SM42A-01 INVITED
Ionospheric Structuring in Alfven waves
We show observations from the FAST satellite indicating that dispersive Alfven waves erode the topside auroral ionosphere. Observations from a case study example show that Alfven waves become focussed into density depressions and through a cascade to small scales drive mass outflow from these depressions to form deep cavities where the ionospheric plasma may be depelted by more than 90 percent. This process is self sustaining in the sense that deeper cavities stimulate more rapid production of small scale waves and hence further mass outflow. The intense density gradients within these cavities are shown to support double layers driven by the Alfven wave current which is intensified through ionospheric feedback. Observations show that this process can occur over an extended latitudinal range leading to a highly depleted/fragmented density profiles over the altitude range traversed by the FAST sateliite (350-4100 km). We present statistical measurements indicating that this process commonly occurs whenever Alfven waves on small scales are observed from the FAST satellite in and above the topside ionosphere.
SM42A-02 INVITED
Searching for ULF signatures of the Cusp: Observations from search coil magnetometers and auroral imagers in Svalbard
Spacecraft traveling through the cusp at altitudes ranging from near the magnetopause to just above the ionosphere have consistently found the cusp to be filled with intense but irregular power in both electric and magnetic fields in the upper ULF frequency range (up to at least 4 Hz). Ground-based induction magnetometers have for many years observed Pc 1-2 (electromagnetic ion cyclotron) waves in this same frequency range at various latitudes including near the footpoint of the cusp, but it has not been possible with magnetometers alone to either confirm or deny a cusp source for any of these waves, partly because of the occurrence of horizontal ducting of these waves in the ionosphere. We report here on the first simultaneous, collocated observations of a set of induction magnetometers installed at three sites on Svalbard (Longyearbyen, Ny Alesund, and Hornsund), and auroral imagers also located at the first two of these sites. Data during northern winter 2006-2007, when the cusp footpoint was in darkness, showed occasional narrowband Pc 1-2 wave events, frequent broadband noise when energetic particle precipitation occurred overhead, but the consistent absence of broadband ULF power above the noise level when only soft cusp precipitation was overhead. The intensity of narrowband Pc 1-2 wave events most often increased toward lower latitudes, consistent with wave sources on closed field lines. However, on one day when the cusp was observed to be equatorward of Longyearbyen, band-limited Pc 1-2 wave power was strongest at Ny Alesund, the northernmost station. This latter observation is consistent with waves originating in the plasma mantle just poleward of the cusp, as was found in a recent study using the Polar satellite and more widely spaced ground-based ULF observations by Engebretson et al. [2005], and again suggests that the cusp proper is not the source of any narrowband Pc 1-2 wave activity that can be observed on the ground, and little if any broadband activity as well.
SM42A-03 INVITED
Intense Ion Outflow and its Association With Auroral Zone Alfven Wave Regions
In situ observations show that ion outflow occurs in three regions in the auroral zone. These three regions are the downward current region, upward current region, and Alfven wave region. Intense ion outflow in the Alfven wave region consists of ion conics with energies from ~10 to several 100 eV. The intensity of this outflow is typically an order of magnitude greater than the outflow from the other two auroral regions. Statistical studies of in situ data show that the outflow from the Alfven wave region extends over a broad range of local times that is approximately centered at ~0900 Magnetic Local Time. These studies require a minimum of several months of spacecraft in situ data to cover all local times in the auroral zone. This study investigates the instantaneous (~10 min time resolution) local time extent of ion outflow using neutral atom imaging from the IMAGE/Low Energy Neutral Atom imager. The instantaneous local time extent is compared to the results from statistical studies of in situ ion outflow data. In particular, the bias of the outflow to the dawnside (i.e., at the peak occurrence frequency of Alfven wave acceleration observations) is investigated as is the association between outflow and the location of the ionospheric footprint of the cusp.
SM42A-04 INVITED
Characteristics of Nightside Auroral Arcs
Auroral arcs have been studied by in-situ measurements over 30 years and these measurements have revealed a wide variety of plasmaphysical processes taking place in association with aurorae. This talk summarises some of the observations made in the vicinity of nightside auroral arcs in the ionosphere, at middle altitudes (thousands of km) and at high altitudes (a couple of ten thousand km) as observed by the EISCAT radar, Viking and Cluster satellites, respectively. Also, ground-based optical measurements have played an important role. Arc-associated current systems, electric fields at different altitudes and particle signatures are discussed for medium and small scale auroral arcs.
