Elizabeth A. Lucek

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

Joint observations by Cluster satellites of bursty bulk flows in the magnetotail

[1] Using the observations of three satellites of Cluster (C1, C3, and C4) during the periods July to October 2001 and July to October 2002, we study 209 active time bursty bulk flows (BBFs), the difference between single satellite observations and multisatellite observations, and the difference among three selection criteria (two about BBFs and one about rapid convection event). Single satellite observations show that the average duration of BBFs selected by the criterion of Angelopoulos et al. is 604 s, while multisatellite observations show that the average duration of BBFs is 1105 s. Single satellite sometimes misses the BBFs. The missing ratio of single satellite is 22.4% for the criterion of Angelopoulos et al. and 44.9 % for the criterion of Raj et al. Therefore the single satellite observations cannot tell the true number of BBFs. The multisatellite observations are more important for the criterion of Raj et al. The single satellite observations also show that 22% of substorms are not accompanied by BBFs, while multisatellite observations show that only 4.5% of substorms are not accompanied by BBFs. Thus it seems possible that all substorms are accompanied by BBFs. The occurrence frequency of RCEs in the central plasma sheet obtained by multisatellites is 12.2%. The occurrence frequency of BBFs in the central plasma sheet is 9.5% for single satellite observations and 19.4% for multisatellite observations. So BBFs may contribute more to the transport of magnetic flux, mass, and energy than what was estimated by previous studies based on single satellite observations.

Ion composition and pressure changes in storm time and nonstorm substorms in the vicinity of the near-Earth neutral line

[i] Using CLUSTER/CODIF data from close to ∼ 19 Re in the magnetotail, we have performed a superposed epoch analysis of storm time and nonstorm substorms to determine how the ion composition changes during a substorm. We find that the median O + density and pressure in the plasma sheet are a factor of 5 higher during storm times than during nonstorm times. However, we do not observe significant changes in the composition during a substorm that would indicate that ionospheric outflow is playing a dynamic role in loading the plasma sheet or triggering the substorm at this location. There are differences between the storm time and nonstorm substorms, and it is intriguing to consider whether the composition differences play a role. The storm time substorms exhibit more loading and faster unloading than the nonstorm substorms. In addition, we observe differences in the H + and O + behavior at onset in the storm time substorms that we attribute to the different dynamics of the two ion species at the reconnection site and during the field reconfiguration due to their different gyroradii. The H + density and pressure decrease over the whole energy range at substorm onset, while the O + density and pressure decrease less, and the O + temperature increases. That more O + is left after substorm onset indicates that either the O + is more quickly replenished from O + in the lobes and/or that the more energetic O + , due to its larger gyroradius, is not depleted when the field reconfigures and is accelerated in the thin current sheet.

Electron flat-top distributions around the magnetic reconnection region

[1] Cluster multisatellite observations provide snapshots of electron distributions around the magnetic neutral line. An isotropic flat-top-type electron distribution in phase space is frequently observed around the X line, together with large ion velocities and a Hall quadrupole-like magnetic field inside the hot and tenuous plasma sheet in the magnetotail. The flat-top distributions are also associated with a finite magnetic field in the direction normal to the neutral sheet, and the cross-tail current density is sometimes very small. These results indicate that the flat-top-type distribution is mainly located near the outer boundary of the ion diffusion region in the plasma sheet outflow region, before reaching the pileup region with large normal component of the magnetic field. Simultaneously with the flat-top distributions, strong field-aligned electron beams mainly toward the X line are occasionally observed. This type of beam is mainly observed in the off-equatorial plasma sheet and also appears well inside the plasma sheet. Typical energies of the beam are 4–10 keV, which is comparable to the upper edge of flat-top energy. These highly accelerated electron distributions have a steep decrease in phase space density at the high-energy end, and it is found that they are not correlated with the increase of the higher-energy electrons related to suprathermal acceleration (>30 keV). This result indicates that the electron acceleration processes for the flat-top-type distributions are different from the suprathermal components, both of which are beyond the conventional MHD outflow acceleration and considered to be associated with some kinetic processes.

