SH34A-01 16:00h
Hemispherical and Longitudinal Asymmetries in the Heliospheric Magnetic Field: Flip-flops of a Bashful Ballerina
Several studies during many decennia have studied possible longitudinal and hemispherical asymmetries in various forms of solar activity. E.g., there are well known periods when one of the solar hemispheres has dominated the other in sunspot numbers, flare occurrence or some other form of solar activity. However, the solar asymmetries have not been found to be very conclusive, or to form any clear systematical patterns (e.g., relation to solar cycle). On the contrary, recent studies of similar longitudinal and hemispherical asymmetries in the heliospheric magnetic field have shown a very clear and systematic behaviour. E.g., it was found recently that the dominance of the two HMF sectors experiences an oscillation with a period of about 3.2 years. This new flip-flop periodicity in the heliospheric magnetic field is most likely related to a similar periodicity recently found in sunspots. Also, it has recently been found that the HMF sector coming from the northern solar hemisphere systematically dominates at 1AU during 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 also implies that the Sun has a large-scale quadrupole magnetic moment. Here we review these recent developments concerning the longitudinal and hemispherical asymmetries in the heliospheric magnetic field and study their inter-connection.
SH34A-02 16:15h
High Latitude Magnetic Field Polarity Reversals: A Solar Source or In Situ Generation?
High latitude observations of the magnetic field by the Ulysses spacecraft have shown a significant number of cases where the radial magnetic field polarity is reversed with respect to the dominant polarity of the coronal hole from which the wind emanates. It has been suggested that such reversals are due to reconnection of closed and open field lines in the lower corona which would launch a large amplitude Alfven wave into the solar wind. Here we carry out 2.5D simulations of such a wave and show that the kink in the field line tends to be unstable, disappearing within a few dynamical time-scales. We suggest an alternative mechanism for the generation of polarity reversals, namely, the coupling of standard large amplitude Alfven turbulence propagating away from the sun with the micro-stream shears observed in the high speed solar wind. Numerical simulations show that reversals generated by shears in the wind tend to be stable, and the time-scale required for such generation is compatible with the transport time out to 1 AU and beyond. It remains to be seen whether any correlation between microstream structure and polarity reversals can be found in the Ulysses data.
SH34A-03 16:30h
A Comparison of Heliospheric Magnetic Fields in the Ab Initio Approach to Cosmic-ray Modulation
In the ab initio approach to cosmic-ray modulation much attention has been given to examination of models for turbulence and for diffusion. In this paper we shift the emphasis somewhat to consider the effect of a heliospheric magnetic field (HMF) other than the standard Parker field. The Parker field scales as one over heliocentric radial distance in the equatorial regions of the heliosphere, but as one over heliocentric radial distance squared in the polar regions, thus making the magnitude of the field much smaller in polar regions than in equatorial regions. Modification to the Parker field, as well as more recent three-dimensional models of the HMF, typically has an azimuthal component that also scales as one over heliocentric radial distance in the polar regions of the heliosphere, thus increasing the magnitude of the field compared to the Parker model. An increase in the magnitude of the magnetic field increases the QLT parallel mean free path at low energies, but leaves it unchanged at high energies, while the perpendicular mean free path predicted by non-linear guiding center theory increases at all energies. The drift coefficient decreases with increasing magnetic field magnitude. We discuss the effect of this disparate behavior of the transport coefficients on cosmic ray modulation and illustrate it with our ab initio modulation model.
SH34A-04 16:45h
The Sheared Sub-Parker Spiral
Recent studies have shown that, in selected cases, the heliospheric magnetic field is much more radial than the standard Parker spiral in the rarefactions of co-rotation interaction regions (CIRs). This reveals that open field footpoints at the Sun move through the coronal hole boundary, which causes magnetic field lines to be connected and stretched across the fast-slow wind transition. Here we include footpoint motion at the Sun in a magneto-hydrodynamic (MHD) model to describe the formation and evolution of a CIR out to 5 AU. We compare results with Ulysses observations in CIR rarefactions. In comprehensive observations of CIRs observed by Ulysses from 1992 through 1997, we find that the more radial field, the "sheared sub-Parker spiral", is almost always present in CIR rarefactions. These observations show striking evidence for both systematic footpoint motions strongly consistent with differential rotation, and random footpoint motions, consistent with interchange reconnection that continually reconfigures the open magnetic field. Thus, we reveal the sheared sub-Parker spiral of the inner heliospheric magnetic field, and discuss its broad implications for the origin of solar wind, the nature of coronal holes, and the magnetic structure of the heliosphere.
SH34A-05 17:00h
On shock strength and electric fields in the inner heliosphere
I explore the scaling of shock strength, cross-shock and convection electric field with heliospheric radius in the inner heliosphere. In the low $\beta_e$ inner heliosphere, the primary contribution to the cross-shock field may be due to Hall currents, which are shown to scale non-monotonically with Mach number and exhibit a local minimum near the Alfv\`en speed peak at several solar radii. This allows that the shock and cross-shock component of the convection fields are nearly equal $|\vec{E}_{shock}| \approx (\vec{v} \times \vec{B})_x$, which is a condition for high efficiency of the shock-surfing mechanism of ion acceleration. Shock-surfing is thought to be a injection mechanism to diffusive acceleration and, indeed, observations show that energetic ion events are often accelerated at near this same heliospheric radius.
SH34A-06 17:15h
Upstream Diffuse Ions and Association With Low Frequency Hydromagnetic Waves: Cluster Observations at the Quasi-Parallel Earth's Bow Shock
We investigate quasi-linear theory of ion acceleration at Earth's quasi-parallel bow shock using Cluster magnetic field and particle measurements during an upstream diffuse ion event (February 18, 2003). Simultaneous magnetic field power spectra far upstream obtained by ACE show that the interplanetary fluctution level in the region of resonant frequencies was orders of magneitude lower than near the bow shock, so that the fluctuations near the bow shock were excited by backstreaming ions. In particular, we determine the energetic ion spatial profile, the dependence of the upstream ion scalelength upon energy, the behavior of the energetic ion anisotropy upstream and downstream of the shock, the spatial scale lenghts of the low frequency upstream waves that resonate with them, and the dependence of the integrated wave energy density upon the ion energy density at the shock.
SH34A-07 17:30h
Characteristics of Langmuir electric field waveforms and power spectra exhibiting nonlinear behavior in Earth's foreshock
The characteristics of Langmuir waves observed by the Cluster WBD Plasma Wave Receiver in Earth's foreshock region are presented with respect to nonlinear wave behavior. Our previous work on waves in the foreshock has shown that the electric field amplitude probability distributions for Langmuir waves observed by Cluster can follow a combination of the log-normal statistics predicted by stochastic growth theory and power law statistics. Power law deviations from log-normal statistics are often the result of spatial averaging, but can also be found for large amplitude waves near the edge of the foreshock, which may show evidence of nonlinearity. This raises questions about the possible effects of nonlinear processes on the amplitude statistics. We examine the significance of nonlinear processes in the foreshock by considering the various types of Langmuir wave power spectra observed in this region. Far away from the foreshock boundary, Langmuir waves have been observed to shift up and down in frequency from the local plasma frequency. Evidence for nonlinear processes such as parametric decay, has also been found in Earth's foreshock in the form of wave spectra with double peaks near the plasma frequency. For selected cases, we discuss the occurrence rates of these types of Langmuir wave spectra for different solar wind conditions and the possible influence of the associated physical processes on the amplitude probability distributions.
SH34A-08 17:45h
Angle-averaged Efficiency of Linear Mode Conversion of Langmuir waves into Radiatio
Langmuir waves propagating in density irregularities can have a fraction of their energy converted linearly into radio waves near the plasma frequency. Linear mode conversion (LMC) is potentially relevant to the Langmuir waves and associated radio emissions in planetary foreshocks and the source regions of type II and III solar radio bursts. Recent idealised 2-D calculations show that LMC can have an efficiency of order $50 - 90%$ for optimum choices of parameters. In order to compare the efficiencies of LMC and nonlinear processes it is necessary to average over the Langmuir wavevector spectrum and distribution of density irregularities. Here it is shown analytically and numerically that averaging over 2-D and 3-D density irregularities with a uniform distribution of orientation angles reduces the overall LMC conversion efficiency to values $\approx 10^{-3}$ and $10^{-5}$, respectively, for 1 AU parameters. Moreover, the average efficiency decreases as the square root of the electron temperature. The important qualitative conclusion is that averaging over more realistic source conditions strongly reduces the efficiency of LMC.