One way to obtain information about the state of astrophysical plasma is by studying the radiation they emit and that we remotely observe. Therefore, it is of essential importance to understand the mechanisms that contribute to the generation of radiation. Due to large-scale shear movements, shock waves, jets, etc., astrophysical plasma (Hajivassiliou, 1992; Arzoumanian et al., 2011; Brandenburg and Nordlund, 2011;) and near-Earth plasma (Saur, 2004; Uritsky et al., 2011; Alexandrova et al., 2013; Bruno and Carbone, 2013; Von Papen et al., 2014) enter a turbulent state. At the same time, remote observations of turbulence and the evolution of plasma energy in astrophysical plasma can provide only integrated results, and are mostly considered only within the framework of one or another model. In this regard, the near-Earth environment is a preferred laboratory for analysing turbulent processes, since we can use in situ observations to obtain information about the dynamical state of the plasma and the variation of the characteristic plasma parameters. In turbulent regions, energy enters the system on large scales, and then it is transferred to smaller scales through nonlinear interactions (turbulent energy cascade). Since energy dissipation becomes significant on small scales, where the scale of turbulent fluctuations becomes comparable to the particle gyroradius, the nature of turbulent processes and the scale of their changes depend significantly on the content of heavy ions (Kozak et al., 2018). In turn, the dissipation of turbulent fluctuations leads to plasma heating and acceleration of charged particles.

The question of how heating occurs due to turbulent motions remains open. Another important question is whether the heating will be associated only with localized structures, or is distributed more evenly throughout the volume. Both possibilities are related to the redistribution of particles by energies through a sequence of both linear and non-linear processes (Servidio et al., 2009; Chandran et al., 2010; Camporeale and Burgess, 2011; Servidio et al., 2012; Karimabadi et al., 2013; TenBarge and Howes, 2013). At the same time, localized dissipation is associated with the phenomenon of “intermittency”, which is an uneven distribution of energy within a turbulent environment.

Observations of solar wind plasma show that heavier ions (in particular, alpha particles) provide a much more efficient heating than protons (Kasper et al., 2008). Dissipation of kinetic Alfvén waves (KAW) by stochastic heating is also more efficient for heavier ions, indicating a privileged channel for alpha particle heating and energy dissipation in the solar wind (Chandran et al., 2013). Turbulent heating and acceleration of alpha particles due to the dissipation of cyclotron waves are considered in the study by (Maneva et al., 2013). Such waves can be effective for feeding heavier ions and for generating complex velocity distribution functions and temperature anisotropy (Telloni et al., 2007; Grigorenko et al., 2015; Malykhin et al., 2018; Malykhin et al., 2019). Wave damping also plays an important role in heating and accelerating heavier ions in shock wave regions (Kronberg et al., 2009).

Both the solar wind and ionosphere can serve as sources for ions and electrons in the magnetosphere (Delzanno et al., 2021). The source of ions can be determined based on the property that ionospheric particles are singly ionized, while solar wind ions are almost completely ionized. The number of ions entering the magnetosphere will depend on the level of geomagnetic activity. It can be determined by a significant number of factors, including solar wind ion heating at the front of the shock wave, the modulation in the region of magnetic field reconnection, etc.

The presence of heavy ions in the magnetosphere will significantly change the physics of the processes taking place there. Since the main characteristics of the plasma (density, temperature, pressure, Alfvén velocity, thickness of the current/plasma layer) are changing, the conditions and rate of development of instabilities (in particular the Kelvin-Helmholtz instability and reconnection) will also change.

According to our current understanding, the main factors affecting the dynamics of electron and ion fluxes in the Earth’s magnetosphere include the EUV radiation from the Sun, reconnection events in the magnetopause region, particle acceleration, etc. (Kronberg et al., 2014). As a result of such dynamical events, charge particles may be subject to prominent energization up to the hundreds of keV range (e.g., (Ipavich et al., 1984; Keika et al., 2011)). Repeated enhancements of the energetic O+ content in conjunction with repeated auroral intensifications were observed by Mitchell et al. (2003). Evidence of a mass-dependent energization in the ∼ 10–210 keV/e range during substorms in the near-Earth plasma sheet was also presented by Möbius et al. (1987) and Kistler et al. (1990), Retinò et al. (2007), Chasapis et al. (2015), Malykhin et al. (2018), Malykhin et al. (2019).

The energy of charged particles in the Earth’s magnetosphere changes significantly with a sudden change in the reconfiguration of the magnetic field from a stretched to more dipole configuration (dipolarization). Observations from space vehicles showed that the rapid changes of B _z component of the magnetic field represent the spatial structures, i.e., the fronts of dipolarization (DF) moving towards the Earth (Nakamura et al., 2002; Runov et al., 2011). The DFs are often associated with bursty bulk flows (BBFs) (Angelopoulos et al., 1992). These flows can be responsible for energy and mass transfer from far regions of the magnetospheric tail to distances of about 10 Earth’s radii, where the flows are decelerated (Shiokawa et al., 1997). The dipolarization fronts can be detached as (i) isolated (observed over a few minutes) (Nakamura et al., 2002; Runov et al., 2009; Runov et al., 2011) and (ii) “secondary” (with duration up to hours), which are associated with the system of currents (wedge) during substorm generation (Sergeev et al., 2012). The properties and behaviour of various ion fluxes during magnetic field dipolarization, which are characterized by turbulence and significant variability of plasma parameters, were considered in the studies by Kozak et al. (2018b), Kozak et al. (2018a), Kozak et al. (2018), Kozak et al. (2020), Kozak et al. (2021).

