Climatologists study records of Earth's weather to calculate averages and find trends and patterns.I wanted to do the same with space weather to find out how geomagnetic activity varies by year, month, and day. The British Geological Survey (BGS) has been keeping records of geomagnetic activity since 1932. So I imported this 86 years worth of data into a spreadsheet to analyze it, and break it down into graphs and charts. The Royal Observatory of Belgium - SILSO has been recording sunspot numbers for even longer since the 1700's. The effects of the solar cycle become clear in the first 6 graphs below.

What is the Solar Cycle?

For centuries it has been known that the sun follows a regular pattern of high and low solar activity. This period averages 11 years in length, but can vary from 9 to 13 years long. Solar maximum is defined as the month with the highest sunspot number. Conversely, solar minimum is defined as the month with the lowest sunspot number. Learn how the sunspot number is counted here. Once solar minimum is reached, that marks the end of one solar cycle and the beginning of the next. As of right now, we are approaching the end of solar cycle 24, which started in December of 2008. The maximum occurred in April of 2014 and the next minimum is expected in 2019 or 2020, marking the start of solar cycle #25.

Visit the Kp Index Explained page to learn how it relates to seeing the aurora.

By Year

After looking at this data, a few things become apparent:

  1. The current solar cycle (#24) is very lame. Solar maximum in 2014 was the weakest since 1906. Solar minimum in 2008 was the quietest since 1823 (the Dalton Minimum). No other solar cycle on record has had so few geomagnetic storms. What does this mean for the future and the next solar cycle? Many predictions have been made, but no one really knows for certain.
  2. The years of greatest geomagnetic activity do not coincide with solar maximum. Instead they occur 3 years afterward on average. The years of lowest geomagnetic activity almost always occur 1 year after solar minimum.
  3. The last truly active year was in 2003. According to this map the northern lights would have been visible from the latitudes of Calgary, the Minnesota-Canada border, and southern Scandinavia on an average night. From locations like Vancouver, Minneapolis, Montreal, northern UK, and southern New Zealand the aurora would have appeared 1 out 3 nights on average (not considering weather).
  4. There are plenty of anomalies. The sun isn't always predictable. Some solar cycles have double peaks, and some years are more or less active than might be expected.

By Month

The numbers above will obviously vary depending on the solar cycle. But still, it's clear that geomagnetic storms are more common during the spring and fall months than they are in summer and winter. For instance a strong geomagnetic storm of Kp7 or above is almost 4x more likely to occur in April compared to December. The reason for this likely has to do with Earth's tilt. During the equinoxes, Earth's axis is 0° relative to the ecliptic, and the sun's magnetic field lines can better connect with Earth's magnetic field. But during the solstices the Earth is tilted 23.4° which makes it harder for their magnetic fields to link up.

Personally, the end of September has always been my best time of year to see the aurora (9/27/2017)

This seasonal variation matches my personal experience as well while aurora hunting. I've never managed to see the northern lights in December, January, or July. But since 2012 I've seen the aurora every single year at the end of September or beginning of October, with a total of 10 sightings during that time period.

Most Active Month On Record

 February of 1982 had an average daily Kp index of 5.79. There were geomagnetic storms every single day of the month except for 4. Twice there were 10 days in a row with geomagnetic storms. But oddly enough the Kp index never exceeded 7.

Least Active Month On Record

December of 2009 had an average daily Kp index of just 0.06. There were 25 days in a row in which the Kp index never rose above 0. During this time the northern lights would have been virtually non-existent unless you were directly underneath the auroral oval. And even there it would have been difficult to see.

By Day

Most Geomagnetically Active Days of the Year

    Date.........................Average Kp Index
  1. April 4......................3.70
  2. March 27.................3.53
  3. April 3......................3.52
  4. March 29.................3.48
  5. March 1...................3.47
  6. April 1......................3.45
  7. September 27.........3.44
  8. September 26.........3.43
  9. May 2......................3.42
  10. March 26................3.38

Least Geomagnetically Active Days of the Year

    Date.........................Average Kp Index
  1. June 21...................1.94
  2. December 24..........2.01
  3. December 6............2.05
  4. July 3......................2.06
  5. January 9................2.07
  6. August 8..................2.07
  7. December 7.............2.07
  8. December 9.............2.07
  9. June 22....................2.09
  10. July 2........................2.09

On April 4, there is a 38% chance of a geomagnetic storm occuring. On June 21st, there is only a 12% chance. But don't read too much into exact dates. One of the best aurora displays I've ever seen happened to occur on one of the least active days historically.

If you are planning a trip to see the aurora hopefully this data will help you decide when to go and have the highest success rate. Of course there are other factors to consider as well such as climate and length of day/night. Watch out for my next post, "10 Best Places in the World to See the Aurora."

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