Y. F. Gao1, P. J. Chi2, G. Le2, C. T. Russell2, D. M. Yang1, X. Zhou1, S. F. Yang3, V. Angelopoulos4, and F. K. Chun5
The Sino-Magnetic Array at Low Latitudes (SMALL) is an ongoing project for the development of a two-dimensional (2-D) ground-based magnetometer array in China. The ultimate array will consist of 24 stations spanning over 3 hours in local time in the low-latitude (L < 2) region. We present some initial results from the Beijing station as well as several U.S. sister sites using the same instrumentation. It is shown that the coverage, high sensitivity, and accurate timing of the SMALL array will advance our understanding of low-latitude geomagnetic phenomena.
The SMALL array is the first permanent, high temporal resolution 2-D magnetometer array deployed at low latitudes. Even though 70% of the Earth's surface is below L = 2, no such 2-D array has been previously deployed in this region. Before the establishment of SMALL array, there was only one north-south chain passing through this region and providing the high temporal resolution needed to resolve phenomena such as waves and sudden impulses. This chain was the 210° magnetic meridian chain [Yumoto et al., 1992] which effectively includes the eastern Australian chain (described by Ziesolleck et al. ). There are also a number of sparsely located stations associated with this chain but they are not close enough together or sufficient in number to provide 2-D information. Ground stations provide crucial observations in the low-latitude regions where there are few spacecraft measurements.
The SMALL project is a joint venture by the China Seismological Bureau (CSB), Chinese Academy of Sciences (CAS), and the University of California. The purpose of this project is to build a modern magnetometer array across China at low latitudes (L = 1-2). The instruments will be installed at the existing observatories managed by the CSB and CAS. In particular, the magnetometer network of CSB consists of more than 30 observatories in China, and the oldest one among them, the Sheshan observatory near Shanghai, has a 124-year history of geomagnetic field measurements. UCLA designs and manufactures the magnetometer system and shares the responsibility for disseminating and analyzing data. The instruments are installed and operated by CSB who also share in the dissemination and analysis process.
Figure 1 shows the existing ground-based observatories in China. The stations selected for participation in the SMALL array are represented in solid circles. Their full names and locations are also listed in Table 1. These stations were selected based on the requirements of a stable 220 V/50 Hz electrical power supply, phone connection, minimum magnetic field gradient and minimum environment noise level. Furthermore, all these stations are equipped with classical instruments, such as variometers and photographic recordings, measuring the absolute magnitude and variations of the geomagnetic fields. The four groups of magnetometers in Table 1 indicate the stations at different areas as well as the stages of deployment, and altogether they form a roughly rectangular grid including at least 6 chains extending from 7.8o to 42.7o in geomagnetic latitudes and from 150.9o to 194.2o in geomagnetic longitudes.
|Fig. 1. Map of the SMALL stations in geographic coordinates.|
Table 1. Selected Sites for Sino-Magnetic Array at Low Latitudes (SMALL)
Beijing Meridian Chain
Western and Southern Chains
The design for the SMALL array consists of a highly accurate fluxgate magnetometer, based on UCLA's successful magnetometers on numerous spacecraft [e.g, Russell, 1978, Russell et al., 1980, 1995] coupled to a 20-bit analogue to digital converter in a low noise electronic environment. This system has three sensors all with a dynamic range of ±5000 nT, and the magnetic vectors are returned with a digitization of 9.5 pT. Not only can such wide measurement range record all types of magnetic perturbations, but it also makes the installation very easy. In order to measure the geomagnetic field which has a stronger magnitude, prescribed offset currents are applied on the three components according to the location of the station. This system provides a low noise level for all the three components. The noise power is ~10-3 nT2/Hz at 1 Hz or the peak-peak noise level less than 0.1 nT.
The data are sampled at 1 Hz, and the variations having frequencies ranging from DC to 0.5 Hz can be studied. The precise timing is provided by a GPS receiver with millisecond accuracy. This timing allows the synchronization of all stations and enables an accurate measurement of phase differences between stations.
Utilizing a Personal Computer for power and data acquisition has enabled the costs per unit to be kept to roughly $6000 per station. Using the PC also simplifies maintenance and repair of a major component of the system. The data are stored temporarily on the internal disk of a PC capable of storing 500 Mbytes, or up to 1 year of 1 Hz data. The PC will also provide a modem or internet link to the central data collection and data dissemination facility. For sites without an internet or phone-line connection, the data will be collected via portable disks from the internal disks of the sites at least twice per year. Accurate baselines are maintained by mounting the sensors on deep-set piers, and also by keeping both the electronics and sensors in thermally controlled environments, isolated from other noise sources.
