How to use Auroral in a sentence
Sunlight is not the only disturbing cause in estimates of auroral frequency.
In auroral displays the brightness often varies greatly over the illuminated area and changes rapidly.
During some displays, auroral light appears in irregular areas or patches, which sometimes bear a very close resemblance to illuminated detached clouds.
The mean sun-spot frequency for the group of years of few sun-spots is almost exactly the same for the two subperiods, but the auroral frequency for the later group is nearly 40% in excess of that for the earlier, and even exceeds the auroral frequency in the years of many sun-spots in the earlier sub-period.
Rubenson (14), from whom Tromholt derives his data for Sweden, seems to accept this view, assigning the apparent increase in auroral frequency since 1860 to the institution by the state of meteorological stations in 1859, and to the increased interest taken in the subject since 1865 by the university of Upsala.Advertisement
Greenland lies to the north of Fritz's curve of maximum auroral frequency, and the suggestion has been made that the zone of maximum frequency expands to the south as sun-spots increase, and contracts again as they diminish, the number of auroras at a given station increasing or diminishing as the zone of maximum frequency approaches to or recedes from it.
Auroral Meridian.-It is a common belief that the summit of an auroral arc is to be looked for in the observer's magnetic meridian.
In temperate latitudes auroral arcs are seldom near the zenith, and there is reason to believe them at very great heights.
Very elaborate observations have been made during several Arctic expeditions of the azimuths of the summits of auroral arcs.
An auroral curtain travelling with considerable velocity would approach from the south, pass right overhead and retire to the north.Advertisement
The behaviour of the needle, as Paulsen points out, is exactly what it should be if the space occupied by the auroral curtain were traversed by electric currents directed upwards from the ground.
At Tasiusak the auroral curtain after reaching the zenith usually retired in the direction from which it had come.
Birkeland (19), who has made a special study of magnetic disturbances in the Arctic, proceeding on the hypothesis that they arise from electric currents in the atmosphere, and who has thence attempted to deduce the position and intensity of these currents, asserts that whilst in the case of many storms the data were insufficient, when it was possible to fix the position of the mean line of flow of the hypothetical current relatively to an auroral arc, he invariably found the directions coincident or nearly so.
Auroral displays generally cover a considerable area, and are constantly changing, so the figures are necessarily somewhat rough.
Of course the phenomenon might be due to actual change in the arc, but it is at least consistent with the view that arcs are of two kinds, one form constituting a layer of no great vertical depth but considerable real horizontal width, the other form having little horizontal width but considerable vertical depth, and resembling to some extent an auroral curtain.Advertisement
If an auroral arc represented a definite selfluminous portion of space of small transverse dimensions at a uniform height above the ground, its height could be accurately determined by observations made with theodolites at the two ends of a measured base, provided the base were not too short compared to the height.
If a very long base is taken, it becomes increasingly open to doubt whether the portions of space emitting auroral light to the observers at the two ends are the same.
Heights have been calculated in various less direct ways, by observing for instance the angular altitude of the summit of an arc and the angular interval between its extremities, and then making some assumption such as that the portion visible to an observer may be treated as a circle whose centre lies over the so-called auroral pole.
The height has also been calculated on the hypothesis that auroral light has its source where the atmospheric pressure is similar to that at which most brilliancy is observed when electric discharges pass in vacuum tubes.
If the Godthaab observations can be trusted, auroral discharges must often occur within a few miles of the earth's surface in Arctic regions.Advertisement
In some cases changes of intensity take place round the auroral zenith, simulating the effect that would be produced by a cyclonic rotation of luminous matter.
Thus a bright auroral ray may seem red towards the foot and green at its summit, with yellow intervening.
One line, in the yellow green, is so dominant optically as often to be described as the auroral line.
In the spectrograms three auroral rays-including the principal one mentioned above-were pre-eminent.
In addition, he assigns wave-lengths for 156 other auroral lines between wave-lengths 5205 and 3513.Advertisement
Out of a total of 146 auroral lines, with wave-lengths longer than 3684 tenth-metres, Westman identifies 82 with oxygen or nitrogen lines at the negative pole in vacuum discharges.
Amongst the lines thus identified are the two principal auroral lines having wave-lengths 4276.4 and 3913.5.
The interval considered by Westman contains at least 300 oxygen and nitrogen lines, so that approximate coincidence with a number of auroral lines was almost inevitable, and an appreciable number of the coincidences may be accidental.
C. C. Baly (21), making use of the observations of the Russian expedition in Spitsbergen in 1899, accepts as the wave-lengths of the three principal auroral lines 557 o, 4276 and 3912; and he identifies all three and ten other auroral lines ranging between 5570 and 3707 with krypton lines measured by himself.
In addition to these, he mentions other auroral lines as very probably krypton lines, but in their case the wave-lengths which he quotes from Paulsen (22) are given to only three significant figures, so that the identification is more uncertain.Advertisement
The majority of the krypton lines which Baly identifies with auroral lines require for their production a Leyden jar and spark gap.
If, as is now generally believed, aurora represents some form of electrical discharge, it is only reasonable to suppose that the auroral lines arise from atmospheric gases.
Many of the auroral lines seen in any single aurora are exceedingly faint, so that even their relative positions are difficult to settle with high precision.
Whether or not auroral displays are ever accompanied by a characteristic sound is a disputed question.
If sound waves originate at the seat of auroral displays they seem hardly likely to be audible on the earth, unless the aurora comes very low and great stillness prevails.Advertisement
If it exists, it is presumably confined to cases when the auroral discharge comes unusually low.
Upsala, however, is a place where the auroral spectrum can often be observed in the sky, even when no aurora is visible, and it has generally been believed that what Angstrom really saw was an auroral and not a zodiacal spectrum.
In this instance plasma convection appeares to play the dominant role in forming the large-scale spatial structure of the nightside auroral ionosphere.
The familiar features of the auroral oval are evident in this presentation.
To my imagination it retained throughout the day more or less of this auroral character, reminding me of a certain house on a mountain which I had visited a year before.Advertisement
At several stations in Greenland auroral curtains have been observed when passing right overhead to narrow to a thin luminous streak, exactly as a vertical sheet of light would seem to do to one passing underneath it.
The auroral data are from Table E of Tromholt's catalogue (5), with certain modifications.
Wolf and Wolfer have, however, aimed persistently at securing a definite standard, and there are several reasons for believing that the change of unit has been in the auroral rather than the sun-spot frequency.
Even smaller mean values have been found for the angle between the auroral and magnetic " zeniths " - as the two directions have been called - e.g.
A bright aurora visible over a large part of Europe seems always accompanied by a magnetic storm and earth currents, and the largest magnetic storms and the most conspicuous auroral displays have occurred simultaneously.Advertisement
This has been more especially the case when the auroral light has been of a diffused character, showing only minor variability.
In the Arctic, auroral displays seem sometimes to be very local, and this may be the explanation.
The fact that at most places the morning shows a marked decay of auroral frequency and intensity as compared to the evening, the maximum preceding midnight by several hours, is certainly favourable to theories which postulate ionization of the atmosphere by some cause or other emanating from the sun.
At Cape Thorsden diffused auroral light had percentages e.