Earth Planets Space, 53, 981–992, 2001

Characteristics of Pi 2 magnetic pulsations observed at the CPMN stations:
A review of the STEP results
Kiyohumi Yumoto and the CPMN Group
Department of Earth and Planetary Sciences, Kyushu University 33, Fukuoka 812-8581, Japan(Received July 14, 1999; Revised June 13, 2001; Accepted August 9, 2001)

From analyses of magnetic data from the Circum-pan Pacific Magnetometer Network (CPMN) stations, we canfind the following wave characteristics of Pi 2 magnetic pulsations. (1) Pi 2 pulsations observed on the ground are
an ensemble of various hydromagnetic modes excited at high, middle, low, and equatorial latitudes. Each mode
exhibits its own propagation (timing) and spatial illumination. (2) There are apparent longitudinal propagation andlatitudinal time delays of the maximum Pi 2 magnetic energy among the CPMN stations within 100 sec. (3) Whena single low-latitude station is located in the sectors of 00–24 hr LT, 20–03 hr local time (LT), and 23–01 hr LT,
respectively, 62%, 79%, and 100% of auroral breakups identified by the Polar satellite have associated Pi 2s. 66%of the low-latitude Pi 2s occur within 1 min of the auroral breakups, and 85% within 2 min. Pi 2s are still a good
indicator of substorm onsets, but they should be used with care.

1.

Introduction

variations at mid- and low-latitudes. Ohtani et al. (1999)
asserted that the tail current disruption is a unique processrather than a direct consequence of the near-Earth neutralline formation, and took over the near-Earth reconnection
process as a major role in the substorm dynamics.
After the Circum-pan Pacific Magnetometer Network(CPMN) have been constructed along the 210◦ magnetic
meridian (MM) (Yumoto and the 210◦ MM Magnetic Observation Group, 1996) and the magnetic equator (Tachihara etal., 1996) as shown in Fig. 1, it is now possible to investigate
global characteristics of Pi 2 pulsations and one-to-one correspondence between Pi 2s and substorm onsets (i.e. auroral breakups), and then examine if nighttime Pi 2 pulsations
are just an excited oscillation/wave, and/or have a role oftrigger/guide for magnetospheric substorms. In order to reexamine whether Pi 2 pulsations are still a good indicator ofmagnetospheric substorm onsets or not, we analyzed magnetic data from the CPMN stations. We will present global
wave characteristics of Pi 2s observed at the CPMN stations,show timings of the maximum Pi 2 magnetic energy, examine one-to-one correspondence between the observed Pi 2s
and auroral breakups identified by the Polar satellite, andthen draw conclusions in the final section.

At the onset of magnetospheric substorms, impulsive hydromagnetic oscillations with periods of 40–150 sec, so
called Pi 2 magnetic pulsations, occur globally in the magnetosphere. Pi 2 magnetic pulsations recorded on the induction magnetogram at low latitudes appear at the beginning part of positive H-componet bays in the ordinary magnetogram at low latitudes (see figure 39 of Saito, 1969),
and its onsets show one-to-one correspondence to individual bays (see figure 42 of Saito, 1969). Low-latitude Pi 2
magnetic pulsations have been regarded as a good indicatorof substorm onsets for the past two decades (e.g., Saito etal., 1976; Sakurai and Saito, 1976; Yeoman et al., 1994).
But recently, Liou et al. (2000) compared Pi 2 onsets observed at a single station (Kakioka, L = 1.07) with theauroral breakups identified by the Polar ultraviolet images.
The identifying substorm onset with the Pi 2 alone is often
ambiguous, and a Pi 2 onset observed at Kakioka is sometimes delayed by as long as 2 min from an auroral breakup
identified by the Polar satellite. They suggested that Pi 2 observed at a single low-latitude station may not be as a good
indicator of the substorm onset as it was thought to be.
The source of nighttime Pi 2 pulsations is believed to
be a sudden change in magnetospheric configuration orconvection during the substorm expansion phase. Thischange may be caused by earthward plasma flows from thereconnection region and/or a sudden generation of fieldaligned currents (FACs) between the polar ionosphere andthe magnetospheric plasmasheet in association with the disruption of cross-tail currents (see references in Yumoto,1986, 1988). The formation of the substorm current wedge(SCW; McPherron et al., 1973) also causes the magnetic bay

2.

Global Characteristics of Pi 2s

During the past two decades, many workers have studied Pi 2 magnetic pulsations observed on the ground and inspace (see references in Olson, 1999), but the generation andpropagation mechanisms of the pulsations are only partly
understood. After the international Solar Terrestrial Energy
Program (STEP) period (1990–1997), 1-sec magnetic fielddata from coordinated ground-based network stations madeit possible to (1) study magnetospheric processes by distinguishing between temporal changes and spatial variations in
the phenomena, (2) clarify global structures and propagation
characteristics of magnetospheric variations from higher toc The Society of Geomagnetism and Earth, Planetary and Space Sciences
Copy right
(SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan;
The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.

