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Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions
Accepted Manuscript Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions Md Golam Mostafa, Haris Haralambous, Christina Oikonomou PII: DOI: Reference:
S0273-1177(17)30767-6 https://doi.org/10.1016/j.asr.2017.10.029 JASR 13461
To appear in:
Advances in Space Research
Received Date: Revised Date: Accepted Date:
6 July 2017 15 October 2017 19 October 2017
Please cite this article as: Golam Mostafa, M., Haralambous, H., Oikonomou, C., Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions, Advances in Space Research (2017), doi: https://doi.org/10.1016/j.asr.2017.10.029
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Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions
Md Golam Mostafa PhD Student, Department of Electrical Engineering, Frederick University, 7 Filokyprou St, Palouriotissa, Nicosia, 1036, Cyprus [email protected], [email protected]
Haris Haralambous Assistant Professor, Department of Electrical Engineering, Frederick University, 7 Filokyprou St, Palouriotissa, Nicosia, 1036, Cyprus [email protected]
Christina Oikonomou Research Associate, Frederick Research Centre, 7 Filokyprou St, Palouriotissa, Nicosia, 1036, Cyprus [email protected]
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Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions Md Golam Mostafa a,*, Haris Haralambous a, Christina Oikonomoub a
Department of Electrical Engineering, Frederick University, 7 Filokyprou St, Palouriotissa, Nicosia 1036, Cyprus b
Frederick Research Centre, 7 Filokyprou St, Palouriotissa, Nicosia 1036, Cyprus
Abstract This paper reports the diurnal, seasonal, and long term variability of the E layer critical frequency (foE) and peak height (hmE) derived from Digisonde measurements from 2009—2016 at the low-middle latitude European station of Nicosia, Cyprus (geographical coordinates: 35◦N, 33◦E, geomagnetic lat. 29.38◦N, I = 51.7◦). Manually scaled monthly median values of foE and hmE are compared with IRI-2012 predictions with a view to assess the predictability of IRI. Results show that in general, IRI slightly overestimates foE values both at low and high solar activity. At low solar activity, overestimations are mostly limited to 0.25 MHz (equivalent electron density, 0.775×103 el/m-3) but can go as high as 0.5 MHz (equivalent electron density, 3.1×10 3 el/m-3, during noon) around equinox. In some months, underestimations, though sporadic in nature, up to 0.25 MHz are noted (mostly during sunrise and sunset). At high solar activity, a similar pattern of over-/underestimation is evident. During the entire period of study, over-/under estimations are mostly limited to 0.25 MHz. In very few cases, these exceed 0.25 MHz but are limited to 0.5 MHz. Analysis of hmE reveals that: (1) hmE remains almost constant during ±2 to ±4 hours around local noon, (2) hmE values are higher in winter than in spring, summer and autumn, (3) there are two maxima near sunrise and sunset with a noontime minimum in between. During the entire period of study, significant differences between observed hmE and the IRI predictions have been noted. IRI fails to predict hmE and outputs a constant value of 110 km, which is higher than most of the observed values. Over- and under estimations range from 3 to 13 km and from 0 to 3 km respectively.
Keywords- Ionosphere; International Reference Ionosphere (IRI) Model; E layer; E layer critical frequency (foE); E layer peak height (hmE)
1. Introduction Over the past years, various national and international organizations have developed an array of different ionospheric models for the prediction of essential ionospheric parameters. These models include theoretical, empirical, semi-empirical, and were developed on a local, regional and global scale. International Reference Ionosphere (IRI) is widely accepted as the most mature empirical model for predicting the global and temporal mean behaviour of the ionospheric parameters (Bilitza et al., 2014). The Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) jointly established IRI in the late sixties. Since its initiation, this model is continuously upgraded with newer ground and space based observations and improved modelling techniques. As a result, several major milestone editions of the model have evolved (Rawer et al., 1978, 1981; Bilitza, 1990, 1997, 2003; 2
Bilitza and Reinisch, 2008; Bilitza et al., 2014). IRI-2016 is the most recent version of the model (Bilitza et al., 2016). To evaluate the predictability of IRI, observed ionospheric characteristics are often compared with modelled outputs. Ground based Digisonde measurements are the most vital source of information about long-term variations in E and F regions (Bremer, 2008). The E layer extending from an altitude of 90-150 km with its critical frequency (foE) located at 105110 km (Davies, 1990), has traditionally been considered as an ideal case of application of simple Chapman theory (Chapman, 1931a, 1931b). The E layer properties are well known: the ionisation time-constant is only a few minutes, movement effects are small, the conditions are always near to photochemical equilibrium, and fluctuations due to diffusion, winds and electric fields are of negligible importance (Titheridge, 1996). Here, X-rays in the 8-10.4 nm range and UV radiation from 80 nm to L-beta (102.6 nm) are the source of ionisation, producing the principal NO +, O2+ and secondary O+, N2+ ion components (Schunk and Nagy, 2009). The E layer exhibits a daily maximum at local noon, seasonal maximum in summer, and solar cycle dependence. It does not entirely disappear at night and remains weakly ionised (Zolesi and Cander, 2014). The E layer can provide relatively stable modes of propagation (Xinan et al., 2006) and foE is an important characteristic, which directly affects radio wave propagation through the ionosphere (Wongcharoen et al., 2015). Kouris and Muggleton (1973a) has studied the diurnal variation in the E layer using foE data over a period of 11 years, from 45 ionospheric stations and concluded that the value of diurnal exponent, p in the expression does not vary with season and does not exhibit any systematic variation with latitude. In another study (Kouris and Muggleton, 1973b), the world morphology of the Appleton E layer seasonal anomaly was investigated and it was shown that the anomaly depends not only on latitude but also on longitude and hemisphere. Furthermore, it could be the result of seasonal variations in the solar quiet current system. Kouris (1977) has investigated the solar cycle variations of foE for radio-propagation predictions as well as an index of the changes in the solar photoionising radiation that produces the E layer. Most indices of solar activity showed similar changes through a solar cycle, and the correlations between different indices were high. These facts determine the form of the function of foE, to be relatively proportional to the photoionising radiation as observed by the earth as a whole.
