Because the northern lights appear in pitch-black conditions, your phone needs to keep its shutter open for several seconds to collect enough light. If you hold the phone in your hands, the microscopic shaking of your muscles will blur the stars and turn the aurora into a hazy green smudge.
To avoid this, use a pocket tripod, clamp the phone to a railing, or prop it up securely against a solid surface (like a fence post or backpack).