SM22A-01 10:20h
Multipoint Observations of Pi2 pulsations During Substorm Recovery
Pi2 pulsations are usually discussed in relation to the expansion phase onset of magnetospheric substorms. Pulsations in the Pi2 band are observed at other times but the wave mode and excitation mechanism of these quiet-time Pi2 pulsations have not been identified. In this paper we examine Pi2 pulsations observed during the recovery phase of a substorm, 1300-1600 UT, January 7, 1997. The pulsations occurred as the auroral luminosity, measured by the UVI experiment on the Polar satellite, steadily declined. What is remarkable is that pulsations in the Pi2 band were present continuously. In the last hour of the 3-hour period the overall intensity of the aurora became very low (less than 1 GW) and no localized intensification of aurora is discernible in individual UVI images. The properties of the pulsations, as observed at mid- to low-latitude ground stations on the nightside, are similar to those of Pi2 pulsations associated with substorm expansions. Pulsations in the Pi2 band were also observed on the dayside. The Geotail spacecraft was located at (-30, -5, 3) Re GSE, in the plasma sheet near the Sun-Earth line, but it did not detect strong plasma flows. The observed properties of the Pi2 pulsations suggest that they originated from the same oscillation mode as usual Pi2 pulsations, but the stimulation mechanism of the mode is yet to be identified.
SM22A-02 10:35h
Low-latitude ELF Whistler-like Events Observed in Taiwan
Whistler-like events between 60 to 100 Hz have been detected in the ELF station operated by NCKU-ISUAL team at the Lulin Observatory, Taiwan (120$^{o}$52'E, 23$^{o}$28' N, 2862m high) from August 20, 2003 to July 29, 2004. The most distinguished feature is the frequency descent in the frequency-time spectrograms, resembling terrestrial whistlers. Other features of the ELF events include(a) a long event duration up to 2 minutes, (b) a daytime diurnal maximum occurring around 10 am, (c) a dominant magnetic field polarization in the north-south direction with strength at a few to tens of pT, and (d) no detection of vertical electric fields...etc. For the past 20 years, similar events were only reported twice, one at the auroral latitude (Heacock, 1974) and the other at the mid-latitude (Sentman and Ehring, 1994). Possible source mechanisms have been discussed are (a) magenetosheath lion roars propagating along the TEM-mode waveguide in the earth-ionosphere cavity, and (b) hiss detected in the equator. Lightning-generated whistlers are not favored in previous studies because of being unable to interpret the observed long dispersions with the electron density environment but all the mechanisms remain open questions. References Heacock, R.R., Whistler-like pulsation events in the frequency range 20-200Hz,$\em Geophys. Res. Lett.$, 2, 77, 1974. Sentman, D. D., D. A. Ehring, Midlatitude detection of ELF whistlers.$\em J. Geophys. Res.$, 99, 2183, 1994.
SM22A-03 10:50h
ELF/VLF Wave-Injection Experiments and Magnetospheric Probing with the HAARP HF Ionospheric Heater
Magnetospheric amplification of ducted ELF/VLF signals has been known and documented for over three decades. Research at Siple Station, Antarctica during the period 1972-89 illustrated many cases of ducting, amplification and emission triggering from waves launched on magnetic field lines and reflected from the ionospheric boundary in the conjugate hemisphere. Once reflected (one-hop) and twice reflected (two-hop) signals were found to occur for approximately 45% of transmissions on field lines ranging from L = 3 to L = 5. ELF/VLF wave generation via HF heating of D-region electrojet currents offers the potential for further controlled wave injection experiments. Signals generated by the High Frequency Active Auroral Research Program (HAARP) facility in Alaska have been observed extensively on the ground and in space. Recent observation of one-hop and two-hop echoes generated by HAARP [Inan et al., 2004] offers the ability to study cyclotron resonance wave amplification and emission triggering mechanisms. We report on the characteristics of the echoes and present a survey of statistical occurence of ducted magnetospherically amplified signals excited by the HAARP HF heater. The survey encompasses both geomagnetic activity and local ionospheric conditions.