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K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

Fig. 1. A station map of the Circum-pan-Pacific Magnetometer Network (CPMN).

equatorial latitudes, and (3) understand the global generation mechanisms of the solar-terrestrial phenomena (seeYumoto and the 210◦ MM Magnetic Observation Group,
1995, 1996).
The latitudinal profiles of Pi 2 amplitudes and phase relations obtained at the CPMN stations in the STEP period
imply that Pi 2 pulsations observed on the ground consistof different mode oscillations (see Yumoto et al., 1994; Yumoto and the 210◦ MM Magnetic Observation Group, 1995;
Osaki et al., 1996; Shinohara et al., 1997; Li et al., 1998a;
Cheng et al., 2000; Li et al., 2000; Uozumi et al., 2000;
Solovyev et al., 2000). Recently. Olson (1999) reviewed
more than 100 Pi 2 papers and concluded that a bouncing of
an impulsive field-aligned current (and/or a kinetic Alfvénwave) between the plasma sheet and the auroral ionosphereand an oscillation of the substorm current wedge are detected as high- and mid-latitude Pi 2s, respectively, whilea surface wave excited at the plasmapause and a global oscillation in the inner magnetosphere are observed as midand low-latitude Pi 2s, respectively.
To explain latitude-dependent magnetic pulsations at thehigher latitudes, the field line resonance (FLR) was widelybelieved to be a crucial mechanism in the wave coupling
as distant source waves tunnel to excite the field line oscillations in the inner magnetosphere (Chen and Hasegawa,1974a; Southwood, 1974). Due to the orientation of the major axis of Pi 2 polarization directed to the center of the current wedge, the substorm current wedge is a possible source
for mid-latitude Pi 2 pulsations (Lester et al., 1989 and ref-erences there in). Because of the rapid change of the Alfvénspeed on a boundary, there may exist surface waves of which
the frequency is determined by the Alfvén velocities on theboth sides of the boundary (Chen and Hasegawa, 1974b).
Thus, another source mechanism of mid-latitude Pi 2 pulsations might be associated with the surface waves at theplasmapause. The D-component Pi 2s having almost maximum amplitudes around L = 3.0 as shown in figure 1 of
Yumoto et al. (1994) might be associated with the surface
wave at the plasmapause and/or a localized field-line oscillation just inside the plasmapause. However, by comparing the AMPTE/CCE and ground-based observations, statistical analysis of Pi 2 pulsations in the inner magnetosphere by Takahashi et al. (1995) showed no evidence tosupport a view that ground Pi 2 pulsations at mid-latitudes(2 < L < 5) are field-line resonant oscillations in responseto the source waves on the auroral zone field line. Hence,
more dense ground network observations near the plasmapause with satellite observations are needed to confirm thegeneration and propagation mechanisms of mid-latitude Pi 2
pulsations.
On the other hand, early observations (e.g., Samson and
Rostoker, 1972) reported that Pi 2 pulsations had the characteristics of discrete spectrum, and suggested that there
might exist global cavity modes. Stuart (1974) proposedthat the secondary amplitude maximum of Pi 2 occurring
between L = 2.5 and L = 4 is due to plasmaspheric cavity resonances. Moreover, using the magnetometer arrayfrom L = 2.38 to L = 6.23, Yeoman and Orr (1989) sug-

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

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Fig. 2. A schematic model of Pi 2 magnetic pulsations observed on the ground, consisting of five HM wave components: a bouncing mode of impulsive
field-aligned current at the auroral latitude, a substorm current wedge oscillation, a surface wave at the plasmapause, a cavity-like oscillation in the innermagnetosphere, and a transmission of the Pi 2 electric field from the nightside auroral latitude to the dayside equatorial region.

Fig. 3. One example of H- and D-component, bandpass-filtered (25–250 sec) amplitude-time records of Pi 2 pulsations observed on February 17, 1995,
along the 210◦ MM stations from high latitudes to the dip equator at KTN ( = 70.0◦ , L = 8.5), TIX (65.7◦ , 5.9), CHD (64.7◦ , 5.5), ZYK (59.6◦ , 3.9),
MGD (53.5◦ , 2.8), MSR (37.3◦ , 1.6), GUA (5.6◦ , 1.0), and POH (0.1◦ , 1.0) in nighttime, at the three Brazilian stations ALC (1.2◦ , 1.0), EUS (−0.8◦ ,1.0), and SMA (−18.7◦ , 1.1) in daytime, and at LAQ (36.2◦ , 1.5) in Italy in daytime.

gested that plasmaspheric cavity resonance is the most likely
mechanism for the secondary amplitude maximum of midlatitude Pi 2. Other observations also pointed out that global
cavity modes might be responsible for Pi 2 pulsations atvery low latitudes and in the inner magnetosphere (Yumoto
et al., 1989; Yumoto, 1990; Lin et al., 1991; Sutcliffe andYumoto, 1991). However, using the AMPTE/CCE satellite,

