S. T. Lepri

Ion heating resulting from pickup in magnetic reconnection exhausts

Received 25 August 2008; revised 20 January 2009; accepted 19 March 2009; published 30 May 2009. (1) The heating of ions downstream of the x-line during magnetic reconnection is explored using full-particle simulations, test particle simulations, and analytic analysis. Large-scale particle simulations reveal that the ion temperature increases sharply across the boundary layer that separates the upstream plasma from the Alfvenic outflow. This boundary layer, however, does not take the form of a classical switch-off shock as discussed in the Petschek reconnection model, so the particle heating cannot be calculated from the magnetohydrodynamic, slow-shock prediction. Test particle trajectories in the fields from the simulations reveal that ions crossing the narrow boundary into the exhaust instead behave like pickup particles: they gain both a directed outflow and an effective thermal speed given by the flow speed v0 of the exhaust. The detailed dynamics of these particles are explored by taking 1-D cuts of the simulation data across the exhaust, transforming to the deHoffman-Teller frame, and calculating explicitly the increment in the temperature, miv0 /3, with mi, the ion mass. We compare the model predictions with the temperature increment in solar wind exhausts measured by the ACE and Wind spacecraft, confirming that the temperature increment is proportional to the ion mass. The Wind data from 22 high-shear exhaust encounters confirm the scaling of the proton temperature increment with the square of the exhaust velocity. However, the temperature increments are consistently lower than the model prediction. Implications for understanding the production of high-energy ions in flares and the broader universe are discussed.

Iron charge distribution as an identifier of interplanetary coronal mass ejections

We present solar wind Fe charge state data measured on the Advanced Composition Explorer (ACE) from early 1998 to the middle of 2000. Average Fe charge states in the solar wind are typically around 9 to 11. However, deviations from these average charge states occur, including intervals with a large fraction of Fe 16 which are consistently associated with interplanetary coronal mass ejections (ICMEs). By studying the Fe charge state distribution we are able to extract coronal electron temperatures often exceeding 2 10 6 kelvins. We also discuss the temporal trends of these events, indicating the more frequent appearance of periods with high Fe charge states as solar activity increases.

Solar Wind Heavy Ions over Solar Cycle 23: ACE/SWICS Measurements

Solar wind plasma and compositional properties reflect the physical properties of the corona and its evolution over time. Studies comparing the previous solar minimum with the most recent, unusual solar minimum indicate that significant environmental changes are occurring globally on the Sun. For example, the magnetic field decreased 30% between the last two solar minima, and the ionic charge states of O have been reported to change toward lower values in the fast wind. In this work, we systematically and comprehensively analyze the compositional changes of the solar wind during cycle 23 from 2000 to 2010 while the Sun moved from solar maximum to solar minimum. We find a systematic change of C, O, Si, and Fe ionic charge states toward lower ionization distributions. We also discuss long-term changes in elemental abundances and show that there is a ~50% decrease of heavy ion abundances (He, C, O, Si, and Fe) relative to H as the Sun went from solar maximum to solar minimum. During this time, the relative abundances in the slow wind remain organized by their first ionization potential. We discuss these results and their implications for models of the evolution of the solar atmosphere, and for the identification of the fast and slow wind themselves.

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.

EVOLUTION OF THE RELATIONSHIPS BETWEEN HELIUM ABUNDANCE, MINOR ION CHARGE STATE, AND SOLAR WIND SPEED OVER THE SOLAR CYCLE

The changing relationships between solar wind speed, helium abundance, and minor ion charge state are examined over solar cycle 23. Observations of the abundance of helium relative to hydrogen (A He ≡ 100 × n He/n H) by the Wind spacecraft are used to examine the dependence of A He on solar wind speed and solar activity between 1994 and 2010. This work updates an earlier study of A He from 1994 to 2004 to include the recent extreme solar minimum and broadly confirms our previous result that A He in slow wind is strongly correlated with sunspot number, reaching its lowest values in each solar minima. During the last minimum, as sunspot numbers reached their lowest levels in recent history, A He continued to decrease, falling to half the levels observed in slow wind during the previous minimum and, for the first time observed, decreasing even in the fastest solar wind. We have also extended our previous analysis by adding measurements of the mean carbon and oxygen charge states observed with the Advanced Composition Explorer spacecraft since 1998. We find that as solar activity decreased, the mean charge states of oxygen and carbon for solar wind of a given speed also fell, implying that the wind was formed in cooler regions in the corona during the recent solar minimum. The physical processes in the coronal responsible for establishing the mean charge state and speed of the solar wind have evolved with solar activity and time.