Cluster observations of an ion‐scale current sheet in the magnetotail under the presence of a guide field

We report on Cluster observations of a thin current sheet interval under the presence of a strong vertical bar B-Y vertical bar during a fast earthward flow interval between 1655 UT and 1703 UT on 17 August 2003. The strong vertical bar B-Y vertical bar in the tail could be associated with a strong IMF vertical bar B-Y vertical bar, but the large fluctuations in B-Y, not seen in the IMF, suggest that a varying reconnection rate causes a varying transport of B-Y-dominated magnetic flux and/or a change in B-Y due to the Hall-current system. During the encounter of the high-speed flow, an intense current layer was observed around 1655: 53 UT with a peak current density of 182 nA/m(2), the largest current density observed by the Cluster four-spacecraft magnetic field measurement in the magnetotail. The half width of this current layer was estimated to be similar to 290 km, which was comparable to the ion-inertia length. Its unique signature is that the strong current is mainly field-aligned current flowing close to the center of the plasma sheet. The event was associated with parallel heating of electrons with asymmetries, which suggests that electrons moving along the field lines can contribute to a strong dawn-to-dusk current when the magnetotail current sheet becomes sufficiently thin and active in a strong guide field case.

Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock

The downstream region of a collisionless quasiparallel shock is structured containing bulk flows with high kinetic energy density from a previously unidentified source. We present Cluster multispacecraft measurements of this type of supermagnetosonic jet as well as of a weak secondary shock front within the sheath, that allow us to propose the following generation mechanism for the jets: The local curvature variations inherent to quasiparallel shocks can create fast, deflected jets accompanied by density variations in the downstream region. If the speed of the jet is super(magneto)sonic in the reference frame of the obstacle, a second shock front forms in the sheath closer to the obstacle. Our results can be applied to collisionless quasiparallel shocks in many plasma environments.

Do BBFs contribute to inner magnetosphere dipolarizations: concurrent cluster and double star observations

[1] We examined the relationship between bursty bulk flow (BBF) events observed by Cluster between -19 R E -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.

Global view of dayside magnetic reconnection with the dusk-dawn IMF orientation: A statistical study for Double Star and Cluster data

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.

The solar origin of corotating interaction regions and their formation in the inner heliosphere, Report of Working Group 1

Corotating Interaction Regions (CIRs) form as a consequence of the compression of the solar wind at the interface between fast speed streams and slow streams. Dynamic interaction of solar wind streams is a general feature of the heliospheric medium; when the sources of the solar wind streams are relatively stable, the interaction regions form a pattern which corotates with the Sun. The regions of origin of the high speed solar wind streams have been clearly identified as the coronal holes with their open magnetic field structures. The origin of the slow speed solar wind is less clear; slow streams may well originate from a range of coronal configurations adjacent to, or above magnetically closed structures. This article addresses the coronal origin of the stable pattern of solar wind streams which leads to the formation of CIRs. In particular, coronal models based on photospheric measurements are reviewed; we also examine the observations of kinematic and compositional solar wind features at 1 AU, their appearance in the stream interfaces (SIs) of CIRs, and their relationship to the structure of the solar surface and the inner corona; finally we summarise the Helios observations in the inner heliosphere of CIRs and their precursors to give a link between the optical observations on their solar origin and the in-situ plasma observations at 1 AU after their formation. The most important question that remains to be answered concerning the solar origin of CIRs is related to the origin and morphology of the slow solar wind.

Mirror mode structures observed in the dawn‐side magnetosheath by Equator‐S

The Equator-S satellite was ideally positioned to make magnetic field observations in the dawn-side magnetosheath, relatively close to the magnetopause. The magnetosheath data were particularly rich in compressional signatures, consistent with mirror mode structures, which occurred during ∼30% of orbits crossing into the magnetosheath. In most, although not all cases, strongly compressive signatures extended up to the magnetopause boundary, with no increase in the underlying magnetic field magnitude on the time scale of ten to thirty minutes. The proximity and character of mirror-like fluctuations near the magnetopause suggest that in the dawn-side magnetosheath the plasma depletion layer (PDL) is of narrower extent than is generally observed closer to the subsolar point, or is absent.