arXiv subjects

Roberto Lionello

Publications and source records attributed to Roberto Lionello.

At least 19 recordsLinked to original sources

arXiv2026

Tracking Magnetic Topological Change in a Time-Dependent Coronal Model

We apply the slip-back mapping method of Titov et al. 2009 and Lionello et al. 2020 to a thermodynamic MHD simulation to track topological changes in the magnetic field at a range of temporal cadences. The method constitutes the logical successor to a simple open-field map for a steady-state model, as it tracks changes in the open and closed fields for a time-dependent model by tracking individual magnetic elements as they advect across the map, rather than simply tracing field line connectivity from each cell. Through careful categorization of the slip-back mapping values and analysis of the flux changes, we not only effectively track the open flux but can recover the flux processed through interchange reconnection as well. The field lines involved in these processes are shown to follow lines of high squashing factor, as proposed by interchange reconnection-driven slow solar wind theory. The time-dependent model, which is scaled to solar minimum-like activity, projects that a median value of 3.5% of the total open flux in any given 24-hour interval has been processed through interchange reconnection. This corresponds to a relatively high proportion of the total open flux changes over time in the heliosphere. Our results show that not only is this method a useful tool for accurately tracking topological change in time-dependent simulations, but that its inherent complexity can be visually reduced into an intuitive 2D plot that simply and effectively communicates temporal changes.

astro-ph.SR↗

arXiv2026

Magnetic switchback formation: a review of proposed mechanisms

Magnetic switchbacks are large amplitude deflections of the magnetic field within the solar wind. They are Alfv\'enic in character and so are associated with a spike in velocity and a generally small variation in local plasma density. Early orbits of Parker Solar Probe revealed that the solar wind near the Sun is dominated by these structures, and therefore, they may be playing an important role in the energy budget and acceleration of the young solar wind. In this review, we present an overview of different mechanisms that have been proposed for how switchbacks could be formed. We group the mechanisms by whether they predominantly act in the low solar atmosphere or within the solar wind (in situ). We focus on mechanisms that can create reversals of the ambient magnetic field direction and, thus, account for the most extreme perturbations. The general consensus is that mechanisms in the lower solar atmosphere do not form such reversals on their own but provide the seed perturbations, flows, or particle beams necessary for in situ mechanisms to create switchbacks within the solar wind. Switchback observations thus likely contain an imprint of the coronal source of the seed perturbation or flow, which is evolved further locally by one of several plausible in situ mechanisms. We discuss the strengths and weaknesses of each mechanism and outline future observational and theoretical tests that could help differentiate between them.

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arXiv2026

Magnetic Connectivity in the Time-Dependent Corona and Heliosphere

Magnetic flux fills the heliosphere, expands outward from the solar corona, and is fundamentally related to the structure and dynamics of the solar corona and solar wind. Open magnetic flux and the fast wind are thought to originate from open magnetic field lines in coronal holes. Less understood processes in the streamer belt and the boundaries of coronal holes, associated with the more variable slow wind, may be formed by interchange reconnection between open and closed magnetic flux. Interchange reconnection is thought to give rise to field lines that are "folded," i.e. that turn back on themselves. The properties of strahl electrons measured in the solar wind give clues to the heliospheric magnetic connectivity. Unidirectionally outward strahl indicates open field lines, while bidirectional strahl is associated with closed magnetic flux and CMEs. Inward directed, unidirectional strahl is believed to indicate folded flux. We use two time-dependent, flux-evolutionary MHD models of the combined corona and heliosphere, one for a solar-minimum configuration, one for the 2024 total solar eclipse, to investigate the magnetic connectivity of the corona/heliosphere system. We examine how magnetic connectivity varies with distance from the Sun in the two configurations. We evaluate the evolutionary effects by contrasting time-dependent results with the corresponding steady-state calculations, and compare the model connectivities with statistical studies of strahl. The connectivities in the time-evolving simulations are roughly consistent with observed strahl occurrence rates, while those from the steady-state models are not. Our results suggest that complex magnetic connectivities are ubiquitous in the heliosphere.

