Dusk Flank Research Articles

Highly periodic stormtime activations observed by THEMIS prior to substorm onset

On March 24, 2007 THEMIS observed near the dusk flank several 10 minute quasi‐periodic flow and magnetic field oscillations followed by the onset of a strong substorm (AL ∼ −1000 nT). The substorm occurred during an interval of strongly southward IMF, near the start of the recovery phase of a small storm (SYM‐H near −80 nT). Each magnetic oscillation was accompanied by a rapid flow variation, auroral intensification, energetic particle injection, and Pi2 pulsations. For several hours both prior to and following the substorm THEMIS observed highly periodic flow oscillations, with the same 10 minute periodicity. The average of these flow oscillations was non‐zero and positive, indicating net sunward transport. We suggest that the long interval of oscillatory flow constituted a periodic convective mode of the magnetosphere, and further suggest that the quasi‐periodic activations were associated with reconnection near the THEMIS location.

The storm‐time access of solar wind ions to the nightside ring current and plasma sheet

We investigated the storm‐time entry of solar wind ions into the magnetosphere by carrying out a large‐scale kinetic particle tracing calculation for the 24–25 September 1998 and the 17 April 2002 geomagnetic storms. For each storm, we simulated the magnetosphere by using a global magnetohydrodynamic (MHD) code that was driven by solar wind and interplanetary magnetic field (IMF) data from spacecraft upstream of Earth. We then launched ions in the solar wind in the global time‐dependent fields obtained from the MHD simulation, beginning prior to storm onset and extending into the main phase. We collected those ions that successfully reached the nightside plasma sheet and ring current, and then determined the mechanisms of entry and transport responsible for the injection of these ions into the magnetotail. We found that, in agreement with quiet time entry results, the IMF By component strongly influenced whether ions entered through the dawn or dusk flanks, and changes in IMF By were immediately reflected on the ion entry pattern. Also, in addition to the usual dayside merging source, ion entry into the magnetosphere was facilitated by dynamic pressure enhancements, during which entry occurred over a wide swath of latitudes and extended from the dayside to locations far downtail. More significantly, because of the warmer storm‐time ion population in the magnetosheath, ions were more readily affected by inhomogeneities and rapid changes in the IMF, which caused solar wind ions to gradient drift onto open field lines and reach the inner magnetosphere.

Solar wind driving of magnetospheric ULF waves: Pulsations driven by velocity shear at the magnetopause

We present results from global, three‐dimensional magnetohydrodynamic (MHD) simulations of the solar wind/magnetosphere interaction. These MHD simulations are used to study ultra low frequency (ULF) pulsations in the Earth's magnetosphere driven by shear instabilities at the flanks of the magnetopause. We drive the simulations with idealized, constant solar wind input parameters, ensuring that any discrete ULF pulsations generated in the simulation magnetosphere are not due to fluctuations in the solar wind. The simulations presented in this study are driven by purely southward interplanetary magnetic field (IMF) conditions, changing only the solar wind driving velocity while holding all of the other solar wind input parameters constant. We find surface waves near the dawn and dusk flank magnetopause and show that these waves are generated by the Kelvin‐Helmholtz (KH) instability. We also find that two KH modes are generated near the magnetopause boundary. One mode, the magnetopause KH mode, propagates tailward along the magnetopause boundary. The other mode, the inner KH mode, propagates tailward along the inner edge of the boundary layer (IEBL). We find large vortical structures associated with the inner KH mode that are centered on the IEBL. The phase velocities, wavelengths, and frequencies of the two KH modes are computed. The KH waves are found to be fairly monochromatic with well‐defined wavelengths. In addition, the inner and magnetopause KH modes are coupled and lead to a coupled oscillation of the low‐latitude boundary layer. The boundary layer thickness, d, is computed and we find maximum wave growth for kd = 0.5–1.0, where k is the wave number, consistent with the linear theory of the KH instability. We comment briefly on the effectiveness of these KH waves in the energization and transport of radiation belt electrons.

THEMIS observations of a hot flow anomaly: Solar wind, magnetosheath, and ground‐based measurements

The THEMIS spacecraft encountered a Hot Flow Anomaly (HFA) on the dusk flank of the Earth’s bow shock on 4 July 2007, observing it on both sides of the shock. Meanwhile, the THEMIS ground magnetometers traced the progress of the associated Magnetic Impulse Event along the dawn flank of the magnetosphere, providing a unique opportunity to study the transmission of the HFA through the shock and the subsequent downstream response. THEMIS‐A, in the solar wind, observed classic HFA signatures. Isotropic electron distributions inside the upstream HFA are attributed to the action of the electron firehose instability. THEMIS‐E, just downstream, observed a much more complex disturbance with the pressure perturbation decoupled from the underlying discontinuity. Simple calculations show that the pressure perturbation would be capable of significantly changing the magnetopause location, which is confirmed by the ground‐based observations.

