Y. F. Gao, and T. W. Wang
Institute of Geophysics
China Seismological Bureau
Center for Space Science and Applied Research, Chinese Academy of Science
C. T. Russell, G. Le, and P. J. Chi
Sino-Magnetic Array at Low Latitudes (SMALL) is a joint venture between
China Seismological Bureau (CSB) and UCLA. The purpose of this project
is to build a modern magnetometer array across China at low latitudes (L=1-2).
It is the first non-temporary 2-D magnetometer array deployed at low latitudes.
Upon completion, it will include more than 20 stations spanning over 3
hours in local time and over 36 degrees in latitudes. Figure
1 shows the locations of the SMALL stations.
The magnetometers for SMALL array are designed and fabricated by UCLA and installed at the existing observatories managed by CSB. The magnetometer has three sensors all with a dynamic range of 5000 nT. The peak-to-peak noise level is less than 0.1 nT. The data are sampled at 1 Hz. The precise timing is provided by a GPS receiver with millisecond accuracy. A personal computer is used for controlling the system. The data are stored temporarily on the internal hard disk capable of storing up to one year of 1 Hz data. The PC also provide a modem or internal link to the central data collection and data dissemination facility.
In November, 1998, seven magnetometers were installed in the first phase of the deployment (blue sites in Figure 1). The next six magnetometers in the second phase are expected to be installed in October, 1999 (red sites in Figure 1).
The locations of the seven observatories in the first phase are shown in Table 1. The seven magnetometers operate smoothly and return the first batch of geomagnetic field data since the deployment. These data are now being processed and analyzed at IGCSB (Institute of Geophysics, China Seismological Bureau) and UCLA.
To ensure accurate and stable baselines, both the electronics and sensors
are kept in thermally controlled environments in the design of the magnetometer
system. Moreover, extra measure has been taken to ensure the stability
and precise direction of each sensor. The CSB designed and manufactured
a cylinder sleeve, as shown in Figure 2a, which
holds the sensor assembly and can be adjusted in both vertical and horizontal
directions freely and precisely. The integrated sensor assembly is placed
on a stable pier in the recording room (Figure 2b).
This measure can increase the stability of the baseline values. We are
now testing the stability of baseline values in comparison with the absolute
measurements of the geomagnetic field at Beijing (BJI) station (Figure
3). The test will be finished after one year operation of the magnetometer.
If the baseline is proven to be stable, the SMALL magnetometers can be
used not only to record pulsations but also as regular geomagnetic recording
instrument at permanent observatories.
Table 1. Magnetometer Sites in SMALL Phase 1
** The magnetometer at Chengdu was moved to Tonghai in April 1999 due
to large interference found at Chengdu station.
Observations of Pi 2 Events
We present two examples of large amplitude Pi 2 events recorded at these stations in the initial survey of the data. We found that both the Pi 2 events occurred during quiet days and were not associated with substorm activities.
The February 9, 1999, Pi 2 Event
The first example of Pi 2 event appeared at 1300-1400 UT on February 9 1999. China was in the pre-midnight nightside sector during this time interval (LT~UT+8). The data are available at four of the seven observatories, including BJI, JIH, THJ and QGZ. The maximum latitude span is about 20 degrees and the local time difference is less than one hour.
Figure 4a shows the H-component (north-south) of the magnetic field data recorded at these four stations. All the stations recorded Pi 2 waves occurring at 1318 UT and 1346 UT.
Figure 4b shows the band-pass filtered data
(5-15 mHz) for the Pi 2 wave starting at 1318 UT. The waves recorded at
the four stations have similar amplitudes and are nearly in phase with
each other. Since the timing is very precise at each station, even the
very small phase differences among stations can be determined accurately
by cross-correlation analysis. Figure 4c shows the results of cross-correlation
analysis between station pairs BJI-JIH, BJI-THJ,
and BJI-QGZ. The phase difference is in the order
of 1 second.
The April 24, 1999, Pi 2 Event
The second example of the Pi 2 events occurs at 1500-1600 UT on April 24, 1999. China was within one hour from the midnight. The data are available from six stations, including MZL, BJI, JIH, THJ, YON and QGZ.
Figure 5a shows the stack plot of the H-component of the magnetic field data at the six station. All stations recorded the wave. Figure 5b shows the bandpass (8-15 mHz) filtered data. In this event, the wave amplitude is smaller at the lower latitude station. This seems to be different from the Pi 2 waves commonly associated with substorms.
Figure 5c shows the results of cross-correlation analysis between pairs
and MZL-THJ for the interval between the two
dashed lines in Figure 5b. The wave at lower latitude
station (THJ or BJI) leads the wave at higher latitude station MZL.
Observations of Pc 3 Waves
The April 20, 1999, Pc 3 Waves
Figure 6 shows an example of Pc 3 waves, which were observed near the local noon on April 20, 1999. The data are available for four stations including MZL, BJI, JIH, and THJ. The stack plot of the H-component for the four stations is shown in Figure 6a, and the band-pass filtered data are shown in Figure 6b. All the stations recorded the Pc 3 waves. The wave amplitude becomes smaller toward lower latitudes. At THJ (13 N), the wave amplitude is less than 1 nT.
Figure 6c shows the results of cross-correlation analysis between pairs
of MZL-BJI, MZL-JIH,
and MZL-THJ for the interval between 0233 and
0238 UT. It appears that the Pc 3 waves are propagating mainly in the longitudinal
direction. The largest time delay, 4.7 seconds, occurred between the pair
MZL-THJ, separated about 13 deg longitudinally. The stations MZL, BJI and
JIH have nearly same longitudes and the time delay between pairs MZL-BJI
and MZL-JIH is less than 1 second.
SMALL is the first non-temporary 2-D magnetometer array deployed at low latitudes. Seven magnetometers have been installed successfully and are returning data in phase 1. The next six magnetometers will be installed in October in phase 2.
Time delay of waves among stations can be determined accurately for waves with amplitude less than 1 nT.
Other sites using UCLA magnetometers include the east coast MEASURE array (M. Moldwin, PI) and IGPP/LANL array (V. Angelopoulos/G. Le/G. Reeves, PIs).