Dispersive Alfvén Wave Control of O⁺ Ion Outflow and Energy Densities in the Inner Magnetosphere

Abstract

The relationship between dispersive Alfvén waves (DAWs), magnetospheric activity, and O+ ion outflow/energy density is examined using measurements from the Van Allen Probes. We show that correlated DAW activity and O+ outflow/energization is a characteristic feature of the inner magnetosphere during active conditions and during storms persists for several hours over large L‐shell and azimuthal ranges of the plasma sheet. Though enhanced during substorm and storm active periods, these correlated features are most intense during geomagnetic storms. Comparisons show a linear relationship between DAW electric (and magnetic) field energy density and outflowing O+ energy. Statistical measurements from a large number of storms also reveal a linear relationship between DAW energy density and gross enhancements in energetic O+ energy densities. These observations support the notion that DAWs play an important role in the energization of O+ ions into and within the inner magnetosphere.

Correlations Between Dispersive Alfvén Wave Activity, Electron Energization, and Ion Outflow in the Inner Magnetosphere

“…This population is easily distinguished from the plasma sheet electrons, which are isotropic (green) near apogee or have preferential perpendicular anisotropy (blue to black) at lower L‐shell. The field‐aligned electron maximum energies in the anisotropy spectra also have variations that are correlated with the electric and magnetic field energy densities of intense long‐duration bursts of low‐frequency waves identified as DAWs in the study by Hull et al (2019). The electric and magnetic field energy densities are displayed in Figures 1f and 1g for SC‐A and Figures 1n and 1o for SC‐B.…”

“…Figure 1 shows plasma and field data from the Van Allen Probes (denoted SC-A and SC-B) during an intense storm that commenced on 28 June 2013. This event was reported in an earlier study, which was focused on relationships between DAWs and ion outflow and energy densities (Hull et al, 2019). Here, we examine spatial and temporal interconnections between DAW activity and electrons in the inner magnetosphere.…”

“…Correspondences between earthward Alfvénic Poynting fluxes and O + outflow deep in the inner magnetosphere have been observed (Gkioulidou et al, 2019). Correlations between DAW energy densities and outflowing O + energies and energy densities have also been reported (Hull et al, 2019). Using particle tracings, Chaston et al (2016) demonstrated that the action of nonadiabatic particle motion in the thin fields of DAW field line resonance structure can explain the O + outflow energies observed in the equatorial inner magnetosphere.…”

Filamentary Currents and Alfvénic Vortices in the Inner Magnetosphere

“…These counterstreaming ions follow the arrival of multi-keV electrons at the spacecraft and the onset of more intense wave activity. The observation of enhanced field-aligned electron fluxes and accelerated ionospheric O + ions are characteristic of particle acceleration in Alfvénic wavefields in the inner magnetosphere (Chaston, Bonnell, Wygant, Kletzing, Reeves, Gerrard, Lanzerotti, & Smith, 2015;Hull et al, 2019). This acceleration is a result of wave-particle interactions dependent on the details of the wavefield topology.…”

Episodic occurrence of field-aligned energetic ions on the dayside

“…In general, the energy of ionospheric outflow is only several eV near the topside ionosphere. In order to accelerate the ions to hundreds of eV or even into the keV range, two means of acceleration have been proposed: (1) parallel electric fields (Evans, 1974;Yau et al, 1985;Newell et al, 1996;Gkioulidou et al, 2019); (2) the dispersive Alfven wave (DAWs) (Chaston et al, 2004;2005;2007;Hull et al, 2019). Gkioulidou et al (2019) have observed field-aligned O + ions near the equator with energies below ~1 keV This article is protected by copyright.…”