SH53A-1046
The Structure and Dynamics of the Quiet Corona from Observations with the Extreme ultraviolet Imaging Spectrometer
The goal of the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode satellite is to measure such physical parameters as the velocity and density of the solar corona in order to provide an observational basis to understand how coronal plasmas are heated and accelerated. On 2007 January 20, EIS performed a raster of a 128 x 512 arc-sec. area of a quiet region near Sun center. The observing program recorded spectra of He II λ256, formed at 9 × 104 K, and lines of Fe VIII-XV, formed at temperatures spanning the range from 5 × 105 through 2 × 106 K. Maps of intensities, velocities and electron densities derived from these observations are presented and discussed. Intensity maps in He II λ256 show the chromospheric network. Line intensities of Fe X-XIV show small-scale bright points and more extended structures. The intensity map of Fe VIII shows a transition between the two temperatures. The coronal lines reveal regions of high outflow velocities on the order of 100 km s-1 in a compact region and 12 km s-1 in an extended region. The presence of these high velocities in the quiet corona is an entirely new and unexpected result. Electron densities derived from density sensitive line ratios of Fe XII and XIII are typically about 3 - 20×108 cm-3. The highest densities are found in bright, compact areas. For the first time, explosive events in the quiet sun have been observed in the extreme-ultraviolet in He II λ256 profiles and have properties similar to those previously reported.
SH53A-1047
Are EIT waves slow mode blast waves?
We describe properties and characteristics of EIT waves and dimmings as observed by SOHO, and we show that they better correspond to propagating waves than to any diffusion process. We show that they have clearly pronounced similarities with the blast waves. There is a density and temperature increase around leading edge of the wave front and decrease behind. Presumably, they propagate from the region where the magnetic field decreases due to reconnection. However the velocities of these waves are typically small with respect to the Alfven speed. Blast waves are commonly associated with explosions or rapid energy releases. They were intensively studied theoretically and self-similar solutions of equations were found for sound waves without a magnetic field and for fast magnetosonic waves. We show that similar types of solutions exist for slow mode waves when they propagate obliquely to the magnetic field. Several observational features argue in favour of the hypothesis that EIT waves are slow mode blast waves.
SH53A-1048
Non-Linear Force-Free Modeling of AR NOAA 10930 Based on Vector Magnetogram Observation with Hinode/SOT
Since the detection of three-dimensional (3D) magnetic field in active regions is crucially important to understand the trigger mechanism of solar flares, the methodologies to reconstruct the 3D magnetic field from magnetgram observation is quickly developed recently. In this study, we have developed a new Non-linear Force-Free (NLFF) field extrapolation method, based on the extended magnetofrictional model and the divergence field cleaning technique, and applied it onto the magnetograms of Active Region NOAA10930, which were observed by Hinode/SOT. The data covers the time span before and after the X-class flare arising on December 13, 2006 in this active region, and Hinode satellite clearly observed the typical features of flare; two ribbon and post flare loop structures by SOT, and sigmoidal structure before the onset and the cusp loop structures after that by XRT. As a result of the NLFF extrapolation, we found that, before the onset of flare, strong sheared structures were formed on the neutral line, whereas the field overlying magnetic neutral line was potential-like. Furthermore, we revealed that a sigmoid structure was not formed of a single sheared loop, but was composed of strong multiple sheared fields. It is also detected that, after the flare, elongated magnetic flux is partially formed but a part of magnetic shear was released. The results indicate that the NLFF relaxes towards the potential field on average during the flaring phase, although some sigmoidal structure remains. Also, the long term evolution of the active region and the storage process of magnetic energy and helicity in the active region are investigated.
