|Session:||Poster session: Magnetic Field Transport from the Sun to the Heliosphere (12)|
|Date:||Monday, June 13, 2005|
|Time:||00:00 - 00:00|
The rigidity dependence of galactic cosmic ray modulation
University of New Mexico, UNITED STATES
Observed modulations of galactic cosmic rays (GCRs), on all time scales, contain a wealth of information about GCR mode of transport in the heliosphere. One way to extract some of this information from the data is to study the rigidity dependence of the modulations. For this purpose, we use data obtained with a variety of detectors on ground and mountain locations of the global network as well as the data obtained with detectors carried on balloons, satellites and space probes. For such studies to be meaningful, it is very important to have a very clear understanding of the response characteristics of the detectors involved to GCR spectrum. Sadly, there is a great deal of confusion on this topic even in papers published in the refereed journals. For example, the median rigidity of response (Rm) to GCR spectrum for a neutron monitor at Mt. Washington is stated as ranging from 5.4 GV to 14 GV, by the same authors, at different times. We define Rm as the GCR rigidity below which lies 50 % of detector counting rate. It is easily calculated from the latitude survey data obtained at sea level and at higher altitudes with detectors carried on aircrafts. Several such surveys have been carried out, in the last five decades, in different parts of the world. We discuss some of the values of Rm computed by us for some observing sites of the worldwide network of detectors. We use them to study the rigidity dependence of some modulations and discuss the physical significance of our results.
Investigation of the Stream Structure and Coronal Sources of the solar Wind Using Improved, Updated Mount Wilson Photospheric Field synoptic Maps: The April 7th and May 12th, 1997 Halo CMEs Events
Arge, Charles1; de Toma, G.2; Odstrcil, D.3
1Air Force Reseach Laboratory, UNITED STATES; 2NCAR/HAO, UNITED STATES; 3CU/CIRES & NOAA/SEC, UNITED STATES
We have developed a generalized code that can assemble synoptic maps of various types (e.g., standard Carrington, updated, as well as other specialized types) using virtually any type of solar disk data. With this code, we have constructed new, 0.5° resolution, updated Mount Wilson Solar Observatory (MWO) photospheric field synoptic maps for the two Carrington rotation (CR) time interval encompassing the April 7 and May 12, 1997 halo coronal mass ejections (i.e., CR1921-1922). The new maps are made from 0.5° resolution remapped MWO magnetograms that have been corrected for line saturation effects as well as having the line-of-sight field measurements converted to radial orientation. A set of 0.2° SOHO/EIT 28.4nm and 19.5nm EUV maps have also been constructed for these two rotations using our assembly routine and calibrated EIT disk data. We use these different synoptic maps in combination to better understand the coronal sources and stream structure around the times of the two CME events. The new, updated MWO maps are used as input to the Wang-Sheeley-Arge (WSA) model, which is used to calculate the global coronal field configuration and the ambient solar wind speed and IMF polarity near Earth. The results generated by the WSA model are then compared with the WIND satellite observations near Earth as well as the EIT coronal observations (both individual images as well as synoptic maps). We also use the new MWO updated maps to drive the coupled WSA+ENLIL model during this same time interval in an effort to obtain a more accurate global prediction of the background solar wind structure, as it plays an important role in simulations of transient interplanetary disturbances.
TEARING AND KELVIN-HELMHOLTZ INSTABILITIES IN THE HELIOSPHERIC PLASMA
Bettarini, Lapo1; Rappazzo, F. A.2; Landi, S.1; Velli, M.1
1Universitŕ di Firenze, ITALY; 2Universitŕ di Pisa, ITALY
Free energy, in the form of gradients in velocity and magnetic fields are able to start macroscopic instabilities in a stellar wind, such as Kelvin-Helmholtz and tearing modes. These instabilities drive changes in the initial magnetic topology of the system as well as energy conversion and, ultimately, dissipation. Here we analyze the magnetohydrodynamic instabilities arising from an initial equilibrium configuration consisting of a plasma jet or wake in the presence of a magnetic field with strong transverse gradients, such as for example surrounding the current sheet in the solar wind, using 2.5D simulations. Our analysis extends previous results by considering two different configurations of equilibrium for the jet in the heliospheric plasma sheet: a ``force-free'' configuration of the magnetic field which doesn't vanish anywhere but rather ``rotates'' through the flow an changes polarity; a ``pressure'' balanced configuration where we suppose that the magnetic field has a null point in correspondence of the fluid jet. In both cases we account for the possible non-alignment of the field with the solar/stellar wind stream structures due to solar, or in general stellar, rotation. A limit-case configuration in the second case leads us to analyze the stability of a fluid jet with a transverse magnetic field, which has been found to originate at the edge of the solar system in correspondence of the Termination Shock on the solar equatorial plane and propagate through the Heliosheath and the Heliopause (Opher M. et al, ApJ, 591, L61, 2003): we confirm its evolution to follow a typical Kelvin-Helmholtz instability, calculate the growth rate and compare the evolution with the fully 3D simulations.
