During some displays, auroral light appears in irregular areas or patches, which sometimes bear a very close resemblance to illuminated detached clouds.
Sunlight is not the only disturbing cause in estimates of auroral frequency.
At Cape Thorsden diffused auroral light had percentages e.
- The frequency of auroral displays is much greater in some years than others.
The auroral data are from Table E of Tromholt's catalogue (5), with certain modifications.
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.
„ 55.8 „ I12.2 The mean sun-spot frequencies in the two periods differ by only I %, but the auroral frequency in the later period is 45% in excess of that in the earlier.
The association of high auroral and sun-spot frequencies shown in Table V.
It is shown, for instance, in Loomis's auroral data, which are based on observations at a variety of European and American stations (Ency.
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.
Azimuths were also measured at Jan Mayen for 338 auroral bands, the mean being 22.0° W., or 7.9° to the east of the magnetic meridian.
Combining the results from arcs and bands, Carlheim-Gyllenskäld gives the " anomaly " of the auroral meridian at Jan Mayen as 5.7° E.
At Godthaab in 1882-1883 the auroral anomaly was, according to Paulsen, 15.5° E., the magnetic meridian lying 57.6° W.
- Another auroral direction having apparently a close relation to terrestrial magnetism is the imaginary line drawn to the eye of an observer from the centre of the corona - i.e.
The point to which the auroral rays converge.
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.
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.
An auroral curtain travelling with considerable velocity would approach from the south, pass right overhead and retire to the north.
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.
That man who does not believe that each day contains an earlier, more sacred, and auroral hour than he has yet profaned, has despaired of life, and is pursuing a descending and darkening way.