Accurate, precise and traceable measurements of total and spectral solar irradiance measurements are fundamental for solar energy applications, climate studies, and satellite validation. In this study, we assess the performance and the quality of the data from a commercially available, compact Bi-Tec Sensor (BTS) Spectroradiometer system, by comparing its spectrally integrated total solar irradiance (TSI) values with an electric substitution cavity radiometer (PMO2), which is traceable to the World Radiometric Reference (WRR). The resulting ratio between BTS Spectroradiometer system and WRR-traceable TSI is 0.9975 with a standard deviation of 0.0050. Applying a correction factor of (–) 0.34 % to PMO2, accounting for the known offset between WRR and the International system of Units (SI) results in a relative difference between the BTS Spectroradiometer system derived TSI and PMO2 of +0.09 % with a standard deviation of 0.0050 demonstrating good consistency between BTS derived TSI and the cavity radiometer. This comparison confirms the precision and accuracy of the BTS spectroradiometer system, and its capability to deliver SI traceable TSI from spectrally resolved solar irradiance measurements. Its spectral resolution enables accurate measurements of spectral solar irradiance, which are essential, not only for determining total solar irradiance but also for retrieving key atmospheric gases such as water vapor, ozone, and aerosols, establishing its relevance as a compact instrument for atmospheric and climate research.

The Earth Radiation Budget at the Top of the Atmosphere (ToA) governs the status of climate change on our planet. The ERB is the balance between the incoming Total Solar Irradiance (TSI) and total outgoing radiation at the ToA. If more energy is stored in the system the Earth Energy Imbalance is positive and the temperature in the system rises. The Compact Lightweight Absolute RAdiometer (CLARA) experiment onboard the Norwegian micro satellite NorSat-1 is an SI traceable radiometer with the primary science goal to measure TSI from space. Besides TSI, CLARA also measures the terrestrial Outgoing Longwave Radiation (OLR) at the ToA on the night side of Earth. We present the latest status of the data and degradation correction obtained with this SI-traceable radiometer and compare the CLARA TSI and OLR time series with other available observations and reanalysis data. Furthermore, we explore potential synergies with the upcoming TRUTHS mission as well as the Earth Explorer 12 candidate mission ECO.

A positive Earth Energy Imbalance (EEI) is the energy, which is continuously stored by the Earth and will ultimately released to the atmosphere, causing global warming. The "imperative to monitor Earth’s energy imbalance” (von Schuckmann et al., 2016) has been continuously reported by the Earth’s climate community. The EEI has been identified to be around 0.5 to 1.0 Wm−2. To determine its exact value both the Total Solar Irradiance (TSI) and the Top of the Atmosphere (ToA) Outgoing Radiation (TOR) need to be measured with unprecedented accuracy and precision.However, so far, the EEI could not be determined as the measurements were not sufficiently accurate. This calls for improved instrument technologies as well as a traceable calibration chain of the space instrumentation. To pave the way in that direction, the ISSI International Team "Towards Determining the Earth Energy Imbalance from Space" has been established. We collect the current knowledge of ERB measurements and identify missing elements for measuring EEI from space. Specifically, we collect past and ongoing measurements of the ERB components obtained with instruments such as CLARA, RAVAN, SIMBA, GERB, and CERES. The goal is to evaluate the performance and uncertainty of each of the instruments to identify observational challenges that need to be overcome to be able to measure both TSI and the Earth’s outgoing radiation with the required accuracy to ultimately be able to determine the absolute level of EEI from space.

The Joint Total Solar Irradiance Monitor (JTSIM) onboard the Fengyun-3E meteorological satellite has been launched successfully on 4th of July 2021. It aims at measuring the Total Solar Irradiance (TSI) from the Low Earth Orbit. The instruments on the Fengyun-3E/JTSIM include the Digital Absolute Radiometer (DARA) from the Physikalisch Meteorologisches Observatorium, Davos and World Radiation Center (PMOD/WRC) and the Solar Irradiance Absolute Radiometer (SIAR) from the Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences (CIOMP/CAS). The first light measurements and TSI value determined from DARA and SIAR are compared with other active missions (SOHO-VIRGO,TSIS-1).

Since the late 1970s, successive satellite missions have been monitoring solar activity and recording Total Solar Irradiance (TSI) data. The Digital Absolute Radiometer (DARA) on board the Chinese FY3E spacecraft was launched on July 4, 2021, and has since been recording TSI observations. Here, we analyze these observations and assess the performance of DARA, including sensor degradation of 5 ppm after 2 years in orbit, resulting from exposure to ultraviolet and extreme ultraviolet radiation. Comparing the new dataset’s mean values with observations from active instruments on other spacecraft (i.e., PMO6 on board the VIRGO/SOHO and the TIM/TSIS), along with the Solar Irradiance Absolute Radiometer (SIAR) also on board FY3E/JTSIM, we find that DARA observations closely align with TIM/TSIS, with a difference of approximately 0.07 W/m2. Based on these findings, we generate a new TSI dataset (JTSIM-DARA product) at a 6-hour sampling interval. Finally, we have incorporated this new dataset into the TSI composite time series released by the PMOD/WRC. The results indicate that the inclusion of DARA-recorded observations does not alter the consistency, reliability, and stability of the time series.

