The Vorticity of Solar Photospheric Flows on the Scale of Granulation

We employ time sequences of images observed with a G-band filter (4305{\AA}) by the Solar Optical Telescope (SOT) on board of Hinode spacecraft at different latitude along solar central me-ridian to study vorticity of granular flows in quiet Sun areas during deep minimum of solar activity. Using a feature correlation tracking (FCT) technique, we calculate the vorticity of granular-scale flows. Assuming the known pattern of vertical flows (upward in granules and downward in inter-granular lanes), we infer the sign of kinetic helicity of these flows. We show that the kinetic helicity of granular flows and intergranular vortices exhibits a weak hemispheric preference, which is in agreement with the action of the Coriolis force. This slight hemispheric sign asymmetry, however, is not statistically significant given large scatter in the average vorticity. The sign of the current he-licity density of network magnetic fields computed using full disk vector magnetograms from the Synoptic Optical Long-term Investigations of the Sun (SOLIS) does not show any hemispheric preference. The combination of these two findings suggests that the photospheric dynamo operating on the scale of granular flows is non-helical in nature.Comment: 11 pages, 5 figures, in press Geomagnetism and Aeronom

Magnetic helicity of global field in cycles 23 and 24

For the first time we reconstruct the magnetic helicity density of global axisymmetric field of the Sun using method proposed by Brandenburg et al. (2003) and Pipin et al. (2013). To determine the components of the vector potential, we apply the gauge which is typically employed in mean-field dynamo models. This allows for a direct comparison of reconstructed helicity with the predictions from the mean-field dynamo models. We apply the method to two different data sets: the synoptic maps of line-of-sight (LOS) magnetic field from the Michelson Doppler Imager (MDI) on board of Solar and Heliospheric Observatory (SOHO) and vector magnetic field measurements from Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) system. Based on the analysis of MDI/SOHO data, we find that in solar cycle 23 the global magnetic field had positive (negative) magnetic helicity in the northern (southern) hemisphere. This hemispheric sign asymmetry is opposite to helicity of solar active regions, but it is in agreement with the predictions of mean-field dynamo models. The data also suggest that the hemispheric helicity rule may have reversed its sign in early and late phases of cycle 23. Furthermore, the data indicate an imbalance in magnetic helicity between the northern and southern hemispheres. This imbalance seem to correlate with the total level of activity in each hemisphere in cycle 23. Magnetic helicity for rising phase of cycle 24 is derived from SOLIS/VSM data, and qualitatively, its latitudinal pattern is similar to the pattern derived from SOHO/MDI data for cycle 23.Comment: 25 pages, 9 figures, accepted for Ap

Signature of Differential Rotation in Sun-as-a-Star Ca II K Measurements

The characterization of solar surface differential rotation (SDR) from disk-integrated chromospheric measurements has important implications for the study of differential rotation and dynamo processes in other stars. Some chromospheric lines, such as Ca II K, are very sensitive to the presence of activity on the disk and are an ideal choice for investigating SDR in Sun-as-a star observations. Past studies indicate that when the activity is low, the determination of Sun's differential rotation from integrated-sunlight measurements becomes uncertain. However, our study shows that using the proper technique, SDR can be detected from these type of measurements even during periods of extended solar minima. This paper describes results from the analysis of the temporal variations of Ca II K line profiles observed by the Integrated Sunlight Spectrometer (ISS) during the declining phase of Cycle 23 and the rising phase of Cycle 24, and discusses the signature of SDR in the power spectra computed from time series of parameters derived from these profiles. The described methodology is quite general, and could be applied to photometric time series of other Main-Sequence stars for detecting differential rotation.Comment: Accepted for publication in The Astrophysical Journal. 38 pages, 10 figure

Bimodal Distribution of Magnetic Fields and Areas of Sunspots

We applied automatic identification of sunspot umbrae and penumbrae to daily observations from the Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The results confirm a previously known logarithmic relationship between the area of sunspots and their maximum flux density. In addition, we find that the relation between average magnetic flux (Bavg) and sunspot area shows a bimodal distribution: for small sunspots and pores (A < 20 millionth of solar hemisphere, MSH), Bavg = 800 G (gauss), and for large sunspots (A > 100 MSH), Bavg is about 600 G. For intermediate sunspots, average flux density linearly decreases from about 800 G to 600 G. A similar bimodal distribution was found in several other integral parameters of sunspots. We show that this bimodality can be related to different stages of sunspot penumbra formation and can be explained by the difference in average inclination of magnetic fields at the periphery of small and large sunspots.Comment: 13 pages, 5 figures, accepted for publication in Solar Physic

Latitude of Ephemeral Regions as Indicator of Strength of Solar Cycles

Digitized images of full disk CaK spectroheliograms from two solar observatories were used to study cycle variation of ephemeral regions (ERs) over ten solar cycles 14-23. We calculate monthly averaged unsigned latitude of ERs and compare it with annual sunspot number. We find that average latitude of ERs can be used as a predictor for strength of solar cycle. For a short-term prediction (dT about 1-2 years), maximum latitude of ephemeral regions (in current cycle) defines the amplitude of that cycle (higher is the latitude of ERs, larger are the amplitudes of sunspot cycle). For a long-term prediction (dT about 1.5 solar cycles), latitude of ERs at declining phase of n-th cycle determines the amplitude of (n+2)-th sunspot cycle (lower is the latitude of ERs, stronger is the cycle). Using this latter dependency, we forecast the amplitude of sunspot cycle 24 at W=92 +/- 13 (in units of annual sunspot number).Comment: Memorie della Societ\`a Astronomica Italiana, in pres