Combined EISCAT/ESR/MERLIN Interplanetary Scintillation Observations of the Solar Wind
Bisi, M. M.1; Breen, A. R.1; Fallows, R. A.1; Thomasson, P.2; Jordan, C.2; Jones, R. A.1; Wannberg, G.3
1Institute of Mathematical and Physical Sciences, University of Wales, Aberystwyth, UNITED KINGDOM; 2Jodrell Bank Observatory, University of Manchester, UNITED KINGDOM; 3EISCAT Scientific Association, Kiruna, SWEDEN
Observations of interplanetary scintillation (IPS) provide a view of the solar wind at all heliographic latitudes inside of any current in-situ measurements down to the C2 LASCO field of view. Long baseline (several hundred kilometres) IPS observations have provided increased velocity resolution in the line-of-site and an indication of off-radial solar wind flow (e.g. EISCAT observations as reported in Moran et. al., Annales Geophysicae, 16(10), 1998). Here, we provide a report of the ongoing combined campaigns using the EISCAT, ESR and MERLIN systems with maximum baselines of around 2000 km. The sensitivity of the observations is increased dramatically by such long baselines which allows for an improved accuracy of detecting intra-stream structure and off-radial flows of the solar wind. The 2004 EISCAT-MERLIN campaign results provided an accurate indication of the direction of the solar wind in the inner heliosphere and further work will be carried out with the April/May 2005 campaign. This information allows a greater understanding of the overall structure of solar wind streams and thus the large-scale structure of the magnetic field of the solar wind at IPS distances.
Radial evolution of cross helicity at low and high latitudes in the solar wind
Breech, Ben1; Matthaeus, W.H1; Minnie, J.2; Parhi, S.1; Oughton, S.3; Bieber, J.W.1; Bavassano, B.4
1Bartol Research Institute, UNITED STATES; 2North-West University, SOUTH AFRICA; 3University of Waikato, NEW ZEALAND; 4Istituto di Fisica dello Spazio Interplanetario, ITALY
We employ a turbulence transport theory to the radial evolution of the solar wind at both low and high latitudes. The theory includes cross helicity, magnetohydrodynamic (MHD) turbulence, and driving by shear and pickup ions. The radial decrease of cross helicity, observed in both low and high latitudes, can be accounted for by including sufficient shear driving to overcome the tendency of MHD turbulence to produce Alfvenic states. The shear driving is weaker at high latitudes leading to a slower evolution. Model results are compared with observations from Ulysses and Voyager.
Physical Models to Forecast Dst Index: a Comparison of Results
Cid, Consuelo; Saiz, E.; Cerrato, Y.; Sequeiros, J.
University of Alcala, SPAIN
One of the applications of space weather is to forecast storm events. With this aim, a great effort has been made in the development of models that let to forecast the Dst index, as indicator of geomagnetic storms. In this work we show a comparison of different physical models for predicting the Dst index. We use Dst data provided by World Data Center for Geomagnetism (Kyoto) and solar wind plasma and magnetic field from ACE spacecraft, to analyze different storm events of this solar cycle. We use the Bastille day storm (July 2000) as a test case for several models (Cerrato et al., Burton et al. and RAM code).
Linking coronal to heliospheric magnetic helicity: A new model-independent technique to compute helicity in magnetic clouds.
Dasso, Sergio1; Mandrini, C.H.2; Gulisano, A. M.2; Luoni, M. L.2; Demoulin, P.3
1Instituto de Astronomia y Fisica del Espacio (IAFE), ARGENTINA; 2Instituto de Astronomia y Fisica del Espacio (IAFE)., ARGENTINA; 3Observatoire de Paris, LESIA., FRANCE
Magnetic Clouds (MCs) are a subset of interplanetary coronal mass ejections, which carry a significant amount of large scale magnetic helicity (MH) away from the solar corona as they travel to the external heliosphere. From a theoretical point of view, it is expected that MH be preserved in the solar corona and the heliosphere. In particular, it will be preserved in MCs during their evolution through the interplanetary medium. Thus, MH plays a key role to link the magnetic properties of MCs with their solar active region sources, helping us to improve the knowledge of the ejection mechanisms in the solar corona. MCs present a large and coherent rotation of the magnetic field vector to a heliospheric observer; thus, the large scale magnetic structure of MCs is consistent with a helical flux rope. However, since in situ spacecraft measurements are obtained only along a linear cut of the cloud, their global magnetic shape and, consequently, their MH content are not yet known. We present and apply here a new method/technique that only assume a circular section for the cloud. This method provides values for the cloud MH per unit length (L) along the tube axis using the observed interplanetary magnetic field. We apply this method to determine the content of MH in two magnetic clouds, one of the smallest and one of the biggest ever observed, and compare them with the MH ejected from their respective solar sources. We find that the accumulative MH/L computed from the observations may also help us to improve the determination of the cloud boundaries.
