Full-disk Telescope Research Articles

Comparison of magnetic data products from Solar Orbiter SO/PHI-FDT and SDO/HMI

Context. The Polarimetric and Helioseismic Imager (SO/PHI), on board the Solar Orbiter mission, is the first photospheric magnetograph and tachograph to observe the Sun from outside the Sun-Earth line. The Full Disc Telescope (FDT) of SO/PHI, images the whole solar disk with a spatial resolution that varies with the distance between the Sun and the spacecraft. Aims. We check for consistency between the magnetic field strength (B), the field inclination (γ), the line-of-sight (LoS) magnetic component (BLoS) and the field azimuth (ϕ), inferred by SO/PHI-FDT and the Helioseismic and Magnetic Imager (HMI), on board Solar Dynamics Observatory (SDO), and obtain linear correlation coefficients among them. Methods. We use data from both instruments obtained on 8 March 2022, when the angle between SDO and Solar Orbiter was 3.4° and the solar disk showed four developed active regions. Before comparing the magnetic field products of both instruments we perform a precise alignment of the data, including a matching of the plate scale. Further, in order to improve the homogeneity of the compared data products, the SDO/HMI data were convolved with the SO/PHI-FDT point spread function (PSF). The linear correlation coefficients are obtained through a linear regression of SDO/HMI to SO/PHI-FDT. Results. The two instruments yield comparable magnetic field data products. The slope coefficients for a linear fit are 1.37 for B, 1.11 for γ, 1.35 for BLoS and 1 for the azimuth. The corresponding fit offsets are −94 G, −9.8°, 5.2 G and 0.1°, respectively. The agreement between both instruments is significantly better when we take into account the different spatial resolution of both instruments. The fitting results vary slightly depending on the analyzed active region except for one of the four active regions, which shows larger differences and has been excluded from the comparison. The comparison of the LoS magnetic field products from SDO/HMI at 45 s and 720 s with SO/PHI-FDT shows a slope value of 1.17, with the offset less than 6 G, in both cases.

The Polarimetric and Helioseismic Imager on Solar Orbiter

Aims.This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, while hosting the potential of a rich return in further science.Methods.SO/PHI measures the Zeeman effect and the Doppler shift in the Fe I617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders. The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope, covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope, can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.Results.SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission’s highly elliptical orbit.

Full-disk synoptic observations of the chromosphere using Hα telescope at the Kodaikanal Observatory

This paper reports on the installation and observations of a new solar telescope installed on 2014 October 7 at the Kodaikanal Observatory. The telescope is a refractive type equipped with a tunable Lyot Hα filter. A CCD camera with size 2k×2k acquires images of the Sun and has a pixel size of 1.21″ pixel−1 and a full field-of-view of 41′. The telescope is equipped with a guiding system which keeps the image of the Sun within a few pixels throughout the observations. The FWHM of the Lyot filter is 0.4Å and the filter is motorized, capable of scanning the Hα line profile at a smaller step size of 0.01Å. Partial-disk imaging covering about 10′ is also possible with the help of a relay lens kept in front of the CCD camera. In this paper, we report the detailed specifications of the telescope, filter unit, the installation, observations and the procedures we have followed to calibrate and align the data. We also present preliminary results with this new full-disk telescope.

Analysis and evaluation of the Full Disk Telescope refocusing mechanism for the Solar Orbiter mission

The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. It will provide full-disk measurements of the photospheric magnetic field vector and line-of-sight velocity, as well as the continuum intensity in the visible wavelength range. Along this mission, it is expected that thermal drifts will induce image focus displacements. Consequently, providing an autofocus system is mandatory to prevent image degradation. The refocusing system is based on an autonomous image contrast analysis and it allows for a lens displacement in order to locate the best focus position. The figure of merit chosen for the image quality evaluation is presented. The influences of attitude instability and mechanical uncertainties are considered in a refocusing process simulation. In addition, an engineering model of the mechanism is tested at flight operating conditions. To check its performance, an optical interrogation system is set up. Determination of accuracy and repeatability of the mechanism positioning is experimentally evaluated and discussed according to the ISO standard. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities.

