Solar flare effects on D-region ionosphere using VLF measurements during low- and high-solar activity phases of solar cycle 24
© The Author(s) 2018
Received: 4 August 2017
Accepted: 30 January 2018
Published: 12 February 2018
Solar flares, mainly having X-ray wavelengths of the order of tenths of nanometers, permeate the D-region of the ionosphere and enhance the electron density by extra ionization (e.g., Mitra 1974). The D-region, acting as a reflecting layer, is important for long-wave communication and navigation systems. The very low-frequency (VLF, 3–30 kHz) radio waves generated by strong lightning discharges and navigational transmitters propagate through the waveguide bounded by the Earth’s surface and the lower ionosphere (D-region), known as the Earth–ionosphere waveguide (EIWG) with relatively low attenuation of about 2–3 dB/Mm (Davies 1990, p. 367). Under normal conditions in the absence of solar flares, D-region ionization is mainly produced by the Sun’s Lyman-α radiation. During solar flares, the X-ray flux from the sun increases considerably, which penetrates to the D-region and increases the electron density via extra ionization of the neutral constituents, particularly nitrogen and oxygen.
McRae and Thomson (2004) studied the ionospheric D-region parameter changes, H′ and β, as a function of solar X-ray flux by using solar flare-induced perturbations to the VLF amplitude and phase of the Omega, NPM (USA) and NLK (USA) signals received at Dunedin, New Zealand. H′ is a measure of the ionospheric reference height in kilometer and β is the electron density sharpness in km−1 of the D-region. McRae and Thomson (2004), using the Long Wave Propagation Capability (LWPC) and ModeFinder computer codes, established that during solar flare-associated D-region ionization, H′ decreases in proportion to the logarithm of the X-ray flux intensity. They found that the typical reduction in H′ can be from a midday unperturbed value of about 71 km to about 58 km for an X5 class solar flare; β can increase from 0.39 km−1 to a saturation level of about 0.52 km−1. Thomson et al. (2004, 2005) found that the phase perturbations for X class solar flares to NDK (USA), NLK, NPM and Omega (Australia) signals recorded at Dunedin were more closely correlated with the X-ray flux than was the amplitude. From the measurements of the GQD (Anthorn, UK, 22.1 kHz) transmitter signal at Belgrade, Grubor et al. (2005) found that flares of C to X classes perturbed subionospheric VLF signals during a period of low-solar activity period between 2004 and 2005. Raulin et al. (2006) studied solar flares using VLF sudden phase anomalies (SPAs) on NDAK (North Dakota, USA, 13.1 kHz), HAI (Haiku, USA, 13.6 kHz) and ARG (12.9 kHz) transmitter signals received at Atibaia (ATI), Brazil. Their results suggest that the occurrence of SPAs due to weak solar flares, of class C and below, was higher during solar minima, but the occurrence of SPAs was not dependent on the solar activity for stronger class flares. Recently, Tan et al. (2014) studied the effects of 26 solar flare, of classes C2.6 to X3.2, on the amplitude of the NWC (Australia) signal recorded at a low-latitude site at Tay Nguyen University, Vietnam. They found that during the solar flares, β increased from 0.3 to 0.506 km−1, while H′ decreased from 74 to 60 km.
