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C/1995 O1 (Hale-Bopp)
the 'Porcupine' Structure of August 1996

R-scaling on/off

Date:18.8.1996 Time:18:07 UT Exposure:20s
Field of View:7.2' x 4.7' Receiver:576 x 387 CCD Filter:RX (red Continuum)
Instrument: D=2m, f=5.6m Observatory: Pik Terskol, Caucasus Observer:T. Credner, K. Jockers

Dust Morphology

In 1996 the appearance of Comet Hale-Bopps shape was dominated by the so called 'porcupine' structure. These jetlike features in the dust tail remained very stable for months until about November 96 and then changed more and more to the shell like structures of spring 1997.

Possible Explanation

One scientific explanation of this 'porcupine' and the change to the shell structure is the following: On the cometary surface are active spots which shoot out collimated beams of gas and dust. Due to the rotation of the nucleus, the ejected material is distributed on a spiral that is lying on a cone surface in space. If we see the nucleus rotation axis edge on, we also see this cone edge on with brighter edges, that are our 'porcupine' spikes above.
In 1997 the perspective changed more to a view ontop of the rotation axis. So the cones vanished and due to the closer geocentric distance the spiral structure could be resolved.

R-scaling

R-scaling is a very useful and easy processing for cometary dust images to show the bright inner and the faint outer structures simultaneously. Each pixel intensity is multiplied with the radial distance r to the nucleus.
The idea is the following: If you have an idealized sphere of expanding dust, then the density and intensity drops with 1/r2. In the two-dimensional projection, as in images, this becomes a 1/r decrease of intensity. This can be used as the most simple model for the radial dependence of the dust distribution and removed by the R-scaling.
Some more informations can be found on the special project page of the Max-Planck-Institut für Aeronomie.

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Max-Planck-Institut für Aeronomie