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We investigate the detailed properties of electron inflow in an electron-only reconnection event observed by the four Magnetospheric Multiscale (MMS) spacecraft in the Earth's turbulent magnetosheath downstream of the quasi-parallel bow shock. The lack of ion coupling was attributed to the small-scale sizes of the current sheets, and the observed bidirectional super-Alfvénic electron jets indicate that the MMS spacecraft crossed the reconnecting current sheet on both sides of an active X-line. Remarkably, the MMS spacecraft observed the presence of large asymmetries in the two electron inflows, with the inflows (normal to the current sheet) on the two sides of the reconnecting current layer differing by as much as a factor of four. Furthermore, even though the four MMS spacecraft were separated by less than seven electron skin depths, the degree of inflow asymmetry was significantly different at the different spacecraft. The asymmetry in the inflow speeds was larger with increasing distances downstream from the reconnection site, and the asymmetry was opposite on the two sides of the X-line. We compare the MMS observations with a 2D kinetic particle-in-cell (PIC) simulation and find that the asymmetry in the inflow speeds stems from in-plane currents generated via the combination of reconnection-mediated inflows and parallel flows along the magnetic separatrices in the presence of a large guide field.

We have performed a statistical survey to investigate possible diamagnetic drift suppression of magnetic reconnection using Magnetospheric Multiscale observations of interlinked magnetic field line events at Earth’s magnetopause. Our goal is to investigate a possible cause for the observed magnetic field pileup on the two sides of the thin reconnecting current sheets, at the interface of the converging field lines. We compare whether the diamagnetic drift condition for the suppression of reconnection is satisfied before and after magnetic field pileup. We find that for a majority of events in our data set, the pre-pileup plasma β gradient and magnetic shear values were in the reconnection-suppressed regime, whereas the post-pileup values were in the reconnection-allowed regime. A possible interpretation is that reconnection was suppressed under the pre-pileup conditions, and this led to magnetic field pileup. The magnetic field pileup subsequently generated boundary conditions at the interlinked current sheet that overcame the diamagnetic drift suppression condition, allowing reconnection to happen in the interlinked field line structure. However, in one third of the events, the pre-pileup conditions were already in the reconnection-allowed regime, indicating that in such events, the diamagnetic suppression condition was probably not responsible for field pileup.

Electrons in earth's magnetotail are energized significantly both in the form of heating and in the form of acceleration to non-thermal energies. While magnetic reconnection is considered to play an important role in this energization, it still remains unclear how electrons are energized and how energy is partitioned between thermal and non-thermal components. Here, we show, based on in situ observations by NASA's Magnetospheric Multiscale mission combined with multi-component spectral fitting methods, that the average electron energy [Formula: see text] (or equivalently temperature) is substantially higher when the locally averaged electric field magnitude [Formula: see text] is also higher. While this result is consistent with the classification of “plasma-sheet” and “tail-lobe” reconnection during which reconnection is considered to occur on closed and open magnetic field lines, respectively, it further suggests that a stochastic Fermi acceleration in 3D, reconnection-driven turbulence is essential for the production and confinement of energetic electrons in the reconnection region. The puzzle is that the non-thermal power-law component can be quite small even when the electric field is large and the bulk population is significantly heated. The fraction of non-thermal electron energies varies from sample to sample between ∼20% and ∼60%, regardless of the electric field magnitude. Interestingly, these values of non-thermal fractions are similar to those obtained for the above-the-looptop hard x-ray coronal sources for solar flares.

We present observations of two types of auroral forms located at different latitudes in the cusp region. Type 1 (south) auroras are located at ∼71° – 75° MLAT and occur during intervals of southward directed interplanetary magnetic field (IMF). Higher-latitude (77°–78°) type 2 (north) auroras are associated with northward IMF (Bz > 0). Intervals are studied which are characterized by transitions from one auroral form to the other in response to IMF switches from north (clock angle 90°) and vice versa. These observations are found to be consistent with the interpretation that type south auroras are a signature of low-latitude magnetopause reconnection and that the type north auroras are associated with high-latitude reconnection, tailward of the cusp. The latter is supported by satellite (DMSP F11) observations of particle precipitation and ionospheric convection. The simultaneous existence of type north and type south auroras is observed during intermediate IMF states (clock angle ∼60°). Observations of type north auroras may be used to study the relationship between the IMF Bx component and high-latitude reconnection. We find that type north auroras occur in the northern hemisphere for both Bx polarities. Enhanced emission intensity of the auroral green line seems to be a feature occurring predominantly during negative Bx conditions. The enhanced green line intensity is considered to be an indication of low-altitude particle acceleration in regions of strong field-aligned current intensities. Such particle accelerations (green line emission) may occur predominantly in regions of upward directed IMF By-related currents in association with the type north cusp aurora.

