Full-disk Images Research Articles

The Progress of the Taiwan Oscillation Network Project

We describe the present status of the project of the Taiwan Oscillation Network (TON) and discuss a scientific result using the TON data. The TON is a ground-based network to measure solar intensity oscillations for the study of the solar interior. Four telescopes have been installed in appropriate longitudes around the world. The TON telescopes take K-line full-disk solar images of diameter 1000 pixels at a rate of one image per minute. The data has been collected since October of 1993. The TON high-spatial-resolution data are specially suitable for the study of local properties of the Sun. In 1997 we developed a new method, acoustic imaging, to construct the acoustic signals inside the Sun with the acoustic signals measured at the solar surface. From the constructed signals, we can form intensity map and phase-shift map of an active region at various depths. The direct link between these maps and the subsurface wave-speed perturbation suffers from the poor vertical resolution of acoustic imaging. Recently an inversion method has been developed to invert the measured phase travel time perturbation to estimate the distribution of wave-speed perturbation based on the ray approximation. This technique of acoustic imaging has been used to image the far-side of the Sun that could provides information on space weather prediction.

The Siberian Solar Radio Telescope: the current state of the instrument, observations, and data

The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in 1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec. Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive promin- ences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument. We compare SSRT observations at 5.7 GHz, including computed 'light curves', both morphologically and quantatively, with observations made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray observations, and dynamic radio spectra.

Optimizing the beam pattern of a forward-viewing ring-annular ultrasound array for intravascular imaging

Intravascular ultrasound (IVUS) imaging systems using circumferential arrays mounted on cardiac catheter tips fire beams orthogonal to the principal axis of the catheter. The system produces high resolution cross-sectional images but must be guided by conventional angioscopy. A real-time forward-viewing array, integrated into the same catheter, could greatly reduce radiation exposure by decreasing angiographic guidance. Unfortunately, the mounting requirement of a catheter guide wire prohibits a full-disk imaging aperture. Given only an annulus of array elements, prior theoretical investigations have only considered a circular ring of point transceivers and focusing strategies using all elements in the highly dense array, both impractical assumptions. In this paper, we consider a practical array geometry and signal processing architecture for a forward-viewing IVUS system. Our specific design uses a total of 210 transceiver firings with synthetic reconstruction for a given 3-D image frame. Simulation results demonstrate this design can achieve side-lobes under -40 dB for on-axis situations and under -30 dB for steering to the edge of a 80 degrees cone.

On the intensity contrast of solar photospheric faculae and network elements

Sunspots, faculae and the magnetic network contribute to solar irradiance variations. The contribution due to faculae and the network is of basic importance, but suers from considerable uncertainty. We determine the contrasts of active region faculae and the network, both as a function of heliocentric angle and magnetogram signal. To achieve this, we analyze near-simultaneous full disk images of photospheric continuum intensity and line- of-sight magnetic eld provided by the Michelson Doppler Interferometer (MDI) on board the SOHO spacecraft. Starting from the surface distribution of the solar magnetic eld we rst construct a mask, which is then used to determine the brightness of magnetic features, and the relatively eld-free part of the photosphere separately. By sorting the magnetogram signal into dierent bins we are able to distinguish between the contrasts of dierent concentrations of magnetic eld. We nd that the center-to-limb variation (CLV) of the contrast changes strongly with magnetogram signal. Thus, the contrasts of active region faculae (large magnetogram signal) and the network (small signal) exhibit a very dierent CLV, showing that the populations of magnetic flux tubes that underly the two kinds of features are dierent. The results are compatible with, on average, larger flux tubes in faculae than in the network. This implies that these elements need to be treated separately when reconstructing variations of the total solar irradiance with high precision. We have obtained an analytical expression for the contrast of photospheric magnetic features as a function of both position on the disk and spatially averaged magnetic eld strength, by performing a 2-dimensional t to the observations. We also provide a linear relationship between magnetogram signal and the =c os(), where is the heliocentric angle, at which the contrast is maximal. Finally, we show that the maximum contrast per unit magnetic flux decreases rapidly with increasing magnetogram signal, supporting earlier evidence that the intrinsic contrast of magnetic flux tubes in the network is higher.

