LASCO observations of the coronal rotation and morphology of tracers seen at solar maximum and comparison with solar minimum

We present data obtained from the Large Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory spacecraft (SOHO). We compare the rotation of the white-light corona as seen during a period approaching the maximum of the solar 11-year activity cycle with that observed in a previous study made at solar minimum (Lewis et al., 1999). We find no fundamental difference in the rotation characteristics and again find the white-light corona to be radially rigid. The rotation has been observed at altitudes from 2.5 R _⊙ to beyond 15 R _⊙ and as predicted in the previous study, the greater level of complexity in the coronal structures and their relatively rapid evolution has not allowed periods to be determined as accurately as at solar minimum. Our best estimate of the mean synodic rotation period during the period of study (7 March 1999 to 6 March 2000) is 27.5±0.3 days. This is consistent with the relatively small scale structures associated with the surface activity imposing their rotation signature on an otherwise axisymmetric background corona. The short-lived nature of the small scale coronal morphologies at this epoch has made a thorough analysis of the latitudinal variation difficult, although we again find some evidence for the white light corona's increased latitudinal rigidity when compared to the underlying photosphere. However, we again note how projection effects create difficulties in confirming the exact degree of rigidity in the corona at these altitudes and a very simple coronal model is used to highlight how the appearance of lower latitude features in projection can contaminate the coronal signal observed at other latitudes. We also note evidence for a sudden and apparently fundamental change to the global coronal morphology on the approach to solar maximum and suggest this may represent the time beyond which the classical solar dipole ceases to dominate the coronal field.     

An investigation of the fine ray structure of the coronal streamer belt using LASCO data

It is shown that within R>3–4 Rfrom the solar center the coronal streamer belt consists in a sequence of radial brightness rays. A minimum angular size of the individual ray d2.0°–2.5°, which is about the same in the directions normal to and along the streamer-belt, is independent of the distance from the Sun at R=4–6 R. The lifetime of the rays can exceed 10 days. From time to time, inhomogeneities of material inside the rays begin to move in the antisunward direction. Plots of increase in their velocity with the distance from the Sun are similar to those obtained by Sheeley et al. (1997) for inhomogeneities that are carried by a quasi-stationary solar wind in streamers. It is concluded that the phenomena discussed in this paper and by Sheeley et al. (1997) share a common origin. It is suggested that a different origin of solar wind flows in streamers and in coronal holes may be associated with a different character of flows in microtubes of the magnetic field comprising a total solar wind flow. These tubes are observed as brightness rays in streamer belts and plumes in coronal holes.     

Differential rotation of coronal holes

Using KPNO helium 10830 Å synoptic charts of Carrington rotations 1716 through 1739, and by assembling a time sequence representing single latitude zone, rotational properties of coronal holes for five zones of latitudes (±10°, ±20° – ±40°, and ±40° – ±60°) have been examined. It seems that the rotation period of coronal holes is a function of latitude, thus reflecting differential rotation of coronal holes.     

Extreme-ultraviolet observations of coronal holes

The disk boundaries of coronal holes have been systematically determined from XUV observations taken during the manned Skylab missions (June 1973–January 1974). The resulting Atlas was used to find the sizes, global distributions, differential rotation rates, growth/decay rates and lifetimes of holes during this period. The polar cap holes together covered 15% of the Sun's total surface area, a number which remained surprisingly constant throughout Skylab despite the fact that each pole was independently evolving in time. Lower latitude holes contributed another 2 to 5%. The anomalous differential rotation law derived for a large north-south hole by Timothy et al. (1975) has been confirmed. However, other Skylab holes were too low in latitude to demonstrate the generality of this result. The average growth/decay rate for holes was 1.5 × 10⁴ km² s^-1, in excellent agreement with the value used by Leighton (1964) for his successful treatment of the surface transport of solar magnetic fields. The lifetimes of lower-latitude holes are found to regularly exceed 5 solar rotations, in good agreement with the lifetimes of recurrent geomagnetic storms with which holes are now known to be associated.     

