排序方式： 共有50条查询结果，搜索用时 15 毫秒
11.
Markus J. Aschwanden Jong Kwan Lee G. Allen Gary Michael Smith Bernd Inhester 《Solar physics》2008,248(2):359-377
The three-dimensional (3D) modeling of coronal loops and filaments requires algorithms that automatically trace curvilinear
features in solar EUV or soft X-ray images. We compare five existing algorithms that have been developed and customized to
trace curvilinear features in solar images: i) the oriented-connectivity method (OCM), which is an extension of the Strous pixel-labeling algorithm (developed by Lee,
Newman, and Gary); ii) the dynamic aperture-based loop-segmentation method (developed by Lee, Newman, and Gary); iii) unbiased detection of curvilinear structures (developed by Steger, Raghupathy, and Smith); iv) the oriented-direction method (developed by Aschwanden); and v) ridge detection by automated scaling (developed by Inhester). We test the five existing numerical codes with a TRACE image
that shows a bipolar active region and contains over 100 discernable loops. We evaluate the performance of the five codes
by comparing the cumulative distribution of loop lengths, the median and maximum loop length, the completeness or detection
efficiency, the accuracy, and flux sensitivity. These algorithms are useful for the reconstruction of the 3D geometry of coronal
loops from stereoscopic observations with the STEREO spacecraft, or for quantitative comparisons of observed EUV loop geometries
with (nonlinear force-free) magnetic field extrapolation models. 相似文献
12.
13.
STEREO/EUVI observed 185 flare events (detected above the GOES class C1 level or at >?25 keV with RHESSI) during the first two years of the mission (December 2006?–?November 2008), while coronal mass ejections (CMEs) were reported in about a third of these events. We compile a comprehensive catalog of these EUVI-observed events, containing the peak fluxes in soft X rays, hard X rays, and EUV, as well as a classification and statistics of prominent EUV features: 79% show impulsive EUV emission (coincident with hard X rays), 73% show delayed EUV emission from postflare loops and arcades, 24% represent occulted flares, 17% exhibit EUV dimming, 5% show loop oscillations or propagating waves, and at least 3% show erupting filaments. We analyze an example of each EUV feature by stereoscopic modeling of its 3D geometry. We find that EUV emission can be dominated by impulsive emission from a heated, highly sheared, noneruptive filament, in addition to the more common impulsive EUV emission from flare ribbons or the delayed postflare EUV emission that results from cooling of the soft-X-ray-emitting flare loops. Occulted flares allow us to determine CME-related coronal dimming uncontaminated from flare-related EUV emission. From modeling the time evolution of EUV dimming we can accurately quantify the initial expansion of CMEs and determine their masses. Further, we find evidence that coronal loop oscillations are excited by the rapid initial expansion of CMEs. These examples demonstrate that stereoscopic EUV data provide powerful new methods to model the 3D aspects in the hydrodynamics of flares and kinematics of CMEs. 相似文献
14.
Markus J. Aschwanden 《Solar physics》1994,152(1):53-59
The solar 0.5–8 soft X-ray flux was monitored by the NOAA Geostationary Operational Environmental Satellites (GOES) from 1974 to the present, providing a continuous record over two solar activity cycles. Attempts have been made to determine a soft X-ray (SXR) background flux by subtracting out solar flares (using the daily lowest flux level). The SXR background flux represents the quiescent SXR flux from heated plasma in active regions, and reflects similar (intermediate-term) variability and periodicities (e.g. 155-day period) as the SXR or hard X-ray (HXR) flare rate, although it is determined in non-flaring time intervals. The SXR background flux peaks late in Solar Cycle 21 (2–3 years after the sunspot maximum), similar to the flare rate measured in SXR, HXR, or gamma rays, possibly due the increasing complexity of coronal magnetic structures in the decay phase of the solar cycle. The SXR background flux appears to be dominated by postflare emission from the dominant active regions, while the contributions from the quiet Sun are appreciable in the Solar Minimum only (A1-level). Comparisons with full-disk integrated images from YOHKOH suggest that the presence of coronal holes can decrease the quietest SXR irradiance level by an additional order of magnitude, but only in the rare case of absence of active regions.Presented at IAU Colloquium No. 143, The Sun as a Variable Star: Solar and Stellar Irradiance Variations, Boulder, CO, June 20–25, 1993 相似文献
15.
Hurford G.J. Schmahl E.J. Schwartz R.A. Conway A.J. Aschwanden M.J. Csillaghy A. Dennis B.R. Johns-Krull C. Krucker S. Lin R.P. McTiernan J. Metcalf T.R. Sato J. Smith D.M. 《Solar physics》2002,210(1-2):61-86
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observes solar hard X-rays and gamma-rays from 3 keV to
17 MeV with spatial resolution as high as 2.3 arc sec. Instead of focusing optics, imaging is based on nine rotating modulation
collimators that time-modulate the incident flux as the spacecraft rotates. Starting from the arrival time of individual photons,
ground-based software then uses the modulated signals to reconstruct images of the source. The purpose of this paper is to
convey both an intuitive feel and the mathematical basis for this imaging process. Following a review of the relevant hardware,
the imaging principles and the basic back-projection method are described, along with their relation to Fourier transforms.
Several specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable to RHESSI imaging are briefly described. The
characteristic strengths and weaknesses of this type of imaging are summarized. 相似文献
16.
Aschwanden Markus J. Metcalf Thomas R. Krucker Säm Sato Jun Conway Andrew J. Hurford G.J. Schmahl Edward J. 《Solar physics》2004,219(1):149-157
We compare the photometric accuracy of spectra and images in flares observed with the Ramaty High-Energy Solar Spectroscopic
Imager (RHESSI) spacecraft. We test the accuracy of the photometry by comparing the photon fluxes obtained in different energy
ranges from the spectral-fitting software SPEX with those fluxes contained in the images reconstructed with the Clean, MEM,
MEM-Vis, Pixon, and Forward-fit algorithms. We quantify also the background fluxes, the fidelity of source geometries, and
spatial spectra reconstructed with the five image reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field of view, and time intervals on the quality of image reconstruction. The detailed parameters and statistics
are provided in an accompanying CD-ROM and web page. We find that Forward-fit, Pixon, and Clean have a robust convergence
behavior and a photometric accuracy in the order of a few percent, while MEM does not converge optimally for large degrees
of freedom (for large field of view and/or small pixel sizes), and MEM-Vis suffers in the case of time-variable sources. This
comparative study documents the current status of the RHESSI spectral and imaging software, one year after launch.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/B:SOLA.0000021801.83038.aa 相似文献
17.
18.
Markus J. Aschwanden 《Solar physics》2013,287(1-2):323-344
We derive an analytical approximation of nonlinear force-free magnetic field solutions (NLFFF) that can efficiently be used for fast forward-fitting to solar magnetic data, constrained either by observed line-of-sight magnetograms and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph data. The derived NLFFF solutions provide the magnetic field components B x (x), B y (x), B z (x), the force-free parameter α(x), the electric current density j(x), and are accurate to second-order (of the nonlinear force-free α-parameter). The explicit expressions of a force-free field can easily be applied to modeling or forward-fitting of many coronal phenomena. 相似文献
19.
Markus J. Aschwanden 《Solar physics》2011,274(1-2):99-117
We analyze the occurrence-frequency distributions of peak fluxes [P], total fluxes [E], and durations [T] of solar flares over the last three solar cycles (during 1980??C?2010) from SMM/HXRBS, CGRO/BATSE, and RHESSI hard X-ray data. From the synthesized data we find powerlaw slopes with mean values of ?? P =1.73±0.07 for the peak flux, ?? E =1.62±0.12 for the total flux, and ?? T =1.99±0.35 for flare durations. We find a tendency of an anti-correlation of the powerlaw slope of peak fluxes with the flare rate or sunspot number as a function of the solar cycle. The occurrence powerlaw slope is always steeper by ??????0.1 during a solar-cycle minimum compared with the previous solar-cycle maximum, but the relative amplitude varies for each cycle or instrument. Since each solar cycle has been observed with a different instrument, part of the variation could be attributed to instrumental characteristics and different event selection criteria used in generating the event catalogs. The relatively flatter powerlaw slopes during solar maxima could indicate more energetic flares with harder electron-energy spectra, probably due to a higher magnetic complexity of the solar corona. This would imply a non-stationarity (or solar-cycle dependence) of the coronal state of self-organized criticality. 相似文献
20.
The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ18O, high 87Sr/86Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ18OVPDB decrease from ?1·0‰ at the top to ?6·7‰ at the base and an 87Sr/86Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ18OVSMOW = 0‰; 87Sr/86Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ18OVSMOW = ?4·0‰; 87Sr/86Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ18O and high 87Sr/86Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps. 相似文献