On the exchange of kinetic and magnetic energy between clouds and the interstellar medium

We investigate, through two-dimensional MHD numerical simulations, the inte raction of a uniform magnetic field oblique to a moving interstellar cloud. In particular we explore the transformation of cloud kinetic energy into m agnetic energy as a result of field line stretching. Some previous simulati ons have emphasized the possible dynamical importance of a "magnetic shield " formed around clouds when the magnetic field is perpendicular to the clou d motion. It was not clear, however, how dependent those findings were on t he assumed field configuration and cloud properties. To expand our understa nding of this effect, we examine several new cases by varying the magnetic field orientation angle with respect to the cloud motion (theta), the cloud -background density contrast, and the cloud Mach number. We show that in tw o dimensions and with theta large enough, the magnetic field tension can be come dominant in the dynamics of the motion of high density contrast, low M ach number clouds. In such a case, a significant fraction of the cloud's ki netic energy can be transformed into magnetic energy with the magnetic pres sure at the cloud's nose exceeding the ram pressure of the impinging flow. We derive a characteristic timescale, tau(ma), for this process of energy " conversion." We find also that unless the cloud motion is highly aligned wi th the magnetic field, reconnection through tearing-mode instabilities in t he cloud wake limits the formation of a strong flux-rope feature following the cloud. Finally we attempt to interpret some observed properties of the magnetic field in view of our results.