SM33A-1740
Using MHD/particle simulations to study the origin of EMIC waves during the compression event of 29 June 2007
On 29 June 2007, EMIC waves were observed on the ground by the CARISMA network of magnetometers and by three THEMIS spacecraft between L=5 and L=6.5. There are two theoretical pictures for the generation for these waves: Olson and Lee [1982] suggested that such waves may be generated by an adiabatic compression of the dayside magnetosphere, which causes the particles to preferentially gain energy perpendicular to the magnetic field. This, in turn, leads to the beam instability generally thought to produce EMIC waves. Summers et al. [1998] put forward the idea that substorm-injected ring current ions driven by enhanced convection excite EMIC waves near the plasmapause. We use the LFM global MHD model to simulate the event, with upstream boundary conditions determined by observations at the ACE (Advanced Composition Explorer) spacecraft. We then use a Lorentz solver to calculate test particle trajectories, and use these results to compute phase space densities of keV protons. We examine temperature anisotropies in ring current ions and compute EMIC wave growth rates for this event. Results are compared with the observations of Usanova et al. [2008], and with results from the two competing theories.
SM33A-1741
Frequencies and polarizations of ULF waves in the magnetosphere: effects of ionospheric Pedersen and Hall conductances.
We extended a recently developed Field line Resonance model based on covariant-contravariant formalism to include finite ionospheric conductivities. The nature of our model allows a straightforward incorporation of both Pedersen and Hall conductivities, while earlier studies were restricted to using Pedersen conductance only. In agreement with previous studies we find that Pedersen conductance results primarily in attenuation of the standing wave; its effect on the frequency and polarization of the wave is generally small. In contrast, Hall conductance by itself does not lead dissipation of the wave, but modifies wave's polarization. Thus, including ionospheric Hall conductance is extremely important for realistic modeling polarizations of Ultra Low Frequency wave, which have significance for electron energization in the magnetosphere. We present results applying our new ULF wave model to various background magnetic fields, such as idealized "compressed dipole" and Tsyganenko fields.
SM33A-1742
MINIS Observations During the Geomagnetic Storm of 21 January 2005: ULF Waves
The MINIS balloon campaign was successfully conducted in January 2005 to investigate relativistic electron loss mechanisms. The MINIS campaign provided multi-point measurements of electron precipitation up to MeV energies, including simultaneous measurements at different longitudes and hemispheres. Two balloons, each carrying an X-ray spectrometer for measuring the bremsstrahlung produced as electrons precipitate into the atmosphere, were launched from Churchill, Manitoba. Four balloons, each carrying an X- ray spectrometer, and a 3-axis electric field instrument providing DC electric field and VLF measurements in 3 frequency bands, were launched from the South African Antarctic Station (SANAE IV). An X 7.1 solar flare occurred at 0636 UT on 20 January 2005. A CME from this flare arrived at the Earth 34 hours later. An SSC began at ~1650 UT followed by a geomagnetic storm with a Dst perturbation of ~-100nT. The arrival of the CME initiated an interval of very strong relativistic electron precipitation. The second and third Southern payloads and the first Northern payload made observations in both hemispheres of several extensive relativistic electron precipitation events that occurred from 1700 to 2000 UT on 21 January 2005. ULF waves are thought to play a significant role in the acceleration and precipitation of these particles. This paper will present a preliminary review of the ULF spectra observed by the electric and magnetic field detectors. The status of efforts to determine the payload motion well enough to obtain ULF wave data from the on-board magnetometer will also be discussed.
SM33A-1743
Formation of the Ring Current as a Function of Solar Wind Conditions
Single particle tracking using time-dependent global magnetic and electric fields from multifuild simulations is used to investigate the generation of the ring current from ionospheric outflows of both hydrogen and oxygen ions. Observations of the energization and trapping of different ionospheric ions show that the energization of the particles is highly dependent of the location of their source. Losses from convection towards the dayside magnetopause is one of the major factors inhibiting the formation of a symmetric ring current, and in order to enhance the access of the energetic particles into the inner magnetosphere, it is shown that a northward turning of the IMF is needed in addition to the preliminary southward IMF that initially accelerates the particle. The magnitude of northward turning does not significantly affect ring current formation as only subtle differences are observed between the northerly turning case (-5 nT to 0 nT) and the strongly northward turning case (-5 nT to 5 nT ). This result indicates that a reduction is the cross-polar cap potential of about 50% is need to reduce dayside convection and allow the formation of a symmetric ring current. We also show that the particles that form the ring current are accelerated by small scale processes within a thin (< 1000 km) current sheet and that simulations with coarse grid resolution underestimate the energization of the ring current particles.
