2022 THEMIS SCIENCE NUGGETS
Embedded Region 1 and 2 currents: a frequent feature of the active-time Birkeland current system
by Jiang Liu
Department of Earth, Planetary, and Space Sciences
Department of Atmospheric and Oceanic Sciences
University of California, Los Angeles
Introduction
Earth’s magnetosphere is electromagnetically coupled to its ionosphere by a large-scale field-aligned current system known as Region 1 and Region 2 (R1 and R2) currents, which occupy the ~±60o-75o invariant latitude range at all magnetic local times (Figure 1a). In the ionosphere, these currents largely overlap the auroral oval, the ionospheric footprint of the plasma sheet. Region 1 currents, which are poleward of Region 2 currents, flow downward into the ionosphere in the dawn sector and upward out of the ionosphere in the dusk sector. Region 2 currents flow in opposite directions. Manifestation of large-scale plasma motion in the magnetosphere and ionosphere, R1 and R2 currents are critical for understanding the transmission of energy and magnetic stress between the ionosphere and the magnetosphere. Region 1 and Region 2 currents often contain smaller-scale FAC structures. This paper focuses on one such recently-recognized structure, embedded Region 1 and Region 2 (ER1 and ER2) FACs (Liu et al., 2021). An embedded Region 1 (2) FAC is within the ~3o-8o latitudinal range of R1 (R2) FAC, has a ~0.5o-3o, mesoscale latitudinal range smaller than the entire R1 (R2) latitudinal range, and flows in the same direction as, but is much more intense than the background R1 (R2) FAC. Lying adjacent to or near the interface between the R1 and R2 currents, ER1 and ER2 FACs are embedded in the middle of the total latitudinal range covered by R1 and R2 currents (darker colors in Figure 1b). Embedded Region 1 and 2 FACs have been suggested to be important for heating and recombination of the ionosphere, plasma motion, and instabilities in the magnetosphere-ionosphere system, so we need to understand how ER1 and ER2 FACs arise. Based on simulations of strong FAC generation in the magnetotail, Liu et al. (2021) suggested that ER1 and ER2 FACs arise from plasma disturbances in the central plasma sheet caused by strong plasma motion during active times. This idea needs to be tested by a comprehensive statistical study.
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| Figure 1. (a) Schematic illustration of the locations of Region 1 and 2 currents in the ionosphere. Light red/blue: upward/downward field-aligned currents. Magenta line and diamond: schematic track of DMSP-18 giving the measurements in (c). (b) Similar to (a), but showing embedded Region 1 and 2 FACs as darker colors in the evening sector. The magenta line and diamond give the track of DMSP-18 giving the measurements in (d). (c and d) Components of the perturbation magnetic field vector. Vertical dashed lines indicate the boundaries of R1 and R2 currents; the magenta dashed line marks their interface. Vertical dotted lines in (d) indicate the boundaries of ER1 and ER2 FACs. Green curves are the approximately eastward component.. |
Results
To conduct this statistical study, we first need to identify embedded FACs. The best way to for this identification is to examine the horizontal magnetic field measured by low-altitude (i.e., <1000 km) spacecraft transecting the auroral zone. Figures 1c and 1d show two examples of horizontal magnetic field measurements. In Figure 1c, the poleward-moving DMSP-18 (840 km altitude) transects R1 and R2 currents in the evening sector when ER1 and ER2 FACs are absent. Region 2 and 1 currents are indicated by the gradual increase and decrease of the eastward magnetic field component (green) curve, respectively. Figure 1d shows an example of embedded R1 and embedded R2 field-aligned currents observed by DMSP-18, which was moving equatorward in the evening sector. R1 and R2 currents, whose ranges are marked by vertical dashed lines, there are two steep slopes of the eastward field component (green curve). These two slopes indicate the ER1 and ER2 currents (boundaries marked by vertical dotted lines). To identify ER1 and ER2 currents statistically, we design an explicit selection algorithm mimicking how FACs are identified in Figure 1. Figure 2 shows examples of identified ranges of R1, R2, ER1, and ER2 FACs using our algorithm. The algorithm did a good job of identifying their boundaries and cases when the embedded FACs are present or absent. Using the identified database of thousands of events, we conduct a statistical study. We found that the ER1 and ER2 FACs are present for >7% of the time when R1 and R2 FACs are well-defined, so they are a frequent feature of the M-I system. The occurrence rate of the conditions when the ER1 and ER2 currents are present or not is shown in Figure 3 as a function of geomagnetic activity, as represented by the aurora low index (AL). A more negative value of AL indicates a more active condition. The statistical result clearly shows that the occurrence rate of ER1 and ER2 currents (red histogram) is much higher during active times (AL<−250 nT) than during quiet conditions (AL>−100 nT). The occurrence rate of their absence (green histogram) shows the opposite trend—during quiet times, the embedded FACs are more likely to be absent.
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| Figure 2. Examples of our automated FAC identification algorithm. Black curves: original approximately eastward magnetic field data. Green lines with red diamonds: fitted line segments and nodes by our algorithm. Magenta vertical dashed lines: the R1/R2 interface determined by our algorithm. Black vertical dashed lines: the boundaries of R1 and R2 currents determined by our algorithm. (a-d) Data determined by our algorithm to contain both ER1 and ER2 signatures. Vertical dotted lines: the separation between background and embedded signatures, as determined by the algorithm. (e-h) Transects determined by our algorithm to contain neither ER1 nor ER2 FAC. |
Conclusion
Our statistical study on an automatically selected database indicates that embedded Region 1 and embedded Region 2 field-aligned currents occur frequently and have a higher chance to appear when the geomagnetic condition is more active. This active-time preference supports the scenario that ER1 and ER2 FACs arise from plasma disturbances caused by enhanced magnetospheric plasma motion. Such active-time enhanced FACs have been suggested by previous simulations as important players in active-time processes such as substorms but were unable to be resolved by magnetospheric spacecraft such as THEMIS. Our study has shown that these FACs are observed in low altitudes as ER1 and ER2 currents, so they can be conveniently observed by low-altitude spacecraft such as DMSP and ELFIN.
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| Figure 3. The statistical occurrence rates of conditions with (red) and without (green) as a function of activity level as represented by the AL index. Please ignore the dotted histograms.. |
References
Liu, J., Lyons, L. R., Wang, C., Ma, Y., Strangeway, R. J., Zhang, Y., et al. (2021). Embedded Regions 1 and 2 Field‐Aligned Currents: Newly Recognized From Low‐Altitude Spacecraft Observations. Journal of Geophysical Research: Space Physics, 126(6). https://doi.org/10.1029/2021JA029207Liu, J., Higuchi, T., Lyons, L. R., Ohtani, S., Wu, J., Zou, Y., et al. (2022). The Occurrence of Embedded Region 1 and 2 Currents Depends on Geomagnetic Activity Level. Journal of Geophysical Research: Space Physics, 127, e2022JA030539. https://doi.org/10.1029/2022JA030539
Biographical Note
Jiang Liu's research interest covers the Earth's magnetosphere and ionosphere, and outer planets' magnetosphere. My research focuses on: Transient energy carriers at the dayside magnetopause and in the magnetotail, especially their role in global energy transport during geomagnetic storms and substorms. How the current system of the magnetosphere and the ionosphere change in response to geomagnetic activities. I investigate these subjects using data from THEMIS, Van Allen Probes, MMS, and Cassini missions. In addition, I simulate the response of the Energetic Particle Detector onboard the Electron Losses and Fields Investigation (ELFIN) CubeSat and participate in the Europa Clipper misson.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu