[Show abstract][Hide abstract] ABSTRACT: The solar cycle variation of F2 region winter anomaly is related to solar cycle changes in the latitudinal winter-to-summer difference of O/N2. Here we use the National Center for Atmospheric Research–Global Mean Model to develop a concept of why the latitudinal winter-to-summer difference of O/N2 varies with solar cycle. The main driver for these seasonal changes in composition is vertical advection, which is expressed most simply in pressure coordinates. Meridional winds do not change over the solar cycle, so the vertical winds should also not change. The other component of vertical advection is the vertical gradient of composition. Is there any reason that this should change? At solar maximum vertical temperature gradients between 100 and 200 km altitude are strong, whereas they are weak at solar minimum. To maintain the same pressure, the weak vertical temperature gradients at solar minimum must be balanced by weak density gradients and the strong temperature gradients at solar maximum must be balanced by strong density gradients to obtain the same pressure profile. Changes in the vertical density gradients are species dependent: heavy species change more and light species change less than the average density change. Hence, vertical winds act on stronger O/N2 gradients at solar maximum than they do at solar minimum, and a stronger winter-to-summer difference of O/N2 occurs at solar maximum compared with solar minimum.
Journal of Geophysical Research: Space Physics 07/2015; 120(7). DOI:10.1002/2015JA021220 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC), Ionosonde and Global Ultraviolet Imager (GUVI) data have been used to investigate the solar cycle changes in the winter anomaly (the winter anomaly is defined as the enhancement of the F2 peak electron density in the winter hemisphere over that in the summer hemisphere) in the last solar cycle. There is no winter anomaly in solar minimum, and an enhancement of about 50 % in winter over summer ones on the same day of the year at solar maximum. This solar cycle variation in the winter anomaly is primarily due to greater winter to summer differences of [O]/[N2] in solar maximum than in solar minimum, with a secondary contribution from the effects of temperature on the recombination coefficient between O+ and the molecular neutral gas. The greater winter increases in electron density in the northern hemisphere than in the southern hemisphere appear to be related to the greater annual variation of [O]/[N2] in the north than in the south.
Journal of Geophysical Research: Space Physics 06/2014; 119(6). DOI:10.1002/2013JA019552 · 3.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this chapter, the authors perform numerical simulations of equatorial spread F (ESF), assessing the roles of collisional shear instability (CSI) and generalized Rayleigh-Taylor (gRT) and comparing the results with different kinds of space weather diagnostics. The simulation advances the plasma number density and electrostatic potential forward in time by enforcing the constraints of quasineutrality and momentum conservation. Simulations are performed with realistic background conditions including bottomside plasma shear flow and the attendant vertical current. The results are evaluated using computed numerical proxies for ESF observations, including in situ observations from magnetometers on board satellites and remote sensing observations made by coherent/incoherent scatter radar and airglow imagers. The diagnostic codes can be used to validate the numerical simulation, which is evidently able to reproduce the salient characteristics of ESF observed by these.
[Show abstract][Hide abstract] ABSTRACT: Most intercrater plains were emplaced volcanically based on the unit distribution and the stratigraphic relationships between secondary craters and smooth plains.
Lunar and Planetary Science Conference, The Woodlands, TX; 03/2014
[Show abstract][Hide abstract] ABSTRACT: A large pyroclastic deposit on Mercury is strongly depleted in S. Sulfur is a key volatile for explosive volcanism and sulfides play a role in darkening the surface.