ELF whistler events with a reduced intensity observed by the DEMETER spacecraft

International audienceA survey of VLF frequency-time spectrograms obtained by the DEMETER spacecraft (2004-2010, altitude about 700 km) revealed that the intensity of fractional hop whistlers is sometimes significantly reduced at specific frequencies. These frequencies are typically above about 3.4 kHz (second cutoff frequency of the Earth-ionosphere waveguide), and they vary smoothly in time. The events were explained by the wave propagation in the Earth-ionosphere waveguide, and a resulting interference of the first few waveguide modes. We analyze the events whose frequency-time structure is rather similar, but at frequencies below 1 kHz. Altogether, 284 events are identified during the periods with active Burst mode, when high resolution data are measured by DEMETER. The vast majority of events (93%) occurs during the nighttime. All six electromagnetic field components are available, which allows us to perform a detailed wave analysis. An overview of the properties of these events is presented, and their possible origin is discussed

ELF whistler events with a reduced intensity observed by the DEMETER spacecraft

International audienceA survey of VLF frequency-time spectrograms obtained by the DEMETER spacecraft (2004-2010, altitude about 700 km) revealed that the intensity of fractional hop whistlers is sometimes significantly reduced at specific frequencies. These frequencies are typically above about 3.4 kHz (second cutoff frequency of the Earth-ionosphere waveguide), and they vary smoothly in time. The events were explained by the wave propagation in the Earth-ionosphere waveguide, and a resulting interference of the first few waveguide modes. We analyze the events whose frequency-time structure is rather similar, but at frequencies below 1 kHz. Altogether, 284 events are identified during the periods with active Burst mode, when high resolution data are measured by DEMETER. The vast majority of events (93%) occurs during the nighttime. All six electromagnetic field components are available, which allows us to perform a detailed wave analysis. An overview of the properties of these events is presented, and their possible origin is discussed

Quasiperiodic Saturn Auroral Hiss Observed During a Cassini Proximal Orbit

Saturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1‐hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L‐shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam‐generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions

Anticorrelation between whistler occurrence and MLR and QP emissions

International audienceWe investigate a possible influence of lightning-generated whistlers on the occurrence of selected whistler mode emissions in the inner magnetosphere. Specifically, we focus on Magnetospheric Line Radiation (MLR) and Quasiperiodic (QP) emissions, i.e., electromagnetic waves at frequencies of a few kHz with a clear frequency/time modulation of the wave intensity. We use the data from the low altitude satellite DEMETER (2004-2010) to demonstrate that the occurrence of both these emissions exhibits a clear seasonal dependence, with a minimum during the northern summer. We argue that this dependence follows the global distribution of lighting-generated whistlers. Further, we use the whistler occurrence rate data obtained by the neural network on board DEMETER to directly compare whistler occurrence in the presence and in the absence of MLR/QP emissions. It is shown that the whistler occurrence rate as detected by the neural network is significantly lower in the presence of MLR/QP emissions than normally. We discuss whether this is due to a lower efficiency of whistler identification in the presence of MLR/QP emissions or whether this is a real effect suggesting a possible controlling role of whistlers for the occurrence of other electromagnetic emissions

Evaluating Whistler Influence on the VLF Wave Intensity in the Inner Magnetosphere

International audienceLightning activity was shown to be one of the most significant contributors to the overall intensity of electromagnetic waves in the Earth's inner magnetosphere, especially on the nightside and at frequencies on the order of a few kilohertz. In the present study, we combine electromagnetic wave intensity measurements performed by the Van Allen Probes and DEMETER spacecraft with ground-based lightning activity observations by the World Wide Lightning Location Network (WWLLN) to evaluate this contribution in detail. Different spacecraft orbits allow us to compare measurements performed by the Van Allen Probes at various radial distances near the equatorial plane with DEMETER spacecraft measurements at low altitudes. Lightning times and locations obtained by the WWLLN then make it possible to estimate lightning activity levels corresponding to individual data points from spacecraft measurements. The difference between wave intensities measured at the times of high and low lightning activity is evaluated as a function of wave frequency, spacecraft location, and level of geomagnetic activity

Anticorrelation between whistler occurrence and MLR and QP emissions

International audienceWe investigate a possible influence of lightning-generated whistlers on the occurrence of selected whistler mode emissions in the inner magnetosphere. Specifically, we focus on Magnetospheric Line Radiation (MLR) and Quasiperiodic (QP) emissions, i.e., electromagnetic waves at frequencies of a few kHz with a clear frequency/time modulation of the wave intensity. We use the data from the low altitude satellite DEMETER (2004-2010) to demonstrate that the occurrence of both these emissions exhibits a clear seasonal dependence, with a minimum during the northern summer. We argue that this dependence follows the global distribution of lighting-generated whistlers. Further, we use the whistler occurrence rate data obtained by the neural network on board DEMETER to directly compare whistler occurrence in the presence and in the absence of MLR/QP emissions. It is shown that the whistler occurrence rate as detected by the neural network is significantly lower in the presence of MLR/QP emissions than normally. We discuss whether this is due to a lower efficiency of whistler identification in the presence of MLR/QP emissions or whether this is a real effect suggesting a possible controlling role of whistlers for the occurrence of other electromagnetic emissions

Evaluating Whistler Influence on the VLF Wave Intensity in the Inner Magnetosphere

International audienceLightning activity was shown to be one of the most significant contributors to the overall intensity of electromagnetic waves in the Earth's inner magnetosphere, especially on the nightside and at frequencies on the order of a few kilohertz. In the present study, we combine electromagnetic wave intensity measurements performed by the Van Allen Probes and DEMETER spacecraft with ground-based lightning activity observations by the World Wide Lightning Location Network (WWLLN) to evaluate this contribution in detail. Different spacecraft orbits allow us to compare measurements performed by the Van Allen Probes at various radial distances near the equatorial plane with DEMETER spacecraft measurements at low altitudes. Lightning times and locations obtained by the WWLLN then make it possible to estimate lightning activity levels corresponding to individual data points from spacecraft measurements. The difference between wave intensities measured at the times of high and low lightning activity is evaluated as a function of wave frequency, spacecraft location, and level of geomagnetic activity