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1.
We compare the shape and position of some plasma formations visible in the polar corona with the cyclic evolution of the global
magnetic field. The first type of object is polar crown prominences. A two-fold decrease of the height of polar crown prominences
was found during their poleward migration from the middle latitudes to the poles before a polar magnetic field reversal. The
effect could be assigned to a decrease of the magnetic field scale. The second type of object is the polar plumes, ray like
structures that follow magnetic field lines. Tangents to polar ray structures are usually crossed near some point, “a magnetic
focus,” below the surface. The distance q between the focus and the center of the solar disk changes from the maximum value about 0.65 R
⊙ at solar minimum activity to the minimum value about 0.45 R
⊙ at solar maximum. At first glance this behaviour seems to be contrary to the dynamics of spherical harmonics of the global
magnetic field throughout a cycle. We believe that the problem could be resolved if one takes into account not only scale
changes in the global magnetic field but also the phase difference in the cyclic variations of large-scale and small-scale
components of the global field. 相似文献
2.
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
3.
Xing-Xiang Wang 《Astrophysics and Space Science》2004,293(4):433-440
Some locally rotationally symmetric (LRS) Bianchi type I cosmological models for a cloud string with bulk viscosity and magnetic
field are presented. Where an equation of state ρ = kλ and a relation between metric potential R = AS
n are considered. The solution describes a shearing and nonrotating model with a big bang start. In the absence of magnetic
field it reduces to a string model with bulk viscosity, where the relation between the coefficient of bulk viscosity and energy
density is ζ ∝ ρ1/2. After choosing k =
, it further reduces to a string model without viscosity and magnetic field.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds
(MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs.
Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that
the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field,
is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing
magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected
to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection
occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar
coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic
field reversal process. These results will be further constrainable with the rising phase of solar cycle 24. 相似文献
5.
Comparisons of solar magnetic-field measurements made in different spectral lines are very important, especially in those
lines in which observations have a long history or (and) specific diagnostic significance. The spectral lines Fe i 523.3 nm and Fe i 525.0 nm belong to this class. Therefore, this study is devoted to a comprehensive analysis using new high-precision Stokes-meter
full-disk observations. The disk-averaged magnetic-field strength ratio R=B(523.3)/B(525.0) equals 1.97±0.02. The center-to-limb variation (CLV) is R=1.74−2.43μ+3.43μ
2, where μ is the cosine of the center-to-limb angle. For the disk center, we find R=2.74, and for near-limb areas with μ=0.3, R equals 1.32. There is only a small dependence of R on the spatial resolution. Our results are rather close to those published three decades ago, but differ significantly from
recent magnetographic observations. An application of our results to the important SOHO/MDI magnetic data calibration issue
is discussed. We conclude that the revision of the SOHO/MDI data, based only on the comparison of magnetic-field measurements
in the line pair Fe i 523.3 nm and Fe i 525.0 nm (increasing by a factor of 1.7 or 1.6 on average according to recent publications) is not obvious and new investigations
are urgently needed. 相似文献
6.
Makarov V.I. Tlatov A.G. CALLEBaUT D.K. Obridko V.N. Shelting B.D. 《Solar physics》2001,198(2):409-421
Hα magnetic synoptic charts of the Sun are processed for 1915–1999 and the spherical harmonics are calculated. It is shown
that the polarity distribution of the magnetic field on Hα charts is similar to the polarity distribution of the Stanford
magnetic field observations during 1975–1999. The index of activity of the large-scale magnetic field A(t), representing the sum of the intensities of dipole and octupole components, is introduced. It is shown that the cycle of
the large-scale magnetic field of the Sun precedes on the average by 5.5 years the sunspot activity cycle, W(t). This means that the weak large-scale magnetic fields of the Sun do not result from decay and diffusion of strong fields
from active regions as it is supposed in all modern theories of the solar cycle. On the basis of the new data the intensity
of the current solar cycle 23 is predicted and some aspects of the theory of the solar cycle are discussed. 相似文献
7.
Yu. V. Glagolevskij 《Astrophysical Bulletin》2010,65(1):34-41
We model the magnetic fields of four magnetic stars using published longitudinal (Be) field measurements. The structure of
the magnetic field of each of the four stars is close to that of the central dipole. Unfortunately, the number of measurements
for each star is insufficient for accurate finding of the field parameters, and therefore we find no dipole shift exceeding
its error Δa ≈ 0.1, expressed as a fraction of the stellar radius. Our data support the opinion that the results of modeling depend most
strongly on the adopted inclination of the star’s rotation axis i. 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.
I. N. Toptygin 《Astrophysics and Space Science》1973,20(2):351-371
The acceleration of fast particles by Alfvén and magnetic sound waves of small amplitude is considered. The waves exist against the background of a strong, uniform magnetic field. We take into accunnt the contributions to acceleration from a large scale random field (harmonics withk<R
–1, whereR is the Larmor radius), as well as from a small scale field (k>R
–1). The small scale field was considered by perturbation theory, while large scale random field-in an adiabatic approximation. The energy dependence of the diffusion coefficient in momentum space, and the time of acceleration are estimated. The possible anisotropy of angular distribution is taken into account.If the Alfvén waves have spectral power index >2 and wave amplitude is small enough, then the energy dependance of the diffusion coefficient is stronger than in the case of the Fermi acceleration. For magnetic sound waves with 2 the energy dependance of the diffusion coefficient is the same as for the Fermi acceleration, but for <2 this dependance is less.The space diffusion coefficient of particles across the regular magnetic field is estimated. It is shown that this diffusion is due mainly to the large scale random field. 相似文献
10.
Lepping R.P. Berdichevsky D.B. Burlaga L.F. Lazarus A.J. Kasper J. Desch M.D. Wu C.-C. Reames D.V. Singer H.J. Smith C.W. Ackerson K.L. 《Solar physics》2001,204(1-2):285-303
The energetic charged particle, interplanetary magnetic field, and plasma characteristics of the `Bastille Day' shock and
ejecta/magnetic cloud events at 1 AU occurring over the days 14–16 July 2000 are described. Profiles of MeV (WIND/LEMT) energetic
ions help to organize the overall sequence of events from the solar source to 1 AU. Stressed are analyses of an outstanding
magnetic cloud (MC2) starting late on 15 July and its upstream shock about 4 hours earlier in WIND magnetic field and plasma
data. Also analyzed is a less certain, but likely, magnetic cloud (MC1) occurring early on 15 July; this was separated from
MC2 by its upstream shock and many heliospheric current sheet (HCS) crossings. Other HCS crossings occurred throughout the
3-day period. Overall this dramatic series of interplanetary events caused a large multi-phase magnetic storm with min Dst lower than −300 nT. The very fast solar wind speed (≥ 1100 km s−1) in and around the front of MC2 (for near average densities) was responsible for a very high solar wind ram pressure driving
in the front of the magnetosphere to geocentric distances estimated to be as low as ≈ 5 R
E, much lower than the geosynchronous orbit radius. This was consistent with magnetic field observations from two GOES satellites
which indicated they were in the magnetosheath for extended times. A static force-free field model is used to fit the two
magnetic cloud profiles providing estimates of the clouds' physical and geometrical properties. MC2 was much larger than MC1,
but their axes were nearly antiparallel, and their magnetic fields had the same left-handed helicity. MC2's axis and its upstream
shock normal were very close to being perpendicular to each other, as might be expected if the cloud were driving the shock
at the time of observation. The estimated axial magnetic flux carried by MC2 was 52×1020 Mx, which is about 5 times the typical magnetic flux estimated for other magnetic clouds in the WIND data over its first
4 years and is 17 times the flux of MC1. This large flux is due to both the strong axially-directed field of MC2 (46.8 nT
on the axis) and the large radius (R
0=0.189 AU) of the flux tube. MC2's average speed is consistent with the expected transit time from a halo-CME to which it
is apparently related. 相似文献
11.
During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the Magnetic Field Investigation (MFI) team
using Wind (1995 – 2003) solar wind plasma and magnetic field data from solar minimum through the maximum of cycle 23. The average occurrence
rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were
during the early part of solar cycle 23: (i) only four MCs were observed in 1999, and (ii) an unusually large number of MCs
(17 events) were observed in 1997, just after solar minimum. We also discuss the relationship between MCs, coronal mass ejections
(CMEs), and geomagnetic storms. During the period 1996 – 2003, almost 8000 CMEs were observed by SOHO-LASCO. The occurrence
frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number.