The Polarimetric and Helioseismic Imager for Solar Orbiter: SO/PHI

AbstractThe Solar Orbiter is the next solar physics mission of the European Space Agency, ESA, in collaboration with NASA, with a launch planned in 2018. The spacecraft is designed to approach the Sun to within 0.28 AU at perihelion of a highly eccentric orbit. The proximity with the Sun will also allow its observation at uniformly high resolution at EUV and visible wavelengths. Such observations are central for learning more about the magnetic coupling of the solar atmosphere. At a later phase in the mission the spacecraft will leave the ecliptic and study the enigmatic poles of the Sun from a heliographic latitude of up to 33○.A central instrument of Solar Orbiter} is the Polarimetric and Helioseismic Imager, SO/PHI. It will do full Stokes imaging in the Landé g = 2.5 Fe I 617.3 nm line. It is composed of two telescopes, a full-disk telescope and a high-resolution telescope, that will allow observations at a resolution as high as 200 km on the solar surface. SO/PHI will also be the first solar polarimeter to leave the Sun-Earth line, opening up new possibilities, such as stereoscopic polarimetry (besides stereoscopic imaging of the photosphere and stereoscopic helioseismology). Finally, SO/PHI will have a unique view of the solar poles, allowing not just more precise and exact measurements of the polar field than possible so far, but also enabling us to follow the dynamics of individual magnetic features at high latitudes and to determine solar surface and sub-surface flows right up to the poles.In this paper an introduction to the science goals and the capabilities of SO/PHI will be given, as well as a brief overview of the instrument and of the current status of its development.

On the co-alignment of solar telescopes. A new approach to solar pointing

Helioseismological measurements require long observing times and thus may be adversely affected by lateral image drifts as caused by pointing instabilities. At the Vacuum Tower Telescope VTT, Tenerife we have recorded drift values of up to 5" per hour under unstable thermal conditions (dome opening, strong day-to-day thermal gradients). Typically drifts of 0.5" – 1.0" per hour may be encountered under more favorable conditions.Past experience has shown that most high-resolution solar telescopes may be affected by this problem to some degree. This inherent shortcoming of solar pointing is caused by the fact that the guiding loop can be closed only within the guiding beam but not within the telescope's main beam.We have developed a new approach to this problem. We correlate continuum brightness patterns observed from within the telescope main beam with patterns originating from a full disk telescope. We show that brightness patterns of sufficient size are unique with respect to solar location at any instant of time and may serve as a location identifier. We make use of the fact that averaged location information of solar structures is invariant with respect to telescope resolution. We have carried out tests at the VTT together with SDO. We have used SDO as a full disk reference. We were able to reduce lateral image drifts by an order of magnitude.

The Big Bear Solar Observatory Ca II K‐line index for solar cycle 23

AbstractWe present an analysis of 2634 Ca II K‐line full‐disk filtergrams obtained with the 15‐cm aperture photometric full‐disk telescope at Big Bear Solar Observatory during the period from 1996 January 1 to 2005 October 24. Using limb darkening corrected and contrast enhanced filtergrams, solar activity indices were derived, which are sensitive to the 11‐year solar activity cycle and 27‐day rotational period of plages around active regions and the bright chromospheric network. The present work extends an earlier study (solar cycle 22), which was based on video data. The current digital data are of much improved quality with higher spatial resolution and a narrower passband ameliorating photometric accuracy. The time series of chromospheric activity indices cover most of solar cycle 23. One of the most conspicuous features of the Ca II K indices is the secondary maximum in late 2001/early 2002 after an initial decline of chromospheric activity during the first half of 2001. We conclude that a secular trend exists in the Ca II K indices, which has its origin in the bright chromospheric network and brightenings related to decaying active regions. Superposed on this secular trend are the signatures of recurring, long‐lived active regions, which are clusters of persistent and continuously emerging magnetic flux. Such features are less visible, when the activity belts on both side of the equator are devoid of the brightenings related to decaying active regions as was the case in October/November 2003 at a time when a superactivity complex including several naked‐eye sunspots emerged (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The ADAHELI solar mission: Investigating the structure of Sun’s lower atmosphere

