Monday, December 21 2020
16:00 - 17:00

IMSc Webinar

Pulsar radio emission and non-resonant hydrodynamic Langmuir mode instability in pulsar plasma

Sk Minhajur Rahaman; google meet link:


Pulsars are a class of fast rotating (~ milliseconds to seconds) and strongly magnetized (10^8 to 10^12 gauss) neutron stars that emit beamed electromagnetic radiation received as periodic pulses of light at the rotational frequency of the pulsar. The origin of radio emission from pulsars is fundamentally different from emission processes in other wavebands. In radio astronomy, we define the brightness temperature for a radio source as the equivalent blackbody temperature that would give the same intensity at a given radio wavelength. The brightness temperature of pulsar radio emission is around 10^25 - 10^27 K. This points not only to a non-thermal process but a highly coherent emission process as well. Coherent radiation involves in-phase emission of a collection of charges acting as a single entity referred to as a ``charge'' bunch. The physics of the emergence of these "charge bunches'' has remained an open problem since the discovery of pulsars. However, there is sufficient observational evidence that radio emission in normal period radio pulsars is due to coherent curvature radiation (hereafter CCR) which originates and escapes within a few hundred to thousand km from the neutron star surface. This requires excitation of electrostatic and longitudinal Langmuir waves in a pulsar beam-plasma system. Pulsar beam-plasma consists of a hot and dense electron-positron pair plasma tens of millions of kelvin hot; along with tenuous and charged beams of positron and ion. This composite of pair plasma and beams is restricted to a one-dimensional ultra-relativistic outflow strictly along the super-strong ambient magnetic field in the radio emission region. The plasma theory of CCR charge bunch formation involves both the linear and the non-linear stage of Langmuir wave evolution. The non-linear stage necessarily requires high growth rates in the linear regime. This has been a challenge since most of the works in the literature make use of the Landau regime where only a small portion of the particle distribution function contributes to wave growth. In this talk, I will explore two models of two-stream instability viz., longitudinal drift and cloud-cloud overlap. The former involves the relative separation of the bulk velocity of the electron-positron distribution functions of pair plasma in momentum space. The latter involves the spatial overlap of successive pair plasma clouds due to intermittent pair cascade discharges at the polar gap. For both models, we numerically solve the hot plasma Langmuir dispersion relation for a non-resonant hydrodynamic branch for realistic pulsar parameters. The use of a non-resonant hydrodynamic form is novel as in this regime all the particles in the distribution function contribute to wave amplification. I will show that this approach provides very high growth rates for both models of the two-stream condition. This provides the necessary justification for exploring the non-linear phase of Langmuir mode evolution.

google meet link:

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