We study periods of elevated energetic particle intensities observed by STEREO-A when the partial pressure exerted by energetic (≥83 keV) protons (PEP) is larger than the pressure exerted by the interplanetary magnetic field (PB). In the majority of cases, these periods are associated with the passage of interplanetary shocks. Periods when PEP exceeds PB by more than one order of magnitude are observed in the upstream region of fast interplanetary shocks where depressed magnetic field regions coincide with increases of energetic particle intensities. When solar wind parameters are available, PEP also exceeds the pressure exerted by the solar wind thermal population (PTH). Prolonged periods (12 hr) with both PEP PB and PEP PTH may also occur when energetic particles accelerated by an approaching shock encounter a region well upstream of the shock characterized by low magnetic field magnitude and tenuous solar wind density. Quasi-exponential increases of the sum PSUM = PB + PTH + PEP are observed in the immediate upstream region of the shocks regardless of individual changes in PEP, PB, and PTH, indicating a coupling between PEP and the pressure of the background medium characterized by PB and PTH. The quasi-exponential increase of PSUM implies a radial gradient ?PSUM/?r 0 that is quasi-stationary in the shock frame and results in an outward force applied to the plasma upstream of the shock. This force can be maintained by the mobile energetic particles streaming upstream of the shocks that, in the most intense events, drive electric currents able to generate diamagnetic cavities and depressed solar wind density regions.
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