SM23A-0464 1340h
Observations of Narrow-Band $\sim$60-100 Hz Waves at High Auroral Latitudes
ELF magnetic pulsations have been measured using a new induction coil magnetometer having a frequency response in the ELF (Extremely Low Frequency) range ($\sim$0.1-250 Hz), which has recently been installed at South Pole Station. An initial survey of the first month of data revealed very interesting phenomena, including narrow-banded ELF pulsations with slowly varying frequencies and ELF 'hiss' in the $\sim$10-130 Hz frequency range. The narrow-band signatures maintain their bandwidth as their frequencies change slowly, typically from 100 to 60 Hz over the course of the order of $\sim$40-90 seconds. Although the narrow-band waves have a signature somewhat similar to VLF (Very Low Frequency) whistlers in the sense that their frequencies typically decrease with time, their duration and frequency range are much different than whistlers; they also show an unusual characteristic where their frequency often increases before it decreases. An initial survey of these events shows a peak in their occurrence distribution near the pre-midnight region, implying a possible association with substorm occurrences, a result that contrasts sharply with the only two other published studies of these waves. In this presentation, we show results of a more complete study, including polarization characteristics of these waves, comparison with other data to determine the geophysical conditions present when they occur and a more complete statistical result of their occurrence distribution.
SM23A-0465 1340h
Interpretation of Vector Electric Field Measurements by RPI Using Three Dipole Antennas on IMAGE
RPI is a multi-mode instrument using three orthogonal thin-wire antennas - two long (nominal length of 500-m tip-to-tip) dipoles in the spin plane (X and Y antenna) and one short (nominal length of 20-m tip-to-tip) dipole along the spin axis (Z antenna). The X antenna is used for transmission and all three antennas are used for reception to measure three components of electric field. Unfortunately, on October 3, 2000, one of the X-axis monopoles was partially severed, apparently by a micrometeorite, reducing the dipole length to 340 m. On September 18, 2001 an unknown section of the Y antenna was lost. Therefore, it is difficult to estimate three components of wave electric field. Measurement of three components of electric field is important to determine wave normal directions of the received echoes and to estimate the radiated and received powers. We have used known source locations, relations for polarization and refractive index as a function of wave normal direction, and antenna orientations in order to interpret the amplitudes of electric field components as measured by three antennas. The RPI transmission frequency ranges from 3 kHz to 3 MHz, which allows multiple modes of propagation including whistler, Z, and free space modes. We analyzed two kinds of signals: 1) whistler mode waves from the ground transmitter observed at the IMAGE satellite, 2) discrete Z mode echoes propagating within the Z mode cavities. In both cases the wave normal direction is presumed to be known - vertical in the case of ground transmitter signal and parallel to the geomagnetic field in the case of discrete Z mode echoes. Ground transmitter signals in the frequency range 10-30 kHz are commonly observed when IMAGE is in the northern hemisphere. In one case, observed on October 2, 2002, NAA signal at 24 kHz was observed on three antennas. Voltages induced across X and Y antenna terminals were consistently $\sim$30-40 dB higher than those measured across the Z antenna. These observations can be explained by taking into account the differences in the three antenna lengths, their orientation, and the vertical wave normal direction. Discrete Z mode echoes result from reflections at the Z mode lower cutoff frequency ($f_{z}$) when waves propagate in both upward and downward directions from the satellite. The expected polarization for these echoes based on parallel propagation is circular. The measured polarization showed significant deviations from circular polarization. We show that multipath propagation in Z mode cavities can explain these deviations from circular polarization. A further evidence of multipath propagation is provided by multiple time delays at each frequency for the discrete Z mode traces.
SM23A-0466 1340h
Local Electron Density Measurements from Sounding Experiments by RPI on IMAGE
RPI sounding experiments lead to a variety of echoes, propagating in various plasma wave modes, and local resonances. Characteristic frequencies of these echoes and resonances can be used to determine the local plasma frequency and thus the local electron density. In this work we have estimated plasma frequency by two methods: (1) using upper hybrid frequency measured from the diffuse Z mode echo upper cutoff and gyro-frequency measured from a gap in the diffuse Z mode echo or from resonances at the multiples of gyrofrequency, (2) upper hybrid frequency from the diffuse Z mode and the free space cutoff frequency $f_{R=0}$ from the $R-X$ mode echo. Broadband diffuse Z-mode echoes occur 90$%$ of the time at high latitudes ($\lambda$m$>$45$^\circ$S) near perigee in the southern hemisphere, where $f_{pe} << f_{ce}$. In the middle and low latitudes ($\lambda$m$<$45$^\circ$S), where $f_{pe} >> f_{ce}$, Z-mode echoes are narrowband and are often accompanied by Qn and Dn resonances. The free space $R-X$ mode echoes are commonly observed at both high and low latitudes. Multiples of gyrofrequency are typically observed at mid- to low-latitude in both the northern and southern hemisphere and at high latitude in the northern hemisphere. RPI plasmagrams were analyzed for three orbits (apogee to apogee) in the year 2002. These three orbits were selected because suitable sounding programs, those that can cover Z mode bandwidth over a wide range of latitude, were used, and also because a large number of diffuse Z mode echoes were actually observed. Electron densities as low as 10 el/cc and as high as 9000 el/cc were measured. The transmission frequencies place a limitation on the upper and lower limits of measurable $f_{pe}$. The measured $f_{pe}$ values showed good agreement with measurements made from the thermal noise but showed large deviations when compared with model $f_{pe}$ values. For a particular orbit on August 26, 2002, $N_e$ measured was as low as $\sim$20 el/cc at higher altitudes outside the plasmasphere ($\lambda$m $>$ 60$^\circ$N, altitude $>$7000 km, MLT=1.89) and increased as IMAGE approached the plasmasphere. A maximum of $\sim$8900 el/cc was measured well within the plasmasphere (L = 1.56, $\lambda$m = 17$^\circ$N, altitude $=$2700 km, MLT = 2.44). As the satellite left the plasmasphere, measured electron density decreased to a minimum of about 55 el/cc near the auroral zone (L = 6.83, $\lambda$m = 57$^\circ$S, altitude = 6277 km, MLT=13.66) and then started to rise again. A sharper change in $N_e$ was seen at both the inbound and outbound crossings of the plasmapause. As the satellite again entered the plasmasphere (L = 3.94, $\lambda$m = 21$^\circ$S, altitude = 15500 km, MLT = 14.34) at a higher altitude the maximum value of Ne measured was lower ($\sim$520 el/cc) as expected. Our results demonstrate that magnetospheric sounding experiments employing Z mode and free space modes provide a powerful means of making local plasma density measurements.
