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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. WG1 Session: RECONCILING OBSERVED AND MODELED CORONAL HOLES Holly Gilbert Chair, Thursday, Aug 3, 9:45.
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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
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
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.
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
27 August 1996 NSO 1083 nm SOHO/EIT 17.1 nm
27 August 1996 NSO Coronal Hole Map NSO Magnetogram
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
2000 Apr 17 Intensity Normalized Intensity + width Normalized Intensity - width
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
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.
What people remember • Coronal hole ≡ Open field ≡ High speed wind
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.
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
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
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.
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
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
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.
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.
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
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
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
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
Mid Latitude Near N pole Ne VIII Intensity Limb zero Ne VIII Velocity Lab zero
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.
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.
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.
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.
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.
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