The International Heliophysical Year
Davila, Joseph1; Crooker, N.2; Gopalswamy, N.3; Thompson, B. J.3; Thompson, B. J.3
1NASA, UNITED STATES; 2BU, UNITED STATES; 3GSFC, UNITED STATES
The International Heliophysical Year (IHY), an international program of scientific collaboration to understand the external drivers of planetary environments, will be conducted in 2007. The IHY will involve the deployment of new instrumentation, new observations from the ground and in space, and an education component. The IHY 2007 will coincide with the fiftieth anniversary of the International Geophysical Year (IGY) in 1957. The IGY was organized to study global phenomena of the Earth and Geospace involving about 60,000 scientists from 66 nations, working at thousands of stations, around the world to obtain simultaneous, global observations from the ground and space. Building on results obtained during IGY 1957, the IHY will expand to the study of universal processes in the solar system that affect the interplanetary and terrestrial environments. The study of energetic events in the solar system will pave the way for safe human space travel to the Moon and planets in the future, and it will serve to inspire the next generation of space physicists. Overarching goals of the IHY are to: (1) Provide benchmark measurements of the response of the magnetosphere, the ionosphere, the lower atmosphere and Earth surface to identify global processes and drivers which affect the terrestrial environment and climate, (2) Global study of the Sun-heliosphere system outward to the heliopause, the new frontier, (3) Foster international scientific cooperation in the study of Heliophysical phenomena now and in the future, and (4) Communicate the unique scientific results of the IHY to the scientific community and to the general public A major thrust of the IHY is to deploy arrays of small instruments such as magnetometers, radio antennas, GPS receivers, all-sky cameras, etc. around the world to provide global measurements of heliospheric phenomena. Scientific teams will be organized through United Nations Basic Space Science (UNBSS) program. Each will consist of a lead scientist who will provide the instruments or fabrication plans for instruments in the array. Scientists from UNBSS member states will participate in the instrument operation, data collection, analysis, and publication of scientific results. Support for local scientists, facilities and data acquisition will be provided by the host nation. The ground based data will be complemented by from space based missions directly measuring near-Earth and heliospheric phenomena. In addition a history program has been established to preserve the record and accomplishments of the IGY, and the IGY Gold Club was established to honor IGY participants.
Observed and Modeled Coronal Holes
de Toma, Giuliana1; Arge, C.N2; Riley, P.3
1NCAR/HAO, UNITED STATES; 2AFRL, UNITED STATES; 3SAIC, UNITED STATES
We combine relative intensity images of the solar chromosphere and corona with magnetograms to identify coronal hole regions on the Sun. We compare them with the coronal holes - defined as the foot-points of magnetically open field lines - derived with two different coronal models: a PFSS model (Arge and Pizzo, 2000) and a 3D MHD model (Riley, Linker, and Mikic, 2001) that use the same magnetic field maps as photospheric boundary. The fraction of the observed coronal holes found by coronal models gives us a means to validate how well models reproduce observations. Cases for different level of solar activity during solar cycle 23 are presented and discussed.
On the diamagnetic action of thermal and nonthermal solar wind ions on the distant heliospheric magnetic field
Fahr , Hans Joerg; Scherer, K.
University of Bonn, GERMANY
Ions and electrons of a magnetized plasma by their gyrations around the local magnetic field produce magnetic moments which are oriented antiparallel to the prevailing field and thus partly compensate its magnitude. To include this diamagnetic action of plasma particles one has to differentiate between the magnetic vacuum field and the magnetic induction. We show that at distances larger than 20 AU this diamagnetic action mainly is due to suprathermal ions, like pick-up ions (PUI`s), in the solar wind. We derive an expression for the magnetic induction B as function of the vacuum magnetic field H and the PUI pressure which, when integrated with respect to solar distance, describes the radial variation of B, and shows that the B-values to be expected from these calculations systematically fall below the Parker expectations. We also show that the diamagnetic action of energized ions change the compression ratio of the magnetic induction B at the solar wind termination shock. Different from classical MHD shock relations the compression value for B strongly depends on the Alfvenic and sonic Mach numbers of the upstream solar wind. This may shed new light on the VOYAGER-1 shock traverse controversy.