SM23A-0467 1340h
Further Studies of Flickering Auroral Roar
In December 2002, a Versatile Electromagnetic Wave Receiver (VIEW) was deployed at South Pole station. This system records HF waveforms continuously for up to 6 hours/day. Summary files are examined weekly by Dartmouth personnel, and interesting time intervals are saved to CD-rom. This interactive experimental method provides extremely high time- and frequency-resolution measurements of auroral radio emissions, while discarding data from times when no events occur. The motivation was to measure three types of auroral radio emissions: Auroral Roar, a relatively narrowband (δf/f <0.1) emission near 2 and 3 times the F region ionospheric electron cyclotron frequency (fce); Auroral Hiss, a whistler mode wave emission with frequencies lower than 1MHz. ; and Auroral medium frequency (MF) burst, broadband impulsive radio emissions observed at ground level during the breakup phase of auroral substorms. In the year of 2003, we recorded about 80 minutes of auroral roar emission, consisting of 40 different events, at South Pole station. Hughes and LaBelle [2001] observed the first flickering auroral roar, with a ~10 Hz pulsation in emission strength, in Greenland. They proposed that these pulsations are related to the electron flux modulations similar to those which cause flickering aurora. By examining all 80 minutes (40 events) of auroral roar captured in 2003, we found more than 10 cases of flickering auroral roar from 10 different days. However, most instances were brief, sometimes only a few seconds. The total time of flickering auroral roar was a few minutes (a few percent of the total time of occurrence of auroral roar emissions). We also observed the first ever example of higher frequency flickering auroral roar, with a modulation frequency around 100 Hz. We investigate these events by taking time series of the strength of the auroral roar emissions, taking Fourier transforms to determine the frequencies of the flickering. In this poster, we show statistics of the auroral roar flickering frequency and compare these to optical flickering aurora statistics in the published literature. We also show examples of variations of the flickering frequency during flickering auroral roar events. Hughes, J.M., and J. LaBelle, First observations of flickering auroral roar, Geophys. Res. Lett., 28, 123-126, 2001.
SM23A-0468 1340h
A Search for Signatures of Ion Acoustic Shoulders in the SERSIO sounding rocket data set
Although first predicted in the early 1960's, enhanced Ion Acoustic Shoulders have only been observed by incoherent scatter radars since the late 1980's. The signature of an IAS is seen as a positive and negative frequency shift about the center radar frequency. These features occur at altitudes of 150 to over 600 km, peaking at 500 km, with spatial extent (perpendicular to the magnetic field) the order of 10 km. The occurrence distribution shows a maximum in the pre-midnight region, with a secondary peak on the dayside (Rietveld et al 1995). Observations of strong (1000 mA/${\rm m}^2$), localized currents by EISCAT have led to theories based on current-driven instabilities as the source of these waves (Forme, 1993; St.-Maurice et al., 1996). The SERSIO (Svalbard EISCAT Rocket Study of Ion Outflows) sounding rocket mission was launched into CME-driven dayside aurora on the 22nd of January 2004 at 0857 UT (0436 MLT) from Ny-Alesund ($78^\circ$ 55' 11" N, $11^\circ$ 56' 60" E) and reached an apogee of 782 km. During the flight, the EISCAT incoherent scatter radar network supported the mission by monitoring altitude profiles of electron and ion density, velocity and temperature. From Longyearbyen, located approximately 50 km south east of Ny-Alesund and near the trajectory of SERSIO, the 32m ESR dish was tracking the ionospheric footprint of the payload while the 42 m dish was making local field-aligned measurements. The data from these radars clearly indicated the presence of enhanced ion acoustic shoulders, suggesting that SERSIO flew through a 'field' of Ion Acoustic Shoulders. In fact, the plasma wave environment observed by SERSIO was composed of traditional VLF hiss and Broad Band ELF hiss with wavelengths less than the order of 6m. Here we present the result of our search for Ion Acoustic Shoulders in the SERSIO data set.
SM23A-0469 1340h
Observation of the Reactive Component of Langmuir Wave-Electron Phase Bunching
Observations from the Rocket Auroral Correlator Experiment (RACE), launched on February 6, 2002 from Poker Flat, Alaska, show show clear evidence of phase bunching of the electrons by Langmuir waves. The measurements were made by a new wave-particle correlator during a period when the Langmuir waves were very monochromatic and the amplitude varied slowly in time. The electrons, which are at energies well below the inverted-V peak energy, are found to be bunched at 90 degrees with respect to the wave field indicating that they are the reactive component of the perturbed distribution function. This is in agreementwith theory that predicts this phase relation for the transit of electrons through a very long wave field.
