ar033.leka01 Posted: 19-Sep-92 Updated: 06-Apr-93, 06-Nov-93, 08-Mar-94, 08-Nov-94, 13-Feb-95 Events specified: AR 7260 (mid-August 1992)
K.D. Leka, with: R.C. Canfield, D.L. Mickey, T.R.Metcalf, J.-P. Wuelser, H.S. Hudson, any interested collaborators from Japan...
Primary Objective: To determine if the magnetic flux in the trailer section of AR7260 emerged into the photosphere inherently carrying electric current.
Motivation: It has long been thought that photospheric motions contribute to the formation of electric currents in active regions, which are known to be associated with flares (Heyvaerts, 1974; Tanaka & Nakagawa, 1973; Krall, et al., 1982, Wang, 1991). However there have been questions raised concerning the energy available in the photosphere, and whether enough energy can be derived from the photospheric motions in a short enough time to power the solar flares observed (McClymont & Fisher, 1989).
NOAA AR7260 grew substantially in area, magnetic flux and complexity in the period 16-19 August 1992. We have obtained good coverage both spatially and temporally of the photospheric magnetic fields with the new Imaging Vector Magnetograph at Mees Solar Observatory for the days 18-20 August. As part of her doctoral thesis work, Leka proposes to use the IVM data and SXT high-resolution images to study the degree of non-potentiality in this region as it emerged (SXT images will be extremely important for the period prior to 18 August, i.e. prior to good coverage by the IVM data). She proposes to examine the hypothesis that the non-potential configuration and the currents implied were generated prior to the flux's photospheric appearance, i.e. in the deeper layers of the convection zone.
Update 13-Feb-95
This topic has been completed in the form of my dissertation (see under 'theses') and will be submitted soon to Solar PHysics.
Update 08-Nov-94
Flare-productive active regions exhibit non-potential magnetic field structures, oft described as `sheared' or `twisted' fields. This morphology indicates that electric currents are present. In this thesis I test whether surface flows generate observed active-region currents, or whether these currents are produced prior to their appearance at the surface as sunspots, \ie\ deep in the solar convection zone.
To study this question I observed emerging magnetic flux in a uniquely rapidly growing active region. First I undertook an exhaustive study of the more than 50 bipoles which appeared in a sunspot group visible in August 1992. I determined the time of emergence, magnetic connectivity and patterns of overall development of this young active region. Then, four independent analysis methods were used to determine whether the emerging flux was carrying the electric current prior to its appearance, or if the observed strong currents were generated by plasma flows in the photosphere.
The four approaches gave consistent results. For a few young bipoles, I show that the morphology of chromospheric and coronal loops were definitively non-potential, that those same dipoles had proper motions which reflected twisted subsurface flux bundles, that electric current existed in greater abundance than could be generated given the observed characteristics and finally that the electric current increased as the magnetic flux itself increased with no substantial delay. All evidence was also consistent with a direction of twist defined by $\Jz / \Bz < 0.$ This twist direction was also present in the older flux of this active region.
I conclude that the electric currents observed in this solar active region were not produced by plasma motions in the photosphere. Rather, the evidence presented in this thesis supports the hypothesis that active region electric currents are generated either deep in the convection zone or are produced with solar magnetic fields in a dynamo process.
Update 08-Mar-94
A manuscript titled "The Magnetic Evolution of AR7260: A Roadmap" has been submitted to Solar Physics.
This is the abstract for a paper recently submitted to Solar Physics. It is the first part of the analysis of 7260, i.e. a general history of the evolution of the region, to be used by others studying (flares, coronal structures, etc.), e.g. the participants of the last Hawaii CDAW. The analysis of the inherent twist is forthcoming, and the abstract will be forwarded upon submission to a journal.
THE MAGNETIC EVOLUTION OF AR 7260: A ROADMAP
K. D. LEKA {1}, L. van DRIEL-GESZTELYI {2}, N. NITTA {3}, R. C. CANFIELD {1}, D.L. MICKEY {1}
{1} Institute for Astronomy, University of Hawaii, U.S.A. {2} Kiso Observatory, Institute of Astronomy, University of Tokyo, Japan {3} Lockheed Palo Alto Research Laboratory, U.S.A.
{Abstract}
The active region NOAA 7260 rotated onto the disk as a mature bipole: a dominant negative-polarity sunspot with trailing plage and small scattered spots in attendance. For a period of seven days beginning 14 August 1992 the region displayed substantial evolution: no fewer than 50 magnetic bipoles emerged, increasing the magnetic flux by more than $10^{22}$Mx. The most substantial flux emergence occurred in the trailing plage of the large existing P spot. At least seven bipoles appeared in that area forming a complex $\beta\gamma\delta$ configuration. Close to eighty flares with soft X-ray counterparts, including eight M-class flares, accompanied this growth. The dominant P spot itself had extremely strong magnetic fields and covered almost 400 millionths of a solar hemisphere.
This region was very well observed by Yohkoh and various ground-based observatories. Several investigations of flux emergence and flaring are underway by the authors and others. Hence, for the benefit of these later studies we present in this report a description of the global characteristics of this active region and a detailed `roadmap' of its evolution during disk passage, including the development of a complex $\delta$ region.
Update 06-Nov-93
The following are abstracts for meetings in which an analysis of the twist in the emerging flux tubes in AR7260 were presented, namely the SPD meeting in Stanford, CA July 1993, and at the SPO summer workshop on active region evolution, Aug 1993.
I have finished a paper entitled 'Roadmap to a Delta-configuration' with co-authors van Driel-Geszsetyi, Nitta, Canfield and Mickey which will be submitted soon, and which documents the emerging bipoles and general evolution of this active region as it crossed the solar disk Aug. 1992. I am currently writing a paper on the evidence for the emergence of twisted flux tubes and the global twist observed in this active region, and will be performing a quantitative analysis of the currents in this active region for the CDAW on AR7260 which is taking place in Hawaii early December, 1993.
(SPD abstract)+++++++++++++++++++++++++++++++++++++++++++++++++++++ -K D Leka, U. Hawaii
K.D. Leka (IfA/U. Hawaii), L. van Driel-Gesztelyi (Kiso Obs/U. Tokyo), \\ R.C. Canfield (IfA/U. Hawaii), B. Anwar (Indonesian Nat'l Inst. of Aeron. \& Space), \\ T.R. Metcalf (IfA/U. Hawaii), D.L. Mickey (IfA/U. Hawaii), and N. Nitta (LPARL) \\
We present evidence that the origin of shear and twist in an active region's magnetic flux tubes is not photospheric motions. Rather, we find that the magnetic flux which emerged in NOAA AR7260 over the period of 15-20 August 1993 was in a non-potential configuration at the time it appeared.
This active region rotated onto the disk as a mature bipole: a dominant negative-polarity spot with trailing plage and small scattered spots in attendance. For a period of seven days beginning 15 August 1992 the area displayed substantial evolution: at least fifteen bipoles emerged, increasing the magnetic flux by more than $10^{22}$Mx. The most substantial flux emergence occurred in the trailing plage of the large existing P spot, forming a complex $\beta\gamma\delta$ configuration. Other emerging flux regions were scattered and short-lived. Close to eighty flares with soft X-ray counterparts, including eight M-class flares, accompanied this growth.
To study the emerging flux in this region we utilized white-light and Soft X-ray (SXR) observations from the Yohkoh satellite, University of Hawaii Mees Observatory data, (specifically vector magnetograms from both the Stokes Polarimeter and the Imaging Vector Magnetograph instruments and Mees CCD \ha\ Imaging Spectroscopy observations), and finally \ha\ images from Hida Observatory. We analyzed the photospheric magnetic fields and sunspot proper motions, \ha\ fibril patters and arch filament systems and coronal images the new flux systesm as well as the older sunspot group in AR7260.
>From the data we conclude that a) the new flux tubes emerged into the solar photosphere, chromosphere and corona with an inherent counter-clockwise twist; b) the old flux system exhibits the same sense of twist; c) this non-potential configuration was generated by a more global and organized mechanism than photospheric motions can provide.
(SPO abstract) +++++++++++++++++++++++++++++++++++++++++++++++++
K.D. Leka (IfA/U. Hawaii), L. van Driel-Gesztelyi (Kiso Obs/U. Tokyo), \\ R.C. Canfield (IfA/U. Hawaii)
The large active region NOAA 7260 showed striking non-potential configurations when viewed in soft X-rays, H-alpha and photospheric magnetograms. Shear and twist were observable in both the large leading sunspot and in the many emerging flux regions which appeared as it transited the disk late August 1992. We investigate the question of whether photospheric motions generated this configuration, or if another source of the electric current must be considered.
