Fast-mode Waves Research Articles

Propagation of Pi2 pulsations in a dipole model of the magnetosphere

AbstractLocalized fast flows that impinge on the inner magnetosphere from the plasma sheet are observed to oscillate on time scales of minutes. The compression ahead of these flows will launch fast mode waves, while the velocity shears at the edges of these flows directly excite shear Alfvén waves. These waves, which are coupled by gradients in the Alfvén speed, have been suggested as a source for the Pi1 and Pi2 waves that are observed at both high and low latitudes in the ionosphere. A new three‐dimensional simulation of the propagation of ULF waves in the dipolar region of the magnetosphere has been developed to study these coupled wave modes. This model includes a height‐resolved ionospheric conductivity so that ionospheric fields can be more realistically determined, as well as a direct calculation of ground magnetic fields to compare with ground magnetometers using an inductive ionosphere model. Results from this model show that a plasmaspheric resonance can be set up by waves with periods about 1 min and that field line resonances can be excited both inside and outside the plasmasphere. The use of the inductive ionosphere model leads to the conclusion that even a uniform Hall conductivity can break the dawn‐dusk symmetry of the convection pattern. Waves from a source at 10 Earth radii reach the ionosphere with time delays between high and low latitudes of tens of seconds, with implications for the timing of substorm phenomena observed by spacecraft and by ground magnetometers and radars.

Modeling radiation belt electron acceleration by ULF fast mode waves, launched by solar wind dynamic pressure fluctuations

AbstractWe investigate the magnetospheric MHD and energetic electron response to a Storm Sudden Commencement (SSC) and subsequent magnetopause buffeting, focusing on an interval following an SSC event on 25 November 2001. We find that the electron flux signatures observed by LANL, Cluster, and GOES spacecraft during this event can largely be reproduced using an advective kinetic model for electron phase space density, using externally prescribed electromagnetic field inputs, (herein described as a “test‐kinetic model”) with electromagnetic field inputs provided by a 2‐D linear ideal MHD model for ULF waves. In particular, we find modulations in electron flux phase shifted by 90° from the local azimuthal ULF wave electric field (Eφ) and a net enhancement in electron flux after 1.5 h for energies between 500 keV and 1.5 MeV near geosynchronous orbit. We also demonstrate that electrons in this energy range satisfy the drift resonance condition for the ULF waves produced by the MHD model. This confirms the conclusions reached by Tan et al. (2011), that the energization process in this case is dominated by drift‐resonant interactions between electrons and MHD fast mode waves, produced by fluctuations in solar wind dynamic pressure.

Global simulation of extremely fast coronal mass ejection on 23 July 2012

The July 23, 2012 CME was an extremely fast backside event, reaching ∼1AU (STEREO-A) within 20h as compared to ∼3–6 days for typical CME events. Here, we present results from a simulation study of the CME and its driven shock using a combined kinematic and magnetohydrodynamic (MHD) simulation model, H3DMHD. In general, the model results match well with in situ measurements in the arrival time of the CME-driven shock and the total magnetic field strength, assuming an initial CME speed of 3100km/s. Based on extrapolation of an empirical model, the fast CME and its large magnetic field (|B|∼120nT) are capable of producing an extremely large geomagnetic storm (Dst∼−545nT), comparable to the well-known Halloween storm in 2003, if the CME had made a direct impact to the Earth. We investigated the effect of the adiabatic index (γ). It is found that the shock tends to arrive slightly later for a smaller γ value, and γ=5/3 provides the best agreement for the shock arrival time. We also demonstrate that the strength (the Mach number) of the CME-driven fast-mode shock is not the largest at the “nose” of the CME. This is mainly due to the manifestation of fast-mode wave speed upstream of the shock.

