Results from Ulysses that Motivate the Solar Probe Mission

"Bimodality"

Perhaps the most striking result from Ulysses is the simple graphic picture of solar wind bimodality that it gave us - meaning slow solar wind and fast solar wind have fundamentally differing origins. Evidence for bimodality is outlined in the table that follows. 

The graphic picture of bimodality is the “dial plot”, shown below, of solar wind speed versus heliographic latitude measured by Ulysses starting at the Jupiter encounter (right side, south of the equator), going around clockwise to the south polar passage, then the fast latitude scan from 80o S to 80o N latitude between 1994 and 1995  (Ulysses was ~2.2 AU over the poles and ~1.4 AU at perihelion, at the equator), and finally back towards the equator and the second aphelion (right side, just north of the equator). This plot shows that fast wind is steady and that the transition to slow wind is nearly discontinuous, occurring here at latitudes of about +/-15o. Seen here is the configuration near solar minimum, the same as shown in the solar image on the introductory Solar Probe web page (it is expected that near solar maximum the region of steady, fast wind will be much smaller or absent). High temporal resolution measurements show that fast wind contains a field of evolving MHD turbulence while fluctuations in the slow wind are of longer period and more characteristic of a transient source than in the fast wind. 

This beautiful picture shown here first appeared on the cover of GRL (January 1, 1998) and is sometimes referred to as "the ultimate dial plot" since several simpler, less colorful versions appeared earlier. 
dial plot
Solar wind speed and magnetic polarity measured by Ulysses, as a function of heliolatitude, overlaid with three concentric images taken with the NASA/GSFC EIT instrument (center), the HAO Mauna Loa coronagraph (inner ring), and the NRL LASCO C2 coronagraph (outer ring). Each 1-hour averaged speed measurement has been color coded to indicate the orientation of the observed interplanetary magnetic field; red for outward pointing and blue for inward. 
Property (1 AU)
Slow Wind
Fast Wind
Flow Speed 400 km/s
Variance ~50%		 750 km/s
Variance ~5%
Density 7 cm-3
Variance "large",  >50%		 3 cm-3
Variance "small", <50%
Temperature T(proton, 1AU) ~ 200,000 K
Variance "large", >50%		 T(proton, 1 AU) ~ 50,000 K
Variance "large" >50% 
Composition  Depends on First Ionization Potential (FIP)  Independent of FIP 
"Freezing-In" Temperature  ~1.5 x 106 K ~106 K

Composition and "Freese-In" Temperature

Ulysses observations reveal that the composition and charge state of fast wind are also relatively simple. 

The charge state distribution is characterized by a single, low freezing-in coronal temperature of ~1x106 K for each element, as shown for Oxygen and Carbon in the top panel of the figure at the right. The composition is least biased in the fast wind (closely resembling photospheric composition) as shown by the abundance of Mg and Fe relative to Oxygen in the bottom panel of the figure at the right. Conversely, Mg and Fe are overabundant and the freezing-in temperatures are high and variable in slow wind. These close correlations with flow speed for a coronal process (freezing-in temperature) and a chromospheric process (composition) show that the boundary between fast and slow wind is a sharp boundary extending all the way down to the chromosphere. This is one reason that it is now believed that slow wind originates in streamers. 

This figure shows the He ion speed, O and C coronal freezing-in temperatures, and Mg/O and Fe/O abundance ratios. These Ulysses/SWICS data are repeated to facilitate recognition of the sharp boundary between fast and slow wind (Geiss et al., 1996). 

The alpha particle speeds are very similar to the proton speeds shown above in the dial plot. The sharp boundary between fast and slow wind is again evident. 
 

 

 Composition and

Proton Temperatures

The proton temperature from the Ulysses fast latitude scan is shown below. There is, yet again, a sharp boundary between fast and slow wind but the variance in the fast wind is ~50% rather than the 5% variance in speed. This is a true variance that is difficult to reconcile with the smooth flow speed shown in the dial plot. It may the consequence of filamentary structures in the corona such as plumes but this cannot be known until Solar Probe makes the necessary in situ measurements. The slow wind has a comparable variance but with differing statistical properties and with several large spikes which may be due to the small CMEs (e.g. Sheeley et al., 1997) that occur even at sunspot minimum. The proton temperature in the fast wind is also anisotropic (see below), being larger perpendicular to the magnetic  field than parallel to the magnetic field (see below) and this will be seen to have a coronal counterpart in SOHO/UVCS observations. Temperature anisotropy is a diagnostic used to distinguish between suspected coronal heating processes because it tests whether high frequency Alfvén/cyclotron waves may be involved. Solar Probe will measure this parameter as a function of distance all the way into the corona. 
Proton Temperatures
  • What we know as a consequence of Ulysses and other solar wind observations: 
    •  The solar wind is bimodal with differing compositions, temperatures, temperature anisotropies, speeds, small scale fluctuations, and intrinsic variabilities between the two states. The fundamental importance of these differences was only appreciated after Ulysses’ first orbit. 
  • What remains to be answered with SP: 
    •  How the differences above are created in the solar corona. 
    		 contours of solar wind
    Contours of solar wind proton velocity distribution in fast wind at 0.29 AU measured by Helios. Contours are 0.8, 0.6, 0.4, 0.2, 0.1, 0.03, 0.01, 0.003 and 0.001 of the maximum phase space density. The distribution is anisotropic (Tperp > Tparallel), hot, and has a faster component along the magnetic field direction (dashed line) (Marsch et al., 1982). 
    This is page 3; Next: 
  • Page 4: Remote Sensing of the Corona and Photosphere - Fast Wind and the Solar Probe 
  • 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 (steve.suess@msfc.nasa.gov)
    3 June 1999