Extreme Ultraviolet Wavebands Research Articles

Attention-Based Deep Learning Model for Image Desaturation of SDO/AIA

The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) captures full-disk solar images in seven extreme ultraviolet wave bands. As a violent solar flare occurs, incoming photoflux may exceed the threshold of an optical imaging system, resulting in regional saturation/overexposure of images. Fortunately, the lost signal can be partially retrieved from non-local unsaturated regions of an image according to scattering and diffraction principle, which is well consistent with the attention mechanism in deep learning. Thus, an attention augmented convolutional neural network (AANet) is proposed to perform image desaturation of SDO/AIA in this paper. It is built on a U-Net backbone network with partial convolution and adversarial learning. In addition, a lightweight attention model, namely criss-cross attention, is embedded between each two convolution layers to enhance the backbone network. Experimental results validate the superiority of the proposed AANet beyond state-of-the-arts from both quantitative and qualitative comparisons.

Image Desaturation for SDO/AIA Using Mixed Convolution Network

The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) provides full-disk solar images with high temporal cadence and spatial resolution over seven extreme ultraviolet (EUV) wave bands. However, as violent solar flares happen, images captured in EUV wave bands may have saturation in active regions, resulting in signal loss. In this paper, we propose a deep learning model to restore the lost signal in saturated regions by referring to both unsaturated/normal regions within a solar image and statistical probability model of massive normal solar images. The proposed model, namely mixed convolution network (MCNet), is established over conditional generative adversarial network (GAN) and the combination of partial convolution (PC) and validness migratable convolution (VMC). These two convolutions were originally proposed for image inpainting. In addition, they are implemented only on unsaturated/valid pixels, followed by certain compensation to compensate the deviation of PC/VMC relative to normal convolution. Experimental results demonstrate that the proposed MCNet achieves favorable desaturated results for solar images and outperforms the state-of-the-art methods both quantitatively and qualitatively.

A Search for the Counterparts of Quiet-Sun Radio Transients in Extreme Ultraviolet Data

Recently nonthermal radio transients from the quiet sun have been discovered and it has been hypothesised using rough calculations that they might be important for coronal heating. It is well realized that energy calculations using coherent emissions are often subject to poorly constrained parameters and hence have large uncertainties associated with them. However energy estimates using observations in the extreme ultraviolet (EUV) and soft X-ray bands are routinely done and the techniques are pretty well established. This work presents our first attempt to identify the EUV counterparts of these radio transients and then use the counterpart to estimate the energy deposited into the corona during the event. I show that the group of radio transients studied here was associated with an brightening observed in the extreme ultraviolet waveband and was produced due to a flare of energy $\sim 10^{25}$ ergs. The fact that the flux density of the radio transient is only $\sim 2\,$mSFU suggests that it might be possible to do large statistical studies in future for understanding the relationship between these radio transients and other EUV and X-ray counterparts and also for understanding their importance in coronal heating.

Design of Pt/C X-ray supermirror by needle optimization

Needle optimization is widely used in visible and infrared region, but it is rarely utilized in X-ray and extreme ultraviolet waveband. We use Needle optimization to design Pt/C X-ray supermirrors, and the grazing incidence angles are 1.0, 1.4, and 1.7 degree, respectively. A modified target is introduced into optilayer software. A good result is obtained by Optilayer software combined with IMD software. The difference of design results between block method and needle optimization is discussed.

The Bipolar Magnetic Emergence in the Solar Polar Region

Abstract Due to projection effect and polarization sensitivity, it is still unknown whether the bipolar magnetic emergence (BME) exists in the solar polar region. In this study, combining the magnetic observations from the Helioseismic and Magnetic Imager and imaging observations of the 211 Å waveband from the Atmospheric Imaging Assembly, we try to identify and diagnose the BME. Considering the magnetic evolution and the corresponding intensity variation in the extreme ultraviolet waveband, we find definite events of the BMEs in the solar polar region. In total, we identify more than 300 BMEs in the solar polar region in the whole year from 2010 June to 2011 May. We find that whether on the southern or northern polar region, the leading polarity of the BMEs is almost half-positive and half-negative, and the BMEs’ magnetic axes show random distribution in that direction. These results imply the possible existence of local dynamo in the solar polar region.

