A Vlasov-Maxwell Equilibrium for the Force-Free Harris Sheet
The first non-linear force-free Vlasov-Maxwell equilibrium is presented. One component of the equilibrium magnetic field has the same spatial structure as the Harris sheet, but whereas the Harris sheet is kept in force balance by pressure gradients, in the force-free solution presented here force balance is kept by magnetic shear. Magnetic pressure, plasma pressure and plasma density are constant. The method used to find the equilibrium is based on the analogy of the one-dimensional Vlasov-Maxwell equilibrium problem to the motion of a pseudo-particle in a two-dimensional conservative potential. This potential is equivalent to one of the diagonal components of the plasma pressure tensor. After finding the appropriate functional form for this pressure tensor component, the corresponding distribution functions can be found using a Fourier transform method. The force-free solution can be generalized to a complete family of equilibria that describe the transition between the purely pressure- balanced Harris sheet to the force-free Harris sheet. The properties of the distribution function will be discussed together with implications for stability.
Upward Propagating Alfvén Wave and its Connection With Magnetic Reconnection in the Chromosphere as Observed by SOT
We identify high-frequency Alfvén waves propagating upward in the solar chromosphere and transition region from observational data obtained by the Solar Optical Telescope (SOT) onboard Hinode. We find that in four cases the spicules are modulated by high-frequency (>0.02~Hz) transverse fluctuations, which are inferred to be associated with Alfvén waves. They apparently propagate upward along the spicules with phase speeds ranging from 50 to 150~km/s. Three of these modulated spicules show clear wave-like shapes, with short wavelengths being less than 8~Mm. In our analysis we thus identified directly upward propagation of Alfvén waves in the solar chromosphere and transition region. In addition to the recently reported Alfvén waves with very long wavelengths and wave periods, we find here four examples of Alfvén waves with shorter wavelengths and periods. Moreover, we report observational evidence for Alfvén-wave excitation due to magnetic reconnection. These findings shed new light on the wave origin and on the coronal and solar-wind heating by waves.
Intermittency Analysis and Spatial Dependence of Magnetic Field Disturbances in the Fast Solar Wind
The intermittency properties of the magnetic field fluctuations in the fast solar wind are investigated here at various heliocentric distances based on the Helios measurements from 0.29 to 1.0 AU during 1974-1981. Seven quantities of normalized PDF (Probability Distribution Function) associated with the magnetic field and its disturbances and the magnetic energy are utilized to characterize the intermittency by fitting the PDF with a Castaing distribution and by using the idea of ˇ§Flatnessˇ¨. The magnetic field fluctuations are found to be more intermittent at farther distances from the sun and at smaller time scales. For all the quantities considered, the intermittency decreases with increasing time scales, with their PDF eventually approaching Gaussian distributions. Such a transition occurs at a relatively small time scale for the perpendicular component of the perturbed magnetic field in particular. Moreover, the increase in the degree of intermittency with decreasing time scale is more obvious farther from the sun. Based on the finding of a good power-law relationship between the magnetic energies of the perturbed and mean fields and the feature of lower mean magnetic field at a larger heliocentric distance, the trend of spatial variation of magnetic energy is found to have a power-law scaling at different time scales. By considering the ion cyclotron resonance condition for the Helios measurements presented here, we suppose that the perturbed magnetic fields in the fast solar wind in the frequency range considered are mainly affected by cross-scale redistribution of the wave energy.
Multi-spacecraft study of interplanetary magnetic field depressions Multi-spacecraft Study of Interplanetary Magnetic Field Depressions
Magnetic holes were observed in the interplanetary field by the four Cluster spacecraft at different times when the separation between the satellites was ranging from ~500 km to ~10000 km. In several cases the holes appeared in wave trains like quasi-sinusoidal oscillations suggesting that they are not the remnants of long time evolving magnetic dips. The estimation of the size of the magnetic structures was based on the correlation between the magnetic field profiles measured across the holes by the four spacecraft. In case when the depressions could be identified by all four spacecraft, the velocity of the structures was determined by triangulation. This study confirmed that the holes are likely to be frozen in the solar wind flow. The relation between solar wind parameters and the characteristic features of the magnetic holes were also investigated in cases when series of events were observed. Inside several holes, the magnetic field was found to be close to zero.
On the Formation of Ion-Acoustic Waves, Solitons, and Double Layers in the Vicinity of the Electrostatic Shocks
Mixing of hot and cold electrons along the magnetic field line in the electron foreshock region can result in electron-electron two-stream instability and generate electron-acoustic waves. But recent observations showed that some of the upstream waves are ion acoustic waves. In this study, we use the ion-acoustic shock as an example to study the formation of nonlinear electrostatic waves and field-aligned potential jumps in the space plasma. Our Vlasov simulation results indicated that when the upstream ion beam energy is greater than the potential energy established by the hot downstream electrons, the shock ramp is characterized by an ion-acoustic soliton-like overshoot potential structure at the shock ramp. When the upstream ion beam energy is less than the potential energy established by the hot downstream electrons, the shock ramp is characterized by a monotonic-increase potential jump with an extended potential foot structure upstream from the shock ramp. The foot structure can accelerate the upstream cold electrons over a wide region and provide an excellent environment for the growth of ion acoustic waves from the ion-electron two-stream instability in this region. Long-term evolution of these ion acoustic waves into ion acoustic solitons and ion-acoustic double layers will also be discussed.
Characteristics of Solar Proton Events Associated with Cosmic Ray Ground Level Enhancements
Previous studies on Solar Proton Events (SPEs) used the definition that a SPE is an event with more than 10 pfu (Particle Flux Unit; 1 particle cm-2 sr-1 s-1) for the peak fluxes of > 10 MeV integral intensity of protons. Even though SPEs are major events with great peak intensity, they do not have a one-to- one association with Ground Level Enhancement (GLEs). On the contrary, minor SPEs are associated with GLEs sometimes. To investigate this puzzle, we examined the peak intensities of 83 SPEs after 1986 using the intensity of proton energy channels (P6-P11) (not integral intensity) from GOES. SPEs associated with GLE comprise 31 events and have well-defined profiles with large increase and clear peak for each proton channel. They have larger peak intensity, fluence, and shorter delay time between onset and peak than SPEs without GLE. Fluence and peak intensities of SPEs have a good correlation with percent increases of GLE, with the best correlation coefficients obtained for the peak intensities of P8, P9, and P10. For these energy channels (spanning 350-700 MeV), we find that there are critical threshold values for GOES fluence and peak intensity. Most SPEs above the thresholds are associated with GLE, while almost none below the thresholds is not.
The Energy Spectra of Suprathermal Tails in Solar Wind Iron
High speed suprathermal tails with a fixed energy spectrum have been observed in solar wind H+ and He2+, as well as in He+ pickup ions (e.g. Gloeckler et al., 2007). The presence of the tails have implications for particle injection into the interplanetary shock acceleration process. The suprathermal tails of solar wind Fe have been investigated with the STEREO/PLASTIC mass spectrometer. The energy spectra will be presented for periods of slow and fast solar wind, and for the entire STEREO mission.
An Analysis of Future In-Flight Calibration of the GOES-O Solar X-Ray Imager using the Crab Nebula
A potential in-flight calibration method for NOAA's Solar X-ray Imager (SXI), to be flown on the O-series Geostationary Operation Environmental Satellites (hereafter referred to as SXI-O), which uses the Crab nebula, is analyzed. Calculations of the Crab nebula's signal, Poisson noise and electronics noise are performed to determine the feasibility of future in-flight calibrations. Such calibration is necessary to aid in understanding instrument performance changes over time. Performance degradation is possible due to thin film filter failure, high voltage system changes, optical and detector contamination, and mechanism failures. Entrance filter and voltage system degradations have occurred on the SXI instrument on-board GOES-12, highlighting the need for accurate calibration techniques. Solar flares, which emit 1-8 angstrom X-rays, and subsequent solar phenomena, like coronal mass ejections, emit energy in the 6-60 angstrom X-ray range. This is also SXI-O's wavelength measurement requirement, and within the range of emission of one of the brightest astronomical X-ray sources, the Crab Nebula. Previous measurements of the Crab Nebula flux in the 6-60 angstrom range were made by the focal plane crystal spectrometer on the Einstein Observatory, and are used in this analysis. These signal and noise calculations integrate the wavelength dependant flux, taking into account components of SXI-O, which include the primary mirror, filters, detectors, and detector electronics. The sources of noise that are analyzed include the shot noise from the signal, the detector's dark current and charge transfer efficiency noise, the electronics read noise, and the quantization noise. The results of these analysis show that in-flight calibration of SXI-O using the Crab nebula is possible, and the longest single exposure of which the instrument is capable (65 sec) yields a signal to noise ratio somewhat insufficient to meet SXI-O's 20% photometric accuracy requirement. However, summing several 65 second images would increase the signal to noise ratio, making such a calibration readily possible.