Takahashi et al. (1992, 1995) found that compressional Pi 2
oscillations can be detected only inside a region of ±3 hrs
of local midnight and primarily at L < 5 in the inner magnetosphere. They suggested that the L dependence of thepower and phase of these components is such that can be
explained by a simple cavity mode resonance with a nodein compressional component located near L = 4. With

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K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

Fig. 4. The spectral peak powers of Pi 2 magnetic pulsations at 6 mHz and 17 mHz from high latitude to the dip equator in nighttime and in daytime. The
upper and lower panels show H- and D-component of the spectral peak power at 6 mHz and 17 mHz, respectively.

availability of magnetic fields at the Akebono satellite andground stations, Osaki et al. (1998) recently confirmed that
Pi 2 pulsations propagate to the ground from the magnetotail. By assuming that observed Pi 2 pulsations represent
the cavity mode, they found that the escaping Poynting flux
could damp a cavity mode oscillation in ∼10–20 s, and thatthe transverse Pi 2 pulsations at the Akebono satellite lastfor 400 s. Since the nightside ionosphere is not a good reflector and the plasmapause is also neither a good reflector
nor a good transmitter, it is not yet well accepted that cavity
resonances are driven by impulsive sources out of the plasmasphere. These observations of Takahashi et al. (1992,
1995) and Osaki et al. (1998) may suggest that the lowerlatitude Pi 2s observed on the ground must be associatedwith the compresional cavity-like oscillations excited in thelongitudinally limited region of ±3 hrs of local midnight,
where the inner and outer boundaries for the compressional
cavity wave between two walls may be the solid Earth of agood reflector and the plasmapause of a not-perfect reflector, respectively. More observations and theoretical studies
are needed to further clarify the generation and propagation
mechanisms of Pi 2 pulsations at lower latitudes.
Recent results from the CPMN project showed that highlatitude Pi 2s which are mainly caused in association withthe westward auroral electrojets, start with a decrease rather
than an increase at higher latitude, while low-latitude Pi 2
at the beginning part of positive bay at low latitude startwith an increase of H-component magnetic field. We cansee almost out-of-phase relationship of H-component Pi 2s
between high and low latitudes as shown in figures 1 and
8 of Yumoto et al. (1994). The H-component Pi 2s having
almost same wave forms and amplitudes at all lower latitudes as shown in figure 1 of Yumoto et al. (1994) might
be associated with sudden formations of the substorm cur-rent wedge and/or the magnetospheric compressional cavity wave. Cheng et al. (2000) found that the ratio of thefirst four harmonic frequencies of low-latitude Pi 2 pulsations observed at the 210◦ MM stations is consistent withthat of plasmaspheric cavity modes simulated in the box
model magnetosphere. Shinohara et al. (1997) further indicated that daytime and nighttime Pi 2 pulsations observedat the CPMN stations in the equatorial and low-latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar tothe dayside equatorial ionosphere, and a direct incidence ofcompressional oscillations from the nightside inner magnetosphere (e.g. Takahashi et al., 1999), respectively.
From the review of Olson (1999) and the results from the
CPMN project, it can be suggested that Pi 2 magnetic pulsations observed on the ground consist of at least five components: (1) a bouncing mode of impulsive FAC at the auroral
latitude, (2) a substorm current wedge oscillation, (3) a surface wave at the plasmapause, (4) a cavity-like oscillationin the inner magnetosphere, and (5) a transmission of Pi 2
electric field from the nightside auroral latitude to the dayside equatorial ionosphere, as summarized in Fig. 2. The
ground Pi 2 pulsations must be an ensemble of these fivemodes, which show their own propagations (or timings) andspatial illuminations.
In the following figures, we will demonstrate how Pi 2
pulsations observed at the CPMN stations have complex
wave characteristics. Figure 3 shows one example of H- andD-component, bandpass-filtered (25–250 sec) amplitudetime records of Pi 2 pulsations observed on February 17,
1995, along the 210◦ MM stations from high latitudes to thedip equator at Kotelnyy (KTN:  = 70.0◦ , L = 8.5), Tixe
(TIX: 65.7◦ , 5.9), Chokurdahk (CHD: 64.7◦ , 5.5), Zyryanka(ZYK: 59.6◦ , 3.9), Magadan (MGD: 53.5◦ , 2.8), Moshiri

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

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Fig. 5. Local time dependence of one-month-averaged horizontal Pi 2 amplitudes of the 249 events observed at CHD at high latitude, ZYK near theplasmapause, MSR at low latitude, GUA near the magnetic equator, and POH at the dip equator. Each solid circle indicates averaged amplitude at eachstation (cf. figure 2 of Li et al., 2000).