Mechanisms for the rigid rotation of coronal holes

We show that the rotation of coronal holes can be understood in terms of a current-free model of the coronal magnetic field, in which holes are the footpoint locations of open field lines. The coronal field is determined as a function of time by matching its radial component to the photospheric flux distribution, whose evolution is simulated including differential rotation, supergranular diffusion, and meridional flow. We find that ongoing field-line reconnection allows the holes to rotate quasi-rigidly with their outer-coronal extensions, until their boundaries become constrained by the neutral line of the photospheric field as it winds up to form stripes of alternating magnetic polarity. This wind-up may be significantly retarded by a strong axisymmetric field component which forces the neutral line to low latitudes; it is also gradually halted by the cross-latitudinal transport of flux via supergranular diffusion and a poleward bulk flow. We conclude that a strong axisymmetric field component is responsible for the prolonged rigid rotation of large meridional holes during the declining phase of the sunspot cycle, but that diffusion and flow determine the less rigid rotation observed near sunspot maximum, when the holes corotate with their confining polarity stripes.     

Monochromatic observations of a coronal loop

An isophotal map of a small coronal loop, obtained from a coronagraph observation through a solid Fabry-Perot interferometer, is used to estimate the variation of emission per unit volume and the pressure gradient at the top and sides of the loop. The magnitude of the magnetic field necessary to maintain the estimated pressure gradients is found to be ¦H ²¦ = 30 G².     

Meterwave observations of a coronal hole

We present meterwave maps showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in He i l10830 spectroheliograms.Using the model of coronal holes by Dulk et al. (1977) we derive the electron density from the radio observations of the brightness temperature. The discrepancy between the density value derived from the Skylab EUV data and that computed from our radio data is even larger than in Dulk et al. 's comparison at similar and higher frequencies.     

MHD simulations of sunspot rotation and the coronal consequences

A simplified magnetic configuration is used to model some aspects of observations of a rotating sunspot and its overlying coronal loops. In the observations a large sunspot rotates over a few days and two smaller pores spiral into it. The coronal loops become sigmoidal in shape and flares are seen in Yohkoh/SXT and GOES. We have modeled the sunspot, one of the pores and the loops connecting these to a diffuse region of plasma of the opposite polarity. Two sets of MHD simulations are considered: (i) rotation of the sunspot and pore alone and (ii) rotation of the sunspot with inflow of the pore. Rotation alone can trigger the ideal kink instability in the loops but only for a rotation that is much greater than the observed value. There is no build-up of current which is needed for magnetic reconnection to occur. However, when inflow is included a strong build-up of current is seen as the pore merges with the sunspot. Comparing these results from the simulations with the observations, we find that the observed merging of the pores coincides with the timing of the flare. Therefore, we suggest that the merging of the pores with the large sunspot may be responsible for the flaring.     

XUV observations of coronal magnetic fields

Spectroheliograms obtained with the Naval Research Laboratory's Extreme Ultraviolet Spectrograph (S082A) on Skylab are compared with Kitt Peak National Observatory magnetograms. A principal result is the characteristic reconnection of flux from an emerging bipolar magnetic region to previously existing flux in its vicinity. Examples of the disappearance of magnetic flux from the solar atmosphere are also shown. The results of a particularly simple, potential field calculation are shown for comparison with the Skylab observations.     

Extreme ultraviolet observations of coronal holes

Extreme-ultraviolet Skylab and ground-based solar magnetic field data have been combined to study the origin and evolution of coronal holes. It is shown that holes exist only within the large-scale unipolar magnetic cells into which the solar surface is divided at any given time. A well-defined boundary zone usually exists between the edge of a hole and the neutral line which marks the edge of its magnetic cell. This boundary zone is the region across which a cell is connected by magnetic arcades with adjacent cells of opposite polarity. Three pieces of observational evidence are offered to support the hypothesis that the magnetic lines of force from a hole are open. Kitt Peak magnetograms are used to show that, at least on a relative scale, the average field strengths within holes are quite variable, but indistinguishable from the field strengths in other quiet parts of the Sun's surface.Finally it is shown that the large, equatorial holes characteristic of the declining phase of the last solar cycle during Skylab (1973–74) were all formed as a result of the mergence of bipolar magnetic regions (BMR's), confirming an earlier hypothesis by Timothy et al. (1975). Systematic application of this model to the different aspects of the solar cycle correctly predicts the occurrence of both large, equatorial coronal holes (the M-regions which cause recurrent geomagnetic storms) and the polar cap holes.     

On 5694 Å coronal line observations

Coronal yellow line emission was observed by the Lyot coronagraph at the Abastumani Astrophysical Observatory. Line intensity is I = 45 erg cm^?2 s^?1 sr^?1 Å^?1, its half-width Δλ = 1.3 Å, electronconcentration n _e = 7.5 × 10⁹ cm^?3.     

Coronagraphic observations of an enhanced coronal region

Ratios of emission line intensities are used to calculate the variation of temperature and the variation of electron density as a function of ion class for differing paths through a coronal enhancement. The data indicate (a) a peak mean electron density of 2.3 × 10⁹ cm^–3, (b) a temperature maximum greater than 2.3 × 10⁶ K, and (c) the non-coincidence of the peak temperature and peak mean electron density. The latter demonstrates the invalidity of the assumption of symmetric models for coronal enhancements.The abundance of Ni was found to be equal to 0.045 that of Fe from the line ratio I( 6702)/ /I( 7059) and a density model based on the variation of the ratio I( 8024)/I( 6702).     

Green line observations of the March 1970 coronal enhancements

Monochromatic observations of a double coronal enhancement were obtained on 3 March and 17 March 1970. These data are used, along with the assumption of cylindrical symmetry, to calculate the emittance of the Fexiv 5303 Å lines as a function of height. These estimates are then used to find the electron density and the density of ions in the ² P _3/2 level of the ground state as a function of height above the limb.     

Three-dimensional coronal structures using clark lake observations

We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph. Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with related observations using white-light coronagraphs.     

An observational study of coronal hole rotation over the sunspot cycle

An analysis of the rotation of coronal holes (CHs) spanning 18 years was done based on data from theCatalogue of Coronal Holes (Sanchez-Ibarra and Barraza-Paredes, 1992). A differential rotation of CHs is confirmed for the totality of CHs, but a different behavior was found when those were separated as equatorial or isolated, and polar hole extensions, such as in theCatalogue. Isolated CHs show a typical differential rotation, but polar hole extensions display two different types of behavior: a rotation rate below 40° ± 5° of heliographic latitude, increasing to the equator, and a rotation rate above the same heliographic latitude but increasing to the poles. Also discussed here is how this last result agrees with other studies that indicate the mostly rigid rotation of the corona at higher latitudes.     

The differential rotation of the corona as indicated by coronal holes

The rotation of the corona can be determined either directly by using Doppler methods or indirectly by using tracers, i.e., structures within the corona. In this study the rotational characteristics of the corona are determined using coronal holes as tracers, for the period 1978–1991. The coronal data used here are from an atlas of coronal holes mapped in Hei 10830 data. A comparison is made between our results and previous determinations of the coronal rotation rate, e.g., by Sime (1986), using white-light K-coronameter observations, by Timothy, Krieger, and Vaiana (1975), using soft X-ray observations, and by Shelke and Pande (1985) and Navarro-Peralta and Sanchez-Ibarra (1994), using Hei 10830 data. For the atlas of coronal holes used in this study the nature of the coronal hole distributions in number and latitude, in yearly averages, has been determined. These distributions show that at solar minimum the polar coronal holes dominate and the few non-polar holes are confined to a narrow band near the equator. At solar maximum, however, mid-latitude coronal holes dominate, with a large spread in latitudes. Given these distributions we consider the differential rotation data only as an average over a solar cycle. This removes spurious effects caused by having only a small number of coronal holes contributing to the results, or by having a narrow latitude band for the observations, thus limiting the results to that narrow latitude band. By considering these coronal holes as tracers of the differential rotation we show that the mid-latitude corona rotates more rigidly than the photosphere, but still exhibits significant differential rotation, with an equatorial rate of 13.30 ± 0.04° day^–1, and at 45° latitude a rate of 12.57 ± 0.13° day^–1. These results are comparable, within errors, to the Sime (1986) results which have an equatorial rate of approximately 13.2 ± 0.2° day^–1 and a rate of approximately 12.9 ± 0.3° day^–1 at 45° latitude.     