Turbulence and Third Moment of Fluctuations: Kolmogorov's 4/5 law and its MHD Analogues in the Solar Wind
Forman, Miriam1; Smith, CW2; MacBride, BT2
1Stony Brook University, UNITED STATES; 2University of New Hampshire, UNITED STATES
The assertion that the third moment of turbulent velocity fluctuations at lag L, is proportional to L times the energy dissipation rate per unit mass, ε, is fundamental to Kolmogorov’s early and late theories of intermittent inertial-range turbulence. All subsequent models of the structure function of intermittent inertial-range hydrodynamic turbulence force the third moment to be proportional to L. This is usually thought of as the scaling of the third moment of the absolute value of the fluctuations, since in the symmetric distribution of fluctuations (the “pdf”) theoretical model cascades produce, the odd moments of the signed fluctuations are identically zero. However, Kolmogorov’s “4/5 law” more specifically states that the third moment of signed longitudinal fluctuations is not only not zero, but quite importantly equal to minus 4/5 εL. Longitudinal fluctuations are the component of velocity fluctuations parallel to the lag direction. For solar wind measured by a single spacecraft, the longitudinal fluctuation (using the Taylor hypothesis) is –[Vr(t+L)- Vr(t)]. The 4/5 law then states that <[Vr(t+L/V)- Vr(t)]3> = + 4/5 εL. This implies that the pdf of the fluctuations in radial component of the solar wind cannot be symmetric in an energy-conserving cascade, i.e., in an inertial range. We are not aware of a theory of the shape and scaling of the pdf that would produce the sign of the third moment.
Simulations of solar wind turbulence
Goldstein, Melvyn; Roberts, D. Aaron
NASA Goddard Space Flight Center, UNITED STATES
Simulations of magnetofluid turbulence using a variety of techniques for solving the equations of magnetohydrodynamics have elucidated many aspects of the evolution and properties of turbulence in the solar wind. In particular, the simulations have shown the important role of velocity shear in driving the evolution of turbulence. MHD simulations have demonstrated the initiation of turbulent cascades arising from an initial packet of Alfv\'en waves, the evolution of cross helicity (the correlation between the fluctuating magnetic and velocity field components), and the formation and evolution of corotating interaction regions. Puzzles remain, however. Simulations to date have not provided a satisfactory explanation of the observed fact that the direction of minimum variance of magnetic fluctuations tends to be oriented along the direction of the background magnetic field. In addition, the observation that the Alfv\'en ratio (the ratio of plasma pressure to magnetic pressure) is typically less than unity has not been simulated satisfactorily. We report recent studies of these phenomena using a high resolution finite difference code.
Large scale properties of magnetic clouds using different
approaches to estimate their orientation and impact parameter.
Gulisano, Adriana Maria1; Gulisano, A. M.2; Dasso, S.3; Mandrini, C. H.2; Demoulin, P.4
1Instituto de Astronomia y Fisica del Espacio (IAFE), ARGENTINA; 2IAFE, ARGENTINA; 3IAFE, Departamento de Fisica FCEyN (UBA), ARGENTINA; 4Obsevatoire de Paris, Meudon Cedex, FRANCE
Magnetic clouds are the interplanetary manifestation of solar ejecta that at 1 astronomical unit (AU) present to a heliospheric observer distinctive properties when compared to the typical solar wind. We study a set of 20 magnetic clouds observed by the spacecraft Wind at 1 AU during a solar minimum. From in situ magnetic observations we estimate their (large scale) magnetic helicity content per unit length (along the axis) and fluxes. We assume a cylindrical symmetry for the magnetic structure of the cloud. For every cloud we analyze the variation of the computed amounts of flux and helicity using different approaches to determine the orientation of the cloud axis and its minimum distance to the spacecraft (i.e., the impact parameter). We also explore different models (force free and non-force free fields) to reconstruct the magnetic structure of the clouds in the 2D section perpendicular to the cloud axis from the observations, which only give information along the (1D) spacecraft trajectory. Estimations of global magnitudes of magnetic clouds are one of the major keys used to link solar and interplanetary events; thus, the results presented here will help to refine the finding of this link from the interplanetary point of view.
The Dance of the Bashful Ballerina in 1926-2004
Hiltula, T.; Mursula, K.
