On induction effects at EISCAT and IMAGE magnetometer stations

Geophys. J . in(. (1995) 121,893-906

On induction effects at EISCAT and IMAGE magnetometer stations

Ari Viljanen,’ Kirsti Kauristie’ and Kari Pajunpaa’ I Finnish Meteorological insritute, Depuriment of Geophysics, PO Box 503, FIN-00101 Helsinki, Finland

Finnish Meteorological Institute, Nurmijarui Geophysical Observatory. FIN-05 100 Roykka, Finlnnd

Accepted 1994 December 16. Received 1994 November 13; in original form 1993 December 21

S U M M A R Y When ground-based magnetometer data are used for ionospheric/magnetospheric studies, it is important to know the external (ionospheric/magnetospheric) and internal (telluric) contributions to the observed total variation. To study them, we calculate geomagnetic induction vectors, and apply the Siebert-Kertz separation method using data from the EISCAT and IMAGE magnetometer stations in Fennoscandia. Induction vectors in the period range 20-2560 s show the expected, and dominating, ocean effect near the Arctic Ocean. At Masi, a small-scale inland anomaly is detected in addition to the ocean effect. At Hankasalmi a man-made anomaly due to the nearby STARE radar is present. The source effect on induction vectors at high latitudes is briefly investigated. By separating the magnetic variations of several bay events at IMAGE stations into external and internal parts, we study the importance of the large-scale induction in the recordings. Depending very much on the particular event, the internal horizontal part is 10-30 per cent of the total field in the auroral electrojet region, and its relative contribution increases in more distant regions. The internal contribution of the vertical field at inland stations is 10-40 per cent (having an opposite sign to the total field), or 10-30 per cent (having the same sign as the total field), depending on the location with respect to the ionospheric currents and to the ocean. Results are also discussed using a thin-sheet model.

Key words: conductivity anomalies, external and internal fields, induction vectors, ionospheric currents, ocean effect.

1 INTRODUCTION

Induction effects at ground magnetometer stations are studied for two reasons.

(1) ‘Solid earth research’: determine the Earth’s conductivity, and especially anomalies, i.e. deviations from a layered structure.

(2) ‘Ionospheric/magnetospheric physics’: find out the contribution of induced currents in the Earth to the total variation. There are two subitems: (i) contribution of the normal induction, i.e. induction in a layered earth; (ii) anomalous induction effects due to lateral inhomogeneities of the Earth’s conductivity.

This paper is mainly addressed to ionospheric(/ magnetospheric) physicists using ground-based magnetometer data from Fennoscandia. We also try to fill the gap that seems to exist between the ‘traditional’ induction studies and ionospheric studies, i.e. we want to remind both

groups about the coupling between the Earth and the ionosphere. Owing to our aim to support ionospheric studies, we are more interested in the effect of the Earth on magnetometer recordings than in the details of the Earth’s conductivity structure. The latter cannot be investigated in detail without a denser magnetometer network or additional recordings of the geoelectric field.

To examine induction effects is a basic study that is necessary for any magnetometer station. It is important to know whether the data are distorted by anomalies, making the station non-representative of the larger area. Such investigations have been performed at the permanent Finnish geomagnetic observatories, Nurmijarvi (Jankowski, Pirjola & Ernst 1986) and Sodankyla (Kauristie et a/ . 1990).

Owing to spatially inhomogeneous ionospheric currents at high latitudes, solid earth research with induction methods is difficult there. The contribution of induced currents to the total variation may be hidden by an inhomogeneous ionospheric primary field. A careful selection of events for studying the Earth’s conductivity is necessary to avoid

893

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894 A . Viljanen, K. Kauristie and K. Pajunpaa

Figure 1. EISCAT (PEL, MUO, KAU, KIL, KEV, ALT, SOR) and IMAGE (NUR, HAN, OUJ, PEL, MUO, KIL, MAS, KEV, SOR) magnetometer stations. The Sodankyla Observatory (SOD) is also shown.

unwanted source effects (Beamish 1979; Quon et al. 1979; Gough & de Beer 1980; Wait 1980; Mareschal 1981, 1986; Pajunpaa 1984; Osipova, Hjelt & Vanyan 1989; Kauristie 1991; Pirjola 1992).

We study the induction effects using the data of the EISCAT and IMAGE magnetometers in Fennoscandia at and near auroral regions (Fig. 1) (Hakkinen, Sucksdorff & Pirjola 1989; IMAGE 1992). We use geomagnetic induction vectors to study the anomalous induction. Results are connected to and supplement the extensive magnetovaria- tional and magnetotelluric studies in Fennoscandia (Jones 1980, 1981; Jones, Olfasdottir & Tiikkainen 1983; Pajunpaa, Heikka & Korja 1983; Hjelt et af. 1986; Korja, Zhang & Pajunpaa 1986; Korja et af. 1989; Korja & Koivukoski 1990; Pajunpaa 1984, 1987; Hjelt 1991). Further interpretation is left to a future work, e.g. induction vectors give a starting point to select sites for magnetotelluric studies with denser arrays.

