A New Observational Constraint on Interplanetary Suprathermal He+ and He++ Acceleration : Evidence for a Multi--Stage Process

Details A New Observational Constraint on Interplanetary Suprathermal He+ and He++ Acceleration : Evidence for a Multi--Stage Process A New Observational Constraint on Interplanetary Suprathermal He+ and He++ Acceleration : Evidence for a Multi--Stage Process

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsh41a..14h&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #SH41A-14

Physics

2114 Energetic Particles (7514), 2152 Pickup Ions, 7807 Charged Particle Motion And Acceleration

Scientific paper

We made intensity measurements of 2--60 keV/nucleon He+ and He++ with the MIMI / CHEMS (Magnetospheric Imaging Investigation / Charge Energy Mass Spectrometer) instrument during the 1999--2004, 1.5--9 AU portion of the Cassini probe's flight to Saturn. At 40 keV/nuc, well in the suprathermal tail spectrum, the composition ratio He+/He++ of the intensities of the interstellar pickup ion vs. solar wind alpha particles is approximately proportional to r2, the square of the distance from the Sun. This is not the direct r proportionality that would be naively expected based on the number density ratio, but agrees very well (< 5%), in radial profile and absolute magnitude, with a straightforward analytical treatment and the detailed EPREM (Energetic Particle Radiation Environment Module) code we've developed. The EPREM simulation includes most transport and acceleration effects and in particular includes a stochastic acceleration term, which we find cannot be constant with radius, but must decrease as r-1 or r-2. However, despite the excellent agreement for the composition, the radial profiles of the absolute fluxes disagree profoundly; the measured fluxes vary from r0.5 to r1.5 for He+, and proportionally to r-0.4 for He++, compared to r-1.16 and r-3.31 dependence, predicted by the simulation. This suggests that the gradual stochastic acceleration is responsible for the compositional variations but that there is a second velocity-dependent acceleration process that boosts the intensities of both species. Thus, we present new observations from Cassini, which appear to require multiple stages of particle acceleration for He ions in the inner heliosphere.

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