THE STEP 210° MAGNETIC MERIDIAN NETWORK PROJECT

K. Yumoto* and the 210° MM Magnetic Observation Group

J. Geomagn. Geoelectr., 48, 1297-1309, 1996
The first draft of June 10, 1995
Revised on March 11, 1995
Accepted on February 5, 1996

ABSTRACT

Imaging the Earth's magnetosphere by using ground-based magnetometer
arrays is still one of the major techniques for investigating the dynamical
features of solar wind-magnetosphere interactions. The organized ground
network data of magnetic fields make it possible (1) to study the magnetospheric
processes by distinguishing between temporal changes and spatial variations
in the phenomena, (2) to clarify the global latitudinal structures and
propagation characteristics of magnetic variations from high to equatorial
latitudes along the magnetic meridian (MM), and (3) to understand the
global generation mechanism of magnetospheric phenomena. During the international
Solar Terrestrial Energy Program (STEP) period of 1990-1997, multinationally
coordinated magnetic observations are being conducted along the 190o,
210o, and 250o MMs from high latitudes through middle and low latitudes
to the equatorial region, spanning L=8.50-1.00, in cooperation with 29
organizations in Australia, Indonesia, Japan, Papua New Guinea, Philippines,
Russia, Taiwan, and the United States. In this paper, we review the 210o
MM Magnetic Observation Project and its initial results.

1. INTRODUCTION

A major new international scientific program, the Solar Terrestrial Energy
Program (STEP) commenced in 1990 and will continue through 1997. Its
purpose is to trace the flow of energy and plasma from the upstream solar
wind through the magnetosphere and ionosphere to the biosphere. The ionospheric
signatures of the magnetospheric energy transfer process can be recorded
on the ground by using appropriate magnetometer networks. Topical Group
2.2 (project leader, S. Kokubun, Solar-Terrestrial Environment Laboratory
(STEL), Japan), set up by Working Group 2 for the STEP, is concerned
with Coordinated Ground-Based Magnetic Observations for Studies on Response
of the Magnetosphere and Magnetosphere-Ionosphere Coupling (COMOSM).
Japanese ground-based observation teams proposed a globally coordinated
magnetic observation program during the STEP period to study energy and
plasma transfer processes and global auroral dynamics (e.g., STEP_GBRSC_News,
August 1991). In order to organize the observations efficiently, working
plans were grouped into four categories based on region: polar region,
high-latitude conjugate region, middle and low latitudes, and equatorial
zone.
During the STEP period, the STEL, Nagoya University (K. Yumoto, K. Shiokawa,
Y. Tanaka), conducts multinationally coordinated magnetic observations
along the 190o, 210o, and 250o magnetic meridians (MMs) in cooperation
with and/or courtesy of the following institutes and organizations (names
of co-investigators and assistants are in parentheses): University of
Newcastle (B.J. Fraser, F.W. Menk), Electronics Research Laboratory,
DSTO, Salisbury (K.J.W. Lynn, D.M. Sutton), CSIRO Tropical Ecosystems
Research Centre (L.K. Corbett, Tony Hertog), Learmonth Solar Observatory
and Canberra Observatory of IPS Radio and Space Services (D.G. Cole,
J.A. Kennewell, R. Marshall), Weipa North State School (Michael and Chrisella
Sbrizzi), Albatross Hotel (Ron Doherty), Birdsville Police Station (J.G.
Guan, Owen T. Harms), Dalby Agriculture College (L.R. Harris), Katanning
Research Station, Department of Agriculture (D.H. Ryall, T. Bell), and
Australian Antarctic Division (R.J. Morris) in Australia; National Institute
of Aeronautics and Space (LAPAN) (S.L. Manurung, Obay Sobari, Mamat Ruhimat,
Sukmadradjat) in Indonesia; Tohoku University (T. Takahashi, T. Tamura,
T. Saito), Tohoku Institute of Technology (M. Seto, Y. Kitamura), Kakioka
Magnetic Observatory (S. Tsunomura), Tokai University (T. Sakurai), University
of Tokyo (K. Hayashi), and Kyushu University (T.A. Kitamura, O. Saka,
H. Tachihara) in Japan; Paradise Wewak Hotel (S. Kawabata) and University
of Papua New Guinea (D. Yeboah-Amankwah) in Papua New Guinea; Coast &
Geodetic Survey Department, National Mapping and Resource Information
Authority (Commodore Renato B. Feir, V.C. Dandoy, A.A. Algaba) in the
Philippines; Institute of Cosmphysical Research and Radiowaves Propagation
(IKIR) (E.F. Vershinin, A. Buzevich, V. Filimonov), Institute of Cosmophysical
Research and Aeronomy (IKFIA) (G.F. Krymsky, S.I. Solovyev, N. Molochushkin),
and Institute of Physics of the Earth (IFZ) (V.A. Pilipenko, L. Baransky)
in Russia; Lunping Observatory, Telecommunication Training Institute
(Y.-H. Huang, S.-W. Chen) and Institute of Space Science, National Central
University (J.K. Chao, J.Y. Liu) in Taiwan; and U.S. Geological Survey
(A.W. Green, D.C. Herzog), Guam Magnetic Observatory (P. Hattori), Pacific
Tsunami Warning Center (M. Blackford, W.J. Mass, R.K. Nygard), Koror
Observatory of the U.S. National Weather Service (Hirao Kloulchad), and
University of Alaska, Fairbanks (S.-I. Akasofu, J. Olson, D. Osborne)
in the United States.

