Claus Fröhlich

Claus Fröhlich
Arts &Science

Dr.sc.nat

About

222
Publications
23,641
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
8,496
Citations
Citations since 2016
2 Research Items
2185 Citations
20162017201820192020202120220100200300
20162017201820192020202120220100200300
20162017201820192020202120220100200300
20162017201820192020202120220100200300

Publications

Publications (222)
Article
Many observational records critically rely on our ability to merge different (and not necessarily overlapping) observations into a single composite. We provide a novel and fully-traceable approach for doing so, which relies on a multi-scale maximum likelihood estimator. This approach overcomes the problem of data gaps in a natural way and uses data...
Article
Full-text available
Aims. The existing records of total solar irradiance (TSI) since 1978 differ not only in absolute values, but also show different trends. For the study of TSI variability these records need to be combined and three composites have been devised; however, the results depend on the choice of the records and the way they are combined. A new composite s...
Article
Full-text available
Several attempts have been made to merge all total solar irradiance (TSI) observations into a single composite, which is crucial for assessing the presence of long-term trends in the solar radiative output. There exist presently three composites, which are based on different assumptions on the data, and also exhibit differing trends. This problem...
Article
Full-text available
The last solar activity minimum during 2008/09 was unusually long and with extended periods without sunspots. During this period the total solar irradiance (TSI) was much lower than during the previous minima and no solar activity proxies show similarly low values. Proxy models for TSI use a measure for the darkening of sunspots, the so-called phot...
Article
Full-text available
The record of total solar irradiance (TSI) during the past 35 years has overlapping observations from space which can be merged to a composite, and three are available, namely the PMOD, the ACRIM and the IRMB composites. There are important differences between them, which are discussed in detail in order to find the best representation of solar var...
Article
Full-text available
The record of total solar irradiance (TSI) during the past 35 years shows similarities of the three solar cycles, but also important differences. During the recent minimum with an unusually long periods with no sunspots, TSI was also extremely low, namely 25% of a typical cycle amplitude lower than in 1996. Together with the values during the previ...
Article
The early increase, first observed by the PMO6V radiometer on VIRGO/SOHO, indicates that aperture heating may be a problem for solar radiometry, not only in space, but also on ground. Heating of the precision aperture in front of the receiver cavity increases the irradiance in proportion to the incoming solar radiation and the emitted radiation fro...
Article
The Sun is the main source of energy for the Earth, and its activity has changed between states of high and low activity during the Holocene. From paleo records it is known that climate changes coincide with these changes in solar activity, raising the question about the solar radiative forcing in past, present, and future climate. To answer this q...
Article
The classical radiometry for total solar irradiance (TSI) measurements is described using examples of the four types of radiometers currently used in space. The design, characterization and operation of these radiometers are described. Besides the instrumental characteristics determining the measurement uncertainties, an important issue is possible...
Article
Full-text available
The central theme of the book "Observing Photons in Space" is the detection and characterization of photons with instruments aboard spacecraft. This chapter presents a global overview of the fundamental processes that accompany photons all the way from their origin in the source region to their detection in our instruments. The radiation of the Sun...
Article
The Sun is the main source of energy for the Earth. There is growing evidence that climate changes in the past coincide with changes in solar activity. This raises the question about the Sun's role in past, present, and future global change. To answer this question, the total solar irradiance (TSI) has to be known not only for the present period of...
Article
Full-text available
Solar gravity modes (or g modes) -- oscillations of the solar interior for which buoyancy acts as the restoring force -- have the potential to provide unprecedented inference on the structure and dynamics of the solar core, inference that is not possible with the well observed acoustic modes (or p modes). The high amplitude of the g-mode eigenfunct...
Article
For the first time a record of total solar irradiance covering 9300 years is presented, which covers almost the entire Holocene. This reconstruction is based on a recently observationally derived relationship between total solar irradiance and the open solar magnetic field. Here we show that the open solar magnetic field can be obtained from the co...
Article
Full-text available
Aims. During the solar minimum of 2008, the value of total solar irradiance at 1 AU (TSI) was more than 0.2 Wm$^{-2}$ lower than during the last minimum in 1996, indicating for the first time a directly observed long-term change. On the other hand, chromospheric indices and hence solar UV irradiance do not exhibit a similar change. Methods. Compari...
