G. Allen Gary

Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5) × 106 K

The three-dimensional structure of solar active region NOAA 7986 observed on 1996 August 30 with the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) is analyzed. We develop a new method of dynamic stereoscopy to reconstruct the three-dimensional geometry of dynamically changing loops, which allows us to determine the orientation of the mean loop plane with respect to the line of sight, a prerequisite to correct properly for projection effects in three-dimensional loop models. With this method and the filter-ratio technique applied to EIT 171 and 195 A images we determine the three-dimensional coordinates [x(s), y(s), z(s)], the loop width w(s), the electron density ne(s), and the electron temperature Te(s) as a function of the loop length s for 30 loop segments. Fitting the loop densities with an exponential density model ne(h) we find that the mean of inferred scale height temperatures, Tλe=1.22 ± 0.23 MK, matches closely that of EIT filter-ratio temperatures, TEITe=1.21 ± 0.06 MK. We conclude that these cool and rather large-scale loops (with heights of h≈30-225 Mm) are in hydrostatic equilibrium. Most of the loops show no significant thickness variation w(s), but we measure for most of them a positive temperature gradient (dT/ds>0) across the first scale height above the footpoint. Based on these temperature gradients we find that the conductive loss rate is about 2 orders of magnitude smaller than the radiative loss rate, which is in strong contrast to hot active region loops seen in soft X-rays. We infer a mean radiative loss time of τrad≈40 minutes at the loop base. Because thermal conduction is negligible in these cool EUV loops, they are not in steady state, and radiative loss has entirely to be balanced by the heating function. A statistical heating model with recurrent heating events distributed along the entire loop can explain the observed temperature gradients if the mean recurrence time is 10 minutes. We computed also a potential field model (from SOHO/MDI magnetograms) and found a reasonable match with the traced EIT loops. With the magnetic field model we determined also the height dependence of the magnetic field B(h), the plasma parameter β(h), and the Alfven velocity vA(h). No correlation was found between the heating rate requirement EH0 and the magnetic field Bfoot at the loop footpoints.

Transformation of vector magnetograms and the problems associated with the effects of perspective and the azimuthal ambiguity

Off-center vector magnetograms which use all three components of the measured field provide the maximum information content from the photospheric field and can provide the most consistent potential field independent of the viewing angle by defining the normal component of the field. The required transformations of the magnetic field vector and the geometric mapping of the observed field in the image plane into the heliographic plane have been described. Here we discuss the total transformation of specific vector magnetograms to detail the problems and procedures that one should be aware of in analyzing observational magnetograms. The effect of the 180-deg ambiguity of the observed transverse field is considered as well as the effect of curvature of the photosphere. Specific results for active regions AR 2684 (23 September, 1980) and AR 4474 (26 April, 1984) from the Marshall Space Flight Center Vector Magnetograph are described which point to the need for the heliographic projection in determining the field structure of an active region.

Eruption of a Multiple-Turn Helical Magnetic Flux Tube in a Large Flare: Evidence for External and Internal Reconnection that Fits the Breakout Model of Solar Magnetic Eruptions

We present observations and an interpretation of a unique multiple-turn spiral flux tube eruption from active region 10030 on 2002 July 15. The TRACE C IV observations clearly show a flux tube that is helical and erupting from within a sheared magnetic field. These observations are interpreted in the context of the breakout model for magnetic field explosions. The initiation of the helix eruption, as determined by a linear backward extrapolation, starts 25 s after the peak of the flares strongest impulsive spike of microwave gyrosynchrotron radiation early in the flares explosive phase, implying that the sheared core field is not the site of the initial reconnection. Within the quadrupolar configuration of the active region, the external and internal reconnection sites are identified in each of two consecutive eruptive flares that produce a double coronal mass ejection (CME). The first external breakout reconnection apparently releases an underlying sheared core field and allows it to erupt, leading to internal reconnection in the wake of the erupting helix. This internal reconnection releases the helix and heats the two-ribbon flare. These events lead to the first CME and are followed by a second breakout that initiates a second and larger halo CME. The strong magnetic shear in the region is compatible with the observed rapid proper motion and evolution of the active region. The multiple-turn helix originates from above a sheared-field magnetic inversion line within a filament channel, and starts to erupt only after fast breakout reconnection has started. These observations are counter to the standard flare model and support the breakout model for eruptive flare initiation.

Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK

In this paper we study the three-dimensional structure of hot MK) loops in solar active (T e B 1.5¨2.5 region NOAA 7986, observed on 1996 August 30 with the Extreme-ultraviolet Imaging Telescope (EIT ) on board the Solar and Heliospheric Observatory (SOHO). This complements a —rst study (Paper I) on cooler MK) loops of the same active region, using the same method of Dynamic Stereo- (T e B 1.0¨1.5 scopy to reconstruct the three-dimensional geometry. We reconstruct the three-dimensional coordinates x(s), y(s), z(s), the density and temperature pro—le of 35 individual loop segments (as a function n e (s), T e (s) of the loop coordinate s) using EIT 195 and 284 images. The major —ndings are as follows. (1) All Ae loops are found to be in hydrostatic equilibrium, in the entire temperature regime of MK. T e \ 1.0¨2.5 (2) The analyzed loops have a height of 2¨3 scale heights, and thus only segments extending over about one vertical scale height have sufficient emission measure contrast for detection. (3) The temperature gra- dient over the lowest scale height is of order dT /ds B 1¨ 10 Kk m~1. (4) The radiative loss rate is found to exceed the conductive loss rate by about two orders or magnitude in the coronal loop segments, implying that the loops cannot be in quasi-static equilibrium, since standard steady-state loop models show that radiative and conductive losses are comparable. (5) A steady state could only be maintained if the heating rate matches exactly the radiative loss rate in hydrostatic equilibrium, requiring a heat E H deposition length of the half density scale height j. (6) We —nd a correlation of p P L~1 between loop j H base pressure and loop length, which is not consistent with the scaling law predicted from steady-state models of large-scale loops. All observational —ndings indicate consistently that the energy balance of the observed EUV loops cannot be described by steady-state models. Subject headings: Sun: activitySun: coronaSun: magnetic —eldsSun: UV radiation ¨ techniques: image processing

Oriented connectivity-based method for segmenting solar loops

A method based on oriented connectivity that can automatically segment arc-like structures (solar loops) from intensity images of the Suns corona is introduced. The method is a constructive approach that uses model-guided processing to enable extraction of credible loop structures. Since the solar loops are vestiges of the solar magnetic field, the model-guided processing exploits external estimates of this fields local orientations that are derived from a physical magnetic field model. Empirical studies of the methods effectiveness are also presented. The oriented connectivity-based method is the first automatic method for the segmentation of solar loops.

Off disk-center potential field calculations using vector magnetograms

We investigate a potential field calculation for off disk-center vector magnetograms that uses all the three components of the measured field. There is neither any need for interpolation of grid points between the image plane and the heliographic plane nor for an extension or a truncation to a heliographic rectangle. Hence, the method provides the maximum information content from the photospheric field as well as the most consistent potential field independent of the viewing angle. The introduction of polarimetric noise produces a less tolerant extrapolation procedure than using the line-of-sight extrapolation, but the resultant standard deviation is still small enough for the practical utility of this method.

Instrumentation for the Advanced Technology Solar Telescope

The 4-m aperture Advanced Technology Solar Telescope (ATST) is the next generation ground based solar telescope. In this paper we provide an overview of the ATST post-focus instrumentation. The majority of ATST instrumentation is located in an instrument Coude lab facility, where a rotating platform provides image de-rotation. A high order adaptive optics system delivers a corrected beam to the Coude lab facility. Alternatively, instruments can be mounted at Nasmyth or a small Gregorian area. For example, instruments for observing the faint corona preferably will be mounted at Nasmyth focus where maximum throughput is achieved. In addition, the Nasmyth focus has minimum telescope polarization and minimum stray light. We describe the set of first generation instruments, which include a Visible-Light Broadband Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters, Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter & Spectrometer and a UV-Polarimeter. We also discuss unique and efficient approaches to the ATST instrumentation, which builds on the use of common components such as detector systems, polarimetry packages and various opto-mechanical components.

