M. W. Dunlop

Beijing University of Aeronautics and Astronautics (Beihang University), Peping, Beijing, China

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Publications (598)913.7 Total impact

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    ABSTRACT: During the first several months of the three-spacecraft Swarm mission all three spacecraft came repeatedly into close alignment, providing an ideal opportunity for validating the proposed dual spacecraft method for estimating current density from the Swarm magnetic field data. Two of the Swarm spacecraft regularly fly side-by-side in closely similar orbits, while the third at times approaches the other two. This provides a data set which under certain assumptions of stationarity of the magnetic field, can produce 2, 3, 4, 5 (or more) point measurements, which can be cross-compared. We find that at low Earth orbit the use of time-shifted positions allow stable estimates of current density to be made and can verify temporal effects as well as validating the interpretation of the current components as arising predominantly from field aligned currents. In the case of four-spacecraft configurations we can resolve the full vector current and therefore can check the perpendicular as well as parallel current density components directly, together with the quality factor for the estimates directly (for the first time in situ at low earth orbit).
    Journal of Geophysical Research: Space Physics 10/2015; DOI:10.1002/2015JA021707 · 3.44 Impact Factor
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    L.Q. Zhang · J.Y. Wang · W. Baumjohann · H. Rème · L. Dai · M. W. Dunlop · T. Chen · Y. Huang
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    ABSTRACT: Utilizing associated observations of Geotail and ACE satellites from the year of 1998 to 2005, we investigated the X-lines in the near-Earth tail under different interplanetary magnetic field (IMF) conditions. The X-lines are recognized by the tailward fast flows (TFF) with negative Bz. Statistically, the X-lines in the tail can be observed for southward as well as northward IMF, but more frequently observed for southward IMF. A typical case on 26 Apr, 2005 showed clear evidence that the X-line can occur for northward IMF while the geomagnetic activity is particularly quiet. Further analysis showed that the X-line-related solar wind (SW) has stronger Ey and Bz components for southward than northward IMF. In addition, the X-line-related geomagnetic activities are stronger for southward than northward IMF.
    Journal of Geophysical Research: Space Physics 09/2015; DOI:10.1002/2015JA021503 · 3.44 Impact Factor
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    DESCRIPTION: L. Q. Zhang, W. Baumjohann, J. Y. Wang, H. Rème, M. W. Dunlop, and T. Chen
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    ABSTRACT: Electromagnetic ion cyclotron (EMIC) waves are an important mechanism for particle energization and losses inside the magnetosphere. In order to better understand the effects of these waves on particle dynamics, detailed information about the occurrence rate, wave power, ellipticity, normal angle, energy propagation angle distributions, and local plasma parameters are required. Previous statistical studies have used in situ observations to investigate the distribution of these parameters in the magnetic local time versus L-shell (MLT-L) frame within a limited magnetic latitude (MLAT) range. In this study, we present a statistical analysis of EMIC wave properties using 10 years (2001–2010) of data from Cluster, totaling 25,431 min of wave activity. Due to the polar orbit of Cluster, we are able to investigate EMIC waves at all MLATs and MLTs. This allows us to further investigate the MLAT dependence of various wave properties inside different MLT sectors and further explore the effects of Shabansky orbits on EMIC wave generation and propagation. The statistical analysis is presented in two papers. This paper focuses on the wave occurrence distribution as well as the distribution of wave properties. The companion paper focuses on local plasma parameters during wave observations as well as wave generation proxies.
    Journal of Geophysical Research: Space Physics 07/2015; 120(7). DOI:10.1002/2015JA021333 · 3.44 Impact Factor
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    L.Q. Zhang · W. Baumjohann · J.Y. Wang · H. Rème · M. W. Dunlop · T. Chen
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    ABSTRACT: In this paper, we statistically analyzed and compared the earthward flow (EF) and the tailward flow (TF) in the plasma sheet. It is found that the properties of the EF/TF in the central plasma sheet (CPS) of β>1 and the outer plasma sheet (OPS) of 0.1<β<1 are distinctly different. The main conclusions include that: 1) the EF occur in both the CPS and the OPS while the TF mainly occur in the OPS; 2) both flows are dominantly convective in the CPS, and parallel in the OPS; 3) in the CPS, the EF and the TF have similar characteristics, including their bulk velocities and ion densities and Ey components. Both flows tend to have isotropic temperatures; 4) in the OPS, the EF tend to have higher ion velocity, density and Ey than the TF. The EF tend to have anisotropic temperatures, while the TF tend to have more isotropic temperatures. As a whole, combined characteristics of the EF and the TF are consistent with (1) reflection at the “magnetic mirror point” near the Earth for parallel flows in the OPS, and (2) bouncing off/back from the dipolar field closer to the Earth for convective flows in the CPS.
