J. P. Thayer

Altitude variations in the thermosphere mass density response to geomagnetic activity during the recent solar minimum

Accelerometer data from coplanar orbits of Challenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites were used to study the complex altitude and latitude variations of the thermosphere mass density response to geomagnetic activity during 1–10 December 2008 near 09 LT. Helium number densities near 500 km altitude were extracted from the CHAMP and GRACE measurements and clearly show the presence of a winter hemisphere helium bulge. This recent extreme solar minimum indicates that wintertime helium concentrations exceed NRLMSISE-00 model estimates by 30%–70% during quiet geomagnetic activity after adjusting F10.7 input into MSIS. The perturbation in mass density from quiet to active conditions is found to be less enhanced in the winter hemisphere at the higher GRACE altitudes (25%) than at the lower CHAMP altitudes (60%) and is attributed to dynamic behavior in the helium/oxygen transition. The investigation revealed the maximum storm time density perturbation to occur near the He/O transition region with a much weaker maximum near the O/N2 transition region. The altitude of maximum density perturbation occurs where the perturbation in the weighted pressure scale height is equal and opposite to the perturbation in the weighted mean molecular weight scale height. The altitude structure of density scale height perturbation is significantly influenced by the changes in the molecular weight scale height and can account for 50% of the change in mass density scale height in a region correspondingly close to the He/O transition during the 2008 solar minimum period.

Statistical analysis of spatial and temporal variations in atmospheric electric fields from a regional array of field mills

Electric fields and currents of the global electric circuit have been measured at different locations and time periods around the globe, but a long-term analysis from a regional array of electric field mills has never been performed. Kennedy Space Center (KSC) in Florida has an array of over 30 electric field mills that have been continuously operating and archiving data for over 18 years. KSC is also instrumented with many meteorological towers. With this initial statistical analysis of a long-term dataset four unique observations are reported that encompass global as well as local effects. To quantitatively describe the effects, a model to determine near-surface electric fields, incorporating space charge and conductivity perturbations, is developed. Statistical autonomous grouping of the mills using the spatial array demonstrates a greater than 50 V/m spatial variation from coastal to inland mills caused by wind-advected space charge generated near the coast. A temporal analysis identified a strong, globally generated, diurnal signature but only a weak annual signal is found. A realistic limit on conductivity reductions within clouds is estimated by analyzing overhead cloud cover in relation to near-surface electric field strengths. The estimated in-cloud conductivity reduction of approximately 1/3 is similar to observations but appreciably less than values estimated from microphysical cloud estimates. Finally, an enhancement in the electric field at sunrise is statistically well correlated with low wind speeds and high relative humidities. This paper provides a statistical description of local environmental effects on near-surface electric fields by which to base future studies.

Modeling and mechanisms of polar winter upper stratosphere/lower mesosphere disturbances in WACCM

This work focuses on the characteristics and mechanisms of upper stratosphere/lower mesosphere (USLM) disturbances in the polar winter as simulated by a free-running 40 year run of the Whole Atmosphere Community Climate Model (WACCM) version 4. USLM disturbances have been shown to precede the development of every major sudden stratospheric warming (SSW), thus potentially increasing the predictability of SSWs. WACCM4 is shown to internally and spontaneously generate polar USLM disturbances that are consistent with an established USLM climatology based on the UK Meteorological Office stratospheric assimilation. Arctic USLM disturbances in WACCM4 occur on average 2.65 times per winter season; are most frequently generated in the months of December, January, and February; and are preferentially located in the longitude band between 30°E and 120°E, poleward of 40° latitude, all in good agreement with observations. Analysis of composite USLM events corroborates the underlying mechanism responsible for their formation as planetary wave breaking between ~45 km and 65 km that decelerates the mean flow, induces vertical air motion, and causes regions of adiabatic heating in a limited longitude band. These conditions are suitable for the growth of a baroclinic instability at the stratopause level. Using the Trenberth localized three-dimensional Eliassen-Palm flux along with the Charney-Stern criteria, a WACCM4 case study of an independent USLM event implicates baroclinic instability as a critical process in the development of the USLM thermal structure.