When you get out of the atmosphere top boundary, we switch to a mode not using the fast Look-Up-Tables but instead per pixel tracing (because LUTs are less accurate when not inside the atmosphere). To reduce banding coming from earth shadows for instance, we jitter the ray position which results in this temporal noise pattern. TAA should eat it up completely and make the pattern invisible. My guess is maybe you have turned off TAA?
Make sure you also have a read at Sky Atmosphere | Unreal Engine Documentation, Section “Moving from Ground to Outer Space”.
Otherwise it is an unknown problem (for such simple view of the atmosphere I would expect the signal to be smooth). An expensive way (workaround) to get rid of the noise is to increase the number of sample taken when raymarching the atmosphere (TraceSampleCountScale on the SkyAtmosphere component).
I hope this helps.