INTERNATIONAL JOURNAL OF GEOMAGNETISM AND AERONOMYVOL. 5, GI2001, doi:10.1029/2003GI000062, 2004

On the relation of giant pulsations (Pg) to pulsations inthe PC1 band (the “pearl” series)

N. A. Kurazhkovskaya, B. I. Klain, B. V. Dovbnya, and O. D. Zotov

Borok Geophysical Observatory, Yaroslavl Region, Russia,

Received 4 December 2003; revised 20 May 2004; accepted 18 June 2004; published 22 October 2004.

[1] Using the data of several observatories in the Northern and Southern Hemispheresdistanced by latitude and longitude, the analysis of simultaneous observations of geomagneticpulsations in two frequency ranges ((7.0–22.0) mHz, the giant pulsations (Pg), and (0.5–2.0) Hz, the pulsations of the Pc1 (“pearls”) type) is performed. It is found that in 70%cases the Pg pulsations observed at auroral and middle latitudes are registered againsta background of a global excitation of pearls series. The Pg pulsation period (T ) isproportional to the period of repetition of pearls (τ), and the T/τ ratio is controlled by thevalue of the Dst variation. The intensity of the latter is governed by protons injected fromthe tail to the daytime side of the magnetosphere during the recovery phase of a storm.The results obtained are an indirect experimental confirmation of the fact that protonswith energy of 10–80 keV participate in generation of the Pg pulsations. INDEX TERMS:

2431 Ionosphere: Ionosphere/magnetosphere interactions; 2716 Magnetospheric Physics: Energetic particles,

precipitating; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: Giant pulsations; Pulsations

of the “pearl” series; Protons and Pg pulsations.

Citation: Kurazhkovskaya, N. A., B. I. Klain, B. V. Dovbnya, and O. D. Zotov (2004), On the relation of giant pulsations (Pg) to

pulsations in the PC1 band (the “pearl” series), Int. J. Geomagn. Aeron., 5, GI2001, doi:10.1029/2003GI000062.

1. Introduction

[2] The giant pulsations (Pg) are known since the begin-ning of the last century [Birkeland, 1901; Rolf, 1931] andhave been studied in a huge number of publications. Duringthe recent 25 years the Pg pulsations were actively studiedusing the data of magnetometer chains [Chisham and Orr,1991; Chisham et al., 1990, 1997; Green, 1979, 1985; Ros-toker et al., 1979], auroral radars [Poulter et al., 1983], andspacecraft [Engebretson et al., 1988; Takahashi et al., 1992].The relation of Pg to other phenomena has been also studied[Chisham et al., 1990; Taylor et al., 1989].

[3] The Pg pulsations belong to rarely encountered phe-nomena localized by latitude and longitude of observations,and they differ from other types of geomagnetic pulsationsby the regularity of their periods and by drop-like shape.The highest frequency of Pg occurrence corresponds to thedawn sector of the auroral oval [Rostoker et al., 1979]. ThePg are mainly registered in the conditions of a quiet magne-tosphere (Kp ∼ 0−2), in the equinox seasons in the years ofsolar activity minimum [Brekke et al., 1987]. As a rule, the

Copyright 2004 by the American Geophysical Union.

1524–4423/04/2003GI000062$18.00

mean duration of a Pg regime is 30–150 min, their typicalperiods belong to the Pc4 range, and the amplitude is ofthe order of 10–30 nT. The most typical feature of the Pg istheir dominant polarization at the D component [Chishamet al., 1997; Green, 1979].

[4] In spite of the fact that spatial, time, spectral, andpolarization characteristics of the Pg pulsations are stud-ied in detail, till now there is no commonly accepted pointof view on their origin. The Alfven waves generated as aresult of a drift instability at the outer boundary of thering current are considered as one of Pg excitation mech-anisms [Green, 1979]. The other mechanism of Pg genera-tion they often relate to bounce resonance of the protons ofthe ring current [Chisham, 1996; Chisham and Orr, 1991;Poulter et al., 1983; Takahashi et al., 1992]. Since the Pgfrequency coincides with the bounce oscillations of protons[Chisham, 1996], some authors suggested that some role inthe Pg pulsation generation may be played by protons andpresented theoretical estimates of the proton energy. Forexample, Poulter et al. [1983] suggested that Pg may bea result of the bounce resonance instability of the protonswith energies of ∼10 keV. Chisham [1996] showed an ex-istence of possible relation between the Pg pulsations andprotons with energies of about 5–30 keV. However, as faras we know, no measurements of the energy of the precipi-

