Rotational Modulation on Total Solar Irradiance
Hari Om Vats [vats@prl.res.in], Astronomy Astrophysics Division, Physical Research
Laboratory, Ahmedabad, India
The Sun –weather relationship is becoming increasingly important. It is true that our
understanding of the Sun and solar processes has increased dramatically during recent years,
however, it is realized that the Sun affects the Earth’s environment in a much more
complicated manner than we have imagined. It is impossible to describe the effects of the
Sun on Earth by just a few parameters. The most important solar parameter is the total power
as irradiance received from the Sun at Earth. The solar angular rotation velocity is a function
of latitude, time and height above or depth below the solar photosphere. This phenomenon is
known as the solar differential rotation. Earlier we had used VIRGO experiment on board
ESA/NASA collaborative SOHO mission measures the total solar irradiance (TSI) with two
absolute radiometers (DIARAD and PMO6) for the period 1996- 2001. The modulation of
this TSI by solar rotation was investigated. The solar irradiance is essentially the contribution
from the small scale solar surface structures integrated over the solar disk. The modulation
will represent rotation period. Recent data of SORCE is used for the period 2003-2011 to
look for the rotational modulation on TSI. It is found that the rotational modulation varies
from 9 – 42 % and it apparently varies in opposition of sunspot numbers. The derived
sidereal rotation period varies from 22.5 to 30.5 days. These results are compared with other
investigation e.g. (1) disk integrated radio emission (2) IMF and (3) solar radio and X-ray
images.