Article
Full-text available
The uninterrupted measurement of the total solar irradiance during the last three solar cycles and an increasing amount of solar spectral irradiance measurements as well as solar imaging observations (magnetograms and photometric data) have stimulated the development of models attributing irradiance variations to solar surface magnetism. Here we re...
Article
Full-text available
Observations made by the Michelson Doppler Imager (MDI) and Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliospheric Observatory (SOHO) and by the ground-based Birmingham Solar Oscillations Network (BiSON) and Global Oscillations Network Group (GONG) have been used in a concerted effort to search for solar grav...
Article
Full-text available
We have previously placed the solar contribution to recent global warming in context using observations and without recours to climate models. It was shown that all solar forcings of climate have declined since 1987. The present paper extends tha analysis to include the effects of the various time constants with which the Earth's climate system mig...
Article
Total solar irradiance (TSI) measurements from different satellites are available since late 1978 and may be extended by reliable models back to the minimum between cycles 19 and 20 around 1976. This time series shows a downward trend of the cycle averaged data since the eighties. Although this trend is also present in other solar activity related...
Article
Full-text available
There is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposit...
Chapter
Full-text available
Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 25 years. Presently, there are three TSI composites available, called PMOD, ACRIM and IRMB, which are all constructed from the same original data, but use different procedures to correct for sensitivity changes. The PM...
Article
The primary objective of the ISS-SOLAR mission on Columbus (to be launched in 2006) is the quasi-continuous measurement of the solar irradiance variability with highest possible accuracy. For this reason the total spectral range will be recorded simultaneously from 3000 to 17 nm by three sets of instruments: SOVIM is combining two types of absolute...
Article
Full-text available
Aims.We present a reconstruction of total solar irradiance (TSI) back to 1974, i.e. from the minimum of cycle 21 to the declining phase of cycle 23. We also present a cross-calibration between the magnetograms obtained by the 512 channel magnetograph and the spectromagnetograph at Kitt Peak. Methods: .The TSI reconstruction is carried out using dat...
Article
Full-text available
Variations in the Sun's total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. In this Review, we show that d...
Article
Full-text available
The Phoebus group is an international collaboration of helioseismologists, its aim being to detect low-frequency solar g modes. Here, we report on recent work, including the development and application of new techniques based on the detection of coincidences in contemporaneous datasets and the asymptotic properties of the g-mode frequencies. The le...
Article
Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 25 years. Presently, there are three TSI composites available, called PMOD, ACRIM and IRMB, which are all constructed from the same original data, but use different procedures to correct for sensitivity changes. The PM...
Article
Full-text available
The Phoebus group is an international collaboration of helioseismologists, its aim being to detect low-frequency solar g modes. Here, we report on recent work, includ- ing the development and application of new techniques based on the detection of coincidences in contemporane- ous datasets and the asymptotic properties of the g-mode frequencies. Th...
Article
Full-text available
Measurements of total solar irradiance (TSI) by VIRGO on SOHO cover the solar cycle 23 cover almost completely. The continuous measurements and the very quiet thermal environment of SOHO allowed to improve our understanding of the long-term behaviour of room-temperature radiometers in space. This not only improves the quality of the VIRGO record it...
Chapter
The solar-constant variations which were measured from Feb. 16-Nov. 7, 1980 by means of the NASA ACRIM experiment on board the SMM satellite are analyzed. It is shown that if the radiative deficit produced by the passage of sunspots on the solar disk and the radiative flux excess produced by the passage of faculae at the edge of the solar disk are...
Chapter
The results of the IPHIR experiment on the USSR planetary mission to Phobos presented here are from data gathered during 160 days of the cruise phase of PHOBOS II to Mars, launched on 12 July 1988. The long uninterrupted observation produces a spectrum of the solar p-mode oscillations in the 5-minute range with a very high signal-to-noise ratio. Fr...
Article
A sunphotometer was used to measure the solar irradiance at three wavelengths from rockets and a balloon over a period of three years. The sunphotometer was developed at the Physikalisch-Meteorologisches Observatorium Davos (PMOD) and calibrated by an irradiance standard lamp. The measured solar irradiance agrees within + 1,5% and - 2,0% with Necke...
Article
The history of solar radiometry since the first pyrheliometer of Pouillet is presented. After the invention of the Ångström and the Smithsonian pyrheliometers around the turn of this century two different "scales" were in use. Comparisons with absolute cavity radiometers developed in America and Europe have been performed since about 1910 which sho...
