Pre-eruption RPWs in relation to magnetic field in the sunspot. Panels a, b: field inclination θ and horizontal component, B_h, of the photospheric magnetic field. Panel c: RPW front in an AIA 171 Å difference image. The blue contour marks the outer penumbral boundary. The circle in panel b indicates the circular slit used to construct the time-distance maps in panels d–f. The azimuthal directions of 0 and 240 deg along the circle are marked by two arrows. The latter direction is also marked by arrows on the right side in panels d–f.