An Observational Review of Coronal Holes and their Connection to the Heliosphere

1. An Observational Review of Coronal Holes and their Connection to the Heliosphere Harrison P. Jones National Solar Observatory Invited Talk, SHINE 2006 Workshop 02 August 2006

2. WG1 Session: RECONCILING OBSERVED AND MODELED CORONAL HOLESHolly Gilbert Chair, Thursday, Aug 3, 9:45 • how to observationally identify coronal holes • Are models correct in what they are predicting? • instrumentation (current and future) • using models and observations to help identify/clarify sources of the solar wind • magnetic flux balance issues • Invited Speakers • Carl Henney SOLIS/VSM Coronal Hole Maps • Guiliana de Toma Multiwavelength observations and identification • Roberto Lionello MHD and potential field models

3. History of Coronal Hole Observations • Perhaps first noticed by Waldmeir from coronagraph data in mid 1950’s. • Later seen in early EUV & XUV rocket observations. • More extensive study in late 60’s from OSO 4, AS&E XUV images. • First systematic studies from Skylab and ground-based He I 1083 nm spectroheliograms in 1970’s.

4. What are coronal holes? • Areas conspicuously lacking in coronal emission • Usually seen in soft Xray/EUV spacecraft images or He I 1083 nm ground-based images • Occur above unipolar, but not particularly strong, areas in photospheric magnetograms • Chromospheric network in He I 1083 nm has less contrast than in quiet Sun • Boundaries tend to be sharp • Anybody can see them, but precise agreement between two people is rare

5. 27 August 1996 NSO 1083 nm SOHO/EIT 17.1 nm

6. 27 August 1996 NSO Coronal Hole Map NSO Magnetogram

7. Coronal Hole Recognition • See tomorrow’s talks by Henney and de Toma • Malanushenko and Jones (2005, Sol Phys226, 3) • Looked at scatterplots (correlative properties) of He I 1083 nm central intensity, half width, and velocity. • Found that width and intensity correlate differently in coronal holes and quiet sun • Formed linear combination of quantities such that coronal hole is clearly separated from quiet Sun

9. 2000 Apr 17 Intensity Normalized Intensity + width Normalized Intensity - width

13. What’s Important about Coronal Holes? • Skylab Solar Workshop I • Coronal Holes and High Speed Wind Streams (1976, J. B. Zirker, ed.) • Many excellent articles (Bohlin, Krieger, Levine, Withbroe, Kopp, Hundhausen, Gilman, Svalgaard) • Skylab and SHINE 06 workshop participants • Uri Feldman • Bernie Jackson • Leif Svalgaard • S.T. Wu

14. Skylab Solar Workshop I Paradigm • Open fields originate in coronal holes and near active regions. • High-speed solar wind (~700 km/s) is associated with polar coronal holes and some equatorial coronal holes. • Lower speed wind occurs near heliospheric neutral lines and is associated with active regions and helmet streamers.

15. What people remember • Coronal hole ≡ Open field ≡ High speed wind

16. What is an open magnetic region? • A spatial domain in which magnetic lines of force extend from the solar surface far (hundreds of solar radii) into the heliosphere and cannot be traced back to the Sun. • Provides a direct connection for plasma flow to the Earth and other planets. • No one has ever directly observed an open field line all the way from the surface into the distant heliosphere.

17. Recognizing Open-Field Structures • Mapping in-situ magnetic fields back to the solar surface • Extrapolation of measured surface fields into the heliosphere • Common practice to map inward to and extrapolate out to a “source surface” at ~ 2R • Intensity or density patterns as tracers of field lines • Correlation of structural patterns on the Sun and in the heliosphere

18. McComas 2003

19. Coronal Holes as Sources of (High-speed) Solar Wind • See WG1/2session on Origin and Evolution of the Solar Wind following this talk. • See poster by Zhao and Zurbuchen

20. Solar Wind Sources at MinimumNeugebauer et al. 1998,JGR 103, 14,587-14,599 • Compared mapping techniques • Good general agreement between SS and MHD methods (SS with current sheet not so good-but see poster by Hughes, Arge, and Owens) • Predicted open field regimes matched pretty well with He 1083 nm coronal holes with notable exceptions. • Near solar min, all high-speed streams from Ulysses and Wind mapped back to polar coronal holes • Lower-speed streams (300-600 km/s) mapped back to equatorial coronal holes or equatorward of polar coronal holes. • Heliospheric and solar polarities did not match for 4 of 38 identified streams.

21. Solar Wind Sources at MaximumNeugebaueret al., 2002JGR 107, 1488Liewer & Neugebauer, 2004, Solar Phys 223, 209 • Used Ulysses and ACE data • Good agreement with observed and mapped source region polarities for spacecraft latitudes < 60o • Solar wind slower than at min, mapped to both coronal holes and active regions. • O7+/O6+ lower from coronal holes

23. Solar Wind Sources at MaximumWang and Sheeley, 2003, ApJ 587, 818. • Used ACE measurements • Mostly low proton speeds, high oxygen charge state ratios • Originates from small, sheared open-field regions near active regions with large expansion factors • Occasional high-speed streams have small expansion factors and low charge state ratios

24. MHD Simulations • Many papers at this meeting regarding MAS and related algorithms • Riley, Linker, Mikic, & Zurbuchen (2003), Solar Wind 10, 79. • Used MHD model based on NSO synoptic charts to map the field to 30 R, ballistic mapping inward from ACE and Ulysses • Compared mapping to computed coronal holes • At solar min, inferred a supergranule-sized boundary layer giving rise to slower wind speeds, and increasing wind speed away from boundary layer.

