Pergamon Phys. Chem. Earth, Vol. 22, No. 7-8, pp. 685-689, 1997

© 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

0079-1946/97 $17.00 + 0.00

PII: S0079-1946(97)00196-1

Semiannual Variation of Geomagnetic Activity in the Greenland Magnetometer Chain

M. Tavares l,z, E. Fr i i s -Chris tensen 2, T. More t to 2 and S. VennerstrCm 2

IUnivers idade Federa l F luminense , Inst i tuto de, Ffsica, Av. L i to ianea , s/n, Boa Viagem, Niter6i , CEP 24210-340, Rio de Janei ro-Bras i l 2Danish Meteo ro log ica l Inst i tute, Lyngbyve j 100, DK-2100 Copenhagen , Denmark

Received 13 November 1996; accepted 13 March 1997

A b s t r a c t . The semiannual variation of geomagnetic activity is investigated for different conditions of the in- terplanetary magnetic field during one solar cycle using data from the Greenland magnetometer chain. The re- sults confirm that one important parameter is the effec- tive southward component of the interplanetary mag- netic field. But the results also indicate that there re- mains a part of the semiannual variation that may not be explained by external causes and therefore may orig- inate from internal causes.

© 1997 Published by Elsevier Science Ltd

1 I n t r o d u c t i o n

The semiannual variation of geomagnetic activity has been studied for a long time without a definite con- clusion about its origin. Many different theories have been presented but none of them seems to be able to explain all aspects of the phenomena that appear in the observations. One of the early and still vital explana- tions is provided by McIntosh (1959) who proposed the "equinoctial" hypothesis, which attributed the semian- nual variation to the change of tilt of the dipole axis, because larger activity is supposed to take place when the dipole is perpendicular to the solar wind flow. The maximum activity therefore arises twice a year at the equinoxes.

Boller and Stolov (1970) proposed that the semi- annual variation of geomagnetic activity is associated with the variation of Kelvin-Helmholtz instability at the boundary of the magnetosphere. They suggested that Kelvin-Helmholtz instability generated at the magne- topause may modulate the geomagnetic disturbance to have semiannual peaks. Although they did not iden- tify the exact mechanism that could be involved, they suggested that the effect could be through the seasonal variation of substorm occurrence. They did, however,

Correspondence to: M. Tavares

685

indicate that other disturbance phenomena apart from the Kelvin-Helmholtz mechanism might contribute as well.

Russell and McPherron (1973) proposed that the geo- magnetic activity is caused by a semiannual variation in the effective southward component of the interplanetary field. Here the effective southward component means the component strictly antiparallel to the geomagnetic axis. Therefore an additional contribution to the south- ward field arises because of the seasonal and diurnal variation of the geomagnetic axis relative to the solar equatorial coordinate system in which the interplane- tary magnetic field (IMF) is ordered. The interaction between the solar wind and the magnetosphere is con- trolled by the magnetospheric system.

Orlando et al. (1993) studied the geomagnetic activ- ity using solar wind and IMF data, covering 23 years. They referred to the Russell and McPherron (1973) model as primarily controlled by the southward compo- nent B, of the IMF in the GSM system and that the geomagnetic activity itself is strongly dependent on the IMF polarity. Theirstatistical analysis, however, seem to indicate that the recurrent high-speed streams in so- lar wind contribute to the semiannual geomagnetic vari- ation during the late descending phase of the sunspot cycle. They thereforeattributed a considerable effect to the solar wind velocityin contrast to the hypothesis by Russell and McPherron, who did not point at the so- lar wind speed as a principal agent of the geomagnetic variations.

In this paper we examine the semiannual variation of geomagnetic activity in the Greenland magnetometer chain for several different conditions of the interplan- etary magnetic field. Data from the Greenland mag- netometer chain are available in digital form for more than one solar cycle and provide a homogeneous set of data from the same magnetic meridian. This facilitates a direct comparison of the changes in the variation pat- terns according to geomagnetic latitude, without regard

686 M. Tavarcs et al.

, . - • t • Is~-Rad.- , . L~ ~. • Hultdl~am Ri~wlet~ ~

• . ~ ' - ~ . . . . 7 - - - . ~ , s r r : , J : ~ .. • { , ~ x ~ ~ " * ~ • ~ • T.KJ "

• 2 .......... "

• . . . : . i i : ~

Fig. 1. Map of Greenland Magnetometer stations

to possible longitudinal effect. This data set is used to detect to which degree the statistical results of the various planetary geomagnetic activity indices may be reproduced by individual station data and whether the statistical behaviour of these data may elucidate some of the remaining questions regarding the causes of the semiannual variation.

The paper is organised with a section about the deriva- tion and treatment of the data, ground-based as well as IMF and solar wind plasma data. This is followed by a section about the results and their interpretation.

2 D a t a s e t

Figure 1 shows a map of Greenland with the distribution of the stations used in this analysis. In order to have a representative and nearly homogeneous data set for about one solar cycle from 1983 to 1992, we have selected a primary data set from the following stations on the west coast of Greenland: THL (Thule) and UPN (Uper- navik) in the polar cap, UMQ (Umanaq), GDH (God- havn), ATU (Attu), and STF (SOndre Stromford), in the cusp/cleft region,and GHB (Godthaab), FHB (Fred- erikshaab), and NAQ (Narsarsuaq) in the auroral zone. The three stations THL, GDH, and NAQ, are magnetic observatories with absolute control, and the other sta- tions are variation stations without any regular check of the base line. For these stations only limited, and semi-automatic quality control has been performed. All hours indicating possible errors were excluded from the analysis.

Since the objective of this paper is to describe the

seasonal variation of magnetic activity, an appropriate index has to be used. The commonly used global mag- netic activity indices, like I~/p and Ap have been designed to represent a kind of range index during a three-hour interval. In order to facilitate direct comparison with the global indices, while at the same time increasing the time resolution, we have used an hourly range index for each individual station. A range index has an additional advantage in long-term analysis of geomagnetic data be- cause any slowly varying uncertainly in the base line of the magnetic variation will not affect the indices. For this reason, also variation stations without regular base line control may be used directly in such an analysis. The first step in our analysis is to derive tables of hourly ranges for the three components of the magnetic field variation. For further studies we selected the variation in the northward component, because this component normally shows the largest amplitude. This is due to the fact that the ionospheric currents causing the major part of the activity are typically east-west directed along the invariant latitude circles. Finally we binned the data according to the solar wind plasma and magnetic field data. We used the IMF data to divide the hourly range indices into four basic IMF-quadrants, corresponding to the signs of the actual By and Bzcomponents in the Geocentric Solar Magnetospheric system (GSM). Each of these four cases was then further divided into two parts, one with solar wind velocities less than 450 km/s and one with velocities greater than 450 km/s. In or- der to compare with previous studies of the semiannual variation that have been concerned with the different polarities of the IMF, we also used this classification by dividing the data into positive (toward) and nega- tive (away) B, components of the IMF. Since there is a varying time delay between changes in the IMF and the associated effects on the geomagnetic response Bar- gatze (1986), a data point is included in the statistics only when the same IMF condition continues for three consecutive hours. Although, the "memory" of the mag- netosphere maybe longer than three hours during very special conditions (in particular in terms of substorm response) it is believed that this stability criterion does ensure a reasonable homogeneity of the data for each class of IMF conditions.

For each combination of IMF parameters and solar wind velocity classification (a total of 12 different con- ditions) the average hourly range for each month and each UT hour was calculated for each station. In order to check for the effect of a possibly non-uniform distri- bution of solar wind conditions, the corresponding IMF components (Bx, B u, Bz) and solar wind plasma data (V,n) were calculated as well. In this presentation we have concentrated on the seasonal variation and in par- ticular discuss the effect at auroral zone stations. In a further study we will deal with the results for various local time sectors and focus on the high-latitude effects as well.

Semiannual Variation of Geomagnetic Activity 687

Hours: oil; Bx>O V>450 I I I I I I I I I I I I

THL

UPN ~ ~ ~

UMQ 1 . 1 ~

GDH

ATU ~ ~ ~

STF

GHB ~ ~ -

FHB

NAQ ~

I I I I t I I I [ I I [ don Apr dul Oct

F i g . 2 . A v e r a g e hour ly range v a l u e s 1 9 8 1 - 1 9 9 0 for the Greenland m a g n e t o m e t e r s t a t ions for Bx > 0 and V > 450km/s. ( the scale is 100nT between tickmarks.)

THL

UPN

UMQ

GDH

ATU

STF

GHB

Hours: all; Bx<O V>450 ] I I I I I I I I I I I

J ~

j

FHB ~ - - - - ~ ~

NAQ ~

I I I B 1 I I I [ I I I Jan Apt dul Oct

F i g . 3 . A v e r a g e hourly range values 1981-1990 for the Greenland magne tomete r s ta t ions for B~ < 0 and V > 450km/s.(same scale as in Fig. 2}.

3 R e s u l t s

The results are given as time series plots showing the yearly variation for each station for the selected IMF and plasma velocity cases. One of the most pronounced but not unexpected results are, that all the stations show considerably increased geomagnetic activity dur- ing high solar wind conditions, regardless of IMF con- ditions. This shows that even for northward IMF, there is a continued activity present which is affected by the solar wind speed. Another prominent feature is that the stations in the polar cap show a very distinct max- imum in activity during Summer. This is consistent with the fact that the solar EUV produced conductivity is of major importance for the high latitude ionospheric currents. In this study we have concentrated on those cases showing the most prominent semiannual variation. In another paper under preparation other features of the annual and daily variations of the activity in relation to IMF conditions will be discussed.

