STEREO/SEPT Observations of Upstream Particle Events: Almost Monoenergetic Ion Beams

The presence of energetic particles (<~1 MeV) upstream of the Earth's bow-shock, which are preferentially streaming from the magnetosphere in the Sun direction is well known since the 1960s. The origin of upstream ion bursts often associated with energetic electrons (>~30 keV) was explained in two ways: (1) leakage of magnetospheric particles accelerated within the magnetosphere, and (2) acceleration at the bow-shock. Normally, the ion upstream bursts are characterized by power-law energy spectra with negative slope, independent of their origin inside the magnetosphere or at the bow-shock. But the recently observed "Almost Monoenergetic Ion" (AMI) bursts detected in the magnetosphere and near the Earth's bow-shock at INTERBALL-1 show that some events exhibit narrow spectral lines. These lines were explained as acceleration in a burst of electrostatic field inside the magnetosphere. We present observations of such AMI events in the energy range of 100-1000 keV detected with the SEPT instruments onboard both STEREO spacecraft. These AMI events were detected inside the magnetosphere at STEREO-B as well as unexpectedly far upstream of the bow-shock at distances up to 1100 RE (STEREO-B) and 1900 RE (STEREO-A). We discuss the origin of AMI events, the connection to the Earth's bow-shock and to the magnetosphere, and the conditions of the IP medium and magnetosphere under which these AMI bursts occur. There is evidence that the detected lines may be caused by quasi-monoenergetic beams of protons, helium, and heavier ions. Furthermore, we present spatial and temporal distributions and the occurrence rate of all electron and ion upstream events from December 2006 until July 2007.