Y. V. Bogdanova

Formation of the low-latitude boundary layer and cusp under the northward IMF: Simultaneous observations by Cluster and Double Star

On 28 February 2004 the configuration of the Cluster and Double Star TC1 satellites facilitated a simultaneous study of plasma properties inside the low-latitude boundary layer (LLBL) near the subsolar magnetopause and inside the midaltitude cusp during an interval with strong northward IMF. TC1, crossing the dayside magnetopause, observed a complex structure of boundary layers. We suggest that one part of the LLBL, characterized by high fluxes of magnetosheath-like electrons, is formed due to reconnection processes. We can identify three different plasma populations inside this region: on open field lines outside the magnetopause which are reconnected in the northern hemisphere lobe sector; on open field lines inside the magnetosphere which are reconnected in the northern hemisphere lobe sector and sink inside the magnetosphere; and on reclosed field lines, which undergo a second reconnection in the southern hemisphere lobe sector. Another part of the LLBL, characterized by equal fluxes of magnetosheath-like and plasma sheet populations, is formed by diffusion processes as strong pitch angle diffusion and formation of a loss cone are observed inside this region. Cluster, moving from the polar cap toward the dayside magnetosphere via the cusp region, crossed many different sublayers with different plasma properties. Comparison of plasma populations inside the different subregions of the LLBL and cusp shows that the complex LLBL observed at the dayside magnetopause maps into the midaltitude cleft/cusp region and that observed sublayers inside the cusp can be explained by reconnection in the lobe sector of one or both hemispheres and by diffusion processes.

On the formation of the high-altitude stagnant cusp : Cluster observations

Received 24 February 2005; revised 4 May 2005; accepted 16 May 2005; published 16 June 2005. [1] On 16 March 2002, Cluster moved from nightside to dayside, across the high-altitude northern cusp during an extended period of relatively steady positive IMF BY and BZ. Combined Cluster and SuperDARN data imply the existence of two reconnection sites: in the high-latitude northern hemisphere dusk and southern hemisphere dawn sectors. Within the cusp, Cluster encounters 3 distinct plasma regions. First, injections of magnetosheath-like plasma associated with dawnward and sunward convection suggest Cluster crosses newly-reconnected field lines related to the dusk reconnection site. Second, Cluster observes a Stagnant Exterior Cusp (SEC), characterized by nearly isotropic and stagnant plasma. Finally, Cluster crosses a region with significant antifield-aligned flows. We suggest the observed SEC may be located on newly re-closed field lines, reconnected first poleward of the northern hemisphere cusp and later reconnected again poleward of the southern hemisphere cusp. We discuss how the Cluster observations correspond to expectations of ’double reconnection’ model. Citation: Bogdanova, Y. V., A. Marchaudon, C. J. Owen, M. W. Dunlop, H. U. Frey, J. A. Wild, A. N. Fazakerley, B. Klecker, J. A. Davies, and S. E. Milan (2005), On the formation of the high-altitude stagnant cusp: Cluster observations, Geophys. Res. Lett., 32, L12101, doi:10.1029/2005GL022813.

TC1 and Cluster observation of an FTE on 4 January 2005: A close conjunction

Observations of a Flux Transfer Event (FTE) signature at the dayside magnetopause are reported, which was consecutively observed on 4 January 2005 by both the Double Star/TC1 spacecraft and the Cluster quartet, while the spacecraft were traversing through the northern-dusk magnetopause. The event occurred as a magnetosheath FTE first at the Cluster spacecraft at about 07:13 UT on 4 January 2005 and crossed each of the others within 2 minutes. The spatial separations between the Cluster spacecraft were of the order of 200 km. The TC1 signature occurred about 108s after Cluster. All findings including magnetic fluxes, orientations and hot ion velocity distributions strongly suggest that Cluster and TC1 encountered the magnetosheath branch of the same flux tube at two different positions along its length and this is borne out by computation of the expected time delay. Four-spacecraft timing is used to obtain the velocity of FTE.

