Solar Loops Research Articles

Models for Solar Magnetic Loops. V. A New Diagnostic Technique to Compare Loop Models and Observations

We present a new diagnostic technique to compare theoretical models with observations of quiescent magnetic loops from high-resolution imaging spectrometers. The diagnostic technique is primarily suited for the analysis of high-resolution, spatially resolved spectra, but it can also be applied to narrowband images. The diagnostic technique is based on a steady state, dynamic loop model and allows us to unambiguously determine whether the model reproduces the observations and to determine the plasma heating, velocities, footpoint conductive flux, and pressure in the loop.

Solarthermal parabolic trough power plants with integrated storage capacity

Parabolic trough power plants use concentrated direct insulation for steam generation in a Rankine cycle. The fundamentals for direct steam generation in parabolic troughs have been developed in recent years, the feasibility was successfully demonstrated at a solar powered test facility reaching steam temperatures of 400°C at a maximum pressure of 100 bar. Important components for the accelerated market penetration of solarthermal power plants are cost-effective local storage systems. Main benefits of storage systems are: extended utilisation of the power block, improved efficiency of components and facilitated integration into the electricity system. Within the R&D project WESPE, a test facility for solid sensible heat storage is developed and connected to a solar collector loop. A simulation tool for the analysis of the transient performance of the storage system integrated into the power plant was implemented. The results show a significant potential for economic optimisation by applying advanced operation strategies based on an improved adaptation of the storage unit to the characteristics of solar collectors and power cycle.

Velocity of magnetic neutral lines in the magnetotail

The Geotail spacecraft often observed intermittent enhancements of ion density and velocity in the distant (>70 R E) tail lobe [Shirai, H., Takada, T.K., Kamide, Y, Mukai, T. Enhancement of lobe ion density and velocity associated with plasmoids. J. Geophys. Res. 106, 29,935–29,942, 2000]. Each enhancement occurred alter the passage of a plasmoid and was accompanied by a south-to-north variation in B z . These signatures suggest that a magnetic neutral line formed in the near-Earth region moves tailward after the plasmoid is ejected. In this paper, we analyze the timing of the ion enhancement in detail. From the timing, we estimate the average velocity of the neutral line motion. The estimated velocity is about 130–580 km/s, which is around 20–70% of a typical value of the plasmoid velocity (i.e., about 760 km/s). We also compare the neutral line velocity in the Earth ’s magnetotail with the rise velocity of a solar flare loop [Shibata, K., Masuda, S., Shimojo, M, et al., Hot plasma ejection associated with compact-loop solar flares. Ap. J. Lett. 451, L83–L85, 1995]. The neutral line velocity in the magnetotail is larger than the loop velocity (4–20 km/s). However the ratio of the neutral line velocity to the plasmoid velocity in the tail is similar to the ratio of the loop velocity to the plasmoid velocity in solar flares suggesting the presence of the same physical mechanism behind them.

What causes solar active region loops to exist at transition region temperatures?

High-lying, dynamic loops have been observed at transition region temperatures since Skylab observations. The nature of these loops has been debated for many years with several explanations having been put forward. These include that the loops are merely cooling from hotter coronal loops, that they are produced from siphon flows, or that they are loops heated only to transition region temperatures. In this paper we will make use of combined SOHO-MDI (Michelson-Doppler Imager), SOHO-CDS (Coronal Diagnostic Spectrometer) and Yohkoh SXT (Soft X-ray Telescope) datasets in order to determine whether the appearance of transition region loops is related to small-scale flaring in the corona, and to estimate the magnetic configuration of the loops. The latter allows us to determine the direction of plasma flows in the transition region loops. We find that the appearance of the transition region loops is often related to small-scale flaring in the corona and in this case the transition region loops appear to be cooling with material draining down from the loop top.

Models for Solar Magnetic Loops. IV. On the Relation between Coronal and Footpoint Plasma in Active Region Loops

In the present work we analyze several SUMER intensity maps of active region solar loops in order to compare the relative brightnesses of the footpoints and the coronal section of active region loops. We find that the former are barely distinguishable from the background emission of the active region, while the coronal emission of loops is confined in well-identifiable structures that are significantly brighter than the background. This result means that the vast majority of the active region emission in chromospheric and transition region lines is generated by plasma not directly connected with the coronal plasma that constitutes the observed coronal loops. We determine the observed intensities of coronal lines relative to the observed transition region and chromospheric emission and compare them with predictions from loop models having uniform cross section and different heating functions. We find that the loop models overestimate the footpoint emission by orders of magnitude. We discuss the discrepancy in light of the heating function and of the loop cross section. We speculate that nonuniformity in the loop cross section, more specifically a significant decrease of the cross section near the footpoints, is the most likely solution to the discrepancy.

