共查询到20条相似文献，搜索用时 62 毫秒
1.
Previous statistical analyses of a large number of SOHO/MDI full disk longitu-dinal magnetograms provided a result that demonstrated how responses of solar flares to photospheric magnetic properties can be fitted with sigmoid functions. A logistic model reveals that these fitted sigmoid functions might be related to the free energy storage process in solar active regions. Although this suggested model is rather simple, the free energy level of active regions can be estimated and the probability of a solar flare with importance over a threshold can be forecast within a given time window. 相似文献
2.
Tilaye Tadesse T. Wiegelmann P. J. MacNeice B. Inhester K. Olson A. Pevtsov 《Solar physics》2014,289(3):831-845
Measurements of magnetic fields and electric currents in the pre-eruptive corona are crucial to the study of solar eruptive phenomena, like flares and coronal mass ejections (CMEs). However, spectro-polarimetric measurements of certain photospheric lines permit a determination of the vector magnetic field only at 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 above multiple active regions with the help of a potential field and a nonlinear force-free field (NLFFF) extrapolation code over the full solar disk using Helioseismic and Magnetic Imager (SDO/HMI) data as boundary conditions. We compare projections of the resulting magnetic field lines with full-disk coronal images from the Atmospheric Imaging Assembly (SDO/AIA) for both models. This study has found that the NLFFF model reconstructs the magnetic configuration closer to observation than the potential field model for full-disk magnetic field extrapolation. We conclude that many of the trans-equatorial loops connecting the two solar hemispheres are current-free. 相似文献
3.
The synoptic map of the solar wind speed (SWS) estimated by the computer-assisted tomography (CAT) method with interplanetary scintillation observations is constructed for the 1909 Carrington rotation. A similar synoptic map of expansion rate (RBR) of the coronal magnetic field calculated by the so-called 'potential model' with the photospheric magnetic field is also constructed under the radial field assumption (RF model). These maps consist of 64800 (180×360) data points of equal area. We examine for the first time relations between the SWS estimated by the CAT technique and the RBR calculated by the RF model. A highly significant correlation is found between the SWS and the RBR. A simple correlation coefficient is about –0.72; that is, high-velocity winds emanate from photospheric areas corresponding to a low expansion rate of the coronal magnetic field, and low-velocity winds emanate from photospheric areas of high expansion rate. This result suggests that there is some acceleration mechanism relating to the coronal field expansion. 相似文献
4.
Photospheric and heliospheric magnetic fields 总被引：1，自引：0，他引：1
The magnetic field in the heliosphere evolves in response to the photospheric field at its base. This evolution, together with the rotation of the Sun, drives space weather through the continually changing conditions of the solar wind and the magnetic field embedded within it. We combine observations and simulations to investigate the sources of the heliospheric field from 1996 to 2001. Our algorithms assimilate SOHO/MDI magnetograms into a flux-dispersal model, showing the evolving field on the full sphere with an unprecedented duration of 5.5 yr and temporal resolution of 6 hr. We demonstrate that acoustic far-side imaging can be successfully used to estimate the location and magnitude of large active regions well before they become visible on the solar disk. The results from our assimilation model, complemented with a potential-field source-surface model for the coronal and inner-heliospheric magnetic fields, match Yohkoh/SXT and KPNO/He?10830 Å coronal hole boundaries quite well. Even subject to the simplification of a uniform, steady solar wind from the source surface outward, our model matches the polarity of the interplanetary magnetic field (IMF) at Earth ～3% of the time during the period 1997–2001 (independent of whether far-side acoustic data are incorporated into the simulation). We find that around cycle maximum, the IMF originates typically in a dozen disjoint regions. Whereas active regions are often ignored as a source for the IMF, the fraction of the IMF that connects to magnetic plage with absolute flux densities exceeding 50 Mx cm?2 increases from ?10% at cycle minimum up to 30–50% at cycle maximum, with even direct connections between sunspots and the heliosphere. For the overall heliospheric field, these fractions are ?1% to 20–30%, respectively. Two case studies based on high-resolution TRACE observations support the direct connection of the IMF to magnetic plage, and even to sunspots. Parallel to the data assimilation, we run a pure simulation in which active regions are injected based on random selection from parent distribution functions derived from solar data. The global properties inferred for the photospheric and heliospheric fields for these two models are in remarkable agreement, confirming earlier studies that no subtle flux-emergence patterns or field-dispersal properties are required of the solar dynamo beyond those that are included in the model in order to understand the large-scale solar and heliospheric fields.
相似文献5.
Hongqi Zhang 《Monthly notices of the Royal Astronomical Society》2002,332(2):500-512
In this paper we analyse the non-potential magnetic field and the relationship with current (helicity) in the active region NOAA 9077 in 2000 July, using photospheric vector magnetograms obtained at different solar observatories and also coronal extreme-ultraviolet 171-Å images from the TRACE satellite.
We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the 'Bastille Day' 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region. 相似文献
We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the 'Bastille Day' 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region. 相似文献
6.
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
7.
Using high-cadence magnetograms from the SOHO/MDI we have investigated variations of the photospheric magnetic field during
solar flares and CMEs. In the case of a strong X-class flare of May 2, 1998, we have detected variations of magnetic field
in a form of a rapidly propagating magnetic wave. During the impulsive phase of the flare we have observed a sudden decrease
of the magnetic energy in the flare region. This provides direct evidence of magnetic energy release in solar flares. We discuss
the physics of the magnetic field variations, and their relations to the Moreton Hα waves and the coronal waves observed by
the EIT. 相似文献
8.
High-cadence, high-resolution magnetograms have shown that the quiet-Sun photosphere is very dynamic in nature. It is comprised
of discrete magnetic fragments which are characterized by four key processes – emergence, coalescence, fragmentation and cancellation.
All of this will have consequences for the magnetic field in the corona above.
The aim of this study is to gauge the effect of the behavior of the photospheric flux fragments on the quiet-Sun corona. By
considering a sequence of observed magnetograms, photospheric flux fragments are represented by a series of point sources
and the resulting potential field arising from them is examined. It is found that the quiet-Sun coronal flux is generally
recycled on time scales considerably shorter than the corresponding time scales for the recycling of photospheric flux. From
the motions of photospheric fragments alone, a recycling time of coronal flux of around 3 h is found. However, it is found
that the amount of reconnection driven by the motions of fragments is comparable to the amount driven by emergence and cancellation
of flux, resulting in a net flux replacement time for the corona of only 1.4 h.
The technique used in this study was briefly presented in a short research letter (R. M. Close et al., Astrophys. J., 612, L81, 2004); here the technique is discussed in far greater depth. Furthermore, an estimate is made of the currents required
to flow along separator field lines in order to sustain the observed heating rates (assuming separator reconnection is the
key mechanism by which the solar corona is heated). 相似文献
9.
Reversed-polarity structures of chromospheric magnetic fields are magnetic gulfs and islands of opposite polarity relative
to the underlying photospheric fields. In this paper data measured with the Solar Magnetic Field Telescope of the Huairou
Solar Observing Station in Beijing were analyzed. From more than 300 pairs of photospheric magnetograms (in FeI λ5324.19 Å) and relevant chromospheric magnetograms (Hβ λ4861.34 Å), the reality of the reversed-polarity structures is demonstrated. According to an analysis of the fine structure
of the magnetic field in the two layers of active regions, we found that there are probably four different types as follows:
Type A: magnetic islands of opposite polarity corresponding to photospheric fields appear in the chromospheric magnetogram.
Type B: magnetic gulfs of opposite polarity corresponding to photospheric fields appear in the chromospheric magnetogram.
Type C is the reverse of type B. That is, a magnetic gulf of opposite polarity corresponding to the chromospheric field appears
in the photospheric magnetogram.
Type D is the reverse of type A. 相似文献
10.
This article is the third in a series working towards the construction of a realistic, evolving, non-linear force-free coronal-field model for the solar magnetic carpet. Here, we present preliminary results of 3D time-dependent simulations of the small-scale coronal field of the magnetic carpet. Four simulations are considered, each with the same evolving photospheric boundary condition: a 48-hour time series of synthetic magnetograms produced from the model of Meyer et al. (Solar Phys. 272, 29, 2011). Three simulations include a uniform, overlying coronal magnetic field of differing strength, the fourth simulation includes no overlying field. The build-up, storage, and dissipation of magnetic energy within the simulations is studied. In particular, we study their dependence upon the evolution of the photospheric magnetic field and the strength of the overlying coronal field. We also consider where energy is stored and dissipated within the coronal field. The free magnetic energy built up is found to be more than sufficient to power small-scale, transient phenomena such as nanoflares and X-ray bright points, with the bulk of the free energy found to be stored low down, between 0.5?–?0.8 Mm. The energy dissipated is currently found to be too small to account for the heating of the entire quiet-Sun corona. However, the form and location of energy-dissipation regions qualitatively agree with what is observed on small scales on the Sun. Future MHD modelling using the same synthetic magnetograms may lead to a higher energy release. 相似文献
11.
