The effective magnetic field response over the absolute noise level (logarithmic scale) for the COG method (open circles) and the OMP method (solid circles). The response curve shows how much of the noise is erroneously attributed to a magnetic field. On the left side, the response is shown for a noise range between 10^-5 and 10^-3. The noise response of the COG method rapidly increases with the noise level from 2 to 100 G, whereas the OMP method remains largely unaffected by the noise with a flat response not larger than 0.7 G. On the right side, a close-up of the magnetic response curve for the region between 10^-5 to 10^-4 is shown. Even in such a low noise regime, the COG method falsely interprets a significant portion of the noise as magnetic field. The OMP method stays essentially flat at a level close to zero over the entire region. The gray shaded area marks the region of incorrect response for the COG method, i.e. , and defines the lower limit for magnetic field detections