Solar wind excitation of Pc5 fluctuations in the magnetosphere and on the ground

The primary purpose of this paper is to show the importance of solar wind excitation as a source of magnetospheric and ground Pc5 fluctuations. In particular, we show that pressure fluctuations, a nearly ubiquitous feature of the solar wind, drive magnetospheric compressional Pc5, whose amplitude and power increase in direct proportion to the amplitude and power of the solar wind pressure fluctuations. Previous studies of compressional Pc5 have focused primarily on internally generated waves on the dusk flank; here we highlight the pressure fluctuations associated with geoeffective high‐speed streams (HSSs) and interplanetary coronal mass ejections (ICMEs) during a time interval in March and April 2002, in the declining phase of the solar cycle, when there was a favorable alignment of satellites. Using three examples and a statistical survey, we show that for this time interval external forcing due to solar wind dynamic pressure fluctuations was dominant over internal forcing. Fluctuations driven by internal processes, occurring primarily at dusk and on the nightside, accounted for less than 20% of the total power. We also found that multiple external forcing processes were occurring during this interval, including evanescent propagation of compressional waves, field line resonance (FLR), and global mode excitation. We need to enlarge our vision of the source of ULF waves to include excitation by solar wind pressure fluctuations as a significant factor.

Geotail observations of two-component protons in the midnight plasma sheet

Abstract. Through the effort to obtain clues toward understanding of transport of cold plasma in the near-Earth magnetotail under northward IMF, we find that two-component protons are observed in the midnight plasma sheet (−10>XGSM>−30 RE, |YGSM| <10 RE) under northward IMF by the Geotail spacecraft. Since the two-component protons are frequently observed on the duskside during northward IMF intervals but hardly on the dawnside, those found in the midnight plasma sheet are thought to come from the dusk flank. The cold proton component in the midnight region occasionally has a parallel anisotropy, which resembles that in the tail flank on the duskside. The flows in the plasma sheet with two-component protons were quite stagnant or slightly going dawnward, which supports the idea that the observed two-component protons in the midnight region are of duskside origin. Because the two-component protons in the midnight plasma sheet emerge under strongly northward IMF with the latitudinal angle larger than 45 degrees, and because the lag from the strongly northward IMF to the emergence can be as short as a few hours, we suggest that prompt plasma transport from the dusk to midnight region occurs under strongly northward IMF. We propose that the dawnward flows result from viscous interaction between the high-latitude portion of the plasma sheet and the lobe cell. Another candidate for plasma transport process from the dusk to the midnight region is turbulent flow due to vortical structures of the Kelvin-Helmholtz instability that developed around the dusk low-latitude boundary under strongly northward IMF. In addition, we also suggest that gradual cooling of hot protons under northward IMF is a global phenomenon in the near-Earth magnetotail.

“Compression aurora”: Particle precipitation driven by long‐duration high solar wind ram pressure

The transient behavior of shock‐induced auroras reported by previous workers suggests a cause‐and‐effect relationship between the shock front and the auroral transient. However, it is not known if the high solar wind dynamic pressure downstream of a shock can have significant effects on the global auroral morphology. In this brief report we present results from an observational study on global auroral response to large solar wind dynamic pressure, using global auroral images acquired by the ultraviolet imager on board the Polar spacecraft. It is found that the luminosity of the aurora showed a general prompt and lasting increase in the dawn and dusk flanks of the oval. Such an auroral activity is closely related to the compression of the magnetosphere and is thus christened “compression aurora.” The luminosity increase, though predominantly in the dawn sector of the oval, may persist as long as the solar wind dynamic pressure stays high and may disappear ∼10 min after the dynamic pressure drops. A detailed examination of Defense Meteorological Satellite Program particle data on one fortuitous event on 26 September 1999 indicates that the compression aurora is produced mainly by precipitating electrons originating from the central plasma sheet (CPS). Plasma wave observations made from Geotail in the dawn sector of the near‐Earth plasma sheet indicates no expected plasma wave activity that may have been otherwise considered a primary cause of enhanced CPS particle precipitation through a pitch angle loss by wave‐particle interactions. We propose that reductions of the Earth's magnetic field mirror ratio by magnetospheric compression can be a prime mechanism for the compression aurora as long as there is a continuing supply of particle source.