The large-scale normal induction in a layered structure must be studied in other ways, e.g. by direct modelling or by the field separation used in this paper. A magnetic variation observed on the ground can be divided into an external part due to ionospheric/magnetospheric currents, and an internal part due to induced Earth currents. The separation method was originally presented by Siebert & Kertz (1957) (see also Weaver 1964; Frazer 1974). The anomalous induction affects the results, but local small-scale structures are hidden due to the smoothing properties of the separation.

Separation concerning the IMS (International Magneto- spheric Study) Scandinavian magnetometer array (SMA) was performed by Kiippers et al. (1979) and Mersmann et al. (1979). They studied only 2-D events. Porath, Oldenburg &

Gough (1970) performed a full 3-D separation using a 2-D network in the United States. In these studies, the conventional separation procedure of Siebert & Kertz was applied. Richmond & Baumjohann (1984) applied optimal linear estimation to separate a 3-D field observed by SMA.

An extensive review of SMA studies (Untiedt & Baumjohann 1993) also discusses the induction effects. Conclusions are mainly based on the separation works mentioned above. So our study fulfils these results, and due to a remarkably larger number of events than in previous studies, it yields new information.

2 GEOMAGNETIC INDUCTION VECTORS

2.1 General

The induction vectors are a way to present the complex transfer functions A ( o ) and B ( w ) defined by the equation

Z(o) = A ( w ) X ( o ) + B ( w ) Y ( o ) . (1) The usual convention for the north ( X ) , east ( Y ) and vertical ( Z ) magnetic field components is used. We assume a plane-wave model, so Z is due only to currents induced in subsurface anomalies. X and Y are then only smoothly changing over the research area except above anomalous induced currents, where the transfer function estimates may be downward biased. In practice, the observed total variation is used. Eq. (1) can also be written in the time domain as a convolution integral. The inverse Fourier transforms of transfer functions are called impulse responses.

The reversed real induction vector S,, and the unreversed


IMAGE and induction effects 895

imaginary induction vector S,,, are defined by the formulas

S,, = -!a [A(w)]e, - &[B(w)]e,,, (2)

S,, = A [A(w)]e, + [B(w)]e,. ( 3 )

The real vector points towards induced currents in the Earth (Schmucker 1970, p. 23: Berdichevsky & Zhdanov 1984, Section 28). The behaviour of the imaginary vector is more complicated (e.g. Agarwal & Dosso 1990). Generally it is believed to be connected to near-surface anomalies. Results concerning imaginary vectors will not be discussed in this paper, because they d o not seem to be relevant for ionospheric studies.

2.2 Selection of plane-wave events

We assume that the primary magnetic field is a laterally homogeneous plane wave. Then the normal variation connected with a purely layered conductivity structure has no vertical component. Consequently, a non-zero Z is purely due to induced currents in lateral conductivity inhomogeneities.

Selection of plane-wave events is not easy at high latitudes, as was again realized in this study. To find plane-wave-like events (Table l), the following criteria were applied. As a necessary although not a sufficient condition, a high correlation of horizontal components at different stations is required. For X , correlation coefficients are larger than 0.85 between stations in the EISCAT magnetometer

region (PEL and more northern stations). For Y , a poorer correlation is found in some cases, but then Y is much smaller than X . Between the EISCAT region and southern stations (NUR, H A N , OUJ) the correlation is at least 0.7 for all events. Correlation of Z varies greatly from event to event.

Another condition is based on a R value defined by

std ( B - Brc.) r = 100 (per cent),

max (lBrcfl) (4)

where B is a component (as a time series) of the variation field of the event under study at a station, B,, is the variation field at a reference station, and std stands for the standard deviation (Kauristie et al. 1990). For an exactly uniform field R is zero. The reference station M U 0 appeared to be a good choice with small anomalous induction effects. For the selected events R is usually less than 10 per cent for X. For Y , it is sometimes more than 20 per cent, but then Y is much smaller than A'. For 2, no conditions concerning R are required. Typical values are percentages of some lOs, sometimes more than 100 per cent. It must be noted that R depends on the selection of the reference station, so it can be considered only as a semi-quantitative criterion. (However, for bay events of Table 2, R is typically larger than 30 per cent).

As a rule, plane-wave events with a large horizontal field (220 nT) are not typical in auroral regions. An example of an acceptable event is shown in Fig. 2. Note the large

Table 1. Events used for induction vector calculations for the EISCAT (1990) and IMAGE (1992-93) magnetometer stations. The sampling in!erval was 20s for the EISCAT events (18 hr) and 10s for the IMAGE events (17 hr).

EISCAT IMAGE m UT w UT

9.4. 1990 5.00 - 6.00 5.11. 1992 7.00 - 10.00

18.4. 1990 10.20 - 12.20 7.12. 1992 6.00 - 10.00

6.6. 1990 9.00 - 12.00 15.12. 1992 7.00 - 8.00

10.6. 1990 14.20 - 15.20 6.2. 1993 7.00 - 12.00

11.6. 1990 14.20 - 17.20 18.2. 1993 10.00 - 12.00

30.6. 1990 8.40 - 11.40 27.2. 1993 9.30 - 11.30

11.7. 1990 11.20 - 13.20

12.7. 1990 13.00 - 16.00

Table 2. Bay events used for the separation of the field at IMAGE magnetometer stations. Sign of X is that of the northern stations (PEL, MUO, MAS, SOR). In some cases, marked with an asterisk (*), the sign changes in the south, but the amplitude is then much smaller than in the north. One minute mean values were used in the calculations.

m UT time sign of X dw UT time sign of X

22.11. 1992 13.00 - 17.00 + (*) 14.12. 1992 16.30 - 18.00 + (*) 22.11. 1992 20.00 - 22.00 - 19.12. 1992 22.30 - 23.30 -

30.11. 1992 19.15 - 20.15 - 25.1. 1993 15.30 - 17.00 + (*) 6.12. 1992 15.00 - 17.00 + (*) 30.1. 1993 23.00 - 24.00 -

12.12. 1992 23.00 - 13.12. 01.00 - (*) 10.2. 1993 13.30 - 14.30 +


6.2.1993

I 1 I I I

-60 I I I I 1

08:OO 09:OO 1o:oo 11:oo UT

Figure 2. Magnetograms of a plane-wave event used in this study.


IMAGE und induction effects 897

amplitude of Z at SOR and MAS. Amplitudes o f A' and 1' increase smoothly towards north. Anomalous behaviour can be observed in .Y at KIL and in Y at MAS where these fields are clearly increased. Because of the difficulty of finding plane-wave events. we also studied an electrojet event (Section 2.4) to get some idea of the importance of a clear source effect.

2.3 Results

We calculated induction vectors using the algorithm by Wieladek & Ernst (1977) (see the Appendix). The impulse responses are first determined bv the method of least-squares fitting in the time domain. after which the transfer functions are obtained by a Fourier transform. The procedure also gives error estimations. For acceptable events, the impulse responses must vanish in a few minutes as a function of time. Previous results around the Sodankyla Observatory are also included (Kauristie ef ul. 1YYO).

As a rule, the longer the vector the smaller is the estimated error. Typically it is less than 10 per cent for vectors longer than 0.3. Then the accuracy of the direction angle is normally better than *S degrees. The errors for shorter vectors increase rapidly, but on the other hand, short vectors d o not indicate the presence of significant anomalies.

Examples of reversed real vectors are shown in Fig. 3. For clarity. only the northern part of the region is shown. Results of NUR and OUJ agree with earlier studies. and are not shown here. H A N will be discussed below. There are some differences in the vectors computed from the EISCAT and IMAGE data. The qualitative features are similar for

300

200

E 100

9 3 . 0

0

-100

-200

1 reversed real induction vectors, T = 128 s I

SOR unit length

KEV ALT *

, L u M A S 0,

KIL

M U 0 s'

4:; AAP - vuo ' PEL

-200 -100 0 100 200 300

east I km

Figure 3. Reversed real induction vectors from the northern EISCAT and IMAGE magnetometer stations at periods of 128s (a). 427 s ( h ) and 128Y s (c). Thin arrows indicate induction vectors that are calculated using EISCAT events, thick ones those using IMAGE events (Table 1). Vectors from the Sodankyla region are from the study by Kauristie el al. (1990).

300

200

100 E

5 Y . 0

O O

-100

-200

(c) reversed real induction vectors, T = 1280 s I ' I I I

I reversed real induction vectors, T = 427 s 1

SOR unit length

YMAS >KIL ~ K A U

-200 -100 0 100 200 300

east I km

200

unit length

5 0

0 M U 0 3

-200 1 I t

-200 -100 0 100 200 300

east I km

Figure 3. (Conrinrtrd.)

both data sets. The source effect is not believed to produce the differences, because at T = 1280 s the results are nearly equal, and the source effect is most significant at long periods (e.g. Osipova ef ul. 19x9).

For comparison, the induction vectors were also computed for the EISCAT events (Table 1 ) with the frequency-domain method (Jones 19x1 ). There are differences between the results obtained with the methods of Wieladek & Ernst (1977) and of Jones. For the EISCAT events, the directions of real vectors may typically differ by 10"-20" and the magnitudes by 10-20 per cent. The overall


898

features of the results are the same, i.e. in particular the directions vary in the same way as functions of the period. For a detailed comparison between the traditional method and the method applied here, and its advantages, see Ernst (1981).

Jones (1981) calculated induction vectors for SMA using a single 3 hr event. For comparison, we studied the same event using Jones' station SOY (=SOR). At T = I(K)Os, Jones' real vector at S O R points approximately to the NW, and ours to the WNW. Real and imaginary vectors for 450 s are quite similar. but for shorter periods there are again differences if vectors are short. The event selected by Jones fulfils our plane-wave criteria well. Because only a minor part of SMA data is digitized, and mainly of substorm events, additional induction vector calculations using SMA were not performed.

The general features of the EISCAT/IMAGE region are listed below.

