The Magnetic Connectivity of Moss Regions

A novel emission feature resembling moss was first identified in high-resolution TRACE Feix/x 171 Å images by Berger et al. (1999). The moss emission is characterized by dynamic arc-second scale, bright elements surrounding dark inclusions in images of solar active regions. Patches of moss elements, called moss regions, have a scale of 20–30 Mm. Moss regions occur only above some of magnetic plages that underlie soft X-ray coronal loops. Using the potential field extrapolation of the photospheric magnetic field into the corona, we find that the magnetic field lines in moss-associated magnetic plages connect with adjacent plages with opposite polarity; however, all field lines from mossless plages end in surrounding quiet regions. This result is consistent with the idea that the TRACE moss is the emission from the upper transition region due to heating of low-lying plasma by field-aligned thermal conduction from overlying hot plasma (Berger et al., 1999).     

Solar flare activity and the structure of the coronal neutral line

In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R , and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R . This indicates that major flares are strongly associated with coronal structure at the 0.35R level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought.     

The magnetic field in a protostellar cloud

General forms of theB-p relation are investigated in both the isothermal and the non-isothermal regions. The magnetic flux dissipation either by ambipolar diffusion or by Ohmic dissipation has been studied. The rates of heating due to the magnetic dissipation processes have been calculated in comparison with the rate of compressional heating.The magnetic field strength is derived as a function of flux/mass ratio, mass, density, and geometry of the isothermal cloud. In the non-isothermal region, the temperature is added to the above-mentioned variables.It has been found that the magnetic flux starts to dissipate via ambipolar diffusion at neutral density ofn>3×10⁹ cm^–3. Ambipolar diffusion continues effective until reaching densities ofn>10¹¹ cm^–3, where Ohmic dissipation dominates. Under some conditions, the electrons evaporate from the grain surface atn>10¹³ cm^–3, while the ions are still adsorbed on the grain surfce. In this case, the magnetic flux loss returns to be influenced by ambipolar diffusion.The rates of heating by both Ohmic dissipation _OD and ambipolar diffusion _AD are found to be smaller than the rate of compressional heating _C in case of magnetic dissipation. Assuming that the magnetic field is frozen in the medium, then _C is smaller than both _OD and _AD . The above results of heating were found in the non-isothermal region.     

The relationship between the longitudinal magnetic field and the line-of-sight velocity at different angular positions of sunspots

Using observational data on 14 sunspots from the Sayan Observatory vector magnetograph, a study was made of the relationship between the sunspot magnetic field and the Evershed motions. It is shown that the central area of the solar disk is dominated by an anti-correlation of the longitudinal magnetic field B and the line-of-sight velocity V when a maximum of V corresponds to the neutral line of the longitudinal field. Near the limb there usually is a coincidence of the field and velocity neutral lines. There is evidence for the possible asymmetric character of the effect with respect to the central meridian.     

Thermosolutal instability of compressible hall plasma in porous medium

The thermosolutal instability of a plasma in porous medium in the presence of a vertical magnetic field is considered to include the effects of compressibility and Hall currents. The effects of stable solute gradient and compressibility are found to be stabilizing and the Hall currents have a destabilizing effect. The system is stable for (C _p/g)<1;C _p, , andg denoting specific heat at constant pressure, uniform temperature gradient, and acceleration due to gravity, respectively. In contrast to the non-oscillatory modes in the absence of magnetic field and stable solute gradient, the presence of magnetic field (and, hence, Hall currents) and stable solute gradient introduce oscillatory modes for (C _p/g)>1. The case of overstability is also studied wherein the necessary conditions for the existence of overstability are obtained.     

Electromagnetic lower hybrid instability in the solar wind

Low frequency electromagnetic lower hybrid waves (so-called hybrid whistlers) propagating nearly transverse to the magnetic field can be driven unstable by a resonant interaction with halo electron distributions carrying solar wind heat flux. The electromagnetic lower hybrid instability is excited when the halo electron drift exceeds the parallel phase velocity of the wave. The growth rate attains a maxima at a certain value of the wavenumber. The maximum growth rate decrease by an increase in _e (the ratio of electron pressure to magnetic field pressure) and halo electron temperature anisotropy. At 0.3 AU the growth time of the electromagnetic lower hybrid instability is of the order of 25 ms or shorter, whereas the most unstable wavelengths associated with the instability fall typically in a range of 27 to 90 km. The instability would give rise to a local heating of solar wind ions and electrons in the perpendicular and parallel directions relative to the magnetic field, B₀. The observations of low frequency whistlers having high values ofB/E ratios (B andE being the magnitude of the wave magnetic and electric field, respectively) and propagating at large oblique angles to B₀ behind interplanetary shocks, can be satisfactorily explained in terms of electromagnetic lower hybrid instability. The instability is also relevant to the generation mechanism of correlated whistler and electron plasma oscillation bursts detected on ISEE-3.     

