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During early November 2013, the magnetosphere experienced concurrent driving by a coronal mass ejection (CME) during an ongoing high-speed stream (HSS) event. The relativistic electron response to these two kinds of drivers, i.e., HSS and CME, is typically different, with the former often leading to a slower buildup of electrons at larger radial distances, while the latter energizing electrons rapidly with flux enhancements occurring closer to the Earth. We present a detailed analysis of the relativistic electron response including radial profiles of phase space density as observed by both Magnetic Electron and Ion Sensor (MagEIS) and Relativistic Electron Proton Telescope instruments on the Van Allen Probes mission. Data from the MagEIS instrument establish the behavior of lower energy (

From 2 to 5 UT on 14 November 2012, the Van Allen Probes observed repeated particle flux dropouts during the main phase of a geomagnetic storm as the satellites traversed the post-midnight to dawnside inner magnetosphere. Each flux dropout corresponded to an abrupt change in the magnetic topology, i.e., from a more dipolar configuration to a configuration with magnetic field lines stretched in the dawn-dusk direction. Geosynchronous GOES spacecraft located in the dusk and near-midnight sectors and the LANL constellation with wide local time coverage also observed repeated flux dropouts and stretched field lines with similar occurrence patterns to those of the Van Allen Probe events. THEMIS recorded multiple transient abrupt expansions of the evening-side magnetopause ∼20–30 min prior to the sequential Van Allen Probes observations. Ground-based magnetograms and all sky images demonstrate repeatable features in conjunction with the dropouts. We combine the various in situ and ground-based measurements to define and understand the global spatiotemporal features associated with the dropouts observed by the Van Allen Probes. We discuss various proposed hypotheses for the mechanism that plausibly caused this storm-time dropout event as well as formulate a new hypothesis that explains the combined in situ and ground-based observations: the earthward motion of magnetic flux ropes containing lobe plasmas that form along an extended magnetotail reconnection line in the near-Earth plasma sheet.

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

[1] Using an electrically self-consistent kinetic ring current model, we investigate how different conditions of plasma sheet distribution affect the ring current properties. Results include comparative studies in (1) varying the radial distance of the plasma sheet boundary; (2) varying local time distribution of the source population; (3) varying the source spectra, with #2 as our main focus. Our results show that a source that is farther away from Earth leads to a stronger ring current than a source that is closer to the Earth. Local time distribution of the source plays an important role in determining both the radial and azimuthal (local time) location of the ring current peak pressure. We found that postmidnight source locations generally lead to a stronger ring current. This finding is in agreement with Lavraud et al. (2008). However, our results do not exhibit any simple dependence of the local time distribution of the peak ring current (within the lower energy range) on the local time distribution of the source, as suggested by Lavraud et al. (2008). Instead, the local time distribution of source populations has to compete with the dynamically updated self-consistent electric field (through the coupling to the ionosphere) in controlling the ring current intensity and peak location. Our results also show that different source spectra lead to different (though the difference may not be very significant) ring current intensities and that low-energy plasma in the kiloelectron volt range at the boundary constitutes a dominant source population to the ring current. In addition, we found that the radial location of source population is also important to ring current development.

[1] An outstanding flare on 20 January 2005 was accompanied by a coronal mass ejection which arrived at the magnetopause at ∼1712 UT, 21 January, and produced a strong compression-pressure pulse. Enhanced magnetospheric activity was stimulated. The associated development between 02.19 UT on 21–22 January, 2005 of a ring current disturbance in energetic neutral atom (ENA) data, recorded aboard both the Double Star and the IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) spacecraft, is described. A magnetic storm from ∼1712 UT, 21 January, reached minimum Dst = ∼−101 nT at ∼0600 UT, 22 January, and its recovery phase endured until 27 January. ENA data indicate that the ring current experienced a deep injection of H+ and O+ ions at ∼1830 UT when IMF Bz was oriented southward. At this time, the ring current was strongly asymmetric, although later it became more symmetric. Bz turned northward at 1946 UT. From ∼0224 to ∼0612 UT on 22 January, Bz fluctuated such that it intermittently pointed southward (±10 nT). The moderate but extended response of the geomagnetosphere to the strong pressure pulse is explained by a slow evolution in the orientation of Bz under conditions of enhanced plasma sheet density. Modeling of dynamical parameters that represent various current systems that contributed to Dst revealed their individual characteristics. The changing geomagnetic field was also modeled. Comparisons with ENA data show that early asymmetric enhancements recorded in hydrogen and oxygen were accompanied by intensified external current systems that produced a magnetic field related compression of the magnetosphere. The gradual reduction in ring current asymmetry was complemented by the largely symmetrical configuration displayed by the corresponding, still intensified, modeled magnetic field.

