Waiting for the big one


We are now more than 5 years in the current solar cycle (SC), and so far we have not experienced a single extreme geomagnetic storm.

There exist several parameters to gauge the level of disturbance of the Earth's magnetic field. One of the most commonly used is the Kp index. "Kp" is short for "planetarische Kennziffer" (German for "planetary index"). It was introduced by Julius Bartels in 1949 and extended back to 1932. This 3-hour range index is derived from magnetic recordings done in 13 subauroral observatories (see map underneath). Note that over time, a few of the original stations have dropped out, while a few others have been added.

The Kp index is a geomagnetic range index derived from weighted local K indices of contributing geomagnetic observatories. A local K index is obtained by taking the difference between maximum and minimum of the horizontal magnetic field over a three-hour time interval.
It is compensated for diurnal and seasonal variations as observed at times of the quiet Sun, and scaled such that the K occurrence distributions from all observatories are maximally equal.
In contrast to the local K index which takes only integer values between 0 (very quiet) and 9 (extremely disturbed), the Kp index is expressed in fractions of approximately thirds, usually written as 0o,0+,1-,1o,1+,2-, ... 9-,9o.
The scale is open-ended, i.e. no matter how large the geomagnetic variation is, Kp will never exceed 9. K and Kp are quasi-logarithmic indices, therefore computation of certain statistical measures such as average values has no meaning.


A quick analysis of the final Kp indices as archived at the Kyoto World Data Centre (WDC) for geomagnetism reveals that the current solar cycle (SC24) is really underperforming so far. Not only has there not been any day with extreme geomagnetic storming, SC24 also has a lot more "quiet" days compared to the average of the previous 7 solar cycles (SC17-23). Of course, most of those cycles had already passed their maximum for 1-2 years, whereas SC24 is peaking only now and at a much lower solar activity level.

For countries at a moderate or low geomagnetic latitude such as Belgium (46 degrees), one needs a Kp index equal to or near 9 (extreme storm) to appreciate the polar light in all its glory. Such was on several occasions the case during the last solar cycle. Picture underneath, taken in Belgium by Philippe Mollet, shows the polar light during the morning hours of 7 April 2000 when Kp values wear near 9. Note the red color of the aurora which, if present, usually defines the top of the polar light phenomenon (highest in the sky), hence it is often the first sign of aurora visibility from southern locations.

Of course, polar light can also be seen at these locations with a slightly lower Kp value, but the visibility quickly degrades as the aurora hardly rises above the horizon. An observing site far away from city lights and under a moonless, clear sky, might still provide a glimpse. Photography boosts chances as the longer exposure times may allow to catch the faint glow low above the horizon. Such was recently the case during the evening of 27 February, when an all-sky camera near the Belgian coast (Franky Dubois) recorded the diffuse red aurora for just a few minutes at a Kp of hardly 6. The visual display at that location was a non-event, but observers just a few 100 kilometers to the North (middle part of The Netherlands, England, Ireland,...) reported a nice display. With SC24 hardly half way, it is very likely we'll still get some more opportunities on a great and colorful show.

Credits - Data and additional info on the Kp index can be found at GFZ Potzdam.


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