SM23A-0470 1340h
Particle-In-Cell Simulations on Antenna Characteristics in Space Plasma
Antenna properties in plasma have been investigated by many scientists. However, the analysis of the antenna impedance is very complex because the plasma is a dispersive and anisotropic medium. In the previous theories, approximations in the current distribution along the antenna or the sheath structure around the antenna were hired. Meanwhile, recent progress of computer facilities enables us to analyze the antenna properties in vacuum with the FDTD (Finite Difference Time Domain) method which solves the Maxwell equations with spatial and temporal grid points. However, the FDTD method with a dielectric tensor obtained under the cold plasma approximation can not treat plasma kinetic effects on antenna properties. In the present study, we applied three-dimensional PIC electromagnetic simulations to examine the plasma kinetic effects on the dipole antenna properties. We particularly focus on the dependence of antenna resonance on the plasma temperature and sheath size around the antenna. Preliminary results will be also shown on the effects of photo-electron emission on the antenna impedance.
SM23A-0471 1340h
Gyrokinetic Theory and Simualtion of Mirror Instability
Magnetic mirror instability plays an important role in low frequency magnetic turbulences in high-beta space plasmas, e.g., the Earth's magnetosheath, and also potentially in high-beta laboratory mirror or dipole plasmas with large fraction of trapped particles. A quasi-hydrodynamic analysis of kinetic mirror instability in non-Maxwellian plasmas recently found that the maximum growth rate increases with perpendicular wave number. Therefore, finite Larmor radius effects are important in determining the threshold and wavelength of kinetic mirror instability. In this study, we extend the kinetic theory for mirror instability to the short wavelength regime using the gyrokinetic theory and simulation. We have developed a general dispersion relation of the mirror mode with finite Larmor radius effects and arbitrary distribution functions, and have recovered the quasi-hydrodynamic results in the long wavelength limit. We find that the most unstable mode has a wavelength on the order of ion gyroradius. We also find that both the finite beta and coupling to the slow sound wave have stabilizing effects. Gyrokinetic particle simualtion will be used to benchmark these linear results and to identify nonlinear saturation mechanism.
SM23A-0472 1340h
A Comparison of EFW Probe Potentials Observed During Crossings of the Terrestrial Bow Shock
Cluster is able to study scales of electric field processes not only related to the intersatellite separation but also on finer scales such as the satellite/probe separation. S study of the fine properties of waves observed in the EF has been perfomed using Cluster data in the vicinity of the terrestrial bow shock. The results of this study enable a comparison between scales that correspond to the intersatellite separations (hundreds of kilometer) and those of the interprobe separations (hundred meter).
SM23A-0473 1340h
Average properties of wave emissions below 4 kHz in the inner magnetosphere based on the data of the STAFF-SA instruments on board the Cluster spacecraft.
We present a systematic study of whistler-mode emissions observed below 4 kHz in the inner magnetosphere. It is based on the data of the STAFF-SA instruments on board the Cluster spacecraft. The source of the data for our analysis and for our statistical study originated from measurements that the spacecraft made close to the perigee during the first four years of operation (2001-2004). We mainly focus our attention on the whistler-mode chorus emissions. We investigate the power-spectral density, polarization and propagation properties of the observed waves. We calculate the average values of the results obtained from the entire data set. We also calculate the statistical standard deviation of those results to confirm their systematic character. From our estimation of the Poynting flux based of the large data set we can confirm that the source of the chorus always is close to the equatorial plane. The parallel component of the Poynting flux is clearly oriented outwards from the equatorial plane. We also examine the angle deviation of the wave vector direction from the ambient magnetic field, ellipticity and the electromagnetic planarity. We investigate spatial distribution of chorus in the magnetic local time and radial distance.
http://terezka.ufa.cas.cz/santolik/papers/agu2004
SM23A-0474 1340h
Akebono satellite observations of Alfv\'{e}n waves related to dayside small-scale field-aligned currents
In the cusp and its surrounding region, burst-like disturbances of magnetic and electric fields are often observed on Freja, FAST, Akebono, and Cluster spacecraft. Recent studies suggest that these small-scale disturbances are signatures of the dispersive Alfv\'{e}n wave (DAW) and the ionospheric Alfv\'{e}n resonator (IAR) excited in the magnetosphere-ionosphere coupled system [{\itHirano et al.}, 2004]. These small-scale disturbances would also carry small-scale field-aligned currents (FACs). In our recent study, we investigated statistically the polar distributions of small-scale FACs and showed that small-scale FACs are enhanced in the vicinity of the cusp, particularly in the prenoon Region 1 current system. These dayside small-scale FACs are enhanced on sunlit conditions, suggesting that small-scale FACs are significantly affected by ionospheric conductivity. The purpose of this study is to elucidate the characteristics of Alfv\'{e}n waves related to small-scale FACs using Akebono magnetic and electric field data. Comparing with the in-situ electron density data, the role of DAW and IAR on the generation of small-scale FACs are discussed, focusing particularly on the altitude dependence.
