SH12A-01 10:20h
Characterization of Upstream Field-Aligned Beams by Cluster
A quarter century of study has led to a characterization of the wave domains and particle properties of the terrestrial foreshock. Numerous factors might be expected to affect the upstream ion production, including shock geometry, the absence or presence of waves, and possibly the connection of foreshock field lines with the magnetopause. To date, the greatest emphasis has been on $\theta_{Bn} = \cos^{-1}(\mathbf{B \cdot n}/|\mathbf{B \cdot n}|)$, where $\mathbf{B}$ is the interplanetary magnetic field (IMF) and \mathbf{n} is the bow shock normal inferred for the particle source region. Little effort has been made to control for another important geometric parameter, $\theta_{vn} = \cos^{-1}(\mathbf{v_{sw} \cdot n}/|\mathbf{v_{sw} \cdot n}|)$, with $\mathbf{v}_{sw}$ the solar wind velocity. Consequently earlier statistical results have convolved effects and failed to reveal some trends. Here we present results from a study focusing on emergent field-aligned beams when significant local ultra-low freqency waves were absent. We have minimized the effects of $\theta_{vn}$ variation by considering individual foreshock transits due to rotations of the IMF over intervals for which the position of the spacecraft relative to the shock source remained nearly constant. Unusual care has been taken to remove instrument noise and the tail of the solar wind beam, allowing us to consider very low speed and density beams in our statistics. Results include a strong inverse dependence of upstream ion density with $\theta_{Bn}$, and a change in the slope of this trend between 40--50$\deg$. Velocities show a linear dependence upon $\cos\theta_{Vn}/\cos\theta_{Bn}$, consistent with simple reflection kinematics, but the magnitude of the slope can be larger than models predict. The observed ratio of perpendicular to parallel temperature around 4 is expected, but often there is little dependence of either temperature on $\theta_{Bn}$. The theory for field-aligned beam production is far from definitive, and these observations provide new constraints.
http://sprg.ssl.berkeley.edu/~wilber/AGUFall2004
SH12A-02 10:35h
The GENESIS Solar-Wind Sample: Summary of In-Situ Spacecraft Measurements During the Sample Collection Period
The GENESIS mission returned samples of solar wind to earth on September 8. These samples were collected between 30-Nov-2001 and 01-Apr-2004 at the L1 point on ultrapure substrates that are being analyzed using various laboratory techniques. The primary purpose of the GENESIS mission is to determine to unprecedented accuracy (e.g., $\pm$1%-0.1%, 2$\sigma$) the isotopic composition of the solar wind, and by extension, of the Sun, particularly for oxygen, nitrogen, carbon, and the noble gases. In addition to a continuously-collected sample, separate samples of the different regimes-coronal hole (CH), interstream (IS), and coronal mass ejection (CME)-were collected to help deconvolve any fine-scale isotopic fractionation associated with solar-wind ionization and acceleration. The four different samples (3 regimes plus one cumulative sample) are long-term averages over varying solar-wind conditions, and need to be understood within the context of other measurements besides those that can be made on the substrates themselves. These include charge state, v, T, density, and magnetic field data. Additionally, comparisons of in-situ elemental composition measurements with the measurements made on GENESIS samples will be useful in validating the different measurement techniques. For these reasons, we are compiling a database of conditions as measured by in-situ spacecraft instruments stationed at L1 over the time period of the GENESIS sample collection. Comparison with ACE/SWICS elemental and charge-state abundances confirmed early in the GENESIS mission that the GENESIS regime-selection algorithm was successfully isolating the CME material, protecting the CH and IS samples from CME contamination. The data also demonstrate the discrimination between IS and CH flow types based mostly on solar-wind speed, but allowing compensation for evolution effects due to the transit to 1 AU.
SH12A-03 10:50h
Counterstreaming Suprathermal Electrons Within Coronal Hole Fast Flows Measured at 1 AU by ACE and Genesis
Counterstreaming suprathermal electrons are observed on open interplanetary field lines within some coronal hole fast streams observed at 1 AU with ACE and Genesis. An anti-sunward directed electron beam produced by suprathermal electrons escaping from the solar corona, and commonly referred to as the strahl, is continuously present. In addition, a sunward-directed beam is often present in the fast streams. We attribute the sunward-directed beam to electrons that leak from co-rotating interaction regions (CIRs) at greater heliocentric distances and travel sunward along the interplanetary magnetic field. The sunward-directed flux often exceeds the anti-sunward strahl flux, especially at the measurement energies 980 eV and 1.4 keV. The counterstreaming is observed immediately after the passage of CIR, whether it is bounded by a reverse shock, or a reverse wave that has not yet steepened into a shock. The counter-streaming may also be found hours to days after the onset of the fast stream, indicating a more distant magnetic connection to a CIR. Electron counterstreaming within co-rotating high speed flows on field lines which are open, but connected to CIRs, was reported in earlier work for Ulysses measurements made beyond 2 AU. However it was previously thought that such counterstreaming was unlikely to be important near 1 AU. We present observations of counterstreaming electrons within very regular recurrent fast streams observed from December 1999 to March 2000 and from January through March 2002. In addition we show intervals of counterstreaming within comparatively irregular fast streams during 2003. We find that the intensity and energy extent of the sunward beams is more variable from one solar rotation to the next for the 2003 episodes than for the earlier episodes. LA-UR-04-6195
SH12A-04 11:05h
Multi-satellite Observations of Magnetic Holes in the Solar Wind
A record of magnetic holes observed by the WIND satellite in the solar wind from 1994 to 2004 was recently compiled. When permitted by the relative positions of the two satellites, ACE observed many of the same magnetic holes. By comparing the shape and plasma parameters at each satellite, the geometry and propagation of the holes are constained. The coherence of each hole and the associated pressure balance also help us to distinguish the sources of magnetic holes in the ecliptic and discern possible MHD solitons.
