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.     

The magnetic field in the solar atmosphere

This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.     

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)     

On build-up of magnetic energy in the solar atmosphere

The dynamic response of the solar atmosphere is examined with the use of self-consistent numerical solutions of the complete set of nonlinear, two-dimensional, hydromagnetic equations. Of particular interest are the magnetic energy build-up and the velocity field established by emerging flux at the base of an existing magnetic loop structure in a stationary atmosphere. For a plasma with a relatively low beta ( = 0.03) the magnetic energy build-up is approximately twice that of the kinetic energy, while the build-up in magnetic energy first exceeds but is eventually overtaken by the kinetic energy for a plasma with an intermediate beta ( = 3). The increased magnetic flux causes the plasma to flow upward near the loop center and downward near the loop edges for the low beta plasma. The plasma eventually flows downward throughout the lower portion of the loop carrying the magnetic field with it for the intermediate beta plasma. It is hypothesized that this latter case, and possibly the other case as well, may provide a reasonable simulation of the disappearance of prominences by flowing down into the chromosphere (a form of disparition brusque).The National Center for Atmospheric Research is sponsored by the National Science Foundation.Now at the School of Science and Engineering, The University of Alabama in Huntsville, Huntsville, Alabama 35807.     

The rotation of magnetic loop systems in the solar atmosphere

Eclipse photographs indicate that large regions of the inner solar corona are confined in various types of closed magnetic configurations and, as a result, do not participate in the general solar wind expansion. In this paper, the rotation of initially poloidal loop configurations of this type, as influenced by differential rotation of the footpoints, is investigated. The analysis is restricted to axially symmetric fields and it is assumed that the toroidal magnetic field induced by differential rotation is small as compared to the initial poloidal field. This restricts the validity of the analysis to times less than about one month.The most interesting physical situation is that of flux tubes existing in one solar hemisphere only, one end of the tube being fixed in the photosphere at a higher latitude than the other. As a consequence, the lower end of the tube rotates at a faster rate than the upper end. Solution of the pertinent equations reveals that the angular velocity measured along a field line increases monotonically from its value at the poleward footpoint to that at the lower footpoint. The variation of angular velocity along the field depends upon the field geometry only and is not directly related to the variation of angular velocity along the solar surface between the footpoints. Depending upon the field configuration, both outward radial increases and decreases are possible. Using the Newton and Nunn model for the surface differential rotation rate, the angular velocity distribution on two particularly simple types of closed magnetic loop systems is determined analytically. It is shown that the angular velocity increases outward in the polar regions but decreases outward near the equator - leading to a decrease in differential rotation with height.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Numerical simulations of magnetic reconnection in the lower solar atmosphere

Observations indicate that Ellerman bombs (EBs) and chromospheric microflares both occur in the lower solar atmosphere,and share many common features,such as temperature enhancements,accompanying jet-like mass motions,short life-time,and so on.These strongly suggest that EBs and chromospheric microflares could both probably be induced by magnetic reconnection in the lower solar atmosphere.With gravity,ionization and radiation considered,we perform two-dimensional numerical simulations of magnetic reconnecti...     

Electrostatic turbulence in electron temperature jumps of the solar atmosphere

Astronomy Letters - We discuss the connection of the formation and properties of solar atmosphere transition region characterized by a steep electron temperature gradient with electrostatic...     

On the temperature of the helium emission regions in the solar atmosphere

An analysis of the spectral distribution of intensity of the Hei recombination continuum is probably the only direct method for determination of the electron temperature of helium emission regions on the Sun. On the basis of data on the Hei Lyman continuum, obtained by Dupree and Reeves from OSO-4, the electron temperature of undisturbed helium regions is determined: T _e = = 12500 K. Such a low T _e value is a serious argument in favour of the predominant role of UV coronal radiation in the helium ionization on the Sun. Comparison of the Hei Lyman continuum data with results of observations of the 10830 line showed that the visible helium lines and Hei Lyman continuum are produced within the same regions of the undisturbed solar atmosphere at T _e = 12500 K.     

