Adiabatic Modeling of Inner Source Pickup Ions Pran Mukherjee, Susan T. Lepri, and Thomas H. Zurbuchen

Department of Atmospheric, Oceanic, and Space Science, The University of Michigan, Ann Arbor, MI 48109

This work is supported by award numbers NNG04GL44H and NNX07AK29H of the Graduate Student Research Program of the National Aeronautics and Space Administration.

Inner source pickup ions have been treated very similarly to those from interstellar space, but the physics of the two regions differ significantly. The density of electrons close to the Sun, and consequently the rate of electron impact ionization, is many orders of magnitude greater than in the outer heliosphere (Figure 1). The velocity of the neutral source of the pickup ions has, in the solar wind frame, more than the expected -Vsw term (Figure 2). An azimuthal term perpendicular to the solar wind is caused by the Keplerian orbits of the dust grains from which the neutrals arise. In addition, many of these new ions should be picked up where the solar wind is sub-Alfvénic. The Alfvén wave velocity, orbital velocity, and dominant electron-impact ionization terms strongly affect predictions for pickup ion peak locations and densities. This poster describes the adiabatic expansion of inner source pickup ions based on a spherically symmetric Fokker-Planck equation.

Abstract Model

Inputs

Model Results

Figure 1: The rates of photoionization and charge-exchange ionization come from Ruciński. The electron-impact ionization cross section comes from Lotz, using electron density and temperature from the solar wind model.

Conclusions

We considered inner source pickup ion populations throughout the inner heliosphere. Close to the Sun, the pickup process needs to account for a pair of velocity components that are negligible beyond a few dozen solar radii: azimuthal speeds of the dust grains, and the enhanced heating due to increased Alfvén wave speeds. We made a model that predicts the neutral atom and ion populations and adiabatic cooling of the ions. We showed that this model has solutions consistent with inner source number densities and thermal speeds measured at 1 AU, and that the additional velocity components require the pickup process to happen far closer to the Sun than predicted by traditional models. This provides exciting opportunities for future missions close to the Sun.The authors would like to thank for their help Dr. Susan Lepri, Dr. Len Fisk, and Jim Raines of the

University of Michigan and Dr. George Gloeckler of the University of Maryland

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Figure 4

Figure 2

Figure 3: Simple model of dust radial density profiles. The value of λ/α sets the α

peak location, while α determines the peak sharpness. For purposes of this model, α was set at 1 and 2 while λ varied widely.

A numerical integration of the input data results in the number density profiles seen in Figure 4 as well as local ion pickup rates. A spherically symmetric Fokker-Planck equation using the input velocity components results in the adiabatic expansion profile shown in Figure 5. The local PUI “packets”, injected at starting velocities and locations indicated by the green line in Fig. 5, “slide down” the curves to the measurement point at the end. The distribution expands (i.e. lower density) and cools (i.e. narrower distribution) as it moves out ward. The pale blue cut through the graph is a solar-wind frame distribution function, which is converted into a rest frame distribution via the 3D transformation shown in Figure 6. Implicit in this technique is instant pickup isotropisation to shell distributions.

Figure 5 Figure 6

Figure 7

Figure 8Figure 8 shows the “best-match” distributions for the VSW and Vfull cases. The measured distribution (from Gloeckler et al 2000a) was shifted to peak at W=1 to eliminate a systematic error. The mismatch between simulated and measured data may be due to physics not in the model, such as stochastic acceleration, viscosity