ESA Science & Technology - Publication Archive

Double Star/TC-1 and Cluster data show that both component reconnection and anti-parallel reconnection occur at the magnetopause when the interplanetary magnetic field (IMF) is predominantly dawnward. The occurrence of these different features under these very similar IMF conditions are further confirmed by a statistical study of 290 fast flows measured in both the low and high latitude magnetopause boundary layers. The directions of these fast flows suggest a possible S-shaped configuration of the reconnection X-line under such a dawnward dominated IMF orientation.

We analyze Double Star TC-1 magnetic field data from July to September in 2004 and find that plasmoids exist in the very near-Earth magnetotail. It is the first time that TC-1 observes the plasmoids in the magnetotail at X > -13 R_E. According to the difference of the magnetic field structure in plasmoids, we choose two typical cases for our study: the magnetic flux rope on August 6 with the open magnetic field and the magnetic loop on September 14 with the closed magnetic field. Both of the cases are associated with the high speed earthward flow and the magnetic loop is related to a strong substorm. The ions can escape from the magnetic flux rope along its open field line, but the case of the closed magnetic loop can trap the ions. The earthward flowing plasmoids observed by TC-1 indicate that the multiple X-line magnetic reconnection occurs beyond the distance of X=-10 R_E from the Earth.

The NUADU (NeUtral Atom Detector Unit) instrument aboard TC-2 recorded 4pi solid angle images of charged particles (E >180 keV) spiraling around the magnetic field lines in the near-Earth plasma sheet (at ~ -7 R_E, equatorial dawn-to-night side) during a geomagnetic storm (Dst =-219 nT) on August 24, 2005. Energetic ion beam events characterized by symmetrical, ring-like, solid angle distributions around ambient magnetic field lines were observed during a 34-minute traversal of the plasma sheet by the TC-2 spacecraft. Also, observations during these multiple crossings of the plasma sheet were monitored by the magnetometer experiment (FGM) aboard the same spacecraft. During each crossing, a whistler-mode chorus enhancement was observed in the anisotropic area by the TC-2 low frequency electromagnetic wave detector (LFEW/TC-2) at a frequency just above that of the local lower hybrid wave. A comparison of the ion pitch angle distribution (PAD) map with the ambient magnetic field shows that an enhancement in the field aligned energetic ion flux was accompanied by tailward stretching of the magnetic field lines in the plasma sheet. In contrast, the perpendicular ion-flux enhancement was accompanied by a signature indicating the corresponding shrinkage of the magnetic field lines in the plasma sheet.
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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.

Fifty-three substorms measured by Double Star/TC-1 in the near-Earth magnetotail from July to October, 2004 are studied. The main features of these events are: (a) Magnetic flux pileup characterized by continuous enhancement of Bz is observed, which starts almost simultaneously with aurora breakup within 1-3 minutes, indicating that substorm onset is in close relation to flux pileup. (b) Sudden plasma sheet expansion with sharp increases in ion temperature and density is seen in all events, which occurs typically ~11 minutes after the beginning of pileup. The plasma sheet expansion is shown to be in close relation with the primary substorm dipolarization and, hence, can be referred to as 'dipolarization-associated expansion'. (c) Evidence indicates that the substorm current wedge first forms earthward of TC-1 position and, hence, inward of the flow braking region, and then propagates tailward with an expansion in the Z-direction. Possible implications of these observations are briefly discussed.

We present space- and ground-based observations of the signatures of magnetic reconnection during an interval of duskward-oriented interplanetary magnetic field on 25 March 2004. In situ field and plasma measurements are drawn from the Double Star and Cluster satellites during traversals of the pre-noon sector dayside magnetopause at low and high latitudes, respectively. These reveal the typical signatures of flux transfer events (FTEs), namely bipolar perturbations in the magnetic field component normal to the local magnetopause, enhancements in the local magnetic field strength and mixing of magnetospheric and magnetosheath plasmas. Further evidence of magnetic reconnection is inferred from the ground-based signatures of pulsed ionospheric flow observed over an extended interval. In order to ascertain the location of the reconnection site responsible for the FTEs, a simple model of open flux tube motion over the surface of the magnetopause is employed. A comparison of the modelled and observed motion of open flux tubes (i.e. FTEs) and plasma flow in the magnetopause boundary layer indicates that the FTEs observed at both low and high latitudes were consistence with the existence of a tilted X-line passing through the sub-solar region, as suggested by the component reconnection paradigm. While a high latitude X-line (as predicted by the anti-parallel description of reconnection) may have been present, we find it unlikely that it could have been responsible for the FTEs observed in the pre-noon sector under the observed IMF conditions. Finally, we note that throughout the interval, the magnetosphere was bathed in ULF oscillations within the solar wind electric field.
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We report strong repeated magnetic reconnection pulses that occurred deep inside closed plasma sheet flux tubes at r <= 14Re. They have been observed with a fortuitous spacecraft constellation during three consecutive turbulent magnetic dipolarizations, accompanied by localized auroral brightenings near the equatorward edge of a wide auroral oval. The reconnection separatrix was mapped to ~64° CGLat in the ionosphere, where a very energetic and narrow energy-dispersed ion injection with unusually steep dispersion slope was observed. Reconstruction of the reconnection rate from magnetic waveforms at Cluster provided a reconnection pulse duration (~1 min) and peak strength (E_R ~ 8 mV/m) consistent with direct observations in the reconnection outflow region. The magnetic activity was rather weak, although the concurrent solar wind flow pressure was above the norm. We suggest that near-Earth reconnection events may be a phenomenon more frequent than generally thought. We also confirm that reconnection and the growth of strong turbulence in the near tail are strongly coupled together in near-Earth reconnection events.

We examined the relationship between bursty bulk flow (BBF) events observed by Cluster between -19 R_E < X < -12 R_E and dipolarization events observed by Double Star TC1 between -13 R_E < X < -6 R_E. TC1 observed dipolarizations for ~33% of the cases when BBFs were observed by Cluster. During these dipolarization events the TC1 location was closer to the Cluster location and the local B_Z at TC1 was smaller than during events where TC1 observed no clear dipolarization associated with BBFs at Cluster. This result suggests that (1) flow-associated activity dissipates within a limited spatial scale, 4-8 R_E, and that (2) the initial magnetic topology in the inner magnetosphere can contribute strongly to fast flow penetration toward the Earth. The fact that there were no TC1 dipolarization events at X > -8 R_E associated with BBFs at Cluster in our dataset suggests two possibilities: near-geosynchronous dipolarization needs another mechanism in addition to flux pile-up and braking, or during near-geosynchronous dipolarization the near-tail current sheet/plasma sheet is too thin to be observed by Cluster.

Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding of reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.

The Cluster and Double Star satellites recently observed plasma density holes upstream of Earth's collisionless bow shock to apogee distances of ~19 and 13 Earth radii, respectively. A survey of 147 isolated density holes using 4 s time resolution data shows they have a mean duration of ~17.9±10.4 s, but holes as short as 4 s are observed. The average fractional density depletion (delta n/n) inside the holes is ~0.68±0.14. The upstream edge of density holes can have enhanced densities that are five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock. Multispacecraft analyses show the density holes move with the solar wind, can have an ion gyroradius scale, and could be expanding. A small normal electric field points outward. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that backstreaming particles interacting with the solar wind are important.

We examine magnetic flux closure during an extended substorm interval on 29 August 2004 involving a two-stage onset and subsequent re-intensifications. Cluster and Double Star provide observations of magnetotail dynamics, while the corresponding auroral evolution, convection response, and substorm current wedge development are monitored by IMAGE FUV, SuperDARN, and the Greenland magnetometer chain, respectively. The first stage of onset is associated with the reconnection of closed flux in the plasma sheet; this is accompanied by a short-lived auroral intensification, a modest substorm current wedge magnetic bay, but no significant ionospheric convection enhancement. The second stage follows the progression of reconnection to the open field lines of the lobes; accompanied by prolonged auroral bulge and westward-travelling surge development, enhanced magnetic bays and convection. We find that the tail dynamics are highly influenced by ongoing dayside creation of open flux, leading to flux pile-up in the near-tail and a step-wise down-tail motion of the tail reconnection site. In all, 5 dipolarizations are observed, each associated with the closure of ~0.1 GWb of flux. Very simple calculations indicate that the X-line should progress down-tail at a speed of 20 km s^-1, or 6 R_E between each dipolarization.

Editors: C. P. Escoubet, Z.-X. Liu, and Z. Pu

This special issue of Annales Geophysicae presents the mission, the instruments and the first results of the Double Star programme. Double Star is the first mission in collaboration between China and ESA. Double Star has been a great opportunity for the European and Chinese scientists to enhance the knowledge of the Sun-Earth connection. Double Star, together with Cluster, brings six coordinated spacecraft to study small-, medium- and large-scale plasma processes in geospace. This is the first time that European instruments have been flown on a Chinese spacecraft as part of the payload.

The scientific objectives of an advanced NeUtral Atom Detector Unit (NUADU) designed for the Chinese Double Star Polar Mission, which is scheduled for launch in July 2004, are described. The potential during this mission to realize, hitherto unprecedented, integrated studies of global dynamic magnetospheric processes through combining with NUADU data contemporaneous measurements made aboard the CLUSTER II, IMAGE and TWINS spacecraft is also discussed and a short technical account of NUADU provided.