Measurements of the magnetic field and the gradient of temperature in the solar atmosphere above a flocculus using radio observations

Simultaneous observations made at several wavelengths in microwave range using the high spatial resolution of radiotelescope RATAN-600 make it possible to develop methods of measuring the magnetic fields in the solar corona and the chromosphere. In this paper we develop a method of measuring the magnetic fields from thermal bremsstrahlung and demonstrate it, using observations of a flocculus (plage) during August 1–3, 1977. The observations show that the flocculus under investigation possessed bipolar magnetic structure with peak to peak amplitude of magnetic field strength of about 40 G at the level of the upper chromosphere and the transition region (with a r.m.s. error of 5.7 G for favourable conditions). The radio astronomical map of the magnetic field is in agreement with the Mt. Wilson magnetic field map to within the experimental error. It follows that the average longitudinal magnetic field above the flocculus does not drop significantly with height above the photosphere up to the CCTR (chromosphere-corona transition region). An analysis of the spectra of polarized radio emission also gives an opportunity to determine the temperature gradient in the CCTR (which proved to amount to about 1000 K km^-1 and to follow their variation with height.     

A classification of magnetic field configurations associated with solar flares

On the assumption that solar flares are due to instabilities which occur in current sheets in the Sun's atmosphere, one may classify magnetic-field configurations associated with flares into two types. One is characterized by closed current sheets, magnetic-field lines adjacent to these sheets beginning and ending at the Sun's surface. The other is characterized by open current sheets, magnetic-field lines adjacent to these sheets beginning at the Sun's surface but extending out into interplanetary space. Flares associated with open current sheets can produce Type III radio bursts and high-energy-particle events, but flares associated with closed current sheets cannot. The flare of July 6, 1966 apparently consisted of one flare of each type.     

Stability of the solar tachocline with magnetic fields

The stability of magnetic fields in the solar tachocline is investigated. We present stability limits for higher azimuthal wave numbers and results on the dependence of the stability on the location of toroidal magnetic fields in latitude. While the dependence of the wave number with the largest growth rate on the magnetic field strength and the magnetic Prandtl number is small, the dependence on the magnetic Reynolds number Rm indicates that lowest azimuthal modes are excited for very high Rm. Upon varying the latitudinal position of the magnetic field belts, we find slightly lower stability limits for high latitudes, and very large stability limits at latitudes below 10°, with little dependence on latitude in between. An increase of the maximum possible field was achieved by adding a poloidal field. The upper limit for the toroidal field which can be stored in the radiative tachocline is then 1000 G, compared to about 100 G for a purely toroidal field as was found in an earlier work. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the energy release by magnetic field dissipation in the solar atmosphere

The energy release by Joule magnetic-field dissipation in the solar atmosphere is discussed. It is shown that the heating is unimportant in the case of granulation and intergranular space. In the case of spot features the additional temperatures T_r with the accounting of the radiation losses are no more than 30° for small new spots, 1° for the large umbrae and 300° for bright points in large umbrae. This effect gives the possibility to suggest a hypothesis on the source of temperature inhomogeneity in the spot umbra and the nature of bright points. In the chromosphere the dissipation is negligible.On leave from the Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN), U.S.S.R., Moscow region, p/o Academgorodok.     

The temperature anisotropy and adiabatic cooling of the protons in the solar wind

A simple theoretical expression for the mean kinetic temperature of the protons in a steady state as a function of heliocentric distance is derived. The basic assumption is that the temperature anisotropy of the protons is invariant in space where binary encounters are rare. For an assumed base temperature of 5 × 10⁵K at a distance of 0.05 AU, the calculated temperature at a distance of 1 AU is in the range (2–4) × 10⁴K for an average anisotropy factor of 3: this range of temperatures is close to the observed average value under so-called ‘quiet’ conditions. Measurement of the anisotropy factor at different heliocentric distances is required to test the basis of the model.     

