T. A. Kucera

Physics of Solar Prominences: I—Spectral Diagnostics and Non-LTE Modelling

This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (i.e. when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.

THREE-DIMENSIONAL MORPHOLOGY OF A CORONAL PROMINENCE CAVITY

We present a three-dimensional density model of coronal prominence cavities, and a morphological fit that has been tightly constrained by a uniquely well-observed cavity. Observations were obtained as part of an International Heliophysical Year campaign by instruments from a variety of space- and ground-based observatories, spanning wavelengths from radio to soft X-ray to integrated white light. From these data it is clear that the prominence cavity is the limb manifestation of a longitudinally extended polar-crown filament channel, and that the cavity is a region of low density relative to the surrounding corona. As a first step toward quantifying density and temperature from campaign spectroscopic data, we establish the three-dimensional morphology of the cavity. This is critical for taking line-of-sight projection effects into account, since cavities are not localized in the plane of the sky and the corona is optically thin. We have augmented a global coronal streamer model to include a tunnel-like cavity with elliptical cross-section and a Gaussian variation of height along the tunnel length. We have developed a semi-automated routine that fits ellipses to cross-sections of the cavity as it rotates past the solar limb, and have applied it to Extreme Ultraviolet Imager observations from the two Solar Terrestrial Relations Observatory spacecraft. This defines the morphological parameters of our model, from which we reproduce forward-modeled cavity observables. We find that cavity morphology and orientation, in combination with the viewpoints of the observing spacecraft, explain the observed variation in cavity visibility for the east versus west limbs.

Evidence for a Cutoff in the Frequency Distribution of Solar Flares from Small Active Regions

In this paper, we present evidence that active regions with small sunspot areas have an upper limit to the energy of the flares they produce. This result is consistent with predictions of the avalanche model of Lu et al. We used data from the Hard X-Ray Burst Spectrometer on the Solar Maximum Mission to study differences in the frequency distributions of solar flares as grouped by active region characteristics. The active region parameters considered were the total sunspot area, the longitudinal extent, the Mount Wilson class, and the McIntosh class. We find that there are significantly fewer high count rate flares (104 counts s-1 above 60 keV) from regions with small sunspot areas (0-500 microhemispheres) than would be expected from a power-law extrapolation from the frequency distribution of flares with peak rates greater than 50 counts s-1 above 60 keV. This is not found in the distribution of flares produced by regions with large sunspot areas (600-3600 microhemispheres). Using our analysis of the data and the predictions of the avalanche model, we calculated a limit to the energy of a flare that can be produced by an active region with given sunspot area. There are no statistically significant differences between the frequency distributions of flares with peak count rates 103 counts s-1 grouped according to the other region characteristics studied. We also find that, in all cases, large complex regions appear to produce a lower percentage of low-energy events than do smaller, simpler regions. It is possible that this effect is the result of biases against observations of low count rate flares and the determination of their locations.

Observations and Implications of Large-amplitude Longitudinal Oscillations in a Solar Filament

On 2010 August 20, an energetic disturbance triggered large-amplitude longitudinal oscillations in a nearby filament. The triggering mechanism appears to be episodic jets connecting the energetic event with the filament threads. In the present work, we analyze this periodic motion in a large fraction of the filament to characterize the underlying physics of the oscillation as well as the filament properties. The results support our previous theoretical conclusions that the restoring force of large-amplitude longitudinal oscillations is solar gravity, and the damping mechanism is the ongoing accumulation of mass onto the oscillating threads. Based on our previous work, we used the fitted parameters to determine the magnitude and radius of curvature of the dipped magnetic field along the filament, as well as the mass accretion rate onto the filament threads. These derived properties are nearly uniform along the filament, indicating a remarkable degree of cohesiveness throughout the filament channel. Moreover, the estimated mass accretion rate implies that the footpoint heating responsible for the thread formation, according to the thermal nonequilibrium model, agrees with previous coronal heating estimates. We estimate the magnitude of the energy released in the nearby event by studying the dynamic response of the filament threads, and discuss the implications of our study for filament structure and heating.

TEMPERATURE AND EXTREME-ULTRAVIOLET INTENSITY IN A CORONAL PROMINENCE CAVITY AND STREAMER

We analyze the temperature and EUV line emission of a coronal cavity and surrounding streamer in terms of a morphological forward model. We use a series of iron line ratios observed with the Hinode Extreme-ultraviolet Imaging Spectrograph (EIS) on 2007 August 9 to constrain temperature as a function of altitude in a morphological forward model of the streamer and cavity. We also compare model predictions to the EIS EUV line intensities and polarized brightness (pB) data from the Mauna Loa Solar Observatory (MLSO) Mark 4 K-coronameter. This work builds on earlier analysis using the same model to determine geometry of and density in the same cavity and streamer. The fit to the data with altitude-dependent temperature profiles indicates that both the streamer and cavity have temperatures in the range 1.4‐1.7 MK. However, the cavity exhibits substantial substructure such that the altitude-dependent temperature profile is not sufficient to completely model conditions in the cavity. Coronal prominence cavities are structured by magnetism so clues to this structure are to be found in their plasma properties. These temperature substructures are likely related to structures in the cavity magnetic field. Furthermore, we find that the model overestimates the EUV line intensities by a factor of 4‐10, without overestimating pB. We discuss this difference in terms of filling factors and uncertainties in density diagnostics and elemental abundances.

