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Minima, When the Type B Redauroras Are Predominant. SOLAR VOL. 43, 1957 GEOPHYSICS: J. BARTELS 75 are being slowed down much higher in the atmosphere than occurs at sunspot minima, when the Type B red auroras are predominant. From the general physical interpretation of the observations of unusual heights of aurora, it seems probable that a consideration of the degree of ionization of the upper atmosphere may account for the wide range found in auroral heights. For this reason the degree of ionization may become one of the problems of auroral morphol- ogy. The auroral program for the International Geophysical Year, 1957-1958, was de- signed with many of the above-mentioned problems in auroral morphology in mind. Hence we may expect to have answers for some of the problems in the next two or three years. 1 J. A. Van Allen, Phys. Rev., 99, 609, 1955. 2 S. Chapman and C. G. Little, J. Atm. and Terrest. Phys. (in press). 3 J. P. Heppner, J. Geophys. Research, 59, 329, 1954. 4 C. W. Gartlein, Nat. Geographic Mag., 92, 683, 1947. 5 F. T. Davies, Transactions of the Oslo Meeting, August 19-928, 1948 (Washington: Interna- tional Union of Geodesy and Geophysics, 1950). 6 A. B. Meinel, Astrophys. J., 113, 50, 1951; 114, 431, 1951. 7 A. B. Meinel and C. Y. Fan, Astrophys. J., 115, 330, 1952. 8 H. Leinbach, Sky and Telescope, 15, 329, 1956. 9 C. T. Elvey, Seventh Alaska Science Conference, Alaska Division, AAAS, Juneau, September 26-29, 1956. 10 C. St0rmer, The Polar Aurora (Oxford: Clarendon Press, 1955). 1 M. Sugiura, Proceedings of the Third Alaska Science Conference, September 22-97, 1962, p. 32. 12 H. B. Maris and E. 0. Hulburt, Phys. Rev., 33, 412, 1929. 13 C. St0rmer, Photographic Atlas of Auroral Forms and Schemefor Visual Observations of Aurorae (Oslo: International Geodetic and Geophysical Union, 1951). SOLAR INFLUENCES ON GEOMAGNETISM BY JULIus BARTELS GEOPHYSIKALISCHES JNSTITUT, UNIVERSITAT GOTTINGEN, AND MAX-PLANCK-INSTITUT FtR PHYSIK DER STRATOSPHARE (Read by S. E. Forbush) Geomagnetic records provide information respecting various solar influences upon the earth: Corpuscular radiation is effective in producing immediate geo- magnetic activity, as well as in the aftereffects known as "post-perturbation." Wave radiation is effective in causing diurnal variations of the quiet-day types, varying slowly with solar activity in the 11-years cycle, or in the solar rotation period, and occasionally varying rapidly, during solar flares. Magnetograms are of particular value for the continuous measurement of that part of solar corpuscular radiation (P) which has travel times between about 20 hours (in the production of sudden commencements of storms) and about 3 or 4 days (in the medium geomagnetic activity exhibiting 27-day-recurrence tendencies). The 3-hour-range K-index has been devised to measure the effects of P locally, in- cluding its considerable systematic changes with local day-time. A standardized abstract of the K-indices of many magnetic observatories, called the "planetary Downloaded by guest on September 23, 2021 76 GEOPHYSICS: J. BARTELS PROC. N. A. S. three-hour-range index"Kp, provides a summary for the earth as a whole. (SeeEigure 1.) K-indices and Kp are available for the Second International Polar Year, 1932- 1933, and currently since the beginning of 1937. Indices for complete days, such as the equivalent amplitude Ap or the planetary character figure Cp, are derived from Kp. Monthly tables and 27-day-recurrence diagrams for Kp are published around the twentieth of each month, and international data on magnetic disturbances ap- pear quarterly in the Journal of Geophysical Research. [ 1950 9e5 i1 1952 1 195 19s4 I 1955 I Sunspots. Zirich Numbers rIL I -1 I I I I I ,*.NnBy II.; Mri-l ol _ Kp -9 n m . Kp-Uand 9 Hnn mllfnFri fn 10_ Kp-6t9309 . ~n - -Rr,-f , 11_ a - IO _ So _ FIG. 1.-Monthly numbers, January, 1950, to February, 1956, for Zurich relative sunspot numbers and frequencies of magnetic plan- etary 3-hour-range indices, Kp. These K-indices, apart from their immediate value in geomagnetic work itself, are being used as a reliable measure of solar corpuscular radiation in studies on auro- rae and airglow, in work on the ionosphere, on meteors, on comets, on cosmic radia- tion, etc. As to the inference to be drawn about solar events themselves, it is clear Downloaded by guest on September 23, 2021 VOL. 43, 1957 GEOPHYSICS: J. BARTELS 77 AOleanook. Mq/ W3 SodJ4,1 b 933 K Kp , ouro<2 Im 3 AaNJ ...... ... .. , 6 kg^ w -r--LIL;. 614 __lip IRA..§.-.-.......,,,. , ... 1 r s *w j . 9? -.