What is the Evidence for Open Field Lines from Active Regions?

1. What is the Evidence for Open Field Lines from Active Regions? J. Harvey National Solar Observatory

2. Context • Ulysses: Open solar flux is ~8 x 1022 Mx with little latitude or time dependence at ≥ 1 AU (small variance is a hot topic) • How much of it comes from: • Coronal holes • Streamers • Quiet sun • Active regions

3. Scope • Marcia’s talk addresses solar wind properties associated with supposed open field lines from active regions • I consider near-Sun observational evidence for open fields in active regions • Transients mostly ignored

4. Overview • Some definitions • Signatures of open field lines • Synopsis and summary

5. Abbreviations OF = open field AR = active region CH = coronal hole SW = solar wind PFSS = potential field source surface

6. Open Field Lines • Field lines that start in photosphere and “end” in interplanetary space • Conduits for plasma to move inward or outward from or to the solar wind • Field lines that reach β > 1 regime • Field lines cannot be directly observed

7. Active Region The entire time-varying structure produced by emergence of a sunspot-producing magnetic flux rope.

8. Open Fields from ARs? • Presumption of open field lines from ARs is long-standing (Maunder, Birkeland, etc.) • No direct evidence from remote sensing • Conjecture

9. Signatures of Open Fields • Morphology of coronal rays and loops • Coronal holes and voids • Extrapolations of photospheric magnetic fields • Type III metric radio bursts

10. Eclipse Morphology – 1950s • Based on Russian studies of many total eclipse photos • Long, narrow rays often seen over “centers of activity” (i.e. ARs) • Presumption of “corpuscular streams” from ARs • Implies some (weak) evidence for open field lines from ARs Mustel 1963 in The Solar Corona, IAU Symp. 16, p. 331

11. Eclipse Morphology Today

12. EUV Morphology – 1970s • X-ray and EUV AR images from space show dark voids, loop “fountains” and sprays • Interpreted as miniature active region coronal holes • Contain relatively large amounts of magnetic flux, comparable to large coronal holes • Implies ~10% of flux of the AR is open Mag Fe XV negative Mg IX negative Bohlin & Sheeley 1978, SolPhys 56, 125

13. SOLIS/VSM chromospheric magnetogram 12 Nov 2006 EUV Morphology Today XRT/Hinode temperature image 12 Nov 2006 (XRT POW 2007 June 7 by F. Reale and S. Parenti)

14. Coronal Holes in ARs – 1970s Harvey & Sheeley 1979, SpSciRev 23, 139 (27-29 April 1978 NSO 10830 Å images) • X-ray and 10830 Å images showed transient CHs adjacent to ARs • Last up to 1 day after flare • Associated with high-speed winds • Interpreted as high-reaching closed field lines opened as a result of a flare or CME • aka coronal voids, dimmings, depletions

15. Coronal Holes in ARs – 1980s 1826 UT B|| 2106 UT Δt 1982 Dec 17, X10 and M4 flares 10830 Å KPVT Recely & Harvey 1986, in Solar-Terrestrial Predictions, Meudon, P. A. Simon et al, p. 204

16. Coronal Holes in ARs Today • Much improved 10830 Å observations(de Toma, Holzer, Burkepile, Gilbert 2005, ApJ 621, 1109) • Far better EUV and X-ray observations (e.g., EIT: Attrill et al. 2006, SolPhys 238, 117) • Large and growing literature • But, most coronal dimming studies concentrate on events adjacent to ARs not deep inside ARs

17. Field Line Extrapolations: 1967-1970 Aug. 1959 Harvey 1967 extensions to Schmidt program • No source surface or no plotted field lines with source surface • Poor spatial and time resolution • Poor field measurements • Minimal physics • “Unipolar magnetic regions … result in open field lines …” (Schatten 1968, Nature 220, 1211) Newkirk, Altschuler, Harvey 1968, IAU Symp. 35, 379

18. Field Line Extrapolations: 1970-1982 • Advances • Potential field with source surface • Some non-potential experiments • Better spatial resolution (~ x100) • Better field measurements • Still minimal physics (Pneuman, Hansen & Hansen 1978, SoPhy 59, 313) • Selected findings • Open fields inferred from ARs • Strong cycle variation of AR open fields (at max most traceable to ARs; almost none at min) Randy Levine, ca. 2004

19. Field Line Extrapolations: last decade • Selected Modeling Advances • PFSS and MHD model extrapolations of open FL footpoints compare well (Neugebauer et al. 1998, JGR 103, 14587) • Interchange reconnection model (Fisk, Schwadron & Zurbuchen 1999, JGR 104, 19765; see also Marsh 1978, SolPhys 59, 105) • High-resolution “local” PFSS models (e.g. Marsch, Wiegelmann & Xia 2004, A&A 428, 629) • Resolution of PFSS models has “tremendous” influence on open field footpoint locations (Poduval & Zhao 2004, JGR 109, A08102) • “Open Flux Equilibrium” model (Gilbert, Zurbuchen & Fisk 2007, ApJ 663, 583) • Strict CH topology for PFSS and MHD quasi-steady models (Antiochos et al. 2007, ApJ submitted)

20. Field Line Extrapolations: last decade • Selected Observational Advances • High cadence magnetograms 1996- (MDI, GONG) • Snapshot synoptic maps 1998- (Worden & Harvey; Schrijver & DeRosa) • Better calibrations (e.g. old KPVT archive) • Polar field treatment • Regular chromospheric LOS and photospheric vector FD data • PFSS models run in near real time (e.g. NOAA, Lockheed, NSO, etc.)

21. Field Line Extrapolations: last decade • Selected Results: Open Field Lines from Active Regions • IPS mapping to ARs @ min show slow wind from one side (Kojima et al. 1998, JGR 104, 16993) • Type III radio bursts probably originate in ARs (Paesold et al. 2001 A&A 371, 333) • OF at max rooted in small, low-latitude CHs near ARs (Wang & Sheeley 2002, JGR 107, A10, SSH 10-1) • OF at max often rooted in low-latitude strong field regions (Neugebauer et al., 2002, JGR 107, A12, SSH 13-1)

22. Field Line Extrapolations: last decade • Selected Results: Open Field Lines from Active Regions • At max, slow SW comes from AR areas with large expansion factors (Wang & Sheeley 2003, ApJ 587, 818) • OF from AR <1% at min to 30-50% at max (Schrijver & DeRosa 2003, SolPhys 212, 165) • FL from emerging AR may reconnect with CH FL to open AR FL (Neugebaur & Liewer 2003, JGR 108, A1 SSH3-1) • AR outflow in Ne VIII matches OF (Marsch et al. 2004, A&A 428, 629) • Some OF traced to AR at max; not always dark in EUV (Liewer et al. 2004, SolPhys 223, 209)

23. Synopsis and Summary • Compelling (indirect) evidence for open field lines from active regions – even some sunspots • Expected whenever an AR emerges in an area dominated by one polarity • As much as 10% of AR flux may be open • Strong cycle variation: <1% at min to ~50% at max • Better field measurements needed in sunspots • OpenQuestions • What is the role of transients? • Where does the rest of the OF originate?