Abstract
Context. Interstellar and inner-source pickup ions (PUIs) are produced by the ionization of neutral atoms that originate either outside or inside the heliosphere. Just after ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic toroidal features in their velocity distribution functions (VDF). As the plasma parcel moves outwards with the solar wind, the pickup ion VDF gets more and more affected by resonant wave-particle interactions, changing heliospheric conditions, and plasma drifts, which lead to a gradual isotropization of the pickup ion VDF. Past investigations of the pickup ion torus distribution were limited to He+ pickup ions at 1 astronomical unit (AU).Aims. The aim of this study is to quantify the state of anisotropy of the He+ , C+ , N+ , O+ , and Ne+ pickup ion VDF at 1 AU. Changes between the state of anisotropy between PUIs of different mass-per-charges can be used to estimate the significance of resonant wave-particle interactions for the isotropization of their VDF, and to investigate the numerous simplifications that are generally made for the description of the phase-space transport of PUIs.Methods. Pulse height analysis data by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on board the Solar Terrestrial RElations Observatory Ahead (STEREO A) is used to obtain velocity-spectra of He+ , C+ , N+ , O+ , and Ne+ relative to the solar wind, f (w sw ). The w sw -spectra are sorted by two different configurations of the local magnetic field – one in which the torus distribution lies within the instrument’s aperture, φ ⊥ , and one in which the torus distribution lies exclusively outside the instrument’s field of view, φ ∥ . The ratio of the PUI spectra between φ ⊥ and φ ∥ is used to determine the degree of anisotropy of the PUI VDF.Results. The data shows that the formation of a torus distribution at 1 AU is significantly more prominent for O+ (and N+ ) than for He+ (and Ne+ ). This cannot be explained by resonant wave-particle interactions as the sole mechanism for the isotropization of the PUI VDF. The anisotropy of the O+ VDF compared to He+ is highly fluctuating but consistently higher over an observation period of six years and therefore unlikely to be related to either specific heliospheric conditions or solar activity variations. To our surprise, we also found a clear signature of a C+ torus distribution at 1 AU very similar to the one of He+ , although as an inner-source PUI, C+ should have a considerably different spectral and spatial injection pattern than interstellar PUIs.
Highlights
Because the pickup ion torus distribution changes its inclination as a function of the local magnetic field direction (Fig. 1), PLAsma and SupraThermal Ion Composition instrument (PLASTIC) is only capable of observing the pickup ion torus for magnetic configurations in which the direction of the local magnetic field vector, defined by φB and θB, points away from the entrance system of the instrument by at least 90◦
We have derived wsw-spectra, f, of He+, C+, N+, O+, and Ne+ using data accumulated over an observation period of six years from the PLASTIC instrument on board STEREO A (Fig. 4)
Calculating the ratio of pickup ions (PUIs) wsw-spectra, R(wsw) = f⊥/ f, between two different configurations of the solar magnetic field vector φ⊥ and φ (Fig. 5), allowed us to estimate the anisotropy of the PUI velocity distribution functions (VDF) as a function of wsw
Summary
The investigation of heliospheric pickup ions (PUIs) is of particular interest because of their role in the generation of and interaction with electromagnetic waves embedded inside the solar wind plasma (Cannon et al 2014; Saul et al 2009) and because of their diverse origins, which can range from sources that are only several solar radii away from the Sun (Geiss et al 1995), the so-called inner-source of pickup ions, up to a distance of hundreds of astronomical units (au), the local interstellar medium (Moebius et al 1985). After a certain amount of time, the pickup ion VDF is expected to lose its initial torus shape as a result of the continuous influence of several mechanisms acting during the phase space transport between the PUI seed location and the observer These processes are among others: pitch-angle scattering, which changes the PUI’s pitch-angle owing to resonant wave-particle interaction (Fig. 1, bottom right), acceleration and deceleration processes (Fig. 1, bottom left) and local disturbances of the solar magnetic field. Even during conditions of very low solar activity, in which the impact of pitch-angle scattering processes and local disturbances of the global solar magnetic field on the initial torus shape of the VDF are minimal, cooling processes (Fahr 2007; Chen et al 2013) would still act on the VDF of inner-source pickup ions and cause the initial torus distribution of these ions to be observed at significantly lower velocities, i.e. wsw < 0.5, compared to a locally produced pickup ion population (Drews et al 2015), where it would be observed at wsw ≈ 1 (Fig. 1, lower left panel)
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