共查询到20条相似文献,搜索用时 15 毫秒
1.
The Air Force/NASA Solar Mass Ejection Imager (SMEI) will provide two-dimensional images of the sky in visible light with high (0.1%) photometric precision, and unprecedented sky coverage and cadence. To optimize the information available from these images they must be interpreted in three dimensions. We have developed a Computer Assisted Tomography (CAT) technique that fits a three-dimensional kinematic heliospheric model to remotely-sensed Thomson scattering observations. This technique is designed specifically to determine the corotating background solar wind component from data provided by instruments like SMEI. Here, we present results from this technique applied to the Helios spacecraft photometer observations. The tomography program iterates to a least-squares solution of observed brightnesses using solar rotation, spacecraft motion and solar wind outflow to provide perspective views of each point in space covered by the observations. The corotational tomography described here is essentially the same as used by Jackson et al. (1998) for the analysis of interplanetary scintillation (IPS) observations. While IPS observations are related indirectly to the solar wind density through an assumed (and uncertain) relationship between small-scale density fluctuations and density, Thomson scattering physics is more straightforward, i.e., the observed brightness depends linearly on the solar wind density everywhere in the heliosphere. Consequently, Thomson scattering tomography can use a more direct density-convergence criterion to match observed Helios photometer brightness to brightness calculated from the model density. The general similarities between results based on IPS and Thomson scattering tomography validate both techniques and confirm that both observe the same type of solar wind structures. We show results for Carrington rotation 1653 near solar minimum. We find that longitudinally segmented dense structures corotate with the Sun and emanate from near the solar equator. We discuss the locations of these dense structures with respect to the heliospheric current sheet and regions of activity on the solar surface. 相似文献
2.
We describe the application of a multi-scale Laplacian-of-Gaussian (LoG) operator and of an iterative version of Medial Axis
Transform (i-MAT) as tools for the segmentation of both photospheric and chromospheric solar features. We introduce the multi-scale LoG
operator in order to extract compact structures in photospheric intensity or Doppler images. The second method, based on a
i-MAT algorithm applied to gray level images, is introduced to recognize reticulated structures like chromospheric network
or intergranular lanes. The developed numerical procedures allow a non-subjective segmentation of solar images in order to
investigate morphological and topological properties of identified features. We discuss the output of the segmentation procedures
when applied to real images. 相似文献
3.
Principal components analysis (PCA) and independent component analysis (ICA) are used to identify global patterns in solar
and space data. PCA seeks orthogonal modes of the two-point correlation matrix constructed from a data set. It permits the
identification of structures that remain coherent and correlated or that recur throughout a time series. ICA seeks for maximally
independent modes and takes into account all order correlations of the data. We apply PCA to the interplanetary magnetic field
polarity near 1 AU and to the 3.25R
⊙ source-surface fields in the solar corona. The rotations of the two-sector structures of these systems vary together to high
accuracy during the active interval of solar cycle 23. We then use PCA and ICA to hunt for preferred longitudes in northern
hemisphere Carrington maps of magnetic fields. 相似文献
4.
Identifying co-rotating structures in solar wind enables us to predict solar wind variation at the Earth and, hence, geomagnetic disturbances. However, co-rotating structures during solar maximum are sometimes difficult to see. We correlated solar wind data obtained by two spacecraft, Nozomi heading towards Mars and ACE at the L1 point, from late 1999 through early 2002. There were intervals when the solar wind showed specific co-rotating structures even in the midst of the solar maximum, whereas no correlation was found during the other intervals. The coefficient was generally higher between Nozomi and ACE than for the 27-day recurrence at ACE, while there was some correlation, especially when the difference in longitude between the two spacecraft was less than 120°. Although frequency of occurrence of CMEs is partly responsible for the correlation, the results can be interpreted in terms of rapid changes in co-rotating high-speed streams from near-equatorial coronal holes at the solar maximum. 相似文献
5.