2. OBSERVATIONS_AND_DATA

In July 1990, we installed the first fluxgate magnetometer systems [Yumoto
et al., 1992] of the array at Moshiri and Kagoshima in Japan and at Adelaide,
Birdsville, and Weipa in Australia. Fluxgate magnetometer data from the
Chichijima station were obtained courtesy of the Kakioka Magnetic Observatory.
Figure 1 shows these 190o, 210o, and 250o MM stations and their geographic
coordinates. The Birdsville site (L=1.55) is near the magnetic conjugate
point of Moshiri (L=1.59) in Japan. The Chichijima (L=1.14), Weipa (L=1.18),
and Adelaide (L=2.11) sites are near the same meridian as the conjugate
point stations. The Kagoshima site (L=1.22) is situated 2o north of the
conjugate point of Darwin (L=1.18) which is 12o of geomagnetic longitude
west of the Weipa station. At Ewa Beach, Hawaii (L=1.17, = 269.36o),
the magnetometer system was installed in January 1991.
In June 1991, fluxgate magnetometer observations were also started at
Onagawa, Japan (L=1.38), Wewak, Papua New Guinea (L=1.06), and Guam (L=1.01).
We completed installation of magnetometer systems at Dalby (L=1.57),
Darwin (L=1.18), and Learmonth (L=1.46) in Australia in summer 1991 and
at Biak, Indonesia (L=1.05) in May 1992. We are now extending the 210o
MM chain to high northern latitudes in Siberia in cooperation with IKFIA,
IKIR, and IFZ of Russian Academy of Science. The magnetometer systems
were installed at St. Paratunka (L=2.10), Magadan (L=2.83), Chokurdakh
(L=5.46), and Tixie (L=5.89, =197.06) in August 1992. Conjugate magnetic
observations at Kotzebue, Alaska (L=5.40, =249.72o), and Macquarie island,
Australia (5.40, 247.84o), started in November 1993 and November 1992,
respectively. Magnetic observations at Muntinlupa (L=1.00, =191.57o)
near the magnetic equator were also started in cooperation with Coast
& Geodetic Survey Department, Philippines, in July 1993.
In January 1994, magnetic observations were started at Lunping Observatory
(=189.50o, L=1.06), Telecommunication Training Institute in cooperation
with the Institute of Space Science, National Central University, Taiwan.
Installations of the magnetometers at Zyryanka (L=3.91) and Kotel'nyy
island (L=8.50) in Siberia were completed by IKFIA in April 1994 and
October 1994, respectively. To clarify relationships between auroral
electrojets observed at Kotzebue and equatorial electrojets observed
near the magnetic equator, a fluxgate magnetometer was installed at Koror
(L=1.00) by the Geophysical Institute of the University of Alaska in
August 1994.
All-sky television cameras and photometers were also installed at Moshiri,
Japan (L=1.59), in October 1991, at Tixie, Russia (L=5.89), in March
1994, and at Canberra, Australia (L=2.07), and Kotzebue, Alaska (L=5.40),
in August 1994. The scientific objective of the optical instrumentation
is to investigate low- and high-latitude auroras and the relationship
between magnetic and optical variations in them. Optical data from Tixie
and Kotzebue will be analyzed in conjunction with all-sky television
and photometer data from Resolute and Cambridge Bay in the Canadian Arctic.

