Paulett C. Liewer1, Jiong Qiu2, Charles Lindsey3
1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
2. Department of Physics, Montana State University, Bozeman, MT 59717, USA
3. Northwest Research Associates, Boulder, CO 80301, USA
Seismic maps of the Sun’s far hemisphere, computed using Doppler data from the Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO) are now being used routinely to detect strong magnetic field regions on the far side of the Sun ( http:/jsoc.stanford.edu/data/farside/ ). To test the reliability of this technique, the helioseismically inferred active region detections are compared with far-side observation of solar activity from the Solar TErrestrial RElations Observatory (STEREO), using extreme ultraviolet (EUV) intensity as a proxy for magnetic field strength. This work extends previous work testing the reliability of helioseismic far-side detections1,2.
To make the comparisons, we use all-Sun Carrington maps in EUV 304 Å, which combine data from STEREO A, B and SDO/AIA and are created on same twice-daily cadence as the far-side seismic maps at http:/jsoc.stanford.edu/data/farside/ . The comparison of the far-side seismic (FS) regions with EUV plages is a three-step process: (1) Identify major EUV plages at each time (each EUV map); (2) Calculate the area-integrated EUV intensity of each plage, and (3) When possible, associate the plages with corresponding FS regions identified in the JSOC seismic maps (Figure 1). As a fourth step, we also associate the major EUV plages with NOAA-designated magnetic regions. The methodology used is described in Reference 3.
Figure 1| Seismic Carrington map (0 to 360° in longitude, -90° to +90° in latitude) for 2012 January 18 at 12UT. Four strong-field far-side seismic (FS) regions were identified at this time. FS regions are labeled by year and number, e.g., FS-2012-002 was the second strong field region to be detected seismically on the far side in 2012.
Seismic and EUV 304 Å maps for the same date and time are shown in Figures 1 and 2, respectively. Four FS regions are identified in the seismic map in Figure 1, labeled by year and order of detection. Each FS regions can be tracked for several days. The seismic signature strength for each FS region, derived from an area-integrated phase shift, can be found in data files at same web site as the far-side seismic maps. Figure 2 shows the corresponding EUV Carrington map. The white dotted and solid boxes demark major EUV plages identified in this map; they can be tracked for their entire lifecycle using the Carrington EUV maps. Solid white indicates that the plage has been associated with an FS region (Step 3). The outline of FS regions (cf. Figure 1) is also shown on the EUV map.
Figure 2| EUV map for the same time as Figure 1 (2012 January 18 at 12 UT). The major EUV plages are demarked by solid and dashed white boxes; a solid white box indicates the plage has been associated with an FS region. The FS region label is at the centroid’s longitude. One plage box encompasses two FS regions (FS004 and FS005). FS002 and FS003 are associated with future NOAA regions AR 11410 and 11408.
From analysis of nine months of data in 2011 and 2012, we found that 100% (22) of the identified FS regions correspond to major EUV plages. Moreover, 95% of the magnetic regions associated with these plages either became a NOAA-designated active region when reaching the east limb or were one before crossing to the far side hemisphere. The far-side seismic region not associated with a NOAA-designated region could be associated with a low intensity plage and a small magnetic bipole. Thus, there were no false seismic detections of far-side strong field regions among the set of 22 far-side seismic regions studied, an important result for space weather prediction. A low but significant correlation is found between the seismic signature strength and the EUV intensity of a far-side region. The comparison of area-integrated EUV intensity and seismic signature strength is shown in Figure 3. Despite the large scatter in the data, these plots indicate an overall trend that stronger seismic signatures are usually related to EUV plages of larger intensities.
Figure 3| Comparison of the mean area-integrated EUV intensity (normalized to the quiet Sun intensity times the spherical area 0.02 steradian) and mean seismic signal strength (in units of a millionth of a hemisphere radian) for the associated regions, both averaged over the time of the seismic detections. The vertical and/or horizontal bars indicate the standard deviation of the EUV intensity and/or seismic strength. The Spearman’s rank correlation coefficient for this pair of data (21 data points) is 0.52.
Following up on the work in Ref. 2, we also determine whether or not new large east-limb active regions are detected seismically on the far side before they appear Earth side and study how the detectability of these regions relate to their EUV intensity. We find that, while there is a range of EUV intensities for which far-side regions may or may not be detected seismically, there appears to be an intensity level above which they are almost always detected and an intensity level below which they are never detected.
 Liewer, P. C., González Hernández, I., Hall, J. R., W. T. Thompson, W. T., Misrak, A.: 2012, SoPh, 281, 3
 Liewer, P. C., Hall, J. R., Lindsey, C., Lin, X.: 2014, SoPh, 289, 3617
 Liewer, P. C., Qiu, J., Lindsey, C.: 2017, SoPh, 292, 146