arXiv subjects

Nishtha Sachdeva

Publications and source records attributed to Nishtha Sachdeva.

17 recordsLinked to original sources

arXiv2026

The Effects of Energy Conservation in Simulating Solar Eruptions

Strict energy conservation is, perhaps, the most basic principle in all physics, but has proven to be difficult to satisfy in numerical simulations of solar eruptions. The Alfv\'en Wave Solar atmosphere Model (AWSoM) is used to perform a rigorous comparison of CME simulations whose only difference is the use of a conservative vs. non-conservative scheme for the energy equation. A simple, symmetric active region is assumed for the initial magnetic field. As expected, the different numerical schemes result in very different plasma thermal energy, but surprisingly, we also find a factor $>2$ difference in the final kinetic energy, with the energy substantially larger in the energy-conservative scheme. The increase in thermal energy is comparable to the increase in kinetic energy in the conservative simulation. Our analysis reveals that the flare reconnection and increase of kinetic energy terminate earlier with the non-conservative scheme. We conclude that the plasma thermodynamics plays a critical role in the flare reconnection, with the thermal pressure gradient in the current sheet slowing down the reconnection. Our results imply that using strict energy-conservative numerics is critical for space weather modeling of CMEs and for understanding the CME energy budget partitioning.

astro-ph.SR↗

arXiv2026

Effects of the Background Magnetic Field on Flux Rope Eruptions

Solar eruptive events are generally believed to involve magnetic flux ropes (MFR), formed either in the pre-eruptive phase of the event or during the eruption itself. These MFR eruptions exhibit significant complexity and variations due to the interplay of the physical mechanisms involved, in particular magnetic reconnection and ideal instabilities. This work considers the effect of the background magnetic field on the nature of eruptions with pre-existing MFRs. We used a new MHD model to simulate the whole MFR eruption process, including the pre-eruptive stage and the initiation. Three simulations were performed, all of which used an identical bipolar active region, but with different background magnetic fields in the three cases. The simulations resulted in two successful eruptions (CMEs) and one failed eruption (a confined flare). We analyzed the energetics and the acceleration of the MFR in detail, and found a transition to a rapid exponential rise phase in two of the simulations. We also calculated the criterion for the torus instability and the timing of the breakout and flare reconnections. Our results show that the rapid exponential rise phase is likely due to breakout reconnection. We conclude that a background field antiparallel to the active-region field lowers the magnetic free-energy threshold for eruption; but, does not guarantee a successful eruption. We also found that an antiparallel background field leads to faster flare reconnection, but of shorter duration. Our findings underscore the importance of the background magnetic field in understanding CMEs.

astro-ph.SR↗

arXiv2026

Evolution of Coronal Mass Ejections in Different Data-Driven Solar Wind Conditions

Numerical models of the solar wind and coronal mass ejections (CMEs) utilize photospheric magnetic field observations to prescribe the inner boundary conditions for the plasma solutions. These magnetic field data are available to the community through various observational instruments, prepared via different methodologies and/or flux-transport models. The solar wind solution driven by these maps provides the ambient plasma environment into which CMEs travel, coupling, and interacting with the surrounding plasma and governing the CME evolution and propagation in the solar corona and inner heliosphere. In this work, we use different input magnetic field maps for the same time period to drive the global Alfven Wave Solar atmosphere Model (AWSoM). We obtain the ambient solar wind conditions and compare the plasma properties and magnetic morphology in the coronal domain to study the influence of the input maps. To understand how the resulting coronal solutions impact CMEs, we launch eruptions described by analytical flux ropes into these data-driven solutions and compare their evolution in the coronal domain (up to 24 solar radii radially). The CMEs achieve varying speeds, deceleration rates, propagation directions, mass and energies while coupling with the background solar wind. We quantify these differences to show that the different input driving maps can significantly impact the simulated CME propagation in the solar wind plasma. This also highlights the importance of understanding the uncertainties associated with data-driven modeling that become increasingly important in operational models and space weather prediction.