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Table 1. List of Observatories With Codes, Corrected Geomagnetic Coordinates, and L Shells

Observatory Code L Corrected Geomagnetic CoordinatesLatitude, deg Longitude, deg

Molodezhnaya MOL 6.4 −66.70 76.00Novolazarevskaya NVL 4.7 −62.59 51.03Borok BOR 2.8 54.05 113.44Vostok VOS − −85.41 69.01College CMO 5.5 64.83 259.65

tating protons during observations of giant pulsations havebeen conducted. Therefore there is no experimental confir-mation of correlation of the probability of the Pg pulsationobservations to fluxes of energetic protons. At the same timeit is grounded theoretically and proved experimentally thathigh-energy protons participate in excitation of some typesof more high-frequency pulsations (for example, of the Pc1and Pc1,2) [Guglielmi, 1979; Matveeva et al., 1972; Yahn-ina et al., 2002]. From our point of view the detection ofthe relation of Pg to other types of geomagnetic pulsationswould have been very useful for understanding of the mech-anisms of the Pg pulsation generation. However, no studiesof the correlation of Pg with geomagnetic pulsations of otherfrequency ranges have been performed. In this paper an at-tempt to study possible relation between the giant pulsationsand high-frequency pulsations in the range (0.5–2.0) Hz isundertaken.

2. The Data

[5] The magnetograms with the sweeping velocity of 90 mmh−1 of the following observatories: Molodezhnaya for 1976–1992, Novolazarevskaya for 1981–1988, and Borok for 1965–1998 and also analog recordings of the magnetic field tomagnetic carriers in the observatories Borok, Molodezhnaya,Vostok, and College (the data archived at Geophysical Ob-servatory Borok) were used. Table 1 shows the codes, cor-rected geomagnetic coordinates, and L shells of the used ob-servatories. The hourly mean data on the parameters of thesolar wind and interplanetary magnetic field (IMF) from thedigital King catalog (the data from the World Data CenterWDC B) were additionally used.

3. Results

[6] 1. The majority of the studies of Pg known to theauthors have been performed on the basis of observations inobservatories in the Northern Hemisphere. The data of someobservatories of the Southern Hemisphere were attracted toanalyze the conjugacy of the Pg pulsations [Annexstad andWilson, 1968; Green, 1979; Nagata et al., 1963]. Studiesof the Pg pulsations using the data of the auroral observa-tories Molodezhnaya (MOL) and Novolazarevskaya (NVL)

(Antarctic) and midlatitude observatory Borok (BOR) havenever been performed till now. So for the beginning we con-sider briefly the morphological characteristics of the giantpulsations detected at the MOL, NVL and BOR observato-ries.

[7] During the periods analyzed 36 and 10 Pg events weredetected at MOL and NVL, respectively. The average fre-quency of the Pg occurrence is 2.3 and 1.4 events per yearat MOL and NVL, respectively. The comparison with thedata (known from the publications in literature) on the oc-currence frequency of Pg in the Northern Hemisphere showsthat in the Southern Hemisphere the pulsations are morerare events. Actually, it follows from the Pg studies in theNorthern Hemisphere that on the average 3.1 events peryear are observed at the Abisko observatory (L = 5.5) [Rolf,1931], 6 events per year are observed at the Sodankyla ob-servatory (L = 5.2) [Sucksdorff, 1939], 8 events per year areobserved at the Tromso observatory (L = 6.4) [Brekke et al.,1987], and 11 events per year are observed at the chain ofEISCAT magnetometers in Scandinavia (L = 5.01 − 6.66)[Chisham and Orr, 1991].

[8] We have found no cases of simultaneous observation ofthe Pg pulsations at MOL and NVL, though these observa-tories are distanced from each other only by two MLT beltsalong geomagnetic longitude and by 4◦ along geomagneticlatitude. This fact manifests strong localization by longi-tude and latitude of the Pg pulsations also in the SouthernHemisphere.

