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Relationship between auroral electrojet and Pc5 ULF waves
Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1545–1557
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Relationship between auroral electrojet and Pc5 ULF waves V.A. Pilipenkoa; b; ∗ , J. Watermanna , V.A. Popova; c , V.O. Papitashvilia; d a Danish
Meteorological Institute, Copenhagen, DK-2100, Denmark of the Physics of the Earth, B. Gruzinskaya 10, Moscow 123995, Russia c Institute of Terrestrial Magnetism and Radiowave Propagation, Troitsk 142092, Russia d University of Michigan, Ann-Arbor, MI 48109-2143, USA b Institute
Received 10 July 2000; accepted 2 February 2001
Abstract Applying a new data visualization technique to magnetic 6eld observations from the Greenland west coast array we observe that Pc5 wave power spatial=temporal variations in the morning=pre-noon sector are closely related to the location and intensity of the auroral electrojet. This e8ect is not taken into account by existing theories of ULF Pc5 waves, but it could be signi6cant for the development of more adequate models. Consideration of the most evident interpretation schemes shows that there is no simple explanation of this e8ect. An adequate interpretation may require a substantial revision or augmentation of existing c 2001 Published by Elsevier Science Ltd. Pc5 models. Keywords: ULF waves; Auroral electrojet; Ionosphere–magnetosphere interaction
1. Introduction At high latitudes, there are two well-known electrodynamic phenomena which are apparently unrelated: the auroral electrojet (AEJ) and ULF Pc5 pulsations. The AEJ belongs to the ionospheric part of a 3-D current system driven by solar wind-magnetosphere interaction. The AEJ is a latitudinally con6ned Hall current which intensi6es during substorms. Its predominant @ow direction is either westward or eastward. Geomagnetic pulsations in the Pc5 band (f 1:5–7 mHz) are probably the most easily observed ULF waves. Due to their large amplitudes (up to some 100 nT) and long periods (several minutes) Pc5 pulsations can even be detected in magnetograms with low sensitivity and low sampling rate, e.g., 1 min. As Pc5 pulsations re@ect powerful wave processes in geospace they can be observed easily in space (with ∗ Corresponding author. Institute of the Physics of the Earth, B. Gruzinskaya 10, Moscow 123995, Russia. Tel.: +7-095-2544290; fax: +7-095-2556040. E-mail address: [email protected] (V.A. Pilipenko).
@ux-gate magnetometers, electric probes, particle detectors), in the ionosphere (radars, riometers, auroral imagers), and on the ground (magnetometers and telluric probes). Despite a long history of studies into their physical nature and excitation mechanism, their physics is not yet fully understood. Probably, several mechanisms contribute to Pc5 generation in the magnetosphere. Candidate mechanisms which may explain the observations are discussed below. The peculiar spatial amplitude-phase structure of Pc5 waves which has been observed in the morning sector (Samson, 1972; Walker et al., 1979; Ziesolleck et al., 1994) agrees well with predictions from resonant theory, suggesting that these waves are produced by localized Alfven oscillations in the magnetospheric Alfven resonator (AR) resonantly excited by MHD disturbances from remote parts of the magnetosphere. According to this notion the position of the Pc5 amplitude peak corresponds to a latitude where the local Alfven frequency, fA (L), matches the frequency, f, of an external disturbance, i.e. f fA . We note that the latitudes where the AEJ intensity and the Pc5 power reach peak magnitudes are determined by entirely di8erent processes and are not necessarily directly
c 2001 Published by Elsevier Science Ltd. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 1 ) 0 0 0 3 1 - 1
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related to each other. However, Lam and Rostoker (1978) observed the positions of westward AEJ and total Pc5 power in the morning sector in the same latitude range. The Pc5 intensity maximum and the position shifted in latitude in concert such that the Pc5 peak remained within the borders of the AEJ which was ∼8◦ wide. In an accompanying paper, Rostoker and Lam (1978) proposed an interpretation of this relationship, suggesting that Pc5 waves are the eigenmodes of a 3-D magnetosphere– ionosphere current system. Since then, the subject of a possible relationship between AEJ and Pc5 and its signi6cance to generation mechanisms of ULF waves in the magnetosphere has not been investigated further. In this paper we discuss the possibility of coupling between AEJ and Pc5 by examining several typical cases. We developed a new tool for the visualization of latitudinal structure of AEJ and Pc5 which allows to examine the dynamics of AEJ and ULF activity in greater detail as compared to the seminal study of Lam and Rostoker (1978). We 6nally consider the theoretical signi6cance of these observations. 2. Modeling of equivalent ionospheric currents from meridional magnetometer chain data Various approaches have been taken in order to reconstruct the ionospheric current system from ground magnetic observations. Kisabeth and Rostoker (1971) proposed a method with a priori known types of meridional intensity pro6les to calculate magnetic e8ects from extended (along the geomagnetic latitude) ionospheric currents. Colqui et al. (1998) 6tted the current density distribution of the equatorial electrojet to a set of simultaneously recorded ground magnetic 6eld data. Kotikov et al. (1987) developed an inverse scheme for the determination of the 6ne structure of the AEJ utilizing a series of evenly distributed linear ionospheric currents of various intensities to match ground observational data. Popov and Feldstein (1996) further developed the inversion scheme by approximating the AEJ by a series of narrow sheet current strips of various intensities lying side-by-side at a 6xed altitude across the range of geomagnetic latitudes covered by a meridional chain of magnetometers. In a recent paper, Popov et al. (2001) improved upon that scheme by taking the contribution from induced telluric currents into account. A brief description of that technique, which is also used in our paper, follows below. The observed magnetic 6eld disturbances are considered to be a superposition of external and internal parts. The contribution from induced ground currents can be removed from the observed 6elds if the subsurface electrical conductivity distribution is two-dimensional, i.e., it depends only on latitude and depth. It is assumed that the remaining magnetic disturbance is produced by magnetically east-west oriented Hall currents. For a homogeneous ionosphere the ground magnetic perturbation of 6eld-aligned currents is negligi-
ble for a near-vertical geomagnetic 6eld (Tamao, 1986). The range of geomagnetic latitudes covered by a meridional chain of L magnetometers is divided into N longitudinally in6nite current strips, each of them of width 2l and carrying an individual current density ji which is constant across the strip. Using the Biot-Savart law the total magnetic 6eld disturbance at the kth station caused by all available strips can be calculated as follows: Hk =
N xik + l 1 xik − l ji arctan − arctan 2 i=1 h h
Zk =
2 N 1 h + (xik + l)2 ji ln 2 4 i=1 h + (xik − l)2
k = 1; : : : ; L;
(1)
where xik is a distance along the meridian between the kth station and the ground projection of the ith current strip, and h is the height of ionospheric current. Knowing the magnetic 6eld disturbances at L points, the inverse problem of the restoration of current intensities in all N current strips is ill-posed if N ¿ L. It can be solved by a regularization method (Tikhonov and Arsenin, 1977). The above-mentioned procedure can be applied separately to the H and Z magnetic 6eld components. If all assumptions were ful6lled, the H -based and Z-based current distributions would be identical. In reality, this is not always the case. The di8erence between the integrated currents obtained from the horizontal magnetic 6eld and those obtained quasi-independently from the vertical component is used as a quality assessment. If this di8erence is not small compared to the total current intensity the model assumptions must have been violated. In order to avoid confusion with the declination, usually termed “D” and measured in degrees, we denote the magnetic east component, measured in nanoTesla, by “E”.
