Effects of magnetospheric turbulence on electron plasma waves

Our calculations indicate that high frequency plasma waves can be efficiently generated by electrostatic turbulence in the magnetosphere.     

The magnetoheliograph

A method is described, which makes it possible to obtain a magnetic photograph directly in one image without the use of photographic subtraction.Presently Guest Investigator at the Mount Wilson and Palomar Observatories.     

Magnetic-field structure of the photospheric network

A method is developed to determine the physical parameters of the spatially unresolved photospheric network. The apparent magnetic fluxes are recorded simultaneously in the two FeI lines 5250 and 5247 Å, which belong to the same multiplet and have practically the same oscillator strength and excitation potential of the lower level, but differ in the effective Lande factor. By analysing magnetograph recordings in this pair of lines together with simultaneous recordings in the two FeI lines 5250 and 5233 Å, it is possible to separate the effects on the line profiles due to Zeeman splitting and temperature enhancement in the network.From an analysis of the observations the following properties of the photospheric network are obtained: Field strengths of about 2000 G are present in the network in quiet regions. The characteristic size of the magnetic-field structures in the network appears to be in the range 100–300 km. The 5250 Å line is weakened by roughly 50% in the network. If the line had been non-magnetic, the weakening would have been about 20%. The rest of the weakening is caused by the strong Zeeman splitting. The downward velocity at the supergranular cell boundaries is estimated to be of the order of 0.5 km s^-1.     

The polar magnetic fields of July and August 1968

Observations of the polar magnetic fields were made during the period July 3–August 23, 1968, with the Mt. Wilson magnetograph. The scanning aperture was 5 × 5. The magnetic field was found to be ofS polarity near the heliographic north pole and ofN polarity near the south pole. At lower latitudes the polarity was the opposite. The polarity reversal occurred at a latitude of about +70° in the north and -55° in the south hemisphere. This coincides with the position of the polar prominence zones at that time. The observations indicate that the average field strength at the south pole was well above 5 G.Synoptic charts of the magnetic fields have been plotted in a polar coordinate system for two consecutive solar rotations.     

Electromagnetic ion waves in cold relativistic plasmas

The high-energy processes which occur in many astrophysical objects have motivated recent studies (e.g., Tsintsadze and Tsikarishvili, 1976; Ferrariet al., 1978; Sweeney and Stewart, 1978) of large amplitude wave propagation in plasmas. Such investigations are also of interest for laboratory experiments where strong laser radiation interacts with a high-density target. In the present paper we shall show that even rather small amplitude waves in the presence of an external magnetic field can induce particle velocities which are comparable to the velocity of light. In this ultrarelativistic limit we shall demonstrate that two completely new types of circularly polarized waves appear and that under certain conditions, modulational instabilities occur. Finally we look at the possibility to relate the amplitudes and wavelengths of such waves to pulsar data.     

Stimulated scattering of electromagnetic waves in dusty plasmas

The nonlinear coupling between a large amplitude electromagnetic wave and the slow background motion in a dusty plasma is considered. Stimulated scattering instabilities are investigated. The relevance of our investigation to cometary and astrophysical plasmas is pointed out.     

On the small-scale structure of solar magnetic fields

The small-scale structure of solar magnetic fields has been studied using simultaneous recordings in the spectral lines Fe i 5250 Å and Fe i 5233 Å, obtained with the Kitt Peak multi-channel magnetograph. We find that more than 90% of the magnetic flux in active regions (excluding the sunspots), observed with a 2.4 by 2.4 aperture, is channelled through narrow filaments. This percentage is even higher in quiet areas. The field lines in a magnetic filament diverge rapidly with height, and part of the flux returns back to the neighbouring photosphere. Therefore the strong fields within a magnetic filament are surrounded by weak fields of the order of a few gauss of the opposite polarity. The field-strength distribution within a filament, including the surrounding opposite-polarity fields, seems to be almost the same for all filaments within a given active or quiet region.The analysis of a scan made during an imp. 2 flare showed that observations during and after the flare would give a fictitious decrease of the magnetic energy in the region by a factor of 2–3 due to line-profile changes during the flare.Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Measurements of magnetic fields and the analysis of stokes profiles

Recent advances in polarimetry allowing the recording of polarized line profiles with high spectral resolution and signal-to-noise ratio over large portions of the solar spectrum offer rich new diagnostic possibilities. Thus we can now in a systematic way build models of the height variation of the magnetic field, temperature, density, and mass motions in the spatially unresolved subarcsecond magnetic structures. The analysis of the Stokes spectra also allows us to build a foundation for proper diagnostics of vector magnetic fields, a goal that cannot be achieved before the intrinsic properties of the spatially unresolved magnetic fields have been determined. Another new diagnostic tool is the Hanle effect. A recent exploratory survey of coherence effects through the recording of the linear polarization with high spectral resolution throughout the whole visible solar spectrum aims at establishing a foundation for the exploitation of the Hanle effect on the solar disk.This review describes these developments, most of which have taken place in the 1980s, and summarizes the results obtained so far.     

Solar magnetic fields

Since the structuring and variability of the Sun and other stars are governed by magnetic fields, much of present-day stellar physics centers around the measurement and understanding of the magnetic fields and their interactions. The Sun, being a prototypical star, plays a unique role in astrophysics, since its proximity allows the fundamental processes to be explored in detail. The PRL anniversary gives us an opportunity to look back at past milestones and try to identify the main unsolved issues that will be addressed in the future.     

Nonlinear convective motion in a rotating magnetized electron-positron plasma

A set of coupled nonlinear equations is derived which describes the coupling of the vorticity and the external-field aligned flow of a strongly magnetized rotating electron-positron plasma. The possibility of dipole vortex formation is discussed.