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The noontime day-to-day variability of incoherent scatter radar (ISR) measurements in the E layer was reported to be between 5 and 7% at mid-latitudes (Millstone Hill, Massachusetts), whereas polar latitudes (Longyearbyen, Svalbard) demonstrate variability ranging from 7 to 50%, resulting primarily from a combination of photochemical and auroral processes (Moore et al., 2006). Measurements of foE were initiated in 1935 at Tromsø Geophysical Observatory, Norway. Historical records are available for more than 80 years. Hall et al., (2011) have investigated those records and presented updated results confirming the previously identified a negative trend in E region critical frequency and altitude, signifying the warming of the middle atmosphere of the Arctic region and commensurable with a shrinking of the underlying middle atmosphererespectively, giving high credibility to climate change in the lower thermosphere. Chen et al. (2011) have studied the hourly variation in foE over two decades and investigated their residuals between three pairs of ionospheric vertical sounding stations (European and Asian) at nearly the same latitude but different longitude. The investigation revealed that both annual and 11-year variations of foE and their residuals pertaining to each of the pairs are more pronounced, which ascribe to seasonal meteorological effects (Kazimirovsky et al., 1982; Danilov et al., 1987). Abe et al. (2013) investigated the variability of foE with solar activity within the equatorial ionospheric anomaly region. The study uncovered the increasing trend of foE with rising solar activity as expected but also a decreasing trend in its relative variability. Wongcharoenet al. (2015) analysed ionograms at Chumphonstation (10.72◦N, 99.37◦E), Thailand, during 2005–2008 and reported a good agreement of foE the with IRI2012 predictions. The maximum difference between the observed and predicted values was about 0.5 MHz during daytime. Apart from ground based measurements, Chu et al. (2009) used COSMIC-measured E region peak electron density (NmE), to analyse the morphology of GPS radio occultation (RO)-retrieved NmE and reported the presence of three salient crests of the COSMICretrieved noontime NmE enhancements in geomagnetic low/equatorial region 10°N–10°S and in geomagnetic latitude regions ±15°–35°. Wu et al., (2015) used the IRI model to simulate temporal and spatial distributions of global NmE retrieved by the FORMOSAT-3/COSMIC satellites by means of GPS RO technique. The COSMIC measurement and the IRI model simulation both revealed that the magnitude of the percentage error (PE) and root mean-square-error (RMSE) of the relative 4
RO retrieval errors of the NmE values are dependent on local time (LT) and geomagnetic latitude, with a minimum in the early morning at high latitudes and a maximum in the afternoon at middle latitudes. Yang, et al., (2016) investigated the improvements in foE representation for use in the IRI model and compared the results to Digisonde measurements of four stations (Sondrestrom, Athens, Jicamarca, and Port Stanley) at low, mid and high latitudes during solar minimum and solar maximum. One of the stations, Athens, located in the low-mid latitude (Station ID. AT138 geographical coordinates: 38.0◦N, 23.5◦E, geomagnetic lat. 36.3◦N, 102.7◦E) is expected to exhibit similar E region characteristics to Nicosia, Cyprus (Station ID. NI135, geographical coordinates: 35◦N, 33◦E, geomagnetic lat. 29.38◦N, I = 51.7◦). Results show that the IRI model estimates the foE parameter better at low-mid latitudes than at high latitudes, particularly during solar maximum. The poor performance at high latitudes can be attributed to the fact that IRI-2012 does not yet include the contribution from precipitating electrons. The E layer peak height (hmE) is given as a constant equal to 110 km in IRI-2012 model, which is not confirmed by the rocket and ISR data. In fact, hmE at mid-latitude is constant only a certain time around noon (Ivanov-Kholodny et al., 1998). In order to address this issue, several E layer peak height models have been developed based on rocket and radar data (Chasovitin et al., 1983; Fatkullin et al., 1981; Ivanov-Kholodny et al., 1998). The model proposed by Ivanov-Kholodny et al., (1998) on the mid-latitude E layer is based on average hmE measurements made by ISR and can give more accurate values of hmE for all seasons in the vicinity of local noon. Mikhailov et al., (1999) analysed hmE data using Digisonde observations and concluded that winter hmE values exhibit seasonal variations higher than the summer ones and the seasonal differences increase with solar zenith angle (χ). In this paper, we investigate the variability of foE and hmE over a period of 8 years (2009-2016) mostly corresponding to the ascending half of the solar cycle-24. The data are acquired from ionograms recorded in low mid-latitude European Digisonde station located at Nicosia, Cyprus (Station ID. NI135, geographical coordinates: 35◦N, 33◦E, geomagnetic lat. 29.38◦N, I = 51.7◦). This paper comes as a continuation of recently published studies that have been undertaken by the Cyprus ionospheric group to characterise the long-term ionospheric behaviour over Nicosia (Vryonides and Haralambous, 2013; Haralambous and Oikonomou, 2015; Panda and Haralambous, 2016; Mostafa et al., 2017). The observed foE values every 15 min are compared with the IRI-2012 predicted values. We also attempt to investigate the diurnal, seasonal, and long-term variation of foE and hmE during low and high 5
solar activity. According to Wu, K. H et al., (2015) for the morphological study of the global foE for a long-term period, the use of the ionogram data is reasonable and acceptable according to a number of studies. This is also stressed in Perrone, L et al., (2011) according to which foE are very simple, direct measurements with errors of less ∼3%, i.e., less than 6% for NmE.
2. Data and methodology
2.1. Digisonde measurements We obtained ionograms at 15-min interval during 2009–2016 and scaled them manually using SAO Explorer (http://ulcar.uml.edu/Digisonde.html). From these ionograms, the monthly median values of foE and hmE were subsequently extracted. The extracted values were compared with the corresponding predicted values of IRI-2012 model. Months having less than 10 observations in every hour were excluded from the study. For the purpose of comparison, we calculated the absolute bias as follows:
Absolute bias = (Digisonde observed value - IRI predicted value)
(1)
Approximately, 11% of the data are missing, mostly due to equipment failure; and most of those are for consecutive days of roughly one-month duration. Details of missing data are as follows: (a) 2011- May, June, July, and December, (b) 2012- June, July, August, and September, (c) 2013- February, and August, (d) 2015 - April. Furthermore, we have excluded the following data for not having more than 10 observations in an hour: (a) 2010 April and May (only the period 08:00 to 12:00 UT), (b) 2014 - March and November, (c) 2015 - May.
2.2. IRI predictions Based on photochemical approximation, Kouris and Muggleton (1973a, b) developed the model for International Radio Consultative Committee (CCIR) (CCIR, 1973) describing foE as follows:
(2)
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where, A, B, C, and D depend on the 12-months running mean of the solar 10.7 cm radio flux (COV12), on season (χnoon is the solar zenith angle at noon), on geodetic latitude (φ), and on the solar zenith angle (χ) respectively and are given as follows:
(3)
(4)
(5)
(6) (7)
(8)
Different from CCIR (CCIR, 1973), IRI defines foE as follows:
(9)
During all times foE is kept above or equal to the observed minimum value and a modified zenith angle was introduced to improve the nighttime variation (Rawer and Bilitza, 1990) as follows:
(10)
As mentioned in Mikhailov et al., (1999), the circumstances with hmE is not that straightforward because of the issues with hmE determination from the ionogram reduction procedure. Rocket observations reveal that the Ne(h) profile is very irregular at the E region heights (Andreveva et al., 1971), which imposes an uncertainty with hmE identification. As a result, daily, seasonal, and solar cycle variations of hmE predicted by various models differ from one another. A pronounced hmE seasonal variation was detected from incoherent scatter 7
observations with winter higher than summer ones (Taran, 1979). However, the IRI-90 (Bilitza, 1990) and DGR (Di Giovanni and Radicella, 1990) empirical models are unable to determine seasonal and daily variations of hmE. Nevertheless, daily hmE variations should be present in the empirical models as the E layer mostly follows Chapman’s theory and hmE is expected to vary with χ, as:
(11)
where H is the scale height of the neutral atmosphere. In
this
investigation,
we
have
used
IRI2012web
(https://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html) for foE prediction. However, for hmE prediction, IRI adopts a constant value of 110 km, which is a striking point to note.
