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The first implementation of NASA’s Total and Spectral Solar Irradiance Sensor (TSIS-1) launched on December 15th, 2017, and was integrated into the International Space Station (ISS) to measure both the total solar irradiance (TSI) and the solar spectral irradiance (SSI). The direct measurement of the SSI is made by the LASP Spectral Irradiance Monitor (SIM) and provides data essential to interpreting how the Earth system responds to solar spectral variability. Extensive advances in TSIS-1 SIM instrument design and new SI-traceable spectral irradiance calibration techniques have resulted in improved absolute accuracy with uncertainties of less than 0.5% over the continuous 200–2400 nm spectral range. Furthermore, improvements in the long-term spectral stability corrections provide lower trend uncertainties in SSI variability measurements. Here we present the early results of the TSIS-1 SIM measurements covering the first 5 years of operations. This time period includes the descending phase of solar cycle 24, the last solar minimum, and the ascending phase of solar cycle 25. The TSIS-1 SIM SSI results are compared to previous measurements both in the absolute scale of the solar spectrum and the time dependence of the SSI variability. The TSIS-1 SIM SSI spectrum shows lower IR irradiance (up to 6% at 2400 nm) and small visible increases (~0.5%) from some previous reference solar spectra. Finally, initial comparisons are made to current NRLSSI2 and SATIRE-S SSI model results and offer opportunities to validate model details both for short-term (solar rotation) spectral variability and, for the first time, the longer-term (near half solar cycle) spectral variability across the solar spectrum from the UV to the IR.

We present a new solar irradiance reference spectrum representative of solar minimum conditions between solar cycles 24 and 25. The Total and Spectral Solar Irradiance Sensor‐1 (TSIS‐1) Hybrid Solar Reference Spectrum (HSRS) is developed by applying a modified spectral ratio method to normalize very high spectral resolution solar line data to the absolute irradiance scale of the TSIS‐1 Spectral Irradiance Monitor (SIM) and the CubeSat Compact SIM (CSIM). The high spectral resolution solar line data are the Air Force Geophysical Laboratory ultraviolet solar irradiance balloon observations, the ground‐based Quality Assurance of Spectral Ultraviolet Measurements In Europe Fourier transform spectrometer solar irradiance observations, the Kitt Peak National Observatory solar transmittance atlas, and the semi‐empirical Solar Pseudo‐Transmittance Spectrum atlas. The TSIS‐1 HSRS spans 202–2730 nm at 0.01 to ∼0.001 nm spectral resolution with uncertainties of 0.3% between 460 and 2365 nm and 1.3% at wavelengths outside that range. Plain Language Summary: The Sun's irradiance spectrum is used in many applications, such as constraining the solar forcing in climate models and converting measured satellite radiance to reflectance. A growing body of literature has provided evidence that the currently available solar reference spectra differ by more than their reported uncertainties. Such differences lead to biased results when different reference spectra are adopted in the aforementioned applications. This motivates our work to provide a new high‐resolution solar reference spectrum at higher accuracy than any previously reported. Our ability to produce such a data set is due to the state‐of‐the‐art measurements of the Sun's irradiance spectrum made since March 2018 by the next‐generation Spectral Irradiance Monitor (SIM) instrument on the Total and Spectral Solar Irradiance Sensor‐1 (TSIS‐1) satellite mission and the Compact SIM (CSIM) technology demonstration mission. The TSIS‐1 SIM and CSIM have order‐of‐magnitude reduction in uncertainty relative to predecessor instruments primarily because of a first‐of‐its‐kind spectral radiometric calibration facility capable of characterizing the instruments to higher fidelity. We develop this new, high‐resolution, solar irradiance reference spectrum by adjusting high spectral resolution solar line data to the irradiance scale of the more accurate, but lower spectral resolution, TSIS‐1 SIM and CSIM observations. Key Points: The TSIS‐1 Spectral Irradiance Monitor and Compact SIM instruments observe the Sun's irradiance spectrum at high accuracyThe TSIS‐1 Hybrid Solar Reference Spectrum consists of high resolution solar line data normalized to the TSIS‐1 SIM irradiance spectrumThe TSIS‐1 Hybrid Solar Reference Spectrum has at least 0.01 nm spectral resolution, spans 202–2730 nm, and is accurate to 0.3%–1.3% [ABSTRACT FROM AUTHOR]

