An ionospheric F-2 critical frequency database has been assembled to determine the variability of the F region as a function of local time, latitude, season, and geomagnetic activity. The database comprises observations from 75 ionosonde stations covering a range of geomagnetic latitude and includes 43 storm intervals. The database was previously used to develop the Storm-Time Empirical Ionospheric Correction Model (STORM). The mean and standard deviation have been evaluated by sorting the data by local time, season (five intervals centered on equinox, solstice, and intermediate intervals), latitude (four regions each 20 degrees wide in geomagnetic latitude), and up to eight levels of geomagnetic activity. The geomagnetic activity index was based on a weighted integral of the previous 33 hours of ap and is the same as that used by the STORM model. The database covers a full solar cycle, but insufficient information was available to sort by solar activity without compromising the estimates of variability on the other sorting parameters. About half the data were contained in the first level of geomagnetic activity, between 0 and 500 units of filtered ap corresponding to Kp <= 2, and half above that level. When local time dependence was included in the binning, sufficient data were available to sort into two levels of geomagnetic activity, quiet (Kp <= 2(+)) and disturbed (Kp > 3(-)). For all latitudes and levels of geomagnetic activity, the lowest variability was typically found in summer (10-15 (15-40 extremes. The exception was low latitudes at equinox, which had surprising low variability (10 interhemispheric flow at this time of year. At middle and low latitudes, the variability tended to increase with geomagnetic activity in winter and equinox but remained fairly constant in summer. At high latitudes, the surprising result was that in all seasons, and in winter in particular, the variability tended to decrease, probably because of the increased upwelling of neutral molecular species and stronger chemical control of the ionosphere. The data have also been used to build a table of estimated variability suitable for inclusion in the International Reference Ionosphere or any other climatological model. For periods where data were scarce or nonexistent, an estimated variability was provided on the basis of expectations of the consequences o physical processes. This was necessary to fill in the table o values in order to develop a module suitable for inclusion in th International Reference Ionosphere.
The main objective of DIAS (European Digital Upper Atmosphere Server) project is to develop a pan-European digital data collection on the state of the upper atmosphere, based on real-time information and historical data collections provided by most operating ionospheric stations in Europe. A DIAS system will distribute information required by various groups of users for the specification of upper atmospheric conditions over Europe suitable for nowcasting and forecasting purposes. The successful operation of the DIAS system will lead to the development of new European added-value products and services, to the effective use of observational data in operational applications and consequently to the expansion of the relevant European market.
Keywords: Ionosphere; Upper atmosphere; Ionospheric monitoring; Ionospheric nowcasting; Ionospheric forecasting; Digital libraries
Independent of the possible sources (solar activity, geomagnetic activity, greenhouse effect, etc.) of a global change in the upper atmosphere, it is the sign of a long-term trend of temperature that might reveal the cause of a global change.Long-term change of temperature in the F region of the ionosphere has been studied and is assumed to be expressed in terms of thickness of the bottornside F2 layer characterized by the difference between height of the maximum electron density of the F2 layer hmF2 and altitude of the lower boundary of the F region represented by h'F. Using the difference of two ionospheric parameters has the advantage that it reduces the effect of changes resulting from alteration of equipment and scaling personnel. In this study, in summer only night values of the difference hmF2-h'F and in winter both day and night values have been taken into account considering that h'F might indicate the lower boundary of the F region in these periods. The study of the behaviour of hmF2-h'F taking separately the stations and determining yearly the mean measure (trend) of the variation of hmF2-h'F with solar and geomagnetic activities found that this difference increases significantly with enhanced solar activity, but trends of the solar activity effect exerted on this difference themselves do not practically change with increasing sunspot number. Further, hmF2-h'F decreases only insignificantly with growing geomagnetic activity. Trends of the geomagnetic activity effect related to hmF2-h'F change only insignificantly with increasing Ap; however, trends of the geomagnetic activity effect decreased with increasing latitude.
