Three comets were observed by SMEI in 2004 April and May: C/2001 Q4 (NEAT), C/2002 T7 (LINEAR), and C/2004 F4 (Bradﬁeld). Both NEAT and LINEAR passed close by Earth, within ~0.3 AU. At times their plasma tails can be seen extending up to 8 x 107 km (~0.5 AU ), and they exhibit considerable variability in both position and brightness. On the other hand, comet Bradﬁeld passed less close to Earth and at high ecliptic latitude; its tail remained quiescent throughout the observation period. The analysis of disruptions and motion of these comet tails is found in Kuchar et al. (2007) and in Buffington et al. (2008). Jackson (2012) provides an overview and summary of the use of comet tails to study the solar wind.
The ﬁsheye movie that is centered on the Sun is the best choice to view all three comets at once throughout the time period presented here. Numerous CMEs are also visible in this movie as areas of faintly enhanced brightness sweeping across the sky, although only one (see Kuchar et al. 2007) occurs in close proximity to comet-tail activity. Individual comet movies are more suitable for viewing the changing topography of their tail features. Movies of comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) both exhibit considerable tail activity, while the tail of comet C/2004 F4 (Bradﬁeld) remains straight and unstructured until it fades from view. In addition to the plasma tails, shorter dust tails are also visible during portions of the movies for NEAT and LINEAR.
The plasma-tail motions seen for both of these comets are a combination of:
Typically, a kink at ﬁrst appears as an abrupt change. In the majority of cases an increase in aberration angle relative to the unperturbed comet tail is seen, and this angle often increases further as the disturbance propagates down the tail. In this case the material trailing the kink appears to be catching up with the material within and ahead of the kink. Various unrelated artifacts are also visible in the movies, especially in the wide-angle ﬁsheye Sun-centered ecliptic movie. In most cases these artifacts and empty places in the sky do not interfere much with viewing and measuring the comet-tail motions.