astro-ph.SR↗

arXiv2025

Simulated Operational Testing of the Prototype Implementation of the SOFIE Model: The 2025 Space Weather Prediction Testbed Exercise

The CLEAR Space Weather Center of Excellence's solar energetic particle (SEP) model, SOlar wind with FIeld lines and Energetic particles (SOFIE), was run and evaluated on-site during the Space Weather Prediction Testbed (SWPT) exercise at the National Oceanic and Atmospheric Administration's Space Weather Prediction Center (NOAA/SWPC) in May 2025. As a physics-based SEP model, SOFIE simulates the acceleration and transport of energetic particles by the coronal mass ejection (CME)-driven shock in the solar corona and inner heliosphere, and has been validated against historical events. However, questions remain regarding whether a physics-based model, traditionally considered computationally expensive, could meet operational needs. The SWPT exercise offered a valuable opportunity to evaluate SOFIE under simulated operational conditions. On-site interactive feedback from SWPC forecasters, Space Radiation Analysis Group (SRAG) console operators, Community Coordinated Modeling Center (CCMC) personnel, and Moon-to-Mars Space Weather Analysis Office (M2M SWAO) analysts led to significant strategic improvements in the model configuration. The simulation grid was optimized by combining a coarser background grid with higher-resolution regions along the CME path and toward Earth, reducing computational cost without compromising accuracy. In this work, we present the simulated operational performance of SOFIE and its capability to predict SEP fluxes significantly faster than real time. During the SWPT exercise, SOFIE completed a 4-day SEP simulation within 5 hours using 1,000 central processing unit cores, although the earliest SEP forecast was obtained a few hours after CME onset. This marks a milestone in demonstrating SOFIE's operational usefulness and robustness to support future human space exploration.

astro-ph.SR↗

arXiv2025

Evidence of Time-Dependent Diffusive Shock Acceleration in the 2022 September 5 Solar Energetic Particle Event

On 2022 September 5, a large solar energetic particle (SEP) event was detected by Parker Solar Probe (PSP) and Solar Orbiter (SolO), at heliocentric distances of 0.07 and 0.71 au, respectively. PSP observed an unusual velocity-dispersion signature: particles below $\sim$1 MeV exhibited a normal velocity dispersion, while higher-energy particles displayed an inverse velocity arrival feature, with the most energetic particles arriving later than those at lower energies. The maximum energy increased from about 20-30 MeV upstream to over 60 MeV downstream of the shock. The arrival of SEPs at PSP was significantly delayed relative to the expected onset of the eruption. In contrast, SolO detected a typical large SEP event characterized by a regular velocity dispersion at all energies up to 100 MeV. To understand these features, we simulate particle acceleration and transport from the shock to the observers with our newly developed SEP model - Particle ARizona and MIchigan Solver on Advected Nodes (PARMISAN). Our results reveal that the inverse velocity arrival and delayed particle onset detected by PSP originate from the time-dependent diffusive shock acceleration processes. After shock passage, PSP's magnetic connectivity gradually shifted due to its high velocity near perihelion, detecting high-energy SEPs streaming sunward. Conversely, SolO maintained a stable magnetic connection to the strong shock region where efficient acceleration was achieved. These results underscore the importance of spatial and temporal dependence in SEP acceleration at interplanetary shocks, and provide new insights to understand SEP variations in the inner heliosphere.

