Mode selection mechanism and magnetic diffusivity in a non-axisymmetric solar dynamo model

The generation of solar non-axisymmetric magnetic fields is studied based on a linear α²–Ω dynamo model in a rotating spherical frame. The model consists of a solar-like differential rotation, a magnetic diffusivity varied with depth, and three types of α-effects with different locations, i.e. the tachocline, the whole convective zone and the sub-surface. Some comparisons of the critical α-values of axisymmetric (m = 0) and longitude-dependent modes (m = 1,2,3) are presented to show the roles of the magnetic diffusivity in the problem of modes selection. With the changing of diffusivity intensity for the given solar differential rotation system, the dominant mode possibly changes likewise and the stronger the diffusivity is, the easier the non-axisymmetric modes are excited. The influence of the diffusivity and differential rotation on the configurations of the dominant modes are also presented.     

How far are we from a ‘Standard Model’ of the solar dynamo?

Over the last few years, dynamo theorists seem to be converging on a basic scenario as to how the solar dynamo operates. The strong toroidal component of the magnetic field is produced in the tachocline, from where it rises due to magnetic buoyancy to produce active regions at the solar surface. The decay of tilted bipolar active regions at the surface gives rise to the poloidal component, which is first advected poleward by the meridional circulation and then taken below the surface to the tachocline where it can be stretched to produce the toroidal component. The mathematical formulation of this basic model, however, involves the specification of some parameters which are still uncertain. We review these remaining uncertainties which have resulted in disagreements amongst various research groups and have made it impossible to still arrive at something that can be called a standard model of the solar dynamo.     

Element Settling in the Solar Interior

Element settling inside the Sun now becomes detectable from the comparison of the observed oscillation modes with the results of the theoretical models. This settling is due, not only to gravitation, but also to thermal diffusion and radiative acceleration (although this last effect is small compared to the two others). It leads to abundance variations of helium and heavy elements of ≅ 10% below the convective zone. Although not observable from spectroscopy, such variations lead to non-negligible modifications of the solar internal structure and evolution. Helioseismology is a powerful tool to detect such effects, and its positive results represent a great success for the theory of stellar evolution. Meanwhile, evidences are obtained that the element settling is slightly smoothed down, probably due to mild macroscopic motions below the convective zone. Additional observations of the abundances of both 7Li and 3He lead to specific constraints on these particular motions. This revised version was published online in June 2006 with corrections to the Cover Date.     

太阳微波脉冲事件的时变特性

本文利用瑞士Berne大学1988年1月-1989年12月发布的15个太阳微波脉冲事件的观测资料，分析其时间轮廓的3个主要特征：脉冲度、不对称性和半功率宽度频率响应．这些特性，可以定性地用非热电子的连续注入模，和不均匀的源模型给出较合理的解释．     

Source investigation of impulsive He-rich particle events

We have investigated the source characteristic and coronal magnetic field structure of six impulsive solar energetic particle (SEP) events selected from Wang et al. [Wang, Y.-M., Pick, M., Mason, G.M. Coronal holes, jets, and the origin of ³He-rich particle events. ApJ 639, 495, 2006] and Pick et al. [Pick, M., Mason, G.M., Wang, Y.-M., Tan, C., Wang, L. Solar source regions for ³He-rich solar energetic particle events identified using imaging radio, optical, and energetic particle observations. ApJ 648, 1247, 2006]. Some results are obtained: first, 2 events are associated with wide (≈100°) CMEs (hereafter wide CME events), another 4 events are associated with narrow (?40°) CMEs (hereafter narrow CME events); second, the coronal magnetic field configuration of narrow CME events appear more simple than that of the wide CME events; third, the photospheric magnetic field evolutions of all these events show new emergence of fluxes, while one case also shows magnetic flux cancellation; fourth, the EUV jets usually occurred very close to the footpoint of the magnetic field loop, while meter type III bursts occurred near or at the top of the loop and higher than EUV jets. Furthermore, the heights of type III bursts are estimated from the result of the coronal magnetic field extrapolations.     

Active Region Dynamics

New methods of local helioseismology and uninterrupted time series of solar oscillation data from the Solar and Heliospheric Observatory (SOHO) have led to a major advance in our understanding of the structure and dynamics of active regions in the subsurface layers. The initial results show that large active regions are formed by repeated magnetic flux emergence from the deep interior, and that their roots are at least 50 Mm deep. The active regions change the temperature structure and flow dynamics of the upper convection zone, forming large circulation cells of converging flows. The helioseismic observations also indicate that the processes of magnetic energy release, flares and coronal mass ejections, might be associated with strong (1–2 km/s) shearing flows, 4–6 Mm below the surface.     

The hemispheric asynchrony of polar faculae during solar cycles 19–22

We investigate the north–south (N–S) asynchrony of the polar faculae and compare it with the hemispheric asynchrony of solar activity at low latitudes. We find that, (1) both the solar activity at high and low latitudes do not synchronously occur in the northern and southern hemispheres, there is phase shifts between the two hemispheres; (2) the N–S asynchrony of the polar faculae is a function of latitudes, implying that the asynchrony of the polar faculae between the two hemispheres is related to the hemispheric asymmetry and latitudinal distribution of the polar faculae.     

Solar prominence properties derived from the UV-EUV SUMER spectral atlas

In this paper, we summarize the work done to build a UV-EUV spectral atlas of a prominence and we introduce some recent scientific results obtained from these data.     

SSRT: First results of millisecond spike observations

We present the results of the first observations of spike-like phenomena with the Siberian Solar Radio Telescope (SSRT) in the one-dimensional mode at the frequency of 5.7 GHz with high spatial and temporal resolutions.     

Modeling coronal streamers and their eruption

Numerical solutions of the time-dependent MHD equations are used to generate ambient coronal streamer structures in a corona characteristic of that near solar minimum. The streamers are then disrupted by slow photospheric shear motion at the base of magnetic field lines within the closed field region, which is currently believed to be responsible for producing at least some CMEs. In contrast to several other simulations of this phenomena, the polytropic index is maintained at a value of 5/3 through the addition of coronal heating. Observations are used as a guide in determining the thermodynamic structure and plasma beta in the ambient corona. For a shear speed of 2.5 km/sec, the streamer configuration evolves slowly for about 65 hours before erupting outward with the formation of a CME. The bright CME leading edge travels outward at a speed of about 240 km/sec, and the sheared field lines follow at a somewhat slower speed. A closed magnetic field region is ejected as the magnetic field lines that were opened by the CME reconnect and reform the streamer.