AAS 200th meeting, Albuquerque, NM, June 2002
Session 73. Interim Stellar Materials: Gas, Dust and Clouds
Display, Thursday, June 6, 2002, 9:20am-4:00pm, SW Exhibit Hall

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[73.17] Interstellar Absorption of the Galactic Polar Low Frequency Radio Background Synchrotron Spectrum as an Indicator of Clumpiness in the WIM

J.D. Peterson, W.R. Webber (NMSU)

In the standard model of low frequency synchrotron radiation propagation through the Galaxy, the absorbing warm ionized medium (WIM) is considered to be a thick slab of thermal electrons of uniform density. When the calculated polar radio spectrum is compared with the observed galactic background radio spectrum, it is found that this model requires a much higher electron density ne or much lower temperature Te than permitted by current observations. A more realistic plane-parallel model, in which electron density, temperature, and cosmic ray electron emissivity have smooth distributions with height z above the galactic plane, is also found to suffer from the same setbacks as the standard model. However, a plane-parallel model in which the absorbing WIM has a clumpy distribution with clump densities of 0.2 cm-3 and filling factors of 0.08-0.15 agrees both with the low frequency radio synchrotron spectrum and with the approximate observational parameters < ne(z=0) > = 0.025 cm-3, Te = 7000 K, and DM = 23 pc cm-3. The clumpy WIM model also supports the idea of a local interstellar cloud (LIC) which is required to provide adequate absorption below 0.5 MHz. This LIC appears to become optically thick only below 0.1 MHz where future radio measurements may be used to determine the emissivity spectrum and, therefore, the local interstellar cosmic ray electron spectrum at energies of 40 MeV.

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.