Sanjiv K. Tiwari1, David A. Falconer2, Ronald L. Moore1,2, P. Venkatakrishnan3, Amy R. Winebarger1, & Igor G. Khazanov2
1 NASA Marshall Space Flight Center, Huntsville, Alabama, USA
2 Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, Alabama, USA
3 Udaipur Solar Observatory, Physical Research Laboratory, Udaipur, India.
Speed of solar coronal mass ejections (CMEs) is one important parameter, among others, e.g., their direction, width, mass, and the orientation and strength of the magnetic field therein, that determines the severity of geomagnetic and energetic particle storms. Active regions (ARs) on the Sun are the main sources of the biggest flares and the most energetic CMEs1,2. The magnetic non-potentiality of an AR, inferred by, for instance, free magnetic energy proxies and magnetic-twist parameters, most likely determines the initial speed of CMEs emanating from ARs.
Figure 1 | An example event. (a) LASCO C2 image when the CME becomes clearly visible outside the C2 occulting disk. (b) (c) STEREO-A and B images verifying that the CME is Earth-directed. (d) Image taken from the AIA 193 Å movie confirming the position and the NOAA number of the AR responsible for the CME. (e) Deprojected vector magnetogram containing the source AR 11520. The size and direction of the red vectors, overplotted on the grey-scaled vertical-field magnetogram, show the magnitude and direction of the horizontal field. The longest/shortest vector is for 500/100 G field strength.
From the SoHO Large Angle and Spectrometric Coronagraph (LASCO) CME catalog3, we selected all CMEs that took place between the start of the SDO mission (May 2010) through March 2014 (end of the catalog at the time of our analysis) which had a plane-of-sky width greater than 30˚ and a co-temporal flare (tcme − 2 h < tflare < tcme + 30 min) from an NOAA AR (within 45˚ E/W from disk center). We found 946 CMEs following our criteria, for each of which we manually verified that (1) the CME was not seen in LASCO C2 before the start of the flare, (2) the CME occurred in the same quadrant as the source AR, and (3) there was not a second flare occurring in another AR at about the same time. If there was a second flaring AR, we further verified that it was not the source of the CME under investigation. We inspected LASCO-C2 movies and GOES X-ray light curves to make sure that the prospective flaring source AR was present on the front side of the Sun. By inspecting STEREO A and B movies we ensured that the CME was directed toward the Earth. We then used Atmospheric Imaging Assembly (AIA) 193 Å movies to determine which AR flare (out of sometimes several listed) was co-produced with the CME under investigation (see an example in Fig. 1).
This careful selection procedure yielded a sample of 252 CMEs with known flaring source ARs, which was further reduced by the requirement that there was available a definitive Helioseismic and Magnetic Imager4 (HMI) AR Patch (HARP) vector magnetogram that covered the source AR, which was taken within 12 h of the CME flare and had its magnetic flux centroid within 45 heliocentric degrees from disk center. We also required that (1) the source AR is the only NOAA AR in the HARP tile, (2) the values of parameters are mostly (≥ 90%) from the AR and only negligibly (≤ 10%) from other parts of the tile, or (3) if many ARs exist in the HARP, they are close together and can be treated as one AR. This left a sample of 189 CMEs that we finally kept for our study.
Figure 2 | Log-log scatter plots of CME speed versus six different magnetic parameters (out of ten studied in Ref. ) of the source ARs. Plots are for AR magnetic size (total magnetic flux Φ), two whole-AR magnetic twist parameters (αg and MSSA), and three AR free-energy proxies (WLSS, MSSA × Φ, and αg × Φ). The red dashed line in each panel, drawn to guide the eye, suggests an upper bound of speed as a function of the magnetic parameters.
Logarithmic plots of the plane-of-sky speed of CMEs (obtained from the LASCO catalog) versus ten AR-integrated magnetic parameters (magnetic flux, three magnetic-twist parameters, and six free-magnetic-energy proxies, see Fig. 2 for plots of six of these parameters) measured from HMI vector magnetograms show that (1) the speed of the fastest CMEs that an AR can produce increases with each of these magnetic parameters, and that (2) one of the magnetic-twist parameters (αg) and the corresponding free-magnetic-energy proxy (αg × Φ) each determines the CME speed upper limit somewhat better than the other eight parameters. We conclude that the speed of the fastest CMEs that an AR can produce can be predicted from a vector magnetogram of the AR. Since fast CMEs tend to be a greater threat than slower ones, knowing that an AR cannot produce a fast CME would be a useful forecast. Our results can be incorporated in near real time forecasting tools, e.g., MAG4. Further analysis with improved statistics will confirm or modify our results.
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 Tiwari, S. K., Falconer, D. A., Moore, R. L., Venkatakrishnan, P., Winebarger, A. R., & Khazanov, I. G., 2015, GRL, 42, 5702