Coherent transmission of upstream waves to polar latitudes through magnetotail lobes

We studied the transmission of upstream waves to polar latitudes through the Earth’s magnetotail lobes, statistically examining the correspondence between magnetospheric waves detected by Cluster and geomagnetic fluctuations at Dome C, in Antarctica (λ ~ 88.84°S). The analysis was restricted to time intervals when Cluster was in the southern magnetotail lobe, on field lines with footprints close to Dome C. During the day, the coherence between geomagnetic and magnetospheric signals is significantly high, indicating a common source, in the frequency range 20–35 mHz, at typical upstream wave frequencies, and particularly in the early morning sector, when a higher occurrence of upstream waves is typically expected, and Dome C is relatively unaffected by dayside phenomena. The highest coherence is observed between fluctuations in the horizontal components on the ground and transversal components of the magnetospheric field. Such results suggest that upstream waves, convected by the solar wind along the magnetosphere flanks and transmitted as compressional waves into the magnetotail lobes, are partially converted to shear Alfven waves and propagate, traveling along the open field lines, up to the polar cap. A case study provides explicit evidence of the proposed path. It shows that fluctuations in the D component are delayed with respect to the corresponding waves in the transversal field measured by Cluster; the delay time being consistent with the ground propagation of Alfven waves along the field line connected with the satellite position; the weak correspondence with the H component suggests that the local ionosphere does not rotate efficiently the magnetospheric signals.