Solar Limb Research Articles

Data reduction and calibration of the FMG onboard ASO-S

The Full-disk vectorMagnetoGraph (FMG) instrument will carry out polarization observations at one wavelength position of the Fe I 5324.179 °A spectral line. This paper describes how to choose this single wavelength position, the relevant data-processing and the magnetic field calibrations based on the measured polarization signals at one single wavelength position. It is found that solar radial Doppler velocity, which can cause the spectral line to shift, is a disadvantageous factor for the linear calibration at one wavelength position. Observations at two symmetric wavelength positionsmay significantly reduce the wavelength shift effect (∼ 75%), but simulations show that such polarization signals located at the solar limbs (e.g., beyond the longitude range of ±30°) are not free from the effect completely. In future work, we plan to apply machine learning techniques to calibrate vector magnetic fields, or employ full Stokes parameter profile inversion techniques to obtain accurate vector magnetic fields, in order to complement the linear calibration at the single wavelength position.

VLF Measurements and Modeling of the D-Region Response to the 2017 Total Solar Eclipse

In this paper, we report measurements in Colorado and Utah of the disturbed very-low-frequency (VLF) signals from the NML Navy transmitter in North Dakota during the 2017 solar eclipse. Using an occultation mask of solar fluxes together with detailed chemistry and VLF propagation simulations, we quantify the D-region response to the eclipse, in terms of electron density variation, as well as the expected signatures of VLF transmitter signals. The VLF measurements, including an anomalous amplitude enhancement recorded in UT, can be quantitatively explained using the Wait and Spies ionospheric profile with a sharpness parameter of $\beta = 0.3$ km−1 above ~55 km and an increase in the D-region ionosphere height of $\Delta h' \simeq 8$ km. This sharpness parameter is consistent with previously reported rocket measurements and first-principles calculations. The best-fit results suggest a reduction of D-region electron density by ~90% during the eclipse in the D-region, implying an occultation of Lyman- $\alpha $ by nearly 99%. This finding agrees with detailed calculations of time-dependent obscuration factors utilizing the He 30.4-nm images from Solar Dynamics Observatory as a proxy for the distribution of Lyman- $\alpha $ across the solar disk and limb. Moreover, the present results show that subionospheric VLF propagation is sensitive to the sharpness parameter of the electron density profile in the D-region. Previously reported first-principles simulations have shown that the sharpness parameter is mostly controlled by the background concentration of minor neutral species. Thus, the VLF technique can be likely used to remotely sense these neutral species at and below the effective reflection altitudes of VLF waves.

Studying the relationship of localization parameters of solar sources of magnetic clouds with their characteristics and substorm activity

We propose a method for determining location and orientation of extended solar sources of magnetic clouds, using coronagraph data and SOHO EIT/MDI images of the photosphere. To estimate the probability of formation of magnetic clouds, we use a simple cylindrical force-free model. We have established that more extended sources and those having a slight inclination to the solar equator and located on the solar limb as compared to those that are nonextended and strongly inclined can generate expanding clouds, which with high probability can reach the magnetosphere like clouds from a source near the zero meridian and low latitudes.
 We determine the relationship between extreme values of substorm activity and parameters of solar sources under study during the impact of magnetic clouds on Earth’s magnetosphere from the AL index. We note that there are no substorms associated with extended sources outside the heliolatitude range ~5–20°. The established relationship between solar source coordinates and geomagnetic activity of the magnetic cloud sheath and body are consistent with the most probable distribution of magnetoactive regions over the solar disk.

Исследование связи параметров локализации солнечных источников магнитных облаков с их характеристиками и суббуревой активностью

We propose a method for determining location and orientation of extended solar sources of magnetic clouds, using coronagraph data and SOHO EIT/MDI images of the photosphere. To estimate the probability of formation of magnetic clouds, we use a simple cylindrical force-free model. We have established that more extended sources and those having a slight inclination to the solar equator and located on the solar limb as compared to those that are nonextended and strongly inclined can generate expanding clouds, which with high probability can reach the magnetosphere like clouds from a source near the zero meridian and low latitudes.
 We determine the relationship between extreme values of substorm activity and parameters of solar sources under study during the impact of magnetic clouds on Earth’s magnetosphere from the AL index. We note that there are no substorms associated with extended sources outside the heliolatitude range ~5–20°. The established relationship between solar source coordinates and geomagnetic activity of the magnetic cloud sheath and body are consistent with the most probable distribution of magnetoactive regions over the solar disk.

Spatially Resolved Signatures of Bidirectional Flows Observed in Inverted-Y Shaped Jets

Abstract Numerous apparent signatures of magnetic reconnection have been reported in the solar photosphere, including inverted-Y shaped jets. The reconnection at these sites is expected to cause localized bidirectional flows and extended shock waves; however, these signatures are rarely observed as extremely high spatial-resolution data are required. Here, we use Hα imaging data sampled by the Swedish Solar Telescope’s CRisp Imaging SpectroPolarimeter to investigate whether bidirectional flows can be detected within inverted-Y shaped jets near the solar limb. These jets are apparent in the Hα line wings, while no signature of either jet is observed in the Hα line core, implying reconnection took place below the chromospheric canopy. Asymmetries in the Hα line profiles along the legs of the jets indicate the presence of bidirectional flows, consistent with cartoon models of reconnection in chromospheric anemone jets. These asymmetries are present for over two minutes, longer than the lifetimes of Rapid Blue Excursions, and beyond ±1 Å into the wings of the line indicating that flows within the inverted-Y shaped jets are responsible for the imbalance in the profiles, rather than motions in the foreground. Additionally, surges form following the occurrence of the inverted-Y shaped jets. This surge formation is consistent with models, which suggests such events could be caused by the propagation of shock waves from reconnection sites in the photosphere to the upper atmosphere. Overall, our results provide evidence that magnetic reconnection in the photosphere can cause bidirectional flows within inverted-Y shaped jets and could be the driver of surges.

Forward Modeling of a Pseudostreamer

Abstract In this paper, we present an analysis of a pseudostreamer embedding a filament cavity, observed on 2015 April 18 on the solar southwest limb. We use the flux-rope insertion method to construct nonlinear force-free field (NLFFF) models constrained by observed Solar Dynamics Observatory (SDO)/AIA coronal structures and the SDO/Helioseismic Magnetic Imager photospheric magnetogram. The resulting magnetic field models are forward-modeled to produce synthetic data directly comparable to Mauna Loa Solar Observatory/Coronal Multichannel Polarimeter (CoMP) observations of the intensity and linear polarization of the Fe xiii 1074.7 nm infrared coronal emission line using FORWARD. In addition, we determine the location of quasi-separatrix layers in the magnetic models, producing a Q-map from which the signatures of magnetic null points and separatrices can be identified. An apparent magnetic null observed in linear polarization by CoMP is reproduced by the model and appears in the region of the 2D-projected magnetic null in the Q-map. Further, we find that the height of the CoMP null is better reproduced by our NLFFF model than by the synthetic data we produce with potential-field source-surface models, implying the presence of a flux rope in the northern lobe of the pseudostreamer.

Exceptional Extended Field-of-view Observations by PROBA2/SWAP on 2017 April 1 and 3

Abstract On 2017 April 1 and 3, two large eruptions on the western solar limb, which were associated with M4.4- and M5.8-class flares, respectively, were observed with the Sun Watcher with Active Pixels and Image Processing (SWAP) Extreme Ultraviolet (EUV) solar telescope on board the Project for On Board Autonomy 2 (PROBA2) spacecraft. The large field-of-view (FOV) of SWAP, combined with an advantageous off-point, allows us to study the eruptions up to approximately 2 solar radii (Rs), where space-based coronagraph observations begin. These measurements provide us with some of the highest EUV observations of an eruption, giving crucial additional data points to track the early evolution of Coronal Mass Ejections. In SWAP observations, we track the evolution of off-limb erupting features as well as associated on-disk EUV waves, and the kinematics of both are calculated. The first eruption shows a clear deceleration throughout the lower corona into coronagraph observations, whereas the second eruption, which had a lower initial velocity, shows no obvious acceleration or deceleration profile. This paper presents a unique set of observations, allowing features observed in EUV to be traced to greater heights in the solar atmosphere, helping to bridge the gap to the FOV of white-light coronagraphs. Even with these favorable data sets, it remains a challenging task to associate features observed in EUV with those observed in white light, highlighting our urgent need for single-instrument observations of the combined lower and middle corona.