http://pat.geophys.tohoku.ac.jp
SM23A-0475 1340h
Pc 1 Waves and Associated Unstable Distributions of Magnetospheric Protons Observed During a Solar Wind Pressure Pulse
We present observations of Pc 1 waves ($\sim$0.6 Hz) that occurred shortly after a strong ($>$20 nPa) compression of Earth$'$s magnetosphere at 1321 UT March 18, 2002. Intense Pc 1 waves were observed at several high latitude ground stations in Antarctica and Greenland from 1321 UT to beyond 1445 UT. Two wave bursts were recorded at the Polar satellite at 1338 and 1343-44 UT as it passed outbound in the southern hemisphere at 1154 local time and near L = 7.5, in good magnetic conjunction with the Antarctic sites. The pressure increase created a significant population of protons between a few hundred eV and several keV, whose fluxes were mostly perpendicular to B. These protons seem to have replaced the quiescent stream of protons (presumably convected from the plasma sheet) that existed before this increase. There was also a nearly two-order-of-magnitude increase in the population of thermal/suprathermal (0.32-410 eV) protons. Ion cyclotron waves are expected to limit the proton temperature anisotropy A = T$_p_e_r_p$/T$_|_|$-1 roughly according to A*sqrt($\beta_|_|$) $\sim$ 1, where $\beta_|_|$ is the parallel plasma (proton) beta defined as $\beta_|_|$= 8$\pi$nkT_|_|/B$^{2}$. Peaks in the locally observed A*sqrt($\beta_|_|)$ product as a function of time show excellent agreement with the times of the wave bursts. These observations confirm the importance of both the energization and/or increase in population of protons transverse to B in the several keV range, and the presence of greatly increased fluxes of lower energy protons (100s of eV to a few keV), predominantly aligned along B, in determining whether the particle population is unstable at a given time.
SM23A-0476 1340h
Pc 1 Pulsations and Associated Ion Signatures Observed by the Cluster Spacecraft.
We present six examples of $\sim$0.5 Hz Pc 1 (ion cyclotron) wave activity observed by the Cluster satellites during 2002. Near perigee waves were observed as the spacecraft passed latitudinally through the dayside inner and middle magnetosphere. These waves were never observed at all four spacecraft, and hence are rather localized. Each was associated with highly structured plasma sheet / ring current ion distributions. One event, observed near the equator, provides strong evidence of a radial width of L$\sim$0.2. Another event observed when the spacecraft were near -40$\deg$ MLAT and also observed on the ground is associated with ion distributions similar to the "spatial" event category defined by Engebretson et al. [JGR 107, A12, 1451, 2002]. Near apogee in the tail lobes and/or plasma mantle, waves were observed essentially simultaneously (to within $\sim$ 5 min) for intervals of 0.5 - 3 hours at all four spacecraft in association with orders-of-magnitude step increases in ion density.
SM23A-0477 1340h
Pi2 pulsations associated with poleward boundary intensifications during the absence of substorms
Pi2 pulsations during the intervals of extremely quiet geomagnetic conditions (Kp = 0+) have been reported by Sutcliffe and Lyons [2002]. These authors observed that several Pi2 bursts occurred simultaneously at high (magnetic latitude = $71\deg$) and low ($42\deg$) latitudes during the absence of magnetospheric substorms and found that the bursts are strongly correlated with poleward boundary intensifications (PBIs). The authors discussed the correlation between the PBI-associated Pi2 (PBI-Pi2) bursts and enhancements of energetic particle fluxes in the plasma sheet, but they did not focus on the wave properties of the PBI-Pi2 pulsations. In this study we examine whether the PBI-Pi2 pulsations at mid/low latitudes exhibit spatial variations similar to substorm-associated Pi2 pulsations. Using ground-based data from latitudinally and longitudinally extended magnetometer network and spacecraft data in the duskside, we discuss the spatial variation of the frequency, amplitude, phase, and inter-station coherence of the PBI-Pi2 pulsations.
SM23A-0478 1340h
Correlation between Low Frequency AKR and Auroral Structures
Auroral Kilometric Radiation (AKR), a radio emission with a broadband frequency range, has long been associated with auroral activity. Recently we have done a preliminary study of AKR related very low frequency events (hear after referred to as LF AKR) and their association to specific auroral structures. LF AKR events are characterized as a rapid extension of AKR related emissions to 30 kHz or lower in frequency for typically much less than 10 minutes. We have extended the previous study to include all instances of LF AKR events during the nearly 4 years of observations from the IMAGE spacecraft's Radio Plasma Imager (RPI) instrument. We will discuss the possibility of a correlation between low frequency AKR emissions and the bifurcation in the aurora by examining images of the auroral oval from the Far Ultraviolet (FUV) instrument on IMAGE. We will examine the relationship of LF AKR emissions events and other factors including dipole tilt, Dst and AE Index . Although we don't expect LF AKR to be the only factor in the bifurcated auroral structures, we do expect that this bifurcation will occur in every instance when LF AKR is observed. The unique character of LF AKR is a possible indicator of specific processes sometimes occurring during storm-time conditions, resulting from narrow field-aligned density cavities forming at a distance of perhaps 5-6 RE tailward from the Earth.
SM23A-0479 1340h
Phase mixing of Alfven waves in 2D configurations
The phase mixing of shear Alfven waves in various non homogeneous 2D geometries is analysed. An inhomogeneous bounded system may be considered as a collection of oscillators coupled by friction. Each field line oscillates at its own frequency and becomes more and more out of phase with its neighbours. The phase mixing creates large transverse gradients which strongly enhances the energy dissipation. Here, we solve an initial value problem (1) in a X-point configuration and (2) in a Harris sheet with a small transverse magnetic field. Both viscous and resistive terms are taken into account. We compute the rate of dissipation of the initial energy and we discuss the influence of the ratio eta/nu on this rate.
