Solar Probe Plus (SPP) will be the first spacecraft to fly into the low solar corona. SPP's main science goal is to determine the structure and dynamics of the Sun's coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Understanding these fundamental phenomena has been a top-priority science goal for over five decades, dating back to the 1958 Simpson Committee Report. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. The mission design and the technology and engineering developments enable SPP to meet its science objectives to: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. The SPP mission was confirmed in March 2014 and is under development as a part of NASA's Living with a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and will perform 24 orbits over a 7-year nominal mission duration. Seven Venus gravity assists gradually reduce SPP's perihelion from 35 solar radii (R S ) for the first orbit to <10 R S for the final three orbits. In this paper we present the science, B N.J. Fox

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[1] On the basis of a recently developed nonlinear guiding center theory for the perpendicular spatial diffusion coefficient k ? used to describe the transport of energetic particles, we construct a model for diffusive particle acceleration at highly perpendicular shocks, i.e., shocks whose upstream magnetic field is almost orthogonal to the shock normal. We use k ? to investigate energetic particle anisotropy and injection energy at shocks of all obliquities, finding that at 1 AU, for example, parallel and perpendicular shocks can inject protons with equal facility. It is only at highly perpendicular shocks that very high injection energies are necessary. Similar results hold for the termination shock. Furthermore, the inclusion of self-consistent wave excitation at quasiparallel shocks in evaluating the particle acceleration timescale ensures that it is significantly smaller than that for highly perpendicular shocks at low to intermediate energies and comparable at high energies. Thus higher proton energies are achieved at quasiparallel rather than highly perpendicular interplanetary shocks within 1 AU. However, both injection energy and the acceleration timescale at highly perpendicular shocks are sensitive to assumptions about the ratio of the two-dimensional (2-D) correlation length scale to the slab correlation length scale l 2D /l k . Model proton spectra and intensity profiles accelerated by a highly perpendicular interplanetary shock are compared to an identical but parallel interplanetary shock, revealing important distinctions. Finally, we present observations of highly perpendicular interplanetary shocks that show that the absence of upstream wave activity does not inhibit particle acceleration at a perpendicular shock. The accelerated particle distributions closely resemble those expected of diffusive shock acceleration, and observed at oblique shocks, an example of which is shown.

We have studied the injection rate of shock-accelerated protons in long-lasting particle events by tracing back the magnetohydrodynamic conditions at the shock under which protons are accelerated. This tracing back is carried out by Ðtting the observed Ñux and anisotropy proÐles at di †erent energies, considering the magnetic connection between the shock and the observer, and modeling the propagation of the shock and of the particles along the interplanetary magnetic Ðeld. A focused-di †usion transport equation that includes the e †ects of adiabatic deceleration and solar wind convection has been used to model the evolution of the particle population. The mean free path and the injection rate have been derived by requiring consistency with the observed Ñux and anisotropy proÐles for di †erent energies, in the upstream region of the events. We have extended the energy range of previous models down to 50 keV and up to D100 MeV. We have analyzed four proton events, representative of west, central meridian, and east scenarios. The spectra of the injection rate of shock-accelerated protons derived for these events show that for energies higher than 2 MeV the shock becomes a less efficient proton accelerator. We have related the derived injection rates to the evolution of the strength of the shock, particularly to the normalized downstream-upstream velocity ratio (VR), the magnetic Ðeld ratio, and the angle As h Bn . a result, we have derived an empirical relation of the injection rate with respect to the normalized velocity ratio (log Q P VR), but we have not succeeded with the other two parameters. The Q(VR) relation allows us to determine the injection rate of shock-accelerated particles along the shock front and throughout its dynamical expansion, reproducing multispacecraft observations for one of the simulated events. This relation allows us to analyze the inÑuence of the corotation e †ect on the modeled particle Ñux and anisotropy proÐles. Subject headings : acceleration of particles È interplanetary medium È MHD È shock waves È Sun : particle emission