SM33A-1744
Evolution of Electron Phase Space Density Between and During Geomagnetic Storms: SCATHA Observations
Radial profiles of the energetic electron phase space density measured by the SCATHA satellite during the recovery phase of the storm an May 6-7, 1986 exhibited a locally-growing peak at L ~ 6. The peak occurred for large values of the second invariant K, but not for small K. A possible interpretation of this feature is as a signature of localized pitch angle diffusion by waves. As the storm recovery phase evolved, the peak gradually shifted to higher L. One may speculate that this behavior is due to the natural expansion of the plasmapause, which causes the conditions necessary for the causative plasma waves to also shift outward in L. This electron phase space density analysis has now been extended to several more storms and non-storm intervals. The radial profiles of electron phase space density are compared with similar profiles of wave intensity measured by SCATHA. Depending on the local time coverage of the SCATHA orbit, the wave emissions may or may not be accessible by the satellite. For a subset of the events, both phase space density peaks and wave emissions are simultaneously observed, enabling a detailed examination of the effects of waves on the particles. The results are compared with models of electron losses by quasilinear pitch angle diffusion.
SM33A-1745
Global nature of Pc 5 magnetic pulsation during the WHI observation campaign
In conjunction with the activities of IHY(International Heliophysical Year), an international observation campaign was planned and carried out from March 20 to April 16 of 2008. The name of this campaign is Whole Heliosphere Interval (WHI). During WHI, the nations of the world worked together to collect relevant scientific data. As a result, there now exists an exceptionally good data set of multi-point ground-based and satellite magnetometer data for this time frame. There were no clear and outstanding geomagnetic storms during WHI, but there were some moderate geomagnetically active moments. For example, on March 26, Dst index decreased from 25 nT to -41 nT for 10 hours(1000 -1900 UT). The amplitude of Pc 5 pulsation in the frequency band between 1.67 and 6.67 mHz at the MAGDAS stations increased for few days after March 26. Using magnetometer data obtained globally from ULTIMA(Ultra Large Terrestrial International Magnetic Array) stations, we will investigate the occurrence and wave characteristics(amplitude, period and phase) of Pc 5 pulsations. Particularly high-latitude Pc 5 observed at THEMIS (the Time History of Events and Macroscopic Interactions during Substorms), CARISMA(Canadian Array for Realtime Investigations of Magnetic Activity) and McMaC (Mid-continent Magnetoseismic Chain) stations will be compared with equatorial-latitude Pc 5 observed at MAGDAS stations(TIR, DAV, YAP, ANC, EUS, ILR, and UT=LT+5h, +8h, +9h, -5h, -2h and 0h, respectively). Acknowledgment: MAGDAS data used in this paper were obtained in mutual collaborations with the following representatives of various organizations; Prof. Archana Bhattacharya(Indian Institute of Geomagnetism, TIR), Fr. Daniel McNamara(Manila Observatory, DAV), Dr. David Aranug(Weather Service Office YAP, YAP), Dr. Ronald Woodman Pollitt(Instituto Geofisico del Peru, ANC), Dr. Severino L. G. Dutra(Brazilian National Space Research Institute (INPE), EUS), Dr. A. Babatunde Rabiu(Federal University of Technology, ILR).
SM33A-1746
First In-Situ Observations of Waveguide Modes in the Dayside Magnetosphere by Cluster
We present for the first time convincing evidence that the Cluster satellites observed a waveguide mode in the dayside magnetosphere following a minor enhancement in the solar wind dynamic pressure. The wave was observed in the vicinity of the spacecraft's perigee outside the plasmasphere. The waveguide mode lasted for at least one hour and had a frequency of 14 mHz. Poynting flux measurements show that at the beginning of the observations 50% of the wave's energy was reflected at some boundary in the inner magnetosphere. An hour later, when the last satellite observed the wave, only about 10% were reflected, i.e. the wave's character changed from partially reflected to almost purely propagating radially inwards in the course of the event. The theory of waveguide modes predicts that energy of the compressional wave is converted to standing Alfvénic waves where the frequency of the compressional driver matches the local fundamental eigenfrequency of the field line. Ground based magnetometers in the vicinity of the resonant field line indeed registered several pulsations during the event. Latitude profiles of Fourier power and phase show that these are Field Line Resonances (FLRs), supporting the model predictions. We show that the small amplitudes of the FLRs are consistent with the energy input of the compressional mode into the dayside magnetosphere. From considerations about the location of the turning point of the compressional wave we are able to estimate the azimuthal wave number to be small, between 1 and 2.
