We present results from a systematic study of multisatellite samplings from the Defense Meteorological Satellite Program F13, F15, F16, F17, and F18 satellites over the period from 2007 to 2015 that describe the motion of the convection reversal boundary (CRB) and the local plasma flow across it. Focusing on the cases with continuous poleward and equatorward CRB motion sampled by three consecutive satellites within 50 min, 45% of the time the CRB motion may deviate from the local plasma motion near dawn and dusk where the reconnection process is unlikely to be present. Differences in the inferred CRB motion and the local plasma motion may arise from apparent motion induced by the local time displacement of consecutive samples across the CRB that is tilted with respect to a line of constant latitude. The presence of a viscous-like interaction across the CRB can also contribute to the difference in the CRB and plasma motion. Accounting for these processes, the CRB motion and the motion of the plasma at the CRB are consistent only if a back and forth motion over a timescale of a few minutes is superimposed on a monotonic migration of the CRB over longer time periods.

In this paper we present a systematic study of the location of the convection reversal boundary for southward interplanetary magnetic field by using Defense Meteorological Satellite Program (DMSP) F13 and F15 spacecraft measurements during local summer seasons from 2000 to 2007 for both hemispheres. All the convection reversal boundaries are identified pass bypass by locating the highest‐latitude location where the plasma flow shows a large‐scale two‐cell pattern and reverses direction from sunward to antisunward. The location of the convection reversal boundaries are placed into 10 different categories based on B y and the magnitude of southward B z . Observations suggest that (1) the location of the boundary is well organized by the magnitude of B z , being at lower latitudes for stronger negative B z and also organized by the polarity of B y , moving toward the dawnside/duskside when B y changes from negative to positive in the northern/southern hemisphere; (2) the average latitudinal movement of the boundary associated with B y changes is comparable to the average movement of the boundary with B z changes; (3) an initial reconfiguration of the boundary near local noon is redistributed around the dawnside or duskside dependent on the direction of B y ; and (4) the boundary has a general spiral shape, which varies depending on B z and B y .

Sun-aligned auroral arcs (SAAs) are one of the outstanding phenomena in the high-latitude region during periods of northward interplanetary magnetic field (IMF). Smaller scale SAAs tend to occur either in the duskside or dawnside of the polar cap and are known to drift in the dawn-dusk direction depending on the sign of the IMF By. Studies of SAAs are of particular importance because they represent dynamical characteristics of their source plasma in the magnetosphere, for example in the interaction region between the solar wind and magnetosphere or in the boundary between the plasma sheet and tail lobe. To date, however, very little has been known about the spatial structure and/or temporal evolution of the magnetospheric counterpart of SAAs. In order to gain more comprehensive understanding of the field-aligned plasma transport in the vicinity of SAAs, we have investigated an event of SAAs on November 10, 2005, during which multiple SAAs were detected by a ground-based all-sky camera at Resolute Bay, Canada. During this interval, several SAAs were detached from the duskside oval and moved poleward. The large-scale structure of these arcs was visualized by space-based imagers of TIMED/GUVI and DMSP/SSUSI. In addition to these optical observations, we employ the Cluster satellites to reveal the high-altitude particle signature corresponding to the small-scale SAAs. The ionospheric footprints of the 4 Cluster satellites encountered the SAAs sequentially and observed well correlated enhancements of electron fluxes at weak energies (< 1 keV). The Cluster satellites also detected signatures of upflowing beams of ions and electrons in the vicinity of the SAAs. This implies that these ions and electrons were accelerated upward by a quasi-stationary electric field existing in the vicinity of the SAAs and constitute a current system in the magnetosphere-ionosphere coupling system. Ionospheric convection measurement from one of the SuperDARN radars shows an indication that the SAAs are embedded in the lobe cell during northward IMF conditions. In the presentation, we will show the results of detailed comparison between the ground-based radio and optical signatures of the SAAs and those obtained by the Cluster spacecraft at magnetospheric altitudes.

Cluster results are presented from the acceleration region of an auroral surge and connected horn arc, observed during an extended time period of substorm activity. The Cluster spacecraft crossed different magnetic local time (MLT) sectors of the surge and horn, with lag times of 2–10 min. Acceleration potential patterns are derived for the horn arc and for the double arc (surge and horn) at the surge front and deeper into the surge. The parallel potential drop of the horn arc ranged between 4 and 7 kV. At the surge front, two weakly coupled U‐potentials with parallel potential drops of 8 (7) kV and 7 (5) kV were derived for the surge and horn, respectively, from the C3 (C4) data. A similar, more coupled pattern was derived for the region deeper into the surge. We also address how the field‐aligned currents of the surge and horn system close in the ionosphere. The Cluster data allow almost simultaneous estimates of the latitudinal current closure at various MLT sectors. Significant net upward currents are derived for the horn and surge, whereas the currents at the surge front were found to be balanced. The net upward horn current is proposed to be fed by the zonal divergence of the westward Pedersen current in the horn, consistent with the acceleration potential decrease in the westward horn direction. The net upward surge current is proposed to be fed by the divergence of a westward electrojet and by localized downward currents adjacent to the surge.

Transpolar arcs (TPAs) are often assumed to lie on closed field lines that map to the tail plasma sheet or its boundary layer and connected to sunward convecting plasma. We present now a study looking at the plasma flow pattern within TPAs in much detail and comparing it to Earth dipole tilt, solar wind and IMF conditions as well as substorm phases. The study is based on DMSP measurements of 73 TPA crossings. The selected DMSP orbits are approximately along the dawn-dusk meridian. We find that the flow characteristics changes drastically between dark (negative Earth dipole tilt) and sunlit (positive tilt) TPA passages. In darkness the flows vary much stronger with large sunward and anti-sunward flow peaks. In sunlit cases, the flows are in general small and do not vary much. A clear solar wind and IMF dependence is found only for positive dipole tilts: plasma flows on sunlit passages are only high for a strongly northward IMF and high solar wind energy fluxes. Even the flow direction correlates with dipole tilt. The dark passages contain more cases with anti-sunward flow than the sunlit passages. Another parameter that influences the flow direction is connected to the substorm evolution. Nearly all TPA passages during the substorm growth phase show anti-sunward flow on the TPA, while TPAs during many hours quiet times are in general on sunward flow. The results are discussed with respect to effects of UV illumination of the ionosphere, magnetospheric topology changes during transpolar arcs, and plasma convection patterns during substorms and quiet times.

Results are presented from Cluster crossings of the acceleration region of two inverted-V auroras located in the poleward part of an extensive substorm bulge. The particle and field data are used to infer the acceleration potentials of the arcs and their distribution in altitude and latitude. The C1 data are consistent with a symmetric potential pattern, composed of two negative U potentials and one positive U potential in between, and the C3 and C4 data are consistent with an asymmetric pattern, where the dominating potential structure extends deep into the polar cap boundary (PCB) region. The two patterns may either correspond to different stages of evolution of the same double arc system or represent two longitudinally separated double arc systems. For all spacecraft, the potential well of the poleward arc extends into the PCB region, whereas the density cavity does not but remains confined to R1. This suggests that the Alfvenic activity observed within the PCB region prevents the cavity formation, consistent with the associated FACs being roughly balanced over this region. The results show that Alfvenic and quasi-static acceleration operates jointly in the PCB region, varying from being about equally important (on C1) to being predominantly quasi-static (on C3/C4). The presence (absence) of an upward electron beam, associated with a positive potential structure and a downward current, observed by C1 (C4/C3) is expected from its short life time, shorter than the time lag between the Cluster spacecraft. The evolution involves both a broadening and a density reduction of the associated downward current sheet to below the critical current density above which parallel electric fields will form. The deepest potential well of 13 kV observed by C4 was located in Region 1, adjacent to the PCB region and coinciding with the deepest density cavity, with a minimum density of 0.1 cm(-3). The interface between Region 1 and the PCB region, coinciding with the steep density gradient, appears to be the leading edge of the cavity.

We present two transpolar arc events where for the first time we are able to analyze changes in field-aligned currents associated with high-latitude transpolar auroral arcs on time scales of a few minutes. This is accomplished through the use of highly accurate multipoint magnetic field measurements provided by the Space Technology 5 mission, which consists of three microsatellites in low-Earth orbit. In the first event we examine measurements of an arc that is part of a highly dynamic auroral pattern, that of a hook-shaped arc. In the second event, a more stable dusk oval-aligned arc is analyzed. These events illustrate the dynamic nature of arc formation and show the usefulness of high-resolution multipoint measurements. Minimum variance analysis is used to determine the appropriateness of the infinite current sheet approximation and to calculate arc alignment angles which are then compared with those estimated from UV images or precipitating particle data.

