arXiv subjects

Alex S. Hill

Publications and source records attributed to Alex S. Hill.

57 records · Page 4Linked to original sources

arXiv2008

The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations

We present an analysis of the distribution of H-alpha emission measures for the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude classical H II regions, leaving only sightlines that sample the diffuse WIM. The distribution of EM sin |b| for the full sample is poorly characterized by a single normal distribution, but is extraordinarily well fit by a lognormal distribution, with = 0.146 +/- 0.001 and standard deviation 0.190 +/- 0.001. drops from 0.260 +/- 0.002 at Galactic latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The distribution may widen slightly at low Galactic latitude. We compare the observed EM distribution function to the predictions of three-dimensional magnetohydrodynamic simulations of isothermal turbulence within a non-stratified interstellar medium. We find that the distribution of EM sin |b| is well described by models of mildy supersonic turbulence with a sonic Mach number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field strength. The model also successfully predicts the distribution of dispersion measures of pulsars and H-alpha line profiles. In the best fitting model, the turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8 kpc scale height of the layer. The WIM gas has a lognormal distribution of densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also discuss the implications of these results for interpreting the filling factor, the power requirement, and the magnetic field of the WIM.

astro-ph↗

arXiv2007

Density Distribution of the Warm Ionized Medium

Observations of Halpha emission measures and pulsar dispersion measures at high Galactic latitude (|b| > 10 deg) provide information about the density and distribution of the diffuse warm ionized medium (WIM). The diffuse WIM has a lognormal distribution of EM sin |b|, which is consistent with a density structure established by isothermal turbulence. The H+ responsible for most of the emission along high-EM sin |b| sightlines is clumped in high density (> 0.1 cm^{-3}) regions that occupy only a few parsecs along the line of sight, while the H+ along low-EM sightlines occupies hundreds of parsecs with considerably lower densities.

astro-ph↗

arXiv2004

Deflection of Pulsar Signal Reveals Compact Structures in the Galaxy

We have detected a strong deflection of radio waves from the pulsar PSR B0834+06 in scintillation observations. Interference between the undeflected pulsar image and deflected subimages allows single dish interferometry of the interstellar medium with sub-milliarcsecond resolution. We infer the presence of scattering structure(s) similar to those that are thought to cause Extreme Scattering Events in quasar flux monitoring programs: size ~ 0.2 AU (an angular size of 0.1 mas) with an electron density of > 10^3 compared to the warm ionized medium. The deflectors are nearly stationary in a scattering screen that is thin (< 5% of the pulsar-observer distance in extent), is located 70% of the way from the Earth to the pulsar, and has been seen consistently in observations dating back 20 years. The pulsar scans the scattering screen at a velocity of 110 km/s with a detection radius of 15 mas. Pulsar observations such as these -- particularly with a new generation of low-frequency radio telescopes with large collecting areas -- hold promise for improving constraints on the poorly understood physical characteristics and space density of the deflecting structures. Such observations may also prove useful in correcting deviations the deflectors produce in high-precision timing of millarcsecond pulsars.

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