(MSR: 37.3◦ , 1.6), Guam (GUA: 5.6◦ , 1.0), and Pohnpei(POH: 0.1◦ , 1.0) in nighttime, at the three Brazilian stations,
Alcantara (ALC: 1.2◦ , 1.0), Eusebio (EUS: −0.8◦ , 1.0), andSanta Maria (SMA: −18.7◦ , 1.1) in daytime, and at Laquila(LAQ: 36.2◦ , 1.5) in Italy in daytime. The equatorial enhancement of daytime H-component Pi 2s observed at ALC
can be explained by using (5) the transmission of Pi 2 elec-

tric field from the nightside auroral latitude to the daysideequatorial ionosphere. The higher-frequency H-component
Pi 2s observed at nightside lower latitudes (1 < L < 4) correspond to (4) the cavity-like oscillation in the inner magnetosphere. The nightside D-component Pi 2s observed
around L = 3 may be (3) the surface wave at the plasmapause and/or a localized field-line oscillation just inside the986

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

plasmapause. The nighttime higher-latitude Pi 2s at ZYK,
TIX, and CHD must be associated with (2) the substorm
current wedge oscillation. The nighttime Pi 2s with lowfrequency component at KTN at  = 70.0◦ , L = 8.5 may
have characteristics of a bouncing mode of impulsive fieldaligned current at the auroral latitude. It is noteworthy thatthe low-frequency component of Pi 2 can be seen predominantly at higher latitudes in nighttime and near the equatorin daytime, while the higher-frequency component can beseen predominantly at mid- and low-latitudes in nighttime.
We calculated the power spectral density (nT2 /Hz) of the
Pi 2 event observed at 13:35–13:45 UT at those stations asshown in Fig. 3, and found two predominant components at
frequencies of 6 mHz and 17 mHz. Figure 4 shows the peak
power at frequencies of 6 and 17 mHz as a function of thestations from high latitude to the dip equator in nighttime,and in daytime. The upper and lower panels show H- andD-component peak power variations, respectively. The lowfrequency H component is predominant at high latitudes innighttime and at equatorial latitude in daytime, while thehigh-frequency component is larger than the low-frequency
component at lower latitudes in nighttime. The low- andhigh-frequency D components show comparable power atlower latitudes in nighttime. Li et al. (1998a) statisticallyestimated the spectral power of each of 249 Pi 2 events in
February 1995, made averaged predominant frequencies ofthe events in each hour, and then plotted the hourly averaged frequency as a function of local time at the 210◦ MM
stations. At the high-latitude station CHD ( = 64.7◦ ),the averaged predominant frequency is between 8 and 12
mHz with no significant maximum or minimum. At the midand low-latitude stations, a maximum frequency of about 15
mHz can be seen around local midnight, and a lower frequency of about 10 mHz in the evening hours. At the equatorial station POH, the frequency shows the same local timedependence and values as the mid- and low-latitude components in nighttime, while in daytime (06–18 hr LT) the averaged predominant frequency is between 10 and 12 mHz,
which is in the same frequency range as the auroral-latitude
Pi 2s (see figure 5 of Li et al., 2000).
Each diagram in Fig. 5 shows a local time profile of onemonth-averaged horizontal amplitudes of the 249 Pi 2 events
observed at CHD ( = 64.7◦ ) at high latitude, ZYK (59.6◦ )near the plasmapause, MSR (37.3◦ ) at low latitude, GUA(5.6◦ ) near the magnetic equator, and POH (0.1◦ ) at the dip
equator. Each solid circle indicates an averaged maximum
amplitude of 22.4 nT at CHD, 4.2 nT at ZYK, 0.75 nT at
MSR, 0.39 nT at GUA, and 0.78 nT at POH. It is noteworthy
that high-latitude Pi 2 amplitudes show a maximum, appearing in the narrow local midnight sector of 23–01 hr LT, whileat middle, low, and near-equatorial latitudes, two maximum
amplitudes appear around local midnight and around 21 hr
LT. The secondary peak at 21 hr LT is significant, because it
can be obtained for a different one-month-averaged data set.
At the dip equator, the equatorial enhancement of Pi 2 amplitudes can be seen around 10 hr LT. These diagrams andFig. 4 suggest that at least two dominant frequency components of Pi 2 pulsations appear at high latitude in the narrow
local midnight sector of 23–01 hr LT and in the inner magnetosphere of the midnight sector from 21 hr to 03 hr LT.

Fig. 6. (Bottom) Example of the calculated amplitude of Pi 2 magnetic energy (H )2 + (D)2 + (Z )2 µo , amplitude-time records of d H/dt,
and H variations observed at CHD (L = 5.5) on March 1, 1996. (Top)One- to two-minute time difference between Pi 2 onsets and the timingof maximum magnetic energy in the superposed plots, during the intervalfrom February 11 to March 31, 1996 (see Uozumi, 2000).

There are no existing theories that explain why the second
amplitude peak appears around 21 hr LT and why the equatorial amplitude enhancement has a maximum around 10 hr
LT. But, from the global characteristics as shown in Figs. 2
through 5, the high-latitude Pi 2 predominantly appearing in
the midnight sector, the mid- and low-latitude Pi 2s in the
21–02 hr LT sector, and the dayside equatorial enhancement
around 10 hr LT may be respectively interpreted by using
the substorm current wedge oscillation, a cavity-like compressional oscillation in the nightside inner magnetosphere,
and the transmission of electric fields in the magnetosphereionosphere-atmosphere-solid Earth system from the nightside auroral latitude to the dayside equator.

