Ring-diagram Analysis Research Articles

Average Subsurface Flows of Active and Quiet Regions with Large-Tile Ring-Diagram Analysis

Average Subsurface Flows of Active and Quiet Regions with Large-Tile Ring-Diagram Analysis

On Strengthening of the Solar f-Mode Prior to Active Region Emergence Using the Fourier-Hankel Analysis

Recent results of Singh, Raichur, and Brandenburg (2016) show that the emergence of an active region (AR) can be seen in a strengthening of the f-mode power up to two days prior of the region’s formation. In the original work, ring diagram analysis was used to estimate the power evolution. In this study, we make use of the Fourier-Hankel method, essentially testing the aforementioned results with an independent method. The data are acquired from SDO/HMI, studying the ARs 11158, 11072, 11105, 11130, 11242 and 11768. Investigating the total power as a function of time, we find a similar behavior to the original work for four of the six listed ARs, which is an enhancement of f-mode power about one to three days prior to AR emergence. AR 11105 as well as AR 11242 also show detectable f-mode enhancement, although less significant than what was observed before. In addition, the analysis of the absorption coefficient $\alpha $ , yielded by the Fourier-Hankel analysis, shows neither absorption ( $\alpha > 0$ ) nor emission ( $\alpha < 0$ ) of power during the enhancement. Finding no changes of the absorption coefficient (i.e., $\alpha = 0$ ) is an important result, as it narrows down the possible physical interpretation of the original f-mode power enhancement, showing that no directional dependence (in the sense of inward and outward moving waves) is present.

Radial gradient of the solar rotation rate in the near-surface shear layer of the Sun

We study the radial gradient of the solar rotation rate in the near-surface shear layer (NSSL) from about .950 R⊙ to the solar surface and its variation during Solar Cycles 23 and 24 with ring-diagram analysis applied to Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) Dopplergrams. The average radial gradient is ∂ log Ω/∂ log r = − 1.0 ± .1 at .990 R⊙ in agreement with previous studies. The average radial gradient is ∂ log Ω/∂ log r = − .11 ± .01 at the base of the NSSL at .950R⊙, while it is steeper than −1 closer to the surface between .990R⊙ and .997R⊙. The average radial gradient is rather flat within ±40° latitude from about .970 R⊙ to the solar surface. The radial gradient of the solar rotation rate varies with the solar cycle. At locations of high magnetic activity, the radial gradient is more negative than average from about .970 R⊙ to .990 R⊙, while in quiet regions the radial gradient is less negative than average at these depths. Close to the surface at .997 R⊙, this relationship appears to be reversed. Prominent features of the solar-cycle variation of large-scale flows, such as poleward branches or precursor flows, are not obviously present. The variation of the radial gradient thus more likely indicates the presence or absence of magnetic flux above a certain threshold. The temporal variations derived from the different HMI and GONG data sets agree within one error bar at most depths and latitudes, while their amplitudes might be different.

Sub-surface plasma flows and the flare productivity of solar active regions

The extreme space weather conditions resulting from high energetic events likes solar flares and Coronal Mass Ejections demand for reliable space weather forecasting. The magnetic flux tubes while rising through the convection zone gets twisted by the turbulent plasma flows, energizing the system and resulting in flares. We investigate the relationship between the subsurface plasma flows associated with flaring active regions and their surface magnetic flux and current helicity. The near-surface horizontal velocities derived from the ring-diagram analysis of active region patches using Global Oscillation Network Group Doppler velocity measurements are used to compute the fluid dynamics descriptors like vertical divergence, vorticity and kinetic helicity used in this work. The flaring active regions are observed to have large value of vertical vorticity and kinetic helicity. Also, the horizontal flow divergence, vorticity, flux, kinetic and current helicities are observed to be significantly correlated and evolve in phase with each other. We observe that the integrated values of the above flow and magnetic parameters observed 1 day prior to the flare are significantly correlated with the integrated flare intensity of the active region. Hence, we show that strong vorticity/kinetic helicities lead to larger active region twisting, presumably generating high-intensity flares.

