Lars Berger

Rotation velocities of white dwarfs determined from the Ca II K line

Department of Physics & Astronomy and Center for Astrobiology, UCLA, Los Angeles, CA 90095-1562, USAAbstract. We determine projected rotation velocities v sin i in DAZ white dwarfs, for the first time using therotational broadening of the CaII K line. The results confirm previous findings that white dwarfs are very slowrotators, and set even more stringent upper limits of typically less than 10 km/s. The few exceptions include 3stars known or suspected to be variable ZZ Ceti stars, where the line broadening is very likely not due to rotation.The results demonstrate that the angular momentum of the core cannot be preserved completely between mainsequence and final stage.Key words. stars: white dwarfs – stars: rotation

Composition of inner-source heavy pickup ions at 1 AU: SOHO/CELIAS/CTOF observations Implications for the production mechanisms

Context. Pickup ions in the inner heliosphere mainly originate in two sources, one interstellar and one in the inner solar system. In contrast to the interstellar source that is comparatively well understood, the nature of the inner source has not been clearly identified. Former results obtained with the Solar Wind Ion Composition Spectrometer on-board the Ulysses spacecraft revealed that the composition of inner-source pickup ions is similar, but not equal, to the elemental solar-wind composition. These observations su ered from very low counting statistics of roughly one C + count per day. Aims. Because the composition of inner-source pickup ions could lead to identifying their origin, we used data from the Charge-TimeOf-Flight sensor on-board the Solar and Heliospheric Observatory. It o ers a large geometry factor that results in about 100 C + counts per day combined with an excellent mass-per-charge resolution. These features enable a precise determination of the inner-source heavy pickup ion composition at 1 AU. To address the production mechanisms of inner-source pickup ions, we set up a toy model based on the production scenario involving the passage of solar-wind ions through thin dust grains to explain the observed deviations of the inner-source PUI and the elemental solar-wind composition. Methods. An in-flight calibration of the sensor allows identification of heavy pickup ions from pulse height analysis data by their mass-per-charge. A statistical analysis was performed to derive the inner-source heavy pickup ion relative abundances of N + , O + , Ne + , Mg + , Mg 2+ , and Si + compared to C + . Results. Our results for the inner-source pickup ion composition are in good agreement with previous studies and confirm the deviations from the solar-wind composition. The large geometry factor of the Charge-Time-of-Flight sensor even allowed the abundance ratios of the two most prominent pickup ions, C + and O + , to be investigated at varying solar-wind speeds. We found that the O + /C + ratio increases systematically with higher solar-wind speeds. This observation is an unprecedented feature characterising the production of inner-source pickup ions. Comparing our observations to the toy model results, we find that both the deviation from the solar-wind composition and the solar-wind-speed dependent O + /C + ratio can be explained.

Observations of high and low Fe charge states in individual solar wind streams with coronal-hole origin

Context. The solar wind originating from coronal holes is comparatively well-understood and is characterized by lower densities and average charge states compared to the so-called slow solar wind. Except for wave perturbations, the average properties of the coronal-hole solar wind are passably constant. Aims. In this case study, we focus on observations of the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) of individual streams of coronal-hole solar wind that illustrate that although the O and C charge states are low in coronal-hole wind, the Fe charge distribution is more variable. In particular, we illustrate that the Fe charge states in coronal-hole solar wind are frequently as high as in slow solar wind. Methods. We selected individual coronal-hole solar wind streams based on their collisional age as well as their respective O and C charge states and analyzed their Fe charge-state distributions. Additionally, with a combination of simple ballistic back-mapping and the potential field source surface model, transitions between streams with high and low Fe charge states were mapped back to the photosphere. The relative frequency of high and low Fe charge-state streams is compared for the years 2004 and 2006. Results. We found several otherwise typical coronal-hole streams that include Fe charge states either as high as or lower than in slow solar wind. Eight such transitions in 2006 were mapped back to equatorial coronal holes that were either isolated or connected to the northern coronal-hole. Attempts to identify coronal structures associated with the transitions were so far inconclusive.

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

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 (wsw). The wsw-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.

Suprathermal helium in corotating interaction regions: combined observations from SOHO/CELIAS/STOF and ACE/SWICS

Context. Energetic particle enhancements that are associated with corotating interaction regions (CIRs) are typically believed to arise from the sunward propagation of particles that are accelerated by CIR-driven shocks beyond 1 AU. It is expected that these sunward-travelling particles will lose energy and scatter, resulting in a turnover of the energy spectra below ~0.5 MeV/nuc. However, the turnover has not been observed so far, suggesting that the CIR-associated low-energy suprathermal ions are accelerated locally close to the observer. Aims. We investigate the variability of suprathermal particle spectra from CIR to CIR as well as their evolution and variation as the observer moves away from the rear shock or wave. Methods. Helium data in the suprathermal energy range from the Solar and Heliospheric Observatory/Charge, Element, and Isotope Analysis System/Suprathermal Time-of-Flight (SOHO/CELIAS/STOF) were used for the spectral analysis and were combined with data from the Advanced Composition Explorer/ Solar Wind Ion Composition Spectrometer (ACE/SWICS) in the solar wind energies. Results. We investigated sixteen events: nine clean CIR events, three CIR events with possible contamination from upstream ion events or solar energetic particles (SEPs), and four events that occurred during CIR periods that were dominated by SEPs. Six of the nine clean CIR events showed possible signs of a turnover between ~10−40 keV/nuc in the fast solar wind that trails the compression regions. Three of them even showed this behaviour inside the compressed fast wind. The turnover part of the spectra became flatter and shifted from lower to higher energies with increasing connection distance to the reverse shock. The remaining three clean events showed continuous power-law spectra in both the compressed fast wind and fast wind regions, that is, the same behaviour as reported from previous observations. The spectra of the seven remaining events are more variable, that is, they show power law, turnover, and a superposition of these two shapes.