3.

Timings of the Maximum Pi 2 Wave Energy

Sometimes it is difficult to identify the onset time of
Pi 2s, because of a gradual increase in the amplitudes.
The ground Pi 2 pulsations are an ensemble of the fivemodes as shown in Fig. 2, which show their own propagations (or timings) and spatial illuminations. Although it
is not easy to know propagation mechanisms of the fivemodes by using the limited ground stations, Uozumi et al.
(2000) newly defined a Pi 2 magnetic energy variations as

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

987

Fig. 7. Latitudinal profile of timings of the maximum Pi 2 magnetic energy (H )2 + (D)2 + (Z )2 µo observed at 15:55 UT on January 7, 1995, alongthe 210◦ MM from the auroral stations at KTN and CHD, through the plasmapause near ZYK, to the mid- and low-latitudes at MGD, MSR, and KAG,
and to the equatorial region near GUA (figure 1 of Uozumi et al., 2000).

(H )2 + (D)2 + (Z )2 /µo , and then compared timings
when the amplitudes of Pi 2 magnetic energy became maximum. The magnetic energy carried by MHD waves is expressed by (H )2 + (D)2 + (Z )2 µo . That is, the variation of the magnetic energy is proportional to the squared
amplitude of magnetic variations. When using the magnetic
energy perturbation on the ground, the arrival time of Pi 2
magnetic energy can be determined more accurately without depending on phase or polarization changes among thestations. Figure 6 shows one example of propagation characteristics of Pi 2s (or timings of Pi 2 onsets) observed at globally separated CPMN stations. The bottom panel indicatesthe calculated Pi 2 magnetic energy amplitude, amplitudetime records of d H/dt, and H variations observed at CHD(L = 5.5) on March 1, 1996. As shown statistically in the
top panel, a one- to two-minute time difference between the
Pi 2 onsets and timings of the maximum magnetic energy
can be seen in the superposed plots during the interval fromFebruary 11 to March 31, 1996.
Figure 7 shows the latitudinal profile of timings of the
maximum Pi 2 magnetic energy observed at 15:55 UT on
January 7, 1995, along the 210◦ MM from the auroral stations at KTN ( = 70.0◦ , L = 8.5) and CHD (64.7◦ , 5.5),
through the plasmapause near ZYK (59.6◦ , 3.9), to the midand low-latitudes at MGD (53.5◦ , 2.8), MSR (37.3◦ , 1.6),
and Kagoshima (KAG: 25.1◦ , 1.2), and to the equatorial region near GUA (5.6◦ , 1.0). The amplitudes of Pi 2 wave
energy in the figure have been normalized. The solid vertical line is a time when the Pi 2 magnetic energy became
maximum at Guam. At ZYK near the plasmapause, we
can see an 59-second time delay with respect to the lowlatitude Pi 2s. On the other hand, at KTN higher-latitude

Pi 2 leads those at CHD and the low-latitude stations by
about 40 seconds. These time differences at higher latitudeand near the plasmapause can be explained by using the difference between the times-of-flight of Alfvén and fast magnetosonic waves, which propagate along and across, respectively, the ambient magnetic field from the magnetosphericequator of each station to the ionosphere (see Uozumi etal., 2000). Where a compressional Pi 2 source wave aroundL = 9 in the magnetotail was assumed to propagate acrossthe ambient field, and couple into bouncing oscillations (i.e.
Alfvén wave) at the auroral latitude and near the plasmapause, which may be observed as higher-latitude Pi 2 and asurface wave-type mid-latitude Pi 2, respectively.
The left, middle, and right panels of Fig. 8 obtained by
Uozumi (2000) show three examples of longitudinal timedifferences of the maximum Pi 2 magnetic energy observed=in the high-latitude region at KTN ( = 70.0◦ ,201.0◦ ), TIX (65.7◦ , 196.9◦ ), and CHD (64.7◦ , 212.1◦ ) in
Siberia, and Kotzubue (KOT: 64.5◦ , 249.7◦ , L = 5.40)
in Alaska. The figure shows time differences in the premidnight, midnight, and post-midnight sectors. It is foundthat Pi 2s at KTN ( = 70.0◦ ) tend to lead those at highlatitude stations KOT, TIX, and CHD at  ∼ 65.0◦ by about
40 seconds in all sectors. It is also noteworthy that a westward time delay of high-latitude Pi 2s from the midnight
station KOT to the pre-midnight station TIX can be seen in
the pre-midnight sector, while an eastward time delay fromthe midnight station TIX to the post-midnight station KOT
can be seen in the post-midnight sector.
Uozumi (2000) also analyzed timings of the Pi 2 magnetic energy observed at the low-latitude stations at KAG( = 25.1◦ , = 202.2◦ ) in Japan and Ewa Beach (EWA:988

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

Fig. 8. Three examples of longitudinal time differences of the maximum Pi 2 magnetic energy observed in the high-latitude region at KTN ( = 70.0◦ ,= 201.0◦ ), TIX (65.7◦ , 196.9◦ ), and CHD (64.7◦ , 212.1◦ ) in Siberia, and Kotzubue (KOT: 64.5◦ , 249.7◦ , L = 5.40) in Alaska. Time differences areshown in the pre-midnight (left), midnight (middle), and post-midnight (right) sectors (cf. Uozumi, 2000).

