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Accueil > Recherche > Plasmas Spatiaux > Thématiques scientifiques > Acceleration, radiation and turbulence in terrestrial auroral regions

Acceleration, radiation and turbulence in terrestrial auroral regions

 LPP team

R. Pottelette,M. Berthomier

 Selection of publications

 Introduction

The impact of the solar wind on our planet induces spectacular physical phenomena in auroral regions. In these high-latitude regions, the Earth’s magnetic field connects the low ionosphere, connected to the atmosphere by friction, and the external magnetosphere, dynamically connected to the solar wind (such coupling is generally absent at medium and low latitudes because the magnetic field lines do not stretch far enough into space and remain confined to the internal magnetosphere). As a result of this coupling, solar wind electrons are guided along the auroral field lines from the outer magnetosphere to the low altitudes, while being accelerated to energies of the order of 10 keV through various mechanisms which have not yet been fully elucidated. The acceleration of electrons results in the emission of intense electromagnetic radiation, called terrestrial, or auroral, kilometric radiation (AKR or KTR), and in the precipitation, in the upper atmosphere, of the accelerated electrons.

These energetic electrons then collide with the major constituents of the Earth’s atmosphere, thus transferring their energy to the oxygen and nitrogen atoms, which is then radiated into light emissions known as aurora (boreal in the northern hemisphere). The auroral regions therefore play a special role because of their connection to the external borders of the magnetosphere. The figure below, taken with an UV camera mounted on the satellite Polar, shows that the auroras occur on a narrow belt in latitude (a few degrees) in the shape of an oval surrounding each of the poles Magnetic fields. These regions, called the auroral oval, constitute the feet of the field lines from the external surface of the magnetosphere at the low auroral ionosphere.

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Vue de l’ovale auroral prise par une caméra UV à bord du satellite Polar
(Iowa University)

 The auroras and the electron acceleration

Auroras are the "large-scale" manifestation of the acceleration of electrons. As a result of the coupling imposed by the Earth’s magnetic field, the magnetosphere and the ionosphere react simultaneously, but differently, to the perturbations conveyed by the solar wind :
• Along some field lines, energetic electrons from the outer surface of the magnetosphere are accelerated toward Earth at energies of 10 keV. These electrons collide with the major constituents of the Earth’s atmosphere, thus transferring their energy to the oxygen and nitrogen atoms, which is then radiated into light emissions. The more energetic the electrons are and the more deeply they penetrate into the atmosphere. The composition of the atmosphere varies with the altitude, some colors are preferentially emitted at a given altitude, their diversity produces the magnificent spectacle of total illumination of the sky (See picture).

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Photo de draperies lumineuses dans une aurore
(© Dr. Yamauchi (IRF Kiruna))

• Since the plasma has to remain globally neutral, the ionosphere must react to this increase of negative charges from the magnetosphere. Along other field lines, adjacent to the previous ones, the ionospheric electrons are accelerated towards the confines of the magnetosphere in order to compensate for the additional negative charges. These regions are therefore characterized by the absence of light emissions and appear as dark fringes in auroras.

Simple theories show that in the plasma of the magnetosphere, where binary collisions are absent, the conductivity should be infinite along the magnetic field. In other words, since it cannot be supported by collisions, the parallel electric field should be zero, and the experimental data show that this is not the case. They establish clearly that the acceleration of the electrons takes place in a region where the magnetic field lines cease to be equipotential ; an electric field parallel to the magnetic field lines, E//, directed in the opposite direction to the Earth, develops. In these regions the electrons are accelerated parallel to the mangetic field lines, towards the Earth, while the ions are accelerated in the opposite direction. This simple model has the advantage of describing the observations correctly. This apparent simplicity conceals a puzzling problem : how, in the absence of collision, can be maintained a parallel electric field capable of accounting for the observed acceleration ? Recent high-resolution measurements in acceleration regions seem to provide a first response : turbulence reaches a high amplitude level and is organized into small-scale structures. A parallel electric field, not supported by conventional collisions but by wave-particle interactions within localized nonlinear structures, can emerge from this turbulence.

In fact, satellite data reveal that where turbulence is intense, auroral regions are characterized by the presence of localized parallel electric fields. The figure on the left illustrates the presence of a monopolar static electric field of several hundred mV / m which accelerates the electrons towards the Earth. These localized structures are called "Double Layer", they have a spatial extent along the earth’s magnetic field of several tens of kilometers and are therefore carrying difference of potential of several kV. The "double layer" is always associated with the presence of bipolar electric fields represented in the figure on the right. The latter characterize the strong turbulence structures known as "electron holes" in the phase space ; their spatial extent is very small, of the order of a few hundred meters.

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Structures localisées de champ électrique (de Pottelette et al, 2014)

It should be emphasized that almost all the energy previously stored on a large scale by the solar wind in the magnetosphere is dissipated in terrestrial auroral regions. The characterization of auroral turbulence shows that this dissipation is carried out through small scale structures.

  The radio source « Earth »

The emission of electromagnetic radiation is a direct consequence of the parallel acceleration of electrons and appears as a universal mechanism since similar processes occur in the most remote plasmas of our universe. It is now known that the ionized environments of all the magnetized objects of the solar system (including the Sun) possessing a dense atmosphere emit intense electromagnetic radiations. The Earth does not escape from this rule : seen from the interplanetary space our planet behaves like a radio source that emits a power of 10 to 100 Mega Watts in the frequency domain of the kilometric waves (Frequencies centered around 300 kHz). This radiation propagates to distances of several astronomical units (1 ua 150 million kilometers) from the Earth, and is, in a way, the "messenger" of the acceleration processes occurring around our planet. Its existence was ignored until the years 1965 because it is generated at high altitude and cannot cross the dense layers of the ionosphere. In fact, it was necessary to have satellites operating far above the ionosphere to discover that the Earth was a powerful radio source.

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Spectrogramme du Rayonnement Kilométrique Terrestre
(de Pottelette et al, 2001)

Terrestrial Kilometric Radiation (TKR) is generated at altitudes between 3000 and 15000 km within cavities where the density of the plasma particles is very low (<1 cm-3). The size of these cavities is typically of the order of a few tens of kilometers in the transverse direction with respect to the magnetic field. The figure shows a frequency-time dynamic spectrum measured in the source regions. The RKT forms the high-frequency part of the spectrogram ; one can observe that the most intense emission has roughly the shape of a flattened V. The intensity of the radiation reaches its maximum when the minimum frequency (tip of the V) coincides with the local cyclotron frequency of the electrons. The latter is the natural frequency of rotation of the electrons around the earth’s magnetic field ; It is indicated by a black line on the top figure. The RKT appears as an electromagnetic instability generated at a natural plasma frequency. Approximately 1% of the kinetic energy of the precipitating energetic electrons is thus converted into electromagnetic energy.

The bottom figure shows that at high temporal resolution, the spectrum of the TKR appears to consist of a large number of fine structures deriving in frequency. These observations seem to indicate that the source regions are in fact composed of a multitude of point sources that could result from the presence of localized turbulent structures ; the latter would thus play the role of natural antennas immersed in plasma.

 Conclusions

The circumterrestrial plasmas of the auroral regions constitute a privileged laboratory "easily accessible" to address "in situ" the study of the complex chain of mechanisms leading to the acceleration of charged particles and the emission of an intense coherent electromagnetic radiation. The universality of these mechanisms makes them transposable to the study of distant radiosources, bathing in stellar winds, which are and will remain inaccessible to direct measurement. The French scientific community, among which are members of the LPP space plasma team, is actively associated with various international space experiments designed to unravel the mystery of auroras.

Although auroras have been mentioned in texts dating back thousands of years and the physical mechanisms that gave birth to them can be studied recently by experiments carried on board space vehicles, they still retain much of their mysteries. They represent, in fact, the visual manifestation of turbulent interactions between particles and static and fluctuating electric fields. These interactions occur on very different characteristic spatial scales, ranging from a few hundred meters to several thousand kilometers, hence the difficulty of quantifying them with the only point of observation that constitutes a satellite in space. The previous missions have in fact demonstrated the limitations of the analysis of measurements provided by a single satellite. In such a configuration, it is extremely difficult to determine whether the measured variations in the various parameters correspond to the temporal evolution of the medium or to the displacement of the satellite through inhomogeneous structures. The presence of several appropriately coordinated satellites would remove this uncertainty. The experimental device must be designed in such a way that it is possible - for satellites located at different altitudes - to make "rendezvous" along the same magnetic field line. Such an international mission is currently under study and should allow, in the near future, a more in-depth description of the processes described in this article.


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Directeur de la publication : Pascal Chabert (Directeur)