ALBERT

Description: A widely accepted explanation of the location of the inner edge of the electron plasma sheet and its dependence on electron energy is based on drift motions of individual particles. The boundary is identified as the separatrix between drift trajectories linking the tail to the dayside magnetopause (open paths) and trajectories closed around the Earth. A statistical study of the inner edge of the electron plasma sheet using THEMIS Electrostatic Analyzer plasma data from November 2007 to April 2009 enabled us to examine this model. Using a dipole magnetic field and a Volland-Stern electric field with shielding, we find that a steady state drift boundary model represents the average location of the electron plasma sheet boundary and reflects its variation with the solar wind electric field in the local time region between 21:00 and 06:00, except at high activity levels. However, the model does not reproduce the observed energy dispersion of the boundaries. We have also used the location of the inner edge of the electron plasma sheet to parameterize the potential drop of the tail convection electric field as a function of solar wind electric field (Esw) and geomagnetic activity. The range of Esw examined is small because the data were acquired near solar minimum. For the range of values tested (meaningful statistics only for Esw 〈 2 mV/m), reasonably good agreement is found between the potential drop of the tail convection electric field inferred from the location of the inner edge and the polar cap potential drop calculated from the model of Boyle et al. (1997).

Description: Recent observations in the inner magnetotail have shown rapid and significant flux increases (usually an order of magnitude of increase within seconds) of suprathermal electrons (tens of keV to hundreds of keV) associated with earthward moving dipolarization fronts. To explain where and how these suprathermal electrons are produced during substorm intervals, two types of acceleration models have been suggested by previous studies: acceleration that localizes near the reconnection site and acceleration that occurs during earthward transport. We perform an analytical analysis of adiabatic acceleration to show that the slope of source differential fluxes is critical for understanding adiabatic flux enhancements during earthward transport. Observationally, two earthward propagating dipolarization fronts accompanied by energetic electron flux enhancements observed by the THEMIS spacecraft have been analyzed; in each event the properties of dipolarization fronts in the inner magnetosphere (XGSM ≈ −10RE) were well correlated with those further down the tail (XGSM ≈ −15RE or XGSM ≈ −20RE). Coupled with theoretical analysis, this enables us to estimate the relative acceleration that occurred as the electrons propagated earthward between the two spacecraft. During the two events studied, the differential fluxes of supra thermal electrons had steep energy spectra with power law indices of −4 to −6.These spectra were much steeper than those at lower energy, as well as those of the supra thermal electrons observed before the fronts arrived. A compression factor of 1.5 as the electrons propagated earthward induced a flux increase of suprathermal electrons by a factor of 7 to 17. Provided these steep spectra, we demonstrate that adiabatic acceleration from the betatron and Fermi mechanisms simultaneously operating can account for these flux increases. Since both analytical analysis and data from the two events show that adiabatic acceleration during earthward transport does not significantly change the power law indices, the steep spectra were likely to be traced back to their source region, presumably near the reconnection site.

Description: [1]  The Methodology based on the Error Reduction Ratio (ERR) determines the causal relationship between the input and output for a wide class of nonlinear systems. In the present study, ERR is used to identify the most important solar wind parameters, which control the fluxes of energetic electrons at geosynchronous orbit. The results show that for lower energies, the fluxes are indeed controlled by the solar wind velocity, as was assumed before. For the lowest energy range studied here (24.1 keV), the solar wind velocity of the current day is the most important control parameter for the current day's electron flux. As the energy increases, the solar wind velocity of the previous day becomes the most important factor. For the higher energy electrons (around 1 MeV), the solar wind velocity registered two days in the past is the most important controlling parameter. Such a dependence can, perhaps, be explained by either local acceleration processes due to the interaction with plasma waves or by radial diffusion if lower energy electrons possess higher mobility. However, in the case of even higher energies (2.0 MeV), the solar wind density replaces the velocity as the key control parameter. Such a dependence could be a result of solar wind density influence on the dynamics of various waves and pulsations that affect acceleration and loss of relativistic electrons. The study also shows that statistically the variations of daily high energy electron fluxes show little dependence on the daily averaged B z , daily time duration of the southward IMF and daily integral ∫  B s dt (where B s is the southward component of IMF).

