Abstract
The ion foreshock region is permanently filled by ultra-low frequency (ULF) waves and its spatial extent depends on the interplanetary magnetic field (IMF) orientation. When the IMF is nearly parallel to the Earth-Sun line, the foreshock occupies a large volume in front of the bow shock.
The waves are generated through interactions between the back-streaming particles reflected from the bow shock and the incoming solar wind. It is supposed that the waves propagate sunward but they are convected by the solar wind flow back toward the bow shock and their amplitudes grow.
However, our study shows that waves could be growing as well as decaying towards the bow shock under the quasi radial IMF. Nevertheless, a large statistics based on nine years of two point foreshock observations reveals their positive growth rate in a statistical sense.
We demonstrate that the growth rate is larger for compressive variations of the magnetic field strength and density than for components of the magnetic field. We have found that the growth rate depends on the location of observing spacecraft with respect to the Moon; if the spacecraft is close to the lunar surface, the growth rate is close to zero or even negative.
We discuss the role and importance of different spacecraft-Moon geometries and a possible mechanisms that can lead to the observed results.