Statistical Analysis of Large-Scale EUV Waves Observed by STEREO/EUVI

We statistically analyzed the kinematical evolution and wave pulse characteristics of 60 strong large-scale EUV wave events that occurred during January 2007 to February 2011 with the STEREO twin spacecraft. For the start velocity, the arithmetic mean is 312±115 km s−1 (within a range of 100 – 630 km s−1). For the mean (linear) velocity, the arithmetic mean is 254±76 km s−1 (within a range of 130 – 470 km s−1). 52 % of all waves under study show a distinct deceleration during their propagation (a≤−50 m s−2), the other 48 % are consistent with a constant speed within the uncertainties (−50≤a≤50 m s−2). The start velocity and the acceleration are strongly anticorrelated with c≈−0.8, i.e. initially faster events undergo stronger deceleration than slower events. The (smooth) transition between constant propagation for slow events and deceleration in faster events occurs at an EUV wave start-velocity of v≈230 km s−1, which corresponds well to the fast-mode speed in the quiet corona. These findings provide strong evidence that the EUV waves under study are indeed large-amplitude fast-mode MHD waves. This interpretation is also supported by the correlations obtained between the peak velocity and the peak amplitude, impulsiveness, and build-up time of the disturbance. We obtained the following association rates of EUV wave events with other solar phenomena: 95 % are associated with a coronal mass ejection (CME), 74 % to a solar flare, 15 % to interplanetary type II bursts, and 22 % to coronal type II bursts. These findings are consistent with the interpretation that the associated CMEs are the driving agents of the EUV waves.

Coronal wave associated with a non-radial filament eruption observed by the Solar Dynamics Observatory

We analyse a coronal wave that occurred during a non-radial filament eruption observed by the Solar Dynamics Observatory on 2011 August 10. The filament underwent an extended time activation phase followed by an abrupt ejection, and during its evolution it rotated towards the south. The eruption was accompanied by fast-wave and slow-perturbation phenomena. The slow perturbation occurred before the eruption and impulsively accelerated almost simultaneously with the eruption; its final propagation velocity was about 300 km s(-1), approximately equal tothat of the associated coronal mass ejection. The slow perturbation is possibly an indicator of an expanding loop overlying the filament. The fast wave was probably caused by the rapid inflation of the overlying loop. Because of the eruption location close to the limb and the effect of the complex environment, the fast coronal wave showed different characteristics in different directions: the kick-off speed was about 430-480 km s(-1), showing deceleration in some directions, and a high speed of up to 782 +/- 21 km s(-1) in another direction. All the results indicate that the coronal wave was a fast-mode magnetohydrodynamic wave, and the wavelet analysis confirms the periodic wave nature of the coronal wave.

STUDIES OF THE JET IN BL LACERTAE. I. RECOLLIMATION SHOCK AND MOVING EMISSION FEATURES

Parsec-scale VLBA images of BL Lac at 15 GHz show that the jet contains a permanent quasi-stationary emission feature 0.26 mas (0.34 pc projected) from the core, along with numerous moving features. In projection, the tracks of the moving features cluster around an axis at position angle -166.6 deg that connects the core with the standing feature. The moving features appear to emanate from the standing feature in a manner strikingly similar to the results of numerical 2-D relativistic magneto-hydrodynamic (RMHD) simulations in which moving shocks are generated at a recollimation shock. Because of this, and the close analogy to the jet feature HST-1 in M87, we identify the standing feature in BL Lac as a recollimation shock. We assume that the magnetic field dominates the dynamics in the jet, and that the field is predominantly toroidal. From this we suggest that the moving features are compressions established by slow and fast mode magneto-acoustic MHD waves. We illustrate the situation with a simple model in which the slowest moving feature is a slow-mode wave, and the fastest feature is a fast-mode wave. In the model the beam has Lorentz factor about 3.5 in the frame of the host galaxy, and the fast mode wave has Lorentz factor about 1.6 in the frame of the beam. This gives a maximum apparent speed for the moving features 10c. In this model the Lorentz factor of the pattern in the galaxy frame is approximately 3 times larger than that of the beam itself.

Advances in Observing Various Coronal EUV Waves in the SDO Era and Their Seismological Applications (Invited Review)

Global extreme-ultraviolet (EUV) waves are spectacular traveling disturbances in the solar corona associated with energetic eruptions such as coronal mass ejections (CMEs) and flares. Over the past 15 years, observations from three generations of space-borne EUV telescopes have shaped our understanding of this phenomenon and at the same time led to controversy about its physical nature. Since its launch in 2010, the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) has observed more than 210 global EUV waves in exquisite detail, thanks to its high spatio–temporal resolution and full-disk, wide-temperature coverage. A combination of statistical analysis of this large sample, more than 30 detailed case studies, and data-driven MHD modeling, has been leading their physical interpretations to a convergence, favoring a bimodal composition of an outer, fast-mode magnetosonic wave component and an inner, non-wave CME component. Adding to this multifaceted picture, AIA has also discovered new EUV wave and wave-like phenomena associated with various eruptions, including quasi-periodic fast propagating (QFP) wave trains, magnetic Kelvin–Helmholtz instabilities (KHI) in the corona and associated nonlinear waves, and a variety of mini-EUV waves. Seismological applications using such waves are now being actively pursued, especially for the global corona. We review such advances in EUV wave research focusing on recent SDO/AIA observations, their seismological applications, related data-analysis techniques, and numerical and analytical models.

SDO/AIA ANDHINODE/EIS OBSERVATIONS OF INTERACTION BETWEEN AN EUV WAVE AND ACTIVE REGION LOOPS

We present detailed analysis of an extreme ultraviolet (EUV) wave and its interaction with active region (AR) loops observed by the Solar Dynamics Observatory/Atmospheric Imaging Assembly and the Hinode EUV Imaging Spectrometer (EIS). This wave was initiated from AR 11261 on 2011 August 4 and propagated at velocities of 430--910 km\s. It was observed to traverse another AR and cross over a filament channel on its path. The EUV wave perturbed neighboring AR loops and excited a disturbance that propagated toward the footpoints of these loops. EIS observations of AR loops revealed that at the time of the wave transit, the original redshift increased by about 3 km\s, while the original blueshift decreased slightly. After the wave transit, these changes were reversed. When the EUV wave arrived at the boundary of a polar coronal hole, two reflected waves were successively produced and part of them propagated above the solar limb. The first reflected wave above the solar limb encountered a large-scale loop system on its path, and a secondary wave rapidly emerged 144 Mm ahead of it at a higher speed. These findings can be explained in the framework of a fast-mode magnetosonic wave interpretation for EUV waves, in which observed EUV waves are generated by expanding coronal mass ejections.

Multipoint observations of Pi2 pulsations and correlation with dynamic processes in the near-Earth magnetotail on March 18, 2009

Simultaneous measurements from THEMIS spacecraft, GOES-11 and ground stations (Canadian Array for Realtime Investigations of Magnetic Activity or CARISMA, and 210° magnetic meridian or MM) on March 18, 2009 allow the study of dynamic processes in the near-Earth magnetotail and corresponding Pi2 pulsations on the ground in great detail. Fast earthward flows along with traveling Alfven waves and fast mode waves in the Pi2 band were observed by three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes (P3, P4 and P5) in the near-Earth plasmasheet. At the mid- to high-latitude nightside, the CARISMA stations located near the foot points of the three probes recorded Pi2s with two periods, about 80 s after the earthward fast flows observed by the P4 probe. The long-period Pi2 (140–150 s) belongs to the transient response Pi2 (TR Pi2), since the travel time of the Alfven waves between the plasma sheet and CARISMA stations is very close to half the period of the long-period Pi2. The short-period Pi2 (60–80 s) has the same period band as the perpendicular velocity of the fast flows, which indicates that it may relate to the inertial current caused by periodic braking of the earthward fast flows. The 210° MM stations located at the low-latitude duskside also observed Pi2s with the same start time, waveform and frequency, about ∼120 s after the earthward fast flows. Strong poloidal oscillations are shown by GOES-11 (∼23 MLT) and the compressional component (Bb) is highly correlated with H components of the 210° MM stations, whereas the other two components (Br and Be) are not. These results confirm that the low-latitude Pi2s are generated by cavity mode resonance, which is driven by an impulsive broadband source in the near-Earth magnetotail.

Eruption of a plasma blob, associated M-class flare, and large-scale extreme-ultraviolet wave observed by SDO

We present a multiwavelength study of the formation and ejection of a plasma blob and associated EUV waves in AR NOAA 11176, observed by SDO/AIA and STEREO on 25 March 2011. SDO/AIA images clearly show the formation and ejection of a plasma blob from the lower solar atmosphere at ~9 min prior to the onset of the M1.0 flare. This onset of the M-class flare happened at the site of the blob formation, while the blob was rising in a parabolic path with an average speed of ~300 km/s. The blob also showed twisting and de-twisting motion in the lower corona, and the blob speed varied from ~10-540 km/s. The faster and slower EUV wavefronts were observed in front of the plasma blob during its impulsive acceleration phase. The faster EUV wave propagated with a speed of ~785 to 1020 km/s, whereas the slower wavefront speed varied in between ~245 and 465 km/s. The timing and speed of the faster wave match the shock speed estimated from the drift rate of the associated type II radio burst. The faster wave experiences a reflection by the nearby AR NOAA 11177. In addition, secondary waves were observed (only in the 171 \AA channel), when the primary fast wave and plasma blob impacted the funnel-shaped coronal loops. The HMI magnetograms revealed the continuous emergence of new magnetic flux along with shear flows at the site of the blob formation. It is inferred that the emergence of twisted magnetic fields in the form of arch-filaments/"anemone-type" loops is the likely cause for the plasma blob formation and associated eruption along with the triggering of M-class flare. Furthermore, the faster EUV wave formed ahead of the blob shows the signature of fast-mode MHD wave, whereas the slower wave seems to be generated by the field line compression by the plasma blob. The secondary wave trains originated from the funnel-shaped loops are probably the fast magnetoacoustic waves.

