Cavitons And Ultra Low Frequency Waves In the Earth’s Foreshock

Physics – Plasma Physics

Scientific paper

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[2154] Interplanetary Physics / Planetary Bow Shocks, [2159] Interplanetary Physics / Plasma Waves And Turbulence, [2784] Magnetospheric Physics / Solar Wind/Magnetosphere Interactions, [7829] Space Plasma Physics / Kinetic Waves And Instabilities

Scientific paper

In this work we study the properties of foreshock cavitons. The cavitons were first discovered in global hybrid simulations and were later found in data of Cluster I spacecraft for a variety of cone angles. They appear as regions in the foreshock characterized by diminished values of the magnitude of magnetic field and plasma density (up to 90 % of B0 and n0 ), surrounded by enhanced values of these two quantities. Also, the total plasma temperature has been found to rise substantially inside them. The cavitons form in an extended region inside the foreshock and are convected by the solar wind towards the Earth’s bow shock. What makes cavitons different from other similar phenomena (cavities, holes) found in the solar wind is the fact that they are always immersed in a sea of ultra low frequency (ULF) waves. The simulations suggest that the cavitons are formed due to the nonlinear interactions between two types of ULF waves: the parallel propagating sinusoidal waves and obliquely propagating compressive waves. In this work we study the ULF fluctuations that are found in the immediate neighborhood of cavitons. We analyze their properties, such as compressibility, polarization, periods and amplitudes. We find several types of ULF waves in the data. The fluctuations with largest periods (10 - 100 s) have large amplitudes (|δB|/B ≈1) and can be right- or left hand polarized. These fluctuations come in different forms - they can appear as slightly compressive waves or as schocklets. Fluctuations with higher frequencies include discrete wave packets of whistler waves that can appear just next to or even inside the cavitons and have frequencies between 0.3 - 1 Hz.

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