Global Diagnostics of Ionospheric Absorption During X‐Ray Solar Flares Based on 8‐ to 20‐MHz Noise Measured by Over‐the‐Horizon Radars

Abstract

An analysis of noise attenuation during 80 solar flares between 2013 and 2017 was carried out at frequencies 8–20 MHz using 34 Super Dual Auroral Radar Network radars and the EKB ISTP SB RAS radar. The attenuation was determined on the basis of noise measurements performed by the radars during the intervals between transmitting periods. The location of the primary contributing ground sources of noise was found by consideration of the propagation paths of radar backscatter from the ground. The elevation angle for the ground echoes was determined through a new empirical model. It was used to determine the paths of the noise and the location of its source. The method was particularly well suited for daytime situations, which had to be limited for the most part to only two crossings through the D region. Knowing the radio path was used to determine an equivalent vertical propagation attenuation factor. The change in the noise during solar flares was correlated with solar radiation lines measured by GOES/XRS, GOES/EUVS, SDO/AIA, SDO/EVE, SOHO/SEM, and PROBA2/LYRA instruments. Radiation in the 1 to 8 Å and near 100 Å are shown to be primarily responsible for the increase in the radionoise absorption, and by inference, for an increase in the D and E region density. The data are also shown to be consistent with a radar frequency dependence having a power law with an exponent of −1.6. This study shows that a new data set can be made available to study D and E regions.

Noise level forecasts at 8–20 MHz and their use for morphological studies of ionospheric absorption variations at EKB ISTP SB RAS radar

“…When using the noise level for investigations of ionospheric absorption, the space weather effects usually cause lowering noise level [1,2,3,4], so using the minimal noise level prediction makes the absorption identification technique less sensitive to random variations of the noise level.…”

“…Frequency dependence of the detected absorption Traditionally it is believed that the absorption of radio waves vertically propagating in the ionosphere has a power-law frequency dependence [16,17,18,19,2,4,3]:…”

“…where A 1 , A 2 , f 1 , f 2 are the logarithms of noise absorption at two frequencies, and these frequencies, respectively, E 1 , E 2 are the effective noise elevation angles calculated by algorithm presented in [4]. The Fig.4B shows the distribution of α, calculated according to Eq.…”

“…where f sound (t) -sounding frequency and E(t) is elevation calculated by method, described in [4]. • The resulting A(t) is averaged over the 5 neighbor beams and 2 operational frequencies.…”

“…The use of 8-20MHz radio noise, measured by short-wave radars for the diagnosis of the lower part of the ionosphere, is an intensively developed method [1,2,3,4,5]. The physical processes that cause this effect [4] are close to the physical processes that cause the absorption of the signals scattered from the earth's surface (ground scatter, GS) [6,7], so GS signals and noise level both can be used to study the absorption of radiwaves [5]. The first observations of the absorption of 8-20 MHz radio noise during X-ray solar flares at mid-latitude radars [1] showed the efficiency of this method.…”