Transient response of the ionosphere to X-ray solar flares Jaroslav Chum (1), Jaroslav Urbář (1), Jann-Yenq Liu (2) (1) Institute of Atmospheric Physics, Prague, Czech Republic (2) Institute of Space Science, National Central University, Chung-Li 320, Taiwan Continuous Doppler sounding Examples of measurements Conclusions

Continuous Doppler sounding Ionosphere Continuous, highly stable sine wave of frequency f is transmitted. [That is different from the ionosonde -pulses of short (coded) waveforms are transmitted.] Doppler shift DfD of the reflected wave is measured. Makes it possible to study variability on shorter time-scales up to ~10 s. Ionosondes typically sample at 5 to 15 minutes rate. Reflection is from a specific height, f=fp, which changes during the day. Reflection height can be obtained from nearby ionosondes. Movements of the ionosphere (of the reflecting level) and also increase/decrease of electron density cause Doppler shift DfD. f f + DfD

Doppler shift in detail Time change of the phase path of sounding radio wave fD .. Doppler shift; n .. refractive index; N .. electron density, f .. sounding frequency, c .. speed of light, zR .. reflection height (radial distance). Largest contribution to the Doppler shift is in the region of reflection, where n->0, (f ->fp) Terms contributing to electron density changes and hence Doppler shift Advection Compression Production Losses Equation of continuity Movement of reflecting level (GWs, ExB) Photo- ionisation Recombination, Electron attachment Infrasound

Locations of multipoint Doppler sounding systems

Sudden frequency deviation owing to solar X-ray (EUV) flares Dynamic Doppler shift spectra recorded in Taiwan on 5 May 2015 Derivative of X-ray and EUV flux is important ZR ~185 km; LT~6:10, Sun elevation e~10.2o

Event 5 May 2015 Doppler shift (black) and derivative of X-ray flux in two channels (red 0.1-0.8 nm; blue 0.05-0.4 nm), normalized to the same maximum values Doppler shift DfD ~0 at the time of X-ray maximum (derivative is zero) Doppler shift roughly corresponds with derivative of X-ray flux. [importance of X-ray flux derivative was suggested by Liu et al. (1996), but experimental data were with low time resolution. Derivative of EUV ? Ionizing radiation(O), l<91 nm

Event 25 June 2015, Taiwan The Doppler signal attenuates (collisions in lower ionosphere) Derivatives of EUV flux correspond better to DfD than the derivatives of X-ray flux There is more power in EUV ZR ~250 km; LT~16:15, Sun elevation e~31.9o

Event 22 October 2014, Czech Republic; 3.59, 4.65 and 7.04 MHz EUV from Proba2-Lyra Reflection from various reflection heights ZR ~ 160, 183 and 209 km; LT~14:05, Sun elevation e~14.9o Attenuation is lowest for the highest sounding frequency (similar in ionograms)

Attenuation It is estimated that a significant ionization, corresponding to fp ~ 0.1f is at altitudes ~70 km. Ionosondes do not detect the lower ionosphere owing to attenuated signals.

Doppler at various heights Doppler shift depends on frequency Doppler shift are therefore normalized to the same frequency to exclude the frequency dependence. ZR ~ 160, 183 and 209 km; f = 3.59, 4.65 and 7.04 MHz Electron density changes are larger at lower altitudes than at higher altitudes.

Conclusions Continuous Doppler sounding is sensitive to rapid increase of EUV (X-ray) solar flux (time derivative). The Doppler response to large X-ray solar flares is observed even for low Sun elevations. Doppler shifts are ~0 at times of EUV (X-ray) flux maxima -> Loss processes start balancing the photoionization immediately. Attenuation of radio signal is observed during intense EUV (X-ray) solar flares -> information about electron densities at low altitudes (D layer). Doppler response at various heights (different sounding frequencies) -> information about ionization rate at various altitudes. Thank you for your attention