Frequency growth of IPDPs during a substorm as a manifestation of equatorward expansion of particle precipitation and current region

A. G. Yahnin

Polar Geophysical Institute, Apatity, Murmansk Region, Russia

Contents

Abstract

1. Introduction

During geomagnetic disturbances in the dusk sector (~1400-2200 LT) the ULF geomagnetic pulsations called interval of pulsations of diminishing periods (IPDP) [Troitskaya, 1961] are often observed. Pulsation duration is ~15-60 min, and their appearance is accompanied by the enhancement of the westward (eastward) electrojet in the premidnight (dusk) sector [Fukunishi, 1969; Heacock, 1967]. It is now believed that the IPDP pulsations are excited as a result of the development of ion-cyclotron instability (ICI) when hot plasma of the ring current interacts with cold plasma of the plasmasphere [Nishida, 1978], that is, near the plasmapause projection. It is believed that among several possible mechanisms of frequency growth (IPDP), two mechanisms manifesting processes of radial and azimuthal drifts of hot protons during a substorm and magnetic storm are most suitable [Hayashi et al., 1988; Nishida, 1978; Pikkarainen et al., 1983]. Mechanism 1 is the radial drift of the ICI region toward the Earth as a result of enhancement of the westward convection electric field [Barkova and Solovyev, 1987; Gendrin, 1975; Heacock et al., 1976; Kangas et al., 1988; Troitskaya et al., 1968]. Mechanism 2 is the azimuthal drift of energetic protons to the dusk side after their injection into the premidnight sector during substorm onset [Fukunishi, 1969; Gul'el'mi and Zolotukhina, 1978; Hayakawa et al., 1992; Soraas et al., 1980].

At present the contribution of these mechanisms to formation of the IPDP spectrum remains unclear. It is usually believed that for IPDP excited in the premidnight sector the pulsation frequency growth is caused by the radial source drift and in the afternoon-dusk sector the pulsation frequency growth is due to the azimuthal drift [Heacock et al., 1976; Pikkarainen et al., 1983]. According to mechanism 1 there is a close relationship of the pulsation spectrum to the character of the ICI region equatorward drift, that is, to variations of the electric field intensity [Kangas et al., 1988; Troitskaya et al., 1968], and the absence or small delay of an IPDP event relative to the substorm expansion phase onset [Barkova and Solovyev, 1984, 1987]. The frequency growth in mechanism 2 is determined by the velocity dispersion and (or) L shells of a drifting particle cloud, and the IPDP must delay relative to a moment of their injection (substorm onset) in the midnight sector, frequency growth rate decreasing in time [Fukunishi, 1969; Gul'el'mi and Zolotukhina, 1978; Soraas et al., 1980].

Relationship of IPDP to a substorm onset were studied by Fukunishi [1969], Hayakawa et al. [1992], and Hayashi et al. [1988]. It is shown that the IPDP occur 15-30 min after substorm onset, which was determined by the beginning of the negative magnetic bay in the premidnight sector [Fukunishi, 1969; Hayashi et al., 1988] or the AE index increase [Hayakawa et al., 1992]. However, according to modern ideas the enhancement of auroral electrojets is not sufficient signature of the substorm expansion phase onset, and it is also necessary to take into account other signatures of this phase, that is, the poleward expansion of aurora including the formation of the auroral bulge or surge and substorm current wedge, low-latitude positive bays, and Pi2 pulsation excitation [see, e.g., Sergeev and Tsyganenko, 1980]. Besides that, it is also necessary to take into account that the substorm expansion phase often develops in the form of multiple activations (or multiple onsets) but the local activations of substorm or pseudobreakups [Koskinen et al., 1993; Nakamura et al., 1994] often precede the major substorm onset or full-scale substorm [Opgenoorth, 1992].

Barkova and Solovyev [1984] studied an excitation of IPDP during substorms taking into account their multiple onsets. They revealed that IPDP occurrence at 1500-2100 LT meridian is almost always delayed with respect to the first onset of the substorm determined from the start of low-latitude Pi2 pulsations but is simultaneously observed with one of multiple substorm onsets. These results point to the necessity of further research of IPDP appearance relative to the expansion phase onset.

Nowadays the relationship of the IPDP spectrum to the drifting energetic proton parameters and electric field variations during a substorm is poorly known. It is only known that during IPDP the energetic proton fluxes are enhanced [Soraas et al., 1980]. Kangas et al. [1988] revealed the electric field intensity enhancement from the radar observation data for one event of IPDP. Baishev and Solovyev [1994] showed that region of maximum IPDP intensity is localized 1^o-2^o southward of the eastward electrojet center and near the equatorward boundary of diffuse proton precipitations. According to Feldshtein and Galperin [1996], low-latitude (equatorward) boundary of diffuse aurora (LLBDA) is projected into the magnetosphere onto the plasmapause, so one can obtain some information on the electric field variations at the latitudes of ICI development studying the dynamics of this boundary.

In this paper we study (1) The relationship of the IPDP events to substorm expansion phase onsets taking into account its multiple onsets, and (2) the relationship of IPDP spectrum variations to the LLBDA dynamics.

2. Observation

The data on magnetic field variations at the network of stations at 190^o-210^o magnetic meridian (MM) [Yumoto and the 210^o MM Magnetic Observation Group, 1996] and 110^o MM (International Monitor for Auroral Geomagnetic Effects (IMAGE) project [Viljanen and Hakkinen, 1997]) were used here together with data of ULF magnetic pulsations (110^o MM). In addition, auroral observations were carried out in Tixie and Zhigansk (~190^o MM) by the all-sky television (TV) cameras with a resolution of 4 s. The detailed description of all-sky camera was given by Shiokawa et al. [1996]. The names of the stations, their abbreviations, and the geographical and corrected geomagnetic coordinates are given in Table 1.

2.1. IPDP Occurrence and Multiple Substorm Onsets

The IPDP occurrence in the afternoon-dusk sector (1300-2000 LT) was compared with substorm onsets or its intensifications determined by the data on the Pi2 pulsations, low-latitude magnetic bays, and auroral breakup in the premidnight sector.

The list of the IPDP events, a number of substorm activation associated with the IPDP event, and also Kp and Dst magnetic activity indices during IPDP are presented in Table 2. One can see from Table 2 that pulsations duration is from ~10 to ~60 min and IPDP events occur practically simultaneously with the second or third intensifications. We start with a presentation of data for two typical cases (Figures 1 and 2).

2.1.1. October 24, 1995.

Figure 1 shows variations in the H component of the magnetic field observed at the 210^o MM (~2200-2300 LT) and 110^o MM (~1500 LT) stations, the filtered records of Pi2 pulsations at the MSR low-latitude station in the premidnight sector, and dynamic spectra of IPDP event at the LOV station.

The first Pi2 pulsations were registered at ~1200 and 1212 UT (onsets 1 and 2, see Figure 1) and accompanied by weak disturbances of the magnetic field at the auroral zone and low-latitude stations. We identified these events as pseudobreakups. The next Pi2 occurred at ~1230 UT and ~1238 UT (onsets 3 and 4) and were accompanied by H component enhancement at TIX and CHD with the highest intensity DH 350 nT at ~1238 UT (~2300 LT) at CHD. At the 110^o ŒŒ stations only small magnetic disturbances were observed, and at LOV, weak pulsations in the range of Pc1-2 were registered. Apparently, during these intensifications a westward electrojet was localized in the premidnight sector and didnot exert a noticeable influence on the evening sector. With the onset of the fifth intensification at ~1250 UT accompanied by Pi2 pulsations and intense positive bay at the low-latitude station MSR, the enhancement of the westward electrojet in the midnight sector began, and a sharp expansion of the westward electrojet toward the dusk sector with simultaneous enhancement of the eastward electrojet at the SOR, MAS, and MUO (~1500 LT) stations took place. We identify this global enhancement of electrojets as the major substorm onset or full-scale substorm. The IPDP event occurred at 1250 UT simultaneously with the full-scale substorm onset.