3. Results and Discussion
3.1. Variation of critical frequency of E layer
The monthly median values of foE acquired from Digisonde Nicosia (LT = UT + 2) station, corresponding predictionsby IRI-2012 model, and the differences between Digisonde observations and IRI-2012 predictions during 2009–2016 for each month are displayed on the left, centre and right panel of Fig. 1 respectively. Within the entire period of study, 2009 is the year having the lowest solar activity (R = 10). The top subplot in the left panel of Fig 1 displays the diurnal variation of foE for each month of 2009. For every month, the magnitude of foE increases progressively as the sun intensity increases having a peak around local noon and decreases from noon to sunset. Peak values demonstrate seasonal variation: high (around 3.25 MHz; equivalent electron density, 131×103 el/m-3) in June solstice (May, June, July, August), moderate (around 3.00 MHz; equivalent electron density, 111×103 el/m-3) in March equinox (March, April) and September equinox (September, October), low (around 2.75 MHz; equivalent electron density, 93.77×103 el/m-3) in December solstice (November, December, January, and February). Throughout the year, the appearance of E layer shows a progressive pattern, with the longest duration of appearance during June-July (from 03:00 to 16:00 UT) and a gradual decrease in
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the preceding and succeeding months, reaching the shortest duration in December (from 05:30 to 14:00 UT) This is expected as foE exhibits dependence on χ. At this point, we would like to compare our results of Nicosia station with the results found by Yang et al., (2016) relating to station Athens. As expected, in this study envelop of the annual variation plot of foE for each hour within the day during solar minimum (2009) exhibits the similar kind of diurnal variation demonstrated at Nicosia. From the respective plot, we notice that the duration of appearance from around 04:00 to 17:00 LT (LT = UT + 2) with peak value around 3.57 MHz (equivalent electron density, 158×103 el/m-3) during summer and around 2.57 MHz (equivalent electron density, 82×103 el/m-3) during winter. In another study (Wongcharoen et al., 2015) on the equatorial ionosphere over Thailand, the maximum value of foE was found to be 3.6 MHz (equivalent electron density, 160.7×103 el/m-3) in the June solstice 2006 (R = 20) and the minimum value was 2.1 MHz (equivalent electron density, 54.68×103 el/m-3) in December solstice 2008 (R = 4). From the corresponding IRI-2012 predictions and the absolute biases of 2009, we notice that in most of the representative periods, IRI-2012 slightly overestimates foE values. Overestimation is mostly limited to 0.25 MHz but occasionally can go as high as 0.3 MHz (equivalent electron density, 1.116×103 el/m-3, mostly during noon). We also notice some underestimation (around 0.25 MHz) though sporadic in nature. In March and September equinoxes, the duration of appearance is similar. A gradual decrease in duration of appearance in the preceding and succeeding months with respect to summer is also clear in the IRI predictions. IRI predicts longer duration of appearance (around 1-h early appearance and 1-h late disappearance) of foE than those derived from Digisonde observations. In the succeeding years, as the solar activity gradually intensifies, the magnitude of foE also demonstrates a gradual increase, reaching a maximum (up to 4 MHz; equivalent electron density, 198.4×103 el/m-3) in 2014, the year exhibiting thehighest solar activity (R=116) in solar cycle 24. Diurnal and seasonal variations in the observed values of foE gradually turn out to be more distinct in high solar activity year than of low solar activity year. The magnitude of foE gradually declines in the succeeding years 2015 to 2016 as the solar activity declines. These clear changes can beidentified from the observations and predictions depicted in the subplots of left and centre panel of Figure 1. Analysing the absolute biases shown in the subplots of the left panel of Figure 1, in most of the cases, we can notice slight overestimation in the predicted values. However, few predicted values were lower than the observed values, though sporadic in nature. During the entire period of study 9
(from 2009 to 2016) the over-/under predictionismostly limited to 0.25 MHz, and in very few cases increases beyond 0.25 MHz but was limited to 0.4 MHz (equivalent electron density, 19.8×103 el/m-3). As the solar cycle 24 progresses through the years 2009-2016, with the change in solar activity, differences between the magnitude of foE observations and corresponding IRI predictions do not divert significantly. Therefore, we can fairly rely on the foE predictability performance of IRI. Fig. 2 displays the standard deviations of the observed values of foE from their monthly median. As noticed in the subplot for the year 2009, the standard deviation of foE shows a patterned diurnal variation. The magnitude of the deviation is minimum (up to 0.1 MHz) during early morning and late afternoon and slightly increases during noon (up to 0.2 MHz). In the month of January, increased values of standard deviation (up to 0.2 MHz) are found to prevail for a short duration of one hour during noon. But in the succeeding months this duration gradually increases. The longest duration is in the month of March and September, from 08:00-16:00 LT. This pattern is more or less replicated in the succeeding years except 2014, where the months of July and August show increased value of standard deviation. In general, when the observed monthly median value of foE is greater in magnitude, the standard deviation is also found to be greater. This is indicative of the low dispersion of foE measurements by the Digisonde. Fig. 3 displays the diurnal plots of foE for each of the years from 2009 to 2016 observed in the month of October. It reveals the obvious diurnal variation since foE varies in sympathy to χ. We can also note a long-term solar cycle variation, which demonstrates the dependency of foE on monthly mean total sunspot number (SSN). Table 1 shows the monthly mean total SSN for the month of October throughout the years 2009-2016. It may be noted that as the solar cycle 24 progresses from 2009-2014, SSN values gradually increase, depicting increasing solar activity with exceptions in the year 2011 and 2013.October 2011 is characterised by the highest solar activity (SSN = 125.7) followed by October 2013 (SSN = 114.4). Amongst the family of foE curves shown in Fig 2, curve of October 2011 displays the greatest magnitude followed by the curve of October 2013. All other curves display the magnitude of foE in order of their respective SSN, which is a true manifestation of variation of foE with the solar activity. We can conclude that the magnitude of foE at low solar activity is lower than that of high solar activity as expected. Table 1. Monthly mean total sunspot number for the month of October in 2009-2016.
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Year
Monthly mean total sunspot number
2009
7.7
2010
33.6
2011
125.7
2012
76.5
2013
114.4
2014
90
2015
63.6
2016
33.4
3.2. Variation of peak height of E layer
Bounsanto et al., (1995) and Titheridge, (1997) found that the calculated NmE = 1.24×104foE2 using conventional EUV solar flux models and dissociative recombination rate constants for NO+ and O2+ ion is 30-40% less than the observed value. Due to square law loss process, this 40% deficit in NmE implies a 100% increase in the total ionization rate at the E layer maximum. Ivanov-Kholodny and Nusinov (1979) used low αNO+ and αO2+values by Mul and McGoyan, (1979) along with [O2] scale height seasonal variations in the 100-110 km height range to overcome this problem. Later Antonova et al., (1996) took into account vibrationally excited NO+ and O2+ ions, which allowed them to explain NmE and hmE seasonal variations. Mikhailov et al., (1999) interpreted that the higher hmE values observed in winter than the summer results from thermosphere model MSIS-86 [O2] winter concentration being 40% larger than summer in the E-region considering 105 km height. A similar 40% increase takes place for [O], but atomic oxygen does not absorb these UV lines at all. The enhanced absorption of UV radiation in winter results in a general elevation of the E layer hm0 in expression 11. On the other hand, this increase is partly compensated by lower winter neutral temperature (200 K compared to 218 K in summer) as lower Tn gives less neutral scale height H in expression 11. Fig. 4 displays the measured values of the peak height of the E-layer, hmE and their absolute bias from IRI predictions which outputs hmE = 110 km. A close inspection of the observed values of 2009 (low solar activity with R = 10) reveals three distinctive patterns: (1) the magnitude of hmE remains fairly constant (between 98-104 km) during ±2 to ±4 h local noon, (2) hmE values are higher in winter (around 104-107 km) than those in spring, summer and autumn, (3) there are two maxima (around 104-110 km) near sunrise and sunset with a noontime minimum (around 98-101 km) in between. This is in agreement with the previous studies of mid-latitude ISR (Ivanov-Kholodny et al., 1998) and El Arenosillo Digisonde
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(Mikhailov et al., 1999).
Ivanov-Kholodny et al., (1998) investigated the continuous
measurements of hmE with the 15-minute data by ISR and presented the results with statistical accuracy better than 1 km. Measurements show following magnitudes of hmE: (a) during winter: 118 and 121 km (b) during summer: 113 and 117 km at χ = 72◦ and 82◦ respectively. Digisonde observations in (Mikhailov et al., 1999) show that hmE varies from 103-120 km during winter and from 98-110 km during summer as the χ increases from 55◦ to 90◦. The three distinctive patterns described above are valid for all levels of solar activity in the subsequent years (2010-2016), which leads to the conclusion that the variation of hmE is not adequately described by Chapman’s theory of layer formation. During the entire period of study, significant differences between the observed hmE and the IRI predictions have been observed. IRI fails to predict since it outputs a constant hmE value of 110 km, which is higher than most of the observed values. Overestimation and underestimations range from 3 to 13 km and from 0 to 3 km respectively. Fig. 5 displays the standard deviations of the observed monthly median hmE values as a measure of dispersion. The standard deviations of the observed values of hmE are more or less constant (around 6 km from its monthly median value). Fig. 6 displays the diurnal plots of monthly median values of hmE observations for the month of October from 2009 to 2016 within solar cycle 24. During this period, morning time maximum appears around 06:15 to 07:00 LT, which is in good agreement with the October sunrise time in Cyprus and the height varies from 105.6-110 km. The second maximum near sunset appears around 15:30 to 16:15 LT, which is not in good agreement with the October sunset time in Cyprus and the height varies from 103.7-106.5 km. The noontime minimum appears around 10:45 to 11:15 LT and the height varies from 97.6 to 102.4 km. Differences in hmE between the maximum near sunrise and the noontime minimum ranges from 5.6 to 10.8 km. The difference in hmE between the sunset maximum and the noontime minimum ranges from 5 to 6.9 km. Therefore, we can conclude that the diurnal variation of hmE does not show any conspicuous change. As the solar cycle 24 progresses from low to high solar activity during 2009–2004, we notice an increase in the hmE values. During 2015-2016, hmE values decrease in accordance to the decreasing solar activity.
4. Conclusion
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In this study, we have investigated the E layer critical frequency (foE) and peak height (hmE) characteristics derived from 8 years (2009–2016) of manually scaled Digisonde measurements at the low-middle latitude European station in Nicosia, Cyprus and compared those to the corresponding IRI-2012 predictions. Our conclusive remarks are as follows: (a) Following the increase in solar radiation foE gradually increases from sunrise time, attains the maximum value around local noon, and gradually decreases to the minimum value during sunset. This diurnal variation of foE is observed in all seasons in all years under investigation, both at high and low solar activity period. Observed values of foE also demonstrate seasonal variation: high in June solstice, moderate in March and September equinox, low in December solstice. The magnitude of foE shows a clear longterm solar variation having the lowest value in the lowest solar activity year. As the solar activity intensifies, the magnitude of foE also increases and the diurnal and seasonal variations in the Digisonde observations and IRI predictions turn out to be more distinct. (b) In most of the months, IRI-2012 slightly overestimates the foE values, mostly limited to 0.25 MHz but occasionally can go as high as 0.3 MHz (equivalent electron density, 1.116×103 el/m-3). Very few cases of underestimation, though sporadic in nature limited to 0.25 MHz have also been noted.Throughout the year, the appearance of E layer shows a progressive pattern, with the longest duration of presence in June-July and a gradual decrease in the preceding and succeeding months, reaching minimum duration in December. This is expected as foE exhibits dependence on χ. IRI-2012 predictions are similar but longer (around 1-h early appearance and 1-h late disappearance) than those derived from Digisonde observations. Therefore, the time should be extended by 1 h both at the beginning and at the end of the duration of appearance of E layer as presently predicted by IRI-2012. (c) Observed values of hmE reveal three distinctive patterns: (1) the magnitude of hmE remaining fairly constant (between 98-104 km) during ±2 to ±4 h local noon, (2) hmE values being higher in winter (around 104-107 km) than those of spring, summer and autumn, (3) two maxima (around 104-110 km) near sunrise and sunset with a noontime minimum (around 98-101 km) in between.We can substantiate that the behaviour of hmE is not adequately described by Chapman’s theory of layer formation as mentioned in (Ivanov-Kholodny et al., 1998). During the entire period of study, significant differences between the observed hmE and the IRI predictions have been observed. IRI outputs a constant hmE value of 110 km, which is higher than most of the observed values. Therefore an effort to improve IRI model for better hmE representation is needed. 13
(d) The morning time hmE maximum appears around 06:15 to 07:00 LT, which is in good agreement with the October sunrise time in Cyprus. However, the second maximum near sunset appears around 15:30 to 16:15 LT, which is not in good agreement with the October sunset time in Cyprus. The differences in E layer peak height amongst the two hmE maxima and the noontime minimum range from just 5 to 10.8 km. (e) During our period of study, we note an increasing trend in hmE values as the solar cycle progresses from low to high solar activity during 2009-2014. During the rest of the period 2015-2016, hmE values decreases in accordance to the declining solar activity.
Acknowledgements The authors would like to thank the NASA’s National Space Science Data Center (NSSDC) for providing theIRI-2012 model (http://iri.gsfc.nasa.gov/). This work was supported by the Erasmus Mundus INTACT Project.
References Abe, O.E., Rabiu, A.B., Adeniyi, J.O., 2013. Variation of foE in the equatorial ionosphere with solar activity. Adv. Space Res. 51, 69–75. Andreveva, L.A., BurakovYu.B., Katasev L.A., 1971. Rocket investigation of the ionosphere at mid-latitude. Space Res. 11, 1043-1050. Bilitza, D., Altadill, D., Reinisch, B., Galkin, I., Shubin, V., Truhlik, V., 2016. The International Reference Ionosphere: Model Update 2016, Conference: EGU General Assembly, 18. Bilitza, D., Altadill, D., Zhang, Y., Mertens, C., Truhlik, V., Richards, P., McKinnell, L.A., Reinisch, B., 2014. The international reference ionosphere 2012 a model of international collaboration. J. Space Weather Space Clim. 4. Bilitza, D., Reinisch, B., 2008. International reference Ionosphere 2007: improvements and new parameters. Adv. Space Res. 42 (4), 599–609. Bilitza, D., 2003. International Reference Ionosphere 2000: Examples of improvements and new features. Adv. Space Res. 31 (3), 757–767. Bilitza, D., 1997. International reference Ionosphere – status 1995/96. Adv. Space Res. 20 (9), 1751–1754. Bilitza, D. (Ed.), 1990, International Reference Ionosphere-1990, Rep. NSSDC 90–22, World Data Cen. Bremer, J., 2008. Long-term trends in the ionospheric E and F1 regions. Ann. Geophys. 26, 1189–1197. CCIR, 1973. Atlas of Ionospheric Characteristics. Report. Comite´ Consultatif International des Radiocommunications, Gene`ve, 340. Chapman, S., 1931a. The absorption and dissociative or ionizing effect of monochromatic radiation in an atmosphere on a rotating earth. Proc. Phys. Boc. Lond., 45, 26-45. Chapman, S., 1931b. The absorption and dissociative or ionizing effect of monochromatic radiation in an atmosphere on a rotating earth, Proc. Phys. Boc. Lond., 43, 433-501.
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ChasovitinIu.K., SHushkova, T.N., Sikilinda et al. 1983. Empirical Model of Electron Density from 80 to 200 km for Low and Mid Latitudes. Proc. Inst. of Experimental Meteorology, 13 (102), 52-79. Chen, M., Li, C., Ding L., Gu, B., 2011. Two decades variation about ionospheric parameters foF2, foE and M3000F2. Intl. Conf. on Multimedia Tech., Hangzhou, 2769-2772. Chu, Y. H., Kong-Hong, W., Ching-Lun, S., 2009. A new aspect of ionospheric E region electron density morphology, J. Geophys. Res., 114, A12314. Danilov, A. D., Kazimirovsky, E. S., Vergasova, G. V., Khachikjan, G. Ya., 1987. Meteorological Effects in the Ionosphere. Gidrometeoizdat, Leningrad, 271 p. Davies, K., 1990. Ionospheric Radio. Peregrinus on Behalf of the Institution of Electrical Engineers, London. Fatkullin M.N., Zelenova T.E., Kozlov et al. 1981. Empirical Models of Mid-Latitude Ionosphere. Moscow, Nauka, 256. Giovanni D., Radicella, S.M., 1990. An analytical model of the electron density profile in the ionosphere. Adv. Space Res. 10(11), 27-30. Hall, C. M., Rypdal, K., Rypdal, M., 2011. The E region at 69°N, 19°E: Trends, significances, and detectability. J. Geophys. Res. 116 (A05309), 1-13. Haralambous, H., Oikonomou, C., 2015. Comparison of peak characteristics of the F2 ionospheric layer obtained from the Cyprus Digisonde and IRI-2012 model during low and high solar activity period. Adv. Space Res.56(9),1927-1938. Ivanov-Kholodny, G.S., Kishcha, P.V., Zhivolup, T.G., 1998. A Mid-Latitude E-Layer Peak Height Model. Adv. Space Res. 22(6), 767-770. Kazimirovsky, E.S., Vergasova, G.V., 1982. Meteorological effects in the variations of ionospheric parametersI. The lower thermosphere, J. Atmos. Terr. Phys., 44 (11), 913-922. Kouris, S.S., Muggleton, L.M., 1973a. Diurnal variation in E-layer ionization. J. Atmos. Terr. Phys. 35, 133– 139. Kouris, S.S., Muggleton, L.M., 1973b. World morphology of Appleton E layer seasonal anomaly. J. Atmos. Terr. Phys. 35, 141–151. Kouris, S.S., 1977. Summary. Variation of E-layer critical frequency with solar activity. Proc. IEEE, 124 (11), 1007-1008. Mikhailov A.V., Morena, A. de. La. B., Miro, G., Marin, D., 1999. A comparison of foE and hmE model calculations with El ArenosilloDigisonde observations. Seasonal variations. Annali Di Geofisica, 42(4), 691698. Moore, L., Mendillo, M., Martinis, C., Bailey, S., 2006. Day-to-day variability of the E layer. J. Geophys. Res. 111 (A06307), 1-11. Mostafa, M.G., Haralambous, H., Oikonomou, C., 2017. Evaluation of IRI predicted characteristics of ionospheric F1 layer by Digisonde observations in Nicosia, Cyprus. Adv. Space Res. 59(6), 1548-1558. Panda, S. K., Haralambous, H., 2016. Variability of the bottomside B0 and B1 parameters of ionospheric electron density profile over Cyprus and comparison with IRI-2012 model. Adv. Space Res.60 (2),317-328. Perrone, L., A. V. Mikhailov, and L. P. Korsunova 2011. FORMOSAT‐3/COSMIC E region observations and daytime foE at middle latitudes, . J. Geophys. Res. 116. Rawer, K., Lincoln, J.V., Conkright, R.O., 1981, International Reference Ionosphere-IRI 79, World Data Center A for Solar-Terrestrial Physics, Report UAG-82, 245, Boulder, Colorado.
