共查询到20条相似文献,搜索用时 15 毫秒
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We report observations of the large-scale spatial dependence of the Sun's luminosity variations over the period 1993–1995. The measurements were made using a new scanning disk solar photometer at Big Bear Solar Observatory, specially designed to measure large-scale brightness variations at the 10–4 level. Since the level of solar activity was very low for the entire observation period, the data show little solar cycle variation. However, the residual brightness signal I/I (after subtracting the mean, first, and second harmonics) does show a strong dependence on heliocentric angle, peaking near the limb. This is as one would expect if the residual brightness signal (including the excess brightness coming from the active latitudes) were primarily facular in origin. Additional data over the next few years, covering the period from solar minimum to maximum, should unambiguously reveal the large-scale spatial structure of the solar cycle luminosity variations. 相似文献
4.
Theoretical model, explaining a phenomenon of formation of Intensive Magnetic Flux Tube (IMFT) in a converging flow of partially
ionized solar photospheric plasma is considered. Special attention is paid to the fact of weak ionization (n/n
n ∼ 10-4) of plasma in the photosphere. The cases of 2D magnetic slab and cylindric magnetic tube are considered. It was shown that
in a converging flow of photospheric plasma thin magnetic tubes, or slabs with the characteristic scale L
0 ∼ (1 ÷ 5) ċ 107 cm and magnetic field 1000 ÷ 2000 G can be generated. By this 2D magnetic slabs could be unstable with respect to an exchange instability and appear as an intermediate
step during IMFT formation on the boundary of two supergranulation cells. Formation of compact strong magnetic field structures,
and their energy balance are discussed. Stationary Joule energy dissipation taking place on the photospheric levels in the
models of magnetic slab or IMFT under consideration increases towards the periphery of these objects and can exceed radiation
looses. This can cause the occurrence of magnetic tubes with hot external envelopes, and modification of plasma temperature
and density distribution, with respect to ones in a quiet atmosphere.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
The modes of oscillation of a twisted magnetic flux tube in an incompressible medium are investigated analytically. An exact dispersion relation for the case of uniform twist is obtained. In contrast to the case of an untwisted incompressible tube, body, surface, and hybrid (surface-body) modes arise. 相似文献
6.
We consider turbulence generated by galaxies moving transonically through the intracluster gas. We show that neither the gravitational
drag nor the gas stripping from the galaxies are able, by themselves, to generate turbulence at a level required to feed the
dynamo in the intracluster gas. Some implications for cluster radio halos are discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
An Evolving Synoptic Magnetic Flux map and Implications for the Distribution of Photospheric Magnetic Flux 总被引:1,自引:0,他引:1
We describe a procedure intended to produce accurate daily estimates of the magnetic flux distribution on the entire solar surface. Models of differential rotation, meridional flow, supergranulation, and the random emergence of background flux elements are used to regularly update unobserved or poorly observed portions of an initial traditional magnetic synoptic map that acts as a seed. Fresh observations replace model estimates when available. Application of these surface magnetic transport models gives us new insight into the distribution and evolution of magnetic flux on the Sun, especially at the poles where canopy effects, limited spatial resolution, and foreshortening result in poor measurements. We find that meridional circulation has a considerable effect on the distribution of polar magnetic fields. We present a modeled polar field distribution as well as time series of the difference between the northern and southern polar magnetic flux; this flux imbalance is related to the heliospheric current sheet tilt. We also estimate that the amount of new background magnetic flux needed to sustain the `quiet-Sun' magnetic field is about 1.1×1023 Mx d–1 (equivalent to several large active regions) at the spatial resolution and epoch of our maps. We comment on the diffusive properties of supergranules, ephemeral regions, and intranetwork flux. The maps are available on the NSO World Wide Web page. 相似文献
8.
