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The nucleus of Comet 45P/Honda–Mrkos–Pajdusakova was detected during Hubble Space Telescope observations taken with the Wide-Field Planetary Camera 2 (WFPC2) in February 1996 when the comet was 0.17 AU from the Earth. The high spatial resolution (1 pixel projected to 5.6 km at the distance of the comet) allowed us to separate the signal of the nucleus from that of the coma, and images taken through five broadband filters allowed us to determine the UBVRImagnitudes of the nucleus in the Johnson–Kron–Cousins photometric system (sometime known as the Landolt system). Assuming a spherical body whose geometric albedo is 0.04 for the Vband and a phase coefficient of 0.04 mag/deg, we derive an effective radius of 0.34 km. The uncertainty is dominated by the unknown phase law; the radius may be as small as 0.22 km if the phase coefficient is 0.03 mag/deg. The correlated variations of the magnitudes over 1 day suggest an elongated body with an axis ratio of at least 1.3. The strong reddening of the nucleus in the ultraviolet progressively decreases, and its color tends to neutral in the near infrared. We also derive a fractional active area of 11% and a dust production rate of 1.0 kg s−1(86 metric tons day−1).

The near-rigid rotation of the corona above the differential rotation of the photosphere has important implications for the form of the global coronal magnetic field. The magnetic reconfiguring associated with the shear region where the rigidly-rotating coronal field lines interface with the differentially-rotating photospheric field lines could provide an important energy source for coronal heating. We present data on coronal rotation as a function of altitude provided by the Large Angle Spectrometric Coronagraph (LASCO) instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. LASCO comprises of three coronagraphs (C1, C2, and C3) with nested fields-of-view spanning 1.1 R⊙ to 30 R⊙. An asymmetry in brightness, both of the Fe xiv emission line corona and of the broad-band electron scattered corona, has been observed to be stable over at least a one-year period spanning May 1996 to May 1997. This feature has presented a tracer for the coronal rotation and allowed period estimates to be made to beyond 15 R⊙, up to 5 times further than previously recorded for the white-light corona. The difficulty in determining the extent of differential motion in the outer corona is demonstrated and latitudinally averaged rates formed and determined as a function of distance from the Sun. The altitude extent of the low latitude closed coronal field region is inferred from the determined rotation periods which is important to the ability of the solar atmosphere to retain energetic particles. For the inner green line corona (<2 R⊙) we determine a synodic rotation period of (27.4±0.1) days, whereas, for the outer white- light corona, (>2.5 R⊙) we determine a rotation period of (27.7±0.1) days.

Comet P/Faye 1991 XXI (1991 n) was observed with the planetary camera (PC) of the Hubble Space Telescope (HST) between October 29 and November 21, 1991, when its geocentric distance was in the range 0.62–0.67 AU. The resulting high resolution—a single PC pixel projected to a distance of ∼20 km at the comet—made it possible to clearly discriminate the nucleus and to study the dust coma within ∼100 km from the nucleus. The spherical aberration which affected the early operation of the HST has severely complicated the analysis and image restoration using the Richardson–Lucy method has failed to give satisfactory results. The coma is dominated by the point spread function (PSF) of the nucleus up to a radial distance of ∼100 km and cannot be recovered. Beyond, it is affected by the spherical aberration up to approximately 750 km and was analyzed by comparison with a grid of models convolved with appropriate PSFs. Beyond 750 km, it remains unaffected and can be studied directly. From the outer to the innermost regions, the coma presents an elongated shape which may be explained either by an active source on the nucleus or, more likely, by a projection effect of the dust tail. If this second interpretation is correct, then the temporal evolution of the comet is very slow and smooth, suggesting an extended source of dust on the nucleus. The dust production rate, corrected for the projection effect of the tail, amounts to 125 kg sec−1.

The January 6, 1997 halo CME is the subject of continuing research due to the large geomagnetic effects of this event. This paper presents a very simplified model which explains the key observational features. The CME is modelled as a hemispherical shell with a cone shaped extension toward the sun. The observed circular arc is the projection of the “shoulders” of the CME onto the image plane. The low surface brightness and the fuzzy appearance of the halo CME are naturally explained.