SH33B-01 13:40h
Variability of the Nitrogen Abundance in the Solar Wind and Implications for Past Solar Activity
The abundance of nitrogen in the heliosphere is an enigma. Laboratory analysis of lunar soils shows that trapped nitrogen is overabundant in them by about one order of magnitude relative to all noble gases, which in turn are efficiently trapped in the lunar regolith. On the other hand, the Solar Wind Ion Mass Spectrometer (SWIMS) on ACE has successfully measured the elemental abundance of nitrogen in the solar wind, N/O $ \approx0.121 \pm 0.014$, in good agreement with the photospheric value of N/O $\approx 0.123$ and with the SEP-dervied coronal value. In this work we determine the abundance ratio N/Ne and investigate the variability of N/O and of N/Ne in the solar wind. Nitrogen is not readily measured in the solar wind with spaceborn TOF mass/mass per charge spectrometers such as SWICS because it is not very abundant and is neighbored in mass and in mass per charge by the more abundant heavy ions, oxygen and carbon. For this reason, previous elemental abundance determinations of nitrogen in the solar wind have had large intrinsic uncertainties. However, with SWIMS, nitrogen is cleanly separated from its neighbors and its abundance can be accurately measured. Analyzing data from 1998 to 2004, we have found no unexpected variability of N in the solar wind, the ratios N/O and N/Ne are consistent with a constant value throughout this period of dramatically changing solar activity. We apply this finding to different ideas relating nitrogen in lunar soils to widely different solar input in the distant past and find that our result provides further evidence for a non-solar origin of most of nitrogen in lunar soils.
SH33B-02 13:55h
Composition of Interplanetary Coronal Mass Ejections at Very High Latitudes
Composition signatures such as a high average iron charge state, a high O$^{7+}/$O$^{6+}$ ratio, or a high alpha to proton ratio are established tools for the identification of interplanetary coronal mass ejections (ICMEs). However, their relation to the classical signatures such as conterstreaming electrons, low plasma beta, or magnetic field rotation, is far from being one-to-one, and therefore deserves some attention. When Ulysses was traveling to high latitudes it encountered ICMEs both during the first set of polar passes at solar minimum and during the second set, which occurred around the maximum of solar cycle 23. Six high-latitude ICMEs had been observed in the large fast streams from the polar coronal holes at solar minimum, defining a new class of overexpanding ICMEs, but none had a compositional signature whatsoever. In this paper we analyze five ICME events that were also embedded in a polar fast stream, but occurred not far from solar maximum, in October to December 2001 at a heliolatitude of $>70^\circ$ north. These five events show a surprising variety of compositional signatures, from hardly any to strong. They are ideally suited for the study of these signatures since the background fast solar wind is compositionally uniform, making the boundaries appear very sharp. This enables us to compare the two sets of signatures to a high degree of accuracy.
http://www.issi.unibe.ch/~vsteiger/agu_f04.html
SH33B-03 14:10h
Interplanetary Scintillation Observations of the Large-Scale Structure of the Solar Wind Using EISCAT
Measurements of interplanetary scintillation (IPS) taken with the European Incoherent SCATter radar (EISCAT) in northern Scandinavia can be used to study the evolution of the solar wind as it expands through interplanetary space. IPS arises from changes in the apparent brightness of distant, compact radio sources due to scattering by density irregularities in the solar wind and can be used to obtain estimates of the solar wind speed. In this paper we present the results of a study of the large-scale structure of the fast solar wind under near solar minimum conditions, using data taken with the EISCAT system, and the extremely long baseline observations which combine the EISCAT and MERLIN systems. The latter are the best measurements to date of meridional components of velocity in the inner solar wind. In particular, the existence of a gradient in solar wind velocity of the fast wind over the polar crown, at latitudes corresponding to the x-ray and ultra-violet coronal hole boundary, as reported by Habbal and Woo (2001), is also explored.
SH33B-04 14:25h
Propagation And Thermodynamics Of ICMEs In The Heliosphere: Observations And Simulations
We conduct a statistical study of the propagation and thermodynamics of interplanetary coronal mass ejections (ICMEs) in the heliosphere. We base this study on a comprehensive survey of ICMEs from 0.3 to 30 AU observed at Helios 1 and 2, Ulysses, Wind and ACE, and Voyager 2 (Liu et al. 2004; Wang et al. 2004). We find that the radial width of ICMEs increases with distance out to 10-15 AU and then stays roughly constant; the radial width of near-Earth ICMEs has a solar cycle dependence, with a mean value of 0.34 AU. The expansion speed decreases very slowly, suggestive of a quasi-equilibrium expansion. The expansion is governed by a polytrope with $\gamma = 1.1-1.3$, indicating continuous heating in the ICME plasma. Coulomb collisions between alpha particles and protons are shown to play an important role in the heating process within ICMEs; other possible energy sources including heat flux electrons and dissipation of magnetic fluctuations are also investigated. A 1D MHD model including pickup ions is used to simulate the dynamical expansion of ICMEs with diverse artificial inputs at 1 AU; preliminary results predict that ICMEs over-expand, with maximum width near 5 AU. We will compare the model results and observations.