Fig. 9. Timings of the Pi 2 magnetic energy observed at the low-latitude stations KAG in Japan and EWA in Hawaii. Timings are shown for the
pre-midnight (left), midnight (middle), and post-midnight (right) sectors (cf. Uozumi, 2000).

22.7◦ , 269.4◦ ) in Hawaii as shown in Fig. 9. The figure indicates data for the pre-midnight, midnight, and post-midnight
sectors. The low-latitude Pi 2s show a westward time delay
from the midnight-side station EWA to the duskside station
KAG, no time difference when these stations are located in
the local midnight sector of 22 hr to 03 hr LT, and an eastward time delay from the midnight-side station KAG to thedawnside station EWA.
83 Pi 2 events were statistically examined for a 50-day period from February 11 to March 31, 1996, and it is found thathigh-latitude Pi 2s show apparent westward and eastward
propagation in the pre-midnight and post-midnight sector,
respectively. The low-latitude Pi 2s also show apparent eastward propagation in the dawnside, but an unclear, mixed nature in the duskside.

4.

One-to-one Correspondence between Pi 2s and
Auroral Breakups

We next discuss on one-to-one correspondence between
Pi 2s observed at the CPMN stations and auroral breakups

identified by the Polar satellite. Li et al. (1998b) have examined the one-to-one correspondence in detail. In this section
we will briefly summarize their results. The upper and lower
panels of Fig. 10 show the percentage of auroral breakups(identified by the Ultraviolet Imager (UVI) on board the
ISTP Polar satellite) that have associated Pi 2s observed ata single low-latitude station MSR ( = 37.3◦ , L = 1.6),
and at MSR or the high-latitude station CHD (64.7◦ , 5.5),
respectively. The 108 auroral breakups were identified between latitudes 60◦ N and 73◦ N, and between local times 20
hr and 03 hr (see Liou et al., 1998), and are plotted at hourlyintervals in the panels. If we use a single low-latitude magnetometer station, we can see a one-to-one correspondenceof only 62% (with no restriction in local time of the groundstation) or 79% (when the ground station is in the nighttime sector between 20 hr and 03 hr LT) between the auroral
breakups and the low-latitude Pi 2s. But if the single station is located in the midnight sector from 23 to 01 hr LT,
we can see a 100% one-to-one correspondence. If we use
the high- and low-latitude two-magnetometer stations, weK. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

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Fig. 10. 108 auroral breakups identified by the POLAR satellite displayed in hourly intervals. The number of auroral breakup events is shown in eachhour bin. (Top) Percentage of auroral breakups with associated Pi 2s observed at the low-latitude station MSR. (Bottom) Percentage of auroral breakups
with associated Pi 2s observed at MSR or the high-latitude station CHD (Li et al., 1998b).

can obtain a one-to-one correspondence of 78% (with no restriction in local time) or 95% (when the ground stations arein the nighttime sector between 20 hr and 03 hr LT) between
the auroral breakups and the Pi 2 pulsations.
Figure 11 shows three examples of magnetic substorms
with Pi 2 pulsations observed at the 210◦ MM stations at
KTN ( = 70.0◦ , L = 8.5), TIX (65.7◦ , 5.9), CHD (64.7◦ ,5.5), MGD (53.5◦ , 2.8), MSR (37.3◦ , 1.6), LNP (13.8◦ ,1.06), and EWA (22.7◦ , 1.17, +4 hr LT) in the pre-midnight(∼22 hr LT), midnight (∼00 hr LT), and post-midnight(>01 hr LT) sectors of the 210◦ MM. Pi 2 pulsations at
1504 UT in the midnight sector were observed at all the stations from high- to low-latitudes, whereas Pi 2s at 1254 and1618 UT outside the midnight sector (23–01 hr LT) did notappear at lower latitudes. It is noteworthy that when themeridional chain stations are located in the midnight sectorof 23–01 LT, we can see Pi 2 pulsations from high- to lowlatitudes, while when the chain stations are located outside
the midnight sector, we sometimes miss low-latitude Pi 2s.
The not-perfect one-to-one correspondence between the auroral breakups and the Pi 2 pulsations as shown in Fig. 10
can be explained by the observational fact that Pi 2 pulsations outside the midnight sector of 23–01 hr LT sometimes
cannot appear at lower latitudes.
Further study is needed to clarify why Pi 2s sometimes
do not appear at lower latitudes outside the midnight sector.
This phenomenon may be dependent on the magnitude of
the substorm, the location of auroral breakups, the generation mechanism of cavity-like oscillations in the nighttime
inner magnetosphere, the propagation mechanism of Pi 2from high to lower latitudes, etc.

