Erdal Yiğit

Bio: Erdal Yiğit is an academic researcher from George Mason University. The author has contributed to research in topics: Thermosphere & Gravity wave. The author has an hindex of 27, co-authored 93 publications receiving 2417 citations. Previous affiliations of Erdal Yiğit include University College London & University of California, Berkeley.

Parameterization of the effects of vertically propagating gravity waves for thermosphere general circulation models: Sensitivity study

Abstract: [1] A parameterization of gravity wave (GW) drag, suitable for implementation into general circulation models (GCMs) extending into the thermosphere is presented. Unlike existing schemes, the parameterization systematically accounts for wave dissipation in the upper atmosphere due to molecular viscosity, thermal conduction, ion friction, and radiative damping in the form of the Newtonian cooling. This is in addition to using the commonly employed breaking/saturation schemes, based on either linear Hodges-Lindzen instability criteria or its nonlinear extension to multiple-harmonic spectra. The scheme was evaluated in a series of tests of increasing complexity. In the thermosphere, the simulations suggest that the dissipation competes with the instability caused by amplitude growth, and can seriously alter GW propagation and the associated wave drag. Above the mesopause the GW drag is generally created by harmonics with fast horizontal phase velocities, which under favorable conditions can propagate into the F2 layer. The effects of thermospheric dissipation are more complex than a simple exponential decay of GW fluxes above certain levels. We examine the sensitivity of the GW drag profiles to the variations of the source spectra typically employed in GCMs. These results suggest that GWs can provide strong coupling between the meteorological events in the lower atmosphere and the circulation well above the middle atmosphere.

Introduction to Plasma

Abstract: A plasma is an electrically conducting quasi-neutral gas mainly composed of charged particles that exhibit collective motion. The vast majority of observable universe consists of plasma. Closer to Earth, i.e., in the solar-terrestrial environment, the solar wind, the magnetosphere, and the ionosphere are the most studied plasma environments. Plasma processes are greatly influenced by ambient electromagnetic fields and, in analogy with neutral planetary atmospheres, waves are continuously generated in plasmas and their propagation and interaction influence the structure and the evolution of plasma. A naturally occurring magnetic field environment is the geomagnetic field, which has already been detected four centuries ago. In this introductory chapter, a conceptual discussion of plasma is first presented. Then, various space plasma environments are discussed along with a brief introduction to the history of space research. Finally, some key plasma parameters such as density, gyrofrequency, and temperature, and characteristics (collective motion, quasi-neutrality) are discussed concisely.

Penetration of high latitude electric fields effects tolow latitude during Sundial 1984

Abstract: Electric-field-penetration events have been identified using F-region vertical-drift measurements obtained in the October 6-13, 1984 period by the Jicamarcan incoherent-backscatter radar and corresponding h-prime F measurements from ionosondes at Fortaleza, Cachoeira Paulista, and Dakar. Predictions made using the Rice Convection Model for the pattern, strength, and duration of the low-latitude electric field occurring in response to an increasing high-latitude convection agree with observations. The observed 1-2 h duration of the low-latitude response to decreased convection can be explained by the fossil-wind theory of Richmond (1983).

Mars Global Ionosphere‐Thermosphere Model: Solar cycle, seasonal, and diurnal variations of the Mars upper atmosphere

Abstract: A new Mars Global Ionosphere-Thermosphere Model (M-GITM) is presented that combines the terrestrial GITM framework with Mars fundamental physical parameters, ion-neutral chemistry, and key radiative processes in order to capture the basic observed features of the thermal, compositional, and dynamical structure of the Mars atmosphere from the ground to the exosphere (0–250 km). Lower, middle, and upper atmosphere processes are included, based in part upon formulations used in previous lower and upper atmosphere Mars GCMs. This enables the M-GITM code to be run for various seasonal, solar cycle, and dust conditions. M-GITM validation studies have focused upon simulations for a range of solar and seasonal conditions. Key upper atmosphere measurements are selected for comparison to corresponding M-GITM neutral temperatures and neutral-ion densities. In addition, simulated lower atmosphere temperatures are compared with observations in order to provide a first-order confirmation of a realistic lower atmosphere. M-GITM captures solar cycle and seasonal trends in the upper atmosphere that are consistent with observations, yielding significant periodic changes in the temperature structure, the species density distributions, and the large-scale global wind system. For instance, mid afternoon temperatures near ∼200 km are predicted to vary from ∼210 to 350 K (equinox) and ∼190 to 390 k (aphelion to perihelion) over the solar cycle. These simulations will serve as a benchmark against which to compare episodic variations (e.g., due to solar flares and dust storms) in future M-GITM studies. Additionally, M-GITM will be used to support MAVEN mission activities (2014–2016).

Generation of large-scale gravity waves and neutral winds in the thermosphere from the dissipation of convectively generated gravity waves

Abstract: phase speeds of cH � 480–510 m/s, density perturbations as large as jr 0 /r j� 3.6–5% at z = 400 km, relative [O] perturbations as large as � 2–2.5% atz = 300 km, and total electron content perturbations as large as � 8%. This transfer of momentum from local, relatively slow, small scales at the tropopause to global, fast, large scales in the thermosphere is independent of geomagnetic conditions. The various characteristics of these large-scale waves may explain observations of LSTIDs at magnetically quiet times. We also find that this body force creates a localized ‘‘mean’’ horizontal wind in the direction of the body force. For the plume at 2120 UT, the wind is southward with an estimated maximum of vmax �� 400 m s � 1 that is dissipated after � 4h . We also find that the induced body force direction varies throughout the day, depending on the winds in the lower thermosphere.