SM51B-1634
Sounding rocket observations of particle data in the cusp
The winter 2008 Scifer-2 sounding rocket campaign studied ionospheric outflow in the cusp region. The rocket was launched on January 18, 2008 at 0730 UT from the Andoya Rocket Range in Norway, reaching an apogee of 1468 km over the Eiscat Svalbard Radar. The Scifer 2 campaign was designed as a joint case study, involving both ground and in situ observations, of the low altitude signatures of ionospheric outflow. In situ observations show a thermal ion population with temperatures around 0.6 – 0.8 eV, while ESR observes the temperature at lower altitudes to be ~0.2 eV. This difference is a result of calculating the average over all in situ look directions, which would artificially raise the temperature. In addition to the thermal ion population, there are several bursts of a hotter population of ions with temperatures ranging between 12 -20 eV, along with concurrent elevated wave activity. These hotter ions appear to have been accelerated to energies of several hundred eV, and show interesting velocity dispersion signatures with repeated bands at increasing energies. These two populations are not observed simultaneously, but rather are localized in different regions bordering one another. Additionally, the pitch angle distributions for each of these populations are different. Similar signatures have been seen by other nightside low altitude sounding rockets where upgoing low energy ions are seen adjacent to and coincident with higher energy ion precipitation. Neither observed ion population has a clear local relationship to the variations in the ambient electron temperature, which is a tracer for soft precipitation. We will continue to explore these populations and their boundaries as a case study of structuring in particle signatures in the cusp.
SM51B-1635
Development of a Low Energy Particle Electron Spectrum Analyzer (LEP-ESA) onboard the ICI-2 sounding rocket
Strong HF radar backscatter echoes are well-known characteristics of the polar cusp region by the ground- based observation of HF radar in the polar ionosphere. The gradient drift instability is regarded as a dominant mode for producing backscatter targets. According to Moen et al. [2002], decameter scale measurement that cannot be achieved by ground-based and satellite observations is required to understand the generation mechanism. Norwegian sounding rocket experiment ICI-2(Investigation of Cusp Irregularities) is proposed in order to single out the mechanism(s) running cusp ionospheric plasma unstable and facilitate backscatter targets for HF radars. The ICI-2 rocket will be launched into cusp ionosphere from Svalbard, Norway in Nov/Dec 2008. We are responsible for developing a low energy particle electron spectrum analyzer (LEP-ESA) that is one of the science payloads onboard the ICI-2 sounding rocket. LEP-ESA covers the energy range between 10eV and 10keV. We designed LEP-ESA to achieve high spatial resolution of ~10m/energy spectrum (16 energy steps). We have confirmed the performance of LEP-ESA by experiments as well as numerical simulations. In order to realize the high spatial resolution, high time resolution is required. For the purpose of high time resolution measurement of low energy electrons we have newly developed an electron detector that consists of Z-stack MCPs (Micro Channel Pates) and 64-channel multi-anode. An ASIC (Application Specific Integrated Circuit) with 64-channel fast preamplifiers and counters are installed on the backside of the anode. Since the detected electrons are independently counted by 64 separated anodes, multi-anode can achieve the higher time resolution than any other position sensitive anodes. One of the most severe problems in using a multi- anode is the size of the required electronics that becomes unacceptably large for the sounding rocket / satellite instrument when the number of the channels is large. By using the newly developed multi-anode with ASIC we have succeeded in solving this problem. ICI-2 sounding rocket experiment has great importance from the viewpoint of flight verification of the newly developed detector. In the present study, characteristics of the analyzer that has been studied with laboratory experiments as well as numerical simulations and performance of the MCP multi-anode with ASIC are shown. We also expect to present initial report of the flight performance of LEP-ESA.
SM51B-1636
Three Dimensional Simulations of Magnetospheric Cusps - Results of MHD Parameter Variations
The Magnetospheric cusps are a key feature of the Earth's magnetosphere, as they offer the solar wind its most direct entry point. The cusps regions contain diamagnetic cavities identified by significantly depressed magnetic field accompanied by enhanced density and pressure. Many questions remain with regard to these cavities. In order to investigate these regions, we have implemented a high resolution local three dimensional MHD simulation with an initial configuration representing a cusp-like magnetic configuration. We present results from a study of parameter variations in the vicinity of these cavities. Specifically, we address the dependence of local plasma dynamics on various magnetosheath parameters (e.g., flow speed, density, pressure) for varying IMF conditions.
