Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR AND HELIOSPHERIC PHYSICS
293
decadal survey recommended implementation of a solar probe as a large-class NASA mission. 7 SPP, started
in 2009, will begin its voyage of discovery in 2018 and serve as a keystone of the strategy for solar and
heliospheric science in the coming decade.
The goals of SPP are to determine the structure and dynamics of the Sun’s coronal magnetic field, to
understand how the solar corona and wind are heated and accelerated, and to determine what mechanisms
accelerate and transport energetic particles. To accomplish those goals, SPP is equipped with a tailored
payload for the first near-Sun in situ measurements of solar wind ion and electron thermal plasma, suprathermal and energetic particles, and DC to high-frequency electromagnetic fields. Remote observations
include a large-field-of-view white-light imager to provide global context and a directional radio receiver
to locate and track flares and shocks. Multiple Venus encounters will gradually lower perihelion from 35
R S to 9.5 R S , producing more than 1,000 hours inside 20 R S , including substantial time within the Alfvén
critical point and providing samples of all solar wind types.
SPP will trace the flow of energy that heats and accelerates the solar corona and solar wind (SHP
action 2d). Observations of magnetic-reconnection exhausts, jets, shocks, and plasma properties—including wave-particle coupling, heat flux, and mass flux—will directly indicate how the Sun’s convective
motion and magnetic field create its dynamic atmosphere and how magnetic free energy is transmitted
from the photosphere to the corona. Measurements will determine the energy budget of the solar wind as
it evolves from the corona and thereby detect signatures of heating and dissipation responsible for the high
temperature of the outer corona and extended heating of the solar wind. As described in Section 10.3.2,
composition measurements that determine ionic charge states would facilitate this study by identifying the
various types of wind and constraining their solar origin.
SPP will determine the structure and dynamics of the plasma and magnetic fields at the sources of
solar wind. Measurements will reveal the steady-state mapping between photospheric sources and coronal structures and emerging solar wind. Full-sky maps of the suprathermal-electron strahl and pitch-angle
distribution will unambiguously identify when the spacecraft is on closed magnetic-field lines rooted at
both ends in the corona.
The Sun accelerates high-energy particles in solar flares and at CME-driven shocks, where suprathermal particles from multiple sources are the seed particles. Testing particle-acceleration models at 1 AU is
hampered by lack of knowledge of source conditions and by acceleration and transport ambiguities. By
closing within the Alfvén point, SPP will survey plasma-field and seed-particle properties in the primeacceleration region of CME-driven shocks, providing ground truth for acceleration models and revealing the
causes of energetic-particle variability needed to improve SEP forecasts (motivation M2 and SHP actions 3a
and 3d). SPP is expected to observe directly about 10 strong CME-driven shocks within 20 R S , providing
comprehensive shock, turbulence, and seed-particle properties for comparison with accelerated-particle
spectra, composition, and pitch-angle distributions.
Near the Sun, impulsive SEP events associated with flares appear as sharp spikes, enabling subminute
timing comparisons with flares, jets, coronal waves, and radio bursts. Flare-particle studies will test acceleration and charge- and mass-dependent fractionation models, survey neutron-decay protons and electrons,
and discover how flare-accelerated particles escape and are transported in longitude (SHP action 3b).
Measuring near-Sun suprathermal-particle properties will provide breakthroughs in understanding of the
relative importance of local acceleration and solar sources (SHP action 3c).
Sending a spacecraft into the last unexplored region of the heliosphere will produce transformative
results throughout the field of solar and space physics and will illuminate fundamental physical processes
that occur in stellar atmospheres and energetic astrophysical objects across the universe (motivation M3).
7 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003.
Solar and Space Physics: A Science for a Technological Society
REPORT OF THE PANEL ON SOLAR AND HELIOSPHERIC PHYSICS
293
decadal survey recommended implementation of a solar probe as a large-class NASA mission. 7 SPP, started
in 2009, will begin its voyage of discovery in 2018 and serve as a keystone of the strategy for solar and
heliospheric science in the coming decade.
The goals of SPP are to determine the structure and dynamics of the Sun’s coronal magnetic field, to
understand how the solar corona and wind are heated and accelerated, and to determine what mechanisms
accelerate and transport energetic particles. To accomplish those goals, SPP is equipped with a tailored
payload for the first near-Sun in situ measurements of solar wind ion and electron thermal plasma, suprathermal and energetic particles, and DC to high-frequency electromagnetic fields. Remote observations
include a large-field-of-view white-light imager to provide global context and a directional radio receiver
to locate and track flares and shocks. Multiple Venus encounters will gradually lower perihelion from 35
R S to 9.5 R S , producing more than 1,000 hours inside 20 R S , including substantial time within the Alfvén
critical point and providing samples of all solar wind types.
SPP will trace the flow of energy that heats and accelerates the solar corona and solar wind (SHP
action 2d). Observations of magnetic-reconnection exhausts, jets, shocks, and plasma properties—including wave-particle coupling, heat flux, and mass flux—will directly indicate how the Sun’s convective
motion and magnetic field create its dynamic atmosphere and how magnetic free energy is transmitted
from the photosphere to the corona. Measurements will determine the energy budget of the solar wind as
it evolves from the corona and thereby detect signatures of heating and dissipation responsible for the high
temperature of the outer corona and extended heating of the solar wind. As described in Section 10.3.2,
composition measurements that determine ionic charge states would facilitate this study by identifying the
various types of wind and constraining their solar origin.
SPP will determine the structure and dynamics of the plasma and magnetic fields at the sources of
solar wind. Measurements will reveal the steady-state mapping between photospheric sources and coronal structures and emerging solar wind. Full-sky maps of the suprathermal-electron strahl and pitch-angle
distribution will unambiguously identify when the spacecraft is on closed magnetic-field lines rooted at
both ends in the corona.
The Sun accelerates high-energy particles in solar flares and at CME-driven shocks, where suprathermal particles from multiple sources are the seed particles. Testing particle-acceleration models at 1 AU is
hampered by lack of knowledge of source conditions and by acceleration and transport ambiguities. By
closing within the Alfvén point, SPP will survey plasma-field and seed-particle properties in the primeacceleration region of CME-driven shocks, providing ground truth for acceleration models and revealing the
causes of energetic-particle variability needed to improve SEP forecasts (motivation M2 and SHP actions 3a
and 3d). SPP is expected to observe directly about 10 strong CME-driven shocks within 20 R S , providing
comprehensive shock, turbulence, and seed-particle properties for comparison with accelerated-particle
spectra, composition, and pitch-angle distributions.
Near the Sun, impulsive SEP events associated with flares appear as sharp spikes, enabling subminute
timing comparisons with flares, jets, coronal waves, and radio bursts. Flare-particle studies will test acceleration and charge- and mass-dependent fractionation models, survey neutron-decay protons and electrons,
and discover how flare-accelerated particles escape and are transported in longitude (SHP action 3b).
Measuring near-Sun suprathermal-particle properties will provide breakthroughs in understanding of the
relative importance of local acceleration and solar sources (SHP action 3c).
Sending a spacecraft into the last unexplored region of the heliosphere will produce transformative
results throughout the field of solar and space physics and will illuminate fundamental physical processes
that occur in stellar atmospheres and energetic astrophysical objects across the universe (motivation M3).
7 NRC, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, 2003.