Solar Transition Region Research Articles

Diffusion effects on the helium abundance of the solar transition region and corona

The diffusion of helium in the solar transition region is studied by solving the mass and momentum conservation equations for a hydrogen-helium plasma given a representative temperature profile. Steady state solutions show that two distinct atmospheres may result. In cases where the thermal force on alpha-particles is balanced by the partial pressure gradient force, helium is the dominant coronal species. On the other hand, if it is the frictional force between protons and alpha-particles which balances the thermal force on alpha-particles then hydrogen is the major coronal component. In order to explore which of these solutions are attainable within reasonable time scales, the time-dependent equations are solved, starting from an initial state with a uniform helium abundance of 10 percent. The atmosphere as a whole is close to hydrostatic equilibrium, but due the thermal forces the individual elements are not. This force inbalance leads to a differential flow between species. It is found that this differential flow leads to a significant enhancement of the coronal helium abundance. Even for the relatively shallow temperature gradient used the helium abundance in the lower corona increases to 30 percent over a 24 hr period.

Pervasive variability in the quiet solar transition region

Extreme UV spectroheliograms from an experimental time series are employed to investigate the nature of the quiet solar transition region with respect to temporal variability. A statistical treatment is developed to analyze the fraction of spatial elements that yield intensity variations significantly higher than the signal noise. The EUV intensity in every spatial resolution varies on a time scale of minutes by about 10-30 percent, and the fractional amplitude of the temporal variations is found to be nearly independent of mean intensity. The paper concludes that the quiet solar transition region is probably generated and modulated by small-scale magnetic activity, since the temporal variability in the transition region is spatially uniform and rather pervasive.

On the possibility of the existence of solar transition region ions at chromospheric temperatures

Abstract Certain mechanism of helium ionization in the chromosphere under coronal EUV-irradiation used to be extended to ions of the inner chromosphere–corona transition region. It is shown that due to the photoionization by coronal radiation the existence of ions such as C(III)–C(IV) at the temperature ∼104 K is possible if the electron density is not greater than ∼108 cm−3. Such conditions may occur at heights of about 3 to 10 × 103 km inside the cells of the chromospheric network.

NE V line ratios in the EUV spectra of solar flares

Theoretical line ratios involving 2s(2)2p(2)-2s2p3 transitions in Ne v between 359 and 572 angstrom are presented. A comparison of these with solar flare observational data from the Naval Research Laboratory's S082A spectrograph on board Skylab reveals excellent agreement between theory and experiment, with discrepancies that average only 8%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and in addition resolves discrepancies between theory and observations previously found for this species. The potential usefulness of the Ne V line ratios as electron temperature diagnostics for the solar transition region is briefly discussed.

Temperature-sensitive line ratios of the lithium-like ion O vi

The intensity ratios of emission lines from the lithium-like ion Ovi are calculated as a function of electron temperature using recent computations of collision strengths by Zhang, Sampson, and Fontes (1990). The line ratios are strongly dependent on temperature and are appropriate as a diagnostic of hot thin plasmas such as in the solar transition region and corona.

Numerical simulations of microflare evolution in the solar transition region and corona

A series of numerical simulations indicates that quasi-impulsive thermal energy releases of ~ 1026 ergs in a narrow region of the upper chromosphere of a quiet region coronal loop can provide a transient heating source for the corona. Lower energy events radiate away their energy too quickly to generate substantial temperature enhancements. The microflares seen in UV images may generate such energy releases, and may also be the source for some of the dynamic events seen in UV spectra.

Relative abundances in the lower solar transition region

Properties of the lower solar transition region as determined from UV spectral lines recorded by the Naval Research Laboratory HRTS are investigated. The spectra were obtained from a rocket flight in July 1975. Variations of intensity ratios of strong lines of C IV, Si IV, C II, Si III, and O IV over the entire field of view of the HRTS instrument, which extends from disk center to the solar limb are discussed. The largest apparent abundance variations appear to be in the active regions surrounding a sunspot. The C/Si ratio is lower in the active regions than anywhere else in the instrument field of view, implying a lower C/Si abundance ratio in the active regions. There is a possible correlation of line intensity variation in the active region with Doppler shift of the lines; the explanation for this is unclear. The magnitude and reality of intensity ratio variations of all observed solar features are discussed, and monotonic center-to-limb intensity ratio variations due to the opacity of some of the spectral lines are measured.

