Researchers headed by Prof. LIU Linzhong and MA Yuehua Ma, Purple Mountain Observatory, Chinese Academy of Sciences, consider the instability of the cometary plasma tail which is composed of a neutral sheet, two lobes of the ion tail and solar wind. The plasma is assumed to be highly conductive and incompressible. The unstable state yields a magnetic field which is perpendicular to the tail axis. Their result is consistent with findings about plasma from ICE.

Fig. 1 Sketch of parabolic cylindrical coordinates with c = 1000 km.

（image by SHI Jianchun）

They study the motion and stability of cometary plasma modeled by the MHD method and vector analysis in the differential form with exterior derivatives, finding that as the distance to the nucleus becomes longer the growth rate of instability becomes smaller. Therefore, the growth rate is very large near the head of the comet in the neutral sheet. Once the small disturbance occurs, the magnetic field of the disturbance is quickly amplified. If the amplified magnetic field can penetrate the magnetic pile-up region outside the neutral sheet, a magnetic field nearly perpendicular to the tail axis can be added to the steady magnetic field which is nearly parallel to the tail axis and makes the local magnetic field structure of the tail near the cometary head of the comet change. This may be one of the reasons that explains the formation mechanism of the tail rays. The work by LIU Linzhong , MA Yuehua (corresponding author), SHI Jianchun, accepted by Research in Astronomy and Astrophysics, has been published online ( http://iopscience.iop.org/1674-4527/11/5/011/ )