共查询到20条相似文献，搜索用时 31 毫秒
1.
Coronal holes (CH) emit significantly less at coronal temperatures than quiet-Sun regions (QS), but can hardly be distinguished in most chromospheric and lower transition region lines. A key quantity for the understanding of this phenomenon is the magnetic field. We use data from SOHO/MDI to reconstruct the magnetic field in coronal holes and the quiet Sun with the help of a potential magnetic model. Starting from a regular grid on the solar surface we then trace field lines, which provide the overall geometry of the 3D magnetic field structure. We distinguish between open and closed field lines, with the closed field lines being assumed to represent magnetic loops. We then try to compute some properties of coronal loops. The loops in the coronal holes (CH) are found to be on average flatter than in the QS. High and long closed loops are extremely rare, whereas short and low-lying loops are almost as abundant in coronal holes as in the quiet Sun. When interpreted in the light of loop scaling laws this result suggests an explanation for the relatively strong chromospheric and transition region emission (many low-lying, short loops), but the weak coronal emission (few high and long loops) in coronal holes. In spite of this contrast our calculations also suggest that a significant fraction of the cool emission in CHs comes from the open flux regions. Despite these insights provided by the magnetic field line statistics further work is needed to obtain a definite answer to the question if loop statistics explain the differences between coronal holes and the quiet Sun. 相似文献
2.
Spatial and temporal distributions of coronal holes for the rising phase of the solar cycle during 1996–1999 are considered. Connections between the number of non-polar coronal holes on the solar disk and the Wolf number, the mean solar photospheric magnetic field, and the solar flux density at 2800 mHz are analyzed. Peculiarities of the photospheric magnetic field structure of the regions corresponding to coronal hole locations and comparison with `clear' ones are discussed. 相似文献
3.
Rajendra Shelke 《Journal of Astrophysics and Astronomy》2006,27(2-3):101-112
Coronal holes and interplanetary disturbances are important aspects of the physics of the Sun and heliosphere. Interplanetary
disturbances are identified as an increase in the density turbulence compared with the ambient solar wind. Erupting stream
disturbances are transient large-scale structures of enhanced density turbulence in the interplanetary medium driven by the
high-speed flows of low-density plasma trailing behind for several days. Here, an attempt has been made to investigate the
solar cause of erupting stream disturbances, mapped by Hewish & Bravo (1986) from interplanetary scintillation (IPS) measurements
made between August 1978 and August 1979 at 81.5 MHz. The position of the sources of 68 erupting stream disturbances on the
solar disk has been compared with the locations of newborn coronal holes and/or the areas that have been coronal holes previously.
It is found that the occurrence of erupting stream disturbances is linked to the emergence of new coronal holes at the eruption
site on the solar disk.
A coronal hole is indicative of a radial magnetic field of a predominant magnetic polarity. The newborn coronal hole emerges
on the Sun, owing to the changes in magnetic field configuration leading to the opening of closed magnetic structure into
the corona. The fundamental activity for the onset of an erupting stream seems to be a transient opening of pre-existing closed
magnetic structures into a new coronal hole, which can support highspeed flow trailing behind the compression zone of the
erupting stream for several days. 相似文献
4.
High resolution KPNO magnetograph measurements of the line-of-sight component of the photospheric magnetic field over the entire dynamic range from 0 to 4000 gauss are used as the basic data for a new analysis of the photospheric and coronal magnetic field distributions. The daily magnetograph measurements collected over a solar rotation are averaged onto a 180 × 360 synoptic grid of equal-area elements. With the assumption that there are no electric currents above the photospheric level of measurement, a unique solution is determined for the global solar magnetic field. Because the solution is in terms of an expansion in spherical harmonics to principal index n = 90, the global photospheric magnetic energy distribution can be analyzed in terms of contributions of different scale-size and geometric pattern. This latter procedure is of value (1) in guiding solar dynamo theories, (2) in monitoring the persistence of the photospheric field pattern and its components, (3) in comparing synoptic magnetic data of different observatories, and (4) in estimating data quality. Different types of maps for the coronal magnetic field are constructed (1) to show the strong field at different resolutions, (2) to trace the field lines which open into interplanetary space and to locate their photospheric origins, and (3) to map in detail coronal regions above (specified) limited photospheric areas.The National Center for Atmospheric Research is sponsored by the National Science foundation.Kitt Peak National Observatory is operated by the Association of Universities for Research in Astronomy, Inc. Under contract with the National Science Foundation. 相似文献
5.