The results presented in the studies by Kozak et al. (2018) showed that the relative content of heavy ions in the observed regions of magnetic field dipolarization may exert significant influence upon the scale of the transition from the magnetohydrodynamic to the kinetic approach in the analysis of turbulent processes.

The aim of this study is to use measurements of the magnetic field and particle fluxes in various energy channels to estimate the time dependence of these values before and after the start of dipolarization; to find out the energization evolution of protons and heavier ions.

In the first part of our study, we investigate the change in fluxes of charged particles and magnetic field changes for a single event. Later we will provide a statistical analysis of the change in particle fluxes in the region of magnetic field dipolarization observed during 2001–2015 using the superimposed epoch approach.

For our analysis, we used the magnetic field measurements provided by fluxgate magnetometers (FGM) on board four Clusters spacecraft (SC) with resolution 22.4 Hz (Balogh et al., 2001), and fluxes measurements by RAPID (Research with Adaptive Particle Imaging Detectors) spectrometers in the energy range up to 1,500 keV for protons with the resolution 0.25 Hz, and up to 4,000 keV for heavier ions with 0.067 Hz (Wilken et al., 2001).

Unless otherwise noted, we used GSM (geocentric solar magnetospheric) coordinate system everywhere.

Localization of the source of injection can be established by the signal delay between spacecraft. Accordingly, if the flow reinforcement is observed simultaneously in an extensive energy range means that the space vehicle is close to the injection source (non-dispersion injection), while if the satellite is far from the source the delay in the flow change is observed, that is first the more energetic particles come in, then low-energetic ones (also called dispersion injection) (for example (Zaharia et al., 2000; Kronberg et al., 2017; Malykhin et al., 2018;)).

All analysed events are characterized by multiple dipolarization and correspond to criteria (Borovsky et al., 1993):

• sharp fluctuations B _z components of the magnetic field ( ≥ 4 nT);

The beginning of the first dipolarization front for the considered events is shown in Supplementary Appendix Table S1. Initialization of the substorm and dipolarization of the magnetic field for all events takes place in a spatial range −10 > X _GSE > − 17R _E , −5 < Y _GSE < 9R _E and −1 < Z _GSE < 4R _E . As a typical example of ion flux changes in magnetotail regions, we consider in detail the event on 11 September 2014.

In our work, we have applied a set of techniques to Cluster-II magnetic field and particle measurements to estimate the energization time dependence for protons, and heavier ions near the turbulent region of dipolarization (before the onset, and after).

The main results are:

1. After onset, wave activity remains constant for approximately 1 minute, after that gradually decreases.

Recorded flux increase before arriving dipolarization front may result from the reflection of plasma sheet ions at the dipolarization front (Zhou et al., 2011).

When analysing individual events, it was found that changes in electron fluxes inside the plasma sheet are determined by the Bz-component behaviour and indicate the adiabatic acceleration of electrons. This is consistent with results by Malykhin et al. (2018).

The increase in the fluxes of high-energy protons is consistent with the results of numerical modeling conducted by using the Rice Convection Model with an equilibrated magnetic field model for earthward BBFs in the work Yang et al. (2011). In this case, the injection boundary is well coincident with the earthward boundary of the bubble, inside which the depletion of plasma content causes the magnetic field dipolarization, and in return, the magnetic field collapse energizes particles and alters the drift paths dramatically. In turn, according to the works of Khotyaintsev et al. (2011); Grigorenko et al. (2016), Grigorenko et al. (2017) the energy transported by BBFs results in both adiabatic and non-adiabatic particle acceleration.

Superposed analysis for ion fluxes showed that changes in different energy channels differ: larger values of changes are observed for larger energy channels. Moreover, the increase in ion flows is accompanied by a spectral index decrease. This is consistent with the results obtained in the work of Malykhin et al. (2019). A decrease in γ may indicate the presence of non-adiabatic acceleration of ion population during dipolarization in 2D consideration (Pan et al., 2012). Heavy ions have efficient stochastic energization in turbulent electromagnetic fields (Zelenyi et al., 2008) and by inductive electric fields occurring in the dipolarization region. They are less affected by 3D magnetic structures and penetrate more easily into regions with vanishing magnetic fields (in the vicinity of the neutral sheet) being efficiently accelerated (Birn et al., 2012; Kronberg et al., 2014).

In (Birn et al., 2015) paper, the investigation of the ion acceleration in dipolarization events in the magnetotail, using the electromagnetic fields of an MHD simulation of magnetotail reconnection demonstrated that the ion acceleration stems from a net Lorentz force, resulting from reduced pressure gradients within a pressure pile-up region ahead of the front. Suprathermal precursor ions result from, typically multiple reflections at the front. Low-energy ions also become accelerated due to inertial drift in the direction of the small precursor electric field. An inspection of characteristic proton orbits indicated a nonadiabatic acceleration in the vicinity of the reconnection site, as well as quasi-adiabatic, betatron-like, acceleration during earthward motion in the collapsing, dipolarized field (Pan et al., 2014). For a correct description of turbulent regions of the order of proton gyroradius, it is necessary to use the self-consistent Maxwell-Vlasov equations for fields, currents, and distribution functions of charged particles.