John A. Eddy

The Maunder Minimum

zero. In contrast, in the years around a sunspot maximum there is seldom a day when a number of spots cannot be seen, and often hundreds are present. Past counts of sunspot number are readily available from the year 1700 (3), [m and workers in solar and terrestrial studies often use the record as though it were of of uniform quality. In fact, it is not. Thus it is advisable, from time to time, to

The Maunder Minimum: A reappraisal

A number of papers have questioned the reality of the Maunder Minimum of solar activity (A.D. 1645–1715). A recent study by Xu and Jiang (1982) of newly-discovered Chinese provincial records listing 21 naked-eye sunspot sightings in the 17th century is a case in point, as is Landsbergs (1980) report of 52 sunspots found in ‘unexploited’ German diary sources. We demonstrate that neither claim is cause to alter Eddys (1976) quantitative description of the Maunder Minimum. The new Chinese data add at most three possible sunspots to the 600–1000 that were previously known in the Maunder Minimum years; moreover, the number of 17th century naked-eye reports drops after 1640 in the same manner as do known telescopic reports. The historical sunspots found by Landsberg are almost entirely duplicates of spots included in prior analyses, based on earlier studies of the same well-known sources. The principal evidence for secular solar variability is the precision record of tree-ring radioacarbon compiled in 1980 by Stuiver and his colleagues that establishes an unequivocal drop in solar activity during the Maunder Minimum and on three prior occasions in the current millennium.

Solar rotation during the Maunder Minimum

We have measured solar surface rotation from sunspot drawings made in a.d. 1642–1644 and find probable differences from present-day rates. The 17th century sunspots rotated faster near the equator by 3 or 4%, and the differential rotation between 0 and ±20° latitude was enhanced by about a factor 3. These differences are consistent features in both spots and groups of spots and in both northern and southern hemispheres. We presume that this apparent change in surface rotation was related to the ensuing dearth of solar activity (the Maunder Minimum) which persisted until about 1715.

FAR-INFRARED MEASUREMENT OF THE SOLAR MINIMUM TEMPERATURE.

Radiometric measurement of the brightness temperature of the mean solar disk has been made in the wavelength range from 238 μ to 312 μ (42.1 cm-1 to 32.1 cm-1), using a Michelson interferometer of resolution 0.25 cm-1, carried on the NASA research aircraft at altitude 12.6 km. A mean temperature 4370 ± 260 K is obtained. The result is compared with other measurements of the solar minimum temperature and with the prediction of the BCA Model.

Anomalous solar rotation in the early 17th century.

The character of solar rotation has been examined for two periods in the early 17th century for which detailed sunspot drawings are available: A.D. 1625 through 1626 and 1642 through 1644. The first period occurred 20 years before the start of the Maunder sunspot minimum, 1645 through 1715; the second occurred just at its commencement. Solar rotation in the earlier period was much like that of today. In the later period, the equatorial velocity of the sun was faster by 3 to 5 percent and the differential rotation was enhanced by a factor of 3. The equatorial acceleration with declining solar activity is in the same sense as that found in recent Doppler data. It seems likely that the change in rotation of the solar surface between 1625 and 1645 was associated with the onset of the Maunder Minimum.

Planetary tides during the Maunder Sunspot Minimum

CONTROL of the 11-yr and longer cycles of solar activity tides raised on the Sun resulting from the gravitational pulls of the planets has often been suggested. Rudolf Wolf and R. C. Carrington were among the early astronomers who pointed out this possibility in the middle nineteenth century1. The close coincidence of the sidereal period of gravitationally important Jupiter (11.87 yr) with the mean period of the observed annual sunspot means (11.3 yr) raises the possibility of such a relationship; the range of possible configurations of the other planets allows a wide realm of other tidal periods and effects. In daily sunspot numbers, a small but consistent periodicity at the sidereal period of the planet Mercury has been found2, and a possible 178.7-yr period in sunspots (about twice the Gleissberg cycle) has been linked with multi-planet tidal influences3. Wood and Wood4 have applied a dynamical theory which includes all planets but Mars to reinforce their belief in a more than chance relationship. We reconstruct here Sun-centred planetary conjunctions and tidal potentials for the AD 1645–1715 period of sunspot absence (the Maunder Minimum). These are found to be effectively indistinguishable from patterns of conjunctions and power spectra of tidal potential in the modern era of a well-established 11-yr sunspot cycle. This places a new and difficult constraint on any tidal theory of sunspot formation.

Observation of a possible neutral sheet in the corona

A linear coronal ray, centered on a large helmet, is prominent in white-light photographs taken at the 1922 eclipse. It extends from near the limb to 4 R⊙, has minimum width 9″ arc, and persisted for at least 35 min. Examination reveals that the ray marks the cleavage between the domes of a twin-arch streamer which in turn is associated with two large, distinct chromospheric active regions. The ray is interpreted as the edge-on aspect of a coronal neutral sheet which separates areas of presumed opposite magnetic polarity in the two surface regions.

Aspects of Long-Term Variability in Sun and Stars

Evidence of long-term solar variability is reviewed, including historical data and the tree-ring record of radiocarbon. Epochs of suppressed activity like the Maunder Minimum are shown to be frequent occurences of the last several thousand years, but without no obvious period of recurrence. Weak evidence exists for the 11-year cycle as early as Medieval times, although with insufficient accuracy to establish long-term phase stability.

The λ 747 coronal line at the 1966 eclipse

Observations are presented of emission line resonance polarization in Fe xiii λ10747 at the total solar eclipse of 12 November 1966. Useful data, with angular resolution 15″, describe three quadrants of the corona from 1.08 R⊙ to a maximum of 1.6 R⊙. The direction of the electric vector of observed polarization is perpendicular to the solar limb, to the limits of accuracy of measurement, in at least 74% of all cases. Departures in the other points are consistent with the magnetic depolarization expected from the non-radial fields of streamers. Polarizations observed range from near zero at the limb to 80 % and higher at 1.6 R⊙. Averaged polarization is highest in non-streamer regions, where above 1.2 R⊙ it suggests pure radiative excitation of the λ10747 line. Below 1.2 R⊙, and in a dense streamer, the polarization is significantly depressed, indicating dominant collisional excitation of the line wherever the electron density exceeds 50 × 106 cm−3.

Far Infrared Airborne Spectroscopy

Equipment and techniques employed in an airborne far ir (80-400 micro) spectral measurement of the solar brightness temperature are described, with particular attention paid to the scanning Michelson interferometer and the radiometric calibration. The airborne performance of several telescopic guiding systems, a liquid helium cooled bolometer, and golay cells are discussed, summarizing experience gained on twenty-five jet aircraft flights.