SM23A-0480 1340h
Freja Observations of EM-waves Localized in Density Depletions
Waves with frequencies around the lower hybrid frequency localized in density cavities, so called LHC:s, has been observed in space for more than ten years. These waves are generally purely electrostatic, with no detectable enhancement in magnetic wave activity. We have found waves on the gradient of density depletions that seems to be electromagnetic. These EM-waves and the shape of the cavities we observe are similar to observations made in laboratories. We make case studies as well as a small statistical study to investigate these waves, and verify that they are electromagnetic.
SM23A-0481 1340h
CHARACTERISTICS OF ELECTRIC WIRE ANTENNAS ONBOARD SCIENTIFIC SPACECRAFT
The "effective length" is an important parameter of a wire antenna. The output voltage V for an electric antenna subject to an imposed electric field E (measured in V/m) is written as V=$h_{eff} \times E$ where $h_{eff}$ is the effective length. It is very difficult to determine the exact value of an effective length of a wire antenna onboard satellite by ground testing, because a wire antenna which observes low frequency plasma waves down to several 100 Hz becomes very long. For example, "Akebono" spacecraft has 60 m tip-to-tip antennas and "Geotail" has 100m tip-to-tip antennas. Additionally, the antenna characteristics in geospace are different from those in free space. In previous studies, the effective lengths have been usually assumed to be L for DC (static) electric fields and L/2 for AC (wave) electric fields, where L is the tip-to-tip length of a wire dipole antenna. This means that the effective length depends on the frequency of wave. In this study, we determine the characteristics of the effective length of the wire antenna at low frequencies by an experimental measurement called "Rheometry," and then by a theoretical analysis using the equivalent circuit. The result of the measurement and theoretical analysis confirms that the effective length depends on the structure of the antenna at low frequencies. According to the experiment, we can sea the same characteristic as those described above, that the value of effective length is L at low frequencies and L/2 at high frequencies. Theoretically, when the wire antennas is arranged to be parallel to the direction of the electric field, the electrostatic potential increases linearly along the wire. This is simulated as a voltage supplier which is distributed along the wire. When we calculate the voltage output of the antenna using this equivalent circuit, the result of calculation is consistent with the experimental measurement. In particular, the transition frequency of the effective length is well explained. Also we calculate the effective length of the wire antenna aboard GEOTAIL spacecraft by adding the effect of plasma sheath. We will show and discuss the calculation result.
SM23A-0482 1340h
Laboratory Investigation of Nonlinear Properties of Whistler Waves*
There have been a number of interesting in situ and laboratory observations of whistler wave propagation and stimulated emissions over the past few decades. For example, {\it Stenzel} [1975] reported on the self-ducting of large amplitude whistler waves in a laboratory plasma. Those experiments showed that by increasing the driving amplitude, the radiation pattern from a small dipole antenna becomes increasingly narrow, ultimately forming a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. In the space environment, observations of artificially stimulated VLF emissions triggered in the magnetosphere by whistler modes from VLF transmitters have been reported by {\it Stiles and Helliwell} [1975]. Emission radiation is assumed to come from the transverse currents formed by counterstreaming electrons that have been temporarily phase bunched by the constant frequency triggering signal. \newline These observations have prompted a new NRL Space Physics Simulation Chamber investigation of whistler wave dynamics. The ultimate goals of these experiments are to understand the mechanisms responsible for ducting, self-focusing, and amplification of whistler waves, to investigate nonlinear whistler-plasma interactions, and to study the secondary emission of whistler waves. The initial experiments concentrate on the ducting of whistler waves in pre-existing density depletions and enhancements. Density structures with controllable scale size and depth will be created using methods previously developed for a Space Chamber investigation of the dynamics of magnetospheric boundary layers [{\it Amatucci et al.}, 2003]. \newline *Work supported by the Office of Naval Research and DARPA. \newline Stenzel, R.L., {\it Phys. Rev. Lett.}, {\bf 35}, 574 (1975). \newline Stiles, G. S. and R. A. Helliwell, {\it J. Geophys. Res.}, {\bf 80}, 608 (1975). \newline Amatucci, W. E., {\it et al.}, {\it Phys. Plasmas}, {\bf 10}, 1963 (2003).
SM23A-0483 1340h
Plasma Impedance Measurements Using a Small Spherical Probe$^{1}
Using the response of a spherical probe to a swept RF signal generated with a network analyzer, we have begun a study of plasma impedance properties in the large Space Physics Simulation Chamber (SPSC) at the Naval Research Laboratory. From a measurement of the ratio of reflected to incident power we characterize plasma impedance as a function of the probe driving frequency which is much lower than frequencies for detectable electromagnetic radiation. We demonstrate the measurement of electron density and compare results to those of a Langmuir probe. As the capacitive and inductive responses of the plasma are adiabatic, it is only in the presence of a dissipation mechanism that the driving energy can be extracted as work. In plasmas where the total electron collision frequency, $\nu_{e}$, is on the order of or larger than the plasma frequency, $\omega_{p}$, the dissipation often occurs through collisions. In the case of collisionless plasmas the sheath-plasma resonance, which occurs at fractions of $\omega_{p}$, up to $\omega_{p}$/2, is a possible dissipation mechanism. We present observations of energy transfer under collisionless conditions and compare this to predictions of a kinetic approach. $^{1}$Work supported by ONR and DARPA
SM23A-0484 1340h
Effect of cross-field flow on Alfv\'{e}n waves of small transverse scale.
