J. Geomag.

Geoelectr.,

48, 1297-1309,

1996

The STEP 210° Magnetic Meridian Network Project
K. YUMOTOI and THE 2100 MM MAGNETIC OBSERVATION GROUP'Department of Earth and Planetary Sciences, Kyushu University, 33, Hakozaki, Fukuoka 812-81, Japan(Received September 11, 1995; Accepted February 5, 1996)

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 magnetosphericprocesses by distinguishing between temporal changes and spatial variations in the phenomena, (2) toclarify the global latitudinal structures and propagation characteristics of magnetic variations from highto 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 conductedalong the 1900,2 10°, and 250° 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 2100
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 byusing appropriate magnetometer networks. Topical Group 2.2 (project leader, S. Kokubun, SolarTerrestrial Environment Laboratory (STEL), Japan), set up by Working Group 2 for the STEP, isconcerned with Coordinated Ground-Based Magnetic Observations for Studies on Response of theMagnetosphere andMagnetosphere-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, August1991). 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 equatorialzone.

During the STEP period, the Department of Earth and Planetary Sciences, Kyushu University (K.
Yumoto) and the STEL, Nagoya University (K. Shiokawa, Y. Tanaka), conducts multinationally
coordinated magnetic observations along the 190°, 210°, and 250° magnetic meridians (MMs) incooperation 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 Observatoryof 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), Kittanning Research Station, Department ofAgriculture (D. H. Ryall, T. Bell), and Australian Antarctic Division (R. J. Moms) in Australia; National
InstituteofAeronautics and Space(LAPAN) (S. L. Manurung, Obay Sobari, Mamat Ruhimat, Sukmadradjat)
1297

1298

K. YUMOTO and THE 2100 MM

MAGNETIC OBSERVATION GROUP

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.-I. Kitamura, O. Saka, H. Tachihara) inJapan; Paradise New Wewak Hotel (S. Kawabata) and University of Papua New Guinea (D. YeboahAmankwah) in Papua New Guinea; Coast & Geodetic Survey Department, National Mapping andResource Information Authority (Commodore Renato B. Feir, V. C. Dandoy, A. A. Algaba) in thePhilippines; 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; Limping 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; andU.S. Geological Survey (A. W. Green, D. C. Herzog), Guam Magnetic Observatory (P. Hattori), PacificTsunami 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. V.
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 arrayat 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 190°, 210°, and 250° 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 -'2° north of the conjugate point of
Darwin (L = 1.18) which is -12° of geomagnetic longitude west of the Weipa station. At Ewa Beach,
Hawaii (L = 1.17, A = 269.36°), 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 magnetometersystems 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 210° MM chain to high northernlatitudes 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, A = 197.06) in August 1992. Conjugate magnetic observations at Kotzebue,
Alaska (L= 5.40, A=249.72°), and Macquarie island, Australia (5.40,247.84'), started in November 1993
and November 1992, respectively. Magnetic observations at Muntinlupa (L = 1.00, A = 191.57°) near themagnetic 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 (A= 189.50',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 amoral electrojets observed at Kotzebue and equatorial electrojets observed near
the magnetic equator, a fluxgate magnetometer was installed at Koror (L = 1.00) by the GeophysicalInstitute of the University of Alaska in August 1994.
All-sky television cameras andphotometers 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The STEP 2100 Magnetic Meridian Network Project
photometer
Table

data from Resolute and Cambridge
Bay in the Canadian
1 summarizes
station names, geographic
and geomagnetic

1299

Arctic.
coordinates,and L valuesof pro-posed observationsites, including establishedstations, where additional
instrumentswith high time
resolutions will be installed during the STEP period. The IGRF-90 model was used to calculate corrected
geomagnetic
coordinatesand L values for a 100-km altitude at each station on January 1, 1993. Monthsof commencement
and abbreviationsof institutes and organizations
that support or collaborate with the

210°MMSTATIONS

NETWORK90KIN
TIK OCNll
CST
O ZGH
0
YAK
"'

60

~T
BJI

• ZYK

KOT
_

~~

MGD
PTK

PPI

I NSR
ONW

30

LNP
KAG
0ICBI
K0:OYAP.0GUA

WIMUT lkt
PTNJ

N
V

EWA 0 9

BIK

a 0

ILI
DAW
WTK

WEP

LAlT
-30

BSV

0

ASA

DAL

KAT

CAN

ADL
C+

190'

-60

90

Fig. 1. Map showing geographic
definitions of abbreviation.

I
120

coordinates

NCQ

2L0'
260'

150
Longitudeof the 190°, 210°, and 250° magnetic

180

meridian210

chain stations.

See Table t for1300

K. YumoTO

and TEE 2100

MMMAGNETIC OBSERVATION GROUP

210 MM Magnetic Field Data, 1-min Averages
Feb 19 1994 Day 50(a)H Component 300nT/div(TI ,CHD,KOT,ZYK,MCQ)I TF-FT11
11 11111
1
TIK

-

---

--

-------

60nT/div(Others)

----

v-a

CHID
KOT

-V

MGD

-v

ti

~r

PTK

--------------MSR------------

ONW
KAG

CBI
EWA
MUT

------------------

l

GUA
BIK

DAW

-

-----------------

LMT

~"

BSV

-^!>------------~

^- A

DAL
ADL

MCQ

-

0000

----------------

0300

0600

0900

1200
Universal

1500

1600

2100

2400

Time

Fig. 2. Amplitude-time records of (a) H- and (b) D-component ordinary magnetic variations observed on February 19, 1994, atthe 2100 magnetic meridian chain stations of TIK, CHD, KOT, MGD, lTK, 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 I
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.

TheSTEP2100Magnetic
Meridian
Network
Project

1301

210 MM Magnetic Field Data, 1-min Averages
Feb 19 1994 Day 50(b)

D Component 300nT/div(ITK,CHD,KOT,ZYK,MCQ)

6OnT/div(Others)

UK
CHD
KOT
MGD
FrK
MSR
ONW
KAG
CBI
EWA
MUT
GUA
BIK
DAW
LMT
BSV
DAL
ADL
MCQ
0000

0300

0600

0900

1200
Universal

1500
Time

Fig. 2. (continued).

1800

2100

2400

1302

StationK. YIIMOTO and TBE 2100

MM

Table 1. Station names, geographic

and geomagnetic

name-Geographic
coordinates

Tixie
Zhigansk
Yakutsk
IrkutskPopov Island
Beijing
Looping
Muntinlupa
Pontianak
Watukosek
Leannonth
Katanning

Abbreviation

MAGNETIC OBSERVATION GROUP

coordinates,and L values of proposed

Geomagnetic-:
coordinatesL--observation
Associ.
Inst.---

Lat (deg)Long (deg)Lat (deg)Long (deg)

TIK
ZGN
YAK
IRT
PPI

71.59

128.785.89

123.26129.72104.45131.73193.82200.51

4.25
3.21

177.01203.63

2.16
1.55

BJI
LNP
MUT
PTN
WTK
LMT
KAT

40.06
25.00
14.37
-0.05
7.56
-22 .2233.68

116.18121.17121.02109.25112.63114.10117.62

65.67
61.01
56.08
47.13
36.62
34.37
13.80
3.58
-11 .37-18 .52-34 .15-46.63196.88

66.75
62.02
52.17
42.98

188.40189.50191.57180.49183.85185.02
188.24

1.47
1.06
1.00
1.04
1.11
1.46
2.12

IKFIA, IFZ
IKFIA
IKFIA
ISTP
POI
IGCAS
NCU, LNP
CGSD
LAPAN
LAPAN
IPS, UNCKRS, UNC

sites.
Date of
onset

92/895/-95/-

94/193/7-

91/895/8-

210° MM magnetic observation team are also given in Table 1.
Magnetic variation data (AH, AD, AZ, dH/dt, dD/dt, dZ/dt) from all stations except Chichijima wereobtained 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(AH, AD, AZ) 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(Vo). VTDoutputs 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(rootmean square) equivalent. The six magnetic signals (AH, AD, AZ,dH/dt, dD/dt, dZ/dt) and time pulses (1min, 1 h, 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 magnetometerdata 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 digitalcassette 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 2(a) and 2(b) show one example of theHand D components of ordinary magnetograms fromthe 210° MM chain stations Tixie (L = 5.89, A= 197.06°), Chokurdakh (5.46, 212.12°), Kotzebue (5.40,
249.72°), Magadan (L = 2.83), Paratunka (2.10), Moshiri (1.59), Onagawa (1.38), Kagoshima (1.22),
Chichijima (1.14), Ewa Beach (1.17, 269.36°), Muntinlupa (1.00), and Guam (1.01) in the Northern
Hemisphere and stations Biak (1.05), Darwin (1.18), Learmonth (1.46), Birdsville (1.55), Dalby (1.57),
Adelaide (2.11), and Macquarie Island (L = 5.40, A = 247.84°) in the Southern Hemisphere. In latitudes
lower than 101= 55°, the H-component magnetic bay variations around 1800 UT at the northern andsouthern stations show an in-phase relation, while the D components at the northern and southern stations
show a 180° out-of-phase relation. Amplitude ranges of the H components are of almost the same orderat the lower latitudes, but the D components increase exponentially from the magnetic equator to higher
latitudes.