Department of Physical Sciences, University of Oulu, FINLAND
We have recently shown using satellite observations of the heliospheric magnetic field (HMF) that the heliospheric current sheet (HCS) is shifted or coned southward for a few years close to the solar activity minima. This is a regular pattern (also called the bashful ballerina) during the last 40 years of direct HMF observations. The same pattern has also been found more recently for the last 25 years using observations and modelling of the solar magnetic field. Here we extend the study of the heliospheric current sheet by analysing the HMF polarity for the time before direct satellite measurements. We use several HMF polarity datasets inferred from the ground-based observations and compare them to direct HMF observations during the overlapping period. The longest dataset covers the years 1926-2003. We calculate the annually averaged HCS shift and the mean sector widths. The results of the HCS behaviour calculated with the different methods and from the different datasets are compared. We also estimate the reliability of the different datasets.
Observations of solar wind features from spacecraft separated in heliolongitude.
Lazarus, A.J.1; A.J., Lazarus1; Kasper, J.1; Ashmall, J.1; Skoug, R.2; Steinberg, J.2; Ogilvie, K.3; Szabo, A.4; Smith, C.5; Quiang, H.6
1MIT, UNITED STATES; 2LANL, UNITED STATES; 3`NASA/GSFC, UNITED STATES; 4NASA/GSFC, UNITED STATES; 5UNH, UNITED STATES; 6UCR, UNITED STATES
During the period of 2000 to 2002, the Wind spacecraft was able to observe solar wind features from a range in YGSE +/- 300 Re, and to compare propagation times of features between ACE and Wind.. This study completes earlier observations and confirms that the front normals of such features have radial or Parker spiral orientations. The two types of orientations of front normals produce different characteristic delays as the features propagate from ACE to Wind; thus they will significantly influence predictions of the delays expected between the STEREO spacecraft and the arrival of the features in near-Earth space. These results are consistent with recent efforts to characterise front normals through minimum variance studies of the associated magnetic fields.
Nature of the Reconnecting CME/Flare Current Sheet
Lin, Jun1; Li, J.2; Forbes, T. G.3; Ko, Y.-K.1; Raymond, J. C.1; van Ballegooijen, A. A.1
1Harvard-Smithsonian Center for Astrophysics, UNITED STATES; 2IFA, University of Hawaii, UNITED STATES; 3EOS, University of New Hampshire, UNITED STATES
In the present work, we display our results of studying and analyzing the observational data from UVCS/SOHO and other remote sensing instruments for three CME/flare events that obviously developed a long current sheet during the eruptions. These results include the thickness of the current sheets, magnetic diffusivities and electrical conductivities (resistivities) of the plasma inside the current sheets. This is the first time that the electrical conductivity (resistivity) within magnetic reconnection region during the real eruptive processes has been deduced since the theory of magnetic reconnection was applied to the solar eruptions about 6 decades ago. Our results indicate that values of magnetic diffusivity deduced for three different events are within the range of magnitude, and that they are all 10~12 and 6~8 orders of magnitude greater than those of the classical and the anomalous diffusivities, respectively.
Unraveling space and time in the solar wind:
Preliminary results on the 3D magnetic autocorrelation
from ACE and WIND simultaneous data
Weygand, J1; Matthaeus, W H2; Dasso, S3; Milano, L J4; Kivelson, M G1; Smith, C W5
1IGPP, UCLA, UNITED STATES; 2Bartol. University of Delaware, UNITED STATES; 3IAFE, Buenos Aires, ARGENTINA; 4Bartol, University of Delaware, UNITED STATES; 5EOS, University of New Hampshire, UNITED STATES
Statistical characterizations of the space and time structure of solar wind fluctuations have typically been limited by the use of the frozen-in approximation, namely that the solar wind is "rigidly moving," which is made to translate temporal series into spatial series. However in various theoretical and predictive frameworks, including notably classical turbulence theory, the quantities of interest are either second order spatial correlations (two point, single time) or second order time correlations (two time, single point). More generally, 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. With the advent of multiple spacecraft data, various new possibilities arise. First, one can evaluate two point, second order spatial correlation functions directly, in principle greatly expanding the reliability and information content of the observations. Here we present here preliminary estimates of second order spatial correlation functions from simultaneous ACE and WIND magnetic field data at L1. Both radially aligned  and mean magnetic field-aligned coordinate systems are considered. Second, the frozen in approximation can be quantitatively tested, providing practical estimates its accuracy. Finally, we can devise an approximation, analogous to the frozen-in approximation, that allows estimation of the two-time, single point (Eulerian) correlation function that relates directly to particle scattering theory and predictability of solar wind disturbances. We will discuss methods for addressing all three of these objectives, and will present preliminary results for the first.  J. R. Richardson and K. I. Paularena, JGR, 106, 239 (2001)
Long-term observations of HMF sector structure at 1AU: Flip-flops of a bashful ballerina
Mursula, Kalevi; Hiltula, T
University of Oulu, FINLAND
We discuss the recent developments and implications of the long-term observations of the heliospheric magnetic field (HMF) observed at 1 AU. E.g., it has been shown recently that the HMF sector coming from the northern solar hemisphere systematically dominates around solar minimum times. This leads to a persistent southward shift or coning of the heliospheric current sheet at these times that can be picturesquely described by the concept of a bashful ballerina. This result has recently been verified by direct measurements of the solar magnetic field, and implies that the Sun has a large-scale quadrupole magnetic moment (an S0 dynamo mode). Moreover, it has been shown that the global HMF has persistent active longitudes whose dominance depicts an oscillation with a period of about 3.2 years. This new flip-flop periodicity in HMF is most likely related to a similar periodicity recently found in sunspots. This implies the need of a non-axisymmetric dynamo mode.
