Corona III Posters
Presiding: S Bradshaw, Imperial College; D Falconer, Marshall Space Flight Center
SP21B-01 0830h
Thermal and Kinetic Properties of Motions in a Prominence Activation and Nearby Loop
We perform a quantitative analysis of the thermal properties of a prominence activation and motions in a nearby loop. In order to make measurements of the quickly moving features seen in loops and prominences in the UV we use the SOHO/SUMER spectrograph to take a time series of exposures from a single pointing position, providing a measurement of spectral line properties as a function of time and position along the slit. The lines observed cover a broad range of temperatures from 80,000 - 1.6 million K. These measurements are combined with TRACE movies in transition region and coronal temperature bands to obtain more complete information concerning prominence structure and motions. The resulting observations allow us to analyze the thermal and kinetic energy of the moving sources as functions of time. The loop and prominence are most apparent in lines formed at temperatures below 250,000 K. We find that in most cases the temperature distribution of plasma in a moving feature changes relatively little over time periods of about 20 minutes.
SP21B-02 0830h
The Origin of High-Speed Motions and Threads in Solar Prominences
Prominences are among the most spectacular manifestations of both quiescent and eruptive solar activity, yet the origins of their magnetic-field and plasma structures remain poorly understood. We have made steady progress toward a comprehensive model of prominence formation and evolution with our sheared 3D arcade model for the magnetic field and our thermal nonequilibrium model for the cool, dense material suspended in the corona. According to the thermal nonequilibrium model, condensations form readily along long, low-lying magnetic field lines if the heating is localized near the chromosphere. In most cases this process yields a dynamic cycle in which condensations repetitively form, stream along the field line, and ultimately disappear by falling onto the nearest footpoint. Two key observed features were not adequately explained by our earlier simulations of thermal nonequilibrium, however: the thread-like (i.e., elongated) horizontal structure and high-speed motions of many condensations. Here we discuss how simple modifications to our model largely eliminate these discrepancies, strengthening the case for thermal nonequilibrium as the origin of prominence condensations and for low-twist models of prominence magnetic structure. This work was supported by NASA and ONR.
SP21B-03 0830h
Application of a new technique for deriving prominence mass from SOHO/EIT Fe XII (19.5 nm) absorption features
It is presently unclear what role prominences play in the initiation and dynamics of coronal mass ejections (CMEs), although erupting prominences are strongly correlated with CMEs. The masses of prominences involved in CMEs are not generally measured, but the accurate determination of such masses may help in assessing the dynamical importance of prominences in CME events. We apply a new technique for deriving prominence mass to a sample of different types of prominences (eruptive, quiescent, and surges) in which we use observations of coronal radiation in the Fe XII (19.5 nm) spectral line, which is absorbed by prominence material. This new method allows us to consider the effects of both foreground and background radiation in our calculations.
SP21B-04 0830h
Absolute empirical rate coefficient for the excitation of the 117.6 nm line in C III
We have measured the absolute cross sections for electron impact excitation (EIE) of C2+ (2s2p 3P° - 2p2 3P) for energies from below threshold to 17 eV above and derived EIE rate coefficients required for astrophysical applications. The uncertainty in the rate coefficient at a typical solar temperature of formation of C2+ is less than ± 6 %. Ions are produced in a 5 GHz Electron Cyclotron Resonance (ECR) ion source, extracted, formed into a beam, and transported to a collision chamber where they collide with electrons from an electron beam inclined at 45 degrees. The beams are modulated and the radiation from the decay of the excited ions at λ 117.6 nm is detected synchronously using an absolutely calibrated optical system that subtends slightly over Ï€ steradians. The fractional population of the C2+ metastable state in the incident ion beam has been determined experimentally to be 0.42 ± 0.03 (1.65 σ). At the reported ± 15 % total experimental uncertainty level (1.65 σ), the measured structure and absolute scale of the cross section are in fairly good agreement with 6-term close-coupling R-matrix calculations and 90-term R-matrix with pseudo-states calculations, although some minor differences are seen just above threshold. As density-sensitive line intensity ratios vary by only about a factor of 5 as the density changes by nearly a factor of 100, even a 30 % uncertainty in the excitation rate can lead to a factor of 3 error in density. This work is supported by NASA Supporting Research and Technology grants NAG5- 9516 and NAG5-12863 in Solar and Heliospheric Physics and by the Smithsonian Astrophysical Observatory.