Table 1 summarizes station names, geographic and geomagnetic coordinates,
and L values of proposed observation sites, including established stations,
where additional instruments with high time resolutions will be installed
during the STEP period. The IGRF-90 model was used to calculate corrected
geomagnetic coordinates and L values for a 100-km altitude at each station
on January 1, 1993. Months of commencement and abbreviations of institutes
and organizations that support or collaborate with STEL's magnetic observation
team are also given in Table 1.
Magnetic variation data (H, D, Z, dH/dt, dD/dt, dZ/dt) from all stations
except Chichijima were obtained with ring-core fluxgate magnetometers
with identical logging systems (DCR-3, KOSMO Ltd.) and time signal generators,
as shown in detail by Yumoto et al.[1992]. The resolutions of ordinary
analog outputs VO(H, D, Z) in the 0- to 2.5-Hz frequency range are +300,
+1000, and +2000 nT/+10 V for low-, middle-, and high-latitude stations,
respectively. The time-derivative components (VTD; dH/dt, dD/dt, dZ/dt)
were obtained by putting an analog circuit at the output terminals of
the ordinary components (V0). VTD outputs in the frequency range of 0.0-0.1
Hz exhibit essentially the same frequency response as an induction magnetometer.
The noise level of the magnetometer system is lower than 0.1 nT rms(root
mean square) equivalent. The six magnetic signals (H, D, Z, dH/dt, dD/dt,
dZ/dt) and time pulses (1 min, 1h, 24 h) are registered on a digital
cassette tape by means of a digital data logger with a sampling rate
of 1 s and 16-bit resolutions of 0.012, 0.039, and 0.078 nT/LSB(Least
Significant Bit) at low-, middle-, and high-latitude stations, respectively.
Each cassette tape holds 21 days of data. Fluxgate magnetometer data
from the Chichijima station of the Kakioka Magnetic Observatory are obtained
by the same logging system. Time pulses (1 min, 1 h, 24 h) from the time
signal generator are also recorded on the digital cassette tape as a
way of checking the crystal clock inside the data logger. The accuracy
of the time signal generator is maintained to within +25 ms by automatic
comparisons with standard radio transmissions (WWVH, JJY, and WWV) from
Maui, Hawaii; Koganei, Japan; and Boulder, Colorado, respectively.
Figures 2a and b show one example of the H and D components of ordinary
magnetograms from 210o MM chain stations Tixie (L=5.89,=197.06o), Chokurdakh
(5.46, 212.12o), Kotzebue (5.40, 249.72o), Magadan (L=2.83), Paratunka
(2.10), Moshiri (1.59), Onagawa (1.38), Kagoshima (1.22), Chichijima
(1.14), Ewa Beach (1.17, 269.36o), Muntinlupa (1.00), and Guam (1.01)
in the Northern Hemisphere and stations Biak (1.05), Darwin (1.18), Learmonth
(1.46), Birdsville (1.56), Dalby (1.57), Adelaide (2.11), and Macquarie
Island (L=5.40,=247.84o) in the Southern Hemisphere. In latitudes lower
than ||=55o, the H-component magnetic bay variations around 1800 UT at
the northern and southern stations show an in-phase relation, while the
D components at the northern and southern stations show a 180o out-of-phase
relation. Amplitude ranges of the H components are of almost the same
order at the lower latitudes, but the D components increase exponentially
from the magnetic equator to higher latitudes.

