The Energetic Ion Events Measured Upstream the Earth's Bow Shock by STEREO, Cluster, ACE and Geotail: Where is the Origin?

In earlier studies (e.g. Desai et al., 2008}) the observations of upstream events up to 3800 R_E were reported during declining phase of the solar cycle in 2007. These upstream events observed by STEREO-A mainly occurred after the corotating compression region passed the Earth's magnetosphere. We study the relation of these upstream events (from about 130 to 1600 R_E away from the Earth) with the observations in the direct vicinity of the terrestrial bow shock (up to X = 18 R_E). For this purpose simultaneous measurements of energetic ions with energies >30 keV by particle instruments onboard STEREO-A, STEREO-B, ACE (far upstream region), and onboard Cluster and Geotail (the direct vicinity of the bow shock) are used. The upstream events are observed simultaneously mainly when the magnetic field is pointing along the line joining those satellites in far upstream region with those near the terrestrial bow shock. Therefore the connection of the magnetic field to the bow shock plays an important role for the occurrence of the many energetic events far upstream. Nevertheless there is a number of energetic events whose origin seems not to be connected to the terrestrial bow shock. Using the estimation of the propagation time of solar wind discontinuities we show that these energetic events convect together with the solar wind, pass the upstream region of the Earth's bow shock and trigger the upstream events in the vicinity of the bow shock, not the other way around. The ion intensity observed by STEREO-A and STEREO-B in these cases is often significantly higher than that observed in front of the bow shock by Cluster and Geotail. This suggests that, particles are accelerated in the solar wind, possibly by enhanced wave activity in high speed streams and corotating interaction regions (CIR).