First current density measurements in the ring current region using simultaneous multi-spacecraft CLUSTER-FGM data

Abstract

Abstract. The inner magnetosphere's current mapping is one of the key elements for current loop closure inside the entire magnetosphere. A method for directly computing the current is the multi-spacecraft curlometer technique, which is based on the application of Maxwell-Ampère's law. This requires the use of four-point magnetic field high resolution measurements. The FGM experiment on board the four Cluster spacecraft allows, for the first time, an instantaneous calculation of the magnetic field gradients and thus a measurement of the local current density. This technique requires, however, a careful study concerning all the factors that can affect the accuracy of the J estimate, such as the tetrahedral geometry of the four spacecraft, or the size and orientation of the current structure sampled. The first part of this paper is thus providing a detailed analysis of the method accuracy, and points out the limitations of this technique in the region of interest. The second part is an analysis of the ring current region, which reveals, for the first time, the large latitudinal extent of the ring current, for all magnetic activity levels, as well as the latitudinal evolution of the perpendicular (and parallel) components of the current along the diffuse auroral zone. Our analysis also points out the sharp transition between two distinct plasma regions, with the existence of high diamagnetic currents at the interface, as well as the filamentation of the current inside the inner plasma sheet. A statistical study over multiple perigee passes of Cluster (at about 4 R E from the Earth) reveals the azimuthal extent of the partial ring current. It also reveals that, at these distances and all along the evening sector, there isn't necessarily a strong dependence of the local current density value on the magnetic activity level. This is a direct consequence of the ring current morphology evolution, as well as the relative positioning ofCorrespondence to: C. Vallat (claire.vallat@cesr.fr) the spacecraft with respect to the bulk of the ring current. It also proves the existence of a substantial ring current at these distances, all over the evening and the post-midnight sector.

The distribution of the ring current: Cluster observations

“…5a, we can confirm that the average magnitudes of the measured current density at the Cluster ring plane crossing (∼4-4.5 R E ) ranged from 9 to 27 nA m −2 , which are greater than the quiet time averages of ∼1-4 nA m −2 derived using the Parker equation from observed magnetic field and particle pressures (but then estimated over the range of L-values 2-9; see Lui andHamilton, 1992, andDe Michelis et al, 1999). This difference has been accounted for by Vallat et al (2005) who also used the Curlometer technique, deriving similar values of 30 nA m −2 to our study. The profile of |J | also confirms that there is an enhancement in the morning sector and a dip in after noon.…”

“…Their study was limited by the data taken during 2002, but suggested that the ring current can extend from −65 to 65 • in latitude all over the evening and postmidnight sectors about 9 h of MLT. The present paper extends the study of Vallat et al (2005) to survey the distributions of the ring current at all MLT and discuss the locations of the connecting region 2 field-aligned currents (FACs). Figure 1 shows an example orbit of the Cluster spacecraft S/C1 between 05:00 and 18:00 UT on 6 February 2004.…”

“…These values show the expected westward ring current around the equator. The morphology of the ring current system suggested by Iijima et al (1990) and Le et al (2004) partially closes in the ionosphere via up and down Region 2 FACs, and the ring current can extend from −65 to 65 • in latitude all over the evening and post-midnight sectors (Vallat et al, 2005). Considering the effect of the FACs on the accuracy of the current calculations, we focus here on the results in the equatorial plane (−30 to 30 • ).…”

“…Moreover, this method assumes that all measurement points are situated inside or surround the same current sheet. This technique has been recently applied by Vallat et al (2005), using Cluster 4-point magnetic field data to partially survey the ring current in the evening and postmidnight sectors. Their study was limited by the data taken during 2002, but suggested that the ring current can extend from −65 to 65 • in latitude all over the evening and postmidnight sectors about 9 h of MLT.…”

“…The use of Div(B) does not give a direct indication of the actual error in the current estimate and is less relevant for distorted tetrahedral configurations. Because a very long or a very flat tetrahedron (the elongation (E) or planarity (P) of the tetrahedron is greater than 0.9) will lead to an estimated error reaching 10 % and more, estimates of the absolute uncertainty in the calculation of curl B were also made (see discussion in Robert et al, 1998;Dunlop et al, 2002, andVallat et al, 2005). These authors showed that the curlometer results are reliable when the Div(B)/|Curl(B)| < 0.5, depending on the temporal stability of the current.…”