1. Identifying and Modeling Coronal Holes Observed by SDO/AIA, STEREO /A and B Using HMI Synchronic Frames X. P. Zhao, J. T. Hoeksema, Y. Liu, P. H. Scherrer Stanford University AGU Fall Meeting, San Francisco SH11A-1616, Dec. 13, 2010

2. 1. Purpose of the Work The large dark regions in SDO/AIA EUV images are usually believed to be coronal holes. But it may not always be true. This work tries to find ways for identifying coronal holes in AIA EUV images, and to improve the method of modeling coronal holes observed by SDO/AIA, STEREO/A & B.

3. 2. Observation of the 2010.08.25 Sun 2.1 AIA EUV images Six AIA EUV channels are dominated by iron emission lines: 94Ǻ, 131Ǻ, 171Ǻ, 193Ǻ, 211Ǻ and 335Ǻ. The dark regions in the 6 AIA EUV images show similar shape but different size. Are all dark regions, say the dark regions I and II in right middle panel, coronal holes? Fig 1 Six AIA EUV images observed at 2010.08.25_23h. I II 94131 171 193 211335

4. 2.2 STEREO/ A & B Images Fig 2. Images observed by STEREO/A (195 Ǻ), SDO/AIA (193Ǻ), and STEREO/B (195Ǻ) at 2010.08.25_23h. The south & north ends of the dark region I, Is & In in AIA image, are recognized in the images of A & B. Fig 3 Orbit position of the STEREO Ahead & Behind. The separation of A & B from the Earth are 80.0° & -74.0° respectively. Is In I B 195A 195AIA 193

5. 3. Identifying Large Coronal Holes in EUV Images As shown in Fig 1, there are two mid-latitude large dark regions in the northern hemisphere: regions I and II. In the right column the two regions look connected, but in the left column they are separated from each other. Are both coronal holes? Large coronal holes originate in large unipolar regions. By combining magnetograms with EUV images, the above question may be answered (see Fig 4).

6. Fig 4 shows that dark region I occurred in a large unipolar region and it is a coronal hole. But dark region II occurred along the polarity inversion line and it is certainly not a hole, but a filament, as confirmed in the BBSO Hα image (see bottom panel). Fig 4. The AIA images (top row), HMI & WSO magnetograms (middle row), and BBO Hα images (bottom row). All are observed on 2010.08.25. I II I HMIWSO BBSO Hα AIA

7. 4. Modeling Coronal Holes Using An HMI Synchronic Frame 4.1 High resolution synoptic frames In order to improve modeling of the boundaries of coronal holes we have constructed a synoptic frame [Zhao & Hoeksema, 1997]. The top panel of Fig 5 is the 2010.08.25_15h HMI line-of-sight synoptic frame. It shows that the 2010.08.25 STEREO A & B orbit position (the red and blue dashed lines) are located at CT2100:228 & CT2100:078 in the synoptic frame. The two Carrington longitudes correspond to terrestril central meridian passing dates of 2010.08.19 & 2010.08.31, respectively.

8. Because of the differential rotation of the Sun and the time variation of the photospheric magnetic field itself, the synoptic frame is not as good for modeling the holes observed by STREREO A & B. It is well known that coronal structures are governed mainly by their underlying photospheric magnetic field. All successful modeling of coronal holes so far are using synoptic maps or synoptic frames with their underlying photospheric field were observed at the same time as the observational time of the holes. It is a challenge to model the coronal holes observed by STEREO A & B.

9. Fig 5 The top (bottom) panel is the synoptic (synchronic) frame of 2010.08.25_15h. The area between the two black vertical lines is from the HMI magnetogram of 2010.08.25_15h, the areas outside the central area are from synoptic maps. The red, green and blue vertical dashed lines denote the orbit position of the STEREO Ahead, Earth and STEREO Behind on 2010.08.25. The area between the two green (red & blue) lines faces SDO (STEREO A & B). Slight differences between the two frames can be found in the shape of the strong field regions near the edges of two sides. Synoptic frame Synchronic frame

10. Fig 6 The top (bottom) row compares of 2010.08.25 STEREO A (B) 195 Image with 2010.08.19 (2010.08.31) AIA 193 image. 4.2 High resolution synchronic frame If the large scale magnetic field is stable in the time scale of half of a solar rotation, the effect of differential rotation contained in synoptic frames may be corrected on the basis of the differential rotation rate of magnetic fields. To examine the time variation of the photospheric field seven days apart, i.e., between 2010.08.25 and 2010.08.19 (2010.08.31), we compare the STEREO A (B) EUVI images observed at 2010.08.25 with the AIA images observed at 2010.08.19 (2010.08.31), as shown in Fig 6. BAIA A A B

