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Magnetic Activity and Its Relations to Solar Phenomena by J Terrestrial Magnetism ttmospheric Electricity VOLUXtE37 MARCH, 1932 No. 1 TERRESTRIAL-MAGNETIC ACTIVITY AND ITS RELATIONS TO SOLAR PHENOMENA BY J. BARTELS A bstractmA homogeneousseries of monthly means of terrestrial-magnetic activity for the years 1872 to 1930 is derived and extendedbackward, in annual means,to 1835. The annual variation of magnetic activity and of the relative sunspot-numbersis dis- cussedby means of new tests for periods. Only the semi-annual wave in magnetic activity is recognizedas physicallysignificant. Its maxima prefer the times whenthe Sun is in the celestialequator, and not, as has beensuggested, the times when the Sun's axis is most inclined towards the ecliptic. This view is supportedby tests using the harmonic dial and the probable-error circle, and several independent considerations. The close relations between sunspot-numbersand terrestrial-magnetic activity in the annual and monthly means are discussed. Some general statistical aspectsare given for the treatment of the correlationbetween such series with after-effects,for 'which both solar activ.ity and terrestrial-magneticactivity are typical. The homo- geneity of the whole availableseries for relativesunspot-numbers and for areasof sunspotsand faculaeis tested;some inhomogeneities are found, apart from a general lag of terrestrial-magneticactivity that hasoccurred in somesunspot-cycles. A break in the homogeneityof the internationalmagnetic character-figures in recentyears is discovered. The individual 27-day recurrencesin terrestrial-magneticactivity during 1906-31, and their relationsto solar activity are discussedwith the help of a graphicalday-by- day record. They indicatethe existenceof persistentactive areas on the Sun'ssurface, calledM-regions, which, in manycases, cannot be coordinatedto suchsolar phenomena as are observableby directastrophysical methods. This holdsin particularfor the new solar indiceswhich are available for the years 1928-30,and which are found so closely correlatedto sunspot-numbers,that they fail to improvethe correlationbetween solar activityand terrestrial-magnetic activity. Observationsof terrestrial-magneticactivity yield thereforenot only informationabout geophysicalinfluences of suchsolar phe- nomenathat may be tracedin astrophysicalobservations, but supplementthese direct observations themselves. CONTENTS 1--Introduction 2•Magnetic activity, as expressedby the internationalcharacter- figures and other measures 3•The u-measureof activity; monthlymeans 1872 to 1930 4--Annua! means of activity since 1834 5•Activity and energyof disturbance 6--The u•-measure 7--The annual variation of magneticactivity 8---The annualvariation of sunspot-numbers("Earth-effect?") 9--Testsfor explanationsof the equinoctialmaxima of activity 2 J. BARTELS Iv(•. 37,N•,. 51 10---Testsusing harmonic dial and probable-errorcircle 11---Thequestion of annualrecurrences (influence of cometsand meteors) 12--Relations between annual means of solar activity and terrestrial- magneticactivity 13--Tests of homogeneityfor measuresof solar and terrestrial activity 14--The lag of the annualmeans of terrestrial-magneticactivity behind those of solar activity 15---Relationsbetween monthly means of solar activity and terrestrial- magneticactivity 16•General remarks on correlation in serieswith after-effect (monthly and annual means) 17.... The individual 27-day recurrences,!906-31, and their relation to sunspots 18....... The solar indices, 1928-30, compared with terrestrial-magnetic activity 19......... Summary ! ............I•lrod•tction Among lhe geophysicalphenomena which have i>eeninvestigated with regardto solarinfluences of other kind than the regulartliurnal and seasonalvariations, terrestrial magnetism stands out as yielding the most consistentantl reliable re!ationshiI)S•. The derivation of a new long and lmmogeneousseries for terrestrial-magneticactivity, which will comprisethe firstpart of this paper,justifies the reconsiderationof someof thesi•relationships that httve often linen treated with lesssatis- factorymaterial. The statisticalasl)ects will l•e empl•asized,since the relationshipis of a sta!isli(':alnalure inasmuchas eachobserved solar phenomenonseems to t)roducea geoi)hysicaleffect truly with a certain probability,and since all geophysic. al t.he{•ries either gi'•>w rmt r•f statistical results,or mustbe subjectedt{• s!atisticaltests. The meth{•rlsdescribed in this papermay be usedto test r•therrelationshiI•s, such as those supposedin meteorologyor in wirelesstransmission-phenomena. It will appearthat cautionis necessaryin dealingwith short seriesof observations,taken over intervals of a few years only. This holds especiallyfor the recentyears 1928-30, in whichthe trendin the direct solar observationsdiffers const•i{'u(•uslyfrom the trend in terrestrial- magnetit:activity, in contrastl{• the fairly high ('r•rrelationsfound in the greatestpart of the seriessi•ce i872. 