Large‐scale fluctuations in the solar wind at 1 AU: 1978‐1982

Large-scale fluctuations (with spacecraft frame periods between several hours and the solar rotation period) in the magnetic field strength B(t) and bulk speed V(t) observed at 1 AU were analyzed using data from IMP 8 and ISEE 3. Most of the power is in the low-frequency range (3 × 10−7 to 3 × 10−6 Hz). In the intermediate frequency range (3 × 10−6 to ∼3 × 10−5 Hz), the spectra of V(t) and B(t) computed for successive ∼6-month intervals from 1978 to 1982 had the form f−b, where 〈bB〉 = 1.92 ± 0.06 for the magnetic field spectra and 〈bυ〉 = 1.92 ± 0.10 for the bulk speed spectra. There was no systematic variation of the spectral exponent with time during this interval. A detailed analysis of the bulk speed and magnetic field strength signals was carried out for the data from day 226 to day 365, 1978, which is representative of other such intervals. The spectra of the low-frequency fluctuations during this interval are essentially the same as those computed from piecewise linear (jump-ramp) approximations to the speed and magnetic field profiles. The spectra of the intermediate frequency fluctuations during that interval are the result of a superposition of two components: a component due to the jump-ramp series, which falls off faster than f−2, and a component which falls off slower than f−2. The average width of the jumps in B is 2.2 hours. Twenty-two percent of the jumps in B have the signature of a forward or reverse shock. The jumps in V are not all simultaneous with the jumps in B. The average width of the jumps in V is 2.1 hours. Thirteen percent of the speed jumps have the signature of a forward or reverse shock.