arXiv subjects

M. Cristina Rabello Soares

Publications and source records attributed to M. Cristina Rabello Soares.

8 recordsLinked to original sources

arXiv2026

Correlations with Magnetic Activity in the Solar Near-Surface Shear Layer. I. Rotation

We used data from the Helioseismic and Magnetic Imager to determine the rotation rate of the near-surface shear layer and its time variation. We applied the ring-diagram analysis technique allowing us to probe the layer between the depths of 1 Mm and 17 Mm. We find that the rotation rate increases inwards; it reaches values consistent with those inferred from global helioseismic analyses in the deeper layers, however, there are differences in the rotation rate of the northern and southern hemispheres. We show that the time variation of the rotation rate can be determined even without subtracting the time-averaged rotation rate from each epoch; however, such a subtraction is needed to get the canonical ``torsional oscillation'' signal. We find that even at depths as shallow as 1 Mm, the rotation rate shows the typical torsional oscillation pattern. The cumulative zonal displacement inferred from the residual flows exhibits a pronounced high-latitude hemispheric asymmetry and varies on solar-cycle timescales; at $75^\circ$ it shows an apparent temporal association with the polar magnetic field. We find significant correlations between the cumulative displacement and magnetic activity at a subset of latitudes, with multi-year lags: the displacement leads activity by ~5 years near $15^\circ$, whereas at higher latitudes activity leads by ~4 years. At mid to high latitudes, the inferred lags show a hemispheric dependence, with activity tending to lead in the north and lag in the south, suggesting possible hemispheric differences in the timing of cycle evolution and motivating longer time series to test cycle-to-cycle variation.

astro-ph.SR↗

arXiv2026

Correlations with Magnetic Activity in the Solar Near-Surface Shear Layer. II. Radial Shear

Using Helioseismic and Magnetic Imager ring-diagram measurements and building on the rotation-rate $Ω$ inferences presented in Paper I for depths of 1-17 Mm, we examine the properties of the dimensionless radial shear $\partial\lnΩ/\partial\ln r$. In the radial range overlapping global-mode analyses, the inferred shear agrees with previous results. The near-surface shear layer exhibits a three-region shear structure with an enhanced-shear middle layer, and the largest residual variations occur in the two shallowest regions not accessible to global-mode analyses. We parameterize the enhanced-shear layer by the depth of maximum shear, its amplitude, and its width, and find all three to be strongly correlated with a magnetic activity index; increasing activity corresponds to a shallower, stronger, and modestly narrower layer, indicating that the flows and magnetic fields are interconnected in these layers. This behavior is consistent with expectations that sufficiently strong toroidal fields can enhance the near-surface rotational shear and with inferences of a near-surface toroidal-field concentration near the radius where we observe the strongest shear. Moreover, the observed strengthening and upward shift of the strong-shear layer toward solar-cycle maximum suggest a corresponding solar-cycle dependence in the location of the strong toroidal field. We also highlight the importance of finite-resolution effects and instrumental calibration in interpreting small variations in the results.

astro-ph.SR↗

arXiv2025

Structure and Dynamics of the Sun's Interior Revealed by Helioseismic and Magnetic Imager

High-resolution helioseismology observations with the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) provide a unique three-dimensional view of the solar interior structure and dynamics, revealing a tremendous complexity of the physical processes inside the Sun. We present an overview of the results of the HMI helioseismology program and discuss their implications for modern theoretical models and simulations of the solar interior.

astro-ph.SR↗

arXiv2024

Exploring Substructure of the Near-Surface Shear Layer of the Sun

The gradient of rotation in the near-surface shear layer (NSSL) of the Sun provides valuable insights into the dynamics associated with the solar activity cycle and the dynamo. Results obtained with global oscillation mode-splittings lack resolution near the surface, prompting the use of the local helioseismic ring-diagram method. While the Helioseismic and Magnetic Imager ring-analysis pipeline has been used previously for analyzing this layer, default pipeline parameters limit the accuracy of the near-surface gradients. To address these challenges, we fitted the flow parameters to power spectra averaged over one-year periods at each location, followed by additional averaging over 12 years. We find that the NSSL can be divided into three fairly distinct regions: a deeper, larger region with small shear, steepening towards the surface; a narrow middle layer with a strong shear, with a gradient approximately three times larger; and a layer very close to the surface, where the logarithmic gradient is close to zero but becomes steeper again towards the surface. The middle layer appears to be centered at 3 Mm, but the poor resolution in these layers implies that it is potentially located closer to the surface, around 1.5 Mm deep. While our analysis primarily focused on regions along the central meridian, we also investigated systematic errors at longitudes off the center. The east-west antisymmetric component of the gradient reveals a layer of substantial differences between east and west longitude around at 1.7 Mm, and the amplitude of the differences increases with longitude.

