Thinning of the Sun's magnetic layer: the peculiar solar minimum could have been predicted

The solar magnetic activity cycle causes changes in the Sun on timescales that are relevant to human lifetimes. The minimum in solar activity that preceded the current solar cycle (cycle 24) was deeper and quieter than any other recent minimum. Using data from the Birmingham Solar-Oscillations Network (BiSON), we show that the structure of the solar sub-surface layers during the descending phase of the preceding cycle (cycle 23) was very different from that during cycle 22. This leads us to believe that a detailed examination of the data would have led to the prediction that the cycle-24 minimum would be out of the ordinary. The behavior of the oscillation frequencies allows us to infer that changes in the Sun that affected the oscillation frequencies in cycle 23 were localized mainly to layers above about 0.996Rsun, depths shallower than about 3000 km. In cycle 22, on the other hand, the changes must have also occurred in the deeper-lying layers.Comment: To appear in Ap

The hunt for low-frequency modes of oscillation of the Sun: application of statistical

The main focus of this thesis is to search for low-frequency solar acoustic and gravity modes of oscillation of the Sun. Low-frequency modes are sensitive to the structure of the solar core and so provide stringent tests for models of the internal structure of the Sun. In this thesis we develop and use statistical techniques that aim to detect low-frequency modes by searching for statistically significant features in frequency-power spectra. Frequency-amplitude spectra are compared for coincident prominent features. The amount of common noise shared by two data sets affects the probability that a coincident prominent feature is noise and so the statistical tests were non-trivial to derive. The optimum condition for comparing frequency-amplitude spectra is when no common noise is present. The power and source of the common noise is dependent on the data that are compared. Statistical tests are used to search contemporaneous and non-contemporaneous observations made by the Birmingham Solar Oscillations Network (BiSON) and the Global Oscillations at Low Frequencies (GOLF) instrument. We also searched contemporaneous and non-contemporaneous sets of BiSON data. Many different combinations of BiSON data are compared to find contemporaneous BiSON timeseries that contain as little common noise as possible. Detection threshold levels are significantly reduced by searching for coincident prominent features, however, no new mode candidates are detected. To investigate whether the gaps in the list of detected modes are expected, given the reduced threshold levels, predictions on the detectability of low-frequency modes are made, based on estimates of the modes' powers and damping times. The fraction of modes detected in Monte Carlo simulations is small at low frequencies. Significantly more detections would be made if the power signal to noise could be increased by a factor of 2, which could be achieved by lengthening timeseries and by reducing the amount of noise in the data. The quality of solar observations is affected by instrument stability. We consider three effects that mean unresolved Doppler velocity observations are not homogenous across the solar disc; solar rotation, limb darkening and the position of a detector. The results indicate that observations made by BiSON instruments do not represent a uniform average across the solar disc. Different detectors are weighted towards different regions of the solar disc, and so observe different realizations of the solar noise. The calculated weightings are compared to BiSON observations. Agreement between the calculations and the observations is improved by altering various parameters, such as the observed size of the solar image. We find that the weighting varies with epoch and so can have consequences for the quality of BiSON data

GONG p-mode parameters through two solar cycles

We investigate the parameters of global solar p-mode oscillations, namely damping width $\Gamma$, amplitude $A$, mean squared velocity $\langle v^2\rangle$, energy $E$, and energy supply rate $\mathrm{d}E/\mathrm{d} t$, derived from two solar cycles' worth (1996 - 2018) of Global Oscillation Network Group (GONG) time series for harmonic degrees $l=0 - 150$. We correct for the effect of fill factor, apparent solar radius, and spurious jumps in the mode amplitudes. We find that the amplitude of the activity related changes of $\Gamma$ and $A$ depends on both frequency and harmonic degree of the modes, with the largest variations of $\Gamma$ for modes with $2400\,\mu\text{Hz}\le \nu \le 3300\,\mu\text{Hz}$ and $31\le l \le 60$ with a min-to-max variation of $26.6\pm0.3\%$ and of $A$ for modes with $2400\,\mu\text{Hz}\le\nu\le 3300\,\mu\text{Hz}$ and $61\le l \le 100$ with a min-to-max variation of $27.4\pm0.4\%$. The level of correlation between the solar radio flux $F_{10.7}$ and mode parameters also depends on mode frequency and harmonic degree. As a function of mode frequency, the mode amplitudes are found to follow an asymmetric Voigt profile with $\nu_{\text{max}}=3073.59\pm0.18\,\mu\text{Hz}$. From the mode parameters, we calculate physical mode quantities and average them over specific mode frequency ranges. This way, we find that the mean squared velocities $\langle v^2\rangle$ and energies $E$ of p modes are anti-correlated with the level of activity, varying by $14.7\pm0.3\%$ and $18.4\pm0.3\%$, respectively, and that the mode energy supply rates show no significant correlation with activity. With this study we expand previously published results on the temporal variation of solar p-mode parameters. Our results will be helpful to future studies of the excitation and damping of p modes, i.e., the interplay between convection, magnetic field, and resonant acoustic oscillations.Comment: Accepted for publication in Solar Physics. 33 pages, 16 figures, 5 table