SM51B-1637
Simultaneous Measurements of the High-Energy Particle Properties at a Cusp Diamagnetic Cavity and Surrounding Magnetosheath: Evidence for Local Acceleration Mechanism?
On February 2003 four Cluster spacecraft had a tetrahedron separation of 5000 km when they encountered a large cusp diamagnetic cavity characterized by depressed and fluctuating magnetic field and increased fluxes of high energy (up to ~ 160 keV) electrons, protons and helium. The Cluster separation provided a unique opportunity to determine the large scale structure of the cavity as well as to simultaneously calculate properties of the high energy particles in the cavity and surrounding magnetosheath. The RAPID data indicates that most high energy particles in the cavity have pitch angles of 75-100 degrees, and the High Energy Particle (HEP) fluxes drop in the magnetosheath as a function of distance from the cavity boundary, which would be more supportive of the local acceleration mechanism. We have compared our Cluster observations with the test particle simulations in the high resolution MHD cusp model, which indicates that local acceleration at least to energies of ~ 60 keV is possible due to the 'quasi'-potential which is formed due to reconnection. Additional acceleration may arise from the non-MHD processes. But the story may not be so simple, as there are few HEP intervals with lower fluxes in the magnetosheath that have pitch-angles of 15-45 degrees that could originate from the quasi-parallel bow shock. We will also discuss the origin of the 'turbulence' in the cavity.
SM51B-1638
Simultaneous Observation of FUV Aurora with Precipitating Electrons on STSAT- 1
We present the results of far ultraviolet (FUV, 1350-1750 Å) auroral observations made by the Far- ultraviolet IMaging Spectrograph (FIMS) instrument on the Korean microsatellite STSAT-1. The instrument was capable of resolving spatial structures of a few kilometers with the spectral resolution of 2-3Å. The observations were carried out simultaneously with the measurement of precipitating electrons using an electrostatic analyzer (ESA, 100 eV-20 keV) and a solid state telescope (SST, 170 keV-360 keV) on board the same satellite. With a careful mapping of the field lines, we were able to correlate the particle spectrum to the corresponding FUV spectrum of the footprints of the FIMS image that varied significantly in fine scales. We divided the FIMS spectral band into the LBH long (LBHL, 1640-1715Å) and LBH short (LBHS, 1380- 1455Å) bands, and compared the electron energies with the intensities of LBHL and LBHS for the well- defined inverted-V structures. The result shows a strong correlation between the total LBH intensity and the energy flux measured by ESA while the peak energy itself does not correlate well with the LBH intensity. On the other hand, it was observed that the ratio of the LBHL intensity to that of LBHS increased significantly as the peak electron energy increased, primarily due to a smaller absorption by O2 at LBHL than at LBHS.
SM51B-1639
On the Relationship Between Thin Birkeland Current Arcs and Reversed Flow Channels in the Winter Cusp/Cleft Ionosphere
A Reversed flow event (RFE) is a longitudinally elongated, 100-200 km wide channel, in which the flow direction is opposite to the background convection. Reversed Flow Events (RFEs) seem regulated by Birkeland current arcs in the winter cusp ionosphere above Svalbard. There is always a discrete auroral form situated at the clockwise flow reversal of the RFE, a thin Birkeland current arc, consistent with a converging electric field and an upward field-aligned current (FAC). One category of RFEs propagates into the polar cap in tandem with poleward moving auroral forms, while another category of RFEs moves with the cusp/cleft boundary. The RFE phenomenon is addressed to a region void of electron precipitation, and in lack of direct sunlight the E-region conductivity will be very low. We propose two possible explanations: i) The RFE channel may be a boundary transition phenomenon between two MI-current loops mapping to different locations forced by independent voltage generators with a small gap between them. ii) The reversed flow channel may be the ionospheric footprint of an inverted-V type coupling region. Electron beams of <1 keV will not give rise to significant conductivity gradients and the form of a discontinuity in the magnetospheric electric field will be conserved when mapped down to the ionosphere. These two explanations may be related in the sense that the boundary discontinuity in the magnetospheric electric field in i) may be the driver for the inverted-V in ii).
