Episodic occurrence of field-aligned energetic ions on the dayside

Abstract

<p>The tens of keV ion populations observed in the ring current region at L~ 3- 7, generally have pancake-shaped pitch angle distributions (PADs), that is, peaked at 90 degrees. These pancake PADs are formed due to a combination of betatron and Fermi acceleration when they are transported from the tail plasma sheet, where the major ring current plasma originates. However, in this study, by using the Van Allen Probe observations from 2012 to 2018 on the dayside, unexpectedly we have found that about 5% of the time, protons with energies of ~30 to 50 keV show two distinct populations according to their PADs, having an additional population of field-aligned ions overlapping with the original pancake population. The newly appearing field-aligned populations have higher occurrence rates at ~12-16 MLT during geomagnetically active times. In particular, we have studied eight such events in detail and traced back these ions to their source regions according to the energy-dependent dispersion signatures caused by the differences in drift velocities. We found that the source regions are located around 12 to 18 MLT which coincides with the high occurrence rate region of 12-16 MLT. Based on the ionospheric and LANL geosynchronous observations of these eight events, it is suggested that these energetic ions with field-aligned PADs most probably are accelerated in the post-noon sector in association with ionospheric disturbances that are triggered by tail injection. These results provide evidence of another important source of the ring current ions.</p>

Simultaneously Formed Wedge‐Like Structures of Different Ion Species Deep in the Inner Magnetosphere

“…The ionospheric outflows typically in the field‐aligned feature can form energy‐time or energy‐L shell dispersions due to the time‐of‐flight effect (Quinn & McIlwain, 1979) or the velocity filter effect (Winningham et al, 1984). However, although outflows can be energized up to tens of keV in the inner magnetosphere (Chaston et al, 2015; Yue, Bortnik, Zou, et al, 2019), both effects are not available in the explanation of the wedge‐like structure in this study because Van Allen Probe observations in Figure 4 show that the wedge‐like structures consist of the trapped ions rather than outflows.…”

Ring Current Decay During Geomagnetic Storm Recovery Phase: Comparison Between RBSP Observations and Theoretical Modeling

“…A good numerical agreement is shown: the difference between τ ob and τ tp is generally within an order of magnitude. Comparing τ ob and τ tp of H + in Figures 4a and 4c, τ ob is about several times larger than τ tp from 1 keV to ∼50 keV at L < ∼4, which could be explained by the ion source of accelerated protons from the ionospheric outflow (e.g., Daglis et al., 1999; Yue et al., 2020) or the underestimation of the τ tp of H + . In addition, the τ ob of H + is smaller for energies larger than 50 keV at high L shells compared with the model results, suggesting additional loss processes, such as Coulomb collisions, wave (e.g., electromagnetic ion cyclotron waves) particle interaction loss, as well as outward radial diffusion, may play roles.…”

“…Yue et al. (2020) have observed the ionospheric‐originated protons of ∼30–50 keV with field‐aligned pitch angle distributions (i.e., peaked at ∼0° and ∼180°) in ring current, which is likely due to DAWs. However, it is hard to assess the effect of protons ionospheric outflow on the ring current lifetime estimation.…”

“…Regarding to the differences between τ ob and τ tp of H + from 1 keV to ∼50 keV at L < ∼4, one probable reason is the outflow of energized H + from ionosphere. Generally, the energy of protons ionospheric outflow is only several eV, but they can be accelerated to keVs or even tens of keVs by parallel electric fields (e.g., Evans, 1974; Yau et al., 1985; Newell et al., 1996; Gkioulidou et al., 2019) and/or the dispersive Alfven wave (DAWs) (e.g., Chaston et al., 2005, 2007, 2004; Hull et al., 2019; Yue et al., 2020). Coincidently, Chaston et al.…”

Sustained Oxygen Spectral Gaps and Their Dynamic Evolution in the Inner Magnetosphere

“…The inner magnetosphere is a highly dynamic region where multiple particle populations, including the cold plasmasphere, energetic ring current, and relativistic radiation belt particles coexist, overlap and often interact with each other (e.g., Yue, Bortnik, Chen, et al., 2017; Yue, Bortnik, Throne, et al., 2017). These particles originate from the tail plasma sheet and/or the ionosphere (e.g., Chappell, Huddleston, et al., 2008; Gabrielse et al., 2014; Huddleston et al., 2005; Yue, Bortnik, Li, Ma, Gkioulidou, et al., 2018; Yue, Bortnik, Zou, et al., 2020). During their transport into the inner magnetosphere, these particles can be energized through adiabatic transport associated with large‐scale convection and/or mesoscale impulsive dipolarizations, as well as localized nonadiabatic acceleration (e.g., Birn et al., 2017; Gabrielse et al., 2012; Keika et al., 2013; Yue, Li, et al., 2016; Zhou et al., 2010 and references therein).…”