Modelling of the effect of current-carrying jets on the temperature of an astrophysical surrounding

Numerical solution of the effect of current-carrying jets on the temperature of an astrophysical surrounding is carried out using classical magnetohydrodynamic equations. Under the assumption of small hydrodynamic and magnetic Reynolds numbers and invoking a jet magnetic field intensityB , which confines high pressure jets along thez-axis, a non-linear equation is generated and solved by asymptotic approximation. It is found that when the field intensity is large, the temperature of the surrounding is small and vice-versa. The problem is of interest in the astrophysical studies of current-carrying jets or magnetised radio jets.     

On the physical conditions in the photospheric network: An improved model of solar faculae

Semi-empirical models of solar faculae, cospatial with strong photospheric magnetic fields, have been constructed from continuum observations. The center-to-limb contrast of the various models was computed taking into account their geometrical shape. The adopted model whose horizontal size was taken to be 750 km, indicates that, in field regions, the temperature begins to rise outwards at z -125 km (above ₅₀₀₀ = 1) and that the extrapolated temperature at z -400 km is about 1500 K above that of the undisturbed atmosphere; the electron density is higher by a factor of about 30.     

Simultaneous measurements of the polarization in Hα and D3 prominence emissions

Simultaneous observations of the polarization of radiations H and D₃ have been performed in 1979 at Pic du Midi on 60 solar prominences (300 pairs of measurements). For the essential part of the sample, the degree of polarization p(D₃) does not depend strongly upon the intensity E(D₃); on the contrary p(H) decreases steeply when E(H) goes beyond the brightness which corresponds to (H) = 1 (Figure 4); the deviation of the polarization direction (H) does not show such a strong dependance upon E(H) (Figure 6). A crude estimate of the possible role of multiple scattering in an optically thick prominence is in general agreement with observational data but a detailed comparison will be possible only when a complete theory of the Hanle effect in non optically thin layers will be available.Therefore, H polarization measurements are presently useful mainly when (H) < 1 and, in connexion with D₃ measurements (Figure 8), they should make possible to determine the three components of the prominence magnetic vector. Prospects are given for the observation of other lines (H; 10 830 Å) which do not suffer heavy self-absorption effects and would be well-suited to this type of study.     

Propagation of spiral density waves in a magneto-active disk

The propagation of spiral density waves in a differentially rotating, self-gravitating, magnetoactive and highly flattened disk is investigated by using the asymptotic theory for tightly wound spirals developed by Lin and his collaborators. We adopt the continuum fluid model as the primary basis, and our treatment will be largely analytical. The disk plasma is studied in the frozen field approximation and inhomogenceous magnetic fields in the plane of the disk are considered in detail.In a differentially rotating disk with strong magnetic fields, the field lines will be considerably distorted and the mutual influence of magnetic fields and differential rotation is by no means obvious.In this paper we present a new asymptotic dispersion relation for tightly wound spiral density waves with magnetic fields along the spiral armsB (r). The effects of the magnetic fields lead to such terms likek ²(a ² +V _A ² ), wherek is the wave number,a represents the speed of sound,V _A = (B ²/4)^1/2 is the Alfvén speed,B denotes the field strength, and is the plasma density. These terms depict the well-known magnetoacoustic waves and could have been anticipated without a detailed computation. However the interaction of magnetic fields and differential rotation may give rise to other significant terms which are not so easy to foresee.We also present a more exact local dispersion relation by using the WKB approximation and study the effects of magnetic fields on the growth rate through the parametersQ andJ defined in the literature.Although the effects of the magnetic fields are rather insignificant for applications to Galactic dynamics, the effects of the magnetic fields are important for applications to star formation and problems related to the solar nebula.     