26. Contrarian ViewpointWoo, Habbal, Feldman, others; various publications • Cite evidence from Mauna Loa K-Coronameter and radio scintillations as well as coronagraph and eclipse images that point to radial density structures. • Suggest that open flux and fast solar wind emerges from quiet Sun as well as coronal holes; later papers suggest that this is ubiquitous. • Provide simulations which suggest that this idea is consistent with observations. • Anticipated by Hundhausen (1977) ; reduces disagreement between magnetograph observations and photospheric fields inferred from spacecraft data.

29. Chromosphere/Transition Region • Look for preferential outflows in coronal holes in comparison with the quiet Sun; assumption is that these are related to high-speed solar wind—guilt by association • He I 1083 nm asymmetry (Dupree, Penn, Jones, 1996, ApJ 467, L121) • Looked for excess blue absorption at suggestion of Dupree and found it in two polar coronal holes. • Consistent with part of line-forming region moving outward at several km/s • Asymmetry appeared highest in cell centers

30. 1083 nm Asymmetry • Have frequently (but not always) observed effect in coronal holes, including disk center • Some evidence that asymmetry highest near but not on network boundaries • Determination of asymmetry is at limits of analysis capability and is marginally stable • Have found some agreement with 58.4 nm measurements with SOHO/CDS (with Andretta) • Have been working to improve analysis techniques

32. SOHO/SUMER • Warren, Mariska, & Wilhelm (1997, ApJ 490, L187) • No evidence for preferential outflow in cool TR lines (CII, OVI) • Blueshift observed in NeVIII • Peter (1999, ApJ 522, L77) • Full-disk scan in He I 58.4 nm • Showed blue shift in coronal holes at both poles • Attributed to optical depth effects, not Doppler shifts

34. NeVIII • Hassler, Dammash, Lemaire, Brekke, Curdt, Mason, Vial, Wilhelm (Science 283, 810) • SUMER scans NeVIII 77.0 nm and Si II 153.3 nm • Used zero point from limb, also laboratory reference • Found preferential blue shifts of several km/s at network boundaries, particularly at network vertices

35. Mid Latitude Near N pole Ne VIII Intensity Limb zero Ne VIII Velocity Lab zero

36. Other TR Doppler Observations • Wilhelm, Dammasch, Marsch, and Hassler (2000, AA 353, 749) observed outflows of ~3km/s observed in polar coronal holes (but not polar plumes) in He I 58.4 nm, Ne VIII 77.0 nm • Madjarska, Doyle, and van Driel-Gezsztelyi (2004, ApJ 603, L57) observed bidirectional jets near coronal hole boundaries in N IV and Ne VIII.

37. Coronal hole properties in the Corona • Most recent work based on LASCO, UVCS, EIT, and SUMER data from SOHO. • Wealth of information regarding density, temperature, ionization structure, and velocity. • Lack of direct magnetic field measurements (most that do exist are from radio observations). • Comparatively sparse information close to the Sun.

38. Simultaneous Determination of Electron Density and Flow VelocityAntonucci, Dodero, Giordano, Krishnakumar, & Noci, 2003 AA 416, 749. • Requires analysis of a pair of coronal lines of the same ion emitted by collisional and radiative excitation • Assumes a rapidly diverging magnetic geometry and conserved electron flux • Primary uncertainty is anisotropy of ion temperature. • Obtain electron density results consistent with white-light determination of Guhathakurta et al (1999) • Outflow velocities show rapid acceleration in the 1.5-3R height range but depend strongly on anisotropy.

39. LASCO Observations of Coronal OutflowsSheeley et al. 1997, ApJ 484, 472 • Observed in white light above helmut streamers. • Originate at 3-4 R . • Constant acceleration out to ~ 30 R. • Reach speeds of ~300 km/s. • Concluded that they were passive tracers of the slow solar wind.

40. Coronal Hole Dynamics • Coronal holes tend to rotate rigidly • Known at Skylab workshop (e.g. Levine) and since remarked on by many authors (e.g. Sheeley); see talk by Lionello in tomorrow’s WG1 session. • Basis for interchange reconnection (Fisk, many others) • Recent model (Fisk, 2005) for transport of open field lines through interaction with coronal loops and relation to emerging flux—see posters by J. Gilbert et al and Abramenko et al. • Polar holes are long-lived (~8 yrs; Harvey and Recely, 2002) • Evolve from high-latitude isolated holes near polar reversal • Spread rapidly, fade away near first appearance of high latitude active regions. • Magnetic field tends to be organized in large “cells” as shown by H-α synoptic charts • Coronal holes tend to recur in cells successively eastward of predecessor, perhaps leading to observed synodic period of M-regions.

41. Some Unresolved Issues • Still lack a clear observational and theoretical definitions of coronal holes. • How important is the observed fine structure? • How much of the Sun’s open flux originates in coronal holes? • How are equatorial holes related to the solar wind? • Why (how) does the Sun make coronal holes? • How do we observe coronal holes in new and better ways? • Direct observations of coronal magnetic fields • Better and more extensive imaging spectroscopy