Nonequilibrium ionization effects in asymmetrically heated loops

The effects of nonequilibrium ionization on magnetic loop models with a steady siphon flow that is driven by a nonuniform heating rate are investigated. The model developed by Mariska (1988) to explain the observed redshifts of transition region emission lines is examined, and the number densities of the ions of carbon and oxygen along the loop are computed, with and without the approximation of ionization equilibrium. Considerable deviations from equilibrium were found. In order to determine the consequences of these nonequilibrium effects on the characteristics of the EUV emission from the loop plasma, the profiles and wavelength positions of all the important emission lines due to carbon and oxygen were calculated. The calculations are in broad agreement with Mariska's conclusions, although they show a significant diminution of the Doppler shifts, as well as modifications to the line widths. It is concluded that the inclusion of nonequilibrium effects make it more difficult to reproduce the observed characteristics of the solar transition region by means of the asymmetric-heating models.

Energy balance in the solar transition region. II - Effects of pressure and energy input on hydrostatic models

The radiation of energy by hydrogen lines and continua in hydrostatic energy-balance models of the transition region between the solar chromosphere and corona is studied using models which assume that mechanical or magnetic energy is dissipated in the hot corona and is then transported toward the chromosphere down the steep temperature gradient of the transition region. These models explain the average quiet sun and also the entire range of variability of the Ly-alpha lines. The relations between the downward energy flux, the pressure of the transition region, and the different hydrogen emission are described.

Forbidden lines of np(q) ions. II - Line intensities

Ground state forbidden transitions of np(q) ions of C, N, O, Ne, Mg, Si, S, and Fe can provide important information on the state of cosmic ionized gases. The line intensities of 95 transitions of these ions have been tabulated, including those used to calculate the line ratios as well as other strong lines of the ions in the ground state terms in the same temperature and density ranges treated previously. These data can be used for calculations of the absolute line intensities if the ionic abundances are known. These calculations are important for studies of the solar transition region, ionized nebulae, circumstellar nebulae such as found in symbiotic stars, supernova remnants, interstellar bubbles produced by stellar winds, and emission regions in active galactic nuclei.

The effects of mass flow on the temperature and abundance structure of the solar transition region

The density and temperature structure of a multicomponent plasma consisting of electrons, protons, ionized helium, and a trace minor ion species are computed. The equations of force and energy balance for this model are developed and solved. It is found that in the case of downflows the minor ion temperature can be significantly hotter than the electron temperature, and significant abundance enhancements are possible due to the slowdown of the minor species from the effect of the thermal force. A simple physical picture of the source of the thermal force is given.

The magnetic network location of explosive events observed in the solar transition region

Compact short-lived explosive events have been observed in solar transition region lines with the High-Resolution Telescope and Spectrograph (HRTS) flown by the Naval Research Laboratory on a series of rockets and on Spacelab 2. Data from Spacelab 2 are coaligned with a simultaneous magnetogram and near-simultaneous He I 10,380 -A spectroheliogram obtained at the National Solar Observatory at Kitt Peak. The comparison shows that the explosive events occur in the solar magnetic network lanes at the boundaries of supergranular convective cells. However, the events occur away from the larger concentrations of magnetic flux in the network, in contradiction to the observed tendency of the more energetic solar phenomena to be associated with the stronger magnetic fields.

The finite-amplitude behavior of the Joule mode under astrophysical conditions

Magnetized astrophysical plasmas reveal a great deal of structure when spatially resolved. One possible explanation for this structuring is based on the existence of filamenting instabilities driven by radiation. In this paper, previous linear calculations are extended by considering the weakly nonlinear (finite-amplitude) development of such filamenting instabilities in magnetized unstratified plasmas. It is shown that under most conditions these instabilities (in particular, the Jouse model) are unstable at finite amplitude; in particular, in the temperature and plasma-beta domains characteristic of, for example, much of the solar transition region, these modes - which can be linearly stable under these conditions - become unstable to finite-amplitude perturbations. The relevance of this to the problem of heating the solar low transition region by current dissipation is discussed.

Absolute velocity measurements in the solar transition region and corona

Abstract We present an experimental technique to measure absolute velocities of minor ions formed in the solar transition region and corona. A sounding rocket experiment (27 July 1987) obtained high resolution EUV spectra along a solar diameter with spatial resolution of 20 × 20 arcsec. The wavelengths of the λ1533 Si II, λ1548 C IV, and λ770 Ne VIII emission lines were directly compared with wavelengths of known platinum lines generated by an inflight calibration lamp. On the assumption that horizontal motions cancel statistically so that the line-of-sight velocity approaches zero at the limb, we find a net radial downflow of approximately 7.5 ± 1.0 km/s for C IV and upper limits on the radial flow for Si II and Ne VIII. We then test this assumption by direct comparison to our on-board wavelength reference using recently published laboratory rest wavelengths of the solar emission lines. We find agreement within the published uncertainties of the laboratory wavelengths, ± 2 km/s in the case of C IV. Therefore, we suggest that improved laboratory wavelength measurements (± 1 km/s) in conjunction with inflight wavelength calibration would improve constraints on models of transition region and coronal dynamics.

