A Software Package Generating Long Term and Near Real Time Predictions of the Critical Frequencies of the F2 Layer over Europe and Its Applications

Long term prediction and near real time (nowcasting) maps of the critical frequency of the F2 layer (foF2), over the geographic area extending in latitude from 34 ̊N to 60 ̊N and in longitude from −5 ̊W to 40 ̊E, have been provided since 2006 by the DIAS (European Digital Upper Atmosphere Server) system. This work describes the software package PRODUCTION_DATABASE_foF2 & Extract_Real_Time_Grid_foF2 constituted by two original software packages called PRODUCTION_ DATABASE_foF2 and Extract_Real_Time_Grid_foF2 which have been developed in the framework of the ESA SSA Programme P2-SWE-1, in order to provide numerical grids of foF2 prediction on a mapping area more extended than that offered by the DIAS both in latitude (from 34 ̊N to 80 ̊N) and in longitude (from −10 ̊W to 40 ̊E). PRODUCTION_DATABASE_foF2, by means of the CCIR and SIRM models, provides a database of long term predictions of foF2 for all the months and the solar activities characterized by the 12-month smoothed mean value of the monthly sunspots number (R12) ranged between −50 and 150 with step = 1. On the basis of two effective sunspots numbers, R12eff_Northern and R12eff_Southern, which are calculated each hour using the foF2 autoscaled values provided by some reference stations, Extract_Real_Time_Grid_foF2 extracts from the database of long term predictions of foF2 the numerical grids representing the near real time ionospheric conditions for the hour and month under consideration. Some applications of PRODUCTION_DATABASE_ foF2 and Extract_Real_Time_Grid_foF2 in terms of long term forecast and nowcasting maps of foF2 are shown and their usefulness is discussed.


Introduction
The European Digital Upper Atmosphere Server (DIAS) is a particular infrastructure platform operating since 2006 providing digital data collection on the state of the ionized part of the atmosphere which knowledge is relevant for space weather purposes.A detailed information about the products and services offered by the DIAS system can be found at the portal http://dias.space.noa.gr.
Among the many products distributed by the DIAS system there are the long term forecast maps of the critical frequency of the F2 layer (foF2), which are produced from the predictions obtained applying the simplified ionospheric regional model (SIRM) [1] [2], improved under the COST 251 action [3], and the maps of foF2 for near real time ionospheric conditions (nowcasting), based on the SIRMUP predictions [4] [5].The mapping area extends in latitude from 34˚N to 60˚N and in longitude from −5˚W to 40˚E.
In the framework of the Space Situational Awareness (SSA) Programme P2-SWE-1 of the European Space Agency (ESA) (see the portal http://swe.ssa.esa.int/web/),supporting the development of the European Ionosonde Service (EIS) (see the portal http://ssa-be-vm-fe-09p.ssa.esa.int/web/guest/dias-federated),two original software tools were developed in order to provide numerical grids of foF2 on a mapping area more extended than that offered by the DIAS system both in latitude (from 34˚N to 80˚N) and longitude (from −10˚W to 40˚E), so including also the Scandinavian region.
The software package used for the achievement of this goal is PRODUCTION_DATABASE_foF2 & Extract_ Real_Time_Grid_foF2 which is constituted by two software packages called PRODUCTION_DATABASE_foF2 and Extract_Real_Time_Grid_foF2.PRODUCTION_DATABASE_foF2 is the software that by means of the CCIR [6] and SIRM models generates a database of long term predictions of foF2 for all the months and the solar activities represented by the 12-month smoothed mean value of the monthly sunspots number (R 12 ) ranged between −50 and 150 with step = 1.
Extract_Real_Time_Grid_foF2 is the software tool which is used to extract from the database achieved by the PRODUCTION_DATABASE_foF2 procedure, the numerical grid of foF2 values representing the nowcasting ionospheric conditions relative to a given hour and month.
A general description of how the database of long term predictions of foF2 is produced is given in Section 2. An explanation of how the nowcasting numerical grids of foF2 are derived from the database of long term predictions of foF2 is presented in Section 3. A comprehensive description of how PRODUCTION_DATABASE_ foF2&Extract_Real_Time_Grid_foF2 works is given in Section 4. Some applications of PRODUCTION_ DATABASE_foF2&Extract_Real_Time_Grid_foF2 are shown and discussed in Section 5.

