Authors: Shah Mohammad Bahauddin (University of Colorado Boulder)

The high spatial resolution and fast temporal cadence of the current generation of observing instruments, such as those of the Solar Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph (IRIS), have revealed that the corona and the transition region are composed of open and closed magnetic strands, that are impulsively heated by magnetic reconnection and the associated resistive dissipation of electric currents. In image time-series, these heating events are spatiotemporally located with transient brightenings and are often found to be accompanied by broadenings in the differential emission measure (DEM). Spectroscopic observations and advanced modeling indicate that components of the transiently brightened plasma are driven far from thermal equilibrium, and different ion species manifest distinct responses to the particular physics of the heating mechanism. For example, heavier ion species are more readily energized by ion cyclotron turbulence, whereas lighter species resonate more strongly with ion acoustic turbulence. These dependencies have important consequences for interpreting the spectroscopic diagnostics routinely used to analyze the line emission measured by remote sensing observations, and in-situ measurements of the distribution of ion populations in the solar wind. This study reveals the sequence of ion heating based on mass observed in the transition region brightenings frequently observed in active regions and coronal holes. We find that the ion line broadenings in the active region brightenings closely resembles that of ion cyclotron turbulence, indicating a preferential heating of heavier ions. On the other hand, coronal hole brightenings exhibit a distinct order of ion species that do not conform to either ion cyclotron heating or ion acoustic heating. Multi-species modeling framework reveals that the cooling signatures associated with these heating mechanisms exhibit notable differences, which can be effectively characterized using current observational instruments. These findings provide valuable insights into the intricate details of the heating mechanism and its underlying processes.