How can solar irradiance be different at each minimum?

1. How can solar irradiance be different at each minimum? Tom Woods LASP / University of Colorado tom.woods@lasp.colorado.edu

2. How can irradiance be different at each minimum? Map Ca II K line 6/03/1992 • Black: Quiet-Sun • Purple: Active Network • Grey: Enhanced Network • White: Plage • Surface features can be different at each minimum. e.g., active network and quiet network is not zero at minimum • e.g. Worden et al., Ap J, 1998, 1999, 2001 • Quiet Sun radiance could change with solar activity • e.g. Schuhleet al., A & A, 2000 using SOHO SUMER data • Luminosity could change • e.g. Foukalet al., Nature, 2006 • PICARD mission: solar diameter • Dynamo could change magnetic activity • Faster rotation rate and/or meridional flow rate • e.g. Eddy et al., Solar Phys. 1976

3. Conclusions for 1996-2008 Comparison • Surface features can be different at each minimum • Active Network appears to be the same fractional area (2.3%) in 1996 and 2008 (from SOHO EIT images) • Coronal holes are different – less at poles and more at low-latitudes • Polar magnetic field and open magnetic flux are lower • Quiet Sun radiance could change with solar activity • Don’t know ??? • Luminosity could change • Don’t know ??? • Dynamo could change magnetic activity • Fastermeridionalflow rate  lower polar magnetic field • From David Hathaway (SOHO MDI images, SOHO-23 workshop)

4. Trends in Solar Surface Features from EIT Polar Field is 40% less Courtesy of SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA. • Polar coronal hole area and magnetic field are lower for current minimum • Active network area appears to be same between the twominima From EIT 304 Å images: • 1996: 0.0233 ± 0.002 • 2009: 0.0233 ± 0.003 Figure from Kirk et al., Solar Phy., 2009

5. 1996: cycle 22/23 minimum 2008: cycle 23/24 minimum • lpolar coronal holes are smaller (by ~20%) than in 1996 • large, mid- and low-latitude coronal holes persisted for most of the minimum phase Comparison of Coronal Holes from Giuliana de Toma – SOHO EIT Images

6. Coronal Holes Affect EUV Radiation SOHO SEM EUV Irradiance • SOHO SEM 26-34 nm is about 15% less in 2008 than in 1996 (Leonid Didkovsky, ‘09 SPD 18.07) • Uncertainty is ~6% • This minimum has more low-latitude coronal holes than in 1996 • Low-latitude coronal holes start to go away in 2009 (deToma, SOHO-23) • BREAKOUT DISCUSSION (WG-F): Is minimum in solar EUV irradiance in 2008 related to the larger low-latitude coronal hole area?

7. Meridional Flow is Faster During this Cycle Minimumfrom David Hathaway The Meridional Flow slowed from 11.5 m/s in 1996 to 8.5 m/s in 2001 (cf. Basu & Antia, 2003) – this is a solar cycle effect in having more sunspots on the disk at cycle maximum (Komm, Howard, & Harvey, 1993). ThenMeridional Flow rapidly increased in speed to 13.0 m/s by 2004. This is unexpected result, but this can explain why there is lower polar magnetic field now. 15% increase

8. Conclusions for 1996-2008 Comparison • Surface features can be different at each minimum • Active Network appears to be the same fractional area (2.3%) in 1996 and 2008 (from SOHO EIT images) • Coronal holes are different – less at poles and more at low-latitudes • Polar magnetic field and open magnetic flux are lower • Quiet Sun radiance could change with solar activity • Don’t know ??? • Luminosity could change • Don’t know ??? • Dynamo could change magnetic activity • Fastermeridionalflow rate  lower polar magnetic field • From David Hathaway (SOHO MDI images, SOHO-23 workshop)

9. BACKUP SLIDES

10. Zonal Flow Varies with Latitudefrom David Hathaway Differential Rotation wrt Core Slower at Poles (30-33 days) Faster at Equator (25-27 days) Only in Convection Zone From Manfred Küker

11. Meridional (Poleward) Flow Varies with Latitudefrom David Hathaway ~3 yr Meridional Flow ??? 10-30 yr The Meridional Flow is from equator to the poles – peak velocity in the north is 10 m/s at 30˚ while peak velocity in the south is 12 m/s at 50˚