On the coupling of fast and shear Alfvén wave modes by the ionospheric Hall conductance

There are two low frequency, magnetised, cold plasma wave modes that propagate through the Earth’s magnetosphere. These are the compressional (fast) and the shear Alfven modes. The fast mode distributes energy throughout the magnetosphere with the ability to propagate across the magnetic field. Previous studies of coupling between these two modes have often focussed on conditions necessary for mode coupling to occur in the magnetosphere. However, Kato and Tamao (1956) predicted mode coupling would occur for non-zero Hall currents. Recently, the importance of the Hall conductance in the ionosphere for low frequency wave propagation has been studied using one dimensional (1-D) models. In this paper we describe effects of the ionosphere Hall conductance on field line resonance and higher frequency, 0.1–5 Hz waves associated with the Ionospheric Alfven Resonator (IAR). The Hall conductance reduces the damping time of field line resonances and Joule dissipation into the ionosphere. The Hall conductance also couples shear Alfven waves trapped in the IAR to fast mode waves that propagate across the ambient magnetic field in an ionospheric waveguide. This coupling leads to the production of low frequency magnetic fields on the ground that can be observed by magnetometers.

Loop oscillations and an extreme ultraviolet wave associated with a micro-sigmoid eruption

Taking advantage of the high temporal and spatial resolution observations of the Solar Dynamics Observatory, we present an extreme ultraviolet (EUV) wave associated with a micro-sigmoid eruption, which took place on 2012 October 4. The micro-sigmoid underwent a typical sigmoid-to-arcade evolution via tether-cutting reconnection, accompanied by a micro-flare, a filament eruption. The twin coronal dimmings at the footpoints of the sigmoid indicate the existence of the associated micro-coronal mass ejection that likely triggered a small-scale EUV wave. The wave onset was nearly simultaneous with the start of the eruption and the associated flare. The wave had a nearly circular front, and propagated at initial velocities of 300–360 km s−1. Because the sigmoid was located in the non-quiet region, the generated wave interacted with the ambient loops, and triggered transverse loop oscillations, resulting in many loop-shaped dimmings. Moreover, the interaction between the wave and one large loop led not only to the oscillation but also to the downward plasma flow along the loop, inducing loop footpoints to brighten. Another small loop was nearly intact after the wave passed. In some directions, the wave was refracted by the loops, and the refracted speeds increased, decreased, or vanished in different directions. All results provide evidence that the EUV wave was a fast-mode magnetohydrodynamic wave.

AN EXTREME-ULTRAVIOLET WAVE ASSOCIATED WITH A SURGE

Taking advantage of the high temporal and spatial resolution observations from the Solar Dynamics Observatory, we present an extreme-ultraviolet (EUV) wave associated with a surge on 2010 November 13. Due to the magnetic flux cancelation, some surges formed in the source active region (AR). The strongest surge produced our studied event. The surge was deflected by the nearby loops that connected to another AR, and disrupted the overlying loops that slowly expanded and eventually evolved into a weak coronal mass ejection (CME). The surge was likely associated with the core of the CME. The EUV wave happened after the surge deflected. The wave departed far from the flare center and showed a close location relative to the deflected surge. The wave propagated in a narrow angular extent, mainly in the ejection direction of the surge. The close timing and location relations between the EUV wave and the surge indicate that the wave was closely associated with the CME. The wave had a velocity of 310-350 km s(-1), while the speeds of the surge and the expanding loops were about 130 and 150 km s(-1), respectively. All of the results suggest that the EUV wave was a fast-mode wave and was most likely triggered by the weak CME.

