Letter to the Editor

A reinterpretation of the energy balance in active region loops following new results from Hinode EIS

Space and Atmospheric Physics, Blackett Laboratory, Imperial College London, Prince Consort Road, SW7 2BZ, London, UK e-mail: s.bradshaw@imperial.ac.uk

Received: 26 March 2008
Accepted: 27 May 2008

Abstract

Context. New observations made by the Hinode EUV Imaging Spectrometer have revealed persistent redshifts in solar active region loops in the temperature range 10^5.6 ≤ T ≤ 10^6.4 K. The presence of redshifts, interpreted as bulk downflows, indicates that the loops are undergoing radiative cooling rather than continuous heating. This has significant consequences for current ideas regarding the physics of the ubiquitous 1 MK loops observed by instruments such as TRACE and SoHO-EIT.

Aims. The aim of this work is to assess the validity of the radiative cooling interpretation of the physics governing these loops.

Methods. This interpretation requires an enthalpy flux to balance the radiative energy loss in order to avoid catastrophic cooling. An analytical model is developed and used to find the downflows needed to drive an enthalpy flux sufficient to balance the known radiative loss rates, which are then compared with the observationally measured redshifts.

Results. The model results are found to be in good agreement with the observed redshifts.

Conclusions. Based upon the current analysis it appears that the majority of active region loops observed in the temperature range 10^5.6 ≤ T ≤ 10^6.4 K are in the radiative cooling phase of their lifecycle.