3. DATA_EXCHANGES

Routine magnetic observations at the 190o, 210o, and 250o MM chain stations
will continue during the entire STEP period (1990-1997). For effective
data exchanges and analyses, the magnetic data are being compiled at
the STEL, Nagoya University, for STEP investigators. Data catalogue,
one minute-averaged plots, and 4 second-averaged pulsation data for 1990-1995
can be found through the STEL homepage at World Wide Web (http://www.stelab.nagoya-u.ac.jp/,
and click "STEP Database Catalog" and "210 Magnetic Data"). One-min digital
data are open by Internet file transfer through the STEP networks of
a UNIX workstation (NEC EWS/4800). The data are available by means of
3.5- and 5-in. diskettes for NEC or IBM personal computers, magnetic
tapes in the IBM format, copies of an optical disk for NEC personal computers,
and copies of quick-look summary plots.
The one minute-averaged daily magnetogram and pulsation data from the
210o MM network are distributed to STEP investigators. The data are not
calibrated and are only for quick-look. The plots cannot used at any
publication or presentation without the permission by the principal investigator
of the 210o MM team, K. Yumoto (yumoto@geo.kyushu-u.ac.jp). The user
is requested to offer an authorship to the PI and members of the 210o
MM team when the 210o MM data are essential in the publication and presentation.
If no one on the 210o MM team participates as an author, the paper should
acknowledge the 210o MM team and the STEL for the use of the database
and should refer to the following papers: Yumoto et al., J._Geomagn._Geoelectr.,
44, 261-276, 1992 and 47, 1197-1213, 1995. High-time-resolution (1-s)
data can ordinarily be used for collaborative studies with the 210o MM
team. Scientists conducting research using 210o MM data must contact
the PI of the 210o MM Observation Project, who will organize the joint
studies.

4. INITIAL_RESULTS

The 210o MM Magnetic Observation Project was outlined by Tanaka and Yumoto
[1993], Yumoto [1994], and Yumoto et al.[1991, 1992, 1993b]. The importance
and effectiveness of multistation network observations for obtaining
spatiotemporal information on solar-terrestrial phenomena are emphasized.
Using data from the 210o MM network stations and satellites, we are studying
the STEP objective and obtaining preliminary results [see Yumoto, 1995].
The scientific items and related publications are as follows: (1) magnetospheric
response to interplanetary shocks and discontinuities (sudden commencements
[sc] and sudden impulses [si]) [Araki et al., 1994; Petrinec et al.,
this issue; Yumoto et al., 1994b, e, this issue], (2) the global dynamics
of low- and high-latitude auroras [Shiokawa et al., 1994, 1995a, b, 1996a,
b, c, this issue; Yumoto, 1995; Yumoto et al., 1994a, c], (3) relations
between in situ and ground (or separated ground) observations of substorm
phenomena [Kawano et al., 1994, 1996; Kokubun et al., 1996; Nakamura
et al., 1996; Shiokawa et al., this issue], (4) global characteristics
of Pc 3 waves [Matsuoka et al., 1997; Menk and Yumoto, 1994; Pilipenko
et al., 1995; Yumoto et al., 1992] and Pi 2 waves [Osaki et al., this
issue; Shiokawa et al., this issue; Yumoto et al., 1994d], (5) mass loading
effect on Pc 3 waves in the low-latitude ionosphere [Pilipenko et al.,
1996; Yumoto, 1995; Yumoto et al., 1993a], and (6) miscellaneous (earthquake-related
ULF waves [Hayakawa et al., 1996] and geophysical induction current [Seto
et al., 1996; Yumoto and Utada, 1993]).
In particularly, we obtained new findings from analyses of coordinated
ground-based observation data from the 210o MM stations. The main results
[cf. Yumoto, 1995] are summarized here.

