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Continuous Doppler sounding of the ionosphere during solar flares
© The Author(s) 2018
- Received: 13 August 2018
- Accepted: 11 December 2018
- Published: 18 December 2018
- Solar flares
- HF Doppler sounding
- Radio waves
The electromagnetic radiation from the Sun covers a broad range of frequency spectra from radio waves to X-rays, the largest power spectral density being in the visible range of spectrum. The total solar irradiance at the Earth’s orbit is around 1366 W m−2 and its variability through a solar cycle usually does not exceed 0.1% (Fröhlich 2009; Kopp 2016). Fluctuations in the EUV and X-ray part of the solar flux are much larger. The radiated X-ray flux can increase by several orders of magnitude during solar flares. The changes of EUV and X-ray radiation are one of the main causes of ionospheric variability as the EUV and X-ray fluxes are responsible for the photoionization of the upper atmosphere and formation of the ionosphere. Photoionization takes place if the energy of ionizing photons exceeds the ionization threshold energies of atoms and molecules in the upper atmosphere. The ionization thresholds—maximum wavelengths of photons that ionize the most important species in the thermosphere/ionosphere of O2, NO, O, He and H are 102.8, 133.8, 91.03, 50.42, and 91.16 nm, respectively (Shunk and Nagy 2009). The molecular ions O2+ and NO+ prevail in the lower ionosphere (D and E region), the atomic ion O+ is dominant in the F2 layer, including the peak of the ionosphere. At altitudes higher than 600–1000 km, the H+ ions (protons) are the most common ion species.
In this paper, sudden ionospheric disturbances (SIDs) owing to rapid enhancement of solar EUV and X-ray fluxes during solar flares are investigated. The paper is based on experimental measurements of EUV and X-ray flux by satellites and on monitoring of SIDs by continuous Doppler sounding, together with measurements by ionosondes and riometer. It partly builds on previous works, mainly on those by Mitra (1974) and Liu et al. (1996). Contrary to previous studies, the data are presented with much better time resolution (several seconds) and sudden frequency deviations observed by continuous Doppler sounding are investigated together with signal absorption. The signal absorption occurs as a consequence of collisions between neutral particles and electrons owing to enhanced electron density in the lower ionosphere, especially in the D layer. The flare-enhanced ionization in the D layer can also be observed as cosmic noise absorption at frequencies of several tens of MHz measured by riometers, as amplitude and phase changes of signals from VLF transmitters propagating in the Earth-ionosphere waveguide or as sudden wave fadeouts (SWFs) at frequencies of units of MHz (Laštovička 2009; Silber and Price 2016). Atmospherics (low frequency electromagnetic signals generated by lightning) have also been used to probe the response of D layer to solar flares (Han and Cummer 2010). Han and Cummer (2010) found that the reflection height of atmospherics decreased by about 6 km if the X-ray flux measured in the spectral range 0.1–0.8 nm increased ten times. Their results are in agreement with previous studies based on narrow band measurements of signals from VLF transmitters (McRae and Thomson 2004). We will use this estimate of the decrease in electron density profile in D layer in our study. Solar flares can also cause decrease in echo power or short-wave fadeouts of SuperDARN HF radar signals (Watanabe and Nishitani 2013; Chakraborty et al. 2018; Fiori et al. 2018). The paper is organized as follows: Second section describes measurements and discusses data analysis. Third section analyzes the observations in the Czech Republic, Taiwan and Argentina. Fourth section discusses the obtained results. The last section provides a brief summary.
Continuous Doppler sounding
The first term on the right-hand side of Eq. (2) corresponds to the advection, i.e., to radial motion of the reflection region (usually the vertical motion is the most important). The advection term dominates whether the plasma motion is caused by atmospheric gravity waves (GWs) or ExB drift, associated with, e.g., magneto-hydrodynamic waves (Sutcliffe and Poole 1989). The second term represents the plasma compression and rarefaction (mainly in the reflection region), and it is important in the analysis of infrasound waves (Chum et al. 2016). The increase in photoionization during distinct solar flares can cause sudden frequency deviations (SFDs). The observed Doppler shift is then mainly given by the difference (imbalance) between the rapid electron production and losses, p − l, and the contributions of the advection and compression terms to the observed Doppler shift can be usually neglected.