We investigate how substorms with and without growth‐phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE index analysis shows that substorms with growth‐phase pseudobreakups are typically weak and appear as isolated events after hours of low geomagnetic activity. During the hours before onset the average solar wind merging field Em is weaker, and the length of time with enhanced values shorter than for regular substorms. Integrating Em over the last southward IMF period before onset shows an upper limit above which these substorms do not occur. To estimate how much Em reaches the ionosphere, polar cap potential drop and unified PC indices are examined. It is found that substorms with growth‐phase pseudobreakups have on average lower PC index values than regular substorms. The temporal evolution of the PC indices is similar for both substorm groups; the summer index correlates better with Em, the winter index with AE. Also the average polar cap potential drop curves for both types of substorms resemble one other; the dayside and nightside curves are mainly influenced by Em and AE, respectively. Comparing growth‐phase, isolated and recovery pseudobreakups shows that solar wind and ionospheric conditions around the first substorm after a pseudobreakup are similar, independent of whether the last pseudobreakup appeared hours (recovery and isolated pseudobreakups) or minutes before substorm onset (growth‐phase pseudobreakups). Isolated and recovery pseudobreakups are less often associated with a northward IMF rotation than growth‐phase pseudobreakups or substorms.

A series of quasi-periodic magnetopause crossings were recorded by the MESSENGER spacecraft during its third flyby of Mercury on 29 September 2009, likely caused by a train of propagating Kelvin–Helmholtz (KH) vortices. We here revisit the observations to study the internal structure of the waves. Exploiting MESSENGER's rapid traversal of the magnetopause, we show that the observations permit a reconstruction of the structure of a rolled-up KH vortex directly from the spacecraft's magnetic field measurements. The derived geometry is consistent with all large-scale fluctuations in the magnetic field data, establishes the non-linear nature of the waves, and shows their vortex-like structure. In several of the wave passages, a reduction in magnetic field strength is observed in the middle of the wave, which is characteristic of rolled-up vortices and is related to the increase in magnetic pressure required to balance the centrifugal force on the plasma in the outer regions of a vortex, previously reported in computer simulations. As the KH wave starts to roll up, the reconstructed geometry suggests that the vortices develop two gradual transition regions in the magnetic field, possibly related to the mixing of magnetosheath and magnetospheric plasma, situated at the leading edges from the perspectives of both the magnetosphere and the magnetosheath.

Transpolar arcs (TPAs) are often observed during northward interplanetary magnetic field (IMF). When transpolar arcs move in the dawn or dusk direction across the entire polar region in response to IMF By changes they form a “theta” aurora when the arc is aligned along the noon-midnight meridian. Cumnock [2005] selected events from a 4.5-year period when IMF Bz is northward for at least 2 hours before and at least 3 hours after a By sign change. For northward IMF and a By sign change theta aurora are almost always formed in the northern hemisphere, regardless of Bx and dipole tilt. This implies that theta aurorae form simultaneously in both hemispheres. Strongest UV emissions occur in the summer hemisphere and depend strongly on northward IMF Bz, IMF magnitude and solar wind speed. See also Kullen et al. [2008].

We present a method for estimating the portion of the ionospheric high‐latitude potential that maps to the magnetospheric boundary layer during steady northward IMF and global ionospheric 4‐cell convection patterns associated with lobe reconnection, together with the results of a statistical study based on DMSP F13 data from 1996–2004. In comparison with a previous study for steady southward IMF by K. Å. T. Sundberg et al. (2008), the results show significantly larger boundary layer potentials, with a mean value of 10 kV for the 271 events studied, corresponding to roughly 30–35% of the potential generated by the solar wind interaction. In a statistical analysis, the boundary layer potential is also shown to depend significantly on viscous parameters such as the solar wind velocity, density and pressure.

The saturation tendency of the cross‐polar potential for southward interplanetary magnetic fields (IMFs) has been the subject of numerous studies; however, the behavior of the reverse convection potential when the IMF is northward remains less clear. In this study, we present a thorough statistical analysis of the four‐cell convection pattern associated with northward IMF and lobe reconnection based on a large set of DMSP F13 satellite data. Results show a behavior much similar to the southward IMF case, with a clear saturation tendency of the reverse convection potential for strong solar wind electric fields both seen in the data and validated in the statistical analysis. The saturated potential level reaches a limit of about 60 kV, on the order of a fourth of the saturated potential seen for dayside reconnection during southward IMF.