Abstract
Magnetic field dipolarizations are often observed in the magnetotail during substorms. These generally include three temporal scales: (1) actual dipolarization when the normal magnetic field changes during several minutes from minimum to maximum level; (2) sharp Bz bursts (pulses) interpreted as the passage of multiple dipolarization fronts with characteristic time scales < 1 min, and (3) bursts of electric and magnetic fluctuations with frequencies up to electron gyrofrequency occurring at the smallest time scales (≤ 1 s). We present a numerical model where the contributions of the above processes (1)-(3) in particle acceleration are analyzed. It is shown that these processes have a resonant character at different temporal scales. While O+ ions are more likely accelerated due to the mechanism (1), H+ ions (and to some extent electrons) are effectively accelerated due to the second mechanism. High-frequency electric and magnetic fluctuations accompanying magnetic dipolarization as in (3) are also found to efficiently accelerate electrons.
Highlights
Later phases of substorms are characterized by abrupt changes of magnetotail magnetic field and earthward propagation of Dipolarization Fronts (DF) [Runov et al, 2009, 2011, 2012; Yao et al, 2016] that are step-like structures with sharp enhancements of the normal magnetic field Bz [e.g., Nakamura et al, 2002; Runov et al, 2009; Sergeev et al, 2009]
While protons and oxygen ions are essentially affected by the induced electric field in the course of DFs, the most prominent electron acceleration occurs after DFs in conjunction with the electric and magnetic fluctuations
We focused on the effectiveness of particle energization during dipolarization events that have a complex multiscale character, being composed of successive dipolarization fronts followed by pile-up and consequent excitation of electric and magnetic fluctuations
Summary
A number of observational and theoretical investigations have revealed the importance of accelerated particle flows in the Earth’s magnetotail [Sharma et al, 2008; Retino et al, 2008; Yamada et al, 2010; Fu et al, 2011; Runov et al, 2011a; Birn et al, 2012, 2013; Ashour-Abdalla et al, 2015; Grigorenko et al, 2015], that point to specific regions for energy conversion during substorms [Zelenyi et al, 2008; Zelenyi et al, 2011 and references therein; Angelopoulos et al, 2013]. Several mechanisms have been proposed to explain the generation of DFs: (1) BBF-type flux ropes [Slavin et al, 2003], (2) nightside flux transfer events [Sergeev et al, 1992], (3) generation of burstlike magnetic structures by impulsive magnetic reconnection in the magnetotail [Heyn and Semenov, 1996; Semenov et al, 2005; Longcope and Priest, 2007, Sitnov et al, 2009; Sitnov and Swisdak, 2011] Evidences of such DFs have been found in a variety of spacecraft observations [e.g., Nakamura et al, 2002; Sharma et al, 2008; Runov et al, 2009] and their relation with reconnection processes is clearly established. Acceleration of charged particles as a result of magnetic reconnection is considered as one of the most effective mechanisms to accelerate particles to high energies [Yamada et al, 2010]
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