The role of chromospheric mottles in the mass balance and heating of the solar atmosphere

¹ National Observatory of Athens, Institute for Space Applications and Remote Sensing, Lofos Koufos, 15236 Palea Penteli, Greece e-mail: georgia@space.noa.gr
² Sterrekunding Instituut, Utrecht University, Postbus 80000, 3508 TA, The Netherlands e-mail: K.Tziotziou@astro.uu.nl

Received: 3 December 2003
Accepted: 12 May 2004

Abstract

High-resolution observations of a solar region containing several mottles are analyzed. Mottles constitute the fine structure of the quiet solar chromosphere and are found at the boundaries of the network cells, where the magnetic field is mostly concentrated. The driving mechanism for mottles (as well as for spicules) is suggested to be magnetic flux cancellation which most likely involves magnetic reconnection. Magnetic reconnection explains the observed bi-directional flows and, furthermore, allows conversion of part of magnetic energy to heat. We estimate several physical parameters of mottles and report a detailed analysis of their temporal variations. We then consider their impact on the mass balance and the heating of the solar atmosphere. We find that less than 1% of the mass flux injected by these structures into the corona expands outward as solar wind. The major fraction of this flux returns back to the chromosphere and provides an explanation for the red-shifts observed in the transition region spectral lines. The energy released by magnetic dissipation is quantified in terms of different energy components. Using typical values for the parameters of these structures it is found that they can provide ~20% to the energy budget of the solar corona, but only a small part of it goes to heating. This percentage, as well as the part of the energy that goes to heating, can be lower or higher depending on the relative frequency of these events and on their upward velocity. On the other hand, if one assumes that all the potential energy of the downflowing material is converted to heat the amount supplied to the chromosphere is really negligible.