Solar Brightness Temperature Research Articles

Using Partial Solar Eclipse for the 14-Metre Radio Telescope Calibration

Abstract A partial solar eclipse occurred on 25 October 2022, in the central and northern parts of Europe. The partial solar eclipse was observed at Aalto University Metsähovi Radio Observatory, Finland at the radio wavelength of 8 mm (37 GHz). In Finland, the magnitude of the partial solar eclipse was 62.7 %. Solar radio maps at the time cadence of about 9 minutes were observed over the whole eclipse. The solar eclipse observations could be used for instrument calibration purposes. This paper investigates the solar brightness temperature, the limb brightening effect, the height of the chromosphere and the location of radio brightening using the aforementioned partial solar eclipse observations. We got the confirmation that our earlier results are consistent, e.g., the solar brightness temperature matches with 8100 K ± 300 K. It was also possible to detect limb brightening effect. However, the prevailing solar activity might have distorted the final conclusions. The Moon should operate as a focusing element and the location of radio brightenings could be defined more carefully than in the normal conditions. We investigated this feature. Our results are in some parts unexpected and need further investigations.

The Calibration of the 35–40 GHz Solar Radio Spectrometer with the New Moon and a Noise Source

Calibrating solar radio flux has always been a concern in the solar community. Previously, fluxes were calibrated by matching load or the new Moon for relative calibration, and at times with the assistance of other stations’ data. Moreover, the frequency coverage seldom exceeded 26 GHz. This paper reports the upgraded and calibrated Chashan Broadband Solar millimeter spectrometer (CBS) working from 35 to 40 GHz at the Chashan Solar Observatory (CSO). Initially, the calibration of the solar radiation brightness temperature is accomplished using the new Moon as the definitive source. Subsequently, the 35–40 GHz standard flux is achieved by establishing the correlation between the solar radio flux, brightness temperature, and frequency. Finally, the calibration of the solar radio flux is implemented by utilizing a constant temperature-controlled noise source as a reference. The calibration in 2023 February and March reveals that the solar brightness temperature is 11,636 K at 37.25 GHz with a standard deviation (STD) of 652 K. The solar radio flux’s intensity is ∼3000–4000 solar flux units (SFU) in the range of 35–40 GHz with a consistency bias of ±5.3%. The system sensitivity is about ∼5–8 SFU by a rough evaluation, a noise factor of about 200 K, and the coefficient of variation of the system transmission slope of 6.5% @ 12 hr at 37.25 GHz. It is expected that the upgraded CBS will capture more activity during the upcoming solar cycle.

A Multiscale Normalization Method of a Mixed-Effects Model for Monitoring Forest Fires Using Multi-Sensor Data

This paper points out the shortcomings of existing normalization methods, and proposes a brightness temperature inversion normalization method for multi-source remote sensing monitoring of forest fires. This method can satisfy both radiation normalization and observation angle normalization, and reduce the discrepancies in forest fire monitoring between multi-source sensors. The study was based on Himawari-8 data; the longitude, latitude, solar zenith angle, solar azimuth angle, emissivity, slope, aspect, elevation, and brightness temperature values were collected as modeling parameters. The mixed-effects brightness temperature inversion normalization (MEMN) model based on FY-4A and Himawari-8 satellite sensors is fitted by multiple stepwise regression and mixed-effects modeling methods. The results show that, when the model is tested by Himawari-8 data, the coefficient of determination (R2) reaches 0.8418, and when it is tested by FY-4A data, R2 reaches 0.8045. At the same time, through comparison and analysis, the accuracy of the MEMN method is higher than that of the random forest normalization method (RF) (R2=0.7318), the pseudo-invariant feature method (PIF) (R2=0.7264), and the automatic control scatter regression method (ASCR) (R2=0.6841). The MEMN model can not only reduce the discrepancies in forest fire monitoring owing to different satellite sensors between FY-4A and Himawari-8, but also improve the accuracy and timeliness of forest fire monitoring.

A Spectral Data Compression (SDCOMP) Radiative Transfer Model for High-Spectral-Resolution Radiation Simulations

Abstract With the increasing use of satellite and ground-based high-spectral-resolution (HSR) measurements for weather and climate applications, accurate and efficient radiative transfer (RT) models have become essential for accurate atmospheric retrievals, for instrument calibration, and to provide benchmark RT solutions. This study develops a spectral data compression (SDCOMP) RT model to simulate HSR radiances in both solar and infrared spectral regions. The SDCOMP approach “compresses” the spectral data in the optical property and radiance domains, utilizing principal component analysis (PCA) twice to alleviate the computational burden. First, an optical-property-based PCA is performed for a given atmospheric scenario (atmospheric, trace gas, and aerosol profiles) to simulate relatively low-spectral-resolution radiances at a small number of representative wavelengths. Second, by using precalculated principal components from an accurate radiance dataset computed for a large number of atmospheric scenarios, a radiance-based PCA is carried out to extend the low-spectral-resolution results to desired HSR results at all wavelengths. This procedure ensures both that individual monochromatic RT calculations are efficiently performed and that the number of such computations is optimized. SDCOMP is approximately three orders of magnitude faster than numerically exact RT calculations. The resulting monochromatic radiance has relative errors less than 0.2% in the solar region and brightness temperature differences less than 0.1 K for over 95% of the cases in the infrared region. The efficiency and accuracy of SDCOMP not only make it useful for analysis of HSR measurements, but also hint at the potential for utilizing this model to perform RT simulations in mesoscale numerical weather and general circulation models.

