The solar wind from a stellar perspective

Context. Due to the effects that they can have on the atmospheres of exoplanets, stellar winds have recently received significant attention in the literature. Alfvén-wave-driven 3D magnetohydrodynamic models, which are increasingly used to predict stellar wind properties, contain unconstrained parameters and rely on low-resolution stellar magnetograms. Aims. In this paper, we explore the effects of the input Alfvén wave energy flux and the surface magnetogram on the wind properties predicted by the Alfvén Wave Solar Model (AWSoM) model for both the solar and stellar winds. Methods. We lowered the resolution of two solar magnetograms during solar cycle maximum and minimum using spherical harmonic decomposition. The Alfvén wave energy was altered based on non-thermal velocities determined from a far ultraviolet spectrum of the solar twin 18 Sco. Additionally, low-resolution magnetograms of three solar analogues, 18 Sco, HD 76151, and HN Peg, were obtained using Zeeman Doppler imaging and used as a proxy for the solar magnetogram. Finally, the simulated wind properties were compared to Advanced Composition Explorer (ACE) observations. Results. AWSoM simulations using well constrained input parameters taken from solar observations can reproduce the observed solar wind mass loss and angular momentum loss rates. The simulated wind velocity, proton density, and ram pressure differ from ACE observations by a factor of approximately two. The resolution of the magnetogram has a small impact on the wind properties and only during cycle maximum. However, variation in Alfvén wave energy influences the wind properties irrespective of the solar cycle activity level. Furthermore, solar wind simulations carried out using the low-resolution magnetogram of the three stars instead of the solar magnetogram could lead to an order of a magnitude difference in the simulated solar wind properties. Conclusions. The choice in Alfvén energy has a stronger influence on the wind output compared to the magnetogram resolution. The influence could be even stronger for stars whose input boundary conditions are not as well constrained as those of the Sun. Unsurprisingly, replacing the solar magnetogram with a stellar magnetogram could lead to completely inaccurate solar wind properties, and should be avoided in solar and stellar wind simulations. Further observational and theoretical work is needed to fully understand the complexity of solar and stellar winds.

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Evolution of Alfvén wave-driven solar winds to red giants

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308014889 ◽

2007 ◽

Vol 3 (S247) ◽

pp. 201-207

Author(s):

Takeru K. Suzuki

Keyword(s):

Solar Wind ◽

Mode Conversion ◽

Alfven Waves ◽

Alfven Wave ◽

Alfvén Waves ◽

Stellar Winds ◽

Red Giants ◽

Giant Stars ◽

Solar Winds ◽

Red Giant

AbstractIn this talk we introduce our recent results of global 1D MHD simulations for the acceleration of solar and stellar winds. We impose transverse photospheric motions corresponding to the granulations, which generate outgoing Alfvén waves. The Alfvén waves effectively dissipate by 3-wave coupling and direct mode conversion to compressive waves in density-stratified atmosphere. We show that the coronal heating and the solar wind acceleration in the open magnetic field regions are natural consequence of the footpoint fluctuations of the magnetic fields at the surface (photosphere). We also discuss winds from red giant stars driven by Alfvén waves, focusing on different aspects from the solar wind. We show that red giants wind are highly structured with intermittent magnetized hot bubbles embedded in cool chromospheric material.

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Resonant mode conversion of a shear Alfvén wave

Canadian Journal of Physics ◽

10.1139/p87-041 ◽

1987 ◽

Vol 65 (4) ◽

pp. 357-358

Author(s):

Bhimsen K. Shivamoggi

Keyword(s):

Solar Wind ◽

Acoustic Wave ◽

Wave Energy ◽

Mode Conversion ◽

Resonant Absorption ◽

Resonant Mode ◽

Alfven Wave ◽

Ion Acoustic Wave ◽

Ion Acoustic ◽

Kinetic Alfven Wave

A new mechanism for the resonant absorption of a shear Alfvén wave is proposed that involves a direct mode conversion of the latter to an ion-acoustic wave without bringing in the intermediary — the kinetic Alfvén wave. The fraction of the incident Alfvén-wave energy that is mode-converted to an ion-acoustic wave is calculated. This mechanism likely operates in the solar wind, where it might lead to heating of the plasma.

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Stellar Wind Theories

Highlights of Astronomy ◽

10.1017/s1539299600004482 ◽

1980 ◽

Vol 5 ◽

pp. 591-600

Author(s):

A. G. Hearn

Keyword(s):

Solar Wind ◽

Mass Loss ◽

Stellar Wind ◽

Extreme Case ◽

Stellar Atmosphere ◽

Stellar Winds ◽

Mean Flow ◽

Continuous Mass ◽

Flow Through ◽

Energy And Momentum

The term stellar wind is used nowadays to describe any more or less continuous mass loss from a star. With the observations made with satellites in recent years it is becoming clear that most stars are undergoing this form of mass loss, though its magnitude can be very different from one star to another. The term stellar wind does not include the more eruptive forms of mass loss such as novae, the ejection of mass in shells or mass loss as a result of flares.Stellar winds are maintained by energy and momentum deposited in the outer layers of a stellar atmosphere. The deposition of energy causes the heating of a chromosphere and corona, so that the theory of stellar winds cannot really be separated from the theory of coronal heating. Energy and momentum can both be deposited by the same mechanism. For example if a corona is heated by the dissipation of a wave which deposits energy, the same wave can change the momentum of the mean flow through wave pressure and this can happen even in the extreme case of no dissipation of the wave.The foundation of the theory of stellar winds was laid by Parker (1958) in his theory of the solar wind. A useful review of this work has been given by Parker (1965). The theory of the solar wind in its simplest form is deduced from the equation of motion combined with the equations of continuity and state.

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Physical Processes in Nebular Shells and the Interpretation of Nebular Spectra

Symposium - International Astronomical Union ◽

10.1017/s0074180900093694 ◽

1983 ◽

Vol 103 ◽

pp. 219-227

Author(s):

J. Patrick Harrington

Keyword(s):

Charge Transfer ◽

Simple Model ◽

Stellar Wind ◽

High Velocity ◽

Planetary Nebulae ◽

Geometrical Parameters ◽

Stellar Winds ◽

Physical Processes ◽

Rapid Cooling ◽

Particle Distributions

Computed models are now recognized as useful tools for interpretation of the spectra of planetary nebulae. However, even the most detailed models need geometrical parameters such as filling factors which are poorly determined by observations. Some effects may be seen more clearly by modeling the stratification than by just using total fluxes. A simple model for NGC 6720 is presented which reproduces the behavior of (Ne III) λ3869 observed by Hawley and Miller (1977), clearly showing the effects of charge transfer. The behavior of C II λ4267 remains puzzling. Finally, we comment on the interaction of high velocity stellar winds with nebular shells. Non-equilibrium particle distributions at the contact between the shocked stellar wind and the nebula may result in the rapid cooling of the shocked gas.

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