SM23B-1398
A Study of Relationships between Two-Category Parallel Fast Flows and Nightside Auroral Power
In the mganetotail perpendicular flows are generally believed to play an important role in the transport of magnetic flux, and some perpendicular fast flows can cause major auroral activities. However, the influence of parallel fast flows on auroral activities has not been studied extensively. In this study, we investigate relationships between nightside auroral activities and parallel fast flows in the plasma sheet for the period of 1997 and 1998. We first identify fast flow events from Geotail data using a criterion of |Vpara,x| > 300 km/s, where Vpara,x is the X component of the ion velocity parallel to the ambient magnetic fields. Then we quantitatively estimate the rate of change of auroral power over a region of 60-80 degrees MLAT and 2000-0400 hr MLT from Polar Ultraviolet Imager images for each event. We divide all the parallel fast flow events into two categories. Category I is the parallel fast events with the occurrence of the perpendicular fast flows and Category II is without the occurrence of the perpendicular fast flows. It is found that Category I have a better association with the increasing nightside auroral power than Category II. The relationship between Category II and nightside auroral activities is not apparent. It is also found that the rate of change of auroral power associated with the parallel fast flows is independent of the radial distance in the noon-midnight meridian.
SM23B-1399
Occurrence characteristics of MF auroral radio emissions observed in Iceland
In order to study the generation and propagation processes of the MF auroral radio emissions (auroral roar and MF burst), a radio spectrograph ststem was installed at Husafell in Iceland (invariant latitude: 65.3deg). Within the observation period since late 2006, several MF auroral radio emissions have been detected. Based on our observation results, the polarization character of the MF bursts was consistent to the previous result by Shepherd et al. [1997], and identified that 3fce roar is L-O mode wave. It is suggested that auroral roar appears during magnetic storm recovery phase while MF burst is associated with aurora‚Ś breakup. We compared the 3fce roar detected on May 23, 2007 with auroral image observed by the Polar/UVI and auroral particle observed by the DMSP/SSJ4 in the southern hemisphere. These image data show that the auroral oval has multilayer structure and the Husafell station is located between enhanced layers. In a region with relatively low energy electrons' precipitation (several 100 eV), MF radio waves can propagate to the ground because there is weak ionization in the D and E regions. Therefore, it is suggested that the auroral roar is generated by low energy electrons precipitating near the observation site and propagates downward to the ground. To understand the mechanism, the energy spectrum of precipitating electrons is the key parameter not only for plasma instability but also for ionization of low altitude region. This observation result shows important evidence. On the hypothesis that the frequency of auroral roar coincides with harmonics of fce in the source region, the observation frequencies of 3.1-3.5 MHz correspond to the altitude range of 550-880 km, and electron density should be more than 105/c.c. in this altitude range. It is less likely to occur in the polar ionosphere. When we introduce alternative hypothesis that the plasma instability occurs at (n+1/2)fce ESCH waves, the estimated altitude of the source region becomes 250-350 km. Thus, we have to reexamine the proposed generation mechanism of the MF auroral radio emissions.
SM23B-1400
Relationships between field-aligned currents and particle precipitation
The field-aligned currents and particle precipitation play important roles in the magnetosphere-ionosphere coupling. The polarities and densities of large scale field-aligned currents from the DMSP magnetometer data 1983-2005 were determined using an automated algorithm. The particle precipitations often have characteristics that reflect their source regions in the magnetosphere, which led to the development of the automatic region identification schemes. The particle precipitation regions from the DMSP SSJ4 particle data were also determined from 1983-2005. The relationships between the field-aligned currents and the particle precipitation regions on the dayside were explored and identified. In the dayside open region, the solar wind-magnetosphere coupling issues were investigated.
SM23B-1401
Laboratory Experiments on Dispersion and Damping of Kinetic and Inertial Shear Alfven Waves
Experiments to test the dispersion relation of shear Alfvén waves as a function of perpendicular wave number were performed using the LArge Plasma Device (LAPD) at UCLA. The waves are launched using a 48-element antenna which can be "tuned" for the perpendicular structure of the Alfvén wave, allowing for good control over the perpendicular wave numbers of the wave. Amplified magnetic search coil probes placed along the length of the chamber are used to measure the wave across the center of the wave pattern. The measured signals are processed to separate the signal into perpendicular wave number components. Signal components at two different locations for the same wave number are cross-correlated and the propagation time with the probe separation leads to the parallel phase velocity for each wave number. Relative amplitudes of each wave number between two probes are also found and allow the damping to be determined. The parallel phase velocity and the damping rate are then compared to the theoretical dispersion relation and damping for both the kinetic and inertial cases. In both the kinetic and inertial limits, the theoretical dispersion relation and damping curves are taken from warm-plasma theory with the inclusion of charged-particle collisional effects via a Krook collision operator. Experimental results in the inertial limit show good agreement with warm-plasma theory when electron-ion collisions are included. In the kinetic limit, good agreement with warm-plasma theory can be achieved only if the effects of electron-ion collisions are reduced significantly. These experimental results will be shown, and the apparent difference in electron-ion collision effectiveness for the two shear Alfvén wave limits will be discussed.