Description: [1]  Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasma-depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near-Earth tail around X  = −18  R E . The bubble is bounded by both sharp leading (∂ b z /∂ x  〈 0) and trailing (∂ b z /∂ x  〉 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field-aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super-Alfvénic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E × B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of B z , or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field-aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed.

Description: [1]  The cross polar cap potential is considered an instantaneous monitor of the rate at which magnetic flux couples the solar wind to the Earth's magnetosphere-ionosphere system. Studies have shown that the cross polar cap potential responds linearly to the solar wind electric field under nominal solar wind conditions, but asymptotes to the order of 200 kV for large electric field. Saturation of the cross polar cap potential is also found to occur in MHD simulations. Several mechanisms have been proposed to explain this phenomenon. Two well-developed models are those of Siscoe et al . (2002), herein referred to as the Siscoe-Hill model, and of Kivelson and Ridley (2008), herein referred to as the Kivelson-Ridley model. In this study, we compare the mathematical formulas as well as the predictions of the two models with data. We find that the two models predict similar saturation limits. Their difference can be expressed in terms of a factor, which is close to unity during a saturation interval. A survey of the differences in the model predictions show that, on average, the potential of the Kivelson-Ridley model is smaller than that of the Siscoe-Hill model by 10 kV. Measurements of AMIE, DMSP, PC index, and SuperDARN are used to differentiate between the two models. However, given the uncertainties of the measurements, it is impossible to conclude that one model does a better job than the other of predicting the observed cross polar cap potentials.

Description: Magnetic holes filled with isotropic energetic electrons (up to a few 105 eV) have been observed by THEMIS in the vicinity of dipolarization fronts. These structures can partially contribute to the initial seed population of energetic electrons within the magnetosphere; therefore finding their nature is important for understanding of the population of high energy electrons within the magnetosphere. Previously, these structures have been interpreted as the result of the mirror instability due to the similarity in their appearance with mirror dips observed in the terrestrial magnetosheath and solar wind. The THEMIS data shown here prove that the measured properties of these structures contradict to the interpretation as mirror waves. In the present study it is shown that these waves do not exhibit the effects on the ion population that are expected due to mirror wave structures. However, they do have a pronounced effect on the high energy electron population. The evolution of the high energy electron population within these structures is investigated. It is then argued that the tearing instability can be responsible for their generation.

Description: The polar cap, defined as a region of open magnetic flux, is an ideal region in which to investigate how properties of the solar wind directly affect the magnetosphere. For such studies, the polar cap (PC) index provides a useful characterization of the state of the polar ionosphere. In this paper, we study how polar cap properties, quantified by the PC index, depend on solar wind parameters and other geomagnetic indices during intervals of exceptionally large (10 mV/m) merging electric field. Using 53 one to two-day intervals that include such extreme fields, we find that the PC index correlates strongly with the modified electric field (EK-R) proposed by Kivelson and Ridley (2008). Here, EK-R is a form representative of several models in which the electric field imposed on the ionosphere by magnetopause reconnection saturates for extreme solar wind driving. However, there are anomalous events during which the auroral oval expanded poleward to the latitude of the PC index station and the index increased because of proximity to auroral zone currents. It is found that nightside magnetospheric processes, as represented by AL, make a significant contribution to the PC index. A linear regression analysis shows that the portion of the PC index directly driven by the solar wind electric field outweighs the contribution arising from energy release in the magnetotail by roughly a factor of 2. Neither the solar wind dynamic pressure (Pdyn) nor jumps in Pdyn are found to directly contribute to the PC index. However, there exists some correlation between the PC index and Pdyn, because of the common dependence of EK-R and Pdyn on the solar wind number density.

Description: The solar wind is coupled to the magnetosphere-ionosphere system through various interactions, e.g., magnetic reconnection at the dayside magnetopause, and viscous interactions at the low latitude boundary layer. The polar cap, a region of open magnetic flux connecting the magnetic field of the Earth to that of the solar wind, is an ideal region in which to investigate how solar wind drives the magnetosphere-ionosphere dynamo. For such studies, the polar cap (PC) index provides a useful characterization of the state of the polar ionosphere. A previous study by Gao et al. (2012a) found that polar cap dynamics, characterized by the PC index, responds to both solar wind driving quantified by the electric field (EK-R) proposed by Kivelson and Ridley (2008) which is a representative of the electric field imposed on the ionosphere by magnetopause reconnection that takes cross polar cap potential saturation into account, and the energy release in the magnetotail, described by a modified AL index (ALU). In that study, the dependence of the PC index on EK-R and ALU was investigated assuming a linear relationship. In this study, we test the assumption that the relationship is linear by performing a similar analysis applying a more general, nonlinear model to the events of the Gao et al. (2012a) study. A nonlinear relationship can be established by use of a statistical approach referred to as the additive model. We find that the more flexible additive model outperforms the linear model. However, the improvement is small. Provided that EK-R is used to characterize the solar wind input, results obtained from the additive model are very similar to those from the linear model. This result indicates that the linear relation between the PC index and EK-R, ALU obtained by Gao et al. (2012a) represents the data within fluctuations.

Description: Auroral substorms were first described more than 40 years ago, and their atmospheric and magnetospheric signatures have been investigated extensively. However, because magnetic mapping from the ionosphere to the equator is uncertain especially during active times, the magnetospheric source regions of the substorm-associated features in the upper atmosphere remain poorly understood. In optical images, auroral substorms always involve brightening followed by poleward expansion of a discrete auroral arc. The arc that brightens is usually the most equatorward of several auroral arcs that remain quiescent for ∼30 min or more before the break-up commences. In order to identify the magnetospheric region that is magnetically conjugate to this preexisting arc, we combine auroral images from ground-based imagers, magnetic field and particle data from low-altitude spacecraft, and maps of field-aligned currents based on ground magnetometer arrays. We surveyed data from the THEMIS all sky imager (ASI) array and the FAST spacecraft from 2007 to April 2009 and obtained 5 events in which the low altitude FAST spacecraft crossed magnetic flux tubes linked to a preexisting auroral arc imaged by THEMIS ASI prior to substorm onset. The observations show that, in each of the five cases: 1) the precipitating electrons associated with the preexisting arc are accelerated by a field-aligned potential drop, with characteristic energy ranging from a few hundred eV to a few keV. 2) The preexisting arc is located 1°∼2° poleward of the equatorward edge of the 1 keV electron plasma sheet in the ionosphere, and it maps to equatorial locations within the electron plasma sheet and tailward of its inner edge. 3) The preexisting arc is located at or very near the boundary between the Region 1 and Region 2 field-aligned currents. The localization relative to the Region 1/Region 2 current is confirmed by comparison with maps of field-aligned currents inferred from ground magnetometer data.

Description: Of the 3701 flux transfer event signatures that we identified in THEMIS data between May and October of 2007 and 2008 at low-latitudes on the magnetopause, 41 were distinctive in that the north-south flow components reversed direction during the ∼1 min required for THEMIS spacecraft to traverse the structure. We have ruled out the possibility that these 41 “flow reversal events” (FREs) were single X-line structures in motion, and confirmed from their field and plasma properties that they indeed were flux ropes. We have interpreted the plasma flow reversal as evidence that we observed the flux ropes while they were being generated by a pair of X-lines that developed in sequence through component merging, a process that seems to play a significant role in forming flux ropes. Our analysis, which applies only to low latitude flux ropes, provides evidence to modify the updated multiple X-line reconnection scenario with component merging as the dominant associated reconnection process.