SM12A-01 INVITED
Some Consequences of Penetration Electric Fields
During the main phases of geomagnetic storms penetration electric fields profoundly affect the dynamics of the inner magnetosphere as well as the mid- to low-latitude ionosphere. We derive a modified Volland-Stern model driven by plasmas and fields measured near L1 that predicts the strength of electric fields in the inner magnetosphere. A priori, values of the model's shielding parameter are unknown. However the spatial and temporal variability of equatorial plasma bubbles observed by DMSP satellites in the low-latitude ionosphere provide indirect, but compelling evidence that during that main phase of most storms electric fields remain unshielded. Penetration electric fields can persist for hours, not minutes. This empirical conclusion has major implications for estimating drift paths available to ring-current ions and modeling the dynamics of plasma plumes that extend from the dayside plasmasphere to the magnetopause. The presence of cold plasma through unusual locations in the outer magnetosphere in turn affects the growth rates of waves responsible for precipitation into the dayside ionosphere. Finally, we show that time-integrals of Volland-Stern electric fields are highly correlated with the development of main-phase Dst and hence with the total energy budget of current carrying particles in the plasma sheet and inner magnetosphere. .
SM12A-02
Inner Magnetospheric Electric Fields Derived from IMAGE EUV
The local and global patterns of plasmaspheric plasma transport reflect the influence of electric fields imposed by all sources in the inner magnetosphere. Image sequences of thermal plasma distribution obtained from the IMAGE Mission Extreme Ultraviolet Imager can be used to derive plasma motions and, using a magnetic field model, the corresponding electric fields. These motions and fields directly reflect the dynamic coupling of injected plasmasheet plasma and the ionosphere, in addition to solar wind and atmospheric drivers. What is being learned about the morphology of inner magnetospheric electric fields during storm and quite conditions from this new empirical tool will be presented and discussed.
SM12A-03
Equatorial measurement of SAID electric fields and relation with the plasmapause location
In order to investigate the equatorial source of subauroral ion drifts (SAID) and its association with the plasmapause position, multi-spacecraft measurements of SAID are presented using the CRRES, Akebono, and DMSP. Direct measurement of the convection electric field and plasmapause density close to the equator is measured by the electric field instrument onboard the CRRES satellite, and the plasmasheet electrons and low energy part of the ring current ions are measured by the low energy plasma instrument. The CRRES satellite is on the dusk inner magnetosphere, and the DMSP-F8 and Akebono satellites are approximately on the same field line. Associated with a substorm onset at 16:40 UT on February 20, 1991, the DMSP-F8 satellite at 19 MLT measures SAID with a maximum westward velocity of 1,500 m/s. The CRRES satellite is on outbound in the inner magnetosphere at ~21 MLT and ~5 RE at the onset of the substorm. It measures increase of DC electric field with 0.4 mV/m in the plasmasphere just after the substorm onset. Thirty minutes later, injection of ring current ions are observed in the plasmasphere with Bz decrease. After the crossing of the plasmapause, the electric field increases to 0.8 mV/m. At the same time, the spacecraft enters the plasmasheet, and the DC electric field disappears. The same time sequence is also identified in other SAID events detected on the dusk inner magnetosphere. The above CRRES measurement indicates that DC electric field is intensified in a narrow region between the ring current and electron plasmasheet after the onset of the substorm. Although the E*B drift points sunward in this region, this region with enhanced electric field is filled with plasmaspheric plasma without abrupt density change. The position where the convection electric field is equal to the corotation electric field locates inside the plasmapause. The plasmapause coincides with inner edge of the plasmasheet. This association suggests that the plasmaspheric plasma is depleted by the plasmasheet electrons, possibly by the enhanced E*B drift earthward of the plasmasheet. During the SAID event on 16:40 UT on February 20, 1991, the Akebono satellite was approximately on the same field line of the CRRES satellite (21 MLT and 5 RE) 40 minutes later the substorm onset. It measures enhancement of electric field with 2 mV/m between L=5 and 6. The inner edge of the electric field corresponds to the inner edge of ring current ions, and the outer edge coincides with the plasmasheet electrons. This signature of the electric field intensification in the charge-separated region is in accordance with the CRRES measurement. This study has clarified that the equatorial source of SAID electric fields is charge separation of ring current ions and plasmasheet electrons by electric field associated with substorms. This is consistent with the theoretical study by Southwood and Wolf [1978] and low-altitude measurements by Anderson et al. [2001] by that the charge separation provides current and voltage sources and the electric field is increased by the low conductance of the subauroral ionosphere. http://stpp1.geophys.tohoku.ac.jp
SM12A-04
An Inner-Magnetospheric Electron Density Database Determined from IMAGE/RPI Passive Dynamic Spectra
The magnetospheric electron density (Ne) is a fundamental space-physics parameter. It is often difficult to make Ne measurements to an accuracy better than a factor of two in low-density (Ne ~ 1 cm-3 or less) space plasmas due to problems associated with spacecraft/plasma interactions which become enhanced in such an environment. Plasma-wave techniques can provide accurate Ne measurements if the wave modes of the received signals can be properly identified. The amplitudes of the signals received by the Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite during passive observations are displayed as a function of frequency and time to form a dynamic spectrum. A semi-automated fitting technique has been developed to extract Ne from these dynamic spectra. The fitting method considers physical features found in the dynamics spectra, such as the upper-hybrid band, the continuum edge, and banded emissions. This technique is often able to provide in situ Ne measurements along extended portions of the orbit of IMAGE. With nearly five years of data from 2001 to 2005, the 14-hour orbit of IMAGE resulted in ~ 6000 separate dynamic spectra. This semi-automated method allows the dynamic spectra to be analyzed in a timely manner to determine Ne. The results have been used to form a plasmaspheric Ne database that is being made available to the scientific community.
SM12A-05
Multiple Large Scale Energy Bands Inside the low Altitude Radiation Belts Caused by Human and Natural Waves
The study of the radiation belts performed at low altitudes (600 km) onboard the Demeter satellite reveals several discrete large scale energy structures caused by the interaction of electrons with waves. Three of these structures are repeatdly observed: -i) An energy-L dispersed structure with one or two superposed energy bands due to the interaction of electrons with the powerfull NWC VLF transmitter at 19.8 kHz is seen during night, at L values between 1.4 and 2, when the transmitter is on; the energy of the resonating electron changes as a function of the season as expected from the variations of the density of the thermal plasma in the geomagnetic equatorial plane, -ii) A structure with superposed multiple energy bands is measured West of America inside the inner radiation belt (L=1.25-1.6), mainly during very active periods, with the electron energy spectra at a given location showing up to 10 peaks in the energy range from 70 keV to 1 MeV, and similarly, -iii) a structure with up to 12 energy peaks in the energy range from 200 kev to 2.5 MeV is measured west of South Africa at L Value of about 2.7. For these two last structures the multiple electron energy peaks are separated by the same energy at a given location. This is tentatively interpreted as indicating that these structures are due to the high orders resonance of electrons with whistler waves. Other possibilities are also presented and discussed.
SM12A-06
Low-altitude Measurements of 2-6 MeV Electron Trapping Lifetimes at 1.5 < L < 2.5
During the Halloween Storm period (October-November 2003), the Van Allen belt electron population was powerfully accelerated and re-established inward of its normal position. The new inner belt of electrons formed in this process decayed over a period of days to years. We have examined quantitatively the decay rates for electrons seen in the region of 1.5 < L <2.5 using SAMPEX satellite observations. We find that at L=1.5 the e- folding lifetime for 2-6 MeV electrons was τ~ 180 days. On the other hand, for the half-dozen distinct acceleration (or enhancement) events seen during late-2003 through 2005 at L~ 2.0, the lifetimes ranged from τ~ 8 days to τ~ 35 days. We compare these loss rates to those expected from prior observational and theoretical studies. We find that these lifetimes at L=2.0 are much shorter than the average 100-200 days that present theoretical estimates would suggest for the overall L=2 electron population. Additional wave-particle interaction aspects must be included in theoretical treatments. We describe such possibilities in this talk based on careful new analysis of particle scattering lifetimes at low L-values.
SM12A-07
Integration of the Radiation Belt Environment Model Into the Space Weather Modeling Framework
We have integrated the Fok Radiation Belt Environment model (RBE) into the Space Weather Modeling Framework (SWMF). RBE is coupled to the global magnetohydrodynamics component (represented by BATSRUS) of the SWMF. The radiation belt model solves the convection-diffusion equation of the plasma in the range of 10keV to a few MeV. In stand-alone mode RBE uses Tsyganenko's empirical models for the magnetic field. In the SWMF the BATSRUS model provides the time dependent magnetic field by efficiently tracing the closed magnetic field lines and passing the geometrical and field strength information to RBE at a regular cadence. We discuss the coupling algorithm and show some preliminary results with the coupled code. We run our new coupled model for periods of steady northward and southward IMF and compare our results to the radiation belt model using an empirical magnetic field model. We also simulate the radiation belts for an event of active time period.
SM12A-08
A dynamic model of the radiation-belt electron phase-space density based on POLAR/HIST measurements
The response of the energetic-electron phase-space density (PSD) in the radiation belts is subject to a delicate combination of acceleration and loss processes which are strongly determined by the magnetospheric configuration and field disturbance level. We quantify the response of the density to stormtime fields as observed by the HIST detector on board POLAR. Several distinct modes are identified, characterized by peak second- and third- adiabatic invariants and peak delay time. The modes represent quasiadiabatic transport due to ring current activity; high L* (~6), day-long acceleration linked to ULF wave-particle interaction; and low-L* (~3), minute- to hour-long acceleration interpreted to be due to transient inductive fields or VLF wave-particle interaction. The net transport due to these responses is not always or everywhere diffusive, therefore we quantify the degree of departure from diffusive transport for specific storm intervals and radial ranges. Taken together the response modes comprise a dynamic, nonlinear model which allows us to better understand the historic variability of the high-energy tail of the electron distribution in the inner magnetosphere.