Helicity analysis for expanding magnetic clouds: A case study.
Nakwacki, Maria Soledad1; Dasso, S.2; Mandrini, C.H.2; Demoulin, P.3
1Instituto de Astronomia y Fisica del Espacio (IAFE), ARGENTINA; 2Instituto de Astronomia y Fisica del Espacio (IAFE), CONICET-UBA, ARGENTINA; 3Observatoire de Paris, LESIA., FRANCE
Magnetic Clouds (MCs) are the interplanetary counterpart of coronal mass ejections (CMEs). They transport in the interplanetary medium the magnetic flux and helicity released in CMEs by the Sun. At 1 AU from the Sun, MCs are generally modeled as static flux ropes, i.e., considering them as rigid bodies travelling through the solar wind with constant speed. However, the velocity profile of some MCs present clear signatures of a significant expansion while observed in situ by a spacecraft in the heliosphere. We present here an analysis of the magnetic structure of an expanding magnetic cloud observed during the last increasing phase of the solar cycle by the spacecraft Wind. We consider a dynamical model, based on a self-similar behaviour for the radial velocity. We assume a free expansion for the cloud, and a cylindrical linear force free magnetic field (i.e., the Lundquist's field) as the initial condition for its magnetic field configuration. We derive theoretical expressions for the magnetic flux across a surface perpendicular to the cloud axis and for the magnetic helicity per unit length along the tube using the self-similar model. Finally, we compute these magntitudes fitting the free parameters of the expanding model to the in situ magnetic observations, and compare them with those obtained from the linear force free static model.
A Two-Component Phenomenology for the Evolution of MHD Turbulence
Oughton, S.1; Dmitruk, P.2; Matthaeus, W.H.2
1University of Waikato, NEW ZEALAND; 2Bartol Research Institute, University of Delaware, UNITED STATES
Theory, simulations, and observational evidence suggest that incompressible MHD turbulence with a mean magnetic field B0 develops anisotropic spectral structure and can be simply described only by including at least two distinct fluctuation components. These are conveniently referred to as ``waves,'' for which propagation effects are important, and ``quasi-2D'' turbulence, for which nonlinear effects dominate over propagation ones. The quasi-2D component has wavevectors approximately perpendicular to B0. These two idealized ingredients capture the essential physics of propagation (high frequency fluctuations) and strong turbulence (low frequency fluctuations.) Here we present a two-component energy-containing range phenomenology for the evolution of homogeneous MHD turbulence. Account is taken of both the self interactions of each component and also the couplings between the two components. As some of the couplings are resonant they have the potential to dominate the remaining couplings. The model is developed using Elsasser variables and can include forcing terms. Various solutions to the model will be discussed. We expect that such two-component phenomenologies will be of use in models of the evolution of coronal and solar wind fluctuations.
INITIATION OF CORONAL MASS EJECTION BY MAGNETIC FLUX EMERGENCE
Poedts, Stefaan; Dubey, G.; van der Holst, B.
The initiation of solar Coronal Mass Ejections (CMEs) is studied by means of numerical simulations. The initial CME model includes a magnetic flux rope in 2.5D spherical geometry, i.e. axial symmetry is assumed. In the initial equilibrium configuration the magnetic flux rope, suspended in the corona in the presence of a background magnetic field, is balanced by the magnetic tension, compression, gravitational and curvature forces.
Triggering CMEs by Magnetic Footpoint Shearing
Poedts, Stefaan; Jacobs, C.; van der Holst, B.
A numerical parameter study is performed to analyse the efficiency of magnetic footpoint shearing as a possible mechanism to trigger solar Coronal Mass Ejections (CMEs). The starting points of our simulations consist of three different and frequently used 2.5D (i.e. axi-symmetric) solar wind models, viz. the popular polytropic wind model, an MHD wind model with an additional heating term, and a polytropic wind model with an Alfvén wave heating term. These models have been reconstructed with the same numerical code/technique (VAC) on the same numerical grid. Any differences in the results are thus entirely due to the physics.