Correlation between suprathermal electron bursts, broadband extremely low frequency waves, and local ion heating in the midaltitude cleft/low-latitude boundary layer observed by Cluster

with localized extra low frequency (ELF) (1–10 Hz) magnetic field wave power with broadband spectra. Our study shows that strong ion heating was observed only in the region with electron field-aligned anisotropy more than 2. In addition, comparison of particle data from two spacecraft, which crossed the heating region with a time difference of 4 min, shows the correlation between ion outflow fluxes and fluxes of the injected electrons. Whereas ELF electromagnetic waves are localized inside the ion heating region, ELF electrostatic waves are detected throughout the cleft/cusp/mantle regions, where strong ion heating was not observed, suggesting that electromagnetic ELF waves heat ions in the cleft region. Owing to the absence of magnetosheath ions and strong field-aligned currents, we suppose that inside ‘‘electron-only’’ cleft region the suprathermal electron bursts are most likely an energy source for the wave destabilization. We suggest that the location of the heating region and the level of the outflow ion fluxes could be related to electron injection in the cleft in such events. INDEX TERMS: 2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 7807 Space Plasma Physics: Charged particle motion and acceleration; 7867 Space Plasma Physics: Wave/particle interactions; KEYWORDS: cleft electron observations, dayside ion outflow, ion heating in the cleft/cusp, wave/ particle interaction

Simultaneous field‐aligned currents at Swarm and Cluster satellites

We show for the first time, with direct, multispacecraft calculations of electric current density, and other methods, matched signatures of field-aligned currents (FACs) sampled simultaneously near the ionosphere at low (similar to 500km altitude) orbit and in the magnetosphere at medium (similar to 2.5 R-E altitude) orbits using a particular Swarm and Cluster conjunction. The Cluster signatures are interpreted and ordered through joint mapping of the ground/magnetospheric footprints and estimation of the auroral zone boundaries (taken as indication of the boundaries of Region 1 and Region 2 currents). We find clear evidence of both small-scale and large-scale FACs and clear matching of the behavior and structure of the large-scale currents at both Cluster and Swarm. The methodology is made possible through the joint operations of Cluster and Swarm, which contain, in the first several months of Swarm operations, a number of close three-spacecraft configurations.

Cluster observations of the midaltitude cusp under strong northward interplanetary magnetic field

We report on a multispacecraft cusp observation lasting more than 100 min. We determine the cusp boundary motion and reveal the effect on the cusp size of the interplanetary magnetic field (IMF) changing from southward to northward. The cusp shrinks at the beginning of the IMF rotation and it reexpands at the rate of 0.40° invariant latitude per hour under stable northward IMF. On the basis of plasma signatures inside the cusp, such as counterstreaming electrons with balanced fluxes, we propose that pulsed dual lobe reconnection operates during the time of interest. SC1 and SC4 observations suggest a long-term regular periodicity of the pulsed dual reconnection, which we estimate to be ~1–5 min. Further, the distances from the spacecraft to the reconnection site are estimated on the basis of observations from three satellites. The distance determined using SC1 and SC4 observations is ~15 RE and that determined from SC3 data is ~8 RE. The large-scale speed of the reconnection site sunward motion is ~16 km s-1. We observe also a fast motion of the reconnection site by SC1, which provides new information about the transitional phase after the IMF rotation. Finally, a statistical study of the dependency of plasma convection inside the cusp on the IMF clock angle is performed. The relationship between the cusp stagnation, the dual lobe reconnection process, and the IMF clock angle is discussed.

Electron structure of the magnetopause boundary layer: Cluster/Double star observations

We present a comparison of two events, monitored by the Double Star and Cluster spacecraft at separate locations on the dayside magnetopause, which exhibit distinct properties at high and low latitudes in the magnetopause boundary layer during the occurrence of low-latitude reconnection. On 6 April 2004, the four Cluster and TC-1 spacecraft were on near-coincident, outbound transits of the dawnside magnetosphere at north and south midlatitudes, respectively. The observations show a series of oppositely directed flux transfer events (FTEs), fed by a low-latitude reconnection line located between the spacecraft. Although both spacecraft locations were nearly equidistant from the active reconnection region, the associated magnetopause boundary layer was maintained at TC-1 but not at Cluster. We suggest an asymmetric north and south extent of the LLBL so as to be more extensive at TC-1, where the local magnetic shear across the magnetopause is small. On 4 January 2005, the Cluster and TC-1 spacecraft all repeatedly traverse the northern, duskside magnetopause almost simultaneously, before and after a strong reversal in the IMF from northward to southward during a period of turbulent solar wind. Open flux tubes are observed within minutes of the southward turning, arising from the sudden formation of a nearby subsolar reconnection line. Before the IMF change, a complex and energized boundary layer, largely absent at the lower latitudes of TC-1, and containing an energetic (>40 keV) electron population of locally trapped and field-aligned distributions, is present at the high-latitude Cluster locations. Following reconnection onset after the IMF reversal, the boundary layer is seen to extend to TC-1, and the electron distribution, which depends on position through the boundary layer, develops as an energetic, field-aligned (bistreaming) distribution. The analysis is utilizes an extended electron distribution for energies ranging from a few to 400 keV and by reordering the transition through the magnetopause to the electron distribution.

The magnetic configuration of the high-latitude cusp and dayside magnetopause under strong magnetic shears

This paper investigates the structure of the magnetic field near the magnetopause (MP) by analyzing the multiple-point magnetic measurements from the Cluster mission. In this paper, the spatial distribution of the curvature radius of the MP surface at the noon-midnight meridian and for situations with moderate dynamical pressure of solar wind is implied from direct measurements of magnetic field curvature for the first time. The investigation focused on conditions of strong magnetic shear and in which a clear boundary layer is present at the MP. It has been confirmed that the magnetic field lines surrounding the cusp bend sunward at the precusp region and tailward at the postcusp region, implying the existence of a cusp field indentation. The minimum curvature radius of the near-MP field at both precusp and postcusp regions is about 2 R-E. As the latitude decreases, the curvature radius at the MP increases gradually, so that, as the subsolar point is approached, the curvature radius of the MP is nearly equal to the geocentric distance. These results compare well with existing MP models but reveal the limitations inherent in such statistical estimates of local MP curvature, particularly surrounding the cusp regions. The analysis of the magnetic measurements has also verified the existence of the magnetic bottles at both precusp and postcusp regions, which may play a role for the trapping of the charged particles of magnetosphere.

Coordinated Cluster and Double Star observations of the dayside magnetosheath and magnetopause at different latitudes near noon

We present results of a favorable conjunction where the equatorial spacecraft (TC-1) of the Double Star mission exits the dayside magnetopause near the equator, while Cluster is inbound, near the southern cusp. This configuration makes it possible to compare observations of the magnetopause, around the same magnetic local time but at different latitudes. In this paper, we report on the general properties of the magnetosheath plasma at the two latitudes: unlike predictions from gasdynamic modeling, the density is found lower near the nose of the magnetopause than further downstream. Then, we present three interesting events. First, an FTE is observed at TC-1 and not at Cluster; we discuss the implications this has on the evolution of FTEs and on the size of the reconnection site. Then, a structure observed at both spacecraft is interpreted as a bulge progressing along the magnetopause. It is not clear whether this bulge is actually the remnant of an FTE or a running pulse that makes Cluster sense the reconnection layer. In any case, a rotational discontinuity is observed within it. At last, a northward turning of the magnetosheath magnetic field is observed at TC-1 and a reverse FTE is subsequently seen at Cluster, suggesting that magnetic reconnection is very fast to set up following a change in the IMF orientation.