Evaluation of a Selected Case of the Minimum Dissipative Rate Method for Non-Force-Free Solar Magnetic Field Extrapolation |
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G. Allen Gary Submitted: 2009-04-10 09:54
The minimum dissipative rate (MDR) method for deriving a coronal non-force-free
magnetic field solution is partially evaluated. These magnetic field solutions
employ a combination of three linear (constant α force-free-field solutions with
one being a potential field (i.e., α 0). We examine the particular case of the
solutions where the other two α s are of equal magnitude but of opposite signs.
This is motivated by studying the SOLIS vector magnetograms of AR 10987 which show
a global α value consistent with an α 0 value as evaluated by (Curl B)z/Bz
over the region.
Typical of the current state of the observing technology, there is no definitive twist
for input into the general MDR method. This suggests that the special α case, of two α s
with equal magnitudes and opposite signs, is appropriate given the data. Only for an
extensively twisted active region does a dominant, non-zero α normally emerge from a
distribution of local values. For a special set of conditions, we find: (i)
The resulting magnetic field is a vertically inflated magnetic field resulting from
the electric currents being parallel to the photosphere, similar to the results of
Gary and Alexander (1999). (ii) For α ~ α _max/2), the Lorentz force per unit volume
normalized by the square of the magnetic field is on the order of 1.4x10-10 cm-1.
The Lorentz force (L_F) is a factor of ten higher than that of the magnetic
force d(B^2/8pi)/dz, a component of L_F. The calculated photospheric electric current
densities are an order smaller than the maximum observed in all active regions.
Hence both the Lorentz force density and the generated electric current density
seem to be physically consistent with possible solar dynamics. The results imply
that the field could be inflated with an over pressure along the neutral line.
(iii) However, the implementation of this or any other extrapolation method
using the electric current density as a lower boundary condition must be done
cautiously, with the current magnetography.
Authors: G. Allen Gary
Projects: National Solar Observatory (Sac Peak)
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Publication Status: submitted
Last Modified: 2009-04-11 09:44
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Future Possibilities for Doppler and Magnetic Field Measurements in the Extended Solar Atmosphere |
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G. Allen Gary Submitted: 2008-07-08 08:44
For the first time, a vacuum ultraviolet telescope can be built to observe magnetic fields, plasma flows, and heating events in the Sun's atmosphere. These observations can provide key data for space weather models. The vacuum ultraviolet (VUV) region allows remote sensing of the upper levels of the solar atmosphere where the magnetic field dominates the physics. A VUV Fabry-Perot interferometer (FPI) will allow us to observe the magnetic
field, flows, and heating events in the mid-transition region (between the chromosphere and corona). Observations of this region are needed to directly probe the magnetic structure and activity at the base of the corona where the magnetic field is approximately force-free, i.e., where gas pressures are very small. This is a key element in developing accurate models of the Sun's dynamics for space weather. The specific region of interest is the 100 km thick transition region, between the chromosphere and the much hotter corona, which strongly emits at 155 nm from triply ionized carbon (CIV) at 100,000 K. This is best observed by an imaging interferometer that combines the best attributes of a spectrograph and an imager. We present the latest results from the NASA Marshall Space Flight Center (MSFC) FPI. The major elements of thetunable
CIV VUV FP interferometer are the 35mm MgF2 etalon plates with a plate finesse of F>25 at 155 nm, the pi-dielectric coatings, a Hansen mechanical mount in a pressurize canister, and the piezoelectric control system. The control system for the etalon is a capacitance-stabilized Hovemere Ltd. standard system. The special Cascade Optical Corporation reflectance coatings are 25 pi-multilayers of high-low refractive layers paired in phase. This CIV interferometer, when flown above Earth's atmosphere, will obtain narrow-passband images, magnetograms, and Dopplergrams of the transition region in the CIV 155 nm line at a rapid cadence. We recently measured the MSFC VUV FPI using the University of Toronto's fluoride excimer laser as a proxy for CIV 155 nm. The test demonstrated the first tunable interferometer with the passband required for a VUV filter magnetograph. The measured values have a full-width half-maximum (FWHM) passband of 10 pm, a free-spectral range (FSR) of 61pm, and a transmittance of 58% at 157 nm. The resulting VUV interferometer finesse is 5.9. With this success, we are developing an instrument suitable for a flight on an orbiting solar observatory. A description of the interferometer for this mission is described.
