ACCELERATION OF COMETARY H2O GROUP PICKUP IONS BY OBLIQUELY PROPAGATING NONLINEAR MAGNETOSONIC WAVES

The observations made during the encounter with comet Giacobini Zinner show that the character of MHD turbulence is governed by the magnetos onic (MS) waves generated by the pickup ions via a resonant cyclotron instability. Close to the bow shock these waves are highly nonlinear, \DELTAB/B0\ approximately 1, and are propagating obliquely to the magn etic field. The interaction of cometary ions in the mass loaded solar wind with MS waves propagating away from the comet and oblique to the interplanetary magnetic field (IMF) is investigated using the test par ticle approach. Ion trajectories, distribution functions, widths of pi tch angle scattering and energy diffusion are obtained. Because of the MS 'turbulence'', the particle velocity and acceleration are found to increase with increasing wave amplitude, inclination of the wave vect or to the background magnetic field, and the range of resonant mode nu mbers. It is found that the interaction of water group pickup ions wit h MS waves propagating obliquely to the IMF gives larger pitch angle s cattering and acceleration than that in the can of parallel and antipa rallel propagating waves. In particular, obliquely propagating MS wave s at angles greater than 40' to the ambient magnetic field are very ef fective at accelerating particles because of a high phase velocity alo ng the magnetic field. In the case of monochromatic MS waves, Landau d amping is found to play an important role; the particles get electroma gnetically trapped in a potential well due to Landau damping. In the c ase of MS turbulence, the particles are stochastically heated and the temperature continues to grow linearly with time. We have also investi gated the relationship between pitch angle scattering and the three pa rameters, namely, a, the angle between the solar wind flow direction a nd the ambient magnetic field, theta(Bk), the angle between the ambien t magnetic field and the wave propagation vector, and the ion injectio n velocity. The pitch angle scattering rates are obtained using a mono chrometic nonlinear MS wave as well as MS turbulence in both the quasi -parallel (0 < alpha < 60-degrees) and quasi-perpendicular (60-degrees < alpha < 90-degrees) regimes for various values of theta(Bk). In the case of a monochromatic MS wave, pitch angle scattering rates are fou nd to change very slightly with respect to theta(Bk), a and the mode n umber of the MS wave due to their Landau damping. In the case of MHD t urbulence the pitch angle scattering rates decrease with increasing a for theta(Bk) < 40-degrees and increase with increasing a for theta(Bk ) > 40-degrees. They are found to be independent of the injection velo city. The results are in agreement with the observations.