SH53A-1049
Radiometric Calibration of EUNIS-06 With Theoretical Predicted `Insensitive' Line Ratios
The Extreme-Ultraviolet Normal-Incidence Spectrograph (EUNIS) is a sounding-rocket payload that obtains imaged high-resolution spectra of solar active and quiet-Sun regions, providing information about the corona and upper transition region. EUNIS incorporates two independent, co-pointing imaging spectrographs, one covering EUV lines between 300 and 370 Å\ seen in first order (the longwave [LW] channel), and a second covering lines between 170 and 205 Å\ seen in second order (the shortwave [SW] channel). Shortly after the payload's initial successful flight on 2006 April 12, a complete end-to-end radiometric calibration of its LW bandpass was carried out at the Rutherford Appleton Laboratory in England. Here we develop and apply a technique for deriving the absolute radiometric calibration of its SW bandpass from these direct LW results by means of density- and temperature-insensitive line intensity ratios. The first step is to use the EUNIS LW calibration to get absolute intensities for EUV lines recorded from solar positions along its LW slit during the 2006 flight. Then co-registered SOHO/CDS images taken within minutes of the flight are used to transfer these absolute values to solar locations observed by the EUNIS SW slit, spatially offset by about 1 arcmin. Finally, theoretical `insensitive' line ratios obtained from CHIANTI allow us to determine absolute intensities of emission lines within the EUNIS SW bandpass from those observed in its LW channel. A total of 29 ratios composed of 11 LW and 15 SW emission lines from Fe~{\small X} - Fe~{\small XIII} yield an instrumental response curve that matches very well to a relative calibration which relied on combining measurements of individual optical components. The second EUNIS flight, now scheduled for 2007 October 30, will make coordinated observations and provide similar calibration updates for Hinode/EIS. We will also present some preliminary results from the new observations. EUNIS is supported by the NASA Heliophysics Division through its Low Cost Access to Space Program in Solar and Heliospheric Physics.
SH53A-1050
Cross calibration of soft X-ray telescopes between Hinode/XRT and GOES13/SXI
The X-Ray Telescope (XRT) aboard Hinode satellite is a grazing incidence telescope to observe all the coronal features with a wide temperature range from less than 1MK to more than 10MK. And the XRT has 9 X-ray analysis filters which are optimized to observed the almost whole coronal plasma and to derived the coronal temperature distribution. Meanwhile, the GOES13 satellite carries a Solar X-ray Imager (SXI) to monitor the solar X-rays. The SXI is also a grazing incidence telescope and has 7 X-ray filters. The XRT and SXI are similar telescopes to observe the dynamic solar corona. On 24 Nov 2006, the XRT and SXI-team performed the simultaneous observation for the cross calibration between XRT and SXI. In this study, we analyzed this data set and checked the actual characteristics of each X-ray analysis filter.
SH53A-1051
EUV and UV Irradiance Variations and Evolution of Magnetic Fields
Continuous solar EUV measurements started by the SEM experiment on SOHO in late 1995 providing high cadence data in a broad-band channel (0.1 -- 50 nm) and in the vicinity of the ionospherically important 30.4 nm line. In addition to the SEM experiment, the "EUV Grating Spectrometer (EGS)" instrument on the "Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED)" mission has been providing measurements since December 2001 in the 0 -- 200 nm spectral range, including the soft X-ray (XUV) from 0 to 30 nm, the extreme ultraviolet (EUV) from 0 to 120 nm, and the far ultraviolet (FUV) from 120 to 200 nm. In this paper we show results on the observed EUV and UV irradiance variations related to the changing solar magnetic fields, using the physical parameters (area, intensity, and magnetic field strength) of various solar magnetic structures derived from the analysis of the "Michelson Doppler Imager (MDI)" on SOHO. We show the association between XUV and EUV variations and magnetic structures on various time scales:over the solar cycle, on rotational time scales, and on time scales of minutes to hours.