Two IPDP events of ~15 min duration each took place on that day. Figure 2a shows auroral images obtained by an all-sky TV camera at TIX, the Pi2 pulsations and magnetogram of the low-latitude station MSR, and the OUL sonogram. Figure 2b shows magnetograms of the 210^o MM and 110^o MM stations, the IPDP occurrences at OUL being marked by solid rectangles. According to the data on magnetic field variations and Pi2 pulsations the substorm began at ~1306 UT and developed with multiple onsets at Kp = 4 and Dst = -30 nT (Table 2). Because of twilights the aurora registration started at 1326 UT. The analysis of all the data revealed the following. During the first two substorm onsets the region of the westward electrojet was near the midnight sector. In the dusk sector the increase of positive magnetic variations was observed, and at OUL the narrowband emissions with f 0.25 Hz were registered. Before the third substorm onset the auroral arc moved to the south (frames 1-4), manifesting the growth phase of this substorm intensification. Then aurora were activized at 1336-1337 UT and characterized by brightening of the arc situated southward from TIX, appearance of new, more northern arcs (frames 5 and 6), and poleward and equatorward expansion of the aurora (frames 7 and 8). Simultaneously, the intensity of the low-latitude positive bay at MSR increased, and Pi2 and (~1 min later) IPDP pulsations at OUL were registered. This auroral activation was accompanied by noticeable magnetic variations both in the midnight (negative) and early dusk (positive) sectors. It is worth noting that at the high-latitude station HOP a noticeable intensity decrease of the positive bay took place that is apparently connected with expansion of the westward electrojet to the west (Figure 2b).

The next auroral activation began at 1351 UT with the arc brightening on the northern horizon of the all-sky TV camera (frames 9 and 10) and clearly coincided with Pi2 onset. Then (~1400 UT) the aurora expanded equatorward (frames 11 and 12). Probably, an aurora expansion also occurred to the north, but it was out of the TV camera field of view. Thus this aurora activation occurred further poleward as compared to the preceding one. This substorm onset was accompanied by a sharp expansion of the westward electrojet to the dusk sector (station HOP, Figure 2b), and its influence was manifested in the intensity decrease of positive magnetic bays (eastward electrojet) at the stations SOR, MAS, and MUO (Figure 2b). The enhancement of the eastward electrojet at OUJ and the low-latitude positive magnetic bay at BEL were simultaneously observed (Figure 2b). IPDP pulsations at OUL with a sharp frequency growth appeared at ~1400 UT, coincided in time with the onset of the equatorward aurora expansion (frames 10-12, Figure 2a), and perhaps were delayed relative to the westward electrojet enhancement in the dusk sector (Figure 2b). Thus the appearance of two IPDP events was associated with the third and fourth substorm activations, whose localization regions, in contrast to the first two activations, were situated farther westward from the midnight meridian, that is, covered the dusk sector.

Thus the Table 2 and Figures 1 and 2 data show that IPDP events are observed more often simultaneously or with a small delay (  5 min) relative to the substorm onsets characterized by the poleward and equatorward expansion of aurora in the midnight sector and expansion of the westward electrojet into the dusk sector with a simultaneous enhancement there of the eastward electrojet. This substorm onset accompanied by IPDP excitation is of a global character and, probably, corresponds to the major substorm onset.

2.2. Dynamic IPDP Spectra and Equatorward Displacement of the Eastward Electrojet and Boundaries of Diffuse Aurora

The IPDP sonograms given in Figures 1, 2a, and 2b show that the frequency increases with a constant rate in time, i.e., is of a monotonic character. However, the growth frequency rate of IPDP often gradually decreases, i.e., is of a nonmonotonic character [see, e.g., Pikkarainen et al., 1983].

The IPDP spectra with a nonmonotonic character of the frequency growth are usually explained by the azimuthal drift of the IPDP source due to particle cloud "spreading" during their drift [Soraas et al., 1980]. The monotonic IPDP spectra are difficult to explain by azimuthal drift of injected particles [Zolotukhina, 1982]. In the case of the IPDP spectrum formation as a result of the source radial drift, both types of spectra should be connected with variations of the convection electric field intensity near the region of instability (ICI) development.

In introduction we mentioned that the IPDP region is localized close to the equatorward boundary of diffuse precipitations, the more so, as this boundary is projected into the magnetosphere onto the plasmapause [Feldshtein and Galperin, 1996]. If one supposes that the dynamics of the eastward electrojet center and the particle precipitation boundary is determined by the variations of the electric field intensity of the magnetospheric convection, then the relationship between variations of the motion velocity of precipitation and current region and variations of the IPDP spectrum should be observed. Of particular interest are the IPDP spectra with nonmonotonic frequency growth in time.

