Why Kp is a poor indicator for auroral alerts

/ 2014-09-12

A question we often get asked at AuroraWatch UK is why don’t our alerts always match up with the estimated current Kp value. In the following post, which first appeared on his blog, Dr. Steve Marple discusses why Kp can be a poor indicator for auroral alerts.

What is Kp?

Kp is a an index to represent the planetary geomagnetic activity. The name originates from “planetarische Kennziffer” (German for planetary index). It has 10 main levels, numbered from 0 (no geomagnetic activity) to 9 (extreme geomagnetic activity). The intermediate ranges are subdivided with a + or – suffix, making 28 levels in total (0, 0+, 1-, 1, 1+, 2-, 2, 2+, … 8, 8+, 9-, 9).

The Kp index is calculated for three-hourly intervals, beginning at UT midnight. To calculate Kp the daily “solar-quiet” (Sq) variation is removed from the measurements of magnetic field strength. Then the difference between the largest and smallest values is computed. By looking up the difference in a conversion table the local K index can be found. Stations at different latitudes (or more properly magnetic latitudes) have different conversion tables.

There is a pronounced daily variation in the K index at a single station, with intervals close to local midnight being substantially more disturbed in comparison with those centered on local noon; see http://www-app3.gfz-potsdam.de/kp_index/conv_tab.html. For this reason the planetary Kp index is computed from magnetic variations recorded by 13 magnetometers located around the world.

Why Kp is a poor indicator for auroral alerts

The Kp index only indicates geomagnetic activity within a 3 hour interval, which is too long to be useful for auroral alerts. Geomagnetic conditions can have recovered to a calm state long before the next Kp index is computed. This is particularly true for aurora caused by a substorm, as the entire substorm cycle (including the growth, expansion and recovery phases) is typically 2 to 4 hours duration; see http://www.ann-geophys.net/29/2031/2011/angeo-29-2031-2011.html.
It must also be noted that the official Kp values are not available in real-time. Not only must the data from the 13 stations be collected the quiet-day curves for each of the magnetometers must be calculated for that calendar month, a process which cannot begin until the month ends. Therefore any references to Kp for the current month will be to unofficial estimates. The most reliable estimate of Kp is probably obtained from NOAA, which is derived from a worldwide distribution of magnetometers, see http://www.swpc.noaa.gov/products/planetary-k-index.

In summary: Kp is not available in real-time. The 3 hour interval means current auroral activity may be much lower than an estimated Kp value suggests.

Estimated Kp can differ significantly from the local K index

As noted above, the pronounced daily variation of the K index means that there can be considerable difference between the average planetary geomagnetic activity (Kp) and that observed locally (K). The plot below shows the difference between the local K index measured by the British Geological Survey magnetometer at Eskdalemuir and Kp.

Difference between Eskdalemuir local K index and Kp
Difference between K index measured at Eskdalemuir and Kp.

The plot shows that in general there is good agreement but at certain times the K index can be higher or lower than Kp.

In summary: global measurements of Kp are not always valid indications of  regional geomagnetic activity.

What measurements should be used?

One of the better measurements is using the K index as long as it is from a nearby magnetometer or one on a similar geomagnetic longitude. It is still limited by the 3 hour resolution of the K index so it will not always be accurate for real-time measurements.

An alternative measurement is to consider the rate of change of the magnetic field, commonly referred to by its mathematical notation dB/dt (“D B by D T”). This measurement is of particular interest to operators of pipelines and long power distribution networks since it indicates the levels of geomagnetically-induced currents which might be expected. British Geological Survey publish real-time dB/dt measurements. These measurements require magnetically quiet sites as human interference is likely to cause sudden spikes with a high dB/dt value, limiting its usefulness to observatory measurements.

AuroraWatch UK publishes a real-time auroral activity measurement. It is the H component deviation, which is the difference between the current H component magnetic field strength and the expected value for the time of day taken from the “quiet-day curve”. Unlike Kp this measurement is computed hourly and so can indicate a return to low activity values sooner than Kp. An accurately fitted quiet day curve is required to make the deviation measurement.

Data credits

Eskdalemuir K indices were downloaded from the British Geological Survey, https://www.bgs.ac.uk/discoverymetadata/13480122.html. Kp data was downloaded from GFZ-Potsdam, ftp://ftp.gfz-potsdam.de/pub/home/obs/kp-ap/tab/.

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