Pulsar emission: Langmuir modes in a relativistic multicomponent plasma

Abstract

Ions, protons and possibly a small flux of electrons and positrons are accelerated outward from the polar cap of a normal or millisecond pulsar whose rotational spin is antiparallel with its magnetic moment. The Langmuir modes of this relativistic plasma have several properties of significance for the origin of coherent radio emission. The characteristics of the mode are determined by the sequence of singularities in the dielectric tensor at real angular frequencies, which in turn is fixed by the electronpositron momentum distribution. We find that under a certain condition on its momentum distribution, an electron-positron flux two orders of magnitude smaller than the Goldreich-Julian flux stabilizes the plasma and extinguishes the mode. But more generally, both the growth rate and wavenumber of the multi-component Langmuir mode can be as much as an order of magnitude larger than those of the two-component ion-proton mode. It appears to be a further effective source for the plasma turbulence whose decay is probably responsible for the observed emission.

Pulsar electrodynamics: an unsolved problem

“…Moreover, there are alternative models in which the acceleration by occurs in a current sheet (Coroniti 1990; Uzdensky 2003; Wada & Shibata 2007; Chen & Beloborodov 2014; Philippov & Spitkovsky 2014) rather than a gap. There are also alternative electrodynamics models, including an electrosphere (Krause-Polstorff & Michel 1985) rather than a polar-cap model, and an ion proton plasma (Jones 2014), rather than a pair plasma. Pulsar electrodynamics does not give a clear identification of the source region of the radio emission, or of the specific radio emission mechanism.…”

A multicomponent Langmuir-mode source for the observed pulsar coherent emission

“…This was discussed in Jones (2014b) but that paper had the serious deficiency of failing to describe adequately how the transitions occur in what can be a bistable magnetosphere with nulls and mode changes. It has since been found (Jones 2014c) that the dielectric tensor for an ion-proton plasma which also includes a background component of electrons and positrons gives a zero Langmuir-mode growth rate for a certain class of electron-positron number densities and spectra. This can be the case even for densities small compared with the Goldreich-Julian value, and is no more than a consequence of the small electron-proton mass ratio.…”

Simulations of the radio polarization of a precessing pulsar PSR J1906+0746

“…To conclude, we want to emphasize the uniqueness of this pulsar for future tests of theories of the pulsar radio emission mechanism. For example, the fact that the IP is observed both above and below the magnetic axis rules out theories that rely on the presence of ions in the plasma flow (Cheng & Ruderman 1980;Jones 2014), at least as a generic model applicable to all pulsars. This is because ion extraction is only possible in the half of the polar cap that supports a volume return current (the lower half of the polar cap in Figure 4).…”