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Observing the Active Sun - The Mg II Index

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fig. 3-23

Monthly averaged SCIAMACHY retrievals of OH rotational temperatures at about 87 km for January, April, July and October 2009. (Graphics: K.-U. Eichmann, IUP-IFE, University of Bremen)

Retrieval of OH* rotational temperatures is not the only scientific application of SCIAMACHY’s OH Meinel-band emission observations. The data can also be used to study atmospheric wave signatures (Ern et al. 2009) or to determine chemical heating rates associated with the exothermic chemical reaction H + O3 ® OH* + O2, which forms the vibrationally excited OH molecules as mentioned above (Kaufmann et al. 2007).

Observing the Active Sun - The Mg II Index

SCIAMACHY’s scientific objective to explore atmospheric trace constituents is achieved by analysing solar radiation, both in terms of scattered and reflected sunlight, and also by direct viewing for calibration purposes. Due to its high sensitivity and spectral stability, SCIAMACHY is also feasible for retrieving information about those aspects of solar activity which manifests themselves in the emitted radiation. Therefore, solar observations are analysed on a regular time grid offering the possibility to monitor solar variations and their impact on the atmosphere. The solar activity shows some well-known periodic variations such as the 27-day cycle caused by solar rotation. Another is the 11-year solar cycle, coupled with the 22-year magnetic cycle which correlates with changes in sunspots and Fraunhofer lines. During phases of high solar activity, an increase in the number of sunspots in the photosphere and large chromospheric plage areas are observed. The plage areas are hotter than the surrounding areas and cause the enhancement of the emission core within the absorption features of many solar Fraunhofer lines. Thus, solar proxy indicators can be given by the core-to-wing ratio of selected Fraunhofer lines. The Mg II index is defined as the core-to-wing ratio of the Mg II Fraunhofer line centered at 279.9 nm. It can be used as a proxy for spectral variations in the solar extreme UV (EUV, Viereck et al. 2001) and correlates with atmospheric ozone variations and other relevant atmospheric quantities.

For the understanding of the solar-terrestrial climate interaction, the establishment of long time series covering several solar cycles is important. Due to the limited lifetime of spaceborne missions, this has to be constructed from different satellite experiments. Figure 3-26 combines the Mg-II indices for the solar cycles 21 to 23, using data from NOAA missions (Viereck et al. 2004), GOME (Weber 1999) and SCIAMACHY (Skupin et al. 2004). Both the 27-day periodicity and the declining phase of solar cycle 23 are clearly visible. Differences between SCIAMACHY and GOME are mostly below ±0.5%, between SCIAMACHY and NOAA below ±0.25 %. The derived MgII index was used to identify a correlation of stratospheric ozone with the 27-day solar cycle (Dikty et al. 2010). SCIAMACHY is the first spaceborne instrument that observes daily solar spectral irradiance (SSI) continuously between 230 nm and 1750 nm. In order to address how much the irradiance changes in the UV-VIS-NIR and SWIR range on 27-day and 11-year timescales, short-term SSI variations were parameterised in terms of the proxies faculae brightening, i.e. MgII index, and sunspot darkening, i.e. photometric sunspot index (Pagaran et al. 2009). (fig. 3-26)


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