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1.
We study a time – latitudinal distribution of CMEs observed by the SOHO spacecraft, their projected speeds and associated
magnetic fields, as well as the north – south (N – S) asymmetry of solar surface magnetic fields, and the coronal green line
intensities. We have found that (a) there exists an intricate relation between the average projected velocity of CMEs and
the mean value of large-scale magnetic fields; (b) there exists a pronounced N – S asymmetry in both the distribution and
the number of CMEs; (c) this asymmetry is in favor of the northern hemisphere at the beginning of the cycle, and of the southern
hemisphere from 2001 onward, being, in fact, (d) closely related with the N – S asymmetry in the distribution of large-scale
magnetic fields and the coronal green line intensities. 相似文献
2.
We present results of a study of the green corona (530.3 nm, Fexiv) and photospheric magnetic fields over the period 1976–1999 when both quantities were observed by ground-based observatories. By comparing both the limb green-line intensities and photospheric magnetograms we have found a relation between the strength of magnetic field and coronal intensities. This relation enables us to extend solar surface magnetic fields since 1976 back to 1939. From 1947 to 1992 the magnetic field strength grew at the cycle maxima by a factor of 1.5–2. On the other hand, both the green corona intensity and magnetic field strength in the present cycle are smaller compared to cycle 22, by a factor of 2. No relationship was found between the green corona intensities and magnetic field polarity as was previously supposed. Behavior for the green corona intensities is different between high-latitude and mid-latitude regions, and this break occurs at the heliographic latitude of ± 45°. Homogeneous coronal data set cannot be directly used to derive `the tilt angle', even though some similarities between the green coronal holes, poleward branches in the green corona and prominences and the tilt angle can be found. 相似文献
3.
Large-scale coronal structures (helmet streamers) observed in the white-light corona during total solar eclipses and/or with
ground-based coronagraphs are mostly located only above quiescent types of prominences. These helmet streamers are maintained
due to the magnetic fields of the Sun. Time–latitudinal distribution of prominences during a solar cycle, however, shows both
the poleward and equatorward migrations, similar to the 530.3 nm emission corona (the green corona) intensities. Distribution
of observed coronal helmet streamers during total solar eclipses, enlarged with the helmet streamers as were obtained by the
ground-based coronagraph observations, are compared with the heliographic distribution of prominences and the green corona
intensities for the first time. It is shown that the distribution of above-mentioned helmet streamers, reflects – roughly
– the time–latitudinal distribution of prominences and emission corona branches, and migrates together with them over a solar
cycle. 相似文献
4.
Employing the synoptic maps of the photospheric magnetic fields from the beginning of solar cycle 21 to the end of 23, we
first build up a time – longitude stackplot at each latitude between ±35°. On each stackplot there are many tilted magnetic
structures clearly reflecting the rotation rates, and we adopt a cross-correlation technique to explore the rotation rates
from these tilted structures. Our new method avoids artificially choosing magnetic tracers, and it is convenient for investigating
the rotation rates of the positive and negative fields by omitting one kind of field on the stackplots. We have obtained the
following results. i) The rotation rates of the positive and negative fields (or the leader and follower polarities, depending on the hemispheres
and solar cycles) between latitudes ±35° during solar cycles 21–23 are derived. The reversal times of the leader and follower
polarities are usually not consistent with the years of the solar minimum, nevertheless, at latitudes ±16°, the reversal times
are almost simultaneous with them. ii) The rotation rates of the three solar cycles averaged over each cycle are calculated separately for the positive, negative
and total fields. The latitude profiles of rotation of the positive and negative fields exhibit equatorial symmetries with
each other, and those of the total fields lie between them. iii) The differences in rotation rates between the leader and follower polarities are obtained. They are very small near the
equator, and increase as latitude increases. In the latitude range of 5° – 20°, these differences reach 0.05 deg day−1, and the mean difference for solar cycle 22 is somewhat smaller than cycles 21 and 23 in these latitude regions. Then, the
differences reduce again at latitudes higher than 20°. 相似文献
5.
