Solar prominences and geomagnetic disturbance

Research

solar

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and

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disturbance*

geQxwJnetic

(Received 28 December 1954)

Identification of the hypothetical M-regions of the sun earlier postulated (BARTELS, 1932) to explain 27-day recurrent geomagnetic storms, has eluded astronomers for many years. ALLEN (1944) concluded that M-regions, which are most evident in years preceding minimum solar activity, coincide with undisturbed areas of the solar surface. BRUZEK (1952) and BELL and GLAZER (1954) showed that geomagnetic disturbances are, on the average, more severe when pronounced minima of the emission corona are three days of solar rotation, or about 40” of longitude, past central meridian of the sun. PECKER and ROBERTS (1955)

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Course of average prominence on days preceding and following zero ci ~ys repreaent,ing the five most geomagnetically d’isturbed days in each month. Same for zero days representing the five least geomagnetically dist,urbed days.

suggested that solar spicules distributed over the whole solar disc shoot corpuscular streams into the solar atmosphere, and that these streams are favourably deflected towards earth, when they lie in regions of low coronal emission over undisturbed solar areas. In all of these authors’ views geomagnetic disturbances result when bhe corpuscular streams reach earth about three days after leaving the sun. KIEPENHEUER (1947), however, suggested the contrary hypothesis that the corpuscular source is not spicules, but rather the large quiescent prominences found at intermediate To test KIEPENHEUER’S hypothesis and high latitudes in years preceding solar minimum. with material from an independent sample, we have now measured limb prominence areas * ‘l’hk research WM sponsored in part by the Geophysics Research Directorate of the Air Forw Cambridge Research Center, Air Research and Development Command, under contract AF 19(604)-989. 7 One international prominence unit corresponds t,o an area at the solar limb bounded by 1 second of arc of the celestial sphere in height and one degree of angle of t,he solar periphery in length.

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Researchnote8

from maps prepared and distributed by the High Altitude Observatory (TROTTERand ROBERTS, 1952-54) and compared the average prominence areas with selected days of large and small geomagnetic disturbance. KIEPENHEUERstudied dark filament areas on the solar disc, rather than limb prominence areas. It seems to us, however, that the prominence areas, projected to CMP from both limbs, should reflect the effect suggested by KIEPENIIEUER. Our prominence maps have been prepared from photographic limb-prominence observations in Balmer-alpha of hydrogen made with birefringent filters of 4A half-width at the Upper Air Research Observa~ry, Sunspot, New Mexico, by Harvard College Observatory, and at Climax, Colorado, by the High Altitude Observatory. The periods covered by maps now available include May-November 1951; February-October 1953; and February-June 1954. From these maps we measured first the area in international prominence unitst of all prominences in latitudes greater than 20’ and then all lying in latitudes above 30’. We used only those central meridian dates for which we had prominence data from both east, and west limbs. Such measures were available for 79 per cent of the days in our study. For each date we summed all prominence areas within the specified latitude zones. We then subjected the area sums to superposed epoch analysis, using, as zero dates, as shown in Fig. 1: (a) the five most magnetically disturbed dates of each month, and (b) the five most magnetically quiet days (CRPL, 1951-54). Thus the figures show average prominence areas corresponding to the days preceding and following geomagnetically disturbed and quiet days. Mean prominence areas for the whole time-sample are also shown. Each graph contains eighty-three zero dates for each curve. No significant trends of prominence area with respect to the magnetic disturbance dates can be discerned. The results, thus, do not confirm the finding of KIEPENHEUER (1947) for the years of low activity 1910-14 and 192@25, which gave a maximum average disc prominence area on the fourth day preceding disturbed magnetic conditions. The upper curves of Fig. 1 show that the values for these days lie slightly below the mean. Nor do we find any trend with quiet days. We intend soon to repeat this analysis, using instead of limb prominences the projected areas of dark filaments near the solar centre. We conclude, nonetheless, that these results support the supposition that t,heM-regions must be identified with some solar feature other than the large prominences at intermediate and high solar latitudes. WALTER ORR ROBERTS

High, Altitude Observatory

DOROTHY

E. TROTTER

Boulder, Coobrado REFERENCES

ALLEN,c:. ?v. BAI~TELS, ,J. BELL, B., and GLAZXEK, H. BRUZEK,A. CRPL F-Series, monthly Ionospheric Date

KIEPENHEUEH, K. 0. PECICER, J-C., and ROBERTS,W. 0. TROTTER,D. E., and ROBERTS,W. 0.

Mow. Not. Key. A&. sot. 104, 13 Tew. Magn. 37, 1 J. Geophp. &a. (in prese); Harvard Obnorvetory ScientificReport No. 17 Z. Naturforschun+j ?a, 708 1952 1951-54 Issued by U.S. Department of Commerce, National Bureau of Standards, Central Radio Propagation Laboratory, Boulder, Colorado Aslrophy8.J. 105, 408 1947 1956 J. Gwphg8. Re.8. (in press, for March) 1951-54 Quarterly Summarieeof Solar Activity dixtributed quarterly from High Altitude Observatory under sponsorship of the Central Radio Propagation Laboratory of the National Bureau of Standards

1944 1932 1954

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