The scientific motivation for this array arises from previous studies of the IMS data from the British IGS magnetometer array [Odera et al., 1991; Le and Russell, 1993; Chi et al., 1994] and the US AFGL magnetometer arrays [Chi et al., 1996] and the limitations of these arrays as well as the importance of extending these studies to lower latitudes. It is also the technical advantages of the SMALL array that will enable us to address the problems that heretofore remained unsolved because of the lack of appropriate data. Many problems can be addressed with magnetic field measurements from a 2-D array of magnetometers. In the following, we will discuss three areas that we will investigate first, namely the Pi 2 magnetic pulsations, low-latitude Pc 3-4 pulsations and field line resonances, and the equatorial electrojets, as well as the geomagnetic disturbances during earthquakes preparation. Other applications include the current vortices at low latitudes recently discovered by Yumoto et al., subsurface conductivity, 3-dimensional magnetospheric current systems, sudden impulses, ring current development, and other pulsation phenomena.
Pi 2 pulsations are generated during the reconfiguration of the magnetosphere that occurs with the onset of a substorm. They appear to be associated with the substorm current wedge, and they can be seen all the way to the equatorial electrojet [Sastry et al., 1983]. However, little is understood about their generation [Yeoman et al., 1990]. We do not know how the magnetospheric reconfiguration leads to the Pi 2 pulsation, nor do we know how the energy once released propagates through the magnetosphere to cause the observed signature. The SMALL array with its precise magnetic measurements and ultra-precise timing will enable us to track the wave fronts through a significant portion of the magnetosphere.
At low latitudes, dayside Pc 3-4 pulsations are the strongest and most frequent pulsations phenomena seen. It is widely believed now that their major energy source is from the subsolar foreshock region [Troitskaya et al., 1971; Greenstadt and Olson, 1977] and the effect of field line resonance will modify the wave spectrum depending on the location of observation. Waters et al.  found that the dominant peaks in the observed wave spectrum do not occur generally at the eigenfrequencies of local field line and they developed a cross spectral technique to monitor the resonant frequency of the field lines between two stations on a same meridian. Menk et al.  used this technique to monitor the temporal evolution of plasmaspheric properties in one 5-week observing run. This innovative technique for magnetospheric diagnostics will be applied to our SMALL data. Another unresolved problem related to Pc 3-4 is the wave propagation, including the direction of wave fronts and the speed with which pulsation energy moves in local time. The geometry and accurate timing of SMALL array is most suitable to approach this topic.
Although the array is not situated directly under the equatorial electrojet, the station on Hainan Island and three other stations at the southern end of these meridional chains will measure effects due to the equatorial electrojet. On occasion the effects of the electrojet can be observed to 25o [Rastogi, 1991] and hence affecting half of the proposed array which is centered at about 25o latitude. Thus, even though strictly none of the stations are "equatorial" much will be learned about the equatorial electrojet from this array. In short, despite the many years of study of the electrojet and despite the non-ideal location of our array relative to the electrojet, we expect that we will be able to learn much new about it. In particular the rapid (1 s) sampling of the array magnetometers will permit the determination of pulsation wave fronts and a study of how these waves are affected by the electrojet.
In addition to the above scientific issues, one of our major objectives is to study the relationships between earthquakes and magnetic disturbances. A number of electromagnetic phenomena associated with earthquakes have been reported in different frequency ranges [Fraser-Smith et al., 1990; Gokhberg et al., 1982; Kopytenko et al., 1993]. Molchanov et al.  also formulated the wave propagation from seismic sources into the atmosphere and above. It seems that ultra-low frequency (ULF, f = 10-3-1 Hz) electromagnetic emissions can be a useful tool for earthquake prediction, and the SMALL array will be very valuable for monitoring and studying such phenomena.
INITIAL RESULTS FROM SMALL AND U.S. SISTER SITES
After the first SMALL system was installed in Beijing at the end of 1997, many sudden impulse events have been studied in the SMALL data. Figure 2 shows an example of a sudden impulse on December 10, 1997, when the WIND spacecraft measured the solar wind conditions, the POLAR spacecraft saw the compression of the magnetosphere at high altitudes, and the Beijing station observed geomagnetic field variations on the surface of the Earth. Once the magnetometers are deployed across the array (beginning in late 1998) it will be possible to calculate the time delay between the signals at different stations and understand the propagation of sudden impulse in the magnetosphere. This example also demonstrates the potential studies that can be conducted using the SMALL array data together with the data from ISTP satellite fleet.
