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Wilson Bentley's auroral observations
P/met. Space Sci, Vol 31, No. IO, pp. 1131-1135, Pnnted in Great Britain.
WILSON
1983
0
BENTLEY’S
AURORAL S.M.
Physics
Department,
00324633/83%3.00+0.00 1983 Per~amon Press Ltd.
OBSERVATIONS
SILVERMAN
Boston College, Chestnut
Hill, MA 02167, U.S.A.
and D. C. BLANCHARD
Atmospheric
Sciences Research Center, State University Albany, NY 12222, U.S.A. (Received
22 February
of New York at Albany,
1983)
Abstract-
Wilson Bentley’s aurora1 observations in northern Vermont were made over the period from 1883 to 1931, thus encompassing the solar activity minimum at the beginning of the twentieth century. Long term series of aurora1 observations by a single observer in a region of reasonable frequency of occurrence are rare, and are important for the study of solar-terrestrial relationships. Bentley’s data have been abstracted from manuscript materials. The annual variation and the secular variation over the entire period are presented. The results are compared with the variations of sunspot and magnetic activity during this period. The aurora1 activity minima, comparable to the similar sunspot minima, in the periods around 1902 and 1912 are clearly evident. Changes in the spring/fall ratio of aurora1 occurrence frequency over the entire period under study are also evident.
In recent years there has been increasing interest in the use of proxy data for studies of solar variability, the solar wind and other parameters defining the properties of the interplanetary medium. Proxy data is essential for such studies since direct observations from space are only a few decades old and direct, systematic observations of the Sun exist for only the past two or three centuries. Hence studies of solar variability or of the solar wind at the Earth, for example, must rely on aurora1 or magnetic data to provide information for extended periods (see, for example, Silverman and Feynman, 1981; Feynman and Silverman, 1980; Siscoe, 1980; Feynman, 1983). Aurora1 data are especially useful since observations have been reported over the past two millennia at least. Studies of the variability of solar-terrestrial parameters over extended periods of time generally use one of two types of data sets. In the first type as many data points as possible are collected from a diverse array of sources. If the data set is sufficiently dense then errors due to spotty geographic or temporal coverage and inconsistencies in methods ofdata collection can be minimized. Alternatively, a single, self-consistent source may be used. A data set derived in this way will usually be limited in the range of time over which the data is taken, but will be superior to a more randomly selected data base for the period for which the data is available. In this paper we present a data set of aurora1 observations taken by a single observer, Wilson
Bentley, over a period of 49 years in a location where aurora1 occurrence is of reasonable frequency. Comparison of the resulting data set with other geophysical parameters illustrates the usefulness and importance of self-consistent observations of individual, careful observers over an extended period of time. WILSON BENTLEY
WilsonBentley(1865-1931)wasafarmerinnorthern Vermont. His ancestry included a Revolutionary War soldier and a great-aunt had been married to Martin Chittenden, Governor of Vermont from 1813-1815. His grandfather was one of the first settlers in Jericho, Vermont, where Bentley lived and worked for his entire life. His education until the age of 14 came from his mother, a former school teacher. His later education came primarily from his own reading. In his teens Bentley became fascinated by the variety of forms of ice crystals, the building blocks of snowflakes. This led to the major work of his lifetime, the photographing of several thousand ice crystals using techniques he had himself devised. By the 1920’s his superb ice crystal photomicrographs were widely discussed in the popular press and copies of his lantern slides were acquired by almost every major university and college in the United States. A collection of nearly 2500 of his photomicrographs, almost all of them of ice crystals,
1131
S. M. SILVERMAN and D. C. BLANCHARD
1132
was published in 1931, shortly before his death, with an introductory text writtenlargely by W. J. Humphreys of the Weather Bureau (Bentley and Humphreys, 1931; repr. 1962). The book’s enduring value is evidenced by its remaining in print today, more than 50 years after its first publication. Though known today for his ice crystal photomicrographs, Bentley published about ten technical papers, mostly in the Monthly Weather Review, on the generation and modification of ice crystals in the atmosphere and on their role in the formation of snow and rain (Blanchard, 1970). In 1904 he published a long paper. on his measurements of raindrop-size distributions, the first such made in the United States. Bentley was a meticulous and accurate observer. Each day he recorded the temperature, wind, cloud type, precipitation and any interesting or unusual atmospheric phenomena, including auroras. Our interest is in these latter observations.