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arXiv2025

Investigating Solar Wind Outflows from Open-Closed Magnetic Field Structures Using Coordinated Solar Orbiter and Hinode Observations

ESA/NASA's Solar Orbiter (SO) allows us to study the solar corona at closer distances and from different perspectives, which helps us to gain significant insights into the origins of the solar wind. In this work, we present the analysis of solar wind outflows from two locations: a narrow open-field corridor and a small, mid-latitude coronal hole. These outflows were observed off-limb by the Metis coronagraph onboard SO and on-disk by the Extreme Ultraviolet Imaging Spectrometer (EIS) onboard Hinode. Magnetic field extrapolations suggest that the upflow regions seen in EIS were the sources of the outflowing solar wind observed with Metis. We find that the plasma associated with the narrow open-field corridor has higher electron densities and lower outflow velocities compared to the coronal hole plasma in the middle corona, even though the plasma properties of the two source regions in the low corona are found to be relatively similar. The speed of solar wind from the open-field corridor also shows no correlation with the magnetic field expansion factor, unlike the coronal hole. These pronounced differences at higher altitudes may arise from the dynamic nature of the low-middle corona, in which reconnection can readily occur and may play an important role in driving solar wind variability.

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arXiv2025

Ion Charge States from a Global Time-Dependent Wave-Turbulence-Driven Model of the Solar Wind: Comparison with in-situ Measurements

Solar wind charge-state measurements contain a wealth of knowledge related to the properties of the solar corona from where they originated. However, their interpretation has remained challenging because it convolves coronal temperature, density, and velocity along the particles' trajectory through the corona before they ``freeze in'' and are convected outward through the solar wind. In this study, we calculate ion charge states by coupling a non-equilibrium ionization model with a global magnetohydrodynamic model of the corona and inner heliosphere. We present results for two periods characteristic of solar minimum and maximum and compare them with observations from the ACE spacecraft. We find that the model reproduces the essential features of the observations, rectifying an earlier inconsistency that was apparent in 1-D calculations, and allows us to unambiguously trace the evolution of charge states from the base of the corona into the solar wind.

physics.space-ph↗

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

Global MHD Simulations of the Time-Dependent Corona

We describe, test, and apply a technique to incorporate full-sun, surface flux evolution into an MHD model of the global solar corona. Requiring only maps of the evolving surface flux, our method is similar to that of Lionello et al. (2013), but we introduce two ways to correct the electric field at the lower boundary to mitigate spurious currents. We verify the accuracy of our procedures by comparing to a reference simulation, driven with known flows and electric fields. We then present a thermodynamic MHD calculation lasting one solar rotation driven by maps from the magnetic flux evolution model of Schrijver & DeRosa (2003). The dynamic, time-dependent nature of the model corona is illustrated by examining the evolution of the open flux boundaries and forward modeled EUV emission, which evolve in response to surface flows and the emergence and cancellation flux. Although our main goal is to present the method, we briefly investigate the relevance of this evolution to properties of the slow solar wind, examining the mapping of dipped field lines to the topological signatures of the "S-Web" and comparing charge state ratios computed in the time-dependently driven run to a steady state equivalent. Interestingly, we find that driving on its own does not significantly improve the charge states ratios, at least in this modest resolution run that injects minimal helicity. Still, many aspects of the time-dependently driven model cannot be captured with traditional steady-state methods, and such a technique may be particularly relevant for the next generation of solar wind and CME models.

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arXiv2023

Deflection of Coronal Mass Ejections in Unipolar Ambient Magnetic Fields

The trajectories of coronal mass ejections (CMEs) are often seen to substantially deviate from a purely radial propagation direction. Such deviations occur predominantly in the corona and have been attributed to "channeling" or deflection of the eruptive flux by asymmetric ambient magnetic fields. Here, we investigate an additional mechanism that does not require any asymmetry of the pre-eruptive ambient field. Using magnetohydrodynamic numerical simulations, we show that the trajectory of CMEs through the solar corona can significantly deviate from a radial direction when propagation takes place in a unipolar radial field. We demonstrate that the deviation is most prominent below ~15 solar radii and can be attributed to an "effective IxB force" that arises from the intrusion of a magnetic flux rope with a net axial electric current into a unipolar background field. These results are important for predictions of CME trajectories in the context of space weather forecasts, as well as for reaching a deeper understanding of the fundamental physics underlying CME interactions with the ambient fields in the extended solar corona.

astro-ph.SR↗

arXiv2023

Time-Dependent Dynamics of the Corona

We present in this Letter the first global comparison between traditional line-tied steady state magnetohydrodynamic models and a new, fully time-dependent thermodynamic magnetohydrodynamic simulation of the global corona. The maps are scaled to the approximate field distributions and magnitudes around solar minimum using the Lockheed Evolving Surface-Flux Assimilation Model to incorporate flux emergence and surface flows over a full solar rotation, and include differential rotation and meridional flows. Each time step evolves the previous state of the plasma with a new magnetic field input boundary condition. We find that this method is a significant improvement over steady-state models, as it closely mimics the constant photospheric driving on the Sun. The magnetic energy levels are higher in the time-dependent model, and coronal holes evolve more along the following edge than they do in steady-state models. Coronal changes, as illustrated with forward-modeled emission maps, evolve on longer timescales with time-dependent driving. We discuss implications for active and quiet Sun scenarios, solar wind formation, and widely-used steady state assumptions like potential field source surface calculations.

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arXiv2023

Proton and electron temperatures in the solar wind and their correlations with the solar wind speed

The heating and acceleration of the solar wind remains one of the fundamental unsolved problems in heliophysics. It is usually observed that the proton temperature $T_i$ is highly correlated with the solar wind speed $V_{SW}$, while the electron temperature $T_e$ shows anti-correlation or no clear correlation with the solar wind speed. Here we inspect both Parker Solar Probe (PSP) and WIND data and compare the observations with simulation results. PSP observations below 30 solar radii clearly show a positive correlation between proton temperature and wind speed and a negative correlation between electron temperature and wind speed. One year (2019) of WIND data confirm that proton temperature is positively correlated with solar wind speed, but the electron temperature increases with the solar wind speed for slow wind while it decreases with the solar wind speed for fast wind. Using a one-dimensional Alfvén-wave-driven solar wind model with different proton and electron temperatures, we for the first time find that if most of the dissipated Alfvén wave energy heats the ions instead of electrons, a positive $T_i-V_{SW}$ correlation and a negative $T_e-V_{SW}$ correlation arise naturally. If the electrons gain a small but finite portion of the dissipated wave energy, the $T_e-V_{SW}$ correlation evolves with radial distance to the Sun such that the negative correlation gradually turns positive. The model results show that Alfvén waves are one of the possible explanations of the observed evolution of proton and electron temperatures in the solar wind.

astro-ph.SR↗

arXiv2022

Constraining the CME Core Heating and Energy Budget with SOHO/UVCS

We describe the energy budget of a coronal mass ejection (CME) observed on 1999 May 17 with the Ultraviolet Coronagraph Spectrometer (UVCS). We constrain the physical properties of the CME's core material as a function of height along the corona by using the spectra taken by the single-slit coronagraph spectrometer at heliocentric distances of 2.6 and 3.1 solar radii. We use plasma diagnostics from intensity ratios, such as the O VI doublet lines, to determine the velocity, density, temperature, and non-equilibrium ionization states. We find that the CME core's velocity is approximately 250 km/s, and its cumulative heating energy is comparable to its kinetic energy for all of the plasma heating parameterizations that we investigated. Therefore, the CME's unknown heating mechanisms have the energy to significantly affect the CME's eruption and evolution. To understand which parameters might influence the unknown heating mechanism, we constrain our model heating rates with the observed data and compare them to the rate of heating generated within a similar CME that was constructed by the MAS code's 3D MHD simulation. The rate of heating from the simulated CME agrees with our observationally constrained heating rates when we assume a quadratic power law to describe a self-similar CME expansion. Furthermore, the heating rates agree when we apply a heating parameterization that accounts for the CME flux rope's magnetic energy being converted directly into thermal energy. This UVCS analysis serves as a case study for the importance of multi-slit coronagraph spectrometers for CME studies.

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arXiv2021

Using Parker Solar Probe observations during the first four perihelia to constrain global magnetohydrodynamic models

Parker Solar Probe (PSP) is providing an unprecedented view of the Sun's corona as it progressively dips closer into the solar atmosphere with each solar encounter. Each set of observations provides a unique opportunity to test and constrain global models of the solar corona and inner heliosphere and, in turn, use the model results to provide a global context for interpreting such observations. In this study, we develop a set of global magnetohydrodynamic (MHD) model solutions of varying degrees of sophistication for PSP's first four encounters and compare the results with in situ measurements from PSP, Stereo-A, and Earth-based spacecraft, with the objective of assessing which models perform better or worse. All models were primarily driven by the observed photospheric magnetic field using data from Solar Dynamics Observatory's Helioseismic and Magnetic Imager (HMI) instrument. Overall, we find that there are substantial differences between the model results, both in terms of the large-scale structure of the inner heliosphere during these time periods, as well as in the inferred time-series at various spacecraft. The "thermodynamic" model, which represents the "middle ground", in terms of model complexity, appears to reproduce the observations most closely for all four encounters. Our results also contradict an earlier study that had hinted that the open flux problem may disappear nearer the Sun. Instead, our results suggest that this "missing" solar flux is still missing even at 26.9 Rs, and thus it cannot be explained by interplanetary processes. Finally, the model results were also used to provide a global context for interpreting the localized in situ measurements.

