Discrete time--frequency coupling between the interplanetary magnetic field and the magnetosphere

Recent work has shown the existence at geosynchronous orbit of wave power at frequencies substantially below that of fundamental field-line resonances. The existence of spectral power below that of fundamental frequencies indicates that the driver must be external to the magnetosphere, i.e, that the source is contained in the solar wind. Other work has shown that frequencies matching field-line resonances can sometimes also be observed in the solar wind. The combined spectrum that includes power at fundamental frequencies and lower overlaps with p-mode helioseismic oscillations and the question has been raised whether these helioseismic modes may stimulate the magnetospheric fluctuations. In this study, we compute coherences between the magnetometers on {ACE} and {GOES-10}, that is in interplanetary space and inside the magnetosphere. These show several frequencies where all nine coherences are significant. Moreover, many of these frequencies are the same as those reported from Ulysses data in Thomson, Maclennan, and Lanzerotti (1995). Using hourly average data from all 1999, a frequency tolerance of 30 nHz, and retaining only peaks in the coherence above the 95% significance level, about 21 agreements would be expected by chance but 34 are observed. The coherences exhibit seasonal dependence, so the coupling has a complicated time--frequency structure exhibiting both frequency and event dependence.