As a result of this investigation it has been found that hmF2-h'F regarded as thickness of the bottornside F2 layer shows an effect of the change of solar activity during the last three solar cycles, indicating temperature change in the upper atmosphere to be expected on the basis of changing solar activity. Furthermore, though a long-term variation of solar activity considering only years around solar activity minima is relatively small, the difference hmF2-h'F indicates a trend opposing the change of solar activity; that is, it decreases slightly during the first three 20, 21, 22 solar cycle minima (1964-1986), but decreases more abruptly according to the change of solar activity towards the minimum of solar cycle 23 (1986-1996), thus also indicating variation of temperature in the F region. However, this variation cannot be explained by the change of solar and geomagnetic activities alone, but assumes some other source (e.g. greenhouse gases) too. (c) 2005 Elsevier Ltd. All rights reserved.
Intense late-cycle solar activity during October and November 2003 produced two strong geomagnetic storms: 28 October-5 November 2003 (October) and 1923 November 2003 (November); both reached intense geomagnetic activity levels, K-p = 9, and K-p = 8+, respectively. The October 2003 geomagnetic storm was stronger, but the effects on the Earth's ionosphere in the mid-latitude European sector were more important during the November 2003 storm. The aim of this paper is to discuss two significant effects observed on the ionosphere over the mid-latitude European sector produced by the November 2003 geomagnetic storm, using, data from ground ionosonde at Chilton (51.5 degrees N; 359.4 degrees E), Pruhonice (50.0 degrees N; 14.6 degrees E) and El Arenosillo (37.1 degrees N; 353.3 degrees E), jointly with GPS data. These effects are the presence of well developed anomalous storm E, layers observed at latitudes as low as 37 degrees N and the presence of two thin belts: one having enhanced electron content and other, depressed electron content. Both reside over the mid-latitude European evening sector.
The daily variability of the ionosphere can greatly affect HF or SATCOM communications. HF skywave operators plan frequency schedules months in advance, however, they also require daily knowledge of the ionospheric conditions in order to modify assignments. SATCOM operators also require daily information about the levels of scintillation, which are variations in phase, amplitude, polarisation and angle of arrival that can cause severe degradation of the received signal.Using a number of ionosonde measurements and geomagnetic and solar values, a Daily Ionospheric Forecasting Service (DIFS) has been developed, which provides HF and SATCOM operators with daily forecasts of predicted ionospheric conditions. The system uses in-house algorithms and an externally developed Global Ionospheric Scintillation Model (GISM) to generate HF and SATCOM forecasts. HF forecasts consist of a past summary and a forecast section, primarily displaying observed values and predicted categories for the Maximum Usable Frequency (MUF), as well as an Ionospheric Correction factor (ICF) that can be fed into the ionospheric propagation prediction tool, WinHF. SATCOM forecasts give predictions of global scintillation levels, for the polar, mid and equatorial latitude regions. Thorough analysis was carried out on DIFS and the results conclude that the service gives good accuracy, with user feedback also confirming this, as well.
The robustness of the aa geomagnetic index is of critical importance to the debate about the previously reported doubling of the solar coronal magnetic field in the last 100 years, inferred from an increasing trend in this index. To test the trend in aa, we have reconstructed the aa index using two long-running European stations (Sodankyla from 1914 and Niemegk from 1890) to provide data for the northern component of the index that are independent of data from the UK observatories used in the official'' aa index. Both the fully reconstructed'' aa series, based on Sodankyla ( 67 degrees N, L = 5.2 R-E) and Niemegk ( 52 degrees N, L = 2.3 R-E) data in combination with the official aa Southern Hemisphere data, confirm the increasing trend in the index. The Niemegk-based index shows little solar cycle variation in its deviation from the official index, probably because of the midlatitude location of the station. The high-latitude station, Sodankyla, is more affected by active geomagnetic conditions during solar maximum because of the proximity of the auroral oval to the station. Nevertheless, its index also clearly confirms the increasing trend in the aa index and hence supports the idea of a long-term increase in solar coronal magnetic field strength. As an added test, we reconstructed the aa index from a single site using data from two long-running UK stations, Eskdalemuir and Lerwick, applying a technique known as interhourly variation (IHV) proposed by Svalgaard et al. ( 2004). The resulting series is designed to be primarily sensitive to solar wind conditions. Both the reconstructed aa(IHV) also showed an increasing trend with time and high consistency with the official aa index. Overall, we conclude that the robustness of the trend in the aa index supports the idea of a long-term increase in solar coronal magnetic field strength.
