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LIGO Livingston Observatory News
Late-Breaking Livingston Installation News!
Magnetic Observatory in Operation at LIGO Livingston Observatory
Since the last newsletter, we have begun installation of in-vacuum components of the seismic isolation system within the Horizontal Access Module (HAM) vacuum chambers of the LIGO Livingston Observatory (LLO) interferometer. Dennis Coyne and Joe Giaime are alternately overseeing the installation tasks underway here, with Rich Riesen taking primary responsibility for day-to-day installation work and the provision of site support required for the installation effort. The photographs below, Figures 1 through 3, show the work underway earlier this month when the bellows and support tubes were installed in HAM 4. So far, support tubes, tables, and bellows have been installed in HAM chambers 1 and 4.
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The laser table too has now been installed within the Laser Vacuum
Equipment Area (LVEA), and a laser safety enclosure (selected by Jonathan
Kern) was erected around it. (See Figure 4 at right.) We are very pleased
with this enclosure which provides a very roomy space with overhead support
for cable trays and lighting. From Caltech, LIGO's Rich Abbott and Peter
King visited LLO the week of April 5 to kick off the pre-stabilized laser
installation work within the enclosure.
After a very long wait for delivery, the control room furniture finally
arrived and was put into the control room, shown in Figure 5 at left. The
first computers and the data acquisition server have been installed. The
Length Sensing and Control (LSC) control racks have also been placed in the
LVEA and VEA. Since the last newsletter, installation of the optical fiber
communications links between the corner station and the end buildings were
completed as well. We can now monitor and control the vacuum
instrumentation at the end buildings from the control room.
We are actively preparing for the Beam Tube bake out work scheduled to begin this summer. The Beam Tube enclosures have been thoroughly cleaned and thermocouples and grounding wires have been installed along the Beam Tube arms. Installation of the insulation along the west arm of the interferometer is now underway, as can be seen in Figures 6 and 7 below.
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Figure 8 at right is a happy sight! The much anticipated construction of the
main access road connecting State Highway 63 to the LLO corner station is
finally underway by Price Construction Company (Baton Rouge, LA)! We are
hoping that this work is completed by early fall, but we have been
cautioned by the contractor that a lot depends on the weather during that
period.
LIGO Livingston is now host to a magnetometer for magnetospheric research
operated by Southeastern Louisiana University (SLU) physics professor
Gerard Blanchard. The magnetometer, shown embedded in the earth in
Figure 1 at left, is one of a chain of magnetometers
deployed by the University of California's Institute of Geophysics and
Planetary Physics and by the Los Alamos National Laboratory at 40°
geomagnetic latitude. At present there are three operating magnetic
stations in this chain: San Gabriel, CA; Los Alamos, NM; and Livingston,
LA. (See the map in Figure 2 at right.)
The LIGO site was chosen as the venue for this magnetometer because it provides both Internet access and a location away from traffic. The magnetometer is a sensitive metal detector if the metal object is in motion. Therefore, the magnetometer requires a radius of exclusion from bicycles, cars, trucks, trains, etc., and the larger the vehicle, the larger the radius of exclusion necessary. The heavily trafficked SLU campus was thus unsuitable for the instrument.
Installation of the magnetometer occurred in January this year. Special thanks are due to several members of the LIGO staff. Mark Coles provided his support and supervision. Gerry Stapfer helped to determine where to situate the magnetometer which had to be buried underground, where to place the personal computer that operates the magnetometer and that had to housed inside, and how to run cables to connect these two. Allen Sibley provided the exact coordinates of the magnetometer, and answered various other questions. Finally, Tom Evans helped with the Internet connection.
Operations began on January 22, 1999. Data from the first 30 days of operations already show many of the phenomena to be studied. The data are shown in three graphs (Figures 3, 4, and 5 below) of the X (magnetic northward), Y (magnetic eastward), and Z (downward) components of the magnetic field superimposed according to local time (i.e. CST), starting at local dawn, 06:00. The data are displayed in this way since many atmospheric and magnetospheric phenomena occur in a fixed location relative to the sun. The four vertical lines therefore separate the data into four sectors in a coordinate system fixed with respect to the sun, these are referred to as morning, afternoon, premidnight, and postmidnight.
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The average magnetic field measured by the magnetometer is 18160 nT
horizontally and 33630 nT vertically downward (181.6 mG, 336.3 mG). The
average Y of 200 nT indicates a misalignment of the X axis from magnetic
north of 0.63°.
A value less than 1° is considered acceptable. Most of
the traces follow a characteristic diurnal variation, referred to as the
quiet day variation, which shows the effects of the solar-driven
ionospheric dynamo current system. (See Figure 6 at right.) The
ionospheric dynamo is driven by solar heating, and the shape of the dynamo
represents a balance of thermal pressure, Coriolis, Lorentz, and tidal
forces on the ionosphere.
The ionospheric dynamo drives two current cells that are eastward at the equator and westward at higher latitudes. In northern hemisphere winter, the focus of the upper cell passes south of Livingston, therefore the overhead current is southwesterly in the morning sector, westerly at noon, and northwesterly in the afternoon sector. The ionospheric current produces a magnetic perturbation that is rotated 90° to the left of the current. The magnetic perturbation is thus southeasterly in the morning, southerly at noon, and southwesterly in the afternoon, as can be seen in the plots of Figures 3 and 4 (the X and Y graphs respectively) indicated above. The Z component (Figure 5) is more strongly influenced by the eastward equatorial electrojet, and thus displays a upward perturbation that peaks at noon. The +/- 5 nT spread in the quiet-day variations reflects variations in the intrinsic solar luminosity. At night, the effects of the dynamo are much reduced.
Variations other than the quiet-day variations are caused by disturbances in the Earth's magnetosphere. There are two main magnetospheric effects that perturb X, the northward component. An increase of the northward component of the magnetic field is caused by compression of the magnetosphere by an interplanetary shock or high speed stream in the solar wind. A reduction in the northward component of the magnetic field is caused by an increase in the strength of the ring current, a torus-shaped westward current carried by plasma trapped in the magnetosphere. An increase in the ring current is referred to as a magnetic storm, which is significant, since the strength of the ring current is directly proportional to the particle energy of the Van-Allen radiation belts. The radiation belts can be hazardous to satellites in polar orbit, medium Earth orbit, highly elliptical orbit, or in geosynchronous orbit, especially during a magnetic storm.
The perturbations seen in the Y component indicate another type of geomagnetic activity, referred to as substorms. The Y perturbations consist of eastward perturbations in the premidnight sector, giving way to westward perturbations in the postmidnight sector. These perturbations indicate the occasional presence of electric currents from the magnetosphere into the ionosphere at high latitudes in the postmidnight sector, and return currents from the ionosphere to the magnetosphere in the premidnight sector. Notice the rapid change in the magnetic field strength. These perturbations can cause a problem at high latitudes, where the perturbations are much stronger, by inducing voltage in power transmission lines, either reducing the voltage and causing brownout, or increasing the voltage and causing transformer failure.
For a tutorial on the magnetosphere and geomagnetic disturbances, please see http://ssdoo.gsfc.nasa.gov/education/lectures/magnetosphere.html
For the technically-minded, the magnetometer is a low-noise triaxial
fluxgate magnetometer built by UCLA. Precise timing is provided by a
Global Positioning Satellite (GPS) receiver. The magnetometer is
controlled by, and data are stored temporarily on the internal disk of a
personal computer and exported over the Internet. Figure 7 at left
shows the author and LIGO Livingston chief Mark Coles inside the magnetometer's
computer control room. 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. The magnetic vectors
are returned with a digitization of +/- 15 pT. The sensor is buried to
reduce thermal variations in the electronics and contains its own heater
and thermostat.