- Email: [email protected]
Geomagnetic effects of interplanetary sector structure
Planet. SPaCCSCi. 1974, Vol. 22, pp. 193 to 208. Peremon
Press. Printed in Northern Ireland
GEOMAGNETIC EFFECTS OF INTERPLANETARY SECTOR STRUCTURE A. MOLDOVANU Centre of Technical and Physical Research, Ia$i, Rumania (Received injinalform
19 July 1973)
Abstract-In this paper the geomagnetic effects of the interplanetary magnetic sector structure are studied on the basis of some new criteria and working hypotheses. Thus, we assume that the recurrence of geomagnetic disturbances should be understood in a dynamical sense, in connection with the evolution of the full sector structure, and not necessarily as a 27-day recurrence. Accordingly, on the representation of the sector structure during 1968, as deduced by Wilcox and Colburn, we have defined four ‘main recurring lines’, which link the sector boundaries recurrent in successive solar rotations. The term ‘group of SC and SI events’, abbreviated as gr(SC + SI), introduced by us in previous works to designate the mornholoeical arounine of the individual SC and SI events in collective events, is also used. It should Thepc&ted o’lt that the bulk of gr(SC + SI) events are either associated with sector boundaries, or recurrent in successive solar rotations. Part of these events reveal the existence of some ‘secondary recurring lines’, within the magnetic sectors. The above working hypotheses and observations have been checked by the superposed epoch analysis, performed for each main recurring line in part and for all the main recurring lines combined. The following parameters are analysed: the number of SC events, the number of collective events gr(SC + SI), the total number of SC and SI events and the geomagnetic activity index
KP*
The main result of the superposed epoch analysis consists in the appearance of a sharp maximum for all the parameters considered on the day of sector boundary. This fact proves that the effects of the sector boundaries are important and general, in regard to all aspects of geomagnetic activity. Essentially these effects consist of the occurrence of gr(SC + SI) events and of a specific increase in the Kp index, when the sector boundaries pass by the magnetosphere. This suggests that the sector boundaries are accompanied by corotating shocks and magnetohydrodynamical turbulence. The high frequency in the occurrence of the SC events on the days of sector boundaries is also noticeable. Each main recurring line presents a certain ‘individuality’, expressed particularly by secondary specific maxima in all the parameters, corresponding to the ‘secondary recurring lines’. One suggests that these secondary recurring lines might be due to some corotating distortions within the magnetic sectors and might be related to the ‘subs&or’ or ‘filaments’. The distribution of the geomagnetic disturbances near the sector boundaries depends on the direction of the field polarity change. All these results lead to the conclusion that most of the geomagnetic disturbances can be accounted for by the interaction between corotating distortions in the solar wind connected with the sector structure and the magnetosphere, the flare-induced disturbances representing statistically the secondary mechanism.
1.INTRODUCTION
order to explain the occurrence of the geomagnetic disturbances two main physical patterns are now accepted: (a) the magnetohydrodynamic transient shock waves, related to some individual solar flares, account for the occurrence of casual SC storms (Chapman, 1966; Parker, 1963) and (b) the solar corpuscular streams, as long-living structures corotating with the Sun, account for the occurrence of recurring geomagnetic disturbances (Obayashi, 1964). Although in a number of cases the association between the solar flares and SC events is evident, unfortunately this association is generally uncertain (Ballif and Jones, 1969; Halenka, 1968; Moldovanu et al., 1971; Moldovanu et al., 1972). Most of the SC events In
1
193
194
A. MOLDOVANU
and the associated Forbush decreases cannot be explained by the pattern (a) and consequently, a more satisfactory explanation is expected from the pattern (b) (e.g. Ballif and Jones, 1969), or in an alternative, which considers the possible interaction between the transient shocks and the sector structure (Ness et al., 1971). Wilcox and Ness (1965), Wilcox and Colbum (1969); 1970, 1972) and Ness et al. (1971) drew attention to the geomagnetic effects of the sector boundaries. They pointed out that the geomagnetic activity index Kp rose near the sector boundary and that some sector boundaries coincide with the occurrence of SC events. Using some new criteria to systematize the interplanetary and geomagnetic data, we point out in this paper that the geomagnetic effects of the sector structure are more general, especially concerning the occurrence of SC and SI events. 2. PHYSICAL
CONSIDERATIONS
In relation to the pattern of interplanetary corpuscular streams, it is interesting to recall the theoretical works of Dessler (1967, 1968) on the ‘tangential discontinuity’. A tangential discontinuity can appear in the solar wind when a faster plasma stream overtakes a slower stream. As a result, the two streams collide and the gas in both streams is compressed in an intermediate region of higher density, pressure, temperature and magnetic-field strength. At the front and behind the intermediate region, two shocks can be generated, if the relative velocity of the two streams exceeds the magnetoacoustic velocity. The two shocks cause the solar wind to deviate from the radial flow: for a stationary observer, the flow deviates eastward, in front of the interface, and westward, behind it (Dessler, 1968, Fig. 14). At the interface there appears a velocity discontinuity, which is called ‘tangential discontinuity’. Since the magnetic fields are either parallel or antiparallel on the two sides of the discontinuity, a Kelvin-Helmholtz instability should develop at the interface and generate magnetohydrodynamical turbulence. Dessler (1967) suggested that “ . . . M-region geomagnetic storms are due to a KelvinHelmholtz instability that could develop along the velocity discontinuity and to the enhancement of the irregularities in the solar magnetic field between the two shocks”. In the interplanetary space, distortions of the kind of tangential discontinuities were found, connected with the sector structure of the interplanetary magnetic field (Wilcox and Ness, 1965; Wilcox, 1968; Ness et al., 1971). Thus, across a sector boundary the real picture of plasma parameters is entirely similar to that expected in the theoretical pattern of Dessler. The result is that the sector boundaries should be considered as tangential discontinuities for the particular case when the magnetic field has opposite polarities on both sides of the interface. Several other types of discontinuities were assumed to exist in the solar wind: filaments; directional, simple, appreciable discontinuities, etc. After Burlaga (1972a) most of these might be reduced to two main types: tangential and rotational discontinuities. It is not yet clear which of these two types of discontinuities occurs more frequently. Even for the same cases of discontinuities and for the same set of spatial data, different authors come to opposite conclusions, such as the arguments between Burlaga (1972b) and Ivanov (1971, 1972). For the present study it is not so important to dwell on the types of discontinuities associated with the sector boundaries, as it is to learn if there are discontinuities, and if there are, what effects they have on the geomagnetic activity. Therefore, we shall consider the pattern of tangential discontinuity, mentioned above, as being an acceptable basis of discussion.
GEOMAG~TIC
EFFECTS OF ~ERPL~ETARY
SECTOR STRT_JClWRE
195
In order to ascertain the possible geomagnetic effects of the whole structure connected with a tangential discontinuity, two references should be considered. (1) Akasofu (1969) noted: “We now know by satellite observations that both SSC and SI are caused by contact of interplanetary MHD discontinuities with the magnetosphere Thus, it is not necessarily appropriate to distinguish them”. (2) Yoshida and Akasofu (1966) suggested: “ . . . Nishida and Jacobs have shown that they (i.e. SI events) have essentially the same characteristics as those of SSC. Since SSC is now interpreted as a sudden compression of the magnetosphere by an enhanced solar plasma flow, intense SI activity may be interpreted as an indication of ‘turbulent’ ffow; embedded in such a turbulent flow, the maguetospheric cavity expands or contracts, causing fluctuations of the magnetic field inside the cavity”. Taking into account both the pattern of the tangential discontinuity and the last two references, we consider that the encounter between a tangential discontinuity in the solar wind (in particular, a sector boundary) and the magnetosphere might have the following effects: (a) the occurrence of one or two SC (or SI) events, due to the two shocks, in front and behind the collision region; (b) the starting of an intense SI activity related to the magnetohydrodynamical turbulence along the discontinuity; (c) the increase of the geomagnetic activity, also connected with the MHD turbulence at the interface. To summarize, the occurrence of a group of SC and SI events, accompanied by the increase of the geomagnetic activity index Kp, is expected. The nature, the intensity and the duration of the whole geomagnetic disturbance associated with the tangential discontinuity would depend on the physical properties of the two coiliding plasma streams. The geomagnetic effects already found of the sector bounda~es are in good agreement with those expected in the case of tangential discontinuities (Wilcox and Ness, 1965; Wilcox and Colburn, 1969, 1970, 1972; Ness et al., 1971). Moldovanu et al. (1971,1972), taking into account all the SC and SI events, as reported in Solar-Geophysical Data (1967-1969), drew attention to the fact that, in most cases, the SC and SI events appear to be morphologically grouped in collective events, which they called ‘groups of events SC and SI’, abbreviated to gr(SC + SI). Having noticed that the most abundant gr(SC + SI) events were associated with sector boundaries, these authors suggested that the gr(SC + SI) events would reflect the interaction between the tangential discontinuities in the solar wind and the magnetosphere. From a comparison between the interplanetary magnetic sector structure during 1968, as deduced by Wilcox and Coburn (1970), and the occurrence of SC and SI events reported in Sonar-Geophysical Data, Moldovanu and Bradu (1973) pointed out that most gr(SC + SI) events were either associated with sector boundaries, or recurrent in successive solar rotations. They concluded that the main cause of the gr(SC + SI) events was represented by distortions in the solar wind corotating with the Sun. In the present paper we give new arguments supporting the conclusions of our previous works, based on a statistical treatment of the correlation between the interplanetary sector structure and several aspects of the geomagnetic activity. 3. WORKING METHOD
In Fig. 1, the interplanetary magnetic sector structure during 1968 (near the maximum of the sunspot cycle ZO), is plotted (Wilcox and Colburn, 1970). The periods of positive inte~lane~~ magnetic field (ma~etic field directed away from the Sun) are noted by the areas above the time axis, and the periods of negative inte~lane~ry magnetic field
196
A. MOLDOVANU
1840 Jan
Feb
1841 Feb
Mch
1842 Mch
APr
Mw
FIG. 1. SKTORIAL STRUCTUREOF THE INTERPLANETARY MAGNETIC FIELD IN THE SOLAR ROTATIONS Nos.1839-1852 (SUCHASWASDETERMINED BYWILCOXANDCOLBURN,~~~~)ANDTHJZ OCCURRENCEOFSCANDSIEVENTS.
The periods of positive interplanetary magnetic field (magnetic field directed away from the Sun) are noted by the areas above the time axis, and the periods of negative interplanetary magnetic field (magnetic field directed toward the Sun) are shown by the areas below the time axis. The SC events are marked by black triangles and the SI events, by empty ones. The collective events gr(SC + SI) are marked with straight brackets.
(magnetic field directed toward the Sun) are shown by the areas below the time axis. The boundaries between magnetic sectors recurrent in successive solar rotations are linked by continuous lines, which we called ‘main recurring lines’, and marked by the Roman numbers: I, IL, III, IV. The magnetic sectors, obvious in Fig. 1, were designated by Arabic numbers : 1, 2, 3, 4. Since we are especially interested in what happens near the sector boundaries, we do not use the restrictive condition of the ‘well-established sectors’ (Wilcox and Colbum,
GEOMAGNETIC
EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
197
1970, 1972), considering in exchange those sector boundaries which are evidently recurrent in as many solar rotations as possible, in other words, the ‘well-es~blished recurring lines’. In Fig. 1 we note also all the SC and SI events reported in Solar-Geophy~icul Tutu (1967-1969), selected by Romafia from the world-wide network of geomagnetic observatories. The SC events were marked by black triangles and the SI events by empty triangles. With straight brackets, we marked the grouping of the individual SC and SI events in collective gr(SC + SI) events, as determined in our previous works ~oldovanu et al., 1971, 1972; Moldovanu and Bradu, 1973). In grouping the individual SC and SI events in collective gr(SC! + SI) events, we took into account both their time proximity and their effects on the geomagnetic activity. The isolated SC and SI events were each considered to represent a distinctive gr(SC + SI) event with a number of components equal to 1. To identify the SC and SI events reported in Solar-Geophysical Data, we used as reference material the magnetograms from the Geomagnetic Observatory Surlari ~Rumania). The isolated events not included in brackets (Fig. 1) were reported by less than three observatories in Solar-Geophysical Data, and were also not identified on the magnetograms. These events were not considered in the analysis. The examination of Fig. 1 reveals clearly that most sector boundaries coincide with the occurrence of g&SC + SI) events. In Fig. 1 the gr(SC + SI) events which do not coincide with sector boundaries but present obvious recurrence in successive solar rotations are linked by vertical dotted lines. We call them ‘secondary recurring lines’. One can see that some of the secondary recurring lines present an outstanding continuity and tend to evolve parallel to the main recurring lines. Since both the main and the secondary recurring lines follow the evolution of the whole sector structure (Fig. 1), we consider that the recurrence of gr(SC + SI) events should be understood in a dynamical sense, in connection with the evolution of the sector structure, and not necessarily as a 27-day recurrence. We have checked both the geomagnetic effects of the sector boundaries as well as the validity of the concept of ‘dynamic recurrence’, by applying the superposed epoch analysis. We make this analysis for each of the four main recurring lines. In the superposed epoch analysis the ‘zero’ is determined by the time at which the sector boundaries pass the Earth. These moments, corresponding to the four main recurring lines, are given in Table 1. Some of these times are given by Wilcox and Colburn (1970), and the others (denoted by asterisk) were determined from a figure presented by the same authors. The moments determined by interpolation are also specified. The boundaries corresponding to the main recurring lines I and II in the solar rotation No. 1845 lie in large gaps (Fig.l), and are not analyzed. The boundaries corresponding to the beginnings of the main recurring lines III and IV in rotation No. 1842 refer rather to a ‘filament’ than to a sector and are not considered either. The superposed epoch analysis (Figs. 2-6) was performed for the following parameters : (A) the number of SC events; (B) the number of collective gr(SC + SI) events; (C) the total number of SC and SI events and (D) the planetary magnetic 3-hr index Kp. The distributions of SC, gr(SC! + SI) and all the SC and SI events (respectively A, B, and C, in Figs. 2-6) are given in l-day intervals. Thus, a ‘zero day’ corresponds to a 24-hr period around the moment of occurrence of the sector boundary; on both sides of the zero day, 5 other days are considered.
A. MOLDOVANU
198
Main recurring line II (- , +)
Main recurring line I (+ , -) Date 2 January 1968 28 January 1968 26 February 1968 23 March 1968 21 April 1968 17 May 1968 9 July 1968 4 August 1968 2 September 1968
Rate
Interval 2-3 8-l 6-7 E 5-6 8-l 8-1 (int)+ 4-s*
16January1968 11 February 1968 10 March 1968 5 April 1968 2 May 1968 30 May 1968 17 July 1968 13 August 1968
Main recurring line III ( + , -) Date 9 April 1968 6 May 1968 1 June 1968 27 June 1968 25 July 1968 21 August 1968 19 September 1968 16 October 1968 13 November 1968 10 December 1968
Interval 8-l (int)* 3-4 4-5 6-7 l-2 1-2* 4-S 7-8
Main recurring line IV (+, -1
Interval
7-8; 6-7* 2-3* 6-7 (int)* E 2-3 56 2-3 2-3
Date
11 April 1968 10 May 1968 7 June 1968 3 July 1968 31 July 1968 31 August 1968
Interval 6-7* 5-6 (int)* E z*
t Three-hour intervals are used. Some of these times are inferred by Wilcox and Colbura (1970), and the other (denoted by asterisk) are determined from their figure showing the sector structure. The momenta determined by interpolation are denoted by “int”.
The distribution of the individual SC and of all the individual SC and SI events with respect to the zero day was obtained directly. A more complicated situation appeared in the case of collective gr(SC + SI) events, which lasted from a few hours to 1 day, often occurring near the limit between two diurnal periods. In such situations we assigned the gr(SC + SI) event to that diurnal period in which the bulk of the constitutive SC and SI events occurred. The distribution of the 3-hr values of the geomagnetic activity index Kp was considered in the 3-hr periods, comprised within 5$ days on both sides of every sector boundary (part D in the Figs. 249, but in order to facilitate the comparison with the first three parts, the time axis was also divided into ‘zero day’ and 5 days before and after this. Figures 2-5 comprise the results of the superposed epoch analysis for the main recurring lines I-IV respectively, while Fig. 6 refers to all the sector boundaries of these lines, combined. To facilitate the comparisons, the values given in Figs. 2-6 are average values, ascribed to a single sector boundary. 4. RESULTS
The main recurring line I (change of~~~d~~~ari~~:+ , -) The similarity between the statistical distributions of the four parameters analysed is remarkable (Fig. 2). For the total number of SC and SI events (part C) and Kp index (part D), this close similarity relates even to details. The central maximum on the day of sector change is very sharp for all the parameters.
GEOMAGNETIC EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
MAIN RECURRING LINE j-
2.5 2.0 :
. . .. . .
25 1
.
s+’ t + I-1
1.0~ 0.5:
t
t
’
’
’
’
-5 -4 -3 -2 -7
POSlTlON
1.1
0
t
’
12345
’
’
’
WITH RESPECT TO DAYOF SECTOR BOUNDARY, RAYS
FIG. 2. SUPERPOSED EPOCH ANALYSIS WITH RESPECT TO SECTOR BOUNDARIES ~~RRE~~~NDING TO THE MAIN RECURRING LINE I. (A} NUMBER OF SC EVENTS; (B) NUMBER OF @(SC+ Sl) INDEX Kp, EVENTS; (c)TOTAL NUMBER OF SC AND SE EVENTS; (D)GEOMAGNETIC Ac7fWIlY All
the
values are average values, ascribed to a single sector boundary.
199
200
A. MOLDOVANU
This central maximum is preceded by a minimum, on the day - 1 for the first three parameters and on the days -1 to 0 for the Kp index. On both sides of the central maximum, two secondary maxima for all the parameters appear: a weak maximum on the days -3 to -2 (clearer in the Kp index), and another clear maximum on the days +3 to +4. One can see that these secondary maxima correspond exactly to the secondary recurring lines, which we give in Fig. 1 on both sides of the main recurring line I for the gr(SC + ST) events. It is worth noting that these secondary maxima become conspicuous in the distribution of the Kp index, including all the aspects of the geomagnetic activity. During the solar rotation No. 1848 the main recurring line IV gets within only 2 days of the line I, and their geomagnetic effects interfere. The main recurring line II (change of$eldpolarity:
-
, +)
The resemblance between the graphs of all the four parameters is also fair (Fig. 3). The central maximum is evident for all the parameters. The secondary maximum on the days -4 to -5 is the result of some secondary recurring lines. The secondary maximum on the days $4 to +5 is due, in addition, to the emergence of the main recurring line III on the right side, beginning with the rotation No. 1842, which affects the distribution. The minimum which precedes the sector boundary appears, for all the parameters, within about 14 days earlier than in the case of the main recurring line I. For the Kp index this minimum is less clearly expressed. The minimum following the central maximum also occurs within about one day earlier than for the line I. The main recurring line III (change ofjeldpolarity:
+, -)
The resemblance between the graphs of the four parameters is fair, except for the central maximum, which is not clearly defined for the Kp index (Fig. 4). The minimum which precedes the sector boundary is very clearly defined for all the parameters at the day - 1, as for the line I. The secondary maxima at the days -4 to -5, and +3 to +4 do not correspond to secondary continuous recurring lines, being in part the consequence of the closeness of the main recurring lines II and IV (see Fig. 1). The main recurring line IV (change ofjieldpolarity:
- , +)
The resemblance between the graphs of the four parameters is fair, except for the central maximum, which is very sharp for the first three parameters, but is not clearly defined for the Kp index (Fig. 5). The line IV appears to be the most effective for the first three parameters. The minimum which precedes the sector boundary appears approximatively during the day -2, and the minimum which follows the boundary, during the day +l (as for the line II). The wide and intense secondary maximum at the days +2 to +5 is due first of all to the secondary recurring lines, on the right hand side of the line IV. This secondary maximum is also affected partly by the proximity of the line I in the rotation Nos. 1847 and 1848, and by the great disturbance which occurred on 10-11 June 1968. The proximity of the line III on the left side, in the solar rotation No. 1844, contributes to the emergence of the secondary maximum during the days -3 to -5. The average of all the main recurring lines
The maximum on the day of sector change is very sharp for all the parameters (Fig. 6). The frequency in the occurrence of geomagnetic events is very high on the zero day: it is
l
GEOMAGNETIC
.
.
ACTIVITY
INDEX, Kp
R I I I
E
F
I I I I I, I
SC and SI EVENTS,
0 (
.a ,
I
I
I,,
01
3 I,
TOTAL NUMBER
I,
2
%
I I I I ,
grlSC +SIl EVENTS. NUMBER I
SC EVEN T.S. NUMBER
A. MOLDOVANU
202
MAIN RECURRINGLINE fl
L ..
HI. 2.0-
.= .
l l
FIG.
4.
%JPERPOSED
EPOCH
m
l
l l
l ;
.
00
.*.= .
ID)
.
..
0.5 : -
. : :.
1.5 -
1
5.. ,’ =. ’ 0: -. :*.
.
(Cl
+/I I ! I I I I I I I, -h -4 -3 -2 -1 0 12345 POSITION WITH RESPECT TO DAY OF SECTOR BOUNDARY, DAYS
ANALYSIS
TOTmMAINRECuRRIN
WITH
RESPECT
0 LINE
111
TO
SECTOR
(SEE CAPTION
BOUNDARIES OF
CORRESPONDING
Fm. 2).
larger than O-6for the SC events, O-7 for the gr(SC + SI) events and about 1.7 for the total number of SC and SI events. The minimum which precedes the boundary appears on the day -1 for the first three parameters, and during the days 0 to -1 for the Kp index. The Kp index distribution presents a fair resemblance with the distribution of the other parameters and, particularly, with the distribution of the total number of SC and SI events.
MAIN RECURRING LINE Li
.r
(A)
fB)
0.5
/
:
2.5-
2.0-
1.5 -
II
TO-
0.5-
3.5c
. :.
. 3.0: 2.5 2.0:
(0)
7.5 I.01 0.5 : 4
-4 -3 -2 -1 I
I
0
I
12345 I I
I
I
I
POSITION WITH RESPECT TO DAY OF SECTOR BOUNDARY. DAYS
FIG. 5. SUPERPOSEDEPOCH ANALYSISWITH RESPECTTO SECTORBOUNDARIESCORRESPONDING TOTHEMAINREZURRINGLINE (SEECAP~ON OFFIG. 2). 203
204
A.MOLDOVANu
MEAN OFALL MAIN RECURRING LINES
ccl
1
_
2.5 2.0 1.5
k
. . . 8’ . . . % .......... .s'=. 0,;. :.A "Jo .+ '.+p. . . . op.': 2. . . . 08 . l
I
1
9
54321
1
t
1
8
0
1
RJDJRRIN~
1
1
!
12345
POSlTlON WITH RESPECr SECTOR BOUNDARY,
FIG.6.SUPERPOSED EPOCH ANALYSIS
1
fD)
TO DAY OF DAYS
WITH RESPE~TTOSECTORBOLINDAFUJB OF ALLTHEMAIN LINE&COMBINED (SEE CAPTION OF FIG.2).
GEOMAGNETIC EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
205
The distribution of the Kp index, in Fig. 6, graph D, is quite like that found for 1968 by Wilcox and Colburn (1970), though the sector boundaries considered by us are not exactly the same. 5. DISCUSSION
1. From the foregoing analysis the result is that the geomagnetic effects of the sector boundaries are exactly as predicted in Section 2 of this paper, taking into account the tangential discontinuity pattern (Dessler, 1967). Thus, the bulk of sector boundaries coincides with gr(SC + SI) events, being accompanied by a sharp rise in the geomagnetic activity index Kp (parts B and D, in Figs. 2 -6). The high value of the frequency in gr(SC + SI) events on the zero day (~0.7) shows that more than two thirds of sector boundaries considered in our analysis are accompanied by the occurrence of gr(SC + SI). The frequency of occurrence of the SC events during the day of the sector boundary has a remarkable high value: 064 (Fig. 6, graph A). This fact proves that numerous sector changes are really accompanied by corotating shocks: the SC events with which most of the collective gr(SC + SI) events begin seems to reflect exactly the shock in front of the colliding region of the tangential discontinuity. This aspect, which strengthens the leading part of the sector boundaries in the release of SC events, is sensibly different from the earlier results of Wilcox and Colburn (1969, 1972) and Ness et al. (1971): they pointed out that only some sector boundaries coincide with SC events. This difference comes from the different number of the events considered. While these authors took into account only the SC events included by Virginia Lincoln (Solar-Geophysical Data) in her representation of the planetary Kp index, in our analysis we used all the SC events reported to Solar-Geophysical Data by Romaiia. Our analysis, therefore, relied on a larger number of data. The same treatment of the SC and SI events, performed according to suggestions given by Akasofu (1969), led to a considerable increase in the frequency of these events on the boundary day (part C in Figs. 2-6). We think that it is not essential to know how many SC and SI events accompany a sector boundary, but it is important to know if the boundary is or is not accompanied by such events. In order to describe the geomagnetic effects of sector boundaries, we consider that the collective gr(SC + SI) events are more representative than the individual SC and SI events, the first ones offering a more plausible physical interpretation. 2. The dynamical character of the recurrence appears clearly expressed by the secondary maxima in the Figs. 2-5, which reflect the tendency of secondary recurring lines to evolve parallel to the main lines (Fig. 1). These secondary maxima appear in all the parameters, which means that secondary recurring lines have the same geomagnetic consequences as the main ones. The results of the superposed epoch analysis performed separately for the four main recurring lines showed that each of these lines presented a certain ‘individuality’. It is surprising that, although the similarity between the graphs A-D for each main recurring line is quite good, the distributions for each parameters do not offer a clear resemblance (except for the central maxima, Figs. 2-5). We suggest that the secondary recurring lines come from some discontinuities corotating with the Sun, within the magnetic sectors, connected with the ‘subsector’ structure or filaments in the solar wind. Thus, Wilcox (1970) noticed that within a given magnetic sector two or three large-scale increases in solar wind velocity were often observed. A single magnetic sector may consist of two or three adjacent subsectors.
206
A. MOLDOVANU
One would expect that in front of the subsectors, as well as for the sector boundaries, tangential discontinuities might appear, but having less geomagnetic effects than the magnetic boundaries. Geomagnetic effects of subsectors have not yet been studied. For some cases, the subsector structure appears directly in the solar wind velocity. Thus, in the solar rotation No. 1839 (Fig. 7) one can see that the gr(SC + SI) events on 5-6 and 11 January 1968, coincide with the beginnings of the subsectors. The superposed epoch analysis was carried out for the main recurring lines, in comparison with which the secondary recurring lines do not evolve strictly parallel, presenting only a tendency for parallelism. This contributes, probably, to a less sharp exhibition of secondary maxima in Figs. 2-5. The mixing of data from the four main recurring lines in the mean representation in Fig. 6, leads, of course, to the annihilation of the peculiarities due to the secondary maxima. The effects of the sector structure on the geomagnetic activity are thereby very extensive, suggesting that most geomagnetic disturbances are accounted for by the solar wind distortions, corotating with the Sun. This conclusion does not deny the part played by the transient MHD shock waves emitted by the individual solar flares, but statistically they seem to represent a secondary cause. Thus, even though in the period studied, SC events generated by flares undoubtedly occurred (not only on 10-11 June 1968, as mentioned above), their contribution did not affect the regularities determined by the sector and subsector structure. Besides, from the synthetic graphs in our previous work (e.g. Fig. 1 in Moldovanu et al., 1972) it was shown that during 1968 the flare-producing regions were not as clearly concentrated near the sector boundaries as found earlier by Bumba and Obridko (1969), even though we shifted the active regions for 44 days with respect to the interplanetary sector structure. Also, Bumba (1972) noticed that during 1968 the regions of the greater solar activity were connected with the prevailing large-scale negative polarity fields. It follows that during 1968 the distribution of the geomagnetic effects of flares
6
km pz s
4 2 0 't!!!?
DECEMBER
1967
AuA &A
JANUARY
'23124'25126'2?'28'29'30'3f'l '2'3'4
FIO. 7.
ch.iPARISON
OF SOLAR
WIND
PROPERTIES
'-A
4,b
A-A
1968 '5'6'?'8'9'~'ff'i2'f3'%'~5'f6'f?'f8' AND
CWOhfAGNETIC
AClWlTY
IN THE SOLAR
No. 1839. Top graphs: solar wind velocity after Pioneer 6 (P,) and Pioneer 7 (P,) spacecrafts (SolarGeophysical Data, 1967-1968), and polarity of interplanetary magnetic field after Wilcox and Colburn (1970). Bottom graphs: planetary geomagnetic 3-hr indices Kp and SC (black triangles) and SI (empty triangles) events. Straight brackets mark the collective gr(SC + SI) events. ROTATION
GEOMAGNETIC
EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
207
with respect to sector boundaries should be casual. Thereby, the secondary maxima Figs. 2-5 might not be attributed to the effects of individual flares. 3. During the 1 yr period (1968) analyzed, the direction of the field polarity change across the boundary determines different geomagnetic effects in the proximity of the boundary. Thus, for the (+, -) boundaries, such as in the cases of main recurring lines I and III (Figs. 2 and 4), the minimum in front of the boundary appears during the day -1 for the SC, the gr(SC + SI) and the sum of the SC and SI events, and during the days 0 to -1, for the Kp index. This minimum is very narrow for Kp. After the boundary the activity in SC and SI events continues on the day +I, and the minimum beyond the boundary for these events occurs during the day +2. For the (-, +) boundaries, such as in the cases of the main recurring lines II and IV (Figs. 3 and 5), the minimum in front of the sector boundary appears at about l-11 days earlier than for the boundaries (+, -), and it is wider. The activity of SC and SI events presents a sharp minimum on the day + 1, hence also earlier by a day than for the (+, -) boundaries By drawing a parallel between the geomagnetic effects in the proximity of the (+, -) and (-, +) boundaries, one can see a tendency towards the occurrence of a minimum in the geomagnetic activity within the positive sectcr, at an interval of about 1 day from the zero day, and of another minimum within the negative sector, at an interval of about 2 days from the zero day, regardless of the direction of the field polarity change, as the sector boundary passes the Earth. The above discussion refers to year 1968, and it might not be typical for other periods of time. in
6. CONCLUSIONS The
effects of the interplanetary sector structure on the geomagnetic activity are important and general. Essentially these effects consist of the occurrence of gr(SC + SI) events and of a sharp increase in the Kp index, when the sector boundaries pass the magnetosphere, suggesting that sector boundaries are accompanied by corotating shocks and by magnetohydrodynamical turbulence. The frequency of occurrence of SC events during the day of sector boundary is very high, much higher than that found earlier by other authors. The recurrence of the geomagnetic activity should be understood in a dynamical sense, in connection with the evolution of the sector structure. The superposed epoch analysis, performed with respect to the ‘main recurring lines’, brought out distinct peculiarities of the main recurring lines and showed the existence of some ‘secondary recurring lines’ within the magnetic sectors, which have geomagnetic effects similar to those of the main lines. The occurrence of secondary recurring lines might be accounted for by the corotating distortions of the type of tangential discontinuities, and are probably related to the ‘subsectors’ or ‘filaments’ in the solar wind. During 1968 the distribution of the geomagnetic bisturbances near the sector boundaries is influenced by the direction of the field polarity change across the boundary. Most of the geomagnetic disturbances can be explained by corotating distortions in the solar wind, connected with the sector structure. This does not deny the part played by flare-induced shocks, in the release of geomagnetic storms, but statistically these seem to play a secondary role.
208
A. MOLDOVANU
REFERENCES AKASOFU, S.-I. (1969). Diagnostic of the magnetosphere by using geomagnetic, aurora1 and airglow phenomena. Report at IAGA General Science Assembly, Madrid, September 1969 (Preprint). BALLIF, J. R. and JONES, D. E. (1969). Flares, Forbush decreases, and geomagnetic storms. J. geophys. Res. 14.3499. BUMBA, V. (1972). Solar large-scale positive polarity magnetic fields and geomagnetic disturbances. Solar Wind (Eds. C. P. Sonett, P. J. Coleman, Jr. and J. M. Wilcox), (Proceedings of the NASA Conference, Asilomar, March 1971), p. 151. NASA, Washington. BUMBA, V. and OBRIDKO,V. N. (1969). Bartels active longitudes. Sector boundaries and flare activity. Solar Phys. 6, 104. BURLAGA,L. F. (1972a). Microstructure of the interplanetary medium. Solar Wind (Eds. C. P. Sonett, P. J. Coleman, Jr. and J. M. Wilcox) (Proceedings of the NASA Conference, Asilomar, March 1971), p. 309. NASA, Washington. BURLAGA,L. F. (1972b). A note on large velocity discontinuities in the solar wind. Geomag. Aeron. 12,797. CHAPMAN,S. (1966). Foreward. The Solar Wind (Eds. R. J. Macking Jr. and M. Neugebauer). Pergamon, Oxford. DESSLER,A. J. (1967). Solar wind and interplanetary magnetic field. Rev. Geophys. 5, 1. DESSLER,A. J. (1968). Solar wind interactions and the magnetosphere. Physics of the Magnetosphere (Eds. R. L. Carovillano and H. R. Radoski), p. 65. Reidel, Dordrecht, Holland. HALZNKA, J. (1968). On correlating “cosmic” flares with geomagnetic storms. Geofys. Sb. 15, 1967, Praha 1968, p. 367. IVANOV,K. G. (1971). Rotationary discontinuities in solar wind. Geomagn. Aeron. 11, 765. IVANOV, K. G. (1972). Classification, peculiarities of orientation and some examples of rotationary discontinuities in solar wind. Geomagn. Aeron. 12, 984. MOLDOVANU,A., BRADU, E. and TARNAUCEANU,D. (1971). Contributions ii 1’ etude statistique de la correlation entre les centres Bruptifs solaires et les Bvenements geomagnetiques SC et SI. Anal. St. Univ. “Al. I. Cuza”-Iagi, Sect. I Fizid, 17, 189. MOLDOVANU,A., BRADU, E. B. and TARNAUCEANU,D. (1972). A statistical treatment of the relationship between the solar flare-producing regions and the SC + SI events. Beitr. Geophys. 81/l-2, 1. MOLDOVANU,A. and BRADU, E. B. (1973). Comparison of the sectorial structure of the interplanetary magnetic field and the occurrence of the SC and SI events. Beitr. Geophys. 8213, 171. Ness, N. F., HUNDHAUSEN,A. J. and BAME, S. J. (1971). Observations of the interplanetary medium: Vela 3 and Imp 3, 1965-1967. J. geophys. Res. 76,6643. OBAYASHI,T. (1964). Corpuscular streams related to solar M-regions. Rep. Zonosph. Space Res. Japan 18, 245. PARKER.E. N. (1963). Interplanetary Dynamical Processes. Interscience, New York. Solar-Geophysical Data. (1967-1969). U.S. Dept. of Commerce, Environmental Science Services Administration. Wncox, J. M. (1968). The interplanetary magnetic field. Solar origin and terrestrial effects. Space Sci. Rev 8, 258. Wncox. J. M. (1971). Sector structure of the solar magnetic field. Solar Magnetic Fields (Ed. Robert Howard), p. 744. Reidel, Holland. WILCOX, J. M. and COLBURN,D. S. (1969). Interplanetary sector structure in the rising portion of the sunspot cycle. J. geophys. Res. 74,2388. Wncox, J. M. and C~LBURN,D. S. (1970). Interplanetary sector structure near the maximum of the sunspot cycle. J. geophys. Res. 75, 6366. WILCOX, J. M. and COLBURN, D. S. (1972). Interplanetary sector structure at the solar maximum. J. geophys. Res. 77, 751. WILCOX, J. M. and Ness, N. F. (1965). Quasi-stationary corotating structure in the interplanetary medium. J. geophys. Res. IO, 5793. YOSHIDA, S. and AKASOFU,S.-I. (1966). The development of the Forbush decrease and the geomagnetic storm fields. Planet. Space Sci. 14, 979.
Press. Printed in Northern Ireland
GEOMAGNETIC EFFECTS OF INTERPLANETARY SECTOR STRUCTURE A. MOLDOVANU Centre of Technical and Physical Research, Ia$i, Rumania (Received injinalform
19 July 1973)
Abstract-In this paper the geomagnetic effects of the interplanetary magnetic sector structure are studied on the basis of some new criteria and working hypotheses. Thus, we assume that the recurrence of geomagnetic disturbances should be understood in a dynamical sense, in connection with the evolution of the full sector structure, and not necessarily as a 27-day recurrence. Accordingly, on the representation of the sector structure during 1968, as deduced by Wilcox and Colburn, we have defined four ‘main recurring lines’, which link the sector boundaries recurrent in successive solar rotations. The term ‘group of SC and SI events’, abbreviated as gr(SC + SI), introduced by us in previous works to designate the mornholoeical arounine of the individual SC and SI events in collective events, is also used. It should Thepc&ted o’lt that the bulk of gr(SC + SI) events are either associated with sector boundaries, or recurrent in successive solar rotations. Part of these events reveal the existence of some ‘secondary recurring lines’, within the magnetic sectors. The above working hypotheses and observations have been checked by the superposed epoch analysis, performed for each main recurring line in part and for all the main recurring lines combined. The following parameters are analysed: the number of SC events, the number of collective events gr(SC + SI), the total number of SC and SI events and the geomagnetic activity index
KP*
The main result of the superposed epoch analysis consists in the appearance of a sharp maximum for all the parameters considered on the day of sector boundary. This fact proves that the effects of the sector boundaries are important and general, in regard to all aspects of geomagnetic activity. Essentially these effects consist of the occurrence of gr(SC + SI) events and of a specific increase in the Kp index, when the sector boundaries pass by the magnetosphere. This suggests that the sector boundaries are accompanied by corotating shocks and magnetohydrodynamical turbulence. The high frequency in the occurrence of the SC events on the days of sector boundaries is also noticeable. Each main recurring line presents a certain ‘individuality’, expressed particularly by secondary specific maxima in all the parameters, corresponding to the ‘secondary recurring lines’. One suggests that these secondary recurring lines might be due to some corotating distortions within the magnetic sectors and might be related to the ‘subs&or’ or ‘filaments’. The distribution of the geomagnetic disturbances near the sector boundaries depends on the direction of the field polarity change. All these results lead to the conclusion that most of the geomagnetic disturbances can be accounted for by the interaction between corotating distortions in the solar wind connected with the sector structure and the magnetosphere, the flare-induced disturbances representing statistically the secondary mechanism.
1.INTRODUCTION
order to explain the occurrence of the geomagnetic disturbances two main physical patterns are now accepted: (a) the magnetohydrodynamic transient shock waves, related to some individual solar flares, account for the occurrence of casual SC storms (Chapman, 1966; Parker, 1963) and (b) the solar corpuscular streams, as long-living structures corotating with the Sun, account for the occurrence of recurring geomagnetic disturbances (Obayashi, 1964). Although in a number of cases the association between the solar flares and SC events is evident, unfortunately this association is generally uncertain (Ballif and Jones, 1969; Halenka, 1968; Moldovanu et al., 1971; Moldovanu et al., 1972). Most of the SC events In
1
193
194
A. MOLDOVANU
and the associated Forbush decreases cannot be explained by the pattern (a) and consequently, a more satisfactory explanation is expected from the pattern (b) (e.g. Ballif and Jones, 1969), or in an alternative, which considers the possible interaction between the transient shocks and the sector structure (Ness et al., 1971). Wilcox and Ness (1965), Wilcox and Colbum (1969); 1970, 1972) and Ness et al. (1971) drew attention to the geomagnetic effects of the sector boundaries. They pointed out that the geomagnetic activity index Kp rose near the sector boundary and that some sector boundaries coincide with the occurrence of SC events. Using some new criteria to systematize the interplanetary and geomagnetic data, we point out in this paper that the geomagnetic effects of the sector structure are more general, especially concerning the occurrence of SC and SI events. 2. PHYSICAL
CONSIDERATIONS
In relation to the pattern of interplanetary corpuscular streams, it is interesting to recall the theoretical works of Dessler (1967, 1968) on the ‘tangential discontinuity’. A tangential discontinuity can appear in the solar wind when a faster plasma stream overtakes a slower stream. As a result, the two streams collide and the gas in both streams is compressed in an intermediate region of higher density, pressure, temperature and magnetic-field strength. At the front and behind the intermediate region, two shocks can be generated, if the relative velocity of the two streams exceeds the magnetoacoustic velocity. The two shocks cause the solar wind to deviate from the radial flow: for a stationary observer, the flow deviates eastward, in front of the interface, and westward, behind it (Dessler, 1968, Fig. 14). At the interface there appears a velocity discontinuity, which is called ‘tangential discontinuity’. Since the magnetic fields are either parallel or antiparallel on the two sides of the discontinuity, a Kelvin-Helmholtz instability should develop at the interface and generate magnetohydrodynamical turbulence. Dessler (1967) suggested that “ . . . M-region geomagnetic storms are due to a KelvinHelmholtz instability that could develop along the velocity discontinuity and to the enhancement of the irregularities in the solar magnetic field between the two shocks”. In the interplanetary space, distortions of the kind of tangential discontinuities were found, connected with the sector structure of the interplanetary magnetic field (Wilcox and Ness, 1965; Wilcox, 1968; Ness et al., 1971). Thus, across a sector boundary the real picture of plasma parameters is entirely similar to that expected in the theoretical pattern of Dessler. The result is that the sector boundaries should be considered as tangential discontinuities for the particular case when the magnetic field has opposite polarities on both sides of the interface. Several other types of discontinuities were assumed to exist in the solar wind: filaments; directional, simple, appreciable discontinuities, etc. After Burlaga (1972a) most of these might be reduced to two main types: tangential and rotational discontinuities. It is not yet clear which of these two types of discontinuities occurs more frequently. Even for the same cases of discontinuities and for the same set of spatial data, different authors come to opposite conclusions, such as the arguments between Burlaga (1972b) and Ivanov (1971, 1972). For the present study it is not so important to dwell on the types of discontinuities associated with the sector boundaries, as it is to learn if there are discontinuities, and if there are, what effects they have on the geomagnetic activity. Therefore, we shall consider the pattern of tangential discontinuity, mentioned above, as being an acceptable basis of discussion.
GEOMAG~TIC
EFFECTS OF ~ERPL~ETARY
SECTOR STRT_JClWRE
195
In order to ascertain the possible geomagnetic effects of the whole structure connected with a tangential discontinuity, two references should be considered. (1) Akasofu (1969) noted: “We now know by satellite observations that both SSC and SI are caused by contact of interplanetary MHD discontinuities with the magnetosphere Thus, it is not necessarily appropriate to distinguish them”. (2) Yoshida and Akasofu (1966) suggested: “ . . . Nishida and Jacobs have shown that they (i.e. SI events) have essentially the same characteristics as those of SSC. Since SSC is now interpreted as a sudden compression of the magnetosphere by an enhanced solar plasma flow, intense SI activity may be interpreted as an indication of ‘turbulent’ ffow; embedded in such a turbulent flow, the maguetospheric cavity expands or contracts, causing fluctuations of the magnetic field inside the cavity”. Taking into account both the pattern of the tangential discontinuity and the last two references, we consider that the encounter between a tangential discontinuity in the solar wind (in particular, a sector boundary) and the magnetosphere might have the following effects: (a) the occurrence of one or two SC (or SI) events, due to the two shocks, in front and behind the collision region; (b) the starting of an intense SI activity related to the magnetohydrodynamical turbulence along the discontinuity; (c) the increase of the geomagnetic activity, also connected with the MHD turbulence at the interface. To summarize, the occurrence of a group of SC and SI events, accompanied by the increase of the geomagnetic activity index Kp, is expected. The nature, the intensity and the duration of the whole geomagnetic disturbance associated with the tangential discontinuity would depend on the physical properties of the two coiliding plasma streams. The geomagnetic effects already found of the sector bounda~es are in good agreement with those expected in the case of tangential discontinuities (Wilcox and Ness, 1965; Wilcox and Colburn, 1969, 1970, 1972; Ness et al., 1971). Moldovanu et al. (1971,1972), taking into account all the SC and SI events, as reported in Solar-Geophysical Data (1967-1969), drew attention to the fact that, in most cases, the SC and SI events appear to be morphologically grouped in collective events, which they called ‘groups of events SC and SI’, abbreviated to gr(SC + SI). Having noticed that the most abundant gr(SC + SI) events were associated with sector boundaries, these authors suggested that the gr(SC + SI) events would reflect the interaction between the tangential discontinuities in the solar wind and the magnetosphere. From a comparison between the interplanetary magnetic sector structure during 1968, as deduced by Wilcox and Coburn (1970), and the occurrence of SC and SI events reported in Sonar-Geophysical Data, Moldovanu and Bradu (1973) pointed out that most gr(SC + SI) events were either associated with sector boundaries, or recurrent in successive solar rotations. They concluded that the main cause of the gr(SC + SI) events was represented by distortions in the solar wind corotating with the Sun. In the present paper we give new arguments supporting the conclusions of our previous works, based on a statistical treatment of the correlation between the interplanetary sector structure and several aspects of the geomagnetic activity. 3. WORKING METHOD
In Fig. 1, the interplanetary magnetic sector structure during 1968 (near the maximum of the sunspot cycle ZO), is plotted (Wilcox and Colburn, 1970). The periods of positive inte~lane~~ magnetic field (ma~etic field directed away from the Sun) are noted by the areas above the time axis, and the periods of negative inte~lane~ry magnetic field
196
A. MOLDOVANU
1840 Jan
Feb
1841 Feb
Mch
1842 Mch
APr
Mw
FIG. 1. SKTORIAL STRUCTUREOF THE INTERPLANETARY MAGNETIC FIELD IN THE SOLAR ROTATIONS Nos.1839-1852 (SUCHASWASDETERMINED BYWILCOXANDCOLBURN,~~~~)ANDTHJZ OCCURRENCEOFSCANDSIEVENTS.
The periods of positive interplanetary magnetic field (magnetic field directed away from the Sun) are noted by the areas above the time axis, and the periods of negative interplanetary magnetic field (magnetic field directed toward the Sun) are shown by the areas below the time axis. The SC events are marked by black triangles and the SI events, by empty ones. The collective events gr(SC + SI) are marked with straight brackets.
(magnetic field directed toward the Sun) are shown by the areas below the time axis. The boundaries between magnetic sectors recurrent in successive solar rotations are linked by continuous lines, which we called ‘main recurring lines’, and marked by the Roman numbers: I, IL, III, IV. The magnetic sectors, obvious in Fig. 1, were designated by Arabic numbers : 1, 2, 3, 4. Since we are especially interested in what happens near the sector boundaries, we do not use the restrictive condition of the ‘well-established sectors’ (Wilcox and Colbum,
GEOMAGNETIC
EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
197
1970, 1972), considering in exchange those sector boundaries which are evidently recurrent in as many solar rotations as possible, in other words, the ‘well-es~blished recurring lines’. In Fig. 1 we note also all the SC and SI events reported in Solar-Geophy~icul Tutu (1967-1969), selected by Romafia from the world-wide network of geomagnetic observatories. The SC events were marked by black triangles and the SI events by empty triangles. With straight brackets, we marked the grouping of the individual SC and SI events in collective gr(SC + SI) events, as determined in our previous works ~oldovanu et al., 1971, 1972; Moldovanu and Bradu, 1973). In grouping the individual SC and SI events in collective gr(SC! + SI) events, we took into account both their time proximity and their effects on the geomagnetic activity. The isolated SC and SI events were each considered to represent a distinctive gr(SC + SI) event with a number of components equal to 1. To identify the SC and SI events reported in Solar-Geophysical Data, we used as reference material the magnetograms from the Geomagnetic Observatory Surlari ~Rumania). The isolated events not included in brackets (Fig. 1) were reported by less than three observatories in Solar-Geophysical Data, and were also not identified on the magnetograms. These events were not considered in the analysis. The examination of Fig. 1 reveals clearly that most sector boundaries coincide with the occurrence of g&SC + SI) events. In Fig. 1 the gr(SC + SI) events which do not coincide with sector boundaries but present obvious recurrence in successive solar rotations are linked by vertical dotted lines. We call them ‘secondary recurring lines’. One can see that some of the secondary recurring lines present an outstanding continuity and tend to evolve parallel to the main recurring lines. Since both the main and the secondary recurring lines follow the evolution of the whole sector structure (Fig. 1), we consider that the recurrence of gr(SC + SI) events should be understood in a dynamical sense, in connection with the evolution of the sector structure, and not necessarily as a 27-day recurrence. We have checked both the geomagnetic effects of the sector boundaries as well as the validity of the concept of ‘dynamic recurrence’, by applying the superposed epoch analysis. We make this analysis for each of the four main recurring lines. In the superposed epoch analysis the ‘zero’ is determined by the time at which the sector boundaries pass the Earth. These moments, corresponding to the four main recurring lines, are given in Table 1. Some of these times are given by Wilcox and Colburn (1970), and the others (denoted by asterisk) were determined from a figure presented by the same authors. The moments determined by interpolation are also specified. The boundaries corresponding to the main recurring lines I and II in the solar rotation No. 1845 lie in large gaps (Fig.l), and are not analyzed. The boundaries corresponding to the beginnings of the main recurring lines III and IV in rotation No. 1842 refer rather to a ‘filament’ than to a sector and are not considered either. The superposed epoch analysis (Figs. 2-6) was performed for the following parameters : (A) the number of SC events; (B) the number of collective gr(SC + SI) events; (C) the total number of SC and SI events and (D) the planetary magnetic 3-hr index Kp. The distributions of SC, gr(SC! + SI) and all the SC and SI events (respectively A, B, and C, in Figs. 2-6) are given in l-day intervals. Thus, a ‘zero day’ corresponds to a 24-hr period around the moment of occurrence of the sector boundary; on both sides of the zero day, 5 other days are considered.
A. MOLDOVANU
198
Main recurring line II (- , +)
Main recurring line I (+ , -) Date 2 January 1968 28 January 1968 26 February 1968 23 March 1968 21 April 1968 17 May 1968 9 July 1968 4 August 1968 2 September 1968
Rate
Interval 2-3 8-l 6-7 E 5-6 8-l 8-1 (int)+ 4-s*
16January1968 11 February 1968 10 March 1968 5 April 1968 2 May 1968 30 May 1968 17 July 1968 13 August 1968
Main recurring line III ( + , -) Date 9 April 1968 6 May 1968 1 June 1968 27 June 1968 25 July 1968 21 August 1968 19 September 1968 16 October 1968 13 November 1968 10 December 1968
Interval 8-l (int)* 3-4 4-5 6-7 l-2 1-2* 4-S 7-8
Main recurring line IV (+, -1
Interval
7-8; 6-7* 2-3* 6-7 (int)* E 2-3 56 2-3 2-3
Date
11 April 1968 10 May 1968 7 June 1968 3 July 1968 31 July 1968 31 August 1968
Interval 6-7* 5-6 (int)* E z*
t Three-hour intervals are used. Some of these times are inferred by Wilcox and Colbura (1970), and the other (denoted by asterisk) are determined from their figure showing the sector structure. The momenta determined by interpolation are denoted by “int”.
The distribution of the individual SC and of all the individual SC and SI events with respect to the zero day was obtained directly. A more complicated situation appeared in the case of collective gr(SC + SI) events, which lasted from a few hours to 1 day, often occurring near the limit between two diurnal periods. In such situations we assigned the gr(SC + SI) event to that diurnal period in which the bulk of the constitutive SC and SI events occurred. The distribution of the 3-hr values of the geomagnetic activity index Kp was considered in the 3-hr periods, comprised within 5$ days on both sides of every sector boundary (part D in the Figs. 249, but in order to facilitate the comparison with the first three parts, the time axis was also divided into ‘zero day’ and 5 days before and after this. Figures 2-5 comprise the results of the superposed epoch analysis for the main recurring lines I-IV respectively, while Fig. 6 refers to all the sector boundaries of these lines, combined. To facilitate the comparisons, the values given in Figs. 2-6 are average values, ascribed to a single sector boundary. 4. RESULTS
The main recurring line I (change of~~~d~~~ari~~:+ , -) The similarity between the statistical distributions of the four parameters analysed is remarkable (Fig. 2). For the total number of SC and SI events (part C) and Kp index (part D), this close similarity relates even to details. The central maximum on the day of sector change is very sharp for all the parameters.
GEOMAGNETIC EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
MAIN RECURRING LINE j-
2.5 2.0 :
. . .. . .
25 1
.
s+’ t + I-1
1.0~ 0.5:
t
t
’
’
’
’
-5 -4 -3 -2 -7
POSlTlON
1.1
0
t
’
12345
’
’
’
WITH RESPECT TO DAYOF SECTOR BOUNDARY, RAYS
FIG. 2. SUPERPOSED EPOCH ANALYSIS WITH RESPECT TO SECTOR BOUNDARIES ~~RRE~~~NDING TO THE MAIN RECURRING LINE I. (A} NUMBER OF SC EVENTS; (B) NUMBER OF @(SC+ Sl) INDEX Kp, EVENTS; (c)TOTAL NUMBER OF SC AND SE EVENTS; (D)GEOMAGNETIC Ac7fWIlY All
the
values are average values, ascribed to a single sector boundary.
199
200
A. MOLDOVANU
This central maximum is preceded by a minimum, on the day - 1 for the first three parameters and on the days -1 to 0 for the Kp index. On both sides of the central maximum, two secondary maxima for all the parameters appear: a weak maximum on the days -3 to -2 (clearer in the Kp index), and another clear maximum on the days +3 to +4. One can see that these secondary maxima correspond exactly to the secondary recurring lines, which we give in Fig. 1 on both sides of the main recurring line I for the gr(SC + ST) events. It is worth noting that these secondary maxima become conspicuous in the distribution of the Kp index, including all the aspects of the geomagnetic activity. During the solar rotation No. 1848 the main recurring line IV gets within only 2 days of the line I, and their geomagnetic effects interfere. The main recurring line II (change of$eldpolarity:
-
, +)
The resemblance between the graphs of all the four parameters is also fair (Fig. 3). The central maximum is evident for all the parameters. The secondary maximum on the days -4 to -5 is the result of some secondary recurring lines. The secondary maximum on the days $4 to +5 is due, in addition, to the emergence of the main recurring line III on the right side, beginning with the rotation No. 1842, which affects the distribution. The minimum which precedes the sector boundary appears, for all the parameters, within about 14 days earlier than in the case of the main recurring line I. For the Kp index this minimum is less clearly expressed. The minimum following the central maximum also occurs within about one day earlier than for the line I. The main recurring line III (change ofjeldpolarity:
+, -)
The resemblance between the graphs of the four parameters is fair, except for the central maximum, which is not clearly defined for the Kp index (Fig. 4). The minimum which precedes the sector boundary is very clearly defined for all the parameters at the day - 1, as for the line I. The secondary maxima at the days -4 to -5, and +3 to +4 do not correspond to secondary continuous recurring lines, being in part the consequence of the closeness of the main recurring lines II and IV (see Fig. 1). The main recurring line IV (change ofjieldpolarity:
- , +)
The resemblance between the graphs of the four parameters is fair, except for the central maximum, which is very sharp for the first three parameters, but is not clearly defined for the Kp index (Fig. 5). The line IV appears to be the most effective for the first three parameters. The minimum which precedes the sector boundary appears approximatively during the day -2, and the minimum which follows the boundary, during the day +l (as for the line II). The wide and intense secondary maximum at the days +2 to +5 is due first of all to the secondary recurring lines, on the right hand side of the line IV. This secondary maximum is also affected partly by the proximity of the line I in the rotation Nos. 1847 and 1848, and by the great disturbance which occurred on 10-11 June 1968. The proximity of the line III on the left side, in the solar rotation No. 1844, contributes to the emergence of the secondary maximum during the days -3 to -5. The average of all the main recurring lines
The maximum on the day of sector change is very sharp for all the parameters (Fig. 6). The frequency in the occurrence of geomagnetic events is very high on the zero day: it is
l
GEOMAGNETIC
.
.
ACTIVITY
INDEX, Kp
R I I I
E
F
I I I I I, I
SC and SI EVENTS,
0 (
.a ,
I
I
I,,
01
3 I,
TOTAL NUMBER
I,
2
%
I I I I ,
grlSC +SIl EVENTS. NUMBER I
SC EVEN T.S. NUMBER
A. MOLDOVANU
202
MAIN RECURRINGLINE fl
L ..
HI. 2.0-
.= .
l l
FIG.
4.
%JPERPOSED
EPOCH
m
l
l l
l ;
.
00
.*.= .
ID)
.
..
0.5 : -
. : :.
1.5 -
1
5.. ,’ =. ’ 0: -. :*.
.
(Cl
+/I I ! I I I I I I I, -h -4 -3 -2 -1 0 12345 POSITION WITH RESPECT TO DAY OF SECTOR BOUNDARY, DAYS
ANALYSIS
TOTmMAINRECuRRIN
WITH
RESPECT
0 LINE
111
TO
SECTOR
(SEE CAPTION
BOUNDARIES OF
CORRESPONDING
Fm. 2).
larger than O-6for the SC events, O-7 for the gr(SC + SI) events and about 1.7 for the total number of SC and SI events. The minimum which precedes the boundary appears on the day -1 for the first three parameters, and during the days 0 to -1 for the Kp index. The Kp index distribution presents a fair resemblance with the distribution of the other parameters and, particularly, with the distribution of the total number of SC and SI events.
MAIN RECURRING LINE Li
.r
(A)
fB)
0.5
/
:
2.5-
2.0-
1.5 -
II
TO-
0.5-
3.5c
. :.
. 3.0: 2.5 2.0:
(0)
7.5 I.01 0.5 : 4
-4 -3 -2 -1 I
I
0
I
12345 I I
I
I
I
POSITION WITH RESPECT TO DAY OF SECTOR BOUNDARY. DAYS
FIG. 5. SUPERPOSEDEPOCH ANALYSISWITH RESPECTTO SECTORBOUNDARIESCORRESPONDING TOTHEMAINREZURRINGLINE (SEECAP~ON OFFIG. 2). 203
204
A.MOLDOVANu
MEAN OFALL MAIN RECURRING LINES
ccl
1
_
2.5 2.0 1.5
k
. . . 8’ . . . % .......... .s'=. 0,;. :.A "Jo .+ '.+p. . . . op.': 2. . . . 08 . l
I
1
9
54321
1
t
1
8
0
1
RJDJRRIN~
1
1
!
12345
POSlTlON WITH RESPECr SECTOR BOUNDARY,
FIG.6.SUPERPOSED EPOCH ANALYSIS
1
fD)
TO DAY OF DAYS
WITH RESPE~TTOSECTORBOLINDAFUJB OF ALLTHEMAIN LINE&COMBINED (SEE CAPTION OF FIG.2).
GEOMAGNETIC EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
205
The distribution of the Kp index, in Fig. 6, graph D, is quite like that found for 1968 by Wilcox and Colburn (1970), though the sector boundaries considered by us are not exactly the same. 5. DISCUSSION
1. From the foregoing analysis the result is that the geomagnetic effects of the sector boundaries are exactly as predicted in Section 2 of this paper, taking into account the tangential discontinuity pattern (Dessler, 1967). Thus, the bulk of sector boundaries coincides with gr(SC + SI) events, being accompanied by a sharp rise in the geomagnetic activity index Kp (parts B and D, in Figs. 2 -6). The high value of the frequency in gr(SC + SI) events on the zero day (~0.7) shows that more than two thirds of sector boundaries considered in our analysis are accompanied by the occurrence of gr(SC + SI). The frequency of occurrence of the SC events during the day of the sector boundary has a remarkable high value: 064 (Fig. 6, graph A). This fact proves that numerous sector changes are really accompanied by corotating shocks: the SC events with which most of the collective gr(SC + SI) events begin seems to reflect exactly the shock in front of the colliding region of the tangential discontinuity. This aspect, which strengthens the leading part of the sector boundaries in the release of SC events, is sensibly different from the earlier results of Wilcox and Colburn (1969, 1972) and Ness et al. (1971): they pointed out that only some sector boundaries coincide with SC events. This difference comes from the different number of the events considered. While these authors took into account only the SC events included by Virginia Lincoln (Solar-Geophysical Data) in her representation of the planetary Kp index, in our analysis we used all the SC events reported to Solar-Geophysical Data by Romaiia. Our analysis, therefore, relied on a larger number of data. The same treatment of the SC and SI events, performed according to suggestions given by Akasofu (1969), led to a considerable increase in the frequency of these events on the boundary day (part C in Figs. 2-6). We think that it is not essential to know how many SC and SI events accompany a sector boundary, but it is important to know if the boundary is or is not accompanied by such events. In order to describe the geomagnetic effects of sector boundaries, we consider that the collective gr(SC + SI) events are more representative than the individual SC and SI events, the first ones offering a more plausible physical interpretation. 2. The dynamical character of the recurrence appears clearly expressed by the secondary maxima in the Figs. 2-5, which reflect the tendency of secondary recurring lines to evolve parallel to the main lines (Fig. 1). These secondary maxima appear in all the parameters, which means that secondary recurring lines have the same geomagnetic consequences as the main ones. The results of the superposed epoch analysis performed separately for the four main recurring lines showed that each of these lines presented a certain ‘individuality’. It is surprising that, although the similarity between the graphs A-D for each main recurring line is quite good, the distributions for each parameters do not offer a clear resemblance (except for the central maxima, Figs. 2-5). We suggest that the secondary recurring lines come from some discontinuities corotating with the Sun, within the magnetic sectors, connected with the ‘subsector’ structure or filaments in the solar wind. Thus, Wilcox (1970) noticed that within a given magnetic sector two or three large-scale increases in solar wind velocity were often observed. A single magnetic sector may consist of two or three adjacent subsectors.
206
A. MOLDOVANU
One would expect that in front of the subsectors, as well as for the sector boundaries, tangential discontinuities might appear, but having less geomagnetic effects than the magnetic boundaries. Geomagnetic effects of subsectors have not yet been studied. For some cases, the subsector structure appears directly in the solar wind velocity. Thus, in the solar rotation No. 1839 (Fig. 7) one can see that the gr(SC + SI) events on 5-6 and 11 January 1968, coincide with the beginnings of the subsectors. The superposed epoch analysis was carried out for the main recurring lines, in comparison with which the secondary recurring lines do not evolve strictly parallel, presenting only a tendency for parallelism. This contributes, probably, to a less sharp exhibition of secondary maxima in Figs. 2-5. The mixing of data from the four main recurring lines in the mean representation in Fig. 6, leads, of course, to the annihilation of the peculiarities due to the secondary maxima. The effects of the sector structure on the geomagnetic activity are thereby very extensive, suggesting that most geomagnetic disturbances are accounted for by the solar wind distortions, corotating with the Sun. This conclusion does not deny the part played by the transient MHD shock waves emitted by the individual solar flares, but statistically they seem to represent a secondary cause. Thus, even though in the period studied, SC events generated by flares undoubtedly occurred (not only on 10-11 June 1968, as mentioned above), their contribution did not affect the regularities determined by the sector and subsector structure. Besides, from the synthetic graphs in our previous work (e.g. Fig. 1 in Moldovanu et al., 1972) it was shown that during 1968 the flare-producing regions were not as clearly concentrated near the sector boundaries as found earlier by Bumba and Obridko (1969), even though we shifted the active regions for 44 days with respect to the interplanetary sector structure. Also, Bumba (1972) noticed that during 1968 the regions of the greater solar activity were connected with the prevailing large-scale negative polarity fields. It follows that during 1968 the distribution of the geomagnetic effects of flares
6
km pz s
4 2 0 't!!!?