A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s(1). Several mechanisms have been proposed to explain this connection(2-7), and evidence from modelling(8) as well as various types of measurements(9-14) demonstrate that lightning can interact with the lower ionosphere. It has been proposed, on the basis of a few observed events(15), that the ionospheric 'sporadic E' layer - transient, localized patches of relatively high electron density in the mid-ionosphere E layer, which significantly affect radio-wave propagation - can be modulated by thunderstorms, but a more formal statistical analysis is still needed. Here we identify a statistically significant intensification and descent in altitude of the mid-latitude sporadic E layer directly above thunderstorms. Because no ionospheric response to low-pressure systems without lightning is detected, we conclude that this localized intensification of the sporadic E layer can be attributed to lightning. We suggest that the co-location of lightning and ionospheric enhancement can be explained by either vertically propagating gravity waves that transfer energy from the site of lightning into the ionosphere, or vertical electrical discharge, or by a combination of these two mechanisms.
In New Zealand after World War 2 radar techniques were used in various investigations in geophysics and astronomy. Much local expertise had become available from defence laboratories, which had been set up in 1939 and eventually merged into the Radio Development Laboratory, disbanded in 1946. Wartime radar development had in turn been founded on pre-war research in radio propagation and ionospheric research which included the use of pulse ionosondes. Frequent support for the pre-war radio research in both Britain and New Zealand was given by Ernest Rutherford who, throughout his life, retained the interest from his own early researches in radio wave propagation. This paper is a brief survey of events from Rutherford's early experiments in 1894 to present-day research programmes.
Spatial maps of the ionosphere-plasmasphere slab thickness (T) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 1 degrees spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 35 degrees to 70 degrees N, and longitudes -10 degrees to 40 degrees E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS-TEC and ionosonde observations at solar minimum (1995-1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor (omega) is introduced as a ratio of disturbance to quietness representing area mean TEC,foF2 and tau for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and T maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but T enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main,phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate-the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS-T.EC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300km by night. (c) 2005 Elsevier Ltd. All rights reserved.
The long-term continuous increase of greenhouse gas concentration in the atmosphere and other anthropogenic influences represent serious threat for human civilization. Therefore, it is necessary to determine the long-term trends and changes in the atmosphere-ionosphere system. The observed long-term trends in the 20th century might be. however, influenced by contribution of Sun's origin, and the process of determination of anthropoger c trends from observational data may be spoilt by the 11-year solar cycle. The role of solar/geomagnetic activity in long-term trends in various regions of the atmosphere/ionosphere system is briefly reviewed for the first time. The ways; of avoiding or at least diminishing the effect of solar cycle on trend determination are mentioned. As for the possible solar and geomagnetic activity responsibility for part of the observed long-term trends. the two main conclusions are as follows: (i) The role of solar and geomagnetic activity in the observed long-term trends decreases with decreasing altitude from the F-region ionosphere down to the troposphere. (ii) In the 20th century the role of solar and geomagnetic activity in the observed long-term trends/changes was decreasing from its beginning towards its end.
Ionospheric imaging usually involves solving an underdetermined inversion problem. The inversion is performed involving additional constraints to enforce realistic profiles in the vertical. One way to incorporate those vertical profile constraints is to perform the inversion using Empirical Orthogonal Functions (EOFs). The need of defining a sample space spanned by EOFs to obtain ionospheric images yields the possibility to employ ionosonde measurements in ionospheric tomography based on stochastic inversion of GPS data. Here we present a simulation study based on an existing network of GPS ground receivers and ionosondes across Europe. The locations of the transmitters used in the simulation are actual satellite positions. Simulated GPS data, constructed assuming that the ionosphere is the international reference ionosphere, are inverted via the Multi Instrument Data Analysis System. The sample space of this stochastic inversion is constructed employing ionosonde measurements simulated from the same model ionosphere. Such use of ionosonde data to construct the sample space produces better results than without ionosonde data.
