ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print)
IJECT Vol. 5, Issue Spl - 2, Jan - March 2014
Seasonal Variation of Spread-F Occurrence Probability at
Low Latitude and its Relation with Sunspot Number
1
A. B. Bhattacharya, J. Pandit
Dept. of Physics, University of Kalyani, Kalyani, West Bengal, India
Dept. of Physics, JIS School of Polytechnic, Kalyani, West Bengal, India
1
2
Abstract
The paper considers the seasonal variation of spread-F occurrence
probability and its relation with sunspot number as well as
compared average spread F occurrence probability for a solar
maximum and a solar minimum year.We have observed a welldefined inverse relation between spread-F occurrence probabilities
with sunspot activity in each season taking data for the period of
2001 to 2013 for Kalyani showing quantitatively the dependence
of the spread-F occurrence on sunspot number.
Keywords
Spread-F Occurrence, Sunspot Number, Ionosphere
I. Introduction
The ionospheric spread-F has many significant effects on radio
waves that propagation through the atmosphere of the Earth. The
F-layer of the ionosphere is approximately 400 km from the Earth
surface. Spread F is a generic name and caused by irregularities
of electron density in the F2 region of the ionosphere. In this
phenomenon of the ionosphere in which the pulses returned from
the ionosphere are of a much greater duration than the transmitted
pulse. The irregularities may last minutes to hours with a horizontal
extent of a few to hundreds of kilometers [Bhattacharya and Pandit
2013]. For HF radio wave propagation,F2 region is most important
because, it is present 24 hours of the day andits high altitude
allows the longest communication paths. Alsoit reflects the highest
frequencies in the HF rangewith a greater life time of electrons
compared to other ionospheric layers. The life times of electrons
for different ionospheric layers aregiven in Table 1.
Table 1: Life Times of Electrons for the Different Ionospheric
Layers
Approximate
Name of
height from the
ionospheric region
Earth surface
Duration of lifetime
of electrons
D region
50 to 90 km
Unstable, not yet
determined
90 to 140 km
20 second
140 to 210 km
1 minute
over 210 km
20 minutes
E region
F1 region
F2 region
These irregularities are perturbations in the F region and have
many significant effects on radio waves that propagate through
the Earth’s atmosphere, such as refraction, reflection, absorption,
time delay, phase change, amplitude change, Doppler shifts, and
so on [Shi et. al. 2011]. The radio communication with satellites,
positioning/navigation, and spacecraft telemetry and control are
now so sensitive to ionospheric variation which could seriously
affect or disrupt the system performance due to the effects of
ionosphere on radio signals.The irregularities distort signals
passing through the affected region causing flutter fading on HF
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International Journal of Electronics & Communication Technology
and scintillation on higher frequencies used for ground-to-satellite
communications.
The occurrence of spread F varies with longitude, latitude, local
time, season, solar and geomagnetic activities, seasonal meridian
winds and magnetic field declination and so on [Niranjan et. al.
2003, Abdu 2012]. Spread F occurrence is greatest at night at
all latitudes and at the equinoxes. At high latitudes, spread F
can be common during the day. Spread F occurs least at midlatitudes. At low latitudes, spread F can be increased in the post
sunset hours[Alex 2003]. Our observational site Kalyani(22.98°N,
88.46°E) lies in the low latitude (Fig. 1). At all latitudes there is a
tendency for spread F to occur when there is a decrease in F region
frequencies. The spread F is often associated with ionospheric
storms [ips.gov.au].
Fig. 1: Latitudinal Position of Kalyani
In this paperwe analyze seasonal variation of spread-F occurrence
probability and its relation with sunspot number. Also we have
compared average spread F occurrence probability with spread
F occurrence probability for a solar maximum year and spread F
occurrence probability for a solar minimum year.
II. Analyses and Results
In the low latitude region Kalyani, remarkable spread-F occurrence
probability peaks appear from pre monsoon season (March to May)
and post monsoon season (October to February). Considering
seasonal variation, spread-F occurrence probability in pre monsoon
season is found to be larger than in post monsoon season. Again
the spread-F occurrence probability in post monsoon season is
higher than that in monsoon season. Further, spread-F occurrence
probability posses maximum value at the time of just post sunset
hours (17:00 IST to 19:30 IST) and then it is diminished almost
proportionally (fig. 2).