SH53A-1052
The Fe XIV Corona Approaching Solar Minimum
Although the solar disk is often devoid of sunspots around solar minimum, the upper atmosphere never rests. The most prominent features approaching the minimum of coronal activity are dense regions overlying mid-latitude boundaries of the large scale magnetic field. Seen above the limb, these features typically give the impression of "ears" (and "jowls"). Samples of these features will be shown, and the relationship to the "extended solar cycle" running from near the poles to the equator over 18 years will be discussed. http://nsosp.nso.edu/data/corona.html
SH53A-1053
Numerical Simulations of Power Law Heating Functions for Quiescent Loops: Stability and Observables
We present the numerical simulations of quiescent coronal loops with heating functions that are power law functions of pressure and temperature. These simulations are made using a time-dependent, 1D hydrodynamics code with heating functions that are treated as dynamic variables which are constantly re- evaluated during the loops' lifetimes. These numerical simulations provide a stability test for the analytical solutions formulated by Martens (2007, submitted) for the same heating functions. TRACE and XRT datasets are simulated to determine if present observables can provide adequate information to discriminate between power law heating functions.
SH53A-1054
Flare Loop Top Sources: A diagnostic for non-thermal particle injection
X-ray emitting sources have often been observed at the apex of post-flare loops by the Hard X-Ray telescope (HXT) onboard the Japanese Yohkoh satellite and by NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). In order to investigate the physics behind the generation of these loop top sources we have coupled a 1D, time-dependent, hydro code with a non-thermal particle transport code. The hydro code provides temperature and density information of the target plasma to the particle code and the particle code provides the dynamic heating of the plasma to the hydro code. Previous work with a combined has shown that it is possible to create flare loop top emission sources under certain conditions. In this work we test the relationship between the pitch angle distribution of the injected non-thermal particles and the generation of loop top emission source. RHESSI, HXT and XRT datasets are synthesized from the simulations to provide a direct link between theoretical work and observables. This work is supported by NASA grant NAG5-12820.
SH53A-1055
What is There Before a Flare?
The physical parameters in a coronal volume prior to the occurrence of a flare are generally unknown, but may play an important role in identifying the processes involved in flaring or eruption. Now we have observations from Hinode at very high resolution that can provide the best possible values for preflare temperature and density, for example. We make use of Hinode XRT observations of the preflare magnetic structure with the same footpoint locations as a flaring loop, as identified in RHESSI images. We additionally introduce a method based on conductive equilibrium (RTV scaling) to reduce the uncertainty on estimates of physical parameters due to lack of knowledge of the detailed geometry. Preliminary results are consistent with the finding at lower resolution from Yohkoh: in a majority of cases, the preflare conditions are not observable in soft X-rays. We discuss the upper limits that result, which point to low temperatures, densities, and plasma beta values, but high Alfven speeds. We hope also to be able to extend this conclusion with EIS observations.
SH53A-1056
3D simulations of damping of waves in a loop anchored in a dipole active region: does resonant absorption take place in realistic loops?
We present numerical results of three dimensional MHD model of an idealized active region field. The active region is initialized as a force-free dipole magnetic configuration with gravitationally stratified density and contains a loop with a higher density than its surroundings. This study represents an extension to the model of McLaughlin & Ofman (2007). We examine the impact of different density profiles of the loop on damping of kink waves by introducing a velocity pulse which models the impact of a flare on surrounding fields. We compare our results with a straight cylinder model of the loop which can be solved analytically. We study the resulting loop oscillations and compare our results with TRACE observations.
SH53A-1057
Comparing flare temperature distributions from RHESSI and from Hinode
Using flares that were observed by both RHESSI and Hinode (XRT and EIS), as well as available data from STEREO, TRACE and SOHO's EIT, we compare the spatial and temporal temperature distributions of flaring plasma in the X-ray and EUV regimes.
SH53A-1058
The Magnetic Structure of X-Ray Bright Points
The magnetic structure of X-Ray Bright Points (XBPs) observed by Hinode, SOLIS and GONG are well represented by potential field extrapolation models. SOLIS observations are used to compare the magnetic field model with X-ray observations. GONG observation allow us to follow the evolution of the magnetic structures that form the XBPs. Detailed examination of the nulls in the corona around the bright points will presented and discussed.
