Combined helioseismic inversions for 3D vector flows and sound-speed perturbations

Time-distance helioseismology is the method of the study of the propagation of waves through the solar interior via the travel times of those waves. The travel times of wave packets contain information about the conditions in the interior integrated along the propagation path of the wave. We introduce an improved methodology of the time-distance helioseismology which allows us to invert for a full 3D vector of flows and the sound-speed perturbations at once. Using this methodology one can also derive the mean value of the vertical component of flows and the cross-talk between the flows and the sound-speed perturbations. We used the SOLA method with a minimisation of the cross-talk as a tool for inverse modelling. In the forward model, we use Born approximation travel-time sensitivity kernels with the Model S as a background. The methodology was validated using forward-modelled travel times with both mean and difference point-to-annulus averaging geometries applied to a snapshot of fully self-consistent simulation of the convection. We tested the methodology on synthetic data. We demonstrate that we are able to recover flows and sound-speed perturbations in the near-surface layers. We have taken the advantage of the sensitivity of our methodology to entire vertical velocity, and not only to its variations as in other available methodologies. The cross-talk from both the vertical flow component and the sound-speed perturbation has only a negligible effect for inversions for the horizontal flow components. The inversions for the vertical component of the vector flows or for the sound-speed perturbations are affected by the cross-talk from the horizontal components, which needs to be minimised in order to provide valid results. It seems that there is a nearly constant cross-talk between the vertical component of the vector flows and the sound-speed perturbations.Comment: 13 pages, 18 figure

Effects of Solar Active Regions on Meridional Flows

The aim of this paper is to extend our previous study of the solar-cycle variations of the meridional flows and to investigate their latitudinal and longitudinal structure in the subphotospheric layer, especially their variations in magnetic regions. Helioseismology observations indicate that mass flows around active regions are dominated by inflows into those regions. On average, those local flows are more important around leading magnetic polarities of active regions than around the following polarities, and depend on the evolutionary stage of particular active regions. We present a statistical study based on MDI/SOHO observations of 1996-2002 and show that this effect explains a significant part of the cyclic change of meridional flows in near-equatorial regions, but not at higher latitudes. A different mechanism driving solar-cycle variations of the meridional flow probably operates.Comment: 4 pages, 5 figures, accepted for publication in ApJ

Comparison of time-distance inversion methods applied to SDO/HMI Dopplergrams

We compared the results from the JSOC pipeline for horizontal flow components and the perturbations of the speed of sound at set of depths with equivalent results from an independently implemented pipeline using a different time-distance inversion scheme. Our inversion pipeline allows inversion for all quantities at once while allowing minimisation of the crosstalk between them. This gives us an opportunity to discuss the possible biases present in the JSOC data products. For the tests we used the subtractive optimally localised averaging (SOLA) method with a minimisation of the cross-talk. We compared three test inversions for each quantity at each target depth. At first, we used the JSOC setup to reproduce the JSOC results. Subsequently, we used the extended pipeline to improve these results by incorporating more independent travel-time measurements but keeping the JSOC-indicated localisation in the Sun. Finally, we inverted for flow components and sound-speed perturbations using a localisation kernel with properties advertised in the JSOC metadata. We successfully reproduced the horizontal flow components. The sound-speed perturbations are strongly affected by the high level of the cross-talk in JSOC products. This leads to larger amplitudes in the inversions for the sound-speed perturbations. Different results were obtained when a target function localised around the target depth was used. This is a consequence of non-localised JSOC averaging kernels. We add that our methodology also allows inversion for the vertical flow.Comment: 14 pages, 15 figure

Real-time Simulation of 3 Parallel PWM Rectifiers

This paper describes the development of areal-time model up to 3 parallel PWM rectifiers and itsimplementation on FPGA using LabVIEW developmentenvironment. The main benefit of this real-time model is thefact that there is no need for a real device or a test stand fordebugging of traction drive control SW. The Hardware-inthe-Loop testing with similar RT model of an inductionmachine has already brought large financial and timesavings. Moreover, destructive states or states difficult toevoke can be tested using such a real-time model

