Wednesday, May 12, 2021 - 3 p.m. to 4 p.m.
Speaker: David Hartley, Assistant Research Scientist, Department of Physics and Astronomy - University of Iowa
Abstract
Cold plasma theory and parallel wave propagation are often assumed when approximating whistler mode magnetic field wave powers from electric field observations. Here, I'll present results that include the wave normal angle in the conversion factor, thus allowing for the accuracy of these previously implemented assumptions to be quantified. When the assumption of parallel propagation is removed, calculated plasmaspheric hiss wave powers are, on average, a good estimate of those observed, whereas calculated chorus wave powers are persistently and systematically underestimated.
Investigation of the ratio between observed and calculated wave powers as a function of frequency and plasma density, reveals a structure consistent with signal attenuation via the formation of a plasma sheath around the EFW spherical double probes instrument. A simple, density-dependent model is developed in order to quantify this effect of variable impedance between the electric field antenna and the plasma interface. Further analysis reveals that the shorter spin axis antennas may measure ‘too much’ electric field at specific densities. This is accounted for in an improved sheath impedance model by introducing a density-dependent function to describe the relative effective length of the probe separation. This factor also allows for the ‘shorting’ effect to be accounted for at low densities. These models are demonstrated to improve agreement between theory and observations, but can only be used to remove sheath effects from the total electric field wave power.
More recently, a technique has been developed to quantify the uncertainty associated with each antenna direction separately. This is beneficial as many spacecraft, including Van Allen Probes and MMS, fly long wire booms in the spin-plane of the spacecraft, but shorter rigid booms along the axis of spin. These two types of antenna can have sheath impedance responses and shorting factors that are very different. A correction to the total electric field wave power is sufficient for correcting electric field wave amplitude. However, it is necessary to consider the response of each antenna direction separately to correct both amplitude, direction, and phase, and thus correct higher order data products such as the Poynting vector. Using time periods of favorable magnetic field, antenna, and wave geometry allow for this to be achieved.
Whilst the sheath impedance models presented here are only valid for the Van Allen Probes EFW instrument, we have recently received funding to apply this same methodology to MMS observations.
This online seminar will take place on Zoom and will require a password to join. If you wish to watch the seminar and have not received a password via email, please contact Robbin McPherson at robbin.mcpherson@unh.edu.