ADAHELI (ADvanced Astronomy for HELIophysics) is a small-class (500 kg) low-budget (50 MEuro) satellite mission for the study of the solar photosphere and the chromosphere and for monitoring solar flare emission. ADAHELI’s design has completed its Phase-A feasibility study in December 2008, in the framework of ASI’s (Agenzia Spaziale Italiana) 2007 “Small Missions” Program (calling for two missions at 50 MEeuros each, plus the launch budget). ADAHELI’s main purpose is to explore Sun’s lower atmosphere in the near-infrared, a region so far unexplored by solar observations from space. ADAHELI will carry out observations of the solar photosphere and of the chromosphere at high-temporal rate and high spatial and spectral resolutions. ADAHELI will contribute to the understanding of Space Weather through the study of particle acceleration during flares. A radiometer operating in the millimeter radio band will continuously monitor the solar disk, throughout the spacecraft’s life time. ADAHELI’s baseline instruments are a 50-cm high-resolution telescope operating in the visible and the near-infrared, and a lightweight full-disk radiometer operating at millimeter wavelengths (90 GHz). The core of the telescope’s focal plane suite is the spectral imager based on two Fabry-Perot interferometers, flying for the first time on a solar mission. The instrument will return fast-cadence, full bi-dimensional spectral images at high-resolution, thus improving on current slit-scan, mono-dimensional architectures. Moreover, the possibility of working in polarized light will enable full 3D magnetic field reconstruction on the photosphere and the chromosphere. An optional instrumental package is also being proposed to further extend ADAHELI’s scope: a full-disk telescope for helioseismology based on a double Magneto-Optical Filter, a Neutral Particle Analyzer for magnetospheric research, an Extreme Ultraviolet imaging and spectro-radiometry instrument. These options fall outside the prescribed budget. ADAHELI, flying a Sun-Synchronous orbit at 800 km, will perform continuous, long-duration (4-h), daily acquisitions, with the possibility of extending them up to 24 h. ADAHELI’s operating life is two years, plus one extension year. Launch would be nominally planned for 2014.

On the Variability of the Apparent Solar Radius

Full-disk photometric solar images at a wavelength of 672.3 nm have been obtained daily since 1986 using the CFDT1 (Cartesian Full Disk Telescope No. 1). An analysis of these images from 1986 through the end of 2004 December has shown a peak-to-peak variation in the geocentric north-south solar radius of 0.136 ± 0.01, approximately in phase with the solar cycle. The multiple correlation coefficient squared is R2 = 0.0404 (R = 0.2). While this correlation coefficient is small, due to the large number of data points (N = 4042), the level of significance is less than 0.02. The radius had a maximum value near the times of maximum activity for solar cycles 22 and 23.

A Comparison of Feature Classification Methods for Modeling Solar Irradiance Variation

Physical understanding of total and spectral solar irradiance variation depends upon establishing a connection between the temporal variability of spatially resolved solar structures and spacecraft observations of irradiance. One difficulty in comparing models derived from different data sets is that the many ways for identifying solar features such as faculae, sunspots, quiet Sun, and various types of “network” are not necessarily consistent. To learn more about classification differences and how they affect irradiance models, feature “masks” are compared as derived from five current methods: multidimensional histogram analysis of NASA/National Solar Observatory/Kitt Peak spectromagnetograph data, statistical pattern recognition applied to SOHO/Michelson Doppler Imager photograms and magnetograms, threshold masks allowing for influence of spatial surroundings applied to NSO magnetograms, and “one-trigger” and “three-trigger” algorithms applied to California State University at Northridge Cartesian Full Disk Telescope intensity observations. In general all of the methods point to the same areas of the Sun for labeling sunspots and active-region faculae, and available time series of area measurements from the methods correlate well with each other and with solar irradiance. However, some methods include larger label sets, and there are important differences in detail, with measurements of sunspot area differing by as much as a factor of two. The methods differ substantially regarding inclusion of fine spatial scale in the feature definitions. The implications of these differences for modeling solar irradiance variation are discussed.

On the decay rate of sunspots

We have analyzed the decay of 32 sunspots observed during the years 1988 through 2001 at the San Fernando Observatory (SFO). The data are from digital images obtained in the red (672 nm) with the Cartesian Full Disk Telescope No.1 (CFDT1). We find that the rate of decay is strongly correlated with the total sunspot area and the umbral to total area ratio. The multiple correlation coefficient is 0.93. Thus, the unexplained variance from this simple model is (1–0.87). We find that for the sunspots of this study, the decay rate is not a constant and that there is no significant correlation between the decay rate and the square root of the total spot area.

