The acceleration of minor ion species in the solar wind

This paper provides a comprehensive analysis of the dynamics of the flow of minor ion species in the solar wind under the combined influences of gravity, Coulomb friction (with protons), rotational forces (arising from the Sun's rotation and the interplanetary spiral magnetic field) and wave forces (induced in the minor ion flow by Alfvén waves propagating in the solar wind). It is assumed that the solar wind can be considered as a proton-electron plasma which is, to a first approximation, unaffected by the presence of minor ions. In the dense hot region near the Sun Coulomb friction accelerates minor ions outwards against the gravitational force, part of which is cancelled by the charge-separation electric field. Once the initial acceleration has been achieved, wave and rotational forces assist Coulomb friction in further increasing the minor ion speed so that it becomes comparable with, or perhaps even exceeds, the solar wind speed. A characteristic feature of the non-resonant wave force is that it tends to bring the minor ion flow into an equilibrium where the radial speed matches the Alfvén speed relative to the solar wind speed, whereas Coulomb friction and rotational forces tend to bring the flow into an equilibrium where the radial speed of the minor ions equals the solar wind speed. Therefore, provided that there is sufficient wave energy and Coulomb friction is weak, the minor ion speed can be trapped between these two speeds. This inteststing result is in qualitative agreement with observational findings to the effect that the differential flow speed between helium ions and protons is controlled by the ratio of the solar wind expansion time to the ion-proton collision time. If the thermal speeds of the protons and minor ions are small compared to the Alfvén speed, two stable equilibrium speeds can exist because the rapid decrease in the Coulomb cross-section with increasing differential flow speed allows the non-resonant wave force to balance Coulomb friction at more than one ion speed. However, it must be emphasized that resonant wave acceleration and/or strong ion partial pressure gradients are required to achieve radial speeds of minor ions in excess of the proton speed, since, as is shown in Section 4, the non-resonant wave acceleration on protons and minor ions are identical when their radial speeds are the same, with the result that, in the solar wind, non-resonant wave acceleration tends (asymptotically) to equalize minor ion and proton speeds.     

The rotations of 128 Nemesis and 393 Lampetia: The longest known periods to date

Extensive photoelectric lightcurves of the asteroids 128 Nemesis and 393 Lampetia show for both objects extremely long rotational periods, the longest known to date for minor planets. For 393 Lampetia the combined results suggest with high probability a period of 38.7 hr with a maximal amplitude of 0.14 mag; a double-wave characteristic of the lightcurve must be assumed. For 128 Nemesis the complete double-wave lightcurve was observed and a period of 39.0 hr with a total amplitude of only 0.10 mag was deduced. Observations of 128 Nemesis confirm without doubt the presence of small-scale features of amplitude 0.01 to 0.02 mag, corresponding to small topographic features of about 15 km in height and width on the surface.     

Radar observations of asteroid 1580 Betulia

Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 ± 0.8 km² and a total spectral bandwidth of 26.5 ± 1.5 Hz. Combining our bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 ± 0.2 km.     

Global fracture patterns of a despun planet: Application to Mercury

We have determined the global fracture patterns resulting from combinations of stresses due to tidal despinning and contraction or expansion. We find that Mercury's lineament pattern is consistent with a history of despinning and contraction. According to our model, the observed tectonic pattern implies that the despinning process reached completion before the planet ceased contracting. Our model predicts a stress due to contraction which is up to 1.8 times the maximum despinning stress on Mercury. The maximum contractional stress could be as large as 4 times the maximum despinning stress if the oldest fractures on the planet are N-S thrust faults in the equatorial region.     

Dependence of the correlation of small scale photospheric structures upon resolution

Correlations between continuum intensity, velocity, and equivalent widths of two Mn i lines as observed with two different entrance apertures tend to deteriorate with improvement in spatial resolution. The KPNO multichannel magnetograph was used to make area scans at the center of the disk with entrance apertures 3.5×2.5 arc and 1×1 arc. A coherence analysis shows that this effect is caused by marked differences of fluctuations in temperature and temperature gradients as well as in the velocity structure of photospheric elements of various sizes.     

An indirect mechanism for the production of O(1S) in the aurora

Recent laboratory measurements of the deactivation rate constants for O(¹S) have suggested that the dominant production mechanism for the green line in the nightglow is a two-step process. A similar mechanism involving energy transfer from an excited state of molecular oxygen is considered as a potential source of the OI (5577 Å) emission in the aurora. It is shown that the mechanism, $\text{O}{}_2\text{+ e → O}{}_2{}^1\text{+ e O}{}_2{}^1\text{+ O → O}{}_2\text{+ O(}{}^1\text{S}\text{)}$, is consistent with auroral observations; the intermediate excited state has been tentatively identified as the O₂(c¹∑^?_u) state. For the proposed energy transfer mechanism to be the primary source of the auroral green line, the peak electron impact cross-section for O₂¹ production must be approximately 2 × 10^?17 cm².     

Convective collapse of flux tubes

Flux tubes of constant extending vertically through the solar convection zone are unstable to a convective instability if the surface field strength is less than 1270 G. By downward displacement of matter along the tube an unstable tube can transform into a new equilibrium state with lower energy which has a higher field strength. Numerical calculations of these collapsed states are presented. If the collapse starts in a field with a strength corresponding to equipartition with kinetic energy in the convection zone, it yields a surface field strength of about 1650 G. It is proposed that the small scale magnetic field in active regions consists of such tubes. The collapsed state is not in thermal equilibrium. In the deeper layers the heat exchange following the collapse is very slow but the surface layers return rapidly to temperature equilibrium. It is argued that during the gradual thermal evolution of the collapsed state its surface layers may start an overstable oscillation. A brightness-velocity correlation in this oscillation could account for the observed downdraft in the tubes.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Grain motions in the solar nebula

Isotopic analyses of meteorites suggest the possibility that some interaction between supernova ejecta and grains occurred in the solar nebula. In particular, the dynamics of grain motions in the solar nebula can explain the observed mixing of nucleosynthetic components. The effect of a shock wave on the motions of grains are examined. A steady-state, plane shock propagating into a uniform region of gas and dust grains is followed by a zone of gas/grain slip, in which the grains are accelerated by drag forces from the pre-shock to the post-shock gas velocity, i.e. reducing the relative velocity between the gas and grains to zero. On the basis of these calculations, it is estimated that if grains carried the isotopic anomalies investigated by Lee, Papanastassoiu, and Wasserburg (1978), then those grains could be no bigger than 2×10^–4 cm in size. A scenario is suggested in which the sluggishness of grains provides a natural way to concentrate and mix the nucleosynthetic components carried by grains in the ejecta and in the solar nebula.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Number-magnitude count for galaxies at the South Galactic Pole

COSMOS measures on a deep UK Schmidt Telescope Plate have been used to obtain the number-magnitude count for galaxies in a field of 14.6 square degrees near the South Galactic Pole. The results are in excellent agreement with data for the North Galactic Pole for galaxies fainter thanB=18.0, indicating no large-scale differences between north and south. A deficiency in numbers is observed for galaxies withB16.0. This is comparable to the deficiency atB17.5 for counts at the North Galactic Pole and supports the suggested asymmetry of the bright galaxy distribution between north and south galactic poles.