G. D. Parker

Coronal rotation during solar cycle 20

Using K-coronameter observations made by the High Altitude Observatory at Haleakala and Mauna Loa, Hawaii during 1964–1976, we determine the apparent recurrence period of white-light solar coronal features as a function of latitude, height, and time. A technique based on maximum entropy spectral analysis is used to produce rotational period estimates from daily K-coronal brightness observations at 1.125RS and 1.5RS from disk center and at angular intervals of 5° around the Suns limb. Our analysis reaffirms the existence of differential rotation in the corona and describes both its average behavior and its large year-to-year variations. On the average, there is less differential rotation at the greater height. After 1966–1967 we observe a general increase in coronal rotation rate which may relate to similar behavior reported for the equatorial photospheric Doppler rate. However, the coronal rate increase is significantly greater than the photospheric. If K-coronal features reflect the rotation at depth in the Sun, the long-term rate increase and the variable differential rotation may be evidence for dynamically important exchanges of energy and momentum in the upper convection zone.

Radial variation of differential rotation in the solar electron corona

Long-lived brightness structures in the solar electron corona persist over many solar rotation periods and permit an observational determination of coronal magnetic tracer rotation as a function of latitude and height in the solar atmosphere. For observations over 1964–1976 spanning solar cycle 20, we compare the latitude dependence of rotation at two heights in the corona. Comparison of rotation rates from East and West limbs and from independent computational procedures is used to estimate uncertainty. Time-averaged rotation rates based on three methods of analysis demonstrate that, on average, coronal differential rotation decreases with height from 1.125 to 1.5 RS. The observed radial variation of differential rotation implies a scale height of approximately 0.7 RS for coronal differential rotation.Model calculations for a simple MHD loop show that magnetic connections between high and low latitudes may produce the observed radial variations of magnetic tracer rotation. If the observed tracer rotation represents the rotation of open magnetic field lines as well as that of closed loops, the small scale height for differential rotation suggests that the rotation of solar magnetic fields at the base of the solar wind may be only weakly latitude dependent. If, instead, closed loops account completely for the radial gradients of rotation, outward extrapolation of electron coronal rotation may not describe magnetic field rotation at the solar wind source. Inward extrapolations of observed rotation rates suggest that magnetic field and plasma are coupled a few hundredths of a solar radius beneath the photosphere.

Lifetime, age, and rotation of coronal magnetic fields

The rotation of the solar electron corona is determined for intervals when nearly periodic variations dominated the polarization brightness record during 1964–1976. Coronal rotation rates derived for 765 intervals vary with height, latitude, and interval length. These rotation rates show a decrease of differential rotation with height and support earlier rotation studies which included much less stationary data. Analyses of the selected intervals and autocorrelation of the complete K-coronameter data set give quantitative estimates of the rotational effects of magnetic tracer age and lifetime. The principal effects detected are a relatively fast rotation of very long-lived tracers at high latitude and a relatively fast rotation of very short-lived tracers at low latitudes. The observations indicate that high-to-low latitude magnetic connections extending through the corona speed up rotation at high latitudes and retard it at low latitudes.

Galileo, Planetary Atmospheres, and Prograde Revolution

Early in March 1610 Galileo was preoccupied with curious brightness variations of the newly discovered satellites of Jupiter. In formulating an incorrect explanation he advanced important generalizations about the existence of planetary atmospheres and counterclockwise circulation within the solar system.

Short-term variations of the galactic cosmic ray intensity: 1964–1967

Abstract Variations of the ground-level nucleonic intensity and of indices of solar photospheric, chromospheric, and coronal activity at time scales of less than forty days show strong 27-day recurrent structure. Correlation functions for these time series suggest an origin for short-term modulation in impulsive solar activity. Flare-associated shocks originating in long-lived active zones fixed in solar longitude produce recurrent cosmic ray variations during 1964–1967, and corotating solar wind disturbances are of secondary importance to short-term modulation.

