SolACES (SOLAR Auto-Calibrating EUV / UV Spectrophotometers) will (quasi) continously monitor the Extreme UV (EUV) and UV radiation of the sun in the wavelength range between 17 and 220 nm during an 18-month measurement campaign onboard the International Space Station (ISS). The anticipated launch date will be with ESA’s Columbus module during the second half of 2006. As part of the SOLAR space science instrument package SolACES will be mounted into a Coarse Pointing Device (CPD) on the Columbus External Payload Facility (CEPF). The CPD will compensate for the ISS orbital orientation changes and point SolACES together with the two other solar science payloads of SOLAR, the Solar Variability and Irradiance Monitor (SOVIM, Switzerland) and the Solar Spectrum Measurement instrument (SOLSPEC, France) at the sun. This enables simultaneous and complementary measurements of all three instruments. While SOVIM will serve to determine the total irradiance over the whole solar spectrum, SOLSPEC will obtain measurements of the  spectral irradiance longward of the SolACES range and overlapping with it (SOLSPEC wavelength range: 180 nm to 3 μm).

The primary science goal of SolACES is to observe the absolute (spectral) irradiance (the solar “constant” in physical units) of the full disk of the sun in the EUV / UV range and its variation with time. The spectral resolution of these  measurements will vary between 0.5 and 2 nm depending on the wavelength in the spectrum. A novel feature of SolACES, improving all similar space experiments from the past, is its capability to auto-calibrate the instrument repeatedly during the whole mission at a rate of minimum twice a month. This will account for the inevitable efficiency changes of the instrument, and allow EUV flux measurements with an up to now unprecedented absolute radiometric accuracy of better than 10%. So far, EUV flux measurement uncertainties were typically of the order of 20 to 400% or more.


Apart from its primary goal of monitoring the solar EUV / UV radiation, SolACES will follow a number of scientific goals as well as applications in satellite operations and technology. The main goals comprise questions from the fields of solar-terrestrial relations, solar physics, stellar astrophysics (comparison between the sun and stars), and the monitoring of the ISS  (atmospheric) environment. In particular, basic measurements will be expected to substancially improve the thermospheric / ionospheric models of the Earth’s atmosphere. Applications of the anticipated results of SolACES will be the improvement of forecasts of satellite and space debris orbits, as well as new models for telecommunication via satellites and satellite navigation (space weather).
The development of the SolACES instrument is mutually funded by DLR (65%), ESA (30%), and the Fraunhofer-Gesellschaft
(5%), the launch to the ISS being responsibility of ESA. In addition to the development phase, DLR will also financially support the mission preparation, operation and data reduction phases. The SolACES science team is led by the Principal Investigator from the Fraunhofer Instiute of Physical Measurement Techniques (IPM) in Freiburg, Germany, which is responsible for the development and the operation of the experiment. Members of the team come from the Kiepenheuer Institute for Solar Physics, Freiburg, the Institute for Meteorology, Univ. of Leipzig, the Astrophysical Institute Potsdam (AIP), DLR / DFD, Neustrelitz (all Germany), Space Environment Technologies, Los Angeles, CA (USA), the Lab. for Atmospheric and Space Physics (LASP), Boulder, CO (USA) , the Space Science Center (SSC) of the University of Southern California, Los Angeles, CA (USA), the Service d’aeronomie, TVerrières-le-BuissonT (France, SOLSPEC), and the Physikalisch-Meteorologisches Observatorium Davos / World Radiation Center (PMOD / WRC), Davos (Switzerland, SOVIM).


Science Background and Objectives

SolACES  comprises the experience of  work  of  more  than 40 years in the very complex field of space EUV / UV spectroscopy related to solar-terrestrial relations. Since all the other pioneering groups of the first generation in space research have disappeared in the meanwhile, SolACES will be the first experiment to accomplish the inevitable requirement of re-calibrating its spectrometers in-flight repeatedly at rate of minimum twice per month. If not re-calibrated, the measured variability of the solar irradiance cannot be separated from variations caused by the efficiency changes of the spectrometers. These depend on various  surface  effects, temperatures  variations, angle  of  incident radiation and other effects, leading to time and wavelength dependent efficiency functions.
SolACES will achieve its auto-calibration capability by using two tri-current ionisation chambers, filled with the gases Ne, Xe and NO, as primary detector standards, and silicon diodes as secondary detector standards. The ion currents measured at the chambers are directly related to the incoming solar photon fluxes determined in wavelength subranges to be selected by 43 different filters.
The primary goal of SOLAR is the (quasi-) continuous measurement of the solar “constant” in order to separate solar effects from human influence on climatology. In addition to this, modelling of changing VIS-UV-EUV emissions in dark spots and the surrounding active areas on the sun is one of the SolACES topics in solar physics. It is intended to apply the model results to past and future integral space measurements of the solar constant.
Beyond the primary SOLAR science goals SolACES is expected to contribute to other science areas of strongest interest, because the EUV radiation by the sun is THE primary energy source for the thermospheric / ionospheric (T/I) system of Earth’s atmosphere. Thus, most of the T/I processes are controlled by the solar EUV radiation which strongly changes on short-term (solar flares), medium-term (solar rotation), and long-term time scales (solar cycle). However, due to technological difficulties this important energy source is not yet known to a level of accuracy that is required for today’s state of science (aeronomy of the thermosphere and ionosphere, solar physics, interplanetary and planetary physics), and - even more serious - to the requirements in applied fields such as navigation (especially the GPS technology),
drag analysis of the ISS and satellites, radar measurements and telecommunication. In the past, the given radiometric accuracies of solar EUV flux measurements in the important spectral region of less than 100 nm ranged from 20% up to 400% and even more. The goal of SolACES to lower this level to < 10% will mean a strong improvement. Uncertainties as low as 1 to 3% seem to be achievable depending on the specific sub-wavelength range. This progress will be reached by introducing for the first time an auto-calibrating system with a very high absolute and statistical accuracy. In view of the on-going US missions TIMED and SORCE that neither have this in-flight re-calibration capability, nor the high statistical accuracy, SolACES will contribute new results in a number of different fields of space science and its applications.  In  particular,  SolACES  and  SOLAR will gain new results in the following fields:

  • (Quasi-) continuous monitoring of  the solar EUV / UV radiation (more than 15 spectra per day)
  • Radiometrically  accurate  determination of the solar EUV / UV spectral irradiance between 17 and 220 nm
  • Modelling of the solar EUV / UV spectral / total irradiance
  • Modelling of the terrestrial thermosphere and ionosphere
  • Determination of solar EUV / UV indices
  • Semi-empirical modelling of active regions of the sun
  • Spectroscopy of hydrogen emissions of active regions
  • Investigation of solar-terrestrial relations
  • Investigation of solar-stellar connections
  • Aspects of space weather (impacts on satellite telecommunication and navigation)
  • Interactions of solar EUV radiation with the ISS
  • ISS environment changes with solar activity
  • EUV / UV space instrumentation and its calibration
  • Cross-calibration with other EUV space instrumentation

In order to cooperate with the most important groups working in all these fields the Principal Investigator of SolACES has initiated the international TIGER (Thermospheric-Ionospheric GEospheric Research) program in coordination with the SCOSTEP (Scientific Committee On Solar-TErrestrial Physics) and the COSPAR (Committee on Space Research) international organisations.

Key Characteristics of the SolACES Development and Operation

  • Start of Development:          November 1998

  • Start of Operation:               February 2008

  • Launcher:                            Space Shuttle (NASA)

  • Place of launch:                    Cape Canaveral

  • Nominal mission duration:       18 months

  • Potential mission duration:      36 months

  • Orbit characteristics:              ISS orbit (altitude ∼400 km)

  • Mission Control Center:          B.USOC (Belgium)

  • Observation schedule:           max. 20 minutes per orbit

Key Instrument Characteristics

  • Mass:                                       23.0 kg (SolACES instrument)

  • Size:                                        25 x 29 x 60 cm3

  • Electrical power consumption:     < 25 W (typ.), 60 W (max.)

  • Data rate:                                ∼1.0 kbit/s

  • Spectral range:                         17…220 nm (EUV / UV)

  • Spectral resolution:                    0.5…2 nm


SolACES Payload Description and Measurement Principle

SolACES uses two twin spectrophotometers with four different diffraction gratings and channel electron multipliers as detectors, and two ionization chambers equipped with photodiodes, to detect the inciding EUV / UV radiation and to perform the in-flight calibration. A common filter wheel for the spectrometers and the ionization chambers, containing 43 different thin film metallic and crystal filters, serves to select the spectral bandpasses during the calibration procedure.

Standard spectrophotometric measurements will be carried out using the spectrometers without any filters to obtain one or two EUV / UV spectra per orbit, integrated over the full disk of the sun (i.e. more than 15 spectra per day will be recorded). The auto-calibration procedure is planned to be executed every 3 to 15 days depending on the mission phase. During this procedure, the transmissions of the filters are determined by comparison of spectrometer measurements with and without filters. Absolute EUV / UV fluxes integrated over each of the selected filter bandpasses are then obtained by ionization chamber measurements. Taking into account the actual filter transmissions, these flux determinations can then be used to derive calibration factors for the standard measurements. During all standard measurements and calibrations the accurate pointing towards the sun will be ensured by the CPD.