The STEP 210° Magnetic Meridian Network Project

1303

Table 1. (continued).
Stationname*Geographic
coordinates

Abbreviation

Geomagnetic''

L••

coordinates

Associ.

Date

Inst.'"

onsetof

Lat (deg)Long (deg)Lat (deg)Long (deg)

69.94
64.67
59.62
53.56
46.34
37.61
31.65
25.13
20.59
4.57
-0.3
-2 .64-12 .18-13.94-23 .13-46.46201.02212.12
216.72218.66225.91
213.23212.51202.24213.00214.76209.0
205.21207.30215.37202.68214.34212.96226.80226.14213.66

8.50
5.46
3.91
2.83
2.10
1.59
1.38
1.22
1.14
1.01
1.00
1.00
1.05
1.06
1.18
1.18
1.55
1.57
2.07
2.11

IKFIA
IKFIA,IFZ
IKFIA
IKIR
IKIR
STEL
THU
STEL
KMO
USGS
UK
UAF
LAPAN
PWH, UPNG
TERC, UNCWNSS, UNCPOB
DAC, UNC
IPS
DSTO

94/1092/894/492/892/890/791/690/790/791/693/I94/892/591/691/890/790/791/894/890/7-

64.52

249.72

5.40

236.29

5.40

UAF
IKIR

93/11-

64.51
22.67-20 .60-64 .50

269.36

1.17

PTWC/USGS

91/1-

245.05

1.14
5.40

AAD,

92/11-

Kotel'nyy
Chokurdakh
Zyryanka
Magadan
St. Paratunka
MoshiriOnagawa
Kagoshima
Chichijima
Guam
Yap
Koror
Biak
Wewak
DarwinWeipa
Birdsville
Dalby
Canberra
Adelaide

KTN
CUD
ZYK
MGD
PTK
MSR
ONW
KAG
CBI
GUA
YAP
KOR
BIK
WEW
DAW
WEP
BSV
DAL
CAN
ADL

75.94
70.62
65.75
59.97
52.94
44.37
38.43
31.48
27.15
13.58
9.3
7.33
-1 .08-3.33-12.40-12.68-25 .54-27.18-35.30-34.67137.71147.89150.78150.86158.25142.27141.47130.72142.30144.87138.5
134.48136.05143.74130.90141.88139.21151.20149.00138.65

KotzebueCape Shmidt
Ewa Beach
American Samoa
Macquarie Isl.

KOT
CST
EWA
ASA
MCQ

66.88
68.88
21.32
-14 .28-54 .50

197.40180.55202.00170.70158.9522.99-36 .58-37.09-45.98247.84

UNC-Includes established stations at which additional instruments with a high time resolution will be installed during the STEP
period.
"The IGRF-90 model was used to calculate corrected geomagnetic coordinates and L values for 100-km altitude at eachstation on January 1, 1993.
***The Department of Earth and Planetary Sciences , Kyushu University and the Solar-Terrestrial Environment Laboratory, Nagoya University (STEL), are conducting multinationally coordinated magnetic observations in cooperation with and/or
courtesy ofthe following institutes and organizations: University ofNewcastle (UNC), Electronics Research Laboratory (DSTO),CSIRO Tropical Ecosystems Research Centre (TERC), Learmonth Solar Observatory and Canberra Observatory of IPS Radioand Space Services (IPS), Weipa North State School (WNSS), Birdsville Police Station (POD), Dalby Agriculture College(DAC), Katanning Research Station (KRS), and Australian Antarctic Division (AAD) 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 New 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 Propagation (IKIR), Institute of
Cosmophysical Research and Aeronomy (IKFIA), Institute of Physics of Earth (IFZ), and Pacific Oceanological Institute (POI),
Academy of Science in Russia; Limping 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.

3. Data Exchanges
Routinemagnetic

observations

at the 190°, 2 10°, and 250° MM chain stations

will continue

during

1304

K. YQMOTO and THE 210°

MM MAGNETIC OBSERVATION GROUP

the entire STEP period (1990-1997). For effective data exchanges and analyses, the magnetic data arebeing compiled at the STEL, Nagoya University, for STEP investigators. Data catalogue, one minuteaveraged 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 forNEC 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 210° 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 2100
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 210° MM team when the 210° MM data are essential in the publication andpresentation. If no one on the 210° MM team participates as an author, the paper should acknowledge the210° MM team and the STEL for the use of the database and should refer to the following papers: Yumoto
et al., J. Geomag. 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 210° MM team. Scientists conducting
research using the 210° MM data must contact the PI of the 210° MM Network Project, who will organize
the joint studies.
4. Initial Results

The 210° 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 210° MM network stations and satellites, we are studying the STEP objective andobtaining 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., 1997; Yumoto et al., 1994b, 1996a, 1996b; Petrinec
et al., 1996), (2) the global dynamics of low- and high-latitude auroras (Shiokawa et al., 1994, 1995a,
1995b, 1996a, 1996b, 1996c, 1996d; Yumoto et al., 1994a, 1994c; Yumoto, 1995), (3) relations between
in situ and ground (or separated ground) observations of substorm phenomena (Kawano et a1.,1994,1996;
Kokubun et al., 1996; Nakamura et al., 1996; Shiokawa et al., 1996), (4) global characteristics of Pc 3
waves (Yumoto et al., 1992; Menk and Yumoto, 1994; Pilipenko et al., 1995; Matsuoka et al., 1997) andPi 2 waves (Yumoto et al., 1994d; Osaki et al., 1996; Shiokawa et al., 1996c), (5) mass loading effect on
Pc 3 waves in the low-latitude ionosphere (Yumoto et al., 1993a; Yumoto, 1995; Pilipenko et al., 1996),
and (6) miscellaneous (earthquake-related ULF waves (Hayakawa et al., 1996) and geophysical induction
current (Yumoto and Utada, 1993; Seto et al., 1996)).
In particularly, we obtained new findings from analyses of coordinated ground-based observation
data from the 210° MM stations. The main results (cf. Yumoto, 1995) are summarized here.
4.1 North-south asymmetry ofsc/si disturbances
North-south asymmetry of se and si disturbances at low and middle latitudes indicates that the DP
component of se 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., 1996a).
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 interplanetaryshock 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

The STEP 210° Magnetic Meridian Network Project1305

by geomagnetic storms. Although such sc have been called si, there is no difference between the physicalmechanisms 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 se 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 ionosphericcurrents, 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 themagnetopause through the magnetosphere to the Earth's surface and/or from the magnetopause to thepolar ionosphere and thence to the magnetic equator, we analyzed magnetic field data from the 210° 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 210° 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 usedthe 210° 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 explainedby 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., byinvoking enhanced ionospheric conductivities in the summer hemisphere.
4.2 Transfer of solar wind energy to the magnetosphere
Low-latitude aurorae provide evidence that solar wind energy can be transferred into the innermagnetosphere around L = 2.5 during magnetic storms (Yumoto and Utada, 1993; Yumoto et al., 1994a,
1994c; Shiokawa et al., 1994, 1995a, 1996b, 1996d).
Recent optical and magnetic observations at Moshiri (0 = 44°22' N, = 142'16' E) of the STEL
indicate that even during moderate magnetic storms, invisible low-latitude aurorae sometimes occur inconcert with MI 50 nT positive magnetic excursions and large-amplitude Pi magnetic pulsations arounda 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, 1994c). Optical instrumentations (photometers, all-sky television cameras) wereused to identify six events of invisible low-latitude aurorae at Moshiri and Rikubetsu (0 = 43.46°, 0 =143.77°E, 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 atlatitudes 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 mid-latitude thermosphere and the occurrence of low-latitude aurorae
coincide with large values of the List index, which are proportional to the total energy content of thetrapped particles that constitute the ring current.
Shiokawa et al. (1996b, 1996d) 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-

