The occultation of ϵ Gem by Mars as observed from Agassiz Station

Observations of the April 8, 1976 occultation of ? Gem by Mars made at the Agassiz Station of the Harvard College Observatory have been analyzed to yield temperature profiles of the Martian atmosphere for number densities between 10¹³ and 10¹⁵ cm^?3. Pronounced wavelike structure is evident in both immersion and emersion profiles, with a peak-tto-peak variation of up to 50°K and a vertical scale of 20 km.     

Upper limits to trace constituents in Jupiter's atmosphere from an analysis of its 5-μm spectrum

A high-resolution (0.6 cm^?1) spectrum of Jupiter at 5 μm recorded at the Kuiper Airborne Observatory is used to determine upper limits to the column density of 19 molecules. The upper limits to the mixing ratios of SiH₄, H₂S, HCN, and simple hydrocarbons are discussed with respect to current models of Jupiter's atmosphere. These upper limits are compared to expectations based upon the solar abundance of the elements. This analysis permits upper limit measurements (SiH₄), or actual detections (GeH₄), of molecules with mixing ratios with hydrogen as low as 10^?9. In future observations at 5 μm the sensitivity of remote spectroscopic analyses should permit the study of constituents with mixing ratios as low as 10^?10, which would include the hydrides of such elements as Sn and As as well as numerous organic molecules.     

Meridional characteristics of a Pc 4 micropulsation event in the plasmasphere

An analysis was made of a complex large amplitude Pc 4 micropulsation, of four hours duration around local noon, observed at five ground stations in the United Kingdom (2.4? L ?3.8). The final pulsation waveform was shown to be the results of the superposition of wave packets of different periods. The meridional variation of the amplitude of the different period wavepackets was consistent with their being fundamental “toroidal” field line resonances within the plasmasphere, rotated through 90° in their transmission through the ionosphere in accordance with recent theoretical predictions. Other predicted ionospheric effects, such as the loss of the sense-of-polarization reversal across the amplitude maximum, were apparent in the meridional variation of the polarization characteristics.     

Reservoirs of atmospheric chlorine: Prospects for HOCl revisted

We have re-examined the prospects of HOCl as an inert reservoir for atmospheric chlorine in the light of new theoretical calculations and available experimental measurements of its photodissociation cross-sections. The theoretical calculations and most recent laboratory studies imply that the broad maxima 3200 Å observed in two other experimental spectra may not belong to HOCl. On the basis of this implication HOCl could have a long lifetime against photodissociation in the stratosphere, and could, thereby, become a reservoir for atmospheric chlorine comparable to ClONO₂ or even HCl. In this capacity HOCl could reduce the predicted ozone destruction due to any given level of total chlorine burden. We have also examined the difficulties in laboratory measurements of the HOCl absorption spectrum with particular emphasis on identifying the impurities which may be present in the experimental system. It appears that specialized new experiments are needed to clearly establish the nature and strength of HOCl absorption in the neighbourhood of 3200 Å. Some refinements in the theoretical calculations also seem desirable. In view of the difficulties involved in the laboratory determination of HOCl photodissociation cross-sections, it is suggested that a search for possible stratospheric HOCl by atmospheric spectroscopists would be worthwhile.     

On the dissociation of nitrogen by electron impact and by E.U.V. photo-absorption

The dissociation of N₂ by electron impact and by e.u.v. photo-absorption is studied, and it is shown that the forbidden predissociation of the numerous ¹Π_u and ¹Σ_u⁺ valence and Rydberg states of N₂ in the 11–24eV energy range is the dominant mechanism for N atom production. By measuring the absolute emission cross sections for the e.u.v. singlet bands of N₂ and by using the generalized oscillator strength data of Lassettre (1974), it has been possible to construct a detailed model of the total N₂ dissociation cross section which is in good agreement with the measurements of Winters (1966) and Niehaus (1967) and provides some insights into the maximum possible $\text{N(}{}^2\text{D)}$ yield from dissociative excitation. The total cross section for exciting N₂ e.u.v. radiation in the 800Å–1100Å wavelength range has been measured and found to have a value of 3.4 ×10^?17 cm² at 100 eV under optically thin conditions. Although this result implies that large fluxes of e.u.v. photons should be excited in auroral substorms and in the airglow, they are not observed, and we show that this development is a consequence of radiation entrapment and predissociation. The total cross section for dissociating N₂ by electron impact is given for optically thin and thick media. And some questions concerning the energy budget of a magnetospheric storm which are raised by these results, are discussed.     