SM23B-1402
Testing Conventional Assumptions for High-Latitude Electrodynamics by Using a Self- Consistent M-I Coupling Model
Two conventional assumptions have been widely used in the study of high-latitude electrodynamics of the M-I system. One is that the field-aligned currents are solely closed by the Pedersen currents in the ionosphere; the Hall currents are basically divergence-free and their contribution to the field-aligned currents can be ignored. The other is that the ground magnetic disturbances associated with the currents in the ionosphere are caused by the Hall currents only and the combined ground magnetic effects of the field-aligned currents and Pedersen currents are negligible. By using both an M-I coupling model, in which the M-I coupling electrodynamics is fully self- consistent via an Alfven wave approach, and a global magnetic inversion model, we quantitatively tested the validity of the two assumptions. It was found that in disturbed conditions, especially during substorms, these two assumptions can become invalid. Our results show that in the substorm break-up regions, the field-aligned currents are dominantly closed by the Hall currents with small-scale channel structures and sharp spatial gradients. These structured Hall current closures are associated with the rotation of convection field or a significantly distorted localized convection pattern. Our results also show that during substorms, the combined Bz magnetic effects of the Pedersen and field-aligned currents are still negligible, but the By and Bx magnetic effects of them increase significantly and become comparable with those of the Hall currents. These results indicate that care must be exercised when using the two conventional assumptions in model studies and data interpretations of the high-latitude electrodynamics.
SM23B-1403
Generation of Lower Hybrid Waves in an Oxygen Dominated Plasma
A previous work considered the generation of ULF waves in the inner Magnetosphere due to a heavy ion ring distribution such as Lithium, Cesium or Barium [Ganguli et al., 2007]. We extend this analysis by considering the generation of Lower-hybrid waves in an Oxygen dominated plasma at R~600km due to a Barium ion ring distribution. In this situation it is possible to generate ion-Bernstein modes or Lower-hybrid waves. We demonstrate the criteria in which Lower-hybrid waves are produced. The growth rate is found to be strongly dependent on the ion thermal velocity. For a broad range of parameters the growth rate γ > ØmegaBa so that the Barium ions are considered to be unmagnetized which simplifies the analysis. The analysis is compared with space experiments done several decades ago. Furthermore we investigate the generation of whistler waves and their propagation into the radiation belts. Ganguli, G., L. Rudakov, M. Mithaiwala, and K. Papadopoulos (2007), Generation and evolution of intense ion cyclotron turbulence by artificial plasma cloud in the magnetosphere, J. Geophys. Res., 112, A06231, doi:10.1029/2006JA012162. * Supported by ONR.
SM23B-1404
Plasma Sheet and Diffuse Aurora Structuring During Severe Magnetic Storms
We present analysis of comprehensive simulations of a severe magnetic storm on March 31, 2001 with the Rice Convection Model. When a time-dependent storm-time magnetic field model and time variations of the plasma sheet temperature and density are used as inputs to the model (both driven by the solar wind and IMF parameters), the inner magnetosphere is found to undergo severe perturbations, including strong ring current injection, supersonic ionospheric plasma drifts, and highly structured transport of plasma sheet particles into the ring current region. We analyze the properties of this transport in terms of interchange instability, and present signatures in the diffuse aurora that may be related to observed structuring in the optical emissions.
SM23B-1405
Use of Auroral Processes in Spacecraft Propulsion: A VASIMR VX-100 Status Report
Plasma physics has found an increasing range of practical industrial applications, including the development of
electric spacecraft propulsion systems. One of these systems, the Variable Specific Impulse Magnetoplasma
Rocket (VASIMR) engine, applies several important physical processes occurring in the magnetosphere. These
processes include the mechanisms involved in the ion acceleration and heating that occur in the Birkeland
currents of an auroral arc system. Auroral current region processes that are applied in VASIMR include lower
hybrid heating, parallel electric field acceleration and ion cyclotron acceleration. This paper will focus on using
two physics demonstration model VASIMR's to study ion cyclotron heating (ICRH). Prior to
VASIMR, laboratory simulation of electromagnetic ion cyclotron wave heating has been difficult owing to the
difficulty in obtaining efficient antenna coupling for this mode and to the fact that the ions involved only pass
through the acceleration region once. The VX-50 and VX-100 VASIMR's use(d) a helicon
antenna with 20 kW of power to generate plasma. Both devices then use(d) an RF booster stage that uses left
hand polarized slow mode waves launched from the high field side of the resonance. The VX-50 used 2 to 4 MHz
waves with 30 kW of power. The VX-100 operates at ~500 kHz, with up to 100 kW of available ICRH power.
This paper will summarize results from high power ICRH experiments performed on the VX-50 using deuterium,
neon and argon plasma during 2006 and will present preliminary results from the VX-100. Emphasis will be
placed on results obtained since the last Fall meeting We have demonstrated ion cyclotron acceleration of a
dense (>1019/m
SM23B-1406
Direction of Arrival Measurements of Auroral Medium Frequency Burst Radio Emissions at Toolik Lake, AK
MF burst is an impulsive radio emission of auroral origin detected by ground-based instruments approximately between 1,300 and 3,700 kHz, and associated with substorm onsets. Its exact generation mechanism is unknown, though it has been speculated that it arises from mode conversion radiation. To discover the generation mechanism and the relation of MF burst to auroral processes, Dartmouth has deployed radio interferometers in Alaska, Northern Canada, Greenland, and Antarctica, including a three-element interferometer deployed at Toolik Lake Field Station in Alaska in 2006. This instrument measured spectra, amplitudes, and directions of arrival (DOA's) of over 47 MF burst events occurring between November 30, 2006 and May 26, 2007. These represent the first DOA measurements ever reported for the impulsive MF burst phenomenon. Preliminary analysis shows that the events originated from a wide range of directions in the sky, with all azimuths represented in the distribution of DOA's. The DOA of each individual event is well-defined, however. Many events show apparent motion, with southward motions more common than northward among the subset of events analyzed so far. Some of the events were detected simultaneously on an interferometer deployed at Kaktovik, Alaska, 400 km away. The all-sky imager at Toolik Lake was also operational for some events. Further analysis of these data promises to reveal first information about the locations and motions of MF burst sources, a first step towards discovering the generation mechanism of this mysterious radio emission and its relation to auroral processes.