SH21C-0429 0800h
Foundations for a Virtual Heliospheric Observatory: United L1 data sets, prototypical searching and distributed data processing
Recent work involving the L1 data sets has led to the automated synchronization of data among many of the L1 principal investigators. This in turn has led to combined data sets as well as more efficient processing and calibration and ultimately to high quality data products for the community. In addition, this work has been combined with initial and ongoing efforts to develop a Virtual Heliospheric Observatory (VHO). The current capabilities that include multiple search techniques through Simple Object Access Protocol and XML applications will be discussed. Additionally, we present our initial steps in building a prototype data processing environment for use within the VHO. Specifically, we incorporate a peer-to-peer architecture that contains basic services, such as coordinate transformations, and demonstrate its use through high-level data access tools from the ACE Science Center.
http://vho.nasa.gov
SH21C-0430 0800h
Hybrid Simulation Study of Waves Supported by Tangential Discontinuities
In the solar wind, multiple spacecraft have observed abrupt field rotations, called directional discontinuities. Surprisingly, single spacecraft misidentify many discontinuities as rotational ones. Timings from three or more spacecraft reveal that the discontinuity actually has a small magnetic field normal, more in agreement with a tangential discontinuity (TD) which has zero normal field component. The discrepancy at a single spacecraft is interpreted as a direct sign of the TDs supporting surface waves. Hollweg [1982] predicted the existence of linear noncompressive magnetohydrodynamic (MHD) surface wave solutions on solar-wind TDs where the magnetic field rotates across the layer. Because these waves are noncompressive, they would propagate unattenuated by collisionless resonant particle damping. Hollweg showed that such a wave on a TD causes the inferred normal component from a single spacecraft to appear large. Hollweg's analysis was limited to a cold plasma and linear MHD equations wherein the TDs are true discontinuities. We present numerical hybrid simulation results with particle ions and fluid electrons of the nonlinear behavior of finite-amplitude surface waves, in a warm plasma, and on finite-width TDs. We find that the initial surface wave can steepen when ducted between two TDs and generate body waves supported by the TDs. We examine how wave solutions supported by TDs can contribute to observed solar-wind fluctuations.
SH21C-0431 0800h
Multispacecraft study of the topology of magnetic flux ropes at 1 AU
Magnetic flux ropes are commonly observed by spacecraft in the solar wind at 1 AU. In this study, they are characterized by relatively large magnetic field magnitude, and smooth rotation of the magnetic field direction. They are not limited to well-defined magnetic clouds only. The Grad-Shafranov reconstruction technique has been applied to derive, quantitatively, the cross section of a magnetic flux rope directly from single spacecraft measurements. Previous studies have shown the non-axisymmetric cylindrical configuration of a single flux rope, and discovered multiple flux ropes embedded within otherwise a single structure, in an objective and self-consistent way. Applications of the technique to multiple spacecraft data sets are crucial to validate the technique and the results. Case studies are performed for several flux rope events when multi-point measurements from spacecraft ACE, Wind, and IMP8, etc, were available. The configuration of the flux rope structure as seen at each individual spacecraft will be visualized, and physical parameters associated with them will be reported and compared. In particular, we present a means to determine the magnetic helicity from our results. Possible evolution of the structure in the heliosphere will be discussed as well.
SH21C-0432 0800h
Insights into Planar Magnetic Solar Wind Structures Using New Visualization Methods
We examine the structure of the solar wind using one to four spacecraft at a time, applying a kinematic projection of the data to create spatial pictures. The results are visualized using ViSBARD (Visual System for Browsing, Analysis, and Retrieval of Data) that presents the results in 3-D, allowing a variety of manipulations. Specifically, we examine the degree to which a set of ``Planar Magnetic Structures" previously examined with ISEE-3 data are actually planar. Those with very low ratios of the minimum to the intermediate variance eigenvalues do appear quite planar, but as the ratio approaches the typical cutoff of ~2.5, it becomes difficult to identify what might be the plane of variation. We use the Alfv\'enicity of the intervals to show that the smoothly regions around the PMSs (which often form a ``cone" of magnetic vectors, consistent with the idea of a random walk on a sphere) are much more Alfv\'enic than the PMS structures. We also find that frequently the PMS structures show systematic rotations that suggest a small magnetic cloud structure. The evidence thus supports the idea that a large fraction of the PMS structures are non-wavelike flux-tubes that are convected outward from the Sun. The rapid rotations of the field used to characterize the PMS regions are very frequently associated with magnetic holes, consistent with spiral magnetic fields associated with current sheets that have been seen in MHD simulations.
