Solar wind conditions in the outer heliosphere and the distance to the termination shock

Abstract

The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. A least squares fit of proton ram pressure to heliocentric distance R over this distance yields a ram pressure equal to (1.67 × 10−8 dynes cm−2) R−2.00 ± 0.02, where R is measured in astronomical units. Assuming that the interstellar pressure is due to a 5 µG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength, i.e., for a 7 µG field, the termination shock will be located on average at 64 AU. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of days), due primarily to large variations in solar wind density at a given radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. Using a simple kinematic model, we study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.

Termination shock response to large‐scale solar wind fluctuations

“…This lower speed was used so that the termination shock would be located fairly close to the Sun when reasonable (anticipated) values were used for interstellar conditions. The magnitudes of the simulated ram pressure fluctuations in Figure 2 are at the low end of the observed magnitudes in Figure 3 of Belcher et al [1993] and are similar to those of the fluctuations during solar minimum periods.…”

“…The termination shock in the equilibrium solution is located at about 80 AU; hence the fluctuations at 70 AU are only 10 AU upstream of the shock. The average value of the simulated ram pressure is less than the observed average value in Figure 3 of Belcher et al [1993] owing to the lower-than-observed solar wind speed used in the simulations. This lower speed was used so that the termination shock would be located fairly close to the Sun when reasonable (anticipated) values were used for interstellar conditions.…”

“…The fluctuation period is taken to be 180 days. This is somewhat larger than the observed largescale variations in ram pressure occurring on time scales of tens of days reported by Belcher et al [1993], but it will be sufficient to demonstrate the essential physics involved in the response of the interaction to the fluctuations.…”

“…Their effect on the overall interaction is also not well understood. The analysis of data from the plasma science experiment on Voyager 2 as the spacecraft traveled from 1 to 40.4 AU indicates that there are large-scale fluctuations in the solar wind ram pressure occurring on the time scale of tens of days [Belcher et al, 1993]. Data from Pioneers 10 and 11 also show the presence of large variations in the ram pressure [Barnes, 1990].…”

“…Since the ram pressure plays a major role in controlling the location of the termination shock, it seems logical that large-scale fluctuations in the ram pressure may produce relatively large excursions of the termination shock. Belcher et al [1993] used a kinematic model in which they limited the termination shock speed to 200 km s" 1 and estimated that the location may vary by as much as 15 AU in response to the observed ram pressure fluctuations. This excursion amplitude is similar to that computed in other one-dimensional analyses [Barnes, 1993;Suess, 1993].…”