Composition of inner-source heavy pickup ions at 1 AU: SOHO/CELIAS/CTOF observations

et al. 2015

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Context. He+ pickup ions are either born from the ionization of interstellar neutral helium inside our heliosphere, the so-called interstellar pickup ions, or through the interaction of solar wind ions with small dust particles, the so-called inner source of pickup ions. Until now, most observations of pickup ions were limited to reduced 1D velocity spectra, which are insufficient to study certain characteristics of the He + velocity distribution function (VDF). Aims. It is generally assumed that rapid pitch-angle scattering of freshly created pickup ions quickly leads to a fully isotropic He + VDF. In light of recent observations, this assumption has found to be oversimplified and needs to be reinvestigated. Methods. Using He+ pickup ion data from the PLASTIC instrument on board the STEREO A spacecraft, we reconstruct a reduced form of the He + VDF in two dimensions. This allows us to study relative changes of the 2D He + VDF as a function of the configuration of the heliospheric magnetic field. Results. Our observations show that the He + VDF is highly anisotropic and even indicates that, at least for certain configurations of B, it is not fully gyrotropic. Our results further suggest, that the observed velocity and pitch angle of He + depends strongly on the local solar magnetic field vector, B, the ecliptic longitude, λ, the solar wind speed, v sw , and the global distribution of B. Conclusions. We found two distinct signatures that systematically change as a function of the alignment of B: (1) a ring beam distribution that is most pronounced at w sw > 0.5 and likely attributed to interstellar He + ; (2) a beam signature aligned parallel to B that is most pronounced at w sw < 0.5 and attributed to inner-source He + . The strong anisotropy and the aforementioned dependencies of the He + VDF also imply that observations of 1D velocity spectra of He + pickup ions are potentially deceiving.

Section: The Asymmetry Of the He + Ring Beammentioning

confidence: 99%

2D He⁺pickup ion velocity distribution functions: STEREO PLASTIC observations

et al. 2015

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“…The model, I(m/q), was adapted from Taut et al (2015), where it was used to derive the composition of inner-source pickup ions with the charge and timeof-flight (CTOF) instrument on board the SOlar Heliospheric Observatory (SOHO; Hovestadt et al 1995). Because of the very similar measurement principle of CTOF and PLASTIC, choosing the mass-per-charge model from Taut et al (2015), which considers the asymmetry of the mass-per-charge model for an individual species, over a more simple, symmetric model (Drews et al 2010) or a simple mass-per-charge filter (Drews et al 2012), is most likely the best approach to deconvolve the contributions of individual species from the heavy pickup ion m/qspectra. The model I(m/q) of an individual species, which is a combined Gaussian-κ-function, is given by…”

Section: Discussionmentioning

confidence: 99%

“…Of course, the aforementioned parameters for the m/qmodel depend on the ion species, which further complicates a precise determination of I(m/q). Contrary to the analysis by Taut et al (2015), who derived these parameters via a simulation of the time-of-flight characteristic of CTOF, we use a more direct approach via an in-flight observation, in which the m/q-spectra were significantly dominated by either C + , O + , or Ne + . First of all, a time period between day of year (DoY) since 2007 between 127.3 < DoY < 127.5 was chosen to obtain a m/q-spectrum in which, almost exclusively, O + from the Earth's magnetosphere was observed with PLASTIC on STEREO A (Fig.…”

Section: Discussionmentioning

confidence: 99%

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Anisotropy of the He⁺, C⁺, N⁺, O⁺, and Ne⁺pickup ion velocity distribution functions

et al. 2016

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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 waveparticle 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.

“…Surprisingly, the data, with one exception of Venus tail ray O + observations (Grünwaldt et al 1997), has never been used to study inner source PUIs, although CTOFs active phase coincides with the discovery of the inner source. In this work and a companion paper (Taut et al 2015) we investigate inner source PUIs using CTOF data. Here, we address the short-term variability of inner source O + and C + while the companion paper adresses the composition of the inner source.…”

Section: Introductionmentioning

confidence: 99%

Short-term variability of inner-source pickup ions at 1 AU

et al. 2015

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Context. In 1995 a second extended source of pickup ions in the inner heliosphere was discovered. Since then this so-called inner source has been characterised in many studies, and various scenarios for its nature have been proposed. But to this day, the detailed nature of the inner source is still unknown. Aims. Although it seems most likely that an interaction of solar wind and dust plays a key role in the production of the inner source pickup ions, available observations have not provided conclusive evidence for any proposed scenario. By analysing the short-term variability of the inner source, we determine a new observational constraint to address the nature of the inner source. Methods. We used the data set of the charge time-of-flight instrument that operated in 1996 on-board the solar and heliospheric observatory at the first Lagrangian point to analyse inner source O + and C + . The unmatched combination of mass per charge resolution, which is sufficient to definitely resolve O + and C + , and typical high count rates of about 150 counts per day allowed us to address the short-term variability of the inner source for the first time.Results. The comparison of the variability of inner-source and solar-wind ions shows that the flux of inner source pickup oxygen and carbon is directly correlated with the flux of solar wind oxygen, and carbon, respectively. Conclusions. Among the scenarios for the nature of the inner source alone, the scenario of solar-wind neutralisation agrees with this new observational constraint.