Origin of the solar wind: A novel approach to link in situ and remote observations - A study for SPICE and SWA on the upcoming Solar Orbiter mission

Context. During the last decades great progress has been achieved in understanding the properties and the origin of the solar wind. While the sources for the fast solar wind are well understood, the sources for the slow solar wind remain elusive. Aims. The upcoming Solar Orbiter mission aims to improve our understanding of the sources of the solar wind by establishing the link between in situ and remote sensing observations. In this paper we aim to address the problem of linking in situ and remote observations in general and in particular with respect to ESA’s Solar Orbiter mission. Methods. We have used a combination of ballistic back mapping and a potential field source surface model to identify the solar wind source regions at the Sun. As an input we use in situ measurements from the Advanced Composition Explorer and magnetograms obtained from the Michelson Doppler Interferometer on board the Solar Heliospheric Observatory. For the first time we have accounted for the travel time of the solar wind above and also below the source surface. Results. We find that a prediction scheme for the pointing of any remote sensing instrumentation is required to capture a source region not only in space but also in time. An ideal remote-sensing instrument would cover up to ≈50% of all source regions at the right time. In the case of the Spectral Imaging of the Coronal Environment instrument on Solar Orbiter we find that ≈25% of all source regions would be covered. Conclusions. To successfully establish a link between in situ and remote observations the effects of the travel time of the solar wind as well as the magnetic displacement inside the corona cannot be neglected. The predictions needed cannot be based solely on a model, nor on observations alone, only the combination of both is sufficient.

High-time resolution measurements of solar wind heavy ions with SOHO/CELIAS/CTOF

The Charge Time-Of-Flight (CTOF) mass spectrometer as part of the Charge, ELement and Isotope Analysis System (CELIAS) onboard the SOlar and Heliospheric Observatory (SOHO) is designed to measure the kinetic properties and elemental/ionic composition of solar wind ions heavier than protons, which we refer to as heavy ions. This is achieved by the combined measurements of the energy-per-charge, the time-of-flight and the energy of incident ions. The CTOF instrument combines a remarkable time-of-flight resolution with a large effective area and a high measurement cadence. This allows to determine the Velocity Distribution Functions (VDFs) of a wide range of heavy ions with 5-minute time resolution which ensures that the complete VDF is measured under nearly identical solar wind and magnetic field conditions. For the measurement period between Day Of Year (DOY) 150 and 220 in 1996, which covers a large part of the instrument’s short life time, we analyzed VDFs of solar wind iron Fe8+, Fe9+ and Fe10+ for diff...

Observations of the He+ pickup ion torus velocity distribution function with SOHO/CELIAS/CTOF

Interstellar PickUp Ions (PUIs) are created from neutrals coming from the interstellar medium that get ionized inside the heliosphere. Once ionized, the freshly created ions are injected into the magnetized solar wind plasma with a highly anisotropic torus-shaped Velocity Distribution Function (VDF). It has been commonly assumed that wave-particle interactions rapidly destroy this torus by isotropizing the distribution in one hemisphere of velocity space. However, recent observations of a He+ torus distribution using PLASTIC on STEREO showed that the assumption of a rapid isotropization is oversimplified. The aim of this work is to complement these studies. Using He+ data from the Charge Time-Of-Flight (CTOF) sensor of the Charge, ELement, and Isotope Analysis System (CELIAS) on-board the SOlar and Heliospheric Observatory (SOHO) and magnetic field data from the Magnetic Field Investigation (MFI) magnetometer of the WIND spacecraft, we derive the projected 1-D VDF of He+ for different magnetic field confi...

Investigation of solar wind source regions using Ulysses composition data and a PFSS model

In this work we study the source regions for different solar wind types. While it is well known that the fast solar wind originates from inside Coronal Holes, the source regions for the slow solar wind are still under debate. For our study we use Ulysses compositional and plasma measurements and map them back to the solar corona. Here we use a potential field source surface model to model the coronal magnetic field. On the source surface we assign individual open field lines to the ballistic foot points of Ulysses. We do not only consider the photospheric origin of these field lines, but rather attempt to trace them across several height levels through the corona. We calculate the proximity of the field lines to the coronal hole border for every height level. The results are height profiles of these field lines. By applying velocity and charge state ratio filters to the height profiles, we can demonstrate that slow wind is produced close to the coronal hole border. In particular, we find that not only the...

Suprathermal particles in magnetic clouds

Measurements with the Solar Wind Ion Composition Spectrometer (SWICS) have shown ubiquitous and prominent suprathermal tails with a power-law dependence on velocity, ∼ ν−5 [1, 2]. Those and similar studies have mostly concentrated on long-term averages. Several explanations of these power-law velocity distribution functions (VDFs) have been proposed. Many of them rely on statistical arguments, e.g., that these VDFs result from superpositions of many VDFs, from multiple acceleration sites or processes, from successions of compressions and rarefactions, or from jumps from slow to fast wind and vice versa. To limit such proposed explanations we have investigated short-term spectra of such suprathermal particles (STPs) in magnetic clouds (MCs). Because MCs are generally globally expanding and show very low turbulence and only very limited variability in the overall plasma parameters, we do not expect a similar power law VDF to be observed in MCs. However, we found that the VDFs within MCs are similar to the a...