An investigation of solar flare effects on equatorial ionosphere and thermosphere using co-ordinated measurements
KeywordsDayglow Equatorial electrodynamics Solar flare Equatorial Ionization Anomaly (EIA)
Solar flares are sudden burst of radiations coming from the sun, which significantly alter various physical and chemical processes in the terrestrial upper atmosphere. Considerable number of studies have been made in the past, to understand the effects of solar flare over the ionospheric (e.g., Le et al. 2013 ) and thermospheric (e.g., Le et al. 2012, 2015; Pawlowski and Ridley 2008, 2011) altitudes. Multitude of data from ground/space-based platforms primarily using GPS TEC (Afraimovich 2000, Leonovich et al. 2002, 2010; Liu et al. 2006), incoherent scatter radars (Mendillo and Evans 1974) in tandem with theoretical modeling (Chamberlin et al. 2008; Qian et al. 2010) have provided significant results pertaining to the flare-induced effects over the terrestrial upper atmosphere.
Recently, solar flare effects and its repercussions on the electrodynamics of the equatorial ionosphere have got significant attraction across the globe (e.g., Zhang et al. 2017; Manju 2016; Manju et al. 2012). Liu et al. (2007) has shown that Equatorial Ionization Anomaly (EIA) got significantly weakened during the flare due to the dominant role of electrodynamics than the photochemistry. Simulation studies (Qian et al. 2012) also showed that the E × B drift over equator weakens during the flare. This is consistent with the earlier observations of decrease in eastward electric field in the dynamo region during the flare (Manju and Viswanathan 2005). However, in past both increase (Qian et al. 2012) and decrease (Manju and Viswanathan 2005) in the field-line-integrated Hall-to-Pedersen conductivity ratio over equator had been reported. On the other hand, the effects of solar flare on the neutral thermosphere, particularly over equatorial latitudes are not properly understood so far, mainly due to the paucity of observations. Although CHAllenging Mini satellite Payload (CHAMP) observations provided several new insights in this regard (Liu et al. 2007), they are normalized to a higher altitude of 400 km and have poor temporal resolution (~ 93 min.) due to the inherent time for the satellite to revisit the same latitude and local time. Therefore, the need for high cadence data from lower thermosphere is well appreciated for the comprehensive understanding of the ionospheric–thermospheric coupling during such transient events.
In this context, the ground-based neutral thermospheric OI 630.0 nm dayglow emissions are ideal for investigating such ephemeral events like flare, as they emanate from the altitudes, where maximum EUV absorption takes place. These dayglow measurements over Trivandrum, in recent years, have brought out many significant results pertaining to various aspects of coupling processes in the equatorial upper atmosphere (e.g., Sumod et al. 2011a, b, 2012, 2014, 2015 and references therein). In the present work, an attempt is made to investigate the effects of X3.8 solar flare, which occurred on January 17, 2005, over equatorial ionosphere–thermosphere system, using combined high cadence measurements of thermospheric OI 630.0 nm dayglow, GPS-measured ionospheric Total Electron Content (TEC) and the strength of Equatorial Electrojet (EEJ). The study perhaps provides the ‘first time’ experimental evidence for the weakening of EIA during the flare over the Indian longitudinal sector.
List of the GPS receiver stations used along with their geographic co-ordinates
Results and discussion
Solar flare event of January 17, 2005
During the period 17–23 January 2005, solar activity varied from low to high levels. The period began under the influence of a high-speed coronal hole stream, with solar wind speed ranging between 550 and 600 km/s, later increasing up to ~ 1000 km/s. Transient flow, likely associated with CME activity on 15 January, arrived at around 17 January at ~ 07:15 UT. The region 720, which remained as the largest sunspot in the visible disk of the sun during this period, produced twenty five C-class, eight M-class, and three X-class flares. The first significant event was an X3.8/sf proton flare, occurred on January 17 at 09:52 UT and its effects over the equatorial upper atmosphere are studied here.