[1] The present study examines the characteristics of electric fields in the nightside inner magnetosphere during geomagnetically active periods. Electric field and magnetic field measurements made by the Cluster spacecraft on their perigee passes are used. The results are summarized as follows: (1) The duskward electric field component EY tends to be larger in the premidnight sector and off the equator, presumably corresponding to the more frequent occurrence of substorms and boundary layer crossings, respectively. (2) The occurrence distribution of EY is biased positively with an average of 0.6–0.8 mV/m, which reflects enhanced convection at active time. (3) The occurrence distribution of EY is also characterized by extending tails with a standard deviation larger than the twice the average. Although the occurrence ratio decreases sharply with increasing magnitude of EY, ∣EY∣ occasionally exceeds 5 mV/m. (4) The sign of EY is well organized by the change of magnetic field. When the local magnetic configuration becomes more dipolar, EY tends to be positive (duskward), whereas it tends to be negative (dawnward) when the configuration becomes more stretched. (5) As for strong electric fields, EY tends to be proportional to the change of the H magnetic component, and from the induction equation, the typical spatial scale of EY is estimated at 4.2 RE. Results 4 and 5 strongly suggests that those strong electric fields are inductive. However, the corresponding process/phenomenon can be different from event to event. It is also suggested that substorm(-like) processes inside the ring current effectively intensify the ring current.

[1] Through interspacecraft comparison of energetic neutral oxygen (ENO) intensity from two different vantage points provided by IMAGE and Geotail, Lui et al. (2005) showed that viewing perspective plays a very important role in the observed ENO intensity level during a magnetic storm period. Motivated by the findings of Lui et al. (2005), we investigate how viewing perspective influences energetic neutral atom emissions from a modeling perspective. The main results of this paper are that (1) our simulation results, based upon O+ ion fluxes from the Comprehensive Ring Current Model and the subsequent ENO calculation, reproduce the total differential ENO intensity obtained from two spacecraft to a reasonable degree and (2) further analysis of our results indicates that pitch angle anisotropy in ring current ion flux, a crucial physical quantity in ring current dynamics, is one major contributor to the difference in energetic neutral atom intensity from different viewing perspectives.

[1] Subauroral polarization streams (SAPS) are usually associated with geomagnetically disturbed times and considered a manifestation of magnetosphere and ionosphere coupling. Previous research results using radar and satellite measurements have revealed many features of the SAPS events. In this paper we focus on the effects of subauroral trough conductance on the attributes of SAPS and the evolution of the coupled magnetosphere and ionosphere system through the comprehensive ring current model, which includes the coupled electrodynamics of the inner magnetosphere and ionosphere with a self-consistent description of the electric field. Our numerical analysis indicates that low conductance at the subauroral latitudes (due to midlatitude trough) is critical to the large amplitude of SAPS. The model results are generally in good agreement with common characteristics of SAPS and are consistent with their existing generation mechanism.

[1] On 21 January 2005, a fast interplanetary (IP) shock compressed the magnetosphere and caused detached auroras (DA) on the dayside, duskside and nightside ionosphere. The DA were detected by three independent FUV instruments: IMAGE/SI-12, TIMED/GUVI and DMSP/SSUSI. The SI-12 observations show that the dayside detached aurora (DDA) was located between 60° and 68° Mlat and between 06:00 and 15:00 MLT. It lasted for only ∼2 min. Coincident in situ Polar measurements show that sudden bursts of proton EMIC waves (∼2 min) were associated with the DDA. This provides direct evidence of the link between the EMIC waves and the DDA. The DA in the duskside and nightside appeared once the DDA disappeared. GUVI and SSUSI also observed the DA in the duskside and dayside with more details. Ring current simulations show that ∼10 keV protons with sausage-shaped spatial distribution of high anisotropy in flux and temperature were the particle source for the duskside and nightside DA. Compression of the magnetosphere appears to be the driver for both of the DDA and dusk/night DA. The nightside DA was observed for the first time during a sudden commencement. To unify the different terminologies, the detached auroras due to precipitating energetic protons from the ring current are called the ring current auroras.

