Network Activity and the Evaporative Formation of Polar Plumes

It has previously been suggested that polar plumes are generated and maintained by strong, localized heating within coronal holes and that this heating is somehow associated with the interaction between small magnetic bipoles and nearby unipolar flux concentrations. These ideas are used here as the basis of a model for the formation and evolution of coronal plumes. The energy deposition rate in a plume is taken to be proportional to the rate at which magnetic flux becomes reconnected to the nearby monopole as the poles of the bipole are separated by the supergranular flow field. The heat input is conducted down to the base of the newly opened flux tube (located at the majority-polarity end of the bipole), where a small fraction of the energy goes into lifting chromospheric gas into the corona, while the rest is radiated away. The evaporation timescale (τ evap ~6 hr) represents the characteristic time for a new plume to form following the emergence of a bipole. Once flux exchange between the bipole and the unipolar flux concentration ceases, the plume decays on the radiative cooling timescale τ cool ~4 hr. In the absence of new bipole eruptions, the total lifetime of a plume is found to be 12-24 hr. The model accounts for the quiescent nature of polar plumes and the tendency for the diffuse plume emission to strengthen as the underlying bright network features disperse and decay.