HMI Ring-Diagram Analysis: Effects of Tracking, Noise, and Resolution

Ring-diagram analysis is a standard local helioseismic technique. Data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) are routinely used for ring-diagram analysis, and fits to the power spectra as well as inversion results are standard data products. In this article we examine the effects of different tracking rates, noise, and resolution on ring-diagram results. Most of the analysis is for \(15^{\circ}\) tiles, but we also examine the effects of different tile sizes. The largest effect that we find is that of spatial resolution. Doppler noise has very little effect on the results, except perhaps at the deepest regions for which the tiles can give reliable results; variations in the tracking rate have a similar effect.

Subsurface Horizontal Flows During Solar Cycles 24 and 25 with Large-Tile Ring-Diagram Analysis

Subsurface Horizontal Flows During Solar Cycles 24 and 25 with Large-Tile Ring-Diagram Analysis

Effects of flares on solar high-degree helioseismic acoustic mode amplitudes

ABSTRACT Several attempts have been made to observe whether solar flares excite acoustic modes since Charles Wolff suggested this possibility almost 50 yr ago. We look for the impact of flares on the amplitude of solar acoustic modes and other effects that are also affecting the mode amplitude. Solar acoustic mode amplitudes are known to be sensitive to magnetic fields. As flares usually occur in the presence of strong magnetic fields and most likely are the by-product of magnetic reconnection, we show how the magnetic field in and around the flaring region affects the mode amplitude. The mode amplitudes were obtained using ring-diagram analysis, which was first applied to a single event, the largest flare in the space age (the ‘Halloween Flare’, SOL2003-10-28T11:00), using MDI (Michelson Doppler Imager) data. Then, using HMI (Helioseismic and Magnetic Imager) data, the analysis was applied to the regions corresponding to the flares observed during the high-activity phase of cycle 24 and that fall into two groups. These two groups consist of small (10–60 erg cm−2 s−1) and large (&amp;gt;1200 erg cm−2 s−1) peak-flux flares, based on the Heliophysics Event Knowledgebase (HEK). The first set is used as a comparison to the results of the strong flares in the second set. After applying several corrections in order to take into account several sources of bias, we did not find any amplification in the inferred mode amplitude due to flaring activity larger than a 10 per cent uncertainty.

Divergence and Vorticity of Subsurface Flows During Solar Cycles 23 and 24

We study the solar-cycle variation of the divergence and vorticity of subsurface horizontal flows from the surface to a depth of 16 Mm. The flows were derived with ring-diagram analysis applied to Michelson Doppler Imager (MDI) Dynamics Program, Global Oscillation Network Group (GONG), and Helioseismic and Magnetic Imager (HMI) Dopplergrams. We study their variation for the complete data set and for two subsets representing active and quiet regions. All three data sets show alternating bands of diverging and converging flows and bands of cyclonic and anticyclonic flows moving from mid-latitudes toward the equator during a solar cycle. For Solar Cycle 24, these bands are precursors of the magnetic activity appearing several years before magnetic activity is present at a given latitude even leading the fast bands of the flows. The amplitude differences between the cyclonic and anticyclonic and the converging and diverging bands during a solar cycle agree within the error bars between the complete data set and the two subsets. For Solar Cycle 24, the amplitude differences are $6.0 \pm 0.7 \;10^{-8}~\text{s}^{-1}$ for the bands of vorticity and $-4.9 \pm 0.6\;10^{-8}~\text{s}^{-1}$ for those of divergence averaged over 2.0 – 11.6 Mm using the complete data set. The amplitude differences of Solar Cycle 23 are $26 \pm 3$ % smaller than those of Solar Cycle 24. The flows of the active-region subset are more converging and cyclonic than those of the quiet-region subset with an extra vorticity of $1.3 \pm 0.1 \;10^{-8}~\text{s}^{-1}$ and an extra divergence of $-6.7 \pm 0.3\;10^{-8}~\text{s}^{-1}$ averaged over 7.5∘ – 30∘ and all depths and epochs. The amplitude of the extra divergence of active regions is about a factor of 1.3 larger at depths shallower than 6 Mm and decreases with increasing depth, while the extra vorticity is nearly constant with depth.

