The magnetic field in the heliosphere evolves in response to the photospheric field at its base. This evolution, together with the rotation of the Sun, drives space weather through the continually changing conditions of the solar wind and the magnetic field embedded within it. We combine observations and simulations to investigate the sources of the heliospheric field from 1996 to 2001. Our algorithms assimilate SOHO/MDI magnetograms into a flux-dispersal model, showing the evolving field on the full sphere with an unprecedented duration of 5.5 yr and temporal resolution of 6 hr. We demonstrate that acoustic far-side imaging can be successfully used to estimate the location and magnitude of large active regions well before they become visible on the solar disk. The results from our assimilation model, complemented with a potential-field source-surface model for the coronal and inner-heliospheric magnetic fields, match Yohkoh/SXT and KPNO/He?10830 Å coronal hole boundaries quite well. Even subject to the simplification of a uniform, steady solar wind from the source surface outward, our model matches the polarity of the interplanetary magnetic field (IMF) at Earth ～3% of the time during the period 1997–2001 (independent of whether far-side acoustic data are incorporated into the simulation). We find that around cycle maximum, the IMF originates typically in a dozen disjoint regions. Whereas active regions are often ignored as a source for the IMF, the fraction of the IMF that connects to magnetic plage with absolute flux densities exceeding 50 Mx cm?2 increases from ?10% at cycle minimum up to 30–50% at cycle maximum, with even direct connections between sunspots and the heliosphere. For the overall heliospheric field, these fractions are ?1% to 20–30%, respectively. Two case studies based on high-resolution TRACE observations support the direct connection of the IMF to magnetic plage, and even to sunspots. Parallel to the data assimilation, we run a pure simulation in which active regions are injected based on random selection from parent distribution functions derived from solar data. The global properties inferred for the photospheric and heliospheric fields for these two models are in remarkable agreement, confirming earlier studies that no subtle flux-emergence patterns or field-dispersal properties are required of the solar dynamo beyond those that are included in the model in order to understand the large-scale solar and heliospheric fields.
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J. L. Culhane D. H. Brooks L. van Driel-Gesztelyi P. Démoulin D. Baker M. L. DeRosa C. H. Mandrini L. Zhao T. H. Zurbuchen 《Solar physics》2014,289(10):3799-3816
Seeking to establish whether active-region upflow material contributes to the slow solar wind, we examine in detail the plasma upflows from Active Region (AR) 10978, which crossed the Sun’s disc in the interval 8 to 16 December 2007 during Carrington rotation (CR) 2064. In previous work, using data from the Hinode/EUV Imaging Spectrometer, upflow velocity evolution was extensively studied as the region crossed the disc, while a linear force-free-field magnetic extrapolation was used to confirm aspects of the velocity evolution and to establish the presence of quasi-separatrix layers at the upflow source areas. The plasma properties, temperature, density, and first ionisation potential bias [FIP-bias] were measured with the spectrometer during the disc passage of the active region. Global potential-field source-surface (PFSS) models showed that AR 10978 was completely covered by the closed field of a helmet streamer that is part of the streamer belt. Therefore it is not clear how any of the upflowing AR-associated plasma could reach the source surface at 2.5 R⊙ and contribute to the slow solar wind. However, a detailed examination of solar-wind in-situ data obtained by the Advanced Composition Explorer (ACE) spacecraft at the L1 point shows that increases in O7+/O6+, C6+/C5+, and Fe/O – a FIP-bias proxy – are present before the heliospheric current-sheet crossing. These increases, along with an accompanying reduction in proton velocity and an increase in density are characteristic of both AR and slow-solar-wind plasma. Finally, we describe a two-step reconnection process by which some of the upflowing plasma from the AR might reach the heliosphere. 相似文献
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Thomas R. Metcalf Marc L. DeRosa Carolus J. Schrijver Graham Barnes Adriaan A. van Ballegooijen Thomas Wiegelmann Michael S. Wheatland Gherardo Valori James M. McTtiernan 《Solar physics》2008,247(2):269-299
We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional,
and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied
to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model.
When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s
free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric
boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small
spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric
boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive
to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are
well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though,
like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the
Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent
to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least
a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to
solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various
codes will not necessarily converge to the correct, or even the same, solution.