Article
The Proceedings published in this special issue of Metrologia are of New Developments and Applications in Optical Radiometry III, a meeting held in Davos, Switzerland, from 20–22 September 1990, organized by the Physikalisch-Meteorologisches Observatorium Davos (PMOD). As the title indicates it was the third in a series of such meetings: the first...
Article
The paper presents the results of the first comparison between the solar radiometric scale, the World Radiometric Reference (WRR) and a Système International radiometric scale (SI scale) as disseminated by national standards laboratories, in particular, the scale based on the cryogenic radiometer of the National Physical Laboratory (NPL). The compa...
Article
Full-text available
The experiment SOVA (SOlar VAriability) on board EURECA (EUropean REtrievable CArrier) contains two types of radiometers sunphotometers for measuring the total and spectral irradiance at five wavelengths between 335 nm and 862 nm. Besides the direct observation of variations of the solar irradiance, these measurements will also be used for the obse...
Article
Full-text available
For reconstructions of the total solar irradiance (TSI) into the past it is important to understand the relation between the production of cosmogenic isotopes and TSI. The production rate is a function of the cosmic-ray intensity reaching the Earth, which is well represented by the neutron monitors. As theses data are available since well before th...
Article
Full-text available
Electromagnetic radiation from the Sun is Earth» 0.003\approx 0.003 %, and can increase to as much as 0.015 % during the very largest solar flares. Unknown is whether multi-decadal changes in solar activity produce longer-term irradiance variations larger than observed thus far in the contemporary epoch. Empirical associations with solar activity p...
Article
Only one radiometer - called Hickey-Frieden (HF) - is within the ERB package on NIMBUS-7 for the measurement of total solar irradiance (TSI) and thus changes due to exposure to solar radiation cannot be directly determined by comparison with a less exposed radiometer on the same spacecraft. The geometry and optical property of the cavity of HF is,...
Article
Full-text available
Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. This results in a strong correlation between sunspot area and TSI. During the growth phase of a sunspot other physics may affect this correlation; if the physical growth of...
Article
Since February 1996 VIRGO radiometers observe total solar irradiance from SOHO at L1, allowing continuous and uninterrupted observations of the Sun. Besides the 3-month gap during the SOHO vacation in summer 1998 and a few other minor gaps the record covers more than 99% of the almost 8 years of operation. This allows a very detailed assessment of...
Article
The Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) mission offers a novel approach to the provision of key scientific data with unprecedented radiometric accuracy for Earth Observation (EO) and solar studies, which will also establish well-calibrated reference targets/standards to support other EO missions. This paper pr...
Article
Since February 1996 VIRGO filterradiometers on SOHO monitor spectral solar irradiance at 402, 500 and 862nm with a bandwidth of 5nm. SOHO's vantage point at L1 allows uninterrupted observations of the Sun, 24 h a day and 365 days an year. Besides the 3-month gap during the SOHO vacation in summer 1998 and a few other minor gaps the record covers mo...
Article
Full-text available
Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of nearly 25 years. From measurements made by different space radiometers (HF on NIMBUS 7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite recored of TSI can be constructed. This leads to a reliable record of TSI wi...
Article
A few years ago, we compared for the first time the World Radiometric Reference, which is used as reference for meteorological radiometry, and the Système International (SI) radiometric scale realized at the National Physical Laboratory with cryogenic radiometers. We present an improved comparison which used as transfer standard a trap detector cal...
Article
Two sunphotometers with three channels at 862 nm, 500 nm and 335 nm were operated in space for eleven months on board the European Retrievable Carrier (EURECA). Both instruments experienced substantial losses of sensitivity, although the second instrument was exposed 100 times less than the first one. After retrieval of EURECA, the instruments were...
Article
Total solar irradiance (TSI) observations are now available for more than 23 years and the corresponding composite time series allows to study the TSI variability in detail. There are several components of this variability, the long-term change of about 0.1%, positively correlated with the 11-year solar cycle, and the shorter-term changes due the d...
Article
The changes of total solar irradiance during the course of the solar cycle correlate extremely well with changes of low-degree p-mode frequencies as observed in intensity and velocity by VIRGO/SOHO and BISON. Moreover, the slope of the linear regression between the two quantities depend on the degree of the mode, indicating an asphericity of the re...
Article
The determination of the total solar irradiance (TSI) from the SOHO/VIRGO experiment is made by first correcting the time series for all a priori known influences, and then correcting the data for instrumental degradation, using the back-up instruments PMO6-VB and DIARAD-R. The long-term behaviour of PMO6-VA shows an exponential decrease in its sen...
