5 research outputs found
HLâTWiM Empirical Model of HighâLatitude Upper Thermospheric Winds
We present an empirical model of thermospheric winds (Highâlatitude Thermospheric Wind Model [HLâTWiM]) that specifies F region highâlatitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HLâTWiM represents the largeâscale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical highâlatitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 groundâbased stations (FabryâPerot Interferometers and Scanning Doppler Imaging FabryâPerot Interferometers) located at various northern and southern high latitudes and two spaceâbased instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HLâTWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HLâTWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HLâTWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Modelâdata intraâannual comparisons in this study show semiannual oscillationâlike behavior of GOCE winds, rarely observed before in wind data.Key PointsWe developed a comprehensive empirical model of highâlatitude F region thermospheric winds (HLâTWiM)Universal Time variations in highâlatitude winds are stronger in the Southern than Northern HemisphereHLâTWiM provides a necessary benchmark for validating new highâlatitude wind observations and tuning first principal modelsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/1/jgra55363_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/2/jgra55363-sup-0001-Figure_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153588/3/jgra55363.pd
Polar middle atmospheric responses to medium energy electron (MEE) precipitation using numerical model simulations
Abstract
Energetic particle precipitation (EPP) is known to be an important source of chemical changes in the polar middle atmosphere in winter. Recent modeling studies further suggest that chemical changes induced by EPP can also cause dynamic changes in the middle atmosphere. In this study, we investigated the atmospheric responses to the precipitation of medium-to-high energy electrons (MEEs) over the period 2005â2013 using the Specific Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). Our results show that the MEE precipitation significantly increases the amounts of NOâ and HOâ, resulting in mesospheric and stratospheric ozone losses by up to 60% and 25% respectively during polar winter. The MEE-induced ozone loss generally increases the temperature in the lower mesosphere but decreases the temperature in the upper mesosphere with large year-to-year variability, not only by radiative effects but also by adiabatic effects. The adiabatic effects by meridional circulation changes may be dominant for the mesospheric temperature changes. In particular, the meridional circulation changes occasionally act in opposite ways to vary the temperature in terms of height variations, especially at around the solar minimum period with low geomagnetic activity, which cancels out the temperature changes to make the average small in the polar mesosphere for the 9-year period
Responses of nitrogen oxide to highâspeed solar wind stream in the polar middle atmosphere
Abstract
During highâspeed solar wind stream (HSS) events, energetic electrons from the Earthâs inner magnetosphere transfer solar wind energy to the highâlatitude upper atmosphere, which may affect chemical compositions in the region. We conduct a study on the production of nitrogen oxides (NOâ) in the polar middle atmosphere by energetic electron precipitation (EEP) during HSS events in the period of international polar year 2007â2008 northern winter. During this period, the geomagnetic activity was generally quiet and there were no major solar events, which indicates that the EEPs were mostly associated with HSS events. The electron flux immediately increases with the onset of HSS events and remains elevated during the passage of the events. The estimation of the directly produced NOx by EEPs was attempted by using the correlation between NOâ and dynamic tracers such as CO and CHâ. It was found that the direct effect of EEPs on NOâ reaches down to about 55âkm altitude and the amount is estimated to be about 2 ppbv. This result indicates that the variations of polar stratospheric NOâ in winter are mostly associated with dynamical processes such as vertical transport and horizontal mixing. We also found that the middle atmospheric Oâ depletion during HSS events seems to be related to the EEPâinduced NOâ at least in the uppermost stratosphere in the polar region