NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.

Previous wave‐driven solar wind models (Hollweg, 1978) have been extended by including a new hypothesis for the nonlinear wave dissipation. The hypothesis is that the waves dissipate via a turbulent cascade at the rate given by (1) and the waves evolve according to (16). A subhypothesis is that the relevant correlation length scales as the distance between magnetic field lines. This hypothesis allows us to treat the corona and the solar wind on an equal footing; unlike in previous wave‐driven models, we do not assume that the coronal heating takes place below the base of the model. The models exhibit the correct qualitative features, viz., a steep temperature rise (the transition region) to a maximum coronal temperature in excess of 106 K, and a substantial solar wind mass flux in excess of 3.5×108 cm−2 s−1 at 1 AU. However, the model fails in detail. Parameters that yield a high‐speed flow at 1 AU have base pressures that are too low; parameters that yield correct base pressures have low solar wind flow speeds. However, the model “comes close.” Thus although we have not shown that the initial hypothesis is consistent with available data, we feel that there are sufficient uncertainties both in the model and in the data to preclude outright rejection of the hypothesis altogether.

The resonances that appear in the linear compressible MHD formulation of waves are studied for equilibrium states with flow. The conservation laws and the jump conditions across the resonance point are determined for ID cylindrical plasmas. For equilibrium states with straight magnetic field lines and flow along the field lines the conserved quantity is the Eulerian perturbation of total pressure. Curvature of the magnetic field lines and/or velocity field lines leads to more complicated conservation laws. Rewritten in terms of the displacement components in the magnetic surfaces parallel and perpendicular to the magnetic field lines, the conservation laws simply state that the waves are dominated by the parallel motions for the modified slow resonance and by the perpendicular motions for the modified Alfv6n resonance.The conservation laws and the jump conditions are then used for studying surface waves in cylindrical plasmas. These waves are characterized by resonances and have complex eigenfrequencies when the classic true discontinuity is replaced by a nonuniform layer. A thin non-uniform layer is considered here in an attempt to obtain analytical results. An important result related to earlier work by Hollweg et al. (1990) for incompressible planar plasmas is found for equilibrium states with straight magnetic field lines and straight velocity field lines. For these equilibrium states the incompressible and compressible surface waves have the same frequencies at least in the long wavelength limit and there is an exact correspondence with the planar case. As a consequence, the conclusions formulated by Hollweg et al. still hold for the straight cylindrical case. The effects of curvature are subsequently considered.

A basic procedure is presented for dealing with the resonance problems that appear in MHD of which resonant absorption of waves at the Alfv~n resonance point is the best known example in solar physics. The procedure avoids solving the full fourth-order differential equation of dissipative MHD by using connection formulae across the dissipation layer.

Abstract. We develop a series of new analytical expressions describing the physical properties of the kinetic Alfv6n wave. The wave becomes strongly compressive when k_7_ • is of the order of the ion inertial length. Thus, in a low-/3 plasma, the kinetic Alfv6n wave can be compressive at values of k ñ for which the dispersion relation departs only slightly from that of the usual MHD Alfvfin wave. The compression is accompanied by a magnetic field fluctuation •Bll such that the total pressure perturbation •Ptot m 0. Thus the wave undergoes transit-time damping as well as Landau damping; the two effects are comparable if the ion thermal speed is of the order of the Alfv6n speed. We find that the transverse electric field is elliptically polarized but rotating in the electron sense; this surprising behavior of the polarization of the Alfv6n branch was discovered numerically by Gary [1986]. We derive a new dispersion relation which explicitly shows how the kinetic Alfvfin wave takes on some properties of the large-k ñ limit of the slow mode. We also derive approximate dispersion relations valid for a multi-ion plasma with differential streaming. We suggest that the kinetic Alfvfin wave may be responsible for the flattening of density fluctuation spectra observed at large wavenumbers in the corona and in the solar wind. We also find that our derived properties of the kinetic Alternatively, large values of kñ could be the consequence of a turbulent cascade. In any case, at large values of k z, the Alfvfin mode, which is normally noncompressive, will become the kinetic Alfv6n wave, which is compressive. Harmon [1989] has suggested that the compressibility may be recognizable in the power spectra of coronal density fluctuations, which are derived from a variety of radio sounding techniques. Indeed, 14,811