SM33A-1747
Resonant absorption of ULF waves by heavy ion resonances in the inhomogeneous inner-magnetosphere
The inner magnetosphere contains various heavy ions, which play an important role in the storm time ring current. Unlike single ion case, multi-ion plasmas have their own resonances, which are composed of in- phase or anti-phase motions among the ions. In an inhomogeneous plasma such as the inner- magnetospheric region of large magnetic field gradients, these resonances tend to interact with compressional ULF and EMIC waves where frequencies match those of multi-ion resonances. By solving the full coupled wave equations in an exact manner, we present how the ambient EM waves are mode-converted into these resonances of heavy ions i) near the equatorial region where the inhomogeneity lies almost normal to the magnetic field, and ii) near the mid-latitude region where the density gradient has inclination angle with the magnetic field. In both cases, it is found that strong mode conversion into the enhanced ion motion occurs at each resonance frequency, which is determined simply by heavy ion composition. These resonances show linear polarization as well as field-aligned propagation, which can explain many features of previous unexplained statistical observations in the inner magnetosphere.
SM33A-1748
Study of the Characteristics of Whistler-mode Chorus Generation in the Earth's Magnetosphere by Electron Hybrid Simulation
We study the properties of the whistler-mode chorus wave generation by a self-consistent particle simulation using an electron hybrid model. Recently we showed by the simulation that chorus emissions with rising tones are successively generated from the magnetic equator. For the generation mechanism of chorus emissions, the nonliear wave growth theory has been proposed by Omura et al. (2008) showing that the specific wave phase variation satisfying the maximum resonant current anti-parallel to the wave electric field undergoes significant amplification due to the effect of the selective nonlinear growth. Based on the proposed theory, the frequency sweep-rate of a chorus element is related to the wave amplitude at the saturation level of the instability driven by anisotropic velocity distribution of energetic electrons. In the present study, we performed simulations so as to discuss the validation of the theory of the chorus generation process. Simulation results show that the saturation level of the linear wave growth varied proportional to the assumed number density of energetic electrons. We estimated the frequency sweep-rate theoretically using the obtained satulation level and found that the estimated sweep-rates are consistent with those of chorus elements observed in the simulation results. The result of the present study serves an important clue in understanding the generation mechanism of chorus emissions and validates the proposed nonlinear wave growth theory.
SM33A-1749
Diffusive vs. impulsive energetic electron transport during radiation belt storms
Earth's electron radiation belts are continually replenished by inward particle transport (as well as other, local acceleration processes) taking place during radiation belt storms. For some storms the radial transport is primarily diffusive while for others it is impulsive, or characterized by injections. To distinguish between these types of inward transport, we first use a dynamic model of the phase-space density as measured by POLAR/HIST and expressed in terms of adiabatic invariants [Green and Kivelson, 2004]. In a review of storms from 1997 to 2004 the coefficients of the model are peaked at characteristic temporal and phase- space (mu, k, L*) scales during specific storms. The transport is quantified in terms of those invariants which are violated and identified with peaks of the electron distribution in invariant space. Second, we run guiding- center simulations in wave fields fitted to in situ measurements complemented at low and high L by ground ULF pulsations. The modes of response identified in earlier studies from SAMPEX and POLAR electron flux measurements are now associated with primarily diffusive transport in the central range of the outer belt, L=4-8, and primarily impulsive transport near the plasmapause boundary, L=3-4.
SM33A-1750
Effect of EMIC Waves on the Lifetime of Energetic Electrons in the Earth's Inner Radiation Belt
The stably trapped electrons in the inner radiation belt have lifetimes of years and energies higher than a few hundred keV. These energetic electrons can have serious effects on the way spacecrafts and satellites operate and cause significant hazards to low Earth orbit (LEO) satellites. For mitigating these hazards it is necessary to investigate ways for reducing the electron life times, for example, through pitch angle scattering by waves. For these waves, the gyro-resoance condition yields the minimum wavelength requirement for given particle energy and local magnetic field. For example, at the magnetic equator at L = 2 the waves resonant with 1 MeV electrons should have wavelengths less than 10 km. Low frequency Electromagnetic Ion-Cyclotron (EMIC) waves occur in three bands with frequencies below the hydrogen, helium, and oxygen ion gyro-frequencies, respectively. At frequencies close to the ion gyro-frequencies, the EMIC waves can have wavelength short enough to gyro-resonate with energetic electrons, which can lead to significant changes in the lifetimes of electrons in the inner ration belt. However at these altitudes the amplitudes of naturally excited EMIC waves do not yield significant scattering of the energetic electrons and artificial sources are needed. In order to define the characteristics of such sources we investigated the lifetime of inner belt energetic electrons subject to pitch angle scattering with EMIC waves. The resonant wave characteristics are obtained using the global core plasma model (GCPM). The lifetimes of the electrons in the presence of these waves are computed using the pitch angle diffusion coefficient for broadband waves. For several hundred Watts of broadband EMIC waves in the shell volume enclosed by magnetic field lines at L = 2.0 with width dL = 0.1, the lifetime of 1 MeV electrons can be reduced to a few months. This is a considerable reduction compared to the average life times of about years and have important consequences, including remediation of artificially enhanced energetic electron fluxes.
