Neutral solar wind properties: Advance warning of major geomagnetic storms

Abstract

Coronal mass ejections (CMEs) have been identified as a trigger for large geomagnetic storms. A clever idea to provide advance warning of a high‐speed CME approaching the Earth was recently proposed by Hsieh et al. (1992): ejected solar matter decelerates on its way from the Sun to Earth, energetic neutral atoms (ENAs) are formed in the CME plasma due to recombination, ENAs pass the decelerating CME plasma and arrive first at the Earth. We evaluate the idea to use ENAs for advance detection of the high‐speed Earth‐approaching CMEs and consider the processes involved. Charge exchange between solar wind ions and interplanetary neutral atoms contribute effectively to ENA production. Characteristics of neutral gas within Earth's orbit are updated for both neutral atoms of interstellar origin and from outgassing from interplanetary dust. Computer simulation of CME‐produced ENAs shows that the total flux of CME‐produced ENAs is slightly smaller than the intensity of the quiescent neutral solar wind (NSW) while significantly higher ENA energies make them easily distinguishable from NSW atoms. Arrival of the CME‐produced ENAs is expected 3‐4 hours before the start of a large geomagnetic storm, which provides a basis for advance storm warning and prediction of storm magnitude. The progress in development of ENA measurement technique suggests that both the quiescent NSW and CME‐produced ENAs can be reliably measured.