X‐Ray Bright Points and Photospheric Bipoles during Cycles 22 and 23

We use the Yohkoh soft X-ray telescope (SXT) full-disk images made from 1993 to 2000 to manually identify X-ray bright points (XBPs). We also employ the National Solar Observatory (Kitt Peak) full-disk longitudinal magnetograms made between 1992 April and 2001 April and an automatic procedure to identify photospheric bipoles whose magnetic field strength is above 20 G, with a pole size (cross-section) between 5'' and 552, and with pole separation between 55 and 483. We use these data to study statistical properties of XBPs and photospheric bipoles during the declining phase of solar cycle 22 and the rising phase of cycle 23. The XBP number follows well-known anti-cycle variation. The average number of XBPs (~10 per disk image) remained approximately the same from 1993 to 1994. Beginning in 1995 it grew, reached a maximum around 1996 December (~50 XBPs per image), and then dropped back to pre-1995 levels in 1998. By contrast, the average number of photospheric bipoles remained approximately the same (~250 per disk image) between 1992 and 2001, despite sunspot activity changes from high (1992, cycle 22) to low (1996, solar minimum) and a return to high activity again in 2000 (solar maximum, cycle 23). Since we expect that a fraction of photospheric bipoles associated with X-ray bright points is independent of solar activity, we see this as a clear indication that the anti-cycle variation of XBP numbers is not real. Most likely, the variation in XBP numbers is the result of a change in the background brightness of the quiet-Sun corona, which is affected by the presence of active regions. On the other hand, annual latitudinal histograms of XBPs show an excess of coronal bright points at active region latitudes, contrary to the effect of changing background brightness. Photospheric bipoles show no enhancement of their distribution at active latitudes. We consider two alternative explanations for this inconsistency.

Development of an Automatic Filament Disappearance Detection System

This paper presents an efficient and automatic method for detecting filament disappearances. This method was applied to the Big Bear Solar Observatory's (BBSO) full-disk Hα images. The initial step is to detect the filaments in the solar image, then determine if they are growing, stable or disappearing. If a disappearing filament is found, the solar community can be automatically alerted in near real time. This system is proven to be accurate and fast. In addition, three statistical studies of the appearance and disappearance of all filaments in 1999 are presented.

Active-Region Monitoring and Flare Forecasting – I. Data Processing and First Results

This paper discusses a near real-time approach to solar active-region monitoring and flare prediction using the Big Bear Solar Observatory Active Region Monitor (ARM). Every hour, ARM reads, calibrates, and analyses a variety of data including: full-disk Hα images from the Global Hα Network; EUV, continuum, and magnetogram data from the Solar and Heliospheric Observatory (SOHO); and full-disk magnetograms from the Global Oscillation Network Group (GONG). For the first time, magnetic gradient maps derived from GONG longitudinal magnetograms are now available on-line and are found to be a useful diagnostic of flare activity. ARM also includes a variety of active-region properties from the National Oceanic and Atmospheric Administration's Space Environment Center, such as up-to-date active-region positions, GOES 5-min X-ray data, and flare-to-region identifications. Furthermore, we have developed a Flare Prediction System which estimates the probability for each region to produce C-, M-, or X-class flares based on nearly eight years of NOAA data from cycle 22. This, in addition to BBSO's daily solar activity reports, has proven a useful resource for activity forecasting.

Automatic Solar Flare Detection Using Neural Network Techniques

We present a new method for automatic detection of flare events from images in the optical range. The method uses neural networks for pattern recognition and is conceived to be applied to full-disk Hαimages. Images are analyzed in real time, which allows for the design of automatic patrol processes able to detect and record flare events with the best time resolution available without human assistance. We use a neural network consisting of two layers, a hidden layer of nonlinear neurodes and an output layer of one linear neurode. The network was trained using a back-propagation algorithm and a set of full-disk solar images obtained by HASTA (HαSolar Telescope for Argentina), which is located at the Estacion de Altura Ulrico Cesco of OAFA (Observatorio Astronomico Felix Aguilar), El Leoncito, San Juan, Argentina. This method is appropriate for the detection of solar flares in the complete optical classification, being portable to any Hαinstrument and providing unique criteria for flare detection independent of the observer.

The Extreme‐Ultraviolet Structure and Properties of a Newly Emerged Active Region

The structure and properties of a newly emerged solar active region (NOAA Active Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308-381 ? and 513-633 ? wavelength ranges, while EIT recorded full-disk EUV images in the He II (304 ?), Fe IX/X (171 ?), Fe XII (195 ?), and Fe XV (284 ?) bandpasses. Electron density measurements from Si IX, Si X, Fe XII, Fe XIII, and Fe XIV line ratios indicate that the region consists of a central high-density core with peak densities of the order of 1.2 ? 1010 cm-3, which decrease monotonically to ~5.0 ? 108 cm-3 at the active region boundary. The derived electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatures, and the constant pressure assumption was found to be an unrealistic simplification. Indeed, the active region is found to have a high-pressure core (1.3 ? 1016 cm-3 K) that falls to 6.0 ? 1014 cm-3 K just outside the region. CDS line ratios from different ionization stages of iron, specifically Fe XVI (335.4 ?) and Fe XIV (334.4 ?), were used to diagnose plasma temperatures within the active region. Using this method, peak temperatures of 2.1 ? 106 K were identified. This is in good agreement with electron temperatures derived using EIT filter ratios and the two-temperature model of Zhang et al. The high-temperature emission is confined to the active region core, while emission from cooler (1-1.6) ? 106 K lines originates in a system of loops visible in EIT 171 and 195 ? images. Finally, the three-dimensional geometry of the active region is investigated using potential field extrapolations from a Kitt Peak magnetogram. The combination of EUV and magnetic field extrapolations extends the core-halo picture of active region structure to one in which the core is composed of a number of compact coronal loops that confine the hot, dense, high-pressure core plasma while the halo emission emerges from a system of cooler and more extended loops.

Determination of three‐dimensional structure of coronal streamers and relationship to the solar magnetic field

We have determined the location, in three dimensions, of eight quasi‐stable coronal “streamers” from an analysis of Solar and Heliospheric Observatory (SOHO) Large‐Angle and Spectrometric Coronagraph images acquired over approximately one solar rotation. We use the locations to attempt to determine the solar origin of the streamers. Comparison of the streamers' locations (longitude and latitude at the R ≈ 2.5 Rs source surface) with that of the current sheet computed from a potential source surface model show that all of the streamers lie in or near the heliospheric current sheet. We assume that the streamers coincide with magnetic field lines and use a potential source surface magnetic model to map the location of the streamers from the source surface (R ≈ 2.5 Rs) to the photosphere. We find that many of the streamers are associated with strong magnetic field active regions. When a streamer and its associated active region are visible simultaneously, the active region is seen to be bright in the SOHO extreme ultraviolet imaging telescope (EIT) EUV full disk images. This, and other evidence, leads us to conclude that many of the bright streamers are the result of scattering from regions of enhanced density associated with active region outflow, and not a result of line‐of‐sight viewing through folds in a warped current sheet with uniform density.

Precipitation estimations from geostationary orbit and prospects for METEOSAT Second Generation

AbstractFor over two decades operational rainfall estimations from geostationary satellites have represented an ambitious aspiration of scientists and an identified need of operational meteorologists. A wide variety of infrared and combined visible and infrared methods have been proposed for the identification of suitable relationships between satellite‐observed cloud top radiative features and rainfall at the ground. Microwave‐based retrievals, however, correlate rainfall and internal cloud microphysical features more successfully. The most significant limitation, however, is the indirect character of the retrieval that correlates microphysical and dynamical cloud characteristics with rain amounts at ground level. METEOSAT Second Generation signals a new era for geostationary satellites with its new 12 channel imager SEVIRI and 15 minute full‐disk image repeat cycle. SEVIRI is expected to contribute significantly to a better characterisation of clouds and atmospheric stability by means of improved infrared calibration, radiometric performances, imaging frequency and multispectral image analysis. The significant increase of multispectral cloud observations is expected to provide new data for the improvement of rainfall estimations from geostationary orbit. The anticipated progress from enhanced imaging frequency and multispectral data for the definition of new techniques is discussed. Considerations for operational applications, chiefly for nowcasting, are also provided as they are the main goal of the satellite. Future developments and synergies with other geostationary and polar orbiting instruments, passive and active, are finally considered as the ultimate strategy for more accurate instantaneous rainfall estimations at all latitudes. Copyright © 2001 Royal Meteorological Society

