THE PRODUCT SPECTRA OF GRAVITY AND BAROMETRIC-PRESSURE IN EUROPE

A total of 111000 hourly values of gravity and barometric pressure fro m stations in Europe is analysed. The data consist of two sets of reco rds from Brussels, an early set of 36000 h length and a more recent se t of 21000 h length, a set of records from Bad Homburg of 24000 h leng th, and a set of records from Strasbourg of 30000 h length. All of the gravity measurements were made with similar superconducting instrumen ts and the pressure data were recorded simultaneously at each supercon ducting gravimeter site. The four sets of records have different time bases, and to bring out common features and suppress individual statio n systematic errors, the product spectrum is introduced. Spectral dens ity estimates are first computed for a common spectral window for each record, and the product spectrum is formed by multiplying individual spectral estimates across records. The cumulative distribution functio n is found for the product spectrum and confidence intervals are calcu lated from it by iteration. The product spectrum in gravity reveals a triplet of resonances in the subtidal band which are shown by an autom ated computer search to be uniquely associated with the translational modes of the solid inner core. The product spectrum in barometric pres sure clearly reveals the first 10 solar heating tides in the atmospher e, but otherwise does not show common features with the gravity produc t spectrum. In particular, the triplet of resonances in the subtidal b and of the gravity product spectrum do not show up in the product spec trum of barometric pressure, eliminating the atmosphere as their sourc e.