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The aim of the Working Group is to produce an absolutely calibrated meas= ure of the solar EUV irradiance, and to provide a long-term record of the s= olar EUV irradiance and its variability.
This is accomplished by validating the EUV irradiance products from vari= ous instruments, understanding their calibration and degradation.
The solar Extreme Ultraviolet (EUV) radiation is totally absorbed in the Earth's atmosphere and = drives the photo-chemistry of the Earth's (and other planets) upper atmosph= ere. Even though the EUV is only a small fraction of the total solar irradi= ance, it is highly variable on many time scales from minutes (solar flares)= , hours (flares and active region evolution), days (solar rotation modulati= on) and years (the 22 year magnetic cycle). The amount of variability depen= ds on the wavelength (and hence temperature) we are looking at. The Total S= olar Irradiance variation over an 11 year cycle is ~0.1%. In the mid-UV (20= 0 =E2=80=93 300 nm) the variability is ~1.5% , at Lyman-=CE=B1 (121.6 nm) i= t is almost a factor of 2, and at the shorter EUV wavelengths the solar cyc= le variability is a factor of 10 =E2=80=93 100. We do not know yet if there= are longer-term trends, i.e. is a quiet-Sun spectral irradiance a constant= . This is something we hope to help understand with this work.
The Solar radiation below 200 nm consists of emission lines superimposed= on a rapidly declining continuum. These emission lines arise in higher tem= perature layers of the outer solar atmosphere and are strongly related to t= he magnetic activity of the Sun. Understanding this relationship between ma= gnetic morphology and EUV spectral irradiance is one of the overarching goa= ls of this work.
As this radiation is absorbed in the atmosphere it drives the temperatur= e structure and ionization state of the upper atmosphere. In a modern techn= ological world this has impacts on things such as GPS accuracy, high-freque= ncy communications (used by aircraft) and satellite orbits. These types of = effects are know as Space Weather, and understanding these effects is becom= ing increasingly important.
Due partly to the importance of space weather, we now are in the fortuna= te position to have several instruments measuring the solar EUV irradiance = at the same time. The problem is that the irradiances reported by the vario= us instruments do not always agree within the uncertainties claimed by the = instrument teams. This is very similar to the discrepancies found in the to= tal solar irradiance community, though we are not even starting to approach= the 100 ppm quoted accuracies of those measurements.
Measurements in the EUV are very difficult. The high-energy photons tend= to degrade anything they hit, making material choices and instrument / spa= cecraft cleanliness imperative. even so, filters and detectors degrade with= time, and understanding the degradation is an important goal of the presen= t work.
There are few facilities that can provide absolute calibration of the in= struments (the NIST SURF, BESSY and NSLS synchrotron sources being the ones= most often used)
The working group currently comprises members from most of the current a= nd past EUV Instruments, and various standards organizations.