Unipolar streamers (also known as pseudo-streamers) are coronal structures that, at least in coronagraph images, and when
viewed at the correct orientation, are often indistinguishable from dipolar (or “standard”) streamers. When interpreted with
the aid of a coronal magnetic field model, however, they are shown to consist of a pair of loop arcades. Whereas dipolar streamers
separate coronal holes of the opposite polarity and whose cusp is the origin of the heliospheric current sheet, unipolar streamers
separate coronal holes of the same polarity and are therefore not associated with a current sheet. In this study, we investigate
the interplanetary signatures of unipolar streamers. Using a global MHD model of the solar corona driven by the observed photospheric
magnetic field for Carrington rotation 2060, we map the ACE trajectory back to the Sun. The results suggest that ACE fortuitously
traversed through a large and well-defined unipolar streamer. We also compare heliospheric model results at 1 AU with ACE
in-situ measurements for Carrington rotation 2060. The results strongly suggest that the solar wind associated with unipolar streamers
is slow. We also compare predictions using the original Wang–Sheeley (WS) empirically determined inverse relationship between
solar wind speed and expansion factor. Because of the very low expansion factors associated with unipolar streamers, the WS
model predicts high speeds, in disagreement with the observations. We discuss the implications of these results in terms of
theories for the origin of the slow solar wind. Specifically, premises relying on the expansion factor of coronal flux tubes
to modulate the properties of the plasma (and speed, in particular) must address the issue that while the coronal expansion
factors are significantly different at dipolar and unipolar streamers, the properties of the measured solar wind are, at least
qualitatively, very similar. 相似文献
12.
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
13.
Interfacing MHD Single Fluid and Kinetic Exospheric Solar Wind Models and Comparing Their Energetics
An exospheric kinetic solar wind model is interfaced with an observation-driven single-fluid magnetohydrodynamic (MHD) model. Initially, a photospheric magnetogram serves as observational input in the fluid approach to extrapolate the heliospheric magnetic field. Then semi-empirical coronal models are used for estimating the plasma characteristics up to a heliocentric distance of 0.1 AU. From there on, a full MHD model that computes the three-dimensional time-dependent evolution of the solar wind macroscopic variables up to the orbit of Earth is used. After interfacing the density and velocity at the inner MHD boundary, we compare our results with those of a kinetic exospheric solar wind model based on the assumption of Maxwell and Kappa velocity distribution functions for protons and electrons, respectively, as well as with in situ observations at 1 AU. This provides insight into more physically detailed processes, such as coronal heating and solar wind acceleration, which naturally arise from including suprathermal electrons in the model. We are interested in the profile of the solar wind speed and density at 1 AU, in characterizing the slow and fast source regions of the wind, and in comparing MHD with exospheric models in similar conditions. We calculate the energetics of both models from low to high heliocentric distances. 相似文献
14.
Hakamada Kazuyuki Kojima Masayoshi Tokumaru Munetoshi Ohmi Tomoaki Yokobe Atsushi Fujiki Ken'ichi 《Solar physics》2002,207(1):173-185
Relationships between solar wind speed and expansion rate of the coronal magnetic field have been studied mainly by in-ecliptic observations of artificial satellites and some off-ecliptic data by Ulysses. In this paper, we use the solar wind speed estimated by interplanetary scintillation (IPS) observations in the whole heliosphere. Two synoptic maps of SWS estimated by IPS observations are constructed for two Carrington rotations CR 1830 and 1901; CR 1830 starting on the 11th of June, 1990 is in the maximum phase of solar activity cycle and CR 1901 starting on the 29th of September, 1995 is in the minimum phase. Each of the maps consist of 64800 (360×180) data points. Similar synoptic maps of expansion rate of the coronal magnetic field (RBR) calculated by the so-called potential model are also constructed under a radial field assumption for CR 1830 and CR1901. Highly significant correlation (r=–0.66) is found between the SWS and the RBR during CR1901 in the solar minimum phase; that is, high-speed winds emanate from photospheric areas corresponding to low expansion rate of the coronal magnetic field and low speed winds emanate from photospheric areas of high expansion rate. A similar result is found during CR 1830 in solar maximum phase, though the correlation is relatively low (r=–0.29). The correlation is improved when both the data during CR 1830 and CR 1901 are used together; the correlation coefficient becomes –0.67 in this case. These results suggest that the correlation analysis between the SWS and the RBR can be applied to estimate the solar wind speed from the expansion rate of the coronal magnetic field, though the correlation between them may depend on the solar activity cycle. We need further study of correlation analysis for the entire solar cycle to get an accurate empirical equation for the estimation of solar wind speed. If the solar wind speed is estimated successfully by an empirical equation, it can be used as an initial condition of a solar wind model for space weather forecasts. 相似文献
15.
