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Height of shock formation in the solar corona inferred from observations of type II radio bursts and coronal mass ejections
Authors:N Gopalswamy  H Xie  P Mäkelä  S Yashiro  S Akiyama  W Uddin  AK Srivastava  NC Joshi  R Chandra  PK Manoharan  K Mahalakshmi  VC Dwivedi  R Jain  AK Awasthi  NV Nitta  MJ Aschwanden  DP Choudhary
Institution:1. Code 671, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA;2. ARIES Nainital, Manora Peak, Nainital 263 129, India;3. Kumaun University, Nainital 263 129, India;4. TIFR/NCRA Radio Astronomy Center, P.O. Box 8, Ootacamund 643 001, India;5. Physical Research Laboratory, Navrangapura, Ahmedabad 380 009, India;6. Lockheed Martin Solar and Astrophysical Laboratory, 3251 Hanover St., Palo Alto, CA 94304, USA;g California State University, 18111 Nordhoff Street, Northridge, CA 91330, USA
Abstract:Employing coronagraphic and EUV observations close to the solar surface made by the Solar Terrestrial Relations Observatory (STEREO) mission, we determined the heliocentric distance of coronal mass ejections (CMEs) at the starting time of associated metric type II bursts. We used the wave diameter and leading edge methods and measured the CME heights for a set of 32 metric type II bursts from solar cycle 24. We minimized the projection effects by making the measurements from a view that is roughly orthogonal to the direction of the ejection. We also chose image frames close to the onset times of the type II bursts, so no extrapolation was necessary. We found that the CMEs were located in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs), with mean and median values of 1.43 and 1.38 Rs, respectively. We conclusively find that the shock formation can occur at heights substantially below 1.5 Rs. In a few cases, the CME height at type II onset was close to 2 Rs. In these cases, the starting frequency of the type II bursts was very low, in the range 25–40 MHz, which confirms that the shock can also form at larger heights. The starting frequencies of metric type II bursts have a weak correlation with the measured CME/shock heights and are consistent with the rapid decline of density with height in the inner corona.
Keywords:Coronal mass ejections  Shock  Type II radio bursts
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