SM33A-1751
Magnetospheric Cavity Modes Driven by Solar Wind Dynamic Pressure Fluctuations: Initial Results From Global MHD Simulations and Applications to Radiation Belt Electron Dynamics
Magnetospheric ultra-low frequency (ULF) waves play an important role in the dynamics of radiation belt electron populations, enhancing rates of particle transport due to radial diffusion. There is increasing evidence that magnetospheric ULF waves relevant to radiation belt dynamics are ultimately driven by the solar wind. Thus, determining the solar wind parameters that are the most effective in generating magnetospheric ULF waves is an important question for radiation belt physics. Several observational studies suggest that fluctuations in the solar wind dynamic pressure can directly drive dayside magnetospheric ULF waves. To better understand this causal relationship, we present initial results from Lyon-Fedder-Mobarry (LFM) global, three-dimensional MHD simulations of the solar wind-magnetosphere interaction. These simulations are driven with idealized solar wind input conditions, where we introduce monochromatic and broadband ULF fluctuations in the upstream solar wind dynamic pressure. These idealized solar wind input conditions allow us to study only the effect of a fluctuating solar wind dynamic pressure while holding all of the other solar wind driving parameters constant. We present a limited set of results from five LFM simulations: four driven by monochromatic upstream dynamic pressure fluctuations and one by quasi- broadband upstream dynamic pressure fluctuations. We show that these upstream solar wind dynamic pressure fluctuations directly drive dayside magnetospheric ULF pulsations, in a manner similar to that suggested by Kepko and Spence [2003] and others. Moreover, we show that when the frequency of the upstream dynamic pressure fluctuations matches one of the natural frequencies of the magnetosphere, magnetospheric cavity modes are excited near the noon meridian. Our simulations also suggest that only even-mode number cavity oscillations are possible within the dayside magnetosphere. We compute the frequency, azimuthal mode number and power spectral densities of these dynamic pressure-driven ULF waves/cavity modes. This allows us to quantify the effect that these waves could have on radiation belt electrons, via ULF enhanced radial diffusion. We find that these dynamic pressure driven ULF waves could effectively interact with radiation belt electrons ranging from a few keV to energies in excess of 1 MeV.
SM33A-1752
Simulation of EMIC Excitation in a Model Magnetosphere Including Realistic High-density Plumes
The HOTRAY code is used to evaluate the path integrated gain of EMIC waves as a function of frequency in
two propagation bands above the O+ gyrofrequency. Calculations are performed over a range of L-shell
(3
SM33A-1753
Statistical study on the characterization of ULF Pulsations in the Inner Magnetosphere by THEMIS
ULF pulsations (2~25 mHz) have significant influence on the transport of energetic particles trapped in the outer radiation belt, which have drift frequencies comparable to this range. However recent studies indicate that not all classes of ULF waves are equally important for radiation belt electron acceleration. For example, simulations suggest that electrons could be adiabatically accelerated through a drift-resonance interaction with either azimuthal (toroidal) mode or radial (poloidal) mode ULF waves. THEMIS electric field observations provide a good opportunity to study and fully characterize the features of the ULF pulsations in the inner magnetosphere. The ULF polarization study by Sarris et al [2008] focuses on the observations made by one THEMIS probe during its out-bound pass on September 4th, 2007. Here we will present the statistical studies of the polarizations of ULF pulsations, based on the spin-fit electric and magnetic field data from July 2007 to June 2008, with full local time coverage. Through this global coverage, ULF polarizations are characterized at different regions, in terms of radial distance and local times. Preliminary analysis indicates that the polarizations of electric field pulsation are more radial in dawn/dusk sectors and more azimuthal in noon/midnight sectors.