The interaction of short-scale Alfv\'{e}n waves with large-scale, cross-field flows may be a source of nonlinear phenomena that results in electron acceleration and ion heating in the magnetosphere, and the auroral ionosphere. The phenomena can also be studied in laboratory experiments. An electromagnetic particle-in-cell (PIC) code, PARSEC, is used to identify key features of these interactions by simulation of the propagation of a shear Alfv\'{e}n wave having transverse scale on the order of the electron skin-depth through regions with a prescribed ExB plasma flow. The flow is expected to convect the Alfv\'{e}n wave and to cause filamentation of the parallel current for flow speeds that exceed a threshold value. These processes are investigated for the kinetic as well as the inertial regime of propagation.
SM23A-0485 1340h
Hybrid Simulations of Kinetically Excited Global Magnetospheric Alfv\'en Waves
Shear Alfv\'en waves in the Earth's magnetosphere are investigated by a two-dimensional (2D) nonlinear gyrokinetic(GKE)-magnetohydrodynamic (MHD) hybrid simulation code based on the analytical formulation for two-component (core and energetic) plasmas [Frieman and Chen, 1982; Chen and Hasegawa, 1991]. While the core component, as the ideal MHD fluid, supports the Alfv\'en wave oscillations, the energetic component provides the kinetic instability drive via wave-particle resonances. Our preliminary studies have indicated that, given sufficiently strong energetic-particle drive, the excited Alfv\'en-ballooning modes can be radially trapped in the maximal kinetic drive region to form a 2D global unstable eigenmode; contrast to MHD predictions of no radial potential well structures. In the nonlinear state, our present focus is on the saturation mechanism of a coherent single-$m$ (the azimuthal wavenumber) unstable eigenmode. Specifically, we will explore nonlinear saturation via spatial detuning of wave-particle resonances due to the wave-induced radial excursions of energetic particles. Detailed 2D global stability features and saturation processes will be presented. Work supported by NSF and DOE Grants.
SM23A-0486 1340h
Auroral Undulations in the Afternoon/Dusk Sector Observed with the IMAGE FUV/Wideband Imaging Camera and Their Magnetic Connection to Plasmaspheric Plumes
On February 6, 2002 the IMAGE FUV/Wideband Imaging Camera (WIC) detected a periodic series of tongue-like emission features that extended equatorward from the main auroral oval in the afternoon/dusk sector. Although clearly discernible in the electron aurora, these features cannot be distinguished from background in simultaneous images of the proton aurora. The emission tongues were observed during the recovery phase of a moderate magnetic storm and occurred during a substorm expansion phase. Their wavelength, amplitude, and westward velocity are consistent with those reported for giant undulations observed at the equatorward edge of the diffuse aurora. However, the features seen on February 6 are embedded within a broader band of diffuse auroral emissions and in two instances are observed to brighten in conjunction with brief, localized intensifications along the portion of the oval to which they are connected. The emission tongues map to a well-developed plasmaspheric drainage plume, seen in geosynchronous plasma data as well as in near-simultaneous data from the IMAGE Extreme Ultraviolet (EUV) imager. We discuss this event in the context of previous work on undulations at the equatorward edge of the aurora and consider its implications for the various mechanisms that have been proposed to account for such structures.
SM23A-0487 1340h
Propagation Of Nonlinear Magnetosonic Waves
Using one- and two-dimensional MHD codes of Total Variation Diminishing (TVD) scheme, we perform simulations of propagation of nonlinear magnetosonic waves. Magnetosonic wave is a longitudinal wave propagating perpendicularly to the magnetic field lines, and involves compression and rarefaction of the magnetic field lines and the plasma. We first confirm the theoretical solution of Lee and Kim (2000) for the evolution of nonlinear magnetosonic waves in the homogeneous space. We then extend our numerical studies to one-dimensional inhomogeneous space and two-dimensional homogeneous space.
SM23A-0488 1340h
Latitude-independent Pc5 Geomagnetic Pulsations Associated With Field Line Resonance
The latitude-independent Pc5 pulsations with a spectral peak at $\sim$2.8 mHz were observed with IMAGE and SAMNET magnetometer array in the morning sector (0700-1000 local time) on April 29 (Day 119), 2001. The spectral amplitude had a local peak at $\sim$67$\deg$ geomagnetic latitude, where a sudden phase change of $\sim$180$\deg$ appeared. A vortical equivalent ionospheric current structure centered at latitude between 67$\deg$ and 71$\deg$ was observed during the Pc5 pulsations and the rotational sense of the current vortex was reversed for one cycle of the pulsation. During the interval of the enhancement of the Pc5 pulsations, the POLAR spacecraft in the morning side crossed near the magnetic shell (L $\sim$ 8) corresponding to the latitude where the spectral amplitude was maximum, and observed $\sim$2.8 mHz pulsations in the radial electric field and compressional magnetic field components. Since the toroidal mode Alfv\'{e}n waves in the magnetosphere are characterized by an electric field perturbation in the radial direction, the simultaneous presence of the pulsations in both components indicates that a field line resonance (FLR) was driven by compressional Pc5 pulsations. Using solar wind data, we conformed that the compressional Pc5 pulsations at POLAR occurred during an interval of enhanced solar wind dynamic pressure. From the analysis of the ground magnetometer data and POLAR data, we suggest that latitude independent ground magnetic perturbations are caused by the vortical equivalent current generated by FLR-associated field-aligned currents.
SM23A-0489 1340h
Cyclotron interaction for the chorus emissions in a nonuniform magnetic field
The chorus emissions have been frequently observed by the GEOTAIL spacecraft around the equatorial plane in the dayside outer magnetosphere. The chorus emissions consist of the elements with rapid frequency variation in a few seconds, and it is generally considered that they are generated via nonlinear wave-particle interaction between anisotropic electrons and whistler mode waves. The exact mechanisms of the chorus emissions involved in complicated behavior in amplification and nonlinear frequency shift of chorus emissions are not yet well clarified. We have confirmed an evidence of wave-particle interaction of the chorus emissions from comparison between wave and particle data simultaneously observed onboard GEOTAIL. It is found that the resonant electrons are pitch-angle diffused through the linear cyclotron interaction so that the initially unstable pitch-angle anisotropy is rapidly reduced to the small "critical anisotropy." We also analyze the cyclotron growth and propagation mechanisms of the whistler mode waves by using electromagnetic particle simulation. As the simulation model, we inject the whistler mode wave parallel to the magnetic field into hot electrons with large anisotropy in a one-dimensional model. From the simulation result, the resonant electrons with the resonant velocity corresponding to the wave frequency are pitch-angle diffused, then the initially unstable anisotropy decreases down to the stable anisotropy, which is onsistent with the GEOTAIL observation. We will show and discuss the evolution of the resonant electrons and the frequency shift of the chorus emissions in a nonuniform magnetic field.
SM23A-0490 1340h
Effects of field-aligned inhomogeneity on plasma wave coupling: Numerical simulations
Propagation of electromagnetic waves is studied in a magnetized plasma where the inhomogeneity parallel to the ambient magnetic field becomes important. In an inhomogeneous plasma, the plasma waves become complicated owing to the existence of local cutoff, coupling, and resonances. When the nonuniformity lies perpendicular to the magnetic field, it is well known that O- and X-waves are mode-converted to the electrostatic upper hybrid resonances. When the nonuniformity lies along the magnetic field line, R and L waves become also complicated since the wave equations become coupled. We numerically study the R and L wave coupling in a 3-D multi-fluid model. Characteristics of R and L waves are investigated in irregular density profiles. The finite difference method is used in both time and space for computational simulations. We discuss how each mode behaves near the singular locations by analyzing time histories of each component and compare our investigation to an earlier work.
SM23A-0491 1340h
Effects of field-aligned inhomogeneity on plasma wave coupling: Application of invariant imbedding method
In an inhomogeneous plasma, the plasma waves become complicated owing to the existence of local cutoff, coupling, and resonances. Since the plasma wave equations become coupled, it is often difficult to obtain the analytical solution. Approximations such as WKB and Bremmer series also become invalid near the local cutoff and discontinuities, and the singularities often give rise to complexity in interpreting the time-dependent solutions. We introduce a powerful technique of the invariant imbedding method (IIM), which is very useful in these kinds of coupled differential equations. With the aid of IIM, we can have an exact solution of plasma waves under certain conditions. In this work, we attempt to the wave coupling problem when parallel inhomogeneity is assumed in a magnetized plasma. We obtain the exact reflection and transmission as well as absorption coefficients when the wave propagates across such nonuniform region. It is discussed in detail how R and L waves are affected by the field-aligned nonuniform density by considering the mode conversion and reflection/transmission in the inhomogeneous region.
SM23A-0492 1340h
Using Radars in Place of Magnetometers: Detection and Properties of Pc3-5 Wave Fields in HF Radar Data
SuperDARN HF radars are usually used to examine HF echoes from field-aligned ionospheric irregularity structures. However, ground scatter is also often recorded. Because the ground scatter signal is reflected from the ionosphere its Doppler shift is a sensitive indicator of ionospheric motions. We have used the TIGER radar, which operates at relatively low latitudes, to examine ground scatter returns with high time resolution. Ground scatter returns are present virtually every day and wave-like Doppler shift features are evident almost each time. Comparison with ground magnetometer data shows that these are the ionospheric signature of downgoing ULF waves. Several different types of wave features have been observed, including very large scale Pc5, harmonics of field line resonances in the Pc3-4 range, and bandlimited Pc4 at night. This paper presents examples and discusses the wave generation and propagation mechanisms. Furthermore, estimates of the ionospheric transfer function over the 10-110 mHz range are compared with results of numerical and analytical modelling.
SM23A-0493 1340h
The Influence of Geomagnetic Storms on the D-Region at Mid-Latitudes
Subionospherically propagating Very Low Frequency (VLF) signals have long been used to detect perturbations of the upper atmosphere. In this paper, we examine the effects of several geomagnetic storms, including the Halloween storm (late October of 2003), on the D-region of the ionosphere using several VLF/LF signal paths in both the northern and southern hemispheres. The amplitudes of the received VLF signals exhibit rapid fluctuations following increased geomagnetic activity, with the fluctuations persisting several days after the advent of the geomagnetic storm. We examine the frequency content of these rapid fluctuations, as well as the location and timing of the disturbances, using multiple VLF/LF signal paths. The occurrence rate of lightning-induced electron precipitation (LEP) events, as recorded on several VLF/LF signals, was seen to be highly variable with geomagnetic activity. Comparison of LEP event occurrence rates with measurements of energetic electron flux levels from the NOAA-POES satellite supports the notion that this variability in the number of LEP events detected is largely due to increases in the energetic electron population in the slot region with the advent of geomagnetic activity. Finally, the modulation of the amplitude of received VLF/LF signals at ULF frequencies, possibly due to the Field Line Resonance (FLR) effect, is explored.
