Precision Solar Photometric Telescope Research Articles

Rome Precision Solar Photometric Telescope: precision solar full-disk photometry during solar cycles 23–25

The Rome Precision Solar Photometric Telescope (Rome/PSPT) is a ground-based telescope engaged in precision solar photometry. It has a 27-year database of full-disk images of the photosphere and chromosphere beginning in 1996 and continuing to 2022. The solar images have been obtained daily, weather permitting, with approximately 2 arcsec/pixel scale in Ca II K line at 393.3 nm, G-band at 430.6 nm, and continuum in the blue and red parts of the spectrum at 409.4 nm and 607.2 nm, respectively. Regular observations were also performed at the green continuum at 535.7 nm for a period of about 18 months. Since the first-light, Rome/PSPT operations have been directed at understanding the source of short-and long-term solar irradiance changes, spanning from 1 min to several months, and from 1 year to a few solar cycles, respectively. However, Rome/PSPT data have also served to study a variety of other topics, including the photometric properties of solar disk features and of the supergranulation manifested by the chromospheric network. Moreover, they have been unique in allowing to connect series of historical and modern full-disk solar observations, especially the Ca II K line data. Here, we provide an overview of the Rome/PSPT telescope and of the solar monitoring carried out with it from its first light to the present, across solar cycles 23–25. We also briefly describe the main results achieved with Rome/PSPT data, and give an overview of new results being derived with the whole time series of observations covering the period 1996–2022.

On the Application of the Equal-contrast Technique to Ca-K Data from Kodaikanal and Other Observatories

Abstract The “equal-contrast technique” (ECT) methodology, developed by Singh et al. to generate uniform long time series of Ca-K images obtained during the 20th century from the Kodaikanal Observatory (KO), improved the correlation between the plage area and sunspot parameters. The same methodology can also be used on other observatory data taken with different instruments. We can combine such ECT-corrected images to reduce the gaps in the observations and make a long uniform data set to study short- and long-term variations. We apply this procedure to Mount Wilson Observatory (MWO) historical Ca-K data and recent Ca-K filtergrams obtained using narrowband filters at KO and the Mauna Loa Solar Observatory (MLSO). To determine the success of this method, the results of the analysis of the ECT images obtained from KO, MWO, and MLSO are compared. A comparison of the plage and active areas derived from KO and MWO images before and after the ECT procedure indicates an improvement in the correlation coefficients (CCs) between all the data sets after the ECT application. The CC for the combined monthly mean Ca-K plage area derived from the KO, MWO, and Precision Solar Photometric Telescope (at the MLSO) data with sunspot numbers is 0.96 for the period 1905–2015. The paper demonstrates that the time series of Ca-K data obtained from different instruments after applying the ECT procedure becomes uniform in contrast. The combined time series of KO and MWO spectroheliograms has 12 hr intervals compared to the ≈24 hr gap for a time series from a single observatory.

Oran Richard “Dick” White (1932–2020)

White's career was dedicated to synoptic solar observations using a wide range of ground- and space-based observatories. He played a critical role in the NSF Precision Solar Photometric Telescope and was Co-Investigator on NASA's SORCE, TIMED, and SDO missions.

Generation of High-resolution Solar Pseudo-magnetograms from Ca ii K Images by Deep Learning

Abstract In this Letter, we generate realistic high-resolution (1024 × 1024 pixels) pseudo-magnetograms from Ca ii K images using a deep learning model based on conditional generative adversarial networks. For this, we consider a model “pix2pixHD” that is specifically devised for high-resolution image translation tasks. We use Ca ii K 393.3 nm images from the Precision Solar Photometric Telescope at the Rome Observatory and line-of-sight magnetograms from the Helioseismic and Magnetic Imager (HMI) at the Solar Dynamics Observatory from 2011 January to 2015 June. 2465 pairs of Ca ii K and HMI are used for training except for January and July data. The remaining 436 pairs are used for an evaluation of the model. Our model shows that the mean correlation coefficient (CC) of total unsigned magnetic flux between AI-generated and real ones is 0.99 and the mean pixel-to-pixel CC after 8 × 8 binning over the full disk is 0.74. We find that the AI-generated absolute magnetic flux densities are highly consistent with real ones, even to the fine scale structures of quiet regions. On the other hand, the mean pixel-to-pixel correlations of magnetic flux densities strongly depend on a region of interest: 0.81 for active regions and 0.24 for quiet regions. Our results suggest a sufficient possibility that we can produce high-resolution solar magnetograms from historical Ca ii data.

