Polarisation and propagation of lion roars in the dusk side magnetosheath

International audienceWe present observations of "lion roars" obtained in the magnetosheath by the Spectrum Analyser (SA) of the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment aboard Cluster. STAFF-SA calculates, in near real time, the complete auto- and cross-spectral matrix derived from three magnetic and two electric components of the electromagnetic field at 27 frequencies in the range of 8 Hz to 4 kHz. This allows the study of the properties of whistler mode waves and more particularly, the properties of "lion roars", which are intense, short-duration, narrow-banded packets of whistler waves. Their presence is favoured by the magnetic field troughs associated with mirror mode structures. During two short periods of well-defined mirror modes, we study the depth dB/B of the magnetic troughs, and the direction of propagation of the lion roars. During the first period, close to the magnetopause, deep magnetic troughs pass the satellites. Lion roars are then observed to propagate simultaneously in two directions, roughly parallel and anti-parallel to the magnetic field: this probably indicates that during this period, the satellites were within the successive source regions of lion roars. For the second period, far from the magnetopause, the magnetic troughs are less deep. Lion roars are propagating in only one direction, roughly anti-parallel to the magnetic field, suggesting that the source regions are more distant and predominantly on one side of the satellites

Erratum: The solar orbiter radio and plasma waves (RPW) instrument (Astronomy and Astrophysics (2020) 642 (A12) DOI: 10.1051/0004-6361/201936214)

The erratum concerns Fig. 9 entitled "Antenna radio-electrical properties" for which some of the parameters are not correct. The new figure with new parameters is provided in Fig. 1 of this corrigendum. Fig. 1. Corrected Antenna radio-electrical properties. (Figure Presented)

Systematic analysis of equatorial noise below the lower hybrid frequency.

We report results of a systematic analysis of a large number of observations of equatorial noise between the local proton cyclotron frequency and the local lower hybrid frequency. The analysis is based on the data collected by the STAFF-SA instruments on board the four Cluster spacecraft. The data set covers their first two years of measurement in the equatorial magnetosphere at radial distances between 3.9 and 5 Earth radii. Inspection of 781 perigee passages shows that the occurrence rate of equatorial noise is approximately 60%. We identify equatorial noise by selecting data with nearly linearly polarized magnetic field fluctuations. These waves are found within 10° of the geomagnetic equator, consistent with the published past observations. Our results show that equatorial noise has the most intense magnetic field fluctuations among all the natural emissions in the given interval of frequencies and latitudes. Electric field fluctuations of equatorial noise are also more intense compared to the average of all detected waves. Equatorial noise thus can play a non-negligible role in the dynamics of the internal magnetosphere

Initial Results of a Survey of Equatorial Noise Emissions Observed by the Cluster Spacecraft.

International audienceInitial results of a survey of equatorial noise emissions are presented. These plasma wave emissions are observed in the inner magnetosphere close to the geomagnetic equator at frequencies below the local lower hybrid frequency. We use the data recorded by the four Cluster spacecraft during the first 24 months of measurements. The data set was processed in three steps. First, we have selected the data with a nearly linear polarization corresponding to the known properties of the equatorial noise. Second, we have found parameters of a Gaussian model of the frequency-averaged power-spectral density of the selected waves as a function of the geomagnetic latitude. Third, we have analyzed the data as a function of frequency in the latitudinal interval defined by the width of the Gaussian model. Our results show that most intensity peaks of equatorial noise occur within 2o of the magnetic equator and the full-width at half-maximum (FWHM) of these peaks is below 3o in the majority of cases. The most probable frequency of the emissions is between 4 and 5 local proton cyclotron frequencies. The probability density of occurrence of the emissions then slowly decreases toward higher frequencies. Multipoint measurements indicate that the variations of the ratios of amplitudes of the equatorial noise emissions measured on different spacecraft do not increase at spatial scales up to 0.7 Earth radii in the equatorial plane. On the other hand, the variations do increase with time delay between measurements in an interval from tenths to hundreds of minutes

Equatorial noise : statistical study of its localization, and the derived number density.

