ESA Science & Technology11-Jul-2005 10:00:22

Science Results

Measurements of gradients of density

Gradient of electron density measured in XZ, YZ and XZ plane along the Cluster orbit on 26 February 2001 (from Decreau et al., Annales Geophysicae, 2001.). The red arrows indicated the direction of motion of the magnetopause deduced by from the time of crossing of the magnetopause by the four spacecraft.

Sketch of the magnetopause motion and the gradient of density.

The four spacecraft measurements allow also the measurement of many differential quantities. The gradient of the electron density is one of them. This parameter is key in magnetospheric physics since it is involved in the motion of plasma boundaries and structures. The electron density is deduced in this case from the plasma frequency measured by the WHISPER instrument. Then using the value of the density measured simultaneously at the four spacecraft, Grad(ne) can be calculated.

An example of a magnetopause crossing on 26 February 2001 is shown above. Each panel shows the vectors of Grad(ne) measured along the spacecraft center of mass trajectory in three GSE planes. In addition, the magnetopause motion calculated from the first crossing is also shown. At the beginning of the interval, the spacecraft where in the magnetosphere, characterized by tenuous plasma (lower middle panel), then they crossed the magnetopause around 0550 UT and went into the magnetosheath with a density about 15 cm-3 for about 6 minutes and finally re-enter the magnetosphere at about 0601 UT. In this example, the magnetopause motion is opposite to Grad(ne) (see opposite arrows); a sketch of this motion is shown on the right panel of the figure. These results were published in Decreau et al., Annales Geophysicae, 2001.

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