Description of the Long Term Predictions Database of foF2 Produced by PRODUCTION_DATABASE_foF2
The achievement of a database of long term predictions of foF2 is fundamental because it constitutes the starting point from which numerical grids of foF2 related to nowcasting ionospheric conditions are then obtained.Long term predictions of numerical grids of foF2 have been obtained using simultaneously two different prediction models: 1) the CCIR global model developed by Comitè Consultatif International des Radio communications [6] was used to get predictions of foF2 in the northern part of Europe extending in latitude from 61˚N to 80˚N and in longitude from −10˚W to 40˚E, with a 1˚ × 1˚ resolution (high latitude grid); 2) As the performance of the simplified ionospheric regional model (SIRM) [1] [2] has been proven to be successful within the European area between 34˚N and 60˚N and −5˚W and 40˚E, the improved version of SIRM [3] was applied to generate foF2 forecasts in the southern part of Europe extending in latitude from 34˚N to 49˚N and in longitude from −10˚W to 40˚ E, with a 1˚ × 1˚ resolution (middle latitude grid).
A particular consideration was given in the latitude interval 50˚N -60˚N to which we refer hereafter as buffer zone, where foF2 predictions have been obtained applying both CCIR and SIRM model and then performing a simple linear interpolation giving a suitable weight to the prediction according to the latitude under consideration (see Table 1).
The final grid of long term predictions of foF2, is then obtained combining three grids: the high latitude grid, the buffer zone grid and the middle latitude grid (see Figure 1).
The long term predictions of foF2 are obtained providing as input parameters to the CCIR and SIRM model, the 12-month smoothed mean value of the monthly sunspots number (R 12 ) representing the solar activity index, and the month mm of the year.Varying from time to time R 12 from −50 to 150 (with step = 1) and mm from 1  (January) to 12 (December), the whole database formed by 2412 (201 values of R 12 × 12 values of mm) output files named SA_±rrrmm.dat(where ±rrr and mm indicate the sunspot number R 12 and the month respectively) is generated.Each output file includes 24 numerical grids of foF2 relative to the different hour of the day.The reason for which negative values of R 12 are also considered is explained in Section 3. Figure 1 shows an example of how the two output files SA_+00908.datand SA_+01308.datare structured after applying the PRO-DUCTION_DATABASE_foF2 tool.Further details about the procedure followed by PRODUCTION_DATA-BASE_foF2 to generate the 2412 output files SA_±rrrmm.datwill be provided in Section 4.1.