astro-ph.SR↗

arXiv2024

Physics-Based Simulation of the 2013 April 11 Solar Energetic Particle Event

Solar energetic particles (SEPs) can pose hazardous radiation risks to both humans and spacecraft electronics in space. Numerical modeling based on first principles offers valuable insights into the underlying physics of SEPs and provides synthetic observables for SEPs at any time and location in the inner heliosphere. In this work, we present a numerical scheme, which conserves the number of particles based on integral relations for Poisson brackets \citep{sokolov2023high}, to solve the kinetic equation for particle acceleration and transport processes. We implement this scheme within the Space Weather Modeling Framework, developed at the University of Michigan. In addition, we develop a new shock-capturing tool to study the coronal mass ejection-driven shock originating from the low solar corona. These methodological advancements are applied to conduct a comprehensive study of a historical SEP event on April 11, 2013. Multi-spacecraft observations, including SOHO, SDO, GOES and ACE near Earth, and STEREO-A/B, are used for model--data comparison and validation. We show synthetic observables, including extreme ultraviolet and white-light images, proton time--intensity profiles, and energy spectra, and discuss their differences and probable explanations compared to observations. Our simulation results demonstrate the application of the Poisson bracket scheme with a particle solver to simulating a historical SEP event. We also show the capability of extracting the complex shock surface using our shock-capturing tool and understand how the complex shock surface affects the particle acceleration process.

astro-ph.SR↗

arXiv2024

Solar Wind Driven from GONG Magnetograms in the Last Solar Cycle

In a previous study, Huang et al. (2023) used the Alfven Wave Solar atmosphere Model (AWSoM), one of the widely used solar wind models in the community, driven by ADAPT-GONG magnetograms to simulate the solar wind in the last solar cycle and found that the optimal Poynting flux parameter can be estimated from either the open field area or the average unsigned radial component of the magnetic field in the open field regions. It was also found that the average energy deposition rate (Poynting flux) in the open field regions is approximately constant. In the current study, we expand the previous work by using GONG magnetograms to simulate the solar wind for the same Carrington rotations and determine if the results are similar to the ones obtained with ADAPT-GONG magnetograms. Our results indicate that similar correlations can be obtained from the GONG maps. Moreover, we report that ADAPT-GONG magnetograms can consistently provide better comparisons with 1 AU solar wind observations than GONG magnetograms, based on the best simulations selected by the minimum of the average curve distance for the solar wind speed and density.

astro-ph.SR↗

arXiv2024

Adjusting the Potential Field Source Surface Height Based on MHD Simulations

A potential field solution is widely used to extrapolate the coronal magnetic field above the Sun's surface to a certain height. This model applies the current-free approximation and assumes that the magnetic field is entirely radial beyond the source surface height, which is defined as the radial distance from the center of the Sun. Even though the source surface is commonly specified at 2.5 Rs (solar radii), previous studies have suggested that this value is not optimal in all cases. In this study, we propose a novel approach to specify the source surface height, by comparing the areas of the open magnetic field regions from the potential field solution with predictions made by a magnetohydrodynamics model, in our case the Alfven Wave Solar atmosphere Model. We find that the adjusted source surface height is significantly less than 2.5 Rs near solar minimum, and slightly larger than 2.5 Rs near solar maximum. We also report that the adjusted source surface height can provide a better open flux agreement with the observations near the solar minimum, while the comparison near the solar maximum is slightly worse.

astro-ph.SR↗

arXiv2023

Solar Wind with Field Lines and Energetic Particles (SOFIE) Model: Application to Historical Solar Energetic Particle Events

In this paper, we demonstrate the applicability of the data-driven and self-consistent solar energetic particle model, Solar-wind with FIeld-lines and Energetic-particles (SOFIE), to simulate acceleration and transport processes of solar energetic particles. SOFIE model is built upon the Space Weather Modeling Framework (SWMF) developed at the University of Michigan. In SOFIE, the background solar wind plasma in the solar corona and interplanetary space is calculated by the Aflv\'en Wave Solar-atmosphere Model(-Realtime) (AWSoM-R) driven by the near-real-time hourly updated Global Oscillation Network Group (GONG) solar magnetograms. In the background solar wind, coronal mass ejections (CMEs) are launched by placing an imbalanced magnetic flux rope on top of the parent active region, using the Eruptive Event Generator using Gibson-Low model (EEGGL). The acceleration and transport processes are modeled by the Multiple-Field-Line Advection Model for Particle Acceleration (M-FLAMPA). In this work, nine solar energetic particle events (Solar Heliospheric and INterplanetary Environment (SHINE) challenge/campaign events) are modeled. The three modules in SOFIE are validated and evaluated by comparing with observations, including the steady-state background solar wind properties, the white-light image of the CME, and the flux of solar energetic protons, at energies of > 10 MeV.

