were recently installed at both LIGO sites, as described in the Supplementary Material.

![Conservative estimate of correlated magnetic noise. We assume a conservative transfer function (TF) based on measurements as described in the text. The first Schumann resonance at 8 Hz is visible, higher harmonics are below the noise floor. There is a zero of the overlap function at 64 Hz which leads to an apparent feature in $\Omega_{\rm mag}$. Power line harmonics have been removed, as in the cross-correlation analysis. The two trend lines show power law fits to the magnetometer spectra, scaled by the O1 (purple dotted) and end-of-O2 (blue dot-dashed) transfer functions. This demonstrates the effect of reducing the magnetic coupling in O2. The trend for the noise budget lies well below the solid black O2 PI curve, which indicates that correlated magnetic noise is negligible in O2. However magnetic contamination may be an issue in future observing runs.[]{data-label="fig:schumann"}](Noise_Budget_full.pdf){width="49.00000%"}

*Conclusions*— We have presented the results of a cross-correlation search for the isotropic stochastic background using data from Advanced LIGO’s first and second observing runs. While we did not find evidence for the , we obtain the most sensitive upper limits to date in the $\sim$20-100 Hz frequency 