Pc4 giant pulsations recorded in Tromsø, 1929–1985

Journolof Atmospheric and Terrestrrol Printed in Great Britain.

Physics,

Vol. 49, No. 10, pp. 1027-1032,

1987. 0

OoZl-9169/87 S3.00+ .Xl 1987 Pergamon Journals Ltd.

Pc4 giant pulsations recorded in Tromsti, 1929-1985 A. BREKKE, T. FEDER* AND S. BERGER The Aurora1 Observatory, N-9001 Tromse, Norway (Received infinalform

30 March 1987)

Abstract-In

this investigation terrestrial magnetic records for 1929-1985 from the Aurora1 Observatory in Tromse have been examined for giant micropulsations (Pg). This period encompasses five solar cycles and supplements the work of Harang, who studied the records from the time such recordings started in Tromss in March 1929 through March 1941. In agreement with Harang, an anticorrelation with the solar cycle is found in the frequency of occurrence of the micropulsations. A seasonal variation with maxima in March and September is clearly revealed, as already indicated in Harang’s much more limited data set. Also in support of Harang, it is found that the period of oscillation tends to increase for long lasting pulsatipns and for recurring ones. Contrary to Harang, who found a daily maximum between 0100 and 0200 UT, however, we find the maximum to occur somewhere between 0200 and 0300 UT.

INTRODUCTION Pc4 giant pulsations or Pg are very regular oscillations of low amplitude occurring in the magnetic field. The period of the oscillations can vary between 60 and 200 s and has a tendency to increase through an event. The pulsations are most often observed at aurora1 latitudes in the night and morning hours. The amplitude of the oscillations is of the order of a few y and very seldom larger than 50 y. For simplicity, micropulsations in this study shall be referred to collectively as Pg pulsations. A Pg event may last from a few minutes to an hour or more. An example of a typical giant pulsation in the Pc4 range observed in Tromsla is shown in Fig. 1. Since BIRKELAND(1901) first discussed such events they have been studied by several authors. ROLF (1931) observed altogether 28 events between 1921 and 1930 in the records of Abisco in Sweden. From these limited statistics he found no event between 1200 and 2100 UT (1300 and 2200 LT). Twelve out of the 28 events, or 43% occurred between 0200 and 0400 UT (0300 and 0500 LT). Rolf also noticed that 13 of the 28 events occurred in September and October, while none were observed in August, November or December. SUCKSDOR~; (1939) studied 150 events observed at Sodankyll between 1914 and 1938 and confirmed the findings of ROLF (193 1) that the giant pulsations most often occurred in the morning hours. Sucksdorff furthermore showed that these events were most frequent

*Present address: Department versity, Ithaca, NY, U.S.A.

of Physics, Cornell Uni-

around the equinoxes and during years of minimum solar activity. This latter fact has also been confirmed by HARANG (1941), who observed 97 events between 1929 and 1941 in the records of Tromsra and later by ANNEXSTAD and WILSON (1968). SUCKSDORFF(1939) also noticed that the giant pulsations have a tendency to recur on successive days almost 24 h apart. This tendency has also been observed by HARANG (1941). These authors have noted that Pg are generally confined to the aurora1 zones. VELDKAMP(1960), using a large number of stations in Europe, found that the latitudinal distribution of the amplitude of one particular event maximized at N 57” latitude and that the amplitude dropped to the noise level within 5” of the latitude of maximum intensity. Veldkamp also contended that the average period of 105 s observed corresponded well with the eigen oscillation of 100 s predicted by OBAYASHI and JACOBS (1958) for field lines penetrating a geomagnetic latitude of 57”N. ELEMAN (1966), by studying giant pulsation events from 11 high-latitude stations in Scandinavia, claimed that the region of pulsating disturbance has a tendency to remain fixed geographically during the lifetime of the Pc4 giant pulsation event. ROSTOKW et al. (1979) confirmed the earlier results that the Pc4 giant pulsation events are sharply localized at high latitudes and that the power levels drop an order of magnitude within 5” of the latitude of maximum power. Rostoker et al. also found, however, that the pulsation period increases when the latitude of maximum amplitude increases. HARANG (1941) suggested that the Pg were caused by electron clouds mirroring back and forth along magnetic field lines and that the giant pulsations

1027

A.

1028 zooct.

BREKKEet al.

1963

Fig. 1. A typical record of a well developed giant micropulsation event (Pg) observed in Tromss between 0300 and 0930 UT on 20 Oct. 1963.

pulsations is coupled. They further proposed that this region is the large density gradients at the plasmaand ANNEXSTADand WILSON (1968) have demonpause and that the driving mechanism is the magnetostrated that this is the fact. ANNEXSTADand WILSON pause Kelvin-Helmholtz instability. POULTERet al. (1983) have found that the iono(1968), based on the conjugate symmetry of the pulsations, concluded that the giant pulsations are prospheric electric fields as deduced by the STARE radar duced by elliptically polarized transverse hydroexhibited large oscillations (N 20 mvm ‘) during a magnetic waves in the ordinary (even) mode Pc4 giant pulsation event. From polarization studies propagating along field lines to small, high-latitude, of these oscillations they suggested a resonant oscilconjugate areas. GREEN(1979), based on digitized pullation involving a sector of the geomagnetic field shell. sating magnetometer data from several high and The reason for this short review of some of the medium latitude stations in Europe, together with literature concerning Pg pulsations is to indicate that similar data from the two conjugate stations this study has a very long history and that the question St. Anthony and Halley Bay, concluded that the of the source mechanism for these outstanding polarization and phase characteristics of the pulsations features in geomagnetic records is still open to question. are in agreement with a field line resonance, as It is the intention of this communication to show proposed by CHAN and HASEGAWA(1974a,b) and that very long data series of Pg observations in SOUTHWOOD(1974). From the analysis of the conTromss, Norway, confirm the earlier findings based jugate signals GREEN(1979) concluded that, in contraon rather short data series that the Pg occur at very diction to the findings of ANNFXSTADand WILSON specific time periods. (1968), the wave characteristics are that of an odd No matter what theoretical explanations are mode relationship. brought forward for Pg, the theory is not complete ROSTOKERet al. (1979), based on digitized data unless it can also explain why Pg have this tendency from the Canadian array of magnetometers, have to occur at preferred time periods. It is our view that reached the conclusion that the giant pulsations may these characteristics of Pg have not been taken seribe due to field line resonances at the plasma pause in ously into account when such theories have been prothe region where the electric field changes its moted. azimuthal component from westward to eastward. They also suggest that this field line resonance involves a coupling between the poloidal and toroidal modes. PRESENTATIONOF DATA According to CHEN and HA~EGAWA(1974b), such coupling may take place preferentially at a density At the Aurora1 Observatory in Tromss (geodiscontinuity, such as the plasmapause. graphical latitude 69.7”N) we now have in our posIn order to explain why Pc4 giant pulsations are so session about 60 years of continuous geomagnetic confined to limited areas at high latitides, ROSTOKER observations, one of the most extended series at auret al. (1979) indicated that it is something unique about oral latitudes. The observations were obtained with the region into which the energy responsible for the the same instruments throughout these years and therefore netically

should occur simultaneously at geomagconjugate regions. NACATA et al. (1963)

Pc4 giant pulsations Daily occurrence at

80-

70 -

of Pg giant

micropulsations

1029 in D-component

Tromsa

.-•

1929 - 1985

o---o

1929 - 1941

60f 2 502 b 405 k z 308 2010 -

*/*-a -bz...-./-‘8=1200

/ / d/ _A ,

\

\

L---,--o__,

--o*

l

-0

1800

2400

I

0600

l \

l

-0

A.

‘\

P\

a_’

\

\o-: 1200

Time (UT)

Fig. 2. Daily distribution of 523 Pg events observed in the D-component in Troms0 between 1929 and 1985. Also included are the data obtained by HARANG(1941) between 1929 and 1941. therefore represent a unique homogeneous data set. All the data are unfortunately on photographic records only and are therefore not easily accessible for extensive detailed analysis of polarization patterns, etc. HARANG (1941) analysed the data obtained

Seasonal variation of D-component Pg giant micropulsations observed in Tromsb

80 -

.-. ~----o

1929 -1985 1929 - 1941

l

I\

Month

Fig. 3. Seasonal distribution of the 523 Pg events observed in the D-component in Tromss between 1929 and 1985. Also included are the data obtained by HARANG(1941) between 1929 and 1941.