Nicholeen M. Viall Harlan E. Spence Angelos Vourlidas Russell Howard 《Solar physics》2010,267(1):175-202
We present an analysis of small-scale, periodic, solar-wind density enhancements (length scales as small as ≈ 1000 Mm) observed
in images from the Heliospheric Imager (HI) aboard STEREO-A. We discuss their possible relationship to periodic fluctuations of the proton density that have been
identified at 1 AU using in-situ plasma measurements. Specifically, Viall, Kepko, and Spence (J. Geophys. Res.
113, A07101, 2008) examined 11 years of in-situ solar-wind density measurements at 1 AU and demonstrated that not only turbulent structures, but also nonturbulent, periodic
density structures exist in the solar wind with scale sizes of hundreds to one thousand Mm. In a subsequent paper, Viall,
Spence, and Kasper (Geophys. Res. Lett.
36, L23102, 2009) analyzed the α-to-proton solar-wind abundance ratio measured during one such event of periodic density structures, demonstrating that the
plasma behavior was highly suggestive that either temporally or spatially varying coronal source plasma created those density
structures. Large periodic density structures observed at 1 AU, which were generated in the corona, can be observable in coronal
and heliospheric white-light images if they possess sufficiently high density contrast. Indeed, we identify such periodic
density structures as they enter the HI field of view and follow them as they advect with the solar wind through the images.
The smaller, periodic density structures that we identify in the images are comparable in size to the larger structures analyzed
in-situ at 1 AU, yielding further evidence that periodic density enhancements are a consequence of coronal activity as the solar
wind is formed. 相似文献
6.
Richard Woo 《Solar physics》2007,241(2):251-261
In the absence of magnetic field measurements of the solar corona, the density structure of white-light images has provided
important insight into the coronal magnetic field. Recent work sparked by highly sensitive radio occultation measurements
of path-integrated density has elucidated the density structure of unprocessed solar eclipse pictures. This paper does the
same for processed images that reveal low-contrast small-scale structures, specifically Koutchmy’s edge-enhanced white-light
image of the 11 August 1999 solar eclipse. This processed image provides visual evidence for two important results deduced
from radio occultation measurements of small-scale density variations. First, in addition to the closed loops readily seen
at the base of the corona in high-resolution EUV and soft X-ray images, open filamentary structures permeate the corona including
active regions generally thought to be magnetically closed. Observed at the image resolution, the filamentary structures are
1° wide in latitude and an order of magnitude smaller than polar plumes. Second, although inhomogeneities that are convected
along with the solar wind are also present, filamentary structures dominate the image because of their steeper density gradients.
The quantitative profile of polarized brightness (pB) at the base of the corona shows that the filamentary structures have transverse density gradients that are proportional
to their density. This explains why edge-enhanced images, limited in sensitivity to density gradients, tend to detect filamentary
structures more readily in high-density regions (e.g., active regions, streamer stalks, and prominences) than in low-density polar coronal holes, and why filamentary structures
seem more prevalent in solar eclipse pictures during solar maximum. The pB profile at the base of the corona also fills the gap in Doppler measurements there, reinforcing that open ultra-fine-scale
filamentary structures observed by the radio measurements are predominantly radial and that they are an integral part of the
radial expansion of the solar wind. 相似文献
7.
Gary D. Parker 《Solar physics》2009,257(1):155-167
During the descent of Ulysses following the 2001 solar north pole passage, the SOHO LASCO C2 telescope recorded a particularly strong sequence of recurrent
polarization brightness (pB) features at latitudes of around 55°. As Ulysses passed overhead, solar rotation swept the interplanetary extensions of these persistent coronal structures over the spacecraft.
Comparison of solar remote sensing and Ulysses
in situ observations through 2002 reveals the solar wind effects of very bright and recurrent K-coronal structures at high solar
latitudes and of a steeply inclined heliospheric neutral sheet (HNS). Despite the high level of solar activity, the HNS at
high latitude still organizes solar wind stream structure much as it did near the previous solar minimum. The recurrent coronal
streamers originate slow solar wind and mark the northern extremity of a very tilted HNS whose passage at Ulysses is accompanied by slow, dense solar wind, enhanced temperature, depressed α abundance, enhanced magnetic fields, and magnetic field directional changes that evolve with spacecraft latitude. 相似文献
8.