Response of equatorial/low latitude ionosphere
Effects in the dynamo region
The magnetic field measurements at an EEJ station Trivandrum and an off-EEJ station Alibag are used to investigate the solar flare effect in ΔH, i.e., SFE (H) in the dynamo region. The daytime variation of EEJ on January 17, 2005 during the period 12:30–18:30 IST is also shown in Fig. 1. As moderate geomagnetic activity was ongoing on this day, fluctuations due to solar wind/magnetospheric/high latitude origin are seen in the magnetic field measurements. However, at ~ 13:15 IST, a step-like enhancement has been observed in the surface magnetic field measurements. This is prior to the increase in EUV/X-ray radiations and is associated with the sudden increase in the solar wind ram pressure (not shown here). However, in conjunction with the peak X-ray/EUV flux, a clear-cut enhancement is seen in the ΔH at Trivandrum, Alibag and hence in EEJ. The SFE in ΔH (ΔHpeak flare − ΔHpre flare) over Trivandrum and Alibag is ~ 28 and ~ 2 nT, respectively. Therefore, the flare-associated enhancement in EEJ was conspicuous having magnitude of ~ 26 nT. This shows that the SFE in H decreased drastically from equator to low latitudes.
Recent studies showed that the zonal electric field and, hence, the EIA get significantly weakened during the X-class flare event of 28 October 2003 (Liu et al. 2007). To investigate this aspect in detail, the GPS-measured TEC at six different stations in the ~ 77° E meridian have been analyzed and the results are presented below.
Changes as seen in the TEC
However, the latitudinal distribution of VTEC at ~ 13:30 IST showed a gradual increase over the latitudes 12–20°, indicative of the development of the EIA. The subsequent profiles during the period 14:00–15:00 IST exhibited gradual growth of EIA, with the crest location shifting systematically from latitudes 15° to 17°. Although, the flare-associated enhancement in the VTEC throughout the latitudes can be clearly seen on the profiles from 13:30 IST onwards, the additional increase in VTEC within 12–20° as compared with other latitudes clearly indicates the role of EIA-associated electrodynamics. From 15:30 IST onwards, the anomaly again showed a significant weakening, which continued until 17:30 IST. Therefore during the flare period (flare peak time is 15:20 IST), inhibition of the EIA has been observed. These results are consistent with the recent observations using the CHAMP, where a significant weakening of the EIA-associated plasma fountain has been reported during the X17 flare event on 28th October 2003 (Liu et al. 2007).
In an earlier study, it has been shown that on the flare day, the development of anomaly was inhibited till 13:00 IST owing to the effect of the disturbance dynamo (Sreeja et al. 2009). Therefore, the weakening of the anomaly during the pre-flare period (12:00–13:00 IST) can be attributed mainly to the westward electric field associated with disturbance dynamo. However, the development of anomaly crest during the period 13:30–15:00 IST necessitates the presence of an eastward electric field. The positive excursion of EEJ from 13:00 IST as well as the solar wind pressure-induced increase in the EEJ at ~ 13:30 IST vindicates the presence of an eastward electric field. This eastward electric field enhances the plasma fountain and hence the anomaly after certain period, as there involves a characteristic delay of ~ 30–90 min between EEJ and EIA.
However, after the pressure-induced enhancement at ~ 13:20 IST, the EEJ followed in a similar pattern as that of the control day with an additional enhancement due to the flare-induced radiations. This suggests that from 15:30 IST onwards, the EIA should either grow in a similar pattern as that of the control day or develop further due to the additional positive amplitudes seen in the EEJ. Interestingly, contradictory to the expected behavior, it is found that the EIA got completely suppressed from 15:30 IST onwards which continued the rest of the day. This strongly corroborates the flare-associated weakening of the EIA as reported in the recent study (Liu et al. 2007). This weakening of EIA is believed to be associated with the reduction in the eastward electric field due to the flare. As it is well known, the flare is expected to cause the rapid change in the conductivity, especially in the vertical direction. This in turn affects the ratio of Hall to Pedersen conductivity, which is a crucial factor in regulating the dynamo electric field. As the present case is an X3.8 class flare, it can increase the conductivity (particularly below 100 km), due to the increase in ionization at D region by the X-rays. This in turn can decrease the zonal electric field depending on the vertical polarization electric field and ratio of the Hall to Pedersen conductivity. Similarly, the height-varying winds in the vicinity of the dynamo region due to the formation of a highly conducting layer can also modulate the zonal electric field (Liu et al. 2007). Therefore, the flare-induced changes in the EIA can be attributed to the combined effect of the photochemistry and the electrodynamics related to the plasma fountain.
