Nowcast Formulas for Kp from GOES Magnetometer Data

Results from UPOS Project COX

January 2004

Patrick T. Newell

Johns Hopkins University Applied Physics Laboratory

Background:

When the GOES satellites are on the nightside of the Earth, the extent to which the Earth’s magnetic field has been stretched from its natural dipole inclination into a long magnetotail is measured by the GOES magnetometers. The more stretched the Earth’s magnetic field, the more active is the state of the magnetosphere. GOES magnetometer data is available real-time from NOAA’s space data center.

Kp is defined based on ground magnetometer stations, rather than actual space borne measurements. Nonetheless, there is a substantial relationship (correlations between 0.4 and 0.75) between what is inferred from the ground via Kp over a 3-hour interval, and what is measured over the last hour by GOES while on the nightside.

Empirical Formulas for the GOES to Kp Relationship

The following equations were developed under UPOS to convert the GOES nightside magnetometer readings into a nowcast Kp. Note that when GOES is not at one of the listed local times, these formulas cannot be applied. A running average over the previous 1 hour is used for the GOES data to reduce short term fluctuations. (The inclination angles themselves are averaged, rather than an inclination angle being calculated from an average magnetic field).

In terms of representing the state of the magnetosphere, these correlation coefficients understate the quality of the GOES data. Kp itself is only a rough measure of the magnetosphere’s state, while magnetotail stretching is a rather basic property of the Earth’s space environment. Thus the GOES “nowcast Kp” is actually an independent assessment of the magnetosphere’s condition, couched in the familiar Kp nomenclature.

T = magnetotail inclination angle

T = atan(v/h)

v,h are GOES magnetometer measurements

Kp = a*T + b

Kp is a number between 0 and 9

(T is measured in degrees in equations below)

GOES

Local Time a b corr. coeff. with actual Kp

16 -0.110 16.9 -0.75

17 -0.077 13.4 -0.52

18 -0.050 10.4 -0.52

19 -0.136 22.0 -0.69

20 -0.045 8.7 -0.47

21 -0.050 10.0 -0.52

22 -0.040 8.5 -0.16

23 -0.041 8.0 -0.42

00 -0.064 12.0 -0.55

01 -0.041 9.0 -0.16

02 -0.076 12.7 -0.60

03 -0.095 17.2 -0.63

04 -0.098 16.9 -0.53

05 -0.047 8.8 -0.41