A. Marchaudon

Reconnection at the dayside magnetopause: Comparisons of global MHD simulation results with Cluster and Double Star observations

[1]xa0This study uses two conjunctions between Cluster and Double Star TC-1 spacecraft together with global magnetohydrodynamic (MHD) simulations to investigate the large-scale configuration of magnetic reconnection at the dayside magnetopause. Both events involve southward interplanetary magnetic fields with significant By components. The first event occurred on 8 May 2004, while both spacecraft were exploring the dawn flank of the magnetosphere; TC-1 was skimming the magnetopause whereas Cluster was exploring higher latitudes. Results from a global MHD simulation show the formation of an equatorial merging line in the morning sector and suggest that the three-dimensional geometry of the merging region is mostly a radial juxtaposition of planes displaying X-type reconnection geometries. The second conjunction was on 6 April 2004. During this event, Cluster was located at high latitudes and close to the noon-midnight meridian, while TC-1 was exploring the dawnside at low latitudes. Analysis of the simulation reveals that both antiparallel and component merging occurred simultaneously. Three-dimensional rendering of the parallel electric field indicates that component merging initiated in the subsolar magnetopause. Simulation runs carried out using different parameters in the model suggest that the spread of the merging region depends on the local resistivity. The subsolar-merging region grows with increasing resistivity values and becomes patchy when a resistivity threshold is used. However, the region of component merging appears to remain spatially constrained to the subsolar region where stronger parallel electric fields occur and no clear connection with the antiparallel-merging regions is found for the range of parameters surveyed.

A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes

[1] From one year (2004) of thermosphere total density data inferred from CHAMP/ STAR accelerometer measurements, we calculate the global thermosphere response to auroral magnetic activity forcing at middle and low latitudes using a method based on a singular value decomposition of the satellite data. This method allows separating the large-scale spatial variations in the density, mostly related to altitude/latitude variations and captured by the first singular component, from the time variations, down to timescales on the order of the orbital period, which are captured by the associated projection coefficient. This projection coefficient is used to define a disturbance coefficient that characterizes the global thermospheric density response to auroral forcing. For quiet to moderate magnetic activity levels (Kp < 6), we show that the disturbance coefficient is better correlated with the magnetic am indices than with the magnetic ap indices. The latter index is used in all empirical thermosphere models to quantify the auroral forcing. It is found that the NRLMSISE-00 model correctly estimates the main features of the thermosphere density response to geomagnetic activity, i.e., the morphology of Universal Time variations and the larger relative increase during nighttime than during daytime. However, it statistically underestimates the amplitude of the thermosphere density response by about 50%. This underestimation reaches 200% for specific disturbed periods. It is also found that the difference between daytime and nighttime responses to auroral forcing can statistically be explained by local differences in magnetic activity as described by the longitude sector magnetic indices. Citation: Lathuillere, C., M. Menvielle, A. Marchaudon, and S. Bruinsma (2008), A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes,

Spatial distribution of average vorticity in the high‐latitude ionosphere and its variation with interplanetary magnetic field direction and season

[1]xa0We present a technique to measure the magnetic field-aligned vorticity of mesoscale plasma flows in the F region ionosphere using line-of-sight velocity measurements made by the Super Dual Auroral Radar Network (SuperDARN). Vorticity is often used as a proxy for magnetic field-aligned current (FAC) intensity in the ionosphere but also provides information about turbulent processes in the ionosphere and magnetosphere. Using 6 years (2000–2005 inclusive) of vorticity measurements made by six SuperDARN radars in the Northern Hemisphere, we have compiled, for the first time, maps of average vorticity across the northern polar ionosphere. These maps have been subdivided according to different seasonal and interplanetary magnetic field (IMF) conditions. The variations in the morphology of the vorticity maps with IMF direction match very closely those seen in maps of average FAC intensity (determined using different methods and instrumentation), suggesting that vorticity is a good proxy for FAC in an averaged sense. The variations in the morphology of the vorticity maps with season show differences from those seen in the FAC maps, illustrating that ionospheric conductance plays a major role in determining the differences between measurements of vorticity and FAC.

Signatures of complex magnetic topologies from multiple reconnection sites induced by Kelvin-Helmholtz instability

The Magnetospheric Multiscale mission has demonstrated the frequent presence of reconnection exhausts at thin current sheets within Kelvin-Helmholtz (KH) waves at the flank magnetopause. Motivated ...

Magnetopause response to variations in the solar wind: Conjunction observations between Cluster, TC-1, and SuperDARN

How the solar wind affects the location of the magnetopause has been widely studied and excellent models of the magnetopause based on in situ observations in the solar wind and at the magnetopause have been established, while the careful insight into the responses of the magnetopause to the variations in the solar wind can still provide us some new information about the processes in space plasmas. The short distance from Cluster to TC-1 on 9 March 2004, between 06: 10 and 08: 10 UT, gives us a good opportunity to precisely monitor the responses of the magnetopause to the variations in the solar wind. On the basis of the combined observations between Cluster, TC-1, and SuperDARN we analyze the magnetopause crossings associated with magnetopause motion or magnetic reconnection when the solar wind conditions have a series of variations. New results about the time delays for the propagation from the solar wind monitor to the magnetopause of the interplanetary magnetic fields (IMF) and of the solar wind dynamic pressure, respectively, and the intrinsic time for reconnection onset at the magnetopause are obtained. The most important feature of the event is that the dynamic pressure and the IMF in the solar wind do not arrive at the magnetopause at the same time, which will direct us to find out how the variation in the solar wind dynamic pressure is transported from the bow shock to the magnetopause. Another significant feature is that this event presents a shorter intrinsic time, similar to 2 min, for reconnection onset at the dayside magnetopause than that given by the previous work of Le et al. (1993) and Russell et al. (1997).

Cluster observations and numerical modeling of energy-dispersed ionospheric H+ ions bouncing at the plasma sheet boundary layer

[1]xa0The Cluster mission offers a unique opportunity to investigate the origin of the energy-dispersed ion structures frequently observed at 4.5–5 RE altitude in the auroral region. We present a detailed study of the 14 February 2001 northern pass, characterized by the successive observation by three spacecraft of a series of energy-dispersed structures at ∼72–75° ILAT in a region of poleward convection. Equatorward, the satellites also observed a localized, steady, and intense source of outflowing energetic (3–10 keV) H+ and O+ ions. These substructures were modeled by launching millions of H+ ions from this ionospheric source and following them through time-dependent electric and magnetic fields obtained from a global MHD simulation of this event. Despite the complexity of ion orbits, the simulations showed that a large number of ions returned to the Cluster location, poleward of their source, in a number of adjacent or overlapping energy-latitude substructures with the correct dispersion. The first dispersed echo was unexpectedly generated by “half-bouncing” ions that interacted with the current sheet to return to the same hemisphere. The time-shifted observations made by two Cluster (SC1 and SC3) spacecrafts were correctly reproduced. Almost all the ions returning to the spacecraft underwent a ∼2–5 keV nonadiabatic acceleration at each interaction with the current sheet in a very confined resonant region. This acceleration explains the overall energy increase from one structure to the next. This event confirms the importance of the ionospheric source in populating bouncing ion clusters within the magnetosphere, even at high latitudes.