The plasma sheet and boundary layers under northward IMF: A multi-point and multi-instrument perspective

During conditions of northward interplanetary magnetic field (IMF), the near-tail plasma sheet is known to become denser and cooler, and is described as the cold-dense plasma sheet (CDPS). While its source is likely the solar wind, the prominent penetration mechanisms are less clear. The two main candidates are solar wind direct capture via double high-latitude reconnection on the dayside and Kelvin–Helmholtz/diffusive processes at the flank magnetopause. This paper presents a case study on the formation of the CDPS utilizing a wide variety of space- and ground-based observations, but primarily from the Double Star and Polar spacecraft on December 5th, 2004. The pertinent observations can be summarized as follows: TC-1 observes quasi-periodic (∼2 min period) cold-dense boundary layer (compared to a hot-tenuous plasma sheet) signatures interspersed with magnetosheath plasma at the dusk flank magnetopause near the dawn-dusk terminator. Analysis of this region suggests the boundary to be Kelvin–Helmholtz unstable and that plasma transport is ongoing across the boundary. At the same time, IMAGE spacecraft and ground based SuperDARN measurements provide evidence of high-latitude reconnection in both hemispheres. The Polar spacecraft, located in the southern hemisphere afternoon sector, sunward of TC-1, observes a persistent boundary layer with no obvious signature of boundary waves. The plasma is of a similar appearance to that observed by TC-1 inside the boundary layer further down the dusk flank, and by TC-2 in the near-Earth magnetotail. We present comparisons of electron phase space distributions between the spacecraft. Although the dayside boundary layer at Polar is most likely formed via double high-altitude reconnection, and is somewhat comparable to the flank boundary layer at Double Star, some differences argue in favour of additional transport that augment solar wind plasma entry into the tail regions.

Jupiter's Nightside Airglow and Aurora

Observations of Jupiter's nightside airglow (nightglow) and aurora obtained during the flyby of the New Horizons spacecraft show an unexpected lack of ultraviolet nightglow emissions, in contrast to the case during the Voyager flybys in 1979. The flux and average energy of precipitating electrons generally decrease with increasing local time across the nightside, consistent with a possible source region along the dusk flank of Jupiter's magnetosphere. Visible emissions associated with the interaction of Jupiter and its satellite Io extend to a surprisingly high altitude, indicating localized low-energy electron precipitation. These results indicate that the interaction between Jupiter's upper atmosphere and near-space environment is variable and poorly understood; extensive observations of the day side are no guide to what goes on at night.

Sources, transport, and distributions of plasma sheet ions and electrons and dependences on interplanetary parameters under northward interplanetary magnetic field

We have investigated the Geotail data statistically to understand the particle sources, transport, and spatial distributions of the plasma sheet ions and electrons of different energies during northward interplanetary magnetic field (IMF), and their dependences on the solar wind density (Nsw), the solar wind speed (Vsw), and the magnitude of the northward IMF Bz (∣Bz,IMF∣). We find that the plasma sheet becomes colder and denser, indicating a larger increase in the cold than in the hot population, with increasing Nsw or ∣Bz,IMF∣ or with decreasing Vsw. The cold population dominates the region near the flanks while the hot population dominates the near‐midnight region, which is consistent with the plasma sheet plasma being a mixture of cold particles coming from the flanks and hot particles from the distant‐tail. The phase space densities show that the flank source strongly depends on ∣Bz,IMF∣, while the tail source strongly depends on Vsw. Cold particles from the dawn flank to midnight increase significantly with decreasing Vsw, but no significant changes are seen near the dusk flank, suggesting a dependence of the solar wind entry through the dawn flank on Vsw. The comparisons between the distributions of the phase space density and the electric and magnetic drift paths estimated from the observations indicate that the thermal and high‐energy particles are mainly transported by electric and magnetic drift, while other transport mechanisms, such as diffusion, may play a role in transporting the low energy particles from the flank sources to midnight.

