Nowcast Formulas for b2i 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 the magnetosphere. GOES magnetometer data is available real-time from NOAA’s space data center.

The b2i parameter is a DMSP particle-based boundary which is also related to magnetotail stretching. The b2i particle boundary is used as a parameter in OVATION. If b2i can be estimated in the absence of DMSP particle, data, then the oval position can be roughly guessed.

When b2i and the GOES data are measured at the same local time (MLT, or magnetic local time), they have a relatively high correlation (around 0.8). However magnetotail stretching varies with local time, and each local time can change independent of other local times. Ideally, a global measurement of magnetotail stretching would sample many local times. This is not likely anytime soon.

Since both b2i and GOES are operationally measured each at a single (and different) MLT, the correlation is only 0.4 to 0.5). This is still enough to give a guess for the oval position when no DMSP data is present.

Empirical Formulas for the GOES to b2i Relationship

The following equations were developed under UPOS to convert the GOES nightside magnetometer readings into b2i, to be supplied to OVATION. When GOES is not at a listed local time, these formulas cannot be applied. A running average over the previous 1 hour is used for the GOES data.

In terms of estimating the auroral position, these correlation coefficients understate the quality of the GOES data. A good portion of the variance is due to comparing b2i and GOES data from different MLTs. The correlations are substantially higher when b2i and GOES from the same MLT are compared.

That is, the GOES data will predict oval position best at the local time of the measurement (but so does a single DMSP measurement).

T = magnetotail inclination angle in degrees T = atan(v/h)

v,h are GOES magnetometer measurements

b2i = a + b*T Kp is a number between 0 and 9

GOES

Local Time a b corr. coeff. with b2i

16 0.030 60.0 0.17

17 0.061 55.5 0.32

18 0.082 52.3 0.44

19 0.094 50.7 0.51

20 0.093 50.9 0.52

21 0.082 52.6 0.51

22 0.074 53.8 0.48

23 0.065 55.2 0.46

00 0.071 54.3 0.47

01 0.077 53.5 0.47

02 0.080 53.0 0.48

03 0.083 52.5 0.46

04 0.090 51.4 0.47

05 0.076 53.4 0.41