Figure 3 presents the example of IPDP with two intensity maxima, when the frequency growth rate during the second enhancement of IPDP was less than during the first one. The variations presented in Figure 3 of X and Z components of the magnetic field on a number of 110^o MM stations show that appearance of both maxima of IPDP intensity was accompanied by the enhancements of the eastward electrojet and its southward motion. During the first IPDP maximum the electrojet moved from the station KIL ( F = 65.8^o ) to MUO ( F = 64.6^o ), which is approximately a distance of 100 km, during ~7 min, and during the second maximum it shifted up to the station PEL ( F = 63.5^o ) during ~15-20 min. The velocity of the equatorward motion of the eastward electrojet during the first maximum was V 0.25 km s ^-1, and during the second one it decreased at least by a factor of 2.

Auroral observations in 1995-1996 simultaneously at two different latitudes (Tixie, L = 5.6, and Zhigansk, L = 4.1 ) made it possible to follow the position and dynamics of the discrete auroral structures and boundaries of diffuse aurora equatorward from the oval during IPDP excitation.

Figure 4a shows the frames of the TV camera at ZGN. Figure 4b shows location of the high-latitude and low-latitude boundaries of the diffuse aurora (HLBDA and LLBDA, respectively) for two moments (1248 and 1300 UT), Figure 4c shows averaged velocity of the LLBDA equatorward motion, and Figure 4d shows the IPDP pulsation periods at ZGN during the December 15, 1995, substorm.

The IPDP were observed at the Zhigansk and Yakutsk stations from 1150 to 1250 UT but were absent at Tixie. One can see from Figure 4b that both boundaries of diffuse aurora and discrete auroral forms (not shown) during the IPDP excitation moved to the south. The average velocity of the equatorward drift of auroral arcs and HLBDA during the IPDP event remained almost constant (~0.25 km s ^-1 ). The initial velocity of the LLBDA motion from 1150 to 1200 UT was 0.15 km s^-1, and to the end of the IPDP series, the velocity decreased to 0.005 km s^-1 (Figure 4c); that led to a decrease of the diffuse auroral zone width (Figure 4b). One can see from Figure 4b, where the positions of the eastward electrojet center and maximum IPDP intensity region are also presented, that the eastward electrojet center also moved to the south, and the IPDP region at 1248 UT was located at latitudes near the LLBDA. We estimated the meridional position of the eastward electrojet using the magnetic field variations ( H, D, and Z components) at Tixie, Zhigansk, Chokurdakh, and Zyryanka stations. The position of IPDP excitation region is shown with allowance for localization of the maximum IPDP intensity region 1^o-2^o southward from the eastward electrojet center [Baishev and Solovyev, 1994, Figure 1b].

The IPDP period change rate was of a nonmonotonic character. After 1215 UT the pulsation period change rate decreased significantly and was accompanied by a decrease of the equatorward LLBDA motion velocity (Figures 4c and 4d).

3. Discussion

3.1. IPDP and Full-Scale Substorm Onset: Comparison With the Results of Other Observations

The data presented show that, first, IPDP appear almost simultaneously with onsets of substorm expansion phase which are characterized by a sharp expansion of the westward electrojet toward the dusk side (Figures 1, 2a and 2b). Second, during an IPDP event the eastward electrojet is enhanced at the meridian of its registration and the equatorward motion of its center, and LLBDA is observed (Figures 3 and 4). Before discussing possible causes of such relationship of IPDP to development of the substorm expansion phase, we compare our results with observations by other authors.

3.1.1. Observations by Hayashi et al. [1988].

Hayashi et al. [1988] showed that occurrence of IPDP is delayed by ~15-30 min relative to the onset of negative magnetic bay in the premidnight sector. We analyzed one of the events (case 1) for January 18, 1986, considered by Hayashi et al. [1988] for which, besides ULF wave spectrograms, the magnetograms from a number of auroral stations were also available (see their Figures 3 and 4).