Periodicities in the solar differential rotation,surface magnetic field and planetary configurations

Using Greenwich data on sunspot groups during 1874–1976, we have studied the temporal variations in the differential rotation parametersA andB by determining their values during moving time intervals of lengths 1–5 yr successively displaced by 1 yr. FFT analysis of the temporal variations ofB (orB/A) shows periodicities 18.3 ± 3 yr, 8.5 ± 1 yr, 3.9 ± 0.5 yr, 3.1 ± 0.2 yr, and 2.6 ± 0.2 yr at levels 2. This analysis also shows five more periodicities at levels 1–2. The maximum entropy method is used to set narrower limits on the values of these periods. The reality of the existence of all these periodicities ofB (orB/A ) except the one at 2.8 yr is confirmed by analyzing the simulated time series ofB andB/A with values ofA andB randomly distributed within the limits of their respective uncertainties. Four of the prominent periods ofB agree, within their uncertainties, with the known periods in the the large-scale photospheric magnetic field. The deviations from the average differential rotation are larger near the sunspot minima. On longer time scales, the variations in the amount of sunspot activity per unit time are well correlated to the variations in the amplitudes of the torsional oscillation represented by the 22-yr periodicity inB. All the periods inB found here are in good agreement with the synodic periods of two or more consecutive planets. The possibility of planetary configurations providing perturbations needed for the Sun's MHD torsional oscillations is speculated upon and briefly discussed.     

Wave disturbances near the temperature jump in the transition region of the solar atmosphere

We theoretically analyzed the properties of surface waves near the temperature jump in the solar atmosphere whose dispersion relation is identical in form to the equation for waves on deep water. We found an exact solution to the model equation for the vertical velocity of the medium in such a wave. Based on the derived space-time dependence of the vertical velocity of the medium, we quantitatively explained one of the events recorded in the SOI/MDI experiment onboard the SOHO spacecraft that accompanied coronal mass ejection.     

Excitation of non-spherical waves in solar atmosphere in the presence of toroidal magnetic field

It has been shown that non-spherical waves could be excited in rotating stellar convective envelopes. In this paper, the manner of excitation of such waves is examined in the presence of toroidal magnetic field. The result shows that one of these waves might be considered to induce the formation of the observed magnetic unipolar regions of the sun.Visiting Scientist to the High Altitude Observatory on leave of absence from the Department of Astronomy, University of Tokyo, Japan.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The Hanle effect and the diagnostics of turbulent magnetic fields in the solar atmosphere

The theory of the Hanle effect is used to interpret the linear polarization measured in a number of spectral lines on the solar disk near the heliographic north and south poles, in search for a turbulent magnetic field in the solar atmosphere. The Hanle depolarization is separated from a number of other effects, including collisional depolarization and scattering geometry. Although the main aim of the paper is to elucidate the physics of the Hanle effect as applied to the Sun, our results indicate the existence of hidden or turbulent magnetic flux near the temperature minimum of the solar atmosphere, with a field strength between 10 and 100 G. This field is hidden in the sense that it is not seen in measurements of the longitudinal Zeeman effect (solar magnetograms). It carries more total magnetic flux than the kG network fields.     

Thallium in the solar atmosphere

Umbral spectra are shown to contain an absorption feature attributable to the Tl i transition 6p ² P°_3/2–7s ² S _1/2 at 5350 Å. Analysis of the umbral spectrum suggests a solar abundance in the 0.72< log N(Tl)T<1.10 on the standard scale log N(H) = 12.00. Unidentified blends limit the accuracy of the abundance determination.     

Stability of toroidal magnetic fields in the solar tachocline and beneath

Stability of toroidal magnetic field in a stellar radiation zone is considered for the cases of uniform and differential rotation. In the rigidly rotating radiative core shortly below the tachocline, the critical magnetic field for instability is about 600 G. The unstable disturbances for slightly supercritical fields have short radial scales ∼1 Mm. Radial mixing produced by the instability is estimated to conclude that the internal field of the sun can exceed the critical value of 600 G only marginally. Otherwise, the mixing is too strong and not compatible with the observed lithium abundance. Analysis of joint instability of differential rotation and toroidal field leads to the conclusion that axisymmetric models of the laminar solar tachocline are stable to nonaxisymmetric disturbances. The question of whether sun-like stars can posses tachoclines is addressed with positive answer for stars with rotation periods shorter than about two months. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)