3. Simultaneous analysis of dynamics of the AEJ and ULF waves We developed a code to overlay simultaneously observed AEJ distribution and ULF pulsation activity patterns from Greenland west coast magnetometer data in a corrected geomagnetic (CGM) latitude versus UT diagram. The Greenland stations cover a wide range of latitudes from the polar cap across the auroral zone. The same raw data are used to infer quasi-static variations with periods of about 15 min and longer for feeding the electrojet model, and to estimate ULF wave power in the Pc5 band using periods of less than some 10 min. Proper modeling of the AEJ depends critically on well-de6ned magnetic 6eld reference levels for all stations, and much e8ort is spent on determining those reference levels from quiet days. The integrated spectral power in the Pc5 range is calculated using a running time window (width 30 min). The synoptic view of AEJ and ULF
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pulsation activity has the potential to yield new insight into electrojet=ULF wave coupling. 4. Examples of electrojet=Pc5 coincidence We present several typical events, selected rather arbitrarily, to demonstrate the occurrence of the AEJ–ULF coupling in the morning=pre-noon sector. Local magnetic midnight at the Greenland West Coast occurs around 02 : 30 UT and magnetic noon around 14 : 30 UT. 4.1. Recovery phase of a magnetic storm: event 97-01-12 Monochromatic Pc5 waves are often observed two to three days after the onset of a major magnetic storm (Schott
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et al., 1998). This event occurred during the recovery phase of a magnetic storm which commenced on January 10, 1997. This was one of the storms selected for extensive study by the GEM community. H component magnetograms recorded from 07:00 – 14 : 00 UT on January 10, 1997 are shown in Fig. 1. At ∼08 : 00 UT (∼05 : 30 MLT) a substorm expansive phase begins. The disturbance reaches a maximum of MB 700 nT, at about 08 : 30 UT (∼06 MLT). The substorm recovery phase begins at ∼09 UT. After 09 : 30 UT monochromatic Pc5 pulsations start and last until ∼12 : 30 UT (at STF). The peak-to-peak amplitude of the Pc5 waves reaches 100 nT at STF. Even without performing detailed spectral analysis the resonant meridional structure of Pc5 waves is evident: decrease of the apparent wave period with decreasing latitude
Fig. 1. Magnetogram of 97=01=12 (Day 012) of the magnetic northward (H ) component from stations along the Greenlandic west coast.
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Fig. 2.
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Fig. 3. Magnetogram of 99=02=14 (Day 045) of the H component from the Greenland west coast chain.
(cf. STF and GHB), and weak response in the E component (not shown) as compared to the H and Z components. Spectral analysis reveals that the main spectral power density maximum is latitude dependent and concentrated in the bandwidth ∼2:5–4:5 mHz throughout the meridional pro6le. ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Fig. 2. Time evolution of the equivalent ionospheric current (color coded) for the event 97=01=12, 06 –14 UT, reconstructed from the H (upper panel) and Z (center panel) magnetic 6eld components. Contour lines indicate di8erent levels of the total spectral power of ULF waves in the 2–7 mHz frequency band, estimated for the magnetic northward (H , upper panel) and eastward (E, center panel) components. The bottom panel shows the total absolute current and the di8erence in total absolute currents reconstructed from the H and Z components.
To examine simultaneously the dynamics of the ionospheric currents and ULF waves we have superimposed in Fig. 2 the time evolution of the ULF spectral power in the band 2–7 mHz and the AEJ, reconstructed from H and Z magnetic 6eld components. The ULF power is indicated by contour lines whose thickness increases with increasing power; the ionospheric current intensity is color coded. The 6gure shows that the westward AEJ intensi6es 6rst at ∼68◦ CGM latitude and then quickly expands to higher latitudes, up to 79◦ , with its center at ∼73◦ . The apparent intensi6cation of the eastward AEJ between 08 and 09 UT at the poleward and equatorward boundaries of the chain (middle panel) is related to the sensitivity of the method to conditions at the boundaries of the chain. The total westward equivalent ionospheric current reaches a maximum of ∼800 kA (solid line in bottom panel of Fig. 2). The
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di8erence between the latitude-integrated currents obtained from the H component and those obtained from the Z component (dashed line), which may be considered a proxy for the accurateness of the model, is negligible compared to the total current. The east–west orientation of the AEJ, presumed in the model, is consistent with observations. The 6rst ULF intensity intensi6cation with peak at ∼08 : 30 UT evidently corresponds to a burst of irregular quasi-periodic variations during local substorm onset. This burst occupies a wide range of latitudes and is somewhat more intense in the E component than in the H component. A successive enhancement of the ULF spectral power (peak at ∼10 UT) re@ects monochromatic Pc5 pulsations. This Pc5 activity is most intense in a narrow latitude range around 73◦ , and the wave power in the H component exceeds that in the E component. Fig. 2 demonstrates that the Pc5 intensity peak at 73◦ coincides with the westward AEJ, more precisely, with its equatorward edge. 4.2. Moderately disturbed geomagnetic conditions: event 99-02-14 The second event is a typical example of morning Pc5 pulsations under moderately disturbed geomagnetic conditions. The magnetic disturbance started after 04 UT (01 : 30 MLT) and lasted until about 18 UT (Fig. 3). At 10 – 11 UT the negative bay has its deepest de@ection with about ∼ − 300 nT which means a westward AEJ intensi6cation. As compared with the magnetic storm event January 12, 1997, the AEJ intensi6cation is latitudinally more localized and has lower current intensity, ∼300 kA. From ∼12 UT the negative magnetic disturbance gives way to a positive bay. The intensi6cation of the AEJ is accompanied by an increase of ULF activity, which reached Pc5 peak-to-peak amplitude of 60 nT. Spectral analysis shows that the peak power of ULF pulsations falls, at all stations, into the frequency range 2–6 mHz. The spectral power was then calculated separately for H and Z components using the frequency range, 2–7 mHz. Superimposed AEJ–ULF intensity distributions are shown in Fig. 4 where the upper and center panels refer to AEJ and Pc5 computed from the northward and vertical magnetic 6eld, respectively. This 6gure shows an enhanced westward AEJ from 09 till 12 UT which is centered at ∼72◦ CGM latitude. Soon after, between 12 and 18 UT, i.e. centered around magnetic local noon, an eastward AEJ develops at slightly higher latitude, 73–74◦ . Fig. 4 demonstrates that ULF power enhances concurrently with the westward AEJ intensi6cation. The intensi6cation of the eastward AEJ is also accompanied by increase in ULF power, but with smaller peak intensity. Resonant features of ULF waves are known to appear predominantly in the H and Z components. In fact, ULF wave power in both these components (upper and middle panels) intensi6es near the center or equatorward edge of the AEJ.
4.3. Weakly disturbed geomagnetic conditions: event 95-12-26 The third event exhibits a more complicated time-varying spectral structure of Pc5 pulsations (Fig. 5). The enhancement of monochromatic Pc5 (see GHB) is also in this case stimulated by a negative bay with magnitude MB −200 nT. Both H (Fig. 5a) and Z (Fig. 5b) components of Pc5 have nearly the same peak-to-peak amplitudes, 100 nT. Spectral analysis reveals latitude-dependent peaks between 2.0 and 3:5 mHz. In order to get total power estimates we have the spectral density integrated in the 2–7 mHz range. Fig. 6 indicates that a weak (∼200 kA) westward AEJ expands from latitude 72◦ at 08 : 40 UT to 72–74◦ at 10 UT. The dynamic plot also shows that the morning Pc5 wave trains all coincide spatially and temporally with the equatorward section of the westward AEJ. An eastward AEJ intensi6cation of ∼100 kA at ∼15 UT and then again from ∼16 : 30 UT onward, also stimulates ULF response but with a considerably lower intensity than the westward AEJ does. 5. Possible generation mechanisms of Pc5 Occurrence rate and intensity of Pc5 waves have a primary maximum in the morning sector and a weaker secondary in the afternoon sector. The azimuthal phase propagation and the polarization features of the ground magnetic disturbance in the horizontal plane change across the noon meridian (Samson, 1972). In earlier work, these observations led to the conclusion that a Kelvin–Helmholtz (KH) instability at the magnetopause (Kivelson and Pu, 1984; Mishin and Matukhin, 1986) or at the LLBL (Yumoto and Saito, 1980) is a likely candidate. Later, indications were found that impulsive dynamic pressure variations of the solar wind and @ux transfer events (FTE) possibly constitute a source of Pc5 wave packets in the magnetosphere (Nopper et al., 1982; Rostoker and Sullivan, 1987). Wave disturbances generated by dayside variations of the solar wind pressure propagate tailward along both sides of the magnetosphere, thus producing azimuthal phase velocity pattern across the noon meridian similar to the KH instability. Amplitude and phase spatial distributions of Pc5 clearly demonstrate resonance properties (Saka et al., 1982; Ziesolleck and McDiarmid, 1994) which are best seen in the morning hours. The position of the amplitude maximum depends on frequency (frequency increases towards low L values). The meridional phase velocity in the vicinity of the amplitude maximum is poleward directed. The existence of resonance e8ects for Pc5 geomagnetic pulsations points to a wave mechanism of energy transfer from an external source toward localized 6eld line Alfven oscillations. This short summary of current notions on Pc5 generation mechanisms does not indicate that a spatial-temporal
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Fig. 4. The event 99=02=14 04 –19 UT (Day 045) in the same format as Fig. 2.