SM33A-1754
Characterization of ULF Pulsations by THEMIS
This paper reports the first use of THEMIS to determine the polarization properties of ULF waves in a non- dipolar magnetic topology: The instrumentation and the alignment at close distances (less than 1 RE) among some of the THEMIS probes (particularly in the first period of its mission) provides unique opportunities to characterize ULF pulsations in the magnetosphere and enables us to validate and parameterize existing models. In the case study presented, electric and magnetic field fluctuations are identified as field line resonances and their modes of oscillation are compared to model estimates. Out of the various modes predicted by the model, the second fundamental mode, or toroidal mode, appears to have most similarities to the measured polarizations. Furthermore, phase-difference calculations using the probes' small separations allow estimates of mode number and propagation characteristics. It is shown that these observations give an excellent source for the verification of model estimates of frequency and polarization of the various modes of field line resonances in the magnetosphere.
SM33A-1755
3D Diffusion Analysis of Outer Radiation Belt Electrons During the October 9, 1990 Magnetic Storm
A well-known study of the October 9, 1990 magnetic storm [Brautigam and Albert, 2000] found that both decreases and increases of < 1 MeV electrons could be explained by radial diffusion, but that increases of > 1 MeV electrons required an additional mechanism. Energization by enhanced chorus waves was suggested. Since then, statistical models of various waves have been developed, quasilinear diffusion coefficients have been computed, and several simple simulations of electron dynamics have been performed. Here, the methodology of that earlier study -- the use of adiabatic invariants, activity-dependent diffusion coefficients, and time-dependent scaling of outer boundary fluxes -- is augmented with (coupled) pitch angle and energy diffusion by cyclotron-resonant waves, notably chorus. Results will be compared to CRRES data, and to the earlier results.
SM33A-1756
Poloidal Mode Field Line Oscillations in the Inner Magnetosphere
Plasmaspheric flux tubes may execute toroidal and poloidal standing oscillations when suitably stimulated by incoming ULF waves. The flux tubes can also support the field-aligned propagation of VLF whistler mode waves under appropriate conditions. The VLF signals may exhibit small periodic Doppler shifts due to radial flux tube motions. We examined about 100 such ULF/VLF Doppler events recorded over one year at L=2.5 using artificially generated whistler-mode VLF signals and ground magnetometers. Joint peaks in the VLF Doppler spectra and ground magnetometer spectra occurred at frequencies corresponding to the ion- cyclotron instability in the upstream solar wind. The VLF Doppler shifts were most likely due to radial motion of flux tubes of a few kilometers, driven by the east-west electric field of propagating ULF waves. When the frequencies match, these waves may couple to standing poloidal and toroidal field line oscillations, producing resonance signatures in both the H and D components on ground magnetometers.
SM33A-1757
Survey of Magnetospheric Line Radiation Observed by a Low-Altitude Spacecraft
We present results of a large survey of observations of Magnetospheric Line
Radiation (MLR) events. These are electromagnetic waves in the frequency range
about 1 - 8 kHz that, when represented in a frequency-time spectrogram,
have a form of several clearly seen lines, nearly equidistant in
frequency and with a rather slow frequency drift. They have been observed both
by satellites and ground-based instruments; however, their origin remains still
unclear. We have used 3 years of electric field data measured by the DEMETER
spacecraft (circular orbit with an altitude about 700 km) and a manual search for the
presence of MLR events, resulting in a unique data set of more than 600 events.
According to our knowledge, this is the largest satellite database of MLR
events collected to date. The properties of MLR events (frequency range, time
duration, dimension, etc.) have been thoroughly investigated. Moreover, it has
been checked whether the occurrence rate of these events is larger above some
specific (e. g. industrialized) areas. Finally, the most favorable geomagnetic
conditions (characterized by Kp and Dst indices) for the occurrence of MLR
events have been found.
http://os.matfyz.cz/papers/agu2008/
SM33A-1758
Full particle simulation of whistler-mode chorus emissions in the magnetosphere
We perform a numerical simulation by one-dimensional electromagnetic full particle code to study the generation mechanism of whistler-mode chorus emissions in the equatorial region of the magnetosphere. As a model along the ambient magnetic field strength in the vicinity of the equator, we assume a parabolic variation of dipole. We have cold thermal electrons and relatively low anisotropic hot electrons with loss cone as plasma particles. In the initial phase, the whistler-mode waves originated in background thermal noise interact with counter-streaming anisotropic hot electrons. The amplitude growth of the incoherent whistler- mode waves is determined by the linear growth rate. When the wave amplitude reaches a certain level, the chorus elements consisting of coherent phase in the vicinity of the magnetic equator grow rapidly, and their enhanced wave packets propagate from the magnetic equator toward both hemisphere. The frequency sweep rates of the consecutively excited rising chorus elements decrease gradually. We find a threshold of wave amplitude for nonlinear growth of chorus elements. The comparison between the wave amplitude and the frequency sweep rate of rising chorus elements in the simulation fully support the nonlinear wave growth theory for generating of chorus emissions.
