Elenin passes through the image field of view of the STEREO H1 Behind imager on August 19. Click to embiggen. Image processed from NASA/STEREO raw images using ImageJ. Inverted image, dark lines are imaging artifacts.
I wrote earlier that comet C/2010 X1 Elenin was hit by intense solar wind, and that its tail flared up. See the AVI animation of the August 19 images (1 Mb) for the tail dramatically flaring and twisting around. Also see Comet Al's animation of the August 19 event here.
Well, in the wake of that event the brightness of comet Elenin has dropped dramatically, by almost a magnitude (from magnitude 8.1 on the 19th to magnitude 8.9 on the 23rd, the last reported magnitude). You can see before and after images by Rob Kaufmann here showing the drop. Michael Mattiazzo also has images of comet Elenin before and after the flare with estimates of the comet magnitude here.
So what is happening? What is most likely going is that the bast of energetic solar wind particles has eroded exposed "ices" (water and carbon dioxide) creating the brief highly active tail (comet Elein has had a small but nice tail for some time). With these exposed ices gone, the generation of dust and gas drops off and the magnitude drops. UPDATE: Jakub Cerny thinks it is more likely due to depletion of particles in the inner coma, causing dimming that way. Also see Michael Mattiazzo's latest images here, showing what looks to be an elongated nucleus.
Elenin is a new comet, this is the first time it has dropped in to the inner solar system from the Oort Cloud. New comets generally are bright early than drop off quickly as their surface ices are depleted. Older comets tend to be brighter due to fissures and cracks penetrating more deeply into the comet allowing more volatiles to be accessed.
What does it mean for the future? Comets, despite the images of them that look like asteroids, and quite fragile, loose agglomerations of dust and ices smeared together. It has already been suggested that Elenein will not survive perihelion. This recent dimming of Elenin suggests it may be sufficiently fragile to evaporate as it passes close to the Sun, astronomers will be watching this small but cute comet as it plunges towards perihelion for more clues about the structure and dynamics of comets.