Jason A. Gilbert

MESSENGER Observations of the Spatial Distribution of Planetary Ions Near Mercury

The polar regions of Mercury are important sources of material for its ionized exosphere. Global measurements by MESSENGER of the fluxes of heavy ions at Mercury, particularly sodium (Na+) and oxygen (O+), exhibit distinct maxima in the northern magnetic-cusp region, indicating that polar regions are important sources of Mercury’s ionized exosphere, presumably through solar-wind sputtering near the poles. The observed fluxes of helium (He+) are more evenly distributed, indicating a more uniform source such as that expected from evaporation from a helium-saturated surface. In some regions near Mercury, especially the nightside equatorial region, the Na+ pressure can be a substantial fraction of the proton pressure.

Solar Wind Alpha Particles and Heavy Ions in the Inner Heliosphere Observed with MESSENGER

obstructed by the spacecraft sunshade, a data analysis technique has been developed that recovers both bulk and thermal speeds to 10% accuracy and provides the first measurements of solar wind heavy ions (mass per charge >2 amu/e) at heliocentric distances within 0.5 AU. Solar wind alpha particles and heavy ions appear to have similar mean flow speeds at values greater than that of the protons by approximately 70% of the Alfven speed. From an examination of the thermal properties of alpha particles and heavier solar wind ions, we find a ratio of the temperature of alpha particles to that of protons nearly twice that of previously reported Helios observations, though still within the limits of excessive heating of heavy ions observed spectroscopically close to the Sun. Furthermore, examination of typical magnetic power spectra at the orbits of MESSENGER and at 1 AU reveals the lack of a strong signature of local resonant ion heating, implying that a majority of heavy ion heating could occur close to the Sun. These results demonstrate that the solar wind at � 0.3 AU is a blend of the effects of wave–particle interactions occurring in both the solar corona and the heliosphere.

Carbon Ionization Stages as a Diagnostic of the Solar Wind

Oxygen charge states measured by in situ instrumentation have long been used as a powerful diagnostic of the solar corona and to discriminate between different solar wind regimes, both because they freeze in very close to the Sun, and because the oxygen element abundance is comparatively high, allowing for statistically relevant measures. Like oxygen, carbon is also rather abundant and freezes in very close to the Sun. Here, we show an analysis of carbon and oxygen ionic charge states. First, through auditory and Fourier analysis of in situ measurements of solar wind ion composition by ACE/SWICS we show that some carbon ion ratios are very sensitive to solar wind type, even more sensitive than the commonly used oxygen ion ratios. Then we study the evolution of the ionization states of carbon and oxygen by means of a freeze-in code, and find that carbon ions, commonly found in the solar wind, freeze in at comparable coronal distances, while oxygen ions evolve over a much larger range of coronal distances. Finally, we show that carbon and oxygen ion abundance ratios have similar sensitivity to the electron plasma temperature, but the carbon ratios are more robust against atomic physics uncertainties and a better indicator of the temperature of the solar wind source regions.

The Solar Wind Neon Abundance Observed with ACE/SWICS and Ulysses/SWICS

Using in situ ion spectrometry data from ACE/SWICS, we determine the solar wind Ne/O elemental abundance ratio and examine its dependence on wind speed and evolution with the solar cycle. We find that Ne/O is inversely correlated with wind speed, is nearly constant in the fast wind, and correlates strongly with solar activity in the slow wind. In fast wind streams with speeds above 600 km s −1 , we find Ne/O = 0.10 ± 0.02, in good agreement with the extensive polar observations by Ulysses/SWICS. In slow wind streams with speeds below 400 km s −1 , Ne/O ranges from a low of 0.12 ± 0.02 at solar maximum to a high of 0.17 ± 0.03 at solar minimum. These measurements place new and significant empirical constraints on the fractionation mechanisms governing solar wind composition and have implications for the coronal and photospheric abundances of neon and oxygen. The results are made possible by a new data analysis method that robustly identifies rare elements in the measured ion spectra. The method is also applied to Ulysses/SWICS data, which confirms the ACE observations and extends our view of solar wind neon into the three-dimensional heliosphere.

THERMALIZATION of HEAVY IONS in the SOLAR WIND

Observations of velocity distribution functions from the Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer heavy ion composition instrument are used to calculate ratios of kinetic temperature and Coulomb collisional interactions of an unprecedented 50 ion species in the solar wind. These ions cover a mass per charge range of 1–5.5 amu/e and were collected in the time range of 1998–2011. We report the first calculation of the Coulomb thermalization rate between each of the heavy ion (A > 4 amu) species present in the solar wind along with protons (H+) and alpha particles (He2+). From these rates, we find that protons are the dominant source of Coulomb collisional thermalization for heavy ions in the solar wind and use this fact to calculate a collisional age for those heavy ion populations. The heavy ion thermal properties are well organized by this collisional age, but we find that the temperature of all heavy ions does not simply approach that of protons as Coulomb collisions become more important. We show that He2+ and C6+ follow a monotonic decay toward equal temperatures with protons with increasing collisional age, but O6+ shows a noted deviation from this monotonic decay. Furthermore, we show that the deviation from monotonic decay for O6+ occurs in solar wind of all origins, as determined by its Fe/O ratio. The observed differences in heavy ion temperature behavior point toward a local heating mechanism that favors ions depending on their charge and mass.

