SM51A-0265
What is the Alfvenic Dissipation Scale in the Auroral Region?
The auroral acceleration region includes a large amount of turbulence which contributes to acceleration of electrons along the magnetic field and ions perpendicular to the magnetic field. Much of this turbulence is believed to be Alfvénic in nature. However, no theory of turbulence specifically applicable to the auroral acceleration region has been developed, and the present observational understanding of the turbulence seems to be inconsistent with theories of Alfvénic turbulence that have been developed for the interplanetary medium. In particular, the standard theory of Alfvénic turbulence predicts that in the auroral acceleration region the waves should be heavily damped at scales which are long compared with the ion gyroradius, while many models of ion heating require significant wave power at scales comparable to or less than the ion gyroradius in order to heat ions stochastically in the perpendicular direction. We discuss some possible explanations for the apparent discrepancy between standard Alfvénic turbulence theory and the auroral region, and we present a preliminary investigation into the dissipation scale as observed with the FAST satellite.
SM51A-0266
Numerical Modeling of Alfven Waves and Quasistatic Structures in the Plasmasheet Boundary Layer
Significant power is transmitted via Alfven waves and quasistatic current structures from the solar wind/magnetosphere dynamo to the auroral region. This study investigates the nature of the developing turbulence within downward field-aligned current in the nightside plasma sheet boundary layer, as observed by the FAST, Polar and Cluster missions. A non-linear, two-fluid, reduced MHD model for cold, collisionless plasma in dipole geometry with a reactive ionosphere is used to simulate the interaction of a large-scale, slowly-evolving field-aligned current with the ionosphere. As the field-aligned current modifies the ionospheric Pederson conductivity, medium scale (~10km) and small scale (~1 km) structures develop in regions of low conductivity. The multi-scale nature of the developing structures is explained by the presence of multiple resonators within the system. The small-scale structures, which are associated with the classical ionospheric Alfven resonator, are confined to low-altitude and experience a high growth rate. The medium-scale structures, associated with a larger resonator bounded from above by high-altitude gradients in the wave conductivity, experience a lower growth rate, but develop at a higher Pederson conductivity. In addition to the numerical results, dispersion analysis for the multi-resonator system is performed. The numerical solution of the resulting dispersion relation, when comparing growth rates and conductivity thresholds of the multiple resonators, agrees with the results of the numerical model.
SM51A-0267
Ion and Electron Accelerations by Large-Scale Shear Alfvén Waves Via Cross-Field Instabilities
Using two-dimensional particle-in-cell simulations we study the linear and nonlinear effects of a large-scale shear Alfvén wave (LS-SAW) propagating parallel to the ambient magnetic field. We demonstrate that ion polarization drift in the propagating SAW drives cross-field instability (CFI) by ion-electron coupling and produces electrostatic (ES) waves, which accelerate electrons parallel and ions perpendicular to the ambient magnetic field. Even though the large-scale SAW has no parallel electric fields, the parallel electric field of the ES waves embedded in LS-SAW provide a novel acceleration mechanism for the electrons moving with the group velocity of the LS-SAW. Examining data from satellite observations, we find that the mechanism of CFI for plasma acceleration/heating powered by LS-SAWs is distinctly possible on auroral field lines.
SM51A-0268
Properties of the Density Structures associated with Strong Fields and Plasma Outflow within the Cusp/Cleft at 2 Re
Strong electric fields and currents have been associated with ion outflow, higher temperatures, and density structures above the auroral ionosphere. Using observations from the Polar satellite as it traverses the cusp/cleft at 2 Re we will look at these same parameters within the coupled magnetospheric-ionospheric region and study the connection with previous observations from above the dayside aurora. In this narrow latitudinal region of direct solar wind plasma access, we will focus on the widths and other properties of the density depletions, elevated temperatures, and fluctuating fields.
