Coronal loops diagnostics through the parameters of type U burst with equally developed branches
Vladimir Dorovskyy, Institute of Radio Astronomy of National academy od sciences of Ukraine
Solar inverted U-bursts are manifestations of fast electron beams moving along closed magnetic field lines – coronal loops. We report about the solar inverted U-burst with equally developed ascending and descending branches observed with the new Ukrainian radio telescope GURT operated in frequency band 8-75 MHz. Such symmetric U-bursts are extremely rare events. The turning frequency of this burst equaled 57 MHz and absolute frequency drift rates of the ascending and descending branches at frequency 70 MHz were both equal to 4MHz/s. Neighboring type III bursts drifted 5 times of magnitude faster at the same frequency. Assuming the Newkirk coronal density model the velocity of the electrons responsible for the type III burst generation appeared to be 0.25c. Since there was the only active region on the Eastern limb of the Sun (NOAA 12651) we supposed that electron beams associated with inverted U and type III bursts originated from the same source and thus might have equal velocities, i.e. 0.25c. Under this assumption the length of the coronal loop segment which corresponds to the U-burst bounded by points at frequency of 70 MHz was found to be 0.94Rs, where Rs is the solar radius. Then we suppose that the plasma confined within the coronal loop is completely isolated from the ambient solar corona by the magnetic field of the loop. This plasma is supposed to be isothermal and following barometric (Boltzman) density distribution law. The temporal and frequency parameters of the inverted U burst allow us to retrieve the geometry of the part of the coronal loop associated with the observed fragment of the U-burst under given plasma temperature. For temperature of 1.4MK the coronal loop should be as high as 3.5Rs. To satisfy U-burst parameters the shape of the loop should be elliptical with radial to transvers axes ratio of 2:1. According to our analysis two unknown parameters of the loop – its height and confined plasma temperature are connected in such a way that the higher the loop the colder the confined plasma and vice versa. So for unambiguous determination of one of these two parameters another one has to be defined by either method. For example the height of the loop can be defined through source imaging methods realized at LOFAR and NenuFAR radio telescopes.