PMC

Colored circles represent individual magnetic anomaly identifications color-coded by magnetic chron name. Seafloor-spreading between Africa and South America is represented by the magnetic anomaly picks in the South Atlantic, between Africa and Antarctica along the Southwest Indian Ridge, between South America and Antarctica in the Weddell Sea and several episodes of back-arc basin opening in the Scotia Sea.

(A) For a north-seeking bacterium (with the magnetic moment aligned with the velocity of the bacterium, thus swimming with the north of its magnet in front) swimming counterclockwise (CCW); (B) north-seeker swimming clockwise (CW); (C) for a south-seeker (with the magnetic moment opposite to the velocity vector, swimming with the south of its magnet in front) CCW; and (D) for a south-seeker CW. The red color indicates the north of the external magnetic field, and the blue the south. The blue/red arrow shows the external magnetic field direction. The circular arrows in black indicate the direction of the wall torque; in red, the magnetic torque in the north part of the trap, and in blue in the south part. The dot at the center of the torque arrow indicates that the torque vector points out of the image, and the cross that it points into the image. Inside the trap, the black lines with arrows indicate the trajectories of the bacteria. Lower velocities and higher magnetic moments correspond to trajectories with smaller radii, and vice versa, higher velocities and smaller magnetic moments correspond to the trajectories with bigger radii. While the sign of the wall torque is given just by the swimming direction (CCW or CW), the sign of the magnetic moment depends on the position of the bacterium (if it is in the north side or south side of the trap), and on the orientation of the magnetic moment with respect to its velocity. When the wall torque and the magnetic torque point in the same direction, they sum up and push the bacterium off the wall and into the middle of the trap, producing the U-turns.

Geomagnetic field (GMF). The GMF offers different cues for navigation: The polarity (red arrows) allows North to be distinguished from South. The magnetic poles (magnetic North mN, magnetic South mS) are shifted relative to the geographic poles (geographic North gN, geographic South gS). The angle between mN and gN is called declination. The intensity (red triangles) is highest at the poles and lowest at the magnetic equator. The inclination is the angle between the GMF lines and gravity (red bars). It changes gradually from the magnetic poles (± 90°) to the magnetic equator (0°)

Numerical model of the magnetostatic mattress. The magnetic strips were aligned, with south poles at both ends and the north pole in the middle (S1-N-S2) or the south pole in the middle and the north poles at both ends (N1-S-N2) []. In the figure, red represents the south pole, and blue represents the north pole. Yellow represents the position of magnetic field measurement.