SM33C-01
Speed of Compression of Magnetosphere by CME Clouds
The multi-point Cluster observations provide the opportunity to study the speed of compression of the magnetosphere at the impact of extreme solar events such as CMEs. The four-point Cluster FGM (high resolution), CIS and PEACE data during the passage of 17 CME clouds during 2001-2005, together with models of magnetosphere and magnetopause, are used to obtain the speed of compression of the dayside magnetosphere. The study shows that the speed of compression (within three seconds of impact) increases with the dynamic pressure of the CMEs, and that this speed exceeds the speed of the CMEs in some (five) cases (suggesting impulsive response) when the dynamic pressure of the CMEs exceed about 20 nPa. The magnetosphere is also found to undergo damped oscillations for about two minutes after the impact of some extreme CMEs (24 October 2003 and 29 October 2003) until the magnetic pressure outside and inside the magnetopause balances. The speed of compression is also found to increase with the negative IMF Bz of the CME suggesting that part of the compression is due to CME pressure and another part is due to magnetic reconnection. The plasma data (PEACE and CIS), though of low resolution (4 seconds), are being analysed to check if the magnetic field and plasma move together or do they undergo differential motion (important for magnetic field-plasma interactions at short time scales).
SM33C-02 INVITED
Solar disturbances and their geospace impacts
Coronal mass ejections (CMEs) and other impulsive solar disturbances are observed to have significant effects in near-Earth space. Fast CMEs often have strong interplanetary shock waves ahead of magnetic cloud structures. It is important from a Sun-Earth connections standpoint to understand magnetospheric and atmospheric responses to impulsive solar wind changes. Recent CME-driven events have been observed by SOHO sensors and they have subsequently produced intense geomagnetic storms at Earth. The events in late- October and November 2003, for example, were seen by ACE sensors to be co-mingled with solar wind streams. The result was very pronounced substorm events, intense relativistic electron enhancements, and massive buildups of nitric oxide (NO) in the upper atmosphere. Multispacecraft data give a good measure of the many effects in the magnetosphere and the atmosphere. Production of NO, as one illustration, is compared with the spatial extent and time variability of energetic particle inputs to the atmosphere. The combination of available space platforms allows disturbances from the Sun to be traced to the atmosphere and to assess quantitatively the energy transport throughout the Sun-Earth system during major events.
SM33C-03
In situ observation of radiation belt particle response to an interplanetary shock
Interplanetary shock is one of the most powerful drivers of magnetic storms which often result in strong energetic particles enhancements and the Van Allen radiation belt compression in the magnetosphere. The elevated fluxes of the energetic particles have been proven to be number one threat to space technological systems. How these particles are accelerated is an important and active research subject in space physics. In this study we show that after an interplanetary shock impact on the magnetosphere, the acceleration of the energetic electrons started nearly immediately in the radiation belt and lasted a few hours.
SM33C-04
Two-Stage Oscillatory Response of the Magnetopause to a Current/Vortex Sheet Followed by Northward IMF: Cluster Observations
We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the ACE spacecraft on December 7, 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east-west components of the field and flow ,By and Vy, occurred at the discontinuity. These rotations were followed by a period of strongly northward IMF. These two factors elicited a two-stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large-amplitude oscillations of ~3 min period being set up. These are argued to be mainly the result of tangential stresses associated with Δ Vy, the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in an MLT range spanning at least 12 hours. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher frequency waves (~79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of ~3 in the second phase. Applying compressible, linearized MHD theory, we show that these perturbations are consistent with surface waves originating from the Kelvin-Helmholtz instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside.
SM33C-05
Magnetosphere response to sudden solar wind and IMF changes as observed by the Cluster and Double Star spacecraft
The four identical Cluster spacecraft, launched in 2000, orbit the Earth in a tetrahedral configuration and on a highly eccentric polar orbit (4 - 19.6 Re). This allows the crossing of critical layers that develop as a result of the interaction between the solar wind and the Earth's magnetosphere. During the northern hemisphere winter their apogee is in the solar wind, allowing to study the magnetopause and bow shock structure and response to solar wind conditions, whereas during the remaining part of the year they analyze the magnetotail dynamics during storms and substorms. The 4 Re perigee permits them to sample the ring current, the outer radiation belt and the outer plasmasphere from south to north, following almost the same magnetic flux tube (latitudinal profile). Since 2004 the Double Star TC-1 and TC-2 spacecraft provide two additional points of measurement, on a larger scale. The Cluster and Double Star TC-1 orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies in the outer magnetosphere, whereas in the inner magnetosphere they can simultaneously investigate the ring current in opposite MLT sectors. Their observations during sudden interplanetary conditions changes and extreme solar events will be presented, showing unusual dayside magnetosphere compression, detection of high-energy particle populations, and the evolution of the energetic ion populations in the ring current.
SM33C-06 INVITED
Solar Wind Triggers for Substorm Expansion Onsets
One of the greatest challenges in space plasma physics is the physical process or processes responsible for onsets of magnetospheric substorm expansions. As solar wind essentially provides the overall energy input into the magnetosphere, sudden changes in the solar wind and/or its embedded magnetic field are likely candidates to trigger these onsets. In this presentation, we review several prevailing substorm models and assess their predictions on what are identified in the solar wind as substorm triggers. Furthermore, we shall discuss what conditions these solar wind triggers create within the magnetosphere that may lead to the development of substorm expansions.
SM33C-07
Interaction of the Bow Shock with a Tangential Discontinuity and Solar-Wind Density Decrease: Observations of Predicted Fast Mode Waves and Magnetosheath Merging
Shortly after 06:00 UT on 7 April 2000 a tangential discontinuity (TD) in the solar wind passed the Advanced Composition Explorer satellite (ACE). It was characterized by a rotation of the interplanetary magnetic field (IMF) by ~145° and more than a factor-of-two decrease in the plasma density. About 50 minutes later Polar encountered more complex manifestations of the discontinuity near noon in the magnetosheath outside the northern hemisphere cusp. Based on Polar observations, theoretical modeling, and MHD simulations we interpret the event as demonstrating that: (1) a fast-mode rarefaction wave was generated during the TD–bow shock interaction, (2) the fast wave carried a significant fraction of the density change to the magnetopause while the remainder stayed with the transmitted discontinuity, and (3) magnetic merging occurred between IMF field lines within the magnetosheath on opposite sides of the discontinuity's surface as it approached the magnetopause. Before the discontinuity passed the spacecraft, Polar detected ions accelerated anti-parallel to B in the fast wave and perpendicular to B in a weak slow-mode structure located adjacent to and just downstream of the fast wave. The anti-parallel accelerated ions in the fast wave had no measurable ion-velocity dispersion signature, placing their source a few RE equatorward of Polar. Simulation results, a Walén test, detections of wave Poynting flux parallel to B, bi-directional electron heat flux, and ion velocity enhancements all indicate that the three ion bursts associated with the passage of the discontinuity were signatures of time- dependent, magnetic merging events within the magnetosheath.