Solar irradiance

Solar irradiance is a measure of the irradiance (power per unit area on the Earth's surface) produced by the Sun in the form of electromagnetic radiation, which is perceived by humans as sunlight. Solar irradiance in a specific area may be measured as insolation, the solar radiation energy per unit area during a given time, or as direct insolation, insolation which reaches a location after absorption and scattering in the atmosphere. Total solar irradiance (TSI), is a measure of the solar radiative power per unit area normal to the rays, incident on the Earth's upper atmosphere. The solar constant is a conventional measure of mean TSI at a distance of one Astronomical Unit (AU).

The Sun's energy output varies over the solar cycle. Solar irradiance "varies significantly from one place to another and changes throughout the year" as well as by the Earth's changing distance from the sun. Sunlight only reaches the parts of the Earth that are facing the Sun at any given time. The most intense irradiance is experienced by those parts that are approximately normal to the Sun as the Earth turns.

Measurement

All TSI satellite instruments employ a common approach, active cavity electrical substitution radiometry. This technique applies measured electrical heating to maintain an absorptive blackened cavity in thermal equilibrium while incident sunlight passes through a precision aperture of calibrated area. The aperture is modulated via a shutter. In orbit, radiometric calibrations drift for reasons including solar degradation of the cavity, electronic degradation of the heater, surface degradation of the precision aperture and varying surface emissions and temperatures that alter thermal backgrounds. These calibrations require compensation to preserve consistent measurements.

The space-based TSI record comprises measurements from more than ten radiometers spanning three solar cycles. For various reasons, the sources do not always agree. The Solar Radiation and Climate Experiment/Total Irradiance Measurement (SORCE/TIM) TSI values are lower than prior measurements by the Earth Radiometer Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS), VIRGO on the Solar Heliospheric Observatory (SoHO) and the ACRIM instruments on the Solar Maximum Mission (SMM), Upper Atmosphere Research Satellite (UARS), and ACRIMSat. Ground calibrations relied on component rather than system level measurements, since irradiance standards prior to their launches lacked absolute accuracies.

Uncertainties of individual irradiance observations exceed solar irradiance variations (∼0.1%). Thus, instrument stability and measurement continuity are relied upon to compute real variations. Instrument stability involves exposing redundant radiometer cavities to different accumulations of solar radiation to quantify exposure-dependent degradation effects that are then compensated for in final data. Sequential radiometer observation overlaps permits corrections for absolute offsets and validation of instrumental drifts.

Despite the fact that ACRIM I, ACRIM II, ACRIM III, VIRGO, and TIM all track degradation with redundant cavities, notable and unexplained differences remain in irradiance and the modeled influences of sunspots and faculae. Features not easily attributable to solar activity include an annual cycle that is nearly in phase with the Sun-Earth distance in ACRIM III data, and 90-day spikes in the VIRGO data coincident with SoHO spacecraft maneuvers that are most apparent during the 2008 solar minimum. Disagreement among overlapping observations indicates unresolveded drifts that suggest the TSI record is not sufficiently stable to discern solar changes on decadal time scales. Only the ACRIM composite shows irradiance increasing by ∼1 W m−2 between 1986 and 1996; this change is also absent in the model.

Recommendations to resolve the instrument discrepancies include validating optical measurement accuracy by comparing ground-based instruments to laboratory references, such as those at National Institute of Science and Technology (NIST); NIST validation of aperture area calibrations using spares from each instrument; and applying diffraction corrections from the view-limiting aperture.

For ACRIM, NIST determined that diffraction from the view-limiting aperture contributes a 0.13% signal not accounted for in the three ACRIM instruments. This correction lowers the reported ACRIM values, bringing ACRIM closer to TIM. In ACRIM and all other instruments, the aperture is deep inside the instrument, with a larger view-limiting aperture at the front. Depending on edge imperfections this can directly scatter light into the cavity. This design admits two to three times the amount of light intended to be measured; if not completely absorbed or scattered, this additional light produces erroneously high signals. In contrast, TIM's design places the precision aperture at the front so that only desired light enters.

TSI Radiometer Facility

TIM's high absolute accuracy creates new opportunities for measuring climate variables. TSI Radiometer Facility (TRF) is a cryogenic radiometer that operates in a vacuum with controlled light sources. L-1 Standards and Technology (LASP) designed and built the system, completed in 2008. It was calibrated for optical power against the NIST Primary Optical Watt Radiometer, a cryogenic radiometer that maintains the NIST radiant power scale to an uncertainty of 0.02% (1σ). As of 2011 TRF was the only facility that approached the desired <0.01% uncertainty for pre-launch validation of solar radiometers measuring irradiance (rather than merely optical power) at solar power levels and under vacuum conditions.

TRF encloses both the reference radiometer and the instrument under test in a common vacuum system that contains a stationary, spatially uniform illuminating beam. A precision aperture with area calibrated to 0.0031% (1σ) determines the beam's measured portion. The test instrument's precision aperture is positioned in the same location, without optically altering the beam, for direct comparison to the reference. Variable beam power provides linearity diagnostics, and variable beam diameter diagnoses scattering from different instrument components.