-- i1t_ 12 |P t | | * | - _15 1S "OWV NUwaR0, 91t 5 ° 123_rt-1245678.99urE 0 1 234 r r Fig. 2a SO#*n19. Aly 73 -16;9 Il aL*La 17 . _ ;=s ~~~~~~~~17 28 06 *4 19,4 21paIa PNu ~~ ..42? ~./A 22 *-A A& A mA A~~~~~~- 2021 -- 23 s_9 i, **w |^wg, , 24 _-- 24 S ..... 2 LJx4 27 le 27_ 2eL 29 _AX 31_ _:i~~~~~w~30 sv~~~~--;d8L4 - 31-~~~ 3 7 9 ~~~~~~~a as .0 so sos s Fig. 2b Downloaded by guest on September 23, 2021 78 GEOPHYSICS: J. BARTELS PROC. N. A. S. that the significance of Kp-indices is limited in so far as they can record only such radiation that actually affects the earth: there is ample evidence that many clouds of solar gas fail to hit the earth, not only by crossing the earth's orbit somewhere else but sometimes also by shooting beyond the earth, northward or southward. The "narrow-cone" character of the solar streams that affect geomagnetism, as compared with the "wide-angle flares" of cosmic radiation emitted in rare cases from the sun, has again appeared in the cosmic-ray surge of February 23, 1956, ejected from the western edge of the solar disk. Speaking in 1956, at the beginning of a new sunspot cycle, we may recall that the new sources of solar gas clouds, lying in the spot zones appearing in higher helio- graphic latitudes, fail more often to affect the earth than do those sources nearer the solar equator. That the first magnetic storms of the new cycle have their origin in the zones of sunspots seems to be proved by their (weak) recurrence tendency, with an interval of 27.75 days (average for three years after the minimum), corresponding to the rotation period for about 200 heliographic latitude, as compared with the shorter period of 27.03 days (corresponding to the solar equator) in the three years before the minimum. For the International Geophysical Year, additional indices are planned to sub- divide the 3-hour-interval into quarter-hours centered at 00, 15, 30, and 45 minutes of each hour, to agree with the proposed auroral observation schedule. They will presumably be known as Q-indices (quarter-hourly). Only stations poleward of about 580 geomagnetic latitude will measure them. While the K-index measures ranges only, the Q-index will be based on maximal deviations from normal, quiet-day curves, in order to take full account of SD (the solar daily variation on disturbed days), which is so pronounced in polar regions. The following scale will be pro- posed: Q=0123456789; T= 10; E= 11, if the deviation exceeds the following number of gammas (1 gamma = 10-5 cgs): A= .. 10 .. 20 ... 40 ... 70. ..120. .. 330 ... 500... 800... 1200 ... 18007o. In contrast to K-scaling, there will be only one and the same Q-scale for all sta- tions. This makes it necessary to introduce the additional steps Q = 10 and 11, which actually occur, e.g., at College, Alaska; for convenience in tables they will be indicated by one-letter symbols T = 10, E = 11. In Q-scaling, the vertical component Z will be disregarded, because in subauroral latitudes it is much affected by the field of the long line currents of the SD-system along the auroral zone. Thus a strong auroral current near Iceland would raise Q in Central Europe to 2 or 3, while the ionosphere overhead might be less disturbed. Restriction to the horizontal geomagnetic components has the advantage that Q then refers mainly to the ionospheric electric currents overhead of the station. With the usually adopted ratio of exterior and interior parts in the horizontal mag- netic components (about 6 to 4), one may say that every gamma in A is the effect of an equivalent horizontal current sheet in the ionosphere with the current density 1 ampere per kilometer. Q = 11 corresponds to ionospheric current surface den- sities of at least 1,800 amperes per kilometer or more. Downloaded by guest on September 23, 2021 Voi,. 43, 1957 GEOPHYSICS: J. BARTELS 79 The first studies on Q-indices' made on magnetograms of Meanook, Sodankyli, College, Sitka, and Eskdalemuir show the great daily variation in geomagnetic ac- tivity just as in the K-index. At Meanook and Sodankyla, the Q-indices before lo- cal noon lie systematically lower by one to three units than before local midnight. In the two months May and June, 1933, at Meanook, less than 1 per cent of the Q-indices reached Q = 6 or more before noon, while, before midnight, nearly 14 per cent reached Q = 6 or more. (See Figures 2(a) and 2(b).) 1 5 10 15 20 25 30 T§TT TT l --r-rhoI T -T ' l June = 25 -July A 1-o* 50- =~~~~U, 7S-~~~~~~~~~~~~~~~~-U 50-100 23,C 200 I~~~~.,~~I~ 4zIjII4 4 ,5I.L0 I I~~~~~~ 1 5 10 15 20 25 30 Daily Mean Ap and Equatorial Ring-Current-Intensity U 1941, June-August Fig.
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