Abstract
We address the problem of calculating the transverse magnetic field in the solar wind outside of the hypothetical sphere that is called the source surface where the solar wind originates. This calculation must overcome a widely used fundamental assumption about the source surface – the field is normally required to be purely radial at the source surface. Our model rests on the fact that a change in the radial field strength at the source surface is a change in the field line density. Surrounding field lines must move laterally to accommodate this field line density change. As the outward wind velocity drags field lines past the source surface, this lateral component of motion produces a tilt, implying there is a transverse component to the field. An analytic method of calculating the lateral translation speed of the field lines is developed. We apply the technique to an interval of approximately two Carrington rotations at the beginning of 2011 using 2-h averages of data from the Helioseismic Magnetic Imager instrument onboard the Solar Dynamics Observatory spacecraft. We find that the value of the transverse magnetic field is dominated on a global scale by the effects of high-latitude concentrations of field lines that are buffeted by supergranular motions.
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Acknowledgements
We thank Jack Harvey for several helpful suggestions including bringing the perceptive quote from Zurbuchen to our attention. We thank Chen Shi for uncovering an error in the sign of \(B_{\theta}\) in our original equations. This error is corrected in the current paper. The HMI images used in preparation of this article are courtesy of NASA/SDO and the science team. This research has been supported by NASA through award NNX15AF39G to Predictive Science, Inc. and subaward to UCLA.
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11207_2016_882_MOESM2_ESM.mp4
This video shows the series of hybrid photospheric maps like that in Figure 4. The maps are stationary relative to the central meridian as seen by HMI while the solar longitudes are shifted each frame. (MP4 17.8 MB)
11207_2016_882_MOESM3_ESM.mp4
This video shows the projection from above the south pole of the maps seen in Figure 5 and the video of Figure 10. Note that the solar coordinates are stationary in this video while the observed portion of the solar surface moves around the circular image. (MP4 18.1 MB)
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Ulrich, R.K., Tran, T. Generation of a North/South Magnetic Field Component from Variations in the Photospheric Magnetic Field. Sol Phys 291, 1059–1076 (2016). https://doi.org/10.1007/s11207-016-0882-5
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DOI: https://doi.org/10.1007/s11207-016-0882-5