Hypervelocity dust particle impacts observed by the Giotto Magnetometer and Plasma Experiments

We report thirteen very short events in the magnetic field of the inner magnetic pile‐up region of comet Halley observed by the Giotto magnetometer experiment together with simultaneous plasma data obtained by the Johnstone plasma analyzer and the ion mass spectrometer experiments. The events are due to dust impacts in the milligram range on the spacecraft at the relative velocity between the cemetery dust and the spacecraft of 68 km/sec. They are generally consistent with dust impact events derived from spacecraft attitude perturbations by the Giotto camera [Curdt and Keller, private communication]. Their characteristic shape generally involves a sudden decrease in magnetic field magnitude, a subsequent overshoot beyond initial field values and an asymptotic approach to the initial field somewhat reminiscent of the magnetic field signature after the AMPTE releases in the solar wind. These observations give a new way of analyzing ultra‐fast dust particles incident on a spacecraft

Evaluation of magnetic helicity density in the wave number domain using multi-point measurements in space

We develop an estimator for the magnetic helicity density, a measure of the spiral geometry of magnetic field lines, in the wave number domain as a wave diagnostic tool based on multi-point measurements in space. The estimator is numerically tested with a synthetic data set and then applied to an observation of magnetic field fluctuations in the Earth foreshock region provided by the four-point measurements of the Cluster spacecraft. The energy and the magnetic helicity density are determined in the frequency and the wave number domain, which allows us to identify the wave properties in the plasma rest frame correcting for the Doppler shift. In the analyzed time interval, dominant wave components have parallel propagation to the mean magnetic field, away from the shock at about Alfvén speed and a left-hand spatial rotation sense of helicity with respect to the propagation direction, which means a right-hand temporal rotation sense of polarization. These wave properties are well explained by the right-hand resonant beam instability as the driving mechanism in the foreshock. Cluster observations allow therefore detailed comparisons with various theories of waves and instabilities

Titan's magnetic field signature during the Cassini T34 flyby: Comparison between hybrid simulations and MAG data

During the T34 flyby on 19 July 2007, the Cassini spacecraft passed through the magnetic pile-up region at Titan's ramside. The magnetic environment of Titan during this flyby is studied using a three-dimensional hybrid simulation model. This approach treats the electrons of the plasma as a massless, charge-neutralizing fluid, whereas the effects of finite ion gyroradii are taken into account by modeling the ions as individual particles. The simulation results are compared to data collected by the Cassini Magnetometer Instrument. The key features of the measured magnetic field signature have shown to be fully reproducible in the framework of the simulation model. Several signatures in the observed magnetic field can be ascribed to the passage of the Cassini spacecraft through the magnetic barrier upstream of Titan.Fil: Simon, S.. Technische Universitat Braunschweig; AlemaniaFil: Motschmann, U.. Technische Universitat Braunschweig; AlemaniaFil: Kleindienst, G.. Technische Universitat Braunschweig; AlemaniaFil: Glassmeier, K. H.. Technische Universitat Braunschweig; AlemaniaFil: Bertucci, Cesar. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Dougherty, M. K.. Imperial College London; Reino Unid

Field line resonances in discretized magnetospheric models: an artifact study

International audienceFor more than two decades numerical models of the Earth's magnetosphere have been used successfully to study magnetospheric dynamic features such as the excitation of ULF pulsations and the mechanism of field line resonance. However, numerical formulations simplify important properties of the real system. For instance the Alfvén continuum becomes discrete because of a finite grid size. This discretization can be a possible source of numerical artefacts. Therefore a careful interpretation of any observed features is required. Examples of such artefacts are presented using results from a three dimensional dipole model of the magnetosphere, including an inhomogeneous distribution of the Alfvén velocity

Poloidal ULF oscillations in the dayside magnetosphere: a Cluster study

Three ULF wave events, all occurring in the dayside magnetopshere during magnetically quiet times, are studied using the Cluster satellites. The multi-point measurements obtained from Cluster are used to determine the azimuthal wave number for the events by means of the phase shift and the azimuthal separation between the satellites. Also, the polarisation of the electric and magnetic fields is examined in a field-aligned coordinate system, which, in turn, gives the mode of the oscillations. The large-inclination orbits of Cluster allow us to examine the phase relationship between the electric and magnetic fields along the field lines. The events studied have large azimuthal wave numbers (<i>m</i>~100), two of them have eastward propagation and all are in the poloidal mode, consistent with the large wave numbers. We also use particle data from geosynchronous satellites to look for signatures of proton injections, but none of the events show any sign of enhanced proton flux. Thus, the drift-bounce resonance instability seems unlikely to have played any part in the excitation of these pulsations. As for the drift-mirror instability we conclude that it would require an unreasonably high plasma pressure for the instability criterion to be satisfied.<br><br><b>Keywords.</b> Ionosphere (Wave propagation) – Magnetospheric physics (Plasma waves and instabilities; Instruments and techniques