THE OBSERVATIONS
At an early period in his life Bentley began his series of meteorological observations at his home in Jericho, Vermont (geographical coordinates : latitude 44” 27’ N, longitude 72” 56’ W; corrected geomagnetic coordiN, longitude 359.9”). nates : latitude 57.5” Unfortunately the notebooks in which he kept his observations for the period from 1883 through 1920 appear to have been lost. His aurora1 observations for this period, however, have been preserved. On 19 September 1920 Bentley sent a tabulation of all the auroras he had observed during the period from 1883 to Dr C. Brooks ofthe Weather Bureau. In his cover letter Bentley said : “I would be grateful if you will preserve this record, so that it will be easily accessible to investigators of aurora1 phenomena.. . Sharing this record may be of some real value to students of aurora1 phenomena.“The tabulation lists a total of 398 auroras, the earliest on 1 February 1883 and the latest on 8 September 1920, with theobservations continuing to 17 September. Bentley’s manuscript aurora1 listing was bound and placed in the Weather Bureau library and is currently in the library of the successor to the Bureau, the National Oceanic and Atmospheric Administration. Bentley’s meteorological notebooks for the period from 1921 to 7 December 1931, a little over 2 weeks before his death on 23 December, have survived. These were acquired about 1947, together with a collection of nearly 12 000 photographicnegatives and plates, by the Buffalo Museum of Science, their current home. Through the courtesy of the Buffalo Museum we obtained copies of the notebooks and have abstracted
the aurora1 observations from them. This provided an additional 236 auroras, giving a total number observed by Bentley of 634 auroras over a period of 49 years. The two manuscript sources taken together provide an aurora1 record for Jericho, Vermont from 1 February 1883 to 7 December 1931, with the exception of a short period from 18 September through 31 December 1920. In order to provide a record for the entire period from 1883 through 1931 we have used published aurora1 observations from the New England Section of the Weather Bureau. Three stations, Burlington, Enosburg Falls and Northfield are within 30milesofJericho.Fortheperiodfrom 18September to 31 December 1920 auroras were observed on four nights at these stations : 20 and 22 September and 6 and 23 October. The resulting total number of nights on which aurora was observed in 1920, 19, is consistent with an estimate of 18-20 derived from a comparison of the annual number of auroras with the number in the first 8 or 8.5 months of the year using the statistical averages of the Bentley data for the period 1883-1920. We do not believe, in any event, that the error for the entire period of Bentley’s observations can be more than one or two auroras. The dates on which auroras were observed by Bentley are given in Table 1. Bentley’s occasional comments on the brightness, direction and other aspects of the auroras have not been included in the table.
CHARACTER
OF THE AURORA DURING THE PERIOD 188>1931
The annual count of the number of nights on which auroras were observed for the period 1883-1931 is shown in Fig. 1. The general trend over this period is a decline in frequency of occurrence up to the period 1901-1914 and then a fairly rapid increase in occurrence from 1915 to 1931. During the years 1901, 1913 and 1914 no auroras at all were observed. Superimposed on this general trend are other variations which can be correlated with both magnetic activity and sunspot activity. The aurora1 data, in its general trend, closely tracks the magnetic activity, as represented by the Mayaud aa indices (Mayaud, 1973). For much of the entire period the two sets of data track in detail as well. This is perhaps not surprising since the aurora and magnetic activity reflect two different, but closely related, aspects of the solar wind at Earth. In fact, if an extended geographic area1 coverage for aurora1 observations is used the correlation aurora1 and magnetic activity can be remarkably high. For example, comparison of aurora1 data for all of Sweden with the aa indices for the