Hemispheric Helicity Trend for Solar Cycle 23

Applying the same methods we used in solar cycle 22, we study active region vector magnetograms, full-disk X-ray images, and full-disk line-of-sight magnetograms to derive the helicity of solar magnetic fields in the first 4 years of solar cycle 23. We find that these three data sets all exhibit the same two key tendencies—significant scatter and weak hemispheric asymmetry—as were observed in solar cycle 22. This supports the interpretation of these tendencies as signatures of the writhing of magnetic flux by turbulence in the convection zone.

Solar Feature Identification using Contrasts and Contiguity

We present a new technique for the rapid, automatic identification of solar features on full-disk photometric images. The technique permits the detection of features whose contrasts are only slightly above the noise level. Contrast and contiguity criteria are used to identify pixels belonging to an individual feature. The criteria used are simple and objective, and do not require one to guess at the contrast distribution of the features. Comparison of Ca ii K images with magnetograms shows excellent agreement between the identified features and observed magnetic features. In addition, we can now reliably identify faculae on continuum images. Since this technique can be rapidly applied to a large set of images, it allows us to compile a database of the physical and photometric properties of individual solar features.

An Analysis of the Solar Rotation Velocity by Tracing Coronal Features

The solar rotation is determined tracing coronal EUV and soft X-ray bright points, active regions, parts of coronal holes and foot points of coronal loops. Full-disc solar images in the EUV part of the spectrum from the SOHO spacecraft (EIT, Fe XV, 28.4 nm) and from the Yohkoh satellite (SXT, 2 nm) are used.

SWRI/LASP Sounding Rocket Inter-Calibration With The Eit Instrument On Board SOHO

Two successful sounding rocket flights were launched on 15 May 1997 and 2 November 1998 with an objective of providing inter-calibration with several of the instruments on board SOHO and TRACE. We will discuss here the results of the inter-calibration between the SwRI/LASP rocket imaging instruments and the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO. The Multiple XUV Imager (MXUVI) sounding rocket instrument is a multi-layer mirror telescope equipped with an internal occulter and light trap to provide full-disk images of Fe ix/x 17.1 nm and off-limb observations of He ii 30.4 nm. The SOHO/EIT instrument is also a full-disk multi-layer imager with four channels, Fe ix/x 17.1 nm, Fe xii 19.5 nm, Fe xv 28.4 nm and He ii 30.4 nm. By comparison with the EIT observations taken at the same time, we provide new flat-field determinations for EIT which help quantify the sensitivity degradation of the EIT detector, as well as provide a measure of the off-limb stray-light characteristics of the two instruments. We find that the EIT stray-light function is strongly asymmetric, with greater stray light in the 17.1 and 19.5 nm quadrants than the 30.4 and 28.4 nm quadrants. Two possible causes of this asymmetry are the polishing processes of the EIT mirrors and the asymmetric support grid pattern in the foil mesh at the EIT pupil.

Fabry–Perot Imaging of Jupiter's Aurora at 2.1 μm

Jupiter was imaged in the near-infrared K band on July 22–24, 1994, i.e., shortly after the impact of Comet Shoemaker–Levy 9 (S-L9). Using the IRAC-2b near-infrared camera with Fabry–Perot-interferometer at the MPG/ ESO 2.2-m Telescope of the European Southern Observatory on La Silla in Chile, we obtained full disk images of Jupiter's H + 3 and H 2 emission at 2.093 and 2.121 μm, respectively. In two nights almost one full jovian rotation was covered. Our observations represent the first ground-based narrow-band images of Jupiter's H + 3 and H 2 emission in the K band. No impact-related line emission is detected. In the north polar cap the aurora is clearly visible at 2.093 μm. Some emission at 2.121 μm appears close to the limb. It has different morphology and a different λ III dependence and is fainter than the H + 3 emission. Despite H 2 being a major auroral emitter in the far-UV spectral range it is not clear how its infrared emission relates to auroral phenomena. Shortly after the S-L9 impacts the intensity at 2.093 μm is slightly larger and at 2.121 μm the intensity is three times larger as compared to values provided by Drossart et al. (1989, Nature 340, 539–541). No emissions are detected in the south polar cap. This agrees with observations in the L band (Miller et al. 1997, Planet. Space Sci. 45, 1237–1250). The north polar aurora was close to normal and its vibrational temperature close to 1100 K. The Io footprint is not visible.