A . Viljanen, K. Kauristie and K. Pajunpaa

( 1 ) The most significant conductivity anomaly affecting the northern stations is the Arctic Ocean as expected (Kuppers ef al. 1979: Jones 1981). The explanation is the edge effect of the deep ocean (Section 3). The possibility of using Z in ionospheric/magnetospheric studies must be examined carefully and separately for each event.

(2) The most striking inland anomaly affecting Z strongly (and slightly also Y ) is observed at MAS.

( 3 ) Man-made disturbances are present a t HAN. (4) At other stations the anomalous features are weaker

and can be ignored in ionospheric/magnetospheric studies.

Conclusions concerning the real induction vectors at single stations are listed below. Conclusions on the Earth's structure are inevitably quite qualitative due to long distances between stations.

ALT. A clear ocean effect, no local conductivity anomalies.

WAN. The induction vectors computed from our data set point about to NE-NNE at all periods. Lengths increase with decreasing period from 0.1 (hundreds of seconds) to 0.6 (about 20s). According to Pajunpaa (1984, 1987) the vectors in nearby regions point to W-NW or about S with lengths less than 0.5 at the period T = 300 s. We could also use some data of periods during which the nearby radar STARE (about 100m N E ) was not in operation. Then the induction vectors usually point about S-SE being shorter than 0.5, which is in accordance with Pajunpaa's results.

KAU. Quite a clear ocean effect. Some evidence of a local anomaly or a conductivity contrast near the station.

KEV. Probably a weak ocean effect. KIL . In addition to the ocean effect, anomalous currents

probably under or near the station enhancing A'. M A S . Distortions in Z can be almost as large as at SOR

(Fig. 2). Induction vectors point to WSW at all periods, and have lengths increasing from 0.3 ( T =20s ) to 0.8 ( T = 32Os) , and decreasing to 0.6 at T = 1280s. The results of Jones (1981) indicate a local anomaly at the SMA station Mieron east from MAS (geographic latitudes and lon- gitudes: 69.12, 33.27 at Mieron and 69.46. 23.70 at MAS). However, at A L T and KAU the same anomaly is not seen, so the MAS anomalv is of a very small scale. It seems that the anomalous effects are not too serious (thinking of studies of ionospheric currents) if the primary magnetic field

is itself spatially strongly inhomogeneous. On the other hand. especially during pulsations, Z is too much perturbed to be used at all.

MUO. Only minor anomalous effects. NUR. Results agree with those by Jankowski, Pirjola &

Ernst (1986): a large-scale induction effect causes the induction vectors to point SW.

OUJ. The highly conducting Oulu anomaly can be seen (cf Pajunpaa ef al. 1983; Pajunpaa 1984, 1987; Korja et al. 1986).

PEL. At periods longer than 200s, the vectors point nearly in the same direction as those of the SOD region (although they are shorter). However, this is evidently not due to the same anomaly. For shorter periods, the vectors become shorter, and have a clear tendency to rotate clockwise.

SOD and nearby stations AAP, PET, S W , VUO. Rather long vectors pointing approximately in the same direction (SW-WSW) of all periods indicate that quite a significant large-scale induction takes place SW of SOD. This may be due to the highly conducting Karasjok-Kittila Greenstone Belt on which the Sodankyla area is located (Korja et a/. 1989).

SOR. A very clear ocean effect due t o the deep ocean west of SOR. When Z is used in ionospheric- magnetospheric studies. induction effects cannot be neglected. Maxima of IZI are often assumed to indicate the boundaries of electrojets. It is a rule that a maximum occurs at SOR due t o induced currents on the ocean side (Section 3.4).

2.4 Remarks on a non-plane-wave event

To study the source distortion on real induction vectors, we computed them also for the 10 bay events in Table 2. Owing to the dominating induction effect at MAS and SOR, the directions of the real induction vectors are nearly the same as for plane waves. The lengths are not equal, especially at SOR they are clearly smaller for bay events.

To study further the effect of the anomalies on SOR and MAS, we selected a severe magnetic storm on 1992 September 10, 0-4UT (maximum X of about 1000nT at PEL), The lengths of the vectors are clearly shorter at both stations compared with the plane-wave results. Directions at both stations are shifted at most by some 10s of degrees. Therefore the location of anomalies is still detectable, but the information obtained by the lengths of the induction vectors may be misleading.

At NUR and OUJ the source effect (of the bay events) is almost absent. At NUR this is due t o the large distance from the electrojet region. At OUJ, the highly conducting Oulu anomaly still seems to dominate.

At HAN, the direction of the bay induction vectors is about 70" clockwise from that of plane waves, when the period is longer than 100 s. At the longest periods the length of the vector begins to increase ( u p to 0.4 at 1280s). This is evidently due to the source effect; induction in nearby anomalies is not dominating (cf plane-wave results at long periods). Induction effects in the nearby regions seem to be too small to overcome the source contribution. (STARE radar was not in operation.)

At other stations the differences between plane-wave and


I M A G E and induction effects 899

bay events are quite random. Using many bay events does not evidently smooth the source effect if the observation point is in the electrojet region, and local conductivity anomalies are weak. Other methods must be used if non-plane-wave events are used to calculate induction vectors to localize conductivity anomalies. Some averaging methods are proposed by Viljanen, Pirjola & Hakkinen (1993).

Induction vectors could be useful in studies of electrojets, because ionospheric currents can be considered similar anomalies to lateral variations of the Earth’s conductivity. However, single events should then be considered because the total effect of many events is difficult to interpret.

3 SEPARATION OF THE FIELD INTO EXTERNAL A N D INTERNAL PARTS

3.1 General

We have data only from one meridian. So we must look for events in which the magnetic field has only two components (in a rotated frame). This is the case i f

( 1 ) the ionospheric current system is 2-D, i.e. the current

( 5 )

density j(r, t ) is of the form

j(r, t ) =.f(x, 2 , t)e,:

(2 ) the Earth has a layered conductivity structure.

(In fact, an x-dependence is also allowed if j is as above.) Then the induced currents are also of the form ( 5 ) . The anomalous induction effect produces Earth currents not satisfying (5). So we assume implicitly that the anomalous effect does not exceed the normal one during the selected events.

The explicit separation formulae are

where K is a (Hilbert transform) operator defined by

Here P denotes the Cauchy principal value (Weaver 1964). The separation formulae are valid both in the frequency

and time domains. Because the temporal behaviour is usually of interest in ionospheric studies, we will proceed in the time domain.

3.2 Selection of events

Events that satisfactorily fulfil given conditions are listed in Table 2. Practical selection criteria are given below (allowing some flexibility).

( 1 ) The direction of the horizontal variation field does not vary much (&loo) from station to station in the northern region (OUJ, PEL, MUO, MAS, SOR). This was checked by plotting the equivalent current vectors. At NUR and HAN the directions may differ greatly from those in the north. However, the amplitudes of the field are usually

much smaller in the south, so the deviation from 2-D is not serious.

(2) The approximate centre of the current system determined by the zero of Z varies by less than *lo latitudinally (about *lo0 km) from its mean value of the event. This is considered as evidence of a fairly stable ionospheric current system.

Magnetograms of the event of 1992 December 6 are shown in Fig. 4.

3.3 Results

We used 1 min mean values of the chain NUR-HAN- OUJ-PEL-MUO-MAS-SOR (Fig. 1 ) . We rotated the coordinate frame to have, at each instant, th6 x-axis approximately parallel to the horizontal variation. The choice of the rotation angle was not very critical. For example, assuming that the horizontal field equals the northern component, the results changed typically only by a few per cent.

The integration in eq. (8) was performed in a closed form using linear interpolation between data points (e.g. Schmucker 1970). The choice of interpolation method does not essentially change the results. Even a simple broken line interpolation between the data points was found to be acceptable.

A quiet period near each event was visually selected to define the base line. The field was extended out of the chain assuming that ,Y behaves asymptotically like x.-’ and Z like xp3, where x is the distance from the centre of the electrojet system. This is true, for example, for a line current over a homogeneous earth, which is seen by replacing the earth by a perfect conductor at some depth (Viljanen 1992, Section 2.5). (Kuppers et al. (1979) assumed an exponential damping.) The extrapolation method does not usually affect the results obtained inside the chain very much. The field values near the station under study are the most important (see eq. 8). For comparison, we performed the separation assuming that the field outside the chain is exactly zero. Even such a crude assumption yielded qualitatively, and especially for X often also quantitatively (within *lo per cent), the same results as the more sophisticated extrapolation.

However, the induction effect due to the Arctic Ocean is essential. The extrapolation used may fail if the current system is located near the coast. A better method might be based on theoretical estimations of the ocean effect discussed in Section 3.4. The closest magnetometer station north of SOR is Bear Island (Bjarnaya), but this distance is about 400 km. Additionally, the field recorded there is not necessarily connected to the electrojets above Fennoscandia.

Another problem connected to the induction in the ocean (and inland anomalies) is that, even if the external field were 2-D, the induced field could be 3-D. However, the external part normally dominates, so the failure of the 2-D assumption due to induced currents is not too serious (see below).

Richmond & Baumjohann (1984) separated a 3-D field observed by the SMA array. A comparison of their results with ours cannot be made because they considered only two instantaneous field configurations. It is interesting that they


6.12. 1992

400 = I I I I

300

200

-

-

c

-200 -

400 -

300 -

200 -

I I I I -

-

-

_ _ I 1 I I

-200 1 I I I I

14:OO 1500 16:00 17:OO UT

I I J

300

200

-

-

k

N \ = 100-

0:

-200 - I

14:00 15:00 16:00 17:00 UT

Figure 4. Magnetograrns o f a bay event used in this study. A longer time than used in calculations is plotted to show the quite time field too.