Velocity oscillations in solar plage regions

The properties of wave propagation in a perfectly electrically conducting, plane-stratified, inviscid, compressible atmosphere premeated by a horizontal magnetic field which varies with height are investigated. It is shown that a diagnostic diagram can be constructed through a generalization of the propagation equation to account for the presence of a magnetic field.The effect of the magnetic field on the oscillations in solar plages around the temperature minimum is studied and compared with the non-magnetic case based on the Bilderberg Continuum Atmosphere. Due to the joint action of ionization and the magnetic field, a potential well for internal gravity waves is formed. The properties of the trapped waves are in qualitative agreement with the observations.The Brunt-Väisälä frequencies in the presence (N) and absence (N ₀) of the magnetic field around the temperature minimum are shown to obey the relation N/N ₀ (1 + ²)^–1/2, where is the ratio between the Alfvén and the sonic speeds. The modified Brunt-Väisälä frequency (N) is decreased as the strength of the magnetic field increases. The magnetic effect makes the potential well shallower, thus shrinking the domain for the trapped gravity modes.     

Hall effects on natural convective flow in slip flow regime

Hall effects on the flow of electrically conducting rarefied gas due to combined buoyant effects of thermal and mass diffusion past an infinite porous plate with constant suction in the presence of strong transverse magnetic field have been investigated. The equations governing the flow poblem have been solved for primary, secondary velocities and temperature. The effects of Hall current, magnetic field and the effect of rarefication have been discussed graphically followed by a discussion.Nomenclature x,y coordinate system - u velocity inx direction - v ₀ suction velocity - w velocity inz direction - E Eckert number - G, G^* Grashof numbers - h ₁ velocity slip coefficient - h ₂ temperature jump coefficient - h ₃ concentration jump coefficient - M, m magnetic field parameter, Hall parameter - Pr Prandtl number - Sc Schmidt number - T, T _w, T temperature in flow regime, plate temperature, temperature outside the boundary layer very away from the plate - C, C _w, C concentration of the gas in flow, concentration at the plate, concentration far away from the plate - thermal conductivity - D coefficient of chemical molecular diffusion - coefficient of kinematic viscosity - coefficient of viscosity - electrical conductivity - C _p specific heat of gas at constant pressure density     

Expanding and shearing magneto-viscous fluid cosmological model in general relativity

The object of this paper is to investigate the behavior of a magnetic field in a viscous fluid cosmological model. It has been assumed that the expansion () is proportional to the eigenvalue ₁ of the shear tensor _i ^j and the coefficient of shearing viscosity is proportional to the scalar of expansion. The paper also discusses the behavior of the model when the magnetic field tends to zero and comments on some other physical properties.     

A self-consistent model for the storm radio emission from the Sun

A self-consistent theoretical model for storm continuum and bursts is presented. We propose that the Langmuir waves are emitted spontaneously by an anisotropic loss-cone distribution of electrons trapped in the magnetic field above active regions. These high-frequency electrostatic waves are assumed to coalesce with lower-hybrid waves excited either by the trapped protons or by weak shocks, making the observed brightness temperature equal to the effective temperature of the Langmuir waves.It is shown that whenever the collisional damping ( _c ) is more than the negative damping (- _A ) due to the anisotropic distribution, there is a steady emission of Langmuir waves responsible for the storm continuum. The type I bursts are generated randomly whenever the collisional damping ( _c ) is balanced by the negative damping (- _A ) at the threshold density of the trapped particles, since it causes the effective temperature of Langmuir waves to rise steeply. The number density of the particles responsible for the storm radiation is estimated. The randomness of type I bursts, brightness temperature, bandwidth and transition from type I to type III storm are self-consistently explained.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Cyclic Reversal of Magnetic Cloud Poloidal Field

Coronal mass ejections (CMEs) carry magnetic structure from the low corona into the heliosphere. The interplanetary CMEs (ICMEs) that exhibit the topology of helical magnetic fluxropes are traditionally called magnetic clouds (MCs). MC fluxropes with axis of low (high) inclination with respect to the ecliptic plane have been referred to as bipolar (unipolar) MCs. The poloidal field of bipolar MCs has a solar cycle dependence. We report a cyclic reversal of the poloidal field of low inclination MC fluxropes during 1976 to 2009. The MC poloidal field cyclic reversal on the same time scale of the solar magnetic cycle is evident over three sunspot cycles. Approximately 48% of ICMEs are MCs, and 40% of IMCs are bipolar MCs during solar cycle 23. The speed of the bipolar MCs has essentially the same distribution as all ICMEs, which implies that they are not from any special type of CMEs in terms of the solar origin. Although CME fluxropes may undergo a number of complications during the eruption and propagation, a significant group of MCs retains sufficient similarity to the source region magnetic field to posses the same cyclic periodicity in polarity reversal. The poloidal field of bipolar MCs gives the out-of-ecliptic-plane field or B _z component in the IMF time series. MCs with southward B _z field are particularly effective in causing geomagnetic disturbances. During the solar minima, the B _z field IMF sequence within MCs at the leading portion of a bipolar MC is the same with the solar global dipole field. Our finding shows that MCs preferentially remove the like polarity of the solar dipole field, and it supports the participation of CMEs in the solar magnetic cycle.     

