Nightside Field-aligned Currents Research Articles

Solar cycle dependence of nightside field‐aligned currents: Effects of dayside ionospheric conductivity on the solar wind‐magnetosphere‐ionosphere coupling

AbstractIn the present study we observationally address the role of ionospheric conductivity in the solar wind‐magnetosphere coupling in terms of global field‐aligned currents (FACs). Solar EUV irradiance changes during a solar cycle and so does its contribution to the ionospheric conductivity. We statistically examine how, under fixed external conditions, the intensities of the R1 and R2 currents and their demarcation latitude depend on solar activity (F10.7). An emphasis is placed on nightside FACs in the dark hemisphere. The result shows that for fixed ranges of interplanetary electric field, the nightside FACs are more intense for higher solar activity irrespective of their polarities or local times. It is also found that the R1‐R2 pair, therefore the auroral oval, moves equatorward as the solar activity increases. For both current intensity and latitude, the dependence on F10.7 is more sensitive at smaller F10.7 and it levels off with increasing F10.7. The intensities of dayside FACs reveal similar F10.7 dependence as expected from the enhancement of the local ionospheric conductance. Interestingly, they also move equatorward with increasing F10.7. It is expected from force balance that as the dayside R1 current becomes more intense with increasing solar activity, the magnetosphere shrinks on the day side and expands on the night side. This configurational change of the magnetosphere presumably affects the energy transport from the solar wind to the magnetosphere, although its details still remain to be understood. We conclude that the ionospheric conductivity plays an active role in the solar wind‐magnetosphere‐ionosphere coupling.

Field-aligned current distribution and response to interplanetary conditions during a superstorm—CHAMP observation

With geomagnetic measurements on board of CHAMP satellite, the characteristics of global large-scale field-aligned currents (FACs) in the topside ionosphere are investigated along with their responses to interplanetary conditions for the superstorm of November, 2003. It is found that (1) The storm-time FAC densities enhanced greatly in comparison with quiet period and the enhancements show hemispheric asymmetry of both summer-winter and sunlit-dark. (2) For the first time, it is revealed that the latitude-integrated FAC density is controlled mainly by solar wind dynamic pressure rather than IMF. (3) FACs expanded equatorward dramatically, with the lowest latitude being 45° ML at or more; on the day-side this expansion was controlled directly by IMF B z , showing an interaction time scale of about 25 min in the solar wind-magnetosphere-ionosphere coupling system, and a nonlinear saturation of the equatorward expansion when IMF B z < −30 nT; while on the nightside, the expansion and recovery lagged about 3 h behind the IMF changes but nearly in phase with changes of SYM-H index. (4) During the storm main phase, the nightside FAC latitude coverage extended to 25° or wider, appearing multi-sheet current structure with more than 10 sheets.

Field-aligned currents observed by CHAMP during the intense 2003 geomagnetic storm events

Abstract. This study concentrates on the characteristics of field-aligned currents (FACs) in both hemispheres during the extreme storms in October and November 2003. High-resolution CHAMP magnetic data reflect the dynamics of FACs during these geomagnetic storms, which are different from normal periods. The peak intensity and most equatorward location of FACs in response to the storm phases are examined separately for both hemispheres, as well as for the dayside and nightside. The corresponding large-scale FAC peak densities are, on average, enhanced by about a factor of 5 compared to the quiet-time FACs' strengths. And the FAC densities on the dayside are, on average, 2.5 times larger in the Southern (summer) than in the Northern (winter) Hemisphere, while the observed intensities on the nightside are comparable between the two hemispheres. Solar wind dynamic pressure is correlated with the FACs strength on the dayside. However, the latitudinal variations of the FACs are compared with the variations in Dst and the interplanetary magnetic field component Bz, in order to determine how these parameters control the large-scale FACs' configuration in the polar region. We have determined that (1) the equatorward shift of FACs on the dayside is directly controlled by the southward IMF Bz and there is a saturation of the latitudinal displacement for large value of negative Bz. In the winter hemisphere this saturation occurs at higher latitudes than in the summer hemisphere. (2) The equatorward expansion of the nightside FACs is delayed with respect to the solar wind input. The poleward recovery of FACs on the nightside is slower than on the dayside. The latitudinal variations on the nightside are better described by the variations of the Dst index. (3) The latitudinal width of the FAC region on the nightside spreads over a wide range of about 25° in latitude.

Comparison of large‐scale field‐aligned currents under sunlit and dark ionospheric conditions

The present study statistically compares large‐scale field‐aligned currents (FACs) under sunlit and dark ionospheric conditions. A total of ∼74,000 auroral oval crossings are selected from magnetic field measurements from the DMSP F7 and F12 to F15 satellites. For the dayside FAC it is reconfirmed that both current intensity and density are statistically larger in the illuminated events than in the unilluminated events. As for the nightside FAC, in contrast, a few important features become clear for the first time, which can be summarized as follows: (1) At 20 ≤ MLT &lt; 02 both R1 and R2 intensities tend to be larger when the ionosphere is dark than when it is sunlit. (2) Although dependence on the ionospheric condition is less clear for the FAC density, a systematic preference of the occurrence of strong FACs for the dark ionosphere can be found for both R1 and R2 currents in the dusk‐to‐premidnight (16 ≤ MLT &lt; 22) sector and for the R1 current in the postmidnight sector (00 ≤ MLT &lt; 02). (3) For both FAC intensity and density the difference between the illuminated and unilluminated events tends to increase with increasing geomagnetic activity as measured by the IMF BZ component. Result 1 can be partially explained in terms of the interhemispheric asymmetry of the magnetospheric configuration, which, however, should not affect the FAC density. Therefore result 2 strongly suggests that the solar‐induced conductivity controls the M‐I coupling in a different way in certain nightside MLT sectors than in dayside sectors.

Solar zenith angle and merging electric field control of field‐aligned currents: A statistical study of the Southern Hemisphere

High‐resolution and precise vector magnetic field measurements of the CHAMP satellite are used to investigate the field‐aligned currents (FACs) in the southern polar ionosphere. The period considered comprises 2 years, providing a double coverage of the seasons and about a six‐fold coverage of all local times. From more than 11,000 polar passes the average spatial pattern of FACs in the polar ionosphere is derived. The response of features like intensity and positions of large‐scale field‐aligned currents to normal (when merging electric field is less than 2 mV/m) and disturbed (when merging electric field is greater than 2 mV/m) conditions in the Southern Hemisphere are investigated. The influence of the solar illumination‐induced conductivity on the morphology features of field‐aligned currents during normal conditions are also studied. It follows from this analysis that the intensity of field‐aligned currents changes with the merging electric field at all MLT sectors but with the solar radiation‐induced conductivity only at the dayside. Furthermore, a linear relation between the conductivity and the peak FAC density exists, which implies that the dayside FAC densities are directly controlled by the amount of solar radiation. Solar elevation does not affect the nightside FAC density. On the dayside a systematic difference of the footprint latitude between sunlit and dark conditions emerges. Under dark conditions the auroral region retreats about 2° equatorward. On the basis of the above results, we may suggest that the sources of Birkeland currents on the zdayside behave like a voltage source, while on the nightside the response is possibly like a current source.

Application of an MHD simulation to the study of substorms

The substorm mechanism is studied by the numerical solutions obtained from a resistive magnetohydrodynamic (MHD) simulation. After a southward turning of the interplanetary magnetic field (IMF), the simulation results reproduce observed features of the growth phase. The numerical solutions show that the plasma sheet thinning during the growth phase is formed under the dynamic balance between the flux pileup from the midtail and the flux removal toward the dayside controlled by the convection in the magnetosphere-ionosphere (M-I) coupling system. After the growth phase, dipolarization is generated in the near-earth tail accompanied by a plasma injection into the inner magnetosphere, the formation of plasmoid in the midtail, and the enhancement of the nightside field-aligned currents (FACs). The direct cause of this onset is the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organization in the nonlinear system.