DECEMBER
1967
AuA &A
JANUARY
'23124'25126'2?'28'29'30'3f'l '2'3'4
FIO. 7.
ch.iPARISON
OF SOLAR
WIND
PROPERTIES
'-A
4,b
A-A
1968 '5'6'?'8'9'~'ff'i2'f3'%'~5'f6'f?'f8' AND
CWOhfAGNETIC
AClWlTY
IN THE SOLAR
No. 1839. Top graphs: solar wind velocity after Pioneer 6 (P,) and Pioneer 7 (P,) spacecrafts (SolarGeophysical Data, 1967-1968), and polarity of interplanetary magnetic field after Wilcox and Colburn (1970). Bottom graphs: planetary geomagnetic 3-hr indices Kp and SC (black triangles) and SI (empty triangles) events. Straight brackets mark the collective gr(SC + SI) events. ROTATION
GEOMAGNETIC
EFFECTS OF INTERPLANETARY
SECTOR STRUCTURE
207
with respect to sector boundaries should be casual. Thereby, the secondary maxima Figs. 2-5 might not be attributed to the effects of individual flares. 3. During the 1 yr period (1968) analyzed, the direction of the field polarity change across the boundary determines different geomagnetic effects in the proximity of the boundary. Thus, for the (+, -) boundaries, such as in the cases of main recurring lines I and III (Figs. 2 and 4), the minimum in front of the boundary appears during the day -1 for the SC, the gr(SC + SI) and the sum of the SC and SI events, and during the days 0 to -1, for the Kp index. This minimum is very narrow for Kp. After the boundary the activity in SC and SI events continues on the day +I, and the minimum beyond the boundary for these events occurs during the day +2. For the (-, +) boundaries, such as in the cases of the main recurring lines II and IV (Figs. 3 and 5), the minimum in front of the sector boundary appears at about l-11 days earlier than for the boundaries (+, -), and it is wider. The activity of SC and SI events presents a sharp minimum on the day + 1, hence also earlier by a day than for the (+, -) boundaries By drawing a parallel between the geomagnetic effects in the proximity of the (+, -) and (-, +) boundaries, one can see a tendency towards the occurrence of a minimum in the geomagnetic activity within the positive sectcr, at an interval of about 1 day from the zero day, and of another minimum within the negative sector, at an interval of about 2 days from the zero day, regardless of the direction of the field polarity change, as the sector boundary passes the Earth. The above discussion refers to year 1968, and it might not be typical for other periods of time. in
6. CONCLUSIONS The
effects of the interplanetary sector structure on the geomagnetic activity are important and general. Essentially these effects consist of the occurrence of gr(SC + SI) events and of a sharp increase in the Kp index, when the sector boundaries pass the magnetosphere, suggesting that sector boundaries are accompanied by corotating shocks and by magnetohydrodynamical turbulence. The frequency of occurrence of SC events during the day of sector boundary is very high, much higher than that found earlier by other authors. The recurrence of the geomagnetic activity should be understood in a dynamical sense, in connection with the evolution of the sector structure. The superposed epoch analysis, performed with respect to the ‘main recurring lines’, brought out distinct peculiarities of the main recurring lines and showed the existence of some ‘secondary recurring lines’ within the magnetic sectors, which have geomagnetic effects similar to those of the main lines. The occurrence of secondary recurring lines might be accounted for by the corotating distortions of the type of tangential discontinuities, and are probably related to the ‘subsectors’ or ‘filaments’ in the solar wind. During 1968 the distribution of the geomagnetic bisturbances near the sector boundaries is influenced by the direction of the field polarity change across the boundary. Most of the geomagnetic disturbances can be explained by corotating distortions in the solar wind, connected with the sector structure. This does not deny the part played by flare-induced shocks, in the release of geomagnetic storms, but statistically these seem to play a secondary role.
208
A. MOLDOVANU
REFERENCES AKASOFU, S.-I. (1969). Diagnostic of the magnetosphere by using geomagnetic, aurora1 and airglow phenomena. Report at IAGA General Science Assembly, Madrid, September 1969 (Preprint). BALLIF, J. R. and JONES, D. E. (1969). Flares, Forbush decreases, and geomagnetic storms. J. geophys. Res. 14.3499. BUMBA, V. (1972). Solar large-scale positive polarity magnetic fields and geomagnetic disturbances. Solar Wind (Eds. C. P. Sonett, P. J. Coleman, Jr. and J. M. Wilcox), (Proceedings of the NASA Conference, Asilomar, March 1971), p. 151. NASA, Washington. BUMBA, V. and OBRIDKO,V. N. (1969). Bartels active longitudes. Sector boundaries and flare activity. Solar Phys. 6, 104. BURLAGA,L. F. (1972a). Microstructure of the interplanetary medium. Solar Wind (Eds. C. P. Sonett, P. J. Coleman, Jr. and J. M. Wilcox) (Proceedings of the NASA Conference, Asilomar, March 1971), p. 309. NASA, Washington. BURLAGA,L. F. (1972b). A note on large velocity discontinuities in the solar wind. Geomag. Aeron. 12,797. CHAPMAN,S. (1966). Foreward. The Solar Wind (Eds. R. J. Macking Jr. and M. Neugebauer). Pergamon, Oxford. DESSLER,A. J. (1967). Solar wind and interplanetary magnetic field. Rev. Geophys. 5, 1. DESSLER,A. J. (1968). Solar wind interactions and the magnetosphere. Physics of the Magnetosphere (Eds. R. L. Carovillano and H. R. Radoski), p. 65. Reidel, Dordrecht, Holland. HALZNKA, J. (1968). On correlating “cosmic” flares with geomagnetic storms. Geofys. Sb. 15, 1967, Praha 1968, p. 367. IVANOV,K. G. (1971). Rotationary discontinuities in solar wind. Geomagn. Aeron. 11, 765. IVANOV, K. G. (1972). Classification, peculiarities of orientation and some examples of rotationary discontinuities in solar wind. Geomagn. Aeron. 12, 984. MOLDOVANU,A., BRADU, E. and TARNAUCEANU,D. (1971). Contributions ii 1’ etude statistique de la correlation entre les centres Bruptifs solaires et les Bvenements geomagnetiques SC et SI. Anal. St. Univ. “Al. I. Cuza”-Iagi, Sect. I Fizid, 17, 189. MOLDOVANU,A., BRADU, E. B. and TARNAUCEANU,D. (1972). A statistical treatment of the relationship between the solar flare-producing regions and the SC + SI events. Beitr. Geophys. 81/l-2, 1. MOLDOVANU,A. and BRADU, E. B. (1973). Comparison of the sectorial structure of the interplanetary magnetic field and the occurrence of the SC and SI events. Beitr. Geophys. 8213, 171. Ness, N. F., HUNDHAUSEN,A. J. and BAME, S. J. (1971). Observations of the interplanetary medium: Vela 3 and Imp 3, 1965-1967. J. geophys. Res. 76,6643. OBAYASHI,T. (1964). Corpuscular streams related to solar M-regions. Rep. Zonosph. Space Res. Japan 18, 245. PARKER.E. N. (1963). Interplanetary Dynamical Processes. Interscience, New York. Solar-Geophysical Data. (1967-1969). U.S. Dept. of Commerce, Environmental Science Services Administration. Wncox, J. M. (1968). The interplanetary magnetic field. Solar origin and terrestrial effects. Space Sci. Rev 8, 258. Wncox. J. M. (1971). Sector structure of the solar magnetic field. Solar Magnetic Fields (Ed. Robert Howard), p. 744. Reidel, Holland. WILCOX, J. M. and COLBURN,D. S. (1969). Interplanetary sector structure in the rising portion of the sunspot cycle. J. geophys. Res. 74,2388. Wncox, J. M. and C~LBURN,D. S. (1970). Interplanetary sector structure near the maximum of the sunspot cycle. J. geophys. Res. 75, 6366. WILCOX, J. M. and COLBURN, D. S. (1972). Interplanetary sector structure at the solar maximum. J. geophys. Res. 77, 751. WILCOX, J. M. and Ness, N. F. (1965). Quasi-stationary corotating structure in the interplanetary medium. J. geophys. Res. IO, 5793. YOSHIDA, S. and AKASOFU,S.-I. (1966). The development of the Forbush decrease and the geomagnetic storm fields. Planet. Space Sci. 14, 979.