A theory of the aurora polaris is proposed which attempts to explain many features of the complicated morphology of auroral displays. One basis of the theory is the presence, during magnetic disturbance, of additional or enhanced magnetic fields due to electric currents within a distance of several earth radii from the earth's centre. One such field (denoted by DCF) is due to electric currents flowing near the inner surface of the solar stream that then envelopes the earth. A hollow is carved in the stream by the geomagnetic field. The other field (denoted by DR) is that of an electric ring current, additional or enhanced, that flows westward round the earth. This is carried by the particles of the Van Allen belts. A third field (denoted by DP) is that of the disturbance currents that flow in the ionosphere, under the impulsion of electromotive forces generated mainly in polar regions. We consider it likely that during magnetic storms and auroral displays, neutral lines appear in the magnetic field near the earth. These will lie mainly on the dark side of the earth, in or near the equatorial plane, on the nearer side of the ring current. At times these lines may extend over more than 180 degrees of longitude, so that a part of them may lie on the sunward side of the earth. These neutral lines are of two types, which we call O and X; they appear together, in pairs. During disturbed conditions there may be more than one pair. Lines of force cross at points on X neutral lines, but they do not pass through O neutral lines. As Dungey has shown, charged particles will tend to be concentrated near X points (of which the X neutral lines are the locus). Charges drawn toward the neutral line will be discharged into the earth's atmosphere along the lines of magnetic force. We suggest that the location, nature and motions of the auroral forms are determined by the position, form and motion of the X neutral lines, lying in or near the plane of the geomagnetic equator. It seems necessary to suppose, in addition, that an electric field arises sporadically along the X lines. When this is absent, the aurora appears as a quiet arc. The onset of the suggested electric field concentrates the charges more narrowly near the X line and near the lines of force that extend from it to the auroral zone. This produces extremely thin-rayed auroral arcs. The above concentration of electrons near an X neutral line produces a large flux of electrons, while the proton flux is diminished. A dynamical instability due to this flux difference (the space charge density is supposed to be very small) produces a slight separation of protons and electrons along and near the lines of force through the X line. Hence in the auroral ionosphere there is an associated electric field. This is usually directed towards the equator. It drives electric current, usually westward, along the auroral zones, and produces the strong magnetic disturbances (DP) there observed. Birkeland called these polar elementary storms. The rapid auroral changes are ascribed to instabilities of the magnetic field in the region near the X line or lines, to the rear of the earth, where the resultant magnetic field is weak. The ray structure in the auroral arc is ascribed to an instability of the thin sheet of electron flow. Cosmic rockets have shown that the magnetic field, up to and beyond ten earth radii, departs from the values corresponding to the internally produced main geomagnetic field. As yet these explorations do not seem to have disclosed the existence of reversals of the field in or near the magnetic equatorial plane. But on the basis of our auroral hypothesis, we predict with considerable confidence that such reversals will be found to occur, on the dark side of the earth, during great auroral displays. The theory here proposed is discussed in connexion with recent I.G.Y. and I.G.C. auroral, magnetic and other data.