Feb. 7, 1884] 



NA TURE 



in 



proper development at a time when growth is very rapid, and 

 thus cause that difference in the length and strength of the limbs 

 remarked by your correspondents? Sara S. Owen 



4, Soames Street, Grove Park, S.E., February 2 



ON THE HEIGHT OF THE A URORA BORE A LIS 



IT is with pleasure that I respond to the invitation of 

 NATURE to give an account of the work of the 

 Danish Meteorological Station, which was maintained, 

 under the international scheme, at Godthaab in Green- 

 land, in 18S2-83, and of which I had the honour of being 

 the chief. I intend, in the present article, to confine 

 myself to the aurora borealis. 



The results, which have been obtained from calcula- 

 tions of the height of the aurora borealis in the temperate 

 zone, which lies south of the so-called auroral belt, all 

 agree in fixing the minimum height of the aurora very 

 high, as the auront seem to be confined to the part of the 

 atmosphere where its density is only a fraction of that at 

 the surface of the sea. However different the value may 

 have been of the heights of the aurora; observed outside 

 their true zone, the average of the minimum heights is 

 hardly under two hundred kilometres. On the other hand, 

 the observations in the Arctic regions show that the 

 aurorse may descend to much lower elevations above the 

 earth's crust, and that they may even reach down into 

 regions of the atmosphere where the density is about the 

 same as on the surface of the sea. 



Dr. S. Fritz has thus, at Ivigtut in South Greenland, 

 in February and March, 1872, ineasured aurorse the 

 lower edges of which were only from 50 to 200 metres 

 above the level of the sea, while in nearly every mono- 

 graph of the aurora borealis cases are cited in which the 

 aurora; appear to have reached much further down in 

 the atmosphere. I may further mention some instances, 

 which have, by the bye, not been made public before, ob- 

 served by the eminent zoologist. Prof. Steenstrup, and 

 which he has permitted me to publish here. 



During Prof. Steenstrup's sojourn in Iceland, 1839-40, 

 he saw, on several occasions, auror;e which hid the top of 

 the mountain Esia, some 600 metres in height, lying 

 behind Reykjavik. He further states that he has seen 

 auroral streamers between the masts of a ship, in such a 

 manner that they disappeared where there were sails, and 

 reappeared where the space was free. The Professor 

 asserts even that on one occasion, on January 28, 1840, 

 when walking between Reykjavik and Bessastad with the 

 chief magistrate, Herr Tvede, and Judge Jonasson, he, as 

 well as these two gentlemen, saw auroral streamers ap- 

 pearing between themselves. The phenomenon was not 

 a solitary one, but occurred three or four times during 

 this walk, and in spite of the pedestrians keeping about 

 a yard from each other. 



Although many estimates of the low descent of auroras 

 in the Arctic regions may have been due to optical illu- 

 sions, specially through irradiation, one cannot, even 

 from a casual observation of this magnificent phenome- 

 non, but come to the conclusion that, while some aurorae 

 lie in the same great, indefinable distance from the ob- 

 server as those observed in the temperate zone, there are 

 others whose whole appearance has the character of 

 being a phenomenon of a purely local nature. During 

 our stay at Godthaab this point had my special attention, 

 as it appeared to me of importance to demonstrate by 

 measurements as accurate as possible whether this sub- 

 jective impression answered to the true facts. 



To this end the Danish international station at Godt- 

 haab, 64- 10' 36" N. lat. and 51^ 40' o" E. long., has, 

 during October and December, 18S2, effected a series of 

 measurements. The site of the station was particularly 

 suited for the solution of the problem, as it lies just at 

 the northern border of the great Arctic auroral belt, i.e. 

 in a place where the aurora^ appear with all the 

 peculiarities which distinguish them in their true zone. 



The distance between the two points of observation, 

 separated by the Godthaab Fjord, was 5-8 kilometres, and 

 the direction between them coincided with the magnetic 

 meridian. The two instruments need for the measure- 

 ments, exactly similar in construction, were arranged as 

 universal instruments. Instead of a telescope, a tube was 

 employed, which had in one end a small opening, and in 

 the other a metal cross of very fine wires. In order that 

 the errors in the observations should not affect them very 

 much, measurements were only made in the vertical 

 plane between the two points of observation. The placing 

 and reading of the instruments were effected by means of 

 pre-arranged fire-signals, and only those measurements 

 of which the reading signals were instantaneously an- 

 swered, and for which the time of reading exactly coin- 

 cided, were recorded. Only the lower edges of the auroral 

 bands were measured, as these are nearly always the 

 moat clearly defined. 



We have, during our evenings of observation, measured 

 the height of thirty-two auroral bands. The subjoined 

 figures, showing the result of these, demonstrate that the 

 lower edge of the band certainly descends very low. Thus 

 of the thirty-two auror;B measured by this method ten 

 only had a parallax under i', for six the parallax was 

 between i^ and 2°, four had a parallax of between 3" and 

 4°, two one between 5^ and 6°, four one between 7^ and 8°, 

 while we measured six of 10°, 14°, 15", 17°, 85°, and 143° 

 respectively. 



Leaving the aurora; whose parallax was under 1° out 

 of the calculation, I have found the following heights for 

 the other twentv-two lower edges : — 



The three heights of 1-99, 2-87, and 3-22 kilometres 

 belong to the same aurora as that whose edge was mea- 

 sured at an interval of two minutes between each 

 measurement. The two aurora of 1-35 and 061 kilo- 

 metres stood both above the fjord between the observa- 

 tories. From the southern one they were seen at an 

 altitude of 13^-6 and 30--3 respectively above the northern 

 horizon, while at the northern station they were 8o°-5 and 

 7'''25 respectively above the southern horizon. These two 

 and the above-mentioned third, the height of which was 

 measured three times, had the appearance of curtains 

 with large folds, or of bunches or wreaths of streamers 

 lying close together, separated by darker, faintly-shining 

 spaces, but connected below through a common band. 

 The others were bands or arcs without radiating streamers. 

 The edges measured were all nearly at right angles with the 

 magnetic meridian. Only the height of those edges which 

 were distinctly defined, and whose course did not deviate 

 greatly from perpendicularity on the magnetic meridian, 

 were measuied. 



I must further, as regards the small height of the 

 aurora: observed at Godthaab, state that not only three 

 observers besides myself, but even a student as conversant 

 with auroral phenomena as Herr Kleinschmidt, all agree 

 that aurorae were seen belo^u the clouds on several occa- 

 sions during the winter of 1SS2-83. On this point we do 

 not entertain the least doubt. 



In conclusion I will describe some observations made 

 on several occasions during our stay at Godthaab of the 

 peculiar type of the aurora known as phosphorescent 

 auroral clouds. 



On September 21, 18S2, Herr C. Petersen, one of my 