In order to investigate the occurrence region of Pi 2 andits propagation mechanisms in the magnetosphere, Uozumi(2000) analyzed ground magnetometer data from the CPMN
four stations at Kotel’nyy (KTN: MLAT = 69.94◦ ,MLON = 201.02◦ ), Tixie (TIX: 65.67◦ , 196.88◦ ),Chokurdakh (CHD: 64.67◦ , 212.12◦ ) and Kotzebue (KOT:64.52◦ , 249.72◦ ). The times when the Pi 2 magnetic energy
becomes maximum at each station were compared among
the stations, and the ratios of the maximum energy among
the stations were also compared. From the analyzed results,
it is found that high-latitude Pi 2s (having the maximum
magnetic energy around ∼22.5 MLT) show apparent eastward and westward propagation from the center of ∼22.5
MLT. In consideration of the observed time delays among
the stations and numerical estimation of Alfvén transit timefrom a point in the magnetosphere to the ground station,
Uozumi (2000) suggested that high-latitude Pi 2s must be
generated around Xsm = −9R E and 22.5 MLT on the equatorial plane in the magnetotail.
On the other hand, Liou et al. (2001a) recently used Polar UVI and Wind observations to study the location of 648
well-defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field(IMF) orientation and season. The mostly likely onset location is found to be at 2230 MLT and 67◦ magnetic latitudewith half-maximum widths of 3 hrs of MLT and 2◦ magnetic latitude, respectively. The onset longitude depends on
season and IMF By. In summer, substorms tend to occur
in the early evening at ∼2200 MLT, wherese in winter theytend to occur near midnight at ∼2300 MLT. It is noteworthy
that the location of auroral breakups identified statistically990

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

Fig. 11. Three examples of small substorms with Pi 2 pulsations observed by the 210◦ MM stations at KTN, TIX, CHD, MGD, MSR, LNP, and EWA (+4hr LT) in the pre-midnight (∼22 hr LT), midnight (∼00 hr LT), and post-midnight (>01 hr LT) sectors of the 210◦ MM. (Top) Amplitude-time recordsof the H-component magnetic field at KTN during 10–20 hr UT on January 2, 1997. (Bottom) Pi 2 pulsations observed at the 210◦ MM stations at 1224
UT in the pre-midnight, 1504 UT in the midnight, and 1618 UT in the post-midnight sector.

by the Polar satellite is consistent with the estimated location where high-latitude Pi 2s are generated in the magnetotail (Uozumi, 2000). However, the high-latitude Pi 2s havea maximum amplitude in the narrow local midnight sectorof 23–01 hr LT as shown in Fig. 5, while at middle, low,and near-equatorial latitudes, two maximum amplitudes appear around local midnight and around 21 hr LT. The amplitude maximum around the local midnight must be associatedwith the formation of the SCW (McPherron et al., 1973),
while the other maximum around 21 hr LT might be associated with the onset location of auroral breakups around 2230
MLT (Liou et al., 2001a).
Finally, Li et al. (1998b) examined the time difference
between the auroral breakups identified by the Polar satellite
and the Pi 2 onsets identified by using high-resolution datafrom the 210◦ MM stations. The time resolution of theauroral image of the Polar satellite is 38 seconds and Pi 2’s
period is 40 to 150 seconds; therefore the accuracy of thetime difference is 1 min. The upper panel of Fig. 12 shows
the time difference between the auroral breakups and 67
associated Pi 2 events observed at the single low-latitude
station MSR ( = 37.3◦ , L = 1.6). The lower panel
shows a histogram of the time difference in 1-minute bins.
From a comparison of the onset times of low-latitude Pi 2
pulsations at the 210◦ MM station and auroral breakups at
the Polar satellite, it is found that 66% of low-latitude Pi 2s
occur within 1 min of the auroral breakups, and 85% occur
within 2 min.
Liou et al. (2001b) compared the onset of dispersionlessenergetic particle injections, observed as a sudden increaseof energetic electron and ion flux on a timescale of ∼1 min
at L = 6.6, with the start of auroral breakups by the LANL
satellites. They suggested that the averaged lag time forparticle injections compared to the auroral breakups is 1.8
min with a standard deviation of 2.5 min, and concluded thatparticle energization must take place in the magnetotail ∼1
min earlier than the start of the explosive auroral substorm
onset. With respect to the relationship of the onset timeof low-latitude Pi 2s with the auroral breakups as shownin Fig. 12, the comparison between the onset of energetic
particle injections at L = 6.6 and the start of the auroral
breakups (Liou et al., 2001b), and the Alfvén transit time(∼1 min) of Pi 2 waves from the magnetotail around 9
R E into the polar ionosphere, we suggest that the particle
energization and the Pi 2 generation may take place almost
at the same time in the magnetotail.

