Photospheric line-tying conditions for the MHD stability of coronal magnetic fields

Using the localised, ballooning ordering, the effect of a density stratification on the ideal MHD stability of magnetic fields is investigated. It is found that, when the photospheric density is very much greater than the coronal value, the line tying conditons are best simulated by assuming that all coronal disturbances vanish at the photospheric boundary. This is commonly known as the rigid wall conditions.     

Fine structure of a Greenland reverse tip jet: a numerical simulation

Reverse tip jets are strong low-level winds with easterly component that form near the southern tip of Greenland. In the present study, a reverse tip jet case which occurred from 21 to 22 December 2000 was examined to clarify its fine structure using a numerical model with a horizontal resolution of 3 km. The reverse tip jet, showing the supergeostrophic wind speed with a maximum wind speed in excess of 45 m s⁻¹, extended from the east coast of Greenland to the west of Cape Farewell with anticyclonic curvature. A cloud free region coincided with the jet indicated that there was a mesoscale downdraft. Along the eastern edge of the jet, a banded cloud formed between the upstream easterly wind and the colder northerly wind that is a part of the jet and is located along the east coast. This cloud was associated with large gradients in surface wind speed, temperature, moisture, and heat flux. A maximum surface total heat flux of 300 W m⁻² coincided with the location of the jet. It is suggested that the orographic deflection by Greenland's large-scale topography as well as small-scale downslope winds behind mountains with fiords causes the reverse tip jet.     

Rapid neutron capture in supernova explosions

The implications of recent studies of the dynamics of the cores of highly evolved massive stars are considered with regard to the general problems of nucleosynthesis. The typical conditions estimated for these models are shown to be very promising for the process of element synthesis by neutron capture on a fast time scale (ther-process ofBurbidge et al., 1957).     

Collective effects in diffuse matter-antimatter plasma: II

It is shown that even very slightly ionized clouds of matter and anti-matter can interpenetrate only a little on collision. Initial interpenetration produces fast electrons and positrons from annihilation. These, in turn, produce strong magneto-hydrodynamic shocks which give the small ionized component enough energy to ionize the neutral fraction and produce a Leidenfrost layer in about ten years after which interpenetration stops.     

On the size and structure of bright solar Ca+-network cells depending on the heliographic position

From photographic recordings of some hundred bright Ca⁺-network cells on the solar disk we find evidence for a smaller size of polar cells as compared to equatorial cells by a factor of about 0.9. We do not find an indication of a dependence of the structure of the cells on the heliographic position.     

The effect of a normal magnetic field component on current sheet stability

A $\text{2}\text{2}\text{5}\text{-}\text{dimensional}$ particle-mesh computer model for the simulation of the current-sheet region of the geomagnetic tail is described. Important features are (a) the use of Fast Fourier techniques for the efficient solution of Ampere's equation, (b) the incorporation of sources and sinks of particles, (c) facilities for simulating finite width effects and (d) the option of including a normal magnetic field component linking through the sheet.Simulations carried out using this model indicate that current sheets with a non-zero normal magnetic field component and an infinite width are stable. The particles trace out Speiser-like orbits in such a case. Sheets with B_normal = 0 and a finite width are unstable with respect to the ion tearing-mode instability. However the presence of a normal magnetic field stabilises the system provided ρ₀<2L_y where ρ₀ is the characteristic length associated with the normal field and where L_y is the width of the sheet.On the basis of these results it is suggested that a geomagnetic substorm occurs when the normal magnetic field drops below the critical value needed for stability.     

Total current of the auroral electrojet estimated from the IMS Alaska meridian chain of magnetic observatories

Based on magnetic data from the IMS Alaska meridian chain of observatories, the total current of the westward auroral electrojet flowing across the meridian is estimated by using two independent methods. The first one is a simple integration of the north-south component of magnetic perturbations along the meridian, providing the quantity F in units of nT·km. The other is to use the forward method, providing the total current I in units of A. It is shown that F and I have nearly identical time variations. Thus, by normalizing the two quantities and taking the numerical value of F in units of nT·km, it is possible to estimate the total electrojet current flowing across a magnetic meridian, with an accuracy of 90%, by using the latitudinal profile of the H component, namely I (A) = 2.0 F (nT·km).     

Silicon negative ion chemistry in the atmosphere-in situ and laboratory measurements

The results of a rocket-borne mass spectrometer measurement indicate that large concentrations of negative ions exist above the bottom of the atmospheric atomic oxygen layer. A large majority of these ions have a mass greater than 100 amu. In addition, an ion at mass 76 was observed with concentrations too large to be CO₄^?. In order to explain these features, a number of reactions involving silicon oxide negative ions have been measured in a flowing afterglow system. The ion SiO₃^? is produced by reaction of O₃^?, and CO₃^?, with SiO. The SiO₃^? ion is extremely stable and does not react measurably with NO, NO₂, CO, CO₂, O₃ or O. Since meteoroid ablation produces a large silicon input into the atmosphere, it appears possible that the ions observed at mass 76 may be SiO₃^?. Possible production mechanisms for this ion as well as the heavy ions are discussed.     

Dwarf Galaxies in the M 81/82 Group. New Photometric Data

We present a list of 39 dwarf galaxies of the M 81/82 group. Three different methods are applied to determine their apparent integral magnitude: digital processing, equidensitometry, and calculation according to the formulae of FISHER and TULLY . The plates used were obtained with the Tautenburg Schmidt telescope. For a distance of the group of 3.5 Mpc the absolute magnitudes of the dwarf galaxies investigated are -15^m < M < -9^m.     

The delay between solar activity and density changes in the upper atmosphere

Since 1958 it is known that there exists a response time of the upper atmosphere to changes in solar activity. This response time is best described as the lag between the 27-day variation of solar decimeter flux and the observed density changes of the upper atmosphere. Roemer obtained as a mean observational value for this lag 1.0 ± 0.12 days. Volland's simplified version of the Harris-Priester model of the upper atmosphere is used to calculate the delay which can be expected from theory. Only the effect of solar EUV radiation is taken into account. A possible influence of the corpuscular component of the solar radiation is not included in our estimate.

The calculations are carried out for the Harris-Priester model with solar activity index and a variation of . The resulting delay is 0.6 days. The calculated amplitude of the variations of the diurnal average temperatures during the solar 27-days cycle is in very good agreement with Jacchia's empirical formula.