Iannis Dandouras

Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions

Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma. Citation: Pedersen, A., et al. (2008), Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions,

The Dust Halo of Saturn's Largest Icy Moon, Rhea

Saturns moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturns magnetosphere. The Cassini spacecrafts in situ observations reveal that energetic electrons are depleted in the moons vicinity. The absence of a substantial exosphere implies that Rheas magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.

MAGNETOSPHERIC AND PLASMA SCIENCE WITH CASSINI-HUYGENS

Magnetospheric and plasma science studies at Saturn offer a unique opportunity to explore in-depth two types of magnetospheres. These are an ‘induced’ magnetosphere generated by the interaction of Titan with the surrounding plasma flow and Saturns ‘intrinsic’ magnetosphere, the magnetic cavity Saturns planetary magnetic field creates inside the solar wind flow. These two objects will be explored using the most advanced and diverse package of instruments for the analysis of plasmas, energetic particles and fields ever flown to a planet. These instruments will make it possible to address and solve a series of key scientific questions concerning the interaction of these two magnetospheres with their environment.The flow of magnetospheric plasma around the obstacle, caused by Titans atmosphere/ionosphere, produces an elongated cavity and wake, which we call an ‘induced magnetosphere’. The Mach number characteristics of this interaction make it unique in the solar system. We first describe Titans ionosphere, which is the obstacle to the external plasma flow. We then study Titans induced magnetosphere, its structure, dynamics and variability, and discuss the possible existence of a small intrinsic magnetic field of Titan.Saturns magnetosphere, which is dynamically and chemically coupled to all other components of Saturns environment in addition to Titan, is then described. We start with a summary of the morphology of magnetospheric plasma and fields. Then we discuss what we know of the magnetospheric interactions in each region. Beginning with the innermost regions and moving outwards, we first describe the region of the main rings and their connection to the low-latitude ionosphere. Next the icy satellites, which develop specific magnetospheric interactions, are imbedded in a relatively dense neutral gas cloud which also overlaps the spatial extent of the diffuse E ring. This region constitutes a very interesting case of direct and mutual coupling between dust, neutral gas and plasma populations. Beyond about twelve Saturn radii is the outer magnetosphere, where the dynamics is dominated by its coupling with the solar wind and a large hydrogen torus. It is a region of intense coupling between the magnetosphere and Saturns upper atmosphere, and the source of Saturns auroral emissions, including the kilometric radiation. For each of these regions we identify the key scientific questions and propose an investigation strategy to address them.Finally, we show how the unique characteristics of the CASSINI spacecraft, instruments and mission profile make it possible to address, and hopefully solve, many of these questions. While the CASSINI orbital tour gives access to most, if not all, of the regions that need to be explored, the unique capabilities of the MAPS instrument suite make it possible to define an efficient strategy in which in situ measurements and remote sensing observations complement each other.Saturns magnetosphere will be extensively studied from the microphysical to the global scale over the four years of the mission. All phases present in this unique environment — extended solid surfaces, dust and gas clouds, plasma and energetic particles — are coupled in an intricate way, very much as they are in planetary formation environments. This is one of the most interesting aspects of Magnetospheric and Plasma Science studies at Saturn. It provides us with a unique opportunity to conduct an in situ investigation of a dynamical system that is in some ways analogous to the dusty plasma environments in which planetary systems form.

Observations of an active thin current sheet

We analyze observations of magnetotail current sheet dynamics during a substorm between 2330 and 2400 UT on 28 August 2005 when Cluster was in the plasma sheet at [-17.2, -4.49, 0.03] R-E (GSM) wit ...

A statistical study of hot flow anomalies using Cluster data

Abstract Hot flow anomalies (HFAs) are studied using observations of the RAPID suprathermal charged particle detector, the FGM magnetometer, and the CIS plasma detector aboard the four Cluster spacecraft. Previously, we studied several specific features of tangential discontinuities on the basis of Cluster measurements in February–April 2003. In this paper, we confirm the following results: the angle between the Sun direction and the tangentional discontinuity (TD) normal is larger than 45° during HFAs, the magnetic field directional change is large. We then present evidence for a new necessary condition for the formation of HFAs, that is, the solar wind speed is significantly ( about 200 km / s or Δ M f = 2.3 ) higher than the long-term average. The existence of this condition is also confirmed by simultaneous ACE MAG and SWEPAM solar wind observations at the L1 point 1.4 million km upstream of the Earth. The results are compared with recent hybrid simulations.

A global study of hot flow anomalies using Cluster multi-spacecraft measurements

Hot flow anomalies (HFAs) are studied using ob- servations of the magnetometer and the plasma instrument aboard the four Cluster spacecraft. We study several spe- cific features of tangential discontinuities on the basis of Cluster measurements from the time periods of February- April 2003, December 2005-April 2006 and January-April 2007, when the separation distance of spacecraft was large. The previously discovered condition (Facsk´ o et al., 2008) for forming HFAs is confirmed, i.e. that the solar wind speed and fast magnetosonic Mach number values are higher than average. Furthermore, this constraint is independent of the Schwartz et al. (2000)s condition for HFA formation. The existence of this new condition is confirmed by simultane- ous ACE magnetic field and solar wind plasma observations at the L1 point, at 1.4 million km distance from the Earth. The temperature, particle density and pressure parameters observed at the time of HFA formation are also studied and compared to average values of the solar wind plasma. The size of the region affected by the HFA was estimated by us- ing two different methods. We found that the size is mainly influenced by the magnetic shear and the angle between the discontinuity normal and the Sun-Earth direction. The size grows with the shear and (up to a certain point) with the an- gle as well. After that point it starts decreasing. The results are compared with the outcome of recent hybrid simulations.

Energy conversion regions as observed by Cluster in the plasma sheet

In this article we present a review of recent studies of observations of localized energy conversion regions (ECRs) observed by Cluster in the plasma sheet at altitudes of 15–20RE. By examining var ...

Energy deposition processes in titan's upper atmosphere and its induced magnetosphere

Most of Titans atmospheric organic and nitrogen chemistry, aerosol formation, and atmospheric loss are driven from external energy sources such as Solar UV, Saturns magnetosphere, solar wind and galactic cosmic rays. The Solar UV tends to dominate the energy input at lower altitudes of approximately 1100 km but which can extend down to approximately 400 km, while the plasma interaction from Saturns magnetosphere, Saturns magnetosheath or solar wind are more important at higher altitudes of approximately 1400 km, but the heavy ion plasma [O(+)] of approximately 2 keV and energetic ions [H(+)] of approximately 30 keV or higher from Saturns magnetosphere can penetrate below 950km. Cosmic rays with energies of greater than 1 GeV can penetrate much deeper into Titans atmosphere with most of its energy deposited at approximately 100 km altitude. The haze layer tends to dominate between 100 km and 300 km. The induced magnetic field from Titans interaction with the external plasma can be very complex and will tend to channel the flow of energy into Titans upper atmosphere. Cassini observations combined with advanced hybrid simulations of the plasma interaction with Titans upper atmosphere show significant changes in the character of the interaction with Saturn local time at Titans orbit where the magnetosphere displays large and systematic changes with local time. The external solar wind can also drive sub-storms within the magnetosphere which can then modify the magnetospheric interaction with Titan. Another important parameter is solar zenith angle (SZA) with respect to the co-rotation direction of the magnetospheric flow. Titans interaction can contribute to atmospheric loss via pickup ion loss, scavenging of Titans ionospheric plasma, loss of ionospheric plasma down its induced magnetotail via an ionospheric wind, and non-thermal loss of the atmosphere via heating and sputtering induced by the bombardment of magnetospheric keV ions and electrons. This energy input evidently drives the large positive and negative ions observed below approximately 1100 km altitude with ion masses exceeding 10,000 daltons. We refer to these ions as seed particles for the aerosols observed below 300 km altitude. These seed particles can be formed, for example, from the polymerization of acetylene (C2H2) and benzene (C6H6) molecules in Titans upper atmosphere to form polycyclic aromatic hydrocarbons (PAH) and/or fullerenes (C60). In the case of fullerenes, which are hollow spherical carbon shells, magnetospheric keV [O(+)] ions can become trapped inside the fullerenes and eventually find themselves inside the aerosols as free oxygen. The aerosols are then expected to fall to Titans surface as polymerized hydrocarbons with trapped free oxygen where unknown surface chemistry can take place.

Dispersion analysis of ULF waves in the foreshock using cluster data and the wave telescope technique

Cluster provides us with a unique possibility to study ULF waves. We analyze ULF wave activity in the near-Earth upstream solar wind. Using Cluster as a wave telescope we investigate in detail wave propagation directions and wave numbers for various frequencies, obtaining, for the first time, three dimensional dispersion relations experimentally. After Doppler shift correction, we find that the dispersion relations are not linear and the waves are propagating in the sunward direction in the plasma rest frame. Comparison of the experimentally derived dispersion relation with that one for a beam plasma system shows good agreement. We suggest that the ULF waves in the foreshock are generated by a proton population backstreaming from the shock.

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.