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Investigating Alfvénic wave propagation in coronal open-field regions.

Morton RJ, Tomczyk S, Pinto R - Nat Commun (2015)

Bottom Line: The existence of Alfvén waves far from the Sun has been known since the 1970s, and recently the presence of ubiquitous Alfvénic waves throughout the solar atmosphere has been confirmed.However, the presence of atmospheric Alfvénic waves does not, alone, provide sufficient support for wave-based models; the existence of counter-propagating Alfvénic waves is crucial for the development of turbulence.The results enhance our knowledge of Alfvénic wave propagation in the solar atmosphere, providing support and constraints for some of the recent Alfvén wave turbulence models.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Mathematics and Information Sciences, Northumbria University, Newcastle Upon Tyne NE1 8ST, UK. [2] High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado 80307-3000, USA.

ABSTRACT
The physical mechanisms behind accelerating solar and stellar winds are a long-standing astrophysical mystery, although recent breakthroughs have come from models invoking the turbulent dissipation of Alfvén waves. The existence of Alfvén waves far from the Sun has been known since the 1970s, and recently the presence of ubiquitous Alfvénic waves throughout the solar atmosphere has been confirmed. However, the presence of atmospheric Alfvénic waves does not, alone, provide sufficient support for wave-based models; the existence of counter-propagating Alfvénic waves is crucial for the development of turbulence. Here, we demonstrate that counter-propagating Alfvénic waves exist in open coronal magnetic fields and reveal key observational insights into the details of their generation, reflection in the upper atmosphere and outward propagation into the solar wind. The results enhance our knowledge of Alfvénic wave propagation in the solar atmosphere, providing support and constraints for some of the recent Alfvén wave turbulence models.

No MeSH data available.


Related in: MedlinePlus

Propagating Alfvénic waves in the open-field region.(a) A close-up CoMP intensity image of the northern polar region that demonstrates the locations of the box (dashed white lines) and the tracks (black lines) used in the two methods for isolating propagating waves in the open-field region. The predominantly upwardly propagating waves can be seen in the Doppler velocity time–distance diagrams from the box (b) and from the track (c) methods, where the scale bar indicates the velocity (km s−1). The locations of these Doppler velocity time–distance diagrams are highlighted in a by the white dotted line (b) and black arrow (c). The time–distance diagrams are used to derive the corresponding frequency-wavenumber power plots for the box (d) and track (e) methods. A pronounced ridge of power in the negative frequency domain is clearly visible corresponding to the dominance of outward propagating waves. Crucially, the power plots also reveal the existence of inwardly propagating waves, but with reduced power. The scale bar indicates the power (km2 s−2) to the log base 10. The over-plotted white dashed lines show the calculated average inward and outward wave propagation velocities from the Doppler velocity time–distance diagrams.
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f2: Propagating Alfvénic waves in the open-field region.(a) A close-up CoMP intensity image of the northern polar region that demonstrates the locations of the box (dashed white lines) and the tracks (black lines) used in the two methods for isolating propagating waves in the open-field region. The predominantly upwardly propagating waves can be seen in the Doppler velocity time–distance diagrams from the box (b) and from the track (c) methods, where the scale bar indicates the velocity (km s−1). The locations of these Doppler velocity time–distance diagrams are highlighted in a by the white dotted line (b) and black arrow (c). The time–distance diagrams are used to derive the corresponding frequency-wavenumber power plots for the box (d) and track (e) methods. A pronounced ridge of power in the negative frequency domain is clearly visible corresponding to the dominance of outward propagating waves. Crucially, the power plots also reveal the existence of inwardly propagating waves, but with reduced power. The scale bar indicates the power (km2 s−2) to the log base 10. The over-plotted white dashed lines show the calculated average inward and outward wave propagation velocities from the Doppler velocity time–distance diagrams.

Mentions: We focus on the northern open-field region and examine the Doppler velocities in this part of the corona. It is evident from the data that there is a predominantly outward propagating Doppler signal in this region. To analyse the fluctuations in the Doppler velocities, we use two different methods to extract the data in the form of velocity time–distance diagrams (Fig. 2). The two independent methods are used to validate the analysis techniques and results of each other. For the first method, the crude assumption is made that the direction of potential wave propagation is strictly in the North–South direction. Individual Doppler velocity time–distance diagrams are generated from vertical strips of pixels in the boxed region (shown in Fig. 2a). The presence of the outward propagating Doppler velocity signals is visible in these diagrams and so is the periodic nature of these features, implying that the observed signals correspond to MHD waves.


