Date of Award

Winter 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Astrophysical & Planetary Sciences

First Advisor

Sarah Gibson

Second Advisor

Mark Rast

Abstract

The corona is a unique layer in the solar atmosphere where the temperature rapidly increases as a function of radial height. While a majority of the corona exists at temperatures above one million degrees Kelvin, there are embedded regions of much cooler and denser plasma known as prominences. The formation mechanism and force balance of prominences are open questions in solar physics. The energetic equilibrium of the corona is described by a balance of heating, thermal conduction, and radiative cooling. Prominences can be described by the thermal instability of coronal energy balance which leads to the formation of cool condensations. For condensations to exist in force equilibrium in the corona, they must be supported against gravity by the magnetic field. Condensations will fall into the gravitational well provided on dipped magnetic field lines, but these field lines will also contain coronal plasma which surrounds the condensation. These formation and stability considerations require a hydrodynamic energetic connection and a magnetostatic structural connection between the prominence and the surrounding corona. Observationally, the prominence is surrounded by a density depleted elliptical structure known as a cavity. In this dissertation, we use extreme ultraviolet remote sensing observations of the prominence-cavity system to diagnose the static and dynamic properties of these structures. The observations are compared with numerical models for the time-dependent coronal condensation process and the time-independent corona-prominence magnetic field. To diagnose the density of the cavity, we construct a three-dimensional structural model of the corona. This structural model allows us to synthesize extreme ultraviolet emission in the corona in a way that incorporates the projection effects which arise from the optically thin plasma. This forward model technique is used to constrain a radial density profile simultaneously in the cavity and the streamer. We use a &chi2 minimization to find the density model which best matches a density sensitive line ratio (observed with Hinode/Extreme ultraviolet Imaging Spectrometer) and the white light scattered intensity (observed with Mauna Loa Solar Observatory MK4 coronagraph). This diagnostic finds that the observed cavity has a density depletion of 30\% relative to the streamer. The diagnosis of thermodynamic properties of the static cavity provides a measurement of the time-averaged equilibrium condition of the plasma. However, the coronal cooling model of prominence formation requires that there will be a time-dependent change in the coronal region which supplies mass to the prominence. We use extreme ultraviolet spectra and spectral images to diagnose the dynamics of the prominence and the surrounding corona. Based on the doppler shift of extreme ultraviolet coronal emission lines, we find that there are large regions of flowing plasma which appear to occur within cavities. These line of sight flows have speeds of 10 km/s and projected spatial scales of 100 Mm. Using the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA) dataset, we observe dynamic emission from the prominence-cavity system. The SDO/AIA dataset observes multiple spectral bandpasses with different temperature sensitivities. Time-dependent changes in the observed emission in these bandpass images represent changes in the thermodynamic properties of the emitting plasma. We find that the coronal region surrounding the prominence exhibits larger intensity variations (over tens of hours of observations) as compared to the streamer region. This variability is particularly strong in the cool coronal emission of the 171A bandpass. We identify the source of this variability as strong brightening events that resemble concave-up loop segments and extend from the cool prominence plasma. These features, referred to as prominence horns, are observed in multiple SDO/AIA bandpasses which allows us to determine how the characteristics of horns vary as a function of temperature. The most intriguing aspect of horn emission is the strong correlation between cool coronal emission and prominence emission and the weak correlation between hot coronal emission and cool coronal emission. Based on the dynamic observations, we suggest that horns are likely the signature thermal changes along magnetic field lines. Magnetic field lines are the basic structural building block of the corona. Energy and pressure balance in the corona occur along magnetic field lines. The large-scale extreme ultraviolet emission we observe in the corona is a conglomerate of many coronal loops projected along a line of sight. In order to calculate the plasma properties at a particular point in the corona, we use one-dimensional models for energy and pressure balance along field lines. In order to predict the extreme ultraviolet emission along a particular line of sight, we project these one-dimensional models onto the three-dimensional magnetic configuration provided by a MHD model for the coronal magnetic field. The thermal non-equilibrium model describes a method for inducing catastrophic cooling in a coronal loop by distributing chromospheric mass throughout the coronal region of the loop. This model results in the formation of a prominence along dipped magnetic loops. We find that the thermal non-equilibrium model matches our dynamic constraints of prominence horns. Loops undergoing cooling will brighten in the 171A bandpass in SDO/AIA and this coronal emission will be correlated with dynamic emission from prominence plasma. While prominence horns and prominence formation appear compatible based on the one-dimensional modeling, there is a discrepancy when we consider the projected spatial structure of horns. Horns project inside the density-depleted cavity, while the mechanism driving prominence formation requires density-enhancements. The three dimensional corona is composed of the projection of many distinct coronal loops. To diagnose the projection effects of prominence horns, we combine the one-dimensional hydrodynamic model with the three-dimensional flux rope model of the prominence-corona magnetic field. The flux rope model uses dipped and twisted magnetic field lines to support a prominence. The distribution of dipped field lines in the flux rope is not uniform. The segments of dipped field lines which would emit in the coronal bandpasses of SDO/AIA are volumetrically isolated along the topological surface which forms the boundary between the twisted flux rope and the near-potential arcade. The projected emission from these field lines would occur throughout the entirety of the flux rope cross section even though the field lines do not extend throughout most of the flux rope volume. Based on the flux rope model, prominence horns are an indication of prominence formation, but they do not occur within the cavity and they are not responsible for the density depletion in that structure. While the dipped field lines in the flux rope model can be linked to the prominence, they are not linked to the cavity. We use a one-dimensional hydrostatic model for coronal energy balance to solve for the plasma parameters throughout the volume of the flux rope model. Variations of plasma parameters between magnetic field lines occur due to geometric considerations for energy loss and column mass. Using this hydrostatic model, we find that the flux rope interior is density depleted relative to the surrounding arcade. Structurally, we find that the short field lines which dominate the flux rope volume can explain the density depletion of the cavity. By combining the hydrostatic and hydrodynamic results, we find that the flux rope model can describe the connection between prominence horns and the cavity. The cavity is formed by axial field lines which are circumscribed by high-density dipped field lines. The cavity is not directly connected to the prominence and does not supply mass to it. These results have allowed us to the establish the first comprehensive picture on the magnetic and energetic interaction of the prominence and the cavity. While the originally hypothesis that the cavity supplies mass to the prominence proved inaccurate, we cannot simply say that these structures are not related. Rather our findings suggest that the prominence and the cavity are distinct magnetic substructures that are complementary regions of a larger whole, specifically a magnetic flux rope.

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