1133
Wilson Bentley’s auroral observations period from 1868-l 876 gives a correlation
coefficient of 0.97 (Silverman and Feynman, 1981). The correlation of the aurora1 data with sunspot activity is more complex than that with the magnetic activity. While the period around 1900 is a minimum for the sunspot activity over several cycles, there is not the general declining trend prior to 1900 and increasing trend after 1900 which is evident in the aurora1 and magnetic activity data. Furthermore, as noted earlier for Swedish data (Silverman, 1978) the aurora1 activity
typically
lags the sunspot activity. The aurora1 data also contains more fine structure in its variability than does the sunspot data. These differences between the data sets for sunspots and for aurora1 occurrence frequency indicate that both are symptomatic of a solar origin more fundamental than either. The annual variation of aurora1 frequency at Jericho is shown in Fig. 2. The spring and fall maxima are consistent with results generally obtained at other midlatitude stations. The origin of these maxima is not at
TABLE ~.PART 1: DATAFROMMANUSCRIPTIN
NOAA LIBRARY
1883
1885
1888
1892
1895
1897
1903
1907
1913
1917
1920
2.01 2.13 2.28 3.03 3.08 3.26 3.29 3.30 3.31 4.02 4.03 4.04 4.06 4.10 4.18 4.24 4.25 5.01 5.06 6.01 6.30 7.01 7.05 7.08 7.09 7.30 9.05 9.15 10.05 10.06 10.16 10.17 11.19 11.20
1.08 2.07 2.11 3.15 5.13 9.11 9.15 10.31 12.06 12.07
5.07 5.09 7.16
4.29 5.18 6.28 7.16 7.24 7.25 7.26 7.28 9.21 10.11 10.17
2.15 2.16 2.23 3.14 3.15 3.20 3.21 3.22 4.15 8.09 9.14 10.14 10.28 11.11 12.08
5.19 7.21 7.30 8.20 8.28 10.01 10.15 10.20 10.27
12.13
10.01 10.02
None
10.28 11.14
3.23 3.24 4.16 4.17 4.18 4.19 4.20 5.15 5.26 6.08 9.03 9.08
1884 2.23 3.01 3.21 3.27 4.14 4.24 4.26 5.12 9.13 11.09
1886 2.03 3.27 3.31 4.11 4.14 4.20 6.29 6.30 8.17 9.07 9.08 9.20 9.30 10.06 10.21 11.02 1887 2.12 2.13 2.14 3.20 3.23 4.14 4.22 5.12 9.25 11.09 1888 2.09 2.11 3.15 4.04
1889 4.07 9.22 10.16 11.26 1890 4.15 8.18 10.05 10.12 11.10 11.13 1891 2.14 3.30 3.31 4.09 9.02 9.03 9.06 9.28 1892 1.05 2.13 2.24 2.25 2.26 2.28 3.12 3.15 3.25 3.26 3.30 3.31 4.23 4.24 4.25 4.26 4.27
1893 1.06 1.21 2.04 2.05 2.15 2.16 4.11 5.07 6.29 9.08 10.02 10.05 11.01 11.03 12.30 1894 1.02 1.03 2.21 2.22 2.23 2.28 4.06 4.25 6.30 9.22 10.25 1895 1.01 1.19 2.14
1896 1.11 3.13 3.14 4.08 4.09 5.02 5.17 6.05 8.06 8.20 9.09 9.10 9.11 12.27 1897 1.02 1.03 2.02 2.25 3.03 3.04 3.07 3.28 3.29 3.30 4.01 4.19 4.20
1898 1.16 2.13 3.14 3.15 3.20 9.09 9.16 11.21 1899 2.11 2.14 4.10 5.02 5.03 5.15 6.01 6.29 8.29 1900 3.29 4.04 1901 None 1902 7.25 1903 10.13 11.13 11.20 11.21
1904 4.03 5.12 7.06 10.04 10.05 10.06 10.12 11.04 1905 1.04 1.05 3.02 3.06 3.08 3.14 3.31 4.01 8.01 8.02 8.03 9.26
1906 2.15 2.16 2.24 3.12 3.13 3.24 5.03 12.17 1907 2.07 2.08 2.21 2.23 3.09 3.10 3.11 4.13
1908 2.22 2.24 4.07 5.23 5.24 5.25 8.09 8.18 8.24 8.27 9.04 9.05 9.29 10.12 10.13 1909 4.24 4.26 10.08 11.10 11.30 12.01 1910 4.01 4.28 10.02 10.03 10.10 1911 3.01 3.24 3.25 4.01 5.14 10.10 1912 2.08 2.09
1914 None 191.5 3.05 3.08 3.09 3.17 3.19 4.07 4.16 5.11 6.16 6.17 9.28 9.29 10.15 1916 3.24 3.26 3.29 3.31 9.30 10.01 10.10 11.20 12.29 12.30 1917 1.02 1.11 1.16 2.17 3.19 3.24 4.17 5.15 7.21 8.22 9.04 9.05 9.21
1918 1.08 1.29 1.30 2.11 3.07 3.08 3.15 4.04 4.05 4.06 10.03 10.08 11.10 11.11 12.01 1919 1.04 1.05 1.31 2.02 2.19 2.20 2.27 3.20 3.24 3.25 4.21 4.22 6.28 9.17 10.01 10.29 11.16 12.18 12.19 12.20 1920 2.16 2.25 3.22
1134
TABLE 1. PART 2. DATA FROM METEOROLOGICALNOTEBOOKS 1921
1922
4.10 4.11 5.10 5.11 5.13 5.14 6.08 6.09 8.02 9.01 9.03 9.28 9.30 10.07 10.08 10.11 10.27 10.28 11.23 12.22 12.28
5.22 5.26 7.16 7.25 8.11 8.15 8.20 8.26 9.18 10.20
1922 1.23 1.24 1.25 1.30 1.31 2.14 2.17 2.28 3.01 3.18 3.24 4.16 4.22 4.23 4.24 4.25
1923 2.07 3.14 3.24 4.13 5.18 6.12 6.15 7.06 9.09 9.30 10.15 10.16 10.17 10.30 10.31 11.02 12.09
1924 5.22 8.17 9.04 9.07 9.22 10.21 10.22
-
1924
1926
1927
1929
1930
10.23
4.14 4.15 5.04 5.05 5.15 6.08 9.08 9.09 9.10 9.14 10.14 10.15 10.27 11.01 12.06
10.2Y 11.29
7.14 7.15 7.30 7.31 8.01 10.28 11.02 12.04 12.22
10.14 10.18 11.14 11.23