1306

K. YUMOTO and THE 2100 MM

MAGNETIC OBSERVATION GROUP

latitude auroral event. The electron precipitation did not appear in an expanded auroral oval but rather in
an isolated region around 50° magnetic latitude. This precipitation exhibited an unusual acceleration
process in which electrons at all energies measured (32 eV to 30 KeV) were intensified. Nevertheless, weconclude that the intensity variations of optical emissions and magnetic perturbations during low-latitudeauroral 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.
4.3 Cavity-mode-like oscillations
Interplanetary impulses can readily stimulate a standing Pc 3-4 field line oscillation in the innermagnetosphere but don't easily excite cavity resonance oscillations at low latitudes. Only great se and si
can stimulatecavity-mode-like oscillations within the daytime plasmasphere(Yumotoetal.,1992,1994b).
The 210° magnetometer network data were also analyzed to determine whether or not global cavitymode 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 cavitymode-like oscillations with larger amplitudes at lower latitudes (I4PI< 30°) 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 magnetohydrodynamic mode and local time when the events happened showthat cavity-mode-like oscillations tend to be observed only when the 210° MM chain stations are locatedwithin the daytime sector from 0900 to 1500 LT, where the level of magnetic activity is Kp > 7+ and thestimulated power density just after the sc event is larger than 600 nT2/Hz (see Table 2 of Yumoto et al.
(I 994b)). This result implies that the interplanetary impulses must be sufficiently large to drive cavitymode-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 daysideplasmasphere only by great sc and si.
4.4 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 oscillationsin the near-equatorial latitude ionosphere (101< 30°) (Yumoto et al., 1993a; Yumoto, 1995; Pilipenko etal., 1996).
Magnetospheric ULF field line oscillations at middle and high latitudes have been thoroughly studied
for many years with the numerous facilities of modem geophysics, but much less is known about thephysical 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., Yumoto, 1995; Pilipenko et al., 1996). These peculiarities are
related to the primary influence of ionospheric ions on field line oscillations. New facilities for theexperimental study of the spatiospectral structure of ULF pulsations became available after the 210° 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 theresults 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 tomonitor plasma density variations in the plasmasphere, where in situ plasma data are not easily obtainedby satellites (Yumoto et al., 1995).
4.5 Latitudinal profiles of Pi 2 pulsations
The latitudinal profiles of Pi 2 phase relations and amplitudes imply that Pi 2 pulsations observed at

The STEP 2100 Magnetic Meridian Network Project1307

low and high latitudes consist of different mode oscillations (Yumoto et al., 1994d; Osaki et al., 1996;
Shiokawa et al., 1996c).
At the onset of a magnetospheric substorm, transient hydromagnetic oscillations with periods of 40150 s, called Pi 2 magnetic pulsations, are excited globally in the magnetosphere. One possible source ofnighttime Pi 2 pulsations is a sudden change in magnetospheric convection or configuration during thesubstorm expansive phase. This change would be caused by a plasma flow from the reconnection (orcurrent 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 ofthese pulsations are still only partly understood.
In order to check the existing Pi 2 model, we analyzed data from stations along the 210° 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-phaserelation, and D components at the northern and southern stations show a 180° out-of-phase relation;(3) amplitudes of the H components are independent of geomagnetic latitudes and are almost the same atall stations, but D components depend on latitude and increase exponentially from the magnetic equatorto 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 oscillatedwith 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 bya 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 isstill an open question. In order to answer the question, in the near future we will further analyze the 210°
MM chain data, including data from around the plasmapause and simultaneously obtained by the
AKEBONO, GEOTAIL, and CLUSTER satellites.
5. Conclusion

A review of some initial 210° MM network observations demonstrates the usefulness of multistation
observations. Imaging the Earth's magnetosphere by using ground-based magnetometer arrays can bereaffirmed as one of the major techniques for investigating the dynamical features of solar windmagnetosphere interactions. Magnetic field 1-s data from coordinated ground stations make it possible tostudy magnetospheric processes by distinguishing between temporal changes and spatial variations in thephenomena, to clarify global and latitudinal structures and propagation characteristics of magnetosphericvariations from higher to equatorial latitudes, and to understand the global generation mechanisms of thesolar-terrestrial phenomena. Considering the essential importance of simultaneous satellite-groundobservations and also the cost-effectiveness ofexpensive 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 190°, 210°,
and 250° MMs will be continued by adding various remote-sensing techniques. The scientific objectives
ofthe new phase will be to investigate the effects ofhigh-energy particles on the middle atmosphere duringsolar proton and low-latitude auroral events, relationships between the eastward auroral electrojet at highlatitudes and the equatorial electrojet in daytime, and long-term variations of the core magnetic field. The
210° MM observation network will be further connected to the Chinese meridian chain along the 120°
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 stationsin 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 I-s time
resolutions will be also organized for comparison with observations by the multiple satellites.

1308

K. Yorarrro

and THE 2100

MM MAGNETIC OBSERVATION GROUP

My sincere thanks go to H. Oya, Tohoku University, and all the members of the 2100 MM Magnetic
Observation Project for their ceaseless support. The 210° 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 Ministryof 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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