Without the magnetic field, high energy radiation from space would affect the Earth's upper atmosphere much more dramatically – and high-energy radiation reaching the surface would be far higher.
“In 1905, shortly after composing his special relativity paper, Albert Einstein described the origin of the Earth's magnetic field as being one of the great unsolved problems facing modern physicists.”
There maybe a gargantuan revolving molten iron core – the so-called Dynamo Theory – but it’s known from magnetised ‘stripes’ in extruded seabed rock that the Earth’s magnetic field completely (and rapidly) reverses every 200,000 to 300,000 years. There is no clear theory as to how these geomagnetic reversals might have occurred. If the Earth does have an internal ‘Dynamo’ it would seem unlikely that it could spontaneously reverse its rotation, or turn upside down.
Recent calculations regarding the formation of the 'dynamo' conflict with geological evidence :
Evidence is mounting that the dynamo could only have emerged comparatively recently. At the same time, geological clues show that the magnetic field has existed for most of Earth’s 4.5-billion-year history.
see: The paradox powering Earth’s magnetic field New Scientist, Jan 2017