|Fig. 2. A sudden impulse event observed by the WIND spacecraft (top panel), the POLAR spacecraft (middle panel), and a SMALL station at Beijing (bottom panel) on December 10, 1997.|
Even with their large dynamic range, the SMALL magnetometers have enough sensitivity to measure pulsations. Figure 3 shows examples of a dayside Pc 3-4 wave event and a nightside Pi 2 event. One of the major analyses we will be performing is the propagation of pulsations at low latitudes. However, since the magnetometer systems are still in the Beijing station for the final test before they are deployed at other stations, in the following we will present some initial multi-station results from the sister sites in the United States using the same magnetometer design. The SMALL array will provide us with the same resolution and accuracy from more stations.
|Fig. 3. Examples of a dayside Pc 3-4 event and a nightside Pi 2 event observed at Beijing .|
Figure 4 presents the correlation analysis of the pulsations measured at two U.S. stations, Los Alamos (LANL) and San Gabriel Dam (SGD), to identify the time lag between the two signals. The geomagnetic coordinates for LANL and SGD are (40.26°W, 44.34°N) and (52.96°W, 40.94°N), respectively. Figure 4a shows a Pc 3-4 event, for which the data have been rotated to the wave's principal axis coordinates and the wave maximum variance components analyzed. The result of cross correlation shows that the signal at SGD leads the signal at LANL by 4.153 s, which means that the wave was propagating from the subsolar region toward the afternoon sector, if we assume that the latitudinal difference of 3° is negligible. Figure 4b shows a similar analysis for a Pi 2 event observed by the same stations. It shows that the Pi 2 signal at LANL leads the signal at SGD by 2.171 s, which means that the wave was propagating from the post-midnight sector toward midnight.
|Fig. 4. (Left) Cross-correlation analysis applied on filtered signals for a Pc 3-4 event observed simultaneous at Los Alamos (LANL) and San Gabriel Dam (SGD). The wave signal observed at SGD leads the one at LANL by 4.153 s. (Right) Same analysis applied on filtered signals for a Pi 2 event. The wave signal arrived at LANL earlier than at SGD by 2.171 s .|
Finally we illustrate the use of the cross-phase technique which identifies the field line resonance frequencies and enables us to estimate the plasma mass density in the magnetosphere. Figure 5 shows the phase difference between the signals at two stations, Air Force Academy in Colorado (AFA) and LANL, which are roughly on the same longitude but separated by about 3° in latitude. Phase differences of 50°-100° can be seen at about 40 mHz (or log f = -1.4), and it corresponds to the fundamental mode of the field line between the two stations. Knowing the L-value (= 2.1) and assuming a dipole magnetic field and a plasma density distribution ~ r-3, we can estimate that the mass density of ions on the equator is roughly equal to 2580 protons/cm-3. The SMALL array, providing the same accurate timing and longer meridian chains, will allow us to understand the plasma density profile at different latitudes.
|Fig. 5. (Cross-phase spectrogram for the H-component of the magnetic fields observed at Air Force Academy (AFA) and LANL. The horizontal stripe at the center of the diagram implies that the fundamental mode frequency of the field line between AFA and LANL is approximately 40 mHz .|
An array of low cost, high resolution, precisely timed wide bandwidth (DC-0.5 Hz) fluxgate magnetometers are being deployed in China. Sister sites are now in operation in Jicamarca, Mexico City, Los Angeles, Los Alamos, and Colorado Springs (USAFA). Signals with amplitudes greater than 0.1 nT can be routinely detected and analyzed. Scientific investigation will be conducted in the areas of, but not limited to, the propagation of Pi 2 and Pc 3-4 pulsations, low-latitude field line resonance structure probed by the cross-phase technique, the equatorial electrojets, and the relationships between earthquakes and magnetic disturbances.
The development of the SMALL array was supported by the National Science Foundation under research grant ATM 96-23163. The construction of the magnetometers for the San Gabriel Dam, Los Angeles, Colorado Springs and Boulder sites were supported by the Los Alamos National Laboratory branch of the Institute of Geophysics and Planetary Physics. We also acknowledge support from CalSpace and SCOSTEP.
Chi, P. J., C. T. Russell, and G. Le, Pc 3 and Pc 4 activity during a long period of low interplanetary magnetic field cone angle as detected across the Institute of Geological Sciences array, J, Geophys. Res., 99, 11127-11139, 1994.
Chi, P. J., C. T. Russell, G. Le, W. J. Hughes, and H. J. Singer, A synoptic study of Pc 3, 4 waves using the AFGL magnetometer array, J. Geophys. Res., 101, 13215-13224, 1996.