For many years it has been an ambition of the solar and heliophysics communities to obtain a 3-D view of the entire Sun as this is critical for understanding many fundamental processes acting within and around our star; these are key questions to address for all stars, but particularly our own.To understand solar activity, we must study the structure and evolution of the seats of such activity, the so-called active regions that are regions defined by complex and highly dynamic magnetic structure.Currently we do that from a limited set of measurement and view-points constraining the science we can do.The majority of our solar observations are made near or from the Earth, but there have been notable exceptions.Steps towards measuring different views of the Sun were carried out by the Ulysses mission.Despite plans to carry remote sensing instrumentation, the spacecraft only carried in-situ instruments, but flew an extremely successful mission providing the first measurements over the Sun's poles.The NASA STEREO mission provided 2 views of the Sun in the equatorial region, from widely separated platforms but was limited due to the lack of magnetic field data and visibility of the poles.The recent ESA/NASA Solar Orbiter mission will gradually reach out of the equatorial plane and observe from 33 degrees with both remote sensing and in situ instruments providing a major step forward in studies of our star's polar regions.In recent reviews such as the JAXA/NASA/ESA next generation solar physics mission report in 2017, 5 multi spacecraft mission to provide an extended view of the Sun were proposed.In the ESA Voyage 2050 call, polar solar mission concepts were proposed to provide extensive measurements of the mysterious polar regions that have not yet been observed with imaging instruments [1] -this paper describes different technologies that allow an extended view of the poles.ESA's Vigil mission will be stationed at the Lagrange L5 point and will carry both remote sensing and in situ instruments.This white paper indicates the support of the European and Japanese communities for the Firefly mission concept (see white paper led by Nour Raouafi).These communities have expertise from probing the interior of the Sun using helioseismology methods to solar activity and how it feeds the heliosphere.An ambitious mission such as Firefly provides extensive opportunities to answer scientific questions that remain unanswered due to our restricted views of the Sun.There are 4 science questions probing the fundamental processes of what drives the Sun's magnetic activity from the inside of the star to activity in the heliosphere.These topics are of significant consequence to the impact our understanding of every star and hence the understanding of habitability and impacts of space weather on planets.

The Project for On-Board Autonomy-3 (PROBA-3) is the fourth satellite technology development and demonstration precursor mission within ESA's GSTP (General Support Technology Program) series. The primary mission objective is to demonstrate the technologies required for formation flying of multiple spacecrafts. The PROBA-3 mission concept comprises two independent minisatellites in highly-elliptical Earth orbits in precise formation flying, close to one another with the ability to accurately control the attitude and separation of the two satellites. The mission launch is scheduled for end of 2023.PROBA-3 mission consists of a coronograph spacecraft, hosting the coronograph APIICS, and the occulter spacecraft with the Digital Absolute RAdiometer (DARA). The radiometer to record total solar irradiance is mounted on the front satellite pointing to the Sun. DARA is developed and manufactured in Switzerland by the PMOD/WRC. We have done two pre-flight calibration campaigns: one at the World Radiation Center in Davos, Switzerland, and one at the Total Solar Irradiance (TSI) Radiometer Facility of the Laboratory for Atmospheric and Space Physics in Boulder Colorado, USA. We report on the results of the laboratory comparisons and discuss uncertainties of several instrument parameters, which are used to transform the raw measurements, which are voltage and current, into solar irradiance values.

The Earth Radiation Budget at the Top of the Atmosphere (ToA) governs the status of climate change on our planet. The ERB is the balance between the incoming Total Solar Irradiance (TSI) and total outgoing radiation at the ToA. If more energy is stored in the system the Earth Energy Imbalance is positive and the temperature in the system rises. The Compact Lightweight Absolute RAdiometer (CLARA) experiment onboard the Norwegian micro satellite NorSat-1 is an SI traceable radiometer with the primary science goal to measure TSI from space. Besides TSI, CLARA also measures the terrestrial Outgoing Longwave Radiation (OLR) at the ToA on the night side of Earth. We present the latest status of the data and degradation correction obtained with this SI-traceable radiometer and compare the CLARA TSI and OLR time series with other available observations and reanalysis data. The validation of these measurements is key to advance our capability to determine the Earth Energy Imbalance from space.

The Fengyun 3E (FY3E) spacecraft was launched on the 4th of July 2021 at 23h 28min UTC according to CASC (China Aerospace Science and Technology Corp.) on a Long March 4C vehicle from JSLC (Jiuquan Space Launch Center) in China. The orbit is a sun-synchronous near-circular with an altitude of 800 km, and an inclination of 98.7 degrees. The nominal lifetime of the satellite is eight years. The JTSIM experiments belong to the solar activities monitoring package. The solar radiation is absorbed by the black-coated cavity and the induced different heat-flux between the primary and reference cavity is measured, and the electrically calibrated differential heat-flux is used to compute the solar irradiance. SIAR has three identical channels A, B, and C, and each channel has a different solar exposure time to study the instrument’s nonlinear drift due to degradation. DARA also has three cavity radiometers and electrical substitution radiometers (Channel A, Channel B, and Channel C). The difference is that they are aligned in a triangle. Compared to VIRGO/PMO6, DARA inverts the aperture geometry to eliminate stray light. DARA and SIAR absolute radiometers are not operating at the same time due to the different designs and measurement sequences. On August 18, 2021, both instruments successfully passed the first commission phase, and they started to observe the total solar irradiance since then.

The Joint Total Solar Irradiance Monitor (JTSIM) onboard the Fengyun-3E spacecraft has been launched successfully the 4th of July 2021. It aims at measuring the Total Solar Irradiance (TSI) in orbit. The instruments on the Fengyun-3E/JTSIM include the Digital Absolute Radiometer (DARA) from the Physikalisch Meteorologisches Observatorium, Davos and World Radiation Center (PMOD/WRC) and the Solar Irradiance Absolute Radiometer (SIAR) from the Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences (CIOMP/CAS). The JTSIM experiment will use the two different types of TSI radiometers to track the stability of TSI measurements, and to better understand instrumental degradation in space. We will present results from this new experiment at first light. We will compare the measurements from DARA and SIAR over the first few months and relate them to other active missions (SOHO/VIRGO/PMO6v, SORCE/TSIS).

In order to support the analysis of potential long-term trends in the Sun's variability and trace the WRR ground calibration ratio to the measurement on the satellite. We established the total solar irradiance (TSI) calibration model in orbit for the new generation of solar radiation monitor on the FY-3E satellite. First, we measured the non-equivalence of the instrument, and the air to vacuum ratio of the SIAR radiometer is 0.9958-0.9973. Then, we established the temperature correction model for the important parameters, such as the aperture area, the standard voltage and the heating wire resistance. Finally, we calibrated the SIAR with the world radiation reference (WRR) and traced the ground calibration result to the measurement on the satellite. The end-to-end calibration ratio is 0.9991-1.0006, and the uncertainties for three channels are all better than 0.08%.

A new detector for solar absolute radiometers for ground-based applications was developed in a joint project by PMOD/WRC and Davos Instruments AG. New coating technology was applied and characterized, and a flat receiver was designed for the next generation of radiometers. The new detector, which is already commercially available, will also be considered and adapted for future space experiments.

A new method is presented to derive spectrally resolved global and local annual changes in the Earth Energy Imbalance (ΔEEI(λ, Δλ)) from measurements of Total and Spectral Solar Irradiance (TSI and SSI) and Total Outgoing Radiation (TOR) and the Spectral Outgoing Radiation (SOR) of the Earth. Since TSI space radiometers provide data with a long-term absolute accuracy <0.1 W m-2, the Sun should be used as a TSI referenced radiation source to obtain SSI data using the method of the Solar Auto-Calibrating XUV-IR Spectrometer (SOLACER). By repeatedly calibrating the solar and Earth observation instruments, the degradation should be compensated to accurately determine the outgoing flux Φ(λ, Δλ) entering the instrument. If the instruments on a pointing device are moved within the Angular Range of Sensitivity (ARS) in two angular dimensions through the solar disk, the instruments are also regularly calibrated with regard to their dependence of the angular sensitivity. ARS is independent of the environmental conditions. To improve the accuracy of SOR data, a normalization factor Ωa / ARS is used to extend the annual averaged outgoing flux data Φ(λ, Δλ)a to the SOR(λ, Δλ)a. The strength of the method is demonstrated by describing space-evaluated instruments to be adapted for solar and/or Earth observation from a small satellite. In the spectral range from 120 nm to 3000 nm, spectrometers and highly sensitive photometers with signal-to-noise ratios >1:107 are described to generate data records with high statistical accuracy. Given the compactness of the instruments, more than 20 different data sets should be compiled to complement, verify each other and improve accuracy.

Solar radiometers are deployed in many locations on the ground and in space. The radiometers in space are measuring the solar energy input into the Earth system per time and unit area, also known as the Total Solar Irradiance (TSI). TSI radiometers are also used to calibrate Earth Observation instruments and to measure the Total Outgoing Radiation (TOR) at the top of the atmosphere, which is a key component in the Earth Radiation Budget (ERB). Ground-based solar radiometers measure the local irradiance levels, which are used for monitoring of atmospheric properties and solar energy applications. Traceability of the radiation measurements to SI units is crucial in all of these applications. However, calibrating and characterising a solar radiometer is a technically challenging task. Depending on the requirements for a specific application, different calibration concepts can be employed in the calibration and characterization process. We will present the currently available calibration concepts, their advantages and disadvantages, and put special focus on recent technical developments, such as the cryogenic standard radiometers for solar irradiance on the ground and in space.

The Joint Total Solar Irradiance Monitor (JTSIM) is due to fly onboard the Fengyun-3E spacecraft and aims to measure the Total Solar Irradiance (TSI) in orbit. The instruments on the Fengyun-3E/JTSIM include the Digital Absolute Radiometer (DARA) from the Physikalisch Meteorologisches Observatorium, Davos and World Radiation Center (PMOD/WRC) and the Solar Irradiance Absolute Radiometer (SIAR) from the Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences (CIOMP/CAS). Radiometers from Switzerland and China will monitor the TSI variability on the same pointing system for eight years. The scientific data from JTSIM will support the analysis of potential long-term trends in the Sun’s variability. In this article, we describe the sensor box and the electronics box of JTSIM, the measurement principle, and the operation mode of SIAR. Before launch, we accomplished some primary calibrations of SIAR in the CIOMP laboratory, including the aperture area, cavity absorption, non-equivalence, diffraction, etc. Other parameters will be calibrated on orbit. The combined uncertainty of SIAR for characterization is 231 – 233 ppm depending on the measurement channel. The characterization of SIAR is an International System of Units (SI)-native scale calibration. An end-to-end calibration against the World Radiometric Reference (WRR) standard or the Total Irradiance Radiometer Facility (TRF) is a procedure where SIAR is directly calibrated with the WRR reference radiometers. The WRR factor for SIAR is 0.99939 – 1.00092 and the combined measurement uncertainty is 0.074% – 0.099%, depending on the measurement channel.

A mission to view the solar poles from high helio-latitudes (above 60°) will build on the experience of Solar Orbiter as well as a long heritage of successful solar missions and instrumentation (e.g. SOHO Domingo et al. (Solar Phys. 162 (1-2), 1–37 1995 ), STEREO Howard et al. (Space Sci. Rev. 136 (1-4), 67–115 2008 ), Hinode Kosugi et al. (Solar Phys. 243 (1), 3–17 2007 ), Pesnell et al. Solar Phys. 275 (1–2), 3–15 2012 ), but will focus for the first time on the solar poles, enabling scientific investigations that cannot be done by any other mission. One of the major mysteries of the Sun is the solar cycle. The activity cycle of the Sun drives the structure and behaviour of the heliosphere and of course, the driver of space weather. In addition, solar activity and variability provides fluctuating input into the Earth climate models, and these same physical processes are applicable to stellar systems hosting exoplanets. One of the main obstructions to understanding the solar cycle, and hence all solar activity, is our current lack of understanding of the polar regions. In this White Paper, submitted to the European Space Agency in response to the Voyage 2050 call, we describe a mission concept that aims to address this fundamental issue. In parallel, we recognise that viewing the Sun from above the polar regions enables further scientific advantages, beyond those related to the solar cycle, such as unique and powerful studies of coronal mass ejection processes, from a global perspective, and studies of coronal structure and activity in polar regions. Not only will these provide important scientific advances for fundamental stellar physics research, they will feed into our understanding of impacts on the Earth and other planets’ space environment.

Total Solar Irradiance (TSI) is one of the Essential Climate Variables (ECV) identified by the World Meteorological Organization's Global Climate System (GCOS). The Compact Lightweight Absolute RAdiometer (CLARA) experiment onboard the Norwegian micro satellite NorSat-1 is a SI traceable radiometer and was launched July 14, 2017 with the primary science goal to measure TSI from space. We present the latest status of the data and degradation correction obtained with this SI-traceable radiometer. Besides TSI, CLARA also measures the total outgoing radiation (TOR) at the top of the Earth atmosphere on the night side of Earth, which is extremely important to understand the Earth Radiation Budget. It is to our knowledge the first time that TSI and the emitted radiation from Earth are measured simultaneously with one SI-traceable absolute radiometer. We will compare the CLARA TSI and TOR time series with other available datasets. Ultimately, we aim towards determining the Earth Energy Imbalance from space. We will discuss the achievements and limitations in direction of this goal.