共查询到20条相似文献，搜索用时 31 毫秒
1.
Jin－LinHan RichardWielebinski 《中国天文和天体物理学报》2002,2(4):293-294
Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from anal 相似文献
2.
Guang-Li Huang Purple Mountain Observatory Chinese Academy of Sciences Nanjing 《中国天文和天体物理学报》2006,6(1):113-119
Two impulsive microwave bursts observed by Owens-Valley Solar Arrays (OVSA)are studied.The fast time variation of the turnover frequency in these bursts is quite different from the constant value in the earlier conclusion.The observational turnover frequencies are consistent with the calculations using the non-thermal gyro- synchrotron radiation model.We find the turnover frequency may play an important role for calculating the coronal magnetic field on the basis of Dulk and Marsh's ap- proximations. 相似文献
3.
We present the pattern of the polar magnetic reversal for cycle 23 derived from H synoptic charts and have also included the reversals of the earlier cycles 18–22 for comparison. At the beginning of a new cycle (i.e., soon after the polar reversal) the zonal boundaries of unipolar magnetic regions of opposite polarities (seen as filament bands on the synoptic charts) appear close to and on either side of the equator continuing through the years of minimum indicating the onset of the cancellation of flux at these low latitudes. The cycle thus starts with cancellation of flux close to the equator and ends with the polar reversal or flux cancellation near the poles. The filament bands just below the polemost ones migrate and reach latitudes 35°–45° by the time of polar reversal and become the polemost, once the polar reversal has taken place. During the years of minimum that follow, these filament bands remain more or less stagnant at the latitudes 35°–45° except for occasional slow migration towards the equator. The migration to the poles starts at a low speed of 3 m s–1 only when the spot activity has risen to a significant level and then it accelerates to 30 m s–1 at the peak of the activity. It takes 3–4 years for the polemost bands to reach the poles moving at these high speeds. We quantify this possible cause and effect phenomenon by introducing the concept of the `strength of the solar cycle' and represent this by either of a set of three parameters. We show that the velocity of poleward migration is a linear function of the `strength of the solar cycle'. 相似文献
4.
L. K. Harra T. Magara H. Hara S. Tsuneta T. J. Okamoto A. J. Wallace 《Solar physics》2010,263(1-2):105-119
Active region magnetic flux that emerges to the photosphere from below will show complexity in the structure, with many small-scale fragmented features appearing in between the main bipole and then disappearing. Some fragments seen will be absorbed into the main polarities and others seem to cancel with opposite magnetic field. In this paper we investigate the response of the corona to the behaviour of these small fragments and whether energy through reconnection will be transported into the corona. In order to investigate this we analyse data from the Hinode space mission during flux emergence on 1?–?2 December 2006. At the initial stages of flux emergence several small-scale enhancements (of only a few pixels size) are seen in the coronal line widths and diffuse coronal emission exists. The magnetic flux emerges as a fragmented structure, and coronal loops appear above these structures or close to them. These loops are large-scale structures – most small-scale features predominantly stay within the chromosphere or at the edges of the flux emergence. The most distinctive feature in the Doppler velocity is a strong ring of coronal outflows around the edge of the emerging flux region on the eastern side which is either due to reconnection or compression of the structure. This feature lasts for many hours and is seen in many wavelengths. We discuss the implications of this feature in terms of the onset of persistent outflows from an active region that could contribute to the slow solar wind. 相似文献
5.
The Interplanetary Magnetic Field shows complex spatial and temporal variations. Single spacecraft measurements reveal only
a one-dimensional section of this rich four-dimensional phenomenon. Multi-point measurements of the four Cluster spacecraft provide a unique tool to study the spatiotemporal structure of the field. Using Cluster data we determined three-dimensional correlation functions of the fluctuations. By means of the correlation function one
can describe and measure field variations. Our results can be used to verify theoretical predictions, to understand the formation
and nature of solar wind turbulence. We found that the correlation length varies over almost six orders of magnitude. The
IMF turbulence shows significant anisotropy with two distinct populations. In certain time intervals the ratio of the three
axes of the correlation ellipse is 1/2.2/6 while in the remaining time we found extremely high correlation along one axis.
We found favored directions in the orientation of the correlation ellipsoids. 相似文献
6.
The simultaneous photometric and spectroscopic observations of the spotted G dwarf AP149 in the young open cluster α Persei are analyzed here.We reconstruct the observed light curves with a two-starspot model by means of a light curve modeling technique,and find that the active regions shift oppositely along longitude on a time scale of one day.Combining with the observational data obtained by other groups,we discuss the evolution of spotted regions in the photosphere,and find that its starspots evolve not ... 相似文献
7.