Multispacecraft Observation and Simulation of Solar Wind Structure and Dynamics
Roberts, D A1; Goldstein, M. L.2
1NASA Goddard Space Flight Center, UNITED STATES; 2NASA GSFC, UNITED STATES
More than a dozen spacecraft have informed us about the structure and
dynamics of the heliosphere on a wide range of temporal and spatial scales.
Complementing this, MHD simulations can provide closure on mechanisms to
explain the data as well as global pictures to complement local
observations. This talk will review our recent progress, focusing on issues
such as the structure of the heliospheric current sheet; the possible
topological importance of planar magnetic structures and associated magnetic
holes; the paths of heliospheric field lines; and the evolution of
turbulence. Novel visualization methods allow us to see the solar wind
structures with unprecedented clarity.
The hemispheric model for pitch angle scattering of pickup ions: a comparison with SOHO/CTOF observations and IMF wave-power from WIND.
Saul, Lukas; MĂ¶bius, E.; Isenberg, P.; Smith, C.W.
UNH, UNITED STATES
In situ observations in the solar wind have indicated that interstellar pickup ion (PUI) distributions can be strongly anisotropic, indicating larger mean free paths to pitch angle scattering than first predicted [Gloeckler et al., 1995; MĂ¶bius et al., 1998]. One possible reason for this discrepancy is the â€śresonant gapâ€ť which prevents pitch angle scattering past 90° by resonant parallel propagating waves. A hemispheric model of the PUI distribution has been suggested to include a lower scattering rate across 90°, and improve on standard diffusion models [Isenberg, 1997]. Presented are measurements of interstellar pickup He+ velocity distributions from SOHO/CTOF compared to predictions of the hemispheric model. We concentrate on times of near radial IMF, where the transport equations have been solved analytically [Isenberg, 1997]. The dependence of pitch angle scattering rates on IMF wave power are determined using magnetic field data from WIND/MFI. The hemispherical model predictions agree well with the data, across two and a half orders of magnitude of IMF fluctuation power. The cross-hemisphere scattering rate is found to be exponentially dependent on the wave-power. Gloeckler, G., Schwadron, N. A., Fisk, L. A., & Geiss, J. 1995, GRL, 22, 2665 MĂ¶bius, E., Rucinski, D., Lee, M. A., & Isenberg, P. A. 1998, JGR, 103, 257 Isenberg, P. A. 1997, JGR, 102, 4719
Solar Wind from the Coronal Hole Boundaries
Schwadron, Nathan1; Schwadron, N. A.1; McComas, D. J.1; Elliott, H. A.1; Gloeckler, G.2; Geiss, J.3; von Steiger, R.3
1Southwest Res. Institute, UNITED STATES; 2University of Maryland, UNITED STATES; 3International Space Science Institute, SWITZERLAND
Recent studies using in situ observations established that the interface between fast and slow wind in interplanetary space has two distinct parts: a smoothly varying boundary layer flow that flanks fast wind from coronal holes, and a sharper plasma discontinuity between intermediate and slow solar wind. Other studies using in situ observations and modeling have demonstrated the existence of the sub-Parker spiral structure of the heliospheric magnetic field in which the magnetic connection between fast and slow wind created by footpoint motion at the Sun deforms field lines, making them significantly less transverse than the Parker spiral. Here we model the formation of co-rotating interaction regions (CIRs), and by including a coronal hole boundary layer (CHBL) and magnetic footpoint motion across the coronal hole boundary back at the Sun, explain the detailed, characteristic variations in composition and magnetic field orientation observed in interplanetary space. Our model accomplishes this using only two free parameters, with all other quantities derived directly from solar wind observations. Through the model we trace the observed interplanetary variations back to an intrinsic two-part structure in the source of solar wind at the Sun. These parts are (1) a CHBL that encircles the coronal hole and has a smooth transition in the source properties that produce the fast through intermediate speed (~600 km/s) solar wind; and (2) a sharp coronal hole discontinuity separating the distinct sources of solar wind with intermediate speeds and temperatures from slow solar wind. This study establishes the connection between the characteristic variations of the solar wind speed, charge-state composition, and magnetic field orientation observed in situ near 5 AU with their sources in the two-part structure of coronal hole boundaries back at the Sun.