astro-ph.SR↗

arXiv2022

Solar wind modeling with the Alfven Wave Solar atmosphere Model driven by HMI-based Near-Real-Time maps by the National Solar Observatory

We explore model performance for the Alfven Wave Solar atmosphere Model (AWSoM) with near-real-time (NRT) synoptic maps of the photospheric vector magnetic field. These maps, produced by assimilating data from the Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), use a different method developed at the National Solar Observatory (NSO) to provide a near contemporaneous source of data to drive numerical models. Here, we apply these NSO-HMI-NRT maps to simulate three Carrington rotations (CRs): 2107-2108 (centered on 2011/03/07 20:12 CME event), 2123 (integer CR) and 2218--2219 (centered on 2019/07/2 solar eclipse), which together cover a wide range of activity level for solar cycle 24. We show simulation results, which reproduce both extreme ultraviolet emission (EUV) from the low corona while simultaneously matching in situ observations at 1 au as well as quantify the total unsigned open magnetic flux from these maps.

astro-ph.SR↗

arXiv2022

On the specific energy and pressure in near-Earth magnetic clouds

The pressure and energy density of the gas and magnetic field inside solar coronal mass ejections (in relation to that in the ambient solar wind) is thought to play an important role in determining their dynamics as they propagate through the heliosphere. We compare the specific energy (${\rm erg\,g^{-1}}$) [comprising kinetic ($H_{\rm k}$), thermal ($H_{\rm th }$) and magnetic field ($H_{\rm mag}$) contributions] inside MCs and the solar wind background. We examine if the excess thermal + magnetic pressure and specific energy inside MCs (relative to the background) is correlated with their propagation and internal expansion speeds. We ask if the excess thermal + magnetic specific energy inside MCs might make them resemble rigid bodies in the context of aerodynamic drag. We use near-Earth in-situ data from the WIND spacecraft to identify a sample of 152 well observed interplanetary coronal mass ejections and their MC counterparts. We compute various metrics using these data to address our questions. We find that the total specific energy ($H$) inside MCs is approximately equal to that in the background solar wind. We find that the the excess (thermal + magnetic) pressure and specific energy are not well correlated with the near-Earth propagation and expansion speeds. We find that the excess thermal+magnetic specific energy $\gtrsim$ the specific kinetic energy of the solar wind incident on 81--89 \% of the MCs we study. This might explain how MCs retain their structural integrity and resist deformation by the solar wind bulk flow.

astro-ph.SR↗

arXiv2022

Modeling the Solar Wind During Different Phases of the Last Solar Cycle

We describe our first attempt to systematically simulate the solar wind during different phases of the last solar cycle with the Alfv\'en 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 Alfv\'en 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 anti-correlated 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 Alfv\'en 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.

astro-ph.SR↗

arXiv2021

What Sustained Multi-Disciplinary Research Can Achieve: The Space Weather Modeling Framework

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).

