Attenuation of decameter wavelength sky noise during x-ray solar flares in 2013–2017 based on the observations of midlatitude HF radars

Abstract

Based on a joint analysis of the data from 10 midlatitude decameter radars the effects are investigated during 80 x-ray flares in the period 2013-2017. For the investigation nine mid-latitude SuperDARN radars of the northern hemisphere (Adak Island West and East radars, Blackstone radar, Christmas Valley East and West radars, Fort Hays East and West radars, Hokaido East radar and Wallops radar) and Ekaterinburg coherent decameter radar of ISTP SB RAS are used. All the radars work in the same 8-20MHz frequency band and have similar hardware and software. During the analysis the temporal dynamics of noise from each of the radar direction and for each flare is investigated separately. As a result, on the basis of about 13000 daily measurements we found a strong anticorrelation between noise power and x-ray flare intensity, that allows to use shortwave sky noise to diagnose the ionospheric effects of x-ray solar flares. It is shown that in 88.3% of cases an attenuation of daytime decameter radio noise is observed during solar flare, correlating with the temporal dynamics of the solar flare. The intensity of decameter noise anticorrelates well (the Pearson correlation coefficient better than -0.5) with the shape of the X-ray flare in the daytime (for solar elevation angle > 0) in 33% of cases, the average Pearson correlation over the daytime is about -0.34. Median regression coefficient between GOES 0.1-0.8nm x-ray intensity and daytime sky-noise attenuation is about −4.4 · 10 4 [dB · m 2 /W t]. Thus, it has been shown that measurements of the decameter sky noise level at midlatitude decameter radars can be used to study the ionospheric absorption of high-frequency waves in the lower ionosphere during x-ray solar flares. This can be explained by the assumption that the most part of decameter sky noise detected by the radars can be interpreted as produced by ground sources at distances of the first propagation hop (˜3000 km).

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

“…Green sectors in Figures a–h correspond to the field of view of 16 radars that meet the criteria above and used to process the absorption data. As one can see, the obtained results demonstrate the dependence discussed earlier (Berngardt et al, )—the main noise absorption is most often observed in the sunlit region. On the other hand, it is clear that only part of the radars (about half) operates in a sufficient mode for measurement—on the one hand they do not significantly change the operating frequencies, on the other hand, they have a sufficiently high noise level to study its dynamics.…”

“…The next important issue arising in the investigation of noise data by coherent radars is the interpretation of the detailed temporal dynamics of the noise absorption. As shown in Berngardt et al () and seen in Figures a–c, the front of noise absorption at the radar correlates well with the shape of the X‐ray flare according to GOES/XRS 1–8 Å . The rear is substantially delayed with respect to the X‐ray 1‐ to 8‐ Å flare.…”

“…As was shown in Berngardt et al (), during solar X‐ray flares, attenuation of the minimal noise level in the frequency range 8–20 MHz is observed on the dayside by midlatitude coherent radars. The attenuation correlates with the increase of X‐ray solar radiation 1–8 Å and is associated with the absorption of the radio signal in the lower part of the ionosphere.…”

“…However, the temporal dynamics of the absorption sometimes do not precisely track the solar radiation at wavelengths of 1–8 Å , which indicates the presence of mechanisms other than the ionization of the D layer by 1‐ to 8‐ Å solar radiation. An example of such a comparison will be presented here in Figures a–d and was shown by Berngardt et al (, Figure 9).…”

“…The integration was made over the regions to exclude noise saturation effects from consideration. To increase the validity of the retrieved data, we analyzed only the cases where the correlation coefficient between the noise attenuation and the variations of the intensity of solar radiation in the 1‐ to 8‐ Å band exceeded 0.4, which indicates a statistically significant absorption effect (Berngardt et al, ). As a result, we obtained a statistical distribution of the exponent of the power law dependence of the absorption on the frequency by calculating the ratio for every experiment: where f 2, i , f 1, i are the frequencies of noise observation simultaneously on the same beam at the same radar, and A 0, i is the coefficient of regression between the absorption and X‐ray flare dynamics at different sounding frequencies; i is the experiment number.…”