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Rawer, K., Ramakrishnan, S., Bilitza, D., 1978, International Reference Ionosphere-1978, International Union of Radio Science, URSI Special Report, 75, Bruxelles, Belgium. Schlegel, K., Haldoupis, C., 1994. Observation of the modified two-stream instability in the midlatitude E region. J. Geophys. Res. 99, 6219–6226. Schunk, R., Nagy, A., 2009. Ionospheres: Physics, Plasma Physics, and Chemistry, second ed. Cambridge University Press, Cambridge. Taran, V.I., 1979. Incoherent scatter observations of the ionosphere. Sov. Radio. 27, 7-12. Titheridge, J. E., 1997. Model results for the ionospheric E region: solar and seasonal changes, Ann. Geophys., 15 (1), 63–78. Vryonides, P., Haralambous, H., 2013. Comparison of COSMIC measurements with the IRI-2007 model over the eastern Mediterranean region. J. Adv. Res., 4 (3), 297-301. Wongcharoen, P., Kenpankho, P., Supnithi, P., Ishii, M., Tsugawa, T., 2015. Comparison of E layer critical frequency over the Thai station Chumphon with IRI. Adv. Space Res. 55 (8), 2131–2138. Wu, K. H., Su, C. L., Chu, Y. H., 2015. Improvement of GPS radio occultation retrieval error of E region electron density: COSMIC measurement and IRI model simulation. J. Geophys. Res. 120, 2299-2315. Xinan, Y., Wan, W., Liu, L., Ning, B., 2006. An empirical model of ionospheric foE over Wuhan. Earth, Planets and Space, 58 (3), 323-330. Yamamoto, M., Fukao, S., Woodman, R.F., Ogawa, T., Tsuda, T., Kato, K., 1991. Mid latitude E region fieldaligned irregularities observed with the MU radar. J. Geophys. Res. 96, 15943–15949. Yamamoto, M., Fukao, S., Ogawa, T., Tsuda, T., Kato, S., 1992. A morphological study of mid-latitude Eregion field-aligned irregularities observed with the MU radar. J. Atmos. Sol. Terr. Phys. 54, 769–777. Yong, Z., Ssessanga, N., Tran, L.T., Bilitza, D., Kenpankho P., 2016. On improvement in representation of foE in IRI. Adv. Space Res. 60(2),347–356. Zolesi, B., Cander, L.R., 2014. Ionospheric Prediction and Forecasting. Springer, Heidelberg New York Dordrecht London, 42.
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Fig.1. Monthly median values of foE: (a) observations from Nicosia Digisonde (left panels) and (b) IRI-2012 predictions (centre panels), differences in foE: (c) between Digisonde observation and IRI predicted value (right panels), during 2009–16 in solar cycle 24.
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Fig. 2. Standard deviation of the monthly median values of foE observations from Nicosia Digisonde during 2009–16.
Fig. 3. Diurnal plots of monthly median values of foE for the month of October from 2009 to 2016.
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Fig. 4. Monthly median hmE observations from Nicosia Digisonde: (a) during 2009-2012 (left panels) and (c) during 2013-2016 (third panels from left), differences in hmE between Digisonde observation and IRI predicted value: (b) during 2009-2012 (second panel from left), (d) during 20132016 in solar cycle 24.
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Fig. 5. Standard deviation of the monthly median values of hmE observations from Nicosia Digisonde during 2009–16.
Fig. 6. Diurnal plots of monthly median values of hmE observations for the month of October from 2009 to 2016.
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Highlights
IRI slightly overestimates foE values both at low and high solar activity with mostly 0.25 MHz over-/under-prediction.
IRI outputs a constant hmE value of 110 km for all season and years, which is higher than most of the observed values.
Values of hmE are higher in winter than that of spring, summer and autumn.
hmE remains almost constant during ±2 to ±4 hours around local noon.
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S0273-1177(17)30767-6 https://doi.org/10.1016/j.asr.2017.10.029 JASR 13461
To appear in:
Advances in Space Research
Received Date: Revised Date: Accepted Date:
6 July 2017 15 October 2017 19 October 2017
Please cite this article as: Golam Mostafa, M., Haralambous, H., Oikonomou, C., Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions, Advances in Space Research (2017), doi: https://doi.org/10.1016/j.asr.2017.10.029
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Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions
Md Golam Mostafa PhD Student, Department of Electrical Engineering, Frederick University, 7 Filokyprou St, Palouriotissa, Nicosia, 1036, Cyprus [email protected], [email protected]
Haris Haralambous Assistant Professor, Department of Electrical Engineering, Frederick University, 7 Filokyprou St, Palouriotissa, Nicosia, 1036, Cyprus [email protected]
Christina Oikonomou Research Associate, Frederick Research Centre, 7 Filokyprou St, Palouriotissa, Nicosia, 1036, Cyprus [email protected]
1
Statistical ionospheric E layer properties measured with the Cyprus Digisonde and comparisons with IRI predictions Md Golam Mostafa a,*, Haris Haralambous a, Christina Oikonomoub a
Department of Electrical Engineering, Frederick University, 7 Filokyprou St, Palouriotissa, Nicosia 1036, Cyprus b
Frederick Research Centre, 7 Filokyprou St, Palouriotissa, Nicosia 1036, Cyprus
Abstract This paper reports the diurnal, seasonal, and long term variability of the E layer critical frequency (foE) and peak height (hmE) derived from Digisonde measurements from 2009—2016 at the low-middle latitude European station of Nicosia, Cyprus (geographical coordinates: 35◦N, 33◦E, geomagnetic lat. 29.38◦N, I = 51.7◦). Manually scaled monthly median values of foE and hmE are compared with IRI-2012 predictions with a view to assess the predictability of IRI. Results show that in general, IRI slightly overestimates foE values both at low and high solar activity. At low solar activity, overestimations are mostly limited to 0.25 MHz (equivalent electron density, 0.775×103 el/m-3) but can go as high as 0.5 MHz (equivalent electron density, 3.1×10 3 el/m-3, during noon) around equinox. In some months, underestimations, though sporadic in nature, up to 0.25 MHz are noted (mostly during sunrise and sunset). At high solar activity, a similar pattern of over-/underestimation is evident. During the entire period of study, over-/under estimations are mostly limited to 0.25 MHz. In very few cases, these exceed 0.25 MHz but are limited to 0.5 MHz. Analysis of hmE reveals that: (1) hmE remains almost constant during ±2 to ±4 hours around local noon, (2) hmE values are higher in winter than in spring, summer and autumn, (3) there are two maxima near sunrise and sunset with a noontime minimum in between. During the entire period of study, significant differences between observed hmE and the IRI predictions have been noted. IRI fails to predict hmE and outputs a constant value of 110 km, which is higher than most of the observed values. Over- and under estimations range from 3 to 13 km and from 0 to 3 km respectively.
