July 31, 2014

Modelling the Sun’s mechanisms

Solar models account for all of the physical phenomena that are thought to govern the Sun’s mechanisms. To validate a model, we compare its properties with actual measurements of helioseismology, magnetic field intensity, total solar irradiance, the solar spectrum and the distribution of active regions. Any new quantity therefore enables us to advance our understanding of solar physics.

Observations of total solar irradiance show that it diverges from the undecennal cycle. P-mode oscillations exhibit frequency variations that follow this cycle (Fröhlich and Lean, 2004). The Sun’s temperature, density and radius are other very important quantities for modelling, which is why measuring them simultaneously will enable a certain category of solar models to be validated rather than another. So, we need to establish unambiguously whether the Sun’s diameter is variable or constant to help elucidate the origin of solar activity, which is related to magnetic fields at its surface or deeper in the core (Li et al., 2003).

Diameter and brightness variations enable us to deduce w = (r/r)/(L/L), where r is the variation in the solar radius and L is the variation in brightness as a function of solar activity. This term is very poorly defined and its sign even uncertain. The value obtained from measurements by the CERGA geodynamics and astronomy research centre is –2 10-1 (due to the anti-correlation between the diameter and solar activity), but other modellers have found it to be anything between 2 10-4 (Spruit, 1992) and 7.4 10-2 (Sofia et al., 1979). Precise measurement of this parameter is therefore obviously a powerful tool for validating solar models.

Helioseismology is the study of the Sun’s internal structure by observing its oscillation frequencies. The SOHO mission’s MDI and VIRGO instruments, combined with ground-based measurements, have helped scientists to learn a great deal more about the structure of the Sun’s interior. The instruments observed in the acoustic (p-mode) frequency range. Lower-frequency gravity-mode (g-mode) waves are postulated by theory and data from the GOLF and VIRGO instruments on SOHO would appear to confirm this (Appourchaux et al., 2000; Turck-Chièze et al., 2004). Measuring such frequencies would tell us more about the dynamics of the radiative zone and thermonuclear core, in particular for studying the role of magnetic fields and internal waves in transporting matter and angular momentum.