An Estimate of Chromospheric Heating by Acoustic Waves

Several mechanisms may heat the solar chromosphere: acoustic waves, magnetoacoustic waves (slow, fast, and Alfv\u27en waves), and small-scale magnetic reconnections. Based on observations in the Ca II 854.2 nm line, the contribution of acoustic waves to the heating of quiet and plage regions in the chromosphere is discussed. The energy released by radiative losses is compared with the energy deposited by acoustic waves. Radiative losses are computed using a grid of six semi-empirical models VAL A-F. The deposited acoustic flux is calculated using power spectra of Doppler oscillations measured in the Ca~II line core. The comparison shows that the spatial correlation of maps of radiative losses and acoustic flux is 70%. The deposited acoustic flux provides at least 25-30% of the energy radiated in the quiet chromosphere and 50% in plage regions

Reconstruction of Solar Subsurfaces by Local Helioseismology

Local helioseismology has opened new frontiers in our quest for understanding of the internal dynamics and dynamo on the Sun. Local helioseismology reconstructs subsurface structures and flows by extracting coherent signals of acoustic waves traveling through the interior and carrying information about subsurface perturbations and flows, from stochastic oscillations observed on the surface. The initial analysis of the subsurface flow maps reconstructed from the 5 years of SDO/HMI data by time-distance helioseismology reveals the great potential for studying and understanding of the dynamics of the quiet Sun and active regions, and the evolution with the solar cycle. In particular, our results show that the emergence and evolution of active regions are accompanied by multi-scale flow patterns, and that the meridional flows display the North-South asymmetry closely correlating with the magnetic activity. The latitudinal variations of the meridional circulation speed, which are probably related to the large-scale converging flows, are mostly confined in shallow subsurface layers. Therefore, these variations do not necessarily affect the magnetic flux transport. The North-South asymmetry is also pronounced in the variations of the differential rotation ("torsional oscillations"). The calculations of a proxy of the subsurface kinetic helicity density show that the helicity does not vary during the solar cycle, and that supergranulation is a likely source of the near-surface helicity.Comment: 17 pages, 10 figures, in "Cartography of the Sun and the Stars", Editors: Rozelot, Jean-Pierre, Neiner, Corali

Differential rotation in magnetic chemically peculiar stars

Magnetic chemically peculiar (mCP) stars constitute about 10% of upper-main-sequence stars and are characterized by strong magnetic fields and abnormal photospheric abundances of some chemical elements. Most of them exhibit strictly periodic light, magnetic, radio, and spectral variations that can be fully explained by a rigidly rotating main-sequence star with persistent surface structures and a stable global magnetic field. Long-term observations of the phase curves of these variations enable us to investigate possible surface differential rotation with unprecedented accuracy and reliability. The analysis of the phase curves in the best-observed mCP stars indicates that the location and the contrast of photometric and spectroscopic spots as well as the geometry of the magnetic field remain constant for at least many decades. The strict periodicity of mCP variables supports the concept that the outer layers of upper-main-sequence stars do not rotate differentially. However, there is a small, inhomogeneous group consisting of a few mCP stars whose rotation periods vary on timescales of decades. The period oscillations may reflect real changes in the angular velocity of outer layers of the stars which are anchored by their global magnetic fields. In CU Vir, V901 Ori, and perhaps BS Cir, the rotational period variation indicates the presence of vertical differential rotation; however, its exact nature has remained elusive until now. The incidence of mCP stars with variable rotational periods is currently investigated using a sample of fifty newly identified Kepler mCP stars

Comparison of large-scale flows on the Sun measured by time-distance helioseismology and local correlation tracking technique

We present a direct comparison between two different techniques time-distance helioseismology and a local correlation tracking method for measuring mass flows in the solar photosphere and in a near-surface layer: We applied both methods to the same dataset (MDI high-cadence Dopplergrams covering almost the entire Carrington rotation 1974) and compared the results. We found that after necessary corrections, the vector flow fields obtained by these techniques are very similar. The median difference between directions of corresponding vectors is 24 degrees, and the correlation coefficients of the results for mean zonal and meridional flows are 0.98 and 0.88 respectively. The largest discrepancies are found in areas of small velocities where the inaccuracies of the computed vectors play a significant role. The good agreement of these two methods increases confidence in the reliability of large-scale synoptic maps obtained by them.Comment: 14 pages, 6 figures, just before acceptance in Solar Physic