M. R. Kundu 《Solar physics》1996,169(2):389-402
We present a review of selected studies based upon simultaneous radio and X-ray observations of solar flares and coronal transients. We use primarily the observations made with large radio imaging instruments (VLA, BIMA, Nobeyama, and Nançay) along with Yohkoh/SXT and HXT and CGRO experiments. We review the recent work on millimeter imaging of solar flares, microwave and hard X-ray observations of footpoint emission from flaring loops, metric type IV continuum bursts, and coronal X-ray structures. We discuss the recent studies on thermal and nonthermal processes in coronal transients such as XBP flares, coronal X-ray jets, and active region transient brightenings.Dedicated to Cornelis de Jager 相似文献
9.
B. V. Jackson J. A. Boyer P. P. Hick A. Buffington M. M. Bisi D. H. Crider 《Solar physics》2007,241(2):385-396
Interplanetary Scintillation (IPS) allows observation of the inner heliospheric response to corotating solar structures and
coronal mass ejections (CMEs) in scintillation level and velocity. With colleagues at STELab, Nagoya University, Japan, we
have developed near-real-time access of STELab IPS data for use in space-weather forecasting. We use a 3D reconstruction technique
that produces perspective views from solar corotating plasma and outward-flowing solar wind as observed from Earth by iteratively
fitting a kinematic solar wind model to IPS observations. This 3D modeling technique permits reconstruction of the density
and velocity structure of CMEs and other interplanetary transients at a relatively coarse resolution: a solar rotational cadence
and 10° latitudinal and longitudinal resolution for the corotational model and a one-day cadence and 20° latitudinal and longitudinal
heliographic resolution for the time-dependent model. This technique is used to determine solar-wind pressure (“ram” pressure)
at Mars. Results are compared with ram-pressure observations derived from Mars Global Surveyor magnetometer data (Crider et al.
2003, J. Geophys. Res.
108(A12), 1461) for the years 1999 through 2004. We identified 47 independent in situ pressure-pulse events above 3.5 nPa in the Mars Global Surveyor data in this time period where sufficient IPS data were available. We detail the large pressure pulse observed at Mars in
association with a CME that erupted from the Sun on 27 May 2003, which was a halo CME as viewed from Earth. We also detail
the response of a series of West-limb CME events and compare their response observed at Mars about 160° west of the Sun – Earth
line by the Mars Global Surveyor with the response derived from the IPS 3D reconstructions. 相似文献
10.
Berrilli F. Del Moro D. Consolini G. Pietropaolo E. Duvall T.L. Kosovichev A.G. 《Solar physics》2004,221(1):33-45
We investigate spatial dislocation ordering of the solar structures associated with supergranulation and granulation scales.
The supergranular and granular structures are automatically segmented from time-distance divergence maps and from broad-band
images, respectively. The spatial dislocation ordering analysis is accomplished by applying the statistical method of Pair
Correlation Function, g
2(r), to segmented features in the solar fields. We compare the computed g
2(r) functions obtained from both single and persistent, i.e., time-averaged, fields associated with supergranulation and granulation.
We conclude that supergranulation and granulation patterns present a different topological order both in single and persistent
fields. The analysis carried out on single fields suggests that the granulation behaves as an essentially random distribution
of soft plasma features with a very broad distribution in size, while supergranulation behaves as a random distribution of
close packed, coherent stiff features with a rather defined mean size. 相似文献
11.
During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the Magnetic Field Investigation (MFI) team
using Wind (1995 – 2003) solar wind plasma and magnetic field data from solar minimum through the maximum of cycle 23. The average occurrence
rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were
during the early part of solar cycle 23: (i) only four MCs were observed in 1999, and (ii) an unusually large number of MCs
(17 events) were observed in 1997, just after solar minimum. We also discuss the relationship between MCs, coronal mass ejections
(CMEs), and geomagnetic storms. During the period 1996 – 2003, almost 8000 CMEs were observed by SOHO-LASCO. The occurrence
frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number.
When we included “magnetic cloud-like structures” (MCLs, defined by Lepping, Wu, and Berdichevsky, 2005), we found that the
occurrence of the joint set (MCs + MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average
duration of the MCL structures is ~40% shorter than that of the MCs. The MCs are typically more geoeffective than the MCLs,
because the average southward field component is generally stronger and longer lasting in MCs than in MCLs. In addition, most
severe storms caused by MCs/MCLs with Dst
min≤ −100 nT occurred in the active solar period. 相似文献
12.
In this paper, the chromospheric magnetic structures and their relation to the photospheric vector magnetic field in the vicinity of a dark filament in active region 5669 have been demonstrated. Structural variations are shown in chromospheric magnetograms after a solar flare. Filament-like structures in the chromospheric magnetograms occurred after a solar flare. They correspond to the reformation of the chromospheric dark filament, but there is no obvious variation of the photospheric magnetic field. We conclude that (a) some of the obvious changes of the chromospheric magnetic fields occurred after the flare, and (b) a part of these changes is perhaps due to flare brightening in the chromospheric H line.During the reforming process of the dark filament, a part of its chromospheric velocity field shows downward flow, and it later shows upward flow. 相似文献
13.
The Solar TErrestrial RElations Observatory (STEREO) requires powerful tools for the three-dimensional (3D) reconstruction of the solar corona. Here we test such a program
with data from SOHO and TRACE. By taking advantage of solar rotation, a newly developed stereoscopy tool for the reconstruction
of coronal loops is applied to the solar active region NOAA 8891 observed from 1 March to 2 March 2000. The stereoscopic reconstruction
is composed of three steps. First, we identify loop structures in two TRACE images observed from two vantage viewpoints approximately
17 degrees apart, which corresponds to observations made about 30 hours apart. In the second step, we extrapolate the magnetic
field in the corona with the linear force-free field model from the photospheric line-of-sight SOHO/MDI data. Finally, combining
the extrapolated field lines and one-dimensional loop curves from two different viewpoints, we obtain the 3D loop structures
with the magnetic stereoscopy tool. We demonstrate that by including the magnetic modeling this tool is more powerful than
pure geometrical stereoscopy, especially in resolving the ambiguities generated by classical stereoscopy. This work will be
applied to the STEREO mission in the near future. 相似文献
14.
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
15.
We extend the correlation analysis of solar signals and the North Atlantic Oscillation (NAO) back in time by using the aa index (since 1868) and the PC index (since 1948) as a proxy of the solar wind energy imparted to the magnetosphere. Prior
to the 1940s the records of the NAO and the aa index were not closely connected, while after the 1940s their rhythms matched. We compare two distinctive periods with recent
results on the long-scale reconstruction of solar activity. The shift in the NAO–aa interconnection can provide the explanation of a significant increase of solar activity after the 1940s. A strengthening
of the interplanetary magnetic field leads to more intensive variations of the high-latitude ionospheric electric field that
influences the atmospheric circulation. 相似文献
16.
We present calculated Stark-polarized line profiles for a number of H i lines observed in the visible and infrared emission spectrum of solar prominences and other limb activity. For use in measurements of possible electric fields in these structures, we also calculate curves giving the difference in line width between the 1/2 (I ± Q) profiles as a function of electric-field intensity. Our calculations take into account magnetic fields in these structures, and incorporate typical observed values of Doppler broadening. These calculations explicitly consider the H i fine structure neglected in previous work, and thus are more accurate in the range of low to intermediate electric-field intensity likely to be encountered in solar plasmas (E < 103 V cm–1). Our results enable us to compare behavior when E and B are parallel, or perpendicular. We draw particular attention to the high electric-field sensitivity of the transitions between high levels such as 12–8 and 15–9 in H i, observed in prominences at wavelengths around 11. Their sensitivity is roughly an order of magnitude larger than that of the high Paschen-series lines used in solar plasma electric field studies so far. 相似文献
17.
