Abstract
Abstract. In this work we perform a statistical analysis of 92 foreshock cavitons observed with the Cluster spacecraft 1 during the period 2001–2006. We analyze time intervals during which the spacecraft was located in the Earth's foreshock with durations longer than 10 min. Together these amount to ~ 50 days. The cavitons are transient structures in the Earth's foreshock. Their main signatures in the data include simultaneous depletions of the magnetic field intensity and plasma density, which are surrounded by a rim of enhanced values of these two quantities. Cavitons form due to nonlinear interaction of transverse and compressive ultra-low frequency (ULF) waves and are therefore always surrounded by intense compressive ULF fluctuations. They are carried by the solar wind towards the bow shock. This work represents the first systematic study of a large sample of foreshock cavitons. We find that cavitons appear for a wide range of solar wind and interplanetary magnetic field conditions and are therefore a common feature upstream of Earth's quasi-parallel bow shock with an average occurrence rate of ~ 2 events per day. We also discuss their observational properties in the context of other known upstream phenomena and show that the cavitons are a distinct structure in the foreshock.
Highlights
The solar wind (SW) is a magnetized plasma that flows away from the Sun at supersonic speeds
In order for an event to be identified as a caviton, several criteria had to be satisfied: depletions of B and n inside the cavitons had to be deeper than those caused by the surrounding ultra-low frequency (ULF) waves; magnetic field and density during the caviton observations must be highly correlated
We study the foreshock cavitons observed by the Cluster 1 spacecraft during the years 2001–2006
Summary
The solar wind (SW) is a magnetized plasma that flows away from the Sun at supersonic speeds. In addition to ions and waves, there are transient phenomena that populate the Earth’s foreshock Phenomena such as hot flow anomalies (HFAs) (Thomsen et al, 1986; Schwartz et al, 1995; Lucek et al, 2004; Zhang et al, 2010), density holes (Parks et al, 2006, 2008; Wilber et al, 2008) and foreshock cavities (Sibeck et al, 2001, 2002, 2008; Billingham et al, 2008; Schwartz et al, 2006) have been studied extensively in the past. Past numerical simulations predicted B field magnitude and density drops of ∼ 50 % inside the cavitons, with respect to the ambient values These drops would be larger for cavitons closer to the bow shock. We demonstrate that B field magnitude and SW density inside the cavitons are highly correlated and we design a new criteria which enables us to distinguish the cavitons from the surrounding ULF fluctuations
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