On Three-Dimensional Magnetic Skeleton Elements Due to Discrete Flux Sources

The magnetic field in the solar corona plays an important role in coronal heating, flaring activity and many other phenomena studied on the Sun. Magnetic topology is frequently used to understand complicated coronal magnetic fields. By calculating the skeleton of a field, it is possible to build up a sophisticated representation of the key elements of a field’s configuration. This paper determines a simple relation between the numbers of separators (X), coronal null points (N_c), flux domains (D) and flux sources (S) in such a configuration: D=X+S−N_c−1. This equation is used to explain the behaviour of some of the bifurcations found in Magnetic Charge Topology, and to show that a one-to-one relationship exists between the number of circuits in the domain graph and the augmented null graph. Finally, it is shown that in quiet-Sun regions, the number of separators is approximately proportional to the number of flux sources.     

Magnetic Null Points due to Multiple Sources of Solar Photospheric Flux

How common are magnetic null points in the highly complex magnetic field of the solar atmosphere? In this work we seek to model the magnetic structure of quiet regions by placing magnetic sources and sinks on a hexagonal network of supergranule cells to represent the intense magnetic fields that occur at the boundaries of these cells. The resulting potential coronal magnetic field is then computed analytically and searched numerically for magnetic null points, which are classified according to their types and spine directions. Two relations from the theory of vector fields relate the numbers of null points to the numbers of sources and sinks and these are used to check the numerical results. Previous results relating these quantities for monopolar and dipolar magnetic fields are described and a new one for a particular class of quadrupolar fields arising in this study is derived. We model a three-cell configuration and study the effects of increasing the strength of a central sink and of moving the central sink. A twelve-cell configuration is studied in lesser detail.

     

High Brightness Temperatures in IDV Sources

High brightness temperatures are a characteristic feature of IntraDay Variability (IDV) of extragalactic radio sources. Recent studies of the polarization properties of some IDV sources (e.g., 1150 812, PKS 0405-385 and 0716 714) have shown that these sources harbor several compact IDV components with angular sizes of -10-30/uas and very high polarizations (of up to -50%-70%). These results indicate the possibility of the existence of uniform magnetic fields in the IDV components. We investigate the incoherent synchrotron and self- Compton radiation of an anisotropic distribution of relativistic electrons which spin around the magnetic field lines at small pitch angles. The brightness temperature limit caused by second-order Compton losses is discussed and compared to the brightness temperatures derived from energy equipartition arguments. It is found that anisotropic distributions of electrons moving in ordered magnetic fields can raise the equipartition and Compton brightness temperatures by a factor of up to -3-5. This would remove some of the difficulties in the interpretation of extremely high intrinsic brightness temperatures of > 1012 K (or apparent brightness temperatures of - 1014 K with a Doppler factor of -30).     

Coronal Magnetic Topologies in a Spherical Geometry I. Two Bipolar Flux Sources

The evolution of the solar corona is dominated to a large extent by the hugely complicated magnetic field which threads it. Magnetic topology provides a tool to decipher the structure of this field and thus help to understand its behaviour. Usually, the magnetic topology of a potential field is calculated due to flux sources on a locally planar photospheric surface. We use a Green's function method to extend this theory to sources on a global spherical surface. The case of two bipolar flux-balanced source regions is studied in detail, with an emphasis on how the distribution and relative strengths of the source regions affect the resulting topological states. A new state with two spatially distinct separators connecting the same two magnetic null points, called the “dual intersecting“ state, is discovered. An erratum to this article is available at .     

Coronal Magnetic Topologies in a Spherical Geometry II. Four Balanced Flux Sources

The Sun’s magnetic field is the primary factor determining the structure and evolution of the solar corona. Here, magnetic topology is used in combination with a Green’s function method to model the global coronal magnetic field with a spherical photosphere. We focus on the case of three negative flux sources and one positive source, completing our previous categorisation of the topological states and bifurcations that are present in quadrupolar configurations in a spherical geometry. Three fundamental varieties of topological state are found, with three types of bifurcation taking one to the other. A comparison to the equivalent results for a planar photosphere is then carried out, and the differences between the two cases are explained.     

高频VLBI观测射电源的结构

高频VLBI观测揭示出河外射电源在pc或者亚pc尺度上大都有核-喷流的内部结构。尤其是对平谱致密射电源中毫角秒尺度的子源，能以相当高的概率观测到它的视超光速现象。多频观测可监测到一些新喷流子源的出现，特别在射电激变源中这些新喷流子源的出现与宽带内(从光学到γ射线)的爆发相关。且得到的一些源的核区谱指数分布，为确认源的VLBI核提供了支持。     

高海拔宇宙线观测站与超高能伽马射线源

正近日,我国的重大科技基础设施"高海拔宇宙线观测站"(Large High Altitulde Air Shower Observatory, LHAASO)在《Nature》期刊上发表了首批科学结果[1]. LHAASO利用其1/2阵列在2020年1月至11月这段时期的数据,共发现了12个显著性大于7倍标准偏差的超高能伽马射线(指能量大于100 Te V的光子, 1 TeV=10~(12)eV)源.     

Evidence of Magnetic Flux Ropes in the Solar Wind From Sigmoidal and non-Sigmoidal Active Regions

We have examined WIND magnetic field and plasma data during the first half of 1998 in order to find encounters of this spacecraft with magnetic clouds. From the events obtained through this search, we have selected four of them taking into account their solar origin. The four magnetic clouds are related to halo or partial halo CMEs, but the morphology of the active region before the eruption is sigmoidal for three of them and non-sigmoidal for the other one. We have analyzed these events in the solar wind by fitting the experimental data to a non-force-free flux-rope model. We conclude that both kinds of active regions develop in the solar wind an ejection with a flux-rope topology.     

Increase of the Magnetic Flux From Polar Zones of the sun in the Last 120 Years

Lockwood, Stamper, and Wild (1999) argued that the average strength of the magnetic field of the Sun has doubled in the last 100 years. They used an analysis of the geomagnetic index aa. We calculated the area of polar zones of the Sun, A _pz, occupied by unipolar magnetic field on H synoptic magnetic charts, following Makarov (1994), from 1878 to 2000. We found a gradual decrease of the annual minimum latitude of the high-latitude zone boundaries, _2m, of the global magnetic field of the Sun at the minimum of activity from 53° in 1878 down to 38° in 1996, yielding an average decrease of 1.2° per cycle. Consequently the area of polar zones A _pz of the Sun, occupied by unipolar magnetic field at the minimum activity, has risen by a factor of 2 during 1878–1996. This means that the behavior of the index aa and consequently the magnetic flux from the Sun may be explained by an increase of the area of polar caps with roughly the same value of the magnetic field in this period. The area of the unipolar magnetic field at the poles (A _pz) may be used as a new index of magnetic activity of the Sun. We compared A _pz with the aa, the Wolf number W and A^* -index (Makarov and Tlatov, 2000). Correlations based on `11-year' averages are discussed. A temperature difference of about 1° between the Maunder Minimum and the present time was deduced. We have found that the highest latitude of the polar zone boundaries of the large-scale magnetic field during very low solar activity reaches about 60°, cf., the Maunder Minimum. It is supposed that the _2m-latitude coincides with the latitude where _r=0, with (r,) being the angular frequency of the solar rotation. The causes of the waxing and waning of the Sun's activity in conditions like Maunder Minimum are discussed.     

Statistical Tests of the Unification Scheme for High Luminosity Double Radio Sources

We have carried out some statistical tests of relativistic beaming and radio source orientation scenarios using the core dominance parameter Rand linear size D of a recent sample of double-lobed quasars and radio galaxies as orientation indicators. Our results show that the maximum Doppler boosting occurs within a cone angle of ∼ 13^°corresponding to an optimum Lorentz factor of ∼ 5. On the average, quasar cores appear to be boosted by a factor of ∼ 10 relative to those of radio galaxies. In general, we found that quasars lie at closer angles to the line of sight than radio galaxies with median values of 28^° and 51^°respectively, implying a relative foreshortening factor of ∼ 2. These results are consistent with the simple relativistic beaming and orientation-based unification hypotheses in which quasars are the beamed counterparts of powerful radio galaxies which form the isotropic parent population. The results show a strong evidence that orientation of source axis with respect to the line of sight is a crucial parameter in the classification schemes for radio sources. This revised version was published online in July 2006 with corrections to the Cover Date.     

Emerging Flux Regions

All active regions are born as Emerging Flux Regions (EFRs) which appear in H as two small plages of opposite polarity connected by parallel dark arches. After a brief review of the properties of EFRs, we report on new observations of the birth of an EFR and apparent subsequent field reconnection. We review fluxrope theories, predict the appearance of EFRs, then modify this picture on the basis of high resolution observations. We arrive at a model of this phenomenon that encompasses relevant aspects such as the axial tilt of spot groups, the observed rotation of EFRs as they emerge, and the fact that EFR fluxtubes are made up of many discrete strands.We investigate the relation of the positions of emergence of EFRs to the chromospheric network. We find that new EFRs can be much smaller than supergranules.     

星际水脉泽源中高速气体的CO巡测

用射电天文联合实验室紫金山天文台青海站１３．７米望远镜,于１９９７年４月对９７个水脉泽源做了ＣＯ（Ｊ＝１－０）的巡测及部分成图观测,结果表明８１个源的谱线适于作成协星体活动分析．经初步证认有外向流候选者３３个,全部为首次给出高速范围．对其轮廓特点及与Ｈ２Ｏ脉泽辐射的关系作了简要讨论．ＣＯ高速气体和Ｈ２Ｏ脉泽的速度范围和峰值速度在多数情况下分别相符,这说明两者的激发可能相关．     

Coronal Flux Recycling Times

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).     

