On the Persistent Poloidal Alfven Waves

Abstract

AbstractStanding poloidal Alfven waves, radial oscillations with ultralow frequency of 5–20 mHz, are excited by wave‐particle interaction in the magnetosphere. It has been a question over several decades why poloidal oscillations often persist and are not quickly converted into toroidal waves even when there is no corresponding particle source. By adopting magnetohydrodynamic simulations in the dipole model, we first show that long‐lasting poloidal waves are available if the local Alfven frequency gradient becomes negligible. Assuming that initial poloidal modes are excited by driving particles, we investigate two loss mechanisms into either toroidal modes or compressional modes. Both mechanisms enable us to derive two necessary conditions for the existence of persistent poloidal modes. It is found that these conditions are very consistent with statistical feature of observations. Our results will be useful in identifying the persistent poloidal mode in terms of the corresponding particle energy and wavenumbers.