HIRES: a high-resolution stigmatic extreme ultraviolet spectroheliometer for studies of the fine-scale structure of the solar chromosphere, transition region, and corona

We describe the design of a high-resolution stigmatic extremeultraviolet (EUV) spectroheliometer, configured for flight on a Black Brant sounding rocket, which consists of a 45-cm Gregory telescope coupled to a spectrometer employing a single toroidal diffraction grating in a normalincidence Rowland circle mounting and an imaging pulse-counting multianode microchannel array (MAMA) detector system. The toroidal diffraction grating is fabricated by a technique employing an elastically deformable submaster grating that is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The spectroheliometer will produce spatially resolved spectra of the chromosphere, transition region, and corona with an angular resolution of 0.4 arcsec or better, a spectral resolution λ/Δλ of about 10 4 in first order, and a temporal resolution of the order of seconds. Because of the geometric fidelity of the MAMA detector system, the spectroheliometer will be able to determine Doppler shifts to a resolution of at least 2 ma at wavelengths near 600 a (~1.0 km s -1 ), depending on the level of the accumulated signal. The unique characteristics of the spectroheliometer will be used in combination with plasma-diagnostic techniques to study the emperature, density, and velocity structures of specific features in the solar outer atmosphere.

Absolute flows in the solar transition region

It is the objective of the present study to establish an absolute scale for flows in the solar transition region in observations obtained with the UVSP/SMM. By use of the polar limbs as reference one finds that the downflows range between 3 and 10 km s−1. The brighter regions show the largest downward flows.

Dynamic modeling of the solar atmosphere

Over the past 2 decades our observational understanding of the outer layers of the solar atmosphere has increased dramatically. We now know that the solar chromosphere, transition region, and corona are highly structured and dynamic. Magnetic fields, rooted in the subphotospheric layers, provide the structure that channels the plasma in the upper layers of the solar atmosphere.

Electron velocity distribution function in a fully ionized plasma slab with a large temperature gradient: solar flare transition region

The high velocity tail of the electron distribution function has been calculated by numerically solving the high velocity form of the Landau equation for a thermal structure representative of a solar flare. The calculations show an anisotropy and an enhancement of the tail populations above the Maxwellian value throughout the flaring loop.

Energy balance in the solar transition region. I - Hydrostatic thermal models with ambipolar diffusion

The energy balance in the lower transition region is analyzed by constructing theoretical models which satisfy the energy balance constraint. The energy balance is achieved by balancing the radiative losses and the energy flowing downward from the corona. This energy flow is mainly in two forms: conductive heat flow and hydrogen ionization energy flow due to ambipolar diffusion. Hydrostatic equilibrium is assumed, and, in a first calculation, local mechanical heating and Joule heating are ignored. In a second model, some mechanical heating compatible with chromospheric energy-balance calculations is introduced. The models are computed for a partial non-LTE approach in which radiation departs strongly from LTE but particles depart from Maxwellian distributions only to first order. The results, which apply to cases where the magnetic field is either absent, or uniform and vertical, are compared with the observed Lyman lines and continuum from the average quiet sun. The approximate agreement suggests that this type of model can roughly explain the observed intensities in a physically meaningful way, assuming only a few free parameters specified as chromospheric boundary conditions.

Turbulent thermal conduction in the solar transition region

view Abstract Citations (30) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Turbulent Thermal Conduction in the Solar Transition Region Cally, P. S. Abstract Emission measures E(T) derived from EUV observations of the lower solar transition region indicate that there is far more material in that temperature range than can be explained within the steady single flux-tube scenario if the classical energy transfer and loss mechanisms are adopted. This suggests that some mechanism may be operating which reduces the temperature gradient. The inclusion of a turbulent thermal conductivity kappa(T) = phi rho c(p)ul in a one-dimensional model is investigated, where u is the rms microvelocity in one direction, l is the mixing length, and phi is a constant of order unity. If it is assumed that that l is proportional to T exp-alpha, it is shown that near-perfect agreement with the observed emission measures is obtained for alpha = 3/2, both as regards the slope d log E/d log T equal to about -3 in the LTR, and the upturn at the top of this range. Publication: The Astrophysical Journal Pub Date: June 1990 DOI: 10.1086/168802 Bibcode: 1990ApJ...355..693C Keywords: Radiative Transfer; Solar Atmosphere; Solar Corona; Thermal Conductivity; Turbulence Effects; Emission Spectra; Energy Transfer; Far Ultraviolet Radiation; Solar Granulation; Temperature Gradients; Solar Physics; SUN: TRANSITION REGION; TURBULENCE full text sources ADS |