http://nssdcftp.gsfc.nasa.gov/selected_software/visbard/
SH21C-0433 0800h
Multi-Spacecraft Observations of Energetic Heavy Ions Accelerated by Interplanetary Shocks Near Earth
Energetic ion intensity enhancements that occur in association with the arrival of interplanetary (IP) shocks driven by coronal mass ejections (CMEs) near Earth orbit are known as Energetic Storm Particle or ESP events. Such ESP events are believed to occur due to diffusive shock acceleration of solar wind ions. Although shock acceleration theory has been well studied and widely accepted, many key properties of the accelerated particles such as ion composition, peak intensity, and spectral index are at odds with theoretical expectations. During solar cycle 23, the Wind spacecraft and the Advanced Composition Explorer (ACE) have observed nearly $\sim$200 common IP shocks near 1 AU. A sizable number (30-40$%$) of these shocks were also accompanied by increases in the intensities of ions with 100's of keV to a few MeV in energy. These simultaneous observations offer us a unique opportunity to investigate various properties of energetic particles during individual IP shock events. We will examine the differences and similarities in the spatial and temporal evolution of the intensities, spectra, and composition of heavy ions from 4He through Fe during several shock events observed by two sophisticated mass spectrometers, the SupraThermal-through-Energetic Particle Telescope (STEP) on board Wind and the Ultra-Low Energy Isotope Spectrometer (ULEIS) on board ACE. We will also characterize our results in terms of the locally measured IP shock properties. Finally, we will discuss the main challenges that hinder current theoretical and experimental investigations and highlight new opportunities that will enable us to quantitatively model and predict properties of the IP shock accelerated populations.
SH21C-0434 0800h
Particle spectra and upstream turbulence at CME-driven shocks
Particle spectra at a CME-driven shock often exhibit a power law to certain energies, then roll over exponentially. However, there are cases where a spectrum evolves to another power law, often softer, above a certain energy (e.g. the Oct. 29th, 2003 event [Mewaldt et al., 2004]. By introducing an effective ``loss term'' to the transport equation, we show that under certain circumstances, a broken power law can be obtained. We discuss the physical meaning of the loss term and its relationship with the upstream turbulence. We note that observations of particle spectra at CME-driven shocks provide us a complimentary method for studying the upstream turbulence in front of the shock.
SH21C-0435 0800h
Compound and Perpendicular Diffusion of Solar Energetic Particles and Field Line Random Walk
Simulations of energetic charged particle transport in random magnetic fields by Giacalone and Jokipii (1999), Mace et al. (2000) and Qin et al. (2002,2003) demonstrate that the quasilinear theory of diffusion of cosmic rays normal to the magnetic field is somewhat unsatisfactory. A more satisfactory theory for perpendicular diffusion, which better fits the simulations is the nonlinear guiding center theory of Matthaeus et al. (2003); Zank et al. (2004) and Shalchi et al. (2004), but this theory does not account for compound diffusion of particles perpendicular to the field observed in near slab turbulence. In this paper, we use a model for particle transport normal to the random walking mean magnetic field, based on a Chapman Kolmogorov equation, in which the particle is transported relative to the random walking field line. The model indicates that the particle can undergo both compound diffusion and ordinary diffusion perpendicular to the mean magnetic field. If the particle propagates in a ballistic fashion along the field at early times (as described by the telegrapher equation), then the particle diffuses perpendicular to the field at essentially the classical field line random walk (FLRW) value given by Jokipii (1966). We discuss the relation between this model, and the work of Chuvilgin and Ptuskin (1993) on perpendicular and compound diffusion.
SH21C-0436 0800h
Correlation of Solar Wind Features Observed From Spacecraft Well-Separated in YGSE
How can we improve the predictive capabilities of pairs of LWS Sentinal observers? One contribution is the determination of the correlation and time delay of similar features in the solar wind observed from spacecraft well-separated in YSE (e.g., Wind and ACE in 2000 to 2001). Such correlation can yield information about the variability of the solar source of the wind as well as about evolution of the features between the observing points. If the source of the features were time-independent, one would expect features other than shocks to be corotating with the Sun and thus normals to their fronts would be aligned perpendicular to the Parker spiral. But earlier studies by our group as well as by others have indicated that the normals to the fronts lie between radial and the direction expected for corotation. Thus our goal is to.establish the nature of corotating vs. features with radially-oriented normals. By eliminating shocks and by insisting that the features show good, simultaneous correlation in at least three parameters (among velocity, density, thermal speed, and magnetic field) as well as pressure-balance, we have identified a set of features that are clearly corotating. Without such stringent selection criteria, we reproduce the results of earlier studies which show that the average alignment of normals to the fronts of correlated features to be between radial and that expected for corotation. We also examine several events with excellent correlations with delays that fall outside the range expected for either corotating or radial features.
SH21C-0437 0800h
Correlation of Solar Wind Parameters between SoHO and Wind
SoHO was launched in December 1995, and reached L1 in January 1996. Since then it has been in a halo orbit about L1 with a semi-major axis of $103 R_e (\Delta Y_{GSE})$ and a semi-minor axis of $31 R_e$. $\Delta Z_{GSE}$ is approximately $19 R_e$. Wind was launched in November 1994, and has occupied a variety of orbits, spending most of the time in the interplanetary medium at distances greater than $100 R_e$ from Earth. Plasma instruments aboard SoHO and Wind have been operating reliably and stably since the minimum of solar cycle 23 in 1996, and have provided an opportunity to study correlations between their measurements over a period of more than half a solar cycle. We have calculated the maximum correlation coefficients between densities and fluxes measured at both spacecraft as a function of lag for the range $\pm 1$ hour (lag = time of maximum correlation - advection time). We find that the yearly averages of these correlations show a small decrease of 5% over the period 1996 through 2002, while the sunspot number increased by more than an order of magnitude. During the period 2000 through 2003, when Wind was in a distant pro-grade orbit, separation between the spacecraft was up to $\pm 400 R_e$ in the $Y_{GSE}$ direction. The correlation coefficients, averaged over 30 days, on each side of the XGSE axis, were equal to within measurement precision, indicating that the scale length in the interplanetary medium exceeds $800 R_e$. The yearly histograms of correlation coefficients for each of the years, 1996 through 2003, can be very well represented by the sum of two Gaussian functions, suggesting two distinct regimes. Individual events giving large correlation coefficients are due to shocks or discontinuities in the data stream. We suggest that events leading to small correlation coefficients can be associated with small-scale turbulence. These findings will be illustrated in a quantitative way.
SH21C-0438 0800h
Beam Width of Solar Electron Bursts Observed Below 1.3 keV
We present a statistical analysis of the beam width of solar electron bursts at energies below 1.3 keV using electron detectors on board the ACE and Genesis spacecraft. Pitch-angle dispersion is typically observed at the beginning of each event, leading to broadening of the pitch-angle distribution as the event progresses. By the time of peak intensity, some bursts have a pitch-angle distribution which is broader than the preceding strahl, although this is not universally true. We discuss the implication of these results for electron injection/acceleration at the Sun and electron propagation in the heliosphere.
SH21C-0439 0800h
Timing accuracy and variability of the planar propagation of magnetic field structures in the solar wind
The propagation time of the interplanetary magnetic field (IMF) from an upstream monitor to the magnetosphere may be significantly different from what would be predicted using the advection delay time at the solar wind velocity. This influences our ability to correlate magnetosphere-ionosphere phenomena with solar wind variations. Earlier studies revealed that variable lag times and tilted propagation planes in the IMF could account for this timing difference. Using 3 or more satellites the time varying orientation of the tilted propagation planes can be determined. Alternatively, the time varying plane orientations can be derived with data from a single spacecraft using a modified minimum variance approach. The validity of the derived planes for describing the structure and propagation of the IMF is evaluated by predicting the IMF observations at the location of an additional spacecraft not used in the calculation of the time varying plane orientations. The range of orientations and temporal variability of the propagation planes will be presented for several cases, comparing the 3-spacecraft solution with the single spacecraft solution. The orientation of the time varying propagation planes is observed to vary over a wide range of angles with time scales of 10's of minutes.
SH21C-0440 0800h
Propagation of Energetic Charged Particles in the Solar Wind: Effects of intermittency in the Medium
We address the issue of ``dropouts" observed by the Advanced Composition Explorer (ACE) spacecraft. We study the temporal evolution of energetic particle beams in a turbulent solar wind environment using the propagating source method [Zank et al., 1999, 2000], which is based on the separation of the total particle distribution function into a main beam component (unscattered part) and a secondary component produced by the scattered beam particles. Unlike recent approaches that try to explain the ``dropout" phenomenon using steep localized gradients in the medium normal to the background magnetic field, we show here that intermittent changes in the turbulence responsible for scattering particles in the radial direction of the solar wind medium can also contribute to the generation of fine-scale structure in the intensity profiles of impulsive events. Our aim here is to simplify the problem to the extent that it allows us to demonstrate the suggested process. Implications of a more complicated transport equation are also discussed.