We compare coronal holes observed in solar soft X-ray images obtained with rocket-borne telescopes during 1974 to 1981 with holes observed on nearly simultaneous 10830 Å maps. Hole boundaries are frequently poorly defined, and after 1974 the brightness contrast between the large scale structure and holes appears substantially diminished in both X-rays and 10830 Å. We find good agreement between soft X-rays and 10830 Å for large area holes but poor agreement for mid and low latitude small area holes, which are generally of low contrast. These results appear inconsistent with the popular view that the quiet corona is sharply separated into open magnetic field regions consisting of coronal holes and closed field regions consisting of the large scale structure. 相似文献
6.
太阳剩余磁场是指形成于太阳主序星阶段之前，深藏在太阳辐射核内部的原始磁场。由于太阳内部高电导率和准静态等因素，其剩余磁场耗散相当缓慢，而得以保留至今。太阳剩余磁场的存在不仅能够解释太阳活动的很多不对称性现象，如南北不对称性、活动经度与活动穴、低纬度冕洞和Maunder极小期等，还能通过改变自激发发电机模型的边值条件而影响整个太阳表面磁场的分布与演化。从观测结果和理论模型两方面评述了太阳剩余磁场的研究成果及最新进展，并简单讨论了进一步努力的方向。 相似文献
7.
For the 2.5 year period from January 1, 1977 to June 30, 1979, we have correlated the positions of high latitude coronal holes, obtained from the He 10830 Å synoptic maps, with the velocities of solar wind streams, determined from interplanetary scintillation, that would have originated from these coronal holes. From 24 cases analyzed we find that these high latitude coronal holes are often, but not always, correlated with high speed solar wind streams. The lack of a much stronger correlation may be due to uncertainties in the boundaries of the coronal holes and in the velocities of the solar wind streams. It might also be due to the deflection or attenuation of relatively weak solar wind streams in interplanetary space. 相似文献
8.
The solar magnetic field maps every point in the corona to a corresponding place on the solar surface. Identifying the magnetic
connection map is difficult at low latitudes near the heliospheric current sheet, but remarkably simple in coronal hole interiors.
We present a simple analytic magnetic model (‘pseudocurrent extrapolation’) that reproduces the global structure of the corona,
with significant physical advantages over other nearly analytic models such as source-surface potential field extrapolation.
We use the model to demonstrate that local horizontal structure is preserved across altitude in the central portions of solar
coronal holes, up to at least 30 Rs, in agreement with observations. We argue that the preserved horizontal structure may be used to track the magnetic footpoint
associated with the location of a hypothetical spacecraft traveling through the solar corona, to relate in situ measurements of the young solar wind at ∼10–30 Rs to particular source regions at the solar surface. Further, we discuss the relationship between readily observable geometrical
distortions and physical parameters of interest such as the field-aligned current density. 相似文献
9.
Tilaye Tadesse Alexei A. Pevtsov T. Wiegelmann P. J. MacNeice S. Gosain 《Solar physics》2014,289(11):4031-4045
Solar eruptive phenomena, like flares and coronal mass ejections (CMEs), are governed by magnetic fields. To describe the structure of these phenomena one needs information on the magnetic flux density and the electric current density vector components in three dimensions throughout the atmosphere. However, current spectro-polarimetric measurements typically limit the determination of the vector magnetic field to only the photosphere. Therefore, there is considerable interest in accurate modeling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data. In this work, we model the coronal magnetic field for global solar atmosphere using nonlinear force-free field (NLFFF) extrapolation codes implemented to a synoptic maps of photospheric vector magnetic field synthesized from the Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) as boundary condition. Using the resulting three-dimensional magnetic field, we calculate the three-dimensional electric current density and magnetic energy throughout the solar atmosphere for Carrington rotation 2124 using our global extrapolation code. We found that spatially, the low-lying, current-carrying core field demonstrates a strong concentration of free energy in the active-region core, from the photosphere to the lower corona (about 70 Mm). The free energy density appears largely co-spatial with the electric current distribution. 相似文献
10.
Polar Coronal Holes During Cycles 22 and 23 总被引：3，自引：0，他引：3
The National Solar Observatory/Kitt Peak synoptic rotation maps of the magnetic field and of the equivalent width of the He i 1083 nm line are used to identify and measure polar coronal holes from September 1989 to the present. This period covers the entire lifetime of the northern and southern polar holes present during cycles 22 and 23 and includes the disappearance of the previous southern polar coronal hole in 1990 and and formation of the new northern polar hole in 2001. From this sample of polar hole observations, we found that polar coronal holes evolve from high-latitude (60° ) isolated holes. The isolated pre-polar holes form in the follower of the remnants of old active region fields just before the polar magnetic fields complete their reversal during the maximum phase of a cycle, and expand to cover the poles within 3 solar rotations after the reversal of the polar fields. During the initial 1.2–1.4 years, the polar holes are asymmetric about the pole and frequently have lobes extending into the active region latitudes. During this period, the area and magnetic flux of the polar holes increase rapidly. The surface areas, and in one case the net magnetic flux, reach an initial brief maximum within a few months. Following this initial phase, the areas (and in one case magnetic flux) decrease and then increase more slowly reaching their maxima during the cycle minimum. Over much of the lifetime of the measured polar holes, the area of the southern polar hole was smaller than the northern hole and had a significantly higher magnetic flux density. Both polar holes had essentially the same amount of magnetic flux at the time of cycle minimum. The decline in area and magnetic flux begins with the first new cycle regions with the holes disappearing about 1.1–1.8 years before the polar fields complete their reversal. The lifetime of the two polar coronal holes observed in their entirety during cycles 22 and 23 was 8.7 years for the northern polar hole and 8.3 years for the southern polar hole. 相似文献
11.
