Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind

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Abstract

Energy is required to heat the outer solar atmosphere to millions of degrees (refs 1, 2) and to accelerate the solar wind to hundreds of kilometres per second (refs 2–6). Alfvén waves (travelling oscillations of ions and magnetic field) have been invoked as a possible mechanism to transport magneto-convective energy upwards along the Sun’s magnetic field lines into the corona. Previous observations7 of Alfvénic waves in the corona revealed amplitudes far too small (0.5 km s−1) to supply the energy flux (100–200 W m−2) required to drive the fast solar wind8 or balance the radiative losses of the quiet corona9. Here we report observations of the transition region (between the chromosphere and the corona) and of the corona that reveal how Alfvénic motions permeate the dynamic and finely structured outer solar atmosphere. The ubiquitous outward-propagating Alfvénic motions observed have amplitudes of the order of 20 km s−1 and periods of the order of 100–500 s throughout the quiescent atmosphere (compatible with recent investigations7,10), and are energetic enough to accelerate the fast solar wind and heat the quiet corona.

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Figure 1: Ubiquitous Alfvénic motion above the solar limb.
Figure 2: Examining Alfvénic motion in coronal hole (top row) and quiet Sun (bottom row) regions.
Figure 3: Examining Alfvénic motion in an active region of the Sun.
Figure 4: Determining the phase speed of the Alfvénic motions.

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Acknowledgements

SDO is the first mission of NASA’s Living With a Star Program. NCAR is sponsored by the NSF.

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Authors

Contributions

S.W.M. (with B.D.P. and M.C.) performed all image processing and analysis of observations. S.W.M. and M.C. calculated phase speeds. P.B. (with B.D.P. and V.H.H.) designed the special observing sequences. B.D.P. co-aligned the data, performed Monte Carlo simulations (with V.H.H.) and provided density estimates. M.G. assisted with the identification of the wave mode. S.W.M. and B.D.P. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Scott W. McIntosh.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

The file contains Supplementary Figures 1-5 with legends, Supplementary Text and Supplementary Table 1. (PDF 11819 kb)

Supplementary Movie 1

This movie shows SDO/AIA 304Å observations of a polar coronal hole showing the continuous transverse motion of spicules. The movie covers the entire 75 minutes of the observations studied. (MOV 3670 kb)

Supplementary Movie 2

This movie shows unsharp masked SDO/AIA 171Å observations of a polar coronal hole showing the continuous transverse motion of propagating disturbances. The movie covers the entire 75 minutes of the observations studied. (MOV 9909 kb)

Supplementary Movie 3

This movie shows SDO/AIA 304Å (left) and 171Å (right) observations of a small portion of a polar coronal hole showing the continuous transverse motion of spicules and propagating disturbances as we move upward from the limb. Top row shows the raw data and the bottom row show the respective unsharp masked data. (MOV 5858 kb)

Supplementary Movie 4

This movie shows SDO/AIA 304Å (left) and 171Å (right) observations of a small portion of a quiet Sun showing the continuous transverse motion of spicules and propagating disturbances as we move upward from the limb. Top row shows the raw data and the bottom row show the respective unsharp masked data. (MOV 8494 kb)

Supplementary Movie 5

This movie shows SDO/AIA 171Å observations of an active region showing the continuous transverse motion of the coronal loop system. The region in the box is cut-out for closer inspection and presented in SI Movie 7. The movie covers the entire 75 minutes of the observations studied. (MOV 1949 kb)

Supplementary Movie 6

This movie shows the unsharp masked SDO/AIA 171Å observations of an active region showing the continuous transverse motion of the coronal loop system. The region in the box is cut-out for closer inspection and presented in SI Movie 7. The movie covers the entire 75 minutes of the observations studied. (MOV 10838 kb)

Supplementary Movie 7

This movie shows the loop waves and evolution from SDO/AIA 171Å observations of an active region coronal loop system in the raw data (left) and unsharp-masked to remove the background emission (right). The movie covers the entire 75 minutes of the observations studied. (MOV 2487 kb)

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McIntosh, S., De Pontieu, B., Carlsson, M. et al. Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind. Nature 475, 477–480 (2011). https://doi.org/10.1038/nature10235

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