Response of equatorial thermosphere
As mentioned earlier, the eastward electric field increased at ~ 13:20 IST due to the increase in the solar wind dynamic pressure. The associated development of EIA as shown in Fig. 3 further confirms this aspect. This results in the pumping of more ionization over the equator due to the upward E × B drift. This increases the number of effective dissociative recombination at the emission altitudes due to the pumping of enhanced ionization in the emission region. This in turn increases the airglow intensity over Trivandrum as seen in the present case. Therefore, the increase in airglow intensity, a few minutes after the increase in the EEJ at ~ 13:20 IST, can be attributed to the prevailing eastward electric field.
This increase in dayglow continued until ~ 14:30 IST, and showed an abrupt decrease at ~ 14:45 IST, which persisted until ~ 15:00 IST. The reduction in the dayglow intensity during the period 14:30–15:00 IST can be attributed to the decrease in the effective recombination at the emission altitudes due to the transport of more ionization density from equator to off-equatorial latitudes because of the prevailing eastward electric field. When the eastward electric field becomes strong enough to pump the ionization density from equator to off-equatorial latitudes, it would appear as a decrease in the dayglow over equator. The strengthening of EIA profiles, as shown in Fig. 3, further vindicates the presence of strong eastward electric field during this period. Moreover, a gradual decreasing trend in the dayglow is expected in the post-noon sector due the solar zenith angle dependence of photoelectron impact of O and photo-dissociation of O2, which contributes to the production of ~ 70% of the dayglow intensity. Following this reduction, the dayglow intensity increased concomitantly with the X-ray/EUV radiations during the flare event. This increase in dayglow, which showed a peak at ~ 15:30 IST, is attributed to the increased O(1D) production during the flare time due to all the three production mechanisms, viz, photo-electron impact of O, photo-dissociation of O2 and dissociative recombination of O2+, as discussed in Sumod et al. (2015).
However, the dayglow did not show any appreciable signature associated with the secondary peak in the EUV flux. This is due to the fact that, during this period, the EEJ started to recover and flare-associated westward electric field, as discussed early, started dominating. This weakened the plasma fountain over equator, and filled more ionization over equator. This further increased the ionization at the emission altitudes, resulting in an increase in the dayglow intensity. Thus, the electrodynamics-associated plasma dominated over equator, diminishing the flare-induced peak in the dayglow intensity. The further enhancement in the dayglow intensity during the period 16:30–17:30 IST can be attributed to the subsequent weakening of plasma fountain as it is evident from Fig. 3. However, the delayed enhancement in the dayglow due to the time delay in the charge exchange of O2+O+ reaction, as well as the delayed response of neutral density during the flare, cannot be precluded in this context.
The study adduces the effects of X3.8 flare of January 17, 2005 over the equatorial upper atmosphere using combined radio and optical measurements. Prompt responses have been noticed in the magnetic field inferred EEJ and GPS-measured TEC measurements. The latitudinal distribution of TEC revealed a substantial reduction in the EIA. This is consistent with the earlier observations/simulations, and suggests that the flare-induced radiation has altered the ratio of Hall to Pedersen conductivity and subsequently resulted in the weakening of the eastward electric field. Although, the thermospheric dayglow revealed concomitant resolve enhancement during the peak flare period, it did not show any enhancement associated with later increase in the flare (EUV) radiation. This suggests the dominant role of electrodynamical imprints of these dayglow features when compared with the photochemistry. The importance of the study lies in showing the flare-induced changes in equatorial electrodynamics comprehensively in context of ion-neutral coupling, using high cadence coordinated ionospheric and thermospheric measurements.
SGS acknowledges C. Vineeth and M.M. Hossain for the useful discussions and data collection. This work was supported by Department of Space, Government of India. SGS also acknowledges the financial assistance provided by the Indian Space Research Organization (ISRO) through research fellowship. Authors are thankful to the Director, IIG for the magnetic field data. The GOES & SOHO team are also duly acknowledged.
SGS conceived and designed the present study, performed the experiment in addition to collection of other associated data, carried out the data analysis, drafted the manuscript, and coordinated the study. TKP participated in designing the study, data analysis, as well as in the useful discussions concerning the study. Both authors read and approved the final manuscript.
This work was supported by Department of Space, Government of India through the research associateship to SGS (2013).
The authors declare that they have no competing interests.
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