The Glory/TIM and PICARD/PREMOS flight instrument absolute scales are now traceable to the TRF in both optical power and irradiance. The resulting high accuracy reduces the consequences of any future gap in the solar irradiance record.

Difference Relative to TRF

Instrument	Irradiance: View-Limiting Aperture Overfilled	Irradiance: Precision Aperture Overfilled	Difference Attributable To Scatter Error	Measured Optical Power Error	Residual Irradiance Agreement	Uncertainty
SORCE/TIM ground	NA	−0.037%	NA	−0.037%	0.000%	0.032%
Glory/TIM flight	NA	−0.012%	NA	−0.029%	0.017%	0.020%
PREMOS-1 ground	−0.005%	−0.104%	0.098%	−0.049%	−0.104%	∼0.038%
PREMOS-3 flight	0.642%	0.605%	0.037%	0.631%	−0.026%	∼0.027%
VIRGO-2 ground	0.897%	0.743%	0.154%	0.730%	0.013%	∼0.025%

Applications

Solar irradiance is useful for capacity planning for solar power installations.

Solar activity and irradiance measurement is a concern for space travel. For example, the American space agency, NASA, launched its Solar Radiation and Climate Experiment (SORCE) satellite with Solar Irradiance Monitors.

Climate research

Irradiance plays a part in climate modeling and weather forecasting.

Instrument inaccuracies add a significant uncertainty in determining Earth's energy balance. A non-zero average global net radiation at the top of the atmosphere is indicative of Earth's thermal disequilibrium as imposed by climate forcing. Whereas the energy balance is nominally 0.85 W m−2, estimates of this quantity from space-based measurements range from 3 to 7 W m−2. SORCE/TIM's lower TSI value reduces this discrepancy by 1 W m−2. This difference between the new lower TIM value and earlier TSI measurements corresponds to a climate forcing of −0.8 W m−2, which is comparable to the energy imbalance.

The impact of the new lower TSI value on climate models is unknown. A few tenths of a percent change in the absolute TSI level is typically considered to be of minimal consequence for climate simulations. The new measurements require climate model parameter adjustments. Experiments with GISS Model 3 investigated the sensitivity of model performance to the TSI absolute value during present and pre-industrial epochs, and describe, for example, how the irradiance reduction is partitioned between the atmosphere and surface and the effects on outgoing radiation.

Accuracy uncertainties of <0.01% are required to detect long term solar irradiance variations, because expected changes are in the range 0.05 to 0.15 W m−2 per century.

Long-term radiometer drifts can be mistaken for irradiance variations that can be misinterpreted as affecting climate. Examples include the issue of the irradiance increase between cycle minima in 1986 and 1996, evident only in the ACRIM composite (and not the model) and the low irradiance levels in the PMOD composite during the 2008 minimum. Assessing the impact of long-term irradiance changes on climate requires greater instrument stability.

Long-term measurement accuracy combined with reliable global surface temperature observations are necessary for quantifying climate response processes to radiative forcing on decadal time scales. The observed 0.1% irradiance increase imparts 0.22 W m−2 climate forcing, which suggests a transient climate response of 0.6 °C per W m−2. This response is larger by a factor of 2 or more than in the IPCC-assessed 2008 models, possibly appearing in the models' heat uptake by the ocean.

The most probable value of TSI representative of solar minimum is 1360.8 ± 0.5 W m−2, lower than the earlier canonical value of 1365.4 ± 1.3 W m−2 as measured by SORCE/TIM. In addition to the offsets, published irradiance observations lack coherent temporal structure. A regression model-based split of the relative proportion of sunspot and facular influences from SORCE/TIM data accounts for 92% of observed variance and tracks the observed trends to within TIM's stability band. This agreement provides further evidence that TSI variations are primarily due to solar surface magnetic activity.

In 2014 a new ACRIM composite was developed using the updated ACRI M3 record. It added corrections for scattering and diffraction revealed during recent testing and two algorithm updates. The testing was performed at TRF. The algorithm updates were more accurately account for instrument thermal behavior and parsing of shutter cycle data. These corrected a component of the quasi-annual signal and increased the signal to noise ratio, respectively. The net effect of these corrections decreased the average ACRIM3 TSI value from as above without affecting the trending in the ACRIM Composite TSI. Differences between ACRIM and PMOD TSI composites are evident, but the most significant is the solar minimum-to-minimum trends during solar cycles 21-23. ACRIM established an increase of +0.037%/decade from 1980 to 2000 and a decrease thereafter. PMOD instead presents a steady decrease since 1978. Significant differences can also be seen during the peak of solar cycles 21 and 22. These arise from the fact that ACRIM uses the original TSI results published by the satellite experiment teams while PMOD significantly modifies some results to conform them to specific TSI proxy models. . The implications of increasing TSI during the global warming of the last two decades of the 20th century are that solar forcing of climate change may be a significantly larger factor than represented in the CMIP5 general circulation climate models.

See also

• Earth's energy budget
• PI curve (photosynthesis-irradiance curve)
• Irradiance (irradiance)