SM31A-01
Ring Current Dynamics Using Dipolar, Empirical, or Self-Consistent Magnetic Field Models
We use our kinetic ring current-atmosphere interactions model (RAM) that has been recently extended for non- dipolar magnetic field geometry to investigate the effects of various magnetic field models on ring current evolution. In addition, we have coupled our RAM with a 3-D equilibrium model that calculates self-consistently the magnetic field in force balance with the anisotropic ring current distributions. We use this newly improved model to simulate ring current dynamics during several geomagnetic storms and study the mechanisms responsible for the storm-time injection, trapping, and loss of energetic particles. We find that as strong depressions in both the empirical and self-consistent magnetic field develop near Earth on the dusk-to-midnight side during the main phase of a storm, the particles' gradient-curvature drift velocity increases locally several times, while the bounce- averaged hydrogen density decreases significantly. We compare ring current simulations using a self- consistently calculated magnetic field with simulations using either dipolar magnetic field or the empirical magnetic field models of Tsyganenko. In particular, the effect of non-dipolar magnetic field geometry and the feedback of a self-consistently computed magnetic field on ring current dynamics are investigated.
SM31A-02
Field-Line (Euler-Potential) Model of the Ring Current
The equation of a magnetic field line in Dungey's model magnetosphere (dipole field plus uniform southward ΔB parallel to the dipole axis) is r = La[1 + (r3/2b3)]sin2θ, where r is the radial distance from the point dipole, a is the planetary radius, θ is the magnetic colatitude, and b (~ 12a, but possibly time-dependent) is the radius of the circular neutral line in Dungey's model. The dimensionless parameter L is inversely proportional to the amount of magnetic flux enclosed by the corresponding magnetic shell. The model for B thus described is curl-free and therefore current-free. In the present work we explore a formally similar B-field model in which the parameter b is allowed to vary spatially with L and possibly with φ (magnetic local time), so that the added field is no longer uniform nor even necessarily unidirectional. Our purpose is to simulate (in an analytically controllable way) the outward stretching of magnetic field lines associated with the presence of a mainly azimuthal ring current. To obtain a definite result and thereby test the model for reasonableness, we apply this method to the equatorial ring-current field model of Schulz [JGR, 102, 14149~14154, 1997], for which the amount of magnetic flux enclosed (a quantity inversely proportional to L) is expressible analytically as a function of equatorial radial distance r0. This approach yields the parameter b as a function of L for a specified model of equatorial ΔB. It thereby yields the Euler potential α (directly proportional to 1/L) as a function of r0, and therefore (from the equation of a generic field line) as a function of r and θ throughout the model magnetosphere. Moreover, since magnetic field lines are considered to lie in meridional planes for purposes of this construction, the magnetic field B itself is given by B = grad α × grad φ. Representative field-line configurations will be shown graphically for selected values of Dst.
SM31A-03
Ring Current Behaviour Inferred From Ground Magnetic and Space Observations
The precipitation of energetic ions and electrons into the upper atmosphere is a direct manifestation of their acceleration and pitch angle scattering in the magnetosphere. Electric fields inject/convect the particles from the tail plasma sheet towards the earth, and when closer to the Earth they spread in local time due to magnetic field forces. The electrons drift towards the morning sector and the ions towards the evening sector thus creating the RC (ring current). Certain aspects of the RC behaviour can be revealed by the precipitating energetic protons and electrons. From the protons a proxy for the energy injection rate into the RC can be estimated, and a RC-index which correlates highly with the pressure corrected Dst* can be calculated. In the injection/main phase of the storm the spatial structure of the RC in the midnight/evening MLT-sector can be inferred from the precipitation at high latitudes. The magnetic field at the Earth's surface exhibits an appreciable Magnetic Local Time (MLT) dependence in the initial and main phase of the storm. The field depression is very asymmetric, with the largest depression in the evening to midnight MLT sector. During such storms a well defined Storm Time Equatorial Belt (STEB) of Energetic Neutral Atoms (ENA) and ions is found to exist at low altitudes around geomagnetic equator. Ring Current (RC) asymmetry and symmetry inferred from the STEB are in accordance with results from ground based magnetic observations. There is, however, also a difference. The magnetic observations show the storms to be worldwide, displaying essential the same signature all around the equator. The STEB is not worldwide it appears first in the midnight/evening sector and then it appears in the morning sector largely consistent with the drift of the RC ions.
SM31A-04
Comparison of Magnetospheric Ion Temperatures Obtained by Inversion of Energetic Neutral Atom Images and Direct Analysis
Previous analysis of energetic neutral atom (ENA) images obtained during the 12 August 2000 storm yielded equatorial, inner magnetosphere, ion temperatures near midnight MLT that were consistent with in-situ ion temperatures measured with the Magnetospheric Plasma Analyzer instrument. The ENA derived ion temperatures were obtained through direct analysis of medium energy neutral atom images [Scime et al., 2001] and from equatorial ion flux distributions obtained from inversions of high energy neutral atom images [Zhang et al., 2005]. However, although both remote measurement techniques found evidence of a dawn-dusk asymmetry in ion temperature, the sense of the asymmetry was different for the two techniques. In this work we will present an analysis of additional storm intervals and discuss the spatial distribution of ion temperatures obtained with the two different ENA analysis methods.
SM31A-05
Evolution of Remotely Measured Inner Magnetospheric ion Temperatures During a Geomagnetic Storm
Previous studies have demonstrated that with the Medium Energy Neutral Atom (MENA) instrument aboard the IMAGE spacecraft it is possible to remotely measure the ion temperature of the terrestrial magnetosphere during periods of strong geomagnetic activity. However, neutral atom imaging of the magnetosphere during quiet intervals is more difficult. In this work, we show that by mapping neutral atom fluxes obtained over many days of observation to an equatorial plane fixed in Geocentric Solar Magnetospheric (GSM) coordinates it is possible to construct neutral atom images of the quiet time magnetosphere. Enhanced neutral fluxes in the energy range of 1 - 70 keV/nucleon are observed in the quiet-time pre-midnight region. A superposed-epoch analysis of multiple storm intervals also permits imaging of the ion temperature structure as a function of time through a geomagnetic storm. Using nearly forty geomagnetic storms to produce average ion temperature maps as a function of storm phase, we find that there are significant differences between the spatial distribution of neutral fluxes and ion heating during the main phase of storms. Pronounced ion heating (up to 12 keV) is observed on the dayside during storm main phase from 5 to 8 Earth radii. During the early recovery phase, the ion temperature on the dayside drops to approximately 9 keV and a colder region of approximately 6.5 keV persists near pre-dawn. In the late recovery phase of the storm, the ion temperature throughout the inner magnetosphere appears to relax to a nearly uniform 8 keV.
SM31A-06
Simulations of Small-Scale Electric Potential Structures in the Inner Magnetosphere During Storms
The relationship between the partial ring current pressure peak and the electric potential structure is examined. The hot ion pressure tries to limits itself through the electric field associated with the ionospheric closure currents. To investigate this negative feedback of the stormtime ring current, simulation results for several magnetic storms are analyzed and then compared against IMAGE HENA data for a variety of energy channels. In addition, several set-up configurations of the kinetic ring current model are used to assess the influence of the numerical approach. It is found that the small-scale well-and-peak potential pairs, formed when magnetotail plasma is injected or convected in to the inner magnetosphere, significantly change the near-Earth plasma distribution. The main pressure peak is broken into smaller peaks and the flow pattern of the hot ions is altered. One consequence of this is that the pressure peak remains on the nightside rather than shifting towards dusk. Other effects include the creation of subauroral polarization streams and injection flow channels in the potential distributions at various times during the storm, formed when the small-scale electric field is locally aligned with the large-scale electric field. The small-scale structures are not particularly visible in the energetic neutral atom images, but the location of the simulated pressure peak is often closer to the observed location when this effect is included.
SM31A-07
Ring current asymmetry during intense and superintense magnetic storms
It is well known that, during the main phase development of magnetic storms, the ring current encircling Earth at equatorial plane is strongly asymmetric. The degree of this asymmetry, however, should depend on the storm intensity. In order to assess these behavior, we study 15 intense (-250 < Dst -100 nT) and 15 super-intense (Dst -250 nT) magnetic storms. The geomagnetic indices ASY-H and SYM-H are employed and their ratio ASY/SYM is used as an indicator for the storm asymmetry. We have found that super-intense storms have a large degree of asymmetry around the storm peak. The average and median of the ASY/SYM ratio is found to be 15% higher form super-intense storms as compared to intense storms.
SM31A-08
Magnetic field Distortions and Energetic Particle Enhancements in the Southern Dawnside High-Latitude Region During a Geomagnetic Storm Period
The magnetic field distortions are observed by CLUSTER with orders of magnitude enhancements of the energetic (30 keV - 1 MeV) charged particles (electrons and ions) in the southern high-latitude region around 6 MLT for about two hours, from 15:10 UT to 17:20 UT, on May 4, 2006 when a geomagnetic storm (Dst=-62 nT) was detected. During this period, the IMF Bz was pointing northward, and the IMF By was pointing duskward for most of the time. The northward IMF component could reconnect with the geomagnetic field at the southern polar cap region, while the duskward IMF component might reconnect with the geomagnetic field in the dawnside high-latitude region in the southern hemisphere. The measured local magnetic field shows diamagnetic cavities with large fluctuations. The cavities and particles are very similar to those called cusp energetic particle (CEP) events that were previously found by POLAR in the high-altitude dayside cusp region. The geomagnetic storm started at about 11 UT on 5/4/06. The AE index increased from about 200 nT at 12:10 UT to about 1000 nT at 13:50 UT and decreased to less than 200 nT at 16 UT on that day. The geosynchronous particle data indicate that there were two energetic particle injections observed near midnight at 12:10 UT and 13:50 UT. These observations suggest a global effect in the magnetosphere drived by the external solar wind/IMF conditions. A preliminary analysis suggests that the energetic particle injection near midnight are due to substorms and the enhancements of the energetic particles in the southern dawnside high-latitude region are due to some unknown energization mechanisms. These are two different processes even if they have the same external drivers. The relations between the field distortions and the energetic particle populations and the relations between the storms/substorms and the CEP-like events in the southern dawnside high-latitude region will be discussed.
SM31A-09
The Quiet Time Ionospheric Source of Ring Current Plasmas in Boundary Related Coordinates
Almost all of the ring current plasma comes from the plasma sheet, which is in turn supplied by the ionosphere and solar wind. We know that O+ ions from the ionosphere are present in all regions of the magnetosphere at low levels even during geomagnetically quiet intervals. We also know heavy ionospheric ions such as O+ play a role in the evolution of geomagnetic storms, but we are not sure exactly what that role is. Large-scale modeling efforts constrained by observations provide the fastest path forward to increasing our understanding. One of the obstacles to effectively using the extensive information about ion outflow to constrain large-scale magnetospheric models has been the lack of information about the distribution of the ion outflow in relation to large-scale magnetospheric features such as the auroral oval. We have used data from the Polar satellite to determine the average number and energy fluxes of escaping energetic (15 eV < E/q < 33 keV) H+ and O+ ions in boundary related coordinates during geomagnetically quiet times (Dst < -50). The characteristic energy of escaping ions is determined from the ratio of energy and number fluxes. During quiet times, we found that the characteristic energies in the dayside and nightside auroral regions were moderately uniform. Characteristic O+ energies in the dayside and night side auroral zones are 120 and 700 eV respectively. For H+ the energies are 280 eV and 1.2 keV respectively. We found the most energetic and variable characteristic energies in the polar cap region. Comparison with other observations, including those of thermal O+ from Akebono show that the escaping energetic fluxes in the polar cap are a small fraction (2-3%) of those escaping from the auroral zone. If energization processes acting on auroral field lines above our 1 RE observational altitude are important only during geomagnetic storm intervals, the data presented here almost completely characterize the magnetosphere's ionospheric plasma source during geomagnetically quiet times thus providing an important constraint on large-scale magnetospheric models including mass composition that are in development.