11. The top and bottom rows of Fig 6 show that there is no significant time variation between STEREO and AIA images, suggesting that the magnetic field is indeed nearly stable in the period of time. By correcting the affect of differential rotation, we have constructed a synchronic frame [Zhao, Hoeksema, Scherrer, 2010]. As shown in the bottom panel of Fig 5, the abscissa in the synchronic frame is set for the reference time of 2010.08.25_15h, as if the magnetic field on whole surface were observed simultaneously, and it is good for modeling coronal holes observed by SDO/AIA and STEREO A & B.

12. 4.3 The synchronic frame as input to the PFSS model Before inputting the 2010.08.25_15h synchronic frame into the PFSS model, we need to convert the line-of-sight component to radial component (see left panel of Fig 7), to fill up the data gap in the southern polar region (the left second panel), and to reduce the spatial resolution for obtaining large-scale magnetic field (the left third and fourth panels). In calculating foot-points of open field lines or open regions, we tried four values of the principal order, N, in the spherical harmonic expansion, i.e, N=9, 22, 45, & 90.

13. Fig 7 The whole surface distribution of the photospheric field with spatial grid of 3600x1440, 360x180, & 72x30 (7.2, 7.3, 7.4). Fig 8 Calculated open field regions using N=9,22,45,90. Red and blue denote inward and outward polarity. 7.1 Radial 7.2 Polar correction 7.3 Medium resolution 7.4 Low resolution

14. 4.4 Calculating open field regions Based on synchronic frames with spatial grid of 72x30 (360x180) (see the panels in Fig 7), we first calculate the spherical harmonic coefficients with the principal order N=22 (90), then calculated open field regions using PFSS model with N=9 & 22 (N=45 & 90). The calculated open field regions are shown in Fig 8. The green (red & blue) dashed line denotes the SDO (STEREO A & B) orbit position. The surface between two green (red & blue) lines is facing SDO (STEREO A & B) (see also Fig 5). A part of large inward open field regions in the top two panels of Fig 8 occur in outward photospheric field regions, implying that the result obtained with N=9 & 22 may not be acceptable.

15. 4.5 Comparison of calculated open field regions with observed holes By projecting the calculated open field regions between two green vertical lines onto the 2010.08.25 AIA 193 image, we obtain Fig 9. Fig 9 shows that the open field regions obtained with N=45 agree with observations better than N=9 & 22, and are nearly the same as N=90. By projecting the calculated open field regions between two red & blue vertical lines onto the 2010.08.25 STEREO A & B 195 images, we obtain Fig 10. Fig 10 shows that the open field regions obtained with N=45 agree with observations nealy same as N=90.

16. Fig 9. Comparison of calculated open regions with the 2010.08.25_23h AIA image, showing that N=45 is better than N=9, 22 and similar to N=90. This further confirms that the dark region II is not an open region or coronal hole. II I N=9 N=22 N=45 N=90

17. Fig 10. Comparison of calculated open field regions using Nmax=45 & 90 with the dark regions in STEREO A and B EUVI images. The same agreement between calculation and observation suggests the usefulness of the synchronic frame in modeling STEREO holes. A N=45 B N=45 A N=90 B N=90

18. 5. Summary Not all dark regions in AIA EUV images are coronal holes. By combining HMI or WSO magnetograms with AIA images, coronal holes and filaments can be identified in AIA EUV images. By comparing the STEREO EUVI images with SDO EUV images observed at same orbit position but different times, the large-scale field can be shown to be nearly constant in the time interval of around 7 days during ascending phase of solar cycle.

19. Because of around 7-day constancy of the large-scale photospheric magnetic field, the synchronic frame constructed by correcting the effect of differential rotation is indeed a proxy of the instant whole surface distribution of the large-scale photospheric magnetic field. It is expected to be true for all activity phases except the maximum phase. The agreement between calculated and observed coronal holes shows that the 360x180 HMI synchronic frame can be used as an input to the PFSS model and reproduce with N=45 the coronal structures observed simultaneously by SDO and STEREO A & B.

20. A remaining question is how to understand the different size of identified holes in different EUV lines (as shown in Fig 1). We are investigating the issue using synchronic frames. -- END--