Solar east-west flow correlations that persist for months at low latitudes are dominated by active region inflows

Context. Giant-cell convection is believed to be an important component of solar dynamics. For example, it is expected to play a crucial role in maintaining the Sun’s differential rotation. Aims. We reexamine early reports of giant convective cells detected using a correlation analysis of Dopplergrams. We extend this analysis using 19 years of space- and ground-based observations of near-surface horizontal flows. Methods. Flow maps are derived through the local correlation tracking of granules and helioseismic ring-diagram analysis. We compute temporal auto-correlation functions of the east-west flows at fixed latitude. Results. Correlations in the east-west velocity can be clearly seen up to five rotation periods. The signal consists of features with longitudinal wavenumbers up to m = 9 at low latitudes. Comparison with magnetic images indicates that these flow features are associated with magnetic activity. The signal is not seen above the noise level during solar minimum. Conclusions. Our results show that the long-term correlations in east-west flows at low latitudes are predominantly due to inflows into active regions and not to giant convective cells.

Solar-Cycle Variation of the Subsurface Flows of Active- and Quiet-Region Subsets

We study the solar-cycle variation of subsurface flows for both active and quiet solar regions. We derive flows from the surface to a depth of 16 Mm using ring-diagram analysis applied to Dopplergrams obtained with the Michelson Doppler Imager (MDI) Dynamics Program, the Global Oscillation Network Group (GONG), and the Helioseismic and Magnetic Imager (HMI) instrument. We derive the temporal variation of the zonal and meridional flows in a consistent manner for Solar Cycles 23 and 24 combining the flows from the three data sources scaled to match HMI-derived flows. The subsurface flows associated with active and quiet regions show the same variation with the solar cycle with alternating bands of faster- and slower-than-average zonal and meridional flows moving from mid-latitudes toward the equator during the course of a cycle. We derive the differences between the amplitudes of the extrema of the fast and the slow flows. For Cycle 24, the average difference between the fast- and slow-flow amplitude is $9.5 \pm 0.5~\text{m}\,\text{s}^{-1}$ for the zonal flows and $7.0 \pm 0.4~\text{m}\,\text{s}^{-1}$ for the meridional flows of the quiet-region subset averaged over 2 to 12 Mm within $\pm 30^{\circ}$ latitude. For the active-region subset, the average difference is $10.4 \pm 0.9~\text{m}\,\text{s}^{-1}$ for the zonal flows and $9.3 \pm 0.7~\text{m}\,\text{s}^{-1}$ for the meridional flows. We subtract the flows of the quiet-region subset from those of the active-region one to determine the contribution of active regions to the long-term flow pattern. The resulting meridional flow associated with active regions has a maximum amplitude near 3.1 Mm and its amplitude decreases with depth. This implies that the converging flows attributed to active regions are a shallow-layer phenomenon.

Solar Rossby waves observed in GONG++ ring-diagram flow maps

Context. Solar Rossby waves have only recently been unambiguously identified in Helioseimsic and Magnetic Imager (HMI) and Michelson Doppler Imager maps of flows near the solar surface. So far this has not been done with the Global Oscillation Network Group (GONG) ground-based observations, which have different noise properties. Aims. We use 17 years of GONG++ data to identify and characterize solar Rossby waves using ring-diagram helioseismology. We compare directly with HMI ring-diagram analysis. Methods. Maps of the radial vorticity were obtained for flows within the top 2 Mm of the surface for 17 years of GONG++ data. The data were corrected for systematic effects including the annual periodicity related to the B0 angle. We then computed the Fourier components of the radial vorticity of the flows in the co-rotating frame. We performed the same analysis on the HMI data that overlap in time. Results. We find that the solar Rossby waves have measurable amplitudes in the GONG++ sectoral power spectra for azimuthal orders between m = 3 and m = 15. The measured mode characteristics (frequencies, lifetimes, and amplitudes) from GONG++ are consistent with the HMI measurements in the overlap period from 2010 to 2018 for m ≤ 9. For higher-m modes the amplitudes and frequencies agree within two sigmas. The signal-to-noise ratio of modes in GONG++ power spectra is comparable to those of HMI for 8 ≤ m ≤ 11, but is lower by a factor of two for other modes. Conclusions. The GONG++ data provide a long and uniform data set that can be used to study solar global-scale Rossby waves from 2001.