SM42A-05
Nonlinear coupling between density structures, field-aligned ion flows, ULF electromagnetic waves, and ionospheric feedback in the auroral zone
Results from a numerical study of the nonlinear interaction between ultra-low-frequency (ULF) shear Alfvén waves, slow MHD waves, and the high-latitude magnetospheric and ionospheric plasmas are presented. The basic hypothesis motivating this study is that heavy ion acceleration observed in the topside auroral and subauroral ionosphere and low-altitude magnetosphere, and associated inhomogeneities in density including plasma cavities and ducts (magnetic field-aligned density striations), can be produced by intense ULF electromagnetic waves standing and/or propagating along geomagnetic field lines in these regions. This study is based on a novel two-fluid MHD model describing ULF MHD waves in the cold, low-altitude magnetospheric plasma. The model includes nonlinear coupling between shear and slow MHD modes (parallel ion dynamics) as well as effects of E-region ionospheric activity leading to feedback instability. Numerical simulations of the model equations have been performed in dipole magnetic field geometry with realistic parameters of the ambient plasma. The simulations show that the ionospheric feedback is one of the major mechanisms responsible for formation of intense electromagnetic and plasma structures.
SM42A-06
Ionospheric electron upflow along open field lines: Derived from ISIS 2 measurements
Using the mass conservation equation, the parallel electron upflow velocity normalized by the base velocity in the open field line region of the topside ionosphere is derived as a function of electron density and neutral species densities. The normalized parallel velocity profiles are calculated using ISIS 2 electron density profile observations and neutral species density profiles from the MSIS model. From the cases analyzed, the derived velocity profiles show a similar trend of (a) a region of slow acceleration starting at the ionospheric F layer density peak height and up to about 150 km above it, (b) a region of fast acceleration (~ 500 km - 1100 km), and (c) another region of weak or no acceleration (> ~1100 km). A simple analytical function is proposed to describe the normalized parallel velocity profile. The parameters in the function have been determined by a multivariate least-square fit of calculated profiles to the analytical expression. These parameters define the magnitude and the height of maximum acceleration. For the cases studied the maximum acceleration height is in the range of 774 - 924 km. The parallel velocity can increase by a factor of up to 8 within the acceleration region. The results from the cases studied suggest that the chosen analytical function suitably describes the normalized parallel electron velocity in the high-latitude topside ionosphere. Furthermore, the acceleration mechanisms have been investigated. Using an existing model of the electron temperature, our results suggest that the enhanced pressure gradient force at low ionospheric altitudes may be the main driver of the observed acceleration in the electron upflow.
SM42A-07
Further Advances in Formula Representation of High-Latitude O+ Ionospheric Outflows Based on DyFK Simulations
Considerable interest exists in the magnetospheric community in obtaining compact representations of the ionospheric outflow fluxes and their relationships to putative drivers. Recent satellite data analyses by Strangeway et al.[2005] and Zheng et al.[2005] have obtained formula fits for the measurement-based relationships of the outflows levels to parameterizations for electron precipitation and Poynting fluxes, which are expected to be among the principal drivers, or closely related to them, for the ionospheric outflows. In this presentation, we shall use the results of an extensive set of systematic simulation runs with our Dynamic Fluid Kinetic (DyFK) simulation code for ionospheric plasma field-aligned transport to obtain O+ outflow flux levels versus precipitation electron energy flux levels, characteristic energy levels of the precipitating electron, and the peak spectral wave densities for BBELF waves which transversely heat ionospheric ions, and other geophysical conditions. We shall present spectrograms of the relationship of the ion outflow values to these electron energy flux and BBELF wave levels. A preliminary approximate formula representation at this time is: FluxO+ = 3.03*(3.0x105 + 0.02x109 fe1.4)(tanh(8Dwave)+0.2Dwave0.6)*exp([500- En]/390)2.6 where FluxO+ is the O+ number flux in cm-2s-1 at 3RE mapped to 1000 km altitude, fe is the electron precipitation energy flux in ergs cm-2 s-1, and Dwave is the wave spectral density at 6.5 Hz in (mV)2 m-2 Hz-1m and En is the characteristic energy of the precipitating electrons. Strangeway, R. J., R. E. Ergun, Y.-J. Su, C. W. Carlson, and R. C. Elphic, Factors controlling ionospheric outflows as observed at intermediate altitudes, J. Geophys. Res., 110, A03221, doi:10.1029/2004JA010829, 2005. Zheng, Y., T. E. Moore, F. S. Mozer, C. T. Russell, and R. J. Strangeway, Polar study of ionospheric ion outflow versus energy input, J. Geophys. Res., 110, A07210, doi:10.1029/2004JA010995, 2005.
SM42A-08
Wave Acceleration of Auroral Electrons in the Presence of Quasi-static Parallel Electric Fields
Recent observations from the FAST satellite as well as a number of sounding rocket missions have shown two distinct modes of auroral electron acceleration: the classic inverted-V signature consisting of a beam broad in pitch angle but narrowly confined in energy, and a lower energy, field-aligned acceleration that has been attributed to kinetic Alfven waves. Moreover, observations indicate that these two particle populations often co-exist, suggesting that Alfvenic acceleration can occur on field lines with a quasi-static potential drop. Reflected ionospheric electrons can interact with the downward propagating Alfven producing a Fermi-like acceleration mechanism. A non-local kinetic theory including trapped and reflected electrons has been developed to model these wave-particle interactions. Results from this model of the evolution of the electron particle distributions will be presented and compared with test particle simulations.