Solar models with low opacity

Solar Physics - Evolutionary models of the present Sun with standard and artifically low opacity of stellar matter are obtained and adiabatic nonradial oscillations of the models are computed. The...     

HALL-MHD turbulence in the Solar atmosphere

When the velocity field of a magneto-fluid is accorded an intrinsically equal status with the magnetic field, standard magnetohydrodynamics (MHD) must be replaced by the dispersive or Hall MHD which retains some crucial two-fluid effects, in particular the physics on the ion skin depth scale. The larger system has three quadratic invariants: the generalized helicity (a sum of the cross and fluid helicities) is added to the ranks of the standard total energy and the magnetic helicity invariants. Based on this extended set, dimensional arguments à la Kolmogorov are invoked to derive the turbulent spectral distributions of the kinetic and the magnetic energy densities in the inertial range. By using the selective dissipation hypothesis, we construct the spectra on three different scales within the inertial range, and acknowledge the possible role of the dual cascade of the kinetic and the magnetic energy densities. The additional structure imparted to the spectral laws (by the inclusion of the generalized helicity) allows us to reproduce, remarkably well, the essentials as well as details of the observed spectra of the motions and of the magnetic fields of the solar atmosphere on the scales of a few thousand km.     

Solar diameter measurements with Calern Observatory astrolabe and atmospheric turbulence effects

In order to deduce significant astrophysical results from solar diameter measurements it is necessary to take an accurate account of instrumental and atmospheric effects. This paper presents a comparison between visual and CCD camera measurements performed by means of the Calern Observatory solar astrolabe during the last 4 years; this allows us to evaluate visual measurements done previously (from 1975 to 1989). Then, a study of atmospheric effects is developed. From CCD measurements, the image quality, expressed by Fried's seeing parameter,r ₀, is estimated and related to the errors occurring in solar diameter measurements. A statistical analysis gives about 0.26 arc sec (or 0.13 arc sec for the semi-diameter) as the lowest value that this error may reach at Calern Observatory. One conclusion of this work is that it is important in the future to have image quality observations, obtained using a dedicated monitor, in order to evaluate and classify the measurements. A survey of the seeing might so lead to improve the precision of the results by weighting each diameter estimation and eventually to schedule the observations.     

Soliton and strong Langmuir turbulence in solar flare processes

The occurrence of modulational instability in the current sheet is investigated. Particular attention is drawn to the plasma micro-instability in this current sheet (i.e., the diffusion region) and its relation to the flare process. It is found that the solitons or strong Langmuir turbulence is likely to occur in the diffusion region under solar flare conditions in which the electric resistivity could be greatly enhanced by several orders of magnitude in this diffusion region. The result is a significant heating and stochastic acceleration of particles. Physically, the occurrence of soliton and strong Langmuir turbulence can be identified with a sudden eruption of an electric current leading to a local vacuum in which an electric potential is formed and results in the release of a huge amount of free energy. A numerical example is used to demonstrate the transition of the magnetic field, velocity, and plasma density from the outer MHD region into the diffusive (resistive) region and, then, back out again with the completion of the energy conversion process. This is all made possible by an increase of resistivity by 4–5 orders of magnitude over the classical value.     

Solar and terrestrial parameterizations for radiative-convective models

We present a radiative-convective modelling technique with parameterizations, for both solar and terrestrial radiation transfer, which allow the rapid computation of the mean vertical temperature profile from the ground to the thermosphere. Our method has been specifically designed for modelling the evolution of the Earth's mean vertical temperature structure due to changes in atmospheric composition, variations in the solar flux, surface albedo, cloud cover, water vapour and lapse rate, and changes in the temperature of the thermosphere which is associated with solar activity.     

Limitation imposed by strong Langmuir turbulence on the self-consistency of the quasi-linear dynamics

Strong turbulence occurs when Langmuir waves reach Zakharov's (1972) threshold for the formation and collapse of plasma wave solitons. A more precise criterion, derived by Goldman et al. (1980) in the case of broadband Langmuir noise, is used to derive a necessary condition of self-consistency for models of the quasi-linear dynamics. The results suggest that a regime in which quasi-linear relaxation is important but strong turbulence is not may be relevant to type III solar radio bursts.     

Statistical properties of dissipation bursts within turbulence: Solar flares and geomagnetic activity

We investigate some statistical properties of the solar soft X-ray flux and of the Auroral Electrojet (AE) index. For both these signals, the probability density functions (PDFs) of normalized differences are shown to display wide, non-Gaussian tails. The shape of the PDFs is nearly unchanged as the timelag, used to calculate differences, varies. Moreover, the two time series are characterized by a power-law distribution for waiting times between successive bursts and by a clear departure from an hypothesis of locally Poisson statistics. A similar behavior is found in a shell model of magnetohydrodynamic (MHD) turbulence.     

Solar oscillations in strong and weak fraunhofer lines over a quiet region

We have analysed a 35-min-long time sequence of spectra in the Caii H line, Nai D1 and D2 lines, and in a large number of strong and weak Fei lines taken over a quiet region at the center of the solar disk. The time series of these spectra have been observed simultaneously in these lines under high spatial, spectral, and temporal resolution at the Vacuum Tower Telescope (VTT) of the Sacramento Peak Observatory. We have derived the line profiles and their central intensity values at the sites of the chromospheric bright points, which are visible in the H line for easy identification. We have done a power spectrum analysis for all the lines, using their central intensity values to determine the period of oscillations. It is shown that the 3 Fei lines, present 23 Å away from the core of the H line representing the pure photospheric lines, Nai D1 and D2 lines, 6 Fei lines at the wings of H line, and Can H line exhibit 5-min, 4.05-min, 3.96-min, and 3.2-min periodicity in their intensity oscillations, respectively. Since all these lines form at different heights in the solar atmosphere from low photosphere to middle chromosphere and show different periodicities in their intensity oscillations, these studies may give an idea about the spatial and temporal relation between the photospheric and chromospheric intensities. Therefore these studies will help to better understand the physical mechanisms of solar oscillations. It is clearly seen that the period of intensity oscillations decreases outward from the low photosphere to the middle chromosphere. Since we have studied a single feature at a time on the Sun (i.e., bright points seen in the H line) in all these spectral lines simultaneously, this may explain about the footpoints of the bright points, the origin of 3-min oscillations, and the relation to other oscillations pertaining to these locations on the Sun. We have concluded that 80% of the bright points are associated with dark elements in the true continuum, and they may seem to have a relationship with the dark intergranular lanes of the photosphere, after carefully examining the brightness (bright threads) extending from the core to the far wings of the H line at the locations of a large number of bright points, using their time sequence of spectra.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

A comparison of local simulations and reduced models of MRI-induced turbulence

We run mean-field shearing-box numerical simulations with a temperature-dependent resistivity and compare them to a reduced dynamical model. Our simulations reveal the co-existence of two quasi-steady states, a 'quiet' state and an 'active' turbulent state, confirming the predictions of the reduced model. The initial conditions determine on which state the simulation ultimately settles. The active state is strongly influenced by the geometry of the computational box and the thermal properties of the gas. Cubic domains support permanent channel flows, bar-shaped domains exhibit eruptive behaviour, and horizontal slabs give rise to infrequent channels. Meanwhile, longer cooling time-scales lead to higher saturation amplitudes.     

Solar cycle distribution of strong solar proton events and the related solar-terrestrial phenomena

We investigated the solar cycle distribution of strong solar proton events (SPEs, peak flux ≥1000 pfu) and the solar-terrestrial phenomena associated with the strong SPEs during solar cycles 21–23. The results show that 37 strong SPEs were registered over this period of time, where 20 strong SPEs were originated from the super active regions (SARs) and 28 strong SPEs were accompanied by the X-class flares. Most strong SPEs were not associated with the ground level enhancement (GLE) event. Most strong SPEs occurred in the descending phases of the solar cycles. The weaker the solar cycle, the higher the proportion of strong SPES occurred in the descending phase of the cycle. The number of the strong SPEs that occurred within a solar cycle is poorly associated with the solar cycle size. The intensity of the SPEs is highly dependent of the location of their source regions, with the super SPEs (≥20000 pfu) distributed around solar disk center. A super SPE was always accompanied by a fast shock driven by the associated coronal mass ejection and a great geomagnetic storm. The source location of strongest GLE event is distributed in the well-connected region. The SPEs associated with super GLE events (peak increase rate ≥100%) which have their peak flux much lower than 10000 pfu were not accompanied by an intense geomagnetic storm.     

Acceleration covariance in hydromagnetic dusty turbulence in the presence of a very strong uniform magnetic field

We have found the Electric Dipole Radiation Fields and an estimative of the power radiated by an electric dipole in Einstein and de Sitter's cosmological backgrounds. A possible connection with the quasar problem is suggested.     

Solar dynamo theory and the models of Babcock and Leighton

The dynamo theory of the solar cycle as developed by Parker and others, and the observational models of Babcock and Leighton have been examined, with the conclusion that the dynamo theory is not applicable to the Sun and that the models fail.An essential part of the theory is an adequate effective diffusion coefficient. Fields are continuously sheared and amplified and, in this theory, these may not be allowed to accumulate; all subsurface fields of an old cycle must be eliminated. Ohmic diffusion is negligible and turbulent diffusion is invoked. However, this requires that all solar fields are tangled to a small scale, which is contrary to observation; for Hale's polarity laws are strictly observed, and large-scale surface features are common at the end of an 11-yr cycle in the same general area where new fields are appearing.The erupted (sunspot) fields lie generally above the unerupted, toroidal fields so that, even if they are merged as required, the centroid of the new system would be above that of the old. The result is not a steady-state oscillator, as required, but the complete loss of the solar field.It is concluded that for these and other reasons a shallow, reversing field is unacceptable, and that a deeply penetrating field is required. Reference is made to an alternative theory of the solar cycle based on a deep magnetic field.     

The upper atmosphere of Uranus: A critical test of isotropic turbulence models

Observations of the 15 August 1980 Uranus occultation of KM 12, obtained from Cerro Tololo InterAmerican Observatory, European Southern Observatory, and Cerro Las Campanas Observatory, are used to compare the atmospheric structure at points separated by ～140 km along the planetary limb. The results reveal striking, but by no means perfect, correlation of the light curves, ruling out isotropic turbulence as the cause of the light curve spikes. The atmosphere is strongly layered, and any acceptable turbulence model must accommodate the axial ratios of $\text{?60}$ which are observed. The mean temperature of the atmosphere is 150 ± 15°K for the region near number density 10¹⁴ cm^?3. Derived temperature variations of vertical scale ～ 130km and amplitude ±5°K are in agreement for all stations, and correlated spikes correspond to low-amplitude temperature variations with a vertical scale of several kilometers.     

On the atmosphere with exponential turbulence

We present an exact calculation for the photon-average bending angle and intensity produced during occultation of a point source by an isothermal atmosphere with an exponential height dependence of turbulent power. The calculation is performed via an expansion in powers of the gradients of average refractivity and turbulent power. Conditions for the validity of the expansion are discussed. We compare results with other expressions, for the same distribution of refractivity, recently presented by V. R. Eshleman and B. S. Haugstad. Discrepancies are noted. Certain properties of photon averages, which may be of importance for the reduction of occultation data, are discussed.     

Correlation of Solar Indices with Solar euv Fluxes

The paper presents a more extensive comparison of Extreme Ultraviolet (EUV) irradiances during AE-E (1977–1980), Pioneer Venus (1979–1992) and SEM/SOHO (1996 onwards) with other solar indices than has been discussed previously. For long-term changes (solar cycle), all indices had similar trends and inter-correlations were high, so that any one could serve as a proxy for the other. For intermediate time-scales (monthly means), only L, F10 (2800 MHz) and Mgii had reasonably high correlations with EUV. The 2695 MHz radio emission also had a high correlation. For daily values, data for many indices are intermittant and these cannot serve as proxies. Again, only L, F10 (and 2695 MHz), Mgii stand out as possible proxies for EUV, particularly during intervals of strong 27-day sequences.     

Interaction of fast particles with magneto-hydrodynamical turbulence

The acceleration of fast particles by Alfvén and magnetic sound waves of small amplitude is considered. The waves exist against the background of a strong, uniform magnetic field. We take into accunnt the contributions to acceleration from a large scale random field (harmonics withk<R ^–1, whereR is the Larmor radius), as well as from a small scale field (k>R ^–1). The small scale field was considered by perturbation theory, while large scale random field-in an adiabatic approximation. The energy dependence of the diffusion coefficient in momentum space, and the time of acceleration are estimated. The possible anisotropy of angular distribution is taken into account.If the Alfvén waves have spectral power index >2 and wave amplitude is small enough, then the energy dependance of the diffusion coefficient is stronger than in the case of the Fermi acceleration. For magnetic sound waves with 2 the energy dependance of the diffusion coefficient is the same as for the Fermi acceleration, but for <2 this dependance is less.The space diffusion coefficient of particles across the regular magnetic field is estimated. It is shown that this diffusion is due mainly to the large scale random field.