physics.space-ph↗

arXiv2019

Dynamics of solar Coronal Mass Ejections: forces that impact their propagation

We investigate the Sun-Earth dynamics of a set of 38 well-observed Coronal Mass Ejections(CMEs) using data from the STEREO, SOHO missions and WIND instrument. We seek to quantify the relative contributions of Lorentz force and aerodynamic drag on their propagation. The CMEs are 3D reconstructed using the Graduated Cylindrical Shell(GCS) model to derive observed CME parameters.Using a microphysical prescription of the drag coefficient,we find that solar wind aerodynamic drag adequately accounts for the dynamics of the fastest CMEs from as low as 3.5 Rs. For relatively slower CMEs, however, we find that when the drag-based model is initiated below the distances ranging from 12 to 50 Rs,the observed CME trajectories cannot be accounted for.This suggests that aerodynamic drag force dominates the dynamics of slower CMEs only above these heights.To investigate CME dynamics below the heights where aerodynamic drag dominates, we consider the Torus Instability model for the driving Lorentz force. We find that for fast CMEs,Lorentz forces become negligible in comparison to aerodynamic drag as early as 3.5-4 Rs.For slow CMEs, however, they become negligible only by 12-50 Rs.This justifies the success of the drag-only model for fast CMEs. In case of slow CMEs, the Lorentz force is only slightly smaller than the drag force even beyond 12-50 Rs.For these slow events,our results suggest that some of the magnetic flux carried by CMEs might be expended in expansion or heating. These dissipation effects might be important in describing the propagation of slower CMEs. To the best of our knowledge,this is the first systematic study in this regard using a diverse CME sample. A physical understanding of the forces that affect CME propagation and how they compare with each other at various heliocentric distances is an important ingredient in building tools for describing and predicting CME trajectories.

astro-ph.SR↗

arXiv2019

Episodic Jets from Black Hole Accretion Disks

Several active galactic nuclei and microquasars are observed to eject plasmoids that move at relativistic speeds. We envisage the plasmoids as pre-existing current carrying magnetic flux ropes that were initially anchored in the accretion disk-corona. The plasmoids are ejected outwards via a mechanism called the toroidal instability (TI). The TI, which was originally explored in the context of laboratory tokamak plasmas, has been very successful in explaining coronal mass ejections from the Sun. Our model predictions for plasmoid trajectories compare favorably with a representative set of multi-epoch observations of radio emitting knots from the radio galaxy 3C120, which were preceded by dips in Xray intensity.

astro-ph.HE↗

arXiv2017

CME dynamics using STEREO & LASCO observations: the relative importance of Lorentz forces and solar wind drag

We seek to quantify the relative contributions of Lorentz forces and aerodynamic drag on the propagation of solar coronal mass ejections (CMEs). We use Graduated Cylindrical Shell (GCS) model fits to a representative set of 38 CMEs observed with the SOHO and STEREO spacecraft. We find that the Lorentz forces generally peak between 1.65 and 2.45 Rsun for all CMEs. For fast CMEs, Lorentz forces become negligible in comparison to aerodynamic drag as early as 3.5--4 Rsun. For slow CMEs, however, they become negligible only by 12--50 Rsun. For these slow events, our results suggest that some of the magnetic flux might be expended in CME expansion or heating. In other words, not all of it contributes to directed propagation. Our results are expected to be important in building a physical model for understanding the Sun-Earth dynamics of CMEs.

astro-ph.SR↗

arXiv2015

CME propagation: Where does solar wind drag take over?

We investigate the Sun-Earth dynamics of a set of eight well observed solar coronal mass ejections (CMEs) using data from the STEREO spacecraft. We seek to quantify the extent to which momentum coupling between these CMEs and the ambient solar wind (i.e., the aerodynamic drag) influences their dynamics. To this end, we use results from a 3D flux rope model fit to the CME data. We find that solar wind aerodynamic drag adequately accounts for the dynamics of the fastest CME in our sample. For the relatively slower CMEs, we find that drag-based models initiated below heliocentric distances ranging from 15 to 50 $R_{\odot}$ cannot account for the observed CME trajectories. This is at variance with the general perception that the dynamics of slow CMEs are influenced primarily by solar wind drag from a few $R_{\odot}$ onwards. Several slow CMEs propagate at roughly constant speeds above 15--50 $R_{\odot}$. Drag-based models initiated above these heights therefore require negligible aerodynamic drag to explain their observed trajectories.

astro-ph.SR↗