Multipoint observations of the dynamics of the sharp solar wind structure boundaries

In this paper we performed the selection of sharp and large solar wind ion flux changes and comparison of them by simultaneous observations onboard two or three spacecraft. It was shown that these sharp changes survive on the way up to million km or even sharpen on this way.     

Magnetic configurations of streamer structures in the solar atmosphere

We consider two types of streamer structures observed in the solar atmosphere. Structures of the first type are medium-scale configurations with scale lengths comparable to the scale height in the corona, kT/mg = 100 thousand km, which appear as characteristic plasma structures in the shape of a dome surrounding the active region with thin streamers emanating from its top. In configurations of this type, gravity plays no decisive role in the mass distribution. The plasma density is constant on magnetic surfaces. Accordingly, the structure of the configurations is defined by the condition ψ = const, where ψ is the flux function of the magnetic field. Structures of the second type are large-scale configurations (coronal helmets, loops, and streamers), which differ from the above structures in that their scale lengths exceed the scale height in the corona. For them, gravity plays a decisive role; as a result, instead of the magnetic surfaces, the determining surface is BgradΦ = 0. We constructed three-dimensional images of these structures. Some of the spatial curves called “visible contours” of the B_r = 0 surface are shown to be brightest in the corona. We assume that the helmet boundaries and polar plumes are such curves.     

Parametric generation of magnetoacoustic-gravity waves in the solar atmosphere

Based on a plane-parallel isothermal solar model atmosphere permeated by a horizontal magnetic field whose strength is proportional to the square root of the plasma density and in the approximation of a specified field for vertically propagating and nonpropagating magnetoacoustic-gravity waves, we consider the nonlinear interaction between the corresponding disturbances, to within quantities of the second order of smallness. We investigate the efficiency of the nonlinear generation of waves at difference and sum frequencies and of an acoustic flow (wind) as a function of the magnetic-field strength and the excitation frequency of the initial disturbances at the lower atmospheric boundary.     

Numerical simulations of the magnetic field distribution in a solar complex of activity

Powerful solar complexes of activity are supposed to result from the excitation of Rossby vortices within a thin layer beneath the convection zone. Numerical simulations demonstrate that Rossby vortices generate large-scale arc-like magnetic structures. It is shown that the most powerful complex of activity observed in June-July 1982 was likely to be a result of the excitation of a Rossby anticyclone rather than a cyclone.     

The configuration of the loop-like structure of the solar atmosphere

An integral, governing steady flows in an isolated thin magnetic flux tube in the hydrostatic plane-stratified atmosphere, has been obtained. The integral, that we named as the shape integral, is expressed as (1 − M_A²)B cos θ = const. Here M_A² is the Alfven Mach number, B is the magnetic field strength and θ is the flux tube inclination to the horizontal. The shape integral should hold for most loop models because it represents just the momentum balance laws and has no relation to any energy balance mode. Its application to the isothermal and static cases is discussed and illustrated.     

Ambipolar diffusion in the solar atmosphere

The process of non-linear ambipolar diffusion in the region overlying the solar surface can be an effective mechanism for producing sharp magnetic structures and current sheets. These may be the sites responsible for the occurrence of connectivity of magnetic field lines, and the subsequent explosive input of energy for heating of some of the features in the atmosphere of the Sun..     

Toroidal magnetic field structure of the solar convective envelope inferred from the initial observations of the sunspots

For different life spans, we measure the line of sight component of the magnetic field structure of the bipolar sunspots from the SOHO/MDI magnetograms during their initial appearance on the surface and toroidal component of the magnetic field structure is separated. Irrespective of their sizes, strength of the measured line of sight component of the magnetic field structure varies from ∼450 G for the life span of 2 days to ∼300 G for the life span of 12 days. Where as strength of the estimated surface toroidal component of the bipolar spots varies from ∼10 G for the life span of 2 days to ∼700 G for the life span of 12 days. We use rederived Parker’s (1955a) flux tube model in spherical coordinates and Hiremath’s (2002) life span anchoring depth information to infer the strength of line of sight and toroidal components of the magnetic field structures at different anchoring depths of the bipolar spots in the convective envelope and the important findings are: (i) both the line of sight and toroidal components of the magnetic field structures at the sites of sunspots’ different anchoring depths in the convective envelope have a similar radial variation and the strength (∼10⁴ G near base of the convective envelope to ∼100 G near the surface) and, (ii) rate of emergence of toroidal magnetic field structure near base of the convective envelope is estimated to be ∼100 times the rate of emergence of toroidal magnetic field structure near the surface.     

