Propagation of whistler-mode chorus to low altitudes: divergent ray trajectories and ground accessibility

We investigate the ray trajectories of nonductedly propagating lower-band chorus waves with respect to their initial angle θ<sub>0</sub>, between the wave vector and ambient magnetic field. Although we consider a wide range of initial angles θ<sub>0</sub>, in order to be consistent with recent satellite observations, we pay special attention to the intervals of initial angles θ<sub>0</sub>, for which the waves propagate along the field lines in the source region, i.e. we mainly focus on waves generated with &theta<sub>0</sub> within an interval close to 0° and on waves generated within an interval close to the Gendrin angle. We demonstrate that the ray trajectories of waves generated within an interval close to the Gendrin angle with a wave vector directed towards the lower L-shells (to the Earth) significantly diverge at the frequencies typical for the lower-band chorus. Some of these diverging trajectories reach the topside ionosphere having θ close to 0°; thus, a part of the energy may leak to the ground at higher altitudes where the field lines have a nearly vertical direction. The waves generated with different initial angles are reflected. A small variation of the initial wave normal angle thus very dramatically changes the behaviour of the resulting ray. Although our approach is rather theoretical, based on the ray tracing simulation, we show that the initial angle θ<sub>0</sub> of the waves reaching the ionosphere (possibly ground) is surprisingly close - differs just by several degrees from the initial angles which fits the observation of magnetospherically reflected chorus revealed by CLUSTER satellites. We also mention observations of diverging trajectories on low altitude satellites

Statistical investigation of VLF quasiperiodic emissions measured by the DEMETER spacecraft

International audienceWe present a survey of quasiperiodic (QP) ELF/VLF emissions detected onboard the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite (altitude of about 700 km, nearly Sun-synchronous orbit at 10:30/22:30 LT). Six years of data have been visually inspected for the presence of QP emissions with modulation periods higher than 10 s and with frequency bandwidths higher than 200 Hz. It is found that these QP events occur in about 5% of daytime half orbits, while they are basically absent during the night. The events occur predominantly during quiet geomagnetic conditions following the periods of enhanced geomagnetic activity. Their occurrence and properties are systematically analyzed. QP emissions occur most often at frequencies from about 750 Hz to 2 kHz, but they may be observed at frequencies as low as 500 Hz and as high as 8 kHz. Modulation periods of QP events may range from about 10 to 100 s, with typical values of 20 s. Frequency drifts of the identified events are generally positive, but they are lower for events with larger modulation periods. The events are usually limited to higher L values (L > 2). The upper L shell boundary of their occurrence could not be identified using the DEMETER data, but they are found to extend up to at least L ~ 6. The occurrence rate of the events is significantly lower at the longitudes of the South Atlantic anomaly (by a factor of more than 2)

Isolated Electrostatic Structures Observed Throughout the Cluster Orbit: Relationship to Magnetic Field Strength

Isolated electrostatic structures are observed throughout much of the 4 Re by 19.6 Re Cluster orbit. These structures are observed in the Wideband plasma wave instrument's waveform data as bipolar and tripolar pulses. These structures are observed at all of the boundary layers, in the solar wind and magnetosheath, and along auroral field lines at 4.5-6.5 Re. Using the Wideband waveform data from the various Cluster spacecraft we have carried out a survey of the amplitudes and time durations of these structures and how these quantities vary with the local magnetic field strength. Such a survey has not been carried out before, and it reveals certain characteristics of solitary structures in a finite magnetic field, a topic still inadequately addressed by theories. We find that there is a broad range of electric field amplitudes at any specific magnetic field strength, and there is a general trend for the electric field amplitudes to increase as the strength of the magnetic field increases over a range of 5 to 500 nT. We provide a possible explanation for this trend that releates to the structures being Bernstein-Greene-Kruskal mode solitary waves. There is no corresponding dependence of the duration of the structures on the magnetic field strength, although a plot of these two quantities reveals the unexpected result that with the exception of the magnetosheath, all of the time durations for all of the other regions are comparable, wheras the magnetosheath time durations clearly are in a different category of much smaller time duration. We speculate that this implies the structures are much smaller in size.Comment: 24 pages plus 8 figures; paper presented at Spatio-Temporal Analysis and Multipoint Measurements in Space (STAMMS) conference, held in Orleans, France on 12-16 May 200

Quasiperiodic emissions observed by the Cluster spacecraft and their association with ULF magnetic pulsations

International audience[1] Quasiperiodic (QP) emissions are electromagnetic waves at frequencies of about 0.5–4 kHz characterized by a periodic time modulation of the wave intensity, with a typical modulation period on the order of minutes. We present results of a survey of QP emissions observed by the Wide-Band Data (WBD) instruments on board the Cluster spacecraft. All WBD data measured in the appropriate frequency range during the first 10 years of operation (2001–2010) at radial distances lower than 10 R E were visually inspected for the presence of QP emissions, resulting in 21 positively identified events. These are systematically analyzed, and their frequency ranges and modulation periods are determined. Moreover, a detailed wave analysis has been done for the events that were strong enough to be seen in low-resolution Spatio-Temporal Analysis of Field Fluctuations-Spectrum Analyzer data. Wave vectors are found to be nearly field-aligned in the equatorial region, but they become oblique at larger geomagnetic latitudes. This is consistent with a hypothesis of unducted propagation. ULF magnetic field pulsations were detected at the same time as QP emissions in 4 out of the 21 events. They were polarized in the plane perpendicular to the ambient magnetic field, and their frequencies roughly corresponded to the modulation period of the QP events. Citation: Němec , F., O. Santolík, J. S. Pickett, M. Parrot, and N. Cornilleau-Wehrlin (2013), Quasiperiodic emissions observed by the Cluster spacecraft and their association with ULF magnetic pulsations

A Strong Pulsing Nature of Negative Intracloud Dart Leaders Accompanied by Regular Trains of Microsecond-Scale Pulses

We report the first observations of negative intracloud (IC) dart-stepped leaders accompanied by regular trains of microsecond-scale pulses, simultaneously detected by shielded broadband magnetic loop antennas and the radio telescope Low Frequency Array (LOFAR). Four investigated pulse trains occurred during complicated IC flashes on 18 June 2021, when heavy thunderstorms hit the Netherlands. The pulses within the trains are unipolar, a few microseconds wide, and with an average inter-pulse interval of 5–7 μs. The broadband pulses perfectly match energetic, regularly distributed, and relatively isolated bursts of very high frequency sources localized by LOFAR. All trains were generated by negative dart-stepped leaders propagating at a lower speed than usual dart leaders. They followed channels of previous leaders occurring within the same flash several tens of milliseconds before the reported observations. The physical mechanism remains unclear as to why we observe dart-stepped leaders, which show mostly regular stepping, emitting energetic microsecond-scale pulses.</p

The Role of Intense Upper Hybrid Resonance Emissions in the Generation of Saturn Narrowband Emission

Twenty high-inclination ring-grazing orbits occurred in the final period of the Cassini mission. These orbits intercepted a region of intense Z-mode and narrowband (NB) emission (Ye et al., 2010, ) along with isolated, intense upper hybrid resonance (UHR) emissions that are often associated with NB source regions. We have singled out such UHR emission seen on earlier Cassini orbits that also lie near the region crossed by the ring-grazing orbits. These previous orbits are important because Cassini electron phase-space distributions are available and dispersion analysis can be performed to better understand the free energy source and instability of the UHR emission. We present an example of UHR emission on a previous orbit that is similar to that observed during the ring-grazing orbits. Analysis of the observed plasma distribution of the previous orbit leads us to conclude that episodes of UHR emission and NB radiation observed during the ring-grazing orbits are likely due to plasma distributions containing loss cones, temperature anisotropies, and strong density gradients near the ring plane. Z-mode emissions associated with UHR and NB emission can be in Landau resonance with electrons to produce scattering or acceleration (Woodfield et al., 2018, https://doi.org/10.1038/s41467-018-07549-4)