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Abstract
Abstract. In this work we present an analysis of the dynamics of suprathermal ions of different masses (H+, He+, O+) during prolonged dipolarizations in the near-Earth magnetotail (X>-17RE) according to Cluster/RAPID observations in 2001–2005. All dipolarizations from our database were associated with fast flow braking and consisted of multiple dipolarization fronts (DFs). We found statistically that fluxes of suprathermal ions started to increase ∼1 min before the dipolarization onset and continued to grow for ∼1 min after the onset. The start of flux growth coincided with the beginning of a decrease in the spectral index γ. The decrease in γ was observed for protons for ∼1 min after the dipolarization onset, and for He+ and O+ ions for ∼3 and ∼5 min after the onset respectively. The negative variations of γ for O+ ions were ∼2.5 times larger than for light ions. This demonstrates more efficient acceleration for heavy ions. The strong negative variations of γ were observed in finite energy ranges for all ion components. This indicates the possibility of nonadiabatic resonant acceleration of ions in the course of their interaction with multiple DFs during dipolarizations. Our analysis showed that some fraction of light ions can be accelerated up to energies ≥600 keV and some fraction of oxygen ions can be accelerated up to ∼1.2 MeV. Such strong energy gains cannot be explained by acceleration at a single propagating DF and suggest the possibility of multistage ion acceleration in the course of their interaction with multiple DFs during the prolonged dipolarizations.
Highlights
One of the important processes in the dynamics of the Earth’s magnetotail is magnetic dipolarization
The origin of “secondary” dipolarizations is still debated. They can be the consequence of a magnetic flux pileup due to the arrival of multiple bursty bulk flows (BBFs) (e.g., Liu et al, 2013, 2014) or they can be caused by the development of cross-tail current instability in the near-Earth CS (e.g., Lui, 2011)
In this paper we studied the dynamics of fluxes and spectra of suprathermal ions of different masses during dipolarizations in the near-Earth geomagnetic tail (X > −17RE) according to Cluster/Research with Adaptive Particle Imaging Detectors (RAPID) observations made in 2001–2005
Summary
One of the important processes in the dynamics of the Earth’s magnetotail is magnetic dipolarization. During the prolonged “secondary” dipolarizations the increases in fluxes of suprathermal electrons, protons and heavy ions are often observed (Nosé et al, 2000; Apatenkov et al, 2007; Asano et al, 2010; Grigorenko et al, 2017; Malykhin et al, 2018a, b). Malykhin et al (2018b) studied the dynamics of fluxes and energy spectra of suprathermal electrons during the prolonged dipolarizations and showed that electrons can be accelerated by a betatron mechanism up to ∼ 90 keV. We study the dynamics and acceleration of ions of different masses (H+, He+, O+) to suprathermal energies in the multiscale magnetic structure of dipolarizations in the near-Earth magnetotail by using Cluster/RAPID observations (Wilken et al, 2001).
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