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relation exists between the auroral current structure and ULF wave occurrence. The three sample events described above, with AEJ intensi6cation under di8erent geophysical situations, suggest that ULF wave activity is associated with the AEJ. The AEJ may, in fact, play an active role in Pc5 excitation. Ground-based magnetometers have a limited spatial resolution, ∼100 km in the ionosphere. When mapping magnetic @ux from the high-latitude ionosphere to the equatorial plane, two phenomena which occur in magnetospheric boundary layers may appear to be spatially collocated in the ionosphere although their sources may not actually spatially coincide. A better resolved mapping of magnetospheric boundaries to the ionosphere can be obtained from low altitude satellites. Solely from the data available to us we cannot claim that generation of FAC and Pc5 pulsations occur in the same magnetospheric domain. We rather discuss
the most evident interpretation schemes of AEJ-Pc5 coupling which were brought forward in the past, and attempt to evaluate them in the light of our observations. 6. Energetic electron injection into the morning sector ionosphere The magnetic 6eld depression often preceding or accompanying Pc5 activity in the early morning hours may indicate that the ULF wave generation mechanism is related to energetic electron injection. Electron precipitation can increase the ionospheric conductivity and thus enhance the AEJ intensity. Moreover, there are observations showing that morning time Pc5 pulsations might be associated with increased @uxes of energetic electrons in the magnetosphere (Saka et al., 1992).
Fig. 5. Magnetograms of the (a) H component, and (b) Z component during the event of 95=12=26 (Day 360), 07–20 UT.
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Fig. 5. (continued).
However, diPculties arise when attempting to interpret morning Pc5 pulsations with this mechanism. It helps to recall the properties of oscillations in the same frequency band in the afternoon sector. The spatial structure of afternoon ULF pulsations has been examined with STARE and the geostationary GOES-2 satellite (Allan and Poulter, 1984; Walker et al., 1979, 1982). They fall into two di8erent categories. The 6rst of them, called “storm-time” Pc5 (Bar6eld and McPherron, 1972), is often observed at geostationary orbit and is closely connected with intense @uxes of ring current protons. Oscillations propagate predominantly in azimuthal direction, and no clear meridional wave structure is observed. Their transverse wavelengths are small and, correspondingly, the azimuthal wave numbers are large, m 30–50. The generation mechanism of this class of
Pc5 waves is connected with low frequency drift instabilities (see review by Pilipenko (1990) for references). A small lateral scale length of oscillations is a necessary condition for the e8ective interaction between low frequency waves and energetic particles, because in this case the parameter k⊥ (i.e. the ratio of a Larmor radius to a lateral wave length) is 6nite. Oscillations of this type can hardly be recorded by ground based magnetometers (Allan et al., 1983) because high-m oscillations are e8ectively screened by the ionosphere. Oscillations of the other type are caused by impulsive solar wind disturbances and form a sequence of wave packets. The typical resonance structure occurs in the meridional distribution of the wave amplitude, evidencing the transformation of energy from an external source into 6eld line Alfven oscillations. Due to their large lateral scale length, these
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Fig. 6. The event 95=12=26 (Day 360), 07–20 UT, in the same format as Fig. 2.
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oscillations are only weakly screened by the ionosphere and clearly seen in both, satellite and ground records. One might envisage a similar picture for morning Pc5 pulsations, and observations of Saka et al. (1992) indeed suggest that they might be associated with increased @uxes of energetic electrons in the magnetosphere. However, the assumption that substorm-related morning Pc5 pulsations are generated by resonant instabilities of energetic electrons contradicts ground based observations which show low m-values and, respectively, large lateral scale lengths of these oscillations (Olson and Rostoker, 1978). Moreover, no oscillations with large m values in the morning sector were reported to be regularly observed with satellites.
6.1. Oscillations of a three-dimensional current system
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6.2. Enhanced conductivity in the region of AEJ The ionospheric conductivity in the region of AEJ is increased compared to the background ionosphere, due to electron precipitation in the region of upward FAC. Observations by Sutcli8e and Rostoker (1979) suggest an increase of Pc5 pulsation intensity across the dawn terminator, suggesting that the ionospheric conductivity plays an important role in the Pc5 excitation process. When developing this idea further, the following probable mechanism could be suggested. Permanent background ULF disturbances in the magnetosphere do not develop into prominent resonant oscillations because of severe ionospheric damping. An increase of the ionospheric conductance, which occurs just in the region of the AEJ, would stimulate ULF activity. Accordingly, a decrease of the ionospheric conductivity and associated AEJ intensity would quench ULF activity. However, a quantitative estimate shows that the background Pedersen conductance is high enough at auroral latitudes in the morning hours as compared with the effective magnetospheric conductance. Typical numbers are P 5 –10 S and A 1 S, respectively. Alfven wave re@ection at the high-latitude ionosphere is consequently strong, and excessive damping does not prevent the excitation of magnetospheric AR.
A possible mechanism for coupling between Pc5 and the AEJ was described by Rostoker and Lam (1978) who interpreted Pc5 pulsations as oscillations of a three-dimensional current system which involves magnetospheric plasma and 6eld-aligned and ionospheric currents. The electrical parameters of this circuit, i.e. resistance of the ionosphere, capacity of the convective plasma @ow and inductance of 6eld-aligned currents, determine the oscillation regime of the e8ective LRC circuit. Following Rostoker and Lam (1978), we argue that the eigenfrequency of oscillations of this three-dimensional current system turns out to be the common Alfven frequency !A . Let us consider the following equivalent circuit. The plasma volume under consideration in the magnetosphere has a length l along the 6eld line, width b in radial direction, and the azimuthal scale w. The inductance of the volume, uniformly 6lled with a 6eld-aligned current density j, can be estimated via the total magnetic energy stored, Wm = (B2 =20 ) dV = ((B2 =20 )wbl. Comparing it with the elementary formula for the magnetic energy of a current sheet with total current I = jbw, i.e. Wm = Lm (I 2 =2), we obtain Lm = 0 (bl=w). In the same way we estimate the magnetospheric plasma capacitance Cm as a measure of kinetic energy of oscillating plasma particle motion with velocity VE . Equating Wm = (VE2 =2) dV = (=2)(E=B)2 wbl with Wm = Cm (U 2 =2), where U = Eb is electric potential, we get Cm = (=B2 )(wl=b). The resistance R of the ionosphere produces the Joule dissipation of 6eld line oscillations. Finally, we can estimate the expected resonant frequency of this LC circuit to be
Magnetic noise can be produced via the modulation—or self-modulation—of precipitating particles which transport FAC. Variations of particle @uxes cause variations of the ionospheric conductivity thus eventually producing magnetic noise at the ground. This mechanism was suggested for the interpretation of broad-band pulsations at dayside cusp latitudes (Engebretson et al., 1991). One may assume that morning Pc5 pulsations are also caused by the modulation of the @ux, F, of precipitating particles. At high latitudes particle precipitation is the main source for ionization q and enhancement of the conductivity of the ionospheric E layer. The electron density variation, Mn, induced by small periodic variations of the ionization rate Mq with frequency !, can be estimated with the help of the ionization balance equation. Supposing that the ionosphere is an in6nite thin current layer, one can estimate the magnetic 6eld disturbance produced by the ionospheric electric 6eld Eo . The @ux @uctuations MF produce a magnetic disturbance b on top of the quasi-stationary background disturbance MB according to the following relation.
