“Direct Observations of

Traveling Ionospheric Disturbances as Focusers of Solar Radiation: Spectral Caustics”

(Artem Koval, Yao Chen,

Takuya Tsugawa, Yuichi Otsuka, Atsuki Shinbori, Michi Nishioka, Anatoliy Brazhenko,

Aleksander Stanislavsky, Aleksander Konovalenko, Qing-He Zhang,Christian Monstein,

Roman Gorgutsa)

The solar radiation

focusing effect is related to the specific phenomenon of propagation of the

Sun-emitted HF and VHF waves through terrestrial ionosphere. This natural

effect is observed with ground-based radio instruments running within 10-200

MHz range, as distinctive patterns – the Spectral Caustics (SCs) – on the solar

dynamic spectra. On the spectrograms they exhibit unusual morphology and

specific time-frequency features. It has been suggested that SCs are associated

with medium-scale traveling ionospheric disturbances (MSTIDs). Despite the existing

studies on the SCs topic, a major challenge remains, which is to capture

observationally a specific TID perturbation caused a specific SC. In other

words, can we identify a one-to-one – MSTID-SC – correspondence using available

data? So far, direct observations of a TID as the cause of a SC have never been

reported. To address this challenge we analyze the simultaneous-observed solar

dynamic spectra and detrended total electron content (dTEC) maps, which have

been used as an effective tool in TIDs studies.

In this paper, we

present the first direct observations of SCs induced by MSTIDs, using solar

dynamic spectra with SCs obtained by different European radio telescopes on

January 8, 2014 and simultaneous two-dimensional detrended total electron

content (dTEC) maps over Europe. Spatial examination of dTEC maps as well as

precise timing analysis of the maps and the dynamic spectra have been

performed. First, we found several pairs of one-to-one (TID-SC)

correspondences. The study provides strong observational evidence supporting

the suggestion that MSTIDs are the cause of SCs.

The solar dynamic

spectra from different radio instruments located in Europe have been collected

together and presented in Figure 1. In the spectrograms SCs can be recognized

by their distinctive spectral shapes. To pinpoint a “parental” TID that causes

a SC, we use dTEC maps on which the perturbation component of the ionosphere is

represented as a thin layer located at the height of the F2 peak (250 km). For

each the Sun-antenna pair we obtain the position of the ionospheric pierce

point (IPP), that is where the Sun-antenna line of sight crosses the ionosphere

layer.

Figure 1.Composition of the time-aligned solar dynamic

spectra from NDA (Nancy, France) (a), CALLISTO DARO (Dingden, Germany) (b),

CALLISTO GLASGOW (Glasgow, Scotland) (c), CALLISTO BIR (Birr, Ireland) (d),

URAN-2 (Stepanovka, Ukraine) (e), IZMIRAN (Troitsk, Russia) (f), CALLISTO ESSEN

(Essen, Germany) (g), CALLISTO BLENSW (Bleien, Switzerland) (h) on January 8,

2014. The common time axis is in the range 06:52 UT - 15:55 UT. The scales of

frequency axes in dynamic spectra are arbitrary. (i) The map of the antennas

locations and IPP tracks associated with the solar observational sessions. The

observing sites and the corresponding IPP paths are marked by the same color.

Within the life time of

a particular SC, a set of IPP coordinates can be determined and plotted onto

the dTEC maps. This allows us to directly observe the instantaneous dTEC

structure and the position at which the wave front of incident radio wave

penetrates the ionosphere. The focusing can only happen when the solar radio

wave propagates through a valley where the refractive index is higher than that

in the surrounding crests of TEC perturbation. In Figure 2 and Figure 3 we show

several cases where each SC has been identified with specific MSTID, causing

the focusing of solar radiation, observationally with great confidence.

Figure 2.(Top) The CALLISTO ESSEN dynamic spectrum

with the SC recorded on January 8, 2014. The white solid line on the

spectrogram is at 09:44 UT. On the dTEC map on the right, the white asterisk

shows the location of the IPP that is in the middle of the valley enclosed by

two crests C1 and C2 (red arrows). (Bottom) The NDA dynamic spectrum including

the SC detected on January 8, 2014. The purple solid line on the spectrogram is

at 14:50 UT. On the dTEC map on the right, the purple asterisk points the

position of the IPP, which is within the valley between two crests C1 and C2

(red arrows).

Figure 3.The CALLISTO GLASGOW (a) and the CALLISTO BIR

(b) dynamic spectra with two pairs of SCs, SC-G(I,II) and SC-B(I,II),

respectively, observed on January 8, 2014. The dashed horizontal lines on the

spectrogram in panel (b) show the frequency range of the spectrogram in panel

(a). The vertical white, purple, and gray lines indicate three time instants at

09:58 UT, at 10:04 UT, and at 10:16 UT, correspondingly, in both panels (a) and

(b). The IPPs relating to the CALLISTO GLASGOW (asterisk) and the CALLISTO BIR

(plus) spectrograms for these instants were obtained and plotted on dTEC maps

in panels (c-e). The corresponding lines and marks have the same colors. The

crests C1, C2, and C3 are pointed by red arrows on the dTEC maps.

In this study, for the

first time we provided strong observational evidence of cause-effect

relationship between pairs of TIDs and SCs. In all examined cases, the

performed timing analysis allowed us to reveal the couplings between MSTIDs and

SCs with great confidence. We showed that the observed peculiarities of the

spatial structure of TIDs on dTEC maps were consistent with the time-frequency characteristics

of the corresponding SCs in the spectrograms. In addition, the study settled a

controversy about the origin of SCs in the solar radio records, which may be

mistakenly attributed to the Sun. We believe that the SCs topic can be of

interests to communities of solar, ionospheric, and plasma physicists, since

the studied phenomenon is about the solar radio emission that passes the

ionosphere in which TIDs, forming plasma lenses, make the focusing of the

incident radiation.

The paper “Direct Observations

of Traveling Ionospheric Disturbances as Focusers of Solar Radiation: Spectral

Caustics” by Artem Koval, Yao Chen, Takuya Tsugawa, et al. has been accepted by

the Astrophysical Journal and will be published in the near future.