EUV and coronagraphic observations of coronal mass ejections

The Large Angle Spectrometric Coronagraph (LASCO) and Extreme-ultraviolet Imaging Telescope (EIT) onboard Solar and Heliospheric Observatory (SOHO) provide us with unprecedented multi-wavelength observations helping us to understand different dynamic phenomena on the Sun and in the corona. In this paper we discuss the association between post-eruptive arcades (PEAs) detected by EIT and white-light coronal mass ejections (CMEs) detected by LASCO/C2 telescope.     

Coronagraph observations of the coronal condensation of 4 February 1962

Climax coronagraph observations of the accessible Fe lines, as well as the Caxv 5694 line at the time of the 1962 total eclipse, are analyzed. The spectra show that the ionization equilibrium of iron is not substantially changed in an intense coronal condensation, at least for the stages x through xv. The only exception is Fexv 7059, for which density effects are important. The stability of the ionization distribution is explained by the dynamic nature of the Fe ionization, with ions entering on the high side (Fexvi and up) due to rapid heating and then cooling through the visible stages.Comparison of the ionization distributions inferred from radiative and collisional excitation of the iron lines shows that the excitation must be by collisions everywhere at the heights examined (less than 50 000 km).The iron abundance in the corona is found to be 10^–4 that of hydrogen, but this figure would be reduced by the amount of cyclic excitation.The peak electron density in the condensation is 8 × 10⁹, and the peak value of the 5694 line/ continuum ratio is 2.5, in good agreement with calculations by Chevalier and Lambert.The ratio of the infrared Fexiii lines is measured along the limb and found to vary with electron density as expected, the 10 747/10 798 ratio is 7 or less at densities much below 10⁹ and saturates at a value of 2 for densities above that amount.     

Inferences of plasma parameters from coronal hole observations

The temperature in the acceleration region of the solar wind remains one of the most elusive parameters to measure. Knowledge of the temperature as well as its gradient in the inner corona is fundamental for placing constraints on physical mechanisms thought to be responsible for the coronal heating process, as well as for understanding the flow properties of the solar wind. Estimates of the helium abundance is essential for understanding the puzzling behavior of heavier ions in the solar wind. As an illustration of the difficulties and uncertainties involved in the inferences of plasma parameters in the wolar wind acceleration region, The inference of electron temperature and helium abundance will be described. Prospects for future observations will be briefly discussed.     

Ultraviolet spectroscopic observations of coronal streamers in the SOHO era

Measurements made with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory can be used to determine physical parameters in the solar corona such as hydrogen and ion kinetic temperatures, electron densities, and absolute elemental abundances. Hydrogen and ion outflow velocities can be determined by combining the UV spectroscopic measurements with white light polarized brightness measurements. These combined measurements can be used to reveal physical characteristics of coronal streamers. To date we have studied plasma properties, such as the variation of plasma outflows in quiescent streamers, primarily in classic helmet streamers at solar minimum. Out-flows have not been observed in the centers of coronal streamers suggesting that these are closed magnetic field regions. We propose to study all of the coronal streamers in the UVCS synoptic dataset in order to investigate different types of streamers and their long-term evolution.