Investigating Alfvénic wave propagation in coronal open-field regions.

Morton RJ, Tomczyk S, Pinto R - Nat Commun (2015)

Propagating Alfvénic waves in the open-field region.(a) A close-up CoMP intensity image of the northern polar region that demonstrates the locations of the box (dashed white lines) and the tracks (black lines) used in the two methods for isolating propagating waves in the open-field region. The predominantly upwardly propagating waves can be seen in the Doppler velocity time–distance diagrams from the box (b) and from the track (c) methods, where the scale bar indicates the velocity (km s−1). The locations of these Doppler velocity time–distance diagrams are highlighted in a by the white dotted line (b) and black arrow (c). The time–distance diagrams are used to derive the corresponding frequency-wavenumber power plots for the box (d) and track (e) methods. A pronounced ridge of power in the negative frequency domain is clearly visible corresponding to the dominance of outward propagating waves. Crucially, the power plots also reveal the existence of inwardly propagating waves, but with reduced power. The scale bar indicates the power (km2 s−2) to the log base 10. The over-plotted white dashed lines show the calculated average inward and outward wave propagation velocities from the Doppler velocity time–distance diagrams.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525157&req=5

f2: Propagating Alfvénic waves in the open-field region.(a) A close-up CoMP intensity image of the northern polar region that demonstrates the locations of the box (dashed white lines) and the tracks (black lines) used in the two methods for isolating propagating waves in the open-field region. The predominantly upwardly propagating waves can be seen in the Doppler velocity time–distance diagrams from the box (b) and from the track (c) methods, where the scale bar indicates the velocity (km s−1). The locations of these Doppler velocity time–distance diagrams are highlighted in a by the white dotted line (b) and black arrow (c). The time–distance diagrams are used to derive the corresponding frequency-wavenumber power plots for the box (d) and track (e) methods. A pronounced ridge of power in the negative frequency domain is clearly visible corresponding to the dominance of outward propagating waves. Crucially, the power plots also reveal the existence of inwardly propagating waves, but with reduced power. The scale bar indicates the power (km2 s−2) to the log base 10. The over-plotted white dashed lines show the calculated average inward and outward wave propagation velocities from the Doppler velocity time–distance diagrams.
Mentions: We focus on the northern open-field region and examine the Doppler velocities in this part of the corona. It is evident from the data that there is a predominantly outward propagating Doppler signal in this region. To analyse the fluctuations in the Doppler velocities, we use two different methods to extract the data in the form of velocity time–distance diagrams (Fig. 2). The two independent methods are used to validate the analysis techniques and results of each other. For the first method, the crude assumption is made that the direction of potential wave propagation is strictly in the North–South direction. Individual Doppler velocity time–distance diagrams are generated from vertical strips of pixels in the boxed region (shown in Fig. 2a). The presence of the outward propagating Doppler velocity signals is visible in these diagrams and so is the periodic nature of these features, implying that the observed signals correspond to MHD waves.

Bottom Line: The existence of Alfvén waves far from the Sun has been known since the 1970s, and recently the presence of ubiquitous Alfvénic waves throughout the solar atmosphere has been confirmed.However, the presence of atmospheric Alfvénic waves does not, alone, provide sufficient support for wave-based models; the existence of counter-propagating Alfvénic waves is crucial for the development of turbulence.The results enhance our knowledge of Alfvénic wave propagation in the solar atmosphere, providing support and constraints for some of the recent Alfvén wave turbulence models.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Mathematics and Information Sciences, Northumbria University, Newcastle Upon Tyne NE1 8ST, UK. [2] High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado 80307-3000, USA.

ABSTRACT
The physical mechanisms behind accelerating solar and stellar winds are a long-standing astrophysical mystery, although recent breakthroughs have come from models invoking the turbulent dissipation of Alfvén waves. The existence of Alfvén waves far from the Sun has been known since the 1970s, and recently the presence of ubiquitous Alfvénic waves throughout the solar atmosphere has been confirmed. However, the presence of atmospheric Alfvénic waves does not, alone, provide sufficient support for wave-based models; the existence of counter-propagating Alfvénic waves is crucial for the development of turbulence. Here, we demonstrate that counter-propagating Alfvénic waves exist in open coronal magnetic fields and reveal key observational insights into the details of their generation, reflection in the upper atmosphere and outward propagation into the solar wind. The results enhance our knowledge of Alfvénic wave propagation in the solar atmosphere, providing support and constraints for some of the recent Alfvén wave turbulence models.

No MeSH data available.


Related in: MedlinePlus