    Journal of Geophysical Research: Space Physics 07/2015; DOI:10.1002/2015JA021354 · 3.44 Impact Factor
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    L. Q. Zhang · J.Y. Wang · W. Baumjohann · H. Rème · M. W. Dunlop
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    ABSTRACT: Utilizing C3/Cluster satellite observations from the year of 2001 to 2006, we investigated the earthward flow (EF) and tailward flow (TF) at Bz > 0 in the plasma sheet. We found that the EF and the TF have similar spatial distributions. Both characteristics are independent of the distance beyond 14 RE. Both flows are deflected while closer to the Earth. Statistical results further showed that the EF/TF occur in the central plasma sheet as well as the plasma sheet boundary layer and can be observed during quiet times and periods of geomagnetic activity. A typical event reveals that the EF and the TF have different plasma population. A transition region (TR) can be formed at the interface between the EF and TF. Very significant duskward components appeared in bulk velocities for both populations. It appears that the vortical-like structure can be formed near the TR. The magnetic field within the TR is twisted and strongly fluctuates. No clear magnetic flux pileups are observed inside the TR.
    Journal of Geophysical Research: Space Physics 05/2015; 120(6):n/a-n/a. DOI:10.1002/2015JA021154 · 3.44 Impact Factor
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    ABSTRACT: We show for the first time, with direct, multispacecraft calculations of electric current density, and other methods, matched signatures of field-aligned currents (FACs) sampled simultaneously near the ionosphere at low (~500 km altitude) orbit and in the magnetosphere at medium (~2.5 RE altitude) orbits using a particular Swarm and Cluster conjunction. The Cluster signatures are interpreted and ordered through joint mapping of the ground/magnetospheric footprints and estimation of the auroral zone boundaries (taken as indication of the boundaries of Region 1 and Region 2 currents). We find clear evidence of both small-scale and large-scale FACs and clear matching of the behavior and structure of the large-scale currents at both Cluster and Swarm. The methodology is made possible through the joint operations of Cluster and Swarm, which contain, in the first several months of Swarm operations, a number of close three-spacecraft configurations.
    Geophysical Research Letters 04/2015; 42(10):n/a-n/a. DOI:10.1002/2015GL063738 · 4.20 Impact Factor
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    ABSTRACT: On 18 May 2011, the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observed whistler-mode waves associated with a magnetic dip behind a dipolarization front (DF) structure in the bursty bulk flow (BBF) braking region. For the first time, we find that whistler-mode waves are generated at the edges of magnetic dip, rather than at the center (also known as “minimum-B-pocket”). Detailed wave analysis indicates that the waves are likely lower and upper band whistler-mode chorus. We examine electron pitch angle distributions at the edges of dip, and compare them with those at the center and far outside the magnetic dip. Results confirm that the positive temperature anisotropy and pancake distributions at the edges of magnetic dip provide free energy source for growth of the whistler-mode waves. We also interpret the whole physical process of how whistler-mode waves generate in this event.
