Reconnection Tailward of the Cusp Under Negative IMF Bz Conditions 

Farrugia, C J., Sandholt, P.


In the recent statistical work of Twitty et al. (2004) based on Cluster
observations in the high- and mid-altitude cusp at near-Noon local times (MLT),
the prevalent opinion that lobe reconnection is exclusively a northward IMF
($B_z >$ 0) phenomenon is given further support. Contrary to this view, we
present a case of lobe reconnection during a steady south-easterly IMF ($B_z$ =
-5 nT, IMF clock angle = 115°) midway through the passage at Earth of an
interplanetary magnetic cloud on April 17, 1999. Robust signatures of lobe
reconnection are present in particle data obtained from Polar/HYDRA during an
overflight of the southern polar cusp at 08-09 MLT in the form of an inverse
energy-versus-latitude dispersion of precipitating ions. From the ground
magnetometer records of the Svalbard stations of the IMAGE chain we infer the
presence of a long series of pulsed ionospheric flows (PIFs) in both the merging
and lobe convection cells in the northern hemisphere. These data provide strong
evidence for the occurrence of (pulsed) lobe reconnection during this interval
of steady southeast IMF. The interplanetary conditions included a dominant IMF
$B_y$ component of 20 nT, a low proton plasma beta (< 0.1) and a low Alfven Mach
number (~3) near the center of the cloud. We speculate that these conditions may
give rise to a plasma depletion layer next to the high-latitude magnetopause, a
circumstance which favors lobe reconnection. \vspace{0.2 cm} Twitty, C. et al.,
Geophys. Res. lett., 31, L19808, 2004. This work is supported by a grant in 
NASA's SEC-GI Program}

_______________
Fall Meeting, American Geophysical Union, San Francisco, U.S.A., 
5-9 December 2005