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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, mission concept and the baseline vehicle for SPP, and examine how the mission will address the key science questions

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating 1 , 2 and interchange reconnection 3 – 5 . The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft 6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’ 7 , 8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts. Measurements of fast solar wind streams from the Parker Solar Probe spacecraft provide strong evidence for the interchange reconnection mechanism being responsible for accelerating the fast solar wind.

We analyze dust impacts recorded by the S/WAVES radio instrument onboard the two STEREO spacecraft near 1 A.U. during the period 2007–2010. The impact of a dust particle on a spacecraft produces a plasma cloud whose associated electric field can be detected by on‐board electric antennas. For this study we use the electric potential time series recorded by the waveform sampler of the instrument. The high time resolution and long sampling times of this measurement enable us to deduce considerably more information than in previous studies based on the dynamic power spectra provided by the same instrument or by radio instruments onboard other spacecraft. The large detection area compared to conventional dust detectors provides flux data with a better statistics. We show that the dust‐generated signals are of two kinds, corresponding to impacts of dust from distinctly different mass ranges. We propose calibration formulas for these signals and show that we are able to use S/WAVES as a dust detector with convincing results both in the nanometer and micrometer size ranges. In the latter, the orbital motion of the spacecraft enables us to distinguish between interstellar and interplanetary dust components. Our measurements cover the mass intervals ∼10−22–10−20 kg and ∼10−17 − 5 × 10−16 kg. The flux of the larger dust agrees with measurements of other instruments on different spacecraft. Key Points We expose a technique to use radio instruments as dust detectors We analyze the data from the STEREO/WAVES radio and plasma wave instrument We obtain results in agreement with current interplanetary dust flux models

We present observations of electromagnetic precursor waves, identified as whistler mode waves, at supercritical interplanetary shocks using the Wind search coil magnetometer. The precursors propagate obliquely with respect to the local magnetic field, shock normal vector, solar wind velocity, and they are not phase standing structures. All are right‐hand polarized with respect to the magnetic field (spacecraft frame), and all but one are right‐hand polarized with respect to the shock normal vector in the normal incidence frame. They have rest frame frequencies fci < f ≪ fce and wave numbers 0.02 ≲ kρce ≲ 5.0. Particle distributions show signatures of specularly reflected gyrating ions, which may be a source of free energy for the observed modes. In one event, we simultaneously observe perpendicular ion heating and parallel electron acceleration, consistent with wave heating/acceleration due to these waves. Although the precursors can have δB/Bo as large as 2, fluxgate magnetometer measurements show relatively laminar shock transitions in three of the four events. Key Points Whistler precursors observed at supercritical interplanetary shocks They can have amplitudes >/=20 nT and/or dB/B up to 2 They cause perpendicular (parallel) ion (electron) heating (acceleration)

We present the first observations at an interplanetary shock of large-amplitude (> 100 mV/m pk-pk) solitary waves and large-amplitude (approx.30 mV/m pk-pk) waves exhibiting characteristics consistent with electron Bernstein waves. The Bernstein-like waves show enhanced power at integer and half-integer harmonics of the cyclotron frequency with a broadened power spectrum at higher frequencies, consistent with the electron cyclotron drift instability. The Bernstein-like waves are obliquely polarized with respect to the magnetic field but parallel to the shock normal direction. Strong particle heating is observed in both the electrons and ions. The observed heating and waveforms are likely due to instabilities driven by the free energy provided by reflected ions at this supercritical interplanetary shock. These results offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) characterizes the radiation environment to be experienced by humans during future lunar missions. CRaTER measures the effects of ionizing energy loss in matter due to penetrating solar energetic protons (SEP) and galactic cosmic rays (GCR), specifically in silicon solid-state detectors and after interactions with tissue-equivalent plastic (TEP), a synthetic analog of human tissue. The CRaTER investigation quantifies the linear energy transfer (LET) spectrum in these materials through direct measurements with the lunar space radiation environment, particularly the interactions of ions with energies above 10 MeV, which penetrate and are detected by CRaTER. Combined with models of radiation transport through materials, CRaTER LET measurements constrain models of the biological effects of ionizing radiation in the lunar environment as well as provide valuable information on radiation effects on electronic systems in deep space. In addition to these human exploration goals, CRaTER measurements also provide new insights on the spatial and temporal variability of the SEP and GCR populations and their interactions with the lunar surface. We present here an overview of the CRaTER science goals and investigation, including: an instrument description; observation strategies; instrument testing, characterization, and calibration; and data analysis, interpretation, and modeling plans.