Using the data in Figures 1 and 3 of Hayashi et al. [1988], we show the locations of some stations in the evening-midnight sector in Figure 5a and the frequency-time spectrograms of ULF waves in Figure 5b. According to Hayashi et al. [1988] the substorm began at 0030 UT if one takes as its onset the enhancement of the negative magnetic bay (of westward electrojet) at the stations HUS and NAQ located near the midnight meridian [Hayashi et al., 1988, Figure 4]. Judging by spectrograms in Figure 5b, besides IPDP, excitation of three bursts of Pi1B was observed during this substorm (the localization regions are shown in Figure 5 by thick lines denoted 1, 2, and 3). The Pi1B (and also Pi2) pulsation excitations manifest multiple substorm onsets [see, e.g., Yahnin et al., 1983].

The first burst of Pi1B (burst 1) was observed only at the more eastern stations HUS and NAQ located near the midnight meridian. The next bursts of Pi1B (bursts 2 and 3) were also observed at the more western stations FRB and CRH located in the dusk sector. Parkhomov et al. [1976] showed that such a spatial development of Pi1B pulsations is related to the northwestward expansion of the westward electrojet during the substorm expansion phase as was found by Wiens and Rostoker [1975]. Thus, at least, during the third Pi1B a noticeable expansion of the westward electrojet from the midnight sector into the dusk one took place. According to Hayashi et al. [1988], IPDP during this substorm began at SHM and were observed from 0042 UT to 0132 UT (Figure 5b). As seen from Figure 5b, the pulsation frequency at SHM was ~0.1 Hz, and no noticeable frequency growth was observed. We classify such pulsations as Pc1-2-type pulsations [Barkova and Solovyev, 1987]. Besides that, it is also seen in Figure 5b that noticeable frequency growth of IPDP occurred at ~0050 UT at all stations from GWR to SWR (covering 40^o by longitude) simultaneously with the third onset of the Pi1B burst. Thus the appearance of IPDP at 0050 UT was delayed ~20 min relative to the enhancement beginning of the negative magnetic bay near the midnight meridian but coincided with the third substorm onset when the westward electrojet suddenly expanded toward the dusk sector up to the meridian of the CHR-OTT station chain (Figure 5a). Therefore the IPDP events during the substorm expansion phase developed like the events shown in Figures 1, 2a, and 2b of this paper.

3.1.2. Observations by Hayakawa et al. [1992].

Hayakawa et al. [1992] considered the substorm of February 25, 1985, and obtained an IPDP with the onset at ~1600 UT occurring ~30 min after the AE index increase [Hayakawa et al., 1992, Figure 3]. Our analysis of this event with the allowance for multiple substorm onsets according to Pi2 pulsation data showed that IPDP events were observed at the Iceland station chain and at the Lovozero station separated by 40^o by longitude almost simultaneously with the Pi2 onset at 1600 UT [Yumoto et al., 1994, Figure 1]. Thus the observational data by Hayakawa et al. [1992] and by Hayashi et al. [1988] agree with our observations (Figures 1, 2a, and 2b).

3.2. On Possible Cause of the Relationship of IPDP to Major Expansion Onset and IPDP Spectrum Formation

Nakamura et al. [1994] showed that the main difference of the magnetospheric and ionospheric characteristics observed during pseudobreakup and major expansion onset is the increase of the electron and proton injection region in the dusk-midnight sector from 25^o to 90^o (see their Figure 11). Hence one can suppose that the IPDP appearance simultaneously with the major substorm onset can be caused by the global proton injection directly to the dusk sector and development there of ICI [Baishev et al., 1998; Barkova and Solovyev, 1987]. The westward electrojet enhancement in the dusk sector during the substorm is usually connected with the enhancement and expansion of the substorm current wedge. As the observations (see Figures 1, 2a, and 2b) and also Baishev et al. [1998] show, during IPDP, besides the westward electrojet, also the eastward electrojet is enhanced which moves southward manifesting the development of a DP 2 two-vortex equivalent current system. Kamide and Kokubun [1996] suggested that the enhancement of the eastward electrojet is mainly associated with enhancement of the electric field intensity of large-scale magnetospheric convection during a substorm. So simultaneously with the global particle injection during the major substorm onset, perhaps, the enhancement of the convection electric field intensity takes place in the dusk sector which can lead to the drift of the injected or earlier trapped protons in the dusk sector toward the Earth. The radial proton drift or the ICI region will lead to a pulsation frequency growth and IPDP excitation.

