|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:
What is the cause of the fast solar wind?
(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.
|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:
|Table: JGA Reference Mission Event Summary
|This is page 2; Next:
3 June 1999