Achievement of the Nowcasting Numerical Grids of foF2 Produced by Extract_Real_Time_Grid_foF2
The attainment of nowcasting numerical grids of foF2 from the long term database of foF2 predictions, is based on the SIRMUP [4] [5] concept which is nothing but the SIRM model updated in real time when an effective sunspots number R 12eff is used as input parameter instead of R 12 .The real time updating is made on the base of autoscaled values of foF2 in some reference stations.In general, R 12eff is calculated from the autoscaled values of foF2 according to the method developed by Houminer et al. [7].R 12eff gives the best fit between model predictions and real observations of foF2 obtained from the autoscaling performed by the ionosondes positioned within a given area.Therefore R 12eff can be considered as the real sunspots number at the time when foF2 observations are automatically scaled.This means that the application of SIRM by using R 12eff as input parameter (SIRMUP), constitutes a real time updating which generates nowcasting predictions of foF2.
In order to extract nowcasting numerical grids of foF2 from the database constituted by the 2412 files above mentioned, two effective sunspots numbers named R 12eff_Northern and R 12eff_Southern are calculated separately at high and middle latitudes respectively.R 12eff_Northern is the effective sunspots number that gives the best fit between model predictions and real observations of foF2 obtained from the autoscaling performed by the two digisondes located at Sodankyla (67˚4'N, 26˚6'E) and Tromso (69˚6'N, 19˚2'E), which are the new two stations considered to expand the DIAS prediction capabilities to high European latitudes.Analogously, R 12eff_Southern is the sunspots number providing the best fit between background estimates and actual measurements of foF2 which are autoscaled in quasi real time from 8 European digisondes located at the ionospheric stations of Arenosillo (37˚1'N, 353˚3'E), Athens (38˚0'N, 23˚5'E), Ebre (40˚8'N, 0˚5'E), Rome (41˚9'N, 12˚5'E), Pruhonice (50˚0'N, 14˚6'E), Chilton (51˚5'N, 359˚4'E), Juliusruh (54˚6'N, 13˚4'E), and Moscow (55˚5'N, 37˚3'E) situated at middle latitudes and forming the standard Dias net.
It is worth noting that when the Houminer's procedure is applied for determining the effective sunspots number R 12eff on the basis of the autoscaled values of foF2, it emerges that also negative values of R 12eff are possible; this is the reason for which values of R 12 ranged between −50 and −1 with step = 1 were also considered for the creation of the database.R 12eff_Northern and R 12eff_Southern represent the real solar activity in the northern ad southern part of Europe respectively to which we can refer to nowcast foF2.
Therefore, once R 12eff_Northern and R 12eff_Southern have been determined, the Extract_Real_Time_Grid_foF2 software tool, pulls out from the 2412 files SA_±rrrmm.datan output file named RN± R 12eff_Northern _RS± R 12eff_Southern _mmm_hhh.datproviding a grid of nowcasting predictions of foF2 relative to a given month mm and hour hh that is formed by the union of 3 grids: the high latitude grid obtained for R 12eff_Northern , the buffer zone grid calculated interpolating in the latitude interval 50˚N -60˚N foF2 data derived from R 12eff_Northern and R 12eff_Southern , and the middle latitude grid obtained for R 12eff_Southern (see for example Figure 2).More details about the procedure followed by Extract_Real_Time_Grid_foF2.exe to generate the output files RN± R 12eff_Northern _RS± R 12eff_Southern _mmm_hhh.datwill be provided in Section 4.2.

Description of PRODUCTION_DATABASE_foF2&Extract_Real_Time_Grid_foF2
The software package PRODUCTION_DATABASE_foF2&Extract_Real_Time_Grid_foF2 is constituted by two software tools.The first called PRODUCTION_DATABASE_foF2 is lunched to create the database of long term predictions of foF2.The second called Extract_Real_Time_Grid_foF2.exe is launched to extract from the database previously generated numerical grids representing the nowcasting predictions of foF2.

PRODUCTION_DATABASE_foF2 Software Tool
The procedure implemented by PRODUCTION_DATABASE_foF2 outputs for each value of R 12 between −50 and 150 with step = 1 (hence 201 different values of R 12 ), and for each month mm ranged between 1 and 12, a file containing numerical grids of predicted values of foF2 for all the 24 hours in the area extending in latitude from 34˚N to 80˚N and in longitude from −10˚W to 40˚E with a 1˚ × 1˚ resolution.At the end of its execution a database of 201 × 12 = 2412 files named SA_±rrrmm.datis provided.
The PRODUCTION_DATABASE_foF2 software tool includes the file PRODUCTION_DATABASE_foF2.exe that runs a file batch named PRODUCTION_GRD_foF2.bat to produce the whole database.
As starting point the parameters of the input file SRIN must be initialized as follows: R 12 = −50, month mm = 1.After this initialization, PRODUCTION_DATABASE_foF2.exe is run and the procedure PRODUCTION_ GRD_foF2.bat is repeatedly executed for 2412 times.Each time an output file containing numerical grids of predicted values of foF2 for all the 24 hours is obtained for the value of R 12 and the month mm written in file SRIN which is from time to time automatically updated.Therefore, just to better clarify the question, the first time PRODUCTION_GRD_foF2.bat is run, predicted values of foF2 for R 12 = −50 for the month of January (mm = 1) are obtained for each hour and written in SA_-05001.dat,the second time PRODUCTION_GRD_ foF2.bat is run, SRIN is properly updated and predictions of foF2 relative to R 12 = −50 for February (mm = 2) are calculated again for each hour and written in SA_-05002.dat,the thirteenth time PRODUCTION_GRD_ foF2.bat is run, SRIN is updated automatically with R 12 = −49 and mm = 1 and SA_-04901.dat is generated.In this way the whole database is generated.
PRODUCTION_GRD_foF2.bat is constituted by the instructions del SROUT, del CCIR_GRD.dat,del SRmapf2, del SRmapm3, del CCIR.DAT, del buffer_zone.dat,which assure that any temporary file is deleted in order to avoid possible mistakes, and other seven instructions needed to run the executables which, for the sake of clarity, will be described in detail below.Figure 3 shows the procedure implemented by PRODUCTION_DATABASE_foF2.

