Astron. Astrophys. 326, 263-270 (1997)

7. Discussion

We will discuss the faint dust ring of the BPCA silicate and the edge of the dust-free zone of the BCCA silicate from the observational point of view. Expected brightness of the solar corona at a wavelength range to is calculated as

where , , and denote the brightness of thermal emission from dust particles, their scattered sunlight along the line of sight, and the K-corona, respectively. We use a model from Saito et al. (1977) for the electron density at a solar distance r ; is the electron cross section of the classical Thomson scattering; l is a distance of the dust from an observer along the line of sight; is the wave number of ; and is the radius and spatial distribution of dust grains, where is assumed as a product of the number density distribution and the size distribution . The results shown in Figs. 8 and 9 are used for a determination of . We derive from the interplanetary flux model given by Grün et al. (1985) and then set the total cross-sectional area as m² /m³. The Mie intensity functions perpendicular and parallel to the scattering plane are given by and , respectively. We set m and m.

Fig. 10 shows the expected visible ( m) brightness of the solar corona from the silicate-carbon BPCA, and the observations by Blackwell et al. (1967) and Saito et al. (1977).

Fig. 10. Calculated V-band ( m) brightness of the solar corona with the interplanetary flux model from Grün et al. (1985). Dash-dotted line: K-corona ; dotted line: thermal emission ; dashed line: scattered light ; solid line: expected brightness . Open circle: Blackwell et al. (1967); filled circle: Saito et al. (1977).

As a result of the faint solar dust ring, the enhancement of scattered light expected by the solar dust ring model for spherical particles disappears in the visible brightness of the solar corona. The expected visible brightness decreases smoothly with increasing radial distance R, and this expected feature, including its absolute values, agrees with visible observations. Even in the near-infrared ( m), it is hard to observe such a silicate dust ring (Fig. 11), because of the low temperature and therefore weak thermal emission at 4  , compared to the blackbody.

Fig. 11. Calculated K-band ( m) brightness of the solar corona with the interplanetary flux model from Grün et al. (1985). Dash-dotted line: K-corona ; dotted line: thermal emission ; dashed line: scattered light ; solid line: expected brightness . Thick solid line: Hodapp et al. (1992); dots: MacQueen (1968).

Since the temperature of BCCA silicate is lower than that of BPCA and thus farther from the blackbody temperature, the edge of the dust-free zone from BCCA silicate is invisible in the near-infrared brightness.

Furthermore, the number density distribution of dust used in this paper causes the disappearance of a hump in the brightness. In comparison with a power for at a farther distance from the sublimation zone used in Mukai & Yamamoto (1979), an exponent used in this paper diminishes the contribution of the near-solar dust to the total brightness. Consequently, the exponent brings about no feature of the solar dust ring and edge of the dust-free zone in the visible and near-infrared brightness. It is noteworthy that the power is not only supported theoretically by dynamical behaviour of dust under the Poynting-Robertson effect, but also by the analysis of the zodiacal light observations (Lamy & Perrin 1986). The power is also supported by the zodiacal light observations (Leinert et al. 1978). As shown in Fig. 10, the model visible brightness of the F-corona fulfills a relation of for derived by the zodiacal light observations (Koutchmy & Lamy 1985). In addition, the absolute magnitude of the model brightness corresponds to that of the observations from Blackwell et al. (1967) and Saito et al. (1977). Accordingly, an application of will result in a steeper brightness distribution of R than the zodiacal light observations and brighter solar corona than the observed coronal brightness.

Although an adoption of silicate including a large fraction of carbon causes higher thermal emission, they sublimate at a greater solar distance than at 4  , where the thermal emission hump has been observed. The larger solar distance of highly contaminated silicate may correspond to the other near-infrared hump at near 9  (MacQueen 1968). On the other hand, it is seen from Fig. 6 that carbon aggregates with small impurities sublimate at 4  and, furthermore, from Fig. 5 that large ( m) carbon aggregates with small porosity stay in the F-corona because of . As a result of their high temperatures, which lie near the blackbody temperature (  K), large carbon aggregates will contribute to the near-infrared brightness in the F-corona. Moreover, the color temperature of 2160 200 K was derived from the near-infrared hump at 4  measured during the 1970 solar eclipse (Peterson 1971). Accordingly, the appearance of a thermal emission hump at 4  may indicate the existence of irregularly shaped particles consisting of carbon material in the F-corona as opposed to silicate: We conclude, therefore, that the near-infrared hump observed in the F-corona arises from thermal emission of absorbing material, like a blackbody used in Mann (1992).

In order to observe the faint dust ring or the edge of the dust-free zone of fluffy silicate dust, the intermediate infrared is suitable because of their high emissivity near m (Fig. 12).

Fig. 12. Calculated N-band ( m) brightness of the solar corona with the interplanetary flux model from Grün et al. (1985). Dash-dotted line: K-corona ; dotted line: thermal emission ; dashed line: scattered light ; solid line: expected brightness . Thick solid line: Léna et al. (1974).

As shown in Fig. 6, however, the distance dependence of the sublimation zone on the impurities seems to smear the feature of the faint dust ring or the edge of the dust-free zone if silicate aggregates having different amounts of impurities exist in the F-corona at the same time. Although the absolute magnitude of the model brightness is one order smaller than Mankin et al. (1974) and two orders smaller than Léna et al. (1974), their higher brightnesses are questionable because of the high noise level. More low-noise observations in the intermediate infrared might clarify the existence of fluffy silicate dust in the F-corona.

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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