Figures 2 and 3 illustrate the seasonal variation of all the stations for the two conditions corresponding to away (B, < 0) and toward (B, > 0) polarity of the interplanetary magnetic field. The division of the data into these two classes confirm that the data in the 'away 'sector have a northern hemisphere spring max-

imum while the ' toward 'data reveal a northern hemi- sphere fall maximum. This is exactly the result that was predicted by Russell and McPherron (1973) and which has been confirm by analysis of the planetary magnetic index a a Berthelier (1976).

Figure 2 shows a distinct maximum around March- April for B~ > 0. This applies to all stations, except for the most poleward stations (THL and UPN) which are located in the polar cap. Figure 3 shows a similar maxi- mum activity for B~ < 0 during September-October. A comparison with the average IMF components for the selected intervals (not shown here) confirms that the southward IMF component in the GSM system is con- siderably larger during March-April (for B~ > 0) and September-October (for B~ < 0) than during the other months as predicted by Russell and MePherron (1973).

However, several studies in the past have indicated that the Russell- McPherron mechanism does not ac- count for all the semi-annual variation in geomagnetic activity Berthelier (1976); Orlando et al. (1993). A similar conclusion may be reached by examining Figures 4 and 5. In these figures, we have divided the data into the two IMF conditions corresponding to By < 0 and B~ > 0 respectively, during B~ < 0. If the spring peak for Bx > 0 (Fig. 2) and the fall peak for B, < 0 (Fig. 3) are caused by the Russell-McPherron effect, there

688 M. Tavares et al.

Hours : o11; B y > O , B z < O V > 4 5 0 I I t i I I I I I I I I

THL

UPt~

UM( ~ . ~ _

GOF

ATU ~

STF

GHB

FHB

NAQ ~

I I t r I J J r I I I I Jon Apr Jul Oct

Fig. 4. Average hourly range values 1981-1990 for the Greenland magnetometer stations for By > 0, Bz < 0 and V > 450km/s. (same scale as in Figure 2)

Hours : QII; B y < O , B z < O V > 4 5 0 I I I I I I I I I I I I

THL

UPN

UMQ ~

GDH

ATU

STF ~

OH8

FHB

NAC

I I I I [ I I I I I I I Jon Apt Jul Oct

F i g . 5 . A v e r a g e hourly range values 1981-1990 for the Greenland m a g n e t o m e t e r s t a t i o n s for By < 0, Bz < 0 and V > 450km/s. (same scale as in Figure 2)

must be only one peak if we restrict to either By > 0 or By < 0. However, Figures 4 and 5 show two peaks in both spring and fall especially in the auroral zone stations (GHB, FHB, and NAQ) .

The next question is whether the remaining semi- annual variation corresponds to a similar semiannual variation in the effective southward component of the IMF. Figures 6 and 7 show average IMF and solar wind plasma data corresponding to the same conditions as applied in Figures 4 and 5. The parameters include the three IMF components and the southward component B~ defined as B~ for Bz < 0, and 0 for Bz > 0. Fur- thermore, the average values of the plasma density n, and solar wind speed V have been plotted as well as the parameter B s V 2 which was used by Orlando et al. (1993) as an indication of an effective parameter for geo- magnetic disturbances. None of these parameters shows systematic semiannual variation.

This indicates that the semiannual variation of geo- magnetic activity cannot completely be explained by ex- ternal causes only. Rather the effect may be related to internal causes such as the symmetric ionospheric con- ductivity conditions during the equinoxes as compared to the summer and winter seasons, when one of the hemispheric polar caps is characterised by a low con- ductivity. In a model by Kan and Sun (1985), the

westward traveling surge (WTS) and the Pi2 pulsations show a considerable effect of a decrease in ionospheric conductivity in one of the two hemispheres connected by the closed magnetic field lines involved in these pro- cesses. More work is needed in order to assess the effect of the conductivity distribution on the semiannual vari- ation of geomagnetic activity. The extensive set of data from a dense chain of magnetometers will offer an oppor- tunity to examine the latitudinal and local time depen- dencies that may provide a clue to solving this problem. The scope of the present paper, however, is limited to pointing out the effect and possible future directions of research.

