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    24-Jul-2014
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3.3.2 Star properties

Three categories of star brightness are usually defined from the star visual magnitude. A star with a visual magnitude lower than 0.8 is considered as a bright star; a star with a visual magnitude higher than 2 is considered as a dim star; a star with a visual magnitude higher than 0.8 and lower than 2.0 is considered as of medium brightness. Similarly, a star is considered as cold if its effective temperature is lower than 6000 K, as hot if it is higher than 10000 K and as of medium temperature if it ranges between 6000 K and 10000K.

It is recommended to use full dark and straylight occultations only from a subset of stars, which brightness and effective temperature fill criteria specific to each species, and depending on the altitude range.

Vertical profiles of O3 local density:

The retrieval from the occultation of hot stars is of good quality at all altitude levels. Profiles retrieved from the occultation of cold stars (star temperature lower than 6000 K) should not be used in the mesosphere (altitude levels higher than 40 km).

Vertical profiles of NO2 local density:

Profiles retrieved from the occultation of bright stars may be used in the NO2 validity range (between 20 km and 50 km; see table 1.1 in section 1.4.3.3). Profiles retrieved from the occultation of weaker stars should not be used (stars of visual magnitude higher than 2).

Vertical profiles of NO3 local density:

Profiles retrieved from the occultation of bright stars may be used in the NO3 validity range (between 25 km and 45 km; see table 1.1 in section 1.4.3.3). Profiles retrieved from the occultation of weaker stars should not be used (stars of visual magnitude higher than 2).

Vertical profiles of H2O local density:

Profiles of acceptable quality are provided only from the 9 brightest stars in the near-IR (cold bright stars or very bright stars). Those stars are of star ID: 1, 2, 3, 4, 13, 14, 16, 26, 63.

The impact of the star characteristics on the quality of the vertical profiles of O3, NO2, NO3, H2O is summarised in Table 3.2 .

 

Species

Temperature category

Brightness category

cold stars

hot stars

dim stars

bright stars

O3

valid only below 40 km

valid profiles

noisy profiles in the lower stratosphere

valid profiles

NO2

valid profiles (*)

valid profiles (*)

not to be used

valid profiles (*)

NO3

valid profiles (*)

valid profiles (*)

not to be used

valid profiles (*)

H2O

only from stars of ID: 1, 2, 3, 4, 13, 14, 16, 26, 63

Table 3.2: Impact of the star characteristics on the quality of vertical profiles of local density of O3, NO2, NO3, and H2O; (*) in the validity altitude range: 20 km-50 km for NO2 and 25 km-45 km for NO3(see also Table 1.1 of 1.4.3.3). A star is considered as cold if its effective temperature is lower than 6000 K; a star is considered as dim if its visual magnitude is higher than 2. It is supposed here that only occultations measured in full dark and in straylight illumination conditions are considered.