Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms

This paper describes a new 2D model for the photospheric evolution of the magnetic carpet. It is the first in a series of papers working towards constructing a realistic 3D non-potential model for the interaction of small-scale solar magnetic fields. In the model, the basic evolution of the magnetic elements is governed by a supergranular flow profile. In addition, magnetic elements may evolve through the processes of emergence, cancellation, coalescence and fragmentation. Model parameters for the emergence of bipoles are based upon the results of observational studies. Using this model, several simulations are considered, where the range of flux with which bipoles may emerge is varied. In all cases the model quickly reaches a steady state where the rates of emergence and cancellation balance. Analysis of the resulting magnetic field shows that we reproduce observed quantities such as the flux distribution, mean field, cancellation rates, photospheric recycle time and a magnetic network. As expected, the simulation matches observations more closely when a larger, and consequently more realistic, range of emerging flux values is allowed (4e16 - 1e19 Mx). The model best reproduces the current observed properties of the magnetic carpet when we take the minimum absolute flux for emerging bipoles to be 4e16 Mx. In future, this 2D model will be used as an evolving photospheric boundary condition for 3D non-potential modeling.Comment: 33 pages, 16 figures, 5 gif movies included: movies may be viewed at http://www-solar.mcs.st-and.ac.uk/~karen/movies_paper1

Detection of Atmospheric Cherenkov Radiation Using Solar Heliostat Mirrors

The gamma-ray energy region between 20 and 250 GeV is largely unexplored. Ground-based atmospheric Cherenkov detectors offer a possible way to explore this region, but large Cherenkov photon collection areas are needed to achieve low energy thresholds. This paper discusses the development of a Cherenkov detector using the heliostat mirrors of a solar power plant as the primary collector. As part of this development, we built a prototype detector consisting of four heliostat mirrors and used it to record atmospheric Cherenkov radiation produced in extensive air showers created by cosmic ray particles.Comment: 16 latex pages, 8 postscript figures, uses psfig.sty, to be published in Astroparticle Physic

Gamma-Ray Bursts: Jets and Energetics

The relativistic outflows from gamma-ray bursts are now thought to be narrowly collimated into jets. After correcting for this jet geometry there is a remarkable constancy of both the energy radiated by the burst and the kinetic energy carried by the outflow. Gamma-ray bursts are still the most luminous explosions in the Universe, but they release energies that are comparable to supernovae. The diversity of cosmic explosions appears to be governed by the fraction of energy that is coupled to ultra-relativistic ejecta.Comment: Paper presented at "The Restless High-Energy Universe", May 5-8 2003 Royal Tropical Institute, Amsterda

Solar Intranetwork Magnetic Elements: bipolar flux appearance

The current study aims to quantify characteristic features of bipolar flux appearance of solar intranetwork (IN) magnetic elements. To attack such a problem, we use the Narrow-band Filter Imager (NFI) magnetograms from the Solar Optical Telescope (SOT) on board \emph{Hinode}; these data are from quiet and an enhanced network areas. Cluster emergence of mixed polarities and IN ephemeral regions (ERs) are the most conspicuous forms of bipolar flux appearance within the network. Each of the clusters is characterized by a few well-developed ERs that are partially or fully co-aligned in magnetic axis orientation. On average, the sampled IN ERs have total maximum unsigned flux of several 10^{17} Mx, separation of 3-4 arcsec, and a lifetime of 10-15 minutes. The smallest IN ERs have a maximum unsigned flux of several 10^{16} Mx, separations less than 1 arcsec, and lifetimes as short as 5 minutes. Most IN ERs exhibit a rotation of their magnetic axis of more than 10 degrees during flux emergence. Peculiar flux appearance, e.g., bipole shrinkage followed by growth or the reverse, is not unusual. A few examples show repeated shrinkage-growth or growth-shrinkage, like magnetic floats in the dynamic photosphere. The observed bipolar behavior seems to carry rich information on magneto-convection in the sub-photospheric layer.Comment: 26 pages, 14 figure

Accretion, Outflows, and Winds of Magnetized Stars

Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars are radiatively driven. In all of these cases, the magnetic field influences matter flow from the stars and determines many observational properties. In this chapter we review recent studies of accretion, outflows, and winds of magnetized stars with a focus on three main topics: (1) accretion onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and (3) winds from isolated massive magnetized stars. We show results obtained from global magnetohydrodynamic simulations and, in a number of cases compare global simulations with observations.Comment: 60 pages, 44 figure