IMAGE and induction efsects 901

20

n

nT

150

100

50

0

-50

-100

-

15.30 16 16.30 UT / hours

Pello nT

150

100

50

0

-50

-100 15.30 16 16.30

UT / hours

Figure 5. External and internal parts of the northern and vertical components of the magnetic field at PEL on 19% Dccember 6.

found the differences between 2-D and 3-D separations quite negligible, even for a strongly 3-D field configuration. The differences were not greater than the inherent mapping errors in their optimal linear estimation technique. This conclusion together with our experience is encouraging when thinking about using the efficient 2-D separation.

Results of our study can be presented as magnetograms (Fig. 5 ) . or histograms of the ratios of external parts to the total variation (Fig. 6). The histograms show clearly the relative significances of the external and internal contributions. Histograms for the internal parts are obtained simply by a mirror reflection with respect to the 50 per cent line.

The average ratios and standard deviations of the external parts to the total variations at all stations are listed in Tables 3 and 4. Generally, values for X are more peaked than those for Z. In some cases (marked with an asterisk) values are scattered very widely with standard deviations even greater than 500. For Z , this happens typically at stations near the electrojet centre where the total Z is near zero. For .Y, the standard deviations are usually largest at NUR and HAN. This is obviously due to small total X values. I t is also possible that in some cases (especially events marked with an asterisk in Table 7 ) there are field-aligned currents affecting X but not Z. In such cases, separation results for Z may be more reliable than for ,Y (this may be the case for the 1992 December 6 event).

However, we can conclude the following quantitatively in the IMAGE region.

(1) The external contribution to X is 70-90 per cent (internal 30-10 per cent, respectively) at stations near the centre of the ionospheric current. For southern stations, the contribution may be smaller, even less than 50 per cent. (Some care is required when the results of NUR and HAN are considered because they are usually not located in the electrojet region.) Therefore we do not totally agree with Kiippers et al. (1979) and Mersmann et al. (1979) who state that the induction effect on the horizontal field can be neglected as a first approximation. The ocean effect is not seen in the horizontal field on the continent, as can be

understood using a theoretical model (Section 3.4; see also Figs 7 and 8).

(2) The external contribution to Z near the maxima of IZI

OUJ 1

10

n -300 -150 -100 -50 0 50 100 150 200

XextlXtot (I)

OUJ 40

-200 -150 -100 -50 0 50 100 150 200 Zext/Ztot (9%)

PEL 40

201 n ” -200 -150 -100 -50 0 50 100 150 200

XextlXtot (%)

1 1 1 0 -200 -150 -100 -50 0 50 100 150 200

Zext/Ztot (%)

Figure 6. Histograms of the distribution of the external parts of the magnetic field at OUJ and PEL on 1992 December 6. 15- 17 LIT. The vertical scale gives the number o f separated minute values.


902 A. Viljanen, K . Kauristie and K . Pajunpaa

Table 3. Average ratio and standard deviation (per cent) of the external part of X to the total variation at the IMAGE magnetometer stations during events listed in Table 2. An asterisk ( * ) means that the standard deviation exceeds 500 (for an explanation, see Section 3.3) .

h Y

22.11.

22.11.

30.11.

6.12.

12.12.

14.12.

19.12.

25.1.

30.1.

10.2.

NUR

104f11

3 lf22

35f217

97f7

-357+*

128k2 1

15+26 1

1W-23

-302f*

4 1f6

HAN 11 lf17

41f13

7M270

107f13

467f*

137f28

79f133

94f58

21f34

49i4

OUJ

8M60

725~8

81H5

76H8

- 12f463

904**

79f21

89f15

72f4

53+4

PEL 81fll

14f4

71f14

81f193

4&311

35f12

75f5

87+8

74f3

55$7

MU0

91f9

73f4

75f6

85f17

2370

78f5

8 1f5

88+6

77f4

65f9

MAS

84f6

686

71f5

83f5

68f4

76f3

79f8

86+11

75k4

70f6

SOR

67f11

47+7

5a12

67f9

75f12

65f3

54f87

59f26

48i4

56i4

Table 4. Average ratio and standard deviation (per cent) of the external part of Z to the total variation at the IMAGE magnetometer stations during events listed in Table 2. An asterisk ( * ) means that the standard deviation exceeds 500 (for an explanation, see Section 3.3).

day NLJR HAN OUJ PEL MU0 MAS SOR

22.11. 88f18 94f15 94L-10 118i18 104f441 6f* 98f67

22.11. 228+* 138f304 146f176 -292+* 83f94 90-18 87f8

30.11. 177f350 -10-1* 119f130 -168f* 75f281 62f213 96f15

6.12. 104f24 lllf16 116k13 11B15 105f16 127f491 77k85

12.12. 117f34 117f34 108f26 99f 18 8%12 9M16 46f58

14.12. 115f19 114f10 119k13 112f7 10255 31M* 58+62

19.12. 281f96 184f25 140-114 393+* 107f202 75f14 78f9

25.1. 92fll 97f9 94f9 167f91 llM51 90+ 10 95i9

30.1. 153f9 149f8 131f5 188+47 71f72 74f16 88f8

10.2. 184f25 155+17 137f20 155+30 7 lf23 -104** 79f46

(except MAS and SOR) is typically 110-140 per cent (the internal contribution has then an opposite sign). However, very often the external and internal parts have the same signs at MUO, MAS and SOR, and the external contribution is typically about 70-90 per cent. It is clear from magnetograms and theoretical considerations that at short periods the ocean effect is very serious at SOR (c$ Kuppers et al. 1979, p. 441).

I t is interesting to compare separation results of the bay events with those in middle latitudes, where the primary field is usually a plane wave. If the Earth’s conductivity depends only on the depth, the external and internal horizontal fields are then practically equal. However, local conductivity anomalies may make the internal part significantly larger than the external one, as shown, for example, in a field study in the Carpathians by Jankowski et

al. (1985, Fig. 8). Then the vertical field is purely due to induced Earth currents.

3.4 Theoretical interpretation

The main features of the normal induction can be explained by assuming an ionospheric line current flowing at a height h above the y-axis. Induction in the Earth is modelled by an image current at a depth h + 2d (corresponding to a perfect conductor at a depth d). The ratio of the external X to the internal one has a maximum under the current. There is always a point where the internal and external X are equal, and at longer distances the internal part is larger.

The knowledge of that ratio provides the possibility of constructing a rough earth model, in which the earth is replaced by a perfect conductor at a depth d . An example is



given by Kisabeth & Rostoker (1977. p. 669) who consider a spherical earth and a complex 3-D ionospheric current system. A simple estimation for d is obtained assuming a horizontal line current at a height of h above a planar earth surface. Then

7R - 1 7(1 - R ) d=- h, (9)

where R is the ratio of the external horizontal field to the total one just under the line current. If R = 0.75-0.90 then d = h-4h. i.e. typically 100-400 km. This can be used as a first guess for a parameter fitting in a more realistic current model. Note that d depends on the current model, e.g. for a sheet current it is greater than for a line current (with R fixed). R varies greatly from event to event due to the variations in the primary current system, as Kisabeth & Rostoker (1977, Table 1 on p. 669) have shown by theoretical modelling. Their results agree qualitatively with our experimental ones.

An idealized thin-sheet model to estimate the ocean effect is given by Fischer, Schnegg & Usadel (1978) (for correction of an error in this paper, see Raval, Weaver & Dawson 1981). They assume a plane-wave primary field, but an extension to a laterally inhomogeneous field is straightforward (Viljanen 1992, Section 3.3.2). The ocean is described by a perfectly conducting sheet at the Earth’s surface. The ionospheric current is parallel to the coastline (E-polarization). This model roughly corresponds to electrojet events in northern Norway.

Examples of 2 due to a line current are shown in Fig. 7. The sharp increase of / Z / occurs near the continent-ocean boundary. A plane-wave primary field yields similar behaviour (Fischer et a/. 1978). Consequently, a t the coastline the ocean effect has a significant contribution to the total variation field, and the induced part may greatly exceed the primary field. Concerning X, the anomalous

doubling effect occurs in practice only at the perfectly conducting sheet. Very near the coastline, however, the horizontal field may also be affected (Boteler 1978).

Figure 8 explains qualitatively the anomalous behaviour of Z : the primary current is concentrated above the IMAGE chain, and induced currents in the ocean are north of the magnetometers. Then the primary Z (north of the primary current) and the ,--component of the (anomalous) field due to induced currents in the ocean have the same sign. The induced part due to the layered earth is opposite to the anomalous one, but smaller in magnitude. Thus, the external and internal parts have the same sign.

4 CONCLUSIONS

Geomagnetic induction vectors of the EISCAT and IMAGE magnetometer stations show that the induction effect by the Arctic Ocean is significant at northern stations ( ALT. KAU, KEV, KIL, MAS, SOR) for periods longer than a few minutes, and for all periods at the coast (SOR). Inland anomalies were found at all stations at some period ranges, but distortions caused by them are not generally serious from the viewpoint of ionospheric/magnetospheric research. A small-scale anomaly together with the ocean effect at MAS is troublesome. Disturbances at H A N caused by the S T A R E radar are serious at periods of a few minutes. The most dominant anomalies a t MAS and SOR can be detected even when the source field is very inhomogeneous.

Concerning geological interpretations. induction vector results provide the first step for more detailed magnetotelluric studies. Another interesting further topic would be a study of the behaviour of the induction vectors as functions of time as in papers by Anderson, Lanzerotti & Maclennan (1976, 1978). Such a study could be useful not only for earth studies but also for studies of ionospheric/ magnetospheric current systems.

To study large-scale induction non-plane-wave events, we

300

200

100

0 -3 -2.5 -2 -1.5 -1 -0.5

x 1 skin depth

Figure 7. lB,I at the Earth’s surface due to a line current having an amplitude of lo5 A and flowing parallel to the y-axis at a height of 100 km. The half-plane z = 0. x > 0 is a perfect conductor; otherwise the Earth has a constant conductivity. Three locations of the line current are shown: x g 1.5d (solid line), xg = -0.M (dotted line) and x( , = 0% (dashed line) where d is the skin depth in the earth. BZ is shown only for negative values of x , because it is zero for x > O (Viljanen 1992, Section 3.3.2).


904 A. Viljanen, K. Kauristie and K. Pajunpaa

electrojet @

anomalous induced part - duced ocean current

normal induced part

induced earth current @ (normal induction)

Figure 8. Schematic illustration o f the ocean eff'cct. Bctwccn the electrojet and the coast the primary field and the anomalous induced field have the same sign oppositc to that of the normal induced field.

separated the magnetic variation field o f several bay events into external parts using the IMAGE data. We found that the internal (induced) horizontal field near the ionospheric electrojct system is typically 10-30 per cent of the total variation. Farther from the source region the ratio can increase up to more than 50 per cent. The external vertical field is either 0.7-0.Y or 1.1-1.4 times the total variation near thc maxima o f IZI depending on the location of the ionospheric current with respect t o the station under study. A simplified thin-sheet ocean model explains the behaviour of IZI. Although only bay events were considered, similar conclusions evidently hold true also for events of other types.

The combined conclusions about the induction effects a t the EISCAT/IMAGE magnetometer region are given below.

( I ) Near the electrojet region the primary horizontal field is always dominant.

(2) Near the electrojet region the primary vertical field dominates except at MAS and SOR, where anomalous induction must be taken into account. Careful selection of data is required if Z of these stations is used.

( 3 ) Farther from the usual electrojet region (at NUR, HAN and OUJ) the induced horizontal field may be equal to or even larger than the primary part.

(4) Anomalous induction does not distort Z significantly at any stations other than MAS and S O R (and at the EISCAT stations ALT and KAU). However, the ocean effect is observable sometimes also at M U 0 and perhaps even at PEL. A man-made anomaly (STARE radar) is affecting HAN recordings. The normal induction effect on Z must be taken into account at all stations.

magnetospheric studies as well. Suitable starting points are the thin-sheet methods of Vasseur & Weidelt (1977) and Dawson & Weaver (lY7Y). of which the latter has been applied, e.g. by Agarwal & Weaver (1990). A general 3-D ionospheric current model by Hakkinen & Pirjola (1986) should be combined with the thin-sheet method. Thin-sheet models of Fennoscandia are presented by Kaikkonen (1992).

Another and totally independent possibility is to use scaled analogue model experiments. The results of such a Hungarian-Finnish study, which modelled Fennoscandia and the electrojet. will be discussed in a future paper.

ACKNOWLEDGMENTS

The authors wish to thank particularly Professor Christian Sucksdorff and Dr Risto Pirjola (Finnish Meteorological Institute), D r Toivo Korja (University of Oulu) and Dr Tomasz Ernst (Polish Academy of Sciences) for useful comments and advice. The induction vector program written by D r Alan Jones was used in this study. Ville Virrankoski is acknowledged for his contribution to the programming work concerning the field separation. D r R. J. Banks and unknown referees made constructive comments, which considerably improved the manuscript.

EISCAT and I M A G E magnetometer data used in this study were collected through cooperation between Brauns- chweig Technical University (Germany), Adolf-Schmidt Observatory (Niemegk, Germany). the Finnish Meteorolog- ical Institute (Helsinki, Finland), and Sodankyla Geophysi- cal Observatory (Finland).

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29-36,

APPENDIX: TIME-DOMAIN METHOD FOR COMPUTING INDUCTION VECTORS

The time-domain method (TDM) for calculating induction vectors starts with the following equation of discretized convolution integrals:

vA = d + e ( k - k,,) + z a J x k . I + z b,yk , + 5 c,L,-,, where k = m f l , . . . , n ; r s m ; p 5 m : s s m , and vks are errors (assumed to be white noise) and .rJ, y , , and L, are the

,=o ,=I) 1'0

measured values; d and e are coefficients related to possible linear trends, and a,, b,, and c, are the unknown impulse responses to be solved. (Note: following the notation of Wieladek & Ernst (1977), x here corresponds to our B;, y to B,, and L t o By.)

The upper limits of the sums ( r , s, p ) are first determined with a modified maximum entropy method by Wieladek, Bromek & Ostrowski (1975), and then the impulse responses (and coefficients d and e ) are solved with the least-squares method. After Fourier-transforming the impulse responses to transfer functions A,, B, , and C, , an estimation of accuracy is obtained by calculating covariance matrices of the real and imaginary parts of the ratios B , / A , and C , / A , (the ratios actually are the final transfer functions). For a detailed description of the error estimation and determination of the coefficients d, e , r , s , and p , see Wieladek & Ernst (1977).

Ernst (1981) compares TDM with the classical frequency- domain method both theoretically and by using real data. The basic hehaviour of the induction vectors is similar, but the period dependence of T D M vectors is clearly smoother than that of the traditional induction vectors. Ernst also points out that T D M is an objective method as it eliminates automatically the possible linear trends from the data and no choices for whitening coefficients and time lag windows are needed. T D M also reduces the artificial effects caused by abrupt 'cut-offs' in the input data.


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On induction effects at EISCAT and IMAGE magnetometer stations

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