A characteristic sequence of poleward moving auroral events and the associated ground magnetic deflections in the vicinity of cusp-type precipitation in the convection throat are studied. The occurrence, structure, and motion pattern of these events are found to be strongly regulated by the IMF Bz and By components. The present cases are observed in the prenoon sector during positive IMF By and negative Bz conditions. New, improved results on the dynamical structure of the magnetosphere-ionosphere current system of these events are obtained. The current system consists of a central filament or sheet-like field-aligned current (FAC), located on newly opened field lines with oppositely directed return currents on the poleward and equatorward sides. The ground magnetic deflections can be explained by ionospheric Hall currents generated by this FAC system and the associated pattern of ionospheric electric fields. The propagation speed of the events (auroral and magnetic signatures) are found to be comparable with the simultaneously observed antisunward convection speeds (1–2 km/s); that is, they are related to the tailward convection of newly opened magnetic flux. A qualitatively similar current system with opposite current polarity has recently been proposed for the corresponding auroral and magnetic signatures observed during negative IMF By conditions. The events are found to be consistent with an interpretation in terms of pulsed magnetopause reconnection, that is, flux transfer events.

We report specific changes in the dayside auroral morphology in the winter hemisphere which occur in response to sharp transitions between northward and southward-directed interplanetary magnetic fields (IMF). In two case examples we show how a switch between large negative and large positive IMF BZ component was accompanied by a corresponding switch in the location of the 630.0 nm aurora: the cusp aurora situated at ≈ 74° MLAT disappeared and another form this time situated at ≈ 77–78° MLAT appeared simultaneously (within 1 min.). We suggest that the lower- and higher-latitude auroras correspond to injections of magnetosheath plasma associated with, respectively, magnetic reconnection at low and high magnetopause latitudes. They may be called cusp/LLBL and cusp/mantle auroras, respectively. According to this interpretation the cusp/mantle aurora thus corresponds to reconnection tailward of the cusp, the so-called lobe reconnection. The auroral signature is observed to last for a few tens of minutes, indicating that lobe reconnection can occur in a quasi-steady mode. During the 17 December 1992 case event sunward plasma convection in the polar cap was inferred from magnetometer records obtained during the period when the high-latitude aurora occurred.

Dual-site ground-based optical observations were performed January 25, 1995 in support of the SCIFER rocket campaign (Sounding of the Cleft Ion Foundation Energization Region). Two Meridian Scanning Photometers (MSPs) were operative simultaneously at Ny-Alesund (NYA) and Nordlysstasjonen in Adventdalen (LYR) 118 km apart in the geomagnetic meridian. The photometers at each station scanned approximately along the same geomagnetic meridian plane and measured the intensity of auroral wavelengths as a function of zenith angle and time. The principal wavelengths used in the present analysis were the green 5577 A and the red 6300 A emission lines of atomic oxygen. For the magnetospheric cleft auroral activity studied here at 10 hr magnetic local time (MLT), the luminosity maximum for the green line varied in altitude between 125 and 160 km. The red line maximum was in the range 200 to 220 km. The absolute intensity ratio and the height-luminosity profiles obtained using this method were consistent with the near simultaneous rocket measurement of the field-aligned electrons having a characteristic energy of 230 eV by the SCIFER experiment [Lorentzen, et al., 1996]. Geomagnetic disturbance data obtained during the flight included an example of a magnetic impulse event (MIE) coinciding with a bright, discrete arc. The altitude of this arc was >140 km, and it is doubtful that it produced an increase of conductivity in the E Region or developed a current system which could be associated with the MIE. It is proposed that the transient magnetic events were associated with the energetic pulsating aurora which was occurring equatorward of the zenith. In addition, the altitude of three poleward-moving auroral forms (PMAFs) was measured. Within the error of the observations, the PMAFs do not seem to change in altitude with increasing latitude. The sample is too small for a definitive conclusion, but the method shows promise.