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
8.
We present the results of a statistical study of the solar cycle based on the analysis of the superficial toroidal magnetic
field component phase space. The magnetic field component used to create the embedded phase space was constructed from monthly
sunspot number observations since 1750. The phase space was split into 32 sections (or time instants) and the average values
of the orbits on this phase space were calculated (giving the most probable cycle). In this phase space it is shown that the
magnetic field on the Sun’s surface evolves through a set of orbits that go around a mean orbit (i.e., the most probable magnetic cycle that we interpret as the equilibrium solution). It follows that the most probable cycle
is well represented by a van der Pol oscillator limit curve (equilibrium solution), as can be derived from mean-field dynamo
theory. This analysis also retrieves the empirical Gnevyshev – Ohl’s rule between the first and second parts of the solar
magnetic cycle. The sunspot number evolution corresponding to the most probable cycle (in phase space) is presented. 相似文献
9.
The aim of this article is to investigate how the background magnetic field of the Sun behaves in different hemispheres. We used SOHO/MDI data obtained during a period of eight years from 2003 to 2011 to analyze the intensity distribution of the background magnetic field over the solar surface. We find that the background fields of both polarities (signs) are more intense in the southern than in the northern hemisphere. Mixed polarities are observed in the vicinity of the equator. In addition to the main field, a weaker field of opposite polarity is always present in the polar regions. In the declining phase of the cycle, the main field dominates, but at the minimum and in the rising phase of the cycle, it is gradually replaced by the growing stronger secondary field. 相似文献
10.
On the long way to establish reliable physical properties of the solar atmosphere from different kinds of magnetic field measurement, significant progress has been achieved, but many important issues are still waiting for solution. This is essential for the investigation of weak magnetic fields of the quiet Sun, which usually cover most of the solar surface. Weak magnetic fields significantly contribute to the formation of the interplanetary magnetic field. The problem of reliable diagnostics of such fields hardly ever has a simple solution using only single spectral line observations. A better chance is given by multi-spectral line spectro-polarimetric observations, especially with lines having very different properties. In the present study, we use simultaneous high-precision Stokes-meter measurements of the quiet solar magnetic fields in 15 lines in the vicinity of Fe?i?525.0?nm. These measurements cover the whole range of heliocentric distances. Magnetic field strength ratios of different spectral lines with respect to Fe?i 525.0?nm vary between 1.07 and 2.12. This ratio depends also on the heliocentric position, moving closer to the limb it decreases and approaches values of about unity in most cases. To interpret the observations, different model approaches are compared. SIR-inversions (Stokes Inversion based on Response functions) with a two-component atmospheric model approach reproduce the basic observables much better than with one-component atmospheres. Our best fits are connected with field strengths of 1?–?2?kG and filling factors of less than five percent. To check the justification for the recent re-calibration of the data from the Michelson Doppler Imager (MDI) onboard SOHO, we carried out a numerical experiment, and we confirm our former conclusion that there is no need for such a re-calibration. 相似文献
11.
The differential rotation of compact magnetic elements during activity cycles 20 and 21 (1966 – 1986) is studied by using
solar synoptic charts. For each hemisphere the compact magnetic elements with the polarity of the circumpolar magnetic field
have larger rotation rates than the elements with the opposite polarity. This difference in rotation rates is present during
the whole cycle except during the polarity reversal of the circumpolar field. 相似文献
12.
We summarize studies of helical properties of solar magnetic fields such as current helicity and twist of magnetic fields in solar active regions (ARs), that are observational tracers of the alpha-effect in the solar convective zone (SCZ). Information on their spatial distribution is obtained by analysis of systematic mag-netographic observations of active regions taken at Huairou Solar Observing Station of National Astronomical Observatories of Chinese Academy of Sciences. The main property is that the tracers of the alpha-effect are antisymmetric about the solar equator. Identifying longitudinal migration of active regions with their individual rotation rates and taking into account the internal differential rotation law within the SCZ known from helioseismology, we deduce the distribution of the effect over depth. We have found evidence that the alpha-effect changes its value and sign near the bottom of the SCZ, and this is in accord with the theoretical studies and numerical simulations. We discuss 相似文献
13.
