The quiet geomagnetic field at geosynchronous orbit and its dependence on solar wind dynamic pressure

Vector magnetic fields at geosynchronous orbit were measured during 1980‐1984 using the operational GOES 2, GOES 5, and GOES 6 spacecraft magnetometers. We corrected these spacecraft measurements for offsets due to spacecraft state and then used these field estimates to create a data base with 1‐min resolution. Hourly quiet field values were calculated for these years from this data base using the ground‐based geomagnetic index criteria AE < 120 nT and | Dst | < 20 nT. These quiet field components, rotated into dipole HVD coordinates, were approximated by the first two coefficients of a two‐dimensional Fourier series in time of day and season. These Fourier harmonics provide a compact method of approximating the quiet synchronous field at any time of day and any season. The quiet geosynchronous field components, to first order, are given by mean values of about 90 nT, −60 nT, and 5 nT; and sinusoidal diurnal amplitudes of about 21 nT, 5 nT, and 5 nT, respectively, for H , V , and D where the spacecraft magnetometer was located near the geomagnetic meridian. The second harmonic diurnal amplitudes and the first and second harmonic seasonal amplitudes are typically of the order of a few nanoteslas or less except for the D component, which exhibits a larger seasonal variation. Furthermore, a one‐dimensional Fourier series in time of day was used to study the quiet field dependence on solar wind dynamic pressure, P d , by indexing the measurements into five pressure ranges during 1980. The H component of the quiet field increased 4.6 nT from 80.2 to 84.8 nT in its mean amplitude and 20.8 nT from 11.9 to 32.7 nT in its first harmonic amplitude for P d increasing from 0.71 × 10 −8 to 3.31 × 10 −8 dyn/cm². These quiet H measurements, including the pressure dependence, are compared with a first‐order field model (Mead, 1964) superimposed with a tail current, resulting in magnetospheric currents (magnetopause and tail) in agreement with previous model values. The measured field pressure dependence and the Mead model suggest a tail current dependence on pressure.