?68 



NATURE 



[August 20, 1908 



SOLAR VORTICES. 



SOME preliminary investigations of Prof. Hale on 

 the gyratory forms assumed by the hydrogen 

 llocculi on the sun were described in a recent number 

 of Nature (vol. Ixxviii., p. 200). The photography of 

 the sun, through the hydrogen line Ha, using specially 

 bathed red sensitive plates, by means of the 5-foot 

 spectroheliograph of the Mount Wilson Observatory, 

 gave promise of being of great value. Already the 

 promise is kept. A copy of a paper which is to appear 

 as No. 26 in a series of " Contributions from the 

 Mount \\ilson Solar Observatory " has been for- 

 waided, together with illustrative photographs. After 

 realising the advantage of the Ha line over the line 

 H5 previously used, tfie daily programme was modi- 

 fied to allow of a full series of photographs under the 

 new conditions. A distortion due to heating of th;- 

 mirror, now more continuously used, was eliminated 

 bv the use of a smaller aperture. 



After obtaining new slits adapted to the Ha line, the 

 first photograph of the entire solar disc by the modi- 

 fied method was obtained on March 28, igoS. The 

 remarkable solar " vortex " previously reproduced in 

 Nature was secured by Mr. Ellerman, who was in 

 charge of the routine work with the instrument, on 

 April 30. Further information relative to this phe- 

 nomenon appears in the more recent communication. 

 On a less succes.'^ful exposure made on April 29 the 

 same large storm area is fairly well shown. .A com- 

 parison with the afternoon photograph of .\pril 30 

 recently made in the stereocomparator, together with 

 the measurement of the latitude and longitude of 

 objects identified on both dates, " seem to show thi' 

 existence of a gyratory motion, in a direction opposite 

 to that of the hands of a watch (north, east, south, 

 west)." In Prof. Hale's first note on the same object, 

 he remarks (N.ature, vol. Ixxviii., p. 200) of the 

 dark flocculi surrounding this area that " their 

 appearances strongly suggest the effect of a great 

 whirl rotating clockwise." The identification of 

 objects on fioth plates is a matter of great 

 difficulty, and the evidence for direction of rota- 

 tion is admittedly weak. Further discussion of 

 these plates is |3ostponed until additional data become 

 available. 



The present communication from Prof. Hale is con- 

 cerned chiefly with the phenomena recorded, bv the 

 aid of Ho, in the neighbourhood of a spot which 

 reached tiie east limb of the sun at Sh. i6m. a.m. on 

 May 26, 1908. The changes which took place about 

 it in many cases rapid though not especially violent, 

 were followed until June 2, when the attendant 

 " whirl " was very marked. .\ long dark flocculus 

 had persisted near the spot during this period. " On 

 June 3, in an interval of about ten minutes, a remark- 

 able transformation occurred. The long dark floc- 

 culus, which had been graduallv changing in form 

 and position, was suddenly drawn into the spot." 

 The three photographs here reproduced illustrate this 

 rcTiiarkable occurrence. They were taken on July 3 

 at 4h. 58m. i6s., 5h. 13m. 54s., and .^h. 22m. p.m. 

 respectively. The times given refer to the transit of 

 the spots across the primary slit of the spectro- 

 heliograph, while the scale is such that the sun's 

 diameter would be represented by 14 inches. The 

 definite incurving of the eastern end of the flocculus 

 almost precludes the idea that it is a casual change 

 at a level remote from that of the spots. Spectro- 

 scopic evidence of the motion of the flocculus dotci? 

 into the spot, during the period, would h.ave been of 

 interest. The records were obtained bv Dr. C. E. .St. 

 lohn with the 5-foot spectroheliograph during Mr. 

 Ellerman 's absence on vacation. \\'hen the whirl was 



XO. 2025, VOL. 78] 



best seen its radius was about equal to the distance of 

 the western e.xtremity of the flocculus. Apparently the 

 eastern end did not fall definitely under the influence 

 of the whirl until its distance was about 140,000 km. 

 from the spot. The fact that the western extremity 

 lay. during the whole period, outside this magii; 

 radius may account for its escape. The inean of si?4 



«j&.v 



L 



■'i 



FiG. I. — A hydrogen flocculus drawn into a solar whirl. 

 A at 4S. 58m. 16s. p.m. ; B at sh. t3m. 54s. p.m. ; C at 5h. 22in. p.m. 



measures of the velocity of approach of the flocculus 

 gives 106 km. per second, which, it is suggested, 

 is the order of the maximum velocity in the vortex. 

 It is stated that, except in the case of the large 

 flocculus, motion towards the spots, even along 

 apparent lines of flow, has not made itself obvious. 



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