SP21B-05 0830h
Combination Scattering by Anisotropic Langmuir Turbulence With Application to Solar Radar Experiments
We develop a theory for radar signal scattering by anisotropic Langmuir turbulence in the solar corona due to a t+l = t process. Langmuir turbulence is considered (thought) to be generated in a cone by a narrow field-aligned electron beam e.g. by an electron beam responsible for a type III solar burst. Expressions for the scattering altitudes, frequency shifts, cross-sections, efficiency of the process, and optical depth are obtained. We show that the absolute value of the radar echo frequency shift is equal to the local plasma frequency at the scattering spot. The frequency shift is positive for the coalescence case and negative for the decay case. The minimum echo frequency shift is determined by the minimal phase velocity of the Langmuir waves, the maximum shift is determined by the electron beam velocity, but in any case can not exceed -‰t/2 and ‰tt, where ‰t is the frequency of a radar signal. Angular characteristics of the scattered signal differ dramatically for the cases of coalescence and decay. The signal is scattered into a narrow cone high above the specular reflection point (‰p ≪ ‰t), but in the vicinity of ‰p ~ ‰t/2 the red echo is scattered isotropically, while the blue echo is scattered into a even narrower cone. We show that the scattering efficiency increases with increase of the local plasma frequency as well as the radar frequency. One should note, that at ‰p = ‰t/2 and ‰p = ‰t efficiency goes to infinity. Strong peaks of the echo at ‰t/2 and 2‰t, may be observed only if the velocity of electron beam exceeds c/√{3}. The considered t+l = t process is essential and can be successfully used for radar studies of the spectral energy density of anisotropic Langmuir turbulence.
SP21B-06 0830h
Imaging Type IIIdm Burst Trajectories
Type IIIdm radio bursts are important diagnostics of the upward and downward directed beams of nonthermal electrons that originate in the energy release site. Because these bursts have a very high frequency drift rate ( |df/dt|=(0.09±0.03)× f1.35±0.10, Melendez et al., 1999), high time and frequency resolution is a necessary for their observation. Since existing interferometers do not have this combination of spectral and time resolution in the decimetric range, we still do not have detailed information about the location and trajectory of Type IIIdm bursts. We are developing the new FASR Subsystem Testbed (FST) with three existing antennas of Owens Valley Solar Array (OVSA) that will permit imaging with high time and spectral resolution over the 1-9 GHz band. To accomplish this, the received signal is downconverted to 500 MHz bandwidth, then digitized with 1 GHz sampling rate, with digital correlation performed offline. With this three element interferometer, we have the ability to determine the centroid of simple source structures with very high time resolution (10 ms) and frequency resolution (<1 MHz). Such centroids as a function of frequency may outline the trajectories of nonthermal electron beams and so may show the magnetic topology at the energy release site. The FST is expected to be operational in early 2006. This poster presents the FST system configuration and the results of simulations of such trajectories, using a loop model with a simple magnetic geometry, density and temperature profiles given by hydrostatic equilibrium, for different loop heating functions (Aschwanden 2004, Physics of The Solar Corona, Chapter 3).
