Calibration and Analysis of Mount Wilson Magnetograms and Dopplergrams for the Years 1967 to 2013

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The Sun influences the Earth and its environment in a large range of phenomena, such as aircraft radiation levels, atmospheric drag on spacecraft, long-distance power transmission, pipeline charging and radio communication (especially at high latitudes), etc. Severe solar storms can disrupt global communications and damage spacecraft. These effects have been detailed in "Committee on the Societal and Economic Impacts of Severe Space Weather Events: A Workshop, National Research Council (2008)," and are further discussed in "Committee on a Decadal Strategy for Solar and Space Physics (Heliophysics): National Research Council (2013)." The most recent solar cycle 24 was of relatively low strength, and the upcoming cycle 25 is beginning slowly. Cosmic ray records, as recovered by proxy indicators, suggest that the solar cycle goes through long periods of strength and weakness lasting one or more centuries, which are called grand maxima and grand minima. In the past few decades, there has been considerable progress in understanding the mechanisms governing the Sun's magnetic cycle. In this process, comparisons between model output, including predicted cycle variability and the observed characteristics of the solar cycle, is an essential step in the validation of the models. This 3-year research project is aimed at increasing the number of available solar cycles from four to five by carrying out a new calibration process of the unique magnetogram data gathered by the Mount Wilson Observatory (MWO), thus improving their quality for further use by the broader solar-heliospheric community.The MWO, under the direction of G.E. Hale and H.W. Babcock, pioneered in the discovery and measurement of solar surface magnetic fields. From 1964 to 1985, under the direction of R.F. Howard, the visible solar disk magnetic fields were measured and digitally recorded on magnetic tape. The PI of this project, R.K. Ulrich, managed the program from 1986 to 2013. The full digital record spans 46 years, or about four solar cycles. For the years 1967 to 1973, a major part of solar cycle 20, no other set of regular magnetic field measurements is available. In April 1973 a program of daily magnetogram observations was started at the Kitt Peak National Observatory, and in May 1975 daily observations began at the Stanford Solar Observatory, now the Wilcox Solar Observatory (WSO), with data from space-based systems becoming available in 1995. All these data series, except that from the MWO, continue today. During this 3-year project, the PI will carry out a new calibration of the MWO magnetogram and Dopplergram series, which will take into account various changes in the instrument hardware, calibration data acquisition methods and software reduction methods. Initial analysis of the magnetograms from 1967-1969 shows that there are significant irregularities in the nominal calibration process. Also, all the magnetogram measurements depend on complex spectral line formation processes so that the derived fields are different for each of the series referenced above and, within each series, across jumps due to instrument and calibration changes. The resulting dataset of magnetograms consistently calibrated over an extended period of time is important for cross-calibrating (and identifying systematics in) the magnetic field measurements taken with other instruments during the same period of time. The use of solar surface magnetograms is required for the calculation and prediction of conditions in the heliosphere. An "Open Magnetic Field Flux Problem" has been noted in several studies, so that the calibration and interpretation of the magnetogram observations has become a central issue. This project will address this issue following up on the work carried out previously by the project team. The data products from various stages of the recalibration, including the final, best result will be retained and made available to the community and public through partnership with the Stanford Helioseismology Archive which is part of the JSOC system at Stanford University. The project team will work with the broader solar community to incorporate the MWO magnetograms into the ADAPT/WSA pipeline, which yields predicted properties of the solar wind. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.