SM33A-1759
Effect of Ultra-Low Frequency (ULF) Waves on Relativistic Electron Dynamics: An Observational Study
Understanding the drift-resonant interaction between relativistic electrons and ULF waves is a key to understand the acceleration of outer belt electrons. The September 25, 2001 storm sudden commencement (SSC) event presents us a good opportunity to study the effect of ULF waves on relativistic electron dynamics. We analyzed data from the constellation of 14 satellites, ranged among ACE and WIND satellites monitoring upstream solar wind condition, CLUSTER satellites measuring ULF wave and energetic electron flux, LANL and GOES satellites determining relativistic electron distribution around geosynchronous orbit, and NOAA POES satellite detecting radial diffusion of > 300 keV electron flux. We found that there exists a threshold in electron energy, above which the anomalous energization of electrons as described by Degeling et al. (2007) (Planetary and Space Science, 55(2007) 731-742) occurred. At lower energies we see the substorm injection on night side and the electron flux modulation by the local azimuthal electric field (Ephi) of ULF waves on dayside. In contrast, the drift dispersion signature and large enhancement of electron flux are more evident above the threshold. Our observed signatures of compressional waves (for instance, Ephi frequency is nearly independent of L, Ephi amplitude decreases with L, and the phase shift between Bz and Ephi changes from pi/2 to 0 as L increases) are qualitatively consistent with the ULF wave model in Degeling et al. (2007). In addition, the back-tracing of energetic electrons also suggested the disappearance of Ephi on the night side. Therefore, our observational evidences indicate the profound effect of azimuthal localization in wave amplitudes on the electron dynamics as suggested by Degeling et al. (2007).
SM33A-1760
Numerical modeling of radiation belt diffusion equations using layer methods
A new code using layer methods is presented to solve radiation belt diffusion equations, and used to explore effects of cross diffusion on electron fluxes. Previous results indicate that numerical problems arise when solving diffusion equations with cross diffusion using simple finite difference methods. We show that layer methods, which are based on stochastic differential equations, are capable of solving diffusion equations with cross diffusion and are also generalizable to 3D. We use our layer code to explore effects of including cross diffusion using two chorus wave models and a combined magnetosonic wave and hiss wave model (MH wave model). We show that cross diffusion is important for all three wave models but with different effects on electron fluxes. For the chorus wave models, cross diffusion is more important for higher energy particles at lower pitch angles, while for the MH wave model, cross diffusion is more important for higher energy particles at higher pitch angles. These results show that cross diffusion is not ignorable and should be included when calculating radiation belt electron fluxes.
SM33A-1761
Ground Observations of ULF Pulsations During Pulsating Aurora
The ROPA (Rocket Observations of Pulsating Aurora) sounding rocket was launched northward from Poker Flat, crossing the poleward boundary of a region of pulsating aurora. In association with ground support (incoherent scatter radar, ULF search-coil magnetometer, fluxgate magnetometer, and all sky imager), pulsating auroral signatures have been observed on the ground and in space. During or before the periods of pulsating aurora, the observations of ULF waves revealed two signatures. A narrow band signal, which exhibits an abrupt onset and a rising tone from 0.1 to 0.3 Hz over the course of an hour or two, precedes the pulsating aurora in 4 of 5 events studied but is not observed in the 5th event. Following the narrow band signature, or possibly beginning while the narrow band signal is still observed, broad band waves (characterized by noisy spectral structure) are observed that are strongly modulated (in all of the events) with a period on the order of 10 15 minutes. In this presentation, we discuss the nature of these waves such as polarization (predominantly right handed). Some of the events show double band spectral signatures where the upper ones rise and lower ones fall with periodic spectral signatures (4 - 5 min) and harmonic structures. It is also discussed how they are generated and what role they might play in pulsating aurora.
