its physics is Earth's bow shock. This has been reviewed in other places in this volume and therefore is mentioned here only as the bow shock paradigm in passing.

astro-ph↗
arXiv2008

The Heliospheric Termination Shock

The heliospheric Termination Shock is the largest (by dimension) shock in the heliosphere. It is believed that it is also the strongest shock and is responsible for the generation of the Anomalous Cosmic Ray component in the heliosphere. This chapter review the gross properties and observations of the Termination Shock. It is structured as follows: 1. The heliosphere, providing the heliospheric stage for Termination Shock formation, 2. The argument for a heliospheric Termination Shock, 3. The global heliospheric system, 4. Termination Shock properties, 5. Observations: the Voyager passages, radio observations, plasma waves and electron beams, traces of plasma and magnetic field, energetic particles, galactic cosmic rays, Termination Shock particles, the anomalous cosmic ray component, 6. Conclusions.

astro-ph↗
arXiv2008

Fundamentals of Non-relativistic Collisionless Shock Physics: V. Acceleration of Charged Particles

A comprehensive review is given of the various processes proposed for accelerating particles by shocks to high energies. These energies are limited by several bounds: the non-relativistic nature of the heliospheric collisionless shocks to which this review restricts, the finite size of these shocks, the finite width of the downstream region, and to the nature of turbulence. In general, collisionless shocks in the heliosphere cannot accelerate particles to very high energies. As a fundamental problem of the acceleration mechanism the injection of see particles is identified. Some mecchanisms for production of seed particles are invoked. Acceleration of electrons begins to uncover its nature. The following problems are covered in this chapter: 1. Introduction -- first and second order Fermi acceleration, 2. Accelerating ions when they are already fast, diffusive acceleration, convection diffusion equation, Lee's self-consistent quasilinear shock acceleration model, 3. Observations, 4. The injection problem, ion surfing, test particle simulations, self-consistent shock acceleration simulations, downstream leakage, trapped particle acceleration, 5. Accelerating electrons, Sonnerup-Wu mechanism, Hoshino's electron shock surfing on quasi-perpendicular shocks, quasiparallel shock surfing.

astro-ph↗