ExtractReal_Time_Grid_foF2 Software Tool
Extract_Real_Time_Grid_foF2 For the sake of clarity, the main subroutines making up the Extract_Real_Time_Grid_foF2 software tool are described more in detail below.

Step 1: Subroutine North
The subroutine North is the first subroutine to be launched from the procedure implemented by Extract_Real_ Time_Grid_foF2.exe.When North works, it calls two secondary subroutines: 1) the subroutine chiama_settings reads from the input file settings.datand returns the name of the path where the database formed by 2412 files is placed (path_inp) and the name of the path where the user wants to put the output files (path_out) generated by Extract_Real_Time_Grid_foF2.exe; 2) the subroutine chiama_R_month_North which, after reading from the input file SRIN the values of R 12eff_Northern, R 12eff_Southern, the month and hour under consideration, returns the value of R 12eff_Northern and the month mm.R 12eff_Northern and mm are then used to "build" a file name (nomefile_built); for example if R 12eff_Northern = +009 and mm = 08, nomefile_built = SA_+00908.dat,while path_inp is used "to fish" the appropriate input file (nomefile_inp) from the database.The value hh also returned after the call of North will be used afterwards.
At this point a check is made: if nomefile_built = nomefile_inp, nomefile_inp = SA_+00908.datis opened as input file and a new file named N_nomefile_built = N_SA+00908.dat is built and opened as output file.As last step, foF2 predictions read from nomefile_inp = SA_+00908.datin the latitude intervals 80˚N -61˚N (northern part, CCIR foF2 predictions for R 12 = +009), and 60˚N -50˚N (buffer zone, SIRM&CCIR foF2 predictions for R 12 = +009) are written for the 24 hours in the output file N_SA+00908.dat.To further clarify how North works, an example of the procedure implemented by North is sketched in Figure 4.

Step 2: Subroutine South
The subroutine South is the second subroutine to be run from the procedure implemented by Extract_Real_ Time_Grid_foF2.exe.When South works, it calls two secondary subroutines: 1) the subroutine chiama_settings which reads from the input file settings.datand returns the name of the path where the database formed by 2412 files is placed (path_inp) and the name of the path where the user wants to put the output files (path_out) generated by Extract_Real_Time_Grid_foF2.exe; 2) the subroutine chiama_R_month_South which, after reading from the input file SRIN the values of R 12eff_Northern , R 12eff_Southern , the month and hour under consideration, returns the value of R 12eff_Southern and the month mm.R 12eff_Southern and mm are then used to "build" a file name (nome-file_built); for example if R 12eff_Southern = +013 and mm = 08, nomefile_built = SA_+01308.dat,while path_inp is used "to fish" the appropriate input file (nomefile_inp) from the database.
At this point a check is made: if nomefile_built = nomefile_inp, nomefile_inp = SA_+01308.datis opened as input file and a new file named S_nomefile_built = S_SA+01308.dat is built and opened as output file.As last step, foF2 predictions read from nomefile_inp = SA_+01308.datin the latitude intervals 34˚N -49˚N (southern part, SIRM foF2 predictions for R 12 = +013), and 50˚N -60˚N (buffer zone, SIRM&CCIR foF2 predictions for R 12 = +013) are written for the 24 hours in the output file S_SA+01308.dat.To further clarify how South works, an example of the procedure implemented by South is sketched in Figure 5.

Step 3: Subroutines Calc_Buff_Zone_h00-Calc_Buff_Zone_h23
In general, the 24 subroutines calc_buff_zone_h00, calc_buff_zone_h01, calc_buff_zone_h02… calc_buff_ zone_h21, calc_buff_zone_h22, and calc_buff_zone_h23 take as input foF2 predictions from the file N_SA± R 12eff_Northern mm.dat (for example N_SA+00908.dat) and S_SA± R 12eff_Southern mm.dat (for example S_SA+ 01308.dat)calculated in the buffer zone, and after executing a linear interpolation between foF2 predictions obtained with R 12eff_Northern and R 12eff_Southern according the weights shown in Table 2, output the file buffer_zone.datwhere are written the foF2 values for all the 24 hours.Figure 6 shows the procedure that outputs buffer_ zone.dat when the 24 subroutines work all together.
Figure 8 synthesizes the procedure implemented by Extract_Real_Time_Grid_foF2 described above for a case study characterized by R 12eff_Northern = + 009, R 12eff_Southern = + 013 and mm = 08 (August), when the value hh returned after the call of North is hh = 17.

Application of PRODUCTION_DATABASE_foF2&Extract_Real_Time_Grid_foF2: Some Results and Discussion
As already mentioned above, long term and nowcasting maps of foF2 released routinely by the DIAS system represent the final outputs which are generated when the SIRM and SIRMUP methods are respectively applied. Figure 9 shows an example of long term forecast map of foF2 (on the left) and nowcasting map of foF2 (on the right) over the European area extracted from the DIAS system.The web site of the DIAS system (http://dias.space.noa.gr), has a link (see the arrow in Figure 10) through which it is possible to have access, after you have registered, to the European Ionosonde Service (EIS) (see   Long term forecast and nowcasting maps of foF2, covering a more extended area than that provided by the DIAS system both in latitude (from 34˚N to 80˚N), and in longitude (from −10˚W to 40˚E) are routinely released from the EIS system, and represent the final outputs which are produced when the software packages PRO-DUCTION_DATABASE_foF2 and Extract_Real_Time_Grid_foF2 are respectively applied. Figure 12 shows an   example of long term forecast (on the left) and nowcasting map (on the right) of foF2 extracted from the EIS system.
The availability of long term forecast and near real time maps of foF2 is very important because foF2 is the main parameter describing the ionospheric conditions at a particular location, it also corresponds to the maximum radio frequency that can be reflected by the ionospheric F2 region at vertical incidence.This means that the contour plot of foF2 refers to the transmitted signals that are vertically incident on the ionosphere.Frequencies higher than those indicated on the contours pass through the ionosphere and get lost in space.Therefore foF2 prediction maps can be helpful to establish which are the frequencies that will always be returned to the Earth when a radio signal is transmitted vertically or in Near Vertical Incidence Skywave conditions (NVIS).More precisely, under quiet ionospheric conditions, long term predictions maps of foF2 provide good guidelines for the choice of frequencies to be used for all short distance communications on HF, i.e. in case of a station operating in NVIS mode; under disturbed ionospheric conditions, such as those often occurring after a geomagnetic storm event, nowcasting maps of foF2 can be exploited by the user as a guide to NVIS ionospheric frequency support.
Moreover, long term forecast and nowcasting maps of foF2 can also be used to get information about the frequencies to be used for all long-distance radio communications.Contours plot of lower/higher foF2 values indicate regions of the ionosphere capable of reflecting signals describing an oblique path over shorter/longer distances.
Finally, another important application which derives from the maps of foF2, is the possibility to estimate the Maximum Usable Frequencies (MUFs) which assure a radio link between the transmitter (Tx) to the receiver (Rx): knowing foF2 (from the foF2 maps) and the maximum height of reflection of the F2 layer, hmF2, (from the hmF2 maps) in the mid point of a given radio link Tx-Rx which distance is D, the secant of the optimum an-gle at which to broadcast the signal that has to be received at the distance D, (M(D)F2), can be easily calculated, so that MUF(D) can be determined by the user applying the approximate formula MUF(D)= foF2⋅M(D)F2 for any oblique circuit that can be of interest inside the mapping area under consideration.

