Three‐dimensional nature of interaction regions: Pioneer, Voyager, and Ulysses solar cycle variations from 1 to 5 AU

Abstract

We combined in‐ecliptic observations of interaction regions from 1 to 5 AU by four spacecraft that traveled to Jupiter at different phases of the solar cycle: Pioneer 11 (declining phase of cycle 20), Voyager 1 and 2 (ascending phase of cycle 21), and Ulysses (just after solar maximum 22). We used this set of 97 interaction regions to study two different aspects of these events. (1) We analyzed the geometry of 38 stream interfaces, and found that the interaction regions detected by the three missions have different orientations. Some of these results might be associated with solar cycle variations such as the low‐latitudinal inclination of the interaction regions detected by Ulysses and the high‐latitudinal tilts of some interaction regions detected by Pioneer 11. Other results, such as the deviations with respect to the Parker spiral orientation, seem to be related to the persistent variations in the characteristics of the solar winds streams. (2) We studied the bulk speeds and the dynamic, magnetic, and thermal pressures of the fast and slow streams associated with 75 of these interaction regions. As a consequence of solar cycle variations, the bulk speed distributions of the fast and slow streams were different in the three missions. We found that the pressure ratios (dynamic, thermal, and magnetic) between fast and slow streams vary continuously. In about half of the interaction regions the dynamic pressure of the slow stream was higher than the dynamic pressure of the fast stream. We found similar variations in magnetic and thermal pressures. This implies that in many interaction regions the slow stream transfers momentum to the fast stream. The results presented here show that the solar wind streams associated with interaction regions vary at all the phases of the solar cycle, leading to interaction regions with different characteristics.