In a previous study we developed a new technique for deriving prominence mass by observing how much coronal radiation in the Fe XII (19.5 nm) spectral line is absorbed by prominence material. In the present work we apply this new method, which allows us to consider the effects of both foreground and background radiation in our calculations, to a sample of different types of prominences (eruptive, quiescent, and surging) observed during the period 1999 July through 2004 July. The masses of prominences involved in CMEs are not generally measured, but the accurate determination of such masses may help in assessing the dynamical importance of prominences in CME events. In the present study, we find the average mass of our sample of quiescent prominences to be 4.18 × 1014 g, while the average mass of the eruptive prominences is 9.09 × 1014 g, and that of surges is 1.53 × 1014 g.

Narrow coronal mass ejections (CMEs), defined arbitrarily as events whose apparent angular width is 15 degrees or less, are a small subset of all CMEs. Little is known of the properties of these events and whether these properties differ from those of the larger, more typical CMEs. We have included in this study 15 narrow CMEs observed in the period from 1999 March through December, and we have examined their structure, angular size, projected radial velocity (speed), and likely surface associations. We find it useful to break these events into two classes : structured and unstructured. Unstructured narrow events are generally narrower and slower than the structured events, but both classes of narrow CMEs exhibit speeds similar to those of larger CMEs. We found that 11 of 15 events studied may be traced to regions on the solar surface near a relatively sharp bend in a polarity-reversal line, as revealed from H alpha synoptic maps. We contrast the properties of the narrow CMEs with those of the larger CME population.

Observations of the brightness of the outer solar corona from the Solar Maximum Mission (SMM) coronagraph during solar cycle 22 (1980, then 1984-1989) are compared with the occurrence rate and the mass of coronal mass ejections (CMEs) observed during this period. We find that the brightness and, hence, mass of the outer corona increased by more than a factor of 4 from solar minimum (1986) to late 1989, when the SMM ceased operation. The peak brightness (mass) in 1989 was roughly equivalent to that observed in the latter part of 1980. Accompanying a sharp increase in brightness (mass) of the corona in early 1989 was a concomitant increase in both the occurrence rate and the average mass of CMEs.

The intensity of a sample of large, high-contrast and isolated dark points has been observed with full-disk images in the light of He i 1083 nm from the Chromospheric Helium line Intensity Photometer (CHIP) on Mauna Loa, Hawaii. Temporal variations in the intensity encompassing a broad range of time scales have been recorded. Long-term changes in the intensity, although highly variable, are characterized by e-folding times on the order of 5 h. Superposed on these variations are frequent intensity variations, which occur over time scales ranging from the typical observing cadence of 3 min, to tens of minutes. Microflares-involving intensity changes of at least 50% over periods of minutes are observed frequently. Rapid cadence (\(\frac{1}{2}\) min) observations reveal differences between rise and decay times and shorter-term variations in the intensity profiles of these microflares.

We report here the probable detection of an emission line of Si IX that was observed from an open C130 aircraft over the Pacific Ocean during the 1998 total solar eclipse. Although the IR data themselves are inconclusive because of the uncertainty in the precise central wavelengths of the narrowband filters during the eclipse, the consistency of the measured IR limb excess with simultaneous EUV emission measured by SOHO/Coronal Diagnostic Spectrometer and the EUV Imager Telescope support our detection claim. This line appears to be the brightest IR coronal line yet observed, and its existence may significantly improve future prospects for obtaining optical coronal magnetic field measurements.

The solar eclipse on February 26, 1998, was observed from an aircraft operated by the Research Aviation Facility of the National Center for Atmospheric Research. The observations cover the near infrared spectral range and were made at an altitude of almost 6 km above the Pacific about 800 km southwest from Panama City. Aside from spectroscopic observations of coronal emission lines, the solar F-corona was observed in the J and K-band over a field of view of 7°. A preliminary view on the data shows no indication for the existence of pronounced brightness features in the solar F-corona.

We analyze J- and K-band observations of the 1998 solar eclipse and derive the F-corona brightness in the K band between 3 and 7 R. from the center of the Sun and in the J band out to 5 R.. The falloff in the K-band brightness from 3 to 7 R. is fitted with a radial power law with exponent -2.4 +/- 0.1 at the solar equator and with exponent 2.9(-0.1)(+0.2) at the solar pole. This slope is gentler than that derived from observations in 1983 but is steeper than that derived from observations for the 1991 eclipse. The radial profiles agree well with models that explain the F corona with weakly absorbing dust particles. Comparison of the J- and K-band brightness at 3 R. from the center of the Sun indicates a reddening of the F corona with respect to the solar spectrum. The reddening is, however, less pronounced compared to the F corona observed during the 1983 total solar eclipse. This fact may be attributed to a change in the composition of dust near the Sun. As with the radial profiles, the reddening in 1998 is better explained with models that assume weakly absorbing rather than strongly absorbing dust particles in the solar corona. Similar to recent eclipse observations, we do not detect an excess emission feature of near-solar dust in the F corona. We hence can reject the hypothesis that suggests a correlation between the detection of an emission feature and the solar activity cycle.

