1. 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

2. 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

3. 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

4. Locations of multipoint Doppler sounding systems

5. 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

6. Event 5 May 2015
Doppler shift (black) and derivative of X-ray flux in two channels
(red nm; blue 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

7. 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

8. 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)

9. 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.

10. 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, and 209 km;
f = , 4.65 and 7.04 MHz
Electron density changes are larger at lower altitudes than at higher altitudes.

11. 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