Interplanetary Magnetic-Sector Structure,

Transcription

1 , VOL. 77, NO. 22 JOURNL OF GEOPHYSIL RESERH UGUST 1, 1972 Interplanetary Magnetic-Sector Structure, L. SVLGRD 1 Danish Meteorological Institute, Geophysical Section, openhagen, Denm. ark The influence of the direction of the interplanetary magnetic field on the geomagnetic field at high latitudes is used to study the long-term behavior of the sector structure during nearly four solar cycles. It is found that the rotation period of the sector structure varies from about 28.5 days in the beginning of a solgr cycle to 27.0 days in the end. lso it is shown that short-lived sectors rotate more slowly than long-lived ones. Geomagnetic effects related to the interplane- perturbation classfries the day as type. n tary magnetic field. relation between the observer using magnetograms from a southern polarity of the interplanetary magnetic field polar-cap station should adopt the definition and the type of diurnal variation of the geomag- that a positive Z perturbation is directed toward netic field inside the polar cap has been dis- the earth. In general, the regularity of the covered independently by Svalgaard [1968], magnetograms leaves little doubt about the type Mansurov [1969], and Mansurov and Man- of perturbation. With very few exceptions it is surova [1970] and noted by Iwasaki [1971]. possible to classify every day. Figure 2 shows The relationship has been confirmed by Friis- some sequences of Z records from Resolute Bay hristensen et al. [1971]. The most pronounced (84.3 ø invariant latitude). It is clearly seen effect, a broad perturbation of the geomagnetic that the same type persists through several days. field lasting for several hours, is found in the In some of the sequences a change of type is vertical component Z of the field on stations demonstrated. near the invariant poles. Figure i shows mag- This classification was actually developed benetograms from the near-conjugate stations Thule (86.8 ø invariant latitude) and Vostok ( ø invariant latitude). To increase readability, all magnetograms in this paper have been redrawn on the basis of hourly mean U values. When the earth lies in a polarity sector THULE of the interplanetary magnetic field away from the sun, the Z perturbation is directed away ß - OUT v 0 U T MR 11 MR from the earth; in a sector toward the sun the perturbation is directed toward the earth in both hemispheres. s is seen from the magneto- 9 s o SE VOSTOK grams, it is easy by visual inspection to disz, tinguish two types of disturbances. This is the ' 0'UT v -- ( UT basis of the following simple classification scheme. day is classified as type if the Z magnetograms from the near-pole stations for that day show a broad positive perturbation between magnetic noon and local noon; a negative 1 Now at Institute for Plasma Research, Stanford University, Stanford, alifornig opyright 1972 by the merican Geophysical Union % F 0 u. 0 u g. 1. agnetograms showing the geomagnetic effects of the sectoring of the interp]aneta magnetic e]d in the sector away from the sun (top) and in sector toward the sun (bottom) at Thu]e (86.8 ø invarian latitude) and ostok (--.9 ø invarian latitude). The quie undisturbed fie]ds sre indicsted by arrows.

2 UT UT UT UT , UT UT 2? OO T 2oo 1' , / ß () ' 12 2 UT , UT UT UT 12 ' 24UT Fig. 2. Series of consecutive records of the vertical component Z of the geomagnetic field at Resolute Bay. The notation 27.5 means 27, 28.5 is 28, and so forth. fore the relationship with the sector structure was discovered. When the sector structure of the interplanetary magnetic field was discovered [Wilcox and Ness, 1965], it became evident that variations dominateduring away sectors, whereas variations were found during toward sectors. Figure 3 compares the observed sector structure with the - classification as derived from anadian IQSY stations for This classification uses the Z records from stations with an invariant latitude of about 85 ø. The Z perturbations decrease with increasing distance from the invariant poles. Instead they turn into a perturbation of the horizontal component H. For a station such as Godhavn (77.5 ø invariant latitude) the H component increases on days and decreases on days. Figure 4 shows the relation between deviations of daily mean values from the monthly mean for Z at Thule and H at Godhavn. The relation is so close that the H magnetograms from Godhavn may as well be used to infer the type of day. s a matter of fact, during the winter months it is often easier to base the classification work on Godhavn H records than to base it on Thule Z records. The magnetic observatory at Godhavn has been operating continuously since This unique series of magnetograms is the basis of the in- vestigation of the long-term behavior of the sector structure presented in the last part of this paper. Sector polarity since table presenting the result of an - classification of each day since 1, 1926, has been prepared, but because of its great length only the last 10 years are given in this paper; the complete table may be obtained in machine readable form from the author. The full table is also published by Svalgaard [1972]. Several important conclusions may be drawn from this material. n autocorrelation analysis of the class of the day during the whole period is illus- trated by Figure 5. recurrence property is clearly evident. The recurrence peaks have been labeled with the number of recurrence periods that they represent. The recurrence period may be interpreted as the average rotation period of the sector structure. In Figure 6 it seems that the more stable the recurrence (high peak numbers) is, the faster the pattern rotates. In other words, short-lived sectors rotate more slowly than longlived ones. similar effect has been discovered in an analysis of the interplanetary magnetic field observed by spacecr.aft [Wilcox and Tanen-

3 Rot,. no. lsf'day D 196t - 26 ] c c c.. c c c J 22 ]. c c c c... c... c. c c c c c... c ff 18 [. c c c... c c c c c c c [... c c.. c... c c c c c c c c c I 12 [. c. c c... c c c c c c c c c c c c. c.. 179o Pl 9 [... c c. c... c c. c c c c.. c J 5 [. c c c c... c c c c.. e c c. c J 2 [. c c. c.. c c... c c c c c c c c J 29 [ c c c c c... c c c c c c c c c... c. 179ti 25 I c c c c c c... c c. c c c c c c c.. c $ 21 ]. c c c. c c... c c... c c c... c I c c c c c c... c c c c c. c... c N lt I. c c c. c c c... c c c c c. c.. c D [ +.. c c '. 4- c + c c +. c c. c c c c c ,J 7 ]. c.. c... c... c. c c... c +., oo F 3 [ c c c c c... c.. c c ol 1 2 [ c c.. c. c... c.. c c '¾ o2 Pl 29 I... c c... c... c. c... 18o3 25 I. c c. c... c c c c c c c. c c c.. c. c c oti 1 22 c c c c c... c c c c c c c c... c c I 18o5 18o6 ß J 18 I. c c ß c c c... c c c c c c c c.. c c. c c c c J 15 I c c.. c. c c... c c c c c c c c c o7 11 I c c c c c c c... c... c c c c c c c c c c c o8 S 7 [ c c c. c c.. c. c... c c. c c. c c e c c c , o90 t I c c c c c c. e. c c c.. c.. c c... c c. c 181o 031 [ c c. c c c... c c. c. c c c c c..c... c c ` N 27 I c c c.. c ß. c c c c c.. c D 2ti o ß o ß ½ ½ o o Fig. 3. omparison of the observed sector structure with the - classification. Plus indicates interplanetary magnetic field directed away from the sun, minus indicates field directed toward the sun, dots indicate type daily variations, and c indicates type variations of the vertical component Z. The data are ordered in sun rotation periods. (Reproduced from $vcdgaard [ 1968].)

4 4O3O Month 1962 pril ugust November December 1963 pril ugust November December 1964 pril ugust November December 1965 pril ugust November December 1966 pril ugust L. SWL RD TBLE 1. Negative () and Positive () Perturbations for lassification by Day O

5 SETOR STRUTURE, TBLE 1. (continued) 4031 Month lassification by Day November December 1967 pril J y ugust November December 1968 pril ugust November December pril ugust November December 1970 pril ' ust November December 1971 pril

6 4032 L. SV UORD ZTH DYS o o o o o o o oo o oo\ o loo 50 OoOOO o i, ß o k 50 loo -5 0 eel %e e : ee e e -150ñ ß HGo 150 Fig. 4. Relationship between deviations of daily mean values from the monthly mean for Z at Thule and H at Godhavn. Data points are from to ugust Solid circles denote days within inferred sectors away from the sun, and open circles days within inferred sectors toward the sun. baum, 1971] and in an analysis of the photospheric magnetic field [Wilcox et al., 1970]. To examine the variation of the average ¾o \o o\ PEK NUMBER Fig. 6. Recurrence period for the direction of the interplanetary magnetic field compared with the number of the recurrence peak. period of the sector structure through the sunspot cycle, autocorrelations were computed for each year from 1926 to The observation that years with the same phase within the solar cycle have very similar autocorrelation curves then justifies computing an average autocorrelation curve for all years having the same relative position within the sunspot cycle. This set B0 B5 tø i - ø :ø ' ø. '. I 1 I I ' n=5 ',. Doys , DYS Time Log Fig.?. utocorrelations of the direction of the inferred interplanetary magnetic field for dif- Fig. 5. utocorrelation of the direction of the ferent years in the sunspot cycle. The number n inferred interplanetary magnetic field, denotes years following sunspot minimum.