Coronal holes (CHs) are regions of open magnetic field lines in the solar corona and the source of the fast solar wind. Understanding the evolution of coronal holes is critical for solar magnetism as well as for accurate space weather forecasts. We study the extreme ultraviolet (EUV) synoptic maps at three wavelengths (195 Å/193 Å, 171 Å and 304 Å) measured by the Solar and Heliospheric Observatory/Extreme Ultraviolet Imaging Telescope (SOHO/EIT) and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) instruments. The two datasets are first homogenized by scaling the SDO/AIA data to the SOHO/EIT level by means of histogram equalization. We then develop a novel automated method to identify CHs from these homogenized maps by determining the intensity threshold of CH regions separately for each synoptic map. This is done by identifying the best location and size of an image segment, which optimally contains portions of coronal holes and the surrounding quiet Sun allowing us to detect the momentary intensity threshold. Our method is thus able to adjust itself to the changing scale size of coronal holes and to temporally varying intensities. To make full use of the information in the three wavelengths we construct a composite CH distribution, which is more robust than distributions based on one wavelength. Using the composite CH dataset we discuss the temporal evolution of CHs during the Solar Cycles 23 and 24. 相似文献
12.
It is shown that the contrast of coronal holes, just as their size, determines the velocity of the solar wind streams. Fully calibrated EIT images of the Sun have been used. About 450 measurements in 284 Å have been analyzed. The time interval under examination covers about 1500 days in the declining phase of cycle 23. All coronal holes recorded for this interval in the absence of coronal mass ejections (CMEs) have been studied. The comparison with some other parameters (e.g. density, temperature, magnetic field) was carried out. The correlations with the velocity are rather high (0.70?–?0.89), especially during the periods of moderate activity, and could be used for everyday forecast. The contrast of coronal holes is rather small. 相似文献
13.
14.
We quantitatively re-examine the nonlinear viscous damping of surface Alfvén waves in polar coronal holes, using recently reported observational data on electron density and temperature and the magnetic field spreading near the edges. It is found that in the nonlinear regime the viscous damping of surface Alfvén waves becomes a viable mechanism of solar coronal plasma heating when strong spreading of magnetic field is taken into account. Our estimations confirm that coronal heating is more pronounced in the nonlinear case than in the linear one in presence of magnetic field spreading. 相似文献
15.
This paper is an exploration of the possibility that the large-scale equilibrium of plasma and magnetic fields in the solar corona is a minimum energy state. Support for this conjecture is sought by considering the simplest form of that equilibrium in a dipole solar field, as suggested by the observed structure of the corona at times of minimum solar activity. Approximate, axisymmetric solutions to the MHD equations are constructed to include both a magnetically closed, hydrostatic region and a magnetically open region where plasma flows along field lines in the form of a transonic, thermally-driven wind. Sequences of such solutions are obtained for various degrees of magnetic field opening, and the total energy of each solution is computed, including contributions from both the plasma and magnetic field. It is shown that along a sequence of increasingly closed coronal magnetic field, the total energy curve is a non-monotonic function of the parameter measuring the degree of magnetic field opening, with a minimum occurring at moderate field opening.For reasonable choices of model parameters (coronal temperature, base density, base magnetic field strength, etc.), the morphology of the minimum energy solution resembles the observed quiet, solar minimum corona. The exact location energy minimum along a given sequence depends rather sensitively on some of the adopted parameter values. It is nevertheless argued that the existence of an energy minimum along the sequences of solutions should remain a robust property of more realistic coronal wind models that incorporate the basic characteristics of the equilibrium corona- the presence of both open and closed magnetic regions.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
16.