It should be noted that if the gradient ∂N/∂z is approximately constant around the reflection region (with respect to Δh), then Eqs. (4) and (7) give an equivalent shift of electron density profile at the height of reflection as ΔN = − ∂N/∂z · Δh. In other words, for the approximately constant ∂N/∂z around the reflection region, it does not matter which mechanism (advection or p − l imbalance) dominates as the shift of the electron density profile calculated from the observed Doppler shift fD is the same. As discussed before, the derivation of this statement is based on the assumption that there are no regions with similar values of plasma frequency below the reflection region.
Coordinates of transmitters (Tx) and receivers (Rx)
Attenuation of radio signal in the ionosphere
This means that for ω p 2 ≪ ω2 and ν2 ≪ ω2, the attenuation is proportional to the square of the inverse radio frequency, 1/ω2. The inequality ω p 2 ≪ ω2 is satisfied in the D layer, where collisions are dominant for frequencies larger than about 1 MHz. On the other hand, the inequality ν2 ≪ ω2 might not be satisfied in the D layer.
To calculate Im(k) and attenuation A for waves propagating in a real atmosphere, it is necessary to know how the plasma frequency fp, determined by the electron density, and electron–neutral collision frequency change with height. The electron–neutral collision frequency can be estimated by the following formula (Kelley 2009).
Satellite data of EUV and X-ray fluxes
For comparison with the measured Doppler shifts the space environment monitoring (SEM) instruments on geostationary satellite GOES-15, located at 135°W, are used. Specifically, the XRS-EUV (X-Ray and Extreme Ultra-Violet) sensor measuring disk-integrated solar X-ray (2 s cadence) in two channels, ranges of 0.05–0.4 nm and 0.1–0.8 nm, and EUV fluxes (10 s cadence) in five channels. We have analyzed data from three calibrated EUV channels: A, B, and E covering approximately spectral ranges of 5–15 nm, 25–35 nm (includes Fe-XV at 28.4 nm, He-II at 30.4 nm), and 115–127 nm (includes H-Lyman-alpha at 121.6 nm), respectively. Only channels B and E will be presented further, as for these channels it was possible to compute the time derivatives reliably (time derivative of channel A contained too much of digitization noise).
Solar flares are classified according to X-ray flux in the wavelength range from 0.1 to 0.8 nm. The classification taken from the list available at NASA web page (http://hesperia.gsfc.nasa.gov/goes/goes_event_listings/) is used in this paper.
Obviously, solar flares can cause SFDs at specific locations only if the Sun’s elevation is larger than zero, ε > 0. The following examples will demonstrate that the SFDs induced by solar flares can be observed for various solar elevation angles ε and at different ionospheric heights. The larger ε and the more rapid increase in EUV flux, the more distinct Doppler shift is observed in the Doppler shift spectrogram. Large EUV and X-ray fluxes can lead to substantial attenuation or even disappearance of the signal owing to radio absorption in the lower ionosphere.
X1.6 event on 22 October 2014, observation in the Czech Republic, solar elevation ~ 15°
The electron density profile during the solar flare, shown in Fig. 7b by dashed magenta, was not measured. It was roughly estimated as follows: (a) The lowest part of the original profile (solid blue) up to 83 km was shifted downward by 12 km. The value of this downward shift is based on the work by Han and Cummer (2010), and references therein, who reported the lowering of reflection height for atmospherics in D layer by about 6 km if the X-ray flux in the 0.1–0.8 nm channel increased by a factor of 10. They found that the decrease in the reflection height depends practically linearly on the logarithm of X-ray flux. The X-ray flux in this channel increased approximately 100 times in our case (Fig. 4a). (b) The height of the Es layer around 100 km was considered unchanged during the solar flare. This assumption could not be proved experimentally as the minimum frequency detectable in the ionogram is larger than the critical frequency of Es (Fig. 6) during the X-ray flare. However, the height of Es usually does not change rapidly. Also, the rapid change of the height of Es is not expected as the generally accepted main mechanism responsible for the formation of the Es layer is the wind shear (Mathews 1998; Haldoupis 2012). It is more complicated to estimate the change of electron density in the Es layer, which likely increases during the X-ray flare because of the enhanced ionizing flux. We will assume for simplicity that the relative increase in the Es density is the same as the relative increase in electron densities in the layers above that can be estimated from the Doppler shifts. This might be not quite correct and leads to a certain uncertainty. Nevertheless, the influence of this uncertainty on the cumulative attenuation is not dominant because of the relatively narrow height extent of the Es layer as seen in Fig. 7. (c) The profile in the F layer was lowered by Δh = 4.5 km as estimated from Fig. 5b. It is reminded that there is a close relation between the height decrease and electron density increase at a specific height (Eqs. 4, 6, 7). To ensure that there is an electron density increase ΔN also at regions with negative electron density gradient, mainly above the Es layer, the sign of Δh was changed in that region. It should be noted that the changes of electron densities above ~ 110 km and especially in the F layer have only negligible influence on the imaginary part of wave vector and hence on the attenuation. (d) The downward shift of the most critical and important part between 83 km and Es layer (in original profile) was adjusted so that the relative changes of attenuations would correspond with the observed changes presented in Fig. 3a and b. A rough agreement between the observed and simulated changes ΔAdB was obtained for the downward shift of 9 km for this part of the profile. The corresponding imaginary parts of Im(k) and attenuations are shown by dashed lines in Fig. 7c and d, respectively.
It should be noted that the exact treatment of attenuation and more sophisticated comparison between experiment and modeling would require distinction between R-X and L-O mode. The current continuous Doppler sounding does not distinguish the R-X and L-O mode. Also, the downward shift of electron density profile by constant values in a few specific height intervals is a simplification. Measurements operating at significantly different frequencies (dominant attenuation at different heights) could provide further insight into the behavior of D layer dynamics at different heights. Nevertheless, even this simple approach provides a basic idea about the electron density changes and altitudes at which the attenuation dominates. It is partly based on available models, and it is roughly consistent with measurements and previous studies.
M7.9 event on 25 June 2015, observation in Taiwan, solar elevation ~ 32°
The ionospheric response for M7.9 X-ray flare on 26 June 2015 is similar or even larger than for the X1.6 event on 22 October 2014. There are two likely reasons. First, the local solar elevation ε is larger for the M7.9 event (ε ~ 31.9°) than for X1.6 event (ε ~ 14.9°). Second, the flux derivative dFEUV/dt is larger in the case of M7.9 event than in the case of X1.6 event, which contributes to the larger Doppler shift than that observed in the case of X1.6 event.
Another example of ionospheric response to X-ray event, namely to X2.7 flare observed in the ionosphere over Taiwan on 5 May 2015 around 22:10 UT (~ 06:10 LT) was given in the paper reviewing the observation by international network of CDSS by Laštovička and Chum (2017). The corresponding figure can be found in that paper. Here, we will only mention that the ionospheric response was relatively small; maximum Doppler shift was around 1.5 Hz, and the signal remained above the noise background all the time, though attenuated, because of low solar elevation, ε was ~ 10.2°.
