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

Abstract

In particular during the descending phase of the solar cycle, Alfvén waves in the high-speed solar wind streams are a major form of interplanetary disturbances. The fluctuating southward interplanetary magnetic field (IMF) of Alfvén waves has been suggested to induce geomagnetic activities through intermittent magnetic reconnection at the magnetopause. In this study, we provide in situ observational evidence for dayside magnetopause reconnection induced by such interplanetary Alfvén waves. Using multipoint conjunction observations, we show that the IMF B z from interplanetary Alfvén waves is transmitted through and amplified by the Earth’s bow shock. Associated with the intensified southward B z to the magnetopause, in situ signatures of magnetic reconnection are detected. Repetitively, interplanetary Alfvén waves transmit the intensified B z to the magnetosheath, leading to intervals of large magnetic shear angles across the magnetopause and magnetopause reconnection. Such intervals are promptly followed by hundreds of nanoTesla (nT) increases in the auroral electrojet indices (AE and AU) within 10–20 minutes. These observations are confirmed in multiple events in corotating interaction region-driven geomagnetic storms. To put the observations into context, we propose a phenomenological model of a strongly driven substorm. The substorm electrojet is linked to the enhanced magnetopause reconnection in the short timescale of re-establishing the ionosphere electric field and the two-cell convection. These results provide insights on the temporal patterns of solar wind magnetosphere–ionosphere coupling, especially during the descending phase of the solar cycle.

Extreme Geomagnetic Disturbances (GMDs) Observed in Eastern Arctic Canada: Occurrence Characteristics and Solar Cycle Dependence

“…Tsurutani and Gonzalez (1987) and Tsurutani et al (2011) concluded that the major cause of geomagnetic activity during extended HILDCAA intervals, associated with high speed solar wind streams, is large amplitude Alfvén waves. Dai et al (2023) confirmed these findings in multiple events in corotating interaction region-driven geomagnetic storms. They noted that Alfvénic fluctuations in the solar wind associated with repetitive substorms contributed to the extended recovery phases of geomagnetic storms, and that such intervals were promptly followed by hundreds of nT increases in the AE and AU auroral electrojet indices within 10-20 minutes.…”

“…They noted that Alfvénic fluctuations in the solar wind associated with repetitive substorms contributed to the extended recovery phases of geomagnetic storms, and that such intervals were promptly followed by hundreds of nT increases in the AE and AU auroral electrojet indices within 10-20 minutes. Dai et al (2023) also presented a phenomenological model of strongly driven substorms, in which the increase of the AE index is linked to dayside reconnection mainly through the ionosphere (through enhanced two-cell convection) instead of the magnetotail, and suggested that this pattern is expected to be particularly viable and even dominant in the descending phase of the solar cycle.…”

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

“…The average response of SME indicates that Alfvén waves have a general relation with moderate AE activities. As shown in the in situ observations in our accompanying Paper I (Dai et al 2023), the connection between Alfvén waves and AE increases is made possible by dayside magnetopause reconnection. The observations of the SME index suggest that Alfvén waves generally drive enhanced magnetosphere/ionosphere convection through transient magnetopause reconnection.…”

Characteristics of Isolated and Storm‐Time Ion Injections

“…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.…”