To address this question we studied bipoles which emerged during times of good coverage in chromospheric, photospheric and coronal data, utilizing instruments at the University of Hawaii Mees Observatory and the Yohkoh Soft X-ray Telescope. By following a bipole as it emerged, we see that the current was present from the onset of the bipole's growth. We track the proper motion of the bipole's associated spots, and show that it is consistent with the emergence of a twisted flux tube from beneath the photospheric surface. In addition, we compare the vector magnetograms and soft x-ray structures and demonstrate that they are inconsistent with a current-free bipole from the beginning of its growth.
We find that the old flux system (leading sunspot and associated following spots and plage) exhibits the same sense of twist, such that negative magnetic field carried positive electric current. We also note that the currents observed are long-lived, easily identified from one day to the next unless the sunspot's identity is lost.
We conclude that a) new magnetic flux systems emerged into the solar photosphere, chromosphere and corona with an inherent counter-clockwise twist; b) the non-potential configuration observed in both new and old flux systems was generated by a more global and organized mechanism than photospheric motions can provide. We propose that the source of the electric currents observed is at least deep in the convective layers of the solar atmosphere, tens of thousands of kilometers below the surface.
This work will be submitted for publication by Leka et al. in late 1993.
Update 06-Apr-93
We would like to submit the following as a short indication of work in progress, as an update for earlier posting to the TeamBB: This work was presented at the recent YOHKOH science meeting, and will be written up as a detailed paper for publication later in spring, 1993. In addition, Leka will be carrying out a quantitative analysis of the Emerging Flux and Current systems in this region during summer, 1993.
K.D. Leka {1}, L. van Driel-Gesztelyi {2}, B. Anwar {3,4} R.C. Canfield {1}, T.R. Metcalf {1}, D.L. Mickey {1}, and N. Nitta {5}
{1} Institute for Astronomy, University of Hawaii, U.S.A. {2} Kiso Observatory, Institute of Astronomy, University of Tokyo, Japan {3} Kwasan and Hida Observatories, Kyoto University, Japan {4} Indonesian National Institute of Aeronautics and Space {5} Lockheed Palo Alto Research Laboratory, U.S.A.
For a period of 7 days beginning 15 August 1992, we studied the growth and evolution of a sunspot group which emerged embedded in the S polarity trailing plage of NOAA 7260. The data utilized include white-light and Soft X-ray (SXR) observations from the YOHKOH satellite, University of Hawaii Mees Observatory data, specifically vector magnetograms from both the Stokes Polarimeter and the Imaging Vector Magnetograph instruments and Mees CCD H-alpha Imaging Spectroscopy observations, and finally H-alpha images from Hida Observatory. Below we present a short description of the evidence which indicates that this Emerging Flux Region (EFR) possessed an inherent twist which was generated prior to its appearance in the solar photosphere, chromosphere and corona.
The photospheric horizontal magnetic field lines in the vector magnetograms reveal that a strongly sheared configuration existed between the opposite polarity spots of bipoles at an early age. The SXR images and H-alpha fibril pattern confirm not only the existence of this sheared and twisted configuration, but also its early appearance relative to any period of evolution that the EFR underwent. This suggests that the sheared (twisted) configuration was not formed after the emergence, but that these flux tubes rather _emerged_ with an inherent anticlockwise twist.
A second line of argument concerning the non-potential nature of these fluxtubes involves the position of the sunspots as they appeared on the solar photosphere, and the proper motions they displayed in the ensuing hours. All new bipoles but one (the first) were oriented in a NW-SE direction with an angle approximately 45 degrees to the solar equator. Note that sunspots appear at the intersection of a flux tube with the photosphere, which implies that the geometry of the emerging flux tube strongly influences the proper motions of young spots. It supports our argument, thus, that the proper motions of the young opposite polarity spots were not diametrically opposite, but rather perpendicular to each other, which suggests a non-planar flux tube geometry. The pattern of the proper motions as well as the orientation of the bipoles observed in AR 7260 are consistent with the idea that their flux tube emerged with an inherent anticlockwise twist which caused a kink of their flux tube.