[9] Many of the morphological features of the Pg pulsa-tions detected at MOL and NVL are identical. For exam-ple, at both observatories Pg are observed under quiet ge-omagnetic conditions mainly in the morning hours (0400–0700 MLT). The mean duration of Pg is 20–100 min, therange of the pulsation periods is nearly similar: 90–120 s.The amplitude level of Pg at MOL is slightly higher (5–30 nT) than at NVL (4–15 nT), the latter fact being ex-plained by the dependence of the oscillation intensity on thegeomagnetic latitude of the observations. It should be notedthat almost in all the analyzed cases the giant pulsations atMOL and NVL were observed at the recovery phase of smallgeomagnetic storms.

[10] It is widely known that the most probable region ofthe giant pulsation generation lies at the L shells of about5–6. Nevertheless the Pg pulsations can be observed alsoat L < 4 but much more seldom than at auroral latitudes[Green, 1985]. We have detected 14 cases of Pg at the mid-latitude Borok observatory (L = 2.8) during 33 years. Themean occurrence frequency of Pg at BOR is 0.4 events per

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year, the latter value being much lower than the correspond-ing value at high latitudes of the Northern Hemisphere. Theanalysis of observations of midlatitude giant pulsations atBOR shows that the range of their periods (45–60 s), du-ration (20–60 min) and amplitude level (2–4 nT) are signif-icantly lower than for the auroral Pg. The most often Pgat BOR were observed at 4–6 MLT. The D component po-larization of Pg at BOR is less pronounced than at auroralobservatories. The comparison of the D/H ratio at BOR(approximately equal to 1.0–1.9), to the D/H ratio at MOLand NVL (much higher and varies within 1.5–3.0) confirmthe above statement.

[11] 2. Now we come to the results obtained at the compar-ative analysis of the simultaneous observations of geomag-netic pulsations in two frequency ranges: (7.0–22.0) mHzand (0.5–2.0) Hz. For each Pg pulsation event at the MOL,NVL and BOR observatories (using the analog recordings tomagnetic tape of geomagnetic pulsations by sonograph) dy-namical spectra of high-frequency geomagnetic pulsations inthe frequency range (0.5–2.0) Hz were created. The study ofthe structure of the dynamical spectra corresponding simul-taneous observations of the Pg pulsations shows that in 70%cases the auroral and midlatitude Pg are observed againsta background of globally excited structured pulsations ofthe Pc1 type (the “pearl” series). The latter fact makes itpossible to assume that there is a relation between thesetwo phenomena. For example, out of 36 cases of Pg pulsa-tions detected at MOL 25 Pg events were observed againsta background of the “pearl” series registered both at thesame observatory and at other observatories distanced bylongitude and latitude (BOR, VOS, and CMO). Accordingto Guglielmi and Troitskaya [1973], Pc1 observed simultane-ously on a huge territory from high to lower latitudes andenveloped by a large longitudinal interval are called globalPc1. The frequency of the globally excited pulsations Pc1are usually F > 0.5 Hz [Guglielmi and Troitskaya, 1973].The frequency of “pearls” accompanying Pg as a rule ex-ceeded 0.5 Hz. It is an additional confirmation that theexcitation of the “pearl” pulsations has a global character.Figure 1 shows examples of observations of the Pg pulsationsin the D component and fragments of the dynamical spectraof the “pearl” series at middle and high latitudes: a typi-cal event of simultaneous Pg pulsations at MOL and BORon 29 September 1985 (Figure 1a); a typical event of simul-taneous Pg pulsations at NVL and “pearl” series at MOLand BOR on 10 October 1985 (Figure 1b), and a typicalevent of simultaneous Pg pulsations at NVL and pulsationsand “pearl” series at BOR on 24 February 1985 (Figure 1c).The presented examples visually illustrate the fact of the gi-ant pulsation occurrence against a background excitation ofthe “pearl” series.

[12] 3. It is known that the Pg pulsations appear ratherseldom, whereas “pearls” are observed relatively often. Inorder to be convinced that occurrence of the Pg pulsationsagainst a background of the “pearl” series is not casual wetried to reveal the relation between the main characteristicsof these pulsations. To do this one can use the data of anypair of the observatories where the Pg cases are accompaniedby “pearl” series. The largest statistical material we haveavailable, was selected from the observations of the giant

pulsations in MOL and “pearl” series at BOR. Therefore westudied the relation between the characteristics of Pg and“pearls” using the data of these observatories. We consid-ered the oscillation period (T ) and the repetition period of“pearls” (τ) as the main characteristics of the Pg and Pc1pulsations, respectively. The repetition period of “pearls”was determined from the dynamical spectra. Figure 2 showsthe dependence of the giant pulsations period on the “pearls”repetition period. Every point in Figure 2 corresponds to aparticular event of Pg observation against a background of“pearl” series and line shows the linear approximation by theleast squares method. Dotted line indicates events for whichthe Pg pulsation period T is the same as the pearl repetitionperiod τ . The correlation coefficient between these param-eters is 0.70 and indicates to the significance of the foundrelation. The regression equation for T and τ has the form

T = 0.3τ + 82.0

It is important that the tangent (tan α = T/τ) of the angleof the slope of the line approximating the relation betweenT and τ depends significantly of the Dst variation. Figure 3shows the dependence of the ratio of the giant pulsationsperiod to the repetition period of “pearls” (T/τ or tan α) onDst variations. One can see that the value of tan α decreaseswith a decrease of the negative values of the Dst variations.The line shows an approximation of this dependence. Hor-izontal dotted line at T/τ = 1 indicate where the pulsationperiod T and the pearl repetition period τ are equal. Theregression relation between tan α and Dst may be presentedin the form

tan α = −0.03Dst + 0.90

The existence of the correlation between tan α and Dst ismanifested by the relatively high value of the correlationcoefficient equal to −0.79.

[13] Thus the comparison of magnetograms of individualevents of observations of the Pg pulsations and dynamicalspectra of the pulsations in the Pc1 range, as well as theresults of the obtained experimentally relations between thecharacteristics of these pulsations (T and τ) indicate to therelation between the two phenomena and so to the relationbetween the processes developing on the plasmapause duringgeneration of Pg pulsation and “pearls” series.

[14] 4. Now we consider the interplanetary situation dur-ing Pg observations against a background of “pearl” series.Using the King digital catalog (the data from WDC B) con-taining mean hourly values of the parameters of the solarwind plasma and interplanetary magnetic field (IMF) thedependencies of the occurrence frequency of Pg against abackground of “pearl” series on the velocity (V ) and den-sity (n) of the solar wind, IMF magnitude (B), and theIMF component Bz in the solar ecliptic plane were ana-lyzed. It was found that Pg are observed against a back-ground of “pearl” series in the dominating number of casesat V ∼ 350 − 400 km s−1, n ∼ 6 − 8 cm−3, B ∼ 2 − 4 nT,and −2 nT< Bz < 2 nT. The fact draws attention thatusually the pulsations of the Pc1 type are excited at muchhigher density of the solar wind (30–35 cm−3) and typicalvalues of the IMF component Bz: −3 nT> Bz > −5 nT

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and 3 nT< Bz < 5 nT [Matveeva et al., 1972]. At the sametime Pc1 are more often observed at extremely low values ofthe solar wind velocity (200–250) km s−1 [Matveeva et al.,1972], the latter values being considerably lower than at Pgoccurrence against a background of “pearls”. So the abovementioned events of Pg observations against a backgroundof “pearls” are typical for lower values of the density andrelatively higher values of the velocity of the solar wind.

Figure 1. Magnetogram (D component) of giant pulsations(Pg): (a) – recorded on 29 September 1985 at MOL andfragments of the dynamical spectra Pc1 events registeredat MOL and BOR; (b) – recorded on 10 October 1985 atNVL and fragments of the dynamical spectra Pc1 eventsregistered at MOL and BOR; (c) – recorded on 24 February1985 at BOR and fragments of the dynamical spectra Pc1events registered at BOR.

4. Discussion

[15] One should note that many zonal and morphologicalfeatures of the Pg pulsations [Brekke et al., 1987; Chishamand Orr, 1991; Chisham et al., 1997; Green, 1979] and“pearls” [Guglielmi and Troitskaya, 1973] have mutual simi-larity. For example, (1) both pulsations type have a charac-teristic modulation of the amplitudes (view of wave packets)and belong to relatively rare events; (2) the greatest prob-ability of observation of Pg and “pearls” falls on the dawnsector of the auroral zone; (3) the probable region of Pgand “pearls” generation is the vicinity of the plasmapause;(4) the excitation of these pulsation is typical for weak andmoderate magnetic disturbances (Kp ∼ 1−3) at the recoveryphase of geomagnetic storms; and (5) the seasonal and solarcycle behavior of the pulsation observations coincide. Theperformed study showed that the giant pulsations not onlyare observed against a background of excitation of “pearls”series, but there are correlations between some characteris-tics of these pulsations. This fact assumes apparently some

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genetic relation (of the generation mechanisms) between thePg and “pearls” pulsations.