SH53A-1059
Non-thermal Motions in Solar Active Regions
Optically thin ultraviolet, extreme-ultraviolet (EUV), and X-ray spectral lines emitted by ions in the solar transition region and corona usually have Gaussian widths in excess of the ion temperature thermal Doppler width expected assuming the ions are formed in ionization equilibrium. Also, because of the densities in the lower solar corona and transition region, the electron temperature is assumed to be equal to the ion temperature. The excess line widths do not produce asymmetries in the line profiles and are interpreted as a non-thermal random Doppler motion in the plasma producing the lines. First discovered in 1975, these mysterious motions are still unexplained. They may be a signature of wave propagation, magnetic reconnection, random unresolved bulk flows, or some other as yet unspecified physical mechanism. Up to the present, it has not been possible to relate these motions spatially to coronal structures in active regions in considerable detail. However, the recent launch of the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode spacecraft has made it now possible to establish detailed relationships. We analyze EUV coronal (i.e., lines from ions such as Fe XII) spectral lines obtained from EIS spectra of two active regions and discuss the relationship of the non-thermal line widths to the temperatures, densities, and positions in the active region loops and inter-loop regions. We also discuss the relationship of the line widths to line-of-sight flows in the active regions as determined from the centroid wavelengths of the spectral lines.
SH53A-1060
Global, 4D Differential Emission Measure Analysis of EIT 17.1, 19.5 and 28.4 nm Images
We present for the first time the results of a method that combines 3D tomography and differential emission measure (DEM) analysis to determine the 3D local differential measure (LDEM), which is a measure of the amount of plasma as a function of electron temperature within each volume element of the computation grid. The volume elements are (3 deg X 3 deg X 0.02 Rs). The input data are a time series of EUV images taken in the 17.1, 19.5 and 28.4 nm bands. The method, developed theoretically in a previous paper [Frazin et al. 2005, ApJ v. 628, p. 1070], involves a combination of solar rotational tomography (SRT) and classical differential emission measure (DEM) analysis. SRT uses solar rotation to "undo" the line-of-sight integrals, while DEM analysis determines the temperature distribution (LDEM) in each voxel. Temporal variations of the solar corona limit the applicability of SRT to structures that remain relatively stable on the two-week time scale. We show results for certain structures that were judged to be stable by watching the EIT movies. We anticipate dramatic increases in the temperature resolution of this technique with the XRT instrument.
SH53A-1061
Small-scale X-ray/EUV Jets seen in Hinode XRT and TRACE
In this study, we present the morphological and kinematic characteristics of three small-scale X-ray/EUV jets that are simultaneously observed by the X-ray telescope (XRT) onboard Hinode (Solar-B) and the TRACE. For this study, we examined all XRT movies with a thin Al/Poly filter from 2006 October 20 to April 8 to look for small-scale X-ray eruptions and then found 34 eruptions. Next we took a look at the corresponding TRACE 171/195 {Å} images associated with these X-ray eruptions. As a result, we found three simultaneous X-ray and EUV jets and their major characteristics can be summarized as follows : (1) They all have no association with major flares. (2) From the comparison between XRT and TRACE observations, we found that they have similar characteristics in terms of projected speed, lifetime, and size. (3) Their sizes range from 4 × 105 to 5 × 105 km. (4) Their projected speeds are estimated to be 180-310 km s-1 with an average speed of about 250 km s- 1 (5) Their lifetimes lie in the range of 100 to 600 s. (6) From the comparison between the XRT images with the SOHO MDI maps for two events, all they are associated with the parasitic polarity region. These characteristics are similar to the previous observations for X-ray jets by the Yohkoh SXT but quite different from EUV jets associated with Hα surges. In addition, from the EIS four spectral lines for the last event, we found blueshift (up to -64 km s-1) and redshift (up to 20 km s-1) motions as well as nonthermal velocities ranging from 57 to 106 km s-1 at the jet footpoint. We note a tendency : the hotter the maximum ionization temperature is, the larger the area of blueshift region is. These characteristics are consistent with a typical jet model that includes the magnetic reconnection between open coronal fields and emerging magnetic fields.
