Each planet’s magnetic field determines where its auroras are found, because the magnetic field attracts the charged particles from the sun. On Earth, auroras tend to be concentrated near the poles because that’s where the magnetic field is strongest. Although Mars once had a global magnetic field like ours, it degraded over time. Now all that’s left are small magnetic hotspots (called “crustal magnetic anomalies”). Those are mostly located in the southern hemisphere, so that's where the Martian auroras will be concentrated, too.