Transient Events in Galactic Cosmic Rays Observed by Voyager-1 beyond the Heliopause

Abstract:

Following the passage of Voyager-1 through the Heliopause in August 2012, anomalous cosmic rays essentially disappeared while galactic cosmic rays (GCRs) leveled to a remarkably constant flux. There were two notable exceptions: in March 2013 and May 2014 Voyager-1 experienced small increases of GCRs in the few hundred MeV range. Both events lasted a couple of weeks and were followed by detection of plasma oscillation (Gurnett et al., 2013, 2015). We shall discuss the pitch-angle distribution of GCRs, seen by LECP, and the time profiles of these events. We interpreted the first event as the GCRs remotely sensing a disturbance passing through the magnetic field line of Voyager-1: particles gained energy as they were reflected on the approaching region of stronger magnetic field of the disturbance. Here, we show that energy gain is not restricted to reflected particles, and the effect should be present at all pitch angles in the hemisphere facing the disturbance. The maximum increase of GCR intesity is predicted to occur at the critical reflection angle.
The time profile starts with a sharp increase if GCRs are reflected from a sharp shock. However, the disturbance need not be very sharp. For instance, Voyager-1 magnetometer (Burlaga et al., 2013) detected a gradual 5-day increase of the field strength, B, in November 2012. A disturbance with a similar gradual profile would produce a smoother time-profile in the GCR flux at Voyager-1. We shall present numerical simulations and discuss their implications.

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Gurnett, D.A., et al., Astrophys. J., in press (2105)