Abstract. Electric currents flowing through near-Earth space (R ≤ 12 R E ) can support a highly distorted magnetic field topology, changing particle drift paths and therefore having a nonlinear feedback on the currents themselves. A number of current systems exist in the magnetosphere, most commonly defined as (1) the dayside magnetopause Chapman-Ferraro currents, (2) the Birkeland field-aligned currents with highlatitude "region 1" and lower-latitude "region 2" currents connected to the partial ring current, (3) the magnetotail currents, and (4) the symmetric ring current. In the near-Earth nightside region, however, several of these current systems flow in close proximity to each other. Moreover, the existence of other temporal current systems, such as the substorm current wedge or "banana" current, has been reported. It is very difficult to identify a local measurement as belonging to a specific system. Such identification is important, however, because how the current closes and how these loops change in space and time governs the magnetic topology of the magnetosphere and therefore controls the physical processes of geospace. Furthermore, many methods exist for identifying the regions of near-Earth space carrying each type of current. This study presents a robust collection of these definitions of current systems in geospace, particularly in the nearEarth nightside magnetosphere, as viewed from a variety of observational and computational analysis techniques. The influence of definitional choice on the resulting interpretation of physical processes governing geospace dynamics is presented and discussed.
[1] It is shown that the banana current, a current system in the inner magnetosphere closing entirely within the magnetosphere (i.e., not through the ionosphere or on the magnetopause) but not circumflowing around the Earth, is a regular feature of near-Earth space. Closure options for the eastward asymmetric current on the inside of a localized pressure peak were explored, with the conclusion that the current must close via westward current around the outside of the high pressure region. It is a current that encircles a pressure peak and, therefore, whenever there is a pressure peak in the inner magnetosphere, a banana current exists. If multiple pressure peaks exist in the inner magnetosphere, then multiple banana currents will also coexist. Its occurrence rate is equal to that of the partial ring current, defined here as westward magnetospheric current that closes through field-aligned currents into and out of the ionosphere. Its magnitude can reach a few mega-amps during the main phase of storms, but drops to <0.1 MA during extended quiet intervals. The magnetic perturbation related to this current is strong within the region of high plasma pressure that it encircles, but is otherwise very weak outside of the banana current loop because the oppositely-directed current flow on either side of the loop largely cancels each other. In general, its related magnetic field is a few nanotesla of northward perturbation for both ground-based and geosynchronous magnetometers, making it difficult to magnetically detect. The banana current is placed in the context of the other near-Earth nightside current systems.
[1] The relative contribution of storm-time ring current development by convection driven by either potential or inductive electric fields has remained an unresolved question in geospace research. Studies have been published supporting each side of this debate, including views that ring current buildup is entirely one or the other. This study presents new insights into the relative roles of these storm main phase processes. We perform a superposed epoch study of 97 intense (Dst Min < -100 nT) and 91 moderate (-50 nT > Dst Min > -100 nT) storms using OMNI solar wind and ground-based data. Instead of using a single reference time for the superpositioning of the events, we choose four reference times and expand or contract each phase of every event to the average length of this phase, creating a normalized timeline for the superposed epoch analysis.Using the bootstrap method, we statistically demonstrate that timeline normalization results in better reproduction of average storm dynamics than conventional methods. Examination of the Dst reveals an inflection point in the intense storm group consistent with two-step main phase development, which is supported by results for the southward interplanetary magnetic field and various ground-based magnetic indices. This two-step main-phase process is not seen in the moderate storm timeline and data sets. It is determined that the first step of Dst development is due to potential convective drift, during which an initial ring current is formed. The negative feedback of this hot ion population begins to limit further ring current growth. The second step of the main phase, however, is found to be a more even mix of potential and inductive convection. It is hypothesized that this is necessary to achieve intense storm Dst levels because the substorm dipolarizations are effective at breaking through the negative feedback barrier of the existing inner magnetospheric hot ion pressure peak.Citation: Katus, R. M., M. W. Liemohn, D. L. Gallagher, A. Ridley, and S. Zou (2013), Evidence for potential and inductive convection during intense geomagnetic events using normalized superposed epoch analysis,
A new solar wind‐driven global dynamic plasmapause (NSW‐GDP) model has been constructed based on the largest currently available database containing 49,119 plasmapause crossing locations and 3957 plasmapause profiles (corresponding to 48,899 plasmapause locations), from 18 satellites during 1977–2015 covering four solar cycles. This model is compiled by the Levenberg‐Marquardt method for nonlinear multiparameter fitting and parameterized by VSW, BZ, SYM‐H, and AE. Continuous and smooth magnetic local time dependence controlled mainly by the solar wind‐driven convection electric field ESW is also embedded in this model. Compared with previous empirical models based on our database, this new model improves the forecasting accuracy and capability for the global plasmapause. The diurnal, seasonal, and solar cycle variations of the plasmapause can be captured by the new model. The NSW‐GDP model can potentially be used to forecast the global plasmapause shape with upstream solar wind and interplanetary magnetic field parameters and corresponding predicted values of SYM‐H and AE and can also be used as input parameters for other inner magnetospheric coupling models, such as dynamic radiation belt and ring current models and even MHD models.
[1] Several versions of low-to middle-latitude geomagnetic indices are examined throughout a 24 year interval and during storm time with respect to a normalized epoch timeline based on several key storm features. In particular, we conduct a quantitative comparison of the storm time superpositioning of the Dst, SYM-H, and 1 min U.S. Geological Survey Dst indices using error analysis and employing descriptive statistics to assess the similarities and differences between them. The events are then categorized by storm intensity and examined as a function of the storm phase. While the indices are highly correlated with each other, dramatic deviation between the indices exists at certain storm epoch times. In particular, the error increases at storm peak and especially for more intense storms. The differences at storm peak are, on average, 20% of the peak value of the indices. These differences arise from the choice of magnetometer stations to include in each index and the various methodologies used to compile the individual perturbation measurements into a global value. The conclusions are that multiple indices should be considered when determining low-to middle-latitude magnetic perturbations and that the difference between the indices should be considered as an error estimate on these values.Citation: Katus, R. M., and M. W. Liemohn (2013), Similarities and differences in low-to middle-latitude geomagnetic indices,
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