A Measurement of the Adiabatic Cooling Index for Interstellar Helium Pickup Ions in the Inner Heliosphere

Details A Measurement of the Adiabatic Cooling Index for Interstellar Helium Pickup Ions in the Inner Heliosphere A Measurement of the Adiabatic Cooling Index for Interstellar Helium Pickup Ions in the Inner Heliosphere

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009apj...703..325s&link_type=abstract

The Astrophysical Journal, Volume 703, Issue 1, pp. 325-329 (2009).

Astronomy and Astrophysics

Astronomy

Ism: Kinematics And Dynamics, Magnetic Fields, Plasmas, Solar-Terrestrial Relations, Solar Wind

Scientific paper

Interstellar neutral gas enters the inner heliosphere where it is ionized and becomes the pickup ion population of the solar wind. It is often assumed that this population will subsequently cool adiabatically, like an expanding ideal gas due, to the divergent flow of the solar wind. Here, we report the first independent measure of the effective adiabatic cooling index in the inner heliosphere from SOHO CELIAS measurements of singly charged helium taken during times of perpendicular interplanetary magnetic field. We use a simple adiabatic transport model of interstellar pickup helium ions, valid for the upwind region of the inner heliosphere. The time averaged velocity spectrum of helium pickup ions measured by CELIAS/CTOF is fit to this model with a single free parameter which indicates an effective cooling rate with a power-law index of γ = 1.35 ± 0.2. While this average is consistent with the "ideal-gas" assumption of γ = 1.5, the analysis indicates that such an assumption will not apply in general, and that due to observational constraints further measurements are necessary to constrain the cooling process. Implications are discussed for understanding the transport processes in the inner heliosphere and improving this measurement technique.

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