1925 4.15 4.18 4.20 5.18 5.27 6.13 6.16 6.17 6.23 7.14 7.16 8.22 9.14 9.19 9.23 10.12 10.13 11.08 11.09 11.10
1927 1.07 2.28 3.01 3.02 3.09 3.26 3.27 4.24 5.02 5.03 5.07 5.23 7.01 8.02 8.20 9.04 9.06 9.08 9.25 10.23 10.25
1926 1.13 1.14 2.11 2.13 2.24 3.05 3.09 3.13 3.17 3.18 4.05 4.06 4.12
-
28
-
20
-
I2
:
-4 -0
1928 1.27 2.12 2.13 3.22 3.28 4.07 5.07 7.07 7.08 7.12 8.03 8.11 8.12 8.26 9.07 9.08 9.09 9.22 9.24 10.08 10.14 10.23 11.03 11.11 11.12 11.13 11.14 1929 1.08 1.10 3.07 3.10 3.15 6.30
1930 1.04 1.29 2.16 2.22 2.26 2.27 3.13 3.14 3.25 4.02 4.14 4.15 4.16 4.20 5.09 5.11 5.31 6.01 6.27 7.16 7.24 8.05 8.12 8.21 8.22 9.18 9.23
1931 1.08 1.09 2.24 2.25 3.20 3.2 1 5.06 7.12 8.09 8.10 9.15 9.30 10.02 10.03 10.04 10.12 11.08 11.09 11.26
present established with certainty. The most likely factors influencing aurora1 frequency, by analogy with magnetic activity, are the solar wind velocity and the direction and intensity of the interplanetary magnetic field. These, in turn, may be derivative of the heliographic latitudinal dependency of solar wind velocity and a possibly tilted solar magnetic dipole. The combination of these various factors, and others yet to be determined, can produce varying preferences for the spring or fall in aurora1 activity for different solar cycles (see, for example, the discussions by Russell, 1975; Hakamada and Akasofu, 1981; Prabhakaran Nayar and Revathy, 1982). The effect can be seen in the Bentley data for the portions of the solar cycles at the beginning and end of the data set (see Fig. 2).
FIG.~.THEBENTLEYAURORALDATACOMPAREDWITHSUNSPOT N~~~BERANDWITHMAGNETICACTIVI~(~~ INDICES).
Wilson Bentley’s amoral
1135
observations
on the basic characteristics of the data set in order to indicate the utility of the data in studies of the variability of solar-terrestrial parameters.
Acknowledyement-A portion of the research reported in this paper was supported by the National Science Foundation through grant ATM-81 1753.
REFERENCES
FIG. 2. THEANNUALVAR~ATIONOFAURORALOCCURRENCEFOR THE ENTIRE PERIOD OF OBSERVATION (TOP PANEL) AND FOR PORTlONSOFTHESUNSPOTCYCLE,MEASUREDFROMMINIMA,AT THE BEGINNING AND END OF THE OBSERVATIONAL PERIOD (LOWER PANELS).
CONCLUSION
The primary purpose of this paper is to present an important set of amoral observations. The data set is particularly useful because it covers an extended period of time, 49 years, at a single location, by a single observer. The data set may thus be expected to have a high degree of self-consistency. Furthermore, the geomagnetic position of Jericho, Vermont is such as to provide a reasonable frequency of aurora1 occurrence, sufficient for statistical studies. We have briefly touched
Bentley, W. A. and Humphreys, W. J. (1931) Snow Crystals. McGraw-Hill, New York (reprinted by Dover, New York, 1962). Blanchard, D. C. (1970) Wilson Bentley, the snowflake man. Weatherwise 23,260. Feynman, J. (1983) Solar cycle and long term changes in the solar wind. Rev. Geophys. Space Phys. (in press). Feynman, J. and Silverman, S. M. (1980) Amoral changes during the eighteenth and nineteenth centuries and their implications for the solar wind and the long-term variation of sunspot activity. J. geophys. Res. 85, 2991. Hakamada, K. and Akasofu, S-I. (1981) A cause ofsolar wind speed variations observed at 1 A.U. J. geophys. Rex 86,129O. Mayaud, P. N. (1973) A hundred year series of geomagnetic data, 18681967. Bull. 33, Int. Ass. Geomagn. Aeron., Zurich. Prabhakaran Nayar, S. R. and Revathy, P. (1982) On the cause of geomagnetic activity at Hale sector boundary. Planet. Space Sci. 30, 519. Russell, C. (1975) On the possibility ofdeducing interplanetary and solar parameters from geomagnetic records. Solar Phys. 42,259. Silverman, S. M. (1978) Amoral frequency and solar activity. Presented at Yosemite Solar Terrestrial Coupling Conference, 1978. Silverman, S. M. and Feynman, J. (1981) The changing aurora of the past three centuries, in Exploration ofthe Polar Upper Atmosphere (Edited by Deehr, C. S. and Holtet, J. A.). D. Reidel, Dordrecht. Siscoe, G. L. (1980) Evidence in the amoral record for secular solar variability. Rev. Geophys. Space Phys. 18, 547.
