The Shape of the Solar Limb: Models and Observations

In this paper we compare observed, empirical, and modelled solar limb profiles and discuss their potential use to derive physical properties of the solar atmosphere. The PHOENIX, SolMod3D, and COSI radiative transfer codes as well as VAL-C models are used to calculate the solar limb shape under different assumptions. The main properties of each model are shown. The predicted limb shape as a function of wavelength for different features on the solar disk, such as quiet Sun, sunspots, and faculae, is investigated. These models provide overall consistent limb shapes with some discrepancies that are discussed here in terms of differences in solar atmosphere models, opacities, and the algorithms used to derive the solar limb profile. Our analysis confirms that the most common property of all models is limb shapes that are much steeper than what is observed, or predicted by the available empirical models. Furthermore, we have investigated the role of the Fraunhofer lines within the spectral domain used for the solar limb measurements. Our results show that the presence of the Fraunhofer lines significantly displaces the limb inflection point from its position estimated assuming only the photospheric continuum. The PICARD satellite, launched on 15 June 2010, will provide measurements of the limb shape at several wavelengths. This work shows that the precision of these measurements allows for discrimination among the available models.     

IRIS Observations of Spicules and Structures Near the Solar Limb

We have analyzed Interface Region Imaging Spectrograph (IRIS) spectral and slit-jaw observations of a quiet region near the South Pole. In this article we present an overview of the observations, the corrections, and the absolute calibration of the intensity. We focus on the average profiles of strong (Mg?ii h and k, C?ii and Si?iv), as well as of weak spectral lines in the near ultraviolet (NUV) and the far ultraviolet (FUV), including the Mg?ii triplet, thus probing the solar atmosphere from the low chromosphere to the transition region. We give the radial variation of bulk spectral parameters as well as line ratios and turbulent velocities. We present measurements of the formation height in lines and in the NUV continuum from which we find a linear relationship between the position of the limb and the intensity scale height. We also find that low forming lines, such as the Mg?ii triplet, show no temporal variations above the limb associated with spicules, suggesting that such lines are formed in a homogeneous atmospheric layer and, possibly, that spicules are formed above the height of \(2''\). We discuss the spatio-temporal structure of the atmosphere near the limb from images of intensity as a function of position and time. In these images, we identify p-mode oscillations in the cores of lines formed at low heights above the photosphere, slow-moving bright features in O?i and fast-moving bright features in C?ii. Finally, we compare the Mg?ii k and h line profiles, together with intensity values of the Balmer lines from the literature, with computations from the PROM57Mg non-LTE model, developed at the Institut d’ Astrophysique Spatiale, and estimated values of the physical parameters. We obtain electron temperatures in the range of \({\sim}\, 8000~\mbox{K}\) at small heights to \({\sim}\, 20\,000~\mbox{K}\) at large heights, electron densities from \(1.1\times 10^{11}\) to \(4\times 10^{10}~\mbox{cm}^{-3}\) and a turbulent velocity of \({\sim}\, 24~\mbox{km}\,\mbox{s}^{-1}\).     

Initial CHIP He I Observations of Solar Limb Activity

A new instrument capable of 3-min time resolution full-disk and limb observations in the Hei 1083 nm spectral line has been in operation at the High Altitude Observatory's Mauna Loa Solar Observatory (MLSO) since April 1996. We discuss instrument capabilities and performance and present some initial observations of limb activity from the first year of instrument operation. We compare limb Hei and Hα observations of quiescent and active prominences, comment on the role of Doppler shifts in interpreting the Hei observations, and illustrate the use of disk/limb Hei observations of a CME-associated eruptive filament in mass-ejection studies.     

The Measurement of Solar Diameter and Limb Darkening Function with the Eclipse Observations

The total solar irradiance varies over a solar cycle of 11 years and maybe over cycles of longer periods. Is the solar diameter variable over time too? A discussion of the solar diameter and its variations must be linked to the limb darkening function (LDF). We introduce a new method to perform high-resolution astrometry of the solar diameter from the ground, through the observations of eclipses, using the luminosity evolution of Baily’s bead and the profile of the lunar edge available from satellite data. This approach unifies the definition of the solar limb with the inflection point of LDF for eclipses and drift-scan or heliometric methods. The method proposed is applied for the videos of the eclipse on 15 January 2010 recorded in Uganda and in India. The result suggests reconsidering the evaluations of the historical eclipses observed with the naked eye.     