SM51A-0269
Very High Temporal Resolution Measurements of Subauroral Electric fields Associated With Bursts of Pi 1 Pulsations During the Substorm Expansion Phase
Recent innovations in Doppler analysis techniques used by the SuperDARN radars have allowed the temporal resolution of plasma velocity and electric field measurements to be improved by more than an order of magnitude. Tests of this new capability were made using the Wallops Island SuperDARN radar during the post- midnight hours of August 1, 2007. This was a geomagnetically-disturbed period in which a number of substorm expansions occurred. During one of these expansions, the Wallops radar observed a burst of Pi 1 electric-field pulsations with periods in the vicinity of 14 seconds. These electric-field pulsations were observed in the subauroral ionosphere in the vicinity of the plasmapause at 02 MLT. Nearby ground-based magnetometers observed related bursts of Pi 1 magnetic pulsations with the same period. Longer period Pi 2 pulsations were also observed in the magnetic data but were not observed in the electric field measurements suggesting that the currents producing the longer period magnetic pulsations were not local to the overhead ionosphere. Measurements from auroral zone magnetometers in central Canada show similar bursts of Pi 1 and Pi 2 pulsation over central Canada in response to a substorm expansion phase. The Pi 1 pulsations appear to be propagating away from midnight and show evidence of both toroidal and poloidal modes. They are thought to be excited by the injection of fresh plasma into the inner magnetosphere in response to dipolarization of magnetotail field lines. These observations show the value of having electric field data as well as magnetic field data in interpreting short-period MHD wave phenomena.
SM51A-0270
Observations of Pulsating Auroral Kilometric Radiation and its Possible Stimulation by Pc1 Pulsations
Cases of Auroral Kilometric Radiation (AKR) pulsating periodically at frequencies from 1 to 3 Hz have been found in high resolution dynamic spectra obtained from Cluster Wide Band Data instrument. These frequencies are typical for Pc1 (0.2 to 5 Hz) geomagnetic field oscillations. Duration of a single chain of pulses is generally less than one minute. With the WBD bandpass filter operating nominally in the frequency range of 300 Hz to 77 kHz selected at the time of the observations, the pulsating AKR was observed in a frequency range of emission of 50 to 90 kHz, with a modulation depth of about 20 dB. We show one example at a nearly constant frequency of pulse repetition around 2 Hz, lasting for 66 s, and another at a frequency descending from 3 Hz to 1.25 Hz in a time interval of 17 s. We interpret the observed Pc1 AKR pulsations in terms of Pc1 electromagnetic ion cyclotron waves (EMIC), which are stimulated by compression of the dayside magnetosphere [Anderson and Hamilton, 1993; Anderson and Fuselier, 1993; Anderson et al., 1996; Engebretson et al., 2002]. They are produced in the equatorial magnetosphere [Erlandson and Anderson, 1996; Loto'aniu et al., 2005] at or near the ion gyro- frequencies of H+, He+ [Mursula et al., 2001] and O+ [Braysy et al., 1998], and are propagated along geomagnetic field lines [Hansen et al. 1992] to the noon sector of the high latitude ionosphere [Erlandson and Anderson, 1996], where they are observed in space and on the ground [Engebretson et al., 2002]. We propose the scenario, in which the Pc 1 waves convert to inertial Alfven waves in the auroral region and interact with the downward propagating electrons to modulate their "horseshoe" distribution function, which leads to periodic AKR growth by an electron cyclotron maser instability, in a way similar to that described by Xin and Menietti [2007] for EMIC waves.
SM51A-0271
Characteristics of Pi2 Electric Pulsations at the Ionosphere
At the onset of magnetospheric substorms, impulsive hydromagnetic oscillations occur with period range from 40 to 150 seconds. They are called Pi2 magnetic pulsations and occur globally in the magnetosphere. Pi2 has been studied with arrays of magnetometers on the ground and with in-situ observation by satellites. However Pi2 electric pulsation in the low-latitude ionosphere is not yet clarified sufficiently. Therefore we have focused on measuring Pi2 electric pulsations by an FM-CW radar. In order to detect the ionospheric electric fields we have built an FM-CW (HF) radar at Sasaguri, Fukuoka, Japan (Magnetic Latitude: 23.2 degree, Magnetic Longitude: 199.6 degree). The radar provides us Doppler information of the ionosphere by high-time resolution of 10 sec. When the eastward electric field penetrates into the low- latitude ionosphere, it drifts upward owing to the frozen-in effects of the F-region. In contrast to the penetration of the eastward electric field, the ionosphere drifts downward when the westward electric field penetrates. Thus we can measure the east-west ionospheric electric fields. From our ionospheric radar observation, Pi2 electric pulsation of about 0.2 mV/m amplitude can be identified in nightside at Nov.6, 2003. We also compared the Pi2 with geomagnetic field data obtained from Circum-pan Pacific Magnetic Network (CPMN) stations. As a result, we found a phase lag between the Pi2 electric pulsation and mightside magnetic Pi2 pulsation at Kujyu (KUJ; M. Lat. 23.6 degree, M. Lon. 203.2 degree).
SM51A-0272
Mutual Information based clustering of Pi 2 magneic pulsations observed at CPMN stations
Pi 2 magnetic pulsations are observed on the ground as mixed signals of several independent components that are reflecting (1) propagations of fast and shear Alfven wave, (2) resonances of plasmaspheric/magnetospheric cavity and magnetic field lines, and (3) transformations to ionospheric current systems [e.g., Yumoto et al., 2001]. However, it has been unclear how they coupled with each other and how their signals are distributed at different latitudes. We have attempted to separate mathematically ground-observed Pi 2 pulsations by applying Independent Component Analysis (ICA). ICA is one of the multivariate statistical techniques that started to be used in the 1990s in the field of signal processing [e.g., Common, 1994]. With ICA, source signals are assumed to be non- Gaussian and statistically independent of each other and estimated by maximizing their statistical independence. It has been successful in resolving observed mixed signals including brain imaging data and voice signals into source signals. As an initial stage of this study, we applied FastICA algorithm suggested by Hyvarinen and Oja [1997] to an isolated Pi 2 event on a quiet day observed at CPMN (Circum-pan Pacific Magnetometer Network) stations and successfully decomposed them into two components. One was the global oscillation that occurs from nightside high to equatorial latitudes with the common waveform and has an amplitude maximum at nightside high latitude. Another component was localized at nightside high latitudes. Its amplitudes were quite weak at low latitudes, but were enhanced near dayside dip equator [Tokunaga et al., 2007, GRL]. As a second stage of this study, we have attempted to classify ground-observed Pi 2 pulsations into some groups systematically. In this paper, MILCA (mutual information based least-dependent component analysis) suggested by Stogbauer et al., [2004] have been introduced, which are based on crude approximations for MI (mutual information). The numerical values of the MI can be used for (i) estimating residual dependencies between the output components; (ii) estimating the reliability of the output by comparing the pairwise MIs with those of remixed components; and (iii) clustering the output according to the residual interdependencies.
SM51A-0273
Influence of Plasma Wake Around Satellite Body on Characteristics of Electric Field Sensor
Knowledge of the characteristics of wire antennas in magnetized plasma used as sensors for electric field observations by scientific satellites in geospace is necessary to determine the absolute intensity and the phase of the electric field wave because the observation data about electric field are available as voltage signal. Two important characteristics are the effective length and the antenna impedance. Determination of the impedance can be especially difficult since the impedance depends on the medium surrounding the antenna, and the impedance is affected primarily by the plasma sheath created around the antenna. The impedance of the antennas onboard Geotail was measured by Tsutsui et al. by using the calibration function onboard Geotail where a modulated square wave test signal was applied to the antenna elements. They found that the impedance depended mainly on the electron density and the satellite spin. The dependences of the impedance measurement on the satellite spin are caused by the fluctuation of the photoelectron emissions from the antenna surfaces. On the other hand, the impedance onboard Akebono was measured by Hashimoto et al., where the results also depended on the electron density and the satellite spin. However the photoelectrons do not play an important role in the impedance measurement, because the spin axis of Akebono always points to the sun. In this study, according to analysis of measurement results on Akebono, the dependence of the impedance on the angle between the antenna and the direction of satellite movement were found. Considering this phenomenon, the plasma wake is made around Akebono, and the impedance is changed by the electron density change near the antenna caused by the satellite spin. And the theoretical calculation result of the impedance assuming the electron density distribution around the spinning satellite body and the measurement result ware compare.
