The contribution of alpha particles to the solar wind angular momentum flux in the inner heliosphere

Abstract

Context. An accurate assessment of the Sun's angular momentum (AM) loss rate is an independent constraint for models that describe the rotation evolution of Sun-like stars. Aims. In-situ measurements of the solar wind taken by Parker Solar Probe (PSP), at radial distances of ∼ 28 − 55R , are used to constrain the solar wind AM-loss rate. For the first time with PSP, this includes a measurement of the alpha particle contribution. Methods. The mechanical AM flux in the solar wind protons (core and beam), and alpha particles, is determined as well as the transport of AM through stresses in the interplanetary magnetic field. The solar wind AM flux is averaged over three hour increments, so that our findings more accurately represent the bulk flow. Results. During the third and fourth perihelion passes of PSP, the alpha particles contain around a fifth of the mechanical AM flux in the solar wind (the rest is carried by the protons). The proton beam is found to contain ∼ 10 − 50% of the proton AM flux. The sign of the alpha particle AM flux is observed to correlate with the proton core. The slow wind has a positive AM flux (removing AM from the Sun as expected), and the fast wind has a negative AM flux. As with previous works, the differential velocity between the alpha particles and the proton core tends to be aligned with the interplanetary magnetic field. Conclusions. In future, by utilising the trends in the alpha-proton differential velocity, it may be possible to estimate the alpha particle contribution when only measurements of the proton core are available. Based on the observations from this work, the alpha particles contribute an additional 10 − 20% to estimates of the solar wind AM-loss rate which consider only the proton and magnetic field contributions. Additionally, the AM flux of the proton beam can be just as significant as the alpha particles, and so should not be neglected in future studies.

The angular-momentum flux in the solar wind observed during Solar Orbiter’s first orbit

“…2.43 × 10 −10 au 3 g cm −1 s −2 sr −1 (Liu et al 2021) as averages over their whole datasets. As shown in Section 3.3, we find a general trend of decreasing F p with increasing U r , consistent with previous findings in data from Wind and Parker Solar Probe (Finley et al 2019(Finley et al , 2020(Finley et al , 2021 and with estimates based on the observation of the Earth's F p (10…”

“…We find that F p > 0 and F tot > 0 in the observed slow-wind intervals (U r < 400 km s −1 ), while F p < 0 and F tot < 0 in the observed fast-wind intervals (U r > 500 km s −1 ). A similar behaviour has been seen in Parker Solar Probe measurements (Finley et al 2021). In the separated data subsets for slow wind and fast wind, the mean values of |F p | are greater than the mean value of |F p | in our full dataset of all measurements.…”

“…Our measurements neglect the contribution of α-particles to the angular-momentum flux. Given that their contribution to the local momentum flux is on the order of 20% (even greater near the Sun), α-particles can make a significant contribution to the total angular-momentum flux (Pizzo et al 1983;Marsch & Richter 1984;Verscharen et al 2015;Finley et al 2021). Both Finley et al (2021) and Liu et al (2021) suggest a reconstruction of the α-particle component based on the field-alignment of the differential flow between protons and α-particles.…”

“…Modern space instrumentation provides us with higher spacecraft-pointing accuracy and, thus, a more accurate determination of U φ . Moreover, as the average ratio U φ /U r increases with decreasing heliocentric distance, the error in the quantification of the angular-momentum flux is generally smaller when measured at smaller distances from the Sun (Finley et al 2020(Finley et al , 2021. In addition, co-rotating interaction regions have not yet formed (Richter & Luttrell 1986;Allen et al 2020) and interplanetary coronal mass ejections have not expanded enough to disturb nearby wind streams (Möstl et al 2020) at small distances from the Sun.…”

The solar wind angular-momentum flux observed during Solar Orbiter's first orbit

“…The fit parameters are given in Table 2. The magenta squares represent the measurements of F B during the time of the compression region (CR) on 2020-09-06. data from Wind and Parker Solar Probe (Finley et al 2019(Finley et al , 2020(Finley et al , 2021 and with estimates based on the observation of the Earth's magnetotail deflection (Němeček et al 2020). The observation of this trend in near-Sun data suggests that the trend is generated close to the Sun and not by local deflections at large distances.…”