SP21B-07 0830h
Coherent Structures and Rotation Rates in Coronal Activity, from Principal Component Analysis
Principal component analysis (PCA) offers a way to extract those structures that remain spatially coherent throughout a time series. We apply this method to a ~ 28 year time series of Wilcox Solar Observatory Carrington rotation maps (CR) of the 3.25 R coronal source surface field obtained via a potential field extrapolation. We find that over 99% of the variance is contained in the first eight modes. Mode 1, carrying 81.5% of the variance, and modes 2 and 3 containing 13% of the variance, have "dipole" structures. Modes 4-8, with a "quadruple" structure, contain 4.5% of the variance. The principal components (PCs) give the time dependence of the modes. We combine the PCs of modes 2 and 3 to get the amplitude and phase of a structure that behaves essentially as a dipole in the equatorial plane. During activity minima the structure is relatively weak and rotates at the 27.275 day Carrington rate. During the active periods of cycles 21 and 22 the amplitude is large and highly intermittent, and the dipole rotates more rapidly than the Carrington rate with a synodic period of 26.6 days. During cycle 23, however, the dipole moves backward in Carrington longitude with a synodic period of 27.8 days. The average of these is ~ 27.0 days, though this is actually realized only sporadically. The phase changes that occur at shorter time scales and that coincide with intermittent changes in the dipole amplitude seem to represent essentially random effects of the passage of the magnetic field through the convection zone. While the lower modes tend to lock the hemispheres together the higher modes present separate Northern and Southern hemisphere quadrupole-type patterns that drift in Carrington longitude similarly to the equatorial dipole. Over some periods the drift in each hemisphere closely tracks the other over a wide range of timescales. However, there are large, decadal-scale excursions in which first one hemisphere leads in phase by 3 or 4 rotations and then the other leads by a similar amount.
SP21B-08 0830h
Properties of the Solar Corona Outside the West Solar Limb
We discuss the analysis of 36 spectral observations recorded by the SUMER spectrometer on SOHO on April 22-23, 1998, at the onset of the new solar cycle. The observations were made with the 4" x 300" slit, and are distributed over the west hemisphere between 1.02 and 1.5 solar radii along the equatorial direction and between -0.9 solar radii south of the equator and +0.9 solar radii north of it. At the far corners of the studied region the solar distance was 1.74 solar radii. We have found that the physical characteristics of the central part of the region are typical of quiet Sun coronal plasmas. The regions furthest from the equator resemble coronal hole plasmas and the intermediate regions are consistent with a transition between the two. We believe that this is the most comprehensive work of its kind undertaken thus far using UV spectral lines. We use the recorded line intensities and line shapes to describe, as a function of coronal position, electron temperatures, electron densities, elemental abundances, line-of-sight emission measures (EM), nonthermal mass motions, and mass dependent elemental settling. By using intensity ratios between the two component resonance lines of Li-like O VI, Ne VIII and Mg X, we determine the fractional excitation in the Li-like lines produced by electron collisions and the fractional excitation produced by photo-excitation of transition region radiation emitted just above the photosphere.
SP21B-09 0830h
Observations of Unusual "EIT Wave" Dynamics
The unusual level of flare and CME activity observed in late October 2003 also produced particularly remarkable coronal pulse waves (also called "EIT waves.") Among these events is an EIT wave which appears to travel a complicated trajectory across the solar disk over a period of roughly 2.5 hours, remaining coherent and identifiable throughout the time period. The wave front travels through varied topologies, including a filament channel and an active region. We present results describing the wave dynamics, and discuss how this observation both verifies and contradicts the expected behavior of coronal pulse waves.
SP21B-10 0830h
A Physical Description of Coronal Pulse Wave Dynamics
Many of the studies concerning coronal pulse waves (also called "EIT waves") have interpreted these phenomena as fast-mode magnetoacoustic wave fronts. In many respects, this explanation has proved inconsistent with observations. Recently, it has been postulated that coronal pulse waves are actually non-linear MHD solitons. However, one characteristic of the soliton solution has proved problematic: a simple analytic consideration requires that solitons travel at velocities much faster than those observed in coronal pulse waves. We demonstrate that a slow MHD soliton solution is possible; however, the stability of this solution requires very specific MHD geometry. Fortunately, this geometry happens to be consistent with the conditions that support coronal pulse waves.
SP21B-11 0830h
Statistics of the quiet Sun intensity distribution.
The statistics of the quiet Sun intensity distribution present an interesting challenge. Some authors find that the observed distribution is adequately represented by a two component model, which seems to fit well with the notion of a network and internetwork being physically distinct. Other authors find that a single component model fits the intensity distribution data well, which seems to suggest that the network and internetwork emission may in fact be due to the same process. This paper examines the evidence for both these points of view, using CDS EUV data and a mixture modeling technique. Some comments are also made on the nature of the mechanism which may create the observed distributions.