M1.3 and C8.5 events on 7 August 2016, observation in Tucumán, solar elevation ~ 42°
The previous result by Liu et al. (1996) that the Doppler shifts associated with X-ray solar flares are controlled by the time derivative of ionizing flux rather than by the flux itself was confirmed purely on experimental basis. The paper by Liu et al. (1996) was mainly based on simulations, their time resolution of presented experimental examples did not allow to unambiguously distinguish whether the Doppler shifts correspond better with ionizing flux or with its time derivative and indirect arguments were used. The time resolution provided in the current study (6 s for Doppler shift measurements and 10 s for the EUV flux) made it possible to clearly show experimentally that the Doppler shift is associated with the time derivative of ionizing flux. Thus, the time at which the Doppler shift is maximum does not correspond with the time of maximum of ionizing flux. In addition, it was shown that it is better to use the time derivative of EUV flux rather than the time derivative of X-ray flux to get the best mutual timing because the EUV flux dominates the ionization at the heights of Doppler shift measurements, which is usually F layer. The X-ray flux mostly penetrates deeper, and is mainly responsible for the ionization in the D layer and hence for the attenuation of the radio waves. The attenuation correlates well with the ionizing X-ray flux as can be seen from comparison of Figs. 10a and 11. On the other hand, the rate of electron density change and hence the Doppler shift depends on the time change of the ionizing flux. If the ionizing flux does not change, then loss processes start balancing the production rate and the Doppler shift approaches zero. More precisely, processes such as plasma transport, acoustic gravity waves, etc., become dominant factors that are responsible for the observed Doppler shifts. The precise timing between Doppler shift fD and dFEUV/dt or dFX-ray/dt changes from event to event. The maximum of fD usually corresponds to the maximum of dFEUV/dt within about 20 s. The maximum of dFX-ray/dt might be shifted from max(fD) by 1–2 min. Certain differences between the timing of EUV and X-ray flux arise from different locations of generation. EUV flux is usually produced in the solar chromosphere, whereas the X-ray flux usually originates from the solar corona (Smith and Gottlieb 1974). It is interesting to note in this respect that the time derivative of the soft X-rays measured by GOES is often used to estimate the time evolution of hard X-rays (above ~ 10 keV), a so called Neupert effect (Veronig et al. 2005). It should also be noted that the total ionizing flux FI has to be considered to investigate the time shifts between max(dFI/dt) and max(fD) exactly. The usable EUV and X-ray channels, however, do not cover the whole ionizing flux (In “Satellite data of EUV and X-ray fluxes” section). In addition, the relative role of different spectral ranges of the incoming flux on the ionization changes with height.
Summary of presented events and observations
Date and time of X-ray maximum (UT)
Height of SFD observation (km)
max |u p * | (m/s)
max ∂N/∂t (m−3/s)
max dFEUV/dt (W/m2/s)
Solar elev ε (°)
sin(ε)·max dFEUV/dt (W/m2/s)
206 (7.04 MHz)
1.3 × 109
1.0 × 10−6
0.26 × 10−6
178 (4.65 MHz)
0.9 × 109
1.0 × 10−6
0.26 × 10−6
154 (3.59 MHz)
0.5 × 109
1.0 × 10−6
0.26 × 10−6
185 (6.57 MHz)
0.7 × 109
1.6 × 10−6
0.27 × 10−6
250 (6.57 MHz)
2.9 × 109
15 × 10−6
7.9 × 10−6
200 (4.63 MHz)
0.05 × 109
200 (4.63 MHz)
0.05 × 109
The attenuation of Doppler signal was investigated in the present study and compared with theoretical expectations. Most of the attenuation occurs in the lower ionosphere in the D and lower E region. The attenuation of Doppler signal was also compared with the attenuations measured by the nearby riometer in Tucumán. To the best of our knowledge, it was for the first time that attenuations measured by riometer and CDSS were compared. It should be noted that short-wave fadeouts of SuperDARN HF radar signals and their correlation with riometer measurements were recently discussed by Fiori et al. (2018). Systematic investigation and comparison of attenuations measured by different instruments operating at different frequencies during solar flares might potentially provide interesting information about the dynamics of electron densities at the heights of D and lower E layer. Differences in measurements at various frequencies may result from different electron density changes at different heights in the lower ionosphere and thus reduce the uncertainties in the D layer profile and its evolution during solar flares. As the riometer was installed in Tucumán in 2016 at the decreasing phase of the current solar cycle (SC24), it is expected that such a database will be obtained in the next solar cycle. In addition, construction of a riometer at the location of CDSS in the Czech Republic is underway. An update of CDSS is also considered to measure the received power in ordinary (L-O) and extraordinary (R-X) mode separately, which will enable a more exact calculation of the expected attenuation for the modeled profiles of electron densities and comparison with experiment.