Article
Full-text available
Experience with the two types of radiometers in the experiment VIRGO on the ESA/NASA mission SOHO launched in December 1995 has revealed important information about the long-term behaviour of such radiometers in space. Exposure dependent changes can be determined from comparison of radiometers with different periods of exposure to the Sun, as it wa...
Article
Since November 1978 a complete set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 23 years. From measurements made by different space-based radiometers (HF on NIMBUS 7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of TSI is compiled with an overall precision of ord...
Article
Total solar and UV irradiances have been measured from various space platforms for more than two decades. More recently, observations of the "Variability of solar IRradiance and Gravity Oscillations" (VIRGO) experiment on SOHO provided information about spectral irradiance variations in the near-UV at 402 nm, visible at 500 nm, and near-IR at 862 n...
Article
A composite record of solar total irradiance compiled from measurements made by five independent space-based radiometers since 1978 is the basis for an evaluation of the influence of solar activity on total solar irradiance. An empirical model that parameterizes the combined influences of dark sunspots and bright faculae features on solar irradianc...
Article
The purpose of this report is to describe the research activities and plans of Working Group 1: “Solar Energy Flux Study: From the Interior to the Outer Layer” of the International Solar Cycle Study (ISCS), which is an international research organization operating under the auspices of the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP)...
Article
One of the over-arching questions, among others, to be addressed by studying Sun-Earth connections is: “Is the climate changing in a way we can understand and predict?” The Earth's climate is the result of a complex and incompletely understood system of external inputs and interacting parts. Climate change can occur over a range of time scales, may...
Conference Paper
Full-text available
We have applied a 1-D steady-state radiative-convective model with interactive photochemistry to estimate the sensitivity of temperature and ozone mixing ratio to the observed variability of the extraterrestrial spectral solar irradiance. We have carried out several runs using the spectral solar flux for minimum and maximum solar activity condition...
Article
Full-text available
¶The Alpine Surface Radiation Budget (ASRB) project was initiated in 1994 to investigate the altitude dependence of the surface radiation budget with a particular focus on accurate measurements of the longwave downward radiation. Meanwhile broadband short- and longwave radiation fluxes are continuously measured within this network at eleven station...
Article
International Solar Cycle Study,Working Group 1, "Solar Energy Flux Variations: From the Interior to the Outer Atmosphere" is concerned about measuring and analyzing solar irradiance variations and their underlying physical mechanisms. In this paper we report the latest results on WG 1 activities, i.e., the variations of solar total irradiance and...
Article
A 1-D steady-state radiative-convective model with interactive photochemistry has been applied to evaluate the sensitivity of ozone and temperature to the increase of the spectral solar flux. The model allows to simulate heating and photolysis rates in UV and visible parts of the spectrum with rather high spectral resolution (73 spectral bins) much...
Article
In this paper we show the recent result on irradiance variations and their relation to solar magnetic activity over solar cycles 21 to 23. Comparison of the multi-decade long irradiance and magnetic field measurements indicates that the shape and magnitude of irradiance variations are different from that of magnetic indices. Specifically, while mag...
Article
Solar spectral irradiance variations are known to exhibit a strong wavelength dependence with the amount of variability increasing towards shorter wavelengths. The bulk of solar radiation is emitted at visible and infrared wavelengths. Thus, the spectral radiation length of accounts for 99% of the total solar radiative output. Deposited in the Eart...
Article
Several candidates for low-order p modes (n = 5) and possibly g modes were found by applying mode-detection techniques such as multivariate spectral regression analysis and time-frequency analysis to the VIRGO full-disc solar irradiance data. Three out of the candidates for low-order p modes could be confirmed by significant peaks in the un-treated...
Article
The Alps are one of the regions in Europe showing very high erythemal UV irradiance (UVery) which is a consequence of low aerosol levels, high altitude and snow-covered surfaces. Effective albedo plays a key role in understanding the radiative transfer over an Alpine terrain. The difficulty in determining the effective albedo is predominantly due t...
Article
The frequent clinical observation that the course of atopic eczema, a skin disease involving a disturbed cutaneous barrier function, is influenced by climate and weather motivated us to analyse these relationships biometrically. In the Swiss high-mountain area of Davos the intensity of itching experienced by patients with atopic eczema was evaluate...
