Mass flux in the ecliptic plane and near the Sun deduced from Doppler scintillation

Abstract

During the late declining phase of the solar cycle, the tilt of the solar magnetic dipole with respect to the Sun's rotation axis leads to large‐scale organization of the solar wind, such that alternating regions of high‐ and low‐speed solar wind are observed in the ecliptic plane. In this paper, we use Doppler scintillation measurements to investigate mass flux of these two types of solar wind in the ecliptic plane and inside 0.3 AU, where in situ measurements have not been possible. To the extent that Doppler scintillation reflects mass flux, we find that mass flux in high‐speed streams: (1) is lower (by a factor of approximately 2.2) than the mass flux of the average solar wind in the heliocentric distance range of 0.3–0.5 AU, (2) is lower still (by as much as a factor of about 4) than the mass flux of the slow solar wind associated with the streamer belt, and (3) appears to grow with heliocentric distance. These Doppler scintillation results are consistent with the equator to pole decrease in mass flux observed in earlier spectral broadening measurements, and with trends and differences between high‐ and low‐speed solar wind observed by in situ measurements in the range of 0.3–1.0 AU. The mass flux results suggest that the solar wind flow in high‐speed streams is convergent towards the ecliptic near the Sun, becoming less convergent and approaching radial with increasing heliocentric distance beyond 0.3 AU. The variability of mass flux observed within equatorial and polar high‐speed streams close to the Sun is strikingly low. This low variability implies that, as Ulysses currently ascends to higher latitudes and spends more time in the south polar high‐speed stream after crossing the heliospheric current sheet, it can expect to observe a marked decrease in variations of both mass flux and solar wind speed, a trend that appears to have started already.