Magnetic field structures of Hα flares associated with meter-wave type III bursts during periods of low solar activity in
1975 – 1977 and 1985 – 1987 were investigated. In a statistical analysis it was confirmed that the association rate depends
less on flare importance than on brightness. For subflares (95% of the sample), the location of the Hα flare in the bipolar
pattern turned out to be crucial for the association rate. It is almost one order of magnitude larger for flares occurring
at the border of the active regions, compared to flares located inside the general bipolar pattern. For selected typical examples
of flares, extrapolations of the measured magnetic fields were performed. By matching Hα filtergrams and calculated 3-D structures
it was found that the positions at the border where the flares associated with type III bursts occurred were close to open
field lines extending into the corona. In most investigated cases intrusions of parasitic polarity were found in the vicinity
of the flare locations. The extrapolations showed that subflares located inside the bipolar pattern but have not been associated
with type III bursts were covered by dense arcades of magnetic loops. 相似文献
6.
Oscillations of magnetic structures in the solar corona have often been interpreted in terms of magnetohydrodynamic waves.
We study the adiabatic magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal magnetic field, surrounded
by a prominence – corona transition region (PCTR) and a coronal medium. Considering linear small-amplitude oscillations, we
deduce the dispersion relation for the magnetoacoustic slow and fast modes by assuming evanescentlike perturbations in the
coronal medium. In the system without PCTR, a classification of the oscillatory modes according to the polarisation of their
eigenfunctions is made to distinguish modes with fastlike or slowlike properties. Internal and external slow modes are governed
by the prominence and coronal properties, respectively, and fast modes are mostly dominated by prominence conditions for the
observed wavelengths. In addition, the inclusion of an isothermal PCTR does not substantially influence the mode frequencies,
but new solutions (PCTR slow modes) are present. 相似文献
7.
We study the relationship between full-disk solar radiative flux at different wavelengths and average solar photospheric magnetic-flux
density, using daily measurements from the Kitt Peak magnetograph and other instruments extending over one or more solar cycles.
We use two different statistical methods to determine the underlying nature of these flux – flux relationships. First, we
use statistical correlation and regression analysis and show that the relationships are not monotonic for total solar irradiance
and for continuum radiation from the photosphere, but are approximately linear for chromospheric and coronal radiation. Second,
we use signal theory to examine the flux – flux relationships for a temporal component. We find that a well-defined temporal
component exists and accounts for some of the variance in the data. This temporal component arises because active regions
with high magnetic-field strength evolve, breaking up into small-scale magnetic elements with low field strength, and radiative
and magnetic fluxes are sensitive to different active-region components. We generate empirical models that relate radiative
flux to magnetic flux, allowing us to predict spectral-irradiance variations from observations of disk-averaged magnetic-flux
density. In most cases, the model reconstructions can account for 85 – 90% of the variability of the radiative flux from the
chromosphere and corona. Our results are important for understanding the relationship between magnetic and radiative measures
of solar and stellar variability. 相似文献
8.
Solar filaments show the position of large-scale polarity-inversion lines and are used for the reconstruction of large-scale
solar magnetic field structure on the basis of Hα synoptic charts for the periods that magnetographic measurements are not
available. Sometimes crossing filaments are seen in Hα filtergrams. We analyze daily Hα filtergrams from the archive of Big
Bear Solar Observatory for the period of 1999 – 2003 to find crossing and interacting filaments. A number of examples are
presented and filament patterns are compared with photospheric magnetic field distributions. We have found that all crossing
filaments reveal quadrupolar magnetic configurations of the photospheric field and presume the presence of null points in
the corona. 相似文献
9.
