OMI Solar Spectral Irradiance Data ################################## Authors: Sergey Marchenko and Matthew DeLand Contact: sergey.marchenko@ssaihq.com Last revision: 28 April 2025 Data Product ************ This file gives a basic description of the solar spectral irradiance data that we have produced from Aura OMI solar measurements. Detailed discussion of the procedures used and validation steps can be found in the following papers: S. V. Marchenko and M. T. DeLand (2014), "Solar spectral irradiance changes during Cycle 24", Ap. J. 789:117, https://doi.org/10.1088/0004-637X/789/2/117. S. V. Marchenko, M. T. DeLand, and J. L. Lean (2016), "Solar spectral irradiance variability in Cycle 24: Observations and models", J. Space Weather Space Climate, 6, A40, https://doi.org/10.1051/swsc/2016036. S. V. Marchenko, T. N. Woods, M. T. DeLand, S. Mauceri, P. Pilewskie, and M. Haberreiter (2019), Improved Aura/OMI solar spectral irradiances: Comparisons with independent data sets and model predictions, Earth and Space Sci., 6, 2379-2396, https://doi.org/10.1029/2019EA000624. We designate this dataset as Version 7 ('_v7') to recognize three changes from our previous Version 6 product: 1. The analysis of the "common degradation" correction term, determined from the ratio of quartz volume diffuser and aluminum diffuser measurements, uses a revised exposure time-dependent function. 2. The Aura satellite drifted to beta angle values outside the normal goniometric calibration range during October-December 2023 and September- December 2024. Empirical irradiance corrections were developed using data from selected CCD rows that showed minimal changes during these periods. 3. The magnitude of the irradiance values at 279.5 nm and 280.5 nm, at the core of the Mg II line, is slightly supressed in late 2024 and early 2025. We believe that this is due to a non-linearity issue, and plan to address it in our next data release. Other changes from the Version 3 OMI SSI product described in our 2019 paper were incorporated previously, and include the following items: a. The instrument degradation model is based on data from all three OMI channels (UV1, UV2, VIS) for better spectral consistency. b. The averaging period used to define our solar minimum reference spectrum was shifted to cover 1 July 2008 - 30 April 2009. This provides symmetric coverage around the sunspot minimum at ~2008.9 shown in NOAA NGDC (National Geophysical Data Center) data. c. The filtering process to remove instrument noise was revised to improve flexibility. d. The SSI data are provided on a 0.5 nm wavelength grid for consistency with the OMI spectral resolution (~0.4-0.6 nm). More details will be given in an upcoming journal manuscript. The Version 7 OMI SSI product is based on Collection 4 OMI L1B data, as described by Kleipool et al. [2022]. Kleipool, Q., et al. (2022). Ozone Monitoring Instrument (OMI) collection 4: establishing a 17-year-long series of detrended level-1b data. Atmos. Meas. Tech., 15, 3527-3553, https://doi.org/10.5194/amt-15-3527-2022. Data Location ************* The OMI data can be retrieved at the following web site: https://sbuv.gsfc.nasa.gov/solar/omi/ File Contents ************* The OMI irradiance data, corrected for long-term degradation, are provided in a single HDF file named 'omi_ssi_irradiance_v7_20250428.h5'. All data are originally measured in three spectral channels: UV1 channel, wavelength range = 265.0-309.5 nm UV2 channel, wavelength range = 310.0-364.5 nm VIS channel, wavelength range = 365.0-500.0 nm The datasets presented here combine these data into a single spectral dimension. The irradiance data are provided on an 0.5 nm wavelength grid, i.e. 265.0, 265.5,...500.0 nm. A fill value of -999.0 is used to indicate that no valid irradiance data are available for a particular wavelength and date. The first measurement date available in the OMI data product is 2 July 2006, and the last date included in the current data file is 3 March 2025. OMI SSI measurements are normally taken every 15 orbits, so the typical interval between spectra is slightly longer than one day. The HDF file contains the following variables: JulianDateAdj Observation date in adjusted Julian day format [JD-2450000.0] Year Year of measurement DayofYear Day of year of measurement Month Month of measurement Day Day of month of measurement Hour Hour of measurement Minute Minutes of measurement Second Seconds of measurement WavelengthScale Wavelength scale [nm], sample interval = 0.5 nm IrradianceReference Reference irradiance spectrum created by averaging data between July 2008 and April 2009 [mW/m^2/nm] IrradianceDaily Daily spectra [mW/m^2/nm] IrradianceStDev Standard deviation within each spectral and daily bin