Exploring the latitude and depth dependence of solar Rossby waves using ring-diagram analysis

Context. Global-scale equatorial Rossby waves have recently been unambiguously identified on the Sun. Like solar acoustic modes, Rossby waves are probes of the solar interior. Aims. We study the latitude and depth dependence of the Rossby wave eigenfunctions. Methods. By applying helioseismic ring-diagram analysis and granulation tracking to observations by HMI aboard SDO, we computed maps of the radial vorticity of flows in the upper solar convection zone (down to depths of more than 16 Mm). The horizontal sampling of the ring-diagram maps is approximately 90 Mm (∼7.5°) and the temporal sampling is roughly 27 hr. We used a Fourier transform in longitude to separate the different azimuthal orders m in the range 3 ≤ m ≤ 15. At each m we obtained the phase and amplitude of the Rossby waves as functions of depth using the helioseismic data. At each m we also measured the latitude dependence of the eigenfunctions by calculating the covariance between the equator and other latitudes. Results. We conducted a study of the horizontal and radial dependences of the radial vorticity eigenfunctions. The horizontal eigenfunctions are complex. As observed previously, the real part peaks at the equator and switches sign near ±30°, thus the eigenfunctions show significant non-sectoral contributions. The imaginary part is smaller than the real part. The phase of the radial eigenfunctions varies by only ±5° over the top 15 Mm. The amplitude of the radial eigenfunctions decreases by about 10% from the surface down to 8 Mm (the region in which ring-diagram analysis is most reliable, as seen by comparing with the rotation rate measured by global-mode seismology). Conclusions. The radial dependence of the radial vorticity eigenfunctions deduced from ring-diagram analysis is consistent with a power law down to 8 Mm and is unreliable at larger depths. However, the observations provide only weak constraints on the power-law exponents. For the real part, the latitude dependence of the eigenfunctions is consistent with previous work (using granulation tracking). The imaginary part is smaller than the real part but significantly nonzero.

An improved multi-ridge fitting method for ring-diagram helioseismic analysis

Context.There is a wide discrepancy in current estimates of the strength of convection flows in the solar interior obtained using different helioseismic methods applied to observations from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The cause for these disparities is not known.Aims.As one step in the effort to resolve this discrepancy, we aim to characterize the multi-ridge fitting code for ring-diagram helioseismic analysis that is used to obtain flow estimates from local power spectra of solar oscillations.Methods.We updated the multi-ridge fitting code developed by Greer et al. (2014, Sol. Phys., 289, 2823) to solve several problems we identified through our inspection of the code. In particular, we changed the (1) merit function to account for the smoothing of the power spectra, (2) model for the power spectrum, and (3) noise estimates. We used Monte Carlo simulations to generate synthetic data and to characterize the noise and bias of the updated code by fitting these synthetic data.Results.The bias in the output fit parameters, apart from the parameter describing the amplitude of thep-mode resonances in the power spectrum, is below what can be measured from the Monte-Carlo simulations. The amplitude parameters are underestimated; this is a consequence of choosing to fit the logarithm of the averaged power. We defer fixing this problem as it is well understood and not significant for measuring flows in the solar interior. The scatter in the fit parameters from the Monte-Carlo simulations is well-modeled by the formal error estimates from the code.Conclusions.We document and demonstrate a reliable multi-ridge fitting method for ring-diagram analysis. The differences between the updated fitting results and the original results are less than one order of magnitude and therefore we suspect that the changes will not eliminate the aforementioned orders-of-magnitude discrepancy in the amplitude of convective flows in the solar interior.

Time-distance helioseismology of solar Rossby waves

Context. Solar Rossby waves (r modes) have recently been discovered in the near-surface horizontal flow field using the techniques of granulation-tracking and ring-diagram analysis applied to six years of SDO/HMI data. Aims. Here we apply time-distance helioseismology to the combined SOHO/MDI and SDO/HMI data sets, which cover 21 years of observations from May 1996 to April 2017. The goal of this study is to provide an independent confirmation over two solar cycles and in deeper layers of the Sun. Methods. We have measured south-north helioseismic travel times along the equator, which are sensitive to subsurface north-south flows. To reduce noise, the travel times were averaged over travel distances from 6° to 30°; the mean distance corresponds to a p-mode lower turning point of 0.91 R⊙. The 21-year time series of travel-time measurements was split into three seven-year subsets and transformed to obtain power spectra in a corotating frame. Results. The power spectra all show peaks near the frequencies of the classical sectoral Rossby waves for azimuthal wavenumbers in the range 3 ≤ m ≤ 15. The mode frequencies and linewidths of the modes with m ≤ 9 are consistent with a previous study whereas modes with m ≥ 10 are shifted toward less negative frequencies by 10–20 nHz. While most of these modes have e-folding lifetimes on the order of a few months, the longest lived mode, m = 3, has an e-folding lifetime of more than one year. For each mode, the rms velocity at the equator is in the range of 1–3 m s−1, with the largest values for m ∼ 10. No evidence for the m = 2 sectoral mode is found in the power spectrum, implying that the rms velocity of this mode is below ∼0.5 m s−1. Conclusions. This work confirms the existence of equatorial global Rossby waves in the solar interior over the past two solar cycles and shows that time-distance helioseismology is a promising technique to study them deep in the convection zone.