Generating three-color pulses in high-gain harmonic-generation free-electron lasers with atilted electron bunch.

A multi-color light source is a significant tool for nonlinear optics experiments, pump-dump/repump-probe experiments and in other fields. Here, a novel method is proposed to create three-color pulses based on a high-gain harmonic-generation (HGHG) free-electron laser with a tilted electron bunch. In this method, the initial bunch tilt is created by transverse wakefields after the bunch passes through a corrugated structure with an off-axis orbit, and is further enlarged in a following drift section. Then the tilted bunch experiences the off-axis field of a quadrupole magnet to cool down the large transverse velocity induced before. After that, it enters an HGHG configuration adopting a transverse gradient undulator (TGU) as the radiator, where only three separated fractions of the tilted bunch will resonate at three adjacent harmonics of the seed wavelength and are enabled to emit three-color pulses simultaneously. In addition, the use of the natural transverse gradient of a normal planar undulator instead of the TGU radiator to emit three-color pulses is also studied in detail. Numerical simulations including the generation of the tilted bunch and the free-electron laser radiation confirm the validity and feasibility of this scheme both for the TGU radiator and the natural gradient in the extreme-ultraviolet waveband.

Coexisting fast and slow propagating waves of the extreme-UV intensity in solar coronal plasma structures

Context. From 06:15 UT to 08:15 UT on 2011 June 2, a toroidal filament located at the joint of two active regions, 11226 and 11227, appeared to perform two eruptions. During this phenomenon, fast and slow magnetoacoustic waves were detected to propagate simultaneously along a funnel coronal loop system of AR 11227. Aims. We aim to understand the relationship between fast and slow magnetoacoustic waves during their propagations and measure their properties, such as the propagating speed, path, amplitude, and period observed in the extreme ultraviolet (EUV) wavebands. Methods. We analyse time sequences of EUV images acquired by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. By creating time-distance maps along selected directions, we measure the speeds and localisation of EUV intensity waves in different EUV wavebands. We determine the periods of the waves with wavelet analysis. Results. The fast and slow magnetoacoustic waves, apparently propagating along the same path, are found to have different properties. Their apparent propagation speeds, travel distances, and periods are about 900 km s-1 and 100 km s-1, 145 Mm and 36 Mm, and 2 min and 3 min, respectively.

HOMOLOGOUS EXTREME ULTRAVIOLET WAVES IN THE EMERGING FLUX REGION OBSERVED BY THESOLAR DYNAMICS OBSERVATORY

Taking advantage of the high temporal and spatial resolution of the Solar Dynamics Observatory (SDO) observations, we present four homologous extreme ultraviolet (EUV) waves within 3 hr on 2010 November 11. All EUV waves emanated from the same emerging flux region (EFR), propagated in the same direction, and were accompanied by surges, weak flares, and faint coronal mass ejections (CMEs). The waves had the basically same appearance in all EUV wavebands of the Atmospheric Imaging Assembly on SDO. The waves propagated at constant velocities in the range of 280-500 km s(-1), with little angular dependence, which indicated that the homologous waves could be likely interpreted as fast-mode waves. The waves are supposed to likely involve more than one driving mechanism, and it was most probable that the waves were driven by the surges, due to their close timing and location relations. We also propose that the homologous waves were intimately associated with the continuous emergence and cancellation of magnetic flux in the EFR, which could supply sufficient energy and trigger the onsets of the waves.

