Max Kuperus

Thermal instability of coronal neutral sheets and the formation of quiescent prominences

It is argued that the quiscent prominences are a natural consequence of the formation and thermal instability of current sheets in the corona. Thus observation and theory of prominences can give vital information on the presence of currents and the topology of magnetic fields in the corona. Conversely by developing the theory of the structure and evolution of current sheets under coronal conditions we can attempt to gain a comprehensive understanding of the structure, evolution, and mass and energy balance of quiescent prominences. A stability analysis for coronal material permeated by a vertical magnetic field rooted in the photosphere, indicates that a condensation will take the form of a thin vertical wedge of cool matter. The development of a finite condensation is followed and it is shown that photospheric line tying is only important in the initial stages. A perturbation analysis of vertical motions at the neutral sheet shows that thermal instability can lead to overstable oscillations. Cooling of coronal material can lead to both upward and downward mass motions, and gravitational energy release is important to the thermal balance of prominences. Relevant optical and radio observations are discussed. Synoptic observations of the development of active regions and magnetic fields are needed to test the basic hypothesis of the formation of prominences from neutral sheets.

The role of plasma turbulence in the development of solar flares

The amount of energy released during a solar flare and the relatively short timescale in which all the flare associated events occur lead to the conclusion that a solar flare is a magnetohydrodynamic instability taking place in a strongly turbulent plasma.The main effect of the plasma turbulence is the strong reduction of the electrical conductivity resulting in a rapid heating and expansion of the flare plasma. The acceleration of particles during these processes can be understood with present day knowledge of non-linear plasma physics. The advantages of supertwisted flux tube as a model field configuration for a solar flare are discussed.

The double inverse polarity paradigm

A model is presented for the origin of inverse polarity magnetic fields in the perpendicular as well as in the axial direction of quiescent prominences. The model is based on the presence of a discrete coronal arcade structure where magnetic separating surfaces can be identified. On the crossing of these separating surfaces magnetic reconnection driven by photospheric shear and converging motions can create the observed field direction in quiescent prominences.

Magnetohydrodynamics of accretion disks

Abstract Accretion disks in close binary systems originate when mass overflow occurs from the primary star onto the compact star. When the compact star is a neutron star or a black hole the inner parts of the thin disk extend to the Alfven radius respectively a few times the Schwarzchild radius. In the Keplerian rotating highly turbulent inner parts of the accretion disk magnetic fields are strongly amplified and expelled from the disk thus leading to the formation of a magnetically structured accretion disk corona, sandwiching the disk to which it is electrodynamically coupled. The magnetic energy supplied to the corona is radiated by inverse Compton scattering of soft X-ray photons produced in the disk by the viscous heating of the accreting matter. This may explain why certain X-ray sources show a very large fluctuating hard X-ray component. The interaction of the inner parts of an accretion disk with the magnetosphere around a neutron star leads to channeled accretion onto the magnetic poles, resulting in the phenomenon of X-ray pulsars with the associated spin variations due to angular momentum transfer. The interaction of disk coronal structures with the relatively weak magnetic fields of old fast spinning neutron stars lead to a new form of interaction around the so called beat frequency that can be used as a model for quasiperiodic oscillations in low-mass X-ray binaries.

A Magnetic Explanation for the Rapid Burster

The observations of X-ray Type II bursts from the low-mass X-ray binary MXB 1730–335 can be explained by a particular form of magnetic gating in the presence of steady external accretion. The requirements are a strong magnetic field of the neutron star (7 × 1011 – 2 × 1012 gauss at the surface), rotational symmetry and alignment of the field axis with the axis of a steadily accreting disk to within 6°.

Brief report of the meeting

The first week of the XXVIII SCAR biennial meeting was held at the Congress Centre in Bremen, Germany. The centrepiece of the SCAR Science Week was the SCAR Open Science Conference on “Antarctica and the Southern Ocean in the Global System”. The SCAR Standing Scientific Groups and their various sub-groups met around the Conference. The COMNAP XVII meeting was held in parallel with the SCAR meeting and included the SCALOP Symposium “Towards the International Polar Year and Beyond”, a trade exhibition and an exhibition of polar aircraft at Luneort airport.

On the determination of the velocity of the exciters of type II solar radiobursts

Under the assumption that type II bursts are excited by strong shock waves it is shown that the velocity determined from dynamic spectrograms should be corrected by a term proportional to the gradient of the shock strength. The magnitude of the correction strongly depends on the properties of the pre-shock atmosphere and the geometry of the shock propagation.