The shadow is caused by the disk of Jupiter itself. This is the back side of Jupiter which is not illuminated by the Sun. This is like looking at the half Moon from Earth. You see the illuminated part caused by sunlight reflecting off the surface of the Moon from the Sun. You also see the un-illuminated part of the Moon where the lunar surface is in the shadow zone of the Moon. Only during eclipses of the Moon by the Earth, where the Earth gets between the Moon and the Sun, does the shadow of the Earth cover part or all of the Moon. The moons of Jupiter also cast their shadows upon Jupiter when they come between Jupiter and the Sun. The Earth's shadow is far too feeble and distant to cast a shadow out to the distance of Jupiter.