SM33A-1762
A statistical study of the Cluster/CODIF observations of He+ energization in the magnetosphere
It is well known that He+ can be resonantly energized by the electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. The presence of both anisotropic energetic protons and heavy He+ ions is believed to be an important condition of the generation and propagation of the EMIC waves and the wave- particle interactions near the He+ gyrofrequency. From January 2001 to April 2008, the Composition and Distribution Function (CODIF) Analyzer on board the Cluster satellites observed 41 He+ energization events. In this study, we performed a statistical study of these He+ energization intervals. We report the statistical location of the intervals and the typical properties of H+ and the thermalizated He+ ions. In addition, the statistical results of the solar wind/interplanetary magnetic field (IMF) conditions and the levels of geomagnetic activity during the intervals are also presented.
SM33A-1763
Generation of Chorus Waves and Their Effects on Electrons
Chorus emissions that propagate in the whistler wave band are observed in the inner magnetosphere and may play a role in the acceleration of electrons. To understand how these waves are generated and what their effects are on electrons, a study has been carried out using Cluster satellite observations and particle in cell simulations. The WBD, STAFF, and Whisper instruments onboard Cluster have made observations of chorus waves within the source region in the near-Earth magnetosphere and the PEACE electron instrument has shown the presence of multiple electron species with different temperature from cool (10's eV), to warm (100's eV) to hot (> keV). The warm species is highly anisotropic with perpendicular temperature 10 times the parallel temperature, which can provide the free energy for whistler wave generation. Using the wave and electron observations made in the source region, a linear theory and simulation study has been undertaken. Linear theory shows that the observed electron species are unstable to whistler waves at frequencies greater than 0.5 times the electron gyrofrequency, but less the electron gyrofrequency. To understand how the instability saturates and the ensuing wave-particle interactions, a Darwin electromagnetic particle in cell simulation study has been carried out using observed parameters. The effects of the whistler waves (i.e., chorus) on electrons and the implications for the radiation belt region will be discussed.
SM33A-1764
Nonlinear simulation studies of magnetospheric field line resonances
Field line resonances (FLRs) observed in the magnetosphere often have the amplitude of a few nT, which indicates that dB/B roughly satisfies ~0.01. The FLRs are excited by compressional wave energy via mode conversion, and there has been no apparent criterion on the maximum amplitude in the regime of linear approximations. Thus, such limited range of amplitude should be understood by including nonlinear saturation of FLRs, which have not been examined until now. In this study, the nature of nonlinear field line resonances (FLR) is studied by adopting full MHD simulations. The MHD code used here is based on the total variation diminishing (TVD) scheme and we have performed numerical simulations of FLR with its three- dimensional code. If the source perturbation is strongly impulsive and thus the timescale of the initial variations is sufficiently smaller than the convection timescale, FLRs are easily confirmed in these simulations. When the disturbance is sufficiently small, it is shown that linear properties of MHD wave coupling are well reproduced. In order to examine a nonlinear excitation of FLRs, the initial magnitude of disturbances is assumed to increase. Our results suggest that the maximum amplitude of FLRs become saturated at the level of the same order of dB/B as in observations. We present how FLRs behave differently for various disturbances both in both hot and cold regions.
SM33A-1765
The Physical Principles of the Generation of Whistler Mode Echoes by a Radio Sounder (RPI) on a Spacecraft (IMAGE)
This paper presents the physical principles of the whistler mode (WM) echo generation when WM waves are
injected from a spacecraft at low altitudes (< 5000 km). A WM echo is generated when a wave injected
from a
source in the magnetosphere reflects and comes back to the source either by retracing its path or by forming
a loop. There are two reflection mechanisms: 1) magnetospheric reflection (MR) that occurs at altitudes
where
the wave frequency (f) approximately equals the lower hybrid frequency (flh). WM waves that
undergo
magnetospheric reflection and come back to the satellite are called magnetospherically reflected whistler
mode (MR-WM) echoes. The echo ray path in this case is a loop. 2) Specular reflection that occurs at the
Earth-ionosphere boundary. There are two kinds of echoes that are generated via specular reflection: a)
vertically (or normally) incident specularly reflected whistler mode (VI-SR-WM or NI-SR-WM) echo that
retraces its path and returns back to the source. b) Obliquely incident specularly reflected whistler mode
(OI-SR-WM) echo that forms a loop and returns back to the satellite. The changes in topology and size of
the
WM refractive index surface as a function of altitude along the geomagnetic field line and the Snell's law
explain the generation of both retracing and looping types of echoes. The type of reflection that WM wave
undergoes and the properties of WM echoes generated depend on the value of wave frequency relative to
the
plasma frequency (fpe), gyro frequency (fce), and lower hybrid frequency (flh) along the field
line passing through the injection point (satellite). For example, waves injected from an altitude of a few
thousand kilometers lead to the following echoes: (1) In the frequency regime f SM33A-1766 Duskside Relativistic Electron Precipitation (DREP) Versus Microbursts as the Dominant Loss Mechanism From the Outer Belt.