SORCE‐Based Solar Spectral Irradiance (SSI) Record for Input Into Chemistry‐Climate Studies

AbstractWhole atmosphere chemistry‐climate model (CCM) transient studies require a solar spectral irradiance (SSI) time series with very broad wavelength coverage with an uninterrupted daily cadence over a full solar cycle time period. Herein is described the construction of a SSI record valid over Solar Cycles 23 and 24 (SC23 and SC24) specifically designed to provide: (1) a daily solar spectrum from SORCE (Solar Radiation and Climate Experiment) observations from 208–2400 nm over the 2003 to 2015 time frame, (2) a wavelength extension of this spectrum out to 10000 nm from daily spectral synthesis from the Solar Radiation Physical Model (SRPM), (3) SRPM gap‐filled spectra during the extended SORCE safe‐hold time period (Aug 2013–March 2014) and back to Solar Cycle 23 maximum conditions in 2001 and 2002, (4) analysis of solar images generated by the Precision Solar Photometric Telescope (PSPT) network from the Mauna Loa Solar Observatory (MLSO) and Rome Observatory (OAR) (These images act both as input to the SRPM model and as a stand‐alone proxy of solar variability.), (5) documentation of uncertainties and biases in the time series as a function of wavelength, and (6) comparative analysis of the SORCE time series with SRPM over the full wavelength range of the data set.

Recovering the unsigned photospheric magnetic field from Ca II K observations

Context. A number of studies have aimed at defining the exact form of the relation between magnetic field strength and Ca II H and K core brightness. All previous studies have however been restricted to isolated regions on the solar disc or to a limited set of observations.Aims. We reassess the relationship between the photospheric magnetic field strength and the Ca II K intensity for a variety of surface features as a function of the position on the disc and the solar activity level. This relationship can be used to recover the unsigned photospheric magnetic field from images recorded in the core of Ca II K line.Methods. We have analysed 131 pairs of high-quality, full-disc, near-co-temporal observations from the Helioseismic and Magnetic Imager (SDO/HMI) and Precision Solar Photometric Telescope (Rome/PSPT) spanning half a solar cycle. To analytically describe the observationally determined relation, we considered three different functions: a power law with an offset, a logarithmic function, and a power-law function of the logarithm of the magnetic flux density. We used the obtained relations to reconstruct maps of the line-of-sight component of the unsigned magnetic field (unsigned magnetograms) from Ca II K observations, which were then compared to the original magnetograms.Results. We find that both power-law functions represent the data well, while the logarithmic function is good only for quiet periods. We see no significant variation over the solar cycle or over the disc in the derived fit parameters, independently of the function used. We find that errors in the independent variable, which are usually not accounted for, introduce attenuation bias. To address this, we binned the data with respect to the magnetic field strength and Ca II K contrast separately and derived the relation for the bisector of the two binned curves. The reconstructed unsigned magnetograms show good agreement with the original ones. Root mean square differences are less than 90 G. The results were unaffected by the stray-light correction of the SDO/HMI and Rome/PSPT data.Conclusions. Our results imply that accurately processed and calibrated Ca II K observations can be used to reconstruct unsigned magnetograms by using the relations derived in our study.

PHOTOMETRIC TRENDS IN THE VISIBLE SOLAR CONTINUUM AND THEIR SENSITIVITY TO THE CENTER-TO-LIMB PROFILE

Solar irradiance variations over solar rotational time-scales are largely determined by the passage of magnetic structures across the visible solar disk. Variations on solar cycle time scales are thought to be similarly due to changes in surface magnetism with activity. Understanding the contribution of magnetic structures to total solar irradiance and solar spectral irradiance requires assessing their contributions as a function of disk position. Since only relative photometry is possible from the ground, the contrasts of image pixels are measured with respect to a center-to-limb intensity profile. Using nine years of full-disk red and blue continuum images from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory (PSPT/MLSO), we examine the sensitivity of continuum contrast measurements to the center-to-limb profile definition. Profiles which differ only by the amount of magnetic activity allowed in the pixels used to determine them yield oppositely signed solar cycle length continuum contrast trends; either agreeing with the result of Preminger et al. (2011) showing negative correlation with solar cycle or disagreeing and showing positive correlation with solar cycle. Changes in the center-to-limb profile shape over the solar cycle are responsible for the contradictory contrast results, and we demonstrate that the lowest contrast structures, internetwork and network, are most sensitive to these. Thus the strengths of the full-disk, internetwork, and network photometric trends depend critically on the magnetic flux density used in the quiet-sun definition. We conclude that the contributions of low contrast magnetic structures to variations in the solar continuum output, particularly to long-term variations, are difficult, if not impossible, to determine without the use of radiometric imaging.

Identification of large-scale cellular structures on the Sun based on the SDO and PSPT data

Three independent sets of data: i). series of filtergrams obtained in line CaII K (393.416 nm) with the ground-based telescope Precision Solar Photometric Telescope (PSPT) of Mauna Loa Solar Observatory; ii). series of filtergrams of Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO) in {\lambda}160 nm and iii). series of magnetograms of Helioseismic and Magnetic Imager (HMI) of SDO have been processed to reveal reliably the existence of spatial cellular structures on the solar photosphere at scale about of 300 arcsec. This scale is intermediate between supergranules and giant cells (~30,000 and ~300,000 kilometers across, respectively). To identify the different spatial structures the tens of two-dimensional power spectra (2DFFT) have been averaged. For one-dimensional photometric cross sections of frames, the Fourier power spectra (FFT) and wavelet transforms (Morlet 5-th order) have been calculated.

THE INTENSITY PROFILE OF THE SOLAR SUPERGRANULATION

We have measured the average radial (cell center to network boundary) profile of the continuum intensity contrast associated with supergranular flows using data from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO). After removing the contribution of the network flux elements by the application of masks based on Ca II K intensity and averaging over more than 10^5 supergranular cells, we find a ~ 0.1% decrease in red and blue continuum intensity from the supergranular cell centers outward, corresponding to a ~ 1.0 K decrease in brightness temperature across the cells. The radial intensity profile may be caused either by the thermal signal associated with the supergranular flows or a variation in the packing density of unresolved magnetic flux elements. These are not unambiguously distinguished by the observations, and we raise the possibility that the network magnetic fields play an active role in supergranular scale selection by enhancing the radiative cooling of the deep photosphere at the cell boundaries.

Latitudinal Variation of the Solar Photospheric Intensity

We have examined images from the Precision Solar Photometric Telescope (PSPT) at the Mauna Loa Solar Observatory (MLSO) in search of latitudinal variation in the solar photospheric intensity. Along with the expected brightening of the solar activity belts, we have found a weak enhancement of the mean continuum intensity at polar latitudes (continuum intensity enhancement $\sim0.1 - 0.2%$ corresponding to a brightness temperature enhancement of $\sim2.5{\rm K}$). This appears to be thermal in origin and not due to a polar accumulation of weak magnetic elements, with both the continuum and CaIIK intensity distributions shifted towards higher values with little change in shape from their mid-latitude distributions. Since the enhancement is of low spatial frequency and of very small amplitude it is difficult to separate from systematic instrumental and processing errors. We provide a thorough discussion of these and conclude that the measurement captures real solar latitudinal intensity variations.