Solar wind control of plasma number density in the near‐Earth plasma sheet

The plasma number density in the plasma sheet depends on the solar wind number density and the north‐south component of interplanetary magnetic field (IMF Bz) with the time lag of several hours. We examined such dependences as functions of (X, Y) coordinates in the near‐Earth plasma sheet by fitting observations of plasma sheet and solar wind to an empirical model equation. We explored shortest and longest response time lags to IMF Bz by optimizing the correlation coefficient of fit. Analyses were conducted separately for northward and southward IMF dominant conditions. The dependence of plasma sheet number density on solar wind number density is stronger in the near‐tail region (r > 20 RE) under the southward IMF dominant condition. The dependence has weak dawn‐dusk asymmetry under the southward IMF dominant condition, whereas it is stronger in the dusk flank under the northward IMF dominant condition. The dependence on IMF Bz is globally positive under the northward IMF dominant condition, whereas the dependence is negative in the near‐Earth premidnight region under the southward IMF dominant condition. Both shortest and longest time lags increase from the mid‐tail to the near‐Earth premidnight region under the southward IMF dominant condition. On the other hand, the shortest (longest) time lag increases antisunward (sunward) along flanks under the northward IMF dominant condition. These features can be explained in terms of reconnection and Kelvin‐Helmholtz diffusion mechanisms for entry of magnetosheath plasma into the plasma sheet and electric and magnetic drift transport in the plasma sheet.

Magnetospheric convection during intermediate driving: Sawtooth events and steady convection intervals as seen in Lyon‐Fedder‐Mobarry global MHD simulations

We have used global MHD simulations to investigate the magnetospheric response to steady solar wind for two events. The event of 18 April 2002 is characterized by periodic particle injections and magnetic field dipolarizations, or sawtooth activity, at geosynchronous orbit, while 3–4 February 1998 is a period of steady magnetospheric convection (SMC). In our simulations we find for both events that a general system of convection develops, characterized in the magnetotail by large‐scale sunward flows driven by reconnection in region of 30–45 RE downstream. These flows in general divert around the inner magnetosphere to the dawn and dusk flanks and then converge toward the dayside magnetopause. In the sawtooth event we find reconnection is intermittent and patchy, resulting in flow bursts which on average produce the general convection pattern indicated, but a fraction of them penetrate into the inner magnetosphere and are associated with observed plasma injections and field dipolarizations. In the SMC event the convection system is formed by quasi‐steady reconnection in the midtail, which drives steady earthward flows. These flows divert to the flanks, leaving the inner magnetosphere undisturbed. We find the difference in the magnitude of IMF BZ is unlikely to account for difference in activity in the two events, as a simulation of the SMC event with increased ∣BZ∣ produced qualitatively the same steady convection. Solar wind density variations are shown to control the average mass transport but have no correlation with the flow channels responsible for the inner magnetospheric activity in the simulations.

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

The Geotail spacecraft made an inbound passage perpendicular to the dusk equatorial magnetopause on 1 August 1998 when the interplanetary magnetic field had been very northward for more than 10 hours. As the spacecraft moved through the low‐latitude boundary layer, it detected waves with ∼3 min period that caused transitions between cool dense magnetosheath plasma and mixed magnetosheath and magnetosphere plasmas. A region of highly fluctuating magnetic fields with variations up to 50 nT/s was observed on leaving the magnetosheath plasma but not on its return to this region. This observation suggests that the boundary region is not laminar but more likely consists of Kelvin‐Helmholtz vortices. Such fluctuations are probably caused by twisting of the field by the vortices, and they could be important in transporting plasma to the magnetosphere where densities of 5/cc were observed. A global MHD simulation of the event reproduced the observed low‐frequency waves, their counterclockwise polarization, and a region of high‐velocity magnetosheath flow outside the magnetopause. This flow was associated with a decreased density and increased field strength which are characteristics of a plasma depletion layer. When Geotail moved further inward into the magnetosphere, it detected waves that are probably the magnetotail vortices which had been detected earlier by the ISEE spacecraft.

Geotail observations of temperature anisotropy of the two-component protons in the dusk plasma sheet

Abstract. In search for clues towards the understanding of the cold plasma sheet formation under northward IMF, we study the temperature anisotropy of the two-component protons in the plasma sheet near the dusk low-latitude boundary observed by the Geotail spacecraft. The two-component protons result from mixing of the cold component from the solar wind and the hot component of the magnetospheric origin, and may be the most eloquent evidence for the transport process across the magnetopause. The cold component occasionally has a strong anisotropy in the dusk flank, and the sense of the anisotropy depends on the observed locations: the parallel temperature is enhanced in the tail flank while the perpendicular temperature is enhanced on the dayside. The hot component is nearly isotropic in the tail while the perpendicular temperature is enhanced on the dayside. We discuss possible mechanism that can lead to the observed temperature anisotropies.

