ENA observations at Mars (2): Response of the subsolar ENA jet flux caused by an interplanetary shock

The neutral particle detector (NPD) of the ASPERA-3 experiment on board the Mars Express spacecraft provides a survey of the Martian Energetic Neutral Atom (ENA) environment for the first time. Futaana et al.(2005) confirmed that the subsolar region is a very intensive ENA production region, and the observed ENA fluxes sometimes exhibit very sharp boundaries in their outer edges. The signature implies that the sub-solar ENAs are emitted anisotropically, and then the ENA flux forms a jet-like (or cone-line) structure. Since the solar wind can penetrate deeply into the exosphere at the sub-solar region, the charge-exchange is expected to occur more frequently than at other places. The streamlines above the induced magnetosphere boundary (IMB) are highly deflected at the subsolar region and ENA produced in this region move along tangentially to them forming a jet / cone as observed. The sub-solar ENA jet can be seen in almost all orbits with suitable geometry. In one third of the observations, the sharp boundaries at the outer edge of the jet were observed. However, the data obtained on June 7, 2004 shows the sharpest boundary of the sub-solar ENA jet ever. The observed ENA jet flux was 3-4 times higher than the typical flux and abruptly decreases over a very short time of less than 10s when the spacecraft crossed jet's outer boundary. (Typically the spacecraft crossed the boundary approximately 1 min.) This unusual sharp decrease could be interpreted as a spacial feature related to the spacecraft crossing the jet boundary. However, the simultaneous in situ ion and electron observations show that the structure of the interplanetary medium suddenly changed at approximately the same time when the spacecraft crossed the ENA jet boundary. The change in interplanetary medium is most likely due to an interplanetary shock (IS) related to a co-rotational interaction region (CIR). Therefore, there is another interpretation that the sudden ENA jet decrease was caused by the interaction between Mars and the IS. When the IS pushed the IMB toward the planet, the emission region of ENA jet also moved toward Mars. In addition, the direction of the jet would be changed due to changes in the shape of the IMB. These global changes of the ENA jet geometry can result in the sudden change of the ENA jet at the spacecraft location.