A STUDY OF ULF WAVE FORESHOCK MORPHOLOGY .2. SPATIAL VARIATION OF ULF WAVES

Two case studies are used to show the evolution of upstream ULF waves in the Earth's foreshock region. During these two intervals the IMF cone angle was nearly constant over an extended time period and the ISEE spacecraft was in the foreshock region and traveled a large distance through it. Since the solar wind conditions were nearly constant, the change of wave properties in these cases is primarily related to the geometry in the foreshock, e.g. the different depth from the foreshock boundary and different distance from the bow shock. The observations from ISEE magnetic field data show that ULF waves become stronger, more compressional, and more linearly polarized as the spacecraft travels further down-stream from the ULF foreshock boundary and closer to the bow shock. The peak of the ULF wave power spectrum becomes broader. Associated with the increasing intensity of the lower frequency waves, or shocklets, the discrete wave packets become more intense and develop more cycles as the spacecraft gets deeper into the foreshock and closer to the bow shock. We can also estimate the growth rate of discrete wave packets for one of the cases.