Shirley F. Hansen

BRIGHTNESS VARIATIONS OF THE WHITE-LIGHT CORONA DURING THE YEARS 1964--67.

Observations of the white light corona were made on over 900 days during the years 1964–67 at heights between 1.125 and 2.0 R⊙ with the K-coronameter at Mount Haleakala and Mauna Loa, Hawaii. The brightness distribution of the minimum corona was elliptical with average equatorial intensities three times the polar. Coronal features of the new cycle at 1.125 R⊙ occurred predominantly in the sunspot zones at 25–30° latitude and in a high latitude zone which migrated toward the North pole before solar maximum. The brightness of the inner corona doubled over this period and a close association is found between the average corona and 10.7-cm solar radio flux. Electron densities in the equatorial regions were nearly twice those of Van de Hulsts model corona, in agreement with the results of recent eclipse observations.

Long-lived coronal structures and recurrent geomagnetic patterns in 1974

Abstract Daily measurements of the intensity distribution of the Suns white-light corona over the height range 1.1–2.7. R ⊚ show that the global structure became quite stable (constant over periods of several months) in late 1973 and throughout 1974, as flares, ascending prominences and other transient activity became less frequent with the decline of the solar activity cycle. A highly persistent pattern of geomagnetic activity prevailed for much of this time. Bright coronal structures in the ecliptic plane were associated with geomagnetically quiet conditions, and faint coronal regions (“holes”) with geomagnetic disturbance, after a delay of about three days. These results confirm the “cone-of-avoidance” model for M-regions and reinforce the postulate that high-speed streams in the solar wind originate from coronal holes. Identification of coronal holes from ground-based K-coronal observations corresponds well with those made from spacecraft EUV and X-ray experiments on OSO-7 and Skylab.

Differential rotation of the solar electron corona

Autocorrelation analyses of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii, during 1964–1967 have established average yearly rotation rates of coronal features as a function of latitude and height above the limb. At low latitudes the corona was found to rotate at the same rate as sunspots but at higher latitudes was consistently faster than the underlying photosphere. There were differences as large as 3–4% in the rate at specific latitudes from year to year and between the two hemispheres. In 1967 a nearly constant rotation was found for heights ranging from 1.125 to 2.0 R0. For 1966 there was a more complicated pattern of height dependence, with the rate generally decreasing with height at low latitudes and increasing at high latitudes.

Abrupt depletions of the inner corona

During the time period 1970-early 1973, 13 instances of major abrupt depletions of localized regions of the inner solar corona (1.1-2 R0) were detected at Mauna Loa, Hawaii. Because of their almost invariable association with the ascending Ha prominence material (most generally flare sprays and, in one instance the disparition brusque of a large filament near the solar limb) and close correlation in time and position with outward moving Type IV radio sources, we surmise that the coronal material was expelled from the sun. In several of these cases plasma clouds were tracked to 10 R0 by Naval Research Laboratory with a coronagraph aboard 0S0-7. Limited evidence suggests that some aspects of a coronal disturbance, including trajectory of flare spray and depletion of the inner corona, are homologous within 24 hours, so the overall coronal magnetic field configuration is not necessarily permanently altered. Key words: sun - corona - prominences

On the reality of potential magnetic fields in the solar corona

Global magnetic field calculations, using potential field theory, are performed for Carrington rotations 1601–1610 during the Skylab period. The purpose of these computations is to quantitatively test the spatial correspondence between calculated open and closed field distributions in the solar corona with observed brightness structures. The two types of observed structures chosen for this study are coronal holes representing open geometries and theK-coronal brightness distribution which presumably outlines the closed field regions in the corona. The magnetic field calculations were made using the Adams-Pneuman fixed-mesh potential field code based upon line-of-sight photospheric field data from the KPNO 40-channel magnetograph. Coronal hole data is obtained from AS&Es soft X-ray experiment and NRLs Heii observations and theK-coronal brightness distributions are from HAOsK-coronameter experiment at Mauna Loa, Hawaii.The comparison between computed open field line locations and coronal holes shows a generally good correspondence in spatial location on the Sun. However, the areas occupied by the open field seem to be somewhat smaller than the corresponding areas of X-ray holes. Possible explanations for this discrepancy are discussed. It is noted that the locations of open field lines and coronal holes coincide with the locations ofmaximum field strength in the higher corona with the closed regions consisting of relatively weaker fields.The general correspondence between bright regions in theK-corona and computed closed field regions is also good with the computed neutral lines lying at the top of the closed loops following the same general ‘warped’ path around the Sun as the maxima in the brightness. One curious feature emerging from this comparison is that the neutral lines at a given longitude tend systematically to lie somewhat closer to the poles than the brightness maxima for all rotations considered. This discrepancy in latitude increases as the poles are approached. Three possible explanations for this tendency are given: perspective effects in theK -coronal observations, MHD effects due electric currents not accounted for in the analysis, and reported photospheric field strengths near the poles which are too low. To test this latter hypothesis, we artificially increased the line-of-sight photospheric field strengths above 70° latitude as an input to the magnetic field calculations. We found that, as the polar fields were increased, the discrepancy correspondingly decreased. The best agreement between neutral line locations and brightness maxima is obtained for a polar field of about 30 G.

EVOLUTION OF CORONAL HELMETS DURING THE ASCENDING PHASE OF SOLAR CYCLE 20.

The principal polar-crown coronal helmet structures were selected from nearly three years (May, 1965–January, 1968) of K-coronameter observations made at Haleakala and Mauna Loa, Hawaii. Six isolated and long-lived helmet systems were found at latitudes of 45° and above. Their developments are compared with underlying chromospheric and photospheric activity and a simple phenomenological model is presented showing that a coronal system is formed over an active region. Thereafter the center of gravity of the system gradually drifts poleward with the trailing unipolar magnetic region (UMR), and it becomes a high latitude coronal helmet, arched over a polar crown filament.By comparison of these coronal helmets with observations of the outer corona (to circa 4 R⊙) made at solar eclipse, lunar sunset, and with balloon and rocket-borne externally occulted corona-graphs, it appears that ground-based K-coronameter measurements to a distance of 1.5–2.0 R⊙ are sufficient to detect the coronal streamers.

Global distribution of filaments during solar cycle No. 20

By tracing the positions of filaments on the solar disk for a series of consecutive Carrington rotations, one can make a compact representation of the changes in general topology of photospheric magnetic fields during the course of a solar cycle. Examples are shown for the time period 1964–1974, which may provide some insight into the long-term relationship of the mid-latitude diagonal filaments and the high latitude polar crown.

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.

Differential rotation and reconnection as basic causes of some coronal reorientations

A process is suggested by which a coronal structure (with underlying filament) may form between a polar crown structure and a low-latitude bipolar region. During the ascending phase of the solar cycle the identifying underlying filament should lie poleward and westward of the active region, but during the descending phase it should appear as an eastward extension of the filament separating leader and follower photospheric fields within the active region.

K-coronal enhancements and chromospheric plages

A systematic investigation was made of the K-corona immediately overlying the positions of the brightest and most isolated chromospheric plages during the years 1964–1967. In all cases, the corona was found to be enhanced with peak brightness proportional to the plage area. In the absence of plages, the K-coronal brightness remained at a quiet level which was essentially the same thoughout this part of the solar cycle.