North-South_Asymmetry_of_sc/si_Disturbances

North-south asymmetry of sc and si disturbances at low and middle latitudes
indicates that the DP component of sc and si is larger than the DL component;
i.e., electric field penetration into the equatorial ionosphere plays
an important role on energy transfer from high latitudes to the magnetic
equator [Yumoto et al., this issue].
sc are a global magnetospheric phenomenon caused by interplanetary shocks
and other discontinuities. When the interplanetary magnetic field (IMF)
turns southward or becomes turbulent behind an interplanetary shock or
discontinuity, geomagnetic storms can develop following so-called storm
sudden commencements (ssc). When the IMF remains to the north behind
the shocks or discontinuities, sc are not followed by geomagnetic storms.
Although such sc have been called si, there is no difference between
the physical mechanisms producing ssc and si. The sc can be used to study
transient responses of the magnetosphere, ionosphere, and conducting
Earth system to dynamic pressure variations in the solar wind.
The disturbance fields of sc and si observed on the ground can be decomposed
into two subfields, DL and DP. The DL field is produced by electric currents
flowing on the magnetopause and a propagating compressional wave front
in the magnetosphere. DP fields are produced by twin vortex-type ionospheric
currents, which are caused by a dawn-to-dusk electric field transmitted
to the polar ionosphere. In order to examine which components of the
DL and DP fields dominate sc and si magnetic variations on the ground,
i.e., to investigate transfer processes and latitudinal structures of
sc and si disturbances from the magnetopause through the magnetosphere
to the Earth's surface and/or from the magnetopause to the polar ionosphere
and thence to the magnetic equator, we analyzed magnetic field data from
the 210o MM chain of stations. We statistically analyzed 41 events of
sc and si magnetic variations and long-period (around 1-h) variations
in the initial phases of magnetic storms observed along the 210o meridian
during the 15 months from November 1992 through January 1994 and found
that the amplitudes of sc and si at low and middle latitudes are larger
in the summer hemisphere than in the winter hemisphere. We also used
the 210o MM data to confirm the enhancement of sc and si amplitudes near
the dayside equator.
The north-south asymmetry of sc and si disturbances at middle and low
latitudes cannot be explained by invoking the Chapman-Ferraro current
on the magnetopause but can be interpreted by invoking an asymmetry in
the Northern and Southern Hemisphere twin vortex-type ionospheric currents,
i.e., by invoking enhanced ionospheric conductivities in the summer hemisphere.

Transfer_of_Solar_Wind_Energy_to_the_Magnetosphere

Low-latitude aurorae provide evidence that solar wind energy can be transferred
into the inner magnetosphere around L=2.5 during magnetic storms [Shiokawa
et al., 1994, 1995a, 1996b, c; Yumoto and Utada, 1993; Yumoto et al.,
1994a, c].
Recent optical and magnetic observations at Moshiri (=44o22'N, = 142o16'E)
of the STEL indicate that even during moderate magnetic storms, invisible
low-latitude aurorae sometimes occur in concert with H > 50 nT positive
magnetic excursions and large-amplitude Pi magnetic pulsations around
a minimum Dst index of -200 nT and/or in concert with sudden decreases
in Dst at a rate of >30 nT/h [Yumoto et al., 1994a, c]. Optical instrumentations
(photometers, all-sky television cameras) were used to identify six events
of invisible low-latitude aurorae at Moshiri and Rekubetsu (= 43.46o,
=143.77oE, L=1.6) in Hokkaido, Japan, that occurred between February
1992 and September 1993 [Shiokawa et al., 1994].
All low-latitude aurorae observed in 1992 exhibited a vortical equivalent
ionospheric current patterns, and four of them also showed westward movement
[Yumoto et al., 1994a]. Although we don't yet have sufficient evidence
to provide it, we still propose that the magnetic perturbations associated
with low-latitude aurorae correspond to changes in substorms and local
overhead currents that involve strong perturbing forces that act on the
trapped-particle population and precipitate large fluxes of low-energy
electrons into the thermosphere. The large oscillations in the D component
of the magnetic field can be explained by the intensification of upward
(and downward) field-aligned currents (FACs) within an ionospheric Hall
current vortex (or vortices) and their spatial movement, as shown in
Fig. 15 of Yumoto et al.[1994c]. The upward FAC must be associated with
intense precipitation of energetic electrons at latitudes below that
which is normal for the auroral oval. The direct injection of energy
from the ring current in the form of particle precipitation into the
upper thermosphere must be very significant and must affect the dynamics
of the thermosphere and ionosphere during magnetically disturbed times.
Particle precipitations into the low- and midlatitude thermosphere and
the occurrence of low-latitude aurorae coincide with large values of
the Dst index, which are proportional to the total energy content of
the trapped particles that constitute the ring current.
Shiokawa et al.[1996b, c] recently used plasma particle observations
made by the DMSP-F10 satellite to confirm the intense precipitation of
energetic electrons associated with the May 10, 1992, low-latitude auroral
event. The electron precipitation did not appear in an expanded auroral
oval but rather in an isolated region around 50o magnetic latitude. This
precipitation exhibited an unusual acceleration process in which electrons
at all energies measured (32 eV to 30 KeV) were intensified. Nevertheless,
we conclude that the intensity variations of optical emissions and magnetic
perturbations during low-latitude auroral events are associated with
variations of the upward FAC (and/or electron precipitation) in a localized
region of L=2.5 and on a shorter time scale.