Fast and precise light-curve model for transiting exoplanets with rings

ABSTRACT The presence of silicate material in known rings in the Solar system raises the possibility of ring systems existing even within the snow line – where most transiting exoplanets are found. Previous studies have shown that the detection of exoplanetary rings in transit light curves is possible, albeit challenging. To aid such future detection of exoplanetary rings, we present the Polygon + Segments model for modelling the light curve of an exoplanet with rings. This high-precision model includes full ring geometry as well as possible ring transparency and the host star’s limb darkening. It is also computationally efficient, requiring just a 1D integration over a small range, making it faster than existing techniques. The algorithm at its core is further generalized to compute the light curve of any set of convex primitive shapes in transit (e.g. multiple planets, oblate planets, moons, rings, combination thereof, etc.) while accounting for their overlaps. The python source code is made available.

A Transit of Venus Possibly Misinterpreted as an Unaided-Eye Sunspot Observation in China on 9 December 1874

Large sunspots can be observed with the unaided eye under suitable atmospheric seeing conditions. Such observations are of particular value because the frequency of their appearance provides an approximate indication of the prevailing level of solar activity. Unaided-eye sunspot (UES) observations can be traced back well before the start of telescopic observations of the Sun, especially in the East Asian historical records. It is therefore important to compare more modern, UES observations with the results of telescopic sunspot observations, to gain a better understanding of the nature of the UES records. A previous comparison of Chinese UES records and Greenwich photo-heliographic results between 1874 and 1918 indicated that a few of the UES were apparently not supported by direct photographic evidence of at least one sunspot with a large area. This article reveals that one of the Chinese unaided-eye observations had possibly captured the transit of Venus on 9 December 1874. The Chinese sunspot records on this date are compared with Western sunspot observations on the same day. It is concluded that sunspots on the solar disk were quite small and the transit of Venus was probably misinterpreted as a sunspot by the Chinese local scholars. This case indicates that sunspots or comparable "obscuring" objects with an area as large as 1000 millionths of the solar disk could easily have been seen with the unaided eye under suitable seeing conditions. It also confirms the visibility of sunspots near the solar limb with the unaided eye. This study provides an explanation of the apparent discrepancy between the Chinese UES observation on 9 December 1874 and the Western sunspot observations using telescopes, as well as a basis for further discussion on the negative pairs in 1900 and 1911, apparently without sufficiently large area.

Interferometric Fringe Visibility Null as a Function of Spatial Frequency: A Probe of Stellar Atmospheres

We introduce an observational tool based on visibility nulls in optical spectro-interferometry fringe data to probe the structure of stellar atmospheres. In a preliminary demonstration, we use both Navy Precision Optical Interferometer (NPOI) data and stellar atmosphere models to show that this tool can be used, for example, to investigate limb darkening. Using bootstrapping with either multiple linked baselines or multiple wavelengths in optical and infrared spectro-interferometric observations of stars makes it possible to measure the spatial frequency [Formula: see text] at which the real part of the fringe visibility [Formula: see text] vanishes. That spatial frequency is determined by [Formula: see text], where [Formula: see text] is the projected baseline length, and [Formula: see text] is the wavelength at which the null is observed. Since [Formula: see text] changes with the Earth’s rotation, [Formula: see text] also changes. If [Formula: see text] is constant with wavelength, [Formula: see text] varies in direct proportion to [Formula: see text]. Any departure from that proportionality indicates that the brightness distribution across the stellar disk varies with wavelength via variations in limb darkening, in the angular size of the disk, or both. In this paper, we introduce the use of variations of [Formula: see text] with [Formula: see text] as a means of probing the structure of stellar atmospheres. Using the equivalent uniform disk diameter [Formula: see text], given by [Formula: see text], as a convenient and intuitive parameterization of [Formula: see text], we demonstrate this concept by using model atmospheres to calculate the brightness distribution for [Formula: see text] Ophiuchi and to predict [Formula: see text], and then comparing the predictions to coherently averaged data from observations taken with the NPOI.

