Large-amplitude MHD Waves Research Articles

Solar TErrestrial Relations Observatory-A (STEREO-A) and PRoject for On-Board Autonomy 2 (PROBA2) Quadrature Observations of Reflections of Three EUV Waves from a Coronal Hole

We investigate the interaction of three consecutive large-scale coronal waves with a polar coronal hole, simultaneously observed on-disk by the Solar TErrestrial Relations Observatory (STEREO)-A spacecraft and on the limb by the PRoject for On-Board Autonomy 2 (PROBA2) spacecraft on 27 January 2011. All three extreme ultraviolet (EUV) waves originate from the same active region, NOAA 11149, positioned at N30E15 in the STEREO-A field of view and on the limb in PROBA2. For the three primary EUV waves, we derive starting velocities in the range of ≈ 310 km s−1 for the weakest up to ≈ 500 km s−1 for the strongest event. Each large-scale wave is reflected at the border of the extended coronal hole at the southern polar region. The average velocities of the reflected waves are found to be smaller than the mean velocities of their associated direct waves. However, the kinematical study also reveals that in each case the ending velocity of the primary wave matches the initial velocity of the reflected wave. In all three events, the primary and reflected waves obey the Huygens–Fresnel principle, as the incident angle with ≈ 10° to the normal is of the same magnitude as the angle of reflection. The correlation between the speed and the strength of the primary EUV waves, the homologous appearance of both the primary and the reflected waves, and in particular the EUV wave reflections themselves suggest that the observed EUV transients are indeed nonlinear large-amplitude MHD waves.

Generation of short-lived large-amplitude magnetohydrodynamic pulses by dispersive focusing

Large-amplitude MHD waves are routinely observed in space plasmas. We suggest that dispersive focusing, previously proposed for the excitation of freak waves in the ocean, can be also responsible for the excitation of short-lived large-amplitude MHD waves in space plasmas. The DNLS equation describes MHD waves propagating in plasmas at moderate angles with respect to the equilibrium magnetic field. We obtained an analytical solution of the linearised DNLS equation governing the generation of large-amplitude MHD waves from small-amplitude wave trains due to the dispersive focusing. Our numerical solutions of the full DNLS equation confirm this result.

Phase coherence of MHD waves in the solar wind

Large amplitude MHD waves are commonly found in the solar wind. Nonlinear interactions between the MHD waves are likely to produce finite correlation among the wave phases. For discussions of various transport processes of energetic particles, it is fundamentally important to determine whether the wave phases are randomly distributed (as assumed in quasi-linear theories) or they have a finite coherence. Using a method based on a surrogate data technique and a fractal analysis, we analyzed Geotail magnetic field data (provided by S. Kokubun and T. Nagai through DARTS at the Institute of Space and Astronautical Science) to evaluate the phase coherence among the MHD waves in the earth's foreshock region. The correlation of wave phases does exist, indicating that the nonlinear interactions between the waves is in progress.

Phase Coherence of Foreshock MHD Waves: Wavelet Analysis

The earth's foreshock is a region where particularly large amplitude MHD waves are commonly observed. They exhibit various waveforms, suggesting that nonlinear interaction between the waves is in progress. In a previous paper (Hada et al., 2003) we have introduced a method to quantitatively evaluate the strength of phase coherence among the waves from a given time series data. Here we further develop our method by applying wavelet filtering technique. From the analysis it was found that, although the turbulence is consisted of waves with a wide range of plasma rest frame frequencies, only those frequencies lower than the ion gyrofrequency are responsible for generating the phase coherence.

On large amplitude MHD waves in high‐beta plasma

Downstream of quasi‐parallel shocks, the magnetosheath is often observed as a region of high plasma beta, of the order of 10, with large magnetic field fluctuations and relatively weak thermal pressure fluctuations. Since the wave steepening results from the nonlinearity in the plasma thermal properties, these nonlinear magnetic fluctuations that are accompanied by linear pressure fluctuations in a high‐beta plasma do not necessarily lead to wave steepening. We develop a nonlinear theory based on MHD and observational evidence to describe such a system. We first perturb the MHD equations assuming the reciprocal of beta and the perturbed pressure to be small parameters and retaining nonlinear terms for the magnetic arid velocity perturbations. We then Fourier expand the perturbations and retain terms up to the first order. A nonpropagating mode is present that has the characteristics of a series of tangential discontinuities. A purely incompressible mode is present that propagates obliquely to the background field and is nondispersive and independent of the plasma beta. The field perturbations do not need to be coplanar with the background field and the wave vector. A nonlinear compressible mode is also found. In the limit of small field perturbations, the mode is similar to the linear slow mode in high‐beta plasmas. The first harmonic field perturbation is coplanar with the background field and the wavevector. When the mode propagates along the field, the density and field strength fluctuations are small at the primary frequency of the wave and are significant at a frequency twice the primary frequency. This frequency doubling phenomenon is most severe for parallel propagation. The compressibility of the mode makes it possible to couple the energy across the magnetopause into the magnetosphere. We show observational evidence for the existence of such modes.

Acceleration processes in the course of dayside solar wind-magnetopause interaction

Magnetic reconnection is believed to be the dominant coupling process at the Earth's magnetopause. The main features which characterise this process are the conversion of magnetic field energy into plasma energy, an associated change in the magnetic field topology, and the emission of high-energy (suprathermal) charged particles. A review is presented of the development of theoretical concepts of reconnection, starting with steady-state Petschek model. The topics covered include extension of Petschek's model for the time-dependent case, the switch-on and switch-off phases of reconnection, models of flux transfer events (FTEs), steady-state and time-dependent reconnection in compressible plasma, acceleration of plasma through the system of large-amplitude MHD waves, acceleration of charged particles in the diffusion region, the role of the surface waves.

Analytical MHD model of three-dimensional time-dependent reconnection in a compressible plasma

We describe and analyse a model of three-dimensional time-varying reconnection in which the effect of surface waves is neglected. Reconnection is assumed to be caused by a localized decrease of the plasma conductivity inside the diffusion region. The localized dissipation gives rise to an electric field E ∗, which determines the reconnection rate. As a result, the current sheet decays into a system of large-amplitude MHD waves, which propagate along the current sheet and thereby induce perturbations in the surrounding medium. Our model is applied to the case of reconnection at the magnetopause, in particular to the signatures referred to as FTEs (flux transfer events).

Correction to “Large amplitude MHD waves upstream of the Jovian bow shock”

Journal of Geophysical Research: Space PhysicsVolume 89, Issue A10 p. 9161-9161 CorrectionsFree Access Correction to “Large amplitude MHD waves upstream of the Jovian bow shock” Melvyn L. Goldstein, Melvyn L. GoldsteinSearch for more papers by this authorCharles W. Smith, Charles W. SmithSearch for more papers by this authorWilliam H. Matthaeus, William H. MatthaeusSearch for more papers by this author Melvyn L. Goldstein, Melvyn L. GoldsteinSearch for more papers by this authorCharles W. Smith, Charles W. SmithSearch for more papers by this authorWilliam H. Matthaeus, William H. MatthaeusSearch for more papers by this author First published: 1 October 1984 https://doi.org/10.1029/JA089iA10p09161Citations: 14AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Goldstein, M. L., C. W. Smith, W. H. Matthaeus, Large amplitude MHD waves upstream of the Jovian bow shock, J. Geophys. Res., 88, 9989, 1983. Goldstein, M. L., H. K. Wong, A. F. Viñas, C. W. Smith, Large amplitude MHD waves upstream of the Jovian bow shock: Reinterpretation, NASA Tech. Memo.86131, 1984. Smith, C. W., M. L. Goldstein, W. H. Matthaeus, Turbulence analysis of the Jovian upstream “wave” phenomenon, J. Geophys. Res., 88, 5581, 1983. Smith, C. W., M. L. Goldstein, W. H. Matthaeus, andViñas, Correction to “Turbulence analysis of the Jovian upstream ‘wave’ phenomenon,”J. Geophys. Res., 89(A10)1984. Citing Literature Volume89, IssueA101 October 1984Pages 9161-9161 ReferencesRelatedInformation

Large amplitude MHD waves upstream of the Jovian bow shock

Observations of large amplitude MHD waves upstream of Jupiter's bow shock are analyzed. The waves are found to be right circularly polarized in the solar wind frame, which suggests that they are propagating in the fast magnetosonic mode. A complete spectral and minimum variance eigenvalue analysis of the data was performed. The power spectrum of the magnetic fluctuations contains several peaks. The fluctuations at 2.3 mHz have a direction of minimum variance antiparallel to the direction of the average magnetic field. Several harmonics at 6, 9, and 12 mHz are also present. The direction of minimum variance of these fluctuations lies at approximately 40° to the magnetic field. We argue that these fluctuations are waves excited by protons reflected off the Jovian bow shock. The inferred speed of the reflected protons is about two times the solar wind speed in the solar wind frame. A linear instability analysis is presented that suggests an explanation for many of the observed features of the observations. The fluctuations apparently contain a significant fraction of magnetic energy that is linearly polarized and in the Alfvén mode.