Global Magnetospheric Response to an Interplanetary Shock: THEMIS Observations

We investigate the global response of geospace plasma environment to an interplanetary shock at approx. 0224 UT on May 28, 2008 from multiple THEMIS spacecraft observations in the magnetosheath (THEMIS B and C) and the mid-afternoon (THEMIS A) and dusk magnetosphere (THEMIS D and E). The interaction of the transmitted interplanetary shock with the magnetosphere has global effects. Consequently, it can affect geospace plasma significantly. After interacting with the bow shock, the interplanetary shock transmitted a fast shock and a discontinuity which propagated through the magnetosheath toward the Earth at speeds of 300 km/s and 137 km/s respectively. THEMIS A observations indicate that the plasmaspheric plume changed significantly by the interplanetary shock impact. The plasmaspheric plume density increased rapidly from 10 to 100/ cubic cm in 4 min and the ion distribution changed from isotropic to strongly anisotropic distribution. Electromagnetic ion cyclotron (EMIC) waves observed by THEMIS A are most likely excited by the anisotropic ion distributions caused by the interplanetary shock impact. To our best knowledge, this is the first direct observation of the plasmaspheric plume response to an interplanetary shock's impact. THEMIS A, but not D or E, observed a plasmaspheric plume in the dayside magnetosphere. Multiple spacecraft observations indicate that the dawn-side edge of the plasmaspheric plume was located between THEMIS A and D (or E)

Cluster observes formation of high-beta plasma blobs

Late in a sequence of four moderate substorms on 26 July 2001, Cluster observed periods of a few minutes durations of high-beta plasma events (<i>B</i><10nT, β=2-30), connected with dipolarizations of the magnetic field. Cluster was located near 02:45 MLT, at <i>R</i>=19<i>R<sub>E</sub></i> and at about 5°N GSM. These events began late in the recovery phase of the second and about 5min before onset of the third substorm and lasted for three hours, way beyond the recovery phase of the fourth substorm. The most remarkable observation is that the onset coincided with the arrival of energetic (<i>E</i>~7keV) O<sup>+</sup> ions and energetic electrons obviously from the ionosphere, which tended to dominate the plasma composition throughout the remaining time. The magnetic flux and plasma transport is continuously directed equatorward and earthward, with oscillatory east-west movements superposed. Periods of the order of 5-10min and strong correlations between the magnetic elevation angle and log β (correlation coefficient 0.78) are highly reminiscent of the high-beta plasma blobs discovered with Equator-S and Geotail between 9 and 11<i>R<sub>E</sub></i> in the late night/early morning sector (Haerendel et al., 1999). <P style="line-height: 20px;"> We conclude that Cluster observed the plasma blob formation in the tail plasma sheet, which seems to occur predominantly in the recovery and post-recovery phases of substorms. This is consistent with the finding of Equator-S and Geotail. The origin is a pulsed earthward plasma transport with velocity amplitudes of only several tens of km/s

On the retreat of near-Earth neutral line during substorm expansion phase: a THEMIS case study during the 9 January 2008 substorm

The location of magnetic reconnection in the mid-tail during a substorm was studied in many researches. Here we present multi-point THEMIS observations of a reconnection event in the near-Earth magnetotail during substorm. In this event, THEMIS probes stayed in the near-Earth and mid-tail region aligning along the magnetotail. This allows reconnection evolution to be probed simultaneously from about −10 <I>R</I>_E to −23 <I>R</I>_E down tail. The Hall current related electron streams were observed at the same time by two probes far away from the reconnection site. Before near-Earth reconnection involved the tail lobe magnetic field, the reconnection site was restricted in earthward −23 <I>R</I>_E. When reconnection involved into the tail lobe region, the reconnection site started to retreat gradually