Measurements of the Quiet-Sun Level Brightness Temperature at 8 mm

Defining the solar brightness temperature accurately at millimeter wavelengths has always been challenging. One of the main reasons has been the lack of a proper calibration source. New Moon was used earlier as a calibration source. We carried out a new extensive set of observations at 8 mm using the New Moon for calibration. The solar and Moon observations were made using the 14-meter radiotelescope operated by the Aalto University Metsähovi Radio Observatory in Finland. In this article, we present our method for defining the brightness temperature of the quiet-Sun level (QSL). Based on these observations, we found 8100~mbox{K} pm 300~mbox{K} to be the mean value for the QSL temperature. This value is between the values that were reported in earlier studies.

The Brightness Temperature of the Quiet Solar Chromosphere at 2.6 mm

The absolute brightness temperature of the Sun at millimeter wavelengths is an important diagnostic of the solar chromosphere. Because the Sun is so bright, measurement of this property usually involves the operation of telescopes under extreme conditions and requires a rigorous performance assessment of the telescope. In this study, we establish solar observation and calibration techniques at 2.6 mm wavelength for the Nobeyama 45 m telescope and accurately derive the absolute solar brightness temperature. We tune the superconductor–insulator–superconductor (SIS) receiver by inducing different bias voltages onto the SIS mixer to prevent saturation. Then, we examine the linearity of the receiver system by comparing outputs derived from different tuning conditions. Furthermore, we measure the lunar filled beam efficiency of the telescope using the New Moon, and then derive the absolute brightness temperature of the Sun. The derived solar brightness temperature is 7700 pm 310~mbox{K} at 115 GHz. The telescope beam pattern is modeled as a summation of three Gaussian functions and derived using the solar limb. The real shape of the Sun is determined via deconvolution of the beam pattern from the observed map. Such well-calibrated single-dish observations are important for high-resolution chromospheric studies because they provide the absolute temperature scale that is lacking from interferometer observations.

FIRST DETECTION OF THERMAL RADIO EMISSION FROM SOLAR-TYPE STARS WITH THE KARL G. JANSKY VERY LARGE ARRAY

We present the first detections of thermal radio emission from the atmospheres of solar-type stars {\tau} Cet, {\eta} Cas A, and 40 Eri A. These stars all resemble the Sun in age and level of magnetic activity, as indicated by X-ray luminosity and chromospheric emission in calcium-II H and K lines. We observed these stars with the Karl G. Jansky Very Large Array with sensitivities of a few {\mu}Jy at combinations of 10.0, 15.0, and 34.5 GHz. {\tau} Cet, {\eta} Cas A, and 40 Eri A are all detected at 34.5 GHz with signal-to-noise ratios of 6.5, 5.2, and 4.5, respectively. 15.0-GHz upper limits imply a rising spectral index greater than 1.0 for {\tau} Cet and 1.6 for {\eta} Cas A, at the 95% confidence level. The measured 34.5-GHz flux densities correspond to stellar disk-averaged brightness temperatures of roughly 10,000 K, similar to the solar brightness temperature at the same frequency. We explain this emission as optically- thick thermal free-free emission from the chromosphere, with possible contributions from coronal gyroresonance emission above active regions and coronal free-free emission. These and similar quality data on other nearby solar-type stars, when combined with ALMA observations, will enable the construction of temperature profiles of their chromospheres and lower transition regions.

