arXiv subjects

Jon Linker

Publications and source records attributed to Jon Linker.

6 recordsLinked to original sources

arXiv2023

CORHEL-CME: An Interactive Tool For Modeling Solar Eruptions

Coronal Mass Ejections (CMEs) are immense eruptions of plasma and magnetic fields that are propelled outward from the Sun, sometimes with velocities greater than 2000 km/s. They are responsible for some of the most severe space weather at Earth, including geomagnetic storms and solar energetic particle (SEP) events. We have developed CORHEL-CME, an interactive tool that allows non-expert users to routinely model multiple CMEs in a realistic coronal and heliospheric environment. The tool features a web-based user interface that allows the user to select a time period of interest, and employs RBSL flux ropes to create stable and unstable pre-eruptive configurations within a background global magnetic field. The properties of these configurations can first be explored in a zero-beta magnetohydrodynamic (MHD) model, followed by complete CME simulations in thermodynamic MHD, with propagation out to 1 AU. We describe design features of the interface and computations, including the innovations required to efficiently compute results on practical timescales with moderate computational resources. CORHEL-CME is now implemented at NASA's Community Coordinated Modeling Center (CCMC) using NASA Amazon Web Services (AWS). It will be available to the public by the time this paper is published.

astro-ph.SR↗

arXiv2023

Solaris: A Focused Solar Polar Discovery-class Mission to achieve the Highest Priority Heliophysics Science Now

Solaris is a transformative Solar Polar Discovery-class mission concept to address crucial outstanding questions that can only be answered from a polar vantage. Solaris will image the Sun's poles from ~75 degree latitude, providing new insight into the workings of the solar dynamo and the solar cycle, which are at the foundation of our understanding of space weather and space climate. Solaris will also provide enabling observations for improved space weather research, modeling and prediction, revealing a unique, new view of the corona, coronal dynamics and CME eruptions from above.

astro-ph.IM↗

arXiv2020

Energetic Proton Propagation and Acceleration Simulated for the Bastille Day Event of July 14, 2000

This work presents results from simulations of the 14 July 2000 ("Bastille Day") solar proton event. We used the Energetic Particle Radiation Environment Model (EPREM) and the CORona-HELiosphere (CORHEL) software suite within the SPE Threat Assessment Tool (STAT) framework to model proton acceleration to GeV energies due to the passage of a CME through the low solar corona, and compared the model results to GOES-08 observations. The coupled simulation models particle acceleration from 1 to 20 $R_\odot$, after which it models only particle transport. The simulation roughly reproduces the peak event fluxes, and timing and spatial location of the energetic particle event. While peak fluxes and overall variation within the first few hours of the simulation agree well with observations, the modeled CME moves beyond the inner simulation boundary after several hours. The model therefore accurately describes the acceleration processes in the low corona and resolves the sites of most rapid acceleration close to the Sun. Plots of integral flux envelopes from multiple simulated observers near Earth further improve the comparison to observations and increase potential for predicting solar particle events. Broken-power-law fits to fluence spectra agree with diffusive acceleration theory over the low energy range. Over the high energy range, they demonstrate the variability in acceleration rate and mirror the inter-event variability observed solar-cycle 23 GLEs. We discuss ways to improve STAT predictions, including using corrected GOES energy bins and computing fits to the seed spectrum. This paper demonstrates a predictive tool for simulating low-coronal SEP acceleration.

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arXiv2019

Exploring Plasma Heating in the Current Sheet Region in a Three-Dimensional Coronal Mass Ejection Simulation

We simulate a coronal mass ejection (CME) using a three-dimensional magnetohydrodynamic (MHD) code that includes coronal heating, thermal conduction, and radiative cooling in the energy equation. The magnetic flux distribution at 1 R$_s$ is produced by a localized subsurface dipole superimposed on a global dipole field, mimicking the presence of an active region within the global corona. Transverse electric fields are applied near the polarity inversion line to introduce a transverse magnetic field, followed by the imposition of a converging flow to form and destabilize a flux rope, producing an eruption. We examine the quantities responsible for plasma heating and cooling during the eruption, including thermal conduction, radiation, adiabatic effects, coronal heating, and ohmic heating. We find that ohmic heating is an important contributor to hot temperatures in the current sheet region early in the eruption, but in the late phase adiabatic compression plays an important role in heating the plasma there. Thermal conduction also plays an important role in the transport of thermal energy away from the current sheet region throughout the reconnection process, producing a ``thermal halo'' and widening the region of high temperatures. We simulate emission from solar telescopes for this eruption and find that there is evidence for emission from heated plasma above the flare loops late in the eruption, when the adiabatic heating is the dominant heating term. These results provide an explanation for hot supra-arcade plasma sheets that are often observed in X-rays and extreme ultraviolet wavelengths during the decay phase of large flares.

astro-ph.SR↗

arXiv2018

Identifying Observables that can Differentiate Between Impulsive and Footpoint Heating: Time Lags and Intensity Ratios

Observations of coronal loops have identified several common loop characteristics, including that loops appear to cool and have higher than expected densities. Two potential heating scenarios have been suggested to explain these observations. One scenario is that the loops are heated by a series of small-scale impulsive heating events, or nanoflares. Another hypothesis is that the heating is quasi-steady and highly-stratified, i.e., ``footpoint heating'. The goal of this paper is to identify observables that can be used to differentiate between these two heating scenarios. For footpoint heating, we vary the heating magnitude and stratification, for impulsive heating, we vary the heating magnitude. We use one-dimensional hydrodynamic codes to calculate the resulting temperature and density evolution and expected lightcurves in four channels of AIA and one channel of XRT. We consider two principal diagnostics: the time lag between the appearance of the loop in two different channels, and the ratio of the peak intensities of the loop in the two channels. We find that 1) footpoint heating can predict longer time lags than impulsive heating, 2) footpoint heating can predict zero or negative time lags, 3) the intensity ratio expected from impulsive heating is confined to a narrow range, while footpoint heating predicts a wider range of intensity ratios, and 4) the range of temperatures expected in impulsive heating is broader than the range of temperatures expected in footpoint heating. This preliminary study identifies observables that may be useful in discriminating between heating models in future work.

astro-ph.SR↗

arXiv2016

Deriving the properties of coronal pressure fronts in 3-D: application to the 17 May 2012 ground level enhancement

We study the link between an expanding coronal shock and the energetic particles measured near Earth during the Ground Level Enhancement (GLE) of 17 May 2012. We developed a new technique based on multipoint imaging to triangulate the 3-D expansion of the shock forming in the corona. It uses images from three vantage points by mapping the outermost extent of the coronal region perturbed by the pressure front. We derive for the first time the 3-D velocity vector and the distribution of Mach numbers, $M_{FM}$, of the entire front as a function of time. Our approach uses magnetic field reconstructions of the coronal field, full magneto-hydrodynamic simulations and imaging inversion techniques. We find that the highest $M_{FM}$ values appear near the coronal neutral line within a few minutes of the Coronal Mass Ejection (CME) onset; this neutral line is usually associated with the source of the heliospheric current and plasma sheet. We illustrate the variability of the shock speed, shock geometry and Mach number along different modeled magnetic field lines. Despite the level of uncertainty in deriving the shock Mach numbers, all employed reconstruction techniques show that the release time of GeV particles occurs when the coronal shock becomes super-critical ($M_{FM}>3$). Combining in-situ measurements with heliospheric imagery, we also demonstrate that magnetic connectivity between the accelerator (the coronal shock of 17 May 2012) and the near-Earth environment is established via a magnetic cloud that erupted from the same active region roughly five days earlier.

astro-ph.SR↗