4 Conclus ions

The solar wind velocity plays a very important role for the geomagnetic activity in the auroral zone. Both high solar wind speed and a southward component of the IMF enhances the semi-annual variation of geomagnetic ac- tivity in the auroral zone. Dividing the total data set into two parts corresponding to the B~ polarity of the IMF, we could separate the semi-annual variation into two different annual variations, one with an April max- imum and another with a maximum in October. These

Semiannual Variation of Geomagnet ic Activity 689

8z

,il By

~ ~ " J u l OCt

8 z

Bs

i

V

7 ~

zoo

N

15

IO

~ J~¢ J=l OCl

SsV

0 set Jul Oat

8sV2

o

Fig. 6. Corresponding average of the solar wind plasma data and the IMF components for B~ > 0, Bz < O, and V > 450km/s. In this figure the values of the magnetic field com- ponents Bz ,B~ ,Bz , Bs are given in nT, the velocity of the solar wind is in km/s , the value of the density is in particles x cm -3, the units of BsV in lOnT x km/s and the units of BsV 2 is in 104nT x km/s 2.

di f fe rent m a x i m a c o r r e s p o n d to s e a s o n s w h e n t h e aver-

age s o u t h w a r d c o m p o n e n t is m a x i m i z i n g as is p r o p o s e d by R u s s e l l a n d M e P h e r r o n (1973). However , a s i m i l a r

d iv i s ion o f t h e d a t a for B , < 0 in to B~ < 0 a n d By > 0

c a t e g o r i e s s h o w s t h a t t h e s e m i a n n u a l v a r i a t i o n p reva i l s even t h o u g h t h i s effect is n o t ref lected in t h e co r r e spond -

i ng a v e r a g e v a l u e s o f t h e s o u t h w a r d I M F c o m p o n e n t for

t h e s e ca t egor i e s . T h i s is in c o n t r a s t to t h e h y p o t h e s i s o f

R u s s e l l a n d M c P h e r r o n ( 1 9 7 3 ) a n d the re fo re we need a

d i f fe rent m e c h a n i s m w h i c h can a c c o u n t for p a r t o f t h e

s e m i - a n n u a l v a r i a t i o n in g e o m a g n e t i c ac t iv i ty . O n e pos-

s ib i l i ty is a n i n t e r n a l e n h a n c e m e n t o f g e o m a g n e t i c ac t iv - i ty a t t h e e q u i n o x e s w h i c h cou ld invo lve t h e i onosphe r i c

c o n d u c t i v i t y in b o t h h e m i s p h e r e s t h a t are c o n n e c t e d by

t h e g e o m a g n e t i c field l ines .

Acknowledgements. We are grateful to CAPES (Coordena~o de Pessoal de Ensino Superior) which supports one of the authors, M. Tavares. We thank Dr. M. Yamauchi for valuable discussions.

8 x

- 6 ~ , , , , , , , , , , , ,

j ~ ~ j= l Oct

Sy

,il ~ n ~ Ju, oct

8s

J o ~ ~ t J=l o c t

V

7110

J a n ~ " J= l Oc t

N

I 0

dCm ~ Jul Ocl

BsV

2 ~

7 J o . ~ J ~ 0¢t

BsV2

2 ~

Fig. 7. Corresponding average of the solar wind plasma data and the IMF components for By < O, Bz < 0, and V > 450km/s. (The parameters are described in the same way as in Figure 6.)

R e f e r e n c e s

Bargatze L.F., Baker D.N. and McPherron R.L., Magnetospheric response to solar wind variations, in Solar Wind- Magneto- sphere Coupling. Edited by Y.Kamide and J.A.Slavin, 93-100, Terra Scientific Publ. Co., Tokyo, 1986.

Berthelier A., Influence of the polarity of the interplanetary mag- netic field on the annual and the diurnal variations of magnetic activity, J. Qeophys. Res., 81, 25, 4546, 1976.

Boiler B.R. and Stolov H., Kelvin Helmholtz instability and the semiannual variation of geomagnetic activity J. Geophys. Res., 75, 6073, 1970.

Kan J.R. and Sun W., Simulation of the westward traveling surge and Pi2 pulsations during substorms, J. Geophys. Res., 90, 10911, 1985.

McIntosh D.H., On the annual variation of magnetic disturbance, Phil. Trans. Roy. Soc. London, A1 ,251 , 525, 1959.

Orlando M., Moreno G., Parisi M. and Storini M.,Semiannual vari- ation of the geomagnetic activity and solar wind parameters, Geophys. Res. Lett., 20, 2271, 1993.

Russell C.T.and McPherron R.L., Semiannual variation of geo- magnetic activity, J. Geophys. Res., 78, 92, 1973.

Russell C.T., The Universal Time variation of geomagnetic activ- ity, Geophys. Res. Lett., 16,555, 1989,

Spreiter J.R., Summers A.L., and Alksne A.Y., Theoretical proton velocity distributions in the flow around the magnetosphere, Planet. Space Sci., 14, 1207, 1966.