Ring Current and Radiation Belts I
Presiding: N J Fox, Applied Physics Laboratory, Johns Hopkins University; J L Roeder, Aerospace Corporation
SM41B-01 08:30h
Polar Observations of Isotropic Angular Distributions of Energetic Ions During Geomagnetic Storms
Observations of energetic ion pitch angle distributions during several geomagnetic storms are presented as evidence for pitch angle diffusion processes. Measurements by the CAMMICE/MICS and CEPPAD/IPS instruments on the Polar satellite are used to obtain the angular distributions during several storms. MICS provides ion composition of 1-200 keV/q ions of all major species including H+, He+, He++, and O+. These data are supplemented with proton observations in the energy range 20-1500 keV from IPS. The 20 keV H+ fluxes at L>5 in the dusk-to-midnight local time sector during several storms show nearly isotropic distributions with filled loss cones. This implies that pitch angle diffusion or scattering processes were active during the main and early recovery phases of the storms. Such distributions may be caused by two possible phenomena: interactions with electromagnetic ion cyclotron (EMIC) waves, or nonadiabatic motion of the ions in regions near current sheets. The isotropic distributions tend to be observed over a wider range of L than predicted by models of EMIC waves so that at least part of the data is more consistent with current sheet scattering than waves.
SM41B-02 08:45h
3D Ring current mapping based on ENA image inversions and the curlometer technique, using IMAGE/ HENA and Cluster/FGM/CIS data.
The inner magnetosphere's electric currents configuration and mapping is one of the keys for understanding current loop closure inside the whole magnetosphere. Development of the ring current as a function of geomagnetic activity is still one of the most interesting aspects about this region. A method for directly computing current is the multi-spacecraft curlometer technique, which is based on Maxwell-Ampere's law application. This requires the use of four point magnetic field high resolution measurements. The FGM experiment on board the four Cluster spacecraft allows for the first time an instantaneous calculation of the magnetic field gradients and thus a measurement of the local current density. Moreover, ENA image inversions can provide a mapping of the equatorial ion distribution. The HENA/IMAGE neutral atom image inversions allow thus the estimation of pressure gradient values, and the computation of the equatorial ring current distribution. Using these two complementary methods (curlometer technique using in-situ measurements and neutral atom image inversion using remote sensing data), an instantaneous 3D mapping of the current can be made. The ring current distribution has been studied for the 20 April 2002 storm-time event (Dst~ - 101 nT; Kp=6). During this period, Cluster was crossing perigee at R~ 4 RE in the evening sector (MLT~21), while situated inside the HENA/IMAGE field-of view. The Cluster constellation provided a direct measure of the local current density, by applying the curlometer technique. Furthermore, the CIS experiment onboard Cluster allowed a validation of the ion fluxes obtained from HENA/IMAGE data. Our analysis reveals the existence of an important local current density (~ 30 nA/m2) in this MLT sector, even if the Cluster spacecraft are situated eastward of the ring current bulk, as shown by the HENA image inversion. The curlometer results reveal also the existence of an asymmetry between the two hemispheres, characterized by a southward orientation of the current at the equator.
SM41B-03 09:00h
Reformulation of the Rice Convection Model to Include a Realistic Internal Magnetic Field
The Rice Convection Model (RCM) is a multi-fluid model of the plasma and electric field distributions of Earth's inner and middle magnetosphere. For computational ease, the RCM was originally formulated assuming that the planetary magnetic dipole axis was aligned with the rotation axis of the planet. Over the years several people have persisted in pointing out that, contrary to these assumptions, the intrinsic magnetic field of the Earth is not a simple magnetic dipole aligned with the rotation axis. While the planetary magnetosphere we have been modeling with the RCM has many Earthlike features, the assumed internal magnetic field geometry is not one of them. Accordingly, we have reformulated the RCM to accept a more general and realistic internal magnetic field. Such a reformulation is necessary in order to realistically include longitude/UT and seasonal effects in the RCM and to properly interface the RCM with global scale thermosphere/ionosphere models. We describe the reformulation of the model, the Euler-potential-based coordinate system used in the reformulated RCM, and early results of the new code.
SM41B-04 09:15h
Observations of energy spectra and flux isotropization of relativistic electrons during energization events. to relativistic energies.
The various models of electron energization in the Earth's magnetosphere may be broadly classified into two types; models where in-situ processes dominate and models where particle transport dominates. The physics of in-situ processes such as wave-particle resonance and transport processes such as radial diffusion result in distinct features of the energized electrons. These features include pitch angle distributions and timescales for energization and isotropization. Thus, characterization of electron acceleration properties such as electron spectra and flux isotropization are important in understanding acceleration models. We report here on measurements of electron spectra and isotropization time scales select electron energization events. We use sensors onboard SAMPEX and POLAR to measure these properties. SAMPEX measurements cover the entire outer zone for more than a decade from mid 1992 to mid 2004 and POLAR covers the time period from mid 1996 to the present. Pulse height analyzed data from the PET detector onboard SAMPEX are used to measure electron spectra. Flux isotrpization is measured by comparing SAMPEX and POLAR fluxes. We will use global field models such as the Tsyaganenko-96 model, to calculate the L parameter during geomagnetically disturbed times.
SM41B-05 09:30h
The Phase Space Density Distribution of Relativistic Electrons in Two GEM/IMS Selected Storms
What physics processes govern the behavior of relativistic electrons in the Earth's radiation belts in geomagnetic storm times is of keen interest to the space weather community. To address this question, two magnetic storms, during October 21-23, 2001 and September 4-9, 2002, have been selected for a thorough data and model study by the Geospace Environment Modeling (GEM) program as part of the Inner Magnetosphere/Storm (IMS) assessment challenge. This study will conduct a survey of the temporal evolving phase space density (PSD) distribution of relativistic electrons at fixed phase space coordinates in the two storms. Data used in this work include pitch-angle resolved electron and magnetic field measurements from multiple spacecraft, which are the LANL geosynchronous satellites, GOES satellites, POLAR and CLUSTER. With orbits going through all key inner magnetospheric areas, those satellites form a constellation which provides simultaneous measurements at multiple locations so that spatial factor can be easily separated from temporal one. Additionally, the fact that detected electrons have a wide range of adiabatic invariants, covering both equatorial and off-equatorial regions throughout L*~2-9, allows tracing a specified electron population across the adiabatic phase space. To provide the highest fidelity, the errors in the PSD calculation are constrained by employing the Liouville's Theorem to check the reliabilities of the calibrations and optimize the dynamic magnetic field model. This work will establish the PSD distribution map as a function of both universal time and local time, which can be compared later with model output and serve as the reference of differentiating physics mechanisms associated with acceleration, transport and loss of relativistic electron during storm phases.
SM41B-06 09:45h
Quantifying the Role Of Non-Adiabatic Processes In The Creation Of The Outer Radiation Belts
We have recently reported (Fox et al., 2005) strong evidence supporting suggestions that non-adiabatic (first invariant breaking) processes are required to explain the generation of Earth's outer electron radiation belt during intense storm events and also during more typical outer belt conditions. This evidence relies on the most conservative of assumptions that maximize the phase space densities of particle distributions displaced in our models from the source population within the near-Earth magnetotail to the inner magnetospheric regions. With the finding that the phase space densities of measured outer belt electron distributions exceed the phase space densities of these maximized source population distributions, we established qualitatively that non-adiabatic processes are required. The purpose of our follow-on studies is to establish the quantitative levels of the contributions of the non-adiabatic processes to the energization of the outer belt electrons. Here we bound these contributions by generalizing our transport calculations from the consideration of only first adiabatic constraints to the consideration of first and secondary adiabatic invariant constraints, electron scattering, and electron losses. The energies that electrons can acquire via radial transport under these new constraints are determined using data from ISEE and CRRES, and samples of spectra are examined at various L-values to identify the deficiencies in phase space density. Fox, N. J., B. H. Mauk, and J. B. Blake, Establishing the role of non-adiabatic processes in the creation of the Earth's outer electron radiation belt, Geophys. Res. Lett., submitted, 2005.