Numerical study of EUV wave on 13 February 2009

结合STEREO卫星的观测和三维磁流体力学数值模拟方法, 采用WSO (Wilcox Solar Observatory)磁场数据和势场源表面模型建立日冕初始磁场, 并在日面活动区加上时变的压强扰动, 对2009年2月13日05:35UT爆发的CME-EUV波(Coronal Mass Ejections-Extreme Ultraviolet wave, 日冕物质抛射-远紫外波)事件进行研究. 从COR1/STEREO-A图像判断, 此次CME前沿速度约340km·s^-1, 角宽度约60°; 分析EUVI/STEREO-B 195 Å的差分图像, 可以看到, 环形亮环波前从活动区向四周传播, 亮环波前后面是日冕暗化区, 取四个方向的波前位置进行线性拟合可知, 该EUV波速度为247km·s^-1, 数值模拟得到的EUV波速度为245km·s^-1, 将计算结果采用IDL可视化后可以看到明显的亮环和暗区结构, 数值模拟结果与卫星观测相一致, 表明该EUV波现象是快磁声波.

Investigation of diffusivity and viscosity in solar plasma

For diffusive and viscous plasma, the dispersion relation ω2 = k 2 [v 2 − iω(ν + η)] + νηk 4 is applied for the North Polar Coronal Hole, where we have assumed the angular frequency ω to be a real quantity and the wave number k as a complex quantity. For ω = 2π/τ, we have chosen three values 10−2, 10−3 and 10−4 s for τ. For each value of τ, we have considered three situations: (i) where ν = 0 (no viscosity), (ii) where η = 0 (no magnetic diffusivity) and (iii) where both the diffusivity η and viscosity ν are present. For the cases (i) and (ii), we have obtained two solutions, ±(k r + i k i ). But, for the case (iii), we have obtained two pairs of solutions, ±(k r1 + i k i1) and ±(k r2 + i k i2). These two pairs correspond to the fast-mode and slow-mode waves.

SECONDARY WAVES AND/OR THE “REFLECTION” FROM AND “TRANSMISSION” THROUGH A CORONAL HOLE OF AN EXTREME ULTRAVIOLET WAVE ASSOCIATED WITH THE 2011 FEBRUARY 15 X2.2 FLARE OBSERVED WITHSDO/AIA ANDSTEREO/EUVI

For the first time, the kinematic evolution of a coronal wave over the entire solar surface is studied. Full Sun maps can be made by combining images from the Solar Terrestrial Relations Observatory satellites, Ahead and Behind, and the Solar Dynamics Observatory, thanks to the wide angular separation between them. We study the propagation of a coronal wave, also known as "EIT" wave, and its interaction with a coronal hole resulting in secondary waves and/or reflection and transmission. We explore the possibility of the wave obeying the law of reflection of waves. In a detailed example we find that a loop arcade at the coronal hole boundary cascades and oscillates as a result of the EUV wave passage and triggers a wave directed eastwards that appears to have reflected. We find that the speed of this wave decelerates to an asymptotic value, which is less than half of the primary EUV wave speed. Thanks to the full Sun coverage we are able to determine that part of the primary wave is transmitted through the coronal hole. This is the first observation of its kind. The kinematic measurements of the reflected and transmitted wave tracks are consistent with a fast-mode MHD wave interpretation. Eventually, all wave tracks decelerate and disappear at a distance. A possible scenario of the whole process is that the wave is initially driven by the expanding coronal mass ejection and subsequently decouples from the driver and then propagates at the local fast-mode speed.

SDO/AIA and Hinode/EIS observations of interaction between an EUV wave and active region loops

AbstractWe study an EUV wave initiated in active region (AR) 11261 on 2011 August 4 by using the SDO/AIA and the Hinode/EIS. We found that: (1) the EUV wave interacted with AR loops between AR 11261 and AR 11263, and exited mass flows in these loops. (2) EIS observations of AR loops in AR 11263 revealed that at the time of the wave transit, the original red-shifted feature had an increase of about 3 km s−1, and the original blue-shifted feature slightly weakened. These findings could be explained in the framework of a fast-mode magnetosonic wave interpretation for EUV waves.

