Dawnside Wedge Current System Formed During Intense Geomagnetic Storms

Abstract

We investigate the formation and development of a large-scale current system in the dawn sector during intense geomagnetic storms. Four events are selected based on the historical ranking of the westward deflections of the morningside midlatitude magnetic field. For each event the polar distribution of equivalent currents indicates a significant intensification of the westward electrojet (WEJ), which initially takes place at postmidnight and then extends eastward covering the entire dawn sector. The longitudinal confinement of the enhanced WEJ suggests that it closes with downward and upward field-aligned currents (FACs) at its eastern and western ends, respectively, and therefore, the entire system may be envisioned as a wedge current. It is noted, however, that the primary closure of FACs is meridional, and those upward and downward FACs are considered to be unbalanced parts of the R1 and R2 currents. In one event dipolarization was observed in the dawnside plasma sheet near the magnetic conjugate point of the enhanced WEJ, and in another event a major auroral expansion was observed in the entire dawn sector. It is therefore suggested that the formation of the wedge current system and the subsequent expansion toward dayside is an ionospheric projection of the tail current reduction extending toward the dawnside flank. This wedge current system is similar to the substorm wedge current system except that it is centered at dawn. The recurrent formation of this dawnside current wedge in intense storms suggests that this is another distinct constituent of the storm time current system. The source current of this midlatitude B E disturbance is not obvious. Its timescale was of the order of 1 hr, which is comparable to that of substorms. However, this event was observed in the late morning sector, LT = 9.6, where the effect of the nightside substorm current system, if at all, is generally considered insignificant. The magnetopause current is another source of midlatitude geomagnetic disturbances, especially on the day side. However, its effect should be more pronounced in the north-south component rather than in the east-west component. The same remark also applies to the ring current. In general, the meridional component of ionospheric current and the field-aligned current (FAC) are the two main causes of geomagnetic deflections. In either case this GIC event suggests that a certain magnetosphere-ionosphere (M-I) coupling process in the morning sector possibly prevails various processes in other LTs, including nightside substorms, in the magnitude of midlatitude magnetic deflection.

Effects of Nearly Frontal and Highly Inclined Interplanetary Shocks on High‐Latitude Field‐Aligned Currents (FACs)

“…Regarding the high NFS/HIS prenoon Region R1 current at 1.27, Ohtani et al () noticed similar enhancements of dawnside R1 current and strong dawnside current wedge during several intense storms. Our results suggest that this kind of strong dawnside current can happen at times other than during intense storms and maybe a feature of NFS response.…”

“…We have also identified specific regions where the NFS FAC response is stronger than HIS FAC response. Of particular interest is possible post dawn current system (e.g., Ohtani et al, ) subsequent to NFSs. This may be a region to watch for geomagnetically induced currents effects just after the impact of the most extreme shocks.…”

Revisiting the Partial Ring Current Model: Longitudinal Asymmetry of Ground Magnetic Depression During Geomagnetic Storms

“…The dawnside wedge current system reported by Ohtani et al. (2018) is a likely candidate for this process, and the enhancement of global convection may also contribute. Since the dawn‐dusk asymmetry of ground magnetic depression is a characteristic feature of the storm main phase, the formation of the dawnside wedge current system must be prevalent when storms develop.…”

Is the Substorm Current Wedge an Ensemble of Wedgelets?: Revisit to Midlatitude Positive Bays

“…Another possibility is that the premidnight and postmidnight wedge current systems actually represent two separate magnetospheric processes. Interestingly, during geomagnetic storms, an intense wedge current system recurrently forms around midnight and extends dawnward (Ohtani et al, 2018), suggesting that there is a certain process that drives a WEJ in the postmidnight sector. Obviously, the issue is complex and deserves a separate dedicated study in the future.…”