“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
23rdand 24thsolar 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 theJournal of
Geophysical Research: Space Physicsand will be published in the near
future.