WILSON
1983
0
BENTLEY’S
AURORAL S.M.
Physics
Department,
00324633/83%3.00+0.00 1983 Per~amon Press Ltd.
OBSERVATIONS
SILVERMAN
Boston College, Chestnut
Hill, MA 02167, U.S.A.
and D. C. BLANCHARD
Atmospheric
Sciences Research Center, State University Albany, NY 12222, U.S.A. (Received
22 February
of New York at Albany,
1983)
Abstract-
Wilson Bentley’s aurora1 observations in northern Vermont were made over the period from 1883 to 1931, thus encompassing the solar activity minimum at the beginning of the twentieth century. Long term series of aurora1 observations by a single observer in a region of reasonable frequency of occurrence are rare, and are important for the study of solar-terrestrial relationships. Bentley’s data have been abstracted from manuscript materials. The annual variation and the secular variation over the entire period are presented. The results are compared with the variations of sunspot and magnetic activity during this period. The aurora1 activity minima, comparable to the similar sunspot minima, in the periods around 1902 and 1912 are clearly evident. Changes in the spring/fall ratio of aurora1 occurrence frequency over the entire period under study are also evident.
In recent years there has been increasing interest in the use of proxy data for studies of solar variability, the solar wind and other parameters defining the properties of the interplanetary medium. Proxy data is essential for such studies since direct observations from space are only a few decades old and direct, systematic observations of the Sun exist for only the past two or three centuries. Hence studies of solar variability or of the solar wind at the Earth, for example, must rely on aurora1 or magnetic data to provide information for extended periods (see, for example, Silverman and Feynman, 1981; Feynman and Silverman, 1980; Siscoe, 1980; Feynman, 1983). Aurora1 data are especially useful since observations have been reported over the past two millennia at least. Studies of the variability of solar-terrestrial parameters over extended periods of time generally use one of two types of data sets. In the first type as many data points as possible are collected from a diverse array of sources. If the data set is sufficiently dense then errors due to spotty geographic or temporal coverage and inconsistencies in methods ofdata collection can be minimized. Alternatively, a single, self-consistent source may be used. A data set derived in this way will usually be limited in the range of time over which the data is taken, but will be superior to a more randomly selected data base for the period for which the data is available. In this paper we present a data set of aurora1 observations taken by a single observer, Wilson
Bentley, over a period of 49 years in a location where aurora1 occurrence is of reasonable frequency. Comparison of the resulting data set with other geophysical parameters illustrates the usefulness and importance of self-consistent observations of individual, careful observers over an extended period of time. WILSON BENTLEY
WilsonBentley(1865-1931)wasafarmerinnorthern Vermont. His ancestry included a Revolutionary War soldier and a great-aunt had been married to Martin Chittenden, Governor of Vermont from 1813-1815. His grandfather was one of the first settlers in Jericho, Vermont, where Bentley lived and worked for his entire life. His education until the age of 14 came from his mother, a former school teacher. His later education came primarily from his own reading. In his teens Bentley became fascinated by the variety of forms of ice crystals, the building blocks of snowflakes. This led to the major work of his lifetime, the photographing of several thousand ice crystals using techniques he had himself devised. By the 1920’s his superb ice crystal photomicrographs were widely discussed in the popular press and copies of his lantern slides were acquired by almost every major university and college in the United States. A collection of nearly 2500 of his photomicrographs, almost all of them of ice crystals,
1131
S. M. SILVERMAN and D. C. BLANCHARD
1132
was published in 1931, shortly before his death, with an introductory text writtenlargely by W. J. Humphreys of the Weather Bureau (Bentley and Humphreys, 1931; repr. 1962). The book’s enduring value is evidenced by its remaining in print today, more than 50 years after its first publication. Though known today for his ice crystal photomicrographs, Bentley published about ten technical papers, mostly in the Monthly Weather Review, on the generation and modification of ice crystals in the atmosphere and on their role in the formation of snow and rain (Blanchard, 1970). In 1904 he published a long paper. on his measurements of raindrop-size distributions, the first such made in the United States. Bentley was a meticulous and accurate observer. Each day he recorded the temperature, wind, cloud type, precipitation and any interesting or unusual atmospheric phenomena, including auroras. Our interest is in these latter observations.