    Journal of Geophysical Research: Space Physics 03/2015; 120(4). DOI:10.1002/2014JA020786 · 3.44 Impact Factor
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    Tieyan Wang · Jinbin Cao · Huishan Fu · Wenlong Liu · Malcolm Dunlop
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    ABSTRACT: :On April 13 2002, four Cluster spacecraft with separations up to 127 km measured similar turbulence in the exterior cusp during northward interplanetary magnetic field (IMF) Bz. Both the power spectra of magnetic and electric field fluctuations resemble the classical Kolmogorov power law with the scaling f –1.7 under the proton gyrofrequency fcp (~0.3 Hz), breaks near fcp and then steepens with the scalings f –2.8 and f –2.0 up to 10 Hz, respectively. The observed ratio of the electric to magnetic field is in agreement with the theoretical values of |δE/δB| for the quasi perpendicular kinetic Alfvén waves (KAWs), which reflects the features of Alfvén turbulence. The wave vector and dispersion relation of the turbulence are obtained using k-filtering technique. The results show that the waves propagate quasi-perpendicularly to the background magnetic field. The similarity between the experimental and the theoratical dispersion relations indicates that the measured waves are kinetic Alfvén wave. The waves have right handed elliptical polarization in the plane perpendicular to k. The main axis of polarization ellipse is perpendicular to the average magnetic field. These features furthermore indicate that the turbulence properties agree well with those of KAW mode. The observed KAW is much possibly produced through resonance mode conversion. We calculate the density gradient vector using multi-point density data and found that the waves propagate basically towards high density region. The density gradient in the exterior cusp provides a favorable condition for the resonance converted KAW.
    Journal of Geophysical Research: Space Physics 12/2014; DOI:10.1002/2014JA019997 · 3.44 Impact Factor
  • A.Y. Duan · J. B. Cao · M. Dunlop · Z.Q. Wang
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    ABSTRACT: We studied energetic electron (40-250 keV) bursts (EEB) in the plasma sheet (PS) and their relation to bursty bulk flows (BBF) using the data recorded by Cluster from 2001 to 2009. The EEBs in the PS can be classified into four types. Three types of EEBs are dispersionless, including EEBs accompanied with BBFs (V > 250 km/s) but without DF, EEBs accompanied with both dipolarization front (DF) and BBF, and EEBs accompanied with DF and fast flow with V < 250 km/s. One type of EEB, i.e. EEBs not accompanied with BBFs and DFs, is dispersed. The energetic electrons (40 -130 keV) can be easily transported earthward by BBFs due to the strong dawn-dusk electric field embedded in BBFs. The DFs in BBFs can produce energetic electrons (40 to 250 keV). For the EEBs with DF and BBFs, the superposed epoch analyses show that the increase of energetic electron flux has two phases: gradual increase phase before DF and rapid increase phase concurrent with DF. In the PS around x = -18 RE, 60%-70% of EEBs are accompanied with BBFs, indicating that although hitherto there have been various acceleration mechanisms of energetic electrons, most of energetic electrons in the PS are related with magnetic reconnection, and they are produced either directly by magnetic reconnection or indirectly by the DFs within BBFs. In the BBF's braking region of -12 RE < x < -10 RE, 20% of EEBs are accompanied with BBFs. The corresponding ratio between EEBs and BBFs shows a dawn-dusk asymmetry.
    Journal of Geophysical Research: Space Physics 11/2014; 119(11). DOI:10.1002/2014JA020169 · 3.44 Impact Factor
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    ABSTRACT: Knowing the magnetic field distribution in the magnetotail current sheet (CS) is essential for exploring magnetotail dynamics. In this study, using a joint dataset of Cluster/TC-1, the radial profile of the magnetic field in the magnetotail CS with radial distances covering 8<r<20 RE under different geomagnetic activity states (i.e., AE≤100 nT for quiet intervals while AE>100 nT for active times) and solar wind parameters are statistically surveyed. Our new findings demonstrate that, independent of the activity state, the field strength and Bz component (GSM coordinates) start the monotonic increase prominently as r decreases down to ∼11.5RE, which means the dipole field starts to make a significant contribution from there. At least in the surveyed radial range, the Bz component is found to be weaker in the midnight and dusk sectors than that in the dawn sector, displaying a dawn-dusk asymmetry. The occurrence rate of negative Bz in active times also exhibits a similar asymmetric distribution, which implies active dynamics may occur more frequently at midnight and dusk flank. In comparison with that in quiet intervals, several features can be seen in active times: (1) a local Bz minimum between 10.5<r<12.5 RE is found in the dusk region, (2) the Bz component around the midnight region is generally stronger and experiences larger fluctuations, and (3) a sharp positive/negative-excursion of the By component occurs at the dawn/dusk flank regions inside r<10 RE. The response to solar wind parameters revealed that the Bz component is generally stronger under higher dynamic pressure (Pdy>5 nPa), which may support the dawn-dusk squeezing effect as presented by Miyashita et al. (2010). The CS By is generally correlated with the interplanetary magnetic field (IMF) By component, and the correlation quality is found to be better with higher penetration coefficient (the ratio of CS By to IMF By) when IMF Bz is positive. The implications of the present results are discussed.