Keywords: Electromagnetic methods; Ionosphere; Radio propagation; Remote sensing; Radio tomography; Data inversion
The meridional propagation velocities of the ionospheric F2-region response to 268 geomagnetic storms are calculated. Ionospheric vertical sounding data of I h time resolution from several stations located in a longitude sector approximately centred along the great circle that contains both the geomagnetic poles and the geographic poles are used.Most meridional propagation velocities from high to low latitudes are less than 600 m/s. The smaller velocities are typical of global neutral meridional wind circulation and the larger are representative of traveling atmospheric disturbances.
Simultaneous disturbances at several locations are more frequent during positive phases than during negative phases. Negative phase meridional propagation velocities associated with meridional neutral winds are less frequent in the southern hemisphere when compared with corresponding velocities observed in the northern hemisphere. This may be related to the fact that the distance between the geomagnetic pole and the equator is smaller in the northern hemisphere.
Most negative phase onsets are within the 06-10 LT interval. For middle geomagnetic latitudes a forbidden time interval between 11 and 14 LT is present. The positive phase onsets show the dusk effect. (c) 2005 Elsevier Ltd. All rights reserved.
An instantaneous space-weighted ionospheric regional model (ISWIRM) for the regional now-casting of the critical frequency of the F2 layer ( foF2) has been developed. The geographical area of applicability of the model is ranged between 35degreesN - 70degreesN and 5degreesW - 40degreesE. Inside this region the hourly values of foF2 are obtained, correcting the monthly medians values of foF2 predicted by the space-weighted ionospheric local model ( SWILM) on the basis of hourly observations of foF2 coming from four reference stations ( Rome, Chilton, Lycksele, and Loparskaya ( or Sodankyla)). The performance of the model, evaluated at four testing stations ( Tortosa, Juliusruh, Uppsala, and Kiruna) during some periods characterized by strong solar and geomagnetic activity, can be considered satisfactory, given that the hourly values of the residuals are almost always below 1 MHz. A comparison between ISWIRM's performance using manually validated and autoscaled data of foF2 and SWILM's performance was made for two disturbed periods. One example of instantaneous ionospheric mapping of foF2 relative to the selected disturbed periods is also shown.
The increasing demand for upper-atmosphere nowcasting services for operational applications reveals the need for a realistic mapping of the ionosphere over Europe in real-time and especially during storm periods. To meet this need, a real-time updating method of simplified ionospheric regional model (SIRM) with autoscaled ionospheric characteristics observed by four European Digital Portable Sounders (DPS) ionosondes was recently developed. SIRM belongs to the group of ionospheric models for the standard vertical incidence (VI) ionospheric characteristics such as the critical frequency of the ionospheric F2 layer foF2 and the propagation factor M(3000)F2, which oversimplify a number of the ionospheric phenomena of real significance for radio communications applications showing satisfactory performance for median ionospheric condition description in restricted area of mid-latitudes. As a step forward, the rapid conversion of real-time data from four European digisondes to the driving parameters of the SIRM was introduced as the real-time SIRM updating (SIRMUP). In SIRMUP approach, the values of the ionospheric characteristics from first-guess model parameters at measurement points are combined with real-time measurements. The reliability of the real-time SIRM update method has already been tested in terms of the foF2 for various ionospheric conditions and the simulation results were very promising. In this paper, the simulation tests are continued in order to investigate the efficiency of the SIRMUP method in mapping the propagation conditions over Europe as they are expressed by the propagation factor M(3000)F2. In general, the results demonstrate that SIRMUP procedure has the potential to be used in real time for nowcasting the standard ionospheric characteristics over Europe, for operational applications.
Keywords: Ionospheric radio-propagation; Ionospheric mapping; Ionospheric modelling; Mid-latitude ionosphere
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