astro-ph.SR↗

arXiv2022

Blackbody temperature of 200+ stellar flares observed with the CoRoT satellite

We estimated blackbody temperature for 209 flares observed at 69 F-K stars, significantly increasing the number of flare temperature determinations. We used the Blue and Red channels obtained by the 27 cm telescope of the CoRoT satellite at high cadence and long duration. The wavelength limits of the channels were estimated using spectra from the Pickles library for the spectral type and luminosity class of each star, provided by the Exodat Database. The temperatures were obtained from the flare energy Blue-to-Red ratio, using the flare equivalent duration and stellar flux in both channels. The expected value of the analyzed flares is equal to 6,400 K with a standard deviation of 2,800 K, where the mean stellar spectral type, weighted by the number of flares in each spectral subclass, is equal to G6. Contrary to our results, a stellar white-light flare is often assumed to emit as a blackbody with a temperature of 9,000 K or 10,000 K. Our estimates agree, however, with values obtained for solar flares. The GAIA G-band transmissivity is comparable to that of the CoRoT White channel, which allows us to calibrate the flares to the Gaia photometric system. The energy in the G band of the analyzed flares varies between $10^{32}$ and $10^{37}$ erg and the flare area ranges from 30$μ$sh to 3 sh (solar hemisphere). The energy release per area in a flare is proportional to $T_{\rm flare}^{2.6}$, at least up to 10,000 K.

astro-ph.SR↗

arXiv2021

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

Several attempts have been made to observe whether solar flares excite acoustic modes since Charles Wolff suggested this possibility almost fifty years 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 data. Then, using HMI 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) and large (>1200 erg/cm^2/s) 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% uncertainty.

astro-ph.SR↗

arXiv2019

On the saturation of acoustic mode frequencies at high solar activity

Acoustic mode frequencies obtained by applying spherical harmonic decomposition to HMI, MDI and GONG observations were analysed throughout the solar cycle. Evidence of a deviation from a linear relation with solar radio flux was found indicating a saturation effect at high solar activity. The Gompertz model, which is one of the most frequently used sigmoid functions to fit growth data, is used. It is shown that its fitting to MDI and GONG data are statistically significant and a median saturation of four-hundred sfu is estimated. This saturation level is 50% larger than any obtained in the last century, hence the small effect observed in the minimum-to-maximum frequency shift. However, as shown here, it should not be disregarded.

astro-ph.SR↗

arXiv2016

Statistical analysis of acoustic wave parameters near active regions

In order to quantify the influence of magnetic fields on acoustic mode parameters and flows in and around active regions, we analyse the differences in the parameters in magnetically quiet regions nearby an active region (which we call `nearby regions'), compared with those of quiet regions at the same disc locations for which there are no neighboring active regions. We also compare the mode parameters in active regions with those in comparably located quiet regions. Our analysis is based on ring diagram analysis of all active regions observed by HMI during almost five years. We find that the frequency at which the mode amplitude changes from attenuation to amplification in the quiet nearby regions is around 4.2 mHz, in contrast to the active regions, for which it is about 5.1 mHz. This amplitude enhancement (the `acoustic halo effect') is as large as that observed in the active regions, and has a very weak dependence on the wave propagation direction. The mode energy difference in nearby regions also changes from a deficit to an excess at around 4.2 mHz, but averages to zero over all modes. The frequency difference in nearby regions increases with increasing frequency until a point at which the frequency shifts turn over sharply as in active regions. However, this turnover occurs around 4.9 mHz, significantly below the acoustic cutoff frequency. Inverting the horizontal flow parameters in the direction of the neighboring active regions, we find flows that are consistent with a model of the thermal energy flow being blocked directly below the active region.

astro-ph.SR↗