We quantitatively re-examine the nonlinear viscous damping of surface Alfvén waves in polar coronal holes, using recently reported observational data on electron density and temperature and the magnetic field spreading near the edges. It is found that in the nonlinear regime the viscous damping of surface Alfvén waves becomes a viable mechanism of solar coronal plasma heating when strong spreading of magnetic field is taken into account. Our estimations confirm that coronal heating is more pronounced in the nonlinear case than in the linear one in presence of magnetic field spreading. 相似文献
17.
V.A. Kovalenko 《Planetary and Space Science》1988,36(12):1343-1358
An analysis has been made of the origin of long-term variations in flux density of solar wind particles (nv) for different velocity regimes. The study revealed a relationship of these variations to the area of the polar coronal holes (CH). It is shown that within the framework of the model under development, the main longterm variations of nv are a result of the latitude redistribution of the solar wind mass flux in the heliosphere and are due to changes in the large-scale geometry of the solar plasma flow in the corona.
A study has been made of the variations of nv for high speed solar wind streams. It is found that nv in high speed streams which are formed in CH, decreases from minimum to maximum solar activity. The analysis indicates that this decrease is attributable to the magnetic field strength increase in coronal holes.
It has been found that periods of rapid global changes of background magnetic fields on the Sun are accompanied by a reconfiguration of coronal magnetic fields, rapid changes in the length of quiescent filaments, and by an increase in the density of the particle flux of a high speed solar wind. It has been established that these periods precede the formation of CH, corresponding to the increase in solar wind velocity near the Earth and to enhancement of the level of geomagnetic disturbance. 相似文献
18.
Liping Yang Xueshang Feng Changqing Xiang Shaohua Zhang S. T. Wu 《Solar physics》2011,271(1-2):91-110
The observations both near the Sun and in the heliosphere during the activity minimum between solar cycles 23 and 24 exhibit different phenomena from those typical of the previous solar minima. In this paper, we have chosen Carrington rotation 2070 in 2008 to investigate the properties of the background solar wind by using the three-dimensional (3D) Solar?CInterPlanetary Conservation Element/Solution Element Magnetohydrodynamic (MHD) model. We also study the effects of polar magnetic fields on the characteristics of the solar corona and the solar wind by conducting simulations with an axisymmetric polar flux added to the observed magnetic field. The numerical results are compared with the observations from multiple satellites, such as the Solar and Heliospheric Observatory (SOHO), Ulysses, Solar Terrestrial Relations Observatory (STEREO), Wind and the Advanced Composition Explorer (ACE). The comparison demonstrates that the first simulation with the observed magnetic fields reproduces some observed peculiarities near the Sun, such as relatively small polar coronal holes, the presence of mid- and low-latitude holes, a tilted and warped current sheet, and the broad multiple streamers. The numerical results also capture the inconsistency between the locus of the minimum wind speed and the location of the heliospheric current sheet, and predict slightly slower and cooler polar streams with a relatively smaller latitudinal width, broad low-latitude intermediate-speed streams, and globally weak magnetic field and low density in the heliosphere. The second simulation with strengthened polar fields indicates that the weak polar fields in the current minimum play a crucial role in determining the states of the corona and the solar wind. 相似文献
19.
V. M. Malashchuk V. G. Fainshtein N. N. Stepanian G. V. Rudenko Ya. I. Egorov 《Bulletin of the Crimean Astrophysical Observatory》2012,108(1):70-77
We propose the concept of a large-scale complex of solar formations with an isolated magnetic field. The complex involves a group of coronal holes, active regions, and regions with intermediate characteristics between a coronal hole and the undisturbed (calm) region. An interesting feature of these complexes is the weak connection between the magnetic fields inside and outside the complexes. Most of the lines of the magnetic flux that emerge from the complex prove to be either opened or closed inside the complex. 相似文献
20.
Evolution of Coronal Holes and Implications for?High-Speed Solar Wind During the Minimum Between Cycles 23 and 24 总被引：1，自引：0，他引：1
G. de Toma 《Solar physics》2011,274(1-2):195-217
We analyze coronal holes present on the Sun during the extended minimum between Cycles 23 and 24, study their evolution, examine the consequences for the solar wind speed near the Earth, and compare it with the previous minimum in 1996. We identify coronal holes and determine their size and location using a combination of EUV observations from SOHO/EIT and STEREO/EUVI and magnetograms. We find that the long period of low solar activity from 2006 to 2009 was characterized by weak polar magnetic fields and polar coronal holes smaller than observed during the previous minimum. We also find that large, low-latitude coronal holes were present on the Sun until 2008 and remained important sources of recurrent high-speed solar wind streams. By the end of 2008, these low-latitude coronal holes started to close down, and finally disappeared in 2009, while smaller, mid-latitude coronal holes formed in the remnants of Cycle 24 active regions shifting the sources of the solar wind at the Earth to higher latitudes. 相似文献