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

In our previous paper, we have presented a theory to explain the observeduniversal correlation between the emission measure ($EM=n^2 V$) and temperature(T) for solar/stellar flares on the basis of the magnetic reconnection modelwith heat conduction and chromospheric evaporation. Here n is the electrondensity and V is the volume. By extending our theory to general situations, weexamined the EM-T diagram in detail, and found the following properties: 1) Theuniversal correlation sequence (``main sequence flares'') with $EM \proptoT^{17/2}$ corresponds to the case of constant heating flux or equivalently thecase of constant magnetic field strength in the reconnection model. 2) The EM-Tdiagram has a forbidden region, where gas pressure of flares exceeds magneticpressure. 3) There is a coronal branch with $EM \propto T^{15/2}$ for $T< 10^7$K, and $EM \propto T^{13/2}$ for $T> 10^7$ K. This branch is situated left sideof the main sequence flares in the EM-T diagram. 4) There is another forbiddenregion determined by the length of flare loop; a lower limit of flare loop is$10^7$ cm. Small flares near this limit correspond to nanoflares observed bySOHO/EIT. 5) We can plot flare evolution track on the EM-T diagram. A flareevolves from the coronal branch to main sequence flares, then returns to thecoronal branch eventually. These properties of the EM-T diagram are similar tothose of the HR diagram for stars, and thus we propose that the EM-T diagram isquite useful to estimate the physical quantities (loop length, heating flux,magnetic field strength, total energy and so on) of flares and corona whenthere is no spatially resolved imaging observations.Comment: 31 pages, ApJ (2002) Sep. 20 issue, in pres