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Traveling Ionospheric Disturbances as Huge Natural Lenses: Solar Radio Emission Focusing Effect

(Artem Koval, Yao Chen,  Aleksander Stanislavsky, Qing-He Zhang)


The solar radio emission in  meter-decameter wavelengths demonstrates a plentiful variety of solar radio  bursts of different types (with branching to sub-types) as well as events not  belonging to any specific type. The last group includes variable solar emissions  that result from radiation processes affected by propagation effects in the  terrestrial ionosphere. Particularly, low-frequency solar radiation passing  through the ionosphere could be subject to different influences such as  reflection, scattering, and absorption along with refraction. The latter, in  addition to causing a regular positional dispersion of solar radio sources  proportional to the inverse square of the frequency, may also involve focusing  and defocusing of solar radiation. Evidently, the natural focusing of radio  waves from the Sun by the Earth's ionosphere represents one specific aspect of  the solar-terrestrial relationship.

In the paper we present an in-depth study about spectral  perturbations appearing in solar dynamic spectra and being manifestations of the  focusing effect of low-frequency solar emission by the Earth's ionosphere. Such  perturbations, known as Spectral Caustics (SCs), are considered to be the result  of the refraction of radio waves by medium scale traveling ionospheric  disturbances (MSTIDs). Using the Nançay Decametric Array (NDA) data set, we have conducted a statistical  analysis of the SCs in solar dynamic spectra within 10-80 MHz.  

The SCs have been detected confidently in the NDA dynamic  spectra for 129 observational days from 1999 till 2015 inclusive. Figure 1  displays the NDA dynamic spectra comprising the most frequent formations of the  SCs which are taken from broad SCs collection. On spectrograms they appear as  intensity variations different from well-known solar radio bursts. The sharp  edges with enhanced intensity are distinctive characteristics of the structures  for most events. Firstly, we have classified the SCs observed by the NDA as  several types, based on their spectral morphology, namely: inverted V-like,  V-like, X-like, fiber-like, and fringe-like (see Figure  1).

Figure 1.  Solar dynamic spectra of the NDA with  different SC types: (a, c-i) Inverted V-like; (j, n) V-like; (b, c, i) X-like;  (j-m) Fiber-like; (n-p) Fringe-like. Note that some dynamic spectra include  multiple SCs belonging to separate types (see detailed description in the  paper).

We have carried out the statistical analysis of the SCs in solar  dynamic spectra during a 17-year period (1999-2015). This period partly covers  23rd and 24th solar cycles. We found that about 81% of all  days with detected SCs fall on active phases of solar cycles 23 (48% in  1999-2003) and 24 (33% in 2012-2015), respectively (see Figure 2). Only about  18% of days with the SCs were recorded in the solar cycle minimum phase  (2005-2010). This result clearly indicates that the appearance of the SCs in  dynamic spectra depends on the phase of the solar cycle. This is explained in  terms of variations of the occurrence rate of strong solar events along a solar  cycle that results in a corresponding change in the occurrence rate of solar  radio bursts, which accompany the SCs in most  cases.


Figure 2.  Distribution of days with detected SCs vs.  years superimposed by solar cycles sunspot number (red color) and 10.7 cm solar  radio flux (blue color) progressions.

On the basis of the statistical  examination of the SCs, we also establish the seasonal dependence in their  occurrence (see Figure 3). It was found that about 95% of days with SCs belong  to autumn-winter months, whereas only near 5% of days with SCs belong to  spring-summer months. Since the SCs are believed to be caused by MSTIDs, this  seasonal dependence can be related to the similar dependence in the appearance  of MSTIDs. In fact, such variations of seasonal occurrence rate of MSTIDs have  been confirmed in several studies. Thus, the current study presents strong  evidence for a direct connection between MSTIDs and SCs with a quantitative  analysis.


Figure 3.  Seasonal appearances of the SCs in solar  radio records found from the NDA observations for a 17-year period. The squares  indicate number of days in a month in which SCs were detected on the NDA dynamic  spectra. The quantity of SCs days from the lowest to the highest is marked by  color from black to white, respectively.

The paper “Traveling  Ionospheric Disturbances as Huge Natural Lenses: Solar Radio Emission Focusing  Effect” by Artem Koval, Yao Chen, Aleksander Stanislavsky, and Qing-He Zhang has  been accepted by the Journal of  Geophysical Research: Space Physics and will be published in the near  future.


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