SM51A-0274
Physics-based formula representations of high-latitude ionospheric outflows: H+ and O+ densities and flow velocities vs. precipitation, wave-heating, and solar zenith angle effects
For many current global magnetospheric modeling efforts, it is highly desirable to try to incorporate realistic compact representations of the ionospheric outflow bulk parameters and their relationships to putative drivers. Recent satellite data analyses by Strangeway et al. [2005] and Zheng et al.[2005] have obtained formula fits for the measurement-based relationships of the outflow levels to parameterizations for electron precipitation and Poynting fluxes, which are expected to be among the principal drivers, or closely related to them, for the ionospheric outflows. In this presentation, we shall use the results of an extensive set of systematic simulation runs with our Dynamic Fluid Kinetic (DyFK) simulation code for ionospheric plasma field-aligned transport to obtain O+ and H+ densities and flow velocities at altitudes corresponding to typical inner boundary levels for prominent current global magnetospheric models which are moving toward multi-fluid treatments. These O+ and H+ densities and parallel flow velocities are parameterized versus precipitation electron energy flux levels, characteristic energy levels of the precipitating electron, the peak spectral wave densities for BBELF waves which transversely heat ionospheric ions, and solar zenith angle. Strangeway, R. J., R. E. Ergun, Y.-J. Su, C. W. Carlson, and R. C. Elphic, Factors controlling ionospheric outflows as observed at intermediate altitudes, J. Geophys. Res., 110, A03221, doi:10.1029/2004JA010829, 2005. Zheng, Y., T. E. Moore, F. S. Mozer, C. T. Russell, and R. J. Strangeway, Polar study of ionospheric ion outflow versus energy input, J. Geophys. Res., 110, A07210, doi:10.1029/2004JA010995, 2005.
SM51A-0275
Comparison of polar cap electron density enhancement due to solar illumination and geomagnetic activity as measured by IMAGE/RPI
Polar cap electron density (Ne) measurements made between the years 2000 – 2005 by the radio plasma imager (RPI) on board the IMAGE spacecraft are used to study the density enhancements resulting from changes in solar illumination and geomagnetic activity level. This study covers a geocentric distance, R = 1.4 – 5.0 RE and the polar cap is defined by an empirical boundary model that takes into account the dynamic nature of the location and size of the polar cap. The average polar cap electron density profile depends on geomagnetic activity level e.g., measured by the Kp index and solar illumination (solar zenith angle) at the footprints of the geomagnetic field lines. Our analysis of RPI Ne data shows that increase in geomagnetic activity leads to an enhancement in Ne. This enhancement in Ne is found to increase with altitude. At geocentric distance of R = 4.5 RE, an increase in the geomagnetic activity level from Kp < 2 to ~5 results in an Ne increase by a factor of ~5. On the other hand, a strong solar illumination control of Ne at lower altitudes, and not at higher is observed. At geocentric distance of ~ 2 RE, the average Ne is larger on the sunlit side than on the dark side by a factor of 3 – 4 both for quiet and disturbed conditions. At geocentric distance of about 2.5 RE the effects of these two factors on Ne appear to be comparable. Similar to previous polar cap density models, a functional representation of RPI Ne that takes the form of a power law is proposed. While in the previous Ne functional representations the power index is a constant, the power index in our representation of Ne distribution is found to correlate with (and hence is a function of) the Kp index and the solar zenith angle (SZA).
