Impact of Schumann resonances on the Einstein Telescope and projections for the magnetic coupling function

Correlated magnetic noise in the form of Schumann resonances could introduce limitations to the gravitational-wave background searches of future Earth-based gravitational-wave detectors. We consider recorded magnetic activity at a candidate site for the Einstein Telescope, and forecast the necessary measures to ensure that magnetic contamination will not pose a threat to the science goals of this third-generation detector. In addition to global magnetic effects, we study local magnetic noise and the impact it might have on colocated interferometers. We express our results as upper limits on the coupling function of magnetic fields to the interferometer arms, implying that any larger values of magnetic coupling into the strain channel would lead to a reduction in the detectors’ sensitivity. For gravitational-wave background searches below $\sim 30\mathrm{Hz}$ it will be necessary for the Einstein Telescope magnetic isolation coupling to be two to four orders of magnitude better than that measured in the current Advanced LIGO and Virgo detectors.

Figure 1

Sos Enattos magnetic ASD constructed using 48 days of data from November 14, 2019 to December 31, 2019. A one-directional magnetometer was employed to collect the data in the mine approximately 200 m below ground level. The line at 50 Hz is coming from the power mains.

Figure 2

The ET configurations—ET-S, ET-X [25, 34]—and their anticipated sensitivity curves (left panel) as well as their power-law integrated curves after a year-long observation (right panel).

Figure 3

“ASD” and “GWB” magnetic coupling function upper limits for $\mathrm{ET}-\mathrm{X}$ design sensitivity. Also included are the average of the measurements of the coupling functions at LIGO Hanford, LIGO Livingston, and Virgo during the O3 run for comparison.

Figure 4

Variation in the “GWB” magnetic coupling function upper limits for the different ET designs. Also included are the average of the measurements of coupling functions at LIGO Hanford, LIGO Livingston, and Virgo during the O3 run for comparison.

Figure 5

Upper limits on “GWB” magnetic coupling function of ET-S and ET-X at high frequencies. Also included is the average of the measurements of coupling functions at LIGO Hanford, LIGO Livingston, and Virgo during the O3 run for comparison.

Figure 6

Needed improvement factor as a function of frequency for the “ASD” (left panels) and “GWB” (right panels) upper limits on the ET magnetic coupling function. The low-frequency (top panels) magnetic coupling poses a greater challenge for the operation of ET compared to the high-frequency (bottom panels) magnetic coupling, while “GWB” upper limits on the magnetic coupling are more constraining than the “ASD” ones. In all panels, the dash-dotted blue line indicates the line where no improvement is necessary.

Figure 7

“ASD” and “GWB” magnetic coupling function upper limits of all ET design sensitivities in the case the local magnetic noise is the same level as the CEB at Virgo during O3. Also included are the average of the measurements of coupling functions at LIGO Hanford, LIGO Livingston, and Virgo during the O3 run for comparison.