! = (Lm Cm )−1=2 = VA =l:
|b=MB| |M = | |Mn=n|
(2)
Therefore, the mechanism of 3-D current system oscillations cannot be considered an alternative hypothesis but appears to be just another representation of the 6eld line resonance theory.
6.3. FAC =uctuations
(Mq=2q)(1 + !2 $2r )−1=2 ;
(3)
where $r = 1=(2&n) is the typical recombination time determined by the recombination coePcient &. Under the assumption of typical numbers, & 2 × 10−7 cm3 s−1 and
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n 2 × 105 cm−3 , it follows from (3) that ground magnetic signals in the Pc5 band (! 0:01 s−1 ), with relative amplitudes b=MB 0:1 can be produced by oscillating particle @uxes with a modulation MF=F = Mq=q 0:2. If the background ionospheric density n is not high, that is !$r ¿ 1, the amplitude of magnetic variations, produced by the @ux modulation with depth Mq=q, should linearly increase with increasing background precipitation level, that is, b ˙ n(Mq=q), according to observations by Olson et al. (1980). This means that, in principle, modulated precipitation cannot be excluded as Pc5 generation mechanism. However, a suitable periodic modulation factor remains to be identi6ed. Another diPculty with this interpretation exists: a @uctuating ionospheric current should produce synchronous horizontal and vertical magnetic variations on the ground: the H component should be in-phase along the meridian, while the Z component should be out-of-phase at both sides of the ionospheric current. A more close examination of the Pc5 magnetograms shown in Figs. 1 and 5 reveals, however, that the magnetic H and Z components indicate an apparent poleward phase propagation. Such phase characteristics are consistent with a resonant model which is based on the assumption of local excitation of magnetospheric AR by external MHD disturbances. It contradicts the idea of modulated ionospheric currents.
6.4. Feedback excitation of magnetospheric AR An ionospheric conductivity variation M in the presence of an external electric 6eld Eo leads to the generation of a polarization electric 6eld, which then propagates as an Alfven wave along geomagnetic 6eld lines into the magnetosphere. The magnitude of the polarization electric 6eld E can be estimated as E = − Eo
M +
A
(Mal’tsev et al., 1974). For a reasonable set of parameters, the magnitude of E is large enough to contribute signi6cantly to the stimulation of Pc5 pulsations. The tendency to excite magnetospheric AR at the location of the AEJ could possibly be related to a reverse in@uence of the ULF-modulated ionosphere on excitation forcing. The mechanism, similar to the feedback instability (Lyatsky and Mal’tsev, 1983; Lysak, 1991), could operate as follows: FAC modulation → localized variations of M → emission of E and additional FAC into the magnetosphere along the gradient of . With proper phase relations this scenario may explain an enhanced excitation rate of magnetospheric AR in the FAC region compared to other magnetospheric regions. The mechanism remains speculative as AR excitation with FAC feedback has not yet been investigated theoretically.
7. Conclusion The new visualization technique suggested here opens up the possibility to take a synoptic view on time variations not only of the spatial positions of the AEJ and the Pc5 pulsation peak, as in earlier studies, but also of their intensities. Observations from the Greenland west coast magnetometer chain con6rm the 6ndings of Lam and Rostoker (1978): Pc5 power spatial=temporal variations in the morning=prenoon sector are closely related to the location and intensity of the AEJ. In the cases presented here, Pc5 oscillations occur during the recovery phase of a previously intensi6ed AEJ. The intensi6cation of an eastward AEJ also stimulates ULF response, but with considerably lower intensity than the westward AEJ does. From the observed resonant features of the Pc5 wave structure and the relationship between ULF wave power and the AEJ intensity, it may be concluded that the location of the AEJ (predominantly its equatorward edge) is a preferred latitude for magnetospheric AR excitation by external MHD disturbances. This e8ect is not taken into account by existing theories of ULF Pc5 waves, but it might be signi6cant for development of more adequate models. Consideration of the most evident interpretation schemes shows that no simple explanation for this e8ect is known. The explanations discussed above have certain pitfalls and demand more elaborate models. An adequate interpretation may require a substantial revision or augmentation of existing Pc5 models, e.g. development of a resonant model with ionospheric feedback. Acknowledgements We thank E. Fedorov and W. Lyatsky for discussions concerning theoretical aspects of the problem. V.A. thank the Danish Meteorological Institute for supporting their visits. V.O.P. also acknowledges support from the NSF awards OPP-9614175 and OPP-9876473. We appreciate the constructive comments of both referees.
References Allan, W., Poulter, E.M., 1984. The spatial structure of di8erent ULF pulsation types: a review of STARE radar results. Reviews in Geophysics 22, 85. Allan, W., Poulter, E.M., Glassmeier, K.-H., Nielsen, E., 1983. Ground magnetometer detection of a large-m Pc5 pulsation observed with the STARE radar. Journal of Geophysical Research 88, 183. Bar6eld, J.N., McPherron, R.L., 1972. Statistical characteristics of storm-associated Pc5 micropulsations observed at synchronous equatorial orbit. Journal of Geophysical Research 77, 4720. Colqui, R., Yumoto, K., Tachihara, H., Kitamura, T.I., Trivedi, N.B., DaCosta, J.M., 1998. Two-dimensional equatorial electrojet current system deduced from the Brazilian network. I. Based on the H component. Memoirs of the Faculty of Science,
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 1545–1557 Kyushu University, Series D, Earth and Planetary Science XXX (1), 1–14. Engebretson, M.J., Cahill, L.J., Arnoldy, R.L., Anderson, B.J., Rosenberg, T.J., Carpenter, D.L., Inan, U.S., Eather, R.H., 1991. The role of the ionosphere in coupling upstream ULF wave power into the dayside magnetosphere. Journal of Geophysical Research 86, 1527–1542. Kisabeth, J.L., Rostoker, G., 1971. Development of the polar electrojet during polar magnetic substorms. Journal of Geophysical Research 76, 6815. Kivelson, M.G., Pu, Z.Y., 1984. The Kelvin–Helmholtz instability on the magnetopause. Planetary and Space Science 32, 1335. Kotikov, A.L., Latov, Yu.A., Troshichev, O.A., 1987. Structure of auroral electrojets by the data from a meridional chain of magnetic stations. Geophysica 23, 143. Lam, H.L., Rostoker, G., 1978. The relationship of Pc5 micropulsation activity in the morning sector to the auroral westward electrojet. Planetary and Space Science 26, 473. Lyatsky, V.B., Mal’tsev, Yu.P., 1983. Magnetosphere–Ionosphere Interaction, Nauka, Moscow, Russia. Lysak, R.L., 1991. Feedback instability of the ionospheric resonant cavity. Journal of Geophysical Research 96, 1553–1568. Mal’tsev, Y.P., Leontyev, S.V., Lyatsky, W.B., 1974. Pi2 pulsation as a result of evolution of an Alfven impulse originating in the ionosphere during a brightening of aurora. Planetary and Space Science 22, 1519–1533. Mishin, V.V., Matukhin, Yu.G., 1986. Kelvin–Helmholtz instability at magnetopause as possible source of wave energy in the Earth magnetosphere. Geomagnetism and Aeronomy 26, 952. Nopper, R.W., Hughes, W.J., MacLennan, C.G., McPherron, R.L., 1982. Impulse-excited pulsations during the July 29, 1977, event. Journal of Geophysical Research 87, 5911. Olson, J., Rostoker, G., 1978. Longitudinal phase variations of Pc4-5 micropulsations. Journal of Geophysical Research 83, 2481. Olson, J.V., Rostoker, G., Olchowy, G., 1980. A study of concurrent riometer and magnetometer variations in the Pc4-5 pulsation band. Journal of Geophysical Research 85, 1695–1702. Pilipenko, V.A., 1990. ULF waves on the ground and in space. Journal of Atmospheric and Terrestrial Physics 52, 1193. Popov, V.A., Feldstein, Ya.I., 1996. About a new interpretation of “Harang discontinuity”. Geomagnetism and Aeronomy 36, 43.
