Speed of Meridional Flows and Magnetic Flux Transport on the Sun

We use the magnetic butterfly diagram to determine the speed of the magnetic flux transport on the solar surface towards the poles. The manifestation of the flux transport is clearly visible as elongated structures extended from the sunspot belt to the polar regions. The slopes of these structures are measured and interpreted as meridional magnetic flux transport speed. Comparison with the time-distance helioseismology measurements of the mean speed of the meridional flows at the depth of 3.5--12 Mm shows a generally good agreement, but the speeds of the flux transport and the meridional flow are significantly different in areas occupied by the magnetic field. The local circulation flows around active regions, especially the strong equatorward flows on the equatorial side of active regions affect the mean velocity profile derived by helioseismology, but do not influence the magnetic flux transport. The results show that the mean longitudinally averaged meridional flow measurements by helioseismology may not be used directly in solar dynamo models for describing the magnetic flux transport, and that it is necessary to take into account the longitudinal structure of these flows.Comment: 4 pages, 3 figures, accepted in ApJ Letter

Comparison of solar surface flows inferred from time--distance helioseismology and coherent structure tracking using HMI/SDO observations

We compare measurements of horizontal flows on the surface of the Sun using helioseismic time--distance inversions and coherent structure tracking of solar granules. Tracking provides 2D horizontal flows on the solar surface, whereas the time--distance inversions estimate the full 3-D velocity flows in the shallow near-surface layers. Both techniques use HMI observations as an input. We find good correlations between the various measurements resulting from the two techniques. Further, we find a good agreement between these measurements and the time-averaged Doppler line-of-sight velocity, and also perform sanity checks on the vertical flow that resulted from the 3-D time--distance inversion.Comment: 22 pages of the manuscript, 5 figures, 3 tables, accepted for publication in the Astrophysical Journa

Polar cap magnetic field reversals during solar grand minima: could pores play a role?

We study the magnetic flux carried by pores located outside active regions with sunspots and investigate their possible contribution to the reversal of the global magnetic field of the Sun. We find that they contain a total flux of comparable amplitude to the total magnetic flux contained in polar caps. The pores located at distances of 40--100~Mm from the closest active region have systematically the correct sign to contribute to the polar cap reversal. These pores can predominantly be found in bipolar magnetic regions. We propose that during grand minima of solar activity, such a systematic polarity trend, akin to a weak magnetic (Babcock-Leighton-like) source term could still be operating but was missed by the contemporary observers due to the limited resolving power of their telescopes.Comment: 11 pages, 9 figures, accepted for publication in Astronomy&Astrophysic

Atmosphere above a large solar pore

A large solar pore with a granular light bridge was observed on October 15, 2008 with the IBIS spectrometer at the Dunn Solar Telescope and a 69-min long time series of spectral scans in the lines Ca II 854.2 nm and Fe I 617.3 nm was obtained. The intensity and Doppler signals in the Ca II line were separated. This line samples the middle chromosphere in the core and the middle photosphere in the wings. Although no indication of a penumbra is seen in the photosphere, an extended filamentary structure, both in intensity and Doppler signals, is observed in the Ca II line core. An analysis of morphological and dynamical properties of the structure shows a close similarity to a superpenumbra of a sunspot with developed penumbra. A special attention is paid to the light bridge, which is the brightest feature in the pore seen in the Ca II line centre and shows an enhanced power of chromospheric oscillations at 3-5 mHz. Although the acoustic power flux in the light bridge is five times higher than in the "quiet" chromosphere, it cannot explain the observed brightness.Comment: 6 pages, 3 figures, accepted in Journal of Physics: Conference Serie