Solar Activity and the Mg II Index

The broad Mg II absorption feature in the solar UV spectrum is centered at a wavelength of about 279.9 nm. It is the result of the overlap of two Mg II absorption lines in the solar irradiance from the sun's photosphere. By contrast, the two Mg II emission lines located at the base of this composite feature originate from higher in the solar atmosphere in the chromosphere. The behavior of these lines' irradiance these through time has been found to be a convenient proxy for changes in chromospheric and upper photospheric UV flux at other wavelengths. Generally described, the Mg II index is the ratio of the irradiance at the core of the Mg II feature (containing the variable emission lines) to that of its relatively less variable wings. Because the Mg II index is an irradiance ratio from proximate wavelength ranges, unwanted instrumental effects that are present in the basis irradiances tend to cancel. As a result, the Mg II index is relatively uncorrupted by such instrumental effects.

Displayed above is a plot of the latest version of the SUSIM UARS Mg II Index whose time series began on October 11, 1991. Several features of solar behavior are readily apparent. For hundreds of years, the number and area of sunspots visible on the solar disk has been found to vary in 11 year cycles. More recently, sunspot number has been correlated with the presence of so-called "UV active regions", i.e. areas on the solar surface exhibiting enhanced UV emissions relative to their surroundings. The last secondary peak of solar cycle 22 sunspot number occurred in early 1992, shortly after the start of the SUSIM observations. This was followed by a decline to minimum levels by 1996. Not surprisingly, the long-term behavior in the Mg II index is similar to that of sunspot number and other measures of solar activity. The plots also shows often quasi-sinusoidal 27-day period modulation having a variable amplitude, a result of the sun's rotation. In late 1996, the index began a rise in activity associated with a new solar cycle. By early 2000, peak levels of solar cycle 23 were reached. We expect this activity and the Mg II index to remain at peak levels until 2003 when the decline to solar minimum is expected to begin.

Displayed above is the latest version of the Mg II index plotted over the latest 60 days. There the evidence of 27-day modulation associated with solar rotation is more clearly seen.

Mg II Index Implementation

For each operational day solar irradiances are generated from the SUSIM daily 1.1 nm (mid) resolution scans. At that resolution, the two (h & k) individual emission lines at the base of the Mg II absorption feature are not resolved. However, the variations in strength of these emission lines affect the irradiance level at the bottom of the absorption feature consistent with changes in solar activity. As stated earlier, the Mg II index is the ratio of the irradiance at the center of the absorption feature to the average irradiance at the less variable, nearby "continuum" wavelengths. A more detailed description of the Mg II index algorithm is also available. The SUSIM measurements displayed above show that at 1.1 nm resolution, the irradiance at the minimum is a bit more than one quarter of its value at the nearby continuum.

Data Versions

SUSIM irradiances are recalculated from time to time. As the calibration of the instrument improves, reprocessing of the raw data is necessary to produce more accurate irradiances. This is also true for the various derived solar indices, such as the Mg II core-to-wing ratio index. When a new version of the data becomes available, the previous and obsolete version is no longer updated. Care is taken to ensure that any updated version will include as many time periods covered as did the earlier version. The current version fo SUSIM reprocessed irradiance data is V21 and should be used instead of any previous version of the index.

The V21r2 Mg II index uses an algorithm which reduces noise and instrumental systematic trends, especially in the first 100 experiment days. V21r2 differs from the previous V19r5 index only in the base irradiances which were used to construct the index. It is recommended that any studies which use the SUSIM UARS Mg II index use this latest version. It is available via ftp in IDL save file and ASCII formats. In publications, references to the SUSIM Mg II index should also include the version identification used.

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