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Abstract The acceleration and transport of energetic electrons during solar flares is one of the outstanding topics in solar physics. Recent X-ray and radio imaging and spectroscopy observations have provided diagnostics of the distribution of nonthermal electrons and suggested that, in certain flare events, electrons are primarily accelerated in the loop top and likely experience trapping and/or scattering effects. By combining the focused particle transport equation with magnetohydrodynamic (MHD) simulations of solar flares, we present a macroscopic particle model that naturally incorporates electron acceleration and transport. Our simulation results indicate that physical processes such as turbulent pitch-angle scattering can have important impacts on both electron acceleration in the loop top and transport in the flare loop, and their influences are highly energy-dependent. A spatial-dependent turbulent scattering with enhancement in the loop top can enable both efficient electron acceleration to high energies and transport of abundant electrons to the footpoints. We further generate spatially resolved synthetic hard X-ray (HXR) emission images and spectra, revealing both the loop-top and footpoint HXR sources. Similar to the observations, we show that the footpoint HXR sources are brighter and harder than the loop-top HXR source. We suggest that the macroscopic particle model provides new insights into understanding the connection between the observed loop-top and footpoint nonthermal emission sources by combining the particle model with dynamically evolving MHD simulations of solar flares. 

Abstract Decomposition of soil organic matter (SOM) can be stimulated by fresh organic matter input, a phenomenon known as the ‘priming effect’. Despite its global importance, the relationship of the priming effect to mineral weathering and nutrient release remains unclear. Here we show close linkages between mineral weathering in the critical zone and primed decomposition of SOM. Intensified mineral weathering and rock-derived nutrient release are generally coupled with primed SOM decomposition resulting from “triggered” microbial activity. Fluxes of organic matter products decomposed via priming are linearly correlated with weathering congruency. Weathering congruency influences the formation of organo-mineral associations, thereby modulating the accessibility of organic matter to microbial decomposers and, thus, the priming effect. Our study links weathering with primed SOM decomposition, which plays a key role in controlling soil C dynamics in space and time. These connections represent fundamental links between long-term lithogenic element cycling (= weathering) and rapid turnover of carbon and nutrients (= priming) in soil. 

We present a reduced magnetohydrodynamic (MHD) mathematical model describing the dynamical behavior of highly conducting plasmas with frozen-in magnetic fields, constrained by the assumption that there exists a frame of reference, where the magnetic field vector, B, is aligned with the plasma velocity vector, u, at each point. We call this solution “stream-aligned MHD” (SA-MHD). Within the framework of this model, the electric field, E = − u × B ≡ 0, in the induction equation vanishes identically and so does the electromagnetic energy flux (Poynting flux), E × B ≡ 0, in the energy equation. At the same time, the force effect from the magnetic field on the plasma motion (the Ampère force) is fully taken into account in the momentum equation. Any steady-state solution of the proposed model is a legitimate solution of the full MHD system of equations. However, the converse statement is not true: in an arbitrary steady-state magnetic field, the electric field does not have to vanish identically (its curl has to, though). Specifically, realistic three-dimensional solutions for the steady-state (“ambient”) solar atmosphere in the form of so-called Parker spirals can be efficiently generated within the stream aligned MHD (SA-MHD) with no loss in generality. 

Abstract We describe our first attempt to systematically simulate the solar wind during different phases of the last solar cycle with the Alfvén Wave Solar atmosphere Model (AWSoM) developed at the University of Michigan. Key to this study is the determination of the optimal values of one of the most important input parameters of the model, the Poynting flux parameter, which prescribes the energy flux passing through the chromospheric boundary of the model in the form of Alfvén wave turbulence. It is found that the optimal value of the Poynting flux parameter is correlated with the area of the open magnetic field regions with the Spearman’s correlation coefficient of 0.96 and anticorrelated with the average unsigned radial component of the magnetic field with the Spearman’s correlation coefficient of −0.91. Moreover, the Poynting flux in the open field regions is approximately constant in the last solar cycle, which needs to be validated with observations and can shed light on how Alfvén wave turbulence accelerates the solar wind during different phases of the solar cycle. Our results can also be used to set the Poynting flux parameter for real-time solar wind simulations with AWSoM. 

The heating near the coronal hole boundary is included into the AWSoM model via the Alfven wave interaction with the boundary between the closed and open field lines. Although the excessive heating on the Alfven surface wave is possible (Evans et al 2012), we chose to parameterize the surface effect in terms of excessive nonlinear reflection proportional to the transverse gradient of density. The efficiency of such reflection is derived analytically and compared with the solar wind observation data at 1 AU. 

Abstract To simulate solar coronal mass ejections (CMEs) and predict their time of arrival and geomagnetic impact, it is important to accurately model the background solar wind conditions in which CMEs propagate. We use the Alfvén Wave Solar atmosphere Model (AWSoM) within the the Space Weather Modeling Framework to simulate solar maximum conditions during two Carrington rotations and produce solar wind background conditions comparable to the observations. We describe the inner boundary conditions for AWSoM using the ADAPT global magnetic maps and validate the simulated results with EUV observations in the low corona and measured plasma parameters at L1 as well as at the position of the Solar Terrestrial Relations Observatory spacecraft. This work complements our prior AWSoM validation study for solar minimum conditions and shows that during periods of higher magnetic activity, AWSoM can reproduce the solar plasma conditions (using properly adjusted photospheric Poynting flux) suitable for providing proper initial conditions for launching CMEs. 

MHD-based global space weather models have mostly been developed and maintained at academic institutions. While the ``free spirit'' approach of academia enables the rapid emergence and testing of new ideas and methods, the lack of long-term stability and support makes this arrangement very challenging. This paper describes a successful example of a university-based group, the Center of Space Environment Modeling (CSEM) at the University of Michigan that developed and maintained the Space Weather Modeling Framework (SWMF) and its core element, the BATS-R-US extended MHD code. It took a quarter of a century to develop this capability and reach its present level of maturity that makes it suitable for research use by the space physics community through the Community Coordinated Modeling Center (CCMC) as well as operational use by the NOAA Space Weather Prediction Center (SWPC). 

This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere.