Faculty  

Dr. Yi-Jiun Su

Staff   

Dr. Wen Zeng

Graduate Student   

Mr. Lun Ma

(Schematic diagram of the terrestrial magnetosphere. Adapted from http://helios.gsfc.nasa.gov/magneto.jpg)

Because of the Sun's UV radiation, Earth's upper atmosphere is partially (0.1% or less) ionized at altitudes of 70-1500 km. This region is called ionosphere [Space Physics Text Book of Oulu, 2002]. The ionized content of the ionosphere varies with latitude, altitude, solar radiation level, local time, etc.

(Ionosphere plasma density profile. Adapted from http://www.oulu.fi/~spaceweb/textbook/ionosphere.html)

The ionosphere and magnetosphere are closely linked together via magnetic field lines. Magnetospheric electric fields map down to the ionosphere, producing plasma convection, frictional heating and plasma instabilities. Auroral particle precipitation ionizes the high latitude atmosphere also during nighttime, and heat can be conducted from the magnetosphere down to the ionosphere [Space Physics Text Book of Oulu, 1998].

On the other hand, some of the cold ionospheric electrons and ions flow up into the magnetospheric regions above labled as plasmasphere, plasma sheet and tail lobes. The magnetospheric ion composition (especially increased O⁺) influences local and global magnetospheric processes [Space Physics Text Book of Oulu, 1998].

The aurora is one of the most fundamental examples of the interaction between a planet's magnetosphere and ionosphere. They are seen on all of the magnetized planets in our solar system. The magnetosphere-ionosphere coupling phenomena involves many plasma process, such electron acceleration, wave generation, and ion heating.

The mass, energy, and momentum couplings between magnetosphere and ionosphere are explored by the Space Physics group at UTA, using simulation and analysis of data from spacecraft and ground-based instruments.

(Schematic diagram of the DyFK model)

(Contour plots of H⁺ velocity distribution function at different altitudes after auroral energization [eng et al., 2006])

Our research also includes analysis of observations from satellite-borne plasma and field detectors of low-energy plasma transport in the magnetosphere and F-region/topside ionosphere, particularly the Thermal Ion Dynamics Experiment (TIDE) on NASA’s POLAR spacecraft. Current investigations focus on the morphology of O⁺ density troughs within the polar cap at 5000 km altitude [Zeng et al., 2004], and near-simultaneous near-conjunction ion transport measurements made by POLAR/TIDE (at 5000 km altitude) and DMSP (near 840 km altitude) ion detectors at high-latitudes [Zeng et al., 2001], and use of DyFK simulations to model direct spacecraft observations. Here we show evidence of an O⁺ density trough measured by POLAR/TIDE during 12/20/1998, indicated by the absence of counts during the period 9:18-9:27 UT in the spectrogram.

The top panel is a spectrogram of total ion flux vs. UT and spin angle. The bottom panel is a spectrogram of total ion flux vs. UT and energy. O⁺ density trough was observed from 0917 to 0926 UT [Zeng et al., 2004].

 ( Density comparison of DMSP12 (a), DMSP13 (b) with POLAR observations and results of a DyFK simulation on April 13, 1996. The simulation profiles shown were for 0, 12, 24, 36, and 48 minutes following the turnoff of the auroral processes [Zeng et al., 2001].)

A linear gyrofluid code, originally developed in nuclear fusion research, was adapted for use on the Earth's cusp magnetic field line to study Alfvén wave propagation. A test particle scheme is established to load electrons with wave fields generated by the gyrofluid model to study electron behaviors. With this approach, not only were we able to reproduce the energy-time dispersion signature studied extensively by modelers, but we also had the ability to generate the most frequently observed electron bursts. Our simulation results from this effort very closely resemble FAST observations.

(The left and right panels are results from simulations and observation, respectively [Su et al., 2004].)

We have suggested that each of the three types of auroral acceleration regions observed in the Earth's environment by FAST are active MI coupling processes on Jupiter as well. The bright emissions at the Io magnetic footprint are caused by Alfvén-dominated precipitation, while the extended tail emissions in Io's wake are due to electron acceleration from parallel electric field in an upward current region between Jupiter and Io. The downstream electron acceleration is investigated using a static Vlasov code. Results suggest that the proton and the hot electron population in Io's plasma torus control the electron current densities and thus may control the energy flux and the brightness of the aurora downstream from Io's magnetic footprint.

(This illustration is generated by Dr. Robert Ergun at University of Colorado at Boulder.)

An Alfvén eignemode, called the ionospheric Alfvén resonator, can develop between the conducting boundary of Jupiter and the exponential increase in Alfvén velocity on the topside of ionosphere. At Earth, reflections of Alfvén waves give rise to pulsations with a frequency near 0.1-1 Hz. Recently, we suggest that the Alfvén resonator in Jupiter's ionosphere may be associated with the periodicity of S-bursts.

Link to Dr. Yi-Jiun Su's research interests
 

These positions involve numerical simulation and plasma data analysis associated with Magnetosphere-Ionosphere coupling involving: (1) Wave-particle interactions in auroral physics, and/or (2) Ionospheric plasma outflow using a dynamic fluid-kinetic model and analysis of spacecraft and ground-based measurements of ionospheric plasma transport.

Candidates should have a demonstrated capability in programming and graphical display techniques. Applicants must have a Ph.D. degree in Space Physics or a relevant discipline at the time of appointment. Salary and benefits will be competitive. Initial appointment will be for one year, with funding available for extension for one or more further years pending satisfactory performance. For additional information please contact Dr. Yi-Jiun Su at yijiun@uta.edu, telephone 817-272-2460, or Dr. James L. Horwitz at horwitz@uta.edu, 817-272-0991 at the Department of Physics, University of Texas as Arlington, Arlington, Texas 76019.

  817-272-0991

 yijiun@uta.edu

Mailing Address

502 Yates St
Department of Physics
University of Texas at Arlington
Arlington, TX 76019