7 SETOR STRUTURE, o o -180 d "'0,, \ / ?,160 Number of 1/-,0 days per year Recurr. Period Fig. 8. verage recurrence period for the sector structure (solid circles) during the solar cycle. lso shown are the average rotation period for sunspots (crosses) and the number of days, or polarity toward the sun, per year (open circles). of curves is shown in Figure 7. The recurrence period shows a systematic variation through the polarity during 1970 and the first half of This prediction will make possible a strong cycle (Figure 8). Near sunspot minimum the test of the connection between the - effects period is 27.1 days. With the rise of the new and the sector polarity in the sense that the high-latitude solar activity the recurrence period structure predicted is not just an extrapolation increasesharply to 28.3 days; with the progress of that of 1969 but includes the disappearance of the cycle the period decreases and remains of a sector and the creation of a new one having rather constant at about 27.2 days during the a different phase. last half of the cycle. The variation of the cknowledgments. I would like to thank the recurrence period resembles the variation of the staff of the Geophysical Section of the Danish rotation period of sunspots through the solar Meteorological Institute, headed by K. Lassen, cycle. Just around sunspot minimum the number for use of data from the observatories in Greenland and for helpful discussions the subject. of days per year (days with the probability of a sector toward the sun) is significantly smaller The Editor thanks S.-I. kasoœu, S. Matsushita, and J. M. Wilcox for their assistance in evaluating than that during the rest of the cycle. Often this paper. it seems as if the sectors are dying away just at minimum. When the new activity starts, it REFERENES begins rather erratically with no well-developed Friis-hristensen, E., K. Lassen, J. M. Wilcox, sector structure. fter sunspot maximum the W. Gonzalez, and D. S. olburn, Interplanetary sector structure is usually well defined before magnetic sector polarity from polar geomagit disappears again near the next minimum. The netic field observations, Nature, 233, 48, Iwasaki, N., Localized abnormal geomagnetic disdisappearance usually happens within one or turbance near the geomagnetic pole and simultwo solar rotations and may provide an inde- ta neous ionospheric variations, Rep. Ionos. Res. pendent way of determining the time of the Space Res. Jap., 25, 163, beginning of the next cycle. lso near minimum Mansurov, S. M., New evidence of a relationship there is a high probability for the occurrence between magnetic fields in space and on earth, Geomagn. eron., 9, 622, of a four-sector structure, as is seen from the Mansurov, S. M., and L. G. Mansurova, The secondary peaks in Figure 7 for n -- I and relationship between the magnetic fields in n= 10. space and at the earth's surface, nn. Geophys., Table I includes a prediction of the sector 26, 397, 1970.

8 4O34 L. SVLGRD Svalgaard, L., Sector structure of the interplanetary magnetic field and daily variation of the geomagnetic field at high latitudes, Pap. 6, Dan. Meteorol. Inst. Geophys., harlottenlund, Denmark, Svalgaard, L., Interplanetary magnetic sector structure , Pap. 29, Dan. Meteorol. Inst. Geophys., harlottenlund, Denmark, Wilcox, J. M., and N. F. Ness, Quasi-stationary corotating structure in the interplanetary me- dium, J. Geophys. Res., 70, 5793 i Wilcox, J. M., and. S. Tanenbaum, The influence of solar differential and rigid rotation on the interplanetary magnetic field, Space Sci. Lab. Rep., Set. 12, issue 45, Univ. of alif., Berkeley, Wilcox, J. M., K. I-I. Schatten, and. S. Tanenbaum, Photospheric magnetic field rotation: Rigid and differential, Solar Phys., 14, 255, (Received 31, 1972; accepted pril 26., 1972.)