The effects of viewing angle on the inference of magnetic shear in preflare active regions

The magnetic shear at a point within an active region field configuration can be defined (Hagyard et al., 1984b) as the difference in angle between the observed photospheric transverse field and that of a reference potential field calculated using the observed line-of-sight field as a boundary condition. Using analytic models for non-potential (but force-free) fields representative of preflaring active regions, we calculate the degree of magnetic shear along the magnetic neutral line that such fields would exhibit, as a function of the location and orientation of the active region on the solar disk. We find that, except for regions close to disk center, the position of the inferred neutral line (zero line-of-sight field) is significantly different from the actual neutral line (zero radial field), and that the calculated reference potential field also varies significantly with the position of the region. Thus the inferred degree of shear can vary significantly with the position and orientation of the region, due to (a) straightforward geometric projection effects, (b) the shift of the inferred neutral line relative to its true position, and (c) variations in the reference potential field. The significance of these results for flare prediction is considered.

Response to “Comment on ‘Resolving the 180° Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions’ ”

We address points recently discussed in Georgoulis (2011, Solar Phys., doi:10.1007/s11207-011-9819-1) in reference to Leka et al. (2009b, Solar Phys.260, 83). Most importantly, we find that the results of Georgoulis (2011) support a conclusion of Leka et al. (2009b): that limited spatial resolution and the presence of unresolved magnetic structures can challenge ambiguity-resolution algorithms. Moreover, the findings of both Metcalf et al. (2006, Solar Phys.237, 267) and Leka et al. (2009b) are confirmed in Georgoulis (2011): a method’s performance can be diminished when the observed field fails to conform to that method’s assumptions. The implication of boundaries in models of solar magnetic structures is discussed; we confirm that the distribution of the field components in the model used in Leka et al. (2009b) is closer to what is observed on the Sun than what is proposed in Georgoulis (2011). It is also shown that method does matter with regards to simulating limited spatial resolution and avoiding an inadvertent introduction of bias. Finally, the assignment of categories to data-analysis algorithms is revisited; we argue that assignments are only useful and elucidating when used appropriately.

RENDERING THREE-DIMENSIONAL SOLAR CORONAL STRUCTURES

An X-ray or EUV image of the corona or chromosphere is a 2D representation of an extended 3D complex for which a general inversion process is impossible. A specific model must be incorporated in order to understand the full 3D structure. We approach this problem by modeling a set of optically-thin 3D plasma flux tubes which we render these as synthetic images. The resulting images allow the interpretation of the X-ray/EUV observations to obtain information on (1) the 3D structure of X-ray images, i.e., the geometric structure of the flux tubes, and on (2) the internal structure using specific plasma characteristics, i.e., the physical structure of the flux tubes. The data-analysis technique uses magnetograms to characterize photospheric magnetic fields and extrapolation techniques to form the field lines. Using a new set of software tools, we have generated 3D flux tube structures around these field lines and integrated the plasma emission along the line of sight to obtain a rendered image. A set of individual flux-tube images is selected by a non-negative least-squares technique to provide a match with an observed X-ray image. The scheme minimizes the squares of the differences between the synthesized image and the observed image with a non-negative constraint on the coefficients of the brightness of the individual flux-tube loops. The derived images are used to determine the specific photospheric foot points and physical data, i.e., scaling laws for densities and loop lengths. The development has led to computer efficient integration and display software that is compatible for comparison with observations (e.g., Yohkoh SXT data, NIXT, or EIT). This analysis is important in determining directly the magnetic field configuration, which provides the structure of coronal loops, and indirectly the electric currents or waves, which provide the energy for the heating of the plasma. We have used very simple assumptions (i.e., potential magnetic fields and isothermal corona) to provide an initial test of the techniques before complex models are introduced. We have separated the physical and geometric contributions of the emission for a set of flux tubes and concentrated, in this initial study, on the geometric contributions by making approximations to the physical contributions. The initial results are consistent with the scaling laws derived from the Yohkoh SXT data.