Theoretical and experimental studies suggest four main classes of magnetopause/boundary layer processes to explain how particles, momentum and energy transport from the solar wind into the magnetosphere-ionosphere system. Assuming that the IMF B y -related prenoon-postnoon asymmetry of plasma convection is specific to the magnetic merging coupling mode, we determine statistically the local time distribution of transient auroral events as a function of the IMF B y polarity. This is based on concurrent observations of the interplanetary magnetic field components B z and B y and optical auroral observations of cusp/cleft activities within approximately 1000-1400 magnetic local time (MLT) and 71°-81° magnetic latitudes. Intervals of negative IMF B z and nonzero B y are selected for this study. Earlier case studies have shown that transient auroral events moving eastward or westward in the midday sector, depending on the IMF B y polarity, are candidate ionospheric signature of magnetopause flux transfer events (FTEs). An example of this auroral event sequence, illustrating some main characteristics of the expected ionospheric footprints of FTEs, is presented. The statistical study reported here shows an asymmetric prenoon-postnoon auroral occurrence distribution which depends on IMF B y , consistent with the predictions of the reconnection model within a 95% level of confidence. This result confirms the previously existing evidence in favor of an FTE-related interpretation of the actual auroral phenomenon.

We report direct measurements of high-energy particles in a rare crossing of the diffusion region in Earth's magnetotail by the Wind spacecraft. The fluxes of energetic electrons up to approximately 300 keV peak near the center of the diffusion region and decrease monotonically away from this region. The diffusion region electron flux spectrum obeys a power law with an index of -3.8 above approximately 2 keV, and the electron angular distribution displays strong field-aligned bidirectional anisotropy at energies below approximately 2 keV, becoming isotropic above approximately 6 keV. These observations indicate significant electron acceleration inside the diffusion region. Ions show no such energization.

We interpret certain characteristic sequences of ionospheric poleward moving events near the cusp, seen simultaneously in dayside aurora and ground magnetometer observations, as flux transfer event (FTE) signatures and propose associated FTE current systems. This current system consists of a central filament or sheet-like field-aligned current, located on newly opened field lines with oppositely directed return currents on the poleward and equatorward sides. The propagation speed is found to be 1–2 km/s, and the size of the current system 2–300 km in the north-south direction. The FTE current system has opposite polarities corresponding to positive and negative IMF B y component. The events are typically observed in the prenoon sector for positive B y and postnoon during negative B y conditions. In this study we present two case examples, one of each category.

Ground-based observations of auroral, geomagnetic, and radar signatures of solar wind-magnetosphere coupling modes are presented. Two different categories of ionospheric signatures are studied: (1) enhanced geomagnetic deflections and associated equatorward displacements of the cusp/cleft aurora and (2) a sequence of poleward moving auroral and geomagnetic events. It is suggested that signature 1 is mainly caused by directional discontinuities in the interplanetary magnetic field changing the solar wind-magnetosphere coupling geometry and that signature 2 is caused by a recurring modulation of the reconnection rate at the dayside magnetopause (flux transfer events (FTEs)), during the period of strong coupling. The observed auroral and geomagnetic signatures are placed in the context of plasma convection, as derived from magnetometers in Svalbard, Greenland, and Canada, and the European incoherent scatter (EISGAT) radar. A suggestion for an ionospheric current system explaining the sequence of poleward propagating events, i.e., the FTE signatures, is presented.

The energetic ion outflow from the dayside ionosphere has been investigated for different AE and IMF conditions. Viking ion data, 1-min values of the AE index, and 5-min averaged IMF values have been used. An automated algorithm using principal component analysis has been used to categorize and classify the upflowing ion events (UFIs) into beams and conics. The Viking data cover altitudes between 6000 and 13,500 km, 0600 to 1800 magnetic local time, and 65 to 90° invariant latitude. Beams dominate the outflow prenoon and postnoon outside the cusp region, while conics are responsible for most of the cusp region outflow. Generally, the highest average number flux is found in the cusp region. The outflow intensity is higher for southward than for northward IMF, especially in the cusp and postnoon sector. Also, the outflow region increases slightly for southward IMF. However, the difference in outflow intensity and active area is stronger when the data are sorted according to quiet and active geomagnetic conditions using the AE index. The highest average UFI number flux is observed during the expansion phase of substorms, when the flux increase from quiet conditions ( AE ) is by a factor of 2, but even during growth and recovery phase the average UFI number flux is ∼30% higher than for quiet conditions. The results imply that the dayside energetic ion outflow is controlled partly by nightside (driven by the release of stored energy in the magnetotail, as observed in the AE index) and partly by dayside (directly driven, as observed in the polarity in the IMF B z component) activity.