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About the MgII c/w Ratio

Mg II c/wThe NOAA SBUV MgII c/w measurement has been an important input to solar irradiance models, and is one of the best chromospheric time series available for describing solar irradiance variations from daily to solar cycle timescales. Continuous daily values are available from 1978, covering nearly three solar cycles (see below). In many ways, compared to sunspots and the F10.7 2800 MHz data, it is a superior index to solar variability and space weather because it can accurately describe chromosphere variability on time scales ranging from a solar rotation (27 days) to the 11-year solar cycle, and because it describes the solar EUV variability that directly affects the Earth's thermospheric density, unlike F10.7 and sunspots.

The SBUV instrument (Solar Backscatter Ultraviolet) has been on-board all of the NOAA-9 to NOAA-18 polar-orbiting satellites. The main purpose of the SBUV instrument is to monitor terrestrial ozone. However, it also acquires solar UV spectral irradiance. One component of these solar irradiance data is the daily Mg II center-to-wing ratio (MgII c/w), which is a relative photometric measurement at 280 nm between the Mg II h and k lines. Because it is based on the ratio of the lines, and not to an absolute calibration, it is much less susceptible to instrument degradation. However, there are other instrument and satellite effects that need to be accounted for in deriving the ratio.

NOAA National Environmental Satellite, Data, and Information Service (NESDIS) monitors the satellites, and provides the unprocessed data to the NOAA Space Weather Prediction Center (SWPC). In the past, NOAA/SWPC produced a calibrated data file of Mg II c/w data and those data are in the process of being re-calibrated.

Near real-time SET Mg II c/w data are available at:

MgII c/w ratio, long term

Update: 21 Sep 2017

NOAA-19 Mg II data has suddenly gone offline Sep 3, 2017. The source of the problem is unknown at this point, but the SBUV instrument data suggests NOAA-19 MgII data has gone offline permanently. We have back-filled our operational MgII data with GOME data, and will continue to use it until an alternate MgII data source is implmented (possibly LASP or GOES-16)

Update: 27 Jan 2017

NOAA-19 data has been corrected. It was discovered that NOAA-19 data (after 1 Jun 2015) was under-estimated by 0.003. Historical data has been replaced. All future data will include the correction. There may still be issues with NOAA-19 MgII data (a low dynamic rang). However, the background/minimum data is within 1-3% of former solar minima, and is consistent with GOME MgII data. Further investigation will be performed.

Update: 23 Sep 2015

We have been using GOME data since the last update. In June 2015, the NOAA-19 data came on-line. On September 23, new processing code using a Data State Matrix was implemented as a beta-level product. When verified, the NOAA-19 data will replace the GOME data as the primary source, and GOME will be the secondary.

Update: 13 Jun 2014

In Dec 2012, the SBUV instrument failed on NOAA-18, and SBUV data are no longer available from NOAA-18 (permanently). Within several weeks, the NOAA-16 data processing was re-implemented. Note that the SET issued Mg II data during that period used extrapolated estimates, and that introduced a failrly large error (Dec 2012) ).

Then, on 9 Jun 2014, NOAA-16 suddenly failed, and has been decommissioned. Efforts are ongoing to implement the GOME MgII data.

Update: 13 Dec 2011

There have been a number of difficulties in determining an accurate Mg II c/w since approximately February 2009. The SBUV diffuser plate on NOAA-17 increasingly was shadowed due to the spacecraft's attitude, due to it's slow precession in it's orbit. SET Mg II data operations substituted a combination of adjusted SOLSTICE and GOME data, but the data were not consistently accurate (to within the 5% level).

Recently, Matt DeLand at SSAI has graciously provided new processing algorithms for the NOAA-18 data, which has been performing very well. SET has integrated those new algorithms in it's operations 13 Dec 2011. These data have been re-calibrated back to June 1, 2008. Prior to that, the data is unchanged from earlier versions. In the most recent version, NOAA-18 is the primary source, and when those measurments are missing, data from the SOLSTICE instrument are substitued in. Users of the SET V4 Mg II c/w are asked to credit Space Environment Technologies.

Update: 27 Dec 2012

On 12 Dec 2012 NOAA-18 had a sudden SBUV instrument failure, and MgII is no longer available from NOAA-18. Since then, we have switched to NOAA-16. Currently, we are still calibrating the NOAA-16 data to NOAA-18, consequently the relative error may be on the order of 20%, but we expect to complete adjusting the data to historical MgII baselines within 30 days.

Update: 13 Feb 2013

Normal Mg II data processing has been running for five weeks without incident; thus we have resumed normal Mg II data operations, using the NOAA-16 SBUV instrument, and preliminary evaluation indicates less than a 5% error. Work will continue on this.

Update: 22 Mar 2013

Normal Mg II data processing continues. A detailed comparison of the SOSTICE and NOAA-16 data was performed with respect to F10.7. Neither source indicated long-term insturments from 2006-2013, and comparing SOLSTICE to NOAA-16 resulted in a 0.94 correlation coefficient. By simply applying a linear regression SOLSTICE data can be adjusted to NOAA-16, and is done so when NOAA-16 data are missing.

Below are links to miscellaneous plots and data related to operations (and are not intended for official use):

EVE Ly-a Daily 1-min/1hr avg.: eve_Lya_1m.dat.txt eve_Lya.1m.pp.jpg


EVE Ly-a Daily averages: eve_Lya_1d.pp.txt eve_Lya_1d.pp.jpg




Current SOLSTICE vs NOAA-16 comparison. The independent time series is the NOAA MgII, the dependent is SOLSTICE. After the linear regrassion coefficients are applied to the SOLSTICE data, the NOAA-16 MgII measurement and the SOLSTICE estimate is written to:

SOL vs N16