Cavity-mode-like_Oscillations

Interplanetary impulses can readily stimulate a standing Pc 3-4 field
line oscillation in the inner magnetosphere but don't easily excite cavity
resonance oscillations at low latitudes. Only great sc and si can stimulate
cavity-mode-like oscillations within the daytime plasmasphere [Yumoto
et al., 1992, 1994b].
The 210o magnetometer network data were also analyzed to determine whether
or not global cavity-mode and localized field line oscillations can be
excited in the inner magnetosphere by interplanetary impulses (sc/si)
[Yumoto et al., 1994b]. Two types of Pc 3-4 magnetic pulsations were
stimulated at low latitudes just after 13 sc and si events. Most of the
Pc 3-4 pulsations were standing field line oscillations with maximum
power densities around L=1.6 and/or higher latitudes, while only four
global cavity-mode-like oscillations with larger amplitudes at lower
latitudes (||<30o) were stimulated by extremely large sc and si within
the dayside plasmasphere.
Spectral peak power densities of the 38 oscillations measured at Moshiri
(L=1.59) just after the sc and si events as a function of magnetohydromagnetic
mode and local time when the events happened show that cavity-mode-like
oscillations tend to be observed only when the 210o MM chain stations
are located within the daytime sector from 0900 to 1500 LT, where the
level of magnetic activity is KP > 7+ and the stimulated power density
just after the sc event is larger than 600 nT2/Hz (see Table 2 of Yumoto
et al. [1994b]). This result implies that the interplanetary impulses
must be sufficiently large to drive cavity-mode-like waves. We conclude
that Pc 3-4 cavity-mode-like oscillations at low latitudes are not easily
stimulated by the external impulses in the solar wind and tend to be
excited within the dayside plasmasphere only by great sc and si.

Mass_Loading_Effect_at_Near-equatorial_Latitudes

Pc 3 magnetic pulsations at low latitudes indicate an abnormal dependence
of the resonant period on L and a drastic increase in ionospheric damping,
i.e., a so-called mass loading effect on Pc 3 oscillations in the near-equatorial
latitude ionosphere (| |<30o) [Pilipenko et al., 1996; Yumoto, 1995;
Yumoto et al., 1993a].
Magnetospheric ULF field line oscillations at middle and high latitudes
have been thoroughly studied for many years with the numerous facilities
of modern geophysics, but much less is known about the physical nature
of ULF waves at low and near-equatorial latitudes. Low-latitude magnetic
field line oscillations may have a number of characteristic features
that distinguish them from the well-known midlatitude geomagnetic pulsations
[e.g. Pilipenko et al., 1996; Yumoto, 1995]. These peculiarities are
related to the primary influence of ionospheric ions on field line oscillations.
New facilities for the experimental study of the spatiospectral structure
of ULF pulsations became available after the 210o MM project began. In
these studies we have found a peculiarity of Pc 3 pulsations at low latitudes,
i.e., increasing period with decreasing magnetic latitude, and we have
compared the observations with the results of numerical models of the
magnetospheric resonator.
Although a detailed comparison between various ionospheric plasma models
and observational data is required, ULF observations could be used as
a low-latitude extension of whistler observations to monitor plasma density
variations in the plasmasphere, where in situ plasma data are not easily
obtained by satellites.

