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1. An Empirically Driven MHD Model to Predict the Solar Wind at Parker Solar Probe and Solar Orbiter during the Current Solar Minimum

   Kim, T. K.; Pogorelov, N.; Arge, C. N.; Jones-Mecholsky, S. I. (December 2020, American Geophysical Union, Fall Meeting 2020, abstract #SH021-08)

   null (Ed.)

   Since the launch on 2018 August 12, the Parker Solar Probe (PSP) has completed its first five orbits around the Sun, having reached down to ~28 solar radii at perihelion 5 on 2020 June 7. More recently, the Solar Orbiter (SolO) made its first close approach to the Sun at 0.52 AU on 2020 June 15, nearly 4 months after the launch. Using a 3D heliospheric MHD model coupled with the Wang-Sheeley-Arge (WSA) coronal model using the Air Force Data Assimilative Photospheric flux Transport (ADAPT) magnetic maps as input, we simulate the time-varying inner heliosphere, including the trajectories of PSP and SolO, during the current solar minimum period between 2018 and 2020. Above the ADAPT-WSA model outer boundary at 21.5 solar radii, we solve the Reynolds averaged MHD equations with turbulence and pickup ions taken into account and compare the simulation results with the PSP solar wind and magnetic field data, with particular emphasis on the large-scale solar wind structure and magnetic connectivity during each solar encounter. 

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2. Lagrangian Stochastic Model for the Motions of Magnetic Footpoints on the Solar Wind Source Surface and the Path Lengths of Boundary-driven Interplanetary Magnetic Field Lines

   https://doi.org/10.3847/1538-4357/acbd43  

   Li, Gang; Bian, N. H. (March 2023, The Astrophysical Journal)

   Abstract In this work, we extend Leighton’s diffusion model describing the turbulent mixing of magnetic footpoints on the solar wind source surface. The present Lagrangian stochastic model is based on the spherical Ornstein–Uhlenbeck process with drift that is controlled by the rotation frequency Ω of the Sun, the Lagrangian integral timescaleτ_L, and the root-mean-square footpoint velocityV_rms. The Lagrangian velocity and the positions of magnetic footpoints on the solar wind source surface are obtained from the solutions of a set of stochastic differential equations, which are solved numerically. The spherical diffusion model of Leighton is recovered in the singular Markov limit when the Lagrangian integral timescale tends to zero while keeping the footpoint diffusivity finite. In contrast to the magnetic field lines driven by standard Brownian processes on the solar wind source surface, the interplanetary magnetic field lines are smooth differentiable functions with finite path lengths in our model. The path lengths of the boundary-driven interplanetary magnetic field lines and their probability distributions at 1 au are computed numerically, and their dependency with respect to the controlling parameters is investigated. The path-length distributions are shown to develop a significant skewness as the width of the distributions increases. 

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3. Predicting the Solar Wind Using Empirically-driven Data-constrained Heliospheric MHD Model

   Kim, T.; Pogorelov, N (December 2021, AGU Fall Meeting 2021, held in New Orleans, LA, 13-17 December 2021, id. SM53B-08.)

   The Sun emits a stream of charged particles called the solar wind, which is the primary driver of space weather and geomagnetic disturbances. Modeling and observations complement each other to help us identify and understand the physical processes governing the solar wind dynamics on different scales. Numerical models of the solar wind have greatly improved in recent years with advances in computational infrastructure and by employing data-driven or data-assimilative approaches. Designed primarily for modeling the partially ionized space plasma using adaptive mesh refinement technique on Cartesian or spherical grids, the Multi-scale Fluid-kinetic Simulation Suite (MS-FLUKSS) is arguably one of the most sophisticated numerical codes for simulating the solar wind flow. To inform potential users and interested members of the space weather community, we present a brief summary of the current state of the solar wind models developed in the MS-FLUKSS framework, with an emphasis on the 3D heliospheric MHD models driven and constrained by remote/in situ observations and empirical coronal models such as the Wang-Sheeley-Arge model. We also discuss potential scientific and operational applications of our solar wind models on prediction of space weather (e.g., high speed streams, coronal mass ejections, and interplanetary shocks) throughout the solar system. 

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4. AWSoM Magnetohydrodynamic Simulation of a Solar Active Region with Realistic Spectral Synthesis

   https://doi.org/10.3847/1538-4357/ac52ab  

   Shi, Tong; Manchester IV, Ward; Landi, Enrico; van der Holst, Bart; Szente, Judit; Chen, Yuxi; Tóth, Gábor; Bertello, Luca; Pevtsov, Alexander (March 2022, The Astrophysical Journal)

   Abstract For the first time, we simulate the detailed spectral line emission from a solar active region (AR) with the Alfvén Wave Solar Model (AWSoM). We select an AR appearing near disk center on 2018 July 13 and use the National Solar Observatory’s Helioseismic and Magnetic Imager synoptic magnetogram to specify the magnetic field at the model’s inner boundary. To resolve small-scale magnetic features, we apply adaptive mesh refinement with a horizontal spatial resolution of 0°.35 (4.5 Mm), four times higher than the background corona. We then apply the SPECTRUM code, using CHIANTI spectral emissivities, to calculate spectral lines forming at temperatures ranging from 0.5 to 3 MK. Comparisons are made between the simulated line intensities and those observed by Hinode/Extreme-ultraviolet Imaging Spectrometer where we find close agreement across a wide range of loop sizes and temperatures (about 20% relative error for both the loop top and footpoints at a temperature of about 1.5 MK). We also simulate and compare Doppler velocities and find that simulated flow patterns are of comparable magnitude to what is observed. Our results demonstrate the broad applicability of the low-frequency AWSoM for explaining the heating of coronal loops. 

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5. The magnetic topology of the inverse Evershed flow

   https://doi.org/10.1051/0004-6361/202142585  

   Prasad, A; Ranganathan, M; Beck, C; Choudhary, D P; Hu, Q (June 2022, Astronomy & Astrophysics)

   Context. The inverse Evershed flow (IEF) is a mass motion towards sunspots at chromospheric heights. Aims. We combined high-resolution observations of NOAA 12418 from the Dunn Solar Telescope and vector magnetic field measurements from the Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF. Methods. We derived chromospheric line-of-sight (LOS) velocities from spectra of H α and Ca  II IR. The HMI data were used in a non-force-free magnetic field extrapolation to track closed field lines near the sunspot in the active region. We determined their length and height, located their inner and outer foot points, and derived flow velocities along them. Results. The magnetic field lines related to the IEF reach on average a height of 3 megameter (Mm) over a length of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The average field strength difference Δ B between inner and outer foot points is +400 G. The temperature difference Δ T is anti-correlated with Δ B with an average value of −100 K. The pressure difference Δ p is dominated by Δ B and is primarily positive with a driving force towards the inner foot points of 1.7 kPa on average. The velocities predicted from Δ p reproduce the LOS velocities of 2–10 km s −1 with a square-root dependence. Conclusions. We find that the IEF is driven along magnetic field lines connecting network elements with the outer penumbra by a gas pressure difference that results from a difference in field strength as predicted by the classical siphon flow scenario. 

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