SM54A-01 INVITED
How Does Wave Turbulence in the Cusp Affect Ion Precipiation?
The open field-line configuration of the cusp allows solar wind particles to precipitate directly into the ionosphere. Precipitating particles exhibit distinctly different characteristics in energy, density, and temperature depending on local time and latitude. Many of these features (energy dispersion and double cusp distributions) can be predicted based on adiabatic guiding center motion of particles. However, adjacent DMSP spectrograms often show significant variation in flux and energy, and precipitating ion spectra are commonly wider in energy than expected from adiabatic mapping of magnetosheath populations. These observations are suggestive that wave-particle interactions in the cusp may provide an important source of energization of precipitating particles. Indeed, the open-field region of the cusp has access to wide-ranging ULF wave sources originating anywhere from the forshock, bow shock, magnetosheath, and magnetopause as well as waves generated by the energetic particle precipation. Wave fluctuations can energize particles as well as provide pitch angle scattering of energetic particles into the loss cone leading to uneven particle precipitation on adjacent field lines. We estimate particle spectra that would be expected from theoretical models of particle energization due to wave turbulence in the cusp region and compare with observations.
SM54A-02 INVITED
Cusp Structures and their Relation to Magnetic Reconnection
The magnetospheric cusp regions always play an important role in the study of magnetic reconnection at the Earth's magnetopause. Whatever process occurs at the magnetopause, a signature of the process can be found in the precipitating magnetosheath ions observed in the cusp as well as down in the ionosphere where the foot points of all magnetopause field lines converge to a relatively confined region. Plasma observations in the cusp and to in situ observations of field-aligned plasma flows close to the magnetopause (consistent with the occurrence of magnetic reconnection) have demonstrated that the fundamental process of magnetic reconnection occurs somewhere at the magnetopause for any condition of the interplanetary magnetic field (IMF). The distinct dispersion signature of the precipitating ions can be used to gain insights into details of the reconnection process, covering continuity, pulsation frequency, and location of the reconnection site. Observations of the FUV emissions caused by precipitating cusp ions in the ionosphere at the magnetic foot points of the cusp demonstrate the global nature of magnetic reconnection and provide insights into the temporal nature of the process, showing continuous ionospheric emissions during changing IMF conditions. In this review presentation, observational evidence of magnetic reconnection in the cusp will be summarized.
SM54A-03 INVITED
Consequences of Large-scale Reconnection at the Dayside Magnetopause: Results from Global Simulations
It is now well established that the entry of magnetosheath plasma into the dayside magnetosphere is strongly controlled by the occurrence of magnetic reconnection at the magnetopause. While significant progress has been made in understanding the local physics involved in the reconnection process, the topology and global dynamics of the large-scale merging that takes place at the magnetopause boundary are still not well known. Determining the large-scale configuration and time evolution of the merging process is particularly important for predicting the regions of entry of magnetosheath plasma into the magnetosphere and understanding the origin of structures in the plasma precipitation and electrodynamics of the polar ionosphere. In this talk, we review recent progress in the modeling of large-scale reconnection at the dayside magnetopause. In particular, we present the results of global simulations of reconnection and ion precipitation events observed by multiple spacecraft. For each of these case studies, we first used global MHD simulations to determine the global topology of the dayside magnetosphere at different times during the events and then we investigated the topology predicted by the global MHD simulations by computing large samples of ion trajectories in the time-dependent MHD electric and magnetic fields. We examine the results of these simulations and make comparisons with observations to discuss the consequences of large-scale topology of magnetic reconnection.
SM54A-04 INVITED
Polar Rain Gradients and Field-Aligned Polar Cap Potentials
ACE SWEPAM measurements of solar wind field-aligned electrons have been compared with simultaneous measurements of polar rain electrons precipitating over the polar cap and detected by DMSP spacecraft. Such comparisons allow investigation of cross-polar-cap gradients in the intensity of otherwise-steady polar rain. The generally good agreement of the distribution functions from the two data sources confirms that direct entry of solar electrons along open field lines is indeed the cause of polar rain. The agreement between the data sets is typically best on the side of the polar cap with most intense polar rain but the DMSP f's in less intense regions can be brought into agreement with ACE measurements by shifting all energies by a fixed amounts that range from tens to several hundred eV. In most cases these shifts are positive which implies that field-aligned potentials of these amounts exist on polar cap field lines which tend to retard the entry of electrons and produce the observed gradients. These retarding potentials undoubtedly appear in order to prevent the entry of low-energy electrons and maintain charge quasi-neutrality that would otherwise be violated since most tailward flowing magnetosheath ions are unable to follow polar rain electrons down to the polar cap. In more limited regions near the boundary of the polar cap there is sometimes evidence for field-aligned potentials of the opposite sign that accelerate polar rain electrons. A solar electron burst is also studied and it is concluded that electrons from such bursts can enter the magnetotail and precipitate in the same manner as polar rain.