Polarized radiation diagnostics of magnetohydrodynamic models of the solar atmosphere

Solar magnetic elements and their dynamical interaction with the convective surface layers of the Sun are numerically simulated. Radiation transfer in the photosphere is taken into account. A simulation run over 18.5 minutes real time shows that the granular flow is capable of moving and bending a magnetic flux sheet (the magnetic element). At times it becomes inclined by up to 30° with respect to the vertical around the level ₅₀₀₀ = 1 and it moves horizontally with a maximal velocity of 4 km/s. Shock waves form outside and within the magnetic flux sheet. The latter cause a distinctive signature in a time series of synthetic Stokes V-profiles. Such shock events occur with a mean frequency of about 2.5 minutes. A time resolution of at least 10 seconds in Stokes V recordings is needed to reveal an individual shock event by observation.The National Center for Atmospheric Research is sponsored by the National Science Foundation     

A theory of the equilibrium figure and gravitational field of the Galilean satellite Io: The second approximation

We construct a theory of the equilibrium figure and gravitational field of the Galilean satellite Io to within terms of the second order in the small parameter α. We show that to describe all effects of the second approximation, the equation for the figure of the satellite must contain not only the components of the second spherical function, but also the components of the third and fourth spherical functions. The contribution of the third spherical function is determined by the Love number of the third order h₃, whose model value is 1.6582. Measurements of the third-order gravitational moments could reveal the extent to which the hydrostatic equilibrium conditions are satisfied for Io. These conditions are J₃=C₃₂=0 and C₃₁/C₃₃=?6. We have calculated the corrections of the second order of smallness to the gravitational moments J₂ and C₂₂. We conclude that when modeling the internal structure of Io, it is better to use the observed value of k₂ than the moment of inertia derived from k₂. The corrections to the lengths of the semiaxes of the equilibrium figure of Io are all positive and equal to ～64.5, ～26, and ～14 m for the a, b, and c axes, respectively. Our theory allows the parameters of the figure and the fourth-order gravitational moments that differ from zero to be calculated. For the homogeneous model, their values are:\(s_4 = \frac{{885}}{{224}}\alpha ^2 ,s_{42} = - \frac{{75}}{{224}}\alpha ^2 ,s_{44} = \frac{{15}}{{896}}\alpha ^2 ,J_4 = - \frac{{885}}{{224}}\alpha ^2 ,C_{42} = \frac{{75}}{{224}}\alpha ^2 ,C_{44} = \frac{{15}}{{896}}\alpha ^2 \).     

Force-free magnetic fields in the solar atmosphere

Reliable measurements of the solar magnetic field are restricted to the level of the photosphere. For about half a century attempts have been made to calculate the field in the layers above the photosphere, i.e. in the chromosphere and in the corona, from the measured photospheric field. The procedure is known as magnetic field extrapolation. In the superphotospheric parts of active regions the magnetic field is approximately force-free, i.e. electric currents are aligned with the magnetic field. The practical application to solar active regions has been largely confined to constant-α or linear force-free fields, with a spatially constant ratio, α, between the electric current and the magnetic field. We review results obtained from extrapolations with constant-α force-free fields, in particular on magnetic topologies favourable for flares and on magnetic and current helicities. Presently, different methods are being developed to calculate non-constant-α or nonlinear force-free fields from photospheric vector magnetograms. We also briefly discuss these methods and present a comparison of a linear and a nonlinear force-free magnetic field extrapolation applied to the same photospheric boundary data. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rotation cycles of the sector structure of the solar magnetic field and its activity

We study the rotation of the sector structure of the solar magnetic field by using Stanford magnetographic observations from 1975 until 2000 and magnetic synoptic Hα-maps obtained from 1904 until 2000. The two independent series of observations yielded the same rotation periods of the two-sector (26.86 days) and four-sector (13.64 days) structures. We introduce a new index of the solar rotation, SSPM(t). The spectral power density of the sector structure of the magnetic field is shown to exhibit a 22-year cyclicity. The two-and four-sector structures of the magnetic field rotate faster at the maxima of even 11-year sunspot cycles. This phenomenon may be called the Gnevyshev-Ohl rule for the solar rotation. The 11-year sector-structure activity cycles are shown to lead the 11-year sunspot cycles (Wolf numbers) by 5.5 years. A 55-year component with the slowest rotation in the 18th cycle (1945–1955) was distinguished in the sector-structure rotation.     