A coronal hole and its identification as the source of a high velocity solar wind stream 总被引：3，自引：0，他引：3
X-ray images of the solar corona, taken on November 24, 1970, showed a magnetically open structure in the low corona which extended from N20W20 to the south pole. Analysis of the measured X-ray intensities shows the density scale height within the structure to be typically a factor of two less than that in the surrounding large scale magnetically closed regions. The structure is identified as a coronal hole.Since there have been several predictions that such a region should be the source of a high velocity stream in the solar wind, wind measurements for the appropriate period were traced back to the Sun by the method of instantaneous ideal spirals. A striking agreement was found between the Carrington longitude of the solar source of a recurrent high velocity solar wind stream and the position of the hole.Solar wind bulk velocity and photospheric magnetic field data from the period 1962–1970 indicate the possible extension of the result to the interpretation of long term variations in the wind pattern. 相似文献
16.
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
17.
Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at λ=1.76cm are used for estimating the magnetic field strength in the upper chromosphere, based on thermal bremsstrahlung. The results are compared with the magnetic field strength in the photosphere from observations with the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of Beijing Astronomical Observatory. The difference in the magnetic field strength between the two layers seems reasonable. The solar radio maps of active regions obtained with the Nobeyama radioheliograph, both in total intensity (I-map) and in circular polarizations (V-map), are compared with the optical magnetograms obtained with the SMFT. The comparison between the radio map in circular polarization and the longitudinal photospheric magnetogram of a plage region suggest that the radio map in circular polarization is a kind of magnetogram of the upper chromosphere. The comparison of the radio map in total intensity with the photospheric vector magnetogram of an AR shows that the radio map in total intensity gives indications of magnetic loops in the corona, thus we have a method of defining the coronal magnetic structure from the radio I-maps at λ=1.76 cm. Analysing the I-maps, we identified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field. 相似文献
18.
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
19.
Using Fe ix/x 17.1 nm observations from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), we have identified many coronal plumes inside low-latitude coronal holes as they transited the solar limb during the late
declining phase of cycle 23. These diffuse, linear features appear to be completely analogous to the familiar polar plumes.
By tracking them as they rotate from the limb onto the disk (or vice versa), we confirm that EUV plumes seen against the disk appear as faint, diffuse blobs of emission surrounding a brighter core.
When the EIT images are compared with near-simultaneous magnetograms from the SOHO Michelson Doppler Imager (MDI), the low-latitude, on-disk plumes are found to overlie regions of mixed polarity, where small
bipoles are in contact with unipolar flux concentrations inside the coronal hole. The birth and decay of the plumes are shown
to be closely related to the emergence of ephemeral regions, their dispersal in the supergranular flow field, and the cancellation
of the minority-polarity flux against the dominant-polarity network elements. In addition to the faint polar and nonpolar
plumes associated with ephemeral regions, we note the existence of two topologically similar coronal structures: the giant
plume-like features that occur above active regions inside coronal holes, and the even larger scale “pseudostreamers” that
separate coronal holes of the same polarity. In all three cases, the basic structure consists of open field lines of a given
polarity overlying a photospheric region of the opposite polarity; ongoing interchange reconnection at the X-point separating
the open field domains from the underlying double-arcade system appears to result in the steady evaporation of material from
the closed into the open region. 相似文献
20.
Extrapolation of the solar magnetic field within the potential-field approximation from full-disk magnetograms 总被引：3，自引：0，他引：3
G.V. Rudenko 《Solar physics》2001,198(1):5-30
This paper is concerned with the Laplace boundary-value problem with the directional derivative, corresponding to the specific nature of measurements of the longitudinal component of the photospheric magnetic field. The boundary conditions are specified by a distribution on the sphere of the projection of the magnetic field vector into a given direction, i.e., they exactly correspond to the data of daily magnetograms distributed across the full solar disk. It is shown that the solution of this problem exists in the form of a spherical harmonic expansion, and uniqueness of this solution is proved. A conceptual sketch of numerical determination of the harmonic series coefficients is given. The field of application of the method is analyzed with regard to the peculiarities of actual data. Results derived from calculating magnetic fields from real magnetograms are presented. Finally, we present differences in results derived from extrapolating the magnetic field from a synoptic map and a full-disk magnetogram. 相似文献