The lower map appears to show the South Atlantic Anomaly (SAA), an area where the inner Van Allen radiation belt comes closest to the surface of the Earth.

From the linked article:

The effect is caused by the non-concentricity of the Earth and its magnetic dipole. The SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.

The article gives a good side view showing the asymmetry:

Orbiting objects that pass through the SAA are subject to increased radiation flux compared to other portions of their orbit.

The lower map appears to show the South Atlantic Anomaly (SAA), an area where the inner Van Allen radiation belt comes closest to the surface of the Earth.

From the linked article:

The effect is caused by the non-concentricity of the Earth and its magnetic dipole. The SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.

The article gives a good side view showing the asymmetry:

Orbiting objects that pass through the SAA are subject to increased radiation flux compared to other portions of their orbit.

Edit: Here's a picture from my personal collection showing the instructor station of the Shuttle Mission Simulator (from long ago). On the far wall you can see a framed copy of a paper groundtrack map, with the South Atlantic Anomaly marked on it in gray. Wish I had a closeup! But you can see there is no North Atlantic Anomaly.

The lower map appears to show the South Atlantic Anomaly, an area where the inner Van Allen radiation belt comes closest to the surface of the Earth.

From the linked article:

The effect is caused by the non-concentricity of the Earth and its magnetic dipole. The SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.

The article gives a good side view showing the asymmetry:

The lower map appears to show the South Atlantic Anomaly (SAA), an area where the inner Van Allen radiation belt comes closest to the surface of the Earth.

From the linked article:

The effect is caused by the non-concentricity of the Earth and its magnetic dipole. The SAA is the near-Earth region where the Earth's magnetic field is weakest relative to an idealized Earth-centered dipole field.

The article gives a good side view showing the asymmetry:

Orbiting objects that pass through the SAA are subject to increased radiation flux compared to other portions of their orbit.