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Popov, V.A., Papitashvili, V.P., Watermann, J., 2001. Modeling of equivalent ionospheric currents from meridian magnetometer chain data. Earth, Planets and Space 53, 129. Rostoker, G., Lam, H.-L., 1978. A generation mechanism for Pc5 micropulsations in the morning sector. Planetary and Space Science 26, 493. Rostoker, G., Sullivan, B.T., 1987. Polarization characteristics of Pc5 magnetic pulsations in the dusk hemisphere. Planetary and Space Science 35, 429. Saka, O., Kim, J.S., Sugiura, M., 1982. A cross-spectral analysis of high-latitude Pc5 pulsations in the morning sector. Journal of Geophysical Research 87, 2331. Saka, O., Iijima, T., Yamagishi, H., Sata, N., Baker, D.N., 1992. Excitation of Pc5 pulsations in the morning sector by a local injection of particles in the magnetosphere. Journal of Geophysical Research 97, 10,693–10,701. Samson, J.C., 1972. Three-dimension polarization characteristics of high-latitude Pc5 geomagnetic micropulsations. Journal of Geophysical Research 77, 6145. Schott, J.-J., Kleimenova, N.G., Bitterly, J., Kozyreva, O.V., 1998. The strong Pc5 geomagnetic pulsations in the initial phase of the great magnetic storm of March 24, 1991. Earth, Planets and Space 50, 101–106. Sutcli8e, P.R., Rostoker, G., 1979. Dependence of Pc5 micropulsation power on conductivity variations in the morning sector. Planetary and Space Science 27, 631–642. Tamao, T., 1986. Direct contribution of oblique 6eld-aligned currents to ground magnetic 6elds. Journal of Geophysical Research 91, 183–189. Tikhonov, A.N., Arsenin, V.Ya., 1977. Solution of ill-posed problems. Halsted Press, New York. Walker, A.D.M., Greenwald, R.A., Stuart, W.F., Green, C.A., 1979. STARE auroral radar observations of Pc5 geomagnetic pulsations. Journal of Geophysical Research 84, 3373. Walker, A.D.M., Greenwald, R.A., Korth, A., Kremser, G., 1982. STARE and GEOS-2 observations of a storm time Pc5 ULF pulsation. Journal of Geophysical Research 87, 9135. Yumoto, K., Saito, T., 1980. Hydromagnetic wave driven by velocity shear instability in the magnetospheric boundary layer. Planetary and Space Science 28, 789. Ziesolleck, C.W.S., McDiarmid, D.R., 1994. Auroral latitude Pc5 6eld line resonances: quantized frequencies, spatial characteristics, diurnal variation. Journal of Geophysical Research 99, 5817.
www.elsevier.com/locate/jastp
Relationship between auroral electrojet and Pc5 ULF waves V.A. Pilipenkoa; b; ∗ , J. Watermanna , V.A. Popova; c , V.O. Papitashvilia; d a Danish
Meteorological Institute, Copenhagen, DK-2100, Denmark of the Physics of the Earth, B. Gruzinskaya 10, Moscow 123995, Russia c Institute of Terrestrial Magnetism and Radiowave Propagation, Troitsk 142092, Russia d University of Michigan, Ann-Arbor, MI 48109-2143, USA b Institute
Received 10 July 2000; accepted 2 February 2001
Abstract Applying a new data visualization technique to magnetic 6eld observations from the Greenland west coast array we observe that Pc5 wave power spatial=temporal variations in the morning=pre-noon sector are closely related to the location and intensity of the auroral electrojet. This e8ect is not taken into account by existing theories of ULF Pc5 waves, but it could be signi6cant for the development of more adequate models. Consideration of the most evident interpretation schemes shows that there is no simple explanation of this e8ect. An adequate interpretation may require a substantial revision or augmentation of existing c 2001 Published by Elsevier Science Ltd. Pc5 models. Keywords: ULF waves; Auroral electrojet; Ionosphere–magnetosphere interaction
1. Introduction At high latitudes, there are two well-known electrodynamic phenomena which are apparently unrelated: the auroral electrojet (AEJ) and ULF Pc5 pulsations. The AEJ belongs to the ionospheric part of a 3-D current system driven by solar wind-magnetosphere interaction. The AEJ is a latitudinally con6ned Hall current which intensi6es during substorms. Its predominant @ow direction is either westward or eastward. Geomagnetic pulsations in the Pc5 band (f 1:5–7 mHz) are probably the most easily observed ULF waves. Due to their large amplitudes (up to some 100 nT) and long periods (several minutes) Pc5 pulsations can even be detected in magnetograms with low sensitivity and low sampling rate, e.g., 1 min. As Pc5 pulsations re@ect powerful wave processes in geospace they can be observed easily in space (with ∗ Corresponding author. Institute of the Physics of the Earth, B. Gruzinskaya 10, Moscow 123995, Russia. Tel.: +7-095-2544290; fax: +7-095-2556040. E-mail address: [email protected] (V.A. Pilipenko).
@ux-gate magnetometers, electric probes, particle detectors), in the ionosphere (radars, riometers, auroral imagers), and on the ground (magnetometers and telluric probes). Despite a long history of studies into their physical nature and excitation mechanism, their physics is not yet fully understood. Probably, several mechanisms contribute to Pc5 generation in the magnetosphere. Candidate mechanisms which may explain the observations are discussed below. The peculiar spatial amplitude-phase structure of Pc5 waves which has been observed in the morning sector (Samson, 1972; Walker et al., 1979; Ziesolleck et al., 1994) agrees well with predictions from resonant theory, suggesting that these waves are produced by localized Alfven oscillations in the magnetospheric Alfven resonator (AR) resonantly excited by MHD disturbances from remote parts of the magnetosphere. According to this notion the position of the Pc5 amplitude peak corresponds to a latitude where the local Alfven frequency, fA (L), matches the frequency, f, of an external disturbance, i.e. f fA . We note that the latitudes where the AEJ intensity and the Pc5 power reach peak magnitudes are determined by entirely di8erent processes and are not necessarily directly
c 2001 Published by Elsevier Science Ltd. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 1 ) 0 0 0 3 1 - 1
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related to each other. However, Lam and Rostoker (1978) observed the positions of westward AEJ and total Pc5 power in the morning sector in the same latitude range. The Pc5 intensity maximum and the position shifted in latitude in concert such that the Pc5 peak remained within the borders of the AEJ which was ∼8◦ wide. In an accompanying paper, Rostoker and Lam (1978) proposed an interpretation of this relationship, suggesting that Pc5 waves are the eigenmodes of a 3-D magnetosphere– ionosphere current system. Since then, the subject of a possible relationship between AEJ and Pc5 and its signi6cance to generation mechanisms of ULF waves in the magnetosphere has not been investigated further. In this paper we discuss the possibility of coupling between AEJ and Pc5 by examining several typical cases. We developed a new tool for the visualization of latitudinal structure of AEJ and Pc5 which allows to examine the dynamics of AEJ and ULF activity in greater detail as compared to the seminal study of Lam and Rostoker (1978). We 6nally consider the theoretical signi6cance of these observations. 2. Modeling of equivalent ionospheric currents from meridional magnetometer chain data Various approaches have been taken in order to reconstruct the ionospheric current system from ground magnetic observations. Kisabeth and Rostoker (1971) proposed a method with a priori known types of meridional intensity pro6les to calculate magnetic e8ects from extended (along the geomagnetic latitude) ionospheric currents. Colqui et al. (1998) 6tted the current density distribution of the equatorial electrojet to a set of simultaneously recorded ground magnetic 6eld data. Kotikov et al. (1987) developed an inverse scheme for the determination of the 6ne structure of the AEJ utilizing a series of evenly distributed linear ionospheric currents of various intensities to match ground observational data. Popov and Feldstein (1996) further developed the inversion scheme by approximating the AEJ by a series of narrow sheet current strips of various intensities lying side-by-side at a 6xed altitude across the range of geomagnetic latitudes covered by a meridional chain of magnetometers. In a recent paper, Popov et al. (2001) improved upon that scheme by taking the contribution from induced telluric currents into account. A brief description of that technique, which is also used in our paper, follows below. The observed magnetic 6eld disturbances are considered to be a superposition of external and internal parts. The contribution from induced ground currents can be removed from the observed 6elds if the subsurface electrical conductivity distribution is two-dimensional, i.e., it depends only on latitude and depth. It is assumed that the remaining magnetic disturbance is produced by magnetically east-west oriented Hall currents. For a homogeneous ionosphere the ground magnetic perturbation of 6eld-aligned currents is negligi-
