A Solar Cycle Prediction Puzzle s Possible Explanation? Janet Luhmann Space Sciences Laboratory University of California, Berkeley With thanks for many related interactions with my long-time collaborators on PFSS coronal field models: Yan Li, Nick Arge, and the Stanford group (Todd Hoeksema, Xuepu Zhao, Yang Liu) and to Lan Jian at UCLA for use of her pre-publication results SPD 2007 presentation number 111.04 Honolulu, May 2007
Geomagnetic index (e.g. Ap, aa) variations with the solar cycle suggest, but do not exactly follow, the sunspot number cycle
However- for some reason they can be used to predict the next solar maximum! Recent results from Hathaway and Wilson, GRL, 2006, using a technique similar to one applied by Joan Feynman in 1982- showing that the non-sunspot number correlated geomagnetic index cycle seems to predict the SSN peaks ~5.5 years early.
Why should geomagnetic activity portend solar activity? From D. Hathaway and Wilson, GRL, 2006
Causes of geoeffectiveness Large Southward-oriented interplanetary fields High solar wind/interplanetary plasma speeds and/or magnetic field magnitudes High solar wind/interplanetary dynamic pressure But what causes these? (Model results from M. Wiltberger, C. Goodrich and J. Lyon)
Magnetic Clouds or Flux-Rope-like CME Ejecta are responsible for the largest perturbations in Southward field and dynamic pressures (WIND data plot from Yan Li)
Solar wind stream interactions regions (SIRs-or CIRs if repeating due to corotation) can cause southward interplanetary field deflections and intervals of enhanced dynamic pressures (figures from V. Pizzo, JGR 1991 (left), and J. Zhang, ApJ, 2003 (right))
CME Occurrence rates vary with Solar (Sunspot) Cycle phase but SIR/CIRs are not so clearly cyclic Only a few of these CMEs seen at the Sun encounter the Earth (Figure from Webb et al., JGR 1991) CDAW CME catalog SOHO image
What about simply high solar wind speeds? High Speed Polar CH wind Low Speed Streamer Belt, Low Latitude CH, Polar Hole Edge wind UVCS website image
High plasma speeds most often occur outside of ICMEs and SIR/CIRs (figure from SOHO UVCS website) The highest speed solar winds are known to: -Be associated with quiet solar magnetic fields at their base -Most typically arise from centers of large Polar Coronal Holes
The change in dominant solar wind sources produces the well-known cycle changes in solar wind character McComas et al. GRL 2003
Do late cycle CMEs, SIR/CIRs or simply high speed solar winds produce the declining phase and solar minimum geomagnetic activity? -and if so, why a connection to the next solar maximum?
A recent study sorted out current cycle contributions of ICMEs and SIR/CIRs to the solar wind Lan Jian et al., Solar Physics, in press, 2007
Consider the breakdown of contributions of CMEs and SIR/CIRs to geomagnetic activity for the recent cycle The residual activity (green line, right panel) still has a peak in the declining phase even after subtraction of CME and SIR/CIR periods (from the recent analysis by Lan Jian et al., Solar Physics in press, 2007)
NOAA/SEC Geomagnetic Index Statistics for the Cycle also show the same declining phase peak
This is the index peak that most influences the geomagnetically-based next solar max predictions
In-situ data for this cycle (here, from NSSDC) show high rotation-averaged plasma speeds, especially in 2003-2004 Bmag Bz V (solar rotation averages) Pdyn SSN Richardson, Cane and Cliver (JGR, 2002) also found dominant velocity contributions in three previous late phase cycles of interplanetary data
In-situ data for this cycle (here, from NSSDC) show high rotation-averaged plasma speeds, especially in 2003-2004 Bmag Bz V (solar rotation averages) Pdyn SSN What conditions on the Sun produce these high speeds?
Solar Cycle-long Mapping of solar wind sources shows the change from Polar Coronal Holes around solar min to Mid-to- Low Latitude Coronal Holes around solar max (mixed in between) (Luhmann et al., JGR, 2002)
Carrington Rotation coronal hole footpoint maps, reversed and laid end-to-end, give a long-term picture of global solar wind sources over the last few cycles (from Luhmann et al., JGR 2002)
What about the near-ecliptic sources? (blue,green and black show photospheric footpoints of the coronal field lines that map to near the ecliptic)
Inferred velocities at the ecliptic plane source footpoints (based on the Wang-Sheeley formula) show where and when the highest speeds arise
They occur in declining and minimum phases and are rooted in quiet Sun like the polar coronal holes
They occur in quiet regions with the same magnetic polarity as the local pole
They occur when the heliospheric current sheet is relatively flat
Wang and Sheeley have experimented with the flux tube divergence hypothesis using 2-D coronal field models Figure from Wang and Sheeley, Ap J 2003 illustrating cycle progression of coronal field geometry
Zonal harmonics represent those bands of photospheric field that survive differential rotation and evolve subject to meridional flow and flux cancellation at the neutral lines
In one study Sheeley and Wang (Solar Physics, 1991) showed that a combination of these two configurations is particularly good at producing midlatitude open fields with high speed flows + + - + - + - + - P 1 and P 5 field polarity configurations
Such configurations can be found in the declining phase photosphere. Sophisticated source mapping to 1 AU at NOAA/SEC using the WSA model suggests the result- SOHO EIT images WSA model plot from SEC website
These models successfully reproduce observed high speed winds-for example in this recent case, for a number of sequential solar rotations ACE SWEPAM data/wsa model comparisons from the NOAA-SEC website
-and the two high speed wind sources were rooted in quiet Sun fields with the local polar field polarity. This quiet Sun field pattern seems to persist in spite of changes in ARs
Can further insight be gained from solar cycle variations in spherical harmonic contributions to the photospheric field? Low order multipoles clearly dominate the declining phase spectrum (spectrum based on MWO synoptic maps from Nick Arge and Roger Ulrich)
-and when they are dominant the high speed flows occur
Notably, Makarov and Tlatov, like Schatten and Pesnell (1993) and Schatten (2006), have been predicting the next sunspot maximum based on odd low order spherical harmonics of the photospheric field (but no P 5 term) (from Tlatov and Makarov, ASP Conf. Series 346, 2005)
These cycle prediction methods are thus relatedbut the details are TBD Relationship between Max sunspot number in a cycle and the near-solar-minimum geomagnetic index aa (Hathaway and Wilson, GRL, 2006) (left). A similar plot based on the solar dipole+octupole moments at the previous solar minimum (from Makarov et al., Solar Phys., 2001) (right)
Conclusions At least for this cycle, high solar wind speeds are the fundamental geoeffective parameters to blame for the declining phase geomagnetic activity maximum used to predict the next solar maximum. The high speeds appear to be related to the presence of open fields rooted in large areas of quiet photospheric field with the same polarity as the local pole. It is unclear whether These large mid-latitude open fields or polar coronal hole extensions are related to late cycle active region decay or to large scale fields of dynamo origin. The current quiet Sun photospheric field already contains the solar max. We should try to understand it!
Where are we in the cycle now? (Recent Ap index and SSN from NOAA SEC website)