On Limb Observations of Bolides

The LP/OMPS limb sensor of the Suomi satellite detected the atmospheric trace of the Chelyabinsk superbolide with an energy of over 400 kt TNT in 2013. The study of LP/OMPS data shows that it detects several bolides with much less energy each year. The paper considers the cases of observation of three bolides in 2016–2017: on February 6, 2016 (13 kt), March 3, 2016 (0.19 kt), and March 9, 2017 (1 kt). The possibility of improving the detection of bolides using a limb sensor in the form of a microsatellite with an increased number of observation regions along the horizon is discussed. Several such microsatellites will be able to track most of the bodies larger than a meter that burn up in the atmosphere of the Earth or Mars.

     

The Apparent Reversal of Fields at the Solar Limb

We show how the apparent reversal of longitudinal magnetic fields near the limb is a projection effect and may be used to estimate the divergence angle of the magnetic canopy. The limb distance at which the apparent reversal is not seen is the limiting angle of divergence of lines of force emerging from the surface. We have surveyed a number of polar limb magnetograms, where the unipolar field makes this easier, and found the divergence angle to be about 20° from the vertical.     

日食的射电观测(英)

日食为射电天文提供了一维高空间分辨率太阳射电观测机会．日食射电观测在太阳射电物理的发展上起过重要的作用．文中对日食射电观测的若干重要因素作了介绍和分析．日食射电观测在我国太阳射电天文发展上也起了重要作用．文中简要介绍了在我国组织观测的1958年、1968年、1980年及1987年的太阳射电日食观测及其主要结果．     

SDO Observations of Solar Jets

We present an analysis of high cadence observations of solar jets observed in the Extreme Ultraviolet (EUV), at 304 Å, with the Atmospheric Imaging Assembly instrument aboard the Solar Dynamics Observatory (SDO). The jets in our sample lie very close to the solar limb to minimize projection effects. Two of the events show clear helical patterns during ejection. We also find that some of the jets are recurrent and that most of them cannot overcome solar gravity. We investigate the temporal evolution of the jets by measuring the height of their leading edge as a function of time. By fitting the resulting height–time diagrams, we derive the magnitude of their initial ejection speed and plasma acceleration by assuming ballistic motion. Moreover, we calculate the upward acceleration of the jets based on the dynamical velocity of the plasma, without assuming a ballistic motion. In both models, the acceleration profiles suggest the influence of forces other than gravity. In particular, we find indications of an upwards driving force which weakens the decelerating effect of the solar gravitational field along the motion of the jet. This force is larger in the dynamical model, which indicates that the ballistic approximation does not properly determine the rising motion of the plasma jets.     

Probing the Fundamental Physics of the Solar Corona with Lunar Solar Occultation Observations

Imaging and spectroscopy of the solar corona, coupled with polarimetry, are the only tools available at present to capture signatures of physical processes responsible for coronal heating and solar wind acceleration within the first few solar radii above the solar limb. With the recent advent of improved detector technology and image processing techniques, broad-band white light and narrow-band multi-wavelength observations of coronal forbidden lines, made during total solar eclipses, have started to yield new views about the thermodynamic and magnetic properties of coronal structures. This paper outlines these unique capabilities, which until present, have been feasible primarily with observations during natural total solar eclipses. This work also draws attention to the exciting possibility of greatly increasing the frequency and duration of solar eclipse observations with Moon orbiting observatories utilizing lunar limb occultation of the solar disk for coronal measurements.     

On the Performance of Multi-Instrument Solar Flare Observations During Solar Cycle 24

The current fleet of space-based solar observatories offers us a wealth of opportunities to study solar flares over a range of wavelengths. Significant advances in our understanding of flare physics often come from coordinated observations between multiple instruments. Consequently, considerable efforts have been, and continue to be, made to coordinate observations among instruments (e.g. through the Max Millennium Program of Solar Flare Research). However, there has been no study to date that quantifies how many flares have been observed by combinations of various instruments. Here we describe a technique that retrospectively searches archival databases for flares jointly observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE – Multiple EUV Grating Spectrograph (MEGS)-A and -B, Hinode/(EUV Imaging Spectrometer, Solar Optical Telescope, and X-Ray Telescope), and Interface Region Imaging Spectrograph (IRIS). Out of the 6953 flares of GOES magnitude C1 or greater that we consider over the 6.5 years after the launch of SDO, 40 have been observed by 6 or more instruments simultaneously. Using each instrument’s individual rate of success in observing flares, we show that the numbers of flares co-observed by 3 or more instruments are higher than the number expected under the assumption that the instruments operated independently of one another. In particular, the number of flares observed by larger numbers of instruments is much higher than expected. Our study illustrates that these missions often acted in cooperation, or at least had aligned goals. We also provide details on an interactive widget (Solar Flare Finder), now available in SSWIDL, which allows a user to search for flaring events that have been observed by a chosen set of instruments. This provides access to a broader range of events in order to answer specific science questions. The difficulty in scheduling coordinated observations for solar-flare research is discussed with respect to instruments projected to begin operations during Solar Cycle 25, such as the Daniel K. Inouye Solar Telescope, Solar Orbiter, and Parker Solar Probe.     

Observations of a Radio-Quiet Solar Preflare

The preflare phase of the flare SOL2011-08-09T03:52 is unique in its long duration, in that it was covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph, and because it showed three well-developed soft X-ray (SXR) peaks. No hard X-rays (HXR) are observed in the preflare phase. Here we report that no associated radio emission at 17 GHz was found either, despite the higher sensitivity of the radio instrument. The ratio between the SXR peaks and the upper limit of the radio peaks is higher by more than one order of magnitude than the ratio in regular flares. The result suggests that the ratio between acceleration and heating in the preflare phase was different than in regular flares. Acceleration to relativistic energies, if any, occurred with lower efficiency.     

太阳活动区的磁场观测

我们利用北京天文台太阳磁场望远镜在1983年投入试观测期间取得的资料,对该年6月份的一群黑子的磁场以及耀斑作了综合分析,得到一些结论。以光球纵场为边界条件,计算了常α无力场。根据挤压无力场耀斑模式,我们认为耀斑爆发的能量,来自异极性黑子的相互靠近。磁中性线的扭曲程度,反映了无力场的状态。     

Observations of the 21 June 2001 Total Solar Eclipse

Total solar eclipses are rare events, but the publicity surrounding them is generating a lot of interest. These events can be used to popularize astronomy and promote science awareness. The internet is gaining popularity as a medium to view eclipses remotely as is evidenced by the number of websites devoted to the 2001 solar eclipse. More internet webcasts are needed to meet the demand that these events generate. The attempt by a team from the Astronomy Department at the University of South Africa to broadcast a streaming video of the 21 June 2001 total eclipse from Lusaka on the internet are described. All the necessary equipment is commercially available. Lessons learnt from this eclipse can be used to prepare for future events. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-resolution Observations of Plasma Jets in the Solar Corona

We present recent observations of coronal jets, made by TRACE and Yohkoh/SXT on 28 May and 19 August 1998. The high spatial resolution of TRACE enables us to see in detail the process of material ejection; in the line of Fe ix (one million degrees) we see both bright emitting material and dark absorbing/scattering material being ejected, i.e., both hot and cold material, highly collimated and apparently ejected along the direction of the overlying field lines. Bright ejecta are seen simultaneously in Lyman α for one event and Yohkoh/SXT in the other. The jets on the two days are different in that the 19 August jet displays the morphology typical of a one-sided anemone jet while the 28 May jet exhibits a two-sided jet morphology. The 19 August jet shows evidence for rotation and an interesting bifurcation at large distances from the energy release site. We study the physical properties and energetics of these jetting events, and conclude that existing theoretical models capture the essential physics of the jet phenomena. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005213826793     

