Rachel Howe

Context. Acoustic mode frequencies in the Sun and Sun-like stars change due to magnetic activity on timescales that are much longer than that of the star’s rotation and much shorter than that of its evolution. Given the poor S/N of the observed stellar p-modes, it is challenging to measure the changes of individual mode frequencies. Typically, power spectra of different time series segments are cross-correlated to estimate a mean p-mode frequency change, which ends up being an average of the individual mode contributions. Aims. We seek to enhance the cross-correlation method by introducing a novel and computationally cheap method that enables us to disentangle p-mode frequency changes for different spherical harmonic degrees, ℓ . Methods. We designed filters that, if the inclination angle and rotation rate are already measured, enable the isolation of δω _ℓ and frequency changes of modes with a given ℓ while preventing bias creeping in from neighbouring modes. We performed Monte Carlo simulations to quantify the uncertainty in the estimation of δω _ℓ . Results. We validated our method against well-studied solar data (from SOHO/VIRGO and BiSON) and demonstrate its applicability to the solar-like Kepler star KIC 8006161.

Context. Solar-like oscillations have to date been observed in hundreds of main-sequence and sub-giant stars. However, only a select number of detections have been made in K-type dwarfs, using extreme-precision ground-based radial velocity observations and space-based photometric observations made by the NASA Kepler and TESS missions. Whilst the upcoming ESA PLATO Mission promises to add to these individual detections, it will do so only in a similar, modest number of stars. Aims. Here, we propose a new ensemble strategy to exploit the PLATO data, in which frequency power spectra on hundreds of K dwarfs lying in constrained ranges of effective temperature are combined in a weighted manner to significantly improve the detectability of the oscillations. Whilst this approach means it is not possible to extract usable constraints on individual oscillation frequencies, it provides a way to detect and measure the characteristics of the composite envelope of oscillation power given by the ensemble, which in turn provides diagnostics of granulation and magneto-convection and the impact of magnetic activity on the modes. Methods. We used data in the PLATO Input Catalogue (PIC) to make discrete numerical predictions of the detectability of the ensemble spectra. We also derived a simple analytical approximation of our method that obviates the need to perform numerical calculations over a discrete sample of targets, and which serves as a useful tool to make quick predictions for other future or planned missions. Results. Our predictions indicate that PLATO has the potential to provide solid ensemble detections well into the K-dwarf regime. In summary, PLATO offers an ideal opportunity to exploit this new approach.

The Birmingham Solar Oscillations Network (BiSON) provides long-term, high-precision Sun-as-a-star radial-velocity observations for helioseismology. While most BiSON sites operate autonomously, two observatories at Observatorio del Teide, Tenerife, and at Mount Wilson Observatory, USA, have historically required daily on-site operation. We present the automation of these sites using the BiSON:NG next-generation observing platform, incorporating compact fibre-fed instrumentation and a modular, microservice-based control and data acquisition architecture. At both locations, new automated optical feeds and a common software framework have been deployed, enabling autonomous operation of the observing subsystems, including guiding, data acquisition, and instrument control, despite markedly different physical and operational constraints. The upgrades demonstrate that modern automation can be successfully integrated within existing and historic observatory infrastructure. Adoption of the BiSON:NG architecture reduces operational overhead, mitigates risks associated with hardware and software obsolescence, and supports sustainable operation of the network over the multi-decade time-scales required for helioseismic studies. The flexibility of the architecture also provides a pathway for future deployment of additional small-footprint observatories.

High-resolution helioseismology observations with the Helioseismic and Magnetic Imager (HMI) onboard the 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.

We present results from fitting p-mode spectra derived from 7-d segments of Sun-as-a-star helioseismic observations from the Birmingham Solar Oscillations Network covering 32 yr. The results show a clear dependence of the mode frequencies on solar activity, and the frequency dependence of the sensitivity to activity can also be seen. Because we use data segments that cover less than half of a solar rotation, we are able to test for the effect of activity on the solar far side. By fitting with a model that takes into account activity on the far side of the Sun, we show that the frequency shifts are sensitive to activity from the whole Sun, not just the side facing the observer. Our results suggest that there is potential to investigate activity-related asteroseismic frequency shifts in solar-like oscillators using short time series of observations.

We present results from fitting $p$-mode spectra derived from 7-d segments of Sun-as-a-star helioseismic observations from the Birmingham Solar Oscillations Network covering 32 yr. The results show a clear dependence of the mode frequencies on solar activity, and the frequency dependence of the sensitivity to activity can also be seen. Because we use data segments that cover less than half of a solar rotation, we are able to test for the effect of activity on the solar far side. By fitting with a model that takes into account activity on the far side of the Sun, we show that the frequency shifts are sensitive to activity from the whole Sun, not just the side facing the observer. Our results suggest that there is potential to investigate activity-related asteroseismic frequency shifts in solar-like oscillators using short time series of observations.

We describe the defining observations of the solar cycle that provide constraints for the dynamo processes operating within the Sun. Specifically, we report on the following topics: historical sunspot numbers and revisions; active region (AR) flux ranges and lifetimes; bipolar magnetic region tilt angles; Hale and Joy’s law; the impact of rogue ARs on cycle progression and the amplitude of the following cycle; the spatio-temporal emergence of ARs that creates the butterfly diagram; polar fields; large-scale flows including zonal, meridional, and AR in-flows; short-term cycle variability; and helioseismic results including mode parameter changes.