Solar Trends: Total Solar Irradiance Since 1611 – Update

This is the 2nd in a series of  posts I will be doing on solar trends. In this post, I show how to retrieve online daily satellite and reconstructed TSI data,   plot the daily data as well as annual and 11 year moving average smooths for the data series.  Links to the original data sources and my R script Google document are provided. Updated 10/1/09.

Background on Solar Radiation

In this previous post, I showed the sunspot trend and solar cycle numbers from 1750 to 2009. In this post, I want to investigate total solar irradiance (TSI) online data from satellite observations and TSI reconstructions. Let’s start with a definition of TSI.

“Total solar irradiance describes the radiant energy emitted by the sun over all wavelengths that falls each second on 1 square meter outside the earth’s atmosphere …. It measures the solar energy flux in Watts/square meter.” National Geophysical Data Center

“… TSI was previously known as solar constant.  … TSI variability is so weak that earlier measurements from the ground were unable to detect it, the reason the value was called ‘constant’. Since 1978 we know it is not constant, and ‘TSI’ is more common.” E-mail communication – N Krivova, Max Planck Institute

Here’s a link to an excellent tutorial that I have used to gain a rudimentary understanding of solar radiation. The tutorial’s 2nd slide provides a graphic and  definition of the solar constant.

“The amount of solar radiation received by a surface at right angles to the sun at the top of the earth’s atmosphere has an average value of about 1400 watts per square meter, enough to drive a small hairdryer. This value is called the solar constant” Source: Rita D. Haberlin, Geography Professor, Peralta Community College District

It is important to note that the TSI measures the amount of radiation reaching the top of the atmosphere, not at the surface of the earth.

Interested readers can get additional scientific background information on the solar constant and TSI at this Wikipeida link.

TSI Data Sources

There are two sources of TSI data, satellite measurements since November, 1978 and TSI reconstructions dataing back to 1611.

Daily satellite based TSI data since November, 1978 is available at this link maintained by the PMOD – WRC.    Here’s a snapshot of the first few rows of data, after 58 lines of documentation.  There are 3 data columns, the date in “yymmdd” format, number of days since base data, and TSI measurement in w/m^2. Missing values are noted with -99.0000.

There are a number of TSI reconstructions available online. Leif Svalgaard has a downloadable Excel spreadsheet that provides annual data for 10 TSI  reconstructions.  Since I wanted daily values, I selected the Max Planck Institute for Solar System Research website for my reconstructed TSI estimates, their daily TSI ascii file is here.

Here’s a snapshot of the first few rows of the Max Planck TSI reconstruction file:

This file includes 5 columns: year, month, day, TSI estimate (w/m^2), and TSI deviation from 1/1/1700.

TSI Trend Chart Since 1611

Here’s my R based plot of the daily reconstructed and satellite measured TSI values from 1611 until Sept. 6, 2009, with annual average and 11 year moving averages. This is an updated version of my original chart based on e-mail comments from N Krivova of Max Planck Institute. I removed the lowess smooth and added the annual average and 11 year moving average lines.

This chart includes several items:

• Reconstructed TSI estimates –   yellow
• Satellite Based TSI measurements – goldenrod
• Annual average TSI
• 11 year moving average TSI
• Designation of Maunder and Dalton Minimum periods – grey arrow lines

The solar cycles are quite obvious in the chart. Just like in the sunspot chart, each TSI cycle starts at a minimum, rises rapidly and then gradually decreases to a minimum before the start of a new cycle.   We will be charting solar radiation data in future posts to explore the relationship between solar cycles and climate.

The low TSI values in the Maunder Minimum (1645-1715) are quite apparent. TSI’s cyclically rose after the Maunder Minimum, with a noticeable dip during the Dalton Minimum (1790-1830). In my original post, I used a lowess smooth which introduced a discontinuity between the reconstruction and satellite series. The 11 year moving average eliminates the discontinuity.

The satellite series shows a gradual decline in TSI measurements in the 1979-2009.

Online Resources

• Daily Satellite TSI Measurements: PMOD-WRC Data File
• Daily Reconstructed TSI Estimates:  Max Planck institute text file
• R script to generate TSI chart:  Google document

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