Parameters of the Magnetic Flux inside Coronal Holes

Parameters of magnetic flux distribution inside low-latitude coronal holes (CHs) were analyzed. A statistical study of 44 CHs based on Solar and Heliospheric Observatory (SOHO)/MDI full disk magnetograms and SOHO/EIT 284\AA images showed that the density of the net magnetic flux, $B_{{\rm net}}$, does not correlate with the associated solar wind speeds, $V_x$. Both the area and net flux of CHs correlate with the solar wind speed and the corresponding spatial Pearson correlation coefficients are 0.75 and 0.71, respectively. A possible explanation for the low correlation between $B_{{\rm net}}$ and $V_x$ is proposed. The observed non-correlation might be rooted in the structural complexity of the magnetic field. As a measure of complexity of the magnetic field, the filling factor, $f(r)$, was calculated as a function of spatial scales. In CHs, $f(r)$ was found to be nearly constant at scales above 2 Mm, which indicates a monofractal structural organization and smooth temporal evolution. The magnitude of the filling factor is 0.04 from the Hinode SOT/SP data and 0.07 from the MDI/HR data. The Hinode data show that at scales smaller than 2 Mm, the filling factor decreases rapidly, which means a mutlifractal structure and highly intermittent, burst-like energy release regime. The absence of necessary complexity in CH magnetic fields at scales above 2 Mm seems to be the most plausible reason why the net magnetic flux density does not seem to be related to the solar wind speed: the energy release dynamics, needed for solar wind acceleration, appears to occur at small scales below 1 Mm.Comment: 6 figures, approximately 23 pages. Accepted in Solar Physic

Cadmium Telluride Solar Cells on Ultrathin Glass for Space Applications

This paper details the preliminary findings of a study to achieve a durable thin film CdTe photovoltaic device structure onto ultra-thin space qualified cover glass. An aluminium doped zinc oxide (AZO) transparent conducting oxide (TCO) was deposited directly onto cover glass using metal organic chemical vapour deposition (MOCVD). The AZO demonstrated a low sheet resistance of 10 Ω/□ and high optical transparency of 85% as well as an excellent adherence and environmental stability. Preliminary deposition of the photovoltaic layers onto the AZO on cover glass, by MOCVD, showed the possibility of such a structure yielding a device conversion efficiency of 7.2 %. High series resistance (10 Ω.cm2) and low Voc (586 mV) were identified as the limiting factors when compared to the authors platform process on indium tin oxide (ITO) coated aluminosilicate. The coverage of the Cd1-xZnxS window layer along with the front contacting of the device was shown to be the major cause of the low efficiency. Further deposition of the AZO/CdTe employing an oxygen plasma cleaning step to the cover glass and evaporated gold front contacts significantly improved the device performance. A best conversion efficiency of 10.2 % with series resistance improved to 4.4 Ω.cm2 and open circuit voltage (Voc) up to 667 mV and good adhesion has demonstrated for the first time direct deposition of CdTe solar cells onto 100 μm thick space qualified cover glass

CdCl2 treatment related diffusion phenomena in Cd1xZnxS/CdTe solar cells

Utilisation of wide bandgap Cd1_xZnxS alloys as an alternative to the CdS window layer is an attractive route to enhance the performance of CdTe thin film solar cells. For successful implementation, however, it is vital to control the composition and properties of Cd1_xZnxS through device fabrication processes involving the relatively high-temperature CdTe deposition and CdCl2 activation steps. In this study, cross-sectional scanning transmission electron microscopy and depth profiling methods were employed to investigate chemical and structural changes in CdTe/Cd1_xZnxS/CdS superstrate device structures deposited on an ITO/boro-aluminosilicate substrate. Comparison of three devices in different states of completion—fully processed (CdCl2 activated), annealed only (without CdCl2 activation), and a control (without CdCl2 activation or anneal)—revealed cation diffusion phenomena within the window layer, their effects closely coupled to the CdCl2 treatment. As a result, the initial Cd1_xZnxS/CdS bilayer structure was observed to unify into a single Cd1_xZnxS layer with an increased Cd/Zn atomic ratio; these changes defining the properties and performance of the Cd1_xZnxS/CdTe device

Effect of bending test on the performance of CdTe solar cells on flexible ultra-thin glass produced by MOCVD

The development of lightweight and flexible solar modules is highly desirable for high specific power applications, building integrated photovoltaics, unmanned aerial vehicles and space. Flexible metallic and polyimide foils are frequently used, but in this work an alternative substrate with attractive properties, ultra-thin glass (UTG) has been employed. CdTe solar cells with average efficiency reaching 14.7% AM1.5G efficiency have been produced on UTG of 100 μm thickness. Little has been reported on the effects on PV performance when flexed, so we investigated the effects on J-V parameters when the measurements were performed in 40 mm and 32 mm bend radius, and in a planar state before and after the bend curvature was applied. The flat J-V measurements after 32 mm bending test showed some improvement in efficiency, Voc and FF, with values higher than the first measurement in a planar state. In addition, two CdTe solar cells with identical initial performance were subjected to 32 mm static bending test for 168 hours, the results showed excellent uniformity and stability and no significant variation on J-V parameters was observed. External quantum efficiency and capacitance voltage measurements were performed and showed no significant change in spectral response or carrier concentration. Residual stress analysis showed that no additional strain was induced within the film after the bending test and that the overall strain was low. This has demonstrated the feasibility of using CdTe solar cells on UTG in new applications, when a curved module is required without compromising performance

A high resolution imaging detector for TeV gamma-ray astronomy

Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques