Weaker Solar Wind
Over the Protracted
Solar Minimum
Dave McComas
Southwest Research Institute
San Antonio, TX
With input from and thanks to Heather Elliott,
Nathan Schwadron, and Ruedi von Steiger
Ulysses’ Three Polar Orbits
Comparison of
SWOOPS SW
plasma data from
Ulysses’ 1st and 3rd
(Solar Minimum)
Orbits
McComas et al., GRL, 2008
SWOOPS first and third orbits (> 40°)
Parameter
<1st> <3rd> <3rd>-<1st>
<1st>
761 739
-3%
772 746
-3%
2.65 2.19
-17%
0.12 0.10
-17%
2.66 2.30
-14%
vp (km s-1)
vα (km s-1)
npR2 (cm-3)
nαR2 (cm-3)
TpR x105 (K)
Mass Flux (kg m-2 s-1)
ρiviR2 x10-15
3.96 3.17
-20%
Dynamic Pressure (nPa)
ρivi2R2
3.01 2.34
-22%
Proton Thermal Pressure (pPa)
npkTpR3
9.89 7.43
-25%
Alpha to proton ratio, nα/np
0.044 0.044
---- Long term reduction in pressure inflating Heliosphere
McComas et al., GRL, 2008
Heating in Subsonic Region
• Nearly all of the energy of the solar wind is
carried off in the form of bulk flow energy 
excellent measure of the average energy being
introduced in the corona, which ultimately
accelerates the solar wind
• Leer and Holzer [1980] developed a simple
model to examine the effects of solar wind
energization by heating and momentum addition
– addition of energy above the sonic point primarily
increases the wind speed
– addition of energy below the sonic point increases the
mass and momentum flux roughly comparably, leading
to little change in wind speed  higher density
McComas et al., GRL, 2008
Ulysses (black)
compared to
ACE (red)
Shows that
reduction in
SW power is
truly global
phenomenon
McComas et al., GRL, 2008
Electron Properties
• Electron density and temperature deduced using quasithermal noise spectroscopy
• Compared 3rd orbit PCH data with 1st orbit
~19% less dense (confirms SWOOPS ~17% lower protons and
alphas)
~13% lower electron temperature (similar to SWOOPS ~14% lower
proton temperature)
Issautier et al., GRL, 2008
IMF 1st vs 3rd Orbits (Ulysses)
Smith and Balogh, GRL, 2008
IMF Observations
• Earth-based magnetograph observations indicated that
the Sun’s polar cap field strength, Bp, decreased by a
factor of about two between 1st and 3rd polar passes
• Ulysses measurements show that the average value of
r**2 Br decreased from
– 3.6 nT (AU)**2 in 1993.5 to 1995.0
– 2.3 nT (AU)**2 in 2006.1 to 2007.4
– Decreased by factor of 0.64, but not precisely at minimum
• Averages of in-ecliptic Br at the two solar minima are
2.82 and 2.45 nT (13% reduction)
• Decreases contrary to observation based on previous
solar cycles that Br returns to the same value of ~3 nT
at solar minima
Smith and Balogh, GRL, 2008
Magnetic Field Fluctuations Decreased
Smith and Balogh, GRL, 2008
THE SOLAR WIND POWER FROM
MAGNETIC FLUX
• Reduction in high latitude fast SW power in
Ulysses 3rd orbit compared to 1st correlates with
reduction in Sun’s total open magnetic flux
• These observations support models of the SW
energy and particle sources (Schwadron &
McComas, 2003; Schwadron et al., 2006)
– Constant electromagnetic energy input per particle
(assumed)
– Magnetic flux emergence naturally leads to an energy
flux proportional to the strength of large-scale
magnetic field
Schwadron and McComas, ApJ, 2008
Solar wind mass flux vs.
magnetic flux density
Sun delivers ∼600
kW/Wb to power the
solar wind
This power to magnetic
flux relation extremely
stable over Ulysses’
three polar orbits
Schwadron and McComas, ApJ, 2008
SW Power in
Astrophysical context
SW observation in context
of X-ray results of Pevtsov
et al., 2003
Black line is 600 kW / Wb
as shown in previous plot
Red line shows the power
X-ray luminosity of ~1% of
total SW power
(Schwadron et al. 2006)
Schwadron and McComas, ApJ, 2008
So what has happened in the
solar wind since 2007
(2008 publications) ???
Not 2007 – 2010!
Solar Minimum has Extended
Recent Data (and end of Ulysses)
Ram Pressure of typically ~3 nPa
Down by a Factor of ~2
OMNI - CMEs Removed
Summary
• Ulysses’ 3rd orbit showed fast SW was slightly slower,
significantly less dense, cooler, had less m, p, and E flux
and weaker IMF than previous solar minimum (first) orbit
• Combined ACE/Ulysses show that this is a significant,
long-term change in SW output from the entire Sun
• Similar SW speeds with lower power indicates SW
energy fed in below the sonic point
• Charge states indicate coronal temperatures lower
• Significant, long-term trend to lower dynamic pressure 
Heliosphere has been shrinking (Voyager & IBEX)
• Same law that governs energy released in the corona
and from other stars also applies to the total energy in
the solar wind
• Decreasing SW power has continued through 2009
approaching ~1.5 compared to ~3 pPa 20 years ago