Cosmic‐Ray Electrons in Groups and Clusters of Galaxies: Primary and Secondary Populations from a Numerical Cosmological Simulation

We investigate the generation and distribution of high-energy electrons in the cosmic structure environment and their observational consequences by carrying out the first cosmological simulation that includes directly cosmic-ray (CR) particles. Starting from cosmological initial conditions, in addition to the gas and dark matter related quantities, we follow the evolution of CR electrons (primary and secondary) and CR ions along with a passive magnetic field. CR ions and primary electrons are injected in accordance with the thermal leakage model and accelerated in the test-particle limit of diffusive shock acceleration at shocks associated with large-scale structure formation. Secondary electrons are continuously generated through p-p inelastic collisions of the CR ions with the thermal nuclei of the intergalactic medium. The evolution of the CR electrons accounts for spatial transport, adiabatic expansion/compression, and losses due to Coulomb collisions, bremsstrahlung, synchrotron and inverse-Compton emission. The magnetic field is seeded at shocks according to the Biermann battery model, and thereafter amplified by shear flow and gas compression. We compute the emission due to the inverse-Compton scattering of the simulated primary and secondary electrons off cosmic microwave background photons and compare it with the published values of the detected radiation excesses in the hard X-ray and extreme-ultraviolet wavebands. We find that the few instances of detection of hard X-ray radiation excess could be explained in the framework of IC emission from primary electrons in clusters characterized by high accretion/merger activity. On the other hand, with the only exception of measured flux from the Coma Cluster by Bowyer, Berghoefer & Korpela, both primary and secondary CR electrons associated with the cosmic structure formation account at most for a small fraction of the radiation excess detected in the extreme-ultraviolet waveband. Next, we calculate the synchrotron emission after normalizing the magnetic field strength so that for a Coma-like cluster the volume-averaged B21/2 3 ?G. Our results indicate that the synchrotron emission from the secondary CR electrons reproduces several general properties observed in radio halos. These include the recently found P1.4 GHz versus TX relationship, the morphology and polarization of the emitting region, and, to some extent, even the spectral index. In addition, radio synchrotron emission from primary electrons turns out to be large enough to power extended regions of radio emission, resembling radio relics observed at the outskirts of clusters. Once again we find a striking resemblance between the general properties of morphology, polarization, and spectral index of our synthetic maps and those of reported in the literature.

EUV activity in late-type stars during the ROSAT WFC All-Sky Survey -- I. Techniques and initial results

We study 127 active late-type stars, including the well-known flare stars, in the extreme ultraviolet (EUV) region of the electromagnetic spectrum, using the ROSAT Wide Field Camera (WFC) survey observations. Our analysis includes photometry for all the stars in the sample, timing analysis of the observed photon events, and a statistical analysis for possible small-scale, low-level variability. 49 stars in our sample were detected in one or both EUV wavebands. A total of 35 flares were seen, from 23 stars, not all of them well-known flare stars. In addition, about half ofthe detections exhibited low-level variability. Further analysis reveals that the observed low-level variability was not due to rotational or orbital modulation, but rather was related to small-scale activity on the stars, possibly in the form of small­ amplitude, flare-like events which were too faint to be recognized as individual flares at the sensitivity of the WFC. We call this low-level activity 'milliflaring'. We also give estimated upper-limit count rates for the non-detections.

Astronomy and the Extreme Ultraviolet Explorer satellite.

The extreme ultraviolet wave band (100 to 912 angstroms) was thought until recently to be useless to astronomy, primarily because the opacity of the interstellar medium would prevent observations at these wavelengths. However, the interstellar medium has been found to be markedly inhomogeneous in both density and ionization state and the sun is fortunately located in a region of low extreme ultraviolet opacity. The Extreme Ultraviolet Explorer, launched in June 1992, has surveyed the sky in this wave band and has detected a wide variety of astronomical sources at considerable distances, including some extragalactic objects. Studies in the extreme ultraviolet band have already begun to increase our understanding of the contents of the universe.

Carbon and Beryllium Extreme Ultraviolet Filters: Far-ultraviolet Performance

Abstract Designs for extreme-ultraviolet filters using thin carbon and beryllium foils are presented and their far-ultraviolet performance summarized. The transmission data yield values for the linear-absorption coefficients of beryllium and carbon in the wavelength range 1200–2000 A which are compared with other published data in this and in the adjacent extreme-ultraviolet wavebands. Use of multiple foils is demonstrated to be an effective means of eliminating pin-holes, which may contribute significantly to the far-ultraviolet transmission.