SM51B-1640
Double Cusp
When IMF y-component is dominant, a meridionally traveling satellite near noon frequently encounters two cusps that are separated latitudinally (double cusp). Examples of double cusp observations can be found in the satellite particle observations. The lower latitude cusp ion has little energy-latitude dispersion (stagnant) whereas the higher latitude cusp ion exhibits some dispersion. A particle precipitation model calculation of double cusp is presented. The dispersion signature can be explained from the ExB and the spacecraft trajectory. In the model, the lower latitude cusp ion originates from low-latitude magnetopause whereas the higher latitude cusp ion from high-latitude. The result suggests that reconnections simultaneously occur at low- and high-latitude magnetopause. The high-latitude reconnection would have a high magnetic shear but the low-latitude reconnection would have a modest magnetic shear. Examples of double cusps in the UVI images are also presented.
SM51B-1641
Modelling solar wind entry into the magnetosphere using mult-fluid MHD
Multi-fluid MHD modelling is a useful tool in the study of solar wind entry into the magnetosphere. We have shown in an earlier paper that this simulation technique allows solar wind and magnetospheric plasmas to be clearly distinguished, so that the self-consistent entry of the solar wind into the magnetosphere can be traced. Preliminary studies of this process for steady southward IMF were conducted with the multi-fluid version of the Lyon-Fedder-Mobarry global MHD model which currently includes two ion species and a neutralizing massless electron fluid. For this study, the solar wind plasma was specified to be 99.99% ion species A and 0.01% ion species B, with identical mass and temperature of both species at the upstream boundary in the solar wind. The system is then permitted to evolve until a quasi-steady magnetosphere develops. At this point, the fractions of the species are reversed at the upstream simulation boundary and the entry of species B traced. Plasma entry is clearly seen via the cusps, with some indications of entry in the vicinity of the flanks. Here, we examine changes in the dynamics of the previously quasi-steady magnetosphere resulting from entry of the now dominant upstream species B. We also investigate the effect of varying key parameters such as the length of time the system evolves (prior to the reversal of the species fractions) and the fractional concentrations of each species in the solar wind. The global characteristics of these results are contrasted with the evolution of the single-fluid MHD system. Additionally, results from a similar analysis of a steady northward IMF configuration are reported.
SM51B-1642
Cusp Injected Dispersive Ions as a Source for the Cold Dense Plasma Sheet: Simulation Results
The discovery more than a decade ago of a cold, dense plasma sheet near Earth during periods of northward IMF has prompted interest in mechanisms for its origin. While transport of magnetosheath ions across the low-latitude boundary layer via viscous interactions has also been considered, recent attention has focused on the possibility of injection at the high-latitude cusps, with reconnection providing direct access to the magnetosphere. In order for cusp-injected particles to remain trapped, simultaneous dual-lobe reconnection is required. Newly-filled flux tubes can then be expected to shorten as the result of field-line tension and convect around the flanks as they interchange with lower entropy, un-filled magnetospheric flux tubes. Observations of repeated, overlapping dispersive ion signatures near Earth favour this cusp injection scenario. Using particle tracking within electric and magnetic fields generated by the OpenGGCM global MHD model for an event with extended periods of northward IMF, we produce spatially- determined, low-altitude dispersive ion signatures. Direct comparison of simulation results, including synthetic spectrograms, with corresponding observations made by the FAST satellite support the dual-lobe injection picture over competing source mechanisms.
SM51B-1643
Dispersive Ion Injection Signatures Observed at FAST Altitudes during Times of Northward IMF: Characteristics and Origins
Overlapping, dispersive ion signatures seen by low-altitude spacecraft traversing Earth's auroral regions during extended period s of northward IMF can be explained by differing injection source and time of flight considerations, but observations suggest that the dispersion is most often spatially determined, and results from velocity filtering following quasi-steady cusp injection. We present first results from a comprehensive study of dispersive ions seen by FAST, providing statistical occurrence information about associated plasma characteristics. The positive, monotonic relationship between energy and latitude for individual dispersive bursts points to a high- latitude source. Loss cone information from pitch-angle distributions indicates whether these ions have mirrored zero, one or multiple times, and this helps establish that injections are occurring simultaneously in both hemispheres. Successive FAST orbits transect the auroral region at different local times and latitudes as Earth rotates underneath, which allows us to map these newly trapped ions throughout a broad range of the oval. We place these survey results into context with near-Earth cold dense plasma sheet observations and argue that dual-lobe reconnection is a likely source.
SM51B-1644
Polar rain flux variations in northern hemisphere observed by STSAT-1 with IMF geometry
Polar rain is a spatially uniform precipitation of electrons with energies around 100eV that penetrate into the polar cap region where geomagnetic field lines are connected to the Interplanetary Magnetic Fields (IMF). Since their occurrences depend on the IMF sector polarity, they are believed to originate from the field aligned component of the solar wind. However, statistically direct correlation between polar rain and solar wind has not been shown. In this presentation, we examined specifically the IMF strength influence on the polar rain flux variation by classifying of IMF sector polarities. For this study, we employed the polar rain flux data measured by STSAT-1 and compared them with the solar wind parameters obtained from the WIND and ACE satellites. We found the direct mutuality between polar rain flux and IMF strength with correlation coefficient above 0.5. This proportional tendency appears stronger when the northern hemisphere is in the away sector of the IMF, which could be associated with a favorable geometry for magnetic reconnection. Simple particle trajectory simulation clearly shows why polar rain intensity depends on the IMF sector polarity. These results are consistent with the direct entry model of Fairfield et al. [1985], while low correlation coefficient with solar wind density, the similarity between slops of both energy spectra shows that transport process occur without acceleration.
SM51B-1645
Cassini observations of Saturn's polar cusp
In this poster we describe observations from Saturn's southern polar cusp taken by the Cassini spacecraft in January and February 2007. A total of seven cusps were observed in a total of three orbits of the spacecraft. Two of these cusps contained evidence of significant plasma acceleration and heating and associated diamagnetic effects. Double cusps were seen on two separate orbits through the high-latitude dayside region. We present the evidence for the presence of magnetosheath-like electron distributions inside the magnetosphere and show both energy-latitude ion dispersions and low-energy ion cut-offs. We interpret these dispersions and cut-offs in terms of reconnection between the kronian and interplanetary magnetic fields with the solar wind and use the observations to estimate the location of the reconnection X-line. Finally the observations are compared and contrasted with similar observations in Earth's magnetosphere.
SM51B-1646
Cold Dense Magnetopause Boundary Layer Under Northward IMF: Results From THEMIS and MHD Simulations
A layer of nearly stagnant cold dense plasma is observed by THEMIS spacecraft on a closed field region immediately inside the dayside magnetopause near the low latitude boundary layer on 3 June 2007. Using the OpenGGCM global MHD magnetosphere numerical model, we successfully reproduce this observed cold dense plasma layer in the simulation. The simulation results show that reconnection first occurs poleward of the cusp in the northern hemisphere, creating new open field lines extending southward and forming an open field layer; then subsequently occurs in the other hemisphere, creating new closed field lines that capture the magnetosheath plasma and form the dayside cold dense plasma layer. In this event, the open layer and the skin of the cold dense plasma layer have a southward tangential flow while the inner part of the cold dense plasma layer has a more stagnant and more turbulent flow.
SM51B-1647
On the Properties of the Ionospheric Convection Drivers
The study presents an extensive statistical analysis of the sources of the ionospheric convection potential for two different global conditions: (1) Steady southward IMF and ionospheric 2-cell convection patterns, typical for a topology dominated by dayside reconnection. (2) Steady northward IMF and ionospheric 4-cell convection patterns, typical for a general lobe-reconnection topology. The data used are from the DMSP F13 satellite, covering the time period of 1996-2004. The derived potentials are compared with solar wind data from OMNI, and the influence of the solar wind parameters on the different potentials are studied in detail. The results show a significantly larger boundary layer potential for the northward IMF case, where the average contribution is on the order of 10 kV (corresponding to roughly 30-35% of the total energy input during northward IMF conditions), compared to the total boundary layer potential for southward IMF which only measures 1-2 kV on average. An analysis of the reverse convection potential, which functions as a good measure of the present energy input due to lobe reconnection, indicate a saturating behavior of the reconnection process for strong solar wind electric fields during northward IMF, similar to that previously reported for dayside reconnection. The saturated potential reaches a limit of around 60 kV, on the order of a fourth of the maximum potential for southward IMF.