134. The Influence of Different Phases of a Solar Flare on Changes in the Total Electron Content in the Earth’s Ionosphere

Authors: Susanna Bekker and Ryan Milligan at Queen’s University Belfast

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Introduction

Modeling and interpreting geophysical responses to variations in solar plasma parameters is still a relevant problem which also has an important impact on modern communication and navigation systems.

Variations in solar radiation in different emission lines and continua caused by solar flares lead to a non-uniform increase in the electron concentration (Ne) at different altitudes of the Earth’s ionosphere. X-ray emission (λ < 10 nm) generally comes from hot plasma (~107 K) confined to coronal loops. It is known that during flares the X-ray flux increases by orders of magnitude, penetrates into the lower ionosphere layers and becomes the main source of D region ionization (h < 90 km). Extreme ultraviolet (EUV) emission comes predominantly from the loop footpoints formed in the solar chromosphere ~104–105 K). It has more moderate fluctuations but is absorbed at altitudes of more ionized E (90 < h < 120 km) regions of the ionosphere, where the maximum electron density is located. Therefore, it is mostly the EUV range that is responsible for the increase in total electron content (TEC) in the ionosphere during flares [1, 2].

One of the most valuable tools for studying solar-terrestrial connections is Global Navigation Satellite Systems (GNSS). An increase in electron density during solar flares leads to a delay in GPS signals, so GNSS data can be used to quantify changes in the ionospheric total electron content [3, 4].

The value of TEC is integral and does not provide detailed information about the change in the vertical profile of Ne caused by a solar flare. At the same time, the temporal dynamics of the TEC increment contains information about the Ne response to various emissions of the solar spectrum during the impulsive, gradual, and late phases of a solar flare. It is also an experimental source of information about the geoeffectiveness of various lines and continua of the solar spectrum.

The purpose of this study is to experimentally estimate the response of the total electron content to various phases of an X-class solar flare and analyze the temporal structure of TEC variations to determine the most geoeffective solar emissions.

EUV Late Phase of a Solar Flare

As the X-ray emitting plasma cools during the gradual phase of the flare, there can also be successive EUV emission at intermediate temperatures (~105–106 K). Woods et al. [5] showed that one variant of coronal loop reconnection after the impulsive phase of a flare results in warm coronal emissions (such as Fe XVI 33.5 nm, T = 106.43 K) having a second large emission peak that can lag the primary flare by hours. This peak is called the EUV late phase.

According to the definition proposed by Woods et al. [5], EUV late phase must satisfy the following conditions:

  • this should be a second peak of the warm coronal emissions (Fe XV and Fe XVI), which occurs from several minutes to several hours after the X-ray peak;
  • there is no significant enhancement of X-rays or hot coronal emissions (such as Fe X/Fe XIII 13.3 nm) during a late phase;
  • an eruptive event (as coronal dimming in the Fe IX 17.1 nm) should be observed;
  • the second set of post-eruptive coronal loops should be higher than the first set of post-flare loops.

Considering the above criteria, the X2.9 flare on 2011 November 3, which had a noticeable EUV late phase approximately 40 minutes after the main emission peak, was selected for analysis. During the late phase of this flare, pronounced emission were observed in Fe XV 28.4 nm, Fe XVI 33.5 nm, and Fe XVI 36.1 nm lines in addition to commonly observed strong He II 30.4 nm emission.

Ionospheric Total Electron Content Response

To numerically estimate the increment in ∆TEC during different phases of the X2.9 solar flare, parabolic trends associated with satellite trajectory and differential code biases were subtracted from the measured total electron content values. The black line in the left panel of Figure 1 shows the measured relative TEC variations during the main and EUV late phase of the flare. The red line shows background parabolic trends that do not contain any information about changes in solar radiation. In the right panel of Figure 1, we can see ∆TEC value obtained by subtracting the red curve from the black one.

Using the same technique, TEC measurements obtained at 956 illuminated GNSS stations were detrended to calculate clear ∆TEC increment.

Figure 1. Left panel: The relative TEC values (black line) and the subtracted trend (red line) obtained at “bbdm” (34.6°N 120.0°W) GNSS station during the main and EUV late phase of the X2.9 flare on 3 November 2011. Right panel: Corresponding ∆TEC dynamics.

Results and Conclusions

Based on the theoretical estimates and variations in the spectrum of the flare in question (X2.9, 3 November 2011), we selected six EUV lines, which are most likely responsible for the increase in the electron content.

The top panels of Figure 2 show the main and the most geoeffective emissions during the main (left) and late (right) phases of the solar flare. The bottom panels present the resulting averaged dynamics of the ∆TEC for the same time ranges.

As evident from Figure 2, the average response of the ionosphere to the EUV late phase was almost 30% of the response to the significantly more powerful impulsive phase. As we can see, during the impulsive and gradual phases, ∆TEC has four local peaks corresponding to the main EUV and X-ray emissions. The Ne reaction to an increase in radiation fluxes has an expected time delay ∆t, which is ~ 1 minute.

Figure 2. Top panels: Solar flare lightcurves in X-rays and EUV emission lines measured by SDO during the main (left) and EUV late (right) phases of the X2.9 flare on 3 November 2011. Bottom panels: The corresponding TEC response obtained by averaging data from 956 GNSS stations.

During the EUV late phase of the flare, two clear ∆TEC maxima were detected, the second of which is stronger. Such Ne response, both in shape and time, corresponds to the Fe XV 28.4 nm line (purple curve). Thus, the increase in TEC during the late phase is most likely associated with the Fe XV 28.4 nm, rather than the main characteristic of the EUV late phase of a flare – Fe XVI 33.5 nm.

The following conclusions can be drawn:

  • late warm coronal emissions have quite high geoeffectiveness (despite the fact that their absolute flux values are an order of magnitude lower than the flux values of cold chromospheric lines);
  • Fe XV 28.4 nm emission seems to be the main reason for the increase in electron concentration during the late phase of a solar flare;
  • the previously ignored EUV late phase of a solar flare should be considered when modeling and predicting the Ne response to variations in solar radiation, since it also causes a noticeable increase in TEC.

The results presented demonstrates the great promise of analysis of synchronous variations in solar spectrum radiation and experimental measurements of ionospheric parameters for the study of solar-terrestrial connections.

For more details see Bekker et al. (2024) [6].

References

[1] Leonovich, L. A., Afraimovich, E. L., Romanova, E. B., & Taschilin, A. V. 2002, Annales Geophysicae, 20, 1935
[2] Tsurutani, B. T., Verkhoglyadova, O. P., Mannucci, A. J., et al. 2009, Radio Science, 44, RS0A17
[3] Wan, W., Liu, L., Yuan, H., Ning, B., & Zhang, S. 2005, Advances in Space Research, 36, 2465
[4] Yasyukevich, Y., Astafyeva, E., Padokhin, A., et al. 2018, Space Weather, 16, 1013
[5] Woods, T. N., Eparvier, F. G., Hock, R., et al. 2012, SoPh, 275, 115
[6] Bekker, S., Milligan, R. O., Ryakhovsky, I.A. 2024, ApJ, 971, 188