An interpretation of density holes observed by Cluster and Double Star in solar wind

To interpret density holes in the solar wind, which are nonlinear structures observed by Cluster and Double Star, we propose an electrostatic ion fluid model. We derive the Sagdeev potential from the magnetohydrodynamic (MHD) equations and study the characteristics of nonlinear structures in our model. The results show that density depletions (or holes) can develop from linear ion acoustic waves or ion cyclotron waves in space plasmas when parameters such as Mach number, initial electric field and ratio of ion to electron temperature satisfy certain conditions. In our model, the relative density depletion (or density holes) is from 0 to 1, and the time duration of density holes is from 2 s to more than 98 s. These are in good agreement with the observations by Cluster and Double Star in the solar wind. Our model also shows that the density holes should be accompanied by bipolar electric field solitary structures, which have also been observed by Cluster in the solar wind.