Upcoming Seminars

Friday, April 8, 2022, 11am (MDT) Via Zoom

Ami Dubois (NRL)

Compressed Current Sheets in the Magnetotail: Importance of the Ambipolar Electric Field

Micro-scale features are now being resolved by NASA’s Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate thin and non-ideal current sheets (i.e. current sheets that cannot be explained by the Harris equilibrium model) in detail and assess their role in magnetic reconnection. We use MMS satellite data to analyze kinetic-scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Our kinetic equilibrium model shows that as a large scale Harris current sheet is compressed, an ambipolar electric field forms and produces velocity shear near the magnetic null, indicating that velocity shear-driven waves can arise in the center of compressed current sheets. The presence and location of shear-driven waves at the center of current sheets is notable for a couple of reasons. First, because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. Second, using MMS wave data we can calculate the anomalous resistivity directly and show that the resistivity is significant, particularly at the magnetic null. Finally, we show that the electron distribution function is non-gyrotropic, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. Our kinetic equilibrium shows that such non-gyrotropic distribution functions can be generated by a quasi-static electric field and does not necessarily arise from wave induced effects.

This work is supported by the US Naval Research Laboratory Base Program.

Friday, April 15, 2022, 11am (MDT), Gamow Tower F931 and Zoom

 Lauren Blum (CU: APS, LASP)

Multipoint measurements to understand the drivers and structure of waves in Earth’s magnetosphere

Wave-particle interactions provide a primary source of scattering and energization of energetic electrons in the Earth’s radiation belts.  Electromagnetic ion cyclotron (EMIC) waves are one intense wave mode observed in the inner magnetosphere that have been shown to contribute to scattering and loss of both energetic electrons and ions.  Using multi-point measurements from the Heliospheric Systems Observatory, including NASA’s Van Allen Probes, we examine the properties and source regions of EMIC waves in the inner magnetosphere, exploring the formation and evolution of plasma structures as they relate to observed wave growth.  We also investigate the effects of this wave structure on energetic electrons in the outer radiation belt.  Finally, looking forward, we will briefly provide a status on the upcoming GTOSat CubeSat mission which will pave the way for future affordable magnetospheric constellation missions.  These studies aid in the understanding of outer radiation belt dynamics and the relationship between electromagnetic waves, global magnetospheric conditions, and solar wind drivers.


Friday, April 22, 2022, 11am (MDT), Gamow F931 and Zoom

Anna Chashkina (Tel Aviv University)

Jet dissipation mechanisms on different scales

These days jets are observed on very different scales: from few gravitational radii to a few kiloparsecs. The variety of scales and variability times tells us that there are few dissipation mechanisms that transform electromagnetic energy into the observed radiation. In my talk I will discuss two possible mechanisms: (1) the energy dissipation in current sheets in the vicinity of the black hole and (2) the current-driven kink instability as a source of energy dissipation on the scales of 10^5-10^7 gravitational radii from the central engine.