Explores

The Solar Probe Plus Mission: Humanity’s First Visit

to Our Star

Abstract 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 their 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 (RS ) for

the first orbit to <10 RS 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

Keywords Solar Probe Plus · SPP · Corona · Heliophysics · NASA mission · Solar wind

1 Introduction

Solar Probe Plus (SPP) will sample the solar corona to reveal how it is heated and the solar

wind and solar energetic particles are accelerated. Solving these problems has been a top

science goal for over 50 years (see Box 1). During the seven-year mission, seven Venus

gravity assist (VGA) maneuvers will gradually lower the perihelia to <10 RS , the closest

any spacecraft has come to the Sun. Throughout the 7-year nominal mission duration, the

spacecraft will spend a total of 937 hours inside 20 RS , 440 hours inside 15 RS , and 14 hours

inside 10 RS , sampling the solar wind in all its modalities (slow, fast, and transient) as it

evolves with rising solar activity toward an increasingly complex structure. SPP will orbit

the Sun in the ecliptic plane, and so will not sample the fast wind directly above the Sun’s

polar regions (see Fig. 1). However, the current mission design (Lockwood et al. 2012)

compensates for the lack of in-situ measurements of the fast wind above the polar regions

by the relatively long time SPP spends inside 20 RS . This will allow extended measurement

of the equatorial extensions of high-latitude coronal holes and equatorial coronal holes. At

a helioradius ≈35 RS , there are two periods per orbit (one inbound and one outbound)

when SPP will be in quasi-corotation with the Sun and will cross a given longitudinal sector

slowly. In these intervals, known as fast radial scans, the spacecraft will sample the solar

wind over large radial distances within a given flux tube before moving across the sector.

These measurements will yield additional information on the spatial/temporal dependence

of structures in the solar wind and on how they merge in the inner heliosphere. This paper

describes the science, mission concept, and reference vehicle for the SPP mission.

2 Science Overview

The SPP mission targets processes and dynamics that characterize the Sun’s expanding

corona and solar wind. SPP will explore the inner region of the heliosphere through in-

situ and remote sensing observations of the magnetic field, plasma, and energetic particles.

The solar magnetic field plays a defining role in forming and structuring the solar corona and

the heliosphere. In the corona, closed magnetic field lines confine the hot plasma in loops,

while open magnetic field lines guide the solar wind expansion in the inner corona. The

energy that heats the corona and drives the wind derives from photospheric motions, and

is channeled, stored, and dissipated by the magnetic fields that emerge from the convection

zone and expand in the corona where they dominate almost all physical processes therein.

Examples of these are waves and instabilities, magnetic reconnection, and turbulence, which

operate on a vast range of spatial and temporal scales. Magnetic fields play also a critical

role in coronal heating and solar wind acceleration. They are conduits for waves, store energy, and propel plasma into the heliosphere through complex forms of magnetic activity

(e.g., coronal mass ejections (CMEs), flares, and small-scale features such as spicules and

jets). How solar convective energy couples to magnetic fields to produce the multifaceted

heliosphere is central to SPP science.

At times of low solar activity (i.e., solar cycle minima), the solar wind is bimodal. There

is a dominant quasi-steady high-speed wind that originates in open-field polar coronal holes

and a variable, low-speed wind that originates around the equatorial streamer belt (Mc-

Comas et al. 1998, Fig. 1a, c). As solar activity increases and evolves toward solar cycle

maxima, this orderly bimodal configuration of the corona and solar wind breaks down. Po-

lar holes shrink, and the heliospheric current sheet becomes warped due to magnetic flux

emergence whose coronal manifestations are active regions, equatorial coronal holes, and

coronal streamers at higher heliographic latitudes. A mixture of fast flows from smaller coronal holes and transients, embedded in a slow-to-moderate-speed wind, appears at all

latitudes (e.g., McComas et al. 2003, Fig. 1b).

SPP’s closest approach to the Sun (<10 RS from Sun center) will enable it to measure

coronal conditions leading to the nascent solar wind and eruptive transients that create space

weather. The seven-year prime mission will permit observations over a significant portion

(>60 %) of a solar cycle. Direct plasma, magnetic field, and energetic particle measurements

will allow testing of and discrimination among a broad range of theories and models that

describe the Sun’s coronal magnetic field, the heating and acceleration of the solar wind and

energetic particle acceleration.

The primary science objective of the SPP mission is to determine the structure and dynamics of the Sun’s coronal magnetic field and to understand how the corona is heated, the

solar wind accelerated, and how energetic particles are produced and their distributions

evolve. To advance the scientific knowledge needed to characterize the inner heliosphere,

the SPP mission has defined the following three overarching science objectives.

1. Trace the flow of energy that heats the solar corona and accelerates the solar

wind.

1a. How is energy from the lower solar atmosphere transferred to, and dissipated in,

the corona and solar wind?

1b. What processes shape the non-equilibrium velocity distributions observed through-

out the heliosphere?

1c. How do the processes in the corona affect the properties of the solar wind in the

heliosphere?

2. Determine the structure and dynamics of the plasma and magnetic fields at the

sources of the solar wind.

2a. How does the magnetic field in the solar wind source regions connect to the photosphere and the heliosphere?

2b. Are the sources of the solar wind steady or intermittent?

2c. How do the observed structures in the corona evolve into the solar wind?

3. Explore mechanisms that accelerate and transport energetic particles.

3a. What are the roles of shocks, reconnection, waves, and turbulence in the acceleration of energetic particles?

3b. What are the source populations and physical conditions necessary for energetic

particle acceleration?

3c. How are energetic particles transported in the corona and heliosphere?

SPP will make in-situ and remote measurements from <10 RS to at least 0.25 AU

(53.7 RS ). Measurements of the region where the solar wind originates and where the most

hazardous solar energetic particles are energized will improve our ability to characterize

and forecast the radiation environment of the inner heliosphere. SPP will measure local

particle distribution functions, density and velocity field fluctuations, and electromagnetic

fields within 0.25 AU of the Sun. These data will help answer the basic questions of how the

solar corona is powered, how the energy is channeled into the kinetics of particle distribu-

tion functions in the solar corona and wind, and how such processes relate to the turbulence

and wave-particle dynamics observed in the heliosphere. Cross-correlation of velocity, den-

sity, and electromagnetic fluctuations will allow a partial separation of spatial and temporal

effects.

The physical conditions of the region below 20 RS are important in determining large-

scale properties such as solar wind angular momentum loss and global heliospheric structure.The Alfvénic critical surface, where the solar wind speed overtakes the Alfvén speed,

is believed to lie in this region (e.g., Katsikas et al. 2010; Goelzer et al. 2014). This surface

defines the point beyond which the plasma ceases to corotate with the Sun, i.e., where the

magnetic field loses its rigidity to the plasma. In this region solar wind physics changes

because of the multi-directionality of wave propagation (waves moving sunward and anti-

sunward can affect the local dynamics including the turbulent evolution, heating and acceleration of the plasma). This is also the region where velocity gradients between the fast and

slow speed streams develop, forming the initial conditions for the formation, further out, of

corotating interaction regions (CIRs). In the remainder of this section, we summarize SPP

science questions and discuss how the mission design will enable investigators to address

these questions