Knowledge of solar variability and its effects on the Earth is essential since the Sun affects almost every aspect of our lives. Direct observations of the Sun, usually of sunspots, with some continuity, exist only since about 1700. Understanding of long‐term solar variability must then depend on proxy data, such as visual auroral observations, measurements of magnetic activity, and the radiocarbon record. These also give us information on the interaction between the solar wind, the interplanetary field, and the terrestrial magnetosphere, as well as, for the radiocarbon record, heliospheric conditions. This paper uses a data base of visual auroral observations for a period of about 500 years, from 1450 to 1948, comprising about 45,000 observations, in addition to the well‐known sunspot series and the magnetic activity index aa, from 1868 to 1990. The secular variation of the aurora is examined and compared, where possible, to sunspot data and magnetic activity data. Blackman‐Tukey power spectra are used to determine periodicities. The study confirms the variability of the periodicities in both frequency and amplitude. In particular, the well‐known 11.1‐year cycle disappears during the Maunder minimum and at the end of the eighteenth and beginning of the nineteenth century. While the 11.1‐year period is normally strongly dominant for sunspots, other shorter periods become important, and even dominant, for auroras and magnetic activity. Consequently, the temporal behavior of these three variables differs. Prolonged solar activity minima are clearly evident. In addition to the known Spörer, Maunder, Dalton, and 1901–1913 minima, a previously unrecognized minimum about 1765 is clearly evident in the data. Comparison of the depth of these minima shows that the Dalton minimum may be the deepest, or at least rivals the Maunder minimum in importance. This minimum clearly deserves further study. Combining the polar data base with that of mid‐latitudes provides for the first time a globally comprehensive historical record of auroral occurrence. The data provide confirmation of the anticorrelation of auroral occurrence in the polar regions with sunspot activity, as a result of displacement of the auroral oval with changes in solar and magnetic activity. Assuming the validity of some current models of the solar origin of geomagnetic activity, the data provide a basis for understanding the variation over time of the general magnetic field of the Sun, in particular the polar field.
A new predictive engineering model for the interplanetary fluence of protons with energies > 10 MeV and > 30 MeV is described. The data set used is a combination of observations made from the Earth's surface and from above the atmosphere between 1956 and 1963 and observations made from spacecraft in the vicinity of Earth between 1963 and 1985. The data cover a time period three times as long as the period used in earlier models. With the use of this data set the distinction between "ordinary proton events" and "anomalously large events" made in earlier work disappears. This permitted the use of statistical analysis methods developed for "ordinary events" on the entire data set. The > 10 MeV fluences at 1AU (astronomical unit) calculated with the new model are about twice those expected on the basis of models now in use. At energies > 30 MeV, the old and new models agree. In contrast to earlier models, the results do not depend critically on the fluence from any one event and are independent of sunspot number. Mission probability curves derived from the fluence distribution are presented.Nomenclature E = energy per nucleon, MeV F -log of integral proton fluence f p = integral proton fluence, particle /cm" 2 n = number of events; > 10 7 particle cm~2 for > 10 MeV, > 10 6 particle cm~2 for > 30 MeV r = heliocentric distance, AU (astronomical unit) w = average number of events per year I JL -mean of the log fluence a -standard deviation, unitless T = mission length, yr
We review solar=geophysical data relating to the great magnetic storm of 14-15 May 1921, with emphasis on observations of the low-latitude visual aurora. From the reports we have gathered for this event, the lowest geomagnetic latitude of deÿnite overhead aurora (coronal form) was 40 • and the lowest geomagnetic latitude from which auroras were observed on the poleward horizon in the northern hemisphere was 30 •. For comparison, corresponding overhead=low-latitude values of 48 • =32 • and 41 • =20 • were reported for the great auroras on 28-29 August and 1-2 September 1859, respectively. However, for the 1921 event, there is a report of aurora from Apia, Samoa, in the southern hemisphere, within 13 • of the geomagnetic equator. This report by professional observers appears to be credible, based on the aurora description and timing, but is puzzling because of the discrepancy with the lowest latitude of observation in the northern hemisphere and the great implied auroral height (∼ 2000 km, assuming overhead aurora at Auckland, New Zealand). We discuss various possibilities that might account for this observation.
A neglected phenomenon, that of localized, sporadic auroral occurrence at relatively low latitudes during periods of quiet to moderate magnetic activity, is described and discussed. Some 54 cases of such occurrences are tabulated, all, with one exception, from the United States at times from 1880 to 1940.
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