The Earth system responds to solar variability on a wide range of timescales. Knowledge of total solar irradiance (TSI) and solar spectral irradiance (SSI) spanning minutes to centuries is needed by scientists studying a broad array of research applications. For these purposes, the NOAA National Centers for Environmental Information (NCEI) Climate Data Record Program established the Solar Irradiance Climate Data Record. Version 2 of the Naval Research Laboratory's solar variability models that are derived from and demonstrate consistency with irradiance observations specifies TSI and SSI for the Solar Irradiance Climate Data Record. We establish the veracity of the Naval Research Laboratory models on the timescales and over the wavelength range for which the Sun is known to vary and, thereby, specify the utility of these models. Through comparisons with irradiance observations and independent models, we validate NRLTSI2 estimates of TSI on solar rotational (~27‐day), solar cycle (~11‐year), and multidecadal (spacecraft era) variability timescales. Similarly, we validate NRLSSI2 estimates of SSI rotational variability in the ultraviolet through the mid‐visible spectrum. Validation of NRLSSI2 estimates at longer wavelengths, particularly in the near‐infrared, and for the full spectrum at solar cycle timescales and longer is not possible with the current observational record due to instrumental noise and instrument instability. We identify where key new data sets, such as observations from the Total and Spectral Solar Irradiance Sensor‐1, are expected to provide a fuller understanding of total and spectral solar irradiance variability on multiple timescales. Plain Language Summary: An understanding of total and spectral solar irradiance is essential for Earth atmospheric and climate studies because the Sun's energy incident at the top of Earth's atmosphere is the dominant energy source driving a myriad of interactions that establish Earth's climate. We describe how the National Oceanographic and Atmospheric Administration's Solar Irradiance Climate Data Record is meeting the goal to model variability in the Sun's irradiance, identify limitations in our current understanding of solar irradiance variability on multiple timescales and over a broad spectral range, and highlight where new observations are expected to provide a fuller understanding. Key Points: The Naval Research Laboratory's solar variability models, NRLTSI2 and NRLSSI2, establish the Solar Irradiance Climate Data Record (CDR)The CDR total and spectral (between 265 and 500 nm) solar rotational estimates are validated by observations on 27‐day solar rotationsOn solar cycle timescales and longer, particularly in the spectrum, differences in observational data sets preclude model validation [ABSTRACT FROM AUTHOR]

The stability and longevity of the Ozone Monitoring Instrument (OMI), flying on board the (active) Aura Earth‐observing satellite since July 2004, facilitates creation of accurate, long‐term record of relative (normalized to a solar minimum) solar spectral irradiances (SSIs). Here we discuss technical details of the most recent version (V3) of the SSI product that provides approximately daily measurements for the period July 2006 to April 2018 in the 265‐ to 500‐nm domain with average 0.5‐nm resolution. We compare OMI SSIs with concurrent independent observations and model estimates. The short‐term (solar rotational cycle) observations and model predictions mostly agree to ∼0.1–0.2% in the ultraviolet domain, with an excellent, down to ∼0.01% agreement in the visible range. The long‐term (solar cycle) comparisons pose more challenges in the ultraviolet domain, where the differences between observations and models frequently exceed the rather conservative ∼0.1% (both point‐to‐point and long‐term) OMI uncertainties. In the visible range, these differences gradually diminish to <0.05% yet again pointing to reliability and robustness of the amassed SSI data in this domain. Key Points: We describe the new version (V3) of the OMI solar spectral irradiances that provides a record of Solar Cycle 24 variability in the 265‐ to 500‐nm rangeThe short‐term (solar rotation) SSI variations: Comparison with the concurrent independent observations and model predictions show good‐to‐excellent (predominantly, to ∼0.1% in UV, and down to ∼0.01% in the visible) agreement between the various data setsThe long‐term (solar cycle) SSI variations in the UV domain pose serious challenges, with model‐observation and observation‐observation disagreements frequently exceeding 0.5%. In the visible the inter‐comparisons show good agreement, down to ∼0.05% [ABSTRACT FROM AUTHOR]

The response of the Northern Hemisphere winter stratosphere to the Pacific decadal oscillation (PDO) is examined using the Whole Atmosphere Community Climate Model. A 200-yr preindustrial control simulation that includes fully interactive chemistry, ocean and sea ice, constant solar forcing, and greenhouse gases fixed to 1850 levels is analyzed. Based on principal component analysis, the PDO spatial pattern, frequency, and amplitude agree well with the observed PDO over the period 1900-2014. Consistent with previous studies, the positive phase of the PDO is marked by a strengthened Aleutian low and a wave train of geopotential height anomalies reminiscent of the Pacific-North American pattern in the troposphere. In addition to a tropospheric signal, a zonal-mean warming of about 2 K in the northern polar stratosphere and a zonal-mean zonal wind decrease of about 4 m s−1 in the PDO positive phase are found. When compositing PDO positive or negative winters during neutral El Niño years, the magnitude is reduced and depicts an early winter forcing of the stratosphere compared to a late winter response from El Niño. Contamination between PDO and ENSO signals is also discussed. Stratospheric sudden warmings occur 63% of the time in the PDO positive phase compared to 40% in the negative phase. Although this sudden warming frequency is not statistically significant, it is quantitatively consistent with NCEP-NCAR reanalysis data and recent observational evidence linking the PDO positive phase to weak stratospheric vortex events. [ABSTRACT FROM AUTHOR]