Large global transient decreases of >70 MeV cosmic rays observed at Voyager 1 and Voyager 2 in the outer heliosphere in 2001–2003: The onset of heliospheric 2–3 kHz radio emission and the distance to the heliopause

Abstract

We have used the onset of well‐defined large transient decreases of >70 MeV cosmic rays to mark the arrival of strong interplanetary shocks at the locations of Voyager 2 (V2) and Voyager 1 (V1) in the outer heliosphere. Several of the most prominent events in the time period from 1998 to 2003 are studied. The speed of these shocks shows a significant pattern of slowing down beyond the location of V2. These shocks exhibit average speeds of ∼550–600 km s−1 between the Earth and V2 located between 56 and 68 AU; but between V2 and V1, located ∼18 AU farther out, these average shock speeds appear to have decreased to ∼250–400 km s−1. This decreasing shock speed has important implications regarding the distance to the heliopause as inferred from the turn‐on of 2–3 kHz radio emission when these shocks reach this location. A new episode of this radio emission has now been observed by the Iowa group in late 2002, presumably related to strong solar activity occurring ∼570 days earlier in April 2001. The related transient decrease was seen in the >70 MeV cosmic ray data at V2, located at 65 AU, approximately 203 days after the event at the Earth, and at V1, located at 84 AU, approximately 135 days later. These times agree with those inferred from solar wind and magnetic field data. Using these times and a speed profile consistent with the observed slowing down between V2 and V1 puts the heliopause location at 115 ± 5 AU, which is near the inner limit of earlier estimates of its location but well inside the 151–158 AU inferred by the Iowa group for the new 2002 event. This difference is related to the slowing down of the interplanetary shock near the heliospheric termination shock that we find in this paper. Assuming a commonly accepted ratio of 0.75 for the heliospheric termination shock distance relative to the heliopause distance places the average termination shock location in the range ∼83–90 AU or at the distance of V1 between early 2002 and late 2003.