Coronal Magnetic Flux Rope Equilibria and Magnetic Helicity 总被引:1,自引:0,他引:1
1 INTRODUCTIONObservations show that the magnetic helicity of solar magnetic structures has a predominantsign in each hemisphere of the Sun, positive in the southern hemisphere and negative in thenorthern, regardless of the solar cycle (Rust, 1994). The magnetic helicity is strictly conservedin the frame of ideal MHD (WOltjer, 1958), and approximately conserved in the presence ofresistive dissipation and magnetic reconnection in a highly conductive plajsma (Taylor, 1974;Berger, 1984; H… 相似文献
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A.L. Taktakishvili A. Greco P. Veltri G. Zimbardo L.M. Zelenyi A.V. Milovanov 《Astrophysics and Space Science》2001,277(1-2):71-79
The ion dynamics in the Earth's magnetotail is studied in the case when a cross tail electric field E
0 and reconnection-driven magnetic turbulence are present in the current sheet. The magnetic turbulence observed by the Interball
spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. A test particle simulation is performed for
the ions, and the distribution function moments are obtained as a function of the magnetic fluctuation level, δB/B
0, and of the value of the normal component B
n. It appears that even in the presence of magnetic turbulence, the normal component has a marked influence on particle dynamics:
the ion bulk velocity along E
y and ion temperature are almost inversely proportional to B
n. The magnetic turbulence causes the current to split in two layers, and the level of magnetic fluctuations needed to have
splitting is roughly proportional to B
n. It appears that in the relevant range of parameters, B
n and δB/B
0 have opposite effects on the current structure and on ion heating.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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We analyze the role of weak photospheric flux concentrations that evolve in a filament channel, in the triggering of dynamic
changes in the shape of a filament. The high polarimetric sensitivity of THEMIS allowed us to detect weak flux concentrations
(few Gauss) associated with the filament development. The synoptic instruments (MDI, SOLIS) even if their sensitivity is much
less than THEMIS were useful to follow any subsequent strengthening of these flux concentrations after their identification
in the THEMIS magnetograms. We found that (1) the northern part of the filament develops an Hα barb at the same time that
weak minority polarity elements develop near a plage; (2) a section in the southern part of the Hα filament gradually disappears
and later reforms at the same time that several mixed-polarity magnetic elements appear, then subsequently cancel or spread
away from each other. These changes correspond to increases in EUV emission, as observed by TRACE, EIT, and CDS. This suggests
that the plasma is temporarily heated along the filament spine. An idealized sequence of force-free models of this filament
channel, based on plasma-supporting magnetic dips occurring in the windings of a very weakly twisted flux tube, naturally
explains the evolution of its southern part as being due to changes in the topology of the coronal magnetic field as the photospheric
flux concentrations evolve. 相似文献
12.
In this paper we report an analysis of the solar magnetic fluxes estimated in the period 1971–1998. We applied the wavelet
technique to find the significant periodicities of these series. We concentrate particularly in the mid-term quasi-periodicities
(1–2 years). The mid-term periodicity of 1.7 year is the dominant fluctuation for all the types of fluxes analyzed (total,
closed, open, low and high latitude open fluxes) and has a strong tendency to appear during the descending phase of solar
activity. The mid-term fluctuation of 1 year is significantly present in total and closed fluxes, but it is less important
in open fluxes. It is recognizable in the high latitude open flux, but it is absent in the low latitude open flux. Due to
the uncertainties involved in estimating the exact period of the quasi-annual peak, this component may not be different from
the previously-reported 1.3 year periodicity. The high frequency fluctuations of all the fluxes but the high latitude open
flux are in phase with the 11 years solar cycle. The high latitude flux tends to be present all the time, showing that along
the cycle both the low latitude bipolar active regions and the polar coronal holes regulate this flux. These findings rule
out the possibility of a more basic periodicity different from the 11 years cycle. 相似文献
13.
Magnetic flux synoptic charts are critical for a reliable modeling of the corona and heliosphere. Until now, however, these charts were provided without uncertainty estimates. The uncertainties are due to instrumental noise in the measurements and to the spatial variance of the magnetic flux distribution that contributes to each bin in the synoptic chart. We describe here a simple method to compute synoptic magnetic flux maps and their corresponding magnetic flux spatial variance charts that can be used to estimate the uncertainty in the results of coronal models. We have tested this approach by computing a potential-field source-surface model of the coronal field for a Monte Carlo simulation of Carrington synoptic magnetic flux maps generated from the variance map. We show that these uncertainties affect both the locations of source-surface neutral lines and the distributions of coronal holes in the models. 相似文献
14.
The problem of pressure limitations on the rate of flux pile-up magnetic reconnection is studied. We first examine the recent suggestion of Jardine and Allen (1998) for moderating the build-up of magnetic pressure in the current sheet by considering inflows with nonzero vorticity. An analytic argument shows, however, that unbounded magnetic pressures in the limit of small resistivities can be avoided only at the cost of unphysical dynamic pressures in the plasma. Hence, the pressure limitation on the reconnection rate in a low-beta plasma cannot be avoided completely. Nevertheless, we demonstrate that reconnection can be more rapid in a new solution that balances the build-up in dynamic pressure against both the plasma and magnetic pressures. This exact MHD solution has the characteristics of merging driven by the coalescence instability. The maximum energy release rate of the model is capable of explaining a modest solar flare. 相似文献
15.
A. V. Mordvinov 《Solar physics》2007,246(2):445-456
A comparative analysis of solar and heliospheric magnetic fields in terms of their cumulative sums reveals cyclic and long-term
changes that appear as a magnetic flux imbalance and alternations of dominant magnetic polarities. The global magnetic flux
imbalance of the Sun manifests itself in the solar mean magnetic field (SMMF) signal. The north – south asymmetry of solar
activity and the quadrupole mode of the solar magnetic field contribute the most to the observed magnetic flux imbalance.
The polarity asymmetry exhibits the Hale magnetic cycle in both the radial and azimuthal components of the interplanetary
magnetic field (IMF). Analysis of the cumulative sums of the IMF components clearly reveals cyclic changes in the IMF geometry.
