The Solar Wind Angular Momentum Flux as Observed by Parker Solar Probe

<p>he long-term evolution of the Sun\textquoterights rotation period cannot be directly observed, and is instead inferred from trends in the measured rotation periods of other Sun-like stars. Assuming the Sun spins down as it ages, following rotation rate proportional to age(-1/2), requires the current\&nbsp;solar\&nbsp;angular momentum (AM) loss rate to be around 6 x 10(30)erg. Magnetohydrodynamic models, and previous observations of the\&nbsp;solar\&nbsp;wind (from the Helios and Wind spacecraft), generally predict a values closer to 1 x 10(30)erg or 3 x 10(30)erg, respectively. Recently, the\&nbsp;Parker\&nbsp;Solar\&nbsp;Probe\&nbsp;(PSP) observed tangential\&nbsp;solar\&nbsp;wind speeds as high as similar to 50 km s(-1)in a localized region of the inner heliosphere. If such rotational flows were prevalent throughout the corona, it would imply that the\&nbsp;solar\&nbsp;wind AM-loss rate is an order of magnitude larger than all of those previous estimations. In this Letter, we evaluate the AM flux in the\&nbsp;solar\&nbsp;wind, using data from the first two orbits of PSP. The\&nbsp;solar\&nbsp;wind is observed to contain both large positive (as seen during perihelion), and negative AM fluxes. We analyze two\&nbsp;solar\&nbsp;wind streams that were repeatedly traversed by PSP; the first is a slow wind stream whose average AM flux fluctuates between positive and negative values, and the second is an intermediate speed stream that contains a positive AM flux (more consistent with a constant flow of AM). When the data from PSP are evaluated holistically, the average equatorial AM flux implies a global AM-loss rate of around (2.6-4.2) x 10(30)erg (which is more consistent with observations from previous spacecraft).</p>
Year of Publication
The Astrophysical Journal
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Date Published