THE OBSERVATIONS
At an early period in his life Bentley began his series of meteorological observations at his home in Jericho, Vermont (geographical coordinates : latitude 44” 27’ N, longitude 72” 56’ W; corrected geomagnetic coordiN, longitude 359.9”). nates : latitude 57.5” Unfortunately the notebooks in which he kept his observations for the period from 1883 through 1920 appear to have been lost. His aurora1 observations for this period, however, have been preserved. On 19 September 1920 Bentley sent a tabulation of all the auroras he had observed during the period from 1883 to Dr C. Brooks ofthe Weather Bureau. In his cover letter Bentley said : “I would be grateful if you will preserve this record, so that it will be easily accessible to investigators of aurora1 phenomena.. . Sharing this record may be of some real value to students of aurora1 phenomena.“The tabulation lists a total of 398 auroras, the earliest on 1 February 1883 and the latest on 8 September 1920, with theobservations continuing to 17 September. Bentley’s manuscript aurora1 listing was bound and placed in the Weather Bureau library and is currently in the library of the successor to the Bureau, the National Oceanic and Atmospheric Administration. Bentley’s meteorological notebooks for the period from 1921 to 7 December 1931, a little over 2 weeks before his death on 23 December, have survived. These were acquired about 1947, together with a collection of nearly 12 000 photographicnegatives and plates, by the Buffalo Museum of Science, their current home. Through the courtesy of the Buffalo Museum we obtained copies of the notebooks and have abstracted
the aurora1 observations from them. This provided an additional 236 auroras, giving a total number observed by Bentley of 634 auroras over a period of 49 years. The two manuscript sources taken together provide an aurora1 record for Jericho, Vermont from 1 February 1883 to 7 December 1931, with the exception of a short period from 18 September through 31 December 1920. In order to provide a record for the entire period from 1883 through 1931 we have used published aurora1 observations from the New England Section of the Weather Bureau. Three stations, Burlington, Enosburg Falls and Northfield are within 30milesofJericho.Fortheperiodfrom 18September to 31 December 1920 auroras were observed on four nights at these stations : 20 and 22 September and 6 and 23 October. The resulting total number of nights on which aurora was observed in 1920, 19, is consistent with an estimate of 18-20 derived from a comparison of the annual number of auroras with the number in the first 8 or 8.5 months of the year using the statistical averages of the Bentley data for the period 1883-1920. We do not believe, in any event, that the error for the entire period of Bentley’s observations can be more than one or two auroras. The dates on which auroras were observed by Bentley are given in Table 1. Bentley’s occasional comments on the brightness, direction and other aspects of the auroras have not been included in the table.
CHARACTER
OF THE AURORA DURING THE PERIOD 188>1931
The annual count of the number of nights on which auroras were observed for the period 1883-1931 is shown in Fig. 1. The general trend over this period is a decline in frequency of occurrence up to the period 1901-1914 and then a fairly rapid increase in occurrence from 1915 to 1931. During the years 1901, 1913 and 1914 no auroras at all were observed. Superimposed on this general trend are other variations which can be correlated with both magnetic activity and sunspot activity. The aurora1 data, in its general trend, closely tracks the magnetic activity, as represented by the Mayaud aa indices (Mayaud, 1973). For much of the entire period the two sets of data track in detail as well. This is perhaps not surprising since the aurora and magnetic activity reflect two different, but closely related, aspects of the solar wind at Earth. In fact, if an extended geographic area1 coverage for aurora1 observations is used the correlation aurora1 and magnetic activity can be remarkably high. For example, comparison of aurora1 data for all of Sweden with the aa indices for the
1133
Wilson Bentley’s auroral observations period from 1868-l 876 gives a correlation
coefficient of 0.97 (Silverman and Feynman, 1981). The correlation of the aurora1 data with sunspot activity is more complex than that with the magnetic activity. While the period around 1900 is a minimum for the sunspot activity over several cycles, there is not the general declining trend prior to 1900 and increasing trend after 1900 which is evident in the aurora1 and magnetic activity data. Furthermore, as noted earlier for Swedish data (Silverman, 1978) the aurora1 activity
typically
lags the sunspot activity. The aurora1 data also contains more fine structure in its variability than does the sunspot data. These differences between the data sets for sunspots and for aurora1 occurrence frequency indicate that both are symptomatic of a solar origin more fundamental than either. The annual variation of aurora1 frequency at Jericho is shown in Fig. 2. The spring and fall maxima are consistent with results generally obtained at other midlatitude stations. The origin of these maxima is not at
TABLE ~.PART 1: DATAFROMMANUSCRIPTIN
NOAA LIBRARY
1883
1885
1888
1892
1895
1897
1903
1907
1913
1917
1920
2.01 2.13 2.28 3.03 3.08 3.26 3.29 3.30 3.31 4.02 4.03 4.04 4.06 4.10 4.18 4.24 4.25 5.01 5.06 6.01 6.30 7.01 7.05 7.08 7.09 7.30 9.05 9.15 10.05 10.06 10.16 10.17 11.19 11.20
1.08 2.07 2.11 3.15 5.13 9.11 9.15 10.31 12.06 12.07
5.07 5.09 7.16
4.29 5.18 6.28 7.16 7.24 7.25 7.26 7.28 9.21 10.11 10.17
2.15 2.16 2.23 3.14 3.15 3.20 3.21 3.22 4.15 8.09 9.14 10.14 10.28 11.11 12.08
5.19 7.21 7.30 8.20 8.28 10.01 10.15 10.20 10.27
12.13
10.01 10.02
None
10.28 11.14
3.23 3.24 4.16 4.17 4.18 4.19 4.20 5.15 5.26 6.08 9.03 9.08
1884 2.23 3.01 3.21 3.27 4.14 4.24 4.26 5.12 9.13 11.09
1886 2.03 3.27 3.31 4.11 4.14 4.20 6.29 6.30 8.17 9.07 9.08 9.20 9.30 10.06 10.21 11.02 1887 2.12 2.13 2.14 3.20 3.23 4.14 4.22 5.12 9.25 11.09 1888 2.09 2.11 3.15 4.04
1889 4.07 9.22 10.16 11.26 1890 4.15 8.18 10.05 10.12 11.10 11.13 1891 2.14 3.30 3.31 4.09 9.02 9.03 9.06 9.28 1892 1.05 2.13 2.24 2.25 2.26 2.28 3.12 3.15 3.25 3.26 3.30 3.31 4.23 4.24 4.25 4.26 4.27
1893 1.06 1.21 2.04 2.05 2.15 2.16 4.11 5.07 6.29 9.08 10.02 10.05 11.01 11.03 12.30 1894 1.02 1.03 2.21 2.22 2.23 2.28 4.06 4.25 6.30 9.22 10.25 1895 1.01 1.19 2.14
1896 1.11 3.13 3.14 4.08 4.09 5.02 5.17 6.05 8.06 8.20 9.09 9.10 9.11 12.27 1897 1.02 1.03 2.02 2.25 3.03 3.04 3.07 3.28 3.29 3.30 4.01 4.19 4.20
1898 1.16 2.13 3.14 3.15 3.20 9.09 9.16 11.21 1899 2.11 2.14 4.10 5.02 5.03 5.15 6.01 6.29 8.29 1900 3.29 4.04 1901 None 1902 7.25 1903 10.13 11.13 11.20 11.21
1904 4.03 5.12 7.06 10.04 10.05 10.06 10.12 11.04 1905 1.04 1.05 3.02 3.06 3.08 3.14 3.31 4.01 8.01 8.02 8.03 9.26
1906 2.15 2.16 2.24 3.12 3.13 3.24 5.03 12.17 1907 2.07 2.08 2.21 2.23 3.09 3.10 3.11 4.13
1908 2.22 2.24 4.07 5.23 5.24 5.25 8.09 8.18 8.24 8.27 9.04 9.05 9.29 10.12 10.13 1909 4.24 4.26 10.08 11.10 11.30 12.01 1910 4.01 4.28 10.02 10.03 10.10 1911 3.01 3.24 3.25 4.01 5.14 10.10 1912 2.08 2.09
1914 None 191.5 3.05 3.08 3.09 3.17 3.19 4.07 4.16 5.11 6.16 6.17 9.28 9.29 10.15 1916 3.24 3.26 3.29 3.31 9.30 10.01 10.10 11.20 12.29 12.30 1917 1.02 1.11 1.16 2.17 3.19 3.24 4.17 5.15 7.21 8.22 9.04 9.05 9.21
1918 1.08 1.29 1.30 2.11 3.07 3.08 3.15 4.04 4.05 4.06 10.03 10.08 11.10 11.11 12.01 1919 1.04 1.05 1.31 2.02 2.19 2.20 2.27 3.20 3.24 3.25 4.21 4.22 6.28 9.17 10.01 10.29 11.16 12.18 12.19 12.20 1920 2.16 2.25 3.22
1134
TABLE 1. PART 2. DATA FROM METEOROLOGICALNOTEBOOKS 1921
1922
4.10 4.11 5.10 5.11 5.13 5.14 6.08 6.09 8.02 9.01 9.03 9.28 9.30 10.07 10.08 10.11 10.27 10.28 11.23 12.22 12.28
5.22 5.26 7.16 7.25 8.11 8.15 8.20 8.26 9.18 10.20
1922 1.23 1.24 1.25 1.30 1.31 2.14 2.17 2.28 3.01 3.18 3.24 4.16 4.22 4.23 4.24 4.25
1923 2.07 3.14 3.24 4.13 5.18 6.12 6.15 7.06 9.09 9.30 10.15 10.16 10.17 10.30 10.31 11.02 12.09
1924 5.22 8.17 9.04 9.07 9.22 10.21 10.22
-
1924
1926
1927
1929
1930
10.23
4.14 4.15 5.04 5.05 5.15 6.08 9.08 9.09 9.10 9.14 10.14 10.15 10.27 11.01 12.06
10.2Y 11.29
7.14 7.15 7.30 7.31 8.01 10.28 11.02 12.04 12.22
10.14 10.18 11.14 11.23