ble for a near-vertical geomagnetic 6eld (Tamao, 1986). The range of geomagnetic latitudes covered by a meridional chain of L magnetometers is divided into N longitudinally in6nite current strips, each of them of width 2l and carrying an individual current density ji which is constant across the strip. Using the Biot-Savart law the total magnetic 6eld disturbance at the kth station caused by all available strips can be calculated as follows: Hk =
N xik + l 1 xik − l ji arctan − arctan 2 i=1 h h
Zk =
2 N 1 h + (xik + l)2 ji ln 2 4 i=1 h + (xik − l)2
k = 1; : : : ; L;
(1)
where xik is a distance along the meridian between the kth station and the ground projection of the ith current strip, and h is the height of ionospheric current. Knowing the magnetic 6eld disturbances at L points, the inverse problem of the restoration of current intensities in all N current strips is ill-posed if N ¿ L. It can be solved by a regularization method (Tikhonov and Arsenin, 1977). The above-mentioned procedure can be applied separately to the H and Z magnetic 6eld components. If all assumptions were ful6lled, the H -based and Z-based current distributions would be identical. In reality, this is not always the case. The di8erence between the integrated currents obtained from the horizontal magnetic 6eld and those obtained quasi-independently from the vertical component is used as a quality assessment. If this di8erence is not small compared to the total current intensity the model assumptions must have been violated. In order to avoid confusion with the declination, usually termed “D” and measured in degrees, we denote the magnetic east component, measured in nanoTesla, by “E”.
3. Simultaneous analysis of dynamics of the AEJ and ULF waves We developed a code to overlay simultaneously observed AEJ distribution and ULF pulsation activity patterns from Greenland west coast magnetometer data in a corrected geomagnetic (CGM) latitude versus UT diagram. The Greenland stations cover a wide range of latitudes from the polar cap across the auroral zone. The same raw data are used to infer quasi-static variations with periods of about 15 min and longer for feeding the electrojet model, and to estimate ULF wave power in the Pc5 band using periods of less than some 10 min. Proper modeling of the AEJ depends critically on well-de6ned magnetic 6eld reference levels for all stations, and much e8ort is spent on determining those reference levels from quiet days. The integrated spectral power in the Pc5 range is calculated using a running time window (width 30 min). The synoptic view of AEJ and ULF
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pulsation activity has the potential to yield new insight into electrojet=ULF wave coupling. 4. Examples of electrojet=Pc5 coincidence We present several typical events, selected rather arbitrarily, to demonstrate the occurrence of the AEJ–ULF coupling in the morning=pre-noon sector. Local magnetic midnight at the Greenland West Coast occurs around 02 : 30 UT and magnetic noon around 14 : 30 UT. 4.1. Recovery phase of a magnetic storm: event 97-01-12 Monochromatic Pc5 waves are often observed two to three days after the onset of a major magnetic storm (Schott
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et al., 1998). This event occurred during the recovery phase of a magnetic storm which commenced on January 10, 1997. This was one of the storms selected for extensive study by the GEM community. H component magnetograms recorded from 07:00 – 14 : 00 UT on January 10, 1997 are shown in Fig. 1. At ∼08 : 00 UT (∼05 : 30 MLT) a substorm expansive phase begins. The disturbance reaches a maximum of MB 700 nT, at about 08 : 30 UT (∼06 MLT). The substorm recovery phase begins at ∼09 UT. After 09 : 30 UT monochromatic Pc5 pulsations start and last until ∼12 : 30 UT (at STF). The peak-to-peak amplitude of the Pc5 waves reaches 100 nT at STF. Even without performing detailed spectral analysis the resonant meridional structure of Pc5 waves is evident: decrease of the apparent wave period with decreasing latitude
Fig. 1. Magnetogram of 97=01=12 (Day 012) of the magnetic northward (H ) component from stations along the Greenlandic west coast.
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Fig. 2.
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Fig. 3. Magnetogram of 99=02=14 (Day 045) of the H component from the Greenland west coast chain.
(cf. STF and GHB), and weak response in the E component (not shown) as compared to the H and Z components. Spectral analysis reveals that the main spectral power density maximum is latitude dependent and concentrated in the bandwidth ∼2:5–4:5 mHz throughout the meridional pro6le. ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Fig. 2. Time evolution of the equivalent ionospheric current (color coded) for the event 97=01=12, 06 –14 UT, reconstructed from the H (upper panel) and Z (center panel) magnetic 6eld components. Contour lines indicate di8erent levels of the total spectral power of ULF waves in the 2–7 mHz frequency band, estimated for the magnetic northward (H , upper panel) and eastward (E, center panel) components. The bottom panel shows the total absolute current and the di8erence in total absolute currents reconstructed from the H and Z components.
To examine simultaneously the dynamics of the ionospheric currents and ULF waves we have superimposed in Fig. 2 the time evolution of the ULF spectral power in the band 2–7 mHz and the AEJ, reconstructed from H and Z magnetic 6eld components. The ULF power is indicated by contour lines whose thickness increases with increasing power; the ionospheric current intensity is color coded. The 6gure shows that the westward AEJ intensi6es 6rst at ∼68◦ CGM latitude and then quickly expands to higher latitudes, up to 79◦ , with its center at ∼73◦ . The apparent intensi6cation of the eastward AEJ between 08 and 09 UT at the poleward and equatorward boundaries of the chain (middle panel) is related to the sensitivity of the method to conditions at the boundaries of the chain. The total westward equivalent ionospheric current reaches a maximum of ∼800 kA (solid line in bottom panel of Fig. 2). The
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di8erence between the latitude-integrated currents obtained from the H component and those obtained from the Z component (dashed line), which may be considered a proxy for the accurateness of the model, is negligible compared to the total current. The east–west orientation of the AEJ, presumed in the model, is consistent with observations. The 6rst ULF intensity intensi6cation with peak at ∼08 : 30 UT evidently corresponds to a burst of irregular quasi-periodic variations during local substorm onset. This burst occupies a wide range of latitudes and is somewhat more intense in the E component than in the H component. A successive enhancement of the ULF spectral power (peak at ∼10 UT) re@ects monochromatic Pc5 pulsations. This Pc5 activity is most intense in a narrow latitude range around 73◦ , and the wave power in the H component exceeds that in the E component. Fig. 2 demonstrates that the Pc5 intensity peak at 73◦ coincides with the westward AEJ, more precisely, with its equatorward edge. 4.2. Moderately disturbed geomagnetic conditions: event 99-02-14 The second event is a typical example of morning Pc5 pulsations under moderately disturbed geomagnetic conditions. The magnetic disturbance started after 04 UT (01 : 30 MLT) and lasted until about 18 UT (Fig. 3). At 10 – 11 UT the negative bay has its deepest de@ection with about ∼ − 300 nT which means a westward AEJ intensi6cation. As compared with the magnetic storm event January 12, 1997, the AEJ intensi6cation is latitudinally more localized and has lower current intensity, ∼300 kA. From ∼12 UT the negative magnetic disturbance gives way to a positive bay. The intensi6cation of the AEJ is accompanied by an increase of ULF activity, which reached Pc5 peak-to-peak amplitude of 60 nT. Spectral analysis shows that the peak power of ULF pulsations falls, at all stations, into the frequency range 2–6 mHz. The spectral power was then calculated separately for H and Z components using the frequency range, 2–7 mHz. Superimposed AEJ–ULF intensity distributions are shown in Fig. 4 where the upper and center panels refer to AEJ and Pc5 computed from the northward and vertical magnetic 6eld, respectively. This 6gure shows an enhanced westward AEJ from 09 till 12 UT which is centered at ∼72◦ CGM latitude. Soon after, between 12 and 18 UT, i.e. centered around magnetic local noon, an eastward AEJ develops at slightly higher latitude, 73–74◦ . Fig. 4 demonstrates that ULF power enhances concurrently with the westward AEJ intensi6cation. The intensi6cation of the eastward AEJ is also accompanied by increase in ULF power, but with smaller peak intensity. Resonant features of ULF waves are known to appear predominantly in the H and Z components. In fact, ULF wave power in both these components (upper and middle panels) intensi6es near the center or equatorward edge of the AEJ.
