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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

Direct observations of Alfvénic waves with SDO.An example time–distance diagram (location shown in Fig. 1c,d) reveals that the fine-scale magnetic structure in the open-field region sways periodically (a), and this motion is interpreted from MHD wave theory as the kink mode. These Alfvénic waves observed with SDO contribute to both the CoMP Doppler velocities and non-thermal widths. The wave motion is measured directly (b–d) and the mean values with s.e.m. are 590±5 km for displacement amplitude, 470±6 s for the period and 14.7±0.2 km s−1 for the velocity amplitude. Error bars show the sample s.d. for each bin in the histogram.
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f6: Direct observations of Alfvénic waves with SDO.An example time–distance diagram (location shown in Fig. 1c,d) reveals that the fine-scale magnetic structure in the open-field region sways periodically (a), and this motion is interpreted from MHD wave theory as the kink mode. These Alfvénic waves observed with SDO contribute to both the CoMP Doppler velocities and non-thermal widths. The wave motion is measured directly (b–d) and the mean values with s.e.m. are 590±5 km for displacement amplitude, 470±6 s for the period and 14.7±0.2 km s−1 for the velocity amplitude. Error bars show the sample s.d. for each bin in the histogram.

Mentions: Now, combining CoMP and SDO observations, we are able to provide insight and constraints on the energetics of the observed Alfvénic waves. Using SDO, we measure the swaying motions of the fine-scale structures (Fig. 1d) at a height of ∼1.01 (Fig. 6), which is interpreted in terms of the MHD kink wave mode283160. It is likely to be these swaying motions that correspond to the periodic fluctuations of the CoMP Doppler velocities33. The observational signature of torsional motions is oppositely directed Doppler velocities on either side of the fine structure. Owing to large spatial resolution of CoMP and its inability to resolve the finest structure in the corona, these oppositely directed velocities will average to zero and have a negligible contribution to the measured Doppler velocities. The kink motions are observed to be periodic, with the distribution of periods peaking at ∼300 s (3.3 mHz)—coinciding with the frequency range of the enhanced power in the CoMP observations (Fig. 3a). The measured velocity amplitudes have an average value of 14 km s−1. This value is significantly larger than CoMP Doppler velocity amplitudes (<1 km s−1—Fig. 4e), although the poor spatial resolution of CoMP is known to lead to a significant underestimation of the Doppler velocities due to bulk plasma motions3334. Conversely, the measurements from SDO data are almost a third of the value of the non-thermal velocities (Fig. 4e) that CoMP measures (and other instruments2021232425)—although velocity amplitudes up to 40 km s−1 are found in the SDO measurements (Fig. 6). The reasons for the larger amplitudes inferred from the non-thermal widths are still unclear. Should unresolved torsional Alfvénic motions be present along fine structure, it would then be expected that they would contribute to non-thermal line broadening61. In addition, unresolved kink motions with small displacement amplitudes (so as to be also unobservable with SDO) would also contribute.


Investigating Alfvénic wave propagation in coronal open-field regions.

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

Direct observations of Alfvénic waves with SDO.An example time–distance diagram (location shown in Fig. 1c,d) reveals that the fine-scale magnetic structure in the open-field region sways periodically (a), and this motion is interpreted from MHD wave theory as the kink mode. These Alfvénic waves observed with SDO contribute to both the CoMP Doppler velocities and non-thermal widths. The wave motion is measured directly (b–d) and the mean values with s.e.m. are 590±5 km for displacement amplitude, 470±6 s for the period and 14.7±0.2 km s−1 for the velocity amplitude. Error bars show the sample s.d. for each bin in the histogram.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6: Direct observations of Alfvénic waves with SDO.An example time–distance diagram (location shown in Fig. 1c,d) reveals that the fine-scale magnetic structure in the open-field region sways periodically (a), and this motion is interpreted from MHD wave theory as the kink mode. These Alfvénic waves observed with SDO contribute to both the CoMP Doppler velocities and non-thermal widths. The wave motion is measured directly (b–d) and the mean values with s.e.m. are 590±5 km for displacement amplitude, 470±6 s for the period and 14.7±0.2 km s−1 for the velocity amplitude. Error bars show the sample s.d. for each bin in the histogram.
Mentions: Now, combining CoMP and SDO observations, we are able to provide insight and constraints on the energetics of the observed Alfvénic waves. Using SDO, we measure the swaying motions of the fine-scale structures (Fig. 1d) at a height of ∼1.01 (Fig. 6), which is interpreted in terms of the MHD kink wave mode283160. It is likely to be these swaying motions that correspond to the periodic fluctuations of the CoMP Doppler velocities33. The observational signature of torsional motions is oppositely directed Doppler velocities on either side of the fine structure. Owing to large spatial resolution of CoMP and its inability to resolve the finest structure in the corona, these oppositely directed velocities will average to zero and have a negligible contribution to the measured Doppler velocities. The kink motions are observed to be periodic, with the distribution of periods peaking at ∼300 s (3.3 mHz)—coinciding with the frequency range of the enhanced power in the CoMP observations (Fig. 3a). The measured velocity amplitudes have an average value of 14 km s−1. This value is significantly larger than CoMP Doppler velocity amplitudes (<1 km s−1—Fig. 4e), although the poor spatial resolution of CoMP is known to lead to a significant underestimation of the Doppler velocities due to bulk plasma motions3334. Conversely, the measurements from SDO data are almost a third of the value of the non-thermal velocities (Fig. 4e) that CoMP measures (and other instruments2021232425)—although velocity amplitudes up to 40 km s−1 are found in the SDO measurements (Fig. 6). The reasons for the larger amplitudes inferred from the non-thermal widths are still unclear. Should unresolved torsional Alfvénic motions be present along fine structure, it would then be expected that they would contribute to non-thermal line broadening61. In addition, unresolved kink motions with small displacement amplitudes (so as to be also unobservable with SDO) would also contribute.

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