Oran R., E. Landi, B. van der Holst, S. T. Lepri, A. M. Vásquez, F. A. Nuevo, R. Frazin, W. Manchester, I. Sokolov, T. I. Gombosi, (2015), A Steady-state Picture of Solar Wind Acceleration and Charge State Composition Derived from a Global Wave-driven MHD Model, The Astrophysical Journal, 806, 55, doi:10.1088/0004-637x/806/1/55
Abstract
The higher charge states found in slow solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops and is released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using the Alfvén Wave Solar Model (AWSoM), a global MHD model driven by Alfvén waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge state calculation covering all latitudes in a realistic magnetic field. The ratios and are compared to in situ Ulysses observations and are found to be higher in the slow wind, as observed; however, they are underpredicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to Extreme-ultraviolet Imaging Spectrometer (EIS) observations above a coronal hole. The agreement is partial and suggests that all ionization rates are underpredicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest that there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows that it originates from coronal hole boundaries (CHBs), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global and observationally supported by EUV tomography.Authors (sorted by name)
Frazin Gombosi Landi Lepri Manchester Nuevo Oran Sokolov van der HolstJournal / Conference
The Astrophysical JournalAcknowledgments
This work was supported by the NSF grant AGS 1322543. The work of E. Landi is supported by NASA grants NNX10AQ58G, NNX11AC20G, and NNX13AG22G. The authors would like to thank the referee for very constructive and useful suggestions. The simulations performed in this work were made possible thanks to the NASA Advanced Supercomputing Division, which granted us access to the Pleiades Supercomputing cluster. Analysis of radiative processes was made possible through the use of the CHIANTI atomic database. CHIANTI is a collaborative project involving the following universities: Cambridge (UK), George Mason, and Michigan (USA).
Grants
AGS1322543 NNX10AQ58G NNX11AC20G NNX13AG22GBibtex
@article{0004-637X-806-1-55,
author={R. Oran and E. Landi and B. van der Holst and S. T. Lepri and A. M. Vásquez and F. A. Nuevo and R. Frazin and W. Manchester and I. Sokolov and T. I. Gombosi},
title={A Steady-state Picture of Solar Wind Acceleration and Charge State Composition Derived from a Global Wave-driven MHD Model},
journal={The Astrophysical Journal},
volume={806},
doi = {10.1088/0004-637x/806/1/55},
number={1},
pages={55},
url={http://stacks.iop.org/0004-637X/806/i=1/a=55},
year={2015},
abstract = {The higher charge states found in slow solar wind streams compared to fast streams have supported the hypothesis that the slow wind originates in closed coronal loops and is released intermittently through reconnection. Here we examine whether a highly ionized slow wind can also form along steady and open magnetic field lines. We model the steady-state solar atmosphere using the Alfvén Wave Solar Model (AWSoM), a global MHD model driven by Alfvén waves, and apply an ionization code to calculate the charge state evolution along modeled open field lines. This constitutes the first charge state calculation covering all latitudes in a realistic magnetic field. The ratios and are compared to in situ Ulysses observations and are found to be higher in the slow wind, as observed; however, they are underpredicted in both wind types. The modeled ion fractions of S, Si, and Fe are used to calculate line-of-sight intensities, which are compared to Extreme-ultraviolet Imaging Spectrometer (EIS) observations above a coronal hole. The agreement is partial and suggests that all ionization rates are underpredicted. Assuming the presence of suprathermal electrons improved the agreement with both EIS and Ulysses observations; importantly, the trend of higher ionization in the slow wind was maintained. The results suggest that there can be a sub-class of slow wind that is steady and highly ionized. Further analysis shows that it originates from coronal hole boundaries (CHBs), where the modeled electron density and temperature are higher than inside the hole, leading to faster ionization. This property of CHBs is global and observationally supported by EUV tomography.}
}