Introduction
The IGPP/LANL Magnetometer Array is a collaborative effort among the two Universiity of California campuses (Berkeley and Los Angeles), the Los Alamos National Laboratory, and the US Air Force Academy. In this effort, we are installing an array of mid-latitude ground magnetometer stations in the US, as shown in Figure 1. The primary funding for the first four stations of IGPP/LANL has been awarded by Los Alamos branch of Institute of Geophysics and Planetary Physics. The funding to complete the array is still pending.
In the IGPP/LANL Array shown in Figure 1, we will re-instate the AFGL magnetometer network which operated in 1977-1978. The AFGL network consisted of five northern US Air Force base sites and two southern sites. We will extend its southern chain with totally five sites. In addition, one more station will be deployed at Colorado Springs, CO (the Air Force Academy site) and Boulder, CO, in order to familiarize the Air Force students with the installation procedure and normal operations of the magnetometer and another one at Los Alamos (LANL), in order to ensure that pulsation studies and their correlations with LANL spacecraft data can be done internally at LANL. Figure 2a, Figure 2b and Figure 2c show pictures of LANL Fenton Hill site. In the map in Figure 1, we also show the locations of UCLA test site in San Gabriel Dam, and correlative site in Mexico City. When the full deployment is completed the chain will have a 55 and a 40 corrected geomagnetic longitude chain with very good longitudinal resolution for timing of SI propagation, current wedge modeling, and Pi 2 and Pc 3-4 propagation.
Including the associated sites, five sites are now operational: San Gabriel Dam test site, Mexico City site, Jicamarca site, the LANL Fenton Hill site, and the Air Force Academy site. The later two sites can accommodate a fast real time internet connection for rapid dissemination of data. It is our goal to have internet access to all these sites in the near future. The magnetometers for the next two IGPP/LANL sites (Rapid City, SD, and San Antonio, TX) have been assembled at UCLA lab and are being tested at UCLA San Gabriel Dam test site.
Scientific Objectives of the IGPP/LANL Array
Propagation of solar wind disturbances in the magnetosphere is an outstanding problem in magnetospheric physics. Accumulation data from IGPP/LANL Array during the ISTP period provide an unprecedented opportunity for correlative space-ground studies of the propagation of these disturbances. The main scientific objectives are to study following phenomena:
(1) Propagation of sudden impulses (SIs) and storm sudden commencement (SSCs)
(2) Modeling of substorm current wedge
(3) Propagation and generation mechanism of Pi 2 pulsations
(4) Propagation and solar wind control of Pc 3-4 pulsations
Magnetometers for the IGPP/LANL Array
A successful array of magnetometers requires a precise low noise basic magnetometer, precise timing at each location, and a robust data system. UCLA ground-based magnetometer consists of a highly accurate fluxgate magnetometer, based on our successful magnetometers on numerous spacecraft [e.g, ISEE, PVO, POLAR] coupled to a 22 bit analogue to digital converter in a low noise electronic environment.
(1) Low cost ( $6K per station). Utilizing a PC significantly reduces the costs per station and provides a ready source for parts and repair for a large portion of the system.
(2) High resolution and accurate Timing. The data are sampled at 1 Hz. The precise timing is provided by a GPS receiver with 1 millisecond accuracy.
(3) High precision. The noise power is 10-3 nT2/Hz at 1 Hz.
(4) Accurate baseline. Baselines are maintained by keeping both the electronics and sensors in thermally controlled environments, isolated from other noise sources.
(5) High storage capacity. A hard disk of 500 MB can held up to one year's data. The PC will also provide a modem or internet link to the central data collection and data dissemination facility.
(5) High dynamic range . Each sensor has a dynamic range of +/- 5000 nT, that makes the installation very easy.
(7) Broadband.
Examples of Field Data
Magnetic Field During a Quiet Period
Figure 3 shows the three components of the magnetic field measurement at San Gabriel Dam test site during a quiet period (top panel) and their power spectra. The three sensors 1, 2, and 3 are roughly in the north-south, east-west, and vertical direction, respectively. The average amplitude of the noise level is smaller than 0.1 nT peak-to-peak. The noise level is comparable to that of magnetometers in AFGL chain at highest frequency (0.5 Hz) and much lower at low frequencies. Since the magnetometers at AFGL chain had a much smaller dynamic range (+/- 64 nT), this system is approximately 100 times quieter at 100 sec periods than the AFGL circuitry while maintains a dynamic range of +/- 5000 nT, almost 100 times greater than the AFGL units.
Dayside Pc 3-4 Pulsations
Figure 4 shows an example of Pc 3-4 waves observed at Los Alamos site Fenton Hill site (LANL). The waves with a peak-to-peak amplitude of 0.2 nT can be easily seen by the magnetometer. The power of the wave at its peak frequency is two orders of magnitude higher than the noise power.
Cross-Correlation of Waves at Two Sites
The accurate timing provided by the GPS receiver enables us to perform cross-correlation analysis to determine the time lag between the signals at two sites. Below we show examples of Pi 2 and Pc 3-4 pulsations observed at LANL and SGD sites simultaneously.
Nightside Pi 2 Pulsation
Figure 5a shows an example of nightside Pi 2 pulsations observed simultaneously at Los Alamos Fenton Hill site (Top panel) and San Gabriel Dam test site (bottom panel). Figure 5b shows 8-40 mHz bandpass filtered data for the same period.
The top panel of Figure 5c shows the time series of a Pi 2 wave at LANL (blue) and SGD (red) sites. The wave data have been rotated to the wave's principle axis coordinates and the wave maximum variance components are shown in Figure 5c. The bottom panel of Figure 5c shows the cross-correlation coefficient as a function of the time lag between the LANL and SGD sites. The waves seen at both sites are high correlated with a maximum cross-correlation coefficient of 0.975. The wave is seen at LANL site 2.171 seconds earlier than at SGD site. Thus, the phase of the wave propagates towards the midnight in the earlier morning sector (westward).
Dayside Pc 3-4 Pulsation
Figures 6a, Figure 6b, and Figure 6c show an example of Pc 3-4 waves and are in the same format as Figures 5a, b, and c, respectively. In this case, the wave is seen at SGD site 4.153 seconds earlier than at LANL site. Thus the phase of the wave propagates from the subsolar region towards the dusk in the post-noon sector.
Summary
(1) UCLA magnetometer has been successfully installed in five sites (Los Alamos, Colorado Spring, San Gabriel Dam, Mexico City and Jicamarca).
(2) Time delays can be determined accurately for waves with amplitudes as small as 0.1 nT.
(3) Data are available over the internet in near real time.
(4) Future sites using UCLA magnetometers include the east coast MEASURE array (M. Moldwin, PI) and Sino-Magnetic Array at Low Latitudes (SMALL Array) in China.