Balloon missions have observed > 1 MeV electron precipitation on the duskside (DREP), while satellite
observations seem to indicate that microbursts are the main loss mechanism of these electrons. In order to
better understand this discrepancy, we present preliminary results of a study of SAMPEX data focusing
primarily on the 100 millisecond data obtained by the HILT and PET instruments over the entire useful lifetime
of the mission. We make an initial data cut to select only the bounce loss cone, which we set to be between L
= [3,8] and with a conjugate mirroring altitude at or below 90 km, in order to ensure that we are observing
local precipitation and not merely a trapped flux of electrons. We then use a transparent unbiased screen to
find relativistic precipitation events and attempt to resolve their temporal and spatial structure and classify
them as either DREP, microburst, or band precipitation.
SM33A-1767 Heavy ion effects on wave absorption at the magnetosphere
Recent ion composition measurements near the magnetopause have shown that heavy ionospheric ions can
dominate the mass density as much as 30 percent of the time. Heavy ions also significantly modify mode
conversion of low frequency compressional waves to kinetic Alfven waves by introducing the ion-ion hybrid
resonance (IIH) and Buchsbaum (BB) resonance in addition to the Alfven resonance, which increases the
efficiency of mode conversion in the Pc3 frequency range at the magnetopause. By adopting a multi-fluid
wave model which can fully include effects of electrons and multi-ions, we show how resonant absorptions
occur at the BB and IIH resonances in electron-proton-helium/oxygen plasmas. We present wave spectra and
the time histories of the electromagnetic fields at the BB, IIH, and cavity resonances. We also discuss the
role of the BB/IIH resonances on propagation and generation of EMIC waves in the magnetosphere. Our
results demonstrate that (1) incident fast waves are converted into the BB resonance for k∥ = 0 ,
where k∥ is the wavenumber parallel to B0, and IIH resonance for k∥ ≠ 0. (2)
At the BB resonance, the electrostatic component is growing while other electromagnetic components are
damped. (3) At the IIH resonance, for small k∥ value, the electromagnetic transverse components
gain the energy from the other component, which is similar to the FLR in the MHD regime. (4) The
electromagnetic fields show linear polarization at the IIH resonance.
SM33A-1768 Multipoint observations of Pc1-2 waves associated with a cold plasma density enhancement
During mid-September 2003 the Geostationary Operational Environmental Satellite GOES-9 observed a
number of Pc1-2 (0.1-1 Hz) emissions in the dusk sector, in association with the impact of a high-speed solar
wind stream. Localized enhancements in the equatorial cold plasma density were measured by the LANL
MPA instrument. For one of these observations, a Defense Meteorological Satellite Program (DMSP)
spacecraft, F-13, intersected the same magnetic field-line as GOES-9. We present combined observations
from GOES-9 at geostationary orbit, DMSP F-13 in the topside ionosphere, and the magnetic observatory at
Chokurdakh (CHD). All three observations show evidence of Pc1-2 band wave activity across a limited
region. The left-hand polarization of the waves indicates that these are electromagnetic ion-cyclotron (EMIC)
waves. In the region of field-line conjunction DMSP also observed high-energy ion precipitation. Using a
variety of instruments and a 2.5 dimension MHD model we present a scenario for the generation of these
waves and their propagation from geosynchronous orbit, through the ionosphere, to the ground.
SM33A-1769 Global characteristics of Pc5 ULF waves associated with substorms
With the recent success of using Polar ultraviolet imager (UVI) data to probe the global structure of the
ultralow-frequency (ULF) waves in the magnetosphere associated with solar wind dynamic pressure
enhancements, we now explore the global structure of ULF waves associated with magnetospheric
substorms. We investigate ULF waves during the three phases of 5 large isolated substorm events and
found: (1) Multiple monochromatic ULF waves appear in the Polar UVI images, (2) the "auroral pulsation" is
confined on the nightside between 18 and 03 MLT with the maximum wave power coincident with the
westward traveling surge, and (3) some of these ULF waves show the classical field line resonance (FLR)
signature: A phase change of 180 degrees across the wave power maximum. In one event, we found
discrete ULF waves occurring on the dayside from 04 to 17 MLT, with different frequencies from their
nightside counterparts. These dayside ULF waves are likely to be associated with the magnetospheric cavity
mode. We will present detailed analysis results and discuss implications of the results during the presentation.