Propagation of three‐dimensional Alfvén waves in a stratified, thermally conducting solar wind

Orlando, S.; Lou, Yu‐Qing; Rosner, R.; Peres, G.

doi:10.1029/96ja02304

Cited by 19 publications

(16 citation statements)

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“…The initial unperturbed stellar atmosphere is magneto-static and consists of a hot (temperature T ≈ 10 6 K) and tenuous (plasma density n H ≈ 2 × 10 8 cm −3 ) corona that is linked by a steep transition region to an isothermal chromosphere at temperature T = 10 4 K and 1.4 × 10 9 cm thick. The initial distributions of mass density and temperature of the stellar atmosphere are derived, assuming that the plasma is in pressure and energy equilibrium; we adapted the wind model of Orlando et al (1996) to calculate the initial vertical profiles of mass density and temperature from the base of the transition region (T = 10 4 K) to the corona. Initially the stream is in pressure equilibrium with the stellar corona and has a circular cross-section with a radius 1 r str = 10 10 cm.…”

Section: Mhd Modelingmentioning

confidence: 99%

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Orlando

Bonito

Argiroffi

et al. 2013

A&A

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Context. According to the magnetospheric accretion model, hot spots form on the surface of classical T Tauri stars (CTTSs) in regions where accreting disk material impacts the stellar surface at supersonic velocity, generating a shock. Aims. We investigate the dynamics and stability of postshock plasma that streams along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. Methods. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model considers the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation). We explore different configurations and strengths of the magnetic field. Results. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. In the case of weak magnetic fields (plasma β > ∼ 1 in the postshock region), a large component of B may develop perpendicular to the stream at the base of the accretion column, which limits the sinking of the shocked plasma into the chromosphere and perturbs the overstable shock oscillations induced by radiative cooling. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields (β < 1 in the postshock region close to the chromosphere), the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface at the height where the plasma β ≈ 1. In general, we find that a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the postshock plasma at T > 10 6 K lower than when there is uniform magnetic field. Conclusions. The initial magnetic field strength and configuration in the region of impact of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. In addition, the field strength and configuration also influence the energy balance of the shocked plasma with its emission measure at T > 10 6 K, which is lower than expected for a uniform field. The above effects contribute to underestimating the mass accretion rates derived in the X-ray band.

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Section: Mhd Modelingmentioning

confidence: 99%

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Orlando

Bonito

Argiroffi

et al. 2013

A&A

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“…This has long been of interest in wave models of solar coronal heating and dynamics, because reflections tend to limit the amount of energy reaching the corona and solar wind [e.g., An et al, , 1990Barkhudarov, 1991;Bogdan et al, 2003;Charbonneau and MacGregor, 1995;Ferraro and Plumpton, 1958;Grappin et al, 2000Grappin et al, , 2002Hollweg, 1981Hollweg, , 1991Krogulec and Musielak, 1998;Krogulec et al, 1994;Lau and Siregar, 1996;Leroy, 1980;Lou and Rosner, 1994;Moore et al, 1991aMoore et al, , 1991bOfman and Davila, 1995;Orlando et al, 1996Orlando et al, , 1997Rosner et al, 1991;Stark, 1996;Suzuki and Inutsuka, 2005;Turkmani and Torkelsson, 2004;Velli et al, 1991;Velli, 1993;Ventura et al, 1999]. Reflections may also lead to interesting dynamic effects, such as shock formation and spicules [Hollweg, 1982;de Pontieu et al, 2004] (see also the review by Sterling [2000]).…”

Section: Introductionmentioning

confidence: 99%

Reflection of Alfvén waves in the corona and solar wind: An impulse function approach

Hollweg

Isenberg

2007

J. Geophys. Res.

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[1] We consider the reflection of Alfvén waves in the corona and solar wind, using variables f and g which follow sunward and antisunward characteristics, respectively. We show that the basic equations for f and g have the same structure as the Klein-Gordon equation. Unlike previous studies which used a harmonic analysis, we emphasize the impulse response of the system. This is equivalent to finding the Green's function, but it may have direct application to situations where Alfvén waves are launched impulsively. We provide an approximate analysis which can be used to understand most features that appear in detailed numerical solutions. The analysis reveals the origin of a previous result that f and g each has both sunward and antisunward propagating phase in a harmonic analysis, even though f (g) follows only the sunward (antisunward) characteristic. We numerically study the propagation of an antisunward moving impulse in the corona and solar wind. We find that the sunward moving ''wake'' tends to become more important at greater distances beyond the Alfvén critical point, possibly providing a natural explanation of the observation that outward propagating waves become less dominant at greater distances from the Sun. There is an extended region behind the initial impulse in which magnetic energy dominates kinetic energy; it is not clear, however, whether our result can explain the observed dominance of magnetic energy throughout many decades of frequency in the observed power spectrum. We also find that the outgoing wake has a tendency to ''ring,'' with periods of the order of 15-30 min. The ringing is associated mainly with propagation through a structured Alfvén speed profile rather that with the cutoff in the Klein-Gordon equation. These oscillation periods seem too short to explain why Alfvén waves in the solar wind have most power at periods of hours, but other Alfvén speed profiles could yield longer periods. We also investigate whether the same approach can be used for acoustic-gravity waves propagating along magnetic flux tubes in the solar atmosphere.

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“…The radial wave equation has been solved for a uniform radial flow [69], in terms of Gaussian hypergeometric functions, and a cut-off frequency identified [17,92,93,117]; the availability of numerical results in the literature [94,144,143] suggests that this case is chosen first for exact analytical solution for all distances and frequencies. For one-dimensional spherical Alfven waves , the total state of the fluid is assumed to consist of a mean state of radial steady non-uniform magnetic field B{r) and a mean flow ?7(r), upon which transverse and parallel velocity v and magnetic field h perturbations along the parallels are superimposed, depending only on time t and radial distance r, viz.…”

Section: One-and Three-dimensional Alfven Waves In a Radial Uniform Mmentioning

confidence: 99%

“…The cut-off frequency (116b) has been discussed in the literature [69,17,91,92,117], and separates imaginary or propagating waves uj > uj^ from real or standing modes a; < o;^. The differential equation (109) has a singularity at the critical layer, where the mean flow velocity equals the Alfven speed (117a):…”

Section: One-and Three-dimensional Alfven Waves In a Radial Uniform Mmentioning

confidence: 99%

“…Both for the (a) one-and (b) three-dimensional cases, the radial dependence is specified by the same Alfven wave equation, for which exact analytical solutions are given, valid for all wave frequencies and radial distances, for four distinct backgrounds. Each background is consistent with momentum balance and mass conservation, and thus is specified by the profile of mean flow velocity: ( §3.1) the case of uniform mean flow is the only one to which the cut-off frequency applies, discussed in the literature [69,17,91,93,117]; ( §3.2) in the case of mean flow velocity proportional to the radius, the wave fields are specified by Gaussian hypergeometric functions, with different parameters from case §3.1; ( §3.3) the only case without a critical layer is that of a homogeneous medium, or mean flow velocity varying like the inverse square of the radius, which has a transition level; ( §3.4) the closest approximation to the solar wind may be a mean flow velocity proportional to the square root of the radius, in which case there is a critical layer, where the mean flow velocity equals the Alfven speed.…”

Section: Introductionmentioning

confidence: 99%

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Nonlinear MHD Waves and Turbulence

Passot¹,

Sulem²

1999

Lecture Notes in Physics

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The series Lecture Notes in Physics reports new developments in physical research and teaching -quickly, informally, and at a high level. The proceedings to be considered for pubhcation in this series should be hmited to only a few areas of research, and these should be closely related to each other. The contributions should be of a high standard and should avoid lengthy redraftings of papers already pubhshed or about to be pubhshed elsewhere. As a whole, the proceedings should aim for a balanced presentation of the theme of the conference including a description of the techniques used and enough motivation for a broad readership. It should not be assumed that the published proceedings must reflect the conference in its entirety. (A hsting or abstracts of papers presented at the meeting but not included in the proceedings could be added as an appendix.) When applying for publication in the series Lecture Notes in Physics the volume's editor(s) should submit sufficient material to enable the series editors and their referees to make a fairly accurate evaluation (e.g. a complete list of speakers and titles of papers to be presented and abstracts). If, based on this information, the proceedings are (tentatively) accepted, the volume's editor(s), whose name(s) will appear on the title pages, should select the papers suitable for pubhcation and have them refereed (as for a journal) when appropriate. As a rule discussions will not be accepted. The series editors and Springer-Verlag will normally not interfere with the detailed editing except in fairly obvious cases or on technical matters. Final acceptance is expressed by the series editor in charge, in consultation with Springer-Verlag only after receiving the complete manuscript. It might help to send a copy of the authors' manuscripts in advance to the editor in charge to discuss possible revisions with him. As a general rule, the series editor will confirm his tentative acceptance if the final manuscript corresponds to the original concept discussed, if the quality of the contribution meets the requirements of the series, and if the final size of the manuscript does not greatly exceed the number of pages originally agreed upon. The manuscript should be forwarded to Springer-Verlag shortly after the meeting. In cases of extreme delay (more than six months after the conference) the series editors will check once more the timehness of the papers. Therefore, the volume's editor(s) should estabhsh strict deadlines, or collect the articles during the conference and have them revised on the spot. If a delay is unavoidable, one should encourage the authors to update their contributions if appropriate. The editors of proceedings are strongly advised to inform contributors about these points at an early stage. The final manuscript should contain a table of contents and an informative introduction accessible also to readers not particularly famihar with the topic of the conference. The contributions should be in English. The volume's editor(s) should check the contributions f...

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Propagation of three‐dimensional Alfvén waves in a stratified, thermally conducting solar wind

Cited by 19 publications

References 53 publications

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars

Reflection of Alfvén waves in the corona and solar wind: An impulse function approach

Nonlinear MHD Waves and Turbulence

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