Solar barometric distillation for seawater desalting part I: Basic layout and operational/technical features

The report presents an innovative solar barometric distillation technology for seawater desalting (SW-SBD) which has several attractive features such as: (1) efficient vacuum solar collectors of simple construction; (2) a barometric layout for quasi-steady operation at subatmospheric pressure; (3) a demister system of simple mechanical construction; and (4) an electronic control system regulating plant productivity parameters on available solar radiation flux. The proposed seawater desalting technology consists of single-effect distillation with water vapor produced and heated at subatmospheric pressure in a solar collector loop; the basic layout, operational features, and energy efficiency are presented and analysed in detail. Subsequent reports will provide technico-economical data from prototype desalting plants in view of industrial implementation. A promising specific electric energy consumption,∼2 kWm 3 offresh water produced, was found by estimating pumping power requirements of major SW-SBD plant pumps. A sizable reduction in produced water cost with respect to an existing solar seawater desalting plant in Abu Dhabi is expected on the basis of a preliminary economic evaluation for a ∼100 m 3/d plant prototype.

Solar Magnetic Loops Observed with TRACE and EIT

We select some highlights and new results that have been obtained from detailed “microscopic” observations of coronal loop structures with the Transition Region and Coronal Explorer (TRACE) and Extreme Ultraviolet Imager (EIT) instruments, including: (1) the inhomogeneous substructure of EUV loops, (2) the dynamic and non-hydrostatic nature, (3) the non-uniform heating, (4) the magnetic topology at the loop foot-points, (5) the magnetic energy budget for heating, and (6) oscillations and waves in coronal loops.

The Magnetic Corona: Magnetic Reconnection in Solar Flares

The ability of the Transition Region and Coronal Explorer (TRACE) to image the Sun at high spatial resolution and high cadence over a very broad range of temperatures makes it a unique instrument for observing solar flare plasma. TRACE observations have confirmed the reconnection model for solar flares, at least qualitatively. TRACE flare observations show impulsive footpoint brightenings that are followed by the formation of high-temperature loops in the corona. These loops then cool to lower temperatures, forming post-flare loop arcades. Comparisons between TRACE and lower spatial resolution Yohkoh Soft X-Ray Telescope (SXT) observations have revealed that solar flares are composed of a multitude of fine coronal loops. Detailed hydrodynamic modeling of flare light curves shows that this fine scale structuring is crucial to understanding the evolution of the observed emission. Models based on single, isothermal loops are not consistent with the TRACE observations. Models based on the sequential heating of small-scale loops, in contrast, are able to reproduce many of the salient features of the observed light curves. We will discuss the implication of these results for more energetic stellar flares as well as smaller-scale events that may be responsible for the heating of solar active region loops.

The radiative response of solar loop plasma subject to transient heating

In Bradshaw & Mason (2003) we carried out a hydrodynamic simulation of a cooling solar loop and investigated the nonequilibrium response of the population of C VII ions to the changing conditions in the plasma. We also compared equilibrium and nonequilibrium calculations of the total plasma emissivity. In this paper we present two simulations of a solar loop subject to a transient heating process delivering energy on the nanoflare scale at its apex. One simulation treats the ion populations and the energy radiated from the loop plasma entirely as though the system were in equilibrium and the other simulation performs a full nonequilibrium treatment by coupling the time-dependent ion populations to the hydrodynamic equations through the radiative energy loss. Our radiative model accounts for the 15 most abundant elements of the solar atmosphere including C, O, Ne, Mg, Si and Fe. We find some pronounced dierences between the populations of certain transition region ions and the corresponding plasma emissivity curves in the equilibrium and nonequilibrium simulations. Though the apex heating event is relatively weak in comparison to energy released on the microflare and flare scales, nonetheless a significant amount of energy reaches the loop footpoint region to heat the plasma there and we find a nonequilibrium spike in emissivity. However, more surprisingly we find considerable dierences between some of the coronal ions in the equilibrium and nonequilibrium simulations, with important consequences for the plasma emissivity curves. In particular, we find that the total plasma emissivity calculated assuming equilibrium conditions is up to a factor of 5 lower than the nonequilibrium emissivity and this is due almost entirely to the response of the coronal Fe ions. Finally, we suggest possible observational signatures of nonequilibrium ionisation and ways in which one might identify it. This is important because an invalid assumption of equilibrium ion populations may well lead one to incorrect conclusions about the properties of the plasma in both a broad-band and narrow-band/emission line based analysis.

