Comparing and Contrasting the Three Most Recent Solar Minima using MHD Models

Abstract:

Solar cycle 24, which began in December 2008, and likely ended in December 2019, has produced approximately 1,000 spotless days. The last time this occurred was during solar cycle 14, making cycle 24 quieter than any solar period in well over a century. Although the Sun’s magnetic field is undoubtedly responsible for this quiescence, it is not exactly clear which spatial and temporal variations in the magnetic field led to the broad range of features that make this cycle so unique. In this presentation, we develop global MHD model solutions for the past three solar minima (August 1996, December 2008, and December 2019) and compare them with a wide range of remote solar observations and in situ measurements, in an attempt to understand what properties of the observed photospheric magnetic field are responsible for such differences. Our model includes a relatively sophisticated treatment of the flow of energy, including thermal conduction, radiative losses, and a physics-based treatment of coronal heating. It is driven by the observed photospheric magnetic field, and can reproduce both in situ measurements of speed, density, and magnetic field, as well as remote observations, such as EUV and white-light coronagraph, and heliospheric images. Additionally, we compute theoretical constructs, such as the squashing factor, which emphasize the fine spatial structure of the magnetic field and its propensity to reconnect. We compare model results with observations from Ulysses, Wind, ACE, SoHO, STEREO, SDO, PSP, and SolO to constrain the model results, whilst at the same time, providing a global context for interpreting these measurements. Finally, we use the model solutions to compare and contrast the large-scale three-dimensional structure of the inner heliosphere at each of these minima.