East-West Asymmetry of the Yohkoh Soft X-ray Corona L.W. Acton 1, D.E. McKenzie 1, A. Takeda 1, B.T. Welsch 2,and H.S. Hudson 2,3 1 Montana State University, 2 SSL, UC Berkeley, 3 University of Glasgow Fig. 1 (above). Three SXT soft X-ray images from 2000, December 7 ± 4 days. Asymmetry is clear visually in the 3D structure of the coronal hole. SYNOPSIS: Yohkoh/SXT observations reveal a systematic prograde tilt of the lower solar corona. Fig. 2 (right). Synoptic charts produced at different longitudes (top image shows central meridian). Comparison of the E and W maps again shows the asymmetry clearly. This particular Carrington rotation was chosen arbitrarily; the visual asymmetry is pervasive and appears in many such multi-longitude synoptic comparisons. Conclusion from appearance: The large-scale corona has a prograde tilt that is readily visible in soft X-ray images. If this tilt arises from a nonzero average azimuthal magnetic field, then large-scale current systems would must be present, and the field cannot be modeled as potential. Fig. 4 (left) shows an image segmented for accumulation of MDI and SXT data. In each segment, we compare the soft X-ray signal in that lune with the MDI unsigned magnetic flux uniformly shifted in longitude, with each shifted point interpolated to the times of the SXT images. This cross-correlation can be measured as days of rotation. Fig. 5 (below left). Sketch of the geometry, viewing the Sun from above the N pole. The blue dotted lines show photospheric magnetic field, the red lines the overlying soft X-ray corona for both prograde and retrograde tilts. As viewed from below, the two senses of shift have different lead/lag relations. Fig. 6 (right). Results of cross-correlations. The lines show linear fits to the observed displacements as a function of lag. The slope implies a mean height of about 12 Mm, and the intercept is consistent with a prograde tilt of about 4 degrees (blue line). A retrograde tilt would result in the red line. The slope is governed by the mean height. As is well-known from casual inspection of SXT and other soft X-ray images, the coronal X-ray brightness strongly correlates with the intensity of the magnetic field at the photosphere as measured by standard Zeeman-splitting techniques (e.g., Fisher et al., 1998; Pevtsov et al., 2003). The fully reduced “Legacy” database from Yohkoh/SXT fully confirm this (Figure 3 below). The most plausible sources of a true asymmetry in the structure of the solar corona would be stresses systematically imposed from below the photosphere, or torques induced by the acceleration of the solar wind. The latter would seemingly produce a retrograde tilt. Minnaert (1946), following work by E. Maunder and Gleissberg, deduced a small physical tilt in sunspot axes. Such evidence could suggest a systematic stress imposed by deep-seated field structures affected by interior flows. Anthony Yeates (Dundee) has now pointed out that the equatorial acceleration of the photospheric flows (i.e., the differential rotation) may naturally induce a prograde tilt at low altitudes. This results from the inertia of the corona, which must be accelerated to match the apparent westward motion of field anchored at higher latitudes both in the north and in the south. Such an effect would not be present in PFSS models or in any purely mathematical extrapolations of the field, ie those in the low-  limit. Fig. 3 (above). Correlation for year 2000 between SXT soft X-ray flux and MDI total unsigned magnetic flux, using the central segment of each of the 11,456 individual images (see panel C for the geometry and for a spatial cross-correlation analysis). References Acton et al. (2010): Fisher, G., et al., ApJ 508, 885 (1998) Mackay, D. & Van Ballegooijen, A., ApJ 641, 177 (2006) Minnaert, M.G.J., MNRAS 106, 98 (1946) Pevtsov, A., et al. ApJ 598, 1387 (2003) Spatial cross-correlation: Basic geometrical analysis Strong association of soft X-rays and magnetic field The future This explanation makes predictions for the three-dimensional structure of the tilts, which we have not described here. In future developments of this work we aim to compare the observed 3D structure of the observed asymmetries with models such as those of Mackay & Van Ballegooijen (2006). The SXT Legacy database will also allow us to study the evolution of these effects across the full interval , which spans roughly the maxima of Cycles 22 and 23. Note that Yohkoh’s 20 th anniversary is August 31, The mystery and a possible resolution (due to A. Yeates) A B C D