SM43A-1066
Anti-phase oscillation of energetic electron and proton fluxes: Cluster observations
Simultaneous modulation of energetic electron and proton fluxes in the outer radiation belt with the phase difference between them of 180 degrees were observed by the Cluster satellites during a magnetic storm period on Oct. 31, 2003. The "out-of-phase" event, surprisingly, was observed associated with the toroidal ultra-low frequency (ULF) waves instead of the compressional ones, and therefore could not be caused by the mirror effect (Kivelson and Southwood, 1985). To explain the unexpected 180 degree phase difference, we analyze the behaviors of both the electrons and the protons in the toroidal waves, and highlight the role of the background convection. With the effect of the strong convectional electric field, the anti-phase modulation between the electron and the proton fluxes can be understood and predicted. The convection may also affect the modulation amplitude of the ULF waves, and further accelerate particles with a broader energy range by changing the wave-particle resonance conditions.
SM43A-1067
Interplanetary Shock Waves in the Earth Magnetosheath: Cluster Observations
Shock waves collisions are a basic problem in plasma physics and play an important role in many processes occurring in space. In particular, the impacts of the interplanetary shock waves (IPS) on the terrestrial bow shock are relevant to the Space Weather. In fact, understanding how the associated pressure pulses in magnetosheath are shaped after these impacts, can help to model in a more realistic way the perturbed magnetosphere and to gain a deeper knowledge into the fundamental mechanisms causing the geomagnetic activity. Here we present an event, seen by Cluster spacecraft, showing the complex and non-linear nature of the phenomenon. Actually the associated variations in plasma parameters and in the magnetic field are due, besides the transmitted interplanetary shocks, to other secondary (i.e. produced in the impact) discontinuities and waves.
SM43A-1068
Dependence of main phase range of H component on Latitudes for Great Magnetic Storms
We compared the main phase range of H component recorded by ground-based geomagnetic observatories with different latitudes but nearly same geomagnetic longitude during several great storms with Dst < -300nT. The data both from magnetic chain in eastern China and in Central America were analyzed. The result derived from the magnetic data recorded by the magnetic chain in eastern China shows that the magnetic disturbance was strongest in the northernmost observatory MZL but not in QGZ which is nearest to equator during several great storms. The result from magnetic chain in Central America is more complicated. The above-mentioned does not agree with the common knowledge that the recorded main phase range of H component decreases with increasing latitude because magnetic field variation is caused mainly by symmetry equator ring current during storms, especially during main phase of storms. The result indicates that the responsible current systems during great storms are really complicated, and we try to find the possible current to explain our result.
SM43A-1069
Cluster Observations of the Magnetospheric Low-Latitude Boundary Layer and Cusp During Extreme Solar Wind and Interplanetary Magnetic Field Conditions: 10 November 2004 ICME and Statistical Survey.
We present a study of the magnetospheric cusp response to extreme external parameters during passage of the ICME over the Earth on 10th November 2004, based on Cluster observations of the plasma properties inside the low-latitude boundary layer (LLBL)/cusp region in the dawn sector of the southern hemisphere. The Interplanetary Magnetic Field (IMF) is strongly equatorial, mostly due to the dominant dawn-dusk component. Under these conditions, occurring at the same time as pulses of solar wind dynamic pressure, the observations are very complicated. However, we suggest that in the polar region of the southern hemisphere, Cluster cross two LLBLs/cusps, spatially separated by polar cap plasma. The first LLBL/cusp is formed due to anti-parallel reconnection in the dusk sector of the southern hemisphere and the second is formed due to anti-parallel reconnection in the dawn sector of the northern hemisphere. The second LLBL/cusp is located at extremely low latitude, less than ~66.3deg ILAT. A statistical study of the location of the LLBL/cusp equatorward boundary during the ICME events on 28-29 October 2003 and 7-10 November 2004 is performed. During extreme conditions the LLBL/cusp position is off- set by -7deg ILAT from the location under normal conditions, which might be explained by the influence of the high solar wind dynamic pressure. The LLBL/cusp moves equatorward with increasing southward and northward IMF. However, the LLBL/cusp position under strong southward IMF is more poleward than expected from previous studies which could indicate some saturation in the dayside reconnection process or enhancement of the nightside reconnection rate. The LLBL/cusp position under strong northward IMF is extremely low and does not agree with the location predicted in previous studies. For the events with solar wind dynamic pressure more than 10 nPa, the LLBL/cusp position does not depend on the solar wind dynamic pressure. This might indicate some saturation in the mechanism of how the LLBL/cusp location depends on the solar wind dynamic pressure.
