Using transient decreases of cosmic rays observed at Voyagers 1 and 2 to estimate the location of the heliospheric termination shock

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Abstract
We have examined the intensity‐time profiles of outward moving transient decreases of anomalous and galactic cosmic rays observed by Voyager 1 (Vl) and Voyager 2 (V2) in 1998 and 1999 in the outer heliosphere. The goal of this study is to compare these intensity‐time profiles with those obtained by le Roux and Fichtner [1999] using numerical simulations of the time‐dependent cosmic ray modulation produced by the passage of large global merged interaction regions. Their calculations show that when these interaction regions reach the heliospheric termination shock, they weaken rapidly causing the intensity of both galactic and anomalous cosmic rays to increase rapidly. They suggest using this time of rapid increase and the propagation times of these events to determine the location of the termination shock. Three outward propagating transient decreases were observed during the 1998–1999 time period. Because of local temporal variations the first two events did not exhibit a le Roux‐Fichtner type of rapid recovery. However, the third event starting on day 50 of 1999 at V1 exhibited a very sharply defined intensity‐time profile with a very rapid recovery starting on day 88 ± 3 of 1999 that was observed in three energy channels on V1. From this time delay we are able to determine that the termination shock was 9.5–10.7 AU beyond V1 or at a distance of 82.6–83.8 AU at this time. The intensity‐time profile at V2 was less sharp, but the rapid recovery that was observed occurred at the same time (±5 days) as that at V1, thus confirming the conclusions from the V1 data. At its present outward speed of 3.6 AU yr−1, V1 would be expected to encounter the termination shock at this location as early as the last quarter of 2001.