SH12A-05 11:20h
Preliminary results on the evaluation of 3D correlation functions from ACE and WIND simultaneous data
A detailed understanding of second order correlation functions provides a wealth of information about the statistical properties of turbulence, including spectra, correlation functions, cross-correlations, spectral anisotropy, and so on. The evaluation of these correlations in the solar wind from single spacecraft data has been very useful so far, but has typically been limited by the use of the the frozen-in approximation, namely that the solar wind is "rigidly moving," which is made to translate temporal series into spatial series. On the other hand, multiple spacecraft data give the possibility of measuring two point, second order spatial correlation functions directly, in principle greatly expanding the reliability and information content of the observations. We present here preliminary attempts at computing three dimensional correlation functions from simultaneous ACE and WIND data at L1. Both radially aligned [1] and mean magnetic field-aligned coordinate systems will be considered. [1] J. R. Richardson and K. I. Paularena, JGR, 106, 239 (2001)
SH12A-06 11:35h
Modeling Multispacecraft Observations of Flux Dropouts seen in Impulsive Solar Particle Events
We discuss new results from the analysis and modeling of the propagation of energetic particles associated with impulsive solar flares along the braided interplanetary magnetic field. Observations made by ACE/ULEIS have shown that many impulsive solar-flare events (tens of keV to MeV energies) often reveal fine-scale variations in which the intensity flickers on and off during the event. The flux dropouts are non-dispersive. One accepted interpretation of these events consists of intermittent particle fluxes resulting from the passage of the spacecraft through alternately filled and empty flux tubes. These variations are caused by field lines which were either connected or not connected to the flare site. Here we extend our previous analysis of these events (Giacalone et al.~Astrophys.~J.~Lett., {\it 532}, L75, 2000) and simulate the expected flux variations seen by more than one spacecraft with arbitrary orientations. We will compare the model results with observations made by WIND and ACE, an extension of the work by Mazur et al.~(Astropyhs.~J.~Lett., {\it 532}, L79, 2000), whenever possible.
SH12A-07 11:50h
Energetic Particle Acceleration at Near-Earth by Interplanetary Shocks
Interplanetary (IP) shocks provide an excellent opportunity to study energetic particle acceleration. During an IP shock related energetic storm particle (ESP) event, both the shock and the accelerated particles can be measured in-situ simultaneously. Thus, a study of in-situ IP shocks and particle distributions in their vicinity provides a meaningful way to test our theoretical understanding of shock acceleration. In addition, with simultaneous multi-point measurements (such as ACE and Wind), we have for the first time enough data to construct a global picture of IP shock propagation and acceleration from L1 to the Earth. During the current solar cycle, advanced particle instruments on board both the ACE and Wind spacecraft have detected numerous ESP events and therefore provided us with an ideal dataset to study IP shock acceleration in unprecedented detail. In this paper, we will combine simultaneous measurements of ACE/EPAM, ACE/ULEIS, and Wind/STEP during several ESP events and examine the spatial and temporal variations of these events in the Earth's vicinity. We will also investigate the ion composition and energy spectra for some of these events and investigate their relationship with the locally measured properties of the IP shocks.
SH12A-08 12:05h
Four-Spacecraft Observations of Delayed Energetic-Particle Events Near Earth
We consider data from four spacecraft: ACE (plasma, field, and energetic particles), SOHO (plasma and energetic particles), Genesis (plasma and field direction), and Wind (plasma, field, and energetic particles). The study is limited to four typical energetic-particle events following solar east-side flares in which the peak particle flux was observed between 5 and 13 hours after passage of a fast forward shock. The three spacecraft with energetic particle detectors saw approximately the same large-scale intensity profiles, but the localized increase at the shock passage differed from one spacecraft to another. Passage of tangential discontinuities affected the energetic-particle intensities both ahead of and behind the shocks. The shock parameters calculated from Rankine-Hugoniot fits to the ACE data were consistent with only one third of the shock arrival times at the other three spacecraft. This misfit suggests some curvature or spatial variation of the shock front.