Ion nose spectral structures observed by the Van Allen Probes

Abstract

We present a statistical study of nose‐like structures observed in energetic hydrogen, helium, and oxygen ions near the inner edge of the plasma sheet. Nose structures are spectral features named after the characteristic shapes of energy bands or gaps in the energy‐time spectrograms of in situ measured ion fluxes. Using 22 months of observations from the Helium Oxygen Proton Electron instrument onboard Van Allen Probe A, we determine the number of noses observed, and the minimum L shell reached and energy of each nose on each pass through the inner magnetosphere. We find that multiple noses occur more frequently in heavy ions than in H+ and are most often observed during quiet times. The heavy‐ion noses penetrate to lower L shells than H+ noses, and there is an energy‐magnetic local time (MLT) dependence in the nose locations and energies that is similar for all species. The observations are interpreted by using a steady state model of ion drift in the inner magnetosphere. The model is able to explain the energy and MLT dependence of the different types of nose structures. Different ion charge‐exchange lifetimes are the main cause for the deeper penetration of heavy‐ion noses. The species dependence and preferred geomagnetic conditions of multiple‐nose events indicate that they must be on long drift paths, leading to strong charge‐exchange effects. The results provide important insight into the spatial distribution, species dependence, and geomagnetic conditions under which nose structures occur.

Temporal Evolution of Ion Spectral Structures During a Geomagnetic Storm: Observations and Modeling

“…Another feature is that the inner edge of the plasma sheet is more clearly species dependent during quiet times. The observed deeper access of the heavy ion structures (on average more than 1 L value deeper access than the H + structures), which is expected from the effects of charge exchange on slowly drifting ions, is consistent with previous results (Ferradas et al, , ; Kistler & Mouikis, ; Lennartsson, ). On the other hand, since ions drift much faster during active geomagnetic times, appreciable charge exchange effects do not have enough time to take place and different species have a similar range of access.…”

“…The observation of a main nose structure reaching the deepest L values at ~10 keV for all species is a persistent feature at these L values, as confirmed by inspecting previous and latter Probe A orbits. This result confirms the findings from statistical analyses made with Cluster (Ferradas et al, ) and Van Allen Probes (Ferradas et al, ) data of a preferred ion access at these energies during quiet times. Another feature is that the inner edge of the plasma sheet is more clearly species dependent during quiet times.…”

Cross‐scale observations of the 2015 St. Patrick's day storm: THEMIS, Van Allen Probes, and TWINS

“…This type of narrow ring current peak, known as a nose structure, was first identified in Explorer 45 data [Smith and Hoffman, 1974;Ejiri et al, 1980] and is a familiar feature in data from both Van Allen Probes and Cluster [Ferradas et al, 2014[Ferradas et al, , 2015[Ferradas et al, , 2016Burke et al, 2016]. The stable, two-peak spectrum observed by both Van Allen Probes on 17 March, the so-called double nose, has also been previously observed [e.g., in Cluster data by Vallat et al, 2007].…”

Ring/Shell Ion Distributions at Geosynchronous Orbit

“…The occurrence maximizes in the prenoon to postdusk local times and at lower values of Kp . Van Allen Probe observations also show that there are more nose events during quieter magnetospheric conditions than at more active times (Ferradas et al, ). At low Kp , ~20–30% of geosynchronous ion distributions between noon and 18 LT have a logarithmic slope ( f p −1 ·∂f p ( v ⊥ )/ ∂v ⊥ ) greater than +0.3 at 8 keV (Thomsen et al, ).…”

“…As noted by previous authors, there are two plausible mechanisms for forming such velocity distributions: The first arises from discrete nightside injections of plasma sheet ions during substorms; because the ions with higher energies have larger gradient‐ and curvature‐drift speeds, higher‐energy particles outrun their lower energy counterparts as they drift around the inner magnetosphere, leading to an energy‐dispersed enhancement in the dusk/afternoon sector. The second mechanism likewise involves energy‐dependent drifts, which cause newly arriving plasma sheet ions to take energy‐dependent paths through the inner magnetosphere; for a certain energy range, the drift trajectories penetrate deep into the inner magnetosphere, forming so‐called “nose structures” in the ion distribution (e.g., see recent work by Ferradas et al, , and references therein). Moreover, ions passing near the Earth encounter a region where the neutral exospheric density is higher, and thus, the losses to charge exchange are greatly enhanced.…”