Latitudinal_Profiles_of_Pi_2_Pulsations

The latitudinal profiles of Pi 2 phase relations and amplitudes imply
that Pi 2 pulsations observed at low and high latitudes consist of different
mode oscillations [Osaki et al., this issue; Shiokawa et al., this issue;
Yumoto et al., 1994d].
At the onset of a magnetospheric substorm, transient hydromagnetic oscillations
with periods of 40-150 s, called Pi 2 magnetic pulsations, are excited
globally in the magnetosphere. One possible source of nighttime Pi 2
pulsations is a sudden change in magnetospheric convection or configuration
during the substorm expansive phase. This change would be caused by a
plasma flow from the reconnection (or current disruption) region or by
the formation of a substorm current wedge. Many workers have studied
Pi 2 magnetic pulsations on the ground and in space, but the generation
and propagation mechanisms of these pulsations are still only partly
understood.
In order to check the existing Pi 2 model, we analyzed data from stations
along the 210o MM. We found the followings for "low"-latitude Pi 2 pulsations:
(1) Pi 2 pulsations have similar wave forms at lower latitudes; (2) H-component
Pi 2 pulsations at the northern and southern stations show an in-phase
relation, and D components at the northern and southern stations show
a 180o out-of-phase relation; (3) amplitudes of the H components are
independent of geomagnetic latitudes and are almost the same at all stations,
but D components depend on latitude and increase exponentially from the
magnetic equator to higher latitudes. The first two of these observational
facts suggest that the field line resonance theory is inadequate to explain
all of the characteristics of low-latitude Pi 2 pulsations. If each field
line oscillated with its own eigenperiod, then the dominant period of
magnetic pulsations would vary from place to place.
The observed latitudinal characteristics indicate that Pi 2 magnetic
pulsations may be explained by a global cavity-mode oscillation with
phase reversal near the plasmapause, the substorm current wedge model-like
oscillation, and auroral field line oscillations with field-aligned and
ionospheric current variations. However, whether the existing Pi 2 models
are consistent with these observational results is still an open question.
In order to answer the question, in the near future we will further analyze
the 210o MM chain data, including data from around the plasmapause and
simultaneously obtained by the AKEBONO, GEOTAIL, CLUSTER satellites.

5. CONCLUSION

A review of some initial 210o MM network observations demonstrates the
usefulness of multistation observations. Imaging the Earth's magnetosphere
by using ground-based magnetometer arrays can be reaffirmed as one of
the major techniques for investigating the dynamical features of solar
wind-magnetosphere interactions. Magnetic field 1-s data from coordinated
ground stations make it possible to study magnetospheric processes by
distinguishing between temporal changes and spatial variations in the
phenomena, to clarify global and latitudinal structures and propagation
characteristics of magnetospheric variations from higher to equatorial
latitudes, and to understand the global generation mechanisms of the
solar-terrestrial phenomena. Considering the essential importance of
simultaneous satellite-ground observations and also the cost-effectiveness
of expensive satellites, we propose that any in situ measurements by
the GEOTAIL, WIND, POLAR, SOHO, CLUSTER, and INTERBALL satellites should
be coordinated with the COMOSM networks for observing fundamental geophysical
phenomena such as magnetic storms, magnetospheric substorms, and global-mode
ULF waves.
After the STEP period (1990-1997), coordinated ground-based observations
along the 190o, 210o, and 250o MMs will be continued by adding various
remote-sensing techniques. The scientific objectives of the new phase
will be to investigate the effects of high-energy particles on the middle
atmosphere during solar proton and low-latitude auroral events, relationships
between the eastward auroral electrojet at high latitudes and the equatorial
electrojet in daytime, and long-term variations of the core magnetic
field. The 210o observation network will be further connected to the
Chinese meridian chain along the 120o geographic longitude that is organized
by the Institute of Geophysics, Chinese Academy of Science; with the
Yakutsk meridian chain along the Lena river, organized by IKFIA, Russia;
with the AE-index stations in Siberia, Alaska, and Canada; and with island
stations in the Pacific ocean that will be supported by the Earthquake
Research Institute, University of Tokyo. Campaign-based ground observations
with 1-s time resolutions will be also organized for comparison with
observations by the multiple satellites.