When we included “magnetic cloud-like structures” (MCLs, defined by Lepping, Wu, and Berdichevsky, 2005), we found that the
occurrence of the joint set (MCs + MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average
duration of the MCL structures is ~40% shorter than that of the MCs. The MCs are typically more geoeffective than the MCLs,
because the average southward field component is generally stronger and longer lasting in MCs than in MCLs. In addition, most
severe storms caused by MCs/MCLs with Dst
min≤ −100 nT occurred in the active solar period. 相似文献
12.
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
13.
We provide a theory of magnetic diffusion, momentum transport, and mixing in the solar tachocline by considering magnetohydrodynamics (MHD) turbulence on a β plane subject to a large scale shear (provided by the latitudinal differential rotation). In the strong magnetic field regime, we find that the turbulent viscosity and diffusivity are reduced by magnetic fields only, similarly to the two-dimensional MHD case (without Rossby waves). In the weak magnetic field regime, we find a crossover scale (LR) from a Alfvén dominated regime (on small scales) to a Rossby dominated regime (on large scales). For parameter values typical of the tachocline, LR is larger than the solar radius so that Rossby waves are unlikely to play an important role in the transport of magnetic field and angular momentum. This is mainly due to the enhancement of magnetic back-reaction by shearing which efficiently generates small scales, thus strong currents. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
14.
Results of ourmeasurements of the longitudinal magnetic field B
z
for the young star RWAur A are presented. B
z
measured from the so-called narrow component of the He I 5876 line varies in the range from −1.47 ± 0.15 to +1.10 ± 0.15
kG. Our data are consistent with a stellar rotation period of }~5.6 days and the model of two hot spots with opposite magnetic
field polarities spaced about 180° apart in longitude. Relative to the Earth, the spot with B
z
< 0 lies in the hemisphere above the midplane of the accretion disk, while the spot with B
z
> 0 is below the midplane. The upper limit for B
z
(at the 3σ level) obtained by averaging all observations is 180 G for the photosphere and 220 and 230 G for the Hα and [OI] 6300 line formation regions, respectively. We have also failed to detect a field in the formation region of broad
emission line components: the upper limit for B
z
is 600 G. In two of 11 cases, we have detected a magnetic field in the formation region of the blue absorption wing of the
Na I D doublet lines, i.e., in the wind from RW Aur A: B
z
= −180 ± 50 and −810 ± 80 G. The radial velocity of the photospheric lines in RW Aur A averaged over all our observations
is }~+10.5 km s−1, i.e., a value lower than that obtained by Petrov et al. (2001) ten years earlier by 5.5 km s−1. Therefore, we discuss the possibility that RW Aur is not a binary but a triple system. 相似文献
15.
The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through
the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE,
and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate
the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination
of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations.
There are several respects in which the observations differ from what would be expected in the commonly-adopted models for
flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them,
apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate.
Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly,
the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and
EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting
the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection.
We describe the interpretation of our observations within the standard model, and the implications for the distribution of
magnetic fields in this active region. 相似文献
16.
Kenneth H. Schatten 《Solar physics》1970,12(3):484-491
The Faraday rotation of a radio source (Pioneer 6) occulted by the solar corona has been measured by Levy et al. (1969).
During the course of these measurements, three large-scale transient phenomena were observed. These events were preceded by
subflares and class 1 flares. These transient events are interpreted as evidence for a coronal magnetic bottle at 10 R
⊙. The velocity of propagation for the disturbance is set at 200 km/sec; the dimension of the region, 10 R
⊙; field strength at 10 R
⊙, 0.02 G; particle density, 2.0 × 104/cm3; Alfvén speed, 320 km/sec. From the nature of the observations and the lack of related effects from similar flares on the
interplanetary sector pattern observed at 1 AU, it is suggested that such coronal magnetic bottles expand to perhaps 10–30
R
⊙ and then contract to a few solar radii. Such a phenomena is evidence for an expansion of the corona with a sub-Alfvénic velocity.
It is further suggested that such magnetic bottles may be important in the storage and diffusion of solar generated cosmic
ray particles.
