With Claus Fröhlich, who died on 22 February 2019, we lost a physicist, whose enthusiasm, joy, and knowledge concerning the measurement of Total Solar Irradiance (TSI, earlier known as ‘Solar Constant’) made him the clear leader in this important field. He was a brilliant experimenter with a pronounced mathematical flair, who later also became an outstanding science manager. His collaborative spirit and open mind predestined him to bring the ‘Physikalisch-Meteorologisches Observatorium Davos’ (PMOD) and its World Radiation Center (WRC), which he directed from 1975 until 1999, to worldwide recognition.
Claus was born in Zürich on 10 October 1936. His father Max grew up in the Canton of Glarus as the son of an English mother and a father, who directed a weaving mill with branches in Birmingham and Lagos in Western Africa. Max Fröhlich was a gold- and silversmith, whose influence on design extended well beyond Switzerland. Claus’ mother was the daughter of a mechanical engineer who managed companies in France and Switzerland; although she wanted to become an engineer, she had to settle for training as a secretary. Being francophone she preferred the spelling of her son’s first name as Claus.
Claus, whose outgoing personality led everybody to call him by his first name went to school in Zürich and studied physics at ETH. After graduating with a diploma thesis in experimental Solid State Physics in 1961, he remained in this field and chose Georg Busch and Fritz Kneubühl as advisers for his doctoral thesis on ‘Phonon resonance scattering and thermic conduction in a silicon grain boundary’. For this work he received the Silver Medal of ETH.
After he had obtained a stipend from the Swiss National Science Foundation, he moved to Davos in 1969. There he was going to pursue the development of radiometers, specifically of pyrheliometers, i.e. instruments that measure the total solar irradiance; and when the World Meteorological Organisation (WMO) set up the World Radiation Center (WRC) in Davos, Claus became its Head. He later commented that he found the change from Solid State Physics to Atmospheric Physics – with the key aspects Radiation and Climate – inspiring. His thoughtful and innovative approach to the quinquennial International Pyrheliometer Comparisons in Davos assured that he was easily accepted in the solar-radiometry research community.
In 1979 Claus had the opportunity to put his instruments on a balloon in a joint project with the Observatoire de Genève. With the data acquired during the flight he demonstrated that the interior of the Sun may be studied by measuring the coherent oscillations of the solar atmosphere through observing them as radiance oscillations, and not only through Doppler measurements (as it was customary at the time). Claus thus also entered the community of helioseismology.
As a consequence, he was asked in 1981 to participate in the Phase-A Study of the European Space Agency (ESA) for DISCO (Dual Investigation of the Solar Constant and Oscillations) – a satellite that was going to measure the total irradiance of the Sun, as well as the coherent oscillations of its atmosphere from the first Lagrange Point L1. Albeit DISCO was not accepted for implementation as an ESA project, the instruments of particular interest to Claus were selected in 1988 as the VIRGO experiment (Variability of Solar Irradiance and Gravity Oscillations) for flight on the Solar and Heliospheric Observatory (SOHO). It was launched in December 1995 and – also located at L1 – is still operating today. Claus managed the VIRGO experiment as Principal Investigator, and guided its scientific use by a large international collaboration right to the end of his life. VIRGO has by now provided an accurate record of the Total Solar Irradiance (TSI) over more than two solar sunspot cycles and thus more than half of the full TSI record, which started in November 1978. The “TSI composite” (shown below) that Claus put together from various satellites is considered the “gold standard” in the field.
Starting in the 1979, Claus and his collaborators at PMOD/WRC have acquired experience in designing, building, calibrating and managing space experiments by participating in a number of projects, such as IPHIR (Inter-Planetary Helioseismology with IRadiance data). For this major project they collaborated with the French Laboratoire de Physique Stellaire et Planétaire (that later became the Institut d’Astrophysique Spatiale in Orsay) and the Crimean Astrophysical Observatory. For redundancy, they built two IPHIR instruments and placed one each on the two USSR PHOBOS probes. Eventually, only PHOBOS-II reached the Martian moon of the same name, but this still brought the hoped-for result: the first uninterrupted observation of the solar five-minute oscillation over 161 days! Later, the Davos group also designed and built instruments for EURECA (the ‘EUropean REtrievable Carrier’) and for the International Space Station (ISS). These were, moreover, useful for assessing the degradation of instruments owing to their use in space, and contributed further to the seminal literature on TSI and its variation written by Claus and his associates.
Claus was elected member of the Academia Europaea in 1990 and was honoured in 2017 with a prize by the Canton of Grisons in recognition of his scientific achievements, which “brought international fame to the research site Davos”. Many of us knew Kornelia Grassmann, whom Claus married in 1963. She had come to Zürich in 1950, when her father took up a position at ETH. Later on Kornelia graduated as a goldsmith in Max Fröhlich’s class at Kunstgewerbeschule. In Davos the couple and their children lived in a solar house built in 1978, which Claus had designed based on thermal studies performed together with two of his PMOD collaborators. Later, when he was asked to assess energy aspects of a planned printing plant for a Zürich newspaper, he was able to suggest design changes that saved substantial sums, both in investment and use.
When the couple’s children were old enough, Kornelia went back to school, passed the Matura, obtained a Licentiate in Theology, completed her studies at the C. G. Jung Institute in Küsnacht and practised as a psychotherapist until the 1990s, when her interests began to gravitate to painting. It was congenial that two paintings by Kornelia, who had died four years before emblazoned Claus' death notice. The much-admired couple is survived by their three sons, Thomas, Markus and Roman, and by two grandchildren.
For his Davos colleagues, VIRGO team members, and contemporaries: Robert Brusa (Zürich), Wolfgang Finsterle (Davos), Hansjörg Roth (Basel); Bo Andersen (Svalbard), Thierry Appourchaux (Orsay), Bernhard Fleck (Greenbelt), Antonio Jimenez Mancebo (Tenerife); Heidi Blattmann (Zürich), Roger Bonnet (Bern and Paris), Vicente Domingo (Valencia), and Martin Huber (Zürich)