4.3. Weakly disturbed geomagnetic conditions: event 95-12-26 The third event exhibits a more complicated time-varying spectral structure of Pc5 pulsations (Fig. 5). The enhancement of monochromatic Pc5 (see GHB) is also in this case stimulated by a negative bay with magnitude MB −200 nT. Both H (Fig. 5a) and Z (Fig. 5b) components of Pc5 have nearly the same peak-to-peak amplitudes, 100 nT. Spectral analysis reveals latitude-dependent peaks between 2.0 and 3:5 mHz. In order to get total power estimates we have the spectral density integrated in the 2–7 mHz range. Fig. 6 indicates that a weak (∼200 kA) westward AEJ expands from latitude 72◦ at 08 : 40 UT to 72–74◦ at 10 UT. The dynamic plot also shows that the morning Pc5 wave trains all coincide spatially and temporally with the equatorward section of the westward AEJ. An eastward AEJ intensi6cation of ∼100 kA at ∼15 UT and then again from ∼16 : 30 UT onward, also stimulates ULF response but with a considerably lower intensity than the westward AEJ does. 5. Possible generation mechanisms of Pc5 Occurrence rate and intensity of Pc5 waves have a primary maximum in the morning sector and a weaker secondary in the afternoon sector. The azimuthal phase propagation and the polarization features of the ground magnetic disturbance in the horizontal plane change across the noon meridian (Samson, 1972). In earlier work, these observations led to the conclusion that a Kelvin–Helmholtz (KH) instability at the magnetopause (Kivelson and Pu, 1984; Mishin and Matukhin, 1986) or at the LLBL (Yumoto and Saito, 1980) is a likely candidate. Later, indications were found that impulsive dynamic pressure variations of the solar wind and @ux transfer events (FTE) possibly constitute a source of Pc5 wave packets in the magnetosphere (Nopper et al., 1982; Rostoker and Sullivan, 1987). Wave disturbances generated by dayside variations of the solar wind pressure propagate tailward along both sides of the magnetosphere, thus producing azimuthal phase velocity pattern across the noon meridian similar to the KH instability. Amplitude and phase spatial distributions of Pc5 clearly demonstrate resonance properties (Saka et al., 1982; Ziesolleck and McDiarmid, 1994) which are best seen in the morning hours. The position of the amplitude maximum depends on frequency (frequency increases towards low L values). The meridional phase velocity in the vicinity of the amplitude maximum is poleward directed. The existence of resonance e8ects for Pc5 geomagnetic pulsations points to a wave mechanism of energy transfer from an external source toward localized 6eld line Alfven oscillations. This short summary of current notions on Pc5 generation mechanisms does not indicate that a spatial-temporal
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Fig. 4. The event 99=02=14 04 –19 UT (Day 045) in the same format as Fig. 2.
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relation exists between the auroral current structure and ULF wave occurrence. The three sample events described above, with AEJ intensi6cation under di8erent geophysical situations, suggest that ULF wave activity is associated with the AEJ. The AEJ may, in fact, play an active role in Pc5 excitation. Ground-based magnetometers have a limited spatial resolution, ∼100 km in the ionosphere. When mapping magnetic @ux from the high-latitude ionosphere to the equatorial plane, two phenomena which occur in magnetospheric boundary layers may appear to be spatially collocated in the ionosphere although their sources may not actually spatially coincide. A better resolved mapping of magnetospheric boundaries to the ionosphere can be obtained from low altitude satellites. Solely from the data available to us we cannot claim that generation of FAC and Pc5 pulsations occur in the same magnetospheric domain. We rather discuss
the most evident interpretation schemes of AEJ-Pc5 coupling which were brought forward in the past, and attempt to evaluate them in the light of our observations. 6. Energetic electron injection into the morning sector ionosphere The magnetic 6eld depression often preceding or accompanying Pc5 activity in the early morning hours may indicate that the ULF wave generation mechanism is related to energetic electron injection. Electron precipitation can increase the ionospheric conductivity and thus enhance the AEJ intensity. Moreover, there are observations showing that morning time Pc5 pulsations might be associated with increased @uxes of energetic electrons in the magnetosphere (Saka et al., 1992).
Fig. 5. Magnetograms of the (a) H component, and (b) Z component during the event of 95=12=26 (Day 360), 07–20 UT.
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Fig. 5. (continued).
However, diPculties arise when attempting to interpret morning Pc5 pulsations with this mechanism. It helps to recall the properties of oscillations in the same frequency band in the afternoon sector. The spatial structure of afternoon ULF pulsations has been examined with STARE and the geostationary GOES-2 satellite (Allan and Poulter, 1984; Walker et al., 1979, 1982). They fall into two di8erent categories. The 6rst of them, called “storm-time” Pc5 (Bar6eld and McPherron, 1972), is often observed at geostationary orbit and is closely connected with intense @uxes of ring current protons. Oscillations propagate predominantly in azimuthal direction, and no clear meridional wave structure is observed. Their transverse wavelengths are small and, correspondingly, the azimuthal wave numbers are large, m 30–50. The generation mechanism of this class of
Pc5 waves is connected with low frequency drift instabilities (see review by Pilipenko (1990) for references). A small lateral scale length of oscillations is a necessary condition for the e8ective interaction between low frequency waves and energetic particles, because in this case the parameter k⊥ (i.e. the ratio of a Larmor radius to a lateral wave length) is 6nite. Oscillations of this type can hardly be recorded by ground based magnetometers (Allan et al., 1983) because high-m oscillations are e8ectively screened by the ionosphere. Oscillations of the other type are caused by impulsive solar wind disturbances and form a sequence of wave packets. The typical resonance structure occurs in the meridional distribution of the wave amplitude, evidencing the transformation of energy from an external source into 6eld line Alfven oscillations. Due to their large lateral scale length, these
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Fig. 6. The event 95=12=26 (Day 360), 07–20 UT, in the same format as Fig. 2.
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oscillations are only weakly screened by the ionosphere and clearly seen in both, satellite and ground records. One might envisage a similar picture for morning Pc5 pulsations, and observations of Saka et al. (1992) indeed suggest that they might be associated with increased @uxes of energetic electrons in the magnetosphere. However, the assumption that substorm-related morning Pc5 pulsations are generated by resonant instabilities of energetic electrons contradicts ground based observations which show low m-values and, respectively, large lateral scale lengths of these oscillations (Olson and Rostoker, 1978). Moreover, no oscillations with large m values in the morning sector were reported to be regularly observed with satellites.