The phenomenon called flux transfer events (FTEs) is widely accepted as the manifestation of time-dependent reconnection. In this paper, we present observational evidence of a flux transfer event observed simultaneously at low-latitude by Polar and at high-latitude by Cluster. This event occurs on March 21, 2002, when both Cluster and Polar are located near local noon but with a large latitudinal separation. During the event, Cluster is moving outbound from the polar cusp to the magnetosheath, and Polar is in the magnetosheath near the equatorial magnetopause. The observations show that a flux transfer event occurs between the equator and the northern cusp. Polar and Cluster observe the FTE s two open flux tubes: Polar encounters the southward moving flux tube near the equator; and Cluster the northward moving flux tube at high latitude. The low latitude FTE appears to be a flux rope with helical magnetic field lines as it has a strong core field and the magnetic field component in the boundary normal direction exhibits a strong bi-polar variation. Unlike the low-latitude FTE, the high-latitude FTE observed by Cluster does not exhibit the characteristic bi-polar perturbation in the magnetic field. But the plasma data clearly reveal its open flux tube configuration. It shows that the magnetic field lines have straightened inside the FTE and become more aligned to the neighboring flux tubes as it moves to the cusp. Enhanced electrostatic fluctuations have been observed within the FTE core, both at low- and high-latitudes. This event provides a unique opportunity to understand high-latitude FTE signatures and the nature of time-varying reconnection.

[1] The present study examines a sawtooth injection event that took place around 0800 UT on 18 April 2002 when the Cluster spacecraft were located in the inner magnetosphere in the premidnight sector. In association with this injection, Cluster, at a radial distance of 4.6 RE, observed that the local magnetic field became more dipolar and that both ion and electron fluxes increased without notable energy dispersion. These features were accompanied by intensifications of the equatorward component of a double-oval structure and also by an enhancement of the ring-current oxygen ENA flux. The event was also accompanied by large magnetic field (a few tens of nT) and electric field (a few tens of mV/m) fluctuations with characteristic timescales of a few tens of seconds. These observations strongly suggest that this sawtooth injection extended not only widely in local time but also deeply into the inner magnetosphere. Interestingly, Cluster repeatedly observed dipolarization-like signatures afterward, which, however, were not associated with enhancements of local energetic ion flux or with geosynchronous dipolarization or injection signatures. Instead, these magnetic signatures were accompanied by oscillatory plasma motion in the radial direction with a characteristic timescale of about 10 min, which appears to be related to the westward propagation of a spatially periodic auroral structure. The associated azimuthal electric field component was well correlated with the time derivative of the north-south magnetic field component, suggesting that the observed electric field is inductive. These findings suggest that electromagnetic processes far inside geosynchronous orbit play an important role in energization of energetic ions and auroral dynamics during magnetospheric storms.

[1] In this paper we present the analysis and interpretation of a multipoint observation of magnetic field structures at the poleward edge of a premidnight auroral arc by the Enstrophy sounding rocket mission. Four Free-Flying Magnetometers (FFMs) employing autonomous nanospacecraft technology were deployed from the main payload during the flight, and multipoint magnetic field measurements were made. Signatures consistent with both spatial and temporal interpretations were found to be present when large fluctuations in B were seen at the edge of an arc as the rocket flew into the polar cap. Reasons for the interpretation of spatial or temporal features are given and supported by a simple model of multiple payloads crossing through several moving current sheets and a study of the fine structure of this auroral event using multipoint, correlative wavelet analysis to find velocities of structures at different scale sizes. We show that the direct measurement method of current density using multipoint measurement of magnetic fields gives us a different current density than what would be inferred from a single-point measurement and that the multipoint measurement also provides an inherent check on the validity of the measurement through a calculation of the divergence of the measured B.