The intensity of Fe XIV 530.3-nm green coronal line is compared quantitatively with the strength of magnetic fields of small and large scales and also with total sunspot areas for 1977–2001. A degree of similarity of appropriate synoptic maps is evaluated using correlation analysis. The green line intensity maps are constructed from data of its daily monitoring. Strengths of magnetic fields are calculated in a potential approximation using the photosphere observations of Wilcox Solar Observatory for a distance of 1.1 The calculations are performed separately for fields of large and small spatial scales. The total area of sunspots is obtained using data from the Greenwich Catalogue and its continuation by USAF/NOAA. The correlation has been calculated for the aggregate of areas (with a size of 20° in latitude and 30° in longitude) coinciding spatially on all maps. It is found that the most correlation between the green line intensity and coronal fields of small scales is observed in a zone of 0°–20°. The correlation with total sunspot areas (i.e., with local fields at the photosphere level) is substantially less here. In the higher-latitude zone 20°–40°, correlation of the green-line intensity with spot areas and small-scale coronal fields decreases. The large-scale fields have little influence on the green-line emission in the spot-formation zone. These results are the evidence of a complex nature of the effect of different-scale fields, arising as a result of dynamo activity in the subsurface (leptocline) and deep-lying (tachocline) layers of the convective zone, on the processes of the Sun’s corona heating and green coronal line emission. 相似文献
10.
Extreme-ultraviolet data from EIT/SOHO (1996–2002), soft X-ray data from Yohkoh (1991–2001), and magnetic field data from MDI/SOHO (1996–2002) and Kitt Peak Observatory, NSO/NOAO (1991–2002) are analyzed
together in the form of synoptic maps for the investigation of solar cycle variations of the corona and their relation to
the magnetic field. These results show new interesting relations between the evolution of the topological structure of the
corona, coronal heating and the large-scale magnetic field. The long-lived coronal structures are related to complexes of
solar activity and display quasi-periodic behavior (in the form of impulses of coronal activity) with periods of 1.0–1.5 year,
in the axisymmetric distribution of EUV and X-ray fluxes during the current solar cycle 23. In particular, during the second
maximum of this cycle the solar corona became somewhat hotter than it was in the period of the first maximum. 相似文献
11.
To study the quantitative relationship between the brightness of the coronal green line 530.5 nm Fe xiv and the strength of the magnetic field in the corona, we have calculated the cross-correlation of the corresponding synoptic
maps for the period 1977 – 2001. The maps of distribution of the green-line brightness I were plotted using every-day monitoring data. The maps of the magnetic field strength B and the tangential B
t and radial B
r field components at the distance 1.1 R
⊙ were calculated under potential approximation from the Wilcox Solar Observatory (WSO) photospheric data. It is shown that
the correlation I with the field and its components calculated separately for the sunspot formation zone ±30° and the zone 40 – 70° has a cyclic
character, the corresponding correlation coefficients in these zones changing in anti-phase. In the sunspot formation zone,
all three coefficients are positive and have the greatest values near the cycle minimum decreasing significantly by the maximum.
Above 40°, the coefficients are alternating in sign and reach the greatest positive values at the maximum and the greatest
negative values, at the minimum of the cycle. It is inferred that the green-line emission in the zone ±30° is mainly controlled
by B
t, probably due to the existence of low arch systems. In the high-latitude zone, particularly at the minimum of the cycle,
an essential influence is exerted by B
r, which may be a manifestation of the dominant role of large-scale magnetic fields. Near the activity minimum, when the magnetic
field organization is relatively simple, the relation between I and B for the two latitudinal zones under consideration can be represented as a power-law function of the type I ∝ B
q. In the sunspot formation zone, the power index q is positive and varies from 0.75 to 1.00. In the zone 40 – 70°, it is negative and varies from −0.6 to −0.8. It is found
that there is a short time interval approximately at the middle of the ascending branch of the cycle, when the relationship
between I and B vanishes. The results obtained are considered in relation to various mechanisms of the corona heating. 相似文献
12.
Yu Liu 《Solar physics》2008,249(1):75-84
Liu et al. (Astrophys. J.
628, 1056, 2005a) described one surge – coronal mass ejection (CME) event showing a close relationship between solar chromospheric surge ejection
and CME that had not been noted before. In this work, large Hα surges (>72 Mm, or 100 arcsec) are studied. Eight of these
were associated with CMEs. According to their distinct morphological features, Hα surges can be classified into three types:
jetlike, diffuse, and closed loop. It was found that all of the jetlike surges were associated with jetlike CMEs (with angular
widths ≤30 degrees); the diffuse surges were all associated with wide-angle CMEs (e.g., halo); the closed-loop surges were not associated with CMEs. The exclusive relation between Hα surges and CMEs indicates
difference in magnetic field configurations. The jetlike surges and related narrow CMEs propagate along coronal fields that
are originally open. The unusual transverse mass motions in the diffuse surges are suggested to be due to magnetic reconnections
in the corona that produce wide-angle CMEs. For the closed-loop surges, their paths are just outlining stable closed loops
close to the solar surface. Thus no CMEs are associated with them. 相似文献
13.
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
14.
Mutual quasi-periodicities near the solar-rotation period appear in time series based on the Earth’s magnetic field, the interplanetary
magnetic field, and signed solar-magnetic fields. Dominant among these is one at 27.03±0.02 days that has been highlighted
by Neugebauer et al. (J. Geophys. Res.
105, 2315, 2000). Extension of their study in time and to different data reveals decadal epochs during which the ≈ 27.0 days, or a ≈ 28.3 days,
or other quasi-periods dominate the signal. Space-time eigenvalue analyses of time series in 30 solar latitude bands, based
on synoptic maps of unsigned photospheric fields, lead to two maximally independent modes that account for almost 30% of the
data variance. One mode spans 45° of latitude in the northern hemisphere and the other one in the southern. The modes rotate
around the Sun rigidly, not differentially, suggesting connection with the subsurface dynamo. Spectral analyses yield familiar
dominant quasi-periods 27.04±0.03 days in the North and at 28.24±0.03 days in the South. These are replaced during cycle 23
by one at 26.45±0.03 days in the North. The modes show no tendency for preferred longitudes separated by ≈ 180°. 相似文献
15.
Richard Woo 《Solar physics》2005,231(1-2):71-85
The solar magnetic field is key to a detailed understanding of the Sun's atmosphere and its transition to the solar wind.
However, the lack of detailed magnetic field measurements everywhere except at the photosphere has made it challenging to
determine its topology and to understand how it produces the observed plasma properties of the corona and solar wind. Recent
progress based on the synthesis of diversified observations has shown that the corona is highly filamentary, that the coronal
magnetic field is predominantly radial, and that the ability of closed fields to trap plasma at the base of the corona is
a manifestation of how the solar field controls the solar wind. In this paper, we explain how these results are consistent
with the relationship between density structure of white-light images and fields and flow. We point out that the ‘shape’ of
the corona observed in white-light images is a consequence of the steep fall-off in density with radial distance, coupled
with the inherent limitation in the sensitivity of the observing instrument. We discuss how the significant variation in radial
density fall-off with latitude leads to a coronal shape that is more precisely revealed when a radial gradient filter is used,
but which also gives a false impression of the tracing of highly non-radial fields. Instead, the coronal field is predominantly
radial, and the two magnetic features that influence the shape of the corona are the closed fields at the base of the corona,
and the polarity reversal forming the heliospheric current sheet in the outer corona.
An erratum to this article is available at . 相似文献
16.
C. Bouratzis P. Preka-Papadema A. Hillaris P. Tsitsipis A. Kontogeorgos V. G. Kurt X. Moussas 《Solar physics》2010,267(2):343-359
We present a multi-frequency and multi-instrument study of the 20 January 2005 event. We focus mainly on the complex radio
signatures and their association with the active phenomena taking place: flares, CMEs, particle acceleration, and magnetic
restructuring. As a variety of energetic-particle accelerators and sources of radio bursts are present, in the flare – ejecta
combination, we investigate their relative importance in the progress of this event. The dynamic spectra of ARTEMIS-IV – Wind/Waves – HiRAS, with 2000 MHz – 20 kHz frequency coverage, were used to track the evolution of the event from the low corona to
the interplanetary space; these were supplemented with SXR, HXR, and γ-ray recordings. The observations were compared with the expected radio signatures and energetic-particle populations envisaged
by the Standard Flare – CME model and the reconnection outflow termination shock model. A proper combination of these mechanisms seems to provide an adequate model for the interpretation of the observational
data. 相似文献
17.