New observations of solar noise storm radiation at decameter wavelengths

We report multifrequency observations of storm continuum and other radio bursts. Based on their positional study and their correlation with other coronal and photospheric features, we deduce that the storm source is located in the magnetic field lines lying above a single bipolar active region. Energetic electrons trapped in the magnetic structures above the spots must be responsible for the storm radiation. We show that spontaneous emission of Langmuir waves by anisotropic distributions can explain both storm continuum and bursts self-consistently. Whenever the collisional damping ( _c ) is more than the growth (- _A ), there is a steady emission responsible for the continuum, and whenever _c = - _A (which may be satisfied randomly) there is a sudden jump in T _b giving rise to bursts. The number density of energetic particles required to explain the storm continuum at 73.8, 50, and 30.9 MHz frequencies is estimated to lie in the limits n _b /n _e 10^–10–10^–9 in the context of the present observations. The brightness spectrum of the storm continuum is computed and compared with observations.On leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Convective instability of thin flux tubes

The stability of magnetic flux tubes embedded vertically in a convection zone is investigated. For thin tubes, the dominant instability is of the convective type, i.e. it is driven by buoyancy forces associated with displacements along the tube. The stability is determined by = 8P/B ²; if _c the tube is convectively stable, otherwise it is unstable, where the critical value _c depends on the stratification of the convection zone. For a solar convection zone model, _c = 1.83, corresponding to a magnetic field strength of 1350 G at the surface of the Sun. It is concluded that the flux tubes making up the small scale field of the Sun are probably hydrodynamically stable.In tubes with > _c, the instability is expected to transform the tube either into a state of vanishing surface field strength (in the case of an upward flow), or one with a field strength higher than the original value (if the instability sets in as a downward flow). Following Parker, we suggest that this effect is related to the concentrated nature of the observed solar fields.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A spectrophotometric study of GG cassiopeiae

The continuum energy distributions of a totally eclipsing binary system GG Cas at phases 0 _. ^P 903 and 0 _. ^P 003 in the wavelenght range 3200–7600 Å have been presented and discussed. The flux of continuum of the secondary star seems to dominate longward of 4500 Å. The spectral types of the systemic components are found to be as B4+K0.     

The current sheet and Joule heating of a slender magnetic tube in the upper photosphere

Joule heating in a slender magnetic flux tube is investigated. The distribution of the magnetic field and electric sheet current encircling a vertical cylindrical magnetic tube is determined by equating the converging magnetic flux, which results from the converging and downward flow of the granulation, and the dissipative expanding magnetic flux due to Ohmic decay. Here, to ensure the mass flux conservation, an overshooting convective flow pattern resembling recent simulations was assumed. Even with the electrical resistivity from neutral hydrogen, the width of the current sheet was found to be 2 km, being much smaller than the tube diameter of 150 km, either from an exact or approximate (Gaussian) field distribution.The resultant energy flux density due to Joule heating averaged over the cylindrical cross sectional area, is 1 × 10⁹ erg cm^-2 s^-1 for an assumed photospheric magnetic field of 1500 G. This amount may supply enough energy to heat the temperature minimum region of the flux tube by T = 300 K in accord with observations, though our estimation of the excess radiation loss which should be supplied by the Joule heating to keep T = 300 K is rather uncertain.A possible role of the Joule heating on spicule formation is briefly discussed together with discussions on the slab geometry, general flow patterns, and non-constant field distributions inside the flux tube.     

The dependence of the frequency spectra of the interplanetary magnetic-field fluctuations on the direction of the mean field

The frequency spectra of the interplanetary magnetic field fluctuations are the projection of their wavenumber spectra onto one dimension. Only the frequency spectra can be measured by spacecrafts. It is studied how their measured size depends on the direction of the mean fieldB ₀, which structures the symmetry of the fluctuations relative to the solar wind system. It is specialized for the slab model, Alfvén waves, magneto-acoustic waves and the isotropic case. For the slab model the frequency spectra are proportional to , whereq is the spectral index and the angle betweenB ₀ and the radial direction. For the diffusion coefficientK _TT the relation holds.