Comparison of Ground- and Space-based Longitudinal Magnetograms

We compare photospheric line-of-sight magnetograms from the Synoptic Long-term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM) instrument with observations from the 150-foot Solar Tower at Mt. Wilson (MWO), Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO), and Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SOHO). We find very good agreement between VSM and the other data sources for both disk-averaged flux densities and pixel-by-pixel measurements. We show that the VSM mean flux density time series is of consistently high signal-to-noise with no significant zero-offsets. We discuss in detail some of the factors -spatial resolution, flux dependence and position on the solar disk- affecting the determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The VSM flux densities agree well with spatially smoothed data from MDI and HMI, although the scaling factors show clear dependence on flux density. The factor to convert VSM to HMI increases with increasing flux density (from $\approx$1 to $\approx$1.5). The nonlinearity is smaller for the VSM vs. ~SOHO/MDI scaling factor (from $\approx$1 to $\approx$1.2).Comment: Accepted for publication in Solar Physic

First Synoptic Maps of Photospheric Vector Magnetic Field from SOLIS/VSM: Non-Radial Magnetic Fields and Hemispheric Pattern of Helicity

We use daily full-disk vector magnetograms from Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) system to synthesize the first Carrington maps of the photospheric vector magnetic field. We describe these maps and make a comparison of observed radial field with the radial field estimate from LOS magnetograms. Further, we employ these maps to study the hemispheric pattern of current helicity density, Hc, during the rising phase of the solar cycle 24. Longitudinal average over the 23 consecutive solar rotations shows a clear signature of the hemispheric helicity rule, i.e. Hc is predominantly negative in the North and positive in South. Although our data include the early phase of cycle 24, there appears no evidence for a possible (systematic) reversal of the hemispheric helicity rule at the beginning of cycle as predicted by some dynamo models. Further, we compute the hemispheric pattern in active region latitudes (-30 deg \le \theta \le 30 deg) separately for weak (1001000 G) radial magnetic fields. We find that while the current helicity of strong fields follows the well-known hemispheric rule (i.e., \theta . Hc < 0), H_c of weak fields exhibits an inverse hemispheric behavior (i.e., \theta . Hc > 0) albeit with large statistical scatter. We discuss two plausible scenarios to explain the opposite hemispheric trend of helicity in weak and strong field region.Comment: 15 pages, 7 figures, to appear in Astrophysical Journa

Search for a Signature of Twist-Removal in the Magnetic Field of Sunspots in Relation with Major Flares

We investigate the restructuring of the magnetic field in sunspots associated with two flares: the X6.5 flare on 6 December 2006 and the X2.2 flare on 15 February 2011. The observed changes were evaluated with respect to the so-called twist-removal model, in which helicity (twist) is removed from the corona as the result of an eruption. Since no vector magnetograms were available for the X6.5 flare, we applied the azimuthal symmetry approach to line-of-sight magnetograms to reconstruct the pseudo-vector magnetic field and investigate the changes in average twist and inclination of magnetic field in the sunspot around the time of the flare. For the X2.2 flare, results from the full vector magnetograms were compared with the pseudo-vector field data. For both flares, the data show changes consistent with the twist-removal scenario. We also evaluate the validity of the azimuthal symmetry approach on simple isolated round sunspots. In general, the derivations based on the azimuthal symmetry approach agree with true-vector field data though we find that even for symmetric sunspots the distribution of the magnetic field may deviate from an axially symmetric distribution.Comment: accepted for publication in the Ap

Uncertainties in Solar Synoptic Magnetic Flux Maps

Magnetic flux synoptic charts are critical for reliable modeling of the corona and heliosphere. Until now, however, these charts were provided without any estimate of uncertainties. The uncertainties are due to instrumental noise in the measurements and to the spatial variance of the magnetic flux distribution that contributes to each bin in the synoptic chart. We describe here a simple method to compute synoptic magnetic flux maps and their corresponding magnetic flux spatial variance charts that can be used to estimate the uncertainty in the results of coronal models. We have tested this approach by computing a potential-field-source-surface model of the coronal field for a Monte Carlo simulation of Carrington synoptic magnetic flux maps generated from the variance map. We show that these uncertainties affect both the locations of source-surface neutral lines and the distributions of coronal holes in the models.Comment: 15 pages, 7 figures. Accepted for publication in "Solar Physics

Camera Gap Removal in SOLIS/VSM Images

The Vector Spectromagnetograph (VSM) instrument on the Synoptic Optical Longterm Investigations of the Sun (SOLIS) telescope is capable of obtaining spectropolarimetry for the full Sun (or a select latitudinal range) with one arcsecond spatial resolution and 0.05 Angstrom spectral resolution. This is achieved by scanning the Sun in declination and building up spectral cubes for multiple polarization states, utilizing a beamsplitter and two separate 2k x 2k CCD cameras. As a result, the eastern and western hemispheres of the Sun are separated in preliminary VSM images by a vertical gap with soft edges and variable position and width. Prior to the comprehensive analysis presented in this document, a trial-and-error approach to removing the gap had yielded an algorithm that was inconsistent, undocumented, and responsible for incorrectly eliminating too many image columns. Here we describe, in detail, the basis for a new, streamlined, and properly calibrated prescription for locating and removing the gap that is correct to within approximately one arcsecond (one column)