Plasma Turbulence in the Local Bubble

Turbulence in the Local Bubble could play an important role in the thermodynamics of the gas that is there. This turbulence could also determine the transport of cosmic rays and perhaps heat flow through this phase of the interstellar medium. The best astronomical technique for measuring turbulence in astrophysical plasmas is radio scintillation. Scintillation measurements yield information on the intensity and spectral characteristics of plasma turbulence between the source of the radio waves and the observer. Measurements of the level of scattering to the nearby pulsar B0950+08 by Philips and Clegg in 1992 showed a markedly lower value for the line-of-sight averaged turbulent intensity parameter 〈 C N 2 〉 than is observed for other pulsars, qualitatively consistent with radio wave propagation through a highly rarefied plasma. In this paper, we discuss the observational progress that has been made since that time. The main development has been improved measurements of pulsar parallaxes with the Very Long Baseline Array. This provides better knowledge of the media along the lines of sight. At present, there are four pulsars (B0950+08, B1133+16, J0437-4715, and B0809+74) whose lines of sight seem to lie mainly within the local bubble. The mean densities and line of sight components of the interstellar magnetic field along these lines of sight are smaller than nominal values for pulsars, but not by as large a factor as might be expected. Three of the four pulsars also have measurements of interstellar scintillation. The value of the parameter 〈 C N 2 〉 is smaller than normal for two of them, but is completely nominal for the third. This inconclusive status of affairs could be improved by measurements and analysis of “arcs” in “secondary spectra” of pulsars, which contain information on the location and intensity of localized screens of turbulence along the lines of sight. Similar data could be obtained from observations of highly compact extragalactic radio sources which show the “intraday variability” phenomenon.