Auroræ and Clouds.
Auroræ and clouds. Dr. Richardson’s observations. Aurora constantly accompanied by or immediately precedes the formation of cirro-stratus.
Dr. Richardson (‘Sir John Franklin’s Narrative’), so long ago as the years 1819-1822, made many recorded observations on the connexion of clouds with the Aurora Borealis in the Polar regions. Some of these are alluded to in Chapter V., section “Height of the Aurora,” for the purpose of showing the moderate distance he found it to be above the earth; and his inference is there mentioned, “that the Aurora Borealis is constantly accompanied by or immediately precedes the formation of one or other of the various kinds of cirro-stratus.” On the 13th November and 18th December, 1820, the connexion of an Aurora with a cloud intermediate between cirrus and cirro-stratus is mentioned. It is, however, also mentioned that the most vivid coruscations of the Aurora were observed when there were only a few attenuated shoots of cirro-stratus floating in the air, or when that cloud was so rare that its existence was only known by the production of a halo round the moon. (An instance of attenuated streaks of cirro-stratus in connexion with an auroral arc will be found in the Aurora seen at Guildown on the 4th February 1874, a sketch of which is reproduced on Plate VI. fig. 1.)
Polarity discerned in cirro-stratus clouds.
Dr. Richardson goes on to express his opinion that he, on some occasions, discerned a polarity in the masses of clouds belonging to a certain kind of cirro-stratus (approaching cirrus), by which their long diameters, having all the same direction, were made to cross the magnetic meridian nearly at right angles.
Apparent polarity of Aurora might perhaps be ascribed to the clouds themselves.
Dr. Richardson further suggests that if it should be thereafter proved that the Aurora depends upon the existence of certain clouds, its apparent polarity might perhaps be ascribed to the clouds themselves which emit the light; or, in other words, the clouds might assume their peculiar arrangement through the operation of one cause (magnetism, for instance), while the emission of light might be produced by another—a change in their internal constitution perhaps connected with a motion of the electric fluid.
Dr. Richardson further remarks that, generally speaking, the Aurora appeared in small detached masses for some time before it assumed that convergency towards the opposite parts of the horizon which produced the arched form.
Sir John Franklin’s observations.
Sir John Franklin says in his Polar expeditions he often perceived the clouds in the daytime disposed in streams and arches such as the Aurora assumes.