Fast Computational Convolution Methods for Extended Source Effects in Microlensing Light Curves

Abstract Extended source effects can be seen in gravitational lensing events when sources cross critical lines. Such events probe the stellar intensity profile and could be used to measure limb-darkening coefficients to test stellar model predictions. A database of accurately measured stellar profiles is needed to correctly subtract the stellar flux in planetary transient events. The amount of data that is being produced and that will be produced in current and future microlensing surveys, from both space and ground, requires algorithms that can quickly compute light curves for different source-lens configurations. Based on the convolution method, we describe a general formalism to compute those curves for single lenses. We develop approximations in terms of quadratures of elliptic integrals that we integrate by solving the associated first-order differential equations. We construct analytic solutions for a limb darkening and, for the first time, for a parabolic profile that are accurate at the ∼1%–3% and 0.5% level, respectively. These solutions can be computed orders of magnitude faster than other integration routines. They can be implemented in pipelines processing large data sets to extract stellar parameters in real time.

Reconstructing solar magnetic fields from historical observations

Context. Systematic observations of magnetic field strength and polarity in sunspots began at Mount Wilson Observatory (MWO), USA in early 1917. Except for a few brief interruptions, this historical dataset has continued until the present. Aims. Sunspot field strength and polarity observations are critical in our project of reconstructing the solar magnetic field over the last hundred years. We provide a detailed description of the newly digitized dataset of drawings of sunspot magnetic field observations. Methods. The digitization of MWO drawings is based on a software package that we developed. It includes a semiautomatic selection of solar limbs and other features of the drawing, and a manual entry of the time of observations, measured field strength, and other notes handwritten on each drawing. The data are preserved in an MySQL database. Results. We provide a brief history of the project and describe the results from digitizing this historical dataset. We also provide a summary of the final dataset and describe its known limitations. Finally, we compare the sunspot magnetic field measurements with those from other instruments, and demonstrate that, if needed, the dataset could be continued using modern observations such as, for example, the Vector Stokes Magnetograph on the Synoptic Optical Long-term Investigations of the Sun platform.

Structure of the Transition Region and the Low Corona from TRACE and SDO Observations Near the Limb

We examined the structure near the solar limb in TRACE images of the continuum and in the 1600 and 171 A bands as well as in SDO images in the continuum (from HMI) and all AIA bands. The images in different wavelength bands were carefully coaligned by using the position of Mercury for TRACE and Venus for SDO during their transit in front of the solar disk in 1999 and 2012 respectively. Chromospheric absorbing structures in the TRACE 171 A band are best visible 7" above the white light limb, very close to the inner limb, defined as the inflection point of the rising part of the center-to-limb intensity variation. They are correlated with, but are not identical to spicules in emission, seen in the 1600 A band. Similar results were obtained from AIA and SOT images. Tall spicules in 304 A are not associated with any absorption in the higher temperature bands. Performing azimuthal averaging of the intensity over 15 degree sectors near the N, S, E and W limbs, we measured the height of the limb and of the peak intensity in all AIA bands. We found that the inner limb height in the transition region AIA bands increases with wavelength, consistent with a bound-free origin of the absorption from neutral H and He. From that we computed the column density and the density of neutral hydrogen as a function of height. We estimated a height of (2300 $\pm$ 500)km for the base of the transition region. Finally, we measured the scale height of the AIA emission of the corona and associated it with the temperature; we deduced a value of (1.24 $\pm$ 0.25) 10$^6$ K for the polar corona.

Modeling near-surface temperatures of airless bodies with application to the Moon

In this study we present a model to determine surface and sub-surface temperatures of airless bodies in the solar system. To precisely model direct sunlight we incorporated the solar limb darkening effect of the solar disk. Scattered sunlight and thermal re-radiation from nearby planets is also considered in our model. We further consider multiple scattering of reflected sunlight and thermal re-radiation on the modeled object itself. The finite volume method is applied to solve the model for which we present full derivations for the governing equations that control scattering and heat diffusion into the sub-surface. We assessed errors stemming from the chosen discretization of the depth profile, the window size from which scattering is considered, as well as from the chosen integration step-size and the spatial resolution of the Digital Terrain Model (DTM). Exemplarily, we determine surface and sub-surface (2 m depth) temperatures for the lunar polar areas. Topography of the lunar poles is modeled by measurements of the Lunar Orbiter Laser Altimeter (LOLA). We integrated temperatures over a 18.6-year time frame using 180 m pixel−1LOLA DTMs of the poles, a 60 × 60 km window, and a 12 h integration time-step. The resulting preliminary temperature maps for the lunar poles are presented. Further, we show that our model agrees with temperatures obtained by the Diviner lunar radiometer experiment.

