A Bayesian Model for Inferring Total Solar Irradiance From Proxies and Direct Observations: Application to the ACRIM Gap

Differences among total solar irradiance (TSI) estimates are most pronounced during the so-called "ACRIM Gap" of 1989-1991, when available satellite-based observations disagree in trend and no observations exist from satellites with on-board calibration. Different approaches to bias-correcting noisy satellite-based observations lead to discrepancies of up to 0.7 W/m² in the change in TSI during the Gap. Using a Bayesian hierarchical model for TSI (BTSI), we jointly infer TSI during the ACRIM Gap from satellite-based observations and proxies of solar activity. In addition, BTSI yields estimates of noise and drift in satellite-based observations and calibration for proxy records. We find that TSI across the ACRIM Gap changes by only 0.01 W/m², with a 95% confidence interval of [-0.07, 0.09] W/m². Our results are consistent with the PMOD CPMDF and Community Consensus TSI reconstructions and inconsistent with the 0.7 W/m² trend reported in the ACRIM composite reconstruction. Constraints on the trend across the ACRIM Gap are primarily obtained through constraints on the drift in the Nimbus-7 satellite that are afforded by overlapping satellite and proxy observations.