Timescale for the formation of the cold‐dense plasma sheet: A case study

The timescale for the formation of cold‐dense plasma sheet ions was investigated with an event in which the interplanetary magnetic field (IMF) was northward for almost one day. The plasma sheet dawn and dusk flanks appear to reach cold dense states (n > 1 cm−3; T < 2 keV) within a few hours after IMF northward turning. Closer to the center (midnight meridian), the ion temperatures reach < 2 keV within a few hours of IMF northward turning, but the ion densities do not reach above 1 cm−3 for at least ∼8 hours after IMF northward turning. The connection between solar wind ions and plasma sheet cold‐component ions is demonstrated. The plasma sheet dawn flank ions appear to lag the solar wind ions by about 3 hours. This study confirms the previous statistical results: (a) the densification of the plasma sheet can be attributed to the influx of the cold‐component (magnetosheath/solar wind origin) ions; and (b) the cooling of the plasma sheet can be attributed not only to the influx of the solar wind ions, but also to the cooling of the hot components. Order of magnitude calculations of the plasma sheet filling rate from reconnection and diffusion suggest that both entry mechanisms could result in roughly comparable filling rates. Hence, the dawn‐dusk asymmetries would be key in distinguishing the roles of the various proposed entry mechanisms.

Global Pc5 wave activity observed using SuperDARN radars and ground magnetometers during an extended period of northward IMF

Observations are presented of long-lived global Pc5 ULF wave activity observed at a wide range of local times. The event was monitored in the high latitude ionosphere (∼60–80° magnetic latitude) by several SuperDARN HF radars and 5 magnetometer chains in Scandinavia, Greenland, Canada, Alaska and Russia. The event coincided with a protracted period (∼36 h) of northward interplanetary magnetic field (IMF). The study focuses on 4 h during which distinct dawn/dusk asymmetries in the wave characteristics were observed with multiple field line resonance (FLR) structures observed in the dawn flank at 1.7, 2.6, 3.3, 4.2 and 5.4 mHz and compressional oscillations in the dusk flank at 1.7 and 2.3 mHz. The data indicated an anti-sunward propagation in both the dawn and dusk flanks and a low azimuthal m number (∣ m∣∼6) suggesting a generation mechanism external to the Earth's magnetosphere. A sudden increase in the solar wind dynamic pressure followed by a period of strongly northward, B z dominated IMF, coincides with the observations and also a large increase in Pc5 wave power observed in the dawn flank. The observed enhancements in the wave activity and FLR structures are thought to be due to a Kelvin–Helmholtz driven waveguide mode. Additionally, there is no evidence that the frequencies of the FLRs are intrinsic to the solar wind. It thus seems that the frequencies were determined by the dimensions of the magnetospheric cavity.

Single‐spacecraft detection of rolled‐up Kelvin‐Helmholtz vortices at the flank magnetopause

Recent numerical simulations suggest that as soon as the Kelvin‐Helmholtz instability (KHI) has grown nonlinearly to form a highly rolled‐up vortex, plasma mixing is inevitably achieved within the vortex. Identification of rolled‐up vortices by in situ measurements is therefore an important task as a step to establish the mechanism by which solar wind plasmas enter the magnetosphere and to understand conditions under which the vortices form. In the present study we show that the rolled‐up vortices are detectable even from single‐spacecraft measurements. Numerical simulations of the KHI indicate that in the rolled‐up vortex the tailward speed of a fraction of low‐density, magnetospheric plasmas exceeds that of the magnetosheath flow. This feature appears only after a vortex is rolled up and thus can be used as a marker of roll‐up. This signature was indeed found in the Cluster multispacecraft measurements of the rolled‐up vortices at the flank magnetopause. By use of this marker, we have searched for events consistent with the roll‐up from Geotail observations showing quasi‐periodic plasma and field fluctuations in the flank low‐latitude boundary layer (LLBL) under northward interplanetary magnetic field (IMF), presumably associated with KH waves. The survey shows that such rolled‐up events do occur on both dawn and dusk flanks and are not rare for northward IMF conditions. In addition, in all the rolled‐up cases, magnetosheath‐like ions are detected on the magnetospheric side of the boundary. These findings indicate that the KHI plays a nonnegligible role in the formation of the flank LLBL under northward IMF.

