l6o REPORTS ON INVESTIGATIONS AND PROJECTS. 



of the vortex structure, classified as unipolar, bipolar, or multipolar; (b) 

 bright Ha structure about spots; (c) eruptions; (d) filaments, and the angle 

 they make with the equator; (e) angle between the axis of the multipolar 

 Ha flocculi and the equator. 



The characteristic forms of the Ha flocculi and the discovery that the two 

 larger spots lying at opposite ends of a group are usually, if not always, of 

 opposite polarity, led to the classification of the Ha flocculi as (i) unipolar 

 (a single spot or bright flocculus surrounded by simple vortex structure, 

 right-handed, left-handed, or radial) ; (2) bipolar (spot group or two 

 spots — one sometimes absent or replaced by bright flocculus — surrounded 

 by Ha flocculi resembling in form the lines of force connecting the opposite 

 poles of a bar magnet) ; (3) multipolar (spot group surrounded by curved 

 Ha flocculi of special form) ; (4) filaments (very dark, usually long and 

 narrow) ; and (5) eruptions (very bright, rapidly changing in form). This 

 classification with some subdivisions will include most of the more character- 

 istic flocculi. 



A study of 29 spots in the early stage of formation indicates that the first 

 sign of activity is given by the appearance of bright Ha flocculi in the spot 

 region with or without dark filaments, and with or without disturbances in 

 the surrounding region. As the spot develops the surrounding disturbance 

 appears or increases, and the vortex structure usually presents itself. In 

 the bipolar type the bright flocculi are usually seen first between the spots, 

 while in the other cases they are closely massed about the spot or group. 

 This favors the hypothesis, based upon observation of the magnetic phenom- 

 ena, that the two principal spots of a bipolar group are the opposite extremi- 

 ties of a single vortex tube. 



The characteristic structure on which the above system of classification is 

 based appears much more clearly on Ha photographs than on those taken 

 v/ith Hp, Hy, H8, or H,. On H8 plates, made with the 5-foot spectrohelio- 

 graph, the unipolar and bipolar type can be distinguished in some cases, but 

 no good example of the multipolar type has been found as yet. The general 

 form of the bipolar type is roughly indicated on these calcium photographs, 

 and in one or two instances the unipolar type appears, but the multipolar type 

 is absent. On photoheliograms the form of the penumbral filaments recalls 

 the unipolar structure of the Ha flocculi, and the faculae follow roughly the 

 outline of the bipolar type. 



The above results, confirmed by photographs taken with diflferent parts of 

 the Ha, Hj3, Hy, and H8 lines, indicate that the vortex structure of the floc- 

 culi is best shown at comparatively high levels in the solar atmosphere. The 

 relationship of this structure to that of the penumbral filaments is under 

 investigation. 



The long axis of multipolar flocculi has been found, in 38 examples, to be 

 nearly parallel to the solar equator and to coincide very nearly with the axis 

 of the inclosed spot groups. 