Results of a statistical study of equatorial noise emissions are presented. These electromagnetic emissions are observed in the inner magnetosphere in the vicinity of the geomagnetic equator at frequencies below the lower hybrid frequency. We use the data recorded by four Cluster spacecraft during years 2001–2003. The data set was processed in three steps. In the first one, we have selected the data with a nearly linear polarization (ellipticity less than 0.2), corresponding to the known properties of the equatorial noise. Secondly, we have found parameters of a Gaussian model of the frequency-averaged power-spectral density of those selected waves as a function of the geomagnetic latitude. Finally, we have analyzed the data in the latitudinal interval defined by the width of the Gaussian model. Our results show that most intensity peaks of equatorial noise occur exactly at the magnetic equator. Incidental deviations are most probably caused by problems in determination of the true magnetic equator, which is shown by using different magnetic field models. We have estimated the plasma number density at the observation points using the cold plasma theory. These estimates are, within experimental errors, close to the values obtained from the spacecraft potential data measured by the EFW instrument

CLUSTER–STAFF search coil magnetometer calibration – comparisons with FGM

The main part of the Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment consists of triaxial search coils allowing the measurements of the three magnetic components of the waves from 0.1 Hz up to 4 kHz. Two sets of data are produced, one by a module to filter and transmit the corresponding waveform up to either 10 or 180 Hz (STAFF-SC), and the second by the onboard Spectrum Analyser (STAFF-SA) to compute the elements of the spectral matrix for five components of the waves, 3 × <i>B</i> and 2 × <i>E</i> (from the EFW experiment), in the frequency range 8 Hz to 4 kHz. <br><br> In order to understand the way the output signals of the search coils are calibrated, the transfer functions of the different parts of the instrument are described as well as the way to transform telemetry data into physical units across various coordinate systems from the spinning sensors to a fixed and known frame. The instrument sensitivity is discussed. Cross-calibration inside STAFF (SC and SA) is presented. Results of cross-calibration between the STAFF search coils and the Cluster Fluxgate Magnetometer (FGM) data are discussed. It is shown that these cross-calibrations lead to an agreement between both data sets at low frequency within a 2% error. By means of statistics done over 10 yr, it is shown that the functionalities and characteristics of both instruments have not changed during this period

Turbulent cascade in the solar wind at kinetic scales and quasi-parallel whistler waves

International audienceThe nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies 1-400 Hz, during five years (2001-2005) when Cluster was in the free solar wind, i.e. not magnetically connected to the Earth's bow-shock.In most of the analyzed time intervals, the fluctuations are non-polarized and they have a general spectral shape between the ion scales and a fraction of electron scales. The intensity of these spectra is well correlated to the ion thermal pressure. These non-polarized fluctuations seem to have a negligible frequency in the solar wind frame, and a wavevector anisotropy kperp>>k||. In the rest ~10% of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of such waves varies between a few seconds and several hours. We analyze in details the long-lived whistler waves, i.e. with a life time longer than five minutes. We find several conditions for the appearance of such waves: (1) a low level of the background turbulence; (2) a low ion thermal pressure; (3) a slow solar wind speed; (4) an electron heat flux Qe>4muW/m2; (5) an electron mean free path larger than 0.5 AU, i.e., a low collisional frequency; (6) a change in the magnetic field direction. When the level of the background turbulence is high, we cannot affirm that whistler waves do not exist: they can be just masked by the turbulence. The six above conditions for the presence of parallel whistlers in the free solar wind are necessary but are not sufficient. When the electron parallel beta factor betae is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability

Turbulent fluctuations at kinetic scales: from coherent structures to quasi-parallel whistler waves

International audienceThe nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies f &#8712; [1,400] Hz, during five years (2001-2005) when Cluster was in the free solar wind. In most of the analyzed time intervals, the fluctuations have quasi-random polarization and they have a general spectral shape between the ion scales and a fraction of electron scales. The intensity of these spectra is well correlated to the ion thermal pressure. These fluctuations seem to have a negligible frequency in the solar wind frame, and a wavevector anisotropy k&#8869; &#8811; k||. Such time intervals are dominated by coherent structures, propagating with a finit velocity in the plasma frame, in the plane perpendicular to the mean magnetic field. In the rest ~ 10% of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of such waves varies between a few seconds and several hours. We analyze in details the long-lived whistler waves, i.e. with a life time longer than five minutes. When the electron parallel beta factor betae|| is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability