Magnetospheric response to the interplanetary shocks and pressure increases: Cluster observations of sudden impulses in the tail lobes

The sudden impulses (SI) in the tail lobe magnetic field due to interplanetary shocks/pressure increases are examined using Cluster observations for the July-November periods in 2001 and 2002. The SI events were investigated with the minimum variance analysis to study the magnetic field changes in detail. The multispacecraft feature of Cluster allows us to calculate the propagation speed for the disturbance between the spacecraft. Furthermore Cluster plasma data provides us details of the tail compression, e.g. showing the motion of the magnetopause towards Cluster. The SI events in this study were associated with strong interplanetary shocks or substantial enhancements in the solar wind dynamic pressure and clear (> 10 nT) increases in the lobe magnetic field magnitude. The good upstream solar wind coverage (ACE, WIND, Geotail) allowed us to calculate the shock normal orientations, propagation speeds and to perform reliable timing analysis between upstream observations and tail lobe SIs. Also a two-dimensional Cartesian model, (where a vacuum uniform tail lobe magnetic field is compressed by a step-like pressure increase) was applied to the SI events. Examples of the lobe SI events will be presented. The characteristics of the rotation in the magnetic field components and the results of the timing analysis and modeling strongly suggest that the tail lobe SIs are due to the increase in solar wind dynamic pressure outside the tail boundary. The magnetotail is found to remain in constant equilibrium as the pressure enhancement convects down the tail.