Probing the Baryon Density of the Universe using Light Echoes of High Redshift Radio Sources

It is shown that the high redshift intergalactic gas that probably contains most of the baryonic density b = 0.05(Ho = 75kms-1 Mpc-1) in the standard Cold Dark Matter model can be detected through "true" intergalactic Thompson-scattered halos or light echoes around isotropic radio-loud quasars and radio galaxies. These cosmological halos, which form in the uniform ionized medium around "old" non-evolving sources, have a very large degree of polarization, up to pmax 44 percent at a projected distance half the light-age radius, and a plateau in the annular polarized flux density within 1/3 of the light-age radius, which is about 4 arc min for sources older than 107 years at z 4. The optimal wavelength range for halo polarimetric observations is 20 - 30 cm, depending on the Galactic rotation measure in the direction of the source, the spectral index of the source, and the specific maximized parameter of the halo. If observed with large single-dish radiotelescopes, the 21 cm polarized brightness temperatures of some inner halos, for example the halo around the quasar OQ 172 (S1400 = 2.5 Jy,z = 3.53), are of the same order of magnitude as the current total-intensity limits for cosmic background fluctuations on arc min scales. The halo test can be extended to larger volumes of space by concentric co-adding of source-normalized halos around the much more numerous isotropic radio galaxies. The expected ensemble-averaged profile of the polarized flux density around the symmetry center is calculated by integrating over the halo ages, assuming a uniform source-age distribution. In addition to the electron density or a lower limit for b based on the halo brightness, the characteristic life time of the radio emission can be derived based on its angular size. If there is a moderate deviation of the density evolution of the intergalactic gas from the conservative cubic law due to galaxy formation, high redshift radio halos may be on the rising branch of their brightness beyond its minimum, similar to the well known nonmonotonic behaviour of angular sizes.     

Flux expulsion by inhomogeneous turbulence

Flux expulsion is an important consequence of the interaction of magnetic fields with fluid convection and has been well studied for particular cases of steady, single-cell flows. Here we examine a related phenomenon in inhomogeneous turbulence using direct numerical simulations. To understand our numerical results, we analyse average properties of our model, and obtain mean transport coefficients which can be used to describe the approach of the system to its final state. For the kinematic problem these transport coefficients give an excellent prediction of the expulsion process; however, the enhanced transport is suppressed by dynamical back-reaction of the Lorentz force. Finally, we discuss the astrophysical implications for magnetic fields in stellar convection zones. Segregation of magnetic fields from turbulent motion not only allows strong toroidal fields to accumulate in regions of convective overshoot but also permits significant poloidal fields to be maintained by dynamo action in stars like the Sun.     

Clustering of Emerging Magnetic Flux

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.     

The Solar Wind Energy Flux

The solar-wind energy flux measured near the Ecliptic is known to be independent of the solar-wind speed. Using plasma data from Helios, Ulysses, and Wind covering a large range of latitudes and time, we show that the solar-wind energy flux is independent of the solar-wind speed and latitude within 10?%, and that this quantity varies weakly over the solar cycle. In other words the energy flux appears as a global solar constant. We also show that the very high-speed solar wind (V _SW>700?km?s^?1) has the same mean energy flux as the slower wind (V _SW<700?km?s^?1), but with a different histogram. We use this result to deduce a relation between the solar-wind speed and density, which formalizes the anti-correlation between these quantities.     

An Evolving Synoptic Magnetic Flux map and Implications for the Distribution of Photospheric Magnetic Flux

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.     

Rotational Modulation of Microwave Solar Flux

Time series data of 10.7 cm solar flux for one solar cycle (1985–1995 years) was processed through autocorrelation. Rotation modulation with varying persistence and period was quite evident. The persistence of modulation seems to have no relation with sunspot numbers. The persistence of modulation is more noticeable during 1985–1986, 1989–1990, and 1990–1991. In other years the modulation is seen, but its persistence is less. The sidereal rotation period varies from 24.07 days to 26.44 days with no systematic relation with sunspot numbers. The results indicate that the solar corona rotates slightly faster than photospheric features. The solar flux was split into two parts, i.e., background emission which remains unaffected by solar rotation and the localized emission which produces the observed rotational modulation. Both these parts show a direct relation with the sunspot numbers. The magnitude of localized emission almost diminishes during the period of low sunspot number, whereas background emission remains at a 33% level even when almost no sunspots may be present. The localized regions appear to shift on the solar surface in heliolongitudes.