    Planetary and Space Science 11/2014; 103. DOI:10.1016/j.pss.2014.07.014 · 1.88 Impact Factor
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    ABSTRACT: Unambiguous knowledge of magnetic field structure and the electric current distribution is critical for understanding the origin, evolution and related dynamic properties of magnetic flux ropes (MFRs). In this paper, a survey of 13 MFRs in the Earth's magnetotail are conducted by Cluster multi-point analysis, so that their force-free feature, i.e., the kind of magnetic field structure satisfying J × B = 0, can be probed directly. It is showed that the selected flux ropes with the bipolar signature of the South–north magnetic field component generally lie near the equatorial plane, as expected, and that the magnetic field gradient is rather weak near the axis center, where the curvature radius is large. The current density (up to several tens of nA/m2) reach their maximum values as the center is approached. It is found that the stronger the current density, the smaller the angles between the magnetic field and current in MFRs. The direct observations show that only quasi force-free structure is observed and it tends to appear in the low plasma beta regime (in agreement with the theoretic results). The quasi force-free region is generally found to be embedded in the central portion of the MFRs, where the current is approximately field-aligned and proportional to the strength of core field. It is shown that, ~60% of surveyed MFRs can be globally approximated as force-free. The force-free factor α is found to be non-constantly varied through the quasi force-free MFR, suggesting that the force-free structure is non-linear.
    Journal of Geophysical Research: Space Physics 08/2014; 119(8). DOI:10.1002/2013JA019642 · 3.44 Impact Factor
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    ABSTRACT: The penetration of Interplanetary Magnetic Field (IMF) By into the magnetosphere plays an important role in determining magnetospheric configuration and its dynamics. In this paper, using 9 years of Cluster data at the center of neutral sheet, we statistically study the relations of the penetration of IMF By in the neutral sheet (NS) with IMF Bz and the Kp index. The correlation coefficient between NS By and IMF By is enhanced during the periods of southward IMF Bz and large Kp indexes. The penetration efficiency of IMF By, which is defined as the slope of the linear fit of the points in the By -IMF By space, is larger during southward IMF Bz than during northward IMF Bz. The penetration efficiency of IMF By also increases with increasing Kp index. Since the Kp index can be considered as an index of magnetospheric convection, this means that the penetration of IMF By into the magnetosphere is enhanced during the periods of strong magnetospheric convection. These results indicate that the IMF Bz and magnetospheric convection can influence the neutral sheet By and even magnetotail dynamics by changing the penetration of IMF By.
    Journal of Geophysical Research: Space Physics 07/2014; 119(7). DOI:10.1002/2014JA019827 · 3.44 Impact Factor
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    ABSTRACT: We report the in-situ observation of a plasma vortex induced by a solar wind dynamic pressure enhancement in the nightside plasma sheet using multi-point measurements from THEMIS satellites. The vortex has a scale of 5–10 Re and propagates several Re downtail, expanding while propagating. The features of the vortex are consistent with the prediction of the Sibeck [1990] model, and the vortex can penetrate deep (~8Re) in the dawn-dusk direction and couple to field line oscillations. Global magnetohydrodynamics (MHD) simulations are carried out and it is found that the simulation and observations are consistent with each other. Data from THEMIS ground magnetometer stations indicate a poleward propagating vortex in the ionosphere, with a rotational sense consistent with the existence of the vortex observed in the magnetotail.
    Journal of Geophysical Research: Space Physics 06/2014; 119(6). DOI:10.1002/2013JA019551 · 3.44 Impact Factor
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    ABSTRACT: Magnetic disturbances caused by the Earth's ring current, particularly during storm time activity, have a dominant effect on the geomagnetic field. Strong currents and large kinetic and magnetic energies can change considerably local field geometry, and depress the ground geomagnetic field. The multi-spacecraft magnetic measurements of Cluster allow extensive in situ coverage of the ring current, We select 48 storm time Cluster crossing events to investigate the variation of the local current density distribution and magnetic configuration of the ring current. We find direct evidence for the existence of an inner, eastward flowing current in addition to the dominant westward current, in the ring plane. The radius of curvature of the magnetic field lines (MFLs) is found to be increasingly reduced at all local times during increasing storm activity, changing the resulting ring current magnetic geometry considerably, where the MFL configuration and the azimuthal current density distribution, are asymmetric with the local time. During similar storm activity the radius of curvature of the local MFLs, RC, is smallest on the nightside to duskside, medium on the dawnside, and largest on the dayside. This change in geometry may have significant influence on the spatial distribution of the particles with various energies in the plasmasphere, ring current and radiation belts.
