LASP Magnetosphere Seminars

Abstract

Keirra Kernan

The Mystery of Venusian Lightning: How Whistler-mode Wave Origins Differ from Earth to Venus

On Earth, lightning is indicated by three key signatures: optical flashes, broadband radio emission, and whistler-mode waves. Whistler-mode waves in the near Venus space have been observed for over 40 years. The observations of these waves led to the hypothesis that Venus, like Earth, may also produce lightning. Two of the key signatures of lightning from orbit that we see on Earth – optical flashes and broadband radio emissions – are not observed at Venus, leaving whistler-mode waves as the primary evidence for lighting. On its mission to the Sun, Parker Solar Probe (PSP) passed Venus seven times during its Venus Gravity Assists (VGA’s). PSP offered a unique opportunity to analyze the origins of the whistler-mode waves at Venus. The on-board FIELDS search-coil magnetometer (SCM) and electric field power spectral data allowed for a detailed analysis of the wave characteristics and Poynting flux directions. PSP observed whistler-mode waves on VGA 4, one of three fly-bys that pass behind the planet, allowing for these observations. Using the SCM and electric field data, the Poynting flux directions were calculated, revealing propagation towards the planet. VGA 4 allowed for a single observation spanning ~four minutes, during which the spacecraft had one axis of electric field data and two axes of magnetic field data available. The final Venus fly-by, VGA 7, also passed behind the planet, offering a second opportunity to observe and analyze the whistler-mode waves. In this study, the FIELDS SCM and electric field data are used to identify and analyze whistler-mode waves and calculate the Poynting flux directions. Observations of the whistler-mode waves during VGA 7 indicate that planetward propagation is a recurring feature of the near-Venus space. This is consistent with findings from VGA 4, strongly suggesting that the generation of whistler-mode waves at Venus can be caused by mechanisms other than lightning. These findings suggest that, unlike Earth, detecting lightning on Venus may require surface observation.

Lucy Williams

Citizen science identification of EMIC waves using audio-visual analysis: A pilot study

Electromagnetic Ion Cyclotron (EMIC) waves are a critical part of the loss process of high energy radiation belt electrons and ring current ions in the Earth’s magnetosphere, and thus understanding their properties and distributions is crucial for characterizing the near-Earth space environment. Characterizing Heliospheric Resonances in Plasma (CHiRP) is a citizen science project that will use sonification, the technique of listening to numerical data, to identify EMIC waves and look for patterns in their occurrence. Sonification, and more specifically audification, provides a unique approach to data analysis and has proven effective for citizen science projects and engaging the public. Here we present a pilot study of a larger citizen science project where we determine whether different audification processes yield different outcomes, as well as how citizen scientist audio-visual EMIC wave detections compared to more traditional identification methods. This study found that the audio-visual analysis yielded higher EMIC wave detection rates than an automated identification algorithm and that all three methods tested here yielded similar EMIC detection accuracies.

Ben Vaziritabar

Testing for a Filamented Solar Wind Ambipolar Electric Field using Electrostatic Solitary Wave Observations

The solar wind ambipolar electric field, sustained by the electron pressure gradient, integrates to a potential drop of ~50-300 V and contributes substantially to solar wind acceleration. This field is generally treated as smooth and continuous. However, the well-known cross-scale structuring and filamentary nature of solar wind plasma raise the question of whether a measurable fraction of the ambipolar potential is instead carried in kinetic-scale discrete potential drops, possibly associated with electrostatic solitary waves (ESWs). This study uses Parker Solar Probe electric and magnetic field data from 20-85 solar radii to construct three nested ESW catalogs: a broad candidate catalog of ~83,000 detections and progressively stricter filtered subsets of ~29,000 and ~18,000 events. ESW occurrence, normalized by available burst data, rises closer to the Sun, and ESWs are found to systematically occur in the vicinity of solar wind current sheets, linking these structures to solar wind magnetic structure.

Field-aligned potential changes associated with ESWs are estimated using electron temperature and solar wind speed with magnetic geometry quality requirements. Combining these potentials, the summed discrete potential change encountered by a solar wind proton between 25 and 85 solar radii is estimated to be on the same order as the ambipolar potential.

These results indicate that the solar wind ambipolar electric field may possess a measurable discrete component, with the fraction of these discrete potential drops that co-add coherently in the ambipolar direction as the key remaining question.