Supervised neural networks for helioseismic ring-diagram inversions

Context. The inversion of ring fit parameters to obtain subsurface flow maps in ring-diagram analysis for eight years of SDO observations is computationally expensive, requiring ∼3200 CPU hours. Aims. In this paper we apply machine-learning techniques to the inversion step of the ring diagram pipeline in order to speed up the calculations. Specifically, we train a predictor for subsurface flows using the mode fit parameters and the previous inversion results to replace future inversion requirements. Methods. We utilize artificial neural networks (ANNs) as a supervised learning method for predicting the flows in 15° ring tiles. We discuss each step of the proposed method to determine the optimal approach. In order to demonstrate that the machine-learning results still contain the subtle signatures key to local helioseismic studies, we use the machine-learning results to study the recently discovered solar equatorial Rossby waves. Results. The ANN is computationally efficient, able to make future flow predictions of an entire Carrington rotation in a matter of seconds, which is much faster than the current ∼31 CPU hours. Initial training of the networks requires ∼3 CPU hours. The trained ANN can achieve a rms error equal to approximately half that reported for the velocity inversions, demonstrating the accuracy of the machine learning (and perhaps the overestimation of the original errors from the ring-diagram pipeline). We find the signature of equatorial Rossby waves in the machine-learning flows covering six years of data, demonstrating that small-amplitude signals are maintained. The recovery of Rossby waves in the machine-learning flow maps can be achieved with only one Carrington rotation (27.275 days) of training data. Conclusions. We show that machine learning can be applied to and perform more efficiently than the current ring-diagram inversion. The computation burden of the machine learning includes 3 CPU hours for initial training, then around 10−4 CPU hours for future predictions.

Detection of Rossby Waves in the Sun using Normal-mode Coupling

Abstract Rossby waves play a fundamental role in angular momentum processes in rotating fluids. In addition to the potential to shed light on physical mechanisms operating in the solar convection zone, the recent detection of Rossby waves in the Sun also serves as a means of comparison between different helioseismic methods. Time–distance helioseismology, ring-diagram analysis, and other techniques have all proven successful in recovering the Rossby-wave dispersion relation from analyses of Helioseismic and Magnetic Imager data (HMI). In this article, we demonstrate that analyses of two years of HMI global-mode-oscillation data using the technique of normal-mode coupling also show signatures of Rossby waves. In addition to providing an independent means of inferring Rossby waves, this detection lends credence to the methodology of mode coupling and encourages a more complete exploration of its possibilities.

Subsurface Zonal and Meridional Flow During Cycles 23 and 24

We study the solar-cycle variation of subsurface flows from the surface to a depth of 16 Mm. We have used ring-diagram analysis to analyze Dopplergrams obtained with the Michelson Doppler Imager (MDI) Dynamics Program, the Global Oscillation Network Group (GONG), and the Helioseismic and Magnetic Imager (HMI) instrument. We combined the zonal and meridional flows from the three data sources and scaled the flows derived from MDI and GONG to match those from HMI observations. In this way, we derived their temporal variation in a consistent manner for Solar Cycles 23 and 24. We have corrected the measured flows for systematic effects that vary with disk positions. Using time-depth slices of the corrected subsurface flows, we derived the amplitudes and times of the extrema of the fast and slow zonal and meridional flows during Cycles 23 and 24 at every depth and latitude. We find an average difference between maximum and minimum amplitudes of $8.6 \pm0.4~\mbox{m}\,\mbox{s}^{-1}$ for the zonal flows and $7.9 \pm0.3~\mbox{m}\,\mbox{s}^{-1}$ for the meridional flows associated with Cycle 24 averaged over a depth range from 2 to 12 Mm. The corresponding values derived from GONG data alone are $10.5 \pm0.3~\mbox{m}\,\mbox{s}^{-1}$ for the zonal and $10.8 \pm0.3~\mbox{m}\,\mbox{s}^{-1}$ for the meridional flow. For Cycle 24, the flow patterns are precursors of the magnetic activity. The timing difference between the occurrence of the flow pattern and the magnetic one increases almost linearly with increasing latitude. For example, the fast zonal and meridional flow appear $2.1 \pm 0.6$ years and $2.5\pm 0.6$ years, respectively, before the magnetic pattern at $30^{\circ}$ latitude in the northern hemisphere, while in the southern hemisphere, the differences are $3.2 \pm 1.2$ years and $2.6 \pm 0.6$ years. The flow patterns of Cycle 25 are present and have reached $30^{\circ}$ latitude. The amplitude differences of Cycle 25 are about 22% smaller than those of Cycle 24, but are comparable to those of Cycle 23. Moreover, polynomial fits of meridional flows suggest that equatorward meridional flows (counter-cells) might exist at about $80^{\circ}$ latitude except during the declining phase of the solar cycle.