A comparison between global proxies of the sun’s magnetic activity cycle : inferences from Helioseismology

The last solar minimum was, by recent standards, unusually deep and long. We are now close to the maximum of the subsequent solar cycle, which is relatively weak. In this article we make comparisons between different global (unresolved) measures of the Sun’s magnetic activity, to investigate how they are responding to this weak-activity epoch. We focus on helioseismic data, which are sensitive to conditions, including the characteristics of the magnetic field, in the solar interior. Also considered are measures of the magnetic field in the photosphere (sunspot number and sunspot area), the chromosphere and corona (10.7 cm radio flux and 530.3 nm green coronal index), and two measures of the Sun’s magnetic activity closer to Earth (the interplanetary magnetic field and the galactic cosmic-ray intensity). Scaled versions of the activity proxies diverge from the helioseismic data around 2000, indicating a change in relationship between the proxies. The degree of divergence varies from proxy to proxy with sunspot area and 10.7 cm flux showing only small deviations, while sunspot number, coronal index, and the two interplanetary proxies show much larger departures. In Cycle 24 the deviations in the solar proxies and the helioseismic data decrease, raising the possibility that the deviations observed in Cycle 23 are just symptomatic of a 22-year Hale cycle. However, the deviations in the helioseismic data and the interplanetary proxies increase in Cycle 24. Interestingly the divergence in the solar proxies and the helioseismic data are not reflected in the shorter-term variations (often referred to as quasi-biennial oscillations) observed on top of the dominant 11-year solar cycle. However, despite being highly correlated in Cycle 22, the short-term variations in the interplanetary proxies show very little correlation with the helioseismic data during Cycles 23 and 24

Seismic signatures of stellar magnetic activity — what can we expect from TESS?

Asteroseismic methods offer a means to investigate stellar activity and activity cycles as well as to identify those properties of stars which are crucial for the operation of stellar dynamos. With data from CoRoT and Kepler, signatures of magnetic activity have been found in the seismic properties of a few dozen stars. Now, NASA's Transiting Exoplanet Survey Satellite (TESS) mission offers the possibility to expand this, so far, rather exclusive group of stars. This promises to deliver new insight into the parameters that govern stellar magnetic activity as a function of stellar mass, age, and rotation rate. We derive a new scaling relation for the amplitude of the activity-related acoustic (p-mode) frequency shifts that can be expected over a full stellar cycle. Building on a catalog of synthetic TESS time series, we use the shifts obtained from this relation and simulate the yield of detectable frequency shifts in an extended TESS mission. We find that, according to our scaling relation, we can expect to find significant p-mode frequency shifts for a couple hundred main-sequence and early subgiant stars and for a few thousand late subgiant and low-luminosity red giant stars

They do change after all : 25 years of GONG Data reveal variation of p-mode energy supply rates

It has been shown over and over again that the parameters of solar p modes vary through the solar activity cycle: frequencies, amplitudes, lifetimes, energies. However, so far, the rates at which energy is supplied to the p modes have not been detected to be sensitive to the level of magnetic activity. We set out to re-inspect their temporal behaviour over the course of the last two Schwabe cycles. For this, we use Global Oscillation Network Group (GONG) p-mode parameter tables. We analyse the energy supply rates for modes of harmonic degrees l = 0-150 and average over the azimuthal orders and, subsequently, over modes in different parameter ranges. This averaging greatly helps in reducing the noise in the data. We find that energy supply rates are anticorrelated with the level of solar activity, for which we use the F10.7 index as a proxy. Modes of different mode frequency and harmonic degrees show varying strengths of anticorrelation with the F10.7 index, reaching as low as r = -0.82 for low frequency modes with l = 101-150. In this first dedicated study of solar p-mode energy supply rates in GONG data, we find that they do indeed vary through the solar cycle. Earlier investigations with data from other instruments were hindered by being limited to low harmonic degrees or by the data sets being too short. We provide tables of time-averaged energy supply rates for individual modes as well as for averages over disjunct frequency bins

Empirical relations for the sensitivities of solar-like oscillations to magnetic perturbations

Oscillation mode frequencies of stars are typically treated as static for a given stellar model. However, in reality they can be perturbed by time varying sources such as magnetic fields and flows. We calculate the sensitivities of radial p-mode oscillations of a set of models for masses between 0.7–3.0M⊙ from the main sequence to the early asymptotic giant branch. We fit these mode sensitivities with polynomials in fundamental stellar parameters for six stages of stellar evolution. We find that the bestfitting relations differ from thos proposed in the literature and change between stages of stellar evolution. Together with a measure of the strength of the perturbation, e.g., of the level of magnetic activity, the presented relations can be used for assessing whether a star’s observed oscillation frequencies are likely to be close to the unperturbed ground state or whether they should be adjusted