Waves, bursts, and instabilities: a multi-scale investigation of energetic plasma processes in the solar chromosphere and transition region
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Citation
Abstract
The chromosphere and transition region of the solar atmosphere provide an interface
between the cool photosphere (6000 K) and the hot corona (1 million K). Both
layers exhibit dramatic deviations from thermal and hydrostatic equilibrium in the
form of intense plasma heating and mass transfer. The exact mechanisms responsible
for transporting energy to the upper atmosphere remain unknown, but these must
include a variety of energetic processes operating across many spatial and temporal
scales. This dissertation comprises three studies of possible mechanisms for plasma
heating and energy transport in the solar chromosphere and transition region. The
first study establishes the theoretical framework for a collisional, two-stream plasma
instability in the quiet-Sun chromosphere similar to the Farley-Buneman instability
which actively heats the E-region of Earth's ionosphere. After deriving a linear
dispersion relationship and employing a semi-empirical model of the chromosphere
along with carefully computed collision frequencies, this analysis shows that the
threshold electron drift velocity for triggering the instability is remarkably low near
the temperature minimum where convective overshoots could continuously trigger the instability. The second study investigates simultaneous Interface Region Imaging
Spectrograph (IRIS) observations of magnetohydrodynamic (MHD) waves in the
chromospheres and transition regions of sunspots. By measuring the dominant wave
periods, apparent phase velocities, and spatial and temporal separations between
appearances of two observationally distinct oscillatory phenomena, the data show
that these are consistent with upward-propagating slow magnetoacoustic modes tied
to inclined magnetic field lines in the sunspot, providing a conduit for photospheric
seismic energy to transfer upward. The third and final study focuses on intense,
small-scale (1 arcsec) active region brightenings known as IRIS UV bursts. These
exhibit dramatic FUV/NUV emission line splitting and deep absorption features,
suggesting that they result from reconnection events embedded deep in the cool
lower chromosphere. IRIS FUV spectral observations and Solar Dynamics Obser-
vatory/Helioseismic and Magnetic Imager (SDO/HMI) magnetograms of a single
evolving active region reveal that bursts prefer to form during the active region's
emerging phase. These bursts tend to be spatially coincident with small-scale, photospheric,
bipolar regions of upward and downward magnetic flux that dissipate as the active region matures.