Search

Abstract Processes in the radiation belts under extreme geomagnetic conditions involve the interplay between acceleration and loss processes, both of which can be caused by wave‐particle interactions. Whistler mode waves play a critical role in these interactions, and up to now their properties during extreme events remained poorly sampled and understood. We employ extensive databases of spacecraft observations to specify their distribution. We show that under extreme geomagnetic conditions, lower‐band whistler mode chorus waves have a net effect of accelerating ultra‐relativistic electrons, which results in an increase of fluxes at multi‐MeV energies by several orders of magnitude. During future magnetic superstorms, the radiation levels in the outer zone could therefore experience a substantial increase beyond what has been previously observed during the space age.

Type III radio bursts are generated by electron beams accelerated at reconnection sites in the corona. This study, utilizing data from the Parker Solar Probe’s first 17 encounters, closely examines these bursts down to 13 solar radii. A focal point of our analysis is the near-radial alignment (within 5°) of the Parker Solar Probe, STEREO-A, and Wind spacecraft relative to the Sun. This alignment, facilitating simultaneous observations of 52 and 27 bursts by STEREO-A and Wind respectively, allows for a detailed differentiation of radial and longitudinal burst variations. Our observations reveal no significant radial variations in electron beam speeds, radio fluxes, or exponential decay times for events below 50 solar radii. In contrast, closer to the Sun we noted a decrease in beam speeds and radio fluxes. This suggests potential effects of radio beaming or alterations in radio source sizes in this region. Importantly, our results underscore the necessity of considering spacecraft distance in multispacecraft observations for accurate radio burst analysis. A critical threshold of 50 solar radii emerges, beyond which beaming effects and changes in beam speeds and radio fluxes become significant. Furthermore, the consistent decay times across varying radial distances point toward a stable trend extending from 13 solar radii into the inner heliosphere. Our statistical results provide valuable insights into the propagation mechanisms of type III radio bursts, particularly highlighting the role of scattering near the radio source when the frequency aligns with the local electron plasma frequency.

Abstract Our knowledge about the fine structure of lightning processes at Jupiter was substantially limited by the time resolution of previous measurements. Recent observations of the Juno mission revealed electromagnetic signals of Jovian rapid whistlers at a cadence of a few lightning discharges per second, comparable to observations of return strokes at Earth. The duration of these discharges was below a few milliseconds and below one millisecond in the case of Jovian dispersed pulses, which were also discovered by Juno. However, it was still uncertain if Jovian lightning processes have the fine structure of steps corresponding to phenomena known from thunderstorms at Earth. Here we show results collected by the Juno Waves instrument during 5 years of measurements at 125-microsecond resolution. We identify radio pulses with typical time separations of one millisecond, which suggest step-like extensions of lightning channels and indicate that Jovian lightning initiation processes are similar to the initiation of intracloud lightning at Earth.

Abstract Lightning strikes can be routinely observed by the radio receivers that measure electromagnetic signals in the very low frequency range. The acquired pulses called atmospherics provide valuable details about source lightning discharges and also about the Earth‐ionosphere waveguide where they can propagate thousands of kilometers. The automatic acquisition of the events requires also automatic methods for extraction of atmospherics' details to confirm the observed trends with statistical significance. For this purpose, we have developed a method based on a deep learning approach to automatically obtain the type of atmospherics, their exact time, and the frequency range from the frequency‐time spectrograms. The method that employs convolutional neural networks is designed to be adaptable to scientific needs and provide reliable results according to the requirements on the sensitivity to events, computation performance, and precision of extracted details. The efficiency and specific steps of our method are demonstrated for data recorded by the ELMAVAN‐G instrument. The comprehensive description of the method allows its usage for regular characterization of the ionospheric D‐layer or for other similar applications in the future.

Intense electromagnetic impulses induced by Jupiter’s lightning can produce both low-frequency dispersed whistler emissions and non-dispersed radio pulses. Here, the authors show Jupiter dispersed pulses associated with Jovian lightning that are evidence of low density holes in Jupiter’s ionosphere.

Abstract We analyze lightning initiation process using magnetic field waveforms of preliminary breakdown (PB) pulses observed at time scales of a few tens of microseconds by a broad-band receiver. We compare these pulses with sources of narrow-band very high frequency (VHF) radiation at 60–66 MHz recorded by two separate Lightning Mapping Arrays (LMAs). We find that almost none of the observed PB pulses correspond to geo-located VHF radiation sources, in agreement with previous results and with the hypothesis that processes generating VHF radiation and PB pulses are only weakly related. However, our detailed analysis discovers that individual peaks of strong VHF radiation seen by separate LMA stations correspond surprisingly well to the PB pulses. This result shows that electromagnetic radiation generated during fast stepwise extension of developing lightning channels is spread over a large interval of frequencies. We also show that intense VHF radiation abruptly starts with the first PB pulse and that it is then continuously present during the entire PB phase of developing discharges.

The DEMETER satellite was the first satellite specifically dedicated to the recording of electromagnetic phenomena connected with seismic activity. Almost 6.5 years of measurements provide good opportunities to analyze a unique dataset with global Earth coverage. We present the results of a statistical study of the intensity of very low frequency electromagnetic waves recorded in the upper ionosphere. Robust two-step data processing has been used. The expected unperturbed distribution of the power spectral densities of electromagnetic emissions was calculated first. Then, the power spectral densities measured in the vicinities of earthquakes are compared with the unperturbed distribution and are examined for the presence of uncommon effects related to seismic activity. The statistical significance of the observed effects is evaluated. We confirm the previously reported results of a very small, but statistically significant, decrease in wave intensities a few hours before times of main shocks using this much larger dataset. The wave intensity decrease at a frequency of about 1.7 kHz is observed only during the night and only for shallow earthquakes. This can potentially be explained by increases in the cut-off frequency of the Earth ionosphere waveguide caused by imminent earthquakes.

The acceleration of electrons results in observable electromagnetic waves which can be used for remote sensing. Here, we make use of ∼4 Hz–66 MHz radio waves emitted by two consecutive intense positive lightning discharges to investigate their impact on the atmosphere above a thundercloud. It is found that the first positive lightning discharge initiates a sprite where electrons are accelerated during the exponential growth and branching of the sprite streamers. This preconditioned plasma above the thundercloud is subsequently exposed to a second positive lightning discharge associated with a bouncing-wave discharge. This discharge process causes a re-brightening of the existing sprite streamers above the thundercloud and initiates a subsequent relativistic electron beam.