Waves in the Magnetized Solar Atmosphere. II. Waves from Localized Sources in Magnetic Flux Concentrations

Bogdan, Thomas J.; Hansteen, M. Carlsson V.; McMurry, A. D.; Rosenthal, C. S.; Johnson, Michael D.; Petty-Powell, S.; Zita, E. J.; Stein, R. F.; McIntosh, Scott W.; Nordlund, Åke

doi:10.1086/378512

Cited by 264 publications

(325 citation statements)

References 88 publications

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“…Bogdan et al (2003) call this zone the magnetic canopy or the β ≈ 1 layer. Note that here, we use the terminology β for the (true, varying) plasma-β and β 0 for the (constant) value of the plasma-β at a radius of unity.…”

Section: Mhd Wave Behaviour At β = 0 Null Pointsmentioning

confidence: 99%

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

McLaughlin¹,

Hood²,

Moortel³

2010

Space Sci Rev

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We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvén-speed profile. In a β = 0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.

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Section: Mhd Wave Behaviour At β = 0 Null Pointsmentioning

confidence: 99%

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

McLaughlin¹,

Hood²,

Moortel³

2010

Space Sci Rev

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“…Fast and slow waves have differing properties depending on if they are in a high or low β environment (see e.g. Bogdan et al 2003). See Table 1 for a summary of these properties.…”

Section: Mode Conversion Across the C S = V A Layermentioning

confidence: 99%

MHD mode coupling in the neighbourhood of a 2D null point

McLaughlin¹,

Hood²

2006

A&A

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Context. At this time there does not exist a robust set of rules connecting low and high β waves across the β ≈ 1 layer. The work here contributes specifically to what happens when a low β fast wave crosses the β ≈ 1 layer and transforms into high β fast and slow waves. Aims. The nature of fast and slow magnetoacoustic waves is investigated in a finite β plasma in the neighbourhood of a twodimensional null point. Methods. The linearised equations are solved in both polar and cartesian forms with a two-step Lax-Wendroff numerical scheme. Analytical work (e.g. small β expansion and WKB approximation) also complement the work. Results. It is found that when a finite gas pressure is included in magnetic equilibrium containing an X-type null point, a fast wave is attracted towards the null by a refraction effect and that a slow wave is generated as the wave crosses the β ≈ 1 layer. Current accumulation occurs close to the null and along nearby separatrices. The fast wave can now pass through the origin due to the non-zero sound speed, an effect not previously seen in related papers but clear seen for larger values of β. Some of the energy can now leave the region of the null point and there is again generation of a slow wave component (we find that the fraction of the incident wave converted to a slow wave is proportional to β). We conclude that there are two competing phenomena; the refraction effect (due to the variable Alfvén speed) and the contribution from the non-zero sound speed. Conclusions. These experiments illustrate the importance of the magnetic topology and of the location of the β ≈ 1 layer in the system.

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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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Waves in the Magnetized Solar Atmosphere. II. Waves from Localized Sources in Magnetic Flux Concentrations

Cited by 264 publications

References 88 publications

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

MHD mode coupling in the neighbourhood of a 2D null point

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

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