ACKNOWLEDGMENTS---My sincere thanks go to H. Oya, Tohoku University,
and all the members of the 210o MM Magnetic Observation Project for their
ceaseless support. The 210o MM Magnetic Observation Project is also supported
by Y. Tanaka, K. Shiokawa, M. Nishino, T. Kato, M. Sato, Y. Kato, M.
Sera, Y. Ikegami, K. Hidaka, T. Ogawa, T. Watanabe, T. Oguti, and S.
Kokubun of the STEL, Nagoya University. Financial support was given by
the Ministry of Education, Science, and Culture of Japan in the form
of Grants-in-Aid for Overseas Scientific Survey (02041039, 03041061,
04044077, 05041060, 06044094), General Scientific Research (02402015),
Developmental Scientific Research (02504002), and the International STEP
project.

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Yumoto, K., H. Matsuoka, H. Osaki, K. Shiokawa, Y. Tanaka, T. Kitamura,
H. Tachihara, S.I. Solovyev, G.A. Makarov, E.F. Vershinin, A.V. Buzevich,
S.L. Manurung, Obay Sobari, Mamat Ruhimat, Sukmadradjat, R.J. Morris,
B.J. Fraser, F.W. Menk, K.J.W. Lynn, D.G. Cole, J.A. Kennewell, J.V.
Olson, and S.-I. Akasofu, North/south asymmetry of sc/si magnetic variations
observed along the 210o magnetic meridian, this issue.

The Solar-Terrestrial Environment Laboratory, Nagoya University (STEL),
is conducting multinationally coordinated magnetic observations in cooperation
with and/or courtesy of the following institutes and organizations :
University of Newcastle (UNC), Electronics Research Laboratory (DSTO),
CSIRO Tropical Ecosystems Research Centre (TERC), Learmonth Solar Observatory
and Canberra Observatory of IPS Radio and Space Services (IPS), Weipa
North State School (WNSS), Birdsville Police Station (POB), Dalby Agriculture
College (DAC), Katanning Research Station (KRS), and Australian Antarctic
Division (ADD) in Australia; National Institute of Aeronautics and Space
(LAPAN), in Indonesia; Tohoku University (THU), Tohoku Institute of Technology
(TIT), Kakioka Magnetic Observatory (KMO), Tokai University (TKU), University
of Tokyo (GRL), and Kyushu University (UK) in Japan; Paradise Wewak Hotel
(PWH) and University of Papua New Guinea (UPNG) in Papua New Guinea;
Coast & Geodetic Survey Department (CGSD) in the Philippines; Institute
of Space Research and Radiowaves (IKIR), Institute of Cosmophysical Research
and Aeronomy (IKFIA), Institute of Physics of Earth (IFZ), and Pacific
Oceanological Institute (POI), Academy of Science in Russia; Lunping
Observatory (LNP), and National Central University (NCU) in Taiwan; and
Guam Magnetic Observatory, U.S. Geological Survey (USGS), Pacific Tsunami
Warning Center (PTWC), Koror Observatory of the U.S. National Weather
Service and University of Alaska, Fairbanks (UAF) in the United States.

Fig. 1. Map showing geographic coordinates of the 190o, 210o, and 250o
magnetic meridian chain stations. See Table 1 for definitions of abbreviation.

Fig. 2. Amplitude-time records of (a) H- and (b) D-component ordinary
magnetic variations observed on February 19, 1994, at the 210o magnetic
meridian chain stations of TIK, CHD, KOT, MGD, PTK, MSR, ONW, KAG, CBI,
EWA, MUT, and GUA in the Northern Hemisphere, and BIK, DAW,LMT, BSV,
DAL, ADL, and MCQ in the Southern Hemisphere. See Table 1 for definitions
of abbreviations. The magnetic sensitivity at TIK, CHD, KOT, ZYK, and
MCQ and at other stations are 300 and 60 nT/one division, respectively.

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