Retrieval of the Fluid Love Number k2 in Exoplanetary Transit Curves

Abstract We are witness to a great and increasing interest in internal structure, composition, and evolution of exoplanets. However, direct measurements of exoplanetary mass and radius cannot be uniquely interpreted in terms of interior structure, justifying the need for an additional observable. The second degree fluid Love number, k 2, is proportional to the concentration of mass toward the body’s center, hence providing valuable additional information about the internal structure. When hydrostatic equilibrium is assumed for the planetary interior, k 2 is a direct function of the planetary shape. Previous attempts were made to link the observed tidally and rotationally induced planetary oblateness in photometric light curves to k 2 using ellipsoidal shape models. Here, we construct an analytical 3D shape model function of the true planetary mean radius that properly accounts for tidal and rotational deformations. Measuring the true planetary mean radius is critical when one wishes to compare the measured k 2 to interior theoretical expectations. We illustrate the feasibility of our method and show, by applying a Differential Evolution Markov Chain to synthetic data of WASP-121b, that a precision ≤65 ppm/ is required to reliably retrieve k 2 with present understanding of stellar limb darkening (LD), therefore improving recent results based on ellipsoidal shape models. Any improvement on stellar LD would increase such performance.

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope part 5: impacts of enhanced mirror and dichroic coatings on system polarization calibration

The Daniel K. Inouye Solar Telescope (DKIST) is designed to deliver accurate spectropolarimetric calibrations across a wide wavelength range and large field of view for solar disk, limb, and coronal observations. DKIST instruments deliver spectral resolving powers of up to 300,000 in multiple cameras of multiple instruments sampling nanometer scale bandpasses. We require detailed knowledge of optical coatings on all optics to ensure that we can predict and calibrate the polarization behavior of the system. Optical coatings can be metals protected by many dielectric layers or several-micron-thick dichroics. Strong spectral gradients up to 60 deg retardance per nanometer wavelength and several percent diattenuation per nanometer wavelength are observed in such coatings. Often, optical coatings are not specified with spectral gradient targets for polarimetry in combination with both average- and spectral threshold-type specifications. DKIST has a suite of interchangeable dichroic beam splitters using up to 96 layers. We apply the Berreman formalism in open-source Python scripts to derive coating polarization behavior. We present high spectral resolution examples on dichroics where transmission can drop 10% with associated polarization changes over a 1-nm spectral bandpass in both mirrors and dichroics. We worked with a vendor to design dichroic coatings with relatively benign polarization properties that pass spectral gradient requirements and polarization requirements in addition to reflectivity. We now have the ability to fit multilayer coating designs which allow us to predict system-level polarization properties of mirrors, antireflection coatings, and dichroics at arbitrary incidence angles, high spectral resolving power, and on curved surfaces through optical modeling software packages. Performance predictions for polarization at large astronomical telescopes require significant metrology efforts on individual optical components combined with system-level modeling efforts. We show our custom-built laboratory spectropolarimeter and metrology efforts on protected metal mirrors, antireflection coatings, and dichroic mirror samples.

Characteristics of Mars UV Dayglow Emissions From Atomic Oxygen at 130.4 and 135.6 nm: MAVEN/IUVS Limb Observations and Modeling

AbstractWe present an overview of two Martian Years (MYs) oxygen dayglow limb observations of the ultraviolet (UV) emissions at 130.4 and 135.6 nm. The data have been collected with the Imaging Ultraviolet Spectrograph (IUVS) instrument on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We use solar flux measurements of EUVM on board MAVEN to remove the solar‐induced variation and show the variations of the maximum limb brightness and altitude with season, solar zenith angle, and latitude, which reflects the strong variability of the Martian atmosphere. The 130.4‐ and 135.6‐nm peak brightness and altitudes are strongly correlated and behave similarly. Both emissions are modeled for selected data using Monte Carlo codes to calculate emissions arising from electron impact on O and CO2. Additional radiative transfer calculations are made to analyze the optically thick 130.4‐nm emission. Model atmospheres from the Mars Climate Database serve as input. Both simulated limb profiles are in good agreement with the observations despite some deviations. We furthermore show that the observed 130.4‐nm brightness is dominated by resonance scattering of the solar multiplet with a contribution (15–20%) by electron impact on O. Over 95% of the excitation at 135.6 nm arises from electron impact on O. Simulations indicate that the limb brightness is dependent on the oxygen and CO2 content, while the peak emission altitude is mainly driven by the CO2 content because of absorption processes. We deduce [O]/[CO2] mixing ratios of 3.1% and 3.0% at 130 km for data sets collected at LS = 350° in Martian years 32 and 33.

