The Sun, Corona and the Solar Probe Mission

(Current Scientific Understanding and Questions)

The Solar Probe will fly as close to the Sun’s surface as is technologically feasible today. It is being sent to the Sun because the physics of the flow of energy through the Sun’s surface and into its atmosphere and the causes of both slow and fast solar wind are not understood. Both imaging and in situ measurements will provide the first three dimensional view of the corona, high spatial and temporal measurements of the plasma and magnetic fields, and high resolution helioseismology and magnetic field observations of the solar polar photosphere. Two perihelion passes are planned, the first near the 2010 sunspot maximum and the second near the 2015 sunspot minimum - when the solar corona will be similar to what is shown on cover of this report. At its perihelion of 4 solar radii (RS) Solar Probe will be immersed in bright equatorial streamers like those visible on the cover, where the plasma is dense and collision dominated, the plasma b>1 (ratio of thermal pressure to magnetic pressure), the speed is subsonic, and where slow solar wind originates in a way which has so far eluded understanding. Elsewhere, at 5-20 RS, Solar Probe will pass through coronal holes where fast solar wind originates, the plasma is collisionless and non-Maxwellian, and the plasma b<<1. 

The unanswered questions in basic physical phenomena of the Sun that will be addressed by Solar Probe can be summarized as: 

    1. What is the physics of the flow of energy through the Sun’s surface and into the solar atmosphere (corona)? 
    2. What is the cause of the slow solar wind? 

    3.  What is the cause of the fast solar wind? 
    4. What are the properties of the smallest structures in coronal holes and streamers? 
    5. What are the magnetic field and solar rotation like near the poles of the Sun, beneath the polar coronal holes? 
Schematic of evolution of the solar corona over the 11 year sunspot
Schematic of evolution of the solar corona over the 11 year sunspot cycle. 
(a) Solar maximum when the Sun is covered by relatively small streamers with small or nonexistent polar coronal holes. 
(b) Declining phase of the solar cycle, also showing that coronal plumes occur in the coronal holes. Plumes, however, exist at all times in coronal holes. The polar coronal holes are growing in size at this time and the global structure of the corona often appears “tilted” away from the rotation axis (N). 
(c) Solar minimum, similar to the configuration seen on the cover image of this report. At minimum, the polar coronal holes are at their largest. 

There are several alternate scenarios for what may be found on this mission and each scenario is related to specific causes for coronal expansion. The ensemble of instruments on Solar Probe will link the enormous wealth of existing solar and coronal observations to the actual physical state and dynamics of the solar corona and provide the specific information needed to distinguish between these scenarios. This is the overall objective of the Solar Probe mission. This pioneering mission meets basic needs of the NASA Solar Connections Initiative and  is of fundamental significance in astrophysics since the Sun is the prototype for all other stars and is the only example that can be investigated in detail. It is therefore a mission of exploration, discovery, and of comprehension. 

The reference Solar Probe mission uses a Jupiter gravity assist, with launch in February 2007.  The flight duration is approximately 3.7 years to Perihelion 1, and 7.7 years to Perihelion 2.  The following figure illustrates the interplanetary trajectory to Perihelion 1.

The gravity assist flyby at Jupiter (10.5 RJ, retrograde southern target) rotates the trajectory upward to a 90° ecliptic inclination and back toward the Sun for the first of two perihelion encounters at 4 RS.  Quadrature (90° spacecraft-Sun-Earth angle) geometry will exist at the first perihelion to allow real-time communications using spacecraft antenna/shield configuration.

At approximately 10 days prior to perihelion (0.5 AU), periodic high rate (~50 Kbps) real-time telemetry will begin.  Plasma observations wcould begin 10 days prior to perihelion and would continue through to perihelion + 10 days.  Remote sensing observations (imaging) to investigate helioseismology wcould begin when the spacecraft reaches 30° latitude and continue until perihelion + 4 days (60 RS).  The end of the primary observation phase for each of the two perihelia occurs at approximately 10 days past perihelion.
earth at perihelion

The reason Solar Probe will make two full orbits about the Sun is to permit observations to be made in the corona near both solar maximum and solar minimum. This requirement comes from the radically changing nature of the corona over the 11 year solar sunspot cycle and the “bimodality” of the solar wind. The solar cycle changes in the corona are shown schematically in the schematic at the top of this page. 

Near solar maximum (panel (a) above), the large scale magnetic field of the Sun is disordered, coronal mass ejections (CMEs) occur at a rate of several per day, many solar flares occur each day, and radio, EUV, and X-ray emissions from the corona are orders of magnitude higher than at solar minimum. Coronal holes are either absent or very small so that Solar Probe would have a negligible probability of encountering one. At this time Solar Probe would collect information on the active Sun and corona, on the source of the slow wind, on shock waves, and on the acceleration of energetic particles in the corona. 

Near solar minimum (panel (c) above), the Sun’s global magnetic field is well organized and roughly dipolar. The corona is dominated by large equatorial streamers, polar coronal holes which extend down to mid-latitudes at the photosphere and nearly to the equator beyond a few solar radii, and CMEs occur at a rate of approximately one per day. During this time Solar Probe would be certain of passing through a polar coronal hole inside 8RS, and probably inside 5RS. Detailed measurements of the properties of fine structure, waves, and turbulence in the high speed wind would be made and the properties of quiescent equatorial streamers could be determined. This is the portion of the mission which would resolve the many questions about the origin of fast solar wind. 

The Solar Probe mission Class A Objectives, that have been defined from the above unanswered questions and known properties of the corona are as follows: 

  • Determine the acceleration processes and find the source regions of fast and slow solar wind at maximum and minimum solar activity. 
  • Locate the sources and trace the flow of energy that heats the corona. 
  • Construct the three-dimensional density configuration from pole to pole, and determine the subsurface flow pattern, the structure of the polar magentic field and its relationship with the overlying corona. 
  • Identify the acceleration mechanisms and locate the source regions of energetic particles, and determine the role of plasma waves and turbulence in the production of solar wind and energetic particles. 

Because of the large solar cycle dependence of the properties of the corona (see schematic above), it is impossible to meet these Solar Probe mission objectives in a single pass at any single time in the solar cycle. Conversely, the mission plan would meet all the objectives through the use of two passes through the corona at appropriately differing times in the solar cycle. In the following, details are given on what is known of the solar corona and why Solar Probe is necessary to address the unanswered questions. 

Table: JGA Reference Mission Event Summary 
Launch Launch and Interplanetary Injection 15 February 2007
Cruise 1 Earth to Jupiter Cruise L + 30 days to JGA - 90 days
JGA Jupiter Gravity Assist  25 June 2008
Cruise 2 Jupiter to P1 Cruise JGA + 74 days to P1 - 30 days 
Start P1 Primary Mission Begin Primary Science Data Acquisition for P1 P1 - 10 days (0.5 AU) 
Critical Science Data Acquisition Critical Science Data Acquisition for P1 P1 +/- 1 day (+/- 20 RS)
End P1 Primary Mission End Primary Scinece Data Acquisition for P1 P1 + 10 days (0.5 AU) 
Cruise 3 Cruise from P1 to P2 P1+30 days to P2 - 30 days
Start P2 Primary Mission Begin Primary Science Data Acquisition for P2  P2 - 10 days (0.5 AU) 
Critical Science Data Acquisition  Critical Science Data Acquisition for P2  P2 +/- 1 day (+/- 20 RS
End P2 Primary Mission End Primary Science Data Acquisition for P2 P2 + 10 days (0.5 AU) 
EOM End of Mission  TBD

This is page 2; Next: 
  • Page 3: Results from Ulysses that Motivate the Solar Probe Mission
  • Return to the top of the Solar Probe introduction Page. 
  • Return to Page 1: Overview/Executive Summary 
  • Return to Steve Suess' "projects page". 
  • Go to MSFC's Solar Physics Group page. 
  • Prepared by Steve Suess (
    3 June 1999