The accumulated deviations in the IMF spiral angle from its nominal value also demonstrate long-term changes resulting from
a slow increase of the solar wind speed over 1965 – 2006. A predominance of the positive IMF B
z
with a significant linear trend in its cumulative signal is interpreted as a manifestation of the relic magnetic field of
the Sun. Long-term changes in the IMF B
z
are revealed. They demonstrate decadal changes owing to the 11/22-year solar cycle. Long-duration time intervals with a dominant
negative B
z
component were found in temporal patterns of the cumulative sum of the IMF B
z
. 相似文献
16.
胡文瑞 《中国天文和天体物理学报》1991,(4)
本文分析了被等离子体约束由两个间断面组成的有限厚度磁通量片的Helmholtz不稳定性,导出了不稳定条件,若片的厚度趋于无穷则此条件与一个间断面时相同。还给出了两个间断面的色散关系。结果表明,两个间断面时的磁致稳作用比一个间断面时有所减弱,而约束等离子体的致稳作用相对增强,但两种情况具有相同量级的不稳定增长率。 相似文献
17.
Coronal Flux Rope Equilibria in Closed Magnetic Fields 总被引:1,自引:0,他引:1
Zhen Wang You-Qiu HuSchool of Earth Space Sciences University of Science Technology of China 《中国天文和天体物理学报》2003,3(3):241-246
Using a 2.5-dimensional ideal MHD model in Cartesian coordinates,we investigate the equilibrium properties of coronal magnetic flux ropes in background magnetic fields that are completely closed.The background fields are produced by a dipole,a quadrupole,and an octapole,respectively,located below the photosphere at the same depth.A magnetic flux rope is then launched from below the photo-sphere,and its magnetic properties,i.e,the annular magnetic fluxφp and the axial magnetic fluxφz,are controlled by a single emergence parameter.The whole sys-tem eventually evolves into equilibrium,and the resultant flux rope is characterized by three geometrical parameters:the height of the rope axis,the half-width of the rope,and the length of the vertical current sheet below the rope.It is found that the geometrical parameters increase monotonically and continuously with increasing φp and φz:no catastrophe occurs.Moreover,there exists a steep segment in the profiles of the geometrical parameters versus either φp or φz,and the faster the background field decays with height,the larger both the gradient and the growth amplitude within the steep segment will be. 相似文献
18.
In this paper we study the evolution of magnetic fields of a 1F/2.4C solar flare and following magnetic flux cancellation. The data are Big Bear Solar Observatory and SOHO/MDI observations of active region NOAA 8375. The active region produced a multitude of subflares, many of them being clustered along the moat boundary in the area with mixed polarity magnetic fields. The study indicates a possible connection between the flare and the flux cancellation. The cancellation rate, defined from the data, was found to be 3×1019 Mx h–1. We observed strong upward directed plasma flows at the cancellation site. Suggesting that the cancellation is a result of reconnection process, we also found a reconnection rate of 0.5 km s–1, which is a significant fraction of Alfvén speed. The reconnection rate indicates a regime of fast photospheric reconnection happening during the cancellation. 相似文献
19.
As demonstrated by many previous studies, a system consisting of an isolated coronal flux rope and a surrounding background
magnetic field exhibits a catastrophic behavior. In particular, if the magnetic field of the system is force-free and axisymmetric
in spherical geometry, the magnetic energy at the catastrophic point, referred to as the catastrophic energy threshold, is
found to be larger than the corresponding partly or fully open field energy. This paper takes an octapole field as the background
and introduces a flux rope within the central arcade of the octapole field. A relaxation method based on time-dependent ideal
magnetohydrodynamic (MHD) simulations is used to find axisymmetric force-free field solutions in spherical geometry associated
with the flux rope system. With respect to an increase of either the annular flux Φp or the axial flux Φϕ of the rope, the system exhibits a catastrophic behavior as expected, and the catastrophic energy threshold is larger than
that of the corresponding partly open field, in which the central arcade is opened up, but the remainder remains closed. For
a given octapole field, the energy threshold depends on either Φp or Φϕ at the catastrophic point, and it increases with increasing Φp or decreasing Φϕ. On the other hand, the extent to which the central bipolar component of the octapole field is open also affects the energy
threshold. These results differ from those for the bipolar background field case, in which the catastrophic energy threshold
is almost independent of the magnetic properties of the flux rope at the catastrophic points and the extent to which the background
field is open. The reason for such a difference is briefly discussed. 相似文献
20.
由磁绳结构主导、平均尺度约二、三十个小时的行星际磁云是日冕物质抛射在行星际膨胀、传播的体现。最近,Moldwin等人报道在太阳风中还观测到一些尺度在几十分钟的小尺度磁绳结构,并认为太阳风中的磁绳结构在尺度分布上可能具有双峰特征,在全面检视了WIND卫星(1995年-2000年)和ACE卫星(1998年-2000年)的观测资料后,发现了在行星际太阳风中一些尺度为几个小时的中尺度磁绳结构,利用初步整理的其中28个中尺度磁绳结构事件,认为太阳风中的磁绳结构在尺度分布上可能是连续的,这对行星际太阳风中磁绳结构物理起源的研究可能提出重要的物理限制。 相似文献