Instabilities, nulls and subpulse drift in radio pulsars

Abstract

This paper continues a previous study of neutron stars with positive polar‐cap corotational charge density in which free emission of ions maintains the surface electric field boundary condition E·B= 0. The composition of the accelerated plasma on any subset of open magnetic flux lines above the polar cap alternates between two states: protons or positrons and ions, of which the proton state cannot support electron–positron pair creation at higher altitudes. The two states coexist at any instant of time above different moving elements of area on the polar cap and provide a physically consistent basis for a description of pulse nulls and subpulse drift. In the latter case, it is shown that the band separation P3 is determined not by the E×B drift velocity, as is generally assumed, but by the diffusion time for protons produced in reverse‐electron showers to reach the region of the atmosphere from which they are accelerated. An initial comparison is made with the survey of subpulse drift published by Weltevrede et al.

Pulsar State Switching From Markov Transitions and Stochastic Resonance

“…The ansatz for this paper is consistent with similar ideas expressed by other authors (Ruderman & Sutherland 1975;Cordes 1979Cordes , 1983Jones 1982Jones , 1986Jones , 2011Timokhin 2010;Li et al 2012b;van Leeuwen & Timokhin 2012) that the observed state changes on both short and long timescales reflect changes in (1) the global state and energetics of a pulsar's magnetosphere, as implied by the large changes in spin-down rate seen in intermittent pulsars; (2) the accelerating potential directly above the magnetic polar caps (PCs) that generates relativistic particles, as evidenced by mode changes; and (3) the temperature of the hot PC due to particle bombardment as demonstrated by X-ray low and high states that correlate with mode changes. This paper addresses issues in understanding pulsar magnetospheres using the statistical framework of Markov chains:…”

“…This situation applies to an · B < 0 geometry for the OFL region at the NS surface, the situation considered by Ruderman & Sutherland (1975) and Jones (2011Jones ( , 2012Jones ( , 2013, that requires positive charges above the PC. The opposite case with · B > 0 has comparatively uncomplicated electron flows that may not allow pair production and radio emission.…”

“…Depending on their constituency, they can also "dope" the NS surface with ions that differ from those that drift to the top of the thin atmosphere from below (Jones 2011), which are fractionated in charge-to-mass ratio by backflow electrons (Jones 2012). In laboratory diodes with non-SCL current flow, the current density depends on the type of material as well as on the voltage drop (Goncalves et al 2004).…”

“…The heating and cooling times of the PC are estimated to be 1 s and thermostatic regulation from pair production may result in variations on some multiple of this timescale (Cheng & Ruderman 1980). Surface physics introduces the proton drift time through the atmosphere τ p ∼ 0.1-1 s and the "excavation" time for removing one radiation length's amount of material from the PC, τ rl ≈ 2.1 × 10 5 s (P /ZB 12 ; Jones 2011Jones , 2012. Orbital periods around a 1.4 M NS are P orb ≈ 127 s P 3/2 (r/r lc ) 3/2 using r lc as a fiducial radius.…”

“…The exponential increase in P 4 (equivalent to a decrease in drift rate) suggests in the carousel model that, unlike for B0031−07 and B2319+60 discussed above, the E × B drift velocity decreases in B-mode sequences, implying that the PC acceleration region trends toward a force-free state. In the ionic evolution model of Jones (2011), the vertical potential drop in the PC accelerator evolves as high-Z ions are increasingly ionized and disassociated. It is not clear in either of these interpretations why the state changes are not more periodic rather than showing a wide range of durations, but a nonlinear system driven stochastically can account for these.…”