Authors: G. Allen Gary (CSPAR), John M. Davis (MSFC), and Edward A. West (MSFC)
Projects: None
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Publication Status: Submitted to Journal of Advances in Space Research (Elservier 2008)
Last Modified: 2008-09-23 20:57
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SOLAR CONFOCAL INTERFEROMETERS FOR SUB-PICOMETER-RESOLUTION SPECTRAL FILTERS |
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G. Allen Gary Submitted: 2007-03-07 06:20
The confocal Fabry-Perot interferometer allows sub-picometer spectral resolution of Fraunhofer line profiles.
Such high spectral resolution is needed to keep pace with the higher spatial resolution of the new set of large-aperture solar telescopes. The line-of-sight spatial resolution derived for line profile inversions would then track the improvements of the transverse spatial scale provided by the larger apertures. In particular, profile inversion allows improved velocity and magnetic field gradients to be determined independent of multiple line analysis using different energy levels and ions. The confocal interferometer's unique properties allow a simultaneous increase in both etendue and spectral power.The higher throughput for the interferometer provides significant decrease in the aperture, which is important in spaceflight considerations. We have constructed and tested two confocal interferometers. A slow-response thermal-controlled interferometer provides a stable system for laboratory investigation, while a piezoelectric interferometer provides a rapid response for solar observations. In this paper we provide design parameters, show construction details, and report on the laboratory test for these interferometers. The field of view versus aperture for confocal interferometers is comparedwith other types of spectral imaging filters. We propose a multiple etalon system for observing with these units using existing planar interferometers as pre-filters.The radiometry for these tests established that high spectral resolution profiles can be obtained with imaging confocal interferometers. These sub-picometer spectral data of the photosphere in both the visible and near-infrared can provide important height variation information. However, at the diffraction-limited spatial resolution of the telescope, the spectral data is photon starved due to the decreased spectral passband.
Authors: Gary, G. A., Pietraszewski, C., West, E. A. and Dines, T. C.
Projects: None
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Publication Status: A&A (in press)
Last Modified: 2007-03-07 09:45
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CIV Vacuum Ultraviolet Fabry-Perot Interferometer for Solar Research |
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G. Allen Gary Submitted: 2006-10-16 18:14
A tunable, high spectral resolution, high effective finesse, vacuum ultraviolet (VUV) Fabry-Perot interferometer (FPI) is described for obtaining narrow-passband images, magnetograms, and Dopplergrams of the transition region emission line of CIV (155nm). The integral parts of the CIV narrow passband filter package (2-10pm FWHM) consists of a multiple etalon system composed of a tunable interferometer that provides high-spectral resolution and a static low-spectral resolution interferometer that allows a large effective free spectral range. The prefilter for the interferometers is provided by a set of four mirrors with dielectric high-reflective coatings. A tunable interferometer, a VUV piezoelectric-control etalon, has undergone testing using the surrogate F2 eximer laser line at 157nm for the CIV line. We present the results of the tests with a description of the overall concept for a complete narrow-band CIV spectral filter. The fixed-gap interferometer of the filter is envisioned as being built using the coating-test MgF2 plates after being superpolished and recoated. The four-mirror prefilter would have dielectric multilayer Pi-stacks following the design concept used in the Ultraviolet Imager of NASA's Polar Spacecraft. A dual etalon system allows the effective free spectral range to be commensurate with the prefilter profile. With an additional etalon, a triple etalon system would allow a spectrographic resolution of 2pm. The basic strategy has been to combine the expertise of spaceflight etalon manufacturing with VUV coating technology to build a VUV FPI which combines the best attributes of imagers and spectrographs into a single compact instrument. Spectro-polarimetry observations of the transition region CIV emission will allow an increased understanding of the magnetic forces, mass motion, evolution, and energy release within the solar atmosphere at the base of the corona where mainly the magnetic field is approximately force-free. The 2D imaging of the full vector magnetic field at the height of maximum magnetic influence (minimum plasma beta) can be accomplished, albeit difficult, by measuring the Zeeman splitting of the CIV resonance pair. Our designs of multiple VUV FPIs are being developed to integrate within the future orbiting solar observatories to obtain the necessary CIV observations.
Authors: G. Allen Gary, Edward A. West, David Rees, Jack McKay, Maumer Zukic, and Peter Herman
Projects: None
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Publication Status: Accepted/Forthcoming
Last Modified: 2006-10-17 10:57
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