The Milky Way's halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.

Balloon‐borne measurements of atmospheric electrical parameters made during the last several years indicate that there has been a significant change in the average small‐ion mobility. In addition, on some soundings abrupt changes in the ion mobility profile have been noted. These variations provide an unusual opportunity to experimentally study the functional relationships between mobility, ion density, conductivity, and the recombination coefficient in the stratosphere. Within the uncertainties involved it is found that the measurements support the simple, predicted theoretical relationships between these various parameters. If fluctuations in ion mobility are in fact a usual occurrence, then this work indicates that variations frequently observed in the ion concentration are probably related to changes in ion mobility rather than aerosol variations, as is usually assumed. One somewhat surprising result predicted by the functional relationships between the electrical parameters indicates that the ambient conductivity may be relatively insensitive to mobility variations.

Using balloon-borne condensation nu-remainder of the year.

An Atmospheric Electrical Measurements Workshop was conducted at the University of Wyoming in which balloon borne comparisons of ionization, conductivity, ion density, air-earth current, and electric field measurements were made. After some deliberation there now appears to be relatively good agreement between the various experimental groups with a major exception being the ion density measurements. The source of this discrepancy is perhaps the poorly defined flow rate through some types of ion counters. Another important experimental result indicates that the direct measurements of air-earth current are almost exactly one half the value calculated from the total conductivity and electric field profiles. The values of the small ion mobility calculated from the workshop data suggest a relatively constant value to about 30 km, the maximum altitude of the soundings.

Abstract. During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the inflight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than λ/∆λ = 10 000, where ∆λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm 2 near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than ± 0.005 nm.