Helioseismic diagnostics of active regions and their magnetic fields

While two and a half decades of nearly constant observation by the Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory (SDO) spacecraft have yielded key insights into the structure and dynamics of active regions, it is still unclear if active regions can be identified before emerging on the solar surface and, once emerged, whether the subsurface structure of an active region’s magnetic field can be measured. Regarding the dynamical processes associated with active regions, the height and mechanism of sunquake excitation remains poorly understood. To answer these questions, a comprehensive survey of active region magnetic fields and their associated helioseismic signatures for both the pre-emergence and post-emergence phase is completed. For the former, deviations of the mean phase travel time of acoustic waves are used to detect the rise of magnetic flux from the solar interior. Calibration and testing of the time-distance technique is performed using simulations of submerged sound speed perturbations. A detailed case study of select active regions is performed, and the technique is then applied to a collection of 46 active regions to determine the statistical significance of mean travel time perturbations as a signature of active region emergence. Next, a novel technique is developed for the assessment of existing active region magnetic fields. By combining the travel time of acoustic waves propagating in varying directions, perturbations due to subsurface horizontal magnetic fields are isolated from structural changes. The resulting measurements provide a proxy for the magnitude of the horizontal magnetic field as well as a direct measurement of the field’s azimuth. The technique is applied to a sunspot simulation for validation, and is then used to investigate the subsurface magnetic structure of several observed sunspots. Finally, a model of solar acoustic wave propagation is constructed using the compressible form of the mass, momentum, and entropy conservation equations to study the excitation of sunquakes. The constructed model is used to determine at what height sunquakes are excited, what mode of excitation is most energetically favorable, and what properties of particle beams are relevant to sunquake excitation. The excitation height is determined from comparison of observed events with a catalogue of simulated sunquakes for a range of excitation locations and for several excitation mechanisms, which allows the excitation height and energy to be estimated. Additionally, the output of FP proton beam simulations are used to derive forcing functions for the excitation of sunquakes in the model to determine the dependence of wave front amplitude on the low-energy cut-off

Velocity-space sensitivity and inversions of synthetic ion cyclotron emission

This paper introduces a new model to find the velocity-space location of energetic ions generating ion cyclotron emission (ICE) in plasmas. ICE is thought to be generated due to inverted gradients in the v⊥ direction of the velocity distribution function or due to anisotropies, i.e., strong gradients in the pitch direction. Here, we invert synthetic ICE spectra generated from first principles PIC-hybrid computations to find the locations of these ICE-generating ions in velocity space in terms of a probability distribution function. To this end, we compute 2D ICE weight functions based on the magnetoacoustic cyclotron instability, which reveals the velocity-space sensitivity of ICE measurements. As an example, we analyze the velocity-space sensitivity of synthetic ICE measurements near the first 15 harmonics for plasma parameters typical for the Large Helical Device. Furthermore, we investigate the applicability of a least-square subset search, Tikhonov regularization, and Lasso regularization to obtain the locations in velocity space of the ions generating the ICE

Understanding the Impact of Underwater Noise to Preserve Marine Ecosystems and Manage Anthropogenic Activities

Policy makers require a knowledge-based support to identify effective interventions for the socio-economic sustainability of human activities at sea. When dealing with anthropogenic impacts on marine ecosystems, we deal with a complex and faceted system which has high variability in terms of environment, regulation, governance, industrial activities, and geo-political scenarios. We analyzed the conceptual scientific framework adopted to address underwater noise as a polluting component of the marine environment. We identified the scientific paths that can provide useful contributions towards comprehending the impacts on the native ecosystem. In order to furnish relevant clues towards the properties of the interconnection of signals, we briefly reviewed an example from a different discipline (helioseismology). We describe a new approach on how acoustic energy in the sea could be detected and analyzed to understand its role in the functioning of the ecosystem. We propose a change of perspective in the observation strategy of underwater noise, promoting a knowledge transfer from other disciplines, which in turn will enable a better understanding of the system. This will allow researchers and policy-makers to identify feasible and effective solutions to tackle the negative impacts of underwater noise and the conservation of the marine ecosystem

Numerical modelling of waves in the solar atmosphere

The Sun is the closest star to Earth and the bringer of life for all of us. Remove the Sun and Earth is rendered a lifeless, icy rock floating in outer space. It stands to reason that a thorough understanding of the workings of the Sun would be high priority in the scientific world, and beyond. The outer-most layer of the interior of the Sun comprises of what is known as the convection zone. This chaotic zone produces innumerable pressure waves, which propagate through the Sun. These waves carry the energy of the Sun to its atmosphere and beyond, and are thought to be responsible for the infamous ‘coronal heating problem’. The magnetic nature of the Sun allows both acoustic and magnetic waves, or various combinations of the two, to exist. This is where the complexity lies, with so many different types of waves being produced and exchanging energy between themselves, it is extremely difficult to pinpoint which waves are responsible for the observations we make. Observations and mathematical/physical theories of ever improving quality are used to understand the details of waves in the Sun, however they often lack a bridge to connect them, which is where numerical simulations come in. The work presented here provides a combination of 1.5, 2.5 and 3 dimensional simulations looking to explain how a variety of waves propagate and carry energy through the internal and external layers of the Sun. When a wave reaches a layer in the Sun’s atmosphere where the sound and Alfvén speeds coincide, it splits into two ‘modes’, a fast and slow mode. Recent mathematical findings suggested an incoming shock wave would not only split into its fast and slow components, but that both wave modes would be smoothed as they exit this area. Numerical simulations herein show that only the slow wave is smoothed, with the fast wave propagating unhindered. Within the Sun’s atmosphere, various steep gradients of its physical components are found. These gradients have been proposed to act as barriers to incoming waves, which can be partially reflected off them. Multiple reflection sites suggests cavities can be created that acoustic waves can resonate within. Numerical simulations herein show a stark increase in the velocity of frequencies proposed to be characteristic of a cavity within the chromosphere. This suggests cavities can exist within the Sun’s atmosphere and the amplitude of velocity observations from within these areas must be partially attributed to the resonant effects of the cavities themselves. Following the onset of some solar flares, ripples are observed on the Solar surface emanating from the flare site. These ripples show strong anisotropies in their appearance, a characteristic not well studied. Numerical simulations herein show that these anisotropies can be attributed to both the strength and inclination of the magnetic field but even more so by the nature of the impacting source. A source with motion perpendicular to the solar surface causes constructive interference and higher amplitude ripples are created along the axis of motion