SM51A-0276
Io-Jupiter interaction : field aligned potentials revealed by S-bursts analysis
Jovian millisecond (or S-)bursts are intense impulsive Jovian decametric radiations drifting in frequency in tens of milliseconds. Most of the theories about their origin focus on the frequency drift and suggest that S-bursts are cyclotron-maser emission in the flux tube connecting Io's wake to Jupiter. We have performed an automated analysis of high resolution dynamic spectra of S-bursts, from Karkov and Nançay decametric radio telescopes. The analysis of our data show that it is consistent with the above scenario. We find a typical energy of 4 keV for the emitting electrons. But we can go beyhond this scenario: reformulate the adiabatic model including parallel electric fields, and search for correspondig signatures in the data. A statistical analysis suggests the existence of stable localized potential jumps of about 1 keV. Using the Karkov radiotelescope data, the stability and the velocity of these structures is analysed.
SM51A-0277
Auroral Charging at Jupiter
A well-known concern for polar orbiting spacecraft at the Earth is spacecraft charging due to the aurora. Studies of Jupiter reveal the presence of a variety of similar auroral phenomena. In particular, three regions have been identified—a narrow auroral zone at high latitudes, a complex and variable environment over the poles, and aurora-like features associated with the main jovian moons and their magnetic flux tubes. These auroral structures are, like their earthly counterparts, expected to be sources of charging. In combination with models of the jovian plasma environment, these ambient charging currents are then computed with the intention of providing realistic estimates of spacecraft potentials at Jupiter for the polar-orbiting Juno mission. Unlike previous missions to Jupiter, Juno will utilize large solar arrays. Juno's solar arrays may make it sensitive to surface charging if proper mitigation techniques are not implemented in the design. This presentation evaluates the possible effects of the jovian auroral regions on charging. Jovian aurora are a potential threat to Juno and similar missions, but, as will be discussed, an understanding of the environment and of proper mitigation techniques should limit their effects.
SM51A-0278
Magnetospheric Wave-Particle Interactions Excited by the HAARP HF Heater
ELF/VLF wave generation by heated modulation of the ionospheric auroral electrojet currents is used for controlled magnetospheric wave injection experiments. The High Frequency Active Auroral Research Program (HAARP) facility in Alaska is used to efficiently generate electromagnetic radiation in the 500 Hz - 5 kHz frequency range. Electromagnetic waves are injected into the magnetosphere where they experience a wave-particle interaction with radiation belt electrons which is manifested in temporal growth of 15 dB/sec and non-linear triggering of magnetospheric emissions. Amplified and triggered waves called ‘echoes' are observed on the ground at both ends of the magnetic field line and also on the DEMETER satellite. Results are presented both for the original 960 kW HAARP array and the recently upgraded 3.6 MW facility. The observations show that observations of HAARP excited magnetospheric amplification are correlated primarily with geomagnetic conditions but are also heavily dependant on the specific frequency-time formats of the injected ELF/VLF radiation. All cases of observed echoes are shown to propagate inside of the plasmapause and show only weak correlation to the local ELF/VLF signal strength. Echo activity is limited to active frequency bands with widths of ~ 1kHz. The echoes are injected into the magnetosphere primarily directly over the heater.