Asymmetry in Solar Torsional Oscillation and the Sunspot Cycle

Abstract Solar torsional oscillations are migrating bands of slower- and faster-than-average rotation, which are strongly related to the Sun’s magnetic cycle. We perform a long-term study (16 yr) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data for the first time. We study the north–south asymmetry in the velocity using the zonal flow velocities obtained by ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find significant hemispherical asymmetry in the torsional oscillation velocity and explore its variation with respect to depth, time, and latitude. We also calculate the hemispherical asymmetry in the surface velocity measurements from the Mount Wilson Observatory and the zonal flow velocities obtained from the Helioseismic and Magnetic Imager ring diagram pipeline. These asymmetries are found to be consistent with the asymmetry obtained from GONG observations. We show that the asymmetry in near-surface torsional oscillation velocity is correlated with the asymmetry in magnetic flux and sunspot number at the solar surface, with the velocity asymmetry preceding the flux and sunspot number asymmetries. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in sunspot cycles.

Observational Evidence of Shallow Origins for the Magnetic Fields of Solar Cycles

Observational evidence for the origin of active region magnetic fields has been sought from published information on extended solar cycles, statistical distributions of active regions and ephemeral regions, helioseismology results, positional relationships to supergranules, and fine-scale magnetic structure of active regions and their sunspots during their growth. Statistical distributions of areas of ephemeral and active regions blend together to reveal a single power law. The shape of the size distribution in latitude of all active regions is independent of time during the solar cycle, yielding further evidence that active regions of all sizes belong to the same population. Elementary bipoles, identified also by other names, appear to be the building blocks of active regions; sunspots form from elementary bipoles and are therefore deduced to develop from the photosphere downward, consistent with helioseismic detection of downflows to 3-4 Mm below sunspots as well as long-observed downflows from chromospheric/coronal arch filaments into sunspots from their earliest appearance. Time-distance helioseismology has been effective in revealing flows related to sunspots to depths of 20 Mm. Ring diagram analysis shows a statistically significant preference for upflows to precede major active region emergence and downflows after flux emergence but both are often observed together or sometimes not detected. From deep-focus helioseismic techniques for seeking magnetic flux below the photosphere prior major active regions, there is evidence of acoustic travel-time perturbation signatures rising in the limited range of depths of 42-75 Mm but these have not been verified or found at more shallow depths by helioseismic holographic techniques. The development of active regions from clusters of elementary bipoles appears to be the same irrespective of how much flux an active region eventually develops. This property would be consistent with the magnetic fields of large active regions being generated in the same way and close the same depth as small active regions in a shallow zone below the photosphere. All evidence considered together, understanding the origins of the magnetic fields of solar cycles boils down to learning how and where elementary bipoles are generated beneath the photosphere.

Asymmetry in Solar Torsional Oscillation

AbstractSolar torsional oscillations are migrating bands of slower and faster than average rotation, which are thought to be related to the Sun’s magnetic cycle. We perform the first long-term study (16 years) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data. We explore the spatial and temporal variation of North-South asymmetry using zonal flow velocities obtained from ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find a strong correlation between the asymmetries of near-surface torsional oscillation with magnetic flux and sunspot number, with the velocity asymmetry preceding in both the cases. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in the sunspot cycle.