1925 4.15 4.18 4.20 5.18 5.27 6.13 6.16 6.17 6.23 7.14 7.16 8.22 9.14 9.19 9.23 10.12 10.13 11.08 11.09 11.10
1927 1.07 2.28 3.01 3.02 3.09 3.26 3.27 4.24 5.02 5.03 5.07 5.23 7.01 8.02 8.20 9.04 9.06 9.08 9.25 10.23 10.25
1926 1.13 1.14 2.11 2.13 2.24 3.05 3.09 3.13 3.17 3.18 4.05 4.06 4.12
-
28
-
20
-
I2
:
-4 -0
1928 1.27 2.12 2.13 3.22 3.28 4.07 5.07 7.07 7.08 7.12 8.03 8.11 8.12 8.26 9.07 9.08 9.09 9.22 9.24 10.08 10.14 10.23 11.03 11.11 11.12 11.13 11.14 1929 1.08 1.10 3.07 3.10 3.15 6.30
1930 1.04 1.29 2.16 2.22 2.26 2.27 3.13 3.14 3.25 4.02 4.14 4.15 4.16 4.20 5.09 5.11 5.31 6.01 6.27 7.16 7.24 8.05 8.12 8.21 8.22 9.18 9.23
1931 1.08 1.09 2.24 2.25 3.20 3.2 1 5.06 7.12 8.09 8.10 9.15 9.30 10.02 10.03 10.04 10.12 11.08 11.09 11.26
present established with certainty. The most likely factors influencing aurora1 frequency, by analogy with magnetic activity, are the solar wind velocity and the direction and intensity of the interplanetary magnetic field. These, in turn, may be derivative of the heliographic latitudinal dependency of solar wind velocity and a possibly tilted solar magnetic dipole. The combination of these various factors, and others yet to be determined, can produce varying preferences for the spring or fall in aurora1 activity for different solar cycles (see, for example, the discussions by Russell, 1975; Hakamada and Akasofu, 1981; Prabhakaran Nayar and Revathy, 1982). The effect can be seen in the Bentley data for the portions of the solar cycles at the beginning and end of the data set (see Fig. 2).
FIG.~.THEBENTLEYAURORALDATACOMPAREDWITHSUNSPOT N~~~BERANDWITHMAGNETICACTIVI~(~~ INDICES).
Wilson Bentley’s amoral
1135
observations
on the basic characteristics of the data set in order to indicate the utility of the data in studies of the variability of solar-terrestrial parameters.
Acknowledyement-A portion of the research reported in this paper was supported by the National Science Foundation through grant ATM-81 1753.
REFERENCES
FIG. 2. THEANNUALVAR~ATIONOFAURORALOCCURRENCEFOR THE ENTIRE PERIOD OF OBSERVATION (TOP PANEL) AND FOR PORTlONSOFTHESUNSPOTCYCLE,MEASUREDFROMMINIMA,AT THE BEGINNING AND END OF THE OBSERVATIONAL PERIOD (LOWER PANELS).
CONCLUSION
The primary purpose of this paper is to present an important set of amoral observations. The data set is particularly useful because it covers an extended period of time, 49 years, at a single location, by a single observer. The data set may thus be expected to have a high degree of self-consistency. Furthermore, the geomagnetic position of Jericho, Vermont is such as to provide a reasonable frequency of aurora1 occurrence, sufficient for statistical studies. We have briefly touched
Bentley, W. A. and Humphreys, W. J. (1931) Snow Crystals. McGraw-Hill, New York (reprinted by Dover, New York, 1962). Blanchard, D. C. (1970) Wilson Bentley, the snowflake man. Weatherwise 23,260. Feynman, J. (1983) Solar cycle and long term changes in the solar wind. Rev. Geophys. Space Phys. (in press). Feynman, J. and Silverman, S. M. (1980) Amoral changes during the eighteenth and nineteenth centuries and their implications for the solar wind and the long-term variation of sunspot activity. J. geophys. Res. 85, 2991. Hakamada, K. and Akasofu, S-I. (1981) A cause ofsolar wind speed variations observed at 1 A.U. J. geophys. Rex 86,129O. Mayaud, P. N. (1973) A hundred year series of geomagnetic data, 18681967. Bull. 33, Int. Ass. Geomagn. Aeron., Zurich. Prabhakaran Nayar, S. R. and Revathy, P. (1982) On the cause of geomagnetic activity at Hale sector boundary. Planet. Space Sci. 30, 519. Russell, C. (1975) On the possibility ofdeducing interplanetary and solar parameters from geomagnetic records. Solar Phys. 42,259. Silverman, S. M. (1978) Amoral frequency and solar activity. Presented at Yosemite Solar Terrestrial Coupling Conference, 1978. Silverman, S. M. and Feynman, J. (1981) The changing aurora of the past three centuries, in Exploration ofthe Polar Upper Atmosphere (Edited by Deehr, C. S. and Holtet, J. A.). D. Reidel, Dordrecht. Siscoe, G. L. (1980) Evidence in the amoral record for secular solar variability. Rev. Geophys. Space Phys. 18, 547.