A flare-triggered heating of a quiescent filament

Using data obtained with the 20-cm Hα full-disk telescope at Big Bear Solar Observatory and Fe xii 195 A EIT on SOHO, we analyze a sudden disappearance event of a quiescent filament in detail. The filament was located along the common boundary of the active regions NOAA 9672 (S19 E13) and NOAA 9673 (N03 E18). The filament disappeared during a time interval between 17:59 UT and 19:47 UT on 22 October 2001 immediately after the onset of a major flare, which occurred in the active region NOAA 9672. At about 23:23 UT of the same day, the filament began to reappear in Hα and, after about 15 hours, the filament recovered to its steady state with its size being slightly smaller than that before its disappearance. This filament disappearance event belongs to the thermal type of sudden filament disappearances, which is caused by an input of additional heat. The heating mechanism that leads to sudden thermal disappearances of quiescent filaments is still not well understood. This simple event, due to the explicit cause and effect relationship between the flare and the disappearance of the filament, shows us that the flare triggered some kind of heating mechanism which continued several hours. The heat may come from the flare via heat conduction from its ribbon or from the excitation of dissipating Alfven waves. However, from the data analysis, we conclude that the flare triggered an in-situ heating, which is likely caused by magnetic reconnection.

An Improved Determination of the Area Ratio of Faculae to Sunspots

We report new results on the ratio of facular area to sunspot area from a program of continuing photometric observations using the Cartesian Full Disk Telescope No. 1 (CFDT1) at the San Fernando Observatory (SFO). The facular areas are determined from images obtained with a 1 nm bandpass Ca II K line filter, and sunspot areas are determined from red images at 672 nm with a 10 nm bandpass filter. On the K line images faculae were identified by pixels that had a contrast equal to or greater than 4.8% divided by μ. Previously, we found that the average facular-to-spot area ratio was 16.7 ± 0.5 during the latter part of solar cycle 22 and that there was a small but statistically significant rise in the ratio with time. If we take an average from the beginning of the K line data (mid-1988) until the middle of 1996, excluding days of zero sunspot area, the average ratio is 16.4 ± 0.4. The average ratio from mid-1996 to the end of 1999 November is 12.6 ± 0.5. Including days of zero sunspot area for these same intervals we find average ratios of 16.8 ± 0.5 and 13.2 ± 0.6, respectively. We have recently reprocessed our K line images, which have been photometrically "cleaned." We can now reliably identify facular pixels with a contrast criterion of 2.4%, resulting in an increase in the average facular-to-spot ratio of approximately 3. The average facular and sunspot areas for cycle 23 are significantly lower than for cycle 22.

An analysis of full-disk observations of facular contrast in the blue and red

Full-disk images from the Cartesian Full-Disk Telescope no. 2 (CFDT2) were used to study the center-to-limb (CLV) variation of facular contrast in two colors. The CFDT2 images, which have 2.5 arc sec pixels, were obtained during the summer months of 1993, 1994 and 1995. In order to minimize the bias in finding faint facular features in continuum images, we have used coaligned images obtained in the Ca K-line to identify faculae. Faculae were sorted into 20 annular bins of equal width. To reduce the effects of seeing, faculae were not identified closer to the limb than μ=0.2. The facular pixel contrasts were fitted to various trial functions. The contrast in the blue filter (470.6 nm) rose from 0.122% at disk center to 12.2% at μ=0.2. The contrast in the red filter (672.3 nm) rose from 0.13% at disk center to 8.16% at μ=0.2. We have also analyzed the facular contrasts multiplied by their μ-value to obtain an estimate of facular flux tube contrasts. These flux tube contrasts increased roughly linearly from μ=0.95 to 0.25. The blue flux tube contrast reached a maximum of 2.48% near μ=0.25. The red flux tube contrast reached a maximum of 1.59% at μ=0.2. These contrast values are not corrected for the filling factor. The blue curve leveled off slightly betwen μ=0.25 and 0.2 while the red curve showed no deviation from its linear trend. These results may provide some support for the hot wall model of facular 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 Hα 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 Hα 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.

Photometric measurements of solar irradiance variations due to sunspots

A photometric telescope constructed to obtain photometric sunspot areas and deficits on a daily basis is described. Data from this Cartesian full disk telescope (CFDT) are analyzed with attention given to the period between June 4 and June 17, 1985 because of the availability of overlapping sunspot area and irradiance deficit data from high-resolution digital spectroheliograms made with the San Fernando Observatory 28 cm vacuum solar telescope and spectroheliograph. The CFDT sunspot deficits suggest a substantial irradiance contribution from faculae and active region plage.