Multiple cusps during an extended northward IMF period with a significant B_y component

Abstract

[1] On 21-22 March 2001, four cusp-like regions were observed consecutively in about five hours by all four Cluster spacecraft when the interplanetary magnetic field (IMF) was northward with a significant B y component. All four cusp-like encounters were characterized by turbulent magnetic fields, high-density plasma, and plasma flow significantly slower than the magnetosheath level. The cusp-like regions are associated with thermalized, bidirectional distributed plasma electrons. The first encountered cusp is the main cusp; the other three cusp-like regions are temporal effects. The normal velocities v n at boundary interfaces (exit from the cusp) are found to be almost three times larger than that at boundary interfaces (entry into the cusp). The boundary normal, velocity, and timing analysis for six clear boundaries of the last three cusps obtained by all four spacecraft indicates that they are most likely one boundary shifting in the dawn-dusk direction between the dayside magnetosphere/trapping region and the cusp region. So the Cluster spacecraft have been observing the same cusp, and it appeared as four cusp-like regions due to possible magnetospheric oscillations. Oscillations with a period of 22 min are observed by the Cluster spacecraft in the high-latitude region, which is in agreement with the cold-dense plasma sheet fluctuations (20 min period) observed by the Geotail satellite. Multiple cusps observed by Cluster and the wavy-like structures in the dusk low-latitude boundary layer observed by Geotail simultaneously suggest the whole magnetosphere is oscillating during northward IMF.Citation: Zong, Q. -G., et al. (2008), Multiple cusps during an extended northward IMF period with a significant B y component,

Cusp observations during a sequence of fast IMF <I>B<sub>Z</sub></I> reversals

“…Recently, Cluster observations of multiple cusp crossings have been reported, which were interpreted in terms of temporal rather than spatial effects (Zong et al, 2004(Zong et al, , 2008. In the present interval, Cluster crossed the high-altitude cusp twice, both times during steadily northward IMF, before exiting into the magnetosheath, while the IMF B Z component reversed in polarity four times during the period of interest.…”

“…How the cusp reacts to changes of the IMF from southward to northward, or vice versa, is currently a topic of great interest (Bogdanova et al, 2007;Hu et al, 2008;Escoubet et al, 2008). Additionally, "triple" cusp crossings, observed by Cluster have been reported, and explained in terms of the motion of a single cusp due to either changes in the azimuthal component of the solar wind (Zong et al, 2004) or oscillations of the magnetosphere (Zong et al, 2008).…”

Polar observations of ion/electron bursts at the pre-dawn polar cap boundary: evidence for internal reconnection of overdraped lobe flux

“…The observation of bidirectional magnetosheath electron fluxes (see our Fig. 7) has been used to infer the presence of bursty events of newlyclosed field lines under similar IMF conditions as in our case (northward IMF with significant B y component) by Bogdanova et al (2007) and Zong et al (2008). We note that magnetosheath electrons are not bi-directional in general.…”

Variation of dependence of the cusp location at different altitude on the dipole tilt

“…Zong et al [13,17] reported that "multiple" cusps can exist under northward IMF, but it is a very rare phenomenon. In this study, we only take the "single" cusp crossing to do research.…”

“…Based on these expectations and the satellite data that we have, the criteria to identify the cusp region are as follows: a decrease of the magnetic field strength from the background field greater than 1nT associated with some fluctuations; a sudden increase in the proton and electron density (greater than 5 cm ) which signifies a solar wind origin. Some authors have used these conditions to identify the cusp region with the Cluster data and Polar satellite data [9,17]. In our study, we use these conditions to identify the two cusp edges firstly.…”