SAC Seminar - Rosaria Simoniello: The multiple magnetic cycles of the Sun: a window on dynamo symmetry
Oplysninger om arrangementet
Speaker: Rosaria Simoniello, Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/SAp, Centre de Saclay, F-91191, Gif-sur-Yvette, France
Title: The multiple magnetic cycles of the Sun: a window on dynamo symmetry
Since the discovery of multiple stellar magnetic cycle periods in a few stars (Baliunas et al. 1995), there has been great interest in using long term observations to better understand the underlying physical mechanisms.
The CaII observations at Mount Wilson Observatory of more than 100 stars of spectral type G0-K5, show correlated changes in rotation and chromospheric activity on evolutionary timescales: young stars exhibit high levels of activity, rapid rotation rates, no Maunder minimum phase and rarely display cyclic variations; stars of intermediate age (1-2 Gyr for 1 solar mass) have moderate levels of activity and rotation rates, and sometimes exhibit smooth cycles; stars of the Sun’s age and older have the lowest activity levels, slower rotation rates and smooth cycles with occasional Maunder-minimum phases.
Later, Saar & Brandenburg (1999) showed that the cyclic period increases with rotation period along two distinct sequences, the active and inactive sequence. Bohm-Vitense (2007) argued that some stars show that secondary cycles falling on the inactive sequence, might be the result of a second dynamo mechanism. Metcalfe et al. (2010) reported the discovery of the shortest known cycle of 1.6 year for a solar-type star. A comparable timescale magnetic variations has been identified by COROT in HD 49933 (García et al. 2010) and a ~2 year solar variability is also clearly visible in helioseismic velocity and intensity measurements (Simoniello et al. 2012).
To better understand the origin of multiple magnetic cycles, the Sun can play a key role.
Last year we looked for the origin of the solar 2 yr quasi -biennial periodicity (QBP), by means of long high quality helioseismic data provided by the Global Oscillation Network Group (Simoniello et al. 2013). We compared the signatures of the 11 and 2 yr cycle of p-mode frequency shift of low and intermediate degree modes (?=0,120) in the low (1600? ? ?2300) and high (2500? ? ?3300) frequency band at different latitudes and in the subsurface layers, where the magnetic pressure starts to produce visible effects on the mode frequency.
The results provide observational evidences in favor of the beating between the dipolar and quadrupolar component of the magnetic dynamo configuration as the likely mechanism behind the quasi-biennial periodicity (Tobias et al. 1997, Moss et al. 2004, Berdyugina et al. 2003). This mechanism, therefore, might also operate in other solar type stars showing secondary cycles and opens the possibility to use multi-cyclic behavior in stars as a window on stellar magnetic dynamo symmetry.