5.

Summary and Conclusions

From analyses of magnetic field data from the CPMN
stations, the following observational results were obtained:(1) Pi 2 magnetic pulsations observed on the groundare an ensemble of various hydromagnetic modes excited
at high, middle, low, and equatorial latitudes as shown in
Fig. 2. Each mode exhibits its own propagation (timing)and spatial illumination.
(2) There are apparent longitudinal propagation and latitudinal time delays of the maximum Pi 2 magnetic energy among the CPMN stations, suggesting the generation

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

991

Fig. 12. (Top) Time difference between auroral breakup events in Fig. 10 and 67 associated Pi 2 events observed at low latitude (MSR), where a negative
time difference means a delayed Pi 2. (Bottom) A histogram of the time difference in 1-minute bins (Li et al., 1998b).

of high-latitude Pi 2s around geocentric distance at Xsm =−9R E and 22.5 MLT on the equatorial plane in the magnetotail.
(3) When a single low-latitude station is located in the
sectors of 00–24 hr LT, 20–03 hr LT, and 23–01 hr LT, respectively, 62%, 79%, and 100% of auroral breakups identified by the Polar satellite have associated Pi 2s (see Fig. 10).
66% of the low-latitude Pi 2s occur within 1 min of the auroral breakups, and 85% occur within 2 min (see Fig. 11).
From these observational results, when a single groundstation is located at the midnight sector of 23–01 hr LT, almost 100% one-to-one correspondence can be expected between the auroral breakups and low-latitude Pi 2s. But when
the ground station is located outside the midnight sector, we
can see only a 60–80% one-to-one correspondence between
Pi 2 onsets and auroral breakups with a 1–2 min time delay. The Pi 2s are still a good indicator of substorm onsets, butthey should be used with care.
Further observational and theoretical works are needed toclarify why Pi 2s sometimes do not appear at lower latitudes outside the midnight sector (23–01 hr LT), as shownin Fig. 11. This phenomenon may be dependent on the magnitude of the substorms, the location of auroral breakups,
the generation mechanism of cavity-like oscillations in thenighttime inner magnetosphere, the propagation mechanismof Pi 2s from high to lower latitudes, etc. There are no existing theories that explain why the second amplitude peak appears around 21 hr LT at mid- and low-latitudes, and why theequatorial amplitude enhancement has a maximum around10 hr LT as shown in Fig. 5. Theoretical work is also needed
to investigate the transmission of Pi 2 electric fields in themagnetosphere-ionosphere-atmosphere-solid Earth systemfrom the nightside auroral latitude to the dayside equator.
In considerations of the relationship of the onset timesof low-latitude Pi 2s with the auroral breakups as shownin Fig. 12, the comparison between the onset of energetic
particle injections at L = 6.6 and the start of the auroral
breakups (Liou et al., 2001b), and the Alfvén transit time(∼1 min) of Pi 2 waves from the magnetotail around 9
R E into the polar ionosphere, we suggest that the particle
energization and the Pi 2 generation must take place almost
at the same time in the near magnetotail.
This review of Pi 2 magnetic pulsations observed at the
CPMN stations demonstrates the usefulness of multi-station
observations. Imaging the Earth’s magnetosphere by using
ground-based magnetometer arrays is reaffirmed as one of
the major techniques for investigating the dynamic featuresof solar wind-magnetosphere interactions. Magnetic field
1-sec data from coordinated ground-based network stations
make it possible to (1) study magnetospheric processes by
distinguishing temporal changes and spatial variations in thephenomena, (2) clarify the global structures and propagation
characteristics of magnetospheric variations in the latitudinal and longitudinal directions, and (3) understand the generation mechanisms of the global solar-terrestrial phenomena. Considering the essential importance of simultaneous
satellite-ground observations and the cost-effectiveness of
expensive satellites, we propose that any in situ measurements by the ISTP (GEOTAIL, WIND, POLAR, SOHO,
CLUSTER, and INTERBALL) satellites should be coordinated with ground-based network observations for detecting fundamental geophysical phenomena such as magnetic

992

K. YUMOTO AND THE CPMN GROUP: CHARACTERISTICS OF PI 2

storms, magnetospheric substorms, and global-mode ULF
waves.
Acknowledgments. Our sincere thanks go to all members of the
CPMN project for their ceaseless support. The authors would liketo express their thanks to Dr. C.-I. Meng for his kind permission ofcollaborative analysis between the Polar image data and the CPMN
data. The CPMN project is financially supported by the Ministryof Education, Science and Culture of Japan (and Japan Society for
the Promotion of Science) as the Grant-in Aid for Overseas Scientific Survey (05041060, 0841105, 10041122, 12373003). The
editor thanks P. Sutcliffe and Y. Tonegawa for their assistance inevaluating this paper.

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K. Yumoto (e-mail: yumoto@geo.kyushu-u.ac.jp) and the CPMNGroup