NAS-NRC Postdoctoral Resident Research Associate. 相似文献
17.
The applications of the spectral analysis methods discovered by Kirchhoff for the investigation of stellar magnetic fields
are considered. The statistical properties of the mean magnetic fields for OBA stars have been investigated by analyzing data
from two catalogs of magnetic fields. It is shown that the mean effective magnetic field ℬ of a star can be used as a statistically
significant characteristic of its magnetic field. The magnetic field distribution functions F(ℬ) have been constructed for B-type and chemically peculiar (CP) stars, which exhibit a power-law dependence on ℬ. A sharp
decrease in F(ℬ) in the range of weak magnetic fields has been found. The statistical properties of the magnetic fluxes for main-sequence
stars, white dwarfs, and neutron stars are analyzed. 相似文献
18.
A number of independent arguments indicate that the toroidal flux system responsible for the sunspot cycle is stored at the
base of the convection zone in the form of flux tubes with field strength close to 105 G. Although the evidence for such strong fields is quite compelling, how such field strength can be reached is still a topic
of debate. Flux expulsion by convection should lead to about the equipartition field strength, but the magnetic energy density
of a 105-G field is two orders of magnitude larger than the mean kinetic energy density of convective motions. Line stretching by
differential rotation (i.e., the “Ω effect” in the classical mean-field dynamo approach) probably plays an important role, but arguments based on energy considerations show that it does not seem feasible
that a 105-G field can be produced in this way. An alternative scenario for the intensification of the toroidal flux system in the overshoot
layer is related to the explosion of rising, buoyantly unstable magnetic flux tubes, which opens a complementary mechanism for magnetic-field intensification.
A parallelism is pointed out with the mechanism of “convective collapse” for the intensification of photospheric magnetic
flux tubes up to field strengths well above equipartition; both mechanisms, which are fundamentally thermal processes, are
reviewed. 相似文献
19.
R. Grant Athay 《Solar physics》1973,29(2):357-364
Mean density models of the solar corona show evidence for two distinctive density regimes characterized by different density gradients. High density gradients are identified with regions of predominantly open magnetic lines of force and low density gradients are identified with regions of predominantly closed magnetic lines of force. Spectroscopic data yielding equivalent widths of forbidden lines of Fe x and Fe xiv strongly suggest that the coronal temperature for r > 2.5 R
decreases considerably less rapidly in equatorial regions than r
–2/7, which is the decrease predicted by conduction models with open field lines.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
20.
P. Démoulin K.-L. Klein C. P. Goff L. van Driel-Gesztelyi J. L. Culhane C. H. Mandrini S. A. Matthews L. K. Harra 《Solar physics》2007,240(2):301-313
Radio emissions of electron beams in the solar corona and interplanetary space are tracers of the underlying magnetic configuration
and of its evolution. We analyse radio observations from the Culgoora and WIND/WAVES spectrographs, in combination with SOHO/LASCO
and SOHO/MDI data, to understand the origin of a type N burst originating from NOAA AR 10540 on January 20, 2004, and its
relationship with type II and type III emissions. All bursts are related to the flares and the CME analysed in a previous
paper (Goff et al., 2007). A very unusual feature of this event was a decametric type N burst, where a type III-like burst, drifting towards
low frequencies (negative drift), changes drift first to positive, then again to negative. At metre wavelengths, i.e., heliocentric distances ≲1.5R
⊙, these bursts are ascribed to electron beams bouncing in a closed loop. Neither U nor N bursts are expected at decametric
wavelengths because closed quasi-static loops are not thought to extend to distances ≫1.5R
⊙. We take the opportunity of the good multi-instrument coverage of this event to analyse the origin of type N bursts in the
high corona. Reconnection of the expanding ejecta with the magnetic structure of a previous CME, launched about 8 hours earlier,
injects electrons in the same manner as with type III bursts but into open field lines having a local dip and apex. The latter
shape was created by magnetic reconnection between the expanding CME and neighbouring (open) streamer field lines. This particular
flux tube shape in the high corona, between 5R
⊙ and 10R
⊙, explains the observed type N burst. Since the required magnetic configuration is only a transient phenomenon formed by reconnection,
severe timing and topological constraints are present to form the observed decametric N burst. They are therefore expected
to be rare features. 相似文献