Evidence of large scale diverging paths in the solar corona for type III bursts exciters

On 1 July 1971, about ten groups of type III bursts were observed with high time resolution (10^?1 sec) with the 169 MHz Nançay radioheliograph. Each group consists of two or several bursts, appearing successively from E to W in all cases, with very short delays. The analysis of successive E-W profiles allowed us to show that, for each event:

1. the delay between maximum times of the sources was in the range 0.3–0.8 s and that their time profiles were very similar.
2. the mutual distance between sources was ～1.5 × 10⁵ km.

Explanations by simultaneous emission at the fundamental and harmonic of the local plasma frequency, or by reflexion of electromagnetic radiations at boundary at a dense region are shown to be inconsistent with the observations. We suggest that distinct exciters are simultaneously accelerated at low levels in very close regions and propagate upward along very widely diverging magnetic field lines. This divergence could be related to the existence of large scale magnetic connexions in the corona as revealed by XUV observations. We stress the point that this kind of structure in type III bursts groups, visible only with high time resolution, may have led in the past to erroneous conclusions concerning diameter and decay time of type III bursts because of both spatial and temporal overlapping.     

The height variation of granular and oscillatory velocities

Previous observations of spatially-resolved vertical velocity variations in ten lines of Fe i spanning the height range 0 h 1000 km are re-analyzed using velocity weighting functions. The amplitudes and scale heights of granular and oscillatory velocities are determined, as well as those of the remaining unresolved velocities. I find that the optimal representation of the amplitude of the outward-decreasing granular velocities is an exponentially decreasing function of height, with a scale height of 150 km and a velocity at zero height of 1.27 km s^–1. The optimal representation of the same quantities for oscillatory velocities is an exponential increase with height, with a scale height of 1100 km and a velocity at zero height of 0.35 km s^–1. The remaining unresolved velocities decrease with height, with a scale height of 380 km and a velocity at zero height of 2.3 km s^–1.     

Dynamics of coronal hole regions

The hydrodynamic properties of a steadily expanding corona are explored for situations in which departures from spherically symmetric outflow are large, in the sense that the geometrical cross section of a given flow tube increases outward from the Sun faster than r ² in some regions. Assuming polytropic flow, it is shown that in certain cases the flow may contain more than one critical point. We derive the criterion for determining which of these critical points is actually crossed by the transonic solution which begins at the Sun and extends continuously outward. Next, we apply the theory to geometries which exhibit rapid spreading of the flow tubes in the inner corona, followed by more-or-less radial divergence at large distances. This is believed to be the type of geometry found in coronal hole regions. The results show that, if this initial divergence is sufficiently large, the outflow becomes supersonic at a critical point encountered low in the corona in the region of high divergence, and it remains supersonic at all greater heights in the corona. This feature strongly suggests that coronal hole regions differ from other open-field regions of the corona in that they are in a fast, low density expansion state over much of their extent. Such a dynamical configuration makes it possible to reconcile the low values of electron density observed in coronal holes with the large particle fluxes in the associated high speed streams seen in the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Effects of suprathermal grains

Grains ejected from stars at velocities of 10⁷ cm s^–1 and/or grains accelerated by the pressure of starlight in the intercloud medium to velocities in the range 2×10⁶–10⁷ cm s^–1 are slowed to velocities of about 2×10⁵ cm s^–1 in a typical interstellar cloud. The interaction of fast grains with gas atoms as they are slowed in clouds could provide (a) the dominant heat source for interstellar clouds; (b) sites for molecule formation; and (c) a mechanism of providing a pressure balance between clouds and the intercloud medium.Paper presented at the Symposium on Solid State Astrophysics, held at the University College, Cardiff, Wales, between 9–12 July, 1974.