Geoeffectiveness of Interplanetary Alfvén Waves. II. Spectral Characteristics and Geomagnetic Responses

Abstract

Using multipoint observations over 10 yr near 1 au, we investigate the spectra (5 minutes to 2 hr) of interplanetary Alfvén waves and the responses in the geomagnetic activities. We compute the two-point correlations of the wave magnetic field between the ACE and the THEMIS spacecraft, which are separated by ∼200 Earth radius (R E) in the solar wind. Alfvén waves associated with high two-point correlations exhibit steep spectra (spectra index ∼−1.63). Such Alfvén waves occur mostly in slow-speed streams. By contrast, Alfvén waves with low two-point correlations exhibit flatter spectra (spectra index ∼−1.51) with a relative enhancement of power above 2 × 10−4 Hz. The occurrence of Alfvén waves with low two-point correlations is more equally distributed between high-speed and low-speed streams. In general, interplanetary Alfvén waves show correlations with moderate geomagnetic responses in symmetric ring-current intensity, SuperMAG electrojet (SME), and Kp indices. Statistical analyses indicate that the Alfvén waves with flat spectra correspond to stronger responses in the geomagnetic indices than those with steep spectra, suggesting the importance of the tens of minutes (30–90 minutes) Alfvénic power spectra in the generation of SME/Auroral Electrojets. These observations may shed light on the response of the magnetosphere to fluctuating interplanetary magnetic field B z .