Vicki W. Hsu

Effects of the equatorial ionosphere anomaly on the interhemispheric circulation in the thermosphere

We investigate the interhemispheric circulation at the solstices, in order to understand why O/N2 is larger in the northern hemisphere winter than in the southern hemisphere winter. Our studies reveal that the equatorial ionosphere anomaly (EIA) significantly impacts the summer-to-winter wind through plasma-neutral collisional heating, which changes the summer-to-winter pressure gradient, and ion drag. Consequently, the wind is suppressed in the summer hemisphere as it encounters the EIA but accelerates after it passes the EIA in the winter hemisphere. The wind then converges due to an opposing pressure gradient driven by Joule heating in auroral regions and produces large O/N2 at subauroral latitudes. This EIA effect is stronger near the December solstice than near the June solstice because the ionospheric annual asymmetry creates greater meridional wind convergence near the December solstice, which in turn produces larger O/N2 in the northern hemisphere winter than in the southern hemisphere winter.

Formation of the equatorial thermosphere anomaly trough: Local time and solar cycle variations

This paper evaluates the formation and behavior of the equatorial thermosphere anomaly (ETA) trough in neutral temperature and mass density using the National Center for Atmospheric Research thermosphere-ionosphere electrodynamics general circulation model under quiet geomagnetic activity and March equinox conditions. The driving mechanism for the generation of the ETA trough in the model is field-aligned ion drag. In our simulations, during the daytime, field-aligned ion drag on the north-south flanks of the magnetic equator causes a divergence in meridional winds, leading to an upward change in vertical winds, adiabatic cooling, and a reduction in neutral temperature of about 30 K over the magnetic equator near 400 km. This response closely links ETA behavior to variations in the equatorial ionosphere anomaly (EIA) associated with local time and solar cycle. As the EIA begins to disappear in the evening, the processes in the ETA mechanism recede, causing the ETA trough to subside. The ETA trough is not completely eliminated until about after 23:00 LT. In our simulations, the trough becomes more prominent as the solar cycle progresses from low (F10.7=80) to high (F10.7=180), in agreement with observations. The neutral-ion collision frequency (proportional to variations in electron density) controls ETA day-to-night and solar cycle variations, while plasma scale height and gradients in electron number density and plasma temperature produce a secondary structure in ETA local time behavior that varies with solar cycle levels.

D region meteoric smoke and neutral temperature retrieval using the poker flat incoherent scatter radar

Summary form only given. This presentation describes the first measurement of the microphysical properties and variability of meteoric smoke particles (MSPs) at high latitude using the Poker Flat ISR (65.1N, 147.5W). In addition, we present a novel technique for determining height resolved daytime D region neutral temperatures, which takes into account the presence of charged dust. We discuss the temporal/spatial variability and the relation to meteoric input observed and MSP microphysical properties in the polar mesopause region. Further investigation and multi-site measurements in conjunction with global models and neutral wind measurements are required to assess the relative contribution from transport versus local production. This work provides a template for potential use at many other radar sites for the determination of microphysical properties of MSPs and day-time neutral temperature in the D region that show good general agreement with other temperature data during the observing period.

The Great Cold Spot in Jupiter's upper atmosphere

Abstract Past observations and modeling of Jupiters thermosphere have, due to their limited resolution, suggested that heat generated by the aurora near the poles results in a smooth thermal gradient away from these aurorae, indicating a quiescent and diffuse flow of energy within the subauroral thermosphere. Here we discuss Very Large Telescope‐Cryogenic High‐Resolution IR Echelle Spectrometer observations that reveal a small‐scale localized cooling of ~200 K within the nonauroral thermosphere. Using Infrared Telescope Facility NSFCam images, this feature is revealed to be quasi‐stable over at least a 15 year period, fixed in magnetic latitude and longitude. The size and shape of this “Great Cold Spot” vary significantly with time, strongly suggesting that it is produced by an aurorally generated weather system: the first direct evidence of a long‐term thermospheric vortex in the solar system. We discuss the implications of this spot, comparing it with short‐term temperature and density variations at Earth.

Radiation Testing a Very Low-Noise RHBD ASIC Electrometer

We report the results of Single Event Effect (SEE) and Total Integrated Dose (TID) testing of a very-low-noise six-channel electrometer Application Specific Integrated Circuit (ASIC) constructed through the MOSIS service using the ON Semiconductor C5N process. The ASICs were designed using Radiation Hard By Design (RHBD) layout rules, and TID mitigation strategies. This testing provides a verification that the device does not show latch-up behavior, and that performance is not unduly compromised by TID. The SEE testing, using heavy ions, took place at the Single Event Effects Facility at Texas A&M University using the 24.8~MeV/u beam with ions Ar, Kr, and Xe giving a range of Linear Energy Transfer (LET) at the device of 7.5-63.5 MeV cm2/mg. No latch-ups were seen even at an elevated temperature (30C). TID testing using protons was conducted at the Indiana University Cyclotron Facility on the Radiation Effects Research Program RERS2 beamline. Parts were tested to a total dose of 300 krad(Si). As these ASICs are constructed using CMOS technology there will be no Enhanced Low Dose Rate Sensitivity (ELDRS)

New Insights into the Complex Interplay between Drag Forces and its Thermospheric Consequences

Drag forces, ion and viscous, are evaluated as modifiers of global wind and temperature structure in the upper thermosphere, shedding new light on their relative roles in neutral dynamics and energetics. Exploiting the coupling of an ionosphere-thermosphere model, it is discovered that ion and viscous drag forces lead to sustained divergent winds, adjustments in mass, modification of pressure gradients, and a redistribution of the radiatively-forced thermal energy. The interplay between the relative magnitudes of the ion and viscous drag forces and its effect on the ionosphere-thermosphere system has not yet been addressed, and results in diverse behavior in the neutral wind and temperature structures of the upper atmosphere, dependent upon the type of drag acting on the gas. It is found that viscous drag is more efficient in cooling the dayside thermosphere and heating the nightside than the ion drag force in solar maximum and under quiet geomagnetic activity, resulting in a 150 K day-night temperature difference. The ion drag force inhibits this effective day-to-night energy circulation, culminating in a dynamically-induced difference of about 400 K in the day-night temperature difference. It is demonstrated that the resultant wind and thermal structure greatly depends on the type of drag force environment, and a mechanism is introduced whereby ion and viscous drag forces can alter the energy budget of the upper thermosphere system. For example, in solar minimum, viscous drag is significant relative to other forces and more effectively cools the dayside and warms the nightside, thereby reducing the day-night temperature gradient.