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 .

Geoeffectiveness of Interplanetary Alfvén Waves. I. Magnetopause Magnetic Reconnection and Directly Driven Substorms

“…Through the dayside magnetopause reconnection, Alfvén waves drive geomagnetic activities that are measured on the ground. Interplanetary Alfvénic fluctuations are shown to have correlations with the auroral electrojet (AE) index (Tsurutani & Gonzalez 1987;Tsurutani et al 1990;Gonzalez et al 1999;Diego et al 2005;Chian et al 2006;D'Amicis et al 2007;Guarnieri et al 2018;Han et al 2023). High-intensity long-duration continuous AE activities (HILDCAAs) are the effects of Alfvén wave trains in the IMF (Tsurutani & Gonzalez 1987).…”

Characteristics of Isolated and Storm‐Time Ion Injections

“…They performed the superpose epoch analysis on ion fluxes and found storm‐time injections have long‐term durations. Repetitively, the fluctuating IMF Bz from interplanetary Alfven waves leads to the prompt increases in AE/AU index and substorms within 10–20 min (Dai et al., 2023; Han et al., 2023). These repetitive, strongly‐driven substorms inject energetic particles into the inner magnetosphere and cause the enhancement of high‐energy particle fluxes in CIR‐driven storm (Borovsky & Denton 2006; Kataoka & Miyoshi 2006; Xiong et al., 2022).…”

“…The energetic particle injections are commonly produced by storms and substorms (Dai et al., 2023; Han et al., 2023). The differences and similarities in ion flux levels, durations, and their association to AE and Dst indexes between storm‐time and isolated injections have been described in some previous works.…”