The enhanced ionization in the D layer, regardless of the source of ionization, leads to decrease in the signal-to-noise ratio and might cause echoes to not be detected at low frequencies. Therefore, ground-based studies of E and Es layer dynamics, e.g., from ionograms, should consider a possible attenuation of the sounding signal due to radio wave absorption in the D layer. Absorption in the D and lower E regions, using continuous signal of broadcasting stations, was often studied to identify ionospheric effects of solar flares (e.g., Laštovička 2009), or of planetary waves (e.g., Laštovička and Pancheva 1991), or winter anomaly in the lower ionosphere (e.g., Schwentek 1971). However, the broadcast transmitters changed from constant strength of transmitted signal to variable strength to be more energy efficient in the late 1980s and early 1990s, which terminated this application. The strength of radio wave absorption can also be estimated from fmin, where fmin is the lowest detected frequency in the ionogram, as has already been used in several studies (e.g., Fraser and Thorpe 1976; Stanford and Saksena 1989; Vergasova et al. 1995). However, fmin is not applicable for studying rapid processes such as SIDs due to the time resolution of standard ionosonde sounding.
Using high-time resolution data, on the order of seconds, it was experimentally shown that Doppler shift associated with solar flares depends on the time derivative of ionizing flux, especially in the EUV spectral range. The maximum Doppler shift was observed around the time when the rate of change of the ionizing flux was maximal, whereas it was negligible when the ionizing flux reached maximum. The measured increases of relative attenuation of Doppler signals corresponded well with the enhanced ionizing X-ray fluxes and were in rough agreement with calculations of expected attenuations based on modeled electron densities and electron–neutral collision frequencies. The relative attenuation measured by continuous Doppler sounding also correlated well with cosmic noise absorption obtained by riometer. A future systematic analysis of attenuations based on multi-instrument measurements operating at different frequencies could be used for investigation of dynamics at different heights of the lower ionosphere.
JC wrote most of the paper and performed most of the analysis, especially of the Doppler measurements. JU worked with GOES 15 data, JL helped with the text, MAC, FAMB and MF provided the ionosonde and riometer data in Tucumán and were also responsible for the operation of Doppler sounding in Tucumán, JYL provided the ionosonde data and was responsible for the operation of Doppler sounding in Taiwan, JF, and ZM helped with determining of true heights from ionograms. All authors read and approved the final manuscript.
J. Baše and F. Hruška from the Institute of Atmospheric Physics CAS, Czech Republic, and Y. Chen from Taiwan are acknowledged for the development and maintenance of the Doppler system.
The authors declare that they have no competing interests.
Availability of data and materials
GOES-15 satellite data are available at NOAA National Centers for Environmental information http://www.ngdc.noaa.gov/. International Reference Ionosphere model IRI-16 is available at http://www.irimodel.org/. The Doppler data in the form of spectrograms are available at http://datacenter.ufa.cas.cz/ under the link to Spectrogram archive.
Consent for publication
Ethics approval and consent to participate
The support under the grant 18-01969S by the Czech Science Foundation is acknowledged. The support under the Grant PICT 2015/0511 granted by FONCyT – ARG and MOST-18-05 by the Czech Academy of Sciences is also acknowledged.
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