The scientific motivation, design criteria, and specifications for a new ground-based instrument to observe the Sun in the He i 1083-nm spectral line is described. The instrument employs a liquid-crystal tunable Lyot-type spectral filter and an array detector that allows the full solar disk to be observed with a time cadence of minutes. We describe the telescope's optical and mechanical features and discuss computer interface and data-reduction procedures employed. Instrument performance during the initial year of operation of the telescope at its high-altitude site is summarized.

A new instrument capable of 3-min time resolution full-disk and limb observations in the Hei 1083 nm spectral line has been in operation at the High Altitude Observatory's Mauna Loa Solar Observatory (MLSO) since April 1996. We discuss instrument capabilities and performance and present some initial observations of limb activity from the first year of instrument operation. We compare limb Hei and Hα observations of quiescent and active prominences, comment on the role of Doppler shifts in interpreting the Hei observations, and illustrate the use of disk/limb Hei observations of a CME-associated eruptive filament in mass-ejection studies.

In the context of dust measurements on a solar probe, we present an analysis of a 1991 eclipse observation by Hodapp et al. /1/ with respect to the solar F-corona brightness (published in MacQueen and Greeley, /2/). Although the data are limited by observing conditions, we can still gain some information which may be compared with our present knowledge about the interplanetary dust cloud based on the analysis of zodiacal light data, which describe the dust in regions outward from the sun. Past visible light measurements showed that the F-corona has roughly the same brightness in the ecliptic and over the poles, but that the radial gradient of the latter is steeper. In the 1991 infrared observations, the ecliptic radial gradient is flatter than has been observed in the visible, whereas the polar radial gradient is rather similar to past, visible spectral region observations. This appears to point to the existence of different components in the dust cloud as also discussed to explain the zodiacal light /3/. Also in the present data there is no clear signature for the beginning of the dust free zone around the sun, one conclusion being that dust (of some type) possibly approaches the sun to within 4 solar radii. As far as the size distribution of dust in the solar vicinity is concerned we discuss a study by Davidson et al. /4/, which shows that the present F-corona data can be fitted with distinctly different size distributions.

AbstractMotivated by new infrared observations, we examine the contribution of various size intervals of interplanetary dust particles to visible and infrared scattered radiances of the solar F-corona, employing Mie theory and particle size distributions based upon differing interpretations of lunar microcrater evidence and interplanetary flux measurements.

The presence of the solar magnetic field has a profound effect on the structure of the lower chromosphere, and is responsible for the formation of the upper chromosphere and the corona, and the acceleration of the solar wind. The variation of the field induces variations in the chromosphere and the corona on time scales from 0.001 seconds to centries. SOHO, and subsequent approved solar missions such as TRACE will bring powerful observational capabilities to bear on critical questions relating to solar variability. However, the most fundamental question - how energy is transferred from the magnetic field into the solar plasma - will require observations of diagonistic qualtity on a spatial scale of 50 - 100 kilometers; this is an order of magnitude beyond the capability of any planned mission. Our mission concept, the Solar Chromospheric and Coronal Explorer (SCCE) is designed to investigate the mechanisms underlying the variability of the solar atmosphere, by attaining spectroscopic observations of the solar atmosphere over a wide range of temperatures (4,500 K to 100,000,000 K), with very high angular (0.1 arcseconds) and temporal (0.001 seconds) resolution, that will permit models of the physical processes that underlie the phenomena of solar activity to be formulated and tested at the scale, 50 - 75 kilometers that appears to be fundamental. The architecture of the SCCE is based on advances in multilayer optics, which permit broad spectral response, and high angular and spectral resolution to be achieved in a volume, and at a cost that is compatible with deployment within the fiscal and physical constraints of the MIDEX program.

Model calculations are made of the infrared brightness in the solar F-corona motivated by recent infrared solar eclipse observations. Two different approaches are employed to describe the scattering properties of interplanetary dust : Mie scattering theory and diffraction theory, with and without an isotropic scattering term. In addition. two different particle size distributions are used in the calculations and the resultant line of sight brightness is compared with observational data of the solar F-corona between 3 and 8 solar radii (R) in the ecliptic plane. It is found that the use of diffraction theory without an isotropic scattering contribution gives a very poor match with the observations, for both assumed particle size distributions. However, both the diffraction theory including isotropic scattering and the Mie scattering theory agree reasonably with the observed brightness, and especially its radial slope within the corona. for the model size distribution which is dominated by large particles. Only Mie theory may be employed in describing the second model size distribution because diffraction theory poorly describes the scattering due to the small particles which dominate this size distribution. It is concluded that the derivation of particle size distributions from the F-coronal brightness is still ambiguous and a further analysis needs either improved observations, or the application of further reasonable physical assumptions.