In this study, the effects of solar flares during two time periods on the amplitude and phase of subionospheric VLF signals from NWC (19.8 kHz) and NLK (24.8 kHz) monitored at Suva (18.1°S, 178.4°E), Fiji, a low-latitude station in the South Pacific region, were analyzed. The first time interval was from December 2006 to December 2010, a period during the unprecedented solar minimum of solar cycles 23 and 24 (low solar activity, Rz = 9); the second was from January 2012 to December 2013, a period at the peak of solar cycle 24 (moderate solar activity, Rz = 60). The NWC–Suva path is in an almost east–west direction, whereas the NLK–Suva is in a northeast–southwest direction. These two paths have been selected to cover a larger time zone and hence to observe the effect of a greater number of solar flares. The signal perturbations due to A and B class flares are masked by background noise (signal variation), so we considered only C, M and X flares. A comparison was made between the VLF perturbations associated with the same classes of solar flares during both low and moderate levels of solar activity. During the prolonged solar minimum period (2006–2010), the ionosphere shrank to its lowest level (Heelis et al. 2009; Liu et al. 2011), which enabled us to study the low-latitude D-region response to space weather. A greater number of solar flare-associated VLF perturbations were observed during this interval due to the extended time frame and the increased sensitivity of the D-region to solar flares during solar minima. The solar flare-associated signal amplitude and phase enhancements of varying flare intensities and any dependence of the effects of solar flares on solar activity were investigated. Selected VLF perturbation events associated with solar flares for a wide range of flare classes were modeled using LWPC version 2.1 to determine the effect of solar activity on the D-region changes associated with solar flares. The research reported above presented the variations in H′ and β with respect to solar flare power for either low, mid- or high levels of solar activity separately. In the current study, the relationship between flare power and changes in H′ and β is presented using an extensive amount of flare data (6 years) during both low- and moderate (peak of the current solar cycle)-solar activity conditions and a comparison is made between them.
Experimental data and analysis
Occurrence of solar flares of different classes (C, M, X) as observed by GOES for the periods of data analysis (*low solar activity, **moderate solar activity) together with the average sunspot numbers for each year
Average sunspot no. (R z )
Number of flares
*January 2007–December 2007
*January 2008–December 2008
*January 2009–December 2009
*January 2010–December 2010
**January 2012–December 2012
**January 2013–December 2013
Solar flare effects on subionospheric VLF propagation
Solar flares on July 05, 2012
A series of solar flare events, recorded by GOES satellites, perturbed the amplitude and phase of the NWC signal on July 05, 2012. The variation of X-ray flux during the seven flare events of classes M1.1, M2.5, M1.0, C7.1, M2.3, M4.7 and C9.1, together with the amplitude and phase of the NWC signal, is shown in Fig. 2a, b. During the time in which these flares occurred, 00–05 h UT (12–17 h LT), the NWC to Suva TRGCP was completely in daylight. The NLK signal was off-air, so no effect could be observed in it. During these flares, the NWC amplitude and phase enhancements showed clear proportional variation with the X-ray flux intensity. The ΔA values for NWC ranged from 1.4 dB (C7.1) to 2.6 dB (M4.7) for these seven flares, while ∆ϕ ranged from 12° to 54°. Throughout this period, the signal strength remained above the normal daytime value because the next flare occurred before the amplitude recovered fully. The peaks of the amplitude and phase of the signal appear to be well aligned with the peak of the X-ray flux during all the flares. However, upon close observation, time delays (∆t) between amplitude and phase peaks with the X-ray peak ranging from 0.5 to 5 min were estimated for some of these flares, with the C7.1 flare revealing the largest time delay of 5 min. ∆t is an important quantity that can be used to study the ionospheric response to the flares (Zigman et al. 2007); however, it is not the subject of this study.
Solar flares on May 13, 2013
The effects of two solar flares of classes C9.3 and X1.7 that occurred on May 13, 2013, were observed on the phase and amplitude of the NLK and NPM signals simultaneously. The C9.3 flare began at 0:32 UT and finished at 0:46 UT, while the X9.0 flare started at 01:53 UT and ended at 02:32 UT. The NWC signal was inactive; hence, no effect was seen on it. The variations in the X-ray flux together with the NLK and NPM amplitude and phase during both the flares are shown in Fig. 3a–c. For the C9.3 flare, values of ΔA and ∆ϕ of 4.2 dB and 82° and 1.9 dB and 49° were measured for the NLK and NPM signals, respectively. Likewise, values of ΔA and ∆ϕ of 9.8 dB and 220° and 4.8 dB and 133° were measured for the NLK and NPM signals, respectively, for the X1.7 flare. From these data, it can be seen that for the same flare, different signals are perturbed to different extents: In this case, NLK demonstrates stronger perturbations to amplitude and phase than the NPM signal. A zero time delay (Δt) was observed for the C9.3 flare, with the amplitude and phase peaks for both signals occurring at the same time as the solar flare flux peak. The X1.7 flare, however, showed a Δt of about − 3.5 min between the peaks of flux and amplitude, with the amplitude peak occurring before the flare flux peak. However, the peaks of the phase and the flux are in line showing zero Δt between them for both of the signals. (The NPM amplitude and phase data are used here only once to illustrate simultaneous observations of the solar flare-associated perturbations on VLF; we have not used the NPM signal for any further analysis.)