SLOW MAGNETOSONIC WAVES AND FAST FLOWS IN ACTIVE REGION LOOPS

Recent extreme ultraviolet spectroscopic observations indicate that slow magnetosonic waves are present in active region (AR) loops. Some of the spectral data were also interpreted as evidence of fast (∼100–300 km s−1) quasi-periodic flows. We have performed three-dimensional magnetohydrodynamic (3D MHD) modeling of a bipolar AR that contains impulsively generated waves and flows in coronal loops. The model AR is initiated with a dipole magnetic field and gravitationally stratified density, with an upflow-driven steadily or periodically in localized regions at the footpoints of magnetic loops. The resulting flows along the magnetic field lines of the AR produce higher density loops compared to the surrounding plasma by injection of material into the flux tubes and the establishment of siphon flow. We find that the impulsive onset of flows with subsonic speeds result in the excitation of damped slow magnetosonic waves that propagate along the loops and coupled nonlinearly driven fast-mode waves. The phase speed of the slow magnetosonic waves is close to the coronal sound speed. When the amplitude of the driving pulses is increased we find that slow shock-like wave trains are produced. When the upflows are driven periodically, undamped oscillations are produced with periods determined by the periodicity of the upflows. Based on the results of the 3D MHD model we suggest that the observed slow magnetosonic waves and persistent upflows may be produced by the same impulsive events at the bases of ARs.

PROTON, ELECTRON, AND ION HEATING IN THE FAST SOLAR WIND FROM NONLINEAR COUPLING BETWEEN ALFVÉNIC AND FAST-MODE TURBULENCE

In the parts of the solar corona and solar wind that experience the fewest Coulomb collisions, the component proton, electron, and heavy ion populations are not in thermal equilibrium with one another. Observed differences in temperatures, outflow speeds, and velocity distribution anisotropies are useful constraints on proposed explanations for how the plasma is heated and accelerated. This paper presents new predictions of the rates of collisionless heating for each particle species, in which the energy input is assumed to come from magnetohydrodynamic (MHD) turbulence. We first created an empirical description of the radial evolution of Alfven, fast-mode, and slow-mode MHD waves. This model provides the total wave power in each mode as a function of distance along an expanding flux tube in the high-speed solar wind. Next we solved a set of cascade advection-diffusion equations that give the time-steady wavenumber spectra at each distance. An approximate term for nonlinear coupling between the Alfven and fast-mode fluctuations is included. For reasonable choices of the parameters, our model contains enough energy transfer from the fast mode to the Alfven mode to excite the high-frequency ion cyclotron resonance. This resonance is efficient at heating protons and other ions in the direction perpendicular to the background magnetic field, and our model predicts heating rates for these species that agree well with both spectroscopic and in situ measurements. Nonetheless, the high-frequency waves comprise only a small part of the total Alfvenic fluctuation spectrum, which remains highly two-dimensional as is observed in interplanetary space.

EVIDENCE FOR THE WAVE NATURE OF AN EXTREME ULTRAVIOLET WAVE OBSERVED BY THE ATMOSPHERIC IMAGING ASSEMBLY ON BOARD THESOLAR DYNAMICS OBSERVATORY

Extreme Ultraviolet (EUV) waves have been found for about 15 years. However, significant controversy remains over their physical natures and origins. In this paper, we report an EUV wave that was accompanied by an X1.9 flare and a partial halo coronal mass ejection. Using high temporal and spatial resolution observations taken by the {\em Solar Dynamics Observatory} and the Solar-TErrestrial RElations Observatory, we are able to investigate the detailed kinematics of the EUV wave. We find several arguments that support the fast-mode wave scenario: (1) The speed of the EUV wave (570 km/s) is higher than the sound speed of quiet-Sun corona. (2) Significant deceleration of the EUV wave (-130 m/s2) is found during its propagation. (3) The EUV wave resulted in the oscillations of a loop and a filament along its propagation path, and a reflected wave from the polar coronal hole is also detected. (4) Refraction or reflection effect is observed when the EUV wave was passing through two coronal bright points. (5) The dimming region behind the wavefront stopped to expand when the wavefront started to become diffuse. (6) The profiles of the wavefront exhibited a dispersive nature, and the magnetosonic Mach number of the EUV wave derived from the highest intensity jump is about 1.4. In addition, triangulation indicates that the EUV wave propagated within a height range of about 60-100 Mm above the photosphere. We propose that the EUV wave observed should be a nonlinear fast-mode magnetosonic wave that propagated freely in the corona after it was driven by the CME expanding flanks during the initial period.