Article
Full-text available
Total solar irradiance (TSI) observations show a very short solar minimum in early 1996. The increase of TSI starts well before those of other activity parameters such as the magnetic field or the MgII index. Thus the onset of solar cycle 23 seems to be quite different from the last one in 1987. Models based on PSI and MgII-Index are unable to expl...
Article
The m-dependence of the amplitudes of the multiplets of low-degree p modes contains information about the latitudinal distribution of the power in the excitation of the oscillations. We present estimates of those amplitudes from 4 years of VIRGO/LOI observations. Variation of the excitation with magnetic activity is studied.
Article
Full-text available
Since November 1978 a complete set of total solar irradiance (TSI) measurements from space is available, yielding a time series of 21 years. From measurements made by different space radiometers (HF on NIMBUS-7, ACRIMI on SMM, ACRIMII on UARS and VIRGO on SOHO) a composite record of TSI can be compiled. The corrections which are needed for each rad...
Article
Full-text available
A complete set of total solar irradiance measurements from space is available dating from November 1978, yielding a time series of more than twenty years. From measurements made by different space radiometers (HF on NIMBUS-7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of solar total irradiance can be compiled. The correc...
Article
Global, direct and diffuse measurements of erythemal UV radiation with UV-Biometer and radiative transfer calculations are used to estimate the influences of (1) a snow-covered surface, (2) an overcast sky, and (3) a joined snow-covered surface and overcast sky at Davos (1610 m asl, Swiss Alps). The influence of total ozone, zenith angle, and aeros...
Article
Full-text available
Despite 20 years of total solar irradiance measurements from space, the lack of high precision spatially resolved observations limits definitive answers to even simple questions like ``Are the solar irradiance changes caused solely by magnetic fields perturbing the radiative flux at the photosphere?" More subtle questions like how the aspheric stru...
Article
A composite record of the Sun's total irradiance compiled from measurements made by five independent space-based radiometers since 1978 exhibits a prominent 11-year cycle with similar levels during 1986 and 1996, the two most recent minimum epochs of solar activity. This finding contradicts recent assertions of a 0.04% irradiance increase from the...
Article
The more than two years of observations from VIRGO (Variability of solar IRradiance and Gravity Oscillations) yield a unique data set covering the activity minimum and the rising part of solar cycle 23. This allows not only to determine the influence of varying activity on the solar oscillation frequencies and amplitudes, but also to study the rela...
Article
The Variability IRradiance Gravity Oscillation (VIRGO) experiment on SOHO has been observing total solar and spectral irradiances at 402 nm (blue channel), 500 nm (green channel), and 862 nm (red channel) since January 1996. The VIRGO observations have shown that solar active regions influence both total and spectral irradiances in a similar fashio...
Article
PICARD is a CNES micro-satellite mission due for flight by the end of 2002, named after the name of a French astronomer who first observed with consistency the solar diameter changes during the Maunder minimum in the 16th century. It consists of two instruments measuring (i) the solar diameter and differential rotation, and (ii) the total solar irr...
Article
Total solar irradiance has been monitored from space for nearly two decades. These space-borne observations have established conclusively that total solar irradiance changes over a wide range of periodicities—from minutes to the 11-year solar cycle. Since the total energy flux of the Sun is the principal driver for all Earths atmospheric phenomena,...
Article
Full-text available
A composite record of the Sun's total irradi-ance compiled from measurements made by five independent space-based radiometers since 1978 exhibits a prominent 11-year cycle with similar levels during 1986 and 1996, the two most recent minimum epochs of solar activity. This finding contradicts recent assertions of a 0.04% irradiance increase from the...
Article
Helioseismic and precise solar photometric measurements reveal that the Sun varies globally as a start during the source of an 11 year solar cycle. To understand the physical mechanisms of the magnetic cycle in the solar interior we must learn how to measure the tiny changes in the Sun's global properties, like its radius, internal temperature dist...
Article
Full-text available
An accurate determination of the frequencies of low-degree solar p-modes is an important task of helioseis-mology. Using 679 days of solar oscillation data observed in Doppler velocity and continuum intensity from two Solar and Heliospheric Observatory instruments (the Michelson Doppler Imager and the SunPhotoMeter), we show that fitting the spectr...
Article
Full-text available
To support climate research, the World Climate Research Programme (WCRP) initiated a new radiometric network, the Baseline Surface Radiation Network (BSRN). The network aims at providing validation material for satellite radiometry and climate models. It further aims at detecting long-term variations in irradiances at the earth's surface, which are...