http://web.mit.edu/liuxying/www/
SH33B-05 14:40h
Heliospheric CME parameters within the context of solar observations
We describe a study of the compositional properties of heliospheric CME ejecta within the context of solar CME observations. In this study, we examine CME-ICME pairs to determine if a given CME is associated with a flare event. For each of these pairs the charge state ratios are averaged over the event and compared. We find that events originating near the central meridian are more likely to contain significantly higher charge state ratios. The charge states in those events are moderately correlated with flare magnitude, indicating that flare material enters or heats a portion of the CME ejecta. We define "central" events based on the enhanced oxygen charge state and average several in situ quantities over the ICME period. We find that plasma quantities tend to be enhanced in both "central" and "limb" events, but composition quantities are enhanced only in "central" events.
SH33B-06 14:55h
Direct Evidence for Magnetic Reconnection in the Solar Wind Near 1 AU
We have obtained direct evidence for local magnetic reconnection in the solar wind using solar wind plasma and magnetic field data obtained by the Advanced Composition Explorer (ACE). The prime evidence consists of accelerated ion flow observed within a magnetic field reversal region in the interior or at the rear edge of an interplanetary coronal mass ejection (ICME) on November 23, 1997. The observed plasma acceleration was consistent with the Walen relationship, which relates changes in flow velocity to density-weighted changes in the magnetic field vector. Pairs of proton beams having comparable densities and counterstreaming relative to one another along the magnetic field at a speed of ~2 Va, where Va was the local Alfven speed, were observed within portions of the accelerated flow event. We infer from the observations that reconnection occurred sunward of the spacecraft and that the accelerated flow occurred within a reconnection exhaust region bounded by Alfven waves and having a width of ~400,000 km. The counterstreaming ion beams resulted from solar wind plasma entering the exhaust region from opposite directions along the magnetic field. A similar reconnection-associated event was observed within an ICME on October 3, 2000; however, we have not yet identified additional events of this nature, possibly because predicted accelerations are relatively small for typical solar wind densities and magnetic field strengths.
SH33B-07 15:10h
How common are magnetic disconnection events in the solar wind?
Suprathermal electrons (E$>$80eV) in the solar wind carry heat flux away from the Sun. They are typically focussed in a narrow beam called the strahl, streaming away from the Sun along magnetic field lines. The width of the strahl is variable and at times it almost vanishes and there is greatly reduced heat flux. These events, called `heat flux dropouts' have been variously interpreted as due to disconnected magnetic flux in the heliosphere or increased pitch angle scattering or both. Establishing the amount of disconnected magnetic flux in the heliosphere is important for distinguishing between models of solar magnetic flux transport and the solar cycle reversal of the heliospheric magnetic field. Using four years of data from the Wind spacecraft, we perform a comprehensive survey of heat flux dropout events and establish an upper bound for the fraction which might be due to disconnection.
SH33B-08 15:25h
Anisotropy of solar wind fluctuations: fast wind vs slow wind.
The fluctuations in the solar wind are often modeled in terms of two distinct populations: (a) a 'wave-like' population with quasi-parallel wavenumbers and (b) a quasi-two dimensional 'turbulent-like' fluctuations with perpendicular wavenumbers. Here the qualification "quasi-parallel" or "quasi-2D" means that nearby wavevectors are grouped together in an idealzed way, for simplicity. The relative abundance of these two populations is important in gaining insight on the dynamics of waves or turbulence in the solar wind, and also in understanding the transport of energetic particle populations, as turbulence geometry has a major impact on scattering. It has been established in the literature that turbulence is, generally speaking, more developed in the slow solar wind, with power spectra closer to the kolmogorov value at 1AU, while the fast solar wind is more "Alfvenic", typically with higher values of the cross helicity. It seems natural therefore to investigate the anisotropy structure of solar wind fluctuations as a function of wind speed. We present here our preliminary results in this regard, obtained from magnetic and plasma data from the ACE specraft, at 1AU, essentially in the ecliptic plane. We also discuss possible implications for the modeling the evolution of waves and turbulence in the solar wind.