SH53A-1062
Spectral Hardening of Large Solar Flares
We present quantitative studies of the hard X-ray spectral evolution of large solar flares featuring hardening trends, using RHESSI high cadence spectroscopy observations. The temporal evolution of the spectra is compared with the configuration and motion of the hard X-ray sources in RHESSI images. Both soft-hard-soft (impulsive) phases and hardening (gradual) phases are observed during the events and are well described by piecewise linear dependence of the spectral index on the logarithm of the flux. In particular, we investigate whether two different acceleration mechanisms are responsible for the impulsive and gradual phases, finding evidence that points toward a single acceleration mechanism acting in the two phases, rather than two different separated mechanisms, because the impulsive and gradual phases are closely interconnected in time and space.
SH53A-1063
The Connection Between Alfven Waves and the FIP Effect
The FIP (First Ionization Potential) Effect is the by now well known element abundance anomaly in the corona, whereby elements that have FIP lower than about 10 eV (i.e. those that are ionized in large portions of the chromosphere) have their abundance enhanced by a factor typically 3-4 with respect to high FIP elements, the abundances of which are essentially unchanged. Laming (2004) proposed that this abundance modification is due to the action of the ponderomotive force in the chromosphere, arising as Alfven waves propagate through a region where the Alfven speed increases with height. I this talk I will describe some new results, based on a non-WKB treatment of chromospheric Alfven waves. This will include the possible differences in FIP fractionation due to Alfven waves originating as sound waves at the base of the chromosphere, or impinging on the top of the chromosphere from a coronal source, and a consideration of the possible effect on abundances of the generation of Alfven waves at the β = 1 region in the low chromosphere. This work is supported by NASA SEC GI Grant NNG05HL39I, and by basic research funds of the Office of Naval Research.
SH53A-1064
Magnetic and Doppler Observations of the Photosphere and Low Chromosphere with the Solar Optical Telescope on Hinode
We present magnetic field and Doppler shift measurements in the Mg I b line at 517.3 nm obtained with the Solar Optical Telescope's Narrowband Filter Imager on Hinode. The line core forms in the low chromosphere, and the wings where the measurements are made probably form around the temperature minimum. Stokes IQUV images in the red and blue wings are combined to make movies of longitudinal magnetograms and Dopplergrams. The direction of the transverse field component is also measurable in strong field concentrations. These are compared with very accurate photospheric magnetic measurements in Fe I 630.2 nm from the Spectro-Polarimeter. This comparison calibrates the filter longitudinal magnetograms in flux density and shows changes in field geometry with height. The Doppler measurements are calibrated using wavelength scans through the Mg line profile. A number of emerging and canceling magnetic features were observed in AR 10961 during its disk passage in late June and early July. Since these were made during the Hinode eclipse season, the observations have somewhat lower spatial resolution than usual (0.32 arcsecond pixels), but the uniformity and sensitivity are excellent. Intermittent upflows seen between canceling magnetic features are interpreted in terms of reconnection outflows. Strong, persistent downflows are seen adjacent to but not on flux concentrations outside of sunspots and pores. In the sunspot, running penumbral waves are clearly visible, and steady downflows are observed over the light bridges. Hinode is a Japanese mission developed and launched by ISAS/ JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in cooperation with ESA and NSC (Norway).
SH53A-1065
Hinode SOT observations of plume upflows and cascading downflows in quiescent solar prominences
We present several Hinode SOT filtergram movies of quiescent solar prominences that show newly discovered "plume-like" upflows and cascading "waterfall-like" downflows that persist for the entire multi-hour duration of the observations. The flow speeds are on the order of 10 km/sec with typical widths of 400-700 km. Preliminary calculations show that if the upflows are buoyancy driven, the associated thermal perturbation is on the order of 10,000 K, sufficient to explain the dark appearance of the upflows in the interference filter passbands. In addition we observe rotational vortices and body oscillations within the prominences. These new observations challenge current magnetostatic models of solar prominences by showing that prominence plasmas are in constant motion, often in directions perpendicular to the magnetic field lines proposed by the models. TRACE, Hinode/EIS, and Hinode/XRT observations are used to investigate the differential topology of the flows across temperature regimes.
SH53A-1066
Feature Tracking of Hinode Magnetograms
We present results of applying feature tracking to a sequence of Hinode magnetograms. The single line wing Na D 5896 magnetograms have a high signal-to-noise ratio, allowing the detection of flux approximately 30 times weaker than in MDI magnetograms. We find evidence that, even with Hinode's improved resolution and sensitivity, we do not always detect the bipolar emergence of new magnetic flux. This suggests that we have not reached the ultimate resolution to observe the fundamental flux generation processes in the photosphere.
SH53A-1067
Solar He II 30.4 nm Pulsation Frequency Spectrum
We have recently found evidence for solar He II 30.4 nm pulsations from a Fourier transform study of CELIAS/SEM 26-34 nm EUV data. We have now calculated the frequency spectrum of the He II 30.4 nm pulsations during selected days in the solar cycle 23. We will present the CELIAS/SEM EUV flux data and the amplitudes, the frequencies and the phases of the pulsations. The frequencies of the observed pulsations are found to be less than the Brunt-Vaissala frequency. Various pulsation parameters obtained during different phases of the solar cycle will be compared.
SH53A-1068
GONG Synoptic Magnetograms and Coronal Magnetic Field Modeling
Line-of-sight photospheric magnetograms are produced every minute at GONG's six sites. All modulators and driving circuitry have recently been replaced, improving the sensitivity, accuracy and zero point by orders of magnitude. Information on the solar atmospheric field can most reliably be derived from such photospheric data, from which model coronal fields are then extrapolated. Near-real-time synoptic magnetogram are produced by GONG every hour. Potential-field source-surface (PFSS) models are then produced every hour yielding insight into large-scale coronal field changes caused by quasi-static evolution and by flares and coronal mass ejections. We also provide a synoptic map and model summarizing each integral Carrington rotation. Features highlighted in the plots and movies include coronal holes, the streamer belt, magnetic flux open to the ecliptic plane and large-scale changes in the field topology. GONG is the official provider of magnetograms for NASA's STEREO mission.
SH53A-1069
Conical Current Sheets in a Source-Surface Model of the Heliosphere
Different methods of modeling the coronal and heliospheric magnetic field are conveniently visualized and intercompared by applying them to ideally axisymmetric field models. Thus, for example, a dipolar B field with its moment parallel to the Sun's rotation axis leads to a flat heliospheric current sheet. More general solar B fields (still axisymmetric about the solar rotation axis for simplicity) typically lead to cone-shaped current sheets beyond the source surface (and presumably also in MHD models). As in the dipolar case [Schulz et al., Solar Phys., 60, 83-104, 1978], such conical current sheets can be made realistically thin by taking the source surface to be non-spherical in a way that reflects the underlying structure of the Sun's main B field. A source surface that seems to work well in this respect [Schulz, Ann. Geophysicae, 15, 1379-1387, 1997] is a surface of constant F = (1/r)kB, where B is the scalar strength of the Sun's main magnetic field and k (~ 1.4) is a shape parameter. This construction tends to flatten the source surface in regions where B is relatively weak. Thus, for example, the source surface for a dipolar B field is shaped somewhat like a Rugby football, whereas the source surface for an axisymmetric quadrupolar B field is similarly elongated but somewhat flattened (as if stuffed into a cone) at mid-latitudes. A linear combination of co-axial dipolar and quadrupolar B fields generates a somewhat pear-shaped (but still convex) source surface. If the region surrounded by the source surface is regarded as current-free, then the source surface itself should be (as nearly as possible) an equipotential surface for the corresponding magnetic scalar potential (expanded, for example, in spherical harmonics). The solar wind should then flow not quite radially, but rather in a straight line along the outward normal to the source surface, and the heliospheric B field should follow a corresponding generalization of Parker's spiral [Levine et al., Solar Phys., 77, 363-392, 1982]. In particular, heliospheric current sheets (of which there are two if the underlying solar B field is mainly quadrupolar) should emanate from neutral lines on the corresponding source surface. However, because the source surface is relatively flattened in regions where such neutral lines tend to appear, the radial component of the heliospheric B field at r ~} 1 AU and beyond is much more nearly latitude-independent in absolute value than one would expect from models based on a spherical source surface.