STEREO A and B observations of the radial magnetic field between 1 January 2007 and 31 October 2008 show significant evidence
that in the heliosphere, the ambient radial magnetic field component with any dynamic effects removed is uniformly distributed.
Based on this monopolar nature of the ambient heliospheric field we find that the surface beyond which the magnetic fields
are in the monopolar configuration must be spherical, and this spherical surface can be defined as the inner boundary of the
heliosphere that separates the monopole-dominated heliospheric magnetic field from the multipole-dominated coronal magnetic
field. By using the radial variation of the coronal helmet streamers belts and the horizontal current – current sheet – source
surface model we find that the spherical inner boundary of the heliosphere should be located around 14 solar radii near solar
minimum phase. 相似文献
18.
V. A. Sheminova 《Solar physics》2009,254(1):29-50
The properties of solar magnetic fields on scales less than the spatial resolution of solar telescopes are studied. A synthetic
infrared spectropolarimetric diagnostic based on a 2D MHD simulation of magnetoconvection is used for this. Analyzed are two
time sequences of snapshots that likely represent two regions of the network fields with their immediate surroundings on the
solar surface with unsigned magnetic flux densities of 300 and 140 G. In the first region from the probability density functions
of the magnetic field strength it is found that the most probable field strength at log τ
5=0 is equal to 250 G. Weak fields (B<500 G) occupy about 70% of the surface, whereas stronger fields (B>1000 G) occupy only 9.7% of the surface. The magnetic flux is −28 G and its imbalance is −0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3%, 0.3%, −40 G, and −0.10. The distribution of line-of-sight velocities
on the surface of log τ
5=−1 is estimated. The mean velocity is equal to 0.4 km s−1 in the first simulated region. The average velocity in the granules is −1.2 km s−1 and in the intergranules it is 2.5 km s−1. In the second region, the corresponding values of the mean velocities are equal to 0, −1.8, and 1.5 km s−1. In addition the asymmetry of synthetic Stokes V profiles of the Fe i 1564.8 nm line is analyzed. The mean values of the amplitude and area asymmetry do not exceed 1%. The spatially smoothed
amplitude asymmetry is increased to 10% whereas the area asymmetry is only slightly varied. 相似文献
19.
Mark Wardle 《Astrophysics and Space Science》2007,311(1-3):35-45
Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently
transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface.
Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous
response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary
disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in
a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function
of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons
and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have
grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the
disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not,
the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except
at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. For a standard
population of 0.1 μm grains the active surface layers have a combined column Σactive≈2 g cm−2 at 1 AU; by the time grains have aggregated to 3 μm, Σactive≈80 g cm−2. Ionisation in the active layers is dominated by stellar X-rays. In the absence of grains, X-rays maintain magnetic coupling
to 10% of the disk material at 1 AU (i.e. Σactive≈150 g cm−2). At 5 AU the Σactive≈Σtotal once grains have aggregated to 1 μm in size. 相似文献
20.
C. O. Lee J. G. Luhmann D. Odstrcil P. J. MacNeice I. de Pater P. Riley C. N. Arge 《Solar physics》2009,254(1):155-183
We present results of solar-wind parameters generated by 3D MHD models. The ENLIL inner-heliosphere solar-wind model together
with the MAS or Wang – Sheeley – Arge (WSA) coronal models, describe the steady solar-wind stream structure and its origins
in the solar corona. The MAS/ENLIL and WSA/ENLIL models have been tuned to provide a simulation of plasma moments as well
as interplanetary magnetic-field magnitude and polarity in the absence of disturbances from coronal transients. To investigate
how well the models describe the ambient solar wind structure from the Sun out to 1 AU, the model results are compared to
solar-wind measurements from the ACE spacecraft. We find that there is an overall agreement between the observations and the
model results for the general large-scale solar-wind structures and trends, such as the timing of the high-density structures
and the low- and high-speed winds, as well as the magnetic sector structures. The time period of our study is the declining
phase of Solar Cycle 23 when the solar activity involves well-defined stream structure, which is ideal for testing a quasi-steady-state
solar-wind model. 相似文献