Fabien Darrouzet

The Whisper Relaxation Sounder Onboard Cluster: A Powerful Tool for Space Plasma Diagnosis1, 2 around the Earth

The WHISPER relaxation sounder that is onboard the four CLUSTER spacecraft has as its main scientific objectives to monitor the natural waves in the 2 kHz–80 kHz frequency range and, mostly, to determine the total plasma density from the solar wind down to the Earths plasmasphere. To fulfill these objectives, the WHISPER uses the two long double sphere antennae of the Electric Field and Wave experiment as transmitting and receiving sensors. In its active working mode, the WHISPER works according to principles that have been worked out for topside sounding. A radio wave transmitter sends an almost monochromatic and short wave train. A few milliseconds after, a receiver listens to the surrounding plasma response. Strong and long lasting echoes are actually received whenever the transmitting frequencies coincide with characteristic plasma frequencies. Provided that these echoes, also called resonances, may be identified, the WHISPER relaxation sounder becomes a reliable and powerful tool for plasma diagnosis. When the transmitter is off, the WHISPER behaves like a passive receiver, allowing natural waves to be monitored. This paper aims mainly at the resonance identification process description and the WHISPER capabilities and performance highlighting.

Trying to bring the magnetopause to a standstill

Satellite observations of the magnetospheric boundary (magnetopause and boundary layer) show it to be a very dynamic place, in part due to boundary motion. We present a straightforward technique for identifying boundary motion and for recovering magnetopause and boundary layer structure in a reference frame that comoves with the boundary, that is, a frame in which it is at a standstill.

A neural network model of three‐dimensional dynamic electron density in the inner magnetosphere

A plasma density model of the inner magnetosphere is important for a variety of applications including the study of wave-particle interactions, and wave excitation and propagation. Previous empirical models have been developed under many limiting assumptions and do not resolve short-term variations, which are especially important during storms. We present a three-dimensional dynamic electron density (DEN3D) model developed using a feedforward neural network with electron densities obtained from four satellite missions. The DEN3D model takes spacecraft location and time series of solar and geomagnetic indices (F10.7, SYM-H, and AL) as inputs. It can reproduce the observed density with a correlation coefficient of 0.95 and predict test data set with error less than a factor of 2. Its predictive ability on out-of-sample data is tested on field-aligned density profiles from the IMAGE satellite. DEN3Ds predictive ability provides unprecedented opportunities to gain insight into the 3-D behavior of the inner magnetospheric plasma density at any time and location. As an example, we apply DEN3D to a storm that occurred on 1 June 2013. It successfully reproduces various well-known dynamic features in three dimensions, such as plasmaspheric erosion and recovery, as well as plume formation. Storm time long-term density variations are consistent with expectations; short-term variations appear to be modulated by substorm activity or enhanced convection, an effect that requires further study together with multispacecraft in situ or imaging measurements. Investigating plasmaspheric refilling with the model, we find that it is not monotonic in time and is more complex than expected from previous studies, deserving further attention.

Cluster observations of reflected EMIC-triggered emission

On 19 March 2001, the Cluster fleet recorded an electromagnetic rising tone on the nightside of the plasmasphere. The emission was found to propagate toward the Earth and toward the magnetic equator at a group velocity of about 200 km/s. The Poynting vector is mainly oblique to the background magnetic field and directed toward the Earth. The propagation angle θk,B0 becomes more oblique with increasing magnetic latitude. Inside each rising tone θk,B0 is more field aligned for higher frequencies. Comparing our results to previous ray tracing analysis we conclude that this emission is a triggered electromagnetic ion cyclotron (EMIC) wave generated at the nightside plasmapause. We detect the wave just after its reflection in the plasmasphere. The reflection makes the tone slope shallower. This process can contribute to the formation of pearl pulsations.

Detailed Properties of Equatorial Noise With Quasiperiodic Modulation

Equatorial noise (EN) emissions are electromagnetic waves observed routinely in the equatorial region of the inner magnetosphere. Although they are typically continuous in time, they sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity, with modulation periods on the order of minutes. We perform a detailed analysis of 118 EN events with the QP modulation. The events are observed preferentially outside the plasmasphere. We determine the times and frequencies of individual QP elements forming the events. Apart from the event modulation period, this allows us to characterize the intensity and the frequency drift of individual QP wave elements. It is shown that the element intensity peaks at the magnetic equator. The modulation period within a single event is usually quite stable, with variations lower than 25% of the median value in the vast majority of cases. The events with shorter modulation periods are typically more intense, and they tend to have larger frequency drifts. These relations resemble the relations formerly revealed for extra low frequency/very low frequency quasiperiodic emissions, suggesting that the origin of the QP modulation of the wave intensity of EN and ELF/VLF emissions might be similar.

Developing a VLF transmitter for LEO satellites: Probing of plasmasphere and radiation belts — The POPRAD proposal

Recent advances in the monitoring of the plasmasphere (e.g. the PLASMON FP7-Space project, http://plasmon.elte.hu, Lichtenberger et al., Space Weather Space Clim. 3 2013, A23 DOI: 10.1051/swsc/2013045) http) makes the continuous monitoring of the plasmasphere possible. But this monitoring capability totally depends on natural and sporadic phenomena, preventing systematic monitoring required for operational Space Weather models and forecasts.

A new perspective on the Earth's plasmasphere

Sixty years ago, Owen Storey concluded that “whistler” radio waves propagate along the geomagnetic field lines through a dispersive medium, which is now known as the plasmasphere. This was confirmed by Gringauzs plasma measurements on Lunik 2 in 1962. In recent years, satellites such as NASAs Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and the European Space Agencys Cluster probes have offered what previous spacecraft could not: a nonlocal perspective. As IMAGE and Cluster have accumulated more than 6 years of observations, researchers from both communities judged the time was ripe for a review at a workshop organized last September by the Belgian Institute for Space Aeronomy (http.7/ www.aeronomy.be/en/workshop/ plasmasphere/overview.htm). This meeting report summarizes some highlights of the meeting.

Towards statistical and empirical models of the distribution of VLF waves at high latitude from the observations of the viking spacecraft

Abstract It is of considerable interest to compile a model of the low frequency electromagnetic wave intensity across the polar caps, in and around the auroral zones, as well as at lower latitudes. Waves are playing a dynamic role in the auroral region and can be used to characterize the level of activity in the magnetosphere. At middle latitudes, Very Low Frequency (VLF) and Extremely Low Frequency (ELF) waves are known to scatter trapped energetic electrons, alter their angular and energy distributions, and eventually cause their precipitation into the Earths atmosphere where it affects the propagation of radio signals. We report here on the development of a data-based model of the electromagnetic power spectral density in the VLF band (10–46 kHz) as surveyed onboard the Swedish Viking spacecraft in the high-latitude region in the northern hemisphere. The data have been sorted into bins according to spatial location and wave frequency. A preliminary statistical model is presented showing the mean electric power spectral density versus magnetic local time and versus invariant latitude at fixed height intervals and for fixed frequency bands within the VLF range. Our data base includes 1162 data intervals, each of 15 min length, during March–December 1986. As a future goal, an empirical model fitting these averages with simple analytical functions may also be developed. This work also aims to associate observed plasma wave characteristics in a particular region of the magnetosphere with geomagnetic conditions. Progress in these directions is reported.