+----------------------+--------------+--------------+--------------+
| | Annual Term. | 6-Month Term.| 4-Month Term.|
| Station. +-------+------+-------+------+-------+------+
| | Amp. |Phase.| Amp. |Phase.| Amp. |Phase.|
+----------------------+-------+------+-------+------+-------+------+
| | | deg. | | deg. | | deg. |
| Jakobshavn | 10.40 | 123 | 1.13 | 206 | 1.41 | 333 |
| Godthaab | 8.21 | 111 | 1.54 | 316 | 0.64 | 335 |
| St Petersburg | 2.81 | 96 | 5.99 | 309 | 0.57 | 208 |
| Christiania | 4.83 | 116 | 4.99 | 317 | 0.76 | 189 |
| Upsala | 5.41 | 119 | 4.57 | 322 | 0.86 | 296 |
| Stockholm | 3.68 | 91 | 5.80 | 303 | 1.31 | 180 |
| Makerstown (Scotland)| 5.79 | 102 | 4.47 | 310 | 2.00 | 342 |
| Great Britain | 3.87 | 126 | 4.24 | 287 | 0.40 | 73 |
| Toronto | 0.18 | 12 | 2.13 | 260 | 0.52 | 305 |
| Cambridge, Mass. | 1.02 | 262 | 2.84 | 339 | 1.28 | 253 |
| New Haven, Conn. | 0.99 | 183 | 1.02 | 313 | 0.57 | 197 |
| New York State | 1.34 | 264 | 2.29 | 325 | 0.54 | 157 |
+----------------------+-------+------+-------+------+-------+------+
Speaking generally, the annual term diminishes in importance as we
travel south. North of 55 deg. in Europe its phase angle seems fairly
constant, not differing very much from the value 110 deg. in
Lovering's general formula. The 6-month term is small, in the two most
northern stations, but south of 60 deg. N. lat. it is on the whole the
most important term. Excluding Jakobshavn, the phase angles in the
6-month term vary wonderfully little, and approach the value 309 deg.
in Lovering's general formula. North of lat. 50 deg. the 4-month term
is, as a rule, comparatively unimportant, but in the American stations
its relative importance is increased. The phase angle, however, varies
so much as to suggest that the term mainly represents local causes or
observational uncertainties. Lovering's general formula suggests that
the 4-month term is really less important than the 3-month term, but
he gives no data for the latter at individual stations.
6. Sunlight is not the only disturbing cause in estimates of auroral
frequency. An idea of the disturbing influence of cloud may be derived
from some interesting results from the Cape Thorsden (7) observations.
These show how the frequency of visible auroras diminished as cloud
increased from 0 (sky quite clear) to 10 (sky wholly overcast).
Grouping the results, we have:
Amount of cloud 0 1 to 3 4 to 6 7 to 9 10
Relative frequency 100 82 57 46 8
Out of a total of 1714 hours during which the sky was wholly overcast
the Swedish expedition saw auroras on 17, occurring on 14 separate
days, whereas 226 hours of aurora would have occurred out of an equal
number of hours with the sky quite clear. The figures being based on
only one season's observations are somewhat irregular. Smoothing them,
Carlheim-Gyllenskold gives f = 100' - 7.3c as the most probable linear
relation between c, the amount of cloud, and f, the frequency,
assuming the latter to be 100 when there is no cloud.
7. _Diurnal Variation._--The apparent daily period at most stations is
largely determined by the influence of daylight on the visibility. It is
only during winter and in high latitudes that we can hope to ascertain
anything directly as to the real diurnal variation of the causes whose
influence is visible at night as aurora. Table III. gives particulars of
the number of occasions when aurora was seen at each hour of the
twenty-four during three expeditions in high latitudes when a special
outlook was kept.
The data under A refer to Cape Thorsden (78 deg. 28' N. lat., 15 deg.
42' E. long.), those under B to Jan Mayen (8) (71 deg. 0' N. lat., 8
deg. 28' W. long.), both for the winter of 1882-1883. The data under C
are given by H. Arctowski (9) for the "Belgica" Expedition in 1898. They
may be regarded as applying approximately to the mean position of the
"Belgica," or 70-1/2 deg. S. lat., 86-1/2 deg. W. long. The method of
counting frequencies was fairly alike, at least in the case of A and B,
but in comparing the different stations the data should be regarded as
relative rather than absolute. The Jan Mayen data refer really to
Gottingen mean time, but this was only twenty-three minutes late on
local time. In calculating the percentages of forenoon and afternoon
occurrences half the entries under noon and midnight were assigned to
each half of the day. Even at Cape Thorsden, the sun at midwinter is
only 11 deg. below the horizon at noon, and its effect on the visibility
is thus not wholly negligible. The influence of daylight is presumably
the principal cause of the difference between the phenomena during
November, December and January at Cape Thorsden and Jan Mayen, for in
the equinoctial months the results from these two stations are closely
similar. Whilst daylight is the principal cause of the diurnal
inequality, it is not the only cause, otherwise there would be as many
auroras in the morning (forenoon) as in the evening (afternoon). The
number seen in the evening is, however, according to Table III.,
considerably in excess at all seasons. Taking the whole winter, the
percentage seen in the evening was the same for the "Belgica" as for Jan
Mayen, i.e. for practically the same latitudes South and North. At Cape
Thorsden from November to January there seems a distinct double period,
with minima near noon and midnight. The other months at Cape Thorsden
show a single maximum and minimum, the former before midnight. The same
phenomenon appears at Jan Mayen especially in November, December and
January, and it is the normal state of matters in temperate latitudes,
where the frequency is usually greatest between 8 and 10 P.M. An excess
of evening over morning occurrences is also the rule, and it is not
infrequently more pronounced than in Table III. Thus at Tasiusak (65
deg. 37' N. lat., 37 deg. 33' W. long.) the Danish Arctic Expedition
(10) of 1904 found seventy-five out of every hundred occurrences to take
place before midnight.