SM43A-1070
Bifurcation and hysteresis of the magnetospheric structure with a varying Southward IMF: golbal 3D full particle simulations
Using a three-dimensional full electromagnetic particle model (EMPM), we have performed global simulations of the interaction between the solar wind and the terrestrial magnetosphere, and have investigated its asymptotic stability. The distance between the dayside magnetopause subsolar point and the earth center, Rmp , is measured, as the intensity of southward IMF Bz is slowly varying. Based on the field topology theory, one analyzes the variation of Rmp as a reference index of the dynamics of this interaction, when IMF Bz successively increases and decreases to its original value. Two striking results are observed. First, as the IMF Bz increases above a critical value, the variation of Rmp suddenly changes (so called "bifurcation" process in field topology). For this critical value, the overall magnetic field topology changes drastically and is identified as being the signature of magnetic reconnection at the subsolar point on the magnetopause. Second, this subsolar point recovers its original location Rmp by following different paths as the IMF Bz value increases (from zero to a maximum fixed value) and decreases (from this maximum to zero) passing through some critical values. These different paths are the signature of an "hysteresis" effect, and are characteristic of the so-called "subcritical-type" bifurcation. This hysteresis signature indicates that dissipation processes take place via an energy transfer from the solar wind to the magnetosphere by some irreversible way, which leads to a drastic change in the magnetospheric field topology. This hysteresis is interpreted herein as a consequence of the magnetic reconnection taking place at the dayside magnetopause. The field topology reveals to be a very powerful tool to analyze the signatures of three-dimensional magnetic reconnection without the obligation for determining the mechanisms responsible for, and the consequences of the reconnection on the overall magnetospheric dynamics.
SM43A-1071
3D PIC simulation of the magnetosphere during IMF rotation from north to south: signatures of substorm triggering in the magnetotail
Three dimensional PIC simulations are performed in order to analyse the dynamics of the magnetotail as the interplanetary magnetic field (IMF) rotates from northward to southward direction. This dynamics reveals to be quite different within meridan/equatorial planes over two successive phases of this rotation. First , as IMF rotates from North to Dawn-Dusk direction, the X-Point (magnetic reconnection) evidenced in the magnetotail (meridian plane) is moving earthward (from x=-35 Re to x=-17.5 ) distance at which it stabilizes. This motion is coupled with the formation of "Crosstail- S" patterns (within the plane perpendicular to the Sun-Earth mine) through the neutral sheet in the nearby magnetotail. Second, as IMF rotates from dawndusk to South, the minimum B field region is expanding within the equatorial plane and forms a ring . This two-steps dynamics is analyzed in strong association with the cross field magnetotail current Jy, in order to recover the signatures of substorms triggering.
SM43A-1072
Energetic ions modulation by ULF wave in the duskside flank of magnetosphere observed by Double star TC-1
Energetic ion (5eV-30keV) flux variations associated with ultralow frequency (ULF) waves in the duskside flank were observed during a very quite period when Double star TC-1 pass near 0230 MLT on Oct. 17, 2004. A clear association between Pc 5 waves and periodic variations of ion fluxes is observed. The observed magnetic ULF pulsations and ion fluxes are dominated by the poroidal mode. The pitch angle modulations of energetic ions from different energy channels are observed and show different features. The pitch angle modulation suggests that the ULF wave can excite the energetic ions bouncing resonance at around 25 keV.
SM43A-1073
Solar wind ULF wave frequency dependence of the ionospheric response
Using the OpenGGCM global MHD simulations of the magnetosphere-ionosphere system, we study the ionosphere response dependence on the frequency of the ULF waves in the solar wind. This study extends our early study that showed that there is a frequency dependence in a deposited energy distribution at the ionosphere level. In this study we examine the range of frequencies and analyze the ionosphere response dependence in different sectors of the ionosphere. Our goal is to understand at what frequencies the excitation of the field line resonances (FLR) become important and how these results can be quantified. We also conduct event runs to compare how the results of our simulations can be applied.
SM43A-1074
Magnetospheric Current Response to Solar Wind Dynamic Pressure Enhancements during Strong Magnetic Storms
Recent studies have found that solar wind dynamic pressure enhancements can cause clear dawn-dusk asymmetric H perturbations in low-latitude ground magnetometers, particularly when the IMF Bz has been southward for some time before the compression occurs. The asymmetry consists of negative H perturbations on the dusk side and positive H perturbations on the dawn side, a response that is uncharacteristic of what is expected from a typical magnetospheric compression. This response was qualitatively interpreted as the result of the intensification of the existing partial ring current by the pressure enhancement. The assumption is that the intensified partial ring current creates the negative perturbations on the dusk side, overwhelming the effect of all other magnetospheric (Chapman-Ferraro and R1 and R2) currents. The present study quantitatively investigates the response of the different magnetospheric current systems to the solar wind dynamic pressure enhancements and their contribution to the ground H perturbations by combining modeling and observational results during two pressure enhancement events that occurred during strong magnetic storms of similar strength. The magnitude of the pressure in the two events is the differing factor. We used the Tsyganenko storm-time magnetic field models (TS05), which includes separate modules for each magnetospheric current system, to fit and model the ground perturbations that result form the compressions. We first modified the TS05 by adding the present state of dynamic pressure to the parameterization scheme of the R1 and R2 field-aligned current modules and to that of the symmetric and asymmetric ring currents. We then fit the model to the low- and mid-latitude ground magnetometer observations for each of the two selected magnetic compressions. For the pressure enhancement occurring during the main phase of the September 25, 1998 storm, the modeling results show that the primary contributor to the ground asymmetric H perturbation is the intensified partial ring current and its closure field-aligned currents flowing downward in the morning sector and upward in the post-midnight sector. The other currents have much less contribution. The modeling results are consistent with the current pattern inferred from the global ground H and D magnetic perturbations. For the pressure enhancement occurring during the main phase of the May 29, 2003 storm, the modeling results are consistent with an intensified R1 current pattern twisting towards the dawn side, also inferred from the global ground H and D component perturbations. The difference between these two cases is that the pressure enhancement magnitude of the 2003 event was almost twice that of the 1998 event, which resulted in the magnetopause moving much closer to the Earth by comparison. We explore how this difference can explain the different current patterns.
SM43A-1075
Magnetic Structures with Rich Ionospheric Oxygen Ions Observed in the near Earth Plasmasheet
Magnetic structures with predominant ionospheric O+ ions have been observed on November 8, 2004 by the Cluster and Double Star spacecraft during a strong magnetic storm time period. The oxygen densities in two BBFs are found to be 3-5 times larger than the hydrogen densities and the oxygen thermal pressures in the BBFs are 8 times larger than the hydrogen thermal pressure. Moreover, the dynamic ram pressure of the oxygen BBFs is 16 times larger than a normal BBF moving in the same speed and it is inferred that the braking region should be greatly pushed inside the usual pressure balance region when the BBF is dominated by oxygen ions. It is found that in the earthward flowing plasmoid, oxygen ions are relatively cold with a higher kinetic pressure than thermal pressure inside the structure. It is therefore suggested that oxygen ions in the tailward flowing plasmoid might be heated in the near earth current sheet. The observations made in this paper suggest that singly charged oxygen ions embedded in the BBFs may be carried into the inner region during very intense magnetic storms. The Cluster - Double Star constellation offers a good opportunity to ascertain the evolutional signatures of fast moving plasma flow in the near Earth region.