We compare the shape and position of some plasma formations visible in the polar corona with the cyclic evolution of the global
magnetic field. The first type of object is polar crown prominences. A two-fold decrease of the height of polar crown prominences
was found during their poleward migration from the middle latitudes to the poles before a polar magnetic field reversal. The
effect could be assigned to a decrease of the magnetic field scale. The second type of object is the polar plumes, ray like
structures that follow magnetic field lines. Tangents to polar ray structures are usually crossed near some point, “a magnetic
focus,” below the surface. The distance q between the focus and the center of the solar disk changes from the maximum value about 0.65 R
⊙ at solar minimum activity to the minimum value about 0.45 R
⊙ at solar maximum. At first glance this behaviour seems to be contrary to the dynamics of spherical harmonics of the global
magnetic field throughout a cycle. We believe that the problem could be resolved if one takes into account not only scale
changes in the global magnetic field but also the phase difference in the cyclic variations of large-scale and small-scale
components of the global field. 相似文献
18.
The Grad–Shafranov reconstruction is a method of estimating the orientation (invariant axis) and cross section of magnetic
flux ropes using the data from a single spacecraft. It can be applied to various magnetic structures such as magnetic clouds
(MCs) and flux ropes embedded in the magnetopause and in the solar wind. We develop a number of improvements of this technique
and show some examples of the reconstruction procedure of interplanetary coronal mass ejections (ICMEs) observed at 1 AU by
the STEREO, Wind, and ACE spacecraft during the minimum following Solar Cycle 23. The analysis is conducted not only for ideal localized ICME
events but also for non-trivial cases of magnetic clouds in fast solar wind. The Grad–Shafranov reconstruction gives reasonable
results for the sample events, although it possesses certain limitations, which need to be taken into account during the interpretation
of the model results. 相似文献
19.
Using data from the Global Oscillation Network Group (GONG) covering the period from 1995 to 1998, we study the change with solar activity in solar f-mode frequencies. The results are compared with similar changes detected from the Michelson Doppler Imager (MDI) data. We find variations in f-mode frequencies which are correlated with solar activity indices. If these changes are due to variation in solar radius then the implications are that the solar radius decreases by about 5 km from minimum to maximum activity. 相似文献
20.
Plasma and magnetic field parameter variations across fast forward interplanetary shocks are analyzed during the last solar
cycle minimum (1995–1996, 15 shocks), and maximum year 2000 (50 shocks). It was observed that the solar wind velocity and
magnetic field strength variation across the shocks were the parameters better correlated with Dst. Superposed epoch analysis centered on the shock showed that, during solar minimum, B
z
profiles had a southward, long-duration variation superposed with fluctuations, whereas in solar maximum the B
z
profile presented 2 peaks. The first peak occurred 4 hr after the shock, and seems to be associated with the magnetic field
disturbed by the shock in the sheath region. The second peak occurred 19 hr after the shock, and seems to be associated with
the ejecta fields. The difference in shape and peak in solar maximum (Dst peak =−50 nT, moderate activity) and minimum (Dst peak =−30 nT, weak activity) in average Dst profiles after shocks are, probably, a consequence of the energy injection in the magnetosphere being driven by different
interplanetary southward magnetic structures. A statistical distribution of geomagnetic activity levels following interplanetary
shocks was also obtained. It was observed that during solar maximum, 36% of interplanetary shocks were followed by intense
(Dst≤−100 nT) and 28% by moderate (−50≤Dst <−100 nT) geomagnetic activity. During solar minimum, 13% and 33% of the shocks were followed by intense and moderate geomagnetic
activity, respectively. Thus, during solar maximum a higher relative number of interplanetary shocks might be followed by
intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar
cycle a shock has a probability of around 50–60% to be followed by intense/moderate geomagnetic activity. 相似文献