Keywords- Ionosphere; International Reference Ionosphere (IRI) Model; E layer; E layer critical frequency (foE); E layer peak height (hmE)
1. Introduction Over the past years, various national and international organizations have developed an array of different ionospheric models for the prediction of essential ionospheric parameters. These models include theoretical, empirical, semi-empirical, and were developed on a local, regional and global scale. International Reference Ionosphere (IRI) is widely accepted as the most mature empirical model for predicting the global and temporal mean behaviour of the ionospheric parameters (Bilitza et al., 2014). The Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) jointly established IRI in the late sixties. Since its initiation, this model is continuously upgraded with newer ground and space based observations and improved modelling techniques. As a result, several major milestone editions of the model have evolved (Rawer et al., 1978, 1981; Bilitza, 1990, 1997, 2003; 2
Bilitza and Reinisch, 2008; Bilitza et al., 2014). IRI-2016 is the most recent version of the model (Bilitza et al., 2016). To evaluate the predictability of IRI, observed ionospheric characteristics are often compared with modelled outputs. Ground based Digisonde measurements are the most vital source of information about long-term variations in E and F regions (Bremer, 2008). The E layer extending from an altitude of 90-150 km with its critical frequency (foE) located at 105110 km (Davies, 1990), has traditionally been considered as an ideal case of application of simple Chapman theory (Chapman, 1931a, 1931b). The E layer properties are well known: the ionisation time-constant is only a few minutes, movement effects are small, the conditions are always near to photochemical equilibrium, and fluctuations due to diffusion, winds and electric fields are of negligible importance (Titheridge, 1996). Here, X-rays in the 8-10.4 nm range and UV radiation from 80 nm to L-beta (102.6 nm) are the source of ionisation, producing the principal NO +, O2+ and secondary O+, N2+ ion components (Schunk and Nagy, 2009). The E layer exhibits a daily maximum at local noon, seasonal maximum in summer, and solar cycle dependence. It does not entirely disappear at night and remains weakly ionised (Zolesi and Cander, 2014). The E layer can provide relatively stable modes of propagation (Xinan et al., 2006) and foE is an important characteristic, which directly affects radio wave propagation through the ionosphere (Wongcharoen et al., 2015). Kouris and Muggleton (1973a) has studied the diurnal variation in the E layer using foE data over a period of 11 years, from 45 ionospheric stations and concluded that the value of diurnal exponent, p in the expression does not vary with season and does not exhibit any systematic variation with latitude. In another study (Kouris and Muggleton, 1973b), the world morphology of the Appleton E layer seasonal anomaly was investigated and it was shown that the anomaly depends not only on latitude but also on longitude and hemisphere. Furthermore, it could be the result of seasonal variations in the solar quiet current system. Kouris (1977) has investigated the solar cycle variations of foE for radio-propagation predictions as well as an index of the changes in the solar photoionising radiation that produces the E layer. Most indices of solar activity showed similar changes through a solar cycle, and the correlations between different indices were high. These facts determine the form of the function of foE, to be relatively proportional to the photoionising radiation as observed by the earth as a whole.
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The noontime day-to-day variability of incoherent scatter radar (ISR) measurements in the E layer was reported to be between 5 and 7% at mid-latitudes (Millstone Hill, Massachusetts), whereas polar latitudes (Longyearbyen, Svalbard) demonstrate variability ranging from 7 to 50%, resulting primarily from a combination of photochemical and auroral processes (Moore et al., 2006). Measurements of foE were initiated in 1935 at Tromsø Geophysical Observatory, Norway. Historical records are available for more than 80 years. Hall et al., (2011) have investigated those records and presented updated results confirming the previously identified a negative trend in E region critical frequency and altitude, signifying the warming of the middle atmosphere of the Arctic region and commensurable with a shrinking of the underlying middle atmosphererespectively, giving high credibility to climate change in the lower thermosphere. Chen et al. (2011) have studied the hourly variation in foE over two decades and investigated their residuals between three pairs of ionospheric vertical sounding stations (European and Asian) at nearly the same latitude but different longitude. The investigation revealed that both annual and 11-year variations of foE and their residuals pertaining to each of the pairs are more pronounced, which ascribe to seasonal meteorological effects (Kazimirovsky et al., 1982; Danilov et al., 1987). Abe et al. (2013) investigated the variability of foE with solar activity within the equatorial ionospheric anomaly region. The study uncovered the increasing trend of foE with rising solar activity as expected but also a decreasing trend in its relative variability. Wongcharoenet al. (2015) analysed ionograms at Chumphonstation (10.72◦N, 99.37◦E), Thailand, during 2005–2008 and reported a good agreement of foE the with IRI2012 predictions. The maximum difference between the observed and predicted values was about 0.5 MHz during daytime. Apart from ground based measurements, Chu et al. (2009) used COSMIC-measured E region peak electron density (NmE), to analyse the morphology of GPS radio occultation (RO)-retrieved NmE and reported the presence of three salient crests of the COSMICretrieved noontime NmE enhancements in geomagnetic low/equatorial region 10°N–10°S and in geomagnetic latitude regions ±15°–35°. Wu et al., (2015) used the IRI model to simulate temporal and spatial distributions of global NmE retrieved by the FORMOSAT-3/COSMIC satellites by means of GPS RO technique. The COSMIC measurement and the IRI model simulation both revealed that the magnitude of the percentage error (PE) and root mean-square-error (RMSE) of the relative 4
RO retrieval errors of the NmE values are dependent on local time (LT) and geomagnetic latitude, with a minimum in the early morning at high latitudes and a maximum in the afternoon at middle latitudes. Yang, et al., (2016) investigated the improvements in foE representation for use in the IRI model and compared the results to Digisonde measurements of four stations (Sondrestrom, Athens, Jicamarca, and Port Stanley) at low, mid and high latitudes during solar minimum and solar maximum. One of the stations, Athens, located in the low-mid latitude (Station ID. AT138 geographical coordinates: 38.0◦N, 23.5◦E, geomagnetic lat. 36.3◦N, 102.7◦E) is expected to exhibit similar E region characteristics to Nicosia, Cyprus (Station ID. NI135, geographical coordinates: 35◦N, 33◦E, geomagnetic lat. 29.38◦N, I = 51.7◦). Results show that the IRI model estimates the foE parameter better at low-mid latitudes than at high latitudes, particularly during solar maximum. The poor performance at high latitudes can be attributed to the fact that IRI-2012 does not yet include the contribution from precipitating electrons. The E layer peak height (hmE) is given as a constant equal to 110 km in IRI-2012 model, which is not confirmed by the rocket and ISR data. In fact, hmE at mid-latitude is constant only a certain time around noon (Ivanov-Kholodny et al., 1998). In order to address this issue, several E layer peak height models have been developed based on rocket and radar data (Chasovitin et al., 1983; Fatkullin et al., 1981; Ivanov-Kholodny et al., 1998). The model proposed by Ivanov-Kholodny et al., (1998) on the mid-latitude E layer is based on average hmE measurements made by ISR and can give more accurate values of hmE for all seasons in the vicinity of local noon. Mikhailov et al., (1999) analysed hmE data using Digisonde observations and concluded that winter hmE values exhibit seasonal variations higher than the summer ones and the seasonal differences increase with solar zenith angle (χ). In this paper, we investigate the variability of foE and hmE over a period of 8 years (2009-2016) mostly corresponding to the ascending half of the solar cycle-24. The data are acquired from ionograms recorded in low mid-latitude European Digisonde station located at Nicosia, Cyprus (Station ID. NI135, geographical coordinates: 35◦N, 33◦E, geomagnetic lat. 29.38◦N, I = 51.7◦). This paper comes as a continuation of recently published studies that have been undertaken by the Cyprus ionospheric group to characterise the long-term ionospheric behaviour over Nicosia (Vryonides and Haralambous, 2013; Haralambous and Oikonomou, 2015; Panda and Haralambous, 2016; Mostafa et al., 2017). The observed foE values every 15 min are compared with the IRI-2012 predicted values. We also attempt to investigate the diurnal, seasonal, and long-term variation of foE and hmE during low and high 5
solar activity. According to Wu, K. H et al., (2015) for the morphological study of the global foE for a long-term period, the use of the ionogram data is reasonable and acceptable according to a number of studies. This is also stressed in Perrone, L et al., (2011) according to which foE are very simple, direct measurements with errors of less ∼3%, i.e., less than 6% for NmE.
2. Data and methodology
2.1. Digisonde measurements We obtained ionograms at 15-min interval during 2009–2016 and scaled them manually using SAO Explorer (http://ulcar.uml.edu/Digisonde.html). From these ionograms, the monthly median values of foE and hmE were subsequently extracted. The extracted values were compared with the corresponding predicted values of IRI-2012 model. Months having less than 10 observations in every hour were excluded from the study. For the purpose of comparison, we calculated the absolute bias as follows:
Absolute bias = (Digisonde observed value - IRI predicted value)
(1)
Approximately, 11% of the data are missing, mostly due to equipment failure; and most of those are for consecutive days of roughly one-month duration. Details of missing data are as follows: (a) 2011- May, June, July, and December, (b) 2012- June, July, August, and September, (c) 2013- February, and August, (d) 2015 - April. Furthermore, we have excluded the following data for not having more than 10 observations in an hour: (a) 2010 April and May (only the period 08:00 to 12:00 UT), (b) 2014 - March and November, (c) 2015 - May.