On 07/07/2007, the NLFFF team was saddened by the news that Tom Metcalf had died as the result of an accident. We remain grateful
for having had the opportunity to benefit from his unwavering dedication to the problems encountered in attempting to understand
the Sun’s magnetic field; Tom had completed this paper several months before his death, leading the team through the many
steps described above. 相似文献
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Photospheric and heliospheric magnetic fields 总被引：1，自引：0，他引：1
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L. van Driel-Gesztelyi J. L. Culhane D. Baker P. Démoulin C. H. Mandrini M. L. DeRosa A. P. Rouillard A. Opitz G. Stenborg A. Vourlidas D. H. Brooks 《Solar physics》2012,281(1):237-262
During 2??C?18 January 2008 a pair of low-latitude opposite-polarity coronal holes (CHs) were observed on the Sun with two active regions (ARs) and the heliospheric plasma sheet located between them. We use the Hinode/EUV Imaging Spectrometer (EIS) to locate AR-related outflows and measure their velocities. Solar-Terrestrial Relations Observatory (STEREO) imaging is also employed, as are the Advanced Composition Explorer (ACE) in-situ observations, to assess the resulting impacts on the solar wind (SW) properties. Magnetic-field extrapolations of the two ARs confirm that AR plasma outflows observed with EIS are co-spatial with quasi-separatrix layer locations, including the separatrix of a null point. Global potential-field source-surface modeling indicates that field lines in the vicinity of the null point extend up to the source surface, enabling a part of the EIS plasma upflows access to the SW. We find that similar upflow properties are also observed within closed-field regions that do not reach the source surface. We conclude that some of plasma upflows observed with EIS remain confined along closed coronal loops, but that a fraction of the plasma may be released into the slow SW. This suggests that ARs bordering coronal holes can contribute to the slow SW. Analyzing the in-situ data, we propose that the type of slow SW present depends on whether the AR is fully or partially enclosed by an overlying streamer. 相似文献
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The solar photospheric magnetic flux distribution is key to structuring the global solar corona and heliosphere. Regular full-disk photospheric magnetogram data are therefore essential to our ability to model and forecast heliospheric phenomena such as space weather. However, our spatio-temporal coverage of the photospheric field is currently limited by our single vantage point at/near Earth. In particular, the polar fields play a leading role in structuring the large-scale corona and heliosphere, but each pole is unobservable for \({>}\,6\) months per year. Here we model the possible effect of full-disk magnetogram data from the Lagrange points \(L_{4}\) and \(L_{5}\), each extending longitude coverage by \(60^{\circ}\). Adding data also from the more distant point \(L_{3}\) extends the longitudinal coverage much further. The additional vantage points also improve the visibility of the globally influential polar fields. Using a flux-transport model for the solar photospheric field, we model full-disk observations from Earth/\(L_{1}\), \(L_{3}\), \(L_{4}\), and \(L_{5}\) over a solar cycle, construct synoptic maps using a novel weighting scheme adapted for merging magnetogram data from multiple viewpoints, and compute potential-field models for the global coronal field. Each additional viewpoint brings the maps and models into closer agreement with the reference field from the flux-transport simulation, with particular improvement at polar latitudes, the main source of the fast solar wind. 相似文献
8.
A. W. Sandman M. J. Aschwanden M. L. DeRosa J. P. Wülser D. Alexander 《Solar physics》2009,259(1-2):1-11
The Solar Terrestrial Relations Observatory (STEREO) provides the first opportunity to triangulate the three-dimensional coordinates of active region loops simultaneously from two different vantage points in space. Three-dimensional coordinates of the coronal magnetic field have been calculated with theoretical magnetic field models for decades, but it is only with the recent availability of STEREO data that a rigorous, quantitative comparison between observed loop geometries and theoretical magnetic field models can be performed. Such a comparison provides a valuable opportunity to assess the validity of theoretical magnetic field models. Here we measure the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with the Extreme Ultraviolet Imager (EUVI) on STEREO on 30 April, 9 May, and 19 May 2007. We perform stereoscopic triangulation of some 100?–?200 EUVI loops in each active region and compute extrapolated magnetic field lines using magnetogram information from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We examine two different magnetic extrapolation methods: (1) a potential field and (2) a radially stretched potential field that conserves the magnetic divergence. We find considerable disagreement between each theoretical model and the observed loop geometries, with an average misalignment angle on the order of 20°?–?40°. We conclude that there is a need for either more suitable (coronal rather than photospheric) magnetic field measurements or more realistic field extrapolation models. 相似文献
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