Interpretation of Solar Magnetic Field Strength Observations

This study based on longitudinal Zeeman effect magnetograms and spectral line scans investigates the dependence of solar surface magnetic fields on the spectral line used and the way the line is sampled to estimate the magnetic flux emerging above the solar atmosphere and penetrating to the corona from magnetograms of the Mt. Wilson 150-foot tower synoptic program (MWO). We have compared the synoptic program λ5250 Å line of Fe?i to the line of Fe?i at λ5233 Å since this latter line has a broad shape with a profile that is nearly linear over a large portion of its wings. The present study uses five pairs of sampling points on the λ5233 Å line. Line profile observations show that the determination of the field strength from the Stokes V parameter or from line bisectors in the circularly polarized line profiles lead to similar dependencies on the spectral sampling of the lines, with the bisector method being the less sensitive. We recommend adoption of the field determined with the line bisector method as the best estimate of the emergent photospheric flux and further recommend the use of a sampling point as close to the line core as is practical. The combination of the line profile measurements and the cross-correlation of fields measured simultaneously with λ5250 Å and λ5233 Å yields a formula for the scale factor δ ^?1 that multiplies the MWO synoptic magnetic fields. By using ρ as the center-to-limb angle (CLA), a fit to this scale factor is δ ^?1=4.15?2.82sin?²(ρ). Previously δ ^?1=4.5?2.5sin?²(ρ) had been used. The new calibration shows that magnetic fields measured by the MDI system on the SOHO spacecraft are equal to 0.619±0.018 times the true value at a center-to-limb position 30°. Berger and Lites (2003, Solar Phys. 213, 213) found this factor to be 0.64±0.013 based on a comparison using the Advanced Stokes Polarimeter.     

Light Deflection Near the Sun's Limb: Refraction by the Solar Atmosphere

Light refraction by the Sun's atmosphere is calculated.As detected from the Earth, the refraction can deflect a light ray emitted from the Sun's limb by 13 or a starlight ray grazing the solar limb by 26, an effect 15 times larger than the gravitational deflection.     

On the Shape of Force-Free Field Lines in the Solar Corona

This paper studies the shape parameters of looped field lines in a linear force-free magnetic field. Loop structures with a sufficient amount of kinking are generally seen to form S or inverse S (Z) shapes in the corona (as viewed in projection). For a single field line, we can ask how much the field line is kinked (as measured by the writhe), and how much neighbouring flux twists about the line (as measured by the twist number). The magnetic helicity of a flux element surrounding the field line can be decomposed into these two quantities. We find that the twist helicity contribution dominates the writhe helicity contribution, for field lines of significant aspect ratio, even when their structure is highly kinked. These calculations shed light on some popular assumptions of the field. First, we show that the writhe of field lines of significant aspect ratio (the apex height divided by the footpoint width) can sometimes be of opposite sign to the helicity. Secondly, we demonstrate the possibility of field line structures which could be interpreted as Z-shaped, but which have a helicity value sign expected of an S-shaped structure. These results suggest that caution should be exercised in using two-dimensional images to draw conclusions on the helicity value of field lines and flux tubes.     

Reports on Test Observations with the Multi-Channel Solar Telescope

In this paper, we have made a report on the test observations with a Multi-Channel Solar Telescope (MCST), which consists of 60 cm Nine-Channel Solar Telescope (NCST), 35 cm Solar Magnetic Field Telescope (SMFT), 8 cm Full Disc Telescope (FDT), 10 cm Full Disc Magnetic Field Telescope (FDMFT) and 14 cm H telescope. These observations demonstrate that the MCST has the following advantages: (a) It can work at more than nine visible spectral lines simultaneously. In this way, different solar layers of the photosphere and chromosphere can be observed at the same time; (b) every channel of the NCST is entirely equivalent to a videomagnetograph, by means of which the vector magnetic fields and line-of-sight velocity fields can be measured; (c) real-time monochromatic images of the photosphere and chromosphere can be obtained with the FDT, FDMFT, and H Telescope; (d) high-temporal-resolution full-disk magnetic fields can be measured with the FDMFT; (e) spectral profiles over a large field of view can be scanned with the NCST.     

The Shape of Sunspots and Solar Activity Cycles

Solar Physics - A new open-source software, called SunMap, has been developed to obtain synoptic maps in an easy and quick way from multiple full-disc solar images. Our objective is to provide a...     

Application of Theorems on Null-Geodesics on The Solar Limb Effect

A direct and more general calculation of the limb effect connected with red-shift observations is obtained. Result agrees completely, in its general form, with that obtained from observations of the solar spectra but with different value for the maximum effect. This revised version was published online in July 2006 with corrections to the Cover Date. This revised version was published online in July 2006 with corrections to the Cover Date.