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

Verscharen, Daniel; Stansby, David; Finley, Adam J.; Owen, C. J.; Horbury, T. S.; Maksimović, M.; Velli, M.; Bale, S. D.; Fedorov, A.; Bruno, R.; Livi, S.; Khotyaintsev, Yu. V.; Vecchio, A.; Lewis, G. R.; Anekallu, C.; Kelly, Christopher W.; Watson, G.; Kataria, D. O.; O’Brien, H.; Evans, V.; al, et

doi:10.1051/0004-6361/202140956

Cited by 2 publications

(1 citation statement)

References 58 publications

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“…This suggests that the structure in the solar wind angular momentum flux develops in the low to middle corona (1 -20 solar radii), either through the interaction of the magnetic field with rotation, or due to the interaction between neighbouring solar wind streams. An interaction-based mechanism for angular momentum redistribution appears to be supported by the prevalence of the fast solar wind carrying a negative (deflected) angular momentum flux, and the slow wind containing the more dominant net positive angular momentum flux (Finley et al 2021;Verscharen et al 2021), i.e. removing angular momentum from the Sun.…”

Section: Introductionmentioning

confidence: 99%

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Finley¹,

Brun²

2023

A&A

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Context. Sun-like stars shed angular momentum due to the presence of magnetised stellar winds. Magnetohydrodynamic models have been successful in exploring the dependence of this "wind-braking torque" on various stellar properties, however the influence of surface differential rotation is largely unexplored. As the wind-braking torque depends on the rotation rate of the escaping wind, the inclusion of differential rotation should effectively modulate the angular momentum-loss rate based on the latitudinal variation of wind source regions.Aims. Here we aim to quantify the influence of surface differential rotation on the angular momentum-loss rate of the Sun, in comparison to the typical assumption of solid-body rotation. Methods. To do this, we exploit the dependence of the wind-braking torque on the effective rotation rate of the coronal magnetic field, which is known to be vitally important in magnetohydrodynamic models. This quantity is evaluated by tracing field lines through a Potential Field Source Surface (PFSS) model, driven by ADAPT-GONG magnetograms. The surface rotation rates of the open magnetic field lines are then used to construct an open-flux weighted rotation rate, from which the influence on the wind-braking torque can be estimated. Results. During solar minima, the rotation rate of the corona decreases with respect to the typical solid-body rate (the Carrington rotation period is 25.4 days), as the sources of the solar wind are confined towards the slowly-rotating poles. With increasing activity, more solar wind emerges from the Sun's active latitudes which enforces a Carrington-like rotation. Coronal rotation often displays a north-south asymmetry driven by differences in active region emergence rates (and consequently latitudinal connectivity) in each hemisphere.Conclusions. The effect of differential rotation on the Sun's current wind-braking torque is limited. The solar wind-braking torque is ∼ 10 − 15% lower during solar minimum, (compared with the typical solid body rate), and a few percent larger during solar maximum (as some field lines connect to more rapidly rotating equatorial latitudes). For more rapidly-rotating Sun-like stars, differential rotation may play a more significant role, depending on the configuration of the large-scale magnetic field.

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Section: Introductionmentioning

confidence: 99%

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Finley¹,

Brun²

2023

A&A

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The Sun’s Alfvén Surface: Recent Insights and Prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH)

Cranmer,

Chhiber,

Gilly

et al. 2023

Sol Phys

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The solar wind is the extension of the Sun’s hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. The radial distance at which the wind’s outflow speed exceeds the phase speed of Alfvénic and fast-mode magnetohydrodynamic (MHD) waves is called the Alfvén radius. In one-dimensional models, this is a singular point beyond which most fluctuations in the plasma and magnetic field cannot propagate back down to the Sun. In the multi-dimensional solar wind, this point can occur at different distances along an irregularly shaped “Alfvén surface.” In this article, we review the properties of this surface and discuss its importance in models of solar-wind acceleration, angular-momentum transport, MHD waves and turbulence, and the geometry of magnetically closed coronal loops. We also review the results of simulations and data-analysis techniques that aim to determine the location of the Alfvén surface. Combined with recent perihelia of Parker Solar Probe, these studies seem to indicate that the Alfvén surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfvén surface along any radial ray. Thus, in many contexts, it is more appropriate to use the concept of a topologically complex “Alfvén zone” rather than one closed surface. This article also reviews how the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission will measure the properties of the Alfvén surface and provide key constraints on theories of solar-wind acceleration.

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

Cited by 2 publications

References 58 publications

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

The Sun’s Alfvén Surface: Recent Insights and Prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH)

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