Effects of solar activity on solar flare-induced VLF perturbations
D-region modeling for solar flare effects: Wait parameters
VLF signal perturbations due to solar flares during the studied low- and moderate-solar activity periods, measured both on the amplitude and phase of NLK and NWC signals, were used to determine the accompanying changes in D-region parameters (H′ in km and β in km−1) using the LWPC (version 2.1) code. Since the daytime D-region is SZA dependent and the values of H′ and β are different in LWPC-created segments along the paths, it is not possible to find a single undisturbed value of H′ and β along the entire path. Because of this difficulty, values of H′ and β are estimated at the mid-location of TRGCPs (e.g., McRae and Thomson 2000), with respect to which the changes in D-region parameters due to solar flares are obtained by using LWPC modeling.
The initial step for determining the variations in H′ and β with flare power was to establish the unperturbed values of H′ and β for which SZA values for mid-TRGCP paths of the VLF signals at the times of flare occurrence were calculated. The SZA was calculated by the Web site service of http://www.solartopo.com/solar-orbit.htm and unperturbed H′ and β were calculated using the relationships between SZA and D-region parameters given by McRae and Thomson (2000) for low solar activity and by Thomson (1993) for high solar activity. To investigate the solar activity dependence of solar flare-associated D-region changes, we selected flares that occurred within the SZA − 40° < χ < 40°. This essentially minimizes the SZA effect (as shown in Fig. 6) on solar flare-induced VLF perturbations, and hence, D-region changes. The level of perturbations for a narrow range of flare flux intensities (e.g., C3.0–C3.9) was further investigated within this SZA range and it was found that no apparent variation existed with the varying SZA.
The perturbed values of ionospheric H′ and β during solar flares were obtained by varying the values of H′ and β so as to match the observed perturbed amplitude (A + ∆A) and phase (ϕ + ∆ϕ). This was done by adjusting the values of H′ (at intervals of 0.05 km) and β (at intervals of 0.001 km−1). To obtain the observed values of the amplitude and phase, the RANGE model was run using the EXPONENTIAL substring (rexp) in the LWPC. A range of amplitudes and phases were generated as text files for various segments, starting from the transmitter leading to the receiver. In this way, the observed amplitude and phase at the receiver are obtained. In some cases, it was not possible to find the exact match for both the observed amplitude and phase. In such circumstances, H′ and β were varied to obtain the closest values of observed perturbed amplitude and perturbed phase. The values of H′ and β during solar flares were modeled considering the values at mid-path location as ambient values along the entire TRGCP, which is a robust approximation that has also been used in earlier research (e.g., McRae and Thomson 2000).
The panels on the right-hand side of Fig. 7 show that H′ decreases in direct proportion to the increasing logarithm of the X-ray flux. H′ drops to between 56 km for the strong X6.5 flare and 70–71 km for weak C1.0 class flares from normal typical daytime value of about 70.7 km. Classes C1.0 and X6.5 correspond to 0 and 28 dB flare power, respectively. An interesting feature of the variation in H′ with X-ray flux is that there is a difference in the reduction in H′ for the same class of solar flares depending on the level of solar activity. For both signals, it was observed that H′ decreases more during the low-solar activity period than during high-solar activity conditions for the same class of flares, as is evident from the gap separating the best-fit lines of H′ versus flare power in Fig. 7. For the NLK and NWC signals, H′ is estimated to decrease by approximately 1 km more during low levels of solar activity than during moderate levels for strong solar flares of similar class.
The effects of solar flares on subionospheric VLF propagation during low- and moderate-solar activity periods showed enhancements in the amplitude and phase, which were found to be higher during periods of low solar activity than during the period of high solar activity in this study. The solar flare-induced signal enhancement can be explained in terms of electron density enhancements (Grubor et al. 2005) through which propagating VLF signals get reflected from a sharper and lower D-region ionospheric boundary penetrating less into the D-region, and hence undergo less attenuation/absorption than under normal unperturbed conditions. This is consistent with our results in Figs. 2 and 3, where the VLF phase and amplitude always increased during solar flares. But Kolarski and Grubor (2014), based on the amplitude and the phase perturbation of the 22.1 kHz GQD signal trace from Skelton (54.72°N, 2.88°W) to Belgrade (44.85°N, 20.38°E), reported that the pattern of signal perturbation is not always the same for solar flares. They found that for some flares the amplitude and phase delay increased, while for others, the perturbations in amplitude showed a short-duration decrease followed by an increase and a decrease in the phase delay. They stated that solar flares affected the VLF wave propagation in the EIWG by changing the lower ionosphere electron density height profile differently for different solar flare events. Bouderba et al. (2016), using the solar flare-induced perturbations of the VLF signal from the NRK transmitter (37.5 kHz, 63.85°N, 22.45°W) received at Algiers (RALG: 36.7°N, 03.13°E) with a TRGCP of 3495 km and LWPC modeling, calculated the change in the attenuation coefficient and fading displacement and suggested that the sign (negative or positive) of the perturbed signal parameters depends on the propagating distance. Šulic et al. (2016) from the analysis of amplitude and phase data received at Belgrade from four European transmitters during a seven-year period (2008–2014) found a range of amplitude and phase perturbations that varied for different paths. Also, their statistical results showed that the size of amplitude and phase perturbations to the VLF/LF radio signal was correlated with the intensity of the X-ray flux. However, these authors did not state the exact relationship using fit curves between the level of VLF perturbation and the flare flux intensity.
A distinct difference seen between the ΔA and flare power curves for the two signals (Fig. 4) is the level of perturbations for similar class flares during the low- and moderate-solar cycle periods. The solar flares of similar class demonstrate higher levels of ΔA values during periods of low solar activity than during the period of moderate solar activity for both signals. A maximum difference in ΔA of up to 0.5 dB for the NLK and NWC signals is seen at a flare power level of approximately 15 dB. The solar flares of strength C1.0 to C1.3 during the period of low solar activity produced perturbations, whereas flares of same class did not produce any detectable perturbations during the period of moderate solar activity, which is due to the higher sensitivity of the D-region during low solar activity.
The best-fit curves for ΔA versus the flare power for the VLF signals are similar in form to those obtained earlier by McRae and Thomson (2004), Thomson et al. (2005) and more recently by Kumar and Kumar (2014). However, variation between ΔA and the flux intensity during solar flares for the two paths shows that the level of amplitude perturbation is higher for the NLK signal (24.8 kHz, maximum of ~ 11.5 dB) and lower for the NWC signal (19.8 kHz, ~ 3.5 dB). Similar results were obtained by McRae and Thomson (2004) using NLK and NPM transmission to Dunedin. Their results showed NLK displaying ΔA of up to 8 dB, while for NPM, ΔA of up to 4 dB for the same flares was recorded. The authors indicated that the VLF amplitude perturbation level of different signals is frequency dependent. At higher frequencies, the signals exhibit greater amplitude perturbation compared to lower frequencies for the same flares. This increase in amplitude enhancement as a function of frequency was attributed to the second-order TM mode (McRae and Thomson 2004) which becomes comparable to that of the first-order TM mode at the receiver. This dissimilarity in the amplitude perturbation levels could also depend upon the path orientation of NWC and NLK for the same flares because the directions in which the flares are directed and the solar flux to which they are subjected could be different for separate paths. One of the paths could be under more solar radiation (different solar zenith angles) than the other; hence, it may be more susceptible to solar flare-induced perturbations. Unlike the ΔA curves, the Δϕ curves are similar in form and level of perturbation for the two signals (Fig. 5) during both the low- and moderate-solar activity periods. Thus, it is seen that ΔA is the main measurable quantity that can distinguish the solar flare effects on VLF during different levels of solar activity and not Δϕ; however, measurements of both the quantities are essential to accurately model the D-region to determine its flare time parameters H′ and β.
