On the Persistent Poloidal Alfven Waves

Standing 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. Plain Language Summary Ultralow frequency oscillations in the radial direction are often called poloidal Alfven waves. In a theoretical sense, they should be transient or short‐lived since they tend to be absorbed into the east‐west oscillations called toroidal modes. However, many observations show that long‐lasting poloidal modes exist even if there is no corresponding driving source at that moment. We theoretically study how this could happen and what kinds of conditions are required for persistent poloidal waves. We start by showing that constant Alfven frequency at a certain local region can make the poloidal lifetime longer. We derive two new, simple types of very useful necessary conditions, where we can get the conditions in terms of particle energy and wave numbers, which are found to be also consistent with the statistics of observations. Key Points Long‐lasting poloidal Alfven waves with no corresponding particle sources are often observed, but no theories are available yet We derive two new necessary conditions for the existence of persistent poloidal modes in terms of energy and wavenumber We present a statistical database to compare its events with the theoretical conditions, which are very consistent with the observations