Adv. SpaceRes.Vol. 11,No. 1, pp. (l)83—(l)86, 1991 0273—1177/91 $0.00 + .50Printedin GreatBritain. All rightsreserved. Copyright© 1991 COSPAR

HIGH SPEED SOLAR WIND STREAMS INRELATION TO THE SECTOR BOUNDARIESOF THE HELIOSPHERIC CURRENT SHEET

Vasilis P. Tritakis

ResearchCenterfor Astronomyand Applied Mathematics,AcademyofAthens, 145 Anagnostopouloustr., AthensGR-106 73, Greece

ABSTRACT

A greet number of high—speed solar wind streamswhich cover the time period 1964—1982has been analysedend some interesting features of their behaviour have beenrevealed.High—speed streams which occur during positive polarity days of theinterplanetary magnetic field (IMF) prefer to appearmainly in the 1st and the 2ndBartels day of the solar rotation while flare—relatedstreams of the same categoryappear on the 24th and 26th day with a significant frequency. On the otherhand, high—speedstreamswhich occur during negative polarity days prefer to occurmainly between the 13th and 15th Bertels while flare—relatedstreamsof the samecategory appearon the 8th day. In addition high—speedstreams, which occur in bothpositive and negative polarity days of the IMF, show a remarkable tendency to occurvery close to (+,—) and (—,+) sector boundaries of the IMF, respectively.

INTRODUCTION

Properties and various features of the high-speedsolar wind streams (HSS) have beenextensively studied during the past decade. Several authors have reviewed theircharacteristics while interesting points of their behaviour like recurrency.longitudinal preference, correlation with geomagneticdisturbancesetc. have beenunderlined (Intrilligator, 1977; lucci et al.. 1979; Lindblad, 1981; Mavromichalakiand Rangarajan,1989). Lindblad end co—workers in a series of reports have compileda continuous and reliable catalogueof HSS which extendsfrom the beginning of 1964to November 1982 (Lindblad and Lundstedt, 1981, 1983; Lindblad et al. 1989). Inthis catalogue information like the beginning time of a HSS, its duration, thecorrespondedBertels rotation number and day, the polarity of the (IMP), etc. havebeen tabulated. The presentarticle consists of an attempt to find some interestingcharacteristics of the HSS behaviour, like their preferred occurrencein certain daysof the solar rotation as well as to appearvery close to sector boundaries of theheliospheric current sheet.

PREFERRED BARTELS DAYS FOR HIGH-SPEED STREAM OCCURRENCES

All the HSS which occur on days with positive polarity of the IMF in the time span1964—1982 (cases N—315) as well as flare—related HSS of the same category (casesN-60) have been depicted in the upper and low panel of figure 1, respectively. Fromthis figure, it is clear that HSS occur preferentially on the 1st. the 2nd end the24th day of the Bartels rotation. The peaks of these days are 2—3 times higher thenthe averagewhile they exceed the 0.01 confidence level which has been defined to be3a above the averagelevel where a the standard deviation of the total HSS cases ina 27—days typical Bertels solar rotation. In the same way we have defined a 0.05confidence level which stays 2a above the average level. In addition, it is veryinteresting that the flare—related HSS in the low panel of the figure 1 form a veryintense peak on the 24th day and a secondary peak on the 26th day of a Bartelsrotation where both of them surpass the 0.01 confidence level. The main peak on the24th day and the secondery on the 26th on the upper panel without the flare-reisted

(1)84 V. P. Tritakis

The figure 2 has been constructed in the same way as the figure 1 but HSS whichoccur on days with negative polarity of the IMF in the time span 1964—1982 (casesN-321) have been depicted in the upper panel and flare—related HSS of the samecategory (casesN-60) in the lower panel. An extremely high peek in the middle of theBartels rotation that is on the 15th day which surpass the 0.01 confidence level isevident.

A comparison between the upper and the lower panel of figure 2, leads to theconclusion that HSS free from flares preferably occur on Bertels days No. 13, 14and 15 where the caseson the 13th and 14th days stay far above the 0.05 and justbelow the 0.01 confidence level while the caseson the 15th day stay very high abovethe 0.01 confidence level. On the other hand, the flare-related HSS in the low panelof the figure 2 show dinstict peaks on the 8th and the 22nd day of the Bartelsrotation. In the opposite to the figure 1, peaks in the low panel of the figure 2 donot impact somesignificant change on the presentationof the upper panel, so we canlead to the conclusion that HSS which occur in negative polarity days of the IMPprefer to occur on the 13th, 14th and especially the 15th days of the Bertels solarrotation. The flare-related casesof the same categoryshow slightly significantpeeks on the 8th and 22nd days which do not affect essentially the shapeof thecurve on the upper panel.

HIGH-SPEED STREAMS AND SECTOR BOUNDAIUES OF THEHELIOSPHERIC CURRENT SHEET

In the figure 3, all the casesof HSS which occur in days with positive polarity ofthe IMP (in the time span 1964—1975) as well as the (—.+) sector boundaries of theheiospheric current sheet, both in the time span 1964—1975, have been depicted inthe upper and the lower panel, respectively.

The reason we have limited our data set to the span 1964—1975 is that a homogeneous.continuous and reliable data set of sector boundaries has not beencompiled after theyear 1975. it is very interesting that both curves in figure 3 vary in more or lessthe same way. The same effect is more than obvious in the figure 4 where all thecases of HSS which occur in days with negative polarity of the IMF as well as the(+,—) sector boundariesof the heliospheric current sheet have been depicted in theupper and the low panel, respectively. At first glance, the figure 3 and 4 supportthe aspect that HSS tend to occur together or very close to the sector boundariesof the heiospheric current sheet.

An initial check of the 330 casesof sector boundaries and the 310 cases of HSSwhich cover the time span under consideration (1964—1975) showed that 180 cases ofHSS occur within 2 days from sector boundarieswhile 130 casesstay further than 2days. In the following we apply a statistical test which help to decidebetween the hypothesesHo:Pa~P2and Ha:Pa~where P2. P2 the proportions of HSScasesnear to and far from sector boundaries, respectively. The significance of theproportional difference between the N2, N2 casescould be determined in accordancewith the values of the score

Z — (P~ — P2)/c~ (1)

where a., is the standard deviation of the approximately normal distribution of theproportional differences P1—P2 denoted by the relation.

a.. — -flP.q(1/N~+ 1/N2)] (2)

The probability factors P and q ere defined by the relations:

P — (N2P1+N2P2)/(N1+N2) (3)

q—1—P (4)

where N2—180 and N2—130 the samples of the casesnear to and far from sectorboundaries, respectively. On the basis of a two—tailed test at a 0.05 level ofsignificance, we would reject H0 if the score z is

— 1,96 > z > +1.96.

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(1)86 V. P. Tritakis

0.05 significancelevelat least. Finally, the main point of this processing is thatHSS which occur within 2 days time from a sector boundary passageare significantlymore than HSS which occur in a larger than 2 days time far from them.

DISCUSSION

The analysis of the HSS and the sector boundary passagesof the heliospheric currentsheet for the time spans1964—1982 and 1964—1975 respectively, has revealed twointeresting features of their behaviour.

The former of this features underlines and reinforces results which have beenmentionedby Lindblad (1981), that is HSS show a significant preference to appear incertain days of the solar rotation. HSS which occur during positive polarity days ofthe IMP prefer to appear on the first and seconddays of the Bartels rotation. Incontrast, flar8—related HSS which occur in positive IMF days appear mainly on the24th Bartels day.On the other hand HSS which occur during negative polarity days of the IMF prefer toappear on the 13th. 14th and mainly the 15th Bartels days.

The latter of the above mentioned features shows that HSS caseswhich occur duringpositive polarity days of the IMP and (—,+) sector boundariestend to distribute inthe same more or less way within a histogram with 27 bins.

This last property implies that HSS tend to appear close to sector boundarypassages.In fact, a statistical test between casesof HSS which appearwithin 2 daysfrom a sector boundary (N2—180 cases)and HSS caseswhich appearfar from sectorboundaries (N2-130 cases)pointed out that HSS prefer to appear near sectorboundariesin a 0.01 level of significance.

However, the problem of preferred solar longitudes which is linpied by the formerfeature as well as a possible correlation between HSS and sector boundaries appearmore complicated than what they look in this article. Concerning the problem ofthe preferred longitudes, parameters like slippageof active longitudes and thedifferential rotation of the sun should be taken into account. Further work should bealso done in the direction of a possible correlation betweenHSS and sectorboundaries as well as a convincing physical mechanismwhich could justify it.

REFERENCES

D. Intrilligetor,: 1977, in M. Shea et al. (eds.), Study ofTravelling Interplanetary Phenomena,D. Reidel. Publ. Co.Dordrecht, Holland, p. 195.

M. lucci, M. Parisi, M. Storini and G. Villoresi: 1979, Lab,Plasma Spazio. C.N.R. LPS 79—10.

H. Mavromichalaki and G.R. Rangarajan:1989. Solar Physics

]~.,232.

B.A. Lindblad: 1981, Solar Physics ~,187.

B.A. Lindblad and H. Lundstedt: 1981, Solar Physics ~.,197.

B.A. Lindblad and H. Lundstedt: 1983, Solar Physics ~,3fl.

B.A. Lindblad. H. Lundstedt and B. Larsson: 1988, Solar Physicsi~Q,145.