SM22A-04 11:05h
Wave Propagation Characteristics at the Plasma Sheet Boundary Layer
Observations from Polar and Cluster have revealed large-amplitude Alfv\'{e}n waves propagating at the plasma sheet boundary layer during substorms. On the other hand, theories of reconnection have suggested that whistler waves are involved in the reconnection process. To assess the similarities and differences between these wave modes, a kinetic theory of these modes has been developed, including both electron and ion kinetic effects in the dispersion of these waves. Using this model, the observed characteristics of the various wave modes, such as the ratio between electric and magnetic fields, will be determined and compared with observations. These waves occur in the presence of strong gradients across the magnetic field in the boundary layer. The effects of phase mixing, non-local wave structure in the presence of the gradient and the possible excitation of drift-Alfv\'{e}n waves will be assessed.
SM22A-05 11:20h
Analysis of electron distributions associated with the source of auroral roar
In January of 2003, the HIgh Bandwidth Auroral Rocket (HIBAR) passed through two separate regions of strong upper hybrid emission in which the upper hybrid frequency was twice the electron cyclotron frequency $(f_{uh} = 2 f_{ce})$ [\textit{Samara et~al},2004]. These emissions are believed to be the source of HF auroral roar wave emission often observed on ground based receivers. The current theoretical model implies that the electro-magnetic waves observed on the ground are produced through mode conversion of strong emission of upper hybrid waves near 2 and 3 times the cyclotron frequency. The cyclotron maser theory predicts that the Z mode wave undergoes significant growth when the local upper hybrid frequency is just below $(\sim1%)$ twice the electron cyclotron frequency and with the appropriately unstable distribution of energetic electrons [\textit{Yoon et~al.}, 1996,1998,2000]. Though roar is frequently observed from the ground, the source region has rarely been identified in-situ. Analysis of the in-situ electron distributions from HIBAR show qualitative agreement with the theoretical distributions used by \textit{Yoon et~al.},1998. HIBAR encountered three distinctly separate regions where $f_{uh} \sim 2 f_{ce}$, two of these regions include strong upper hybrid emission, while the third is void of upper hybrid wave activity. The measured particle distributions demonstrate that, in the two regions with wave emission, the electron cyclotron maser instability condition is achieved and that, in the third region without wave emission, the instability condition fails. \begin{thebibliography}{} \bibitem{samara:04} Samara, M., J.~LaBelle, C.~A. Kletzing, and S.~R. Bounds, Rocket observations of structured upper hybrid wave at $f_{uh}=2f_{ce}$, \textit{Geophys. Res. Lett.}, submitted August 2004. \bibitem{yoon:96} Yoon, P.~H., A.~T. Weatherwax, and T.~J. Rosenberg, Lower ionospheric cyclotron maser theory: A possible source of $ 2f_{ce} $ and $ 3f_{ce} $ auroral radio emissions, \textit{J. Geophys. Res.}, \textit{101}, 27,015--27,025, 1996. \bibitem{yoon:98} Yoon, P.~H., A.~T. Weatherwax, and T.~J. Rosenberg, On the generation of auroral radio emission at harmonics of the lower ionospheric electron cyclotron frequency: X, O and Z mode maser calculations, \textit{J. Geophys. Res.}, \textit{103}, 4071--4078, 1998. \bibitem{yoon:00} Yoon, P.~H., A.~T. Weatherwax, and J.~LaBelle, Discrete electrostatic eigenmodes associated with ionospheric density structure: generation of auroral roar fine frequency structure, \textit{J. Geophys. Res.}, \textit{105}, 27,589--27,596, 2000. \end{thebibliography}
SM22A-06 11:35h
Remote Sensing of Magnetospheric Plasma Density from the Analysis of Discrete Whistler Mode Echoes Received by RPI on IMAGE
Whistler mode wave injection and reception using the RPI Instrument on IMAGE satellite has led to a new remote sensing method to measure the plasma density. During 2000-2002 period, RPI frequently recorded discrete whistler mode echoes in $\sim$10-400 kHz frequency range when IMAGE was at low altitudes ($<5000$ km) in the inner plasmasphere or near its perigee in the southern hemisphere. In most cases, the whistler mode echoes were accompanied by Z-mode echoes. About 85$%$ of the discrete echoes were observed during the winter time with an occurrence rate of $\sim 5%$ to $10%$ of the total number of transmissions at low altitude. No discrete echoes were observed during the summer time, which could possibly be due to the D region absorbtion. Ray tracing simulations indicate that the discrete echoes may result from reflections of RPI signals from the Earth-ionosphere boundary. By comparing the measured dispersion of discrete echoes with that from ray tracing simulations, it is possible to determine the plasma density along the ray path as well as the nonducted or ducted modes of propagation. When the discrete echoes were accompanied by Z-mode echoes, it was possible to determine the local electron density at the satellite from the Z mode upper hybrid frequency and the local gyrofrequency. This local electron density is used as the reference in the ray tracing model, which includes a diffusive equilibrium model for electron density inside the plasmapause and a $R^{-4.5}$ dependence with altitude outside the plasmapause. The altitude of the base of the diffusive equilibrium model is chosen as 1000 km based on the previous measurements of electron densities from ISIS-A, DE-1 and S3-3 satellites. The ray tracing simulations were carried out for 8 out of 68 cases observed in the year 2002 when discrete echoes were accompanied by Z mode echoes. The 8 cases were chosen to cover the maximum and minimum local electron densities, which varied from $\sim$ 300 - 4000 el/cc at the satellite location as measured from Z mode echoes. For four cases, IMAGE was at relatively low invariant latitudes (47$^\circ$ - 59$^\circ$) and altitude between $\sim$1200 - 1500 km; for other four cases, IMAGE was at higher invariant latitudes ranging from 65$^\circ$ - 82$^\circ$ and altitude from $\sim$1200 - 2300 km, corresponding to the auroral and polar regions. The simulations showed that the electron densities at the F2 layer peak ($\sim$ 250 km altitude)varied from $10^5$ to $7 \times 10^5$ el/cc for all the cases. The interpolated electron density at $\sim$ 4000 km altitude varied from $\sim$ $200$ to $2000$ el/cc for the cases at lower invariant latitudes and $\sim$ $60$ to $1000$ el/cc for the cases in the auroral and polar regions. The interpolated electron density at $\sim$ 8000 km altitude ranged from 100 - 1000 el/cc for the cases at lower invariant latitudes and from 20 - 60 el/cc for the cases in the auroral and polar regions. These results are consistent with previous observations of plasma density.
SM22A-07 11:50h
AKR Activity During Magnetically Quiet Periods
Auroral kilometric radiation (AKR) intensity is well known to be closely related to the substorm activity, indicating that the substorm strength controls the AKR development. In this study, we present the activity of AKR during magnetically quiet periods, when AE index is less than 100 nT, auroral oval is contracted to higher latitude, and particle injection is not detected at the geosynchronous orbit. The investigation showed that the onset of the "quiet time AKR" is triggered by a small substorm in the magnetosphere which is manifested by the small but clear auroral break-up, Pi2 pulsation, and sudden plasma flow in the plasma sheet. It is also shown that the quiet time AKR sometimes appears with an intense amplitude comparable with the usual substorm-time AKR. These results suggest that AKR appearance is related to the onset of substorm but development of AKR does not always depend on the scale of substorm.
SM22A-08 12:05h
Generation of Electrostatic Cyclotron Harmonic (ECH) Waves due to Loss Cone Distribution Functions
Electron cyclotron harmonic (ECH) waves are observed in the Earth's magnetosphere in the nightside near-Earth equatorial region. Observations indicate that a loss cone feature in the electron distribution function generates ECH waves, which result in pitch angle diffusion and the formation of a highly anisotropic "pancake" type of distribution in the low energy electron population. Electrostatic and electromagnetic waves are driven by loss cone distributions in the presence of cold plasma, which is known to strongly affect the linear dispersion properties of ECH waves. To examine the detailed properties of the ECH waves and ensuing wave-particle interactions, two-dimensional (2.5D) electromagnetic Particle-In-Cell (PIC) and Vlasov simulations are employed. The heating of the cold electrons and their effects on the saturation of the ECH waves will be determined. Implications for diffuse auroral precipitation will be discussed.