Woods et al. (2000) studied the variability of solar Lyman α emissions based on satellite observations spanning four and a half solar cycles between 1947 and 1999. They found that the variability ranges between 1% and 37% during one period of solar rotation (27 days), and the average variability during one solar rotation was found to be 9 ± 6%. This result agrees well with the estimated variability of the Lyman α emissions from HD209458. The rotation period of HD209458 is estimated to be ∼10–11 days (Silva-Valio, 2008). The G140M observations covered three different transits and took place within a month and a half. Each observation covered approximately 2 h in time. Thus the data can be affected by short-term variability and it is essential that such variability be properly accounted for. For this reason, we compare our models with the results of Ben-Jaffel (2007, 2008) and Ben-Jaffel and Hosseini (2010) who analyzed the data in the time tag mode and accounted for variability before calculating transit depths.
solar cycles
four and a half
satellite observations
between 1947 and 1999
Lyman α emissions
variability
1% and 37%
Lyman α emissions
variability
9 ± 6%
%
period of solar rotation
one
period of solar rotation
27 days
days
solar rotation
one
HD209458
rotation period
∼10–11 days
days
transits
three
G140M observations
took place
within a month and a half
month
observation
time
approximately 2 h
h