AN OBITUARY: DR. HARVEY CUMMACK

Harvey Cummack, who died December 1 1996, was born March 3 1929 in Auckland. His family moved to Christchurch when Harvey was eleven where he attended Christchurch Boys' High and then studied mathematics, at what was then Canterbury College, to Master of Science level (1951). Subsequently, he devoted his scientific life to the terrestrial ionosphere, first in the Geophysical Observatory (PELGO) within DSIR, and on retirement in 1987 at the Physics and Astronomy Department of the University of Canterbury.
The importance of this region of the Earth's atmosphere, which is partially ionised by solar radiation, was recognised early this century as the agent for making long-distance high-frequency radio propagation possible through multiple radio reflections from the ionosphere and the earth's surface. Remote-sensing radar methods can be used to investigate the ionosphere. The ionosphere is scientifically important both in itself, as a tenuous ionised plasma coupled to the solar and terrestrial magnetic fields, which can support a number of plasma phenomena such as instabilities; and as a tracer, through collisional interaction, of the dynamics of the neutral high atmosphere. Harvey's was first and foremost a mathematician, and his early work involved solutions of the production function for the ionosphere. His work was mainly mathematical as he sought to understand the ionosphere by simulation. In the mid-60s his work on the conjugate ionosphere at low latitudes, using Raratonga and Hawaii data, found wide interest at the time.
A number of experimental radar methods exist for such investigations. Harvey was involved in the mathematical modelling of results from two such techniques: the ionosonde and meteor radar. An ionosonde sweeps a radar signal in frequency from 0.5-20 MHz over about half a minute, and records ionospheric echo traces as an archived (film medium when Harvey started) ionogram. These data can be reduced by standard methods to a profile of electron density as a function of height. Many morphological studies were made to determine electron layer parameters and their dependence on solar activity, season, and time of day, and rules for optimum radio propagation conditions were derived in part from these studies.
About half way through his professional life, Harvey wished to deepen his knowledge of hydromagnetic plasma waves, and studied for a Doctorate of Philosophy degree in Physics at Canterbury.
Harvey's ionogram work was principally with those ionograms that departed from the standard form. The inversion process of ionogram reduction is based on a number of assumptions, of which one of the most important is that of horizontal stratification. A dynamic, moving atmosphere can have time changes of the order of minutes and associated horizontal spatial scales of fifty to a few hundred kilometres. Such features alter the radio ray paths and can cause the ionogram to show features such as traces additional to those expected, "spread" in which the form of a trace is blurred, or the omission of parts of the trace ("lacunae"). Harvey first developed and published results on a fast ray tracing program. He then used this in a series of studies on the calculation of the ionogram traces to be expected from horizontally distorted ionospheres of various forms, with particular emphasis on the changes in the ionogram with alteration of the distortion parameters. It was possible to account for many observations by Harvey's models, indicating that the ionosphere at these times could have taken the forms assumed in the calculations.
The existence of such rapid ionospheric change and distortion begs the question of their cause. Throughout his life Harvey had a deep interest in the short-period atmospheric waves which are one cause of rapid ionospheric change, and in a series of papers and reports investigated how such waves distorted the ionosphere and the effects of such distortions on the ionogram traces obtained.
In his work on meteoric ionisation behaviour, Harvey modelled the evolution of the plasma irregularities controlled by diffusion in the presence of multiple chemical reactions.
Harvey's generosity to others will be remembered by all who knew him. During his time at the Geophysical Observatory, and later in the Physics and Astronomy Department, he was always willing to discuss new ideas and impart the benefit of his experience to co-workers, and to people entering atmospheric and ionospheric physics. It is important that our young students experience the wisdom and guidance of such luminaries as Harvey.
All of us, including many overseas visitors, will always recall the warm welcome to his home that he and his wife Helen always gave. We shared, for example, in his passion for cycling and enjoyed the many generous aspects of this warm-hearted man. We extend our deep feelings of loss to Helen, daughter Robyn, and sons Brent, Paul and Michael.