SM51A-0279
Nightside NEIAL Observations and their Relation to Boundary Auroral Features
We present data from ground camera and radar observations of the aurora conducted from Poker Flat, Alaska during the winters of 2006 and 2007. The Radar observations were conducted using the Poker Flat Advanced Modular Incoherent Scatter Radar (AMISR). Dark auroral signatures in the camera data are found to occur with naturally enhanced ion acoustic lines (NEIALs) in the radar data. Observations of NEIALs are presented from two separate nights, displaying different auroral context. On 08 February 2007, one short-lived NEIAL event was observed with AMISR using high time-resolution (19 ms). These raw data reveal that the large returns associated with the NEIALs come from the dark region immediately adjacent to an active dynamic auroral arc. It was also found that propagation in altitude of the NEIALs occurs at or near the Alfven velocity. Furthermore, it was found that the enhanced up- and down-shifted shoulders of the NEIALs most often occurred independently of one another on a 19 ms timescale. On 31 March 2006, a moderately intense auroral arc, (10 kR at 557.7 nm), was located in the local magnetic zenith. During this event the radar observed 7 distinct periods of NEIAL activity. These times correspond to (a) when the polar cap boundary of the auroral arcs passed through the magnetic zenith and (b) when small-scale filamentary dark structure was present in the magnetic zenith. These observations are consistent with NEIALs occurring within the same auroral morphology which is known to contain broad-band extremely low frequency (BBELF) wave activity. These observations support the hypothesis that NEIALs and BBELF are differently observed aspects of the same auroral phenomenon. A relation between NEIALs and the in situ signature of BBELF wave activity, provides a link between the in situ measurements and the ground-based observations.
SM51A-0280
Temporal Development of Auroral Acceleration Potentials: High-Altitude Evolutionary Sequences, Drivers and Consequences
The region above the auroral acceleration region is an integral part of the auroral zone electrodynamic system. At these altitudes (≥ 3 Re) we find the source plasma and fields that determine acceleration processes occurring at lower altitudes, which play a key role in the transport of mass and energy into the ionosphere. Dynamic changes in these high-altitude regions can affect and/or control lower-altitude acceleration processes according to how field-aligned currents and specific plasma sources form and decay and how they are spatially distributed, and through magnetic configuration changes deeper in the magnetotail. Though much progress has been made, the time development and consequential effects of the high-altitude plasma and fields are still not fully understood. We present Cluster multi-point observations at key instances within and above the acceleration region (> 3 RE) of evolving auroral arc current systems. Results are presented from events occurring under different conditions, such as magnetospheric activity, associations with density depletions or gradients, and Alfvenic turbulence. A preliminary survey, primarily at or near the plasma sheet boundary, indicates quasi- static up-down current pair systems are at times associated with density depletions and other instances occur in association with density gradients. The data suggest that such quasi-static current systems may be evolving from structured Alfvenic current systems. We will discuss the temporal development of auroral acceleration potentials, plasma and currents, including quasi-static system formation from turbulent systems of structured Alfvenic field-aligned currents, density depletion and constituent reorganization of the source and ionospheric plasma that transpire in such systems. Of particular emphasis is how temporal changes in magnetospheric source plasma and fields affect the development of auroral acceleration potentials at lower altitudes.
SM51A-0281
Ion Distribution Functions in Response to Cylindrically Symmetric Electric Fields That Change Linearly With Radius
It is important to study the response of ambient ions to spatially inhomogeneous electric fields for a better understanding of ion velocity distribution data obtained in space, as well as for a clearer understanding of transport properties under such conditions. We have undertaken a kinetic study of the ion response in cylindrically symmetric cases. As a first step, we have determined the response of the ion distribution function and associated transport properties under the idealized case of an ambient cylindrical, radially-inward, inhomogeneous, but otherwise linearly varying electric field as a function of radial distance. We have found that in this particular case we can describe the results with closed-form analytical expressions. For this geometry, we have studied the ion response to the sudden introduction the cylindrically symmetric electric field by solving the attendant Boltzmann equation. In this case, individual ions gyrate in phase, though at a frequency that differs from the conventional gyro-frequency. The phase lock causes the associated velocity distribution to pulsate at a non- steady rate at a frequency roughly comparable to the cyclotron frequency. Nevertheless, for an initial uniform Maxwellian velocity distribution, the distribution remains Maxwellian at all times, although the drift, density and temperature of that distribution oscillate with time (but not with position). In a second phase of our linearly increasing electric fields study, we have also determined the response of the ions after a few collision times, for application to ionospheric F region situations. In this case the distribution no longer pulsates, as the collisions randomize the phases of the various ions. The ion velocity distribution then evolves towards a horseshoe shape in velocity space