6.1. Oscillations of a three-dimensional current system
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6.2. Enhanced conductivity in the region of AEJ The ionospheric conductivity in the region of AEJ is increased compared to the background ionosphere, due to electron precipitation in the region of upward FAC. Observations by Sutcli8e and Rostoker (1979) suggest an increase of Pc5 pulsation intensity across the dawn terminator, suggesting that the ionospheric conductivity plays an important role in the Pc5 excitation process. When developing this idea further, the following probable mechanism could be suggested. Permanent background ULF disturbances in the magnetosphere do not develop into prominent resonant oscillations because of severe ionospheric damping. An increase of the ionospheric conductance, which occurs just in the region of the AEJ, would stimulate ULF activity. Accordingly, a decrease of the ionospheric conductivity and associated AEJ intensity would quench ULF activity. However, a quantitative estimate shows that the background Pedersen conductance is high enough at auroral latitudes in the morning hours as compared with the effective magnetospheric conductance. Typical numbers are P 5 –10 S and A 1 S, respectively. Alfven wave re@ection at the high-latitude ionosphere is consequently strong, and excessive damping does not prevent the excitation of magnetospheric AR.
A possible mechanism for coupling between Pc5 and the AEJ was described by Rostoker and Lam (1978) who interpreted Pc5 pulsations as oscillations of a three-dimensional current system which involves magnetospheric plasma and 6eld-aligned and ionospheric currents. The electrical parameters of this circuit, i.e. resistance of the ionosphere, capacity of the convective plasma @ow and inductance of 6eld-aligned currents, determine the oscillation regime of the e8ective LRC circuit. Following Rostoker and Lam (1978), we argue that the eigenfrequency of oscillations of this three-dimensional current system turns out to be the common Alfven frequency !A . Let us consider the following equivalent circuit. The plasma volume under consideration in the magnetosphere has a length l along the 6eld line, width b in radial direction, and the azimuthal scale w. The inductance of the volume, uniformly 6lled with a 6eld-aligned current density j, can be estimated via the total magnetic energy stored, Wm = (B2 =20 ) dV = ((B2 =20 )wbl. Comparing it with the elementary formula for the magnetic energy of a current sheet with total current I = jbw, i.e. Wm = Lm (I 2 =2), we obtain Lm = 0 (bl=w). In the same way we estimate the magnetospheric plasma capacitance Cm as a measure of kinetic energy of oscillating plasma particle motion with velocity VE . Equating Wm = (VE2 =2) dV = (=2)(E=B)2 wbl with Wm = Cm (U 2 =2), where U = Eb is electric potential, we get Cm = (=B2 )(wl=b). The resistance R of the ionosphere produces the Joule dissipation of 6eld line oscillations. Finally, we can estimate the expected resonant frequency of this LC circuit to be
Magnetic noise can be produced via the modulation—or self-modulation—of precipitating particles which transport FAC. Variations of particle @uxes cause variations of the ionospheric conductivity thus eventually producing magnetic noise at the ground. This mechanism was suggested for the interpretation of broad-band pulsations at dayside cusp latitudes (Engebretson et al., 1991). One may assume that morning Pc5 pulsations are also caused by the modulation of the @ux, F, of precipitating particles. At high latitudes particle precipitation is the main source for ionization q and enhancement of the conductivity of the ionospheric E layer. The electron density variation, Mn, induced by small periodic variations of the ionization rate Mq with frequency !, can be estimated with the help of the ionization balance equation. Supposing that the ionosphere is an in6nite thin current layer, one can estimate the magnetic 6eld disturbance produced by the ionospheric electric 6eld Eo . The @ux @uctuations MF produce a magnetic disturbance b on top of the quasi-stationary background disturbance MB according to the following relation.
! = (Lm Cm )−1=2 = VA =l:
|b=MB| |M = | |Mn=n|
(2)
Therefore, the mechanism of 3-D current system oscillations cannot be considered an alternative hypothesis but appears to be just another representation of the 6eld line resonance theory.
6.3. FAC =uctuations
(Mq=2q)(1 + !2 $2r )−1=2 ;
(3)
where $r = 1=(2&n) is the typical recombination time determined by the recombination coePcient &. Under the assumption of typical numbers, & 2 × 10−7 cm3 s−1 and
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n 2 × 105 cm−3 , it follows from (3) that ground magnetic signals in the Pc5 band (! 0:01 s−1 ), with relative amplitudes b=MB 0:1 can be produced by oscillating particle @uxes with a modulation MF=F = Mq=q 0:2. If the background ionospheric density n is not high, that is !$r ¿ 1, the amplitude of magnetic variations, produced by the @ux modulation with depth Mq=q, should linearly increase with increasing background precipitation level, that is, b ˙ n(Mq=q), according to observations by Olson et al. (1980). This means that, in principle, modulated precipitation cannot be excluded as Pc5 generation mechanism. However, a suitable periodic modulation factor remains to be identi6ed. Another diPculty with this interpretation exists: a @uctuating ionospheric current should produce synchronous horizontal and vertical magnetic variations on the ground: the H component should be in-phase along the meridian, while the Z component should be out-of-phase at both sides of the ionospheric current. A more close examination of the Pc5 magnetograms shown in Figs. 1 and 5 reveals, however, that the magnetic H and Z components indicate an apparent poleward phase propagation. Such phase characteristics are consistent with a resonant model which is based on the assumption of local excitation of magnetospheric AR by external MHD disturbances. It contradicts the idea of modulated ionospheric currents.
6.4. Feedback excitation of magnetospheric AR An ionospheric conductivity variation M in the presence of an external electric 6eld Eo leads to the generation of a polarization electric 6eld, which then propagates as an Alfven wave along geomagnetic 6eld lines into the magnetosphere. The magnitude of the polarization electric 6eld E can be estimated as E = − Eo
M +
A
(Mal’tsev et al., 1974). For a reasonable set of parameters, the magnitude of E is large enough to contribute signi6cantly to the stimulation of Pc5 pulsations. The tendency to excite magnetospheric AR at the location of the AEJ could possibly be related to a reverse in@uence of the ULF-modulated ionosphere on excitation forcing. The mechanism, similar to the feedback instability (Lyatsky and Mal’tsev, 1983; Lysak, 1991), could operate as follows: FAC modulation → localized variations of M → emission of E and additional FAC into the magnetosphere along the gradient of . With proper phase relations this scenario may explain an enhanced excitation rate of magnetospheric AR in the FAC region compared to other magnetospheric regions. The mechanism remains speculative as AR excitation with FAC feedback has not yet been investigated theoretically.
7. Conclusion The new visualization technique suggested here opens up the possibility to take a synoptic view on time variations not only of the spatial positions of the AEJ and the Pc5 pulsation peak, as in earlier studies, but also of their intensities. Observations from the Greenland west coast magnetometer chain con6rm the 6ndings of Lam and Rostoker (1978): Pc5 power spatial=temporal variations in the morning=prenoon sector are closely related to the location and intensity of the AEJ. In the cases presented here, Pc5 oscillations occur during the recovery phase of a previously intensi6ed AEJ. The intensi6cation of an eastward AEJ also stimulates ULF response, but with considerably lower intensity than the westward AEJ does. From the observed resonant features of the Pc5 wave structure and the relationship between ULF wave power and the AEJ intensity, it may be concluded that the location of the AEJ (predominantly its equatorward edge) is a preferred latitude for magnetospheric AR excitation by external MHD disturbances. This e8ect is not taken into account by existing theories of ULF Pc5 waves, but it might be signi6cant for development of more adequate models. Consideration of the most evident interpretation schemes shows that no simple explanation for this e8ect is known. The explanations discussed above have certain pitfalls and demand more elaborate models. An adequate interpretation may require a substantial revision or augmentation of existing Pc5 models, e.g. development of a resonant model with ionospheric feedback. Acknowledgements We thank E. Fedorov and W. Lyatsky for discussions concerning theoretical aspects of the problem. V.A. thank the Danish Meteorological Institute for supporting their visits. V.O.P. also acknowledges support from the NSF awards OPP-9614175 and OPP-9876473. We appreciate the constructive comments of both referees.
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