SM54A-05
Polar rain aurora
The energy flux of polar rain, the precipitating electrons directly from the solar wind, is usually too weak to create observable auroral emissions. However, intense polar rain with mean energy above 1 keV was observed at times by in situ electron spectrometers. The energy flux of the intense keV polar rain is great enough that it can excite detectable optical aurora: we term this the polar rain aurora. Observations of polar rain aurora will help to reveal some new features on solar wind electron entry into the polar cap. Global FUV auroral imagers observed, for the first time, a few polar rain aurora events. In this paper we will present observations and interpretation of the structures (such as dawn-dusk alignment, etc) and anti-sunward motion of polar rain auroras and their associated solar wind and IMF conditions. Possible mechanisms for the observed features will also be discussed.
SM54A-06
Particle Acceleration in Cusp-like Magnetic Trapping Regions
Acceleration of charged particles is a fundamental problem of space plasma systems. Recent test particle computations in MHD magnetic and electric fields have shown significant energization of particles in magnetic trapping regions such as a cusp-like magnetic configuration. Necessary conditions for the particle acceleration are a highly efficient trapping and the presence of an electric field. In such a configuration particle trajectories are mostly a combination of gradient curvature and ExB drift. If the gradient curvature drift has a component along the electric field particles can be energized and an efficient trapping can yield large energy gain. Here we will discuss basic properties of this process, present several examples of different magnetic field geometries, basic scaling laws, current limitations of the model. The results indicate that the process is relevant to regions close to the magnetic separatrix surfaces and magnetic nullpoints where reconnection is operating such as the geomagnetic cusps. However, it is also noted that while trapping and the presence of convection electric field are necessary conditions they are not sufficient conditions for the particle acceleration.
SM54A-07
Ion Energy Steps Observed by Cluster Multi-point Mission in the Polar Cusp: Similarity and Differences
The polar cusps are two regions of the magnetosphere where the influence of the interplanetary magnetic field (IMF) is particularly strong. When the IMF is southward the plasma enters the cusp around the subsolar point or near the equatorward boundary of the cusp and on the other hand when the IMF is northward, the plasma enters through the lobes or the poleward boundary of the cusp. Subsequently the ions precipitating in the cusp are dispersed poleward when the IMF is southward and equatorward when the IMF is northward. This dispersion is caused by the motion of field lines driven by the magnetic tension away from the reconnection point. If reconnection is continuous and operates at constant rate, the ion dispersion is smooth and continuous. On the other hand if the reconnection rate varies, we expect interruption in the dispersion forming energy steps or staircase. Similarly, multiple entries near the magnetopause could also produce steps at low or mid-altitude when a spacecraft is crossing subsequently the field lines originating from these multiple sources. On 23 September 2004 the 4 Cluster spacecraft were crossing the mid-altitude polar cusp within 2-16 minute from each other. The first two spacecraft, separated by about 1min 20s observed a typical IMF southward ion dispersion, where the energy of the ions decreases as latitude increases. The dispersion was not smooth but presented discontinuities that were the same on the two spacecraft. Finally this poleward dispersion changed to an equatorward dispersion, as detected by the last spacecraft, after the abrupt turning of the IMF northward. These results will be discussed in term of temporal or patchy reconnection and we will show that the cusp dispersion takes a few minutes to change direction.
SM54A-08
Modeling of cusp ion structures during southward and northward IMF with particle tracing code
Cusp ion structures have been used to infer the location and properties of reconnection at the Earth's magnetopause. However, satellite movement relative to reconnection sites creates a temporal/spatial ambiguity in the observations, and empirical models have difficulty to account for the constantly changing magnetospheric phenomena. In this study, we model the cusp ion structures during southward and northward IMF by using a particle tracing code with electromagnetic fields obtained from the OpenGGCM global magnetosphere-ionosphere model. Integrating Lorentz equations by using the 4th order Runge-Kutta method, the code traces particles backward in time from given locations along a satellite trajectory to source regions where the particle phase space densities are known, such as magnetosphere or magnetosheath. We then calculate the phase-space density based on information from the source regions using Liouville's theorem. The resulting energy-time spectrograms of the distribution functions can be directly compared with observations. We first show analytical results for an ideal rotational discontinuity as a test case, and then present results for specific events with southward and northward IMF.