Sun-as-a-star observations of the 2017 August 21 solar eclipse

AbstractThe Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) is a state-of-the-art, thermally stabilized, fiber-fed, high-resolution spectrograph for the Large Binocular Telescope (LBT) at Mt. Graham, Arizona. During daytime the instrument is fed with sunlight from the 10-millimeter aperture, fully automated, binocular Solar Disk-Integrated (SDI) telescope. The observed Sun-as-a-star spectra contain a multitude of photospheric and chromospheric spectral lines in the wavelength ranges 4200–4800 Å and 5300–6300 Å. One of the advantages of PEPSI is that solar spectra are recorded in the exactly same manner as nighttime targets. Thus, solar and stellar spectra can be directly compared. PEPSI/SDI recorded 116 Sun-as-a-star spectra during the 2017 August 21 solar eclipse. The observed maximum obscuration was 61.6%. The spectra were taken with a spectral resolution of ≈ 250000 and an exposure time of 0.3 s. The high-spectral resolution facilitates detecting subtle changes in the spectra while the Moon passes the solar disk. Sun-as-a-star spectra are affected by changing contributions due to limb darkening and solar differential rotation, and to a lesser extend by supergranular velocity pattern and the presence of active regions on the solar surface. The goal of this study is to investigate the temporal evolution of the chromospheric Na D doublet during the eclipse and to compare observations with synthetic line profiles computed with the state-of-the-art Bifrost code.

The Sun-Spectroscopic Observations of the Coronal Emission Lines and ADITYA-1 Mission

The sun and the solar atmosphere, known as solar corona have been studied in detail for centuries but still lot need to be understood about the sun. In the late 19th and earlier 20th century, the information gathered about the solar corona was from the brief moments available during the total solar eclipses. In the later part space observations provided a large body of information about the solar corona. We obtained systematic high resolution spectroscopic observations in [Fe x] 637.4 [Fe xi] 789.2, [Fe xiii] 1074.7, and [Fe xiv] 530.3 nm emission lines during the period of 1997-2007 with the 25-cm coronagraph at Norikura, Japan. The variation in line widths with height above the solar limb implies that one pair of lines indicate that top of coronal loops are hotter where as other pair of lines shows that loop top is cooler. To explain these results, we proposed an empirical model as all these results are difficult to explain using the existing models. With this background we planned and designed a visible emission line coronagraph (VELC). This instrument is in advance stage of fabrication and is expected to be launched in 2019-20. The challenges and required technical development for this mission are discussed.

The Sun-Spectroscopic Observations of the Coronal Emission Lines and ADITYA-1 Mission

The sun and the solar atmosphere, known as solar corona have been studied in detail for centuries but still lot need to be understood about the sun. In the late 19th and earlier 20th century, the information gathered about the solar corona was from the brief moments available during the total solar eclipses. In the later part space observations provided a large body of information about the solar corona. We obtained systematic high resolution spectroscopic observations in [Fe x] 637.4 [Fe xi] 789.2, [Fe xiii] 1074.7, and [Fe xiv] 530.3 nm emission lines during the period of 1997-2007 with the 25-cm coronagraph at Norikura, Japan. The variation in line widths with height above the solar limb implies that one pair of lines indicate that top of coronal loops are hotter where as other pair of lines shows that loop top is cooler. To explain these results, we proposed an empirical model as all these results are difficult to explain using the existing models. With this background we planned and designed a visible emission line coronagraph (VELC). This instrument is in advance stage of fabrication and is expected to be launched in 2019-20. The challenges and required technical development for this mission are discussed.