between 1929 and 1941 and found that Pg in Tromss show a distinct daily distribution, with a maximum between 0100 and 0200 UT, and no events were found between 1400 and 2000 UT. In the following we will present new data obtained in Tromsar between 1941 and 1985 in order to complete this picture. We want to compare these new data with the results obtained by Harang and especially investigate whether results obtained during a time span of a little more than one solar cycle are also valid for data averaged over more than 4 such cycles. The magnetic records for the years 1941-1985 were examined extensively, with data, commencement time, duration, amplitude and period noted. As the oscillations are most pronounced in the D-component in Tromss, this element has formed the basis of our research. The pulsations last for from less than one to several hours, often commencing and dying off irregularly, while exhibiting a clear pronounced smooth envelope. The period of oscillations was determined by counting the number of oscillations in 30 min or a minimum of ten swings. Micropulsations in the geomagnetic field are usually categorized by their periods. The pulsations in this investigatioin have periods between 69 and 225 s, the majority in the range 9G-125 s, i.e. mostly in the Pc4 region (45-150 s) and somewhat overlapping into the Pc5 region (150-600 s). The accepted classification system is given in JACOBS (1970, p. 17). We will, however, confine ourselves to presentation of the data

A. BREKKE et al.

1030 Annual

occurrence

number

recorded r Tromsb curve and H-range

1926

-

sunspot

--A-----A

Pg mIcropulsations H-range

32

36

of

D-component

for the years m Tromsb

1929-1965

Pg

m~cropulsatCms

compared

to the

sunspot

CUrYe

40

44

46

52

56

60

64

66

72

76

80

1964

YEW

Fig. 4. Annual Also presented

occurrence numbers of Pg micropulsations as observed in Tromsa between 1929 and 1985. are H-range in y as derived for Tromss throughout the same years. These data are compared with annual sunspot numbers.

in a simple statistical sense, i.e. as to when the Pg events occur with respect to time of day, season and year. In Fig. 2 are shown the daily occurrence of altogether 523 Pg events observed in the years 1929-1985.

Period

vers.us time of day

of long-lasting @Or

in TromsG,

for

or recurnng

D-component Pg mlcropulsationr

1941-1985

160

j

,40-

B C 120a” 100 -

80.

18Or

160 ;; $140

-

B c 120 d _ 100 -

I 00

I1

I

02

I

I

04

06

/

I

/

I

06

10

Time

(UT)

I,

I

12

/

14

I

I

16

I

I

16

Fig. 5. Periods of Pg micropulsation recurring through the same night or lasting for several hours are shown versus time of occurrence.

The pulsations occur most frequently (89%) between 2200 and 0800 UT (2300 and 0900 LT) and rather seldom (11%) between 0800 and 2200 UT. A maximum is found between 0200 and 0300 UT. This is about an hour later than the result found by HARANG (1941) for the years 1929-1941, which also are included in Fig. 2. In Fig. 3 are shown the seasonal variations found in Tromss for the years 1929-1985. Two very pronounced maxima are found at the equinoxes. From the limited data obtained by HARANC (1941), also shown in Fig. 3, similar although very weak maxima can be observed. MATSUSHITAand CAMPBELL(1967, p. 882) and JACOBS(1970, p. 40) mention also “secondary summer enhancement” at aurora1 latitudes, a feature not evident in Fig. 3, although Tromss is at the appropriate latitude. ROSTOKERet al. (1979) have pointed out that geomagnetic substorms may have a tendency to preclude the weaker pulsations. HARANG (1941), however, notes that the time of maximum occurrence of Pg coincides approximately with the seasonal curve of terrestrial storminess in Tromss, making it unlikely that masking of the micropulsations by irregular perturbations is a major concern. In order to look further into this problem, however, we have in Fig. 4 compared the occurrence of Pg with the geomagnetic activity in Tromsnr. As a measure of the geomagnetic activity we have used the hourly range in the H-component and averaged these values for each year. It is seen that the geomagnetic activity in Tromsa is clearly related to the solar sunspot cycle. The maximum geomagnetic

Pc4 giant pulsations

activity, however, lags behind the solar sunspot maximum by about 5 years on average or, said in another way, the geomagnetic maximum occurs on average 2-3 years before the solar minimum. The occurrence of giant pulsations is strongly anticorrelated with the solar sunspot cycle. Pg occur most frequently in years of sunspot minima and least frequently in years of high solar activity. When the occurrence of Pg is compared with the geomagnetic activity it is seen that 3 out of 5 of the Pg occurrence maxima are found in the same years as the geomagnetic activity has maximum. Seen together with the results found by HARANG (1941) it appears that geomagnetic disturbances do not preclude the appearance of giant pulsations in a way that could significantly change the observable occurrence of such pulsations. Curiously, the numbers of Pg in 1985 is more than twice that for any other year investigated. HARANG (1941) found that the period of the Pg is dependent on the time of appearance. Pulsations with short periods show a tendency to occur early in the morning, whereas pulsations with longer periods occur later. In this work we have found that when Pg occur more than once in a day or last several hours, similar increases in the pulsation period are observed. The results obtained on the basis of 50 such events found in the data are shown in Fig. 5. There are 6 cases in which the period remains unchanged and 4 in which a decrease occurs, while in the remaining 40 instances the period increases.

SUMMARY In this investigation terrestrial magnetic records for 1929-1985 from the Aurora1 Observatory in Tromsar have been examined for giant micropulsations (Pg). This period encompasses five solar cycles and supplements the work of HARANG (1941), who studied

1031

the records from the time such recordings started in Tromso in March 1929 through March 1941 (HARANG, 1941). In agreement with Harang, an anticorrelation with the solar cycle is found in the frequency of occurrence of the micropulsations. A seasonal variation with maxima in March and September is clearly revealed, as already indicated in Harang’s much more limited data set. Also in support of HARANG (1941) it is found that the period of oscillation tends to increase for long lasting pulsations and for recurring ones. Contrary to HARANG (1941), who found a daily maximum between 0100 and 0200 UT, however, we find the maximum to occur somewhere between 0200 and 0300 UT. We hope that these results may act to confirm 4 major facts related to the occurrence frequency of the Pg. 1. Pg at aurora1 latitudes tend to occur most frequently at night-time, with a maximum in the occurrence frequency a few hours after midnight. 2. Pg occur most frequently at the equinoxes (70%) and more seldom (30%) at solstices at aurora1 latitudes. 3. The occurrence frequency of Pg is strongly anticorrelated with the occurrence frequency of solar sunspots. Pg tend to occur most frequently about 2 years before sunspot minimum. 4. Geomagnetic disturbance at aurora1 latitudes does not appear to preclude the appearance of Pg in a significant manner. In theories proposed for explaining the occurrence of Pg these 4 observed facts should all be kept in mind. In past literature about Pg we feel that the first three statements especially, which represent very outstanding properties of the giant pulsations, have not been taken seriously into consideration. All these properties must reflect some important characteristics with respect to either the source mechanism or the optimal conditions for creating the pulsations.

REFERENCES ANNEX~TAD J. C. and WILSONC. R. BIRKELANDK. CHEN L. and HASEGAWA A. CHEN L. and HASEGAWA A. ELEMANF. GREENC. A. HARANGL. JACOBSJ. A.

1968 1900 1974a 1974b 1967 1979 1941 1970

MATSUSHITA S. and CAMPBELLW. H.

1967

J. geophys. Res. 73, 1805. Skr. Vidensk Selsk. Christiana 1,7. J. geophys. Res. 79, 1024. J. geophys. Res. 79, 1033. Ark. geofys. 5,23 1. Planet. Space Sci. 27,63. Geofys. Publr XIII, 1. Geomagnetic Micropulsations. Springer Verlag, Berlin. Physics of Groma,gnetic Phenomena, Vol. 11-2. Academic Press, New York.

1032 NAGATAT., KOKUBUNS. and IIJIMAT. OBAYASHIT. and JACOBSJ. A. POULTERE. M., ALLANW. NIELSENE. and GLASSMEIER K.-H. ROLF B. ROSTOKER G., LAM H.-L. and OLSONJ. V. SOUTHWARD D. J. SUCKSDORFF E. VELDKAMPJ.

A. BREKKEet al. 1963 1958 1983

J. geophys. Res. 68,462l. Geophys. J. R. astr. Sot. 1, 53 J. geophys. Res. 47, 5668.

1931 1979 1974 1939 1960

J. geophys. Res. 36,9. J. geophys. Res. 84,5153. Planet Space Sci. 22,483. Terr. Magn. atmos. Elect. 44, 157. J. atmos. terr. Phys. 11, 320.