For many decades, Alfvén waves have been considered to be a source of energy in the solar coronal plasma and supersonic wind. Understanding the possible dissipation mechanisms of Alfvén waves in the heterogeneously structured corona is important. Magnetic singularities enable mode conversion of MHD waves near the equipartition layer (Cs≈CA). In our study, we performed a 2.5D MHD numerical simulation of Alfvén wave packet generation through mode conversion when a fast wave interacts with a localised magnetic null in the corona. Subsequently, the out-of-plane velocity (i.e., Vz) and magnetic field fluctuations (i.e., bz) are generated in the equipartition layer (Cs≈CA) and propagate toward the large-scale heterogeneous corona. Also, the Alfvén wave packet decays in amplitude on the curved magnetic field due to phase mixing from the gradient of the local Alfvén speed across the magnetic field. Moreover, the strength of phase mixing depends on the amplitude of the local transverse gradient of bz, which helps form localised current layers. Thus, our results show that Alfvén waves can be generated from the magnetic null through MHD mode conversion, then damp due to phase mixing and propagate toward another nearby null. We conclude that realistic physical conditions are needed to determine the extent of phase mixing and dissipation in the structured solar corona.
Optical frequency combs are laser devices that provide a broad spectrum of equally-spaced laser lines. This Nobel Prize winning technology has transformed some types of laboratory measurements. But most comb systems are still bulky, complex and require expert operation, somewhat locking them into laboratory environments. Astronomy has been an early adopter of frequency combs, using them to provide precise wavelength calibration for high-resolution spectrographs. However, especially at visible wavelengths, these combs are not well suited to observatory environments and have suffered reliability problems. They are also still very expensive – barely affordable even for the largest observatories. I will describe our work as part of the broader COMBS collaboration in Australia to develop “microcomb” technology for use in astronomy. Microcombs are photonic chips, the size of a fingernail, which can convert single-wavelength laser input into frequency comb output. I will describe our work so far which demonstrates that these microcombs have many properties that are well-suited to high-resolution astronomical spectrographs and observatory environments. We are also attempting to create new methods for extracting the frequency comb spectra from real data from high-resolution spectrographs to ensure the full benefits of the comb are utilised in the calibration process. This is all with a view to enable experiments that can only be achieved with much better calibration, like the “Sandage test” of General Relativity in which the real-time change in redshift of distant objects is measured. I will discuss our work to optimise the measurement techniques for this test.