Land Surface Brightness Temperature Modeling Using Solar Insolation

Retrieval of land surface emissivity and temperature from microwave brightness temperature data is a complex problem. The diurnal variation of temperature due to the diurnal cycle of solar radiation and weather conditions makes this problem even more challenging. In this paper, we use solar radiation in modeling the temporal variation of the brightness temperature state of the surface. Solar insolation modeling is used to estimate the diurnal variation of land surface brightness temperature. Solar radiation and brightness temperature are linked through temperature of the surface which is derived based on the radiation balance equation. The temperature state model behaves consistent to the measured temperature data. The root-mean-square (rms) error of the model and measured temperature during 1999 is 1.47 K with a correlation of 0.98. Brightness temperature is calculated as a product of physical temperature and emissivity. This relationship is used to transform the temperature state model into the temporal model of the brightness temperature. The model is validated using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) brightness temperature observations at 10.65-GHz vertical polarization. The rms error of the modeled and measured brightness temperature during 1999 is 2.15 K with a correlation of 0.98. Physical and brightness temperature models are ordinary differential equations that are solved numerically to estimate model parameters. The model parameters are related to geophysical characteristics that modulate the temporal variation of the physical and brightness temperature. These parameters provide new insight into the thermal characteristics of the land surface. Brightness temperature model is used to retrieve emissivity from TMI measurements. Images of emissivity and other model parameters are spatially coherent and reflect ground geometrical and dielectric conditions. The results confirm that incident solar radiation is an important input in modeling the temporal variation in the physical temperature and brightness temperature.

Clouds-Aerosols-Precipitation Satellite Analysis Tool (CAPSAT)

Abstract. A methodology for representing much of the physical information content of the METEOSAT Second Generation (MSG) geostationary satellite using red-green-blue (RGB) composites of the computed physical values of the picture elements is presented. The physical values are the solar reflectance in the solar channels and brightness temperature in the thermal channels. The main RGB compositions are (1) "Day Natural Colors", presenting vegetation in green, bare surface in brown, sea surface in black, water clouds as white, ice as magenta; (2) "Day Microphysical", presenting cloud microstructure using the solar reflectance component of the 3.9 μm, visible and thermal IR channels; (3) "Night Microphysical", also presenting clouds microstructure using the brightness temperature differences between 10.8 and 3.9 μm; (4) "Day and Night", using only thermal channels for presenting surface and cloud properties, desert dust and volcanic emissions; (5) "Air Mass", presenting mid and upper tropospheric features using thermal water vapor and ozone channels. The scientific basis for these rendering schemes is provided, with examples for the applications. The expanding use of these rendering schemes requires their proper documentation and setting as standards, which is the main objective of this publication.

Solar Irradiance and Effective Brightness Temperature for SWIR Channels of AVHRR/NOAA and GOES Imagers

Abstract Satellite observations in the shortwave infrared (SWIR) part of spectrum between 3.5 and 4.0 μm deliver critically important information for many applications. The satellite signal in this spectral band consists of solar-reflected radiation and thermal radiation emitted by surface, clouds, and atmosphere. Accurate retrievals require precise knowledge of solar irradiance values within a channel's bandwidth. The magnitudes of solar irradiance for shortwave infrared channels (3.7–3.9 μm) for the Advanced Very High Resolution Radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration-7 (NOAA-7) to NOAA-18 satellites and the Geostationary Operational Environmental Satellite-8 (GOES-8) to GOES-12 are considered in this paper. Four recent solar reference spectra [those of Kurucz, Gueymard, the American Society for Testing and Materials (ASTM), and Wehrli] are analyzed to determine uncertainties in the knowledge of solar irradiance values for SWIR channels of the listed sensors. Because thermal radiation is frequently converted to effective blackbody temperature for analysis, computations, and calibration purposes, it is proposed here to express band-limited solar irradiance values in terms of brightness temperature as well. It is shown that band-limited solar irradiance for AVHRR radiometers expressed in terms of blackbody equivalent brightness temperature correspond to the range 355–360 K, and vary around 345 K for the SWIR channels of the GOES imagers. The values of band-limited solar irradiance and brightness temperatures are provided for various reference solar spectra. The relative differences in band-limited solar irradiance computed for the considered reference solar spectra are between 0% and 2.5%. Differences expressed in terms of brightness temperatures may reach 0.8 K. The results for the ASTM and the Kurucz reference spectra agree within 0.1% relative difference. Parameters of linear fits relating effective brightness temperatures and spectral radiance equivalent temperatures are also determined for all sensors. They are required for precise radiance–temperature and temperature–radiance conversion through Planck's functions in the case of the finite spectral response of real sensors.

Solar brightness temperature at submillimeter wavelengths

Solar brightness temperature data obtained at 34.9 km during a balloon flight from the Canadian Space Research Facility on August 28, 1982 are analyzed. A servocontrolled telescope capable of solar image scanning, a Michelson interferometer with 0.015/cm apodized resolution, and an in-flight radiometric calibration source were used to obtain the temperature data. Interferograms and spectra from the sun, the sky background, and blackbody at its equilibrium temperature of 1165 K are studied, and values of solar brightness temperature are derived from the spectra. The relation between solar brightness temperature and wave number is examined. The effects of systematic error on the solar temperature data are investigated. An intrinsic temperature minimum of 4170 K and an observable minimum of 4300 K were obtained. The temperature values are compared with the predictions of Vernazza et al. (1981), and good correlation is detected. 24 references.