Conclusions
The foF2 mapping over Europe provided by the DIAS and EIS system allows the users to know the maximum radio frequency that can be reflected by the ionospheric F2 region at vertical incidence, and it offers a guide both for the NVIS ionospheric frequency support and the estimate of the maximum usable frequencies for a given radio link on relatively long distances, under quiet and disturbed ionospheric conditions.
In particular, nowcasting maps are capable to provide a near real time description of the ionosphere under very disturbed geomagnetic conditions, thus offering an interesting approach to Space Weather forecast in the ionospheric domain.

Figure 4 .
Figure 4. Example of the flow diagram performed by the subroutine North for a case study characterized by R 12eff_Northern = +009 and mm = 08 (August).The output file N_SA+00908.dat is returned.

Figure 5 .
Figure 5. Example of the flow diagram performed by the subroutine South for a case study characterized by R 12eff_Southern = +013 and mm = 08 (August).The output file S_SA+01308.dat is returned.

Figure 8 .
Figure 8.In bold the flow diagram of the main program is summarized for the case study characterized by R 12eff_Northern = +009, R 12eff_Southern = +013, mm = 08 (August), and hh = 17.The output file labelled RN+009_RS+013_m08_h17.dat is returned.

Figure 9 .
Figure 9. (a) Example of long term forecast map of foF2 released by the Dias system for the epoch September 2014 at 11:00 UT over the European area extending in latitude from 34˚N to 60˚N and in longitude from −5˚W to 40˚E after applying SIRM method for R 12 = 75.2;(b) Example of nowcasting map of foF2 released by the Dias system for the epoch 10 September 2014 at 11:00 UT over the European area extending in latitude from 34˚N to 60˚N and in longitude from −5˚W to 40˚E after applying SIRMUP method.On the left are shown the value of R 12eff taken as input by SIRMUP and the autoscaled foF2 data from which R 12eff is calculated.

Figure 11 )
Figure 11), which is part of the Ionospheric Weather Services provided by the ESA SSA SWE (Space WEather) Portal.Long term forecast and nowcasting maps of foF2, covering a more extended area than that provided by the DIAS system both in latitude (from 34˚N to 80˚N), and in longitude (from −10˚W to 40˚E) are routinely released from the EIS system, and represent the final outputs which are produced when the software packages PRO-DUCTION_DATABASE_foF2 and Extract_Real_Time_Grid_foF2 are respectively applied.Figure12shows an

Figure 10 .
Figure 10.Login page of the DIAS system.The black arrow on the left shows the link by means of which the user can access to the EIS system (Figure 11).

Figure 11 .
Figure 11.Home page of the EIS portal: the black arrow at the top on the left panel indicates the products provided by the EIS system.

Figure 12 .
Figure 12.(a) Example of long term forecast map of foF2 released by the EIS system for the epoch September 2014 at 11:00 UT over the European area extending in latitude from 34˚N to 80˚N and in longitude from −10˚W to 40˚E produced by the PRODUCTION_DATABASE_foF2 software tool.(b) Example of nowcasting map of foF2 released by the EIS system on 10 September 2014 at 11:00 UT over the European area extending in latitude from 34˚N to 80˚N and in longitude from −10˚W to 40˚E after applying the Extract_Real_Time_Grid_foF2 procedure.The values of R 12eff_Northern and R 12eff_Southern , indicated in figure respectively as R 12eff high and R 12eff mid , and the autoscaled foF2 data from which R 12eff high and R 12eff mid are calculated are shown on the left.

Table 1 .
Weights given to the SIRM and CCIR predictions to calculate foF2 in the buffer zone.
provides nowcasting grids of foF2 for a given hour hh taking as input from the 2412 output files produced by PRODUCTION_DATABASE_foF2 two appropriate files on the basis of two solar activity indexes R 12eff_Northern and R 12eff_Southern , which are calculated taking into account the foF2 measurements autoscaled at the hour hh in the stations located at high and middle latitudes mentioned in Section 3. Once R 12eff_Northern and R 12eff_Southern have been calculated, they are written in the input file SRIN in which is also written the month mm and the hour hh under consideration.At each hour hh, when new autoscaled data of foF2 are available, new values of R 12eff_Northern and R 12eff_Southern are calculated and written in SRIN.Therefore SRIN is updated each hour so that, from time to time, new nowcasting numerical grids of foF2 are provided by Extract_ Real_Time_Grid_foF2 for the hour hh under consideration.