    Journal of Geophysical Research: Space Physics 04/2014; 119(4). DOI:10.1002/2013JA019460 · 3.44 Impact Factor
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    ABSTRACT: [1] In this paper, we test whether time periods with hot proton temperature anisotropy are associated with EMIC waves, and whether the plasma conditions during the observed waves satisfy the linear theory threshold condition. We identify 865 events observed by the Composition DIstribution Function (CODIF) instrument onboard Cluster spacecraft 4 (SC4) during 1 January 2001 – 1 January 2011 that exhibit a positive temperature anisotropy (Ahp = T⊥ h/T∥ h − 1) in the 10-40 keV protons. The events occur over an L range from 4 to 10 in all magnetic local times and at magnetic latitudes (MLAT) within ±50°. Of these Hot Proton Temperature Anisotropy (HPTA) events, only 68 events have electromagnetic ion cyclotron (EMIC) waves. In these 68 HPTA events, for those at 3.81.0 nT2/Hz mainly appear in the region with fEMIC/fH,eq 0.45 * fEMIC/fH,lo, and Ahp/(Ahp + 1) 0.25. By testing a threshold equation for the EMIC instability based on linear theory, we find that for EMIC waves with |MLAT| ≤ 10° in the He, H and > H bands the percentages that satisfy the predicted conditions for wave growth by the threshold equation are 15.2%, 24.6% and 25.6%. For the EMIC waves with |MLAT| > 10° the percentages that satisfy the wave growth predicted conditions are only 2.8%, 2.6% and 0.0%. Finally, possible reasons for the low forecast accuracies of EMIC waves are suggested.
    Journal of Geophysical Research: Space Physics 02/2014; 119(2). DOI:10.1002/2013JA019541 · 3.44 Impact Factor
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    Z. W. Cheng · J. K. Shi · M. Dunlop · Z. X. Liu
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    ABSTRACT: The influences of the interplanetary magnetic field (IMF) cone angle θ and clock angle ϕ on the field-aligned currents (FACs) at the plasma sheet boundary layers (PSBLs) have been investigated using Cluster Data. The FAC occurrence increases monotonically with IMF cone angle and has two peaks at -90° and +110° clock angle, respectively. The peak at +110° is distinctly larger than that at -90°. Overall, there are more FACs between 0° < ϕ < 180°, indicating that FACs occurrence is closely associated with duskward IMF. More FACs occur when 90° < |ϕ| < 180°, implying that FAC is closely associated with southward IMF. The large FAC densities occur when 60° < |ϕ| < 120°. The density also has two peaks and the peak at +90° clock angle (duskward IMF) is larger than that at -90° (dawnward IMF). These results indicate that the IMF influence on the FACs is from all IMF components and not only from a single component.
    10/2013; 40(20). DOI:10.1002/2013GL056737

Publication Stats

6k Citations
913.70 Total Impact Points


  • 2014–2015
    • Beijing University of Aeronautics and Astronautics (Beihang University)
      • School of Astronautics
      Peping, Beijing, China
  • 1–2015
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 2013
    • Science and Technology Facilities Council
      Swindon, England, United Kingdom
  • 2008
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, CA, United States
  • 1993–2007
    • Technische Universität Braunschweig
      • Institute for Geophysics and Extraterrestrial Physics
      Brunswyck, Lower Saxony, Germany
  • 2002
    • Research Associates Laboratory
      Dallas, Texas, United States
    • KTH Royal Institute of Technology
      • School of Electrical Engineering (EE)
      Tukholma, Stockholm, Sweden
  • 2001
    • The University of Sheffield
      • Department of Automatic Control and Systems Engineering
      Sheffield, England, United Kingdom
  • 1999
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 1990
    • University of Cologne
      • Institute of Geophysics and Meteorology
      Köln, North Rhine-Westphalia, Germany