Sunspot Bright Rings: Evidence from Case Studies

We present evidence, from both the Precision Solar Photometric Telescope and the Advanced Stokes Polarimeter, for a ring of enhanced continuum intensity surrounding large isolated sunspots. We do not attempt to evaluate the frequency of the phenomenon based on a large sample of spots but instead concentrate on illustrative best-case examples. The rings are about 0.5%-1.0% brighter in red and blue continuum (10 K warmer) than the surrounding photosphere and extend about one sunspot radius outward from the outer penumbral boundary. Most of the excess radiation is not directly associated with the strongest regions of Ca II K emission surrounding the spots or with measurable vertical magnetic field when such measurements are available. Moreover, the temporal evolution of the Ca II K and continuum emission in the ring differ, with the continuum intensity evolving on a shorter timescale. This suggests a convective origin for the bright ring, although a role for weaker, more diffuse magnetic fields cannot be ruled out. While we have inferred that only about 10% of the radiant energy missing from the sunspot is emitted through the bright ring, even this small enhancement may be significant to our understanding of subsurface sunspot structure and energy transport.

On the Geometrical Properties of the Chromospheric Network

A sequence of Ca-K images obtained in a period of minimum solar activity, from July to November 1996, at the Rome Observatory with the PSPT (Precision Solar Photometric Telescope) prototype instrument have been used to analyze the geometrical properties of cells identified by the chromospheric network. In particular, we used 256 × 256 sub-arrays of the calibrated full-disk PSPT images. These sub-arrays, centered on the solar disk, are reduced to two-levels (binary) images by means of a suitable threshold after an FFT high-pass filtering. A medial axis transform, better known as skeleton, combined with a cellular automaton, is applied to the two-level images, in order to derive the cell boundaries. The regions corresponding to the cells are then filled by a growing algorithm. In this way we can derive a set of output parameters describing the cells geometry. The size distribution of the identified cells shows a continuous increase toward the smaller scales, rather than a small dispersion around a characteristic scale. Nevertheless the analysis of the inter-cell distances and of the area distribution pointed out a characteristic scale (square root of the area) of ± 24 Mm. To describe the cells irregularity and to probe the nature of solar turbulence, we apply a Mandelbrot fractal analysis to such irregularly shaped features. Examining the cell perimeter–area relationship we found the existence of a ‘critical’ area at which a change in the geometrical properties occurs. This area corresponds to the scale of ± 24 Mm. The estimated fractal dimension for cells with area greater than the ‘critical’ one is 1.35. This value, close to that predicted for isobars in the Kolmogorov 3-D turbulent theory, does not exclude a turbulent origin for such cells. The analysis seems to point to a common origin for solar granulation and supergranulation.

Precision IR and visible solar photometry

A precision Solar Photometric Telescope (SPT) was constructed to study the large-scale thermal structure of the solar photosphere. This instrument does full-disk, broad-band (10 nm FWHM), two-color (500 and 650 nm) imaging of the solar photosphere. Data obtained by the SPT reveals network structures correlated with the supergranulation velocity field, and the CaK network of the chromosphere. Infrared array photometry extends these measurements to 1.6 and 2.2 micron. The observed correlation of the network brightness signal with the CaK network is positive at visible wavelengths. The correlation between the network at the opacity minimum (1.6 micron) and in the higher photosphere (2.2 micron) is positive also. The root-mean-square (r.m.s.) amplitude of the contrast at disk center is (2.34 ± 0.38) × 10-3, (1.83 ± 0.51) × 10-3, (1.02 ± 0.21) × 10-3, and (1.11 ± 0.21) × 10-3 for the green, red, H, and K band, respectively. It is consistent with a brightness temperature modulation of 2.9 K. The r.m.s. amplitude of the contrast of active region network shows a large increase toward the limb, and the quiet region network shows little center-to-limb variation (CLV). Power-spectrum analysis shows that the bright facular points in the active regions appear in the form of enhanced network.