physics.space-ph↗

arXiv2021

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.

astro-ph.SR↗

arXiv2019

Predicting the Structure of the Solar Corona and Inner Heliosphere during Parker Solar Probe's First Perihelion Pass

NASA's Parker Solar Probe (Parker) spacecraft reached its first perihelion of 35.7 solar radii on November 5th, 2018. To aid in mission planning, and in anticipation of the unprecedented measurements to be returned, in late October, we developed a three-dimensional magnetohydrodynamic (MHD) solution for the solar corona and inner heliosphere, driven by the then available observations of the Sun's photospheric magnetic field. Our model incorporates a wave-turbulence-driven (WTD) model to heat the corona. Here, we present our predictions for the structure of the solar corona and the likely {\it in situ} measurements that Parker will be returning over the next few months. We infer that, in the days prior to 1st Encounter, Parker was immersed in wind emanating from a positive-polarity equatorial coronal hole. During the encounter, however, field lines from the spacecraft mapped to another, negative-polarity equatorial coronal hole. Following the encounter, Parker was magnetically connected to the large, positive-polarity northern polar coronal hole. When the Parker data become available, these model results can be used to assist in their calibration and interpretation, and, additionally, provide a global context for interpreting the localized {\it in situ} measurements. In particular, we can identify what types of solar wind Parker encountered, what the underlying magnetic structure was, and how complexities in the orbital trajectory can be interpreted within a global, inertial frame. Ultimately, the measurements returned by Parker can be used to constrain current theories for heating the solar corona and accelerating the solar wind.

physics.space-ph↗

arXiv2019

Coupled MHD-Focused Transport Simulations for Modeling Solar Particle Events

We describe the initial version of the Solar Particle Event (SPE) Threat Assessment Tool or STAT. STAT relies on elements of Corona-Heliosphere (CORHEL) and the Earth-Moon-Mars Radiation Environment Module (EMMREM), and allows users to investigate coronal mass ejection (CME) driven SPEs using coupled magnetohydrodynamic (MHD) and focused transport solutions. At the present time STAT focuses on modeling solar energetic particle (SEP) acceleration in and transport from the low corona, where the highest energy SEP events are generated. We illustrate STAT's capabilities with a model of the July 14, 2000 "Bastille Day" event, including innovative diagnostics for understanding the three-dimensional distribution of particle fluxes and their relation to the structure of the underlying CME driver. A preliminary comparison with NOAA GOES measurements is shown.

astro-ph.SR↗

arXiv2019

Ion Charge States in a Time-Dependent Wave-Turbulence-Driven Model of the Solar Wind

Ion fractional charge states, measured in situ in the heliosphere, depend on the properties of the plasma in the inner corona. As the ions travel outward in the solar wind and the electron density drops, the charge states remain essentially unaltered or "frozen in". Thus they can provide a powerful constraint on heating models of the corona and acceleration of the solar wind. We have implemented non-equilibrium ionization calculations into a 1D wave-turbulence-driven (WTD) hydrodynamic solar wind model and compared modeled charge states with the Ulysses 1994-5 in situ measurements. We have found that modeled charge state ratios of $C^{6+}/C^{5+}$ and $O^{7+}/O^{6+}$, among others, were too low compared with Ulysses measurements. However, a heuristic reduction of the plasma flow speed has been able to bring the modeled results in line with observations, though other ideas have been proposed to address this discrepancy. We discuss implications of our results and the prospect of including ion charge state calculations into our 3D MHD model of the inner heliosphere.

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↗

arXiv2018

The State of the Solar Wind, Magnetosphere, and Ionosphere During the Maunder Minimum

Both direct observations and reconstructions from various datasets, suggest that conditions were radically different during the Maunder Minimum (MM) than during the space era. Using an MHD model, we develop a set of feasible solutions to infer the properties of the solar wind during this interval. Additionally, we use these results to drive a global magnetospheric model. Finally, using the 2008/2009 solar minimum as an upper limit for MM conditions, we use results from the International Reference Ionosphere (ILI) model to speculate on the state of the ionosphere. The results describe interplanetary, magnetospheric, and ionospheric conditions that were substantially different than today. For example: (1) the solar wind density and magnetic field strength were an order of magnitude lower; (2) the Earth's magnetopause and shock standoff distances were a factor of two larger; and (3) the maximum electron density in the ionosphere was substantially lower.

physics.space-ph↗