Evolution of solar wind magnetic field structure between 5 and 10 AU: Ulysses and Cassini
Smith, Edward1; Zhou, X-Y1; McComas, D. J.2; Dougherty, M.K.3; Burton, M.E.1
1Jet Propulsion Laboratory, UNITED STATES; 2Southwest Research Institute, UNITED STATES; 3Imperial College, UNITED KINGDOM
As Cassini approached and then orbited Saturn at @ 10 AU, Ulysses was near aphelion at @ 5AU and at low heliographic latitudes. During 2004-2005, the two spacecraft were also within tens of degrees of one another in heliographic longitude. This fortuitous alignment provides a unique opportunity to study the evolution of the solar wind between 5 and 10 AU during the on-going descent to sunspot minimum. Vector Helium Magnetometers on both spacecraft provide accurate measurements of the weak magnetic fields at the two locations. Using the Ulysses magnetic field and solar wind observations, essential features have been identified including Forward and Reverse shocks, Co-rotating Interaction Regions (CIRs), Co-rotating Rarefaction Regions (CRRs), Coronal Mass Ejections (CMEs) and the Heliospheric Current Sheet-Plasma Sheet. These structures have then been correlated with the same structures at Cassini as it approached Saturn and, subsequently, when bound orbits carried the spacecraft outside the Bow Shock back into the solar wind. A high degree of correlation and an improved understanding of the evolutionary changes should result in Ulysses being able to serve as an “upstream” solar wind monitor when Cassini is orbiting inside Saturn’s magnetosphere and thereby contribute to studies of magnetospheric dynamics.
Hybrid simulation study of surface waves supported by
University of New Hampshire, UNITED STATES
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 tangential discontinuities (TDs). These waves are noncompressive and are related to ones calculated by Hollweg in 1982 using linear magnetohydrodynamic (MHD) equations and assuming a cold plasma. Using a technque of minimum variance at a TD supporting such a surface wave causes the discontinuity to be misclassified as a rotational discontinuity. This comes about because the surface wave rotates the calculated normal by nearly 90 degrees to one nearly along the average magnetic field direction. The actual normal of the TD is at right angles to the average magnetic field. We relate our simulation results to recent observations of directional discontinuities using 3 to 4 spacecraft. In these observations, a single spacecraft very often misclassifies the discontinuity as compared to the type determined with all spacecraft. We consider how surface waves can explain these observations and examine the implications of having surface waves propagate in the solar wind.
Non-local couplings in the solar atmosphere and the heliosphere: methods of their theoretical description
Institute of Nuclear Physics, RUSSIAN FEDERATION
A short review focuses of existing models describing connections between solar and heliospheric processes based on following approaches: 1) dimensionless scaling and similarity; 2) statistics; 3) MHD equations, ideal ones or with dissipation and radiation; 4)self-consistent plasma kinetic and electrodynamic description. Justified and successful applications of these approaches will be outlined as well as their limitations, which are sometimes forgotten when considering energy, momentum and mass transports in the open solar-heliospheric system. Electric currents and magnetic fields play very important roles. They represent governing physical links connecting bottom layers with upper ones on the Sun and in the solar wind formation regions in many instances. Other links and regimes also often play comparable roles and make the situations so complicated and rich in their manifestations. Examples of alternative interpretations of the same phenomena will be presented and discussed. The observational and theoretical ways to solve open issues are considered.
Morphology and Evolution of the Heliospheric Current and Plasma Sheets
from 1 to 5 AU
Zhou, Xiaoyan1; Smith, Edward1; Winterhalter, Daniel1; McComas, Dave2; Skoug, Ruth3; Goldstein, Bruce1; Smith, Charles4
1Jet Propulsion Laboratory, UNITED STATES; 2Southwest Research Institute, UNITED STATES; 3Los Alamos National Laboratory, UNITED STATES; 4Univ. of New Hampshire, UNITED STATES
The heliospheric current sheet (HCS) serves as a magnetic equatorial plane between sectors of opposite IMF polarity. Surrounded by heliospheric plasma sheets (HPS) that are characterized by high-beta and pressure balance, the current sheet stretches throughout the heliosphere. This sector structure was first discovered by Ness and Wilcox about forty years ago. Its signatures at 1 AU were well studied using ISEE-3 by Winterhalter et al. in 1994. With spacecraft reaching out to 5 AU and beyond, questions about how the thickness of these structures will change and what determines the thickness of these structures were raised. To answer these fundamental questions, we have carried out a study of the HCS and HPS using recent Ulysses data near 5 AU. Using data under the same (or very similar) circumstances, we have extended the analysis in two ways. First, the same current-plasma sheets studied at 5 AU have been identified at 1 AU using ACE data. Second, data obtained while Ulysses was en-route to Jupiter near 3 AU have been analyzed. Comparisons of the results at 1 AU using ISEE-3 data and ACE data, and at ~3 and 5 AU using Ulysses data revealed the evolution of the HCS and HPS from 1 to 5 AU and a prediction of beyond.