[Show abstract][Hide abstract] 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 10/2014; · 3.44 Impact Factor
[Show abstract][Hide abstract] 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 10/2014; · 3.44 Impact Factor
[Show abstract][Hide abstract] 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; · 3.44 Impact Factor
[Show abstract][Hide abstract] 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  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; · 3.44 Impact Factor
[Show abstract][Hide abstract] 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 06/2014; · 3.44 Impact Factor
[Show abstract][Hide abstract] 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 03/2014; · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  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 01/2014; · 3.44 Impact Factor
[Show abstract][Hide abstract] 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 01/2014; · 2.11 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: is well accepted that the propagation of electromagnetic ion cyclotron
(EMIC) waves are bidirectional near their source regions and
unidirectional when away from these regions. The generally believed
source region for EMIC waves is around the magnetic equatorial plane.
Here we describe a series of EMIC waves in the Pc1 (0.2-5 Hz) frequency
band above the local He+ cyclotron frequency observed in situ
by all four Cluster spacecraft on 9 April 2005 at midmagnetic latitudes
(MLAT = ~33°-49°) with L = 10.7-11.5 on the dayside (MLT =
10.3-10.4). A Poynting vector spectrum shows that the wave packets
consist of multiple groups of packets propagating bidirectionally,
rather than unidirectionally, away from the equator, while the local
plasma conditions indicate that the spacecraft are entering into a
region sufficient for local wave excitation. One possible interpretation
is that, while part of the observed waves are inside their source
region, the others are either close enough to the source region, or
mixed with the wave packets from multiple source regions at different
Journal of Geophysical Research Atmospheres 10/2013; 118(10):6266-6278. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  Flux transfer events (FTEs) are magnetic flux ropes formed at planetary magnetopauses (MPs). Although evidence suggests that FTEs form through time-dependent magnetic reconnection, details of that process and 3D structure of the flux ropes remain largely unclear. This letter presents Double Star/TC-1 data of an FTE occurred on 7 April 2004 which show that the FTE was separated by two X-lines moving south-dawnward. In particular, the electron energy-pitch angle distribution implies that the FTE was composed of flux ropes of all four possible magnetic topologies, indicating that the field lines must have reconnected multiple times. This is an intrinsic property of FTEs formed by 3D multiple X-line reconnection distinguished from quasi 2D FTE models. This knowledge of FTE magnetic topologies helps to improve our understanding of solar wind- magnetosphere coupling at the MP.
[Show abstract][Hide abstract] ABSTRACT: Electromagnetic ion cyclotron (EMIC) waves are an important mechanism for energy transport and particle interactions inside the magnetosphere. In order to both test and drive models of these waves, detailed information about the wave normal angle, ellipticity, and Poynting vector direction distributions as well as the distribution of plasma conditions necessary for wave excitation is required. Previous statistical studies have used in situ data to investigate the distribution of these parameters in the the L-MLT frame within a limited MLAT range (i.e. [Loto’aniu et al., 2005; Min et al., 2012]). In this study, we present a statistical analysis of these wave parameters as well as the electron plasma/gyrofrequency ratio and a linear theory proxy for wave generation, using ten years (2001-2009) of data from Cluster totaling 2368 minutes of wave activity.
[Show abstract][Hide abstract] ABSTRACT: On 14 June 2007, four Time History of Events and Macroscale Interactions
during Substorms spacecraft observed a flux transfer event (FTE) on the
dayside magnetopause, which has been previously proved to be generated
by multiple, sequential X-line reconnection (MSXR) in a 2-D context.
This paper reports a further study of the MSXR event to show the 3-D
viewpoint based on additional measurements. The 3-D structure of the FTE
flux rope across the magnetospheric boundary is obtained on the basis of
multipoint measurements taken on both sides of the magnetopause. The
flux rope's azimuthally extended section is found to lie approximately
on the magnetopause surface and parallel to the X-line direction; while
the axis of the magnetospheric branch is essentially along the local
unperturbed magnetospheric field lines. In the central region of the
flux rope, as distinct from the traditional viewpoint, we find from the
electron distributions that two types of magnetic field topology
coexist: opened magnetic field lines connecting the magnetosphere and
the magnetosheath and closed field lines connecting the Southern and
Northern hemispheres. We confirm, therefore, for the first time, the
characteristic feature of the 3-D reconnected magnetic flux rope, formed
through MSXR, through a determination of the field topology and the
plasma distributions within the flux rope. Knowledge of the complex
geometry of FTE flux ropes will improve our understanding of solar
Journal of Geophysical Research Atmospheres 05/2013; 118(5):1904-1911. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Modelling plasma entry in the polar cusp has been successful in
reproducing ion dispersions observed in the cusp at low and
mid-altitudes. The use of a realistic convection pattern, when the
IMF-By is large and stable, allowed Wing et al. (2001) to
predict double cusp signatures that were subsequently observed by the
DMSP spacecraft. In this paper we present a cusp crossing where two cusp
populations are observed, separated by a gap around 1° Invariant
Latitude (ILAT) wide. Cluster 1 (C1) and Cluster 2 (C2) observed these
two cusp populations with a time delay of 3 min, and about 15 and 42 min
later Cluster 4 (C4) and Cluster 3 (C3) observed, respectively, a single
cusp population. A peculiarity of this event is the fact that the second
cusp population seen on C1 and C2 was observed at the same time as the
first cusp population on C4. This would tend to suggest that the two
cusp populations had spatial features similar to the double cusp. Due to
the nested crossing of C1 and C2 through the gap between the two cusp
populations, C2 being first to leave the cusp and last to re-enter it,
these observations are difficult to be explained by two distinct cusps
with a gap in between. However, since we observe the cusp in a narrow
area of local time post-noon, a second cusp may have been present in the
pre-noon sector but could not be observed. On the other hand, these
observations are in agreement with a motion of the cusp first dawnward
and then back duskward due to the effect of the IMF-By
[Show abstract][Hide abstract] ABSTRACT: works have emphasized the significant influence of the solar wind
Alfvén Mach number (MA) on magnetospheric dynamics.
Here we report statistical, observational results that pertain to
changes in the magnetosheath flow distribution and magnetopause shape as
a function of solar wind MA and interplanetary magnetic field
(IMF) clock angle orientation. We use all Cluster 1 data in the
magnetosheath during the period 2001-2010, using an appropriate spatial
superposition procedure, to produce magnetosheath flow distributions as
a function of location in the magnetosheath relative to the IMF and
other parameters. The results demonstrate that enhanced flows in the
magnetosheath are expected at locations quasi-perpendicular to the IMF
direction in the plane perpendicular to the Sun-Earth line; in other
words, for the special case of a northward IMF, enhanced flows are
observed on the dawn and dusk flanks of the magnetosphere, while much
lower flows are observed above the poles. The largest flows are adjacent
to the magnetopause. Using appropriate magnetopause crossing lists (for
both high and low MA), we also investigate the changes in
magnetopause shape as a function of solar wind MA and IMF
orientation. Comparing observed magnetopause crossings with predicted
positions from an axisymmetric semi-empirical model, we statistically
show that the magnetopause is generally circular during high
MA, while is it elongated (albeit with moderate statistical
significance) along the direction of the IMF during low MA.
These findings are consistent with enhanced magnetic forces that prevail
in the magnetosheath during low MA. The component of the
magnetic forces parallel to the magnetopause produces the enhanced flows
along and adjacent to the magnetopause, while the component normal to
the magnetopause exerts an asymmetric pressure on the magnetopause that
deforms it into an elongated shape.
Journal of Geophysical Research Atmospheres 03/2013; 118(3):1089-1100. · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An understanding of the transport of solar wind plasma into and throughout the terrestrial magnetosphere is crucial to space science and space weather. For non-active periods, there is little agreement on where and how plasma entry into the magnetosphere might occur. Moreover, behaviour in the high-latitude region behind the magnetospheric cusps, for example, the lobes, is poorly understood, partly because of lack of coverage by previous space missions. Here, using Cluster multi-spacecraft data, we report an unexpected discovery of regions of solar wind entry into the Earth's high-latitude magnetosphere tailward of the cusps. From statistical observational facts and simulation analysis we suggest that these regions are most likely produced by magnetic reconnection at the high-latitude magnetopause, although other processes, such as impulsive penetration, may not be ruled out entirely. We find that the degree of entry can be significant for solar wind transport into the magnetosphere during such quiet times.
[Show abstract][Hide abstract] ABSTRACT: 1] We develop a new simple method for inferring the orientation of a magnetic flux rope, which is assumed to be a time-independent cylindrically symmetric structure via the direct single-point analysis of magnetic field structure. The model tests demonstrate that, for the cylindrical flux rope regardless of whether it is force-free or not, the method can consistently yield the axis orientation of the flux rope with higher accuracy and stability than the minimum variance analysis of the magnetic field and the Grad-Shafranov reconstruction technique. Moreover, the radial distance to the axis center and the current density can also be estimated consistently. Application to two actual flux transfer events observed by the four satellites of the Cluster mission demonstrates that the method is more appropriate to be used for the inner part of flux rope, which might be closer to the cylindrical structure, showing good agreement with the results obtained from the optimal Grad-Shafranov reconstruction and the least squares technique of Faraday's law, but fails to produce such agreement for the outer satellite that grazes the flux rope. Therefore, the method must be used with caution., Method for inferring the axis orientation of cylindrical magnetic flux rope based on single-point measurement, J. Geophys. Res., 118, doi:10.1029/2012JA018079.
Journal of Geophysical Research Atmospheres 01/2013; · 3.44 Impact Factor