Loop-Top Nonthermal Microwave Source in Extended Solar Flaring Loops

Nobeyama Radioheliograph (NoRH) microwave data provide us with unique information about the radio brightness distribution along flaring loops. In particular, it has been found that for several events with extended looplike sources well resolved with NoRH, the brightness maximum at 17 and 34 GHz is located at the top of the corresponding flaring loops. The detailed analysis of these events strongly suggests that the distribution of mildly relativistic electrons along an extended flaring loop must be highly inhomogeneous: accelerated electrons are concentrated in the upper part of the loop. This finding imposes important new constraints on the acceleration/injection mechanisms and on the kinetics of high-energy particles in solar flares.

New numerical tools to study waves and instabilities of flowing plasmas

Studying plasma waves and instabilities is an indispensable part of present thermonuclear fusion and astrophysical magnetohydrodynamics (MHD). Up till recently, spectral analysis was mostly restricted to static plasmas. However, the assumption of a static plasma is unrealistic not only for astrophysical but also for modern fusion research. Plasmas with flow have been shown to have spectra essentially different from those of static plasmas [Phys. Rev. Lett. 84 (2000) 2865]. We present two new numerical tools for spectral studies of plasmas with flow. The first one, a program called FINESSE (Finite Element Solver for Stationary Equilibria), computes equilibria of non-static plasmas for a variety of fusion and astrophysical configurations (tokamaks, solar loops, solar winds, etc.). Ideal and resistive spectra of the computed equilibria are studied with another tool, a program called PHOENIX. In PHOENIX, the large-scale generalized eigenproblems are solved with the recently proposed iterative Jacobi–Davidson method [SIAM J. Matrix Anal. Appl. 17 (1996) 401]. Our numerical examples show how FINESSE and PHOENIX can be used to study the effect of the poloidal flows on Toroidal Alfven Eigenmodes.

Single- and multi-tank energy storage for solar heating systems: fundamentals

A multi-tank liquid-water system for storing low-temperature solar-derived heat is investigated experimentally and analytically. The motivation is a perceived economic advantage of the proposed system over single-tank systems in solar heating systems where the required total volume of water is rather large — say 2000 l or more. In the proposed system, the individual tanks in the multi-tank system (each with a volume of about 200 l) are interconnected by means of two strings of immersed-coil heat exchangers: the first string serially connects exchangers that have been immersed at the bottoms of the tanks, the second connects exchangers that have been immersed at the tops of the tanks. The first string will be connected to the solar collector loop and the second in the load loop. The present work experimentally demonstrates the degree of effective stratification that the multi-tank system can achieve, as well as a thermodynamically advantageous ‘thermal diode’ effect. It also describes a model for a multi-tank system as well as experiments that validate the model. Part of the multi-tank model is a model for a single tank with immersed coil heat exchanger, and this model drew upon a ‘reversion-elimination’ algorithm from the recent literature. The validity of the reversion-elimination algorithm is supported by the present experiments.

Angular and Energy‐dependent Neutron Emission from Solar Flare Magnetic Loops

We have developed new neutron production kinematics and thoroughly updated the neutron production cross sections, and we have included ion pitch-angle scattering and magnetic mirroring in our Monte Carlo simulation programs, to make new calculations of anisotropic neutron emission produced in the solar flare magnetic loop models. The anisotropy in these models arises from the combined effects of converging magnetic field lines and a rapidly increasing ambient density in the portion of the loop below the chromosphere-corona transition. We have carried out new calculations of the depth, time, angle, and energy dependences of the neutron production, the angle distributions and energy spectra of the escaping neutrons, and the energy spectrum of the surviving neutrons at the distance 1 AU from the Sun. These new calculations will now allow much more reliable and detailed analyses of the various solar flare neutron spectral observations.

Electron–Whistler Interaction in Coronal Loops and Radiation Signatures

Interaction of the 30–300 keV electrons with whistlers in solar coronal loops is studied using a quasi-linear approach. We show that the electron–whistler interaction may play a dominant role in the formation of fast electron spectra within the solar flare loops with the plasma temperature ≲ 107 K and plasma density ≲ 1011 cm−3. It is found that Landau damping of whistlers provides weak and intermediate pitch-angle diffusion regimes of fast electrons in coronal loops. The level of whistler turbulence in the weak diffusion regime under flare conditions is estimated as ∼10−7 of the energy density in the thermal particles. The `top – footpoint' relations between the hard X-ray flux densities and spectra are derived. The reason for a `broken' spectrum of the flare microwave emission is discussed.

Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae: assessment of design and performance

A methodology is presented for designing photobioreactors with tubular loop solar receivers in which the fluid is circulated by an airlift device. The design method effectively combines the relevant aspects of external irradiance-dependent cell growth, oxygen accumulation in the solar loop, oxygen removal in the airlift device, and hydrodynamics of the airlift system that determine the flow velocity through the solar receiver. The design approach developed was used to model and build a 0.2 m 3 outdoor photobioreactor. A compact degasser in the airlift section eliminated dead zones and dark zones, while achieving complete separation of gas and liquid. The measured gas–liquid hydrodynamics, mass transfer, and culture productivity were consistent with the model predictions. The reactor was tested with continuous culture of the microalga Phaeodactylum tricornutum at various liquid velocities through the tubular solar receiver. A biomass productivity of 1.20 g l −1 d −1 (or 20 g m −2 d −1 ) was obtained at a dilution rate of 0.050 h −1 . Solar receiver linear liquid velocities of 0.50 and 0.35 m s −1 gave similar biomass productivities, but the culture collapsed at lower velocities. An adverse effect of high dissolved oxygen concentration on productivity was observed. Oxygen accumulation could be reduced by increasing the liquid velocity and this enhanced the biomass yield.

Electron Trapping and Precipitation in Asymmetric Solar Flare Loops

Acceleration, propagation, and energy loss of particles energized in solar flares cannot be studied separately because their radiative signatures observed in the form of hard X-ray bremsstrahlung or radio gyrosynchrotron emission represent a convolution of all these processes. We analyze hard X-ray emission from solar flares using a kinematic model that includes free-streaming electrons (having an energy-dependent time-of-flight delay) as well as temporarily trapped electrons (which are pitch-angle scattered by Coulomb collisional scattering) to determine various physical parameters (trapping times, flux asymmetry, loss-cone angles, magnetic mirror ratios) in flare loops with asymmetric magnetic fields.

Dynamic Models of Optical Emission in Impulsive Solar Flares

Numerical simulations of the dynamics and radiation in a solar flare loop are presented. The heating processes in the lower atmosphere include nonthermal heating by accelerated electrons and thermal soft X-ray irradiation from the flare-heated transition region and corona. Important transitions of hydrogen, helium, and singly ionized calcium and magnesium are treated in non-LTE. The principal results of the analysis are the following:

Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.

SOLAR PHOTOCATALYTIC DEGRADATION OF WATER AND AIR POLLUTANTS: CHALLENGES AND PERSPECTIVES

Solar photocatalytic oxidation processes (PCO) for degradation of water and air pollutants have recently received increasing attention. Some field-scale experiments have demonstrated the feasibility of using a semiconductor (TiO 2) in solar collectors and concentrators to completely mineralize organic contaminants in water and air. Although successful pre-industrial solar tests have been carried out, there are still discrepancies and doubt concerning process fundamentals such as the roles of active components, appropriate modelling of reaction kinetics or quantification of photoefficiency. Challenges to development are catalyst deactivation, slow kinetics, low photoefficiency and unpredictable mechanisms. The development of specific non-concentrating collectors for detoxification and the use of additives such as peroxydisulfate have made competitive use of solar PCO possible. The challenges and perspectives of solar driven PCO as illustrated in the literature and our own results in large solar field loops at the Plataforma Solar de Almeria and CIEMAT laboratories are described.

Single and Multiple Solar Flare Loops: Hydrodynamics and CaxixResonance Line Emission

Studies made so far with one-dimensional hydrodynamic simulations have shown that it is difficult to reproduce the soft X-ray spectral line profile observed in the early phase of solar flares. Simulated line profiles predict a dominant emission from a large blueshifted component, while observations show persistently strong stationary components. We resolve these discrepancies by utilizing a multiple-loop system instead of just a single loop for conductively heated flare simulations. Under a fixed heat input rate, we examine how the heating duration {tau}{sub heat} affects the Ca xix resonance ({ital w}) line emission from single and multiple flare loops. In the multiple-loop model, the flare energy is released into individual loops with a specified time delay, which implicitly mimics the successive formation of flare loops due to continuous reconnection. We find that whether or not {tau}{sub heat} is longer than {tau}{sub {ital c}} affects the hydrodynamic response in an individual flare loop, where {tau}{sub {ital c}} corresponds to the time when the loop is filled with evaporated plasma. The Ca xix spectral line shape is characterized by an intensity ratio of emission from evaporated plasma to emission from accumulated plasma after evaporation. This ratio is mainly determined by the parameter {tau}{submore » heat}/{tau}{sub {ital c}}. Our findings suggest that the following scenario can naturally explain the observed spectral line features. Flare energy is injected into a bundle of loops successively in two steps: in the preflare stage, {tau}{sub heat} {le} {tau}{sub {ital c}} for the inner loops, and then in the main flare stage, {tau}{sub heat} {gt} {tau}{sub {ital c}} for the outer loops. A large initial coronal density is not necessary in this scenario. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}« less