Constraints on Coronal Mass Ejection Evolution from in Situ Observations of Ionic Charge States

We present a novel procedure for deriving the physical properties of coronal mass ejections (CMEs) in the corona. Our methodology uses in situ measurements of ionic charge states of C, O, Si, and Fe in the heliosphere and interprets them in the context of a model for the early evolution of interplanetary CME (ICME) plasma, between 2 and 5 R� . We find that the data are best fit by an evolution that consists of an initial heating of the plasma, followed by an expansion that ultimately results in cooling. The heating profile is consistent with a compression of coronal plasma due to flare reconnection jets and an expansion cooling due to the ejection, as expected from the standard CME/flare model. The observed frozen-in ionic charge states reflect this time history and, therefore, provide important constraints for the heating and expansion timescales, as well as the maximum temperature the CME plasma is heated to during its eruption. Furthermore, our analysis places severe limits on the possible density of CME plasma in the corona. We discuss the implications of our results for CME models and for future analysis of ICME plasma composition.

Ion Charge States in the Fast Solar Wind: New Data Analysis and Theoretical Refinements

We present a further investigation into the increased ionization observed in element charge states in the fast solar wind compared to its coronal hole source regions. Once ions begin to be perpendicularly heated by ion cyclotron waves and execute large gyro-orbits, density gradients in the flow can excite lower hybrid waves that then damp by heating electrons in the parallel direction. We give further analysis of charge-state data from polar coronal holes at solar minimum and maximum, and also from equatorial coronal holes. We also further consider the damping of lower hybrid waves by ions and the effect of non-Maxwellian electron distribution functions on the degree of increased ionization, both of which appear to be negligible for the solar wind case considered here. We also suggest that the density gradients required to heat electrons sufficiently to further ionize the solar wind can plausibly result from the turbulent cascade of MHD waves.

Ion Charge States in Halo Coronal Mass Ejections: What Can We Learn about the Explosion?

We describe a new modeling approach to develop a more quantitative understanding of the charge state distributions of the ions of various elements detected in situ during halo coronal mass ejection (CME) events by the Advanced Composition Explorer (ACE) satellite. Using a model CME hydrodynamic evolution based on observations of CMEs propagating in the plane of the sky and on theoretical models, we integrate time-dependent equations for the ionization balance of various elements to compare with ACE data. We find that plasma in the CME core typically requires further heating following filament eruption, with thermal energy input similar to the kinetic energy input. This extra heating is presumably the result of posteruptive reconnection. Plasma corresponding to the CME cavity is usually not further ionized, since whether heated or not, the low density gives freeze-in close the Sun. The current analysis is limited by ambiguities in the underlying model CME evolution. Such methods are likely to reach their full potential when applied to data to be acquired by STEREO when at optimum separation. CME evolution observed with one spacecraft may be used to interpret CME charge states detected by the other.

Coronal Electron Temperature from the Solar Wind Scaling Law throughout the Space Age

Recent in situ observations of the solar wind show that charge states (e.g., the O7 +/O6 + and C6 +/C5 + abundance ratios) and α-particle composition evolved through the extended, deep solar minimum between solar cycles 23 and 24 (i.e., from 2006 to 2009). Prior investigations have found that both particle flux and magnetic field strength gradually decreased over this period of time. In this study, we find that (for a given solar wind speed) the coronal electron temperature (as derived from O7 +/O6 + and C6 +/C5 + measurements from ACE) likewise decreased during this minimum. We use the Schwadron & McComas solar wind scaling law to show that cooler coronal electron temperatures are naturally associated with lower particle fluxes because downward heat conduction must be reduced to keep the average energy loss per particle fixed. The results of the scaling law should apply to all solar wind models and suggest that the evolution of the solar wind is linked to the solar dynamo, which caused the coronal magnetic field strength to decrease in the deep, extended minimum. We utilize the scaling law to project coronal electron temperatures backward in time throughout the space age and find that these temperatures have been decreasing in successive temperature maxima since 1987 but were increasing in successive temperature maxima from 1969 to 1987. Thus, we show how the solar wind scaling law relates solar wind properties observed at 1 AU back to coronal electron temperatures throughout the space age.

The origin of the local 1/4-keV X-ray flux in both charge exchange and a hot bubble

The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays, coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas, led to a picture of a ‘local cavity’ filled with X-ray-emitting hot gas, dubbed the local hot bubble. This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutral H and He in interplanetary space, potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk. Here we report observations showing that the total solar-wind charge-exchange contribution is approximately 40 per cent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun.