Spherical harmonic decomposition of solar magnetic fields

Abstract

Abstract.We have investigated the temporal evolution of large-scale magnetic fields in the solar photosphere during the time interval 1966-2004 by means of spherical harmonic decomposition and subsequent time series analysis. Two data sets of daily magnetograms recorded at the Mt. Wilson and Kitt Peak observatories were used to calculate the spherical harmonic coefficients of the radial magnetic field for axisymmetric (m = 0) and non-axisymmetric (m 0) modes. Time series analysis was then applied to deduce their temporal variations. A third data set of synoptic Carrington rotation maps from Kitt Peak was also analyzed for completeness. Besides the obvious 22 yr magnetic cycle, we have found evidence for intermittent oscillations with periods of 2.1−2.5 yr, 1.5−1.8 yr and 1.2−1.4 yr. The biennial oscillation occurred during the solar maxima of cycles 20-22 (and likely also during the current cycle 23) and was most pronounced for modes that resemble non-linear dynamo waves (Stix 1972, A&A, 20, 9). The 1.5−1.8 yr period was stronger during the odd cycles 21 and 23 than during the even cycles 20 and 22, whereas the opposite was the case for the 1. (Krivova & Solanki 2002, A&A, 394, 701), and in the large-scale photospheric magnetic field (Knaack et al. 2005, A&A, in press). In agreement with the latter study, we have found additional quasi-periodicities in the range 320−100 d and rotational periods of 29.0 ± 0.1d, 28.2 ± 0.1d, and 26.8 ± 0.1d. Compared to earlier decompositions by Stenflo & Vogel (1986, Nature, 319, 285) and Stenflo & Güdel (1988, A&A, 191, 137), we can confirm the main features of their results, although several modifications need to be considered.

A quasi-biennial signal in western US hydroclimate and its global teleconnections

“…QB variations in sunspots and irradiance however are miniscule and irregular, and any steady QB solar forcing of the atmosphere may perhaps more plausibly involve magneticelectrical forcing of atmospheric waves via the solar wind and interplanetary magnetic field, as proposed for the interannual variability of the North Atlantic Oscillation (NAO) Lundstedt 2002, 2003) and the NAM (Lu et al 2008). Moreover, QB oscillations slightly longer than 2 years are independently established features of the solar magnetic field (Stenflo and Vogel 1986;Stenflo and Güdel 1988;Obridko and Shelting 2001;Knaack and Stenflo 2005). In this regard, it is interesting to note that the initial zones of significant QB activity depicted in Fig.…”

The Dynamics of the Solar Magnetic Field: Polarity Reversals, Butterfly Diagram, and Quasi-Biennial Oscillations

“…The most prominently recognized periods are the quasi-biennial oscillations (QBOs) at timescales around 2 yr. This component of the cycle, although weaker than the main component, has been identified in many activity indices (Rao 1973;Valdés-Galicia et al 1996;Bazilevskaya et al 2000;Kudela et al 2002;Bai 2003;Knaack & Stenflo 2005;Vecchio & Carbone 2008, 2009). The QBO origin is still unknown even if it could be related to the dynamo action in the inner solar layers (Benevolenskaya 1998), being also detected in phenomena directly connected with the solar interior.…”

“…These maps have been used to investigate the main large-scale features of the solar magnetic field, its variability, and periodic behavior (e.g., Knaack & Stenflo 2005;Cadavid et al 2005;). The classic procedure to derive magnetic synoptic maps (one per Carrington Rotation, CR), from full-disk photospheric magnetograms, is as follows (Gaizauskas et al 1983;Harvey & Worden 1998).…”

“…Figure 1, illustrating the latitude-time evolution of the magnetic field B r (θ, t), shows the typical 11 yr butterfly diagram of equatorward magnetic flux with occasional strong poleward "surges" (Wang et al 1989). No well-defined periodicity has been directly associated with these surges, although Knaack & Stenflo (2005), through a Fourier analysis, detected power enhancements, in the QBO range, at the solar latitudes where these magnetic flux concentrations seem to emanate. The time evolution of the magnetic field strongly depends on the latitude.…”

Astrophysics in 2005

“…However, a cycle is not simple in nonlinear dynamics; fluctuations in the Babcock-Leighton dynamo were actually shown to lead to period doubling and to transition to chaos (Charbonneau et al 2005b), possibly explaining the anomaly of the Maunder minimum (Charbonneau 2004(Charbonneau , 2005Charbonneau et al 2004). Spherical harmonic decomposition of magnetic field data revealed also intermittent oscillations with periods of 2.1-2.5 yr, 1.5-1.8 yr, and 1.2-1.4 yr (Knaack & Stenflo 2005;Kane 2005b), as similarly found in cosmic-ray modulations (Starodubtsev et al 2004). While we believed that the magnetic cycle of 22 years (Hale cycle) clocks everything on the Sun, correlations with the equatorial rotation rate actually reveal that the phase of the beginning of a 22 yr cycle in the latitudinal gradients is out of phase by 180Њ (Javaraiah et al 2005).…”