Observations of the total and polarized brightness of the solar corona at wavelength 2.12 mu m during the total solar eclipse of 1991 July 11 are employed to separate the contribution of the electron-scattered component and the remaining, nonpolarized component, the latter dominating in the outer corona. After corrections are applied to account for a two-component sky or instrument background, the brightness of the ecliptic and polar corona are fitted by r(-1.9) and r(-2.3), respectively, over the radial distance range 3-8 Ro from Sun center. The ecliptic outer-coronal brightness is compared with a Mie-scattering model of interplanetary dust particles based upon three particle-size distributions deduced from, respectively, lunar microcrater counts (Lamy and Perrin 1986), interplanetary dust flux measurements (Grun et al. 1985), and for an arbitrary population of large particles (radii > 3 x 10(-5) cm). Particle physical characteristics and spatial distributions are those assumed in past studies of the zodiacal cloud. For reasonable assumed space number densities of particles, the models agree with the magnitude of observed ecliptic coronal brightness. But in all cases, the models predict a steeper brightness fall off with radial distance than that observed, with those models for which the power-law exponent for the space distribution is v = 1.3 being the most discrepant with the observed radial gradient.

Observations of the solar corona in a narrow band filter at wavelength 2.12 μm during the total solar eclipse of 11 July 1991 are described, and compared with results obtained by two observers during the eclipse of November 1966, and shortly thereafter. The lack of any observable signature of thermal emission in the 1991 results suggests that during 1966/67, the near-solar environs were subjected to a locally enhanced dust population, supplied by one or more sungrazer comets. Possible conditions which match the observational circumstances are discussed.

A comparison of 1991 solar eclipse coronal observations over the range of 3R.-8R., at wavelength 2.12 mum, with model calculations of thermal emission and scattering from interplanetary dust particles (IDPs) is presented. The observed brightness in the ecliptic plane shows a monotonic decrease which can be approximated with a radial power law with exponent 2. No signature in the brightness, due to enhanced dust concentration or emission near the Sun can be detected. The modelling attempts result in an average deviation, relative to the observations, of less than 12%, under reasonable assumptions about the spatial distribution and optical and thermal properties of the IDPs. It is only possible to match the observational data with a model of scattering and thermal emission where particles can approach up to 3R. towards the Sun. The data suggest that it is necessary to alter the local variation of number density of dust particles within 10R.. The deduced temperature of the IDPs is about 10% below their blackbody temperature, and the best fit with observations involves a variation in the IDP albedo with distance from the Sun, suggesting physical changes to the near-solar IDP population.

THEORETICAL suggestions 1-3 that there should be a ring of dust in near-ecliptic orbit about the Sun were supported by observations, during a total solar eclipse on 12 November 1966, of enhanced infrared emission 4-6 from the solar corona at a distance of 4 R. from the Sun's centre. The infrared emission was attributed to the sublimation of dust grains as they spiral into the Sun because of the Poynting-Robertson effect, by which solar radiation creates a tangential drag. Two months after the 1966 eclipse, the feature at 4 R. was seen again in observations from a stratospheric balloon-borne coronagraph, as were additional features at 3.5, 8.7 and 9.2 R. (ref. 6). Observations since then, however, have failed unambiguously to corroborate the earlier observations. We searched for excess infrared emission in the solar equatorial plane during the 11 July 1991 eclipse, using a wide-angle infrared camera on Mauna Kea, but failed to find any signature of dust evaporation. We argue that the earlier observations were credible, and therefore that the circumsolar dust ring is a transient feature, perhaps due to the injection of dust into near-solar space by a Sun-grazing comet.

Ten sungrazing comets were discovered by the High Altitude Observatory's white light coronagraph on the NASA Solar Maximum Mission spacecraft during an observing period which covered about 6 years. In addition, observation of another sungrazing comet discovered by the SOLWIND coronagraph was confirmed. The comets exhibited a wide range of brightness near the Sun, spanning roughly 8 astronomical magnitudes. No comets were observed postperihelion, and no discernible coronal effects could be detected as a result of any comet disappearance. The comet apparent motions and orientation of tails are consistent with all of the comets belonging to the Kreutz sungrazer group, a conclusion in support of the orbital analysis of nine of the objects by Marsden (1989). The observing programs employed by the instrument to detect coronal mass ejection events are shown to be well-suited for the detection of sungrazers. The appearance of these comets is seen to be highly episodic over the time period of SMM coronagraph operation. Estimates of the comet head properties, including size, are offered, based upon the observed comet behavior.