Solar Trends: Sunspot Numbers Since 1749

This is the 1st in a series of  posts I will be doing on solar trends. In this post, I show how to retrieve online monthly sunspot data back to 1749, calculate average annual sunspot numbers (SSN),  plot the monthly and annual average SSN as well as a lowess smooth, add the Solar Cycle number to the plot and generate  a csv file that will be used in future posts. Links to the original data source, my annual SSN and cycle date Google spreadsheet files, and my R script Google document file are provided.

Introduction

In 1610, Galileo observed sunspots with his telescope.  He was the first European to record sunspots. Since then, sunspots have been studied extensively with daily records dating back to 1849 and reconstructions going back to 1610.

The sunspot number (SSN) is …

“calculated by first counting the number of sunspot groups and then the number of individual sunspots.”
“The “sunspot number” is then given by the sum of the number of individual sunspots and ten times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see.” Source: Marshall Space Flight Center

Monthly SSN are available on-line as a text file at this NASA  link.  The first few rows of the on-line file are reproduced below.

Each record includes the year, month number, SSN and deviation.

Sunspots Since 1749

Here’s a chart of the SSN since 1749 that I produced from the online file and my R script.

This chart includes several items:

• Monthly SSN -  yellow
• Calculated annual average SSN-  red
• Lowess smooth (f=0.2) of annual SSN – red
• Solar Cycle number

The sunspot cycles are quite obvious in the chart. Each cycle starts at a minimum, rises rapidly and then gradually decreases to a minimum before the start of a new cycle.  The average cycle duration is 11 years.  Sunspot cycles, also called solar cycles, reflect magnetic activity in the sun which ultimately affects radiation that the earth receives from the sun. We will be charting solar radiation data in future posts to explore the relationship between solar cycles and climate.

I obtained the solar cycle start and end months from the solen.info website and added the cycle number to the chart at the cycle midpoint to provide a cross reference between year and solar cycle.

Online Resources

• Monthly SSN: NASA text file
• Calculated annual average SSN:  Google spreadsheet
• Solar Cycle start – end months, mid-point: Google spreadsheet
• R script to generate Sunspot chart:  Google document

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