Solar-cycle dependence of galactic cosmic ray flux. II - The correlation between the nucleonic flux and solar indices

To investigate the relationship between solar activity and cosmic ray modulation, time series of the nucleonic flux and of solar plages, sunspots, centimeter radio noise, and the brightness of the white light corona at 1.1 and 1.5 solar radii from the center of Sun are cross-correlated. Data pertain to the years 1964–1967 during the ascending phase of the current solar cycle. The amplitudes and phases of correlation functions for filtered and unfiltered indices are discussed. The existence of a superior solar index for relating solar activity to long-term modulation is not yet demonstrated conclusively, and the time lag of modulation is too poorly determined to permit its use in estimating the radius of the modulation region.

RECURRENCE: IMPLICATIONS FOR HELIOSPHERIC COSMIC RAY TRANSPORT

ABSTRACT An analysis procedure is developed to quantify recurrence tendencies in lists of events. I apply this technique to 27-day recurrent variations in the intensity of the galactic cosmic radiation. Changes in the ground-level nucleonic intensity are significantly more recurrent than are intensity extrema. The recurrence periods of cosmic ray decreases do not respond to changes in the polarity of the heliospheric magnetic field. Corotation of quasi-static structures is a less important cause of recurrence than are the interplanetary disturbances responsible for abrupt cosmic ray changes.

Spectral properties of hydromagnetic fluctuations near 4 and 5 a.u.

Abstract Power spectra of vector components of interplanetary magnetic field fluctuations near 4–5 a.u. during quiet intervals show a frequency dependence very close to fs over the frequency range 4 × 10−5 to 9 × 10−3 Hz (corresponding to periods of 7 h-2 min). While the spectra are generally very close to power law in frequency, variations in slope among the spectra exceed those expected from random errors and may represent true temporal variations. Mean slopes corrected for systematic error are s = − 1.50±0.02 (Pioneer 10, mean heliocentric distance 5.3 a.u.) and s = − 1.52±0.02 (Pioneer 11, mean heliocentric distance 3.9 a.u.) and are consistent with several determinations of spectral slope for magnetic fluctuations near 1 a.u. Radial evolution of the perturbations is investigated by choosing data samples in which Pioneer 10 and 11 and the sun are nearly colinear. The dependence on heliocentric distance of σc2, the composite vector variance, and of σc/Bmag, where Bmag is the mean magnitude of the magnetic field, show that the radial variation of fluctuation amplitude is highly variable in time with a dependence on heliocentric distance typically in the range R−1 to R−1.5. These observations are compared with theoretical models of outward propagating Alfven waves of solar origin and of MHD turbulence. The mean slopes agree well with that expected for turbulence. The significant variability observed in spectral slopes and in the radial dependence of fluctuation amplitude in data selected specifically for conditions of relative magnetic quiet is noteworthy and urges caution in modeling heliospheric magnetic microstructure in studies of galactic cosmic ray modulation.

Solar-cycle dependence of galactic cosmic ray flux. I. Coronal indices

The coronal green line intensity is inappropriate for correlation studies of galactic cosmic ray variations. Being a non-monotonic function of coronal temperature, the green line intensity is a good index of neither coronal temperature nor solar wind speed. A more appropriate measure of coronal activity is the intensity of the electron corona. Two-dimensional observations of the K-corona trace changes in coronal morphology during the solar cycle. An index based on four years of K-coronal measurements made in Hawaii shows that activity in the lower corona is not better correlated than sunspot number with long-term modulation. Correlation analysis defines the time lag of modulation much too poorly to permit its use in estimating the size of the heliosphere.

Galileo and optical illusion

Galileo’s earliest telescopic measurements are of sufficient quality that their detailed analysis yields scientifically interesting and pedagogically useful results. An optical illusion strongly influences Galileo’s observations of Jupiter’s moons, as published in the Starry Messenger. A simple procedure identifies individual satellites with sufficient reliability to demonstrate that Galileo regularly underestimated satellite brightness and overestimated elongation when a satellite was very close to Jupiter. The probability of underestimation is a monotonically decreasing function of separation angle, both for Galileo and for viewers of a laboratory simulation of the Jupiter ‘‘starfield’’ viewed by Galileo. Analysis of Galileo’s records and a simple simulation experiment appropriate to undergraduate courses clarify the scientific problems facing Galileo in interpreting his observations.