SH53A-1070
The COronal Solar Magnetism Observatory
Measurements of coronal and chromospheric magnetic fields are arguably the most important observables required for advances in our understanding of the emergence of magnetic flux into the solar atmosphere and the processes responsible for the production of solar activity, coronal heating and coronal dynamics. The COronal Solar Magnetism Observatory (COSMO) is a proposed ground-based suite of instruments designed for routine study of coronal and chromospheric magnetic fields and their environment. The facility consists of 3 instruments: 1) a meter-class aperture coronal magnetometer devoted to obtaining the highest quality polarimetric data of forbidden lines of Fe XIII 1074.7 and 1079.8 nm.; 2) a chromosphere and prominence magnetometer devoted primarily to measurements of lines of helium (D3, 1083 nm) and perhaps Halpha, that will provide full disk vector magnetic field observations; 3) a white-light polarized-brightness (pB) coronagraph that will observe down to 1.05 solar radii at very high time cadence (15 seconds) at high signal-to-noise. This new facility will be operated by the High Altitude Observatory of the National Center for Atmospheric Research (HAO/NCAR) in collaboration with the University of Hawaii and the University of Michigan. COSMO will enhance the value of existing and new observatories on the ground (SOLIS, ATST, and FASR) and in space (SOHO, TRACE, GOES, SOLAR-B, STEREO, SDO) by providing unique and crucial observations of the global coronal and chromospheric magnetic field and its evolution. http://www.cosmo.ucar.edu
SH53A-1071
MHD Stability and Damped Oscillations of Coronal Loops
Magnetohydrodynamic (MHD) stability (kink and sausage modes) and damped oscillations of coronal loops are investigated. A coronal loop is treated as a thin toroidal flux rope with two stationary photospheric footpoints, carrying both toroidal and poloidal currents. The forces and the flux-rope dynamics are described within the framework of ideal MHD. The main features of the theory are (1) oscillatory motions are determined by the Lorentz force that acts on curved current-carrying plasma structures and (2) damping is caused by drag that provides the momentum coupling between the flux rope and the ambient coronal plasma. The oscillation is restricted to the vertical plane of the flux rope. The kink- and sausage-mode properties of the initial equilibrium flux rope are investigated. The initial flux rope is set into oscillation by a pulse of upflow of the ambient plasma. The theory is applied to oscillating loops observed by Transition Region and Coronal Explorer (TRACE). It is shown that the component of the ambient magnetic field parallel to the flux rope plays a key role in the stability and that the Lorentz force and drag with a reasonable value of the coupling coefficient (cd) and without anomalous dissipation are able to accurately account for the observed damped oscillations. The analysis shows that the variations in the observed intensity can be explained by the minor radial expansion and contraction. The values of the drag coefficient required to produce the observed damping times are in the range cd ≈~2 - - 5, consistent with a previous MHD simulation study and with values used to reproduce the observed trajectories of coronal mass ejections (CMEs). \medskip Work supported by ONR and NASA
SH53A-1072
An MHD simulation model of the global solar corona with the time-varying boundary magnetic field driven with the current at the lower corona
A time-dependent three-dimensional MHD simulation model to treat the response of the solar corona to the temporal variations of the global solar photospheric magnetic field is proposed. In order to avoid the computational difficulties in calculating the vector quantities of the magnetic field and plasma motion on the boundary surface fully matching the measured variations, we assumed the shallow spherical layer at the lowermost corona for which the "differential potential field" is calculated and superimposed to the existing corona. The differential potential field uses differential synoptic map that is calculated by subtracting two successive synoptic maps of the solar photospheric magnetic field measurement data. The radial component of the differential field at the upper sphere set at 1.1 Rs sphere is zero. The differential potential field confined in a shallow spherical layer can be regarded as the proxy of the global variation of the coronal magnetic field during one Carrington rotation period of about 27 days. By gradually adding the differential potential field to the existing numerical coronal magnetic field, we can simulate the continuous coronal variations in the global scale. The potential field is calculated by the spherical harmonics up to 5th term, which corresponds to about 10-degree spatial resolution to treat only the global scale and to neglect the small-scale quick surface variations. This model assumes the strong current at the upper sphere of the shallow spherical layer that we here chose at 1.1 Rs, which may not represent the real corona. It is, however, beneficial to examine a lot of new features numerically obtained, such as the twisted magnetic loops and the mass condensations along the magnetically neutral lines, and the magnetic reconfigurations at the streamer.
SH53A-1073
Photospheric Magnetic Flux Emergence: A comparative study between Hinode/SOT Observations and MHD simulations
With high angular resolution, high temporal cadence and a stable point spread function, the Solar Optical Telescope (SOT) onboard the Hinode satellite is the ideal instrument for the study of magnetic flux emergence and its manifestations on the solar surface. In this presentation, we focus on the development of ephemeral regions and small active regions. In many instances, SOT has been able to capture the entire emergence process from beginning to end: i.e. from the initial stages of flux appearance in granule interiors, through the intermediate stages of G-band bright point formation, and finally to the coalescence of small vertical flux elements to form pores. To investigate the physics of the flux emergence process, we performed 3D numerical MHD simulations with the MURaM code. The models are able to reproduce, and help us explain, various observational signatures of magnetic flux emergence.
SH53A-1074
Solar Wind Effects of a Persistent, High Latitude Coronal Brightness Enhancement
An analysis of SOHO LASCO C2 images over 1996-2005 defines the years and latitudes at which bright, white light coronal structures are most persistent from rotation to rotation. In the northern hemisphere, these features are most notable for the years 2002-2003 and the latitudes 40-60 degrees. This time interval overlaps that of a north pole passage of Ulysses. As Ulysses passes overhead, solar rotation sweeps the interplanetary extensions of these persistent coronal features over the spacecraft, thereby revealing the interplanetary effects of very bright and recurrent K-corona at high solar latitudes.
SH53A-1075
Center-to-Limb Variation of the Irradiance Contributions of Bright Active Regions.
We investigate the variable contribution to TSI of a typical bright active region as it evolves and transits the solar disk. Bright active regions are manifest as bright faculae on images of the solar photosphere, but are in fact 3- dimensional structures, stretching up to coronal heights. Using spacecraft observations of TSI, and ground- based red and Ca-K line images from the San Fernando Observatory, we compute the center-to-limb variation of the irradiance contributions of bright active regions. We evaluate the red continuum irradiance component that originates in the photosphere and the spectral line irradiance component that originates in the lower chromosphere. We also attempt to infer the bolometric contrast of a bright active region. This work is supported in part by NSF grant ATM-0533511.
SH53A-1076
Solar Shape Measurements from RHESSI: A Large Excess Oblateness
The Solar Aspect System of the RHESSI spacecraft scans the limb at the ~4 sec rotation period of the spacecraft, producing a large quantity of precise differential measurements of the solar radius at optical wavelengths (monochromatic at 670 nm). These data provide the most precise determinations of the oblateness prior in particular to the launch of the Picard mission in 2008. The observation of standing waves in the body of the Sun (helioseismology) provided the first direct way to study the interior of a star. The astrometric shape of the solar limb gives independent constraints on interior structures and flows; the surface rotation predicts an oblate ellipsoid with an equator-pole radius difference of some 8 mas (~0.001%). Here we report the most accurate observations to date of the solar shape, which show a much larger apparent oblateness with an equator-pole radius difference of 13.72± 0.44 mas. This new component can easily be distinguished spatially from the effects of faculae in the active latitude zones. Comparison with earlier observations suggests that this excess oblateness results from solar magnetic activity, as do the frequency variations of the helioseismic modes.