A New Estimate of the Local Interstellar Energy Density and Ionization Rate of Galactic Cosmic Cosmic Rays

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Abstract

Using data from the Voyager and Pioneer spacecraft at distances up to 60 AU from the Sun to help define the effects of solar modulation, we have made a new estimate of the local interstellar cosmic-ray spectra and the associated energy density and ionization rate. The energy density, including the low-energy electron contribution, is ~1.80 eV cm-3, equivalent energetically to that in a 7 μG magnetic field. The ionization rate from this same cosmic-ray population, including a significant contribution from low-energy electrons, is (3-4) × 10-17 s-1 (H atom). This is most likely a minimum value and is not too different from estimates of the typical ionization rate required to heat the interstellar medium and maintain the diffuse ionized gas range from (3-30) × 10-17 s-1 (H atom) as a function of locale in the Galaxy. This suggests that cosmic rays could be a significant contributor to interstellar heating. The cosmic-ray ionization rate will be more or less uniform throughout the Galactic disk and also perpendicular to the disk according to the scale height of cosmic rays which may be ~1 kpc, thus providing a source of ionization at high Z values. The recent observation of low-energy particle acceleration at the heliospheric termination shock, which produces local ionization rates comparable to those due to Galactic cosmic rays, raises the interesting possibility that "hot spots," where the ionization rate due to locally accelerated particles may exceed 10-16 s-1 (H atom), may occur near massive stars with more powerful stellar winds.

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