Geoeffectiveness of magnetic clouds occurred during solar cycle 23

Highly variable conditions prevail in the geospace environment due to the variations in Solar activity. The characteristics of the magnetic clouds (MCs) and their effects on geosphere, which have occurred during the period January 1996 to December 2006; have been investigated. No systematic trend has been observed between MCs and Solar activity cycle which is analyzed on the basis of maximum Sunspot number in that particular year. 85% MCs are found to be geoeffective. MCs are divided into two major classes: unipolar and bipolar. Unipolar MCs are of south (S) or north (N) type while bipolar MCs are of south-north (SN) or north-south (NS) type. During Solar cycle 23, SN-type MCs dominated over NS-type MCs. Highly intense geomagnetic storms (GMSs) of Dst <−300 nT follow from SN or S-type MCs. No preference is observed for right handed (RH) or left handed (LH) clouds for being geoeffective. MCs of very high speed lead to intense GMSs. The correlation coefficient (r) of southward component of magnetic field (Bz), total magnetic field (B) and their products with plasma flow speed (VB and VBz) with Dst are observed to be r=0.78, −0.81, −0.79 and 0.82, respectively, which suggests that these parameters are reliable indicators of the strength of GMS. SN clouds do not always lead to more fall in Dst value (or lead to high strength of GMS) than NS clouds for similar value of Bz minimum associated with both type of MCs.     

The coarse structure of the solar atmosphere

Observations of the quiet sun at wavelengths from 3 Å to 75 cm show (with two exceptions: the Ovi line at 1032 Å and possibly the continuum at 1.2 mm) either no limb brightening or less than had been supposed. On the other hand, the brightness temperature is observed to increase with wavelength in the millimeter and centimeter range. If this increase is due to greater visibility of hot overlying material, that material ought to be evident at the limb at shorter wavelengths, resulting in limb brightening. The only possible explanation for the absence of limb brightening at almost all wavelengths is that the emitting surface is rough at all wavelengths, with a scale of roughness approximately equal to the scale height at each temperature. Contradictions with existing models, along with the additional observations required for an improved model are discussed.     

Polarimetry of solar pores

We address the magnetic field structure of solar pores. The data were obtained at the Gregory Coudè telescope at Izaña using the AT1 CCD camera system to observe pores with three spectral lines: one magnetically sensitive line, recording all 4 Stokes profiles, and two g = 0 lines where only the intensity profiles were measured. The data reduction included the standard procedure (removing dark current and flatfielding) as well as destretching of the polarimetric spectra and removing the non-magnetic straylight by means of a 2-d deconvolution of the observed intensity variation using a Lucy-Richardson restoration algorithm. In the following analysis we first determined the temperature- and pressure stratification of the pore using the g = 0 lines and then applied an inversion of the Stokes profiles to get the parameters of the magnetic field. Across the pore we find a strong variation of the resulting field strength as well as of the inclination and the azimuth, consistent with the assumption of a canopy forming in the higher atmosphere.     

On the possibility of constructing a radiative sunspot model in magnetohydrostatic equilibrium

It is currently believed that it is impossible to construct a radiative sunspot model in magnetohydrostatic equilibrium unless magnetic fields below the surface are excessively large (> 100 kG). This belief is based on results obtained using the mixing length theory of convection. We wish to point out that by using a different theory of convection, due to Öpik (1950), it is possible to compute a radiative sunspot model in which the field becomes no greater than 9000 G. By applying two boundary conditions, (i) depth of spot equals depth of convection zone, (ii) magnetic field has zero gradient at the base of the spot, we show that a radiative spot has a unique effective temperature for a given Wilson depression, . For = 650 km, we find T _e = 3800K ; for = 150 km, T _e = 3950K. According to our model, spots having T _e cooler than these values should not exist.     

日冕三重无力场电流片的磁场重联

采用二维三分量磁流体力学模型,对日冕三重无力场电流片的磁场重联进行了数值研究,揭示了重联过程的基本物理特征．这类重联过程将加热和加速日冕等离子体,并导致多个高温、高密度、高磁螺度的磁岛的形成和向上喷发．这表明,多重无力场电流片的重联可能在日冕磁能释放、上行等离子体团的形成和太阳磁场螺度向行星际空间的逃逸方面起重要的作用．     

Electron density and temperature measurements, and abundance anomalies in the solar atmosphere

Using spectra obtained from the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph on the spacecraft SOHO (Solar and Heliospheric Observatory), we investigate the height dependence of electron density, temperature and abundance anomalies in the solar atmosphere. In particular, we present the behaviour of the solar FIP effect (the abundance enhancement of elements with first ionization potential &amp;lt; 10 eV in the corona with respect to photospheric values) with height above an active region observed at the solar limb, with emphasis on the so-called transition region lines.     

在磁场条件下恒星大气Stokes轮廓特征量的研究

本文根据偏振辐射转移方程组就形成于有磁场的恒星大气中的Stokes轮廓的一些特征量与磁场强度大小的关系从理论上进行了探讨。结果表明:在强场情形下,L,Q,U,V的绝对值在非常靠近Zeeman分裂量±Δλ_H处达到极大;Hale的测量方法给出了总的场强的很好量度。最后用合成轮廓作出了相应的检验。