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D. C. Backer (Astronomy Department, University of California at Berkeley), B. D. G. Chandran (Department of Physics & Astronomy, University of Iowa)
Scintillation of the flux density of pulsars indicates the presence of electron density variations on length scales of 1010-12 cm in the interstellar medium (ISM). Models of the cascade of MHD turbulence in the ISM suggest that the spatial structure of these density perturbations is highly anisotropic: the scale length perpendicular to the magnetic field is much smaller than that parallel to the field. This fundamental anisotropy is typically lost in radio wave propagation effects owing to the random directions of the field along most sight lines. We suggest that the effects of anisotropy may be observable in the semi-annual modulation of the diffractive scintillation time scale for nearby pulsars where the field line randomization is incomplete.
There are several pulsars (J0437-4715, B0655+64, B1855+09) for which measurements exist that allow either possible detection of anisotropy or limits to be placed on the degree of isotropy. New observations with the 85ft Pulsar Monitoring Telescope at NRAO, Green Bank, are in progress with the goal of measuring the anisotropy effect in PSR B0950+08.
This study of anisotropy in turbulent plasma along the paths to nearby pulsars complements other approaches to define the physical state of the local interstellar medium.
This research is partially supported by NSF grant AST-9820662 at Berkeley. We also thank the staff at NRAO, Green Bank for their support of the observational program.
The author(s) of this abstract have provided an email address for comments about the abstract: dbacker@astro.berkeley.edu