A Hertzsprung‐Russell–like Diagram for Solar/Stellar Flares and Corona: Emission Measure versus Temperature Diagram

Abstract

In our previous paper, we presented a theory to explain the observed universal correlation between the emission measure (EM=n2V) and temperature (T) for solar/stellar flares on the basis of the magnetic reconnection model with heat conduction and chromospheric evaporation. Here n is the electron density and V is the volume. By extending our theory to general situations, we examined the EM-T diagram in detail and found the following properties: (1) The universal correlation sequence (``main-sequence flares'') with EM~T17/2 corresponds to the case of constant heating flux or, equivalently, the case of constant magnetic field strength in the reconnection model. (2) The EM-T diagram has a forbidden region, in which gas pressure of flares exceeds magnetic pressure. (3) There is a coronal branch with EM~T15/2 for T<107 K and EM~T13/2 for T>107 K. This branch is situated on the left side of the main-sequence flares in the EM-T diagram. (4) There is another forbidden region determined by the length of flare loop; the lower limit of the flare loop is 107 cm. Small flares near this limit correspond to nanoflares observed by the Solar and Heliospheric Observatory EUV Imaging Telescope. (5) We can plot the flare evolution track on the EM-T diagram. A flare evolves from the coronal branch to main-sequence flares, then returns to the coronal branch eventually. These properties of the EM-T diagram are similar to those of the H-R diagram for stars, and thus we propose that the EM-T diagram is quite useful for estimating the physical quantities (loop length, heating flux, magnetic field strength, total energy, and so on) of flares and coronae when there are no spatially resolved imaging observations.