Pomme-7
Contents:
Overview
The plug-in computes the POMME-7
main magnetic field model at the position of a satellite. This is a QSAS wrapper to the
C code provided by Stefan Maus.
This model performs better than IGRF or Tsyganenko-96 model at Champ's altitudes or Cluster's
position in the auroral acceleration regions.
Below is the important information from the website of the POMME-7 model:
POMME is a scientific main field model representing the geomagnetic field in the region
from the Earth's surface to an altitude of a couple of thousand kilometers. The time
variations of the internal field are given by a piece-wise linear representation of the spherical
harmonic (Gauss) coefficients of the magnetic potential. Pomme-7 was produced from CHAMP satellite
vector magnetic measurements from July 2000 up to September 2010 and Oersted satellite total
field measurements from January 2010 to October 2011. It has the same parametrization of the
magnetospheric field as POMME-6.
The model is more accurate if the present state of the magnetosphere is provided as an input in
the form of magnetic indices. The magnetospheric ring current is tracked by Est/Ist (the default
values for Est and Ist are both 0.0). The Interplanetary magnetic field and F10.7 index are
available from OMNIWEB (default IMF-BY = 0.0). The merging electric field Em is a parameter
derived from the solar wind measurements by: Em (in mV/m) = V * BT * sin(beta/2.0) * sin(beta/2.0) / 1000,
where V is the solar wind flow speed in km/s, BT is the strength of the interplanetary magnetic
field perpendicular to the Sun-Earth axis and beta is the clock angle acos(Bz/BT). A default value
of 0.5 mV/m is to be used if the value of Em is not known. The model uses the 81-day average value
of the F10.7 solar flux, 20 months prior to the given date, to track changes in the ring current base
line with solar cycle progression (F10.7 default value is 120.0).
Input data
The plug-in requires the following inputs:
- the position of a satellite. This variable must be a time series of vectors with the well defined
COORDINATE_SYSTEM attribute. If the coordinate system is different from the geographic (GEOC), then
the CoordRotation plug-in is automatically used to rotate the vectors into the geographic frame.
The variable must also have units of distance.
- the time range of interest. If it is desired to produce the magnetic field for all positions, then
the position variable can be dropped here.
Further parameters describe the influence due to external sources.
- IMF By, the y component of the interplanetary magnetic field in GSM coordinates.
The value must be given in units of nT. OMNIWeb values are given at the magnetopause and the delay of 35 minutes
should be considered. If the measurements from the ACE satellite are taken, then further adjustments should be made.
- Em, the merging electric field 60 minutes earlier and given in units mV/m.
- E_st and I_st indices in units of nT without any time delay.
- F10.7, the 81-day average of the solar flux, taken 20 months before the needed time of the model.
The plug-in does not check the units of the external parameters. If default values are left, then the external contribution
is ignored. The last parameter
- order of the model. POMME-7 can be computed with the order up to 133. Even the order 40 outperforms Tsyganenko'96 model up to a few
Earth radii from Earth's center.
The plug-in returns the modelled magnetic field at the spacecraft position. The magnetic field vector is rotated into the same frame
as the initial frame of the position vector. The model can be directly subtracted from the magnetic field measurement vectors given by
the satellite.
Acknowledgement
We aknowledge the DFG VO 855/3 grant: "Multi-spacecraft analysis toolkit for Swarm".
Eugen Sorbalo, Jacobs University, 21 May 2012