Dawn‐dusk asymmetries, ion spectra, and sources in the northward interplanetary magnetic field plasma sheet

During periods of northward IMF, plasma sheet ions often have two components: hot (magnetospheric origin) and cold (magnetosheath origin). The temperatures of the cold‐component ions are ∼30–40% higher in the dawn sector compared to the dusk sector, implying the dawnside magnetosheath ion heating of ∼30–40%. As a result, the magnetosheath ions are less distinguishable from the hot‐component ions, which have lower temperatures on the dawnside, leading to a higher occurrence of the ions having (apparent) one‐component distribution. As the duration of the hourly averaged IMF being northward (Δt) increases from 1 to 10 hours, the occurrence of two‐component ions increases from 65% to 83% in the dusk flank, but in the dawn flank it remains relatively stable at around 45%. In contrast, the occurrence of ions best characterized by kappa (κ) distribution increases from 25% to 35% in the dawn flank whereas in the dusk flank it remains relatively insensitive to Δt (10%). The occurrence of a one‐component Maxwellian distribution appears to be most pronounced in the region of the plasma sheet close to the midnight meridian, and these ions appear to be characteristic of the nominal plasma sheet (hot component) ions. The densification of the plasma sheet, as Δt increases, mainly results from the influx of the magnetosheath ions. However, the cooling of the plasma sheet ions can be attributed not only to the influx of the cold magnetosheath ions but also to the cooling of the nominal plasma sheet ions. The dawn‐dusk asymmetries observed in the cold magnetosheath ion profiles should provide constraints that can help determine the roles of various proposed magnetosheath ion entry mechanisms.

Survey of energetic O<Sup>+</sup> ions near the dayside mid-latitude magnetopause with Cluster

Abstract. Since December 2000, the Cluster satellites have been conducting detailed measurements of the magnetospheric boundaries and have confirmed the unambiguous presence of ions of terrestrial origin (e.g. O+ in regions adjacent to the dayside, mid-latitude magnetopause. In the present paper, we focus on the statistical properties of the O+ ion component at energies ranging from 30eV up to 40keV, using three years of ion data at solar maximum from the Cluster Ion Spectrometry (CIS) experiment aboard two Cluster spacecraft. The O+ density decreases on average by a factor of 6, from 0.041 to 7x10-3cm-3 when crossing the magnetopause from the magnetosphere to the magnetosheath, but depends on several parameters, such as the geomagnetic activity or the modified disturbed storm time index (Dst*), and on their location. The O+ density is significantly higher in the dusk-side than in the dawn side region, which is consistent with the view that they originate mainly from the plasma sheet. A remarkable finding is that inward of the magnetopause, O+ is the dominant contributor to the mass density 30% of the time on the dusk-side in comparison to 3% in the dawnside and 4% near noon. On an event basis in the dusk flank of the magnetopause, we point out that O+ ions, when dominating the mass composition, lower the threshold for generating the Kelvin-Helmholtz instability, which may allow plasma exchange between the magnetosheath and the plasma sheet. We also discuss the effect of a substantial O+ ion component when present in a reconnection region.

Multipoint observations of transient reconnection signatures in the cusp precipitation: A Cluster‐IMAGE detailed case study

This paper uses 90 min of Cluster multipoint data at ∼5 RE altitude together with global dayside imaging data provided by the IMAGE‐SI‐12 instrument to analyze the northern cusp crossed on 14 July 2001, during a period of high solar wind pressure Psw and strongly duskward interplanetary magnetic field (IMF). Simultaneous observations reveal intense cusp activity in the postnoon sector, characterized by multiple, impulsive energy‐dispersed ion injections, with a recurrence time of ∼8–10 min or less. Most of these transient signatures correspond one to one with repeated Psw enhancements. A multipoint analysis reveals that field‐aligned current sheets associated with ion steps are moving predominantly westward with a velocity, up to ∼20 km/s, in agreement with a flux tube motion controlled by magnetic tension forces when IMF By ≫ 0. These data are used to infer a source region located at ∼7–13 RE from Cluster, that is, on the dusk flank of the compressed magnetosphere, around 17–18 magnetic local time. We interpret these very dynamic and transient features as probable signatures of pulsed magnetic reconnection that is operating in a localized region of the magnetopause centered in the preferential antiparallel merging site. Our results suggest that the reconnection rate is not spontaneously self‐varying but may be directly modulated by either upstream dynamic pressure Psw or changes in the IMF polarity.