Late in 1998 the Solar Probe Science Definition Team prepared its final report. 
These web pages collect some of the material from that report and put it together with material of my own. 
The final report can be seen at

This is purely an informal compilation reflecting my interests and was done independently 
of the other members of the Science Definition Team or NASA sponsors of the Solar Probe Mission. 

- Steve Suess - 
June 1999


Mission to the Sun cover photo Fast Latitude Scan


One of the last unexplored regions of the solar system is the innermost heliosphere - the solar corona and polar photosphere. This region fascinates us because we still don't understand how energy flows into the solar atmosphere, heating the corona and driving the solar wind. In situ measurements offer the opportunity to achieve understanding. It is nowtechnically possible to send a spacecraft to the Sun to explore this last frontier, the inner heliosphere inside 60 Rs and down even to 4 RS
It is time to move beyond vicariously studying the Sun and the Sun's atmosphere by collecting photons emitted by the plasma. It is time to take the next step and actually perform the in situ measurements from a spacecraft flying through the corona - the Solar Probe. 

Solar Probe will be a mission of exploration and discovery.  It is planned to fly from pole to pole to within 3 Rs from the solar surface. It will sample the solar wind in the acceleration region and will take high resolution images of the solar atmosphere and the polar regions of the surface.  This will enable linking the wealth of existing remote observations to the actual physical state and dynamics of the solar corona.  By the design of the mission, Solar Probe will fly directly through regions producing fast and slow wind which fills the solar system, modulates penetrating galactic cosmic rays and, and controls interplanetary space from the Sun to the local interstellar medium beyond the most distant planets.

Polar Wind and the Corona - the Context for Solar Probe

(adapted from the Overview/Executuve Summary of the SDT Report)
Just click on the item to go to that section of the Overview
  • Fast and Slow Solar Wind. 

  • Fast Wind is Steady and Simple. 
  • The Slow Wind is Variable and Complicated. 

  • Boundary Between the Fast and Slow Wind is Sharp. 
  • Coronal Structure and the Solar Cycle. 

  • Characteristics of the Initial Solar Wind in Coronal Holes. 

  • Plumes. 

  • Characterisitcs of the Initial Solar Wind in and above Streamers. 

  • Properties of the Polar Photosphere. 
  • dial plot


    Present knowledge does not resolve fundamental questions of solar wind origin and acceleration or of coronal heating and flow of energy from the solar surface to the corona.  We do not know magnetic field phenomenology and surface and subsurface flow patterns in the polar regions and how they differ from those at lower latitudes.  We have no direct information on the nature of wave turbulence and of wave plasma interactions in the acceleration region.  We have no direct information on the energetic particle populations, their production and acceleration. Turbulence, shocks, and transient events provide the necessary conditions for particle acceleration. Identification of the active mechanisms will depend on knowing the underlying particle population and wave environments, their spatial extent and dynamical evolution. All of these questions will remain unanswered until in situ measurements are made in the solar wind acceleration region near the sun and until high-resolution images of the polar regions of the Sun are taken.  This is the foundation for the simple and strong rationale for the Solar Probe Mission. 

    Current Scientific Understanding and Questions

    (adapted from the "Science Section" of the SDT Report)
    Click on the item to view the information
    The Sun, Corona and the Solar Probe Mission 

    Results from Ulysses that Motivate the Solar Probe Mission

    Remote Sensing of the Corona and Photosphere - Fast Wind and the Solar Probe 

    Remote Sensing of the Corona and Photosphere - Slow Wind, Streamers and the Solar Probe 

    Solar Probe in Context

    How Solar Probe Will Answer the Primary(Category A) Science Questions 

    How Solar Probe Will Answer the Secondary (Category B/C) Science Questions 


    SDT Report image
  • SOHO "sees" the corona and photosphere via the photons that are emitted. 
  • Ulysses collects plasma that originates in the corona. 

    Occasionally, when Ulysses and SOHO are at quadrature relative to the Sun and each 
    other, SOHO can analyze the same plasma on the limb which eventually arrives at Ulysses. 

    Nevertheless, remote sensing is a faint and pale substitute for in situ measurements 
    of coronal properties.

  • Back to the top of the Introduction 
  • Page 1: Overview/Executive Summar Synopsis
  • Page 2: First page of Solar Probe Science Section. 
  • Return to Steve Suess' "projects page". 
  • Go to MSFC's Solar Physics Group page. 
  • NASA OSS Space Physics Home Page
  • NASA OSS Solar Probe Page
  • Prepared by Steve Suess (
    3 June 1999