Multi‐spacecraft observations of diffuse ions upstream of Earth's bow shock

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Abstract
We report on simultaneous measurements of energetic ions upstream of Earth's quasi‐parallel bow shock by Cluster during a 10 hour time period when the inter‐spacecraft separation distance was between ∼1 and ∼1.5 Re. Using a bow shock model we determined for each 32 s averaged data set the distance of the Cluster 1 and Cluster 3 spacecraft from the bow shock along the magnetic field. Using the differences of the partial energetic ion densities observed on the two spacecraft we determined the spatial gradient of partial densities at various distances from the bow shock in 4 energy channels ranging from 10 to 32 keV. The gradients in all four energy channels decrease exponentially with distance. The e‐folding distance of the gradients depends approximately linearly on energy and increases from ∼0.5 Re at ∼11 keV to ∼2.8 Re at ∼27 keV. These values are about half of the values obtained on a statistical basis by Ipavich et al. [1981] and by Trattner et al. [1994].

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