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Mesopause Temperatures

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A good opportunity to study the impact of solar activity on mesospheric ozone fields was a period in October/November 2003 – also known as the ‘Halloween Storm’ – when the Sun exhibited extremely large coronal SPE. Fig. 3-22 presents an analysis of the impact of the solar proton event at the end of October 2003 on the upper atmospheric O3 (Rohen et al. 2005). A strong ozone depletion of more than 50% even deep in the stratosphere is observed at high geomagnetic latitudes in the northern hemisphere, whereas the observed ozone depletion in the more sunlit southern hemisphere is much weaker. SCIAMACHY measurements of the O3 loss due to SPE agree well with model simulations, indicating that the main processes leading to the O3 loss are fairly well understood. (fig. 3-22)


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fig. 3-22

Measured change of ozone concentration at 49 km altitude due to the strong solar proton events end of October 2003 in the northern and southern hemisphere relative to the reference period of 20-24 October 2003. White areas depict regions with no observations. The black solid lines are the Earth's magnetic latitudes at 60 km altitude for 2003. (Graphics: Rohen et al. 2005)

Mesopause Temperatures

A number of atomic and molecular emission signals from the mesosphere and lower thermosphere (MLT) can be detected throughout the SCIAMACHY spectral range, e.g. neutral and ionised magnesium (Mg) lines as well as NO gamma bands in the UV, OH Meinel band emissions in the SWIR, several transitions from excited-state O2 or sodium lines and atomic oxygen in the VIS range. These emission signals can be used to characterise the distribution of the atoms, ions and molecules in the upper atmosphere.

SCIAMACHY measurements of molecular emissions in the upper atmosphere provided for the first time the retrieval of OH* rotational temperatures at the mesopause from satellite measurements during night. OH* is vibrationally excited at the mesopause through the reaction of H and O3. This produces an OH* emission layer centred at about 87 km with a width of 8-10 km. Several of the OH* Meinel emission bands are observable in the SCIAMACHY spectral range, e.g. the OH* (3-1) band at around 1500 nm. This emission band is used for the retrieval of OH* rotational temperatures because it is one of the most intense emission bands. From the relative intensity of two or more rotational lines, the effective temperature of the emitting layer can be retrieved from SCIAMACHY data. For several decades, this method has been applied to retrieve mesopause OH* rotational temperatures from ground measurements (e.g. Bittner et al. 2002). Therefore, coincident ground-based OH* rotational temperature measurements over Germany in Wuppertal and Hohenpeissenberg, as well as over Hawaii, had originally been used to validate the novel SCIAMACHY space-based approach (von Savigny et al. 2004b). A comparison of both methods yielded very good agreement. Fig. 3-23 shows the monthly averaged mesopause temperatures retrieved from SCIAMACHY OH* (3-1) emission measurements for January, April, July and October 2009 as an example. As the observations used for the retrievals are made on the Earth’s nightside, the OH rotational temperatures are only available at higher latitudes during wintertime. (fig. 3-23)


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