[16] According to Guglielmi and Troitskaya [1973] the os-cillation frequency of “pearls” is by the magnitude close tothe gyrofrequency of the ring current protons. Usually as amechanism of “pearls” generation the cyclotron instabilityof the protons of the radiation belt is considered [Guglielmi,1979; Trakhtengerts et al., 2000] and that assumes a relationof Pc1 to energetic proton precipitation. In the current pub-lications there are different points of view on estimation ofthe parameters of the precipitating protons related to Pc1.It follows from the theoretical evaluations and analysis of thesolar wind plasma parameters [Guglielmi, 1979; Matveeva etal., 1972] that the protons with energies of about 10–30 keV,injected from the magnetosphere tail to the daytime side, areresponsible for generation of “pearls”. At the same time thecomparison of the direct satellite measurements of the pro-ton precipitation characteristics to the probability of the Pc1pulsations observations (on the ground) shows that Pc1 areclosely correlated to the proton fluxes with energies >30 keV[Yahnina et al., 2002].

[17] Currently the most popular point of view on the Pgpulsation origin is that they are excited on the plasmapausebecause of the bounce instability of the ring current pro-tons [Chisham, 1996; Chisham and Orr, 1991; Poulter et al.,1983]. If one accepts that the “pearls” excitation is relatedto the energetic protons and the Pg pulsations are observedagainst the background of “pearls”, one may assume that thegiant pulsation generation is caused by protons of the sameenergies as excitation of the structured Pc1. Apparently, thelinear relation between tan α and Dst (Figure 3) manifeststhe relation of these pulsations to the process of the ring

Figure 2. Dependence of the Pg pulsation period (T ) onthe “pearls” repetition period (τ).

Figure 3. The T/τ ratio as a function of the value of Dstvariation.

current decay at the recovery phase of a storm. It is knownthat the intensity of Dst variations at the recovery phase ofa storm is determined by the energy of the protons injectedfrom the tail into the daytime sector of the magnetosphere.

[18] On the basis of the data available it is hard to pro-vide a particular evaluation of the energy of the injectedprotons responsible for simultaneous excitation of Pg andPc1. Taking into account Yahnina et al.’s [2002] results,the observation of the giant pulsations on the background of“pearl” series, and the found relation between the character-istics of Pg and “pearls”, one can assume that both types ofpulsations are related to the protons with the energy in the10–80 keV range. Thus the results obtained in this paperdo not contradict to the conclusions of Poulter et al. [1983]and Chisham [1996] and may serve as an indirect experimen-tal confirmation of the Pg pulsation relation to the protonsinjected during the recovery phase of a storm from the tailinto the daytime side of the magnetosphere.

5. Conclusions

[19] A few conclusions follow from the analysis and com-parison of simultaneous observation of geomagnetic pulsa-tions in two frequency ranges (7.0–22.0 mHz and 0.5–2.0 Hz):

[20] 1. In 70% of giant pulsation recorded at auroral andmiddle latitudes are observed against a background of globalexcitation of the geomagnetic pulsations in the Pc1 band(“pearl” series).

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[21] 2. The Pg pulsation period (T ) is linearly related tothe repetition period of “pearls” (τ), the ratio (T/τ) beingproportional to the value of Dst variation.

[22] 3. The detected interrelation between Pg pulsationand “pearls” is an indirect experimental confirmation of thefact that the protons with energies ∼0–80 keV injected intothe magnetosphere at the nightside and drifting westward tothe dayside participate in generation of Pg.

[23] Acknowledgment. The work was partly supported by

the Russian Foundation of Basic Research (project 03-05-64545).

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B. V. Dovbnya, B. I. Klain, N. A. Kurazhkovskaya, andO. D. Zotov, The Borok Geophysical Observatory, Borok 152742,Yaroslavl Region, Nekouz, Russia. (klain@borok.adm.yar.ru)

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