Previous formulations of heating and transport associated with strong magnetohydrodynamic (MHD) turbulence are generalized to incorporate separate internal energy equations for electrons and protons. Electron heat conduction is included. Energy is supplied by turbulent heating that affects both electrons and protons and is exchanged between them via collisions. Comparison to available Ulysses data shows that a reasonable accounting for the data is provided when (1) the energy exchange timescale is very long and (2) the deposition of heat due to turbulence is divided, with 60% going to proton heating and 40% into electron heating. Heat conduction, determined here by an empirical fit, plays a major role in describing the electron data.

We have examined 178 interplanetary shocks observed by the Wind spacecraft to establish which shock and plasma parameters are favorable for the production of upstream Langmuir waves and therefore to determine which shocks are likely to generate interplanetary Type II radio bursts. Of the 178 shocks included in this study, 43 produced upstream Langmuir waves, as evinced by enhancements in wave power near the plasma frequency. The large number of observed shocks permits the use of statistical tests to determine which parameters control the upstream activity. The best predictor of activity is the de Hoffmann‐Teller speed, a result consistent with the fast Fermi model of electron acceleration. Several other parameters, including the magnetic field strength and the level of solar activity (but not the Mach number), are also correlated with upstream activity. These additional parameters may be associated with an increased level of shock front curvature or upstream structure, leading to the formation of upstream foreshock regions, or with the generation of an upstream electron population favorable for shock reflection.

The prediction of a supersonic solar wind 1 was first confirmed by spacecraft near Earth 2 , 3 and later by spacecraft at heliocentric distances as small as 62 solar radii 4 . These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy 5 – 7 . Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams 8 . Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii 9 – 11 , well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second—considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age 12 – 14 . Data collected by the Parker Solar Probe in the solar corona are used to determine the organization of Alfvén waves, revealing an increasing flow velocity peaking at 35–50 km s −1 .

NASA’s Parker Solar Probe mission1 recently plunged through the inner heliosphere of the Sun to its perihelia, about 24 million kilometres from the Sun. Previous studies farther from the Sun (performed mostly at a distance of 1 astronomical unit) indicate that solar energetic particles are accelerated from a few kiloelectronvolts up to near-relativistic energies via at least two processes: ‘impulsive’ events, which are usually associated with magnetic reconnection in solar flares and are typically enriched in electrons, helium-3 and heavier ions2, and ‘gradual’ events3,4, which are typically associated with large coronal-mass-ejection-driven shocks and compressions moving through the corona and inner solar wind and are the dominant source of protons with energies between 1 and 10 megaelectronvolts. However, some events show aspects of both processes and the electron–proton ratio is not bimodally distributed, as would be expected if there were only two possible processes5. These processes have been very difficult to resolve from prior observations, owing to the various transport effects that affect the energetic particle population en route to more distant spacecraft6. Here we report observations of the near-Sun energetic particle radiation environment over the first two orbits of the probe. We find a variety of energetic particle events accelerated both locally and remotely including by corotating interaction regions, impulsive events driven by acceleration near the Sun, and an event related to a coronal mass ejection. We provide direct observations of the energetic particle radiation environment in the region just above the corona of the Sun and directly explore the physics of particle acceleration and transport.