Mats André

Sources of Ion Outflow in the High Latitude Ionosphere

Ion composition observations from polar-orbiting satellites in the past three decades have revealed and confirmed the occurrence of a variety of ion outflow processes in the high-latitude ionosphere. These processes constitute a dominant source of ionospheric plasma to the Earths magnetosphere. We review the current state of our observational knowledge on their occurrence, energy, composition, variability, interrelationships, and quantitative contributions to the overall mass input to the magnetosphere. In addition, we identify the prevalent sources and the gaps of our current understanding of these sources.

Fermi and betatron acceleration of suprathermal electrons behind dipolarization fronts

Two dipolarization front (DF) structures observed by Cluster in the Earth midtail region (X(GSM) approximate to -15 R(E)), showing respectively the feature of Fermi and betatron acceleration of sup ...

Theories and Observations of Ion Energization and Outflow in the High Latitude Magnetosphere

A review is given of several mechanisms causing outflow at high latitudes of ionospheric ions to the terrestrial magnetosphere. The upward ion motion along the geomaagnetic field can be divided into several categories, including polar wind, bulk ion outflow in the auroral region, upwelling ions and ion conics and beams. More than one ion energization mechanism can be operating within each category, and a combination of categories is important for the total ion outflow.

Ion energization mechanisms at 1700 km in the auroral region

Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations ...

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,

A statistical study of ion energization mechanisms in the auroral region

More than one mechanism is causing ion energization perpendicular to the geomagnetic field in the auroral region. Observations by the Freja satellite obtained during 20 months at altitudes around 1700 km are used to study the most common O+ energization mechanisms. Ion energization can be due to broadband low-frequency waves, causing resonant energization by interaction at frequencies around the ion gyrofrequency. Energization can also be caused by resonant energization by electromagnetic ion cyclotron waves at about half the proton gyrofrequency, or by emissions near the lower hybrid frequency. About 90% of all O+ heating events and about 95% of the total O+ upflow are caused by ion energization associated with broadband low-frequency waves. The dependence of the different energization types on magnetic local time, magnetic latitude, magnetic activity, and season is studied. We find that the prenoon auroral region is a major source of O+ ions energized by broadband low-frequency waves.

Freja observatons of correlated small-scale density depletions and enhanced lower hybrid waves

Localized density depletions filled with electrostatic waves in the lower hybrid frequency range are commonly observed by the wave instrument on the Freja satellite. We refer to these phenomena by the phenomenological name Lower Hybrid Cavities (LHCs). Typically, the amplitude of the density depletion is a few per cent of the ambient plasma density, and its width is around 50 m. The structures are identified at all magnetic latitudes we have searched (60–75 degrees), and at all local times at the satellite altitude (around 1,700 km). Clear examples are found in regions with fairly low or moderate wave activity, but not where the highest wave amplitudes are encountered. We do not investigate the detailed small-scale correlation between LHCs and ion heating in this letter, but note that up to now, we have no LHC observations in intense ion heating regions.

In situ multi-satellite detection of coherent vortices as a manifestation of Alfvénic turbulence

Turbulence in fluids and plasmas is a ubiquitous phenomenon driven by a variety of sources—currents, sheared flows, gradients in density and temperature, and so on. Turbulence involves fluctuations of physical properties on many different scales, which interact nonlinearly to produce self-organized structures in the form of vortices. Vortex motion in fluids and magnetized plasmas is typically governed by nonlinear equations, examples of which include the Navier–Stokes equation, the Charney–Hasegawa–Mima equations and their numerous generalizations. These nonlinear equations admit solutions in the form of different types of vortices that are frequently observed in a variety of contexts: in atmospheres, in oceans and planetary systems, in the heliosphere, in the Earths ionosphere and magnetosphere, and in laboratory plasma experiments. Here we report the discovery by the Cluster satellites of a distinct class of vortex motion—short-scale drift-kinetic Alfvén (DKA) vortices—in the Earths magnetospheric cusp region. As is the case for the larger Kelvin–Helmholtz vortices observed previously, these dynamic structures should provide a channel for transporting plasma particles and energy through the magnetospheric boundary layers.

Ion cyclotron heating in the dayside magnetosphere

Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high-latitude magnetosphere are used to study ion energization. We find that ions, inc ...

Structure of the separatrix region close to a magnetic reconnection X-line: Cluster observations

We use Cluster spacecraft observations to study in detail the structure of a magnetic reconnection separatrix region on the magnetospheric side of the magnetopause about 50 ion inertial lengths away from the X-line. The separatrix region is the region between the magnetic separatrix and the reconnection jet. It is several ion inertial lengths wide and it contains a few subregions showing different features in particle and wave data. One subregion, a density cavity adjacent to the separatrix, has strong electric fields, electron beams and intense wave turbulence. The separatrix region shows structures even at smaller scales, for example, solitary waves at Debye length scale. We describe in detail electron distribution functions and electric field spectra in the separatrix region and we compare them to a numerical simulation. Our observations show that while reconnection is ongoing the separatrix region is highly structured and dynamic in the electric field even if the X-line is up to 50 ion inertial lengths away.