V21r2 Mg II INDEX RELEASE NOTES

The Mg II index is a dimensionless ratio of the core irradiance at the bottom of the Mg II absorption feature at about 279.9 nm to an estimate of the "continuum" irradiance as measured at the surrounding peaks. The SUSIM UARS Mg II core-to-wing ratio index is calculated from level 1 measured signals at SUSIM's middle resolution (1.1 nm FWHM) sampled at approximately 0.24 nm intervals. These signals are corrected for stray light and responsivity degradation using a modified algorithm to produce UV spectral irradiances. The current version, V21r2, is calculated in a way similar to earlier versions. Please note that for each version of the SUSIM Mg II index, the same calculation algorithm is used for all experiment days.

The V21r2 Mg II index differs from the V21r2 version index in that:

1. The responsivity measurements have subtle wavelength- and time-dependent artifacts removed.
2. The estimate of the continuum is based on a weighted sum of the integrated irradiance at the two nearby peaks at about 277 and 283 nm. The weights are chosen so that the resulting index will not be affected responsivity errors which are linear in the region between 276 and 284 nm.
3. A time varying stray light is modeled for the first 90 days of data. The parameters of the stray light model are based on a self consistent evaluation of the time series of the Mg II and Mg I indices.

In general terms, the V21r2 algorithm is as follows:

1. The signals for the region 275-285 nm are corrected for all known instrumental effects and converted to spectral irradiances. These irradiances take the form of a series data points spaced roughly 0.24 nm apart.
2. The data point having the largest irradiance and the two surrounding points in each of the 276.5-278.5 nm and 280.5-282.5 nm wavelength ranges are selected to form the continuum irradiance.
3. The data point having the smallest irradiance and the two surrounding points in the 278.5-280.5 nm wavelength range are selected to form the core irradiance.
4. The irradiance of the two continuum and core regions equal the average irradiance over a given wavelength interval of the parabolas fitted to each three data point set.
5. The continuum irradiance is a weighted average of the two continuum irradiances. The weights are chosen so as to render the resulting index independent of first order responsivity errors in wavelength.
6. The Mg II index is the quotient of the core irradiance over the continuum irradiance.

Last modified on Sep 3, 2002. Please address questions or comments to