Using LWPC simulation, McRae and Thomson (2004) estimated a decrease in H′ of 12 km and an increase in β of 0.13 km−1 relative to their normal daytime values for an X3.0 class flare on the NLK to Dunedin, New Zealand, path. For another class X5 flare for this path, they estimated a 13 km decrease in H′ and the same increase in β. Recently, Kumar et al. (2015) estimated a decrease of 9.4 km in H′ and an increase of 0.126 km−1 in β for an X1.5 class flare on the NPM–Suva path. According to Thomson et al. (2005), the D-region could be used as a solar flare strength indicator for flares greater than class X17 (for which the GOES detector saturates) up to X45 by extrapolating the phase profile.
During solar flares, β increases, which causes an increase in VLF signal amplitudes for paths such as those of NLK and NWC to Suva. According to Thomson and Clilverd (2001), ionization at the lower altitudes (50–65 km) as a result of galactic cosmic rays (e.g., Rishbeth and Garriott 1969), apart from the solar Lyman-α radiation, also contributes to this increase in β. This effect is intensified near solar minima (when compared with solar maximum) by the slightly lower solar Lyman-α flux and enhanced galactic cosmic rays at low latitudes (Thomson and Clilverd 2000), which is the reason why the increase in β is greater for low solar activity than for moderate solar activity for same class flares, as shown by our analysis. If the flare is strong enough to produce extra ionization above a certain threshold of Lyman-α and galactic cosmic rays, the value of β will not increase further, i.e., it will display a saturation effect similar to that of the NWC signal for the period of moderate solar activity, as shown in Fig. 7.
The relationship between the changes in the D-region, H′ and β, and the flare power during solar flares of varying intensities has been reported by a number of researchers (e.g., Thomson and Clilverd 2001; McRae and Thomson 2004; Thomson et al. 2005; Tan et al. 2014). A nearly linear reduction of H′ against the logarithm of flare flux was obtained for weak C1 up to strong X45 class flares. β was found to display sigmoidal variation with respect to flare power. The results presented here show similar variations of H′ and β for the two long VLF paths. The authors above have presented the variations in H′ and β with respect to solar flare power for either low, mid- or high solar activities separately. But in this study, the relationship between the flare power and the changes in the ionospheric D-region H′ and β have been presented using a comparative analysis during low- and moderate-solar activity conditions at a low-latitude station.
A distinct difference in the variation of H′ and β with respect to the flare power is seen during the low- and moderate-solar activity periods. In general, the increase in β is found to be greater for low solar than moderate solar activity for similar class flares. The β value is estimated to be higher by about 0.04 km−1 for the same class of flares during low-solar activity period than during moderate-solar activity period for the two VLF signals. In addition, H′ seems to be reduced further during the low-solar activity period (i.e., ~ 1–2 km) than the moderate-solar activity conditions for same class of flares. This can be clearly seen from the graphs of ΔH′ versus flare power in Fig. 10. This difference in the level of β and H′ is found to be more pronounced for strong M and X class flares than for low C class flares.