2......... Magnetic activity, as expressedby theinternational magnetic character- figures and other measures Terrestrial-magneticactivity at a given station, and in a certain interval,may be definedas an expressionfor the frequencyand intensity of magneticdisturbances in that interval. There are many ways in whichthis generaldefinition may be expressedas a numericalmeasure. Characterization,the simplest, is now widely used. !n this measure everyobservatory assigns, from the character of its photographicrecords, to each interval of 24 hours,between successiveGreenwich midnights, a character-figure,"0" for quiet,"1" for moderatelydisturbed, and "2" for greatlydisturbed days. The averagefor all collaboratingobserva- • Thegeophysical effectsof solar phenomena havebeen summarized bythe author inErgebnisse der exakten Naturw., 9, 37-78 (Berlin 1930). RELATIOiVS MAGNETIC ACTIVITY TO SOLAR PHENOMENA 3 tories (the number of which increased from 30 to about 45, since this measure was begun in 1906) is the international magnetic character- figure• C. The values of C representthe fluctuationsof activity from day to day very satisfactorilyand have been usedwith advantagein selectingthe international quiet and disturbed days. They also have been used in several investigationssuch as those of C. Chree and J. M. Stagg3 on the 27-day recurrencephenomenon, which is due to the rotation of the Sun, and in L. W. Pollak's periodogram-analysis4;they will be used for similar purposesin õõ 17 and 18. Becauseof the complexityof the disturbance-phenomenonand its different appearanceat equatorialand polar stations,Ad. Schmidt• advocatesthe practiceof leavingthe assignmentof the character-figures more or lessto the subjectivejudgment of the observerin chargeat the observatory(though, of course,certain model curves might be selected at eachstation as guidingexamples) and not to give strict prescriptions based on actual measurements. He agreesthat the characteristicsof magneticdisturbance, such as shift in the meanvalue, the magnitude, derivation, number of the single oscillations, etc., are so numerous, that the establishmentof an objectivevalue for the activity of each singleday couldbe gainedonly by very elaborateinvestigations. The time and laborspent in this work wouldbe quite out of proportionto the result, since this characterizationof a single day, or of another interval of time, would be, in any case,only statisticallysignificant, without furtheringessentially a physicalexplanation. On the other hand, just becauseof this limited importanceof the characterization, the simpleprocedure adopted for the internationalcharacter-figures servesits purpose,namely, the relative comparison of the magnetic activity on successivedays within, say, threemonths to a year. That on so many days practicallyall observatoriesreport character "0", and, during greatmagnetic storms, all report"2", provesnot onlythe well-known world-widenature of magneticactivity, but also the satisfactorinessof this method to derive C. There are, however,as Schmidtpoints out, certainlimitations in the useof the averageinternational character-figures C. As statedwhen comparingactivities on twodays close together, we mayuse the values of C with confidence.Thus, we may acceptas about equallydisturbed April I and 18, 1910,since on boththese days C= 1.3,and may regard April 12, 1910,with C= 1.0,as lessdisturbed than the othertwo days, but we shouldhesitate to say that the magneticactivities on April 12, !930, with C= !..3, or April 15, 1930,with C=I.0, wereequivalent to thosefor the daysin April 1910. For, althoughthe arrangedinter- nationalvalues of C are the same,there is no safeguardthat the standard of characterizationmay not haveshifted within the 20 yearsfrom 1910 to 1930. There are severalreasons for suchshifts: (a) Changeof ob- • Publishedby the Royal MeteorologicalInstitute of the Netherlands,and reprintedannually in this JOUKNAL. • Phil. Trans. R. Sot., A, •2•, 21-62 (1927). ' PragerGeophysikalische Studien, 3 [[2echoslovakischeStatist•k Bd. 64 (1930)1;reviewed in Terr. Mag.,36, 110(1931); also Naturw., 18, 343-349 (1930). The results of thisvaluable paper are outside thescope of thispaper and will thereforenot be discussedhere, though an applicationof themethods described in õõ 9 and 10 would seem promising. * ReportInternat. Met. Committee,Berlin Meeting 1910, p. 93 (LondonMet. Off., 1912). See Schmidt'sdiscussion of the character-figures,Met. Zs., 33. 481-492 (1916). 4 J. BART.ELS [v,',•..37, No. il server...... for instance,an observerwho is usedto polar recordswill judge tropical recordsmostly as quiet; (b) shift
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