Over the low latitude area Kalyani, we compare thirteen years
average of spread-F occurrence probability with the spread-F
occurrence probability for a solar maximum period (year 2001) and
a solar minimum period (year 2008). We find from the analysis that
during the solar maximum period (year 2001) and solar minimum
period (year 2008), spread-F occurrences are almost same nature
with average values but the spread-F occurrence in solar maximum
period (year 2001) is larger than solar minimum period (year 2008)
for pre monsoon season and post monsoon season.
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ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print)
IJECT Vol. 5, Issue Spl - 2, Jan - March 2014
area Kalyani. During the solar maximum period (year 2001 and
year 2013) it possesses greater value than the solar minimum
period (year 2008).
III. Discussion
It is well established now that large-scale plasma depletions
are generated first in post-sunset period through RayleighTaylor instability that causes the onset and growth of plasma
density irregularities in the ionosphere The depleted regions rise
fast to cover the entire F region including topside ionosphere
[McKinnellet. al. 2010, Priyadarshi and Singh 2012]. The local
growth rate of the R-T instability can be obtainedby using the
equation [Sreejaet. al. 2009].
Fig. 2: Variation of Spread-F Occurrence Probability for Different
Seasons for the Post Sunset Hours
Specially, for post monsoon season it is remarkably larger than
in the other two seasons (Fig. 3).
< ---Pre monsoon --- ><----------- Monsoon ----------><------------ Post monsoon ------------->
Fig. 3: Compare Average Spread F Occurrence Probability for the
Year from 2001-2013 with Spread F Occurrence Probability for
the Year 2001 (Solar Maximum Year) and Spread F Occurrence
Probability for the Year 2008 (Solar Minimum Year)
We have also analyzed yearly variation of spread-F occurrence
probability and 12-month running mean of sunspot number for
the years from 2001 to 2013 (Fig. 4).
γ = 1/L{[g/νin] + [EX/B] + [WX(νin/Ωi)] − WZ}(1)
where Wx and Wz are the zonal and vertical winds respectively, Ex is
the zonal electric field in the F-region, L is theplasma scale length,
B is the geomagnetic field and Ωi is theion gyro-frequency.
After sunset the F-region may be rapidly rise and developa steep
bottom side gradient due to the combined effectof chemical
recombination of the F-region and E-regions and dueto an actual
increase in the vertical plasma velocity [Pezzopane 2013].
Theseconditions result in an F-region plasma density profile.
The occurrence of Spread F probability showed linear solar
activity dependenceduring solar minimum year and solar
maximum years[Manju, Devasia and Sridharan 2007]. Form fig.
2, we conclude that the average spread-F occurrence probability,
spread-F occurrence probability for the solar maximum period
and solar minimum period have a direct relationship but spread-F
occurrence probability for the solar maximum period always posses
greater value than solar minimum period. Again, a remarkable
occurrence of peaks appear from March to May i.e. pre monsoon
time and also from October to February i.e. at post monsoon
time.
The characteristics and physical mechanism of SF can be
considered as very important aspects in ionospheric studies.
These irregularities are basically perturbations in the F region.
The radio communication with satellites, positioning/navigation,
and spacecraft telemetry and control are in recent years so sensitive
to ionospheric variation that could seriously affect or disrupt the
system performance due to the effects of ionosphere on radio
signals. So, the present research on studying the characteristics
at low latitudes has important significance for science, as well as
for application research.
IV. Acknowledgement
We are thankful to NASA’s International Reference Ionosphere
(IRI) project (iri.gsfc.nasa.gov) for using some of their IRI
parameters.
Fig. 4: Yearly Variation of Spread-F Occurrence Probability and
12-Month Running Mean of Sunspot Number
It is clearly shown that the spread-F occurrence probability is
inverselycorrelated with sunspot number over the low latitude
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IJECT Vol. 5, Issue Spl - 2, Jan - March 2014
ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print)
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