A NEW TECHNIQUE FOR MAPPING OPEN MAGNETIC FLUX FROM THE SOLAR SURFACE INTO THE HELIOSPHERE

The solar wind carries magnetic flux from the photosphere into the heliosphere, making it topologically open in the corona. Open magnetic flux is unevenly distributed at the solar surface, but at some distance in the outer corona it becomes uniformly distributed and approximately radial. Standard potential field models do not provide such uniform distribution of open flux in the heliosphere. A new technique for mapping open magnetic flux is presented here that addresses this deficiency and provides a simple tool to map any initial configuration of photospheric footpoints into the heliosphere. This technique is designed to result in a uniform open flux distribution in the heliosphere and is especially useful for models that include open flux emerging from topologically closed regions. We compare with observations of proton speed to quantify the amount of open flux emerging from these regions. We find that if the slow solar wind originates from topologically closed regions, then the open flux coming from these areas must form a significant component of the heliosphere. We explain this new methodology and discuss its application throughout the solar cycle.

IN SITU PLASMA MEASUREMENTS OF FRAGMENTED COMET 73P SCHWASSMANN–WACHMANN 3

The interiors of comets contain some of the most pristine material in the solar system. Comet 73P/Schwassmann–Wachmann 3, discovered in 1930, is a Jupiter-family comet with a 5.34-year period. This comet split into 5 fragments in 1995 and disintegrated into nearly 70 major pieces in 2006. In 2006 May and June, recently ionized cometary particles originating from fragments including and surrounding some of these major objects were collected with the ACE/SWICS and Wind/STICS sensors. Due to a combination of the instrument characteristics and the close proximity of the fragments passing between those spacecraft and the Sun, unique measurements regarding the charge state composition and the elemental abundances of both cometary and heliospheric plasma were made during that time. The cometary material released from some of these fragments can be identified by the concentrations of water-group pickup ions having a mass-per-charge ratio of 16–18 amu e−1, indicating that while these fragments are small, they are still actively sublimating. We present an analysis of cometary composition, spatial distribution, and heliospheric interactions, with a focus on helium, C+/O+, and water-group ions.

The bird's ear view of space physics: Audification as a tool for the spectral analysis of time series data

The effective navigation, mining, and analysis of large time series data sets presents a recurring challenge throughout heliophysics. Audification, a specific form of auditory analysis commonly used in other fields of research (such as geoseismology), provides a promising technique for the evaluation of spectral features in long heliospheric time series data sets. Following a standard research methodology for the development of new analysis techniques, this paper presents a detailed case study in which audification was introduced into the working process of an experienced heliophysics research scientist and used for the identification and classification of features in high-resolution magnetometer data during a structured analysis task. Auditory evaluation successfully led to the detection of artificial, instrument-induced noise that was not previously observed by the scientist and also the identification of wave activity embedded within turbulent solar wind data. A follow-up interview indicated that the scientist continued using these auditory analysis methods in the assessment of every large data set during the 2 months after the study was completed. These findings indicate that audification can be valuable and enabling for researchers in forming a deeper understanding of both microstructures and macrostructures within large time series. Additionally, as both a standalone methodology and a supplement to visual analysis methods, audification can expedite certain stages of the data survey, analysis, and mining process and provide new qualitative insight into the spectral content of time-varying signals.

Constraining Solar Wind Heating Processes by Kinetic Properties of Heavy Ions

We analyze the heavy ion components (A>4  amu) in collisionally young solar wind plasma and show that there is a clear, stable dependence of temperature on mass, probably reflecting the conditions in the solar corona. We consider both linear and power law forms for the dependence and find that a simple linear fit of the form T_{i}/T_{p}=(1.35±.02)m_{i}/m_{p} describes the observations twice as well as the equivalent best fit power law of the form T_{i}/T_{p}=(m_{i}/m_{p})^{1.07±.01}. Most importantly we find that current model predictions based on turbulent transport and kinetic dissipation are in agreement with observed nonthermal heating in intermediate collisional age plasma for m/q<3.5, but are not in quantitative or qualitative agreement with the lowest collisional age results. These dependencies provide new constraints on the physics of ion heating in multispecies plasmas, along with predictions to be tested by the upcoming Solar Probe Plus and Solar Orbiter missions to the near-Sun environment.

The in-situ manifestation of solar prominence material

Coronal mass ejections observed in the corona exhibit a three-part structure, with a leading bright front indicating dense plasma, a low density cavity thought to be a signature of the embedded magnetic flux rope, and the high density core likely containing cold, prominence material. When observed in-situ, as Interplanetary CMEs (or ICMEs), the presence of all three of these signatures remains elusive, with the prominence material rarely observed. We report on a comprehensive and long-term search for prominence material inside ICMEs as observed by the Solar Wind Ion Composition Spectrometer on the Advanced Composition Explorer. Using a novel data analysis process, we are able to identify traces of low charge state plasma created during prominence eruptions associated with ICMEs. We find that the likelihood of occurrence of cold material in the heliosphere is vastly lower than that observed in the corona but that conditions during the eruption do allow low charge ions to make it into the solar wind, preserving their expansion history. We discuss the implications of these findings.