According to Gonzales et al. [1994] the electric drift of the earlier trapped energetic protons toward the Earth in the dusk sector can be one of the refilling mechanisms of the partial ring current during a magnetic storm. A similar mechanism can also lead to the IPDP excitations which are usually registered in the intense magnetic disturbances including the magnetic storms at Kp = 3-4 and Dst -30 -80 nT (see Table 2). The enhancement of the electric field intensity and proton injection into the region of the partial ring current and their contact with the cold plasma of the plasmasphere (near the plasmapause) are favorable to ICI development and the ICI region motion to the Earth [Nishida, 1978]. The interaction of hot protons with ion-cyclotron waves leads to their precipitation to the ionospheric heights and the ionospheric conductivity increase. It is, perhaps, one of the reasons of the eastward electrojet enhancement and the motion of its center to the equator (Figures 1 and 3). The IPDP relationship with the enhancement of the eastward electrojet due to the proton precipitations at latitudes of ICI development during a magnetic storm was already reported by Baishev and Solovyev [1994].

The ICI region motion to the Earth leads to the frequency growth, that is, to IPDP excitation with a frequency growth rate proportional to the temporal change of the electric field intensity in the instability development region (Figure 4). The electric field intensity of large-scale convection at higher latitudes (in comparison with the plasmapause location) probably remained constant during the development of the December 15, 1995, substorm (Figure 4), since the southward velocity of the motion of the discrete arcs and HLBDA did not appreciably change ( V 0.25 km s ^-1 , see Figure 4b). However, the velocity of LLBDA, which is projected onto the plasmapause latitudes [Feldshtein and Galperin, 1996], decreased from 0.15 to 0.005 km s ^-1 (Figure 4c). That can be due to the shielding effect of the convection electric field in this region because of the Alfvén layer generation [Chen, 1970; Nishida, 1978]. The decrease of the electric field intensity at latitudes of ICI development and of the drift velocity of ICI region to the Earth evidently leads to the gradual reduction of the pulsation frequency growth rate and formation of IPDP with nonmonotonic spectrum character (Figure 4d). The presence of IPDP spectra with constant rate of frequency growth (Figures 1, 2a, and 2b) shows that the shielding effect of the convection electric field perhaps takes place not in all substorms and appears with some delay after the substorm expansion phase onset.

The obtained results agree with the scenario of IPDP spectrum formation suggested by Barkova and Solovyev [1984, 1987]. According to it, the main cause for frequency growth is the radial drift of the source or the ICI region toward the Earth as a result of direct proton injection into the ring current region in the dusk sector and (or) the enhancement of the electric field intensity of the large-scale convection during the substorm expansion phase. This scenario (mechanism 1) easily explains all the known IPDP pecularities including the IPDP spectra with monotonic and nonmonotonic character. Zolotukhina [1982] noted that the monotonic character of IPDP spectrum (Figures 1, 2a, and 2b) is difficult to explain by the azimuthal drift of the IPDP source (mechanism 2). We do not exclude the possibility of partial ring current enhancement in the dusk sector of the magnetosphere due to the azimuthal drift of energetic protons and ICI development under their interaction with the cold plasma of the plasmasphere near the plasmapause. However, we think that these effects lead to emission excitation in the range of Pc1-2 without any noticeable increase or decrease of the pulsation frequency which are usually observed in the afternoon sector during a substorm [Barkova and Solovyev, 1987].

4. Conclusions

One can summarize the results obtained in the following way:

1. It is shown that the beginning of the IPDP excitation or ICI development occurs simultaneously with one of multiple substorm onsets (more often with the second and the third ones) and is accompanied by a sharp expansion of the westward electrojet from the midnight sector into the dusk one; that is, IPDP are excited during a major substorm onset.

2. It is established that the IPDP excitation occurs during the equatorward drift of discrete and diffuse auroral forms and the eastward electrojet center in the dusk sector. The decrease of the frequency growth rate in IPDP series is accompanied by a decrease of the equatorward drift velocity of the low-latitudinal boundary of diffuse aurora, the fact meaning that the electric field intensity at latitudes of ICI development decreases.

3. It follows from the above results that the main contribution to the IPDP spectrum formation is provided by the radial drift of the pulsation source or the ICI region to the Earth as a result of the direct proton injection into the ring current region in the dusk sector and (or) the enhancement of the convection electric field during the major substorm onset.

Acknowledgments

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