4:00 PM - 4:15 PM
[PEM09-21] The response of the Earthʼs ionosphere due to X-ray and EUV emissions from solar flares
Keywords:Dellinger phenomenon, flares, UV radiation
The sudden increase in X-ray to extreme ultraviolet (EUV) emissions of solar flares promote ionization in the ionosphere and it can cause a rapid variation in electron density. The communication failure caused by the absorption of the radio waves, due to the variations in electron density in the ionospheric D region (60-90 km) is called the Dellinger phenomenon (Dellinger 1937). The occurrence of the Dellinger phenomenon can be known from the value of the minimum reflection frequency (fmin) observed by the vertical incident ionosonde. It is known that the fmin value fluctuation depends on the peak X-ray intensity of flare and the solar zenith angle (e.g., Tao et al., 2020), and the estimation of the Dellinger phenomenon is based only on solar X-ray observations. However, there are many cases in which the fmin value is not proportional to the peak flux of X-rays. Thus, it is necessary to consider not only X-rays but also other flare emission wavelengths that affect the electron density in the ionospheric D region. The main candidate for this non-X-ray emission that cause the Dellinger phenomenon is EUV emissions.
Therefore, we first investigated the relationship between the observed X-ray and EUV emissions during a flare and the fmin values obtained from the ionosonde described above. In this study, we analyzed 38 solar flare events of M3 class or larger observed during daytime in Japan (9:00-18:00 JST) between May 2010 and May 2014. We used the GOES/X-ray Sensor (XRS) for X-ray data, the GOES/Extreme Ultraviolet Sensor (EUVS) and the Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE) were used for flare EUV data. The fmin values were obtained from the ionograms which are provided by the National Institute of Information and Communications Technology (NICT) in Wakkanai, Kokubunji, Yamagawa and Okinawa.
Comparing these X-ray and EUV emissions and fmin values, we found that the Lyman-alpha emission from the GOES/EUVS-E did not correlate with fmin values, but X-ray (1-8 Å) and EUV (11-14 nm) emissions did correlate with fmin values, with correlation coefficients of 0.74 and 0.76, respectively. Then next, we performed a detailed event analysis of the X1.7 class flare that occurred on May 13, 2013, to explore which (1-8 Å and/or 11-14 nm) emissions are mainly responsible for the occurrence of the Dellinger phenomenon. The difference in the time at which the flux of these two emissions reaches a maximum value suggests that the EUV (11-14 nm) emission has the main influence on the duration of the Dellinger phenomenon, however, but this was not identified.
Next, we used the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA), a physical model of the Earth's atmosphere, to calculate the ionospheric effects of solar flare emission and compare them with the Dellinger phenomenon. Since GAIA provides the altitude profile of the electron density change at each wavelength of the solar flare spectrum, we chose the electron density in ionospheric D region, which is considered to be the main source of the Dellinger phenomenon and compared it with the observed fmin value. In this paper, we discuss the flare emission spectra that mainly contribute to the occurrence of the Dillinger phenomenon.
Therefore, we first investigated the relationship between the observed X-ray and EUV emissions during a flare and the fmin values obtained from the ionosonde described above. In this study, we analyzed 38 solar flare events of M3 class or larger observed during daytime in Japan (9:00-18:00 JST) between May 2010 and May 2014. We used the GOES/X-ray Sensor (XRS) for X-ray data, the GOES/Extreme Ultraviolet Sensor (EUVS) and the Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE) were used for flare EUV data. The fmin values were obtained from the ionograms which are provided by the National Institute of Information and Communications Technology (NICT) in Wakkanai, Kokubunji, Yamagawa and Okinawa.
Comparing these X-ray and EUV emissions and fmin values, we found that the Lyman-alpha emission from the GOES/EUVS-E did not correlate with fmin values, but X-ray (1-8 Å) and EUV (11-14 nm) emissions did correlate with fmin values, with correlation coefficients of 0.74 and 0.76, respectively. Then next, we performed a detailed event analysis of the X1.7 class flare that occurred on May 13, 2013, to explore which (1-8 Å and/or 11-14 nm) emissions are mainly responsible for the occurrence of the Dellinger phenomenon. The difference in the time at which the flux of these two emissions reaches a maximum value suggests that the EUV (11-14 nm) emission has the main influence on the duration of the Dellinger phenomenon, however, but this was not identified.
Next, we used the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA), a physical model of the Earth's atmosphere, to calculate the ionospheric effects of solar flare emission and compare them with the Dellinger phenomenon. Since GAIA provides the altitude profile of the electron density change at each wavelength of the solar flare spectrum, we chose the electron density in ionospheric D region, which is considered to be the main source of the Dellinger phenomenon and compared it with the observed fmin value. In this paper, we discuss the flare emission spectra that mainly contribute to the occurrence of the Dillinger phenomenon.