Interplanetary Suprathermal He+ and He++ Observations During Quiet Periods from 1 to 9 AU and Implications for Particle Acceleration

Details Interplanetary Suprathermal He+ and He++ Observations During Quiet Periods from 1 to 9 AU and Implications for Particle Acceleration Interplanetary Suprathermal He+ and He++ Observations During Quiet Periods from 1 to 9 AU and Implications for Particle Acceleration

Jul 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009apj...699l..26h&link_type=abstract

The Astrophysical Journal Letters, Volume 699, Issue 1, pp. L26-L30 (2009).

Astronomy and Astrophysics

Astronomy

4

Acceleration Of Particles, Interplanetary Medium, Plasmas, Shock Waves, Solar Wind, Sun: Abundances

Scientific paper

We measured quiet-time differential intensities of ~2-60 keV nucleon-1 He+ and He++ during the 1999-2004, 1-9 AU portion of Cassini's interplanetary cruise to Saturn and found that the He+/He++ composition ratio grows as the distance from the Sun r increases. An increase in the ratio is expected from the theoretical pickup ion and solar wind intensities, but the absolute He+ intensity, counter to the predicted falling r -1 dependence of the density, is actually slightly increasing, and He++ falls off much more slowly than the r -2 dependence one might expect from a population with a solar source. With an approximately r 2.2 radial dependence, our rigorous numerical transport and acceleration model (with stochastic acceleration) matches the higher-energy (>13 keV nucleon-1) measured He+/He++ composition profiles well, as does our analytical theory. Two acceleration processes are likely needed: the composition ratios are explainable by stochastic acceleration while a velocity-dependent mechanism that acts beyond 1 AU equally on He+ and He++ is required to explain the spatial intensity profiles.

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