The Earth's ionosphere at mid-latitudes is produced chiefly by solar EUV-induced photo-ionization of the neutral atmosphere at altitudes of 90 to 300–400 kilometers. Such ionized gas, or plasma—entrained by the Earth's (basically bipolar) magnetic field—flows upward along magnetic lines of force, until the plasma gas pressure is equalized along the entire line of force extending from the northern to the southern ionosphere. The plasma region above the ionosphere on such closed magnetic field lines is called the plasmasphere. In fact, the plasmasphere may be considered as an extension of the ionosphere, for there is no clear distinction between them. In the absence of plasma-removal processes, plasma densities may exceed 104 ions/cm3 even at high altitudes of 12,000 kilometers or more.