Ultraviolet Observations of Prominence Activation and Cool Loop Dynamics

In this paper we investigate the thermal and dynamic properties of dynamic structures in and around a prominence channel observed on the limb on 2003 April 17. Observations were taken with the Solar and Heliospheric Observatorys Solar Ultraviolet Measurements of Emitted Radiation (SOHO SUMER) in lines formed at temperatures from 80,000 K to 1.6 MK. The instrument was pointed to a single location and took a series of 90 s exposures. Two-dimensional context was provided by the Transition Region and Coronal Explorer (TRACE) in the UV and EUV and the Kanzelhohe Solar Observatory in Hα. Two dynamic features were studied in depth: an activated prominence and repeated motions in a loop near the prominence. We calculated three-dimensional geometries and trajectories, differential emission measures, and limits on the mass, pressure, average density, and kinetic and thermal energies. These observations provide important tests for models of dynamics in prominences and cool (~105 K) loops, which will ultimately lead to a better understanding of the mechanism(s) leading to energy and mass flow in these solar features.

Determination of the Formation Temperature of Si IV in the Solar Transition Region

Using spectra obtained with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer flown on the Solar and Heliospheric Observatory spacecraft, we deduce the temperature of formation of the Si IV ion in the solar transition region from the Si IV ultraviolet spectral line intensity ratio, 3p2P3/2-3d2D3/2,5/2/3s2S1/2-3p2P1/2, and compare the result to the temperature predicted under the assumption of ionization equilibrium. The wavelengths are as follows:2D3/2,5/2, 1128.325, 1128.340 A;2P1/2, 1402.770 A. Ratios are derived for typical features of the quiet Sun, such as cell center and network, and are systematically higher than those predicted at the 6.3 × 104 K ionization equilibrium temperature of formation of Si IV. For most solar features the ratios imply a temperature of formation of about 8.5 × 104 K. The ratios for the faintest features imply a temperature of formation of up to 1.6 × 105 K. It is not clear, however, that all the discrepancies between the measured and theoretical ratios are due to a temperature effect. Accurate temperature measurements are important since a large discrepancy from ionization equilibrium has significant implications for the physics of the transition region, such as the possible presence of nonthermal electrons.

Bayesian Analysis of Solar Oscillations

A Bayesian probability-based approach is applied to the problem of detecting and parameterizing oscillations in the upper solar atmosphere for the first time. Due to its statistical origin, this method provides a mechanism for determining the number of oscillations present, gives precise estimates of the oscillation parameters with a self-consistent statistical error analysis, and allows the oscillatory model signals to be reconstructed within these errors. A highly desirable feature of the Bayesian approach is the ability to resolve oscillations with extremely small frequency separations. The code is applied to SOHO CDS O V λ629 observations and resolves four distinct P4,P5,P6, and P7 p-modes within the same sunspot transition region. This suggests that a spectrum of photospheric p-modes is able to propagate into the upper atmosphere of the Sun and Sun-like stars, and places precise observational constraints on models of umbral eigenmodes.

An Observation of Low-Level Heating in an Erupting Prominence

Here we present multiwavelength observations of low-level heating in an erupting prominence observed in the UV and EUV over a wide range of temperatures and wavelengths by the Solar and Heliospheric Observatory (SOHO) Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument and the Transition Region and Coronal Explorer (TRACE), and also in Hα by the Yunnan Astronomical Observatory. The eruption occurred on 2004 April 30. The heating is relatively mild, leading only to the ionization of hydrogen and helium. It is also localized, occurring along the bottom edge of the erupting prominence and in a kinklike feature in the prominence. The heating is revealed as a decrease in the Lyman absorption relative to other parts of the prominence. This decrease results in an apparent increase in emission in all the lines observed by SUMER, especially those formed at temperatures of ~105 K. However, this is due to the disappearance of cooler absorbing material in the prominence rather than to an increase in these higher temperature species. These observations suggest that there may be low-level heating occurring in other erupting prominences that do not show heating to coronal temperatures. They also indicate that the prominence-corona transition region is best modeled with two or more structures along the line of sight. We discuss the results in terms of models of heating in erupting prominences and observations of Lyman absorption in prominences.

Wavelengths of forbidden transitions arising from levels within the Fe+19 2s22p3 ground configuration

In this paper we report the identification of all remaining unidentified forbidden lines arising from transitions within levels of the Fe+19 ground configuration. These lines were identified using data from the SOHO/SUMER spectrograph and Skylab. Adjusted wavelength values are also given for some previously observed lines. Forbidden lines that are the result of transitions within levels of the ground configuration of a highly ionized astrophysically abundant element generally have longer wavelengths than resonance lines emitted by the same ion. Many of these forbidden lines are fairly prominent in low-density plasmas and traditionally have been used in determining properties of high-temperature astrophysical plasmas. The identified Fe+19 forbidden lines span the 300-2665 A wavelength range. Since spontaneous decay rates of forbidden transitions arising from the same upper level are known quite accurately, these lines can be used for calibrating spectrometers over wide wavelength ranges.