Gamma-ray spectroscopy: Status and prospects

Contemporary gamma-ray spectroscopy instruments and their results are reviewed. Sensitivities of 10^?4 to 10^?3 ph/cm²-sec have been achieved for steady sources and 10^?2 to 1 ph/cm²-sec for transient sources. This has led to the detection of gamma-ray lines from more than 40 objects representing 6 classes of astrophysical phenomena. The lines carry model-independent information and are of fundamental importance to theoretical modeling and our understanding of the objects. These results indicate that gamma-ray spectroscopy is relevant to a wide range of astrophysical problems and is becoming a major part of astronomy. The objectives and anticipated results of future instruments are discussed. Several instruments in development will have a factor of ～ 10 sensitivity improvement to certain phenomena over contemporary instruments. A factor of ～ 100 improvement in sensitivity will allow the full potential of gamma-ray spectroscopy to be realized. Instrument concepts which would achieve this with both present and advanced techniques are discussed.     

Validation of the stratospheric sounding unit

The Stratospheric Sounding Unit (SSU) is part of the TOVS (TIROS Operational Vertical Sounder) on NOAA operational meteorological satellites. SSU measurements can be validated by comparison with temperature measurements from colocated rocket sondes. Systematic differences are found which vary with rocket station and sonde and are a function of height. However, these measurements are not adequate to define the performance of individual SSUs to a precision which would allow the observations from different SSUs to be combined in the study of diurnal and semidiurnal tides and of long term trends in stratospheric temperature. Instead this is achieved by detailed radiometric and spectroscopic investigation of each individual SSU, both prior to launch and during its operational life. Using the techniques descirbed, it is demonstrated that measurements from different SSUs can be combined with a relative error of less than 0.2K in equivalent brightness temperature.     

High Power Needs for Commercial Satellites

This overview paper presents estimates of the photovoltaic power systems needed on commercial communications spacecraft in the year 2000. These are developed in the form of power requirements based on extrapolation of the historical growth in communications traffic and are about 5 to 15 kW. The paper also addresses the key technology drivers in these photovoltaic systems. The importance of reducing mass in the power system is described in terms of the tradeoff with communications systems mass to maximize communications revenue. It surveys solar array components and subsystems to meet these future requirements and attempts to identify the development candidates with a large payoff potential and a high probability of successful development.     

A review of long drop tubes as a supplement/alternative to space experiments

The 100 meter high drop tube at the Marshall Space Flight Center has proven to be a viable facility for studies of containerless solidification. Advantages are that experiments are inexpensive and large numbers of specimens can be processed rapidly. It would not be unusual to run ten specimens in a day. Another significant advantage is that the undercooling behavior can be followed with sufficient sensitivity to easily detect the onset of recalescence and subsequent events.Disadvantages are the restrictions on specimen sizes and types of alloys that can be run in a microgravity environment. Practical specimen sizes range between 50 mg and 500 mg depending on the type of furnace being used. Refractory alloys can be processed in a vacuum (about 10^?5 torr) and therefore at microgravity. Non-refractory alloys demand an atmosphere (about 200 torr) to obtain appreciable undercooling before impact at the bottom of the tube. Under these conditions significant g forces result.Because of the present limitations of the 100 meter drop tube, the most definitive work has been done on niobium based alloys. Large amounts of undercooling have been observed routinely and the effects of undercooling on microstructure have been characterized in detail. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been used to determine types of phases, amounts of phases, and compositions of phases. It is clear, as would be expected, that the results bear some resemblance to rapid solidification processing by quenching. However, there are dissimilarities due to the uniqueness of solidification by deep undercooling without quenching in long drop tubes and accompanying recalescence effects.     

50–200 GV cosmic rays at various heliolatitudes

We consider regular motion of 50 – 200 GV particles in a large-scale interplanetary magnetic field model which contains a wavy neutral sheet responsible for the sector-structure. Numerical calculations based upon energy losses along various trajectories are carried out to obtain the predicted omni-directional density and anisotropy of cosmic rays at various solar latitudes. A marked difference is found between odd and even solar cycles. The post-1969 field configuration gives small radial and large latitudinal gradient: cosmic ray density increases toward the poles. The latitudinal gradient turns out smaller and of opposite sense for the pre-1969 epoch. Anisotropy changes dramatically as we move off the solar equator: corotation appears to be restricted to low latitudes.     

ELF radio signals produced in the auroral ionosphere by non-linear demodulation of signals from high-power amplitude-modulated transmitters

ELF and VLF radio signals were recorded in the afternoon to early morning (local time) between 24 March and 4 April 1979, in Northern Scandinavia. Apart from signals of natural origin, timing signals, i.e. six pips of equal duration of 105 ± 8 ms, at 1 kHz ± 0.5 Hz, were observed on the hour UT. Such signals only occur on days of relatively high geomagnetic activity during enhanced auroral electrojet activity. They are believed to be generated by non-linear demodulation (self-detection) of signals from two or more amplitude modulated transmitters in the USSR, operating at 173, 200, 236, 263 and 657 kHz. The simplest explanation for the observations is provided by the three transmitters operating at 173 kHz.