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The detection of the 21 cm signal of neutral hydrogen from the Epoch ofReionization (EoR) is challenging due to bright foreground sources, radiofrequency interference (RFI), the ionosphere, and instrumental effects. Evenafter correcting for these effects in the calibration step and applyingforeground removal techniques, the remaining residuals in the observed 21 cmpower spectra are still above the thermal noise, which is referred to as the"excess variance." We study potential causes of this excess variance based on13 nights of data obtained with the Low-Frequency Array (LOFAR). We focused onthe impact of gain errors, the sky model, and ionospheric effects on the excessvariance by correlating the relevant parameters such as the gain variance overtime or frequency, local sidereal time (LST), diffractive scale, and phasestructure-function slope with the level of excess variance. Our analysis showsthat excess variance has an LST dependence, which is related to the power fromthe sky. And the simulated Stokes I power spectra from bright sources and theexcess variance show a similar progression over LST with the minimum powerappearing at LST bin 6h to 9h. This LST dependence is also present in skyimages of the residual Stokes I of the observations. In very-wide sky images,we demonstrate that the extra power comes exactly from the direction of brightand distant sources Cassiopeia A and Cygnus A with the array beam patterns.These results suggest that the level of excess variance in the 21 cm signalpower spectra is related to sky effects and, hence, it depends on LST. Inparticular, very bright and distant sources such as Cassiopeia A and Cygnus Acan dominate the effect. This is in line with earlier studies and offers a pathforward toward a solution since the correlation between the sky-related effectsand the excess variance is non-negligible.

Statistical analysis of the causes of excess variance in the 21 cm signal power spectra obtained with the Low-Frequency Array