The microwave circular polarization of the active region (AR) NOAA 7260 on 21–23 August 1992 is analyzed. Two-dimensional images at 1.76 cm with spatial resolution of =10 from the Nobeyama radioheliograph and one-dimensional scans at 9 wavelengths in the range of 1.81–3.43 cm and =16.3–31.1 from the radio telescope RATAN-600 were used. An inversion of the sense of circular polarization through the wavelength range was recorded on 22 August. It is shown that both the wavelength and the time dependence of the inversion are consistent with quasi-transverse (QT) propagation of the radiation in the solar corona. From this, the strength of the coronal magnetic field in the active region was found to be H=20–65 G at a height of h= (5.7–8.7)×109 cm above the photosphere on 22 and 30 August and 125 G at the lower height of (3.7–6.4)×109 cm on 23 August. We present a new technique, based on the radio mapping (in both Stokes I and V) of an AR undergoing circular polarization inversion; applying this method to the Nobeyama data we obtained, for the first time, a magnetogram of the coronal magnetic field. For AR 7260 we found values in the range of 70–100 G at heights of (4–6)×109 cm on 23 August, adopting a constant value of NL (where N is the electron density and L is the scale of the coronal field divergence) of 2.5×1018 cm–2. We compare our results with force-free extrapolations of the photospheric magnetic field from a MSFC magnetogram obtained on 20 August. 相似文献
14.
Kinematics of local magnetic features (LMFs) have been investigated by analyzing a 22-year series of synoptic maps of the
radial magnetic field of the Sun (the term ‘local’ refers hereinafter to magnetic features with an effective size of the order
of an arc min). We applied the cross-correlation technique to analyse separately each of the harmonics obtained by using a
one-dimensional Fourier transform of the magnetic field in longitude. Such an approach allowed us to trace the motion of the
LMFs for a time interval as long as 12 Carrington rotations. The analysis also has shown that the effective size of the magnetic
tracers grows significantly with increasing age, which indicates that the local-scale magnetic features undergo diffusion-like
expansion and weakening, in agreement with Leighton's model of magnetic field evolution.
The LMFs emerging at latitudes between 10° and 55° reveal a poleward directed motion with a maximum velocity of about 14 m
s-1 near the latitude of 37°. The profile of the meridional velocity agrees approximately with that derived by Komm, Howard,
and Harvey (1993) for small-scale, short-lived magnetic features.
We have found that the LMFs rotate differentially at latitudes of up to 55°, and do not exhibit the ‘quasi-rigid’ rotation
that is assumed to be characteristic of long-lived magnetic features. This disagrees with the results obtained by Stenflo
(1989) and by Latushko (1994), who applied direct cross-correlation analysis of the synoptic maps. Such a discrepancy may
be treated as being a consequence of inhomogeneity of the large-scale solar magnetic field that consists of several components
with different kinematic characteristics. 相似文献
15.
D. Sokoloff H. Zhang K. M. Kuzanyan V. N. Obridko D. N. Tomin V. N. Tutubalin 《Solar physics》2008,248(1):17-28
A comparison between the two tracers of magnetic field mirror asymmetry in solar active regions – twist and current helicity
– is presented. It is shown that for individual active regions these tracers do not possess visible similarity but averaging
by time over the solar cycle, or by latitude, reveals similarities in their behavior. The main property of the data set is
antisymmetry over the solar equator. Considering the evolution of helical properties over the solar cycle we find signatures
of a possible sign change at the beginning of the cycle, though more systematic observational data are required for a definite
confirmation. We discuss the role of both tracers in the context of solar dynamo theory. 相似文献
16.
An interplanetary shock and a magnetic cloud(MC)reached the Earth on 2012 July 14 and 15 one after another.The shock sheath and the MC triggered an intense geom... 相似文献
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19.
Fluctuations in the solar wind plasma and magnetic field are well described by the sum of two power law distributions. It
has been postulated that these distributions are the result of two independent processes: turbulence, which contributes mainly
to the smaller fluctuations, and crossing the boundaries of flux tubes of coronal origin, which dominates the larger variations.
In this study we explore the correspondence between changes in the magnetic field with changes in other solar wind properties.
Changes in density and temperature may result from either turbulence or coronal structures, whereas changes in composition,
such as the alpha-to-proton ratio are unlikely to arise from in-transit effects. Observations spanning the entire ACE dataset
are compared with a null hypothesis of no correlation between magnetic field discontinuities and changes in other solar wind
parameters. Evidence for coronal structuring is weaker than for in-transit turbulence, with only ∼ 25% of large magnetic field
discontinuities associated with a significant change in the alpha-to-proton ratio, compared to ∼ 40% for significant density
and temperature changes. However, note that a lack of detectable alpha-to-proton signature is not sufficient to discount a
structure as having a solar origin. 相似文献