Overview on past and future UVN missions
UVN observational record
Overview and timelines of past and future UVN (ultraviolet nadir) sounders. The earliest measurements were provided by the BUV instrument starting in 1970. The routine daily atmospheric observations started with SBUV and TOMS aboard Nimbus-7 by the end of 1978. Hyperspectral observations were first provided with GOME in 1995. "Hyperspectral" means that continuous atmospheric spectra across the UV and visible spectral range are recorded that allow the detection of many more trace gases in addition to ozone and SO2 (sulphur dioxide). Current and past missions have been operating in a near polar low-earth orbit (LEO). For most points of the earth at most one measurement per day at nearly identical local times are available. Some of the future missions like Sentinel 4 (UVNS), MP-GEOSat (GEMS), and TEMPO will be observing from a geostationary orbit that will allow local measurements at different times of the day, which will be particularly useful for monitoring air quality. Links to the various UVN satellite missions can be found in the "Link" section.
Typical UV/visible spectra as measured with the hyperspectral GOME spectrometer. The reflectance is obtained by dividing the nadir observed spectrum (I) by the solar spectrum (F). Under cloudy sky condition the reflectance is high across the UV and visible spectral range (white clouds), while the clear-sky reflectance peaks in the near UV (blue). The peaking in the blue part of the spectrum is due to Rayleigh-scattering ("why is the sky blue?") and explains why our planet is called the "blue planet". The reflectance spectra show several major atmospheric absorbers like ozone (O3), water (H2O),and oxygen (O2) that along with the minor absorbers (not directly visible here) can be retrieved from hyperspectral UVN data. The sharp drop below about 340 nm is due to ozone absorption and explains why ozone is vital in protecting the earth surface from the harmful UV radiation. Adapted from Burrows et al. (1999).
GOME Optical layout
Schematics of the GOME spectrometer. A very similar design is used for SCIAMACHY and GOME-2. The only moving part is the scan mirror (red arrow) that can be rotated to pick up the signal from the nadir Earth view, the sun, and the internal Pt/Ne/Cr hallow discharge lamp. Dispersion into four spectral channels occurs with the predisperser- and band-separator prisms. In each of the optical channel gratings leads to further dispersions thus achieving moderately high spectral resolution (0.2-0.4 nm) in the spectrum recorded by a linear Si reticon array. The Pt/Ne/Cr lamp is used to calibrate the instrument. Some of the signal is diverted to Polarisation Measurement Devices (PMD) that are used for polarization corrections and cloud detection. Adapted from Burrows et al. 1999
Bremen composite MGII index
Most UV nadir sounders routinely observe directly the sun (spectral solar irradiance) about once a day. From the emission core of the solar Fraunhofer Mg II doublet observed near 280 nm the so-called Mg II index can be derived. This index is frequently used as a proxy for UV solar spectral irradiance variation. From the series of UVNs (SBUV, SBUV/2, GOME, SCIAMACHY, OMI, and GOME-2) and dedicated solar missions (SOLSTICE, SUSIM) a composite Mg II index (Viereck et al., 2004, Snow et al., 2014) can be derived which shows the variations with the 11-year sunspot cycle (22 year magnetic cycle) and solar rotation (~27 days) . Values indicated are the numbers of days for each satellite contributing to the composite index. Missing values were filled using scaled F30 cm radio flux data obtained from the ground observations. The black curve shows the timeseries twice smoothed with a 55-day boxcar (removing the 27 day solar rotation signal). The solar maximum in solar cycle 24 (reached in 2014) is lower than in the three previous cycles 21-23.