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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.

We have conducted a detailed analysis of a set of events termed short large-arnphtude magnetic structures (SLAMS) observed at an encounter of the quasi-parallel bow shock by the AMPTE UKS and IBM satellites. Both the satellite configuration and the solar wind conditions are favorable for the case study presented here. We have identified isolated SLAMS, surrounded by solar wind conditions, and embedded SLAMS, which lie within or form the boundary with regions of significant heating and deceleration. The duration, polarization, and other characteristics of SLAMS are all consistent with their growth directly out of the ULF wave field, including the common occurrence of an attached whistler as found in ULF shocklets. The plasma rest iYmne propagation speeds, where they can be determined, and two-spacecraft time delays for all cases show that the SLAMS attempt to propagate upstream against the oncoming flow, but are convected back downstream. The speeds and delays vary systematically with SLAMS amphtude in the way anticipated from nonlinear wave theory, as do their polarization features. Inter-SLAMS regions, and boundary regions with the solar wind, contain hot deflected ions of lesser density than within the SLAMS. The amplitude of the SLAMS requires an active growth mechanism. Following earlier inferences about the limited transverse extent of SLAMS, we highlight the importance of determining the thickness of the transition zone over which SLAMS grow and the bulk heating and deceleration is effected. From this case study it appears that, at least under some circumstances, the quasi-parallel shock cannot be regarded as an undulating, cychcally re-forming simply connected surface. Instead, the transition zone is better represented as a set of ULF waves, some of which grow to become SLAMS which gradually decelerate and merge to form the downstream state. conditions and that these instabilities cause the shock to cyclically re-form. pulsations was dictated primarily by the Larmor radii of the backstreaming particles found in abundance upstream of the bow shock but downstream of the foreshock boundary. This boundary is defined by the upstream field line which is tangent to the curved bow shock, or by the trajectory of field-aligned ion beams of a given energy originating at the point of tangency. The turbulence associated with the shock apparently saturates some distance, typically 10 R•, downstream of this ion foreshock boundary [Bonifazi et al., 1983], downstream of which nearly isotropic distributions of "diffuse" energetic ions are found. The curved nature of the bow shock led early workers to suggest that the turbulent appearance of the quasi-parallel shock was due to debris which originated under more quasi-perpendicular configura-4209

Collisionless shocks at quasi‐parallel geometries, i.e., for which the average magnetic field direction upstream of the shock is close to the shock normal, reveal temporally varying quantities, a variety of boundary crossing and kinetic signatures, and magnetic structures, often convecting, of finite extent. These results can be put together by a framework in which the shock can be viewed as an extended region containing three‐dimensional Short Large Amplitude Magnetic Structures (SLAMS) which represent individual semi‐cycles of the ambient upstream low frequency waves associated with diffuse ions in the ULF foreshock. As SLAMS convect with the flow they grow to large amplitudes and entrain inter‐SLAMS regions to form an inhornogeneous downstream state. Their finite transverse extent is probably related to, and interacts with, ion beams, to produce a patchy transition zone which accounts for the variety of spacecraft signatures observed.

The first two orbits of the Parker Solar Probe spacecraft have enabled the first in situ measurements of the solar wind down to a heliocentric distance of 0.17 au (or 36 ). Here, we present an analysis of this data to study solar wind turbulence at 0.17 au and its evolution out to 1 au. While many features remain similar, key differences at 0.17 au include increased turbulence energy levels by more than an order of magnitude, a magnetic field spectral index of −3/2 matching that of the velocity and both Elsasser fields, a lower magnetic compressibility consistent with a smaller slow-mode kinetic energy fraction, and a much smaller outer scale that has had time for substantial nonlinear processing. There is also an overall increase in the dominance of outward-propagating Alfvénic fluctuations compared to inward-propagating ones, and the radial variation of the inward component is consistent with its generation by reflection from the large-scale gradient in Alfvén speed. The energy flux in this turbulence at 0.17 au was found to be ∼10% of that in the bulk solar wind kinetic energy, becoming ∼40% when extrapolated to the Alfvén point, and both the fraction and rate of increase of this flux toward the Sun are consistent with turbulence-driven models in which the solar wind is powered by this flux.

Large scale one-dimensional hybrid simulations with resistive electrons have been carried out of a quasiparallel (0Bn = 30 ø) high Mach number collisionless shock.The shock initially appears stable, but then exhibits cyclic behavior. For the magnetic field the cycle consists of a period when the transition from upstream to downstream is steep and well defined, followed by a period when the shock transition is extended and perturbed. This cyclic shock solution results from upstream perturbations, caused by backstreaming gyrating ions, convecting into the shock.The cyclic re-formation of a sharp shock transition can allow ions, at one time upstream because of reflection or leakage, to contribute to the shock thermalization. The simulations are simplistic in a number of ways for the quasiparallel shock, but their results suggest a model which may explain qualitatively several features of observations.

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional particle-in-cell simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population, etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas. V C 2015 AIP Publishing LLC.

Abstract.Observations of solar wind magnetic field discontinuities using 3 spacecraft allow their orientations to be estimated. During 5 days when Geotail, Wind and IMP 8 were between 6 x 104 and 4 x 105 km apart, 35 events identified using the Tsurutani-Smith method were detected in all 3 magnetic field data sets. Normals estimated from interspacecraft timings showed that very few were unambiguous rotational discontinuities, with 77% likely to be tangential, with < 20% of the magnetic field at the discontinuity threading the normal plane. However, previous single spacecraft studies using minimum variance suggest that most discontinuities are rotational. Minimum variance analysis resulted in many normal estimates lying far from the timing-derived normals. While some of this discrepancy is likely to be due to random errors in minimum variance vectors, there appears to be a class of events with small field magnitude changes where the minimum variance directions and discontinuity normals are approximately perpendicular, probably due to surface waves on the discontinuities.