2.2. IRI predictions Based on photochemical approximation, Kouris and Muggleton (1973a, b) developed the model for International Radio Consultative Committee (CCIR) (CCIR, 1973) describing foE as follows:
(2)
6
where, A, B, C, and D depend on the 12-months running mean of the solar 10.7 cm radio flux (COV12), on season (χnoon is the solar zenith angle at noon), on geodetic latitude (φ), and on the solar zenith angle (χ) respectively and are given as follows:
(3)
(4)
(5)
(6) (7)
(8)
Different from CCIR (CCIR, 1973), IRI defines foE as follows:
(9)
During all times foE is kept above or equal to the observed minimum value and a modified zenith angle was introduced to improve the nighttime variation (Rawer and Bilitza, 1990) as follows:
(10)
As mentioned in Mikhailov et al., (1999), the circumstances with hmE is not that straightforward because of the issues with hmE determination from the ionogram reduction procedure. Rocket observations reveal that the Ne(h) profile is very irregular at the E region heights (Andreveva et al., 1971), which imposes an uncertainty with hmE identification. As a result, daily, seasonal, and solar cycle variations of hmE predicted by various models differ from one another. A pronounced hmE seasonal variation was detected from incoherent scatter 7
observations with winter higher than summer ones (Taran, 1979). However, the IRI-90 (Bilitza, 1990) and DGR (Di Giovanni and Radicella, 1990) empirical models are unable to determine seasonal and daily variations of hmE. Nevertheless, daily hmE variations should be present in the empirical models as the E layer mostly follows Chapman’s theory and hmE is expected to vary with χ, as:
(11)
where H is the scale height of the neutral atmosphere. In
this
investigation,
we
have
used
IRI2012web
(https://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html) for foE prediction. However, for hmE prediction, IRI adopts a constant value of 110 km, which is a striking point to note.
3. Results and Discussion
3.1. Variation of critical frequency of E layer
The monthly median values of foE acquired from Digisonde Nicosia (LT = UT + 2) station, corresponding predictionsby IRI-2012 model, and the differences between Digisonde observations and IRI-2012 predictions during 2009–2016 for each month are displayed on the left, centre and right panel of Fig. 1 respectively. Within the entire period of study, 2009 is the year having the lowest solar activity (R = 10). The top subplot in the left panel of Fig 1 displays the diurnal variation of foE for each month of 2009. For every month, the magnitude of foE increases progressively as the sun intensity increases having a peak around local noon and decreases from noon to sunset. Peak values demonstrate seasonal variation: high (around 3.25 MHz; equivalent electron density, 131×103 el/m-3) in June solstice (May, June, July, August), moderate (around 3.00 MHz; equivalent electron density, 111×103 el/m-3) in March equinox (March, April) and September equinox (September, October), low (around 2.75 MHz; equivalent electron density, 93.77×103 el/m-3) in December solstice (November, December, January, and February). Throughout the year, the appearance of E layer shows a progressive pattern, with the longest duration of appearance during June-July (from 03:00 to 16:00 UT) and a gradual decrease in
8
the preceding and succeeding months, reaching the shortest duration in December (from 05:30 to 14:00 UT) This is expected as foE exhibits dependence on χ. At this point, we would like to compare our results of Nicosia station with the results found by Yang et al., (2016) relating to station Athens. As expected, in this study envelop of the annual variation plot of foE for each hour within the day during solar minimum (2009) exhibits the similar kind of diurnal variation demonstrated at Nicosia. From the respective plot, we notice that the duration of appearance from around 04:00 to 17:00 LT (LT = UT + 2) with peak value around 3.57 MHz (equivalent electron density, 158×103 el/m-3) during summer and around 2.57 MHz (equivalent electron density, 82×103 el/m-3) during winter. In another study (Wongcharoen et al., 2015) on the equatorial ionosphere over Thailand, the maximum value of foE was found to be 3.6 MHz (equivalent electron density, 160.7×103 el/m-3) in the June solstice 2006 (R = 20) and the minimum value was 2.1 MHz (equivalent electron density, 54.68×103 el/m-3) in December solstice 2008 (R = 4). From the corresponding IRI-2012 predictions and the absolute biases of 2009, we notice that in most of the representative periods, IRI-2012 slightly overestimates foE values. Overestimation is mostly limited to 0.25 MHz but occasionally can go as high as 0.3 MHz (equivalent electron density, 1.116×103 el/m-3, mostly during noon). We also notice some underestimation (around 0.25 MHz) though sporadic in nature. In March and September equinoxes, the duration of appearance is similar. A gradual decrease in duration of appearance in the preceding and succeeding months with respect to summer is also clear in the IRI predictions. IRI predicts longer duration of appearance (around 1-h early appearance and 1-h late disappearance) of foE than those derived from Digisonde observations. In the succeeding years, as the solar activity gradually intensifies, the magnitude of foE also demonstrates a gradual increase, reaching a maximum (up to 4 MHz; equivalent electron density, 198.4×103 el/m-3) in 2014, the year exhibiting thehighest solar activity (R=116) in solar cycle 24. Diurnal and seasonal variations in the observed values of foE gradually turn out to be more distinct in high solar activity year than of low solar activity year. The magnitude of foE gradually declines in the succeeding years 2015 to 2016 as the solar activity declines. These clear changes can beidentified from the observations and predictions depicted in the subplots of left and centre panel of Figure 1. Analysing the absolute biases shown in the subplots of the left panel of Figure 1, in most of the cases, we can notice slight overestimation in the predicted values. However, few predicted values were lower than the observed values, though sporadic in nature. During the entire period of study 9
(from 2009 to 2016) the over-/under predictionismostly limited to 0.25 MHz, and in very few cases increases beyond 0.25 MHz but was limited to 0.4 MHz (equivalent electron density, 19.8×103 el/m-3). As the solar cycle 24 progresses through the years 2009-2016, with the change in solar activity, differences between the magnitude of foE observations and corresponding IRI predictions do not divert significantly. Therefore, we can fairly rely on the foE predictability performance of IRI. Fig. 2 displays the standard deviations of the observed values of foE from their monthly median. As noticed in the subplot for the year 2009, the standard deviation of foE shows a patterned diurnal variation. The magnitude of the deviation is minimum (up to 0.1 MHz) during early morning and late afternoon and slightly increases during noon (up to 0.2 MHz). In the month of January, increased values of standard deviation (up to 0.2 MHz) are found to prevail for a short duration of one hour during noon. But in the succeeding months this duration gradually increases. The longest duration is in the month of March and September, from 08:00-16:00 LT. This pattern is more or less replicated in the succeeding years except 2014, where the months of July and August show increased value of standard deviation. In general, when the observed monthly median value of foE is greater in magnitude, the standard deviation is also found to be greater. This is indicative of the low dispersion of foE measurements by the Digisonde. Fig. 3 displays the diurnal plots of foE for each of the years from 2009 to 2016 observed in the month of October. It reveals the obvious diurnal variation since foE varies in sympathy to χ. We can also note a long-term solar cycle variation, which demonstrates the dependency of foE on monthly mean total sunspot number (SSN). Table 1 shows the monthly mean total SSN for the month of October throughout the years 2009-2016. It may be noted that as the solar cycle 24 progresses from 2009-2014, SSN values gradually increase, depicting increasing solar activity with exceptions in the year 2011 and 2013.October 2011 is characterised by the highest solar activity (SSN = 125.7) followed by October 2013 (SSN = 114.4). Amongst the family of foE curves shown in Fig 2, curve of October 2011 displays the greatest magnitude followed by the curve of October 2013. All other curves display the magnitude of foE in order of their respective SSN, which is a true manifestation of variation of foE with the solar activity. We can conclude that the magnitude of foE at low solar activity is lower than that of high solar activity as expected. Table 1. Monthly mean total sunspot number for the month of October in 2009-2016.
