Fig. 8
So what is this strange behavior?
There appears to be at least several possibilities. First, Hale Bopp may be traveling in a group and this may be evidence of a large "chunk" that NASA previously referred. What may be more likely, and in fact may be in addition to this last idea, is that Hale Bopp does in fact possess a field.
This may be an outburst of plasma
and then a retraction. If the size of Hale Bopp is not disputed or disproved,
this may likely be the answer.
Another possibility, is that this may be a reaction of Hale Bopp to the IMF (Interplanetary Magnetic Field). In the Image below, a comet in recent years broke apart as it passed through the IMF, and then unbelievably regrouped itself back on its journey to the Sun.
Fig. 8b
But it is unlikely that this is a variation of what we are seeing with Hale Bopp due to the fact that the comet was not crossing the IMF barrier at the time. Therefore, evidence to support the theory that Hale Bopp does indeed possess an EM field. But what did happened to Hale Bopp when it did cross the IMF?
See HaleBopp's orbit (fig 5), showing a possible effect of the IMF upon the comet.