166 Alaskan Science Conference 



out the Arctic and is essentially unrelated to the auroral zone, 

 the disturbance daily variation has a virtual discontinuity there, 

 and is greatly intensified in the neighboring areas— so much so, 

 indeed, that only on exceptionally calm days can we perceive 

 S Q in Alaska through the overlying effect of the disturbance 

 field. 



To assist in visualizing the facts, we may conceive the Earth 

 to be girdled by a magnetic barrier in the form of a cosmic 

 doughnut that deflects and channels the "flash floods" of 

 ionized matter streaming out intermittently from the sun or its 

 corona. Along that barrier (that is, along certain lines of force 

 of the Earth's magnetic field), these ionized particles form con- 

 centrations pouring into the hole of the doughnut from either 

 side like water running into a drain. The circles where this 

 great barrier meets the globe are the northern and southern 

 auroral zones. During severe disturbances or magnetic storms, 

 they do not remain at precisely their habitual latitude, but 

 advance somewhat toward the temperate regions, receding again 

 with the abatement of the disturbance. (The image invoked 

 here is a provisional one, involving a tentative choice among 

 several theories of magnetic storms, none of which has achieved 

 universal acceptance.) 



Each auroral zone probably represents a sort of curtain of 

 activation, with strong electric currents that are replenished 

 with every new effusion of material from the sun. This curtain 

 terminates in the ionosphere, which it feeds with horizontal 

 currents contributing to the world-wide pattern of S d . The 

 quiet daily variation, on the other hand, is primarily a phe- 

 nomenon arising within the ionosphere— probably at its lower 

 boundary where the ultraviolet absorption yields the needed 

 direct-current conductivity— and stems from tidal and thermal 

 dynamo action in latitudes well below that of the auroral zone. 

 The ionosphere also has an effect on the irregular fluctuations, 

 in that it tends to shield the Earth from the most abrupt fea- 

 tures of the external field changes. These irregular changes, 

 like S d , are greatly intensified in the vicinity of the auroral 

 zones. 