Fraser-Smith, A. C., A. Bernardy, P. R. McDill, M. E. Ladd, R. A. Helliwell, and O. G. Villard Jr., Low frequency magnetic field measurements near the epicenter of the Ms 7.1 Loma Prieta Earthquake, Geophys. Res. Lett., 17, 1465, 1990.
Gokhberg M. B., V. A. Morgunov, T. Yoshino, and I. Tomizawa, Experimental measurements of electromagnetic emission possibly related to earthquakes in Japan, J. Geophys. Res., 87, 7824, 1982.
Greenstadt, E. W., and J. V. Olson, A contribution to ULF activity in the Pc 3-4 range correlated with IMF radial orientation, J. Geophys. Res., 82, 4991, 1977.
Kopytenko, Yu. A., T. G. Matiashvili, P. M. Voronov, E. A. Kopytenko, and O. A. Molchanov, Detection of ultra-low-frequency emissions connected with the Spitak earthquake and its aftershock activity, based on geomagnetic pulsations data at Dusheti and Vardzia observatories, Phys. Earth Planet. Inter., 77, 85, 1993.
Le, G., and C. T. Russell, Effect of sudden solar wind dynamic pressure changes: Time rate of change of magnetic field, Geophys. Res. Lett., 20, 1-4, 1993.
Menk, F. W., B. J. Fraser, C. L. Waters, C. W. S. Ziesolleck, Q. Feng et al., Ground measurements of low latitude magnetospheric field line resonances, in Solar Wind Source of Magnetospheric Ultra-Low-Frequency Waves, edited by M. Engebretson, K. Takahashi and M. Scholer, Geophysical Monograph 81, p.299-308, AGU, Washington DC, 1994.
Molchanov O. A., M. Hayakawa, and V. A. Rafalsky, Penetration characteristics of electromagnetic emissions from an underground seismic source into the atmosphere, ionosphere, and magnetosphere, J. Geophys. Res., 100, 1691, 1995.
Odera, T. J., D. Van Swol, C. T. Russell, and C. A. Green, Pc 3, 4 magnetic pulsations observed simultaneously in the magnetosphere and at multiple ground stations, Geophys. Res. Lett., 18, 1671, 1991.
Rastogi, R. G., Latitudinal extent of the equatorial electrojet effects in the Indian zone, Annal. Geophys., 9, 777-783, 1991.
Russell, C. T., The ISEE 1 and 2 fluxgate magnetometers, IEEE Trans. Geoscience Electronics, GE-16, 239-242, 1978.
Russell, C. T., R. C. Snare, J. D. Means, and R. C. Elphic, Pioneer Venus fluxgate magnetometer, IEEE Trans. on Geoscience and Remote Sensing, GE-18, 32-36, 1980.
Russell, C. T., R. C. Snare, J. D. Means, D. Pierce, D. Dearborn et al., The GGS/POLAR magnetic fields investigation, Space Sci. Rev., 71, 563-582, 1995.
Sastry, T. S., Y. S. Sarma, S. V. S. Sarma, and P. V. Sanker Narayan, Daytime Pi pulsations at equatorial latitudes, J. Atmos. Terr. Phys., 45, 733-741, 1983.
Troitskaya, V. A., T. A. Plyasova-Bakunina, and A. V. Gul'el'mi, Relationship between Pc 2-4 pulsations and the interplanetary field, Dokl. Aka. Nauk. SSSR, 197, 1313, 1971.
Waters, C. L., F. W. Menke, and B. J. Fraser, The resonance structure of low latitude Pc 3 geomagnetic pulsations, Geophys. Res. Lett., 18, 2293-2296, 1991.
Yeoman, T. K., D. K. Milling and D. Orr, Pi 2 pulsation polarization patterns on the UK sub-auroral magnetometer network (SAMNET) Planet Space Sci., 38, 589-602, 1990.
Yumoto, K., Y. Tanaka, T. Oguti, K. Shiokawa et al., Globally coordinated magnetic observations along 210 degrees magnetic meridian during STEP period. 1) Preliminary results of low latitude Pc 3s, J. Geomag. Geoelectr., 44, 261, 1992.
Yumoto, K., K. Shiokawa, T. Endos, Y. Tanaka et al., Characteristics of magnetic variations caused by low-latitude aurorae observed around 210 o magnetic meridian, J. Geomag. Geoelect., 46, 213-229, 1994.
Ziesolleck, C. W. S., B. J. Fraser, F. W. Menk, and P. W. McNabb, Spatial characteristics of low-latitude PC 3-4 geomagnetic pulsations, J. Geophys. Res., 98, 197-207, 1993.