A document by William S Kurth. Click on the document to view its contents.

The Juno Waves instrument measured plasma waves associated with Ganymede’s magnetosphere during its flyby on 7 June, day 158, 2021. Three distinct regions were identified including a wake, and nightside and dayside regions in the magnetosphere distinguished by their electron densities and associated variability. The magnetosphere includes electron cyclotron harmonic emissions including a band at the upper hybrid frequency, as well as whistler-mode chorus and hiss. These waves likely interact with energetic electrons in Ganymede’s magnetosphere by pitch angle scattering and/or accelerating the electrons. The wake is accentuated by low-frequency turbulence and electrostatic solitary waves. Radio emissions observed before and after the flyby likely have their source in Ganymede’s magnetosphere.

In this study, we use the World Wide Lightning Location Network data and investigate properties of more than ninety thousand lightning strokes which hit Northern Europe during an unusually stormy winter 2014/2015. Thunderstorm days with at least two strokes hitting an area of 0.5° x 0.5° occurred 5-13 times per month in the stormiest regions. Such frequency of thunderstorm days is about five times higher than a mean annual number calculated for the same region over winter months in 2008-2017. The number of individual winter lightning strokes was about four times larger than the long-term median calculated over the last decade. In colder months of December, January and February, the mean energy of detected strokes was by two order of magnitude larger than the global mean stroke energy of 1 kJ. We show for the first time that winter superbolts with radiated electromagnetic energies above one mega joule appeared at night and in the morning hours, while the diurnal distribution of all detected lightning was nearly uniform. We also show that the superbolts were often single stroke flashes and that their subsequent strokes never reached MJ energies. The lightning strokes were concentrated above the ocean close to the western coastal areas. All these lightning characteristics suppose an anomalously efficient winter thundercloud charging in the eastern North Atlantic, especially at the sea-land boundary. We found that the resulting unusual production of lightning could not be explained solely by an anomalously warm sea surface caused by a positive phase of the North Atlantic Oscillation and by a starting super El Nino event. Increased updraft strengths, which are believed to accompany the cold to warm transition phase of El Nino, might have acted as another charging driver. We speculate that a combination of both these large-scale climatic events might have been needed to produce observed enormous amount of winter lightning in winter 2014/2015.

Santolík, O., Kolmašová, I., Pickett, J. S., & Gurnett, D. A. (2021). Multi-point Observation of Hiss Emerging from Lightning Whistlers. Journal of Geophysical Research: Space Physics, 126, e2021JA029524. https://doi.org/10.1029/2021JA029524

Isolated breakdown (IBD) process is a lightning phenomenon that was rarely reported in the past. It is characterized by radiowave pulses typical for preliminary breakdown before negative cloud-to-ground flashes, which fail to evolve into return strokes. We identified 128 IBD pulse trains in measurements collected in the Mediterranean by a broadband receiver (0.005 – 37 MHz) in 2015 and 2018. By combining these records with concurrent Lightning Mapping Array measurements of very high frequency radiation (60 – 66 MHz) emitted by in-cloud discharges we investigate the development of each discharge. We identify two scenarios: either the discharges continue to propagate almost horizontally for more than 150 ms (73%), or they disappear sooner, typically within several tens of milliseconds (27%). Using numerical modeling, we verify that a potential barrier inside the thundercloud caused by a strong lower positive charge center could indeed block further propagation of lightning leaders toward the ground.

VLF spectrograms registered at Kannuslehto ground station, after cleaning them from strong sferics, reveal VLF noise suppression by whistlers and whistler echo trains, which consists in significant reduction in the noise spectral power after a strong whistler event. We have found similar effect in the VLF data from Van Allen Probe B taken in the equatorial region on L-shell ~ 3. Detailed analysis of the data shows that the whistler echo train as well as the VLF noise have small wave normal angles. Based on this observation, we limit our analysis to parallel (ducted) whistler wave propagation. The persistence of whistler echo train as well as the VLF noise suggests that in the events under discussion, plasma is unstable in the frequency range corresponding to the observed VLF noise band. In an attempt to explain the effect of VLF noise suppression, we follow up the long-standing idea that relates this effect to the reduction of free energy in the unstable plasma distribution by whistler echo train. To develop this idea into qualitative model, we have studied the motion of energetic electrons, responsible for the noise generation, in the field of ducted whistler echo train. We show that energetic electrons that make the main contribution to the growth rate of VLF noise, during their bounce oscillations in the magnetosphere are subject to multiple resonant impacts from the whistler echo train. These lead to energetic electron diffusion in the phase space, and the corresponding reduction in free energy of the unstable distribution.

Nonlinear wave-particle interactions contribute to the acceleration and precipitation of electrons in the outer radiation belt. Recent simulations and spacecraft observations suggest that oblique whistler-mode chorus can cause loss cone overfilling through nonlinear Landau resonance and thus break the strong diffusion limit of quasilinear theories. Here we show with test-particle simulations that a single element of parallel-propagating chorus can also break the diffusion limit through nonlinear cyclotron resonance, as long as its amplitude remains high. This is due to the strong scattering at low pitch angles caused by individual chorus subpackets. We further demonstrate that the subpacket modulations create a discernible pattern in the precipitating electron fluxes, with peaks correlated with the largest subpackets. Such flux patterns may be connected to weak micropulsations within diffuse auroras.

We study quasi-periodic VLF emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground-based stations on 25 December 2015. Both Van Allen Probes A and B detected quasi-periodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700~km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere, and examine the possibility of their exit to the ground. For the considered event, the wave could exit to the ground in the geomagnetic flux tube with enhanced plasma density, that ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point.

With LOFAR we have been able to image the development of lightning flashes with meter-scale accuracy and unprecedented detail. We discuss the primary steps behind our most recent lightning mapping method. To demonstrate the capabilities of our technique we show and interpret images of the first few milliseconds of two intra-cloud flashes. In all our flashes the negative leaders propagate in the charge layer below the main negative charge. Among several interesting features we show that in about 2 ms after initiation the Primary Initial Leader triggers the formation of a multitude (more than ten) negative leaders in a rather confined area of the atmosphere. From these only one or two continue to propagate after about 30 ms to extend over kilometers horizontally while another may propagate back to the initiation point. We also show that normal negative leaders can transition into an initial-leader like state, potentially in the presence of strong electric fields. In addition, we show some initial breakdown pulses that occurred during the primary initial leader, and even during two “secondary” initial leaders that developed out of stepped leaders.

We present a test particle study of perturbations of a weakly relativistic electron distribution interacting with a rising-tone lower band chorus emission. Trajectories of interacting electrons are traced back in time from the equator to reconstruct the perturbed velocity distribution. The wave field is precalculated from a model based on the nonlinear growth theory and features a realistic subpacket structure. The perturbed distribution reveals a series of stripes of increased and decreased phase space density. This perturbation is associated with the electromagnetic hole structure which exists along the resonance velocity curve of each subpacket. Time-averaging of the final distribution shows that rising-tone lower band chorus emissions produce a sharp decrease in density at low parallel velocities, which might be detectable by spacecraft particle instruments.

The Europlanet H2020 program started on 1/9/2015 for 4 years. It includes an activity to adapt Virtual Observatory (VO) techniques to Planetary Science data called VESPA. The objective is to facilitate searches in big archives as well as sparse databases, to provide simple data access and on-line visualization, and to allow small data providers to make their data available in an interoperable environment with minimum effort. The VESPA system has been hugely improved during the first three years of Europlanet H2020: the infrastructure has been upgraded to describe data in many fields more accurately; the main user search interface (http://vespa.obspm.fr) has been redesigned to provide more flexibility; alternative ways to access Planetary Science data services from VO tools have been implemented; VO tools are being improved to handle specificities of Solar System data, e.g. measurements in reflected light, coordinate systems, etc. Current steps include the development of a connection between the VO world and GIS tools, and integration of Heliophysics, planetary plasmas, and mineral spectroscopy data to support of the analysis of observations. Existing data services have been updated, and new ones have been designed. The global objective is already overstepped, with 42 services open (including ESA’s PSA) and ~15 more being finalized. A procedure to install data services has been documented, and hands-on sessions are organized twice a year at EGU and EPSC; this is intended to favour the installation of services by individual research teams, e.g. to distribute derived data related to a published study. In complement, regular discussions are held with big data providers, starting with space agencies (IPDA). Common projects with PDS have been engaged, with the goal to connect PDS4 and EPN-TAP based on a local data dictionary. In parallel, a Solar System Interest Group has been established in IVOA; the goal is here to adapt existing astronomy standards to Planetary Science. The Europlanet 2020 Research Infrastructure project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654208. [1] Erard et al 2014, Astronomy & Computing 7-8, 71-80. http://arxiv.org/abs/1407.4886

The chorus whistler-mode emission, a major driver of radiation belt electron energization and precipitation, exhibits significant amplitude modulations on millisecond timescales. These subpacket modulations are accompanied by fast changes in the wave normal angle. Understanding the evolution of wave propagation properties inside chorus elements is essential for modeling nonlinear chorus-electron interactions, but the origin of these rapid changes is unclear. We propose that the variations come from propagation inside thin, field-aligned cold plasma enhancements (density ducts), which produce differing modulations in parallel and perpendicular wave magnetic field components. We show that a full-wave simulation on a filamented density background predicts wave vector and amplitude evolution similar to Van Allen Probes spacecraft observations. We further demonstrate that the commonly assumed wide density ducts, in which wave propagation can be studied with ray tracing methods, cannot explain the observed behavior. This indirectly proves the existence of wavelength-scale field-aligned density fluctuations.