McRae and Thomson (2004) showed estimated plots of H′ and β for low-, mid- and high-solar activity conditions. Their estimates were based on the assumption that for a given flare size, such as M3, same amount of X-ray flux is produced in any solar cycle and hence essentially the same total D-region ionization at any time. Thus, their plots revealed that for strong flares, especially X class, the β values were the same during all solar cycle conditions, while their H′ values were also seen to be same for any class of solar flares. Their plots showed the difference at C and weak M class flares, where β was found to be significantly higher during high-solar activity and lower during low-solar activity conditions. For example, it is estimated from the plots of McRae and Thomson (2004) for solar flares around 4 dB strength (equivalent to C2.5 class); the β values were 0.39 km−1 for solar minimum and 0.41 km−1 for moderate solar activity. In contrast, our results indicate greater β values for the solar minimum than for the period of moderate solar activity for the same strength of flares, i.e., 0.35 km−1 and 0.33 km−1, respectively. The results presented here imply greater differences in β and H′ for stronger classes of solar flares during low- and moderate-solar activity conditions. Pacini and Raulin (2006) also investigated the relation between VLF sudden phase anomalies (SPAs) and solar X-ray flares to investigate whether it has a dependence on solar activity cycles. They found a significant correlation between the level of amplitude perturbation and the X-ray flux, F X (time-integrated photon fluxes), in the range 0.5–2.0 Å. They showed that the amplitude perturbation versus F X relation differs depending on the epoch within the solar cycle. Their results showed that for a given strength solar flare, ΔH′ will be approximately 1 km greater during the solar minimum period than during the solar maximum, which is in agreement with our findings. These authors, however, did not study the variation of β for different solar activity cycles.
Summary and conclusions
The solar flare-associated enhancements on both the amplitude and phase of VLF signals were found to vary proportionally with the logarithm of the X-ray flux. A comparison between the amplitude enhancements (ΔA) and the flux intensity for the two paths showed that ΔA is greater for the NLK signal (maximum of ~ 11.5 dB) and less for the NWC signal (~ 3.5 dB).
Solar flares of similar class produced higher values of ΔA during low solar activity than during periods of moderate solar activity. The level of phase perturbations is quite similar during low and moderate solar activities for all the signals, with NLK recording a maximum of up to 200° and NWC showing a maximum of up to 150°.
The solar flares lower the effective reflection height (H′) by creating increased ionization (ref Fig. 11) that is approximately proportional to the logarithm of the X-ray flare flux intensity. The lower-edge ionospheric D-region sharpness (β) also increases significantly showing a sigmoidal variation with the logarithm of X-ray flare intensity.
A distinct difference in the variation of H′ and β with respect to the flare power is seen during the low- and moderate-solar activity periods. In general, the increase in β is found to be greater for low solar activity than for moderate solar activity for similar class flares. In addition, the reduction in H′ (ΔH′) is greater by ~ 1–2 km during the period of low solar activity than during the period of moderate-solar activity conditions for same class of flares. This difference in the level of β and H′ is found to be more pronounced for strong M and X class flares than for low C class flares.
Thus, it can be inferred that strong flares, especially M and X class, can increase the D-region electron density and the redistribution of electron density with height more during low-solar activity conditions, lowering the D-region ionosphere when it is very sensitive, than during higher-solar activity periods.
The author AK analyzed the VLF data for solar flares in consultation with SK. The manuscript was jointly prepared by AK and SK. Both authors read and approved the final manuscript.
The authors sincerely thank the Research Committee of the Faculty of Science, Technology and Environment, University of the South Pacific (USP), for initial financial support to carry out this work. The authors are also thankful to financial support by USP’s main research office under the Strategic Research Theme (Grant No. 6177-3107-Acct-617704) under which a part of work has been carried out. Solar flux data were obtained from http://www.swpc.noaa.gov/ftpmenu/warehouse.html, and subionospheric VLF transmitter data are available from the first author.
The authors declare that they have no competing interests.
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