Launch: 07. February 2008 (with the Columbus Module)
The EUV / UV Radiation of the Sun and Its Influence on the Earth
The radiation of the sun in the wavelength domains from soft X-rays (XUV, wavelengths 1 to 30 nanometers), via the extreme ultraviolett (EUV, 30 to 120 nanometers), to the ultraviolett (UV, 120 to 400 nanometers) represents the main energy source for the system of the Earth’s thermosphere and ionosphere (T/I system). These layers of the atmosphere are located at altitudes of about 85 km above the surface of the Earth, they play an important role in the interaction between the Earth and its interplanetary environment, especially the sun (solar-terrestrial relations). Most of the processes in the T/I system are governed by the solar EUV radiation which can change significantly on short to long-term time scales. These time scales are represented mainly by the solar flares (duration of minutes to hours), the rotation of the sun (25 days), and the solar cycle (about eleven years). Typical variations of the integrated spectral EUV flux of the sun can reach some 10% during flares and over the sun’s rotational period, and up to several 100% during the entire solar activity cycle. Extreme cases are exhibited by highly ionized spectral lines like e.g. Fe XII (elevenfold ionized atoms of iron) in the EUV region which may show flux variations of more than a factor of ten within a few years.
Besides the physics of the sun and the exploration of solar-terrestrial relations, the most important scientific questions in the field of the solar EUV / UV radiation comprise, in particular, the modelling of the terrestrial T/I system and the energy deposition into Earth’s atmosphere by radiation and the solar wind. The changing phenomena in interplanetary space and the environment of the Earth (solar wind, magnetic fields, etc.) with their impacts on the T/I system triggered by the activity of the sun are called "space weather". Important fields of research related to technical applications are the influence of space weather on the orbits of satellites and space debris ("drag analysis"), as well as the telecommunication via satellites, the operation of navigation systems such as the Global Positioning System (GPS), and radar measurements.
Measurement of the solar EUV / UV Flux with high Accuracy
Caused by the big technical challenges to measure the solar EUV radiation this important energy source for many processes in the atmosphere of the Earth was so far unknown with accuracies appropriate for today’s status of science and its applications. Uncertainties of measurements of solar EUV fluxes (in absolute physical units) range between 20 and more than 400%, with corresponding uncertainties of the models and the conclusions derived from it.
The main goal of SolACES ("SOLar Auto-Calibrating EUV / UV Spectrophotometers") was to measure the spectral EUV fluxes of the sun with a relative radiometric accuracy of better than 10%. To this purpose an auto-calibration procedure was applied for the first time, which ensured that the instrument calibrated itself during its operational phase. In this way, the unavoidable degradations in transmissions and efficiency of the instrument could be sufficiently corrected, and the EUV fluxes being determined with an accuracy of better than five to ten per cent.
The SolACES Spectrophotometer on SOLAR
SolACES is part of the scientific instrument package SOLAR which is mounted in the Coarse Pointing Device (CPD) on the Columbus External Payload Facility (CEPF) of the International Space Station (ISS). The CPD compensates for the changing alignment of the ISS, and pointed SolACES together with the other payloads of SOLAR, the "Solar Variability and Irradiance Monitor" (SOVIM, Switzerland) and the "Solar Spectrum Measurement" instrument (SOLSPEC, France), towards the sun. This allowed synchronous and complementary measurements of all three instruments to be performed. While SOVIM determined the total solar flux integrated over the whole electromagnetic spectrum of the sun, SOLSPEC measured the spectral flux in the wavelength range between 180 nanometers and 3 micrometers which overlapped the longwave part of the SolACES range of the solar spectrum (unfortunately, a short time after the launch of SOLAR, SOVIM failed completely, while SolACES lost its longwave channel above about 140 nanometers).
SolACES consists of two twin spectrophotometers with four diffraction gratings and channel electron multipliers as detectors, and two ionization chambers equipped with photodiodes, to detect the incident EUV / UV radiation in the wavelength range between 17 and 220 nanometers, and to carry out auto-calibration during the flight. The spectral resolution ranged from 0.5 to 2 nm depending on wavelength. A filter wheel common to the spectrophotometers and the ionization chambers, containing 43 different thin-film metallic and crystal filters with bandwidths between 5 and 55 nanometers, served to select the spectral bands during the auto-calibration procedure. The fillings of the ionization chambers consisted of neon-, xenon-, or nitrogen monoxyde gases.
Spectrophotometric standard measurements of the solar radiation during the nominal mission operation were carried out by means of the spectrophotometers without a filter in front of the entrance window. In this way, one or two EUV / UV spectra, integrated over the full disk of the sun, could be obtained per orbit, i.e. more than 15 spectra recorded every day at best.
During the auto-calibration procedure, the transmissions of the filters were determined by measurements of the spectrophotometers with and without the filter, at first. The absolute EUV / UV fluxes, integrated over each of the filter bandpasses, could then be derived from measurements with the ionization chambers with the filters applied. Taking into account the actual filter transmissions these measurements were finally used to determine calibration factors that were applied to the standard measurements in order to absolutely calibrate the spectra.
Scientific and Application related Goals
SolACES provided novel and fundamental contributions in different fields of space science and their applications. In particular, these contributions are:
The SolACES science team was led by a principal investigator (PI) from the Fraunhofer Institute for Physical Measurement Techniques (IPM) in Freiburg/Breisgau, Germany. The members of the team come from the following institutions and companies:
In order to establish a cooperation of the most important groups in the research fields mentioned above, an international TIGER programme (Thermospheric-Ionospheric GEospheric Research) in coordination with the international organizations SCOSTEP (Scientific Committee On Solar-TErrestrial Physics) and COSPAR (Committee On SPAce Research) has been initiated.
First Results: EUV Spectra
When SOLAR and SolACES started their scheduled measurements in autumn 2008 the Sun’s eleven-years activity cycle was only a short way from its minimum. This time the minimum turned out to be a specifically long and deep one with longer periods without a single visible sun spot.
On the basis of variations of the XUV and EUV radiation at wavelengths below 45 nanometers, in particular of spectral lines of highly ionized iron atoms in the solar atmosphere (corona), the SolACES science team was able to narrow down the time of the minimum of solar activity quite exactly to an interval in August and September 2009 (ionized atoms lost one or more of their outer electrons, and, thus, appear to be charged electrically).
Moreover, publications have for example been released on the correlation of the solar EUV radiation with the photoionization variability of Earth’s ionosphere. Further observations of the Sun were continued until the end of the missionin February 2017 at regular intervals. In this way, the measurements will accompany the entire increase of the solar activity from its minimum to the maximum expected in 2013/2014 and beyond, and gain further important insights into our Sun and its influence on the Earth’s atmosphere and climate.
Mission Characteristics and Technical Parameters