Geomagnetism and the Emission - Line Corona

This paper summarizes the results of an extensive investigation of the relation between Kp and the intensity of the emission-line corona. We consider the coronal intensity in the northern and the southern solar hemispheres separately, and find that the intensity in the hemisphere on the same side of the solar equator as the earth shows the closer relation to Kp. We discuss the identification of M-regions with areas of unusually weak 5303 corona. Following up results previously reported [see 1 of "References" at end of paper], wehave made a superposed-epoch analysis ofthe relations between Kp and the intensity of the emission-line corona both in the northern and in the southern solar hemispheres. This detailed investigation, which is here only briefly summarized, covers the period 1950-53. To obtain daily indices of the intensity of emission bythe green coronal line, k5303, we averaged the three brightest intensities measured on the east limb of the relevant hemisphere (north or south) by the coronagraph at the Sacramento Peak Station of Harvard Observatory. We used west-limb observations and Climax observations to fill in data for the days when cloudy weather prevented east-limb o servations at Sacramento Peak. We thus obtained two green-line indices daily, one for the northern and one for the southern solar hemisphere. With these two indices we were able to explore further the possibility, previously indicated [1],that (1) the well-known March and September maxima in geomagnetic activity arise from the inclination of the earth's orbital plane to the solar equator, and (2) that geomagnetic activity shows a significantly clearer relation to coronal conditions onthe same (favorable) side of the solar equator as the arth than to coronal conditions on the side opposite the earth (unfavorable). Figure 1 shows the average Kp conditions following east-limb passage (ELP) of the 10 per cent brightest 5303 intensities, in each six-month period for the years 1950-53. The solid and the dashed curves represent, respectively, Kp conditions associated with bright corona in the favorable (f) and in the unfavorable (•) solar hemispheres. Thetwo curves differ conspicuously. The unfavorable curve shows only irregular fluctuations without any significant deviations from the mean. The favorable curve shows a clear weakening of/fp, with a minimum on day 10, or three days after central-meridian passage (CMP). Inspection f the eight alf-years, individually, supports the reality of this difference. The minimum at gp (10) appears much more systematically nd regularly in the favorable curves than in the unfavorable ones.