Multifrequency spectra of solar brightness temperature derived from eclipse observations

Changes in solar radio-brightness temperature were derived at 2.8,19.3 and 22.2 GHz from the observations of radio flux during the total eclipse of 1980 February 16. High-resolution MEM spectra of the brightness temperature fluctuations at the three frequencies showed periodicities ranging from 3.5 min to 128 min. Between 3.5 min and 14.6 min there are several periodicities of comparable significance common to the three operating frequencies. If the corresponding variations in brightness temperature are assumed to result from spatial variations in the solar radio emission, the observed periodicities imply scale sizes in the range 76000 km to 320000 km.

The solar brightness temperature at millimeter wavelengths

The authors have previously discussed an improved method for obtaining the absolute solar brightness temperature using the new Moon as a calibration source. New measurements of the Sun-to-new Moon ratio at three frequencies near 36 GHz (λ = 8 mm) and also at two frequencies near 93 GHz (λ = 3 mm) are reported. The slopes of the solar brightness temperature spectrum based on these ratios are then discussed. The absolute solar brightness spectrum derived from all current available measurements is also presented and discussed.

Solar brightness temperature distribution at 350 and 450 microns

Solar scans, passband limited to effective wavelengths 350 and 450 μ, have been obtained with angular resolution of about 1'. The observations show little evidence of the limb brightening that would be expected from spherically symmetric homogeneous atmospheric models and the observed increase in brightness temperature in the regions above the solar atmosphere temperature minimum. The observations suggest that the complex structure that is observed at higher levels in the solar atmosphere extends down to the level of the temperature minimum.

Far-infrared solar brightness measured with a balloon-borne lamellar-grating interferometer

The solar brightness temperature was measured at wavelengths between 208μ and 660μ with a lamellar-grating interferometer. In order to avoid terrestrial absorption due to water vapor the measurement of the absolute spectral brightness was performed at altitudes of 30 km and 35 km with a balloon borne gondola. A cylindrical black body radiation source served as absolute calibration standard. Its wall temperature lay between 1100 K and 1300 K. Since the box of the interferometer and the black body could not be evacuated for the calibration, the air in the system was replaced by dry nitrogen. Nevertheless, residual water vapor absorption lines hampered the precision of the absolute calibration. The corresponding error could not be reduced to less than ±100 K. The present experimental temperature profile is compared to results obtained from other measurements. The results agree with the empirical HSRA model and with those of other research groups in the spectral range between 208μ and 660μ.

Ultraviolet ion chamber measurements of the solar minimum brightness temperature

The solar ultraviolet flux in the wavelength bands 1580–1640 A and 1430–1470 A (FWHM) has been measured using photon ion chambers carried on the satellite WRESAT I (1967-118A). These observations of the integrated ultraviolet flux from the entire disk indicate a value of (4570 ± 50) K for the solar temperature minimum. The results are compared with other estimates of the minimum value of the solar brightness temperature.

Far Infrared Airborne Spectroscopy

Equipment and techniques employed in an airborne far ir (80-400 micro) spectral measurement of the solar brightness temperature are described, with particular attention paid to the scanning Michelson interferometer and the radiometric calibration. The airborne performance of several telescopic guiding systems, a liquid helium cooled bolometer, and golay cells are discussed, summarizing experience gained on twenty-five jet aircraft flights.

Spectrum measurements of the sun near 1 cm wavelength

The solar brightness temperature was measured at frequencies of 19.0, 21.0, 22.2, 23.5, and 25.5 GHz on 8 days in February and March, 1966. The observed average brightness temperatures were 10 800, 10 900, 11 000, 10 700, and 9 800°K, respectively. The daily values were close to these averages.

Decrement of the Solar Continuum in the Far Infra-red

SOME previous absolute measurements of the solar brightness temperature in the infra-red1,2 have suggested that the Sun is deficient in infra-red radiation. The result reported here lends support to this idea and has allowed a first approximation to a power law for the decrement to be computed.

Measurements of Solar Radiation and Atmospheric Attenuation at 4.3-Millimeters Wavelength

Solar radiation and atmospheric attenuation were measured at 4.3-mm wavelength The sun was scanned with a radio telescope consisting of a 10-foot precision paraboloid antenna (6.7-minute beamwidth) and a Dicke-type radiometer. Atmospheric attenuations were determined from the change in received solar radiation with changing elevation of the sun and from direct measurements of the thermal radiation from the atmosphere. The measured attenuations at the zenith for clear skies were between 1.6 and 2.2 db. At 4.3-mm wavelength, the sun (when it is free of sunspots) appears to be a uniform disk nearly one per cent larger than its optical size. From a large number of measurements over a period of 6 months, the solar brightness temperature was found to be 7000 °K with an uncertainty of about 10 per cent. Sunspot regions are slightly brighter than the quiet areas; the largest observed enhancement is 2 per cent.