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Abstract We present measurements of large-scale cosmic microwave backgroundE-mode polarization from the Cosmology Large Angular Scale Surveyor 90 GHz data. Using 115 det-yr of observations collected through 2024 with a variable-delay polarization modulator, we achieved a polarization sensitivity of $82\mu \mathrm{K}\mathrm{arcmin}$, comparable to Planck at similar frequencies (100 and 143 GHz ). The analysis demonstrates effective mitigation of systematic errors and addresses challenges to large-angular-scale power recovery posed by time-domain filtering in maximum-likelihood map-making. A novel implementation of the pixel-space transfer matrix is introduced, which enables efficient filtering simulations and bias correction in the power spectrum using the quadratic cross-spectrum estimator. Overall, we achieved an unbiased time-domain filtering correction to recover the largest angular scale polarization, with the only power deficit, arising from map-making nonlinearity, being characterized as <3%. Through cross-correlation with Planck, we detected the cosmic reionization at 99.4% significance and measured the reionization optical depth $\tau =0.053_{-0.019}^{+0.018}$, marking the first ground-based attempt at such a measurement. At intermediate angular scales (ℓ > 30), our results, both independently and in cross-correlation with Planck, remain fully consistent with Planck’s measurements. 

Abstract Improved polarization measurements at frequencies below 70 GHz with degree-level angular resolution are crucial for advancing our understanding of the Galactic synchrotron radiation and the potential polarized anomalous microwave emission and ultimately benefiting the detection of primordialBmodes. In this study, we present sensitivity-improved 40 GHz polarization maps obtained by combining the CLASS 40 GHz and Wilkinson Microwave Anisotropy Probe (WMAP)Q-band data through a weighted average in the harmonic domain. The decision to include WMAPQ-band data stems from similarities in the bandpasses. Leveraging the accurate large-scale measurements from the WMAPQband and the high-sensitivity information from the CLASS 40 GHz band at intermediate scales, the noise level atℓ∈ [30, 100] is reduced by a factor of 2–3 in the map space. A pixel domain analysis of the polarized synchrotron spectral index (β_s) using the WMAPKband and the combined maps (mean and 16th/84th percentiles across theβ_smap: $-{3.08}_{-0.20}^{+0.20}$) reveals a stronger preference for spatial variation (probability to exceed for a uniformβ_shypothesis smaller than 0.001) than the results obtained using WMAPKandKabands ( $-{3.08}_{-0.14}^{+0.14}$). The cross-power spectra of the combined maps follow the same trend as other low-frequency data, and validation through simulations indicates negligible bias introduced by the combination method (subpercent level in the power spectra). The products of this work are publicly available onLAMBDA(https://lambda.gsfc.nasa.gov/product/class/class_prod_table.html).