Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm13d2110r&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM13D-2110
Physics
[2744] Magnetospheric Physics / Magnetotail, [2790] Magnetospheric Physics / Substorms
Scientific paper
We performed a global MHD simulation of a substorm event on 14 September 2004 in order to investigate the link between Pi2 generation and dipolarization fronts. Pi2 pulsations (T = 40-150 s) measured by ground-based instruments are typically used as an indicator of substorm onset, therefore, understanding how and where they are generated is vital to understanding the series of events leading up to onset. Kepko et al. [1999] suggested that the compression regions and velocity variations associated with earthward propagating dipolarization fronts directly drive Pi2 pulsations. Similarly, Panov et al. [2011] suggested that Pi2 pulsations are generated by the overshoot and rebound of bursty bulk flows. Dipolarization fronts are step-wise enhancements in Bz which are associated with fast (>100km/s) earthward flows and are followed by tailward expansion due to pile-up at the high pressure region where the magnetic field lines transition from a stretched to a dipolar configuration. Cao et al. [2009] have presented observations from Double Star (TC1), Cluster 4 and Polar of a substorm with onset at 18:22 UT. During this event a dipolarization front was observed by Double Star at ~18:25, and dipolarization associated expansion was observed by Cluster 4 at ~18:50 and Polar at ~18:55 UT. The spacecraft were positioned at (-10.2, -1.6, 1.2), (-16.4, 1.6, 2.2) and (-7.5, -1.8, -4.9) RE in GSM coordinates respectively. The simulation was carried out with the UCLA global MHD code [El-Alaoui (2001), Raeder (1998)], using Geotail, located near the bow shock at ~24 RE, as the solar wind monitor. The solar wind magnetic field data were rotated into a minimum variance frame to be used as input for the simulation. The results from the simulation have been compared to observations and do a good job reproducing the structures observed by all three satellites. Around the time of onset, we have identified a dipolarization front near midnight which originates at ~12 RE. We show that as the dipolarization front begins to travel earthward, Pi2 fluctuations are generated in the pressure and velocity components which propagate along the plasma sheet into the inner magnetosphere. Inside ~-7 RE the frequency seen in the velocity perturbations is matched by perturbations in pressure and magnetic field components. References Ashour-Abdalla, M., et al (2011), Observations and simulations of non-local acceleration of electrons in magnetotail magnetic reconnection events, Nature Physics, vol.7. Cao, X., et al. (2008), Multispacecraft and ground-based observations of substorm timing and activations: Two case studies, J. Geophys. Res., 113, A07S25. El-Alaoui, M. (2001), Current disruption during November 24, 1996 substorm, J. Geophys. Res., 106, 6229- 6245. Kepko, L. and M. Kivelson (1999) Generation of Pi2 pulsations by bursty bulk flows, J. Geophys Res. 104(A11),25,021-25,034. Panov, E. V., et al (2010), Multiple overshoot and rebound of a bursty bulk flow, Geophys. Res. Lett., 37, L08103. Raeder, J., et al. (1998), The Geospace Environment Modeling Grand Challenge: Results from a global geospace circulation model, J. Geophys. Res., 103, 14,787.
Ashour-Abdalla Maha
El-Alaoui Mostafa
Ream J. B.
Walker Ray J.
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