Comparison of 87 GHz solar polar structures with EUV and soft X-ray emission

Polar radio brightenings at 87 GHz (3.5 mm) are compared for the rst time with features seen in EUV and soft X-rays. The data consist of nearly simultane- ous full disk images and maps from Metsahovi Radio Observatory, SOHO/EIT, and Yohkoh/SXT on 9 selected days near the solar minimum (1996-1997). The observed radio brightenings corresponded to var- ious features seen in EUV, such as diuse or localized intensity enhancements (e.g., bright points and bases of polar plumes), and intensity depressions of varying sizes (e.g., coronal holes). Some of these features were also vis- ible in soft X-rays. The visibility of radio bright coronal holes seemed to depend on how much of the polar area was exposed, due to the variation of the B0-angle. The observed radio depressions near the solar poles were very well correlated with coronal holes and other EUV and/or soft X-ray intensity drops. More than half of the coronal holes, or coronal hole-like intensity drops in EUV and soft X-rays, had radio brightenings inside them. Therefore coronal holes do not have uniform radio bright- ness at 87 GHz. Many of the bright points seen at lower latitudes in the EIT and SXT images had no, or just faint, counterparts in the millimeter radio maps. It appears that for an EUV bright point to show up at 87 GHz it has to be bright and/or spatially large also in soft X-rays.

Analysis of NASA/NSO Spectromagnetograph Observations for Comparison with Solar Irradiance Variations

The NASA/NSO Spectromagnetograph (SPM) at the National Solar Observatory/Kitt Peak Vacuum Telescope (NSO/KPVT) has produced daily full-disk solar images of line-of-sight (LOS) magnetic flux, LOS velocity, continuum intensity, equivalent width, and central line depth in either Fe I 550.7 nm or Fe I 868.8 nm since 1992 April. Internally, these five images are strictly cospatial and cotemporal. This paper describes reduction techniques for removing spurious effects (principally from clouds and optical vignetting) from the images and presents summary statistics from the first 2 years of operation. Pairs of bivariate histograms as well as five-dimensional histograms are formed from the cleaned images. Factor analysis of various summary statistics from the histograms shows that mixed-polarity magnetic elements of weak to moderate strength; unipolar magnetic features, including active regions and strong enhanced network; and sunspots are the most important sources of variance in the SPM data. Multiple regressions of spacecraft measurements of bolometric solar irradiance on the dominant factors, however, yield strong correlations with only unipolar regions and sunspots; the weak and mixed-polarity features that account for most of the SPM variance are only weakly correlated with the spacecraft measurements.

Temporal properties of He I 1083 nm dark points

The intensity of a sample of large, high-contrast and isolated dark points has been observed with full-disk images in the light of He i 1083 nm from the Chromospheric Helium line Intensity Photometer (CHIP) on Mauna Loa, Hawaii. Temporal variations in the intensity encompassing a broad range of time scales have been recorded. Long-term changes in the intensity, although highly variable, are characterized by e-folding times on the order of 5 h. Superposed on these variations are frequent intensity variations, which occur over time scales ranging from the typical observing cadence of 3 min, to tens of minutes. Microflares-involving intensity changes of at least 50% over periods of minutes are observed frequently. Rapid cadence ( $$\frac{1}{2}$$ min) observations reveal differences between rise and decay times and shorter-term variations in the intensity profiles of these microflares.

Bias in velocity determinations from full-disk solar observations

Estimates for the global solar surface velocity field can be obtained from time series of full-disk solar images. However, bias is introduced by mistakes in the assumptions about the geometry of the observations, and by imperfections in the optical system. I investigate many sources of bias and determine their first-order influence on the measured velocity field (both the transverse and longitudinal components). Results are presented in analytical and pictorial form. By comparing bias velocities of unknown origin with the results, one may obtain clues to the cause of the bias.