SM23A-0494 1340h
Drift Compressional Modes in Magnetospheric Plasmas
The global eigenmode structure of drift compressional modes (DCM) is examined using gyrokinetic formalism along with a radial WKB procedure. DCM are ultralow frequency trapped-ion modes that occur in high beta plasmas, whose underlying instability mechanism is a kinetic wave-particle resonance between the perturbed magnetic field compression $\delta B_{\|}$ and the magnetic gradient-curvature drifts of ions; tapping the free energy associated with plasma nonuniformities. The coupling of DCM to other polarizations is mitigated by two parameters: 1) cold electrons that decouple the electrostatic mode from the DCM, and 2) small mode localization width along the field line that decouples the DCM from the shear Alfv\'{e}n mode. Exploiting the fact that the most unstable modes typically have azimuthal wavenumbers comparable to the ion gyroradii, the general two-dimensional gryokinetic eigenvalue problem for DCM can be reduced to an integral equation along the field line with the radial structure determined by a WKB procedure. Using realistic equilibria appropriate for magnetospheric plasmas, numerical solutions for the two dimensional mode structure of DCM are presented. Implications of the wave excitation mechanisms and the global radial structures to the nonlinear saturation process will also be discussed. \footnote{ C.C. is supported in part by the U.S. D.O.E. Fusion Energy Postdoctoral Research Program administered by the Oak Ridge Institute for Science and Education. L.C. is supported by NSF Grant ATM-0335279 and U.S. D.O.E. grant DE-FG-94ER54736.}
SM23A-0495 1340h
Observed Relationship Between Ion Energization and the Broadband ELF Spectrum
Many existing theories linking broadband ELF wave activity (BBELF) to ion energization and outflow are based on the gyro-resonant heating of ions. Recent studies on the properties of these broadband waves show that other mechanisms may be more relevant. Using data from the Polar satellite, we will present an analysis of the observed relationship between the broadband electric field spectrum and ion energization across the cusp.
SM23A-0496 1340h
AKR rain: A study of periodically modulated narrow-band drifting AKR emission observed with the Cluster WBD instrument
We have studied a peculiar type of AKR emission detected at 125, 250, and 500 kHz using the WBD instrument on the Cluster spacecraft. The emission ('rain') is characterized by a series of extremely narrowband (50 Hz or less) bursts drifting downward in frequency between 3-12 kHz/sec and often strongly amplitude modulated with a frequency separation 150-250 Hz. Assuming the emission can be identified with the local gyrofrequency, each burst corresponds to a localized emitter of dimension 1 km or less (about 10 Debye lengths) traveling upward along a magnetospheric field line with a speed 200-300 km/sec. Since this speed range corresponds to the local ion acoustic speed, we ascribe the emission to EMIC waves which excite localized AKR emission. We use simultaneous observations of 'rain' bursts from multiple spacecraft to determine the angular distribution of individual bursts.
SM23A-0497 1340h
The role of EMIC waves in scattering electrons
The origin of killer electrons, those with energies greater than 1 MeV, in the Earth's inner magnetosphere is one of the main problems in magnetospheric research. It is theorized that these electrons are created by stochastic acceleration due to whistler waves with pitch angle scattering by electromagnetic ion-cyclotron waves (EMIC). Two-temperature bi-Maxwellian and ion-ring distributions are considered. The particle anisotropy thresholds for destabilizing the electromagnetic ion cyclotron mode are investigated. For the ring distribution, we will determine the condition of instability, the associated parallel wavenumber, and the width of the unstable parallel wavenumber spectrum.
SM23A-0498 1340h
Generation of Filaments by Beam-Plasma Interaction
Filaments have been observed near the bow shock and in other regions of Earth's magnetospheric plasmas. In this study, we carry out two dimensional hybrid simulations to investigate the structures and generation of filaments via interaction between the background plasma and an ion beam, whose velocity is parallel to the ambient magnetic field $B_0$. For the case with beam density $n_b=0.1$ and beam velocity $V_b=10V_A$, the simulation shows that right-hand nonresonant modes are dominant in the earlier linear stage, consistent with the linear theory prediction. In the nonlinear stage, the dynamics are, however, dominanted by two Alfven modes. While one Alfven wave mode propagates along $B_0$, the other is obliquely propagating. In the last stage of nonlinear wave evolution, filaments corresponding to field aligned structures of magnetic field $B$ and density $N$ are formed. The dominant filaments have anti-phase relation between $B$ and $N$, and nearly zero phase speed. Shear Alfven waves, meanwhile, remain coexisting with the filaments. The detailed mechanism for filament formation and nonlinear wave evolution will be presented.
SM23A-0499 1340h
Nonlinear Pulses of Small-scale Alfven Waves
A particle-in-cell code is used to investigate the dynamics of short pulses (on the order of one cycle) of shear Alfven waves having transverse scale comparable to the electron skin-depth or the ion Larmour radius. The signals are excited by pulsing a transverse electric field at a remote boundary across the confinement field at sufficiently large levels that result in travelling pulses with magnetic fluctuations delta $B/B_{0}$ on the order of a few percent . It is found that fast electrons are generated that outrun the pulse and also that secondary waves trail the front of pulse. The differences in behavior for the inertial and kinetic regimes are explored. The interaction of this pulse with a reflecting boundary will also be explored. The phenomena is of relevance to current models of auroral dynamics and to on-going laboratory experiments in which high-power pulses are applied.