Cooper Downs

Studying extreme ultraviolet wave transients with a digital laboratory: Direct comparison of extreme ultraviolet wave observations to global magnetohydrodynamic simulations

In this work, we describe our effort to explore the signatures of large-scale extreme ultraviolet (EUV) transients in the solar corona (EUV waves) using a three-dimensional thermodynamic magnetohydrodynamic model. We conduct multiple simulations of the 2008 March 25 EUV wave (~18:40 UT), observed both on and off of the solar disk by the STEREO-A and B spacecraft. By independently varying fundamental parameters thought to govern the physical mechanisms behind EUV waves in each model, such as the ambient magneto-sonic speed, eruption free energy, and eruption handedness, we are able to assess their respective contributions to the transient signature. A key feature of this work is the ability to synthesize the multi-filter response of the STEREO Extreme UltraViolet Imagers directly from model data, which gives a means for direct interpretation of EUV observations with full knowledge of the three-dimensional magnetic and thermodynamic structures in the simulations. We discuss the implications of our results with respect to some commonly held interpretations of EUV waves (e.g., fast-mode magnetosonic wave, plasma compression, reconnection front, etc.) and present a unified scenario which includes both a wave-like component moving at the fast magnetosonic speed and a coherent driven compression front related to the eruptive event itself.

Magnetohydrodynamic Waves and Coronal Heating: Unifying Empirical and MHD Turbulence Models

We present a new global model of the solar corona, including the low corona, the transition region, and the top of the chromosphere. The realistic three-dimensional magnetic field is simulated using the data from the photospheric magnetic field measurements. The distinctive feature of the new model is incorporating MHD Alfven wave turbulence. We assume this turbulence and its nonlinear dissipation to be the only momentum and energy source for heating the coronal plasma and driving the solar wind. The difference between the turbulence dissipation efficiency in coronal holes and that in closed field regions is because the nonlinear cascade rate degrades in strongly anisotropic (imbalanced) turbulence in coronal holes (no inward propagating wave), thus resulting in colder coronal holes, from which the fast solar wind originates. The detailed presentation of the theoretical model is illustrated with the synthetic images for multi-wavelength EUV emission compared with the observations from SDO AIA and STEREO EUVI instruments for the Carrington rotation 2107.

TOWARD A REALISTIC THERMODYNAMIC MAGNETOHYDRODYNAMIC MODEL OF THE GLOBAL SOLAR CORONA

In this work, we describe our implementation of a thermodynamic energy equation into the global corona model of the Space Weather Modeling Framework and its development into the new lower corona (LC) model. This work includes the integration of the additional energy transport terms of coronal heating, electron heat conduction, and optically thin radiative cooling into the governing magnetohydrodynamic (MHD) energy equation. We examine two different boundary conditions using this model; one set in the upper transition region (the radiative energy balance model), as well as a uniform chromospheric condition where the transition region can be modeled in its entirety. Via observation synthesis from model results and the subsequent comparison to full Sun extreme ultraviolet and soft X-ray observations of Carrington rotation 1913 centered on 1996 August 27, we demonstrate the need for these additional considerations when using global MHD models to describe the unique conditions in the low corona. Through multiple simulations, we examine the ability of the LC model to assess and discriminate between coronal heating models, and find that a relative simple empirical heating model is adequate in reproducing structures observed in the low corona. We show that the interplay between coronal heating and electron heat conduction provides significant feedback onto the three-dimensional magnetic topology in the low corona as compared to a potential field extrapolation, and that this feedback is largely dependent on the amount of mechanical energy introduced into the corona.

Understanding SDO/AIA Observations of the 2010 June 13 EUV Wave Event: Direct Insight from a Global Thermodynamic MHD Simulation

In this work, we present a comprehensive observation and modeling analysis of the 2010 June 13 extreme-ultraviolet (EUV) wave observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). Due to extreme advances in cadence, resolution, and bandpass coverage in the EUV regime, the AIA instrument offers an unprecedented ability to observe the dynamics of large-scale coronal wave-like transients known as EUV waves. To provide a physical analysis and further complement observational insight, we conduct a three-dimensional, time-dependent thermodynamic MHD simulation of the eruption and associated EUV wave, and employ forward modeling of EUV observables to compare the results directly observations. We focus on two main aspects: (1) the interpretation of the stark thermodynamic signatures in the multi-filter AIA data within the propagating EUV wave front, and (2) an in-depth analysis of the simulation results and their implication with respect to EUV wave theories. Multiple aspects, including the relative phases of perturbed variables, suggest that the outer, propagating component of the EUV transient exhibits the behavior of a fast-mode wave. We also find that this component becomes decoupled from the evolving structures associated with the coronal mass ejection that are also visible, providing a clear distinction between wave and non-wave mechanisms at play.

Validating a time-dependent turbulence-driven model of the solar wind

Although the mechanisms responsible for heating the Suns corona and accelerating the solar wind are still being actively investigated, it is largely accepted that photospheric motions provide the energy source and that the magnetic field must play a key role in the process. Verdini et al. presented a model for heating and accelerating the solar wind based on the turbulent dissipation of Alfven waves. We first use a time-dependent model of the solar wind to reproduce one of Verdini et al.s solutions; then, we extend its application to the case where the energy equation includes thermal conduction and radiation losses, and the upper chromosphere is part of the computational domain. Using this model, we explore the parameter space and describe the characteristics of a fast solar wind solution. We discuss how this formulation may be applied to a three-dimensional MHD model of the corona and solar wind.

Probing the solar magnetic field with a Sun-grazing comet.

A Comet in the Sun In 2011, comet Lovejoy plunged into the solar atmosphere and survived its flight through a region of the Sun that has never been visited by spacecraft. Downs et al. (p. 1196) used spacecraft observations of this Sun-grazing comet, combined with advanced magnetohydrodynamic simulations, to constrain the magnetic field of the solar atmosphere—a quantity that has been very difficult to measure directly. Observations of a comets motion through the solar corona constrain this regions magnetic field and plasma properties. On 15 and 16 December 2011, Sun-grazing comet C/2011 W3 (Lovejoy) passed deep within the solar corona, effectively probing a region that has never been visited by spacecraft. Imaged from multiple perspectives, extreme ultraviolet observations of Lovejoys tail showed substantial changes in direction, intensity, magnitude, and persistence. To understand this unique signature, we combined a state-of-the-art magnetohydrodynamic model of the solar corona and a model for the motion of emitting cometary tail ions in an embedded plasma. The observed tail motions reveal the inhomogeneous magnetic field of the solar corona. We show how these motions constrain field and plasma properties along the trajectory, and how they can be used to meaningfully distinguish between two classes of magnetic field models.

MAGNETOHYDRODYNAMIC SIMULATIONS OF INTERPLANETARY CORONAL MASS EJECTIONS

We describe a new MHD model for the propagation of interplanetary coronal mass ejections (ICMEs) in the solar wind. Accurately following the propagation of ICMEs is important for determining space weather conditions. Our model solves the MHD equations in spherical coordinates from a lower boundary above the critical point to Earth and beyond. On this spherical surface, we prescribe the magnetic field, velocity, density, and temperature calculated typically directly from a coronal MHD model as time-dependent boundary conditions. However, any model that can provide such quantities either in the inertial or rotating frame of the Sun is suitable. We present two validations of the technique employed in our new model and a more realistic simulation of the propagation of an ICME from the Sun to Earth.

PARTICLE ACCELERATION AT LOW CORONAL COMPRESSION REGIONS AND SHOCKS

We present a study on particle acceleration in the low corona associated with the expansion and acceleration of coronal mass ejections (CMEs). Because CME expansion regions low in the corona are effective accelerators over a finite spatial region, we show that there is a rigidity regime where particles effectively diffuse away and escape from the acceleration sites using analytic solutions to the Parker transport equation. This leads to the formation of broken power-law distributions. Based on our analytic solutions, we find a natural ordering of the break energy and second power-law slope (above the break energy) as a function of the scattering characteristics. These relations provide testable predictions for the particle acceleration from low in the corona. Our initial analysis of solar energetic particle observations suggests a range of shock compression ratios and rigidity dependencies that give rise to the solar energetic particle (SEP) events studied. The wide range of characteristics inferred suggests competing mechanisms at work in SEP acceleration. Thus, CME expansion and acceleration in the low corona may naturally give rise to rapid particle acceleration and broken power-law distributions in large SEP events.

THE SOLAR CORONA AS PROBED BY COMET LOVEJOY (C/2011 W3)

Extreme-ultraviolet images of Comet Lovejoy (C/2011 W3) from the Atmospheric Imaging Assembly show striations related to the magnetic field structure in both open and closed magnetic regions. The brightness contrast implies coronal density contrasts of at least a factor of six between neighboring flux tubes over scales of a few thousand kilometers. These density structures imply variations in the Alfven speed on a similar scale. They will drastically affect the propagation and dissipation of Alfven waves, and that should be taken into account in models of coronal heating and solar wind acceleration. In each striation, the cometary emission moves along the magnetic field and broadens with time. The speed and the rate of broadening are related to the parallel and perpendicular components of the velocities of the cometary neutrals when they become ionized. We use a magnetohydrodynamic model of the coronal magnetic field and the theory of pickup ions to compare the measurements with theoretical predictions, in particular with the energy lost to Alfven waves as the cometary ions isotropize.

The Open Flux Problem

The heliospheric magnetic field is of pivotal importance in solar and space physics. The field is rooted in the Suns photosphere, where it has been observed for many years. Global maps of the solar magnetic field based on full disk magnetograms are commonly used as boundary conditions for coronal and solar wind models. Two primary observational constraints on the models are (1) the open field regions in the model should approximately correspond to coronal holes observed in emission, and (2) the magnitude of the open magnetic flux in the model should match that inferred from in situ spacecraft measurements. In this study, we calculate both MHD and PFSS solutions using fourteen different magnetic maps produced from five different types of observatory magnetograms, for the time period surrounding July, 2010. We have found that for all of the model/map combinations, models that have coronal hole areas close to observations underestimate the interplanetary magnetic flux, or, conversely, for models to match the interplanetary flux, the modeled open field regions are larger than coronal holes observed in EUV emission. In an alternative approach, we estimate the open magnetic flux entirely from solar observations by combining automatically detected coronal holes for Carrington rotation 2098 with observatory synoptic magnetic maps. This approach also underestimates the interplanetary magnetic flux. Our results imply that either typical observatory maps underestimate the Suns magnetic flux, or a significant portion of the open magnetic flux is not rooted in regions that are obviously dark in EUV and X-ray emission.