Erich Regener and the maximum in ionisation of the atmosphere

TL;DR

This paper reviews Erich Regener's pioneering work on atmospheric ionisation, highlighting his discovery of the maximum ionisation altitude, his early estimates of cosmic ray energy density, and advocating for his recognition in the field.

Contribution

It introduces an alternative naming for the ionisation maximum and emphasizes Regener's overlooked contributions to cosmic ray physics.

Findings

Discovered the altitude of maximum atmospheric ionisation.

Estimated the energy density of cosmic rays.

Influenced subsequent generations of physicists and cosmic ray research.

Abstract

In the 1930s the German physicist Erich Regener (1881-1955) did important work on the measurement of the rate of production of ionisation deep under-water and in the atmosphere. He discovered, along with one of his students, Georg Pfotzer, the altitude at which the production of ionisation in the atmosphere reaches a maximum, often, but misleadingly, called the Pfotzer maximum. Regener was one of the first to estimate the energy density of cosmic rays, an estimate that was used by Baade and Zwicky to bolster their postulate that supernovae might be their source. Yet Regener's name is less recognised by present-day cosmic ray physicists than it should be largely because in 1937 he was forced to take early retirement by the National Socialists as his wife had Jewish ancestors. In this paper we briefly review his work on cosmic rays and recommend an alternative naming of the ionisation maximum. The influence that Regener had on the field through his son, his son-in-law, his grandsons and his students and through his links with Rutherford's group in Cambridge is discussed in an appendix. Regener was nominated for the Nobel Prize in Physics by Schroedinger in 1938. He died in 1955 at the age of 73.