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Year
Monthly mean total sunspot number
2009
7.7
2010
33.6
2011
125.7
2012
76.5
2013
114.4
2014
90
2015
63.6
2016
33.4
3.2. Variation of peak height of E layer
Bounsanto et al., (1995) and Titheridge, (1997) found that the calculated NmE = 1.24×104foE2 using conventional EUV solar flux models and dissociative recombination rate constants for NO+ and O2+ ion is 30-40% less than the observed value. Due to square law loss process, this 40% deficit in NmE implies a 100% increase in the total ionization rate at the E layer maximum. Ivanov-Kholodny and Nusinov (1979) used low αNO+ and αO2+values by Mul and McGoyan, (1979) along with [O2] scale height seasonal variations in the 100-110 km height range to overcome this problem. Later Antonova et al., (1996) took into account vibrationally excited NO+ and O2+ ions, which allowed them to explain NmE and hmE seasonal variations. Mikhailov et al., (1999) interpreted that the higher hmE values observed in winter than the summer results from thermosphere model MSIS-86 [O2] winter concentration being 40% larger than summer in the E-region considering 105 km height. A similar 40% increase takes place for [O], but atomic oxygen does not absorb these UV lines at all. The enhanced absorption of UV radiation in winter results in a general elevation of the E layer hm0 in expression 11. On the other hand, this increase is partly compensated by lower winter neutral temperature (200 K compared to 218 K in summer) as lower Tn gives less neutral scale height H in expression 11. Fig. 4 displays the measured values of the peak height of the E-layer, hmE and their absolute bias from IRI predictions which outputs hmE = 110 km. A close inspection of the observed values of 2009 (low solar activity with R = 10) reveals three distinctive patterns: (1) the magnitude of hmE remains fairly constant (between 98-104 km) during ±2 to ±4 h local noon, (2) hmE values are higher in winter (around 104-107 km) than those in spring, summer and autumn, (3) there are two maxima (around 104-110 km) near sunrise and sunset with a noontime minimum (around 98-101 km) in between. This is in agreement with the previous studies of mid-latitude ISR (Ivanov-Kholodny et al., 1998) and El Arenosillo Digisonde
11
(Mikhailov et al., 1999).
Ivanov-Kholodny et al., (1998) investigated the continuous
measurements of hmE with the 15-minute data by ISR and presented the results with statistical accuracy better than 1 km. Measurements show following magnitudes of hmE: (a) during winter: 118 and 121 km (b) during summer: 113 and 117 km at χ = 72◦ and 82◦ respectively. Digisonde observations in (Mikhailov et al., 1999) show that hmE varies from 103-120 km during winter and from 98-110 km during summer as the χ increases from 55◦ to 90◦. The three distinctive patterns described above are valid for all levels of solar activity in the subsequent years (2010-2016), which leads to the conclusion that the variation of hmE is not adequately described by Chapman’s theory of layer formation. During the entire period of study, significant differences between the observed hmE and the IRI predictions have been observed. IRI fails to predict since it outputs a constant hmE value of 110 km, which is higher than most of the observed values. Overestimation and underestimations range from 3 to 13 km and from 0 to 3 km respectively. Fig. 5 displays the standard deviations of the observed monthly median hmE values as a measure of dispersion. The standard deviations of the observed values of hmE are more or less constant (around 6 km from its monthly median value). Fig. 6 displays the diurnal plots of monthly median values of hmE observations for the month of October from 2009 to 2016 within solar cycle 24. During this period, morning time maximum appears around 06:15 to 07:00 LT, which is in good agreement with the October sunrise time in Cyprus and the height varies from 105.6-110 km. The second maximum near sunset appears around 15:30 to 16:15 LT, which is not in good agreement with the October sunset time in Cyprus and the height varies from 103.7-106.5 km. The noontime minimum appears around 10:45 to 11:15 LT and the height varies from 97.6 to 102.4 km. Differences in hmE between the maximum near sunrise and the noontime minimum ranges from 5.6 to 10.8 km. The difference in hmE between the sunset maximum and the noontime minimum ranges from 5 to 6.9 km. Therefore, we can conclude that the diurnal variation of hmE does not show any conspicuous change. As the solar cycle 24 progresses from low to high solar activity during 2009–2004, we notice an increase in the hmE values. During 2015-2016, hmE values decrease in accordance to the decreasing solar activity.
4. Conclusion
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In this study, we have investigated the E layer critical frequency (foE) and peak height (hmE) characteristics derived from 8 years (2009–2016) of manually scaled Digisonde measurements at the low-middle latitude European station in Nicosia, Cyprus and compared those to the corresponding IRI-2012 predictions. Our conclusive remarks are as follows: (a) Following the increase in solar radiation foE gradually increases from sunrise time, attains the maximum value around local noon, and gradually decreases to the minimum value during sunset. This diurnal variation of foE is observed in all seasons in all years under investigation, both at high and low solar activity period. Observed values of foE also demonstrate seasonal variation: high in June solstice, moderate in March and September equinox, low in December solstice. The magnitude of foE shows a clear longterm solar variation having the lowest value in the lowest solar activity year. As the solar activity intensifies, the magnitude of foE also increases and the diurnal and seasonal variations in the Digisonde observations and IRI predictions turn out to be more distinct. (b) In most of the months, IRI-2012 slightly overestimates the foE values, mostly limited to 0.25 MHz but occasionally can go as high as 0.3 MHz (equivalent electron density, 1.116×103 el/m-3). Very few cases of underestimation, though sporadic in nature limited to 0.25 MHz have also been noted.Throughout the year, the appearance of E layer shows a progressive pattern, with the longest duration of presence in June-July and a gradual decrease in the preceding and succeeding months, reaching minimum duration in December. This is expected as foE exhibits dependence on χ. IRI-2012 predictions are similar but longer (around 1-h early appearance and 1-h late disappearance) than those derived from Digisonde observations. Therefore, the time should be extended by 1 h both at the beginning and at the end of the duration of appearance of E layer as presently predicted by IRI-2012. (c) Observed values of hmE reveal three distinctive patterns: (1) the magnitude of hmE remaining fairly constant (between 98-104 km) during ±2 to ±4 h local noon, (2) hmE values being higher in winter (around 104-107 km) than those of spring, summer and autumn, (3) two maxima (around 104-110 km) near sunrise and sunset with a noontime minimum (around 98-101 km) in between.We can substantiate that the behaviour of hmE is not adequately described by Chapman’s theory of layer formation as mentioned in (Ivanov-Kholodny et al., 1998). During the entire period of study, significant differences between the observed hmE and the IRI predictions have been observed. IRI outputs a constant hmE value of 110 km, which is higher than most of the observed values. Therefore an effort to improve IRI model for better hmE representation is needed. 13
(d) The morning time hmE maximum appears around 06:15 to 07:00 LT, which is in good agreement with the October sunrise time in Cyprus. However, the second maximum near sunset appears around 15:30 to 16:15 LT, which is not in good agreement with the October sunset time in Cyprus. The differences in E layer peak height amongst the two hmE maxima and the noontime minimum range from just 5 to 10.8 km. (e) During our period of study, we note an increasing trend in hmE values as the solar cycle progresses from low to high solar activity during 2009-2014. During the rest of the period 2015-2016, hmE values decreases in accordance to the declining solar activity.
Acknowledgements The authors would like to thank the NASA’s National Space Science Data Center (NSSDC) for providing theIRI-2012 model (http://iri.gsfc.nasa.gov/). This work was supported by the Erasmus Mundus INTACT Project.
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Fig.1. Monthly median values of foE: (a) observations from Nicosia Digisonde (left panels) and (b) IRI-2012 predictions (centre panels), differences in foE: (c) between Digisonde observation and IRI predicted value (right panels), during 2009–16 in solar cycle 24.
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Fig. 2. Standard deviation of the monthly median values of foE observations from Nicosia Digisonde during 2009–16.
Fig. 3. Diurnal plots of monthly median values of foE for the month of October from 2009 to 2016.
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Fig. 4. Monthly median hmE observations from Nicosia Digisonde: (a) during 2009-2012 (left panels) and (c) during 2013-2016 (third panels from left), differences in hmE between Digisonde observation and IRI predicted value: (b) during 2009-2012 (second panel from left), (d) during 20132016 in solar cycle 24.
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Fig. 5. Standard deviation of the monthly median values of hmE observations from Nicosia Digisonde during 2009–16.
Fig. 6. Diurnal plots of monthly median values of hmE observations for the month of October from 2009 to 2016.
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Highlights
IRI slightly overestimates foE values both at low and high solar activity with mostly 0.25 MHz over-/under-prediction.
IRI outputs a constant hmE value of 110 km for all season and years, which is higher than most of the observed values.
Values of hmE are higher in winter than that of spring, summer and autumn.
hmE remains almost constant during ±2 to ±4 hours around local noon.
21