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MFE Science Objectives

Polar Orbit.

The MFE provides measurements of the vector magnetic field in space along the 9 Re X 2 Re highly inclined, elliptical orbit of the Polar spacecraft. This orbit was designed to probe the northern high latitude magnetosphere in the region of the cusp around apogee, and the southern auroral acceleration regions around perigee. The magnetometer is sensitive enough to measure perturbations produced by currents and waves as small as 10 nT in a 20,000 nT field (typical at perigee), and 100 picoTeslas in a 100 nT field (typical at apogee). Its high time resolution mode (100 vectors/s sampling in 'snapshot' intervals) allows spatial structures as narrow as 160m to be resolved near spacecraft perigee where Polar is traveling at ~8 km/s speed. In 2001 the line of apeides has precessed from over the poles to in the ecliptic plane so that POLAR now makes measurements in the near Earth plasma sheet and the subsolar magnetopause regions.

Apogee Measurements

The magnetospheric cusp is the region where the field topology changes from a simple dipole compressed by the incident solar wind to the polar caps where the field is highly distorted by the solar wind interaction. In addition to currents related to these field

Magnetosphere model.

distortions, this region includes current sheets that form as a result of stresses associated with interconnections between the magnetic field of the Earth and the passing interplanetary field, and slippage between the corotating planetary field and fields that are controlled at high altitude by the externally imposed forces. It is thus a place where much of the energy transfer from the solar wind to the magnetosphere occurs. Both the configuration of the magnetosphere and the amount of energy transfer from the solar wind are known to be highly sensitive to the orientation of the interplanetary magnetic field, the solar wind velocity, and the solar wind dynamic pressure. Understanding the physical details of how these control the coupling between the magnetosphere and the solar wind is one of the primary goals of the ISTP. As the Wind spacecraft tells us about the prevailing solar wind conditions, the MFE obtains critical information used for determining the corresponding configuration of the magnetosphere, mapping the associated current systems, and providing contextual information for the particles, electric fields, and waves measurements on Polar.

Perigee Measurements

The acceleration of auroral electrons is thought to occur in the high latitude region between ~1000 km and the 1 Earth radius apogee altitude of the Polar spacecraft. Most previous spacecraft explorations of the acceleration region have focused on the low altitude end of this altitude range. Polar MFE, combined with the particles and electric fields experiments, for the first time conducts a comprehensive reconnaissance of the upper end of this region with concurrent solar wind information coming from the Wind and ACE spacecrafts.

Intermediate Altitudes

At points in the Polar spacecraft orbit removed from apogee and perigee, the MFE measures the effects of the magnetospheric ring current on the magnetospheric field in the middle-magnetosphere as a function of solar wind conditions. It also serves as a monitor of wave activity for those waves that perturb the field and have periods sufficiently long to be resolved at a field sampling rate of .01s. The state of the near-Earth magnetotail (complementary to the ISTP Geotail spacecraft measurements) where ring current injection has its roots and geomagnetic storm and substorm activity may drastically alter the field morphology will also be observed.

Coordinated Measurements and Analyses

Overall, the MFE plays a key role in integrating the Polar measurements into a consistent global picture of how the magnetosphere behaves in response to the solar wind. Coordinated analyses of these data with that from the ground components of the ISTP program and with the data sets from the other ISTP spacecraft give us unique global perspectives. Comparisons with the ISTP theory program global models helps us to visualize these perspectives.

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Last updated: January 5, 2001