Effective Solar Indices for Ionospheric Modeling: A Review and a Proposal for a Real-Time Regional IRI

The first part of this paper reviews methods using effective solar indices to update a background ionospheric model focusing on those employing the Kriging method to perform the spatial interpolation. Then, it proposes a method to update the International Reference Ionosphere (IRI) model through the assimilation of data collected by a European ionosonde network. The method, called International Reference Ionosphere UPdate (IRI UP), that can potentially operate in real time, is mathematically described and validated for the period 9–25 March 2015 (a time window including the well-known St. Patrick storm occurred on 17 March), using IRI and IRI Real Time Assimilative Model (IRTAM) models as the reference. It relies on foF2 and M(3000)F2 ionospheric characteristics, recorded routinely by a network of 12 European ionosonde stations, which are used to calculate for each station effective values of IRI indices \(IG_{12}\) and \(R_{12}\) (identified as \(IG_{{12{\text{eff}}}}\) and \(R_{{12{\text{eff}}}}\)); then, starting from this discrete dataset of values, two-dimensional (2D) maps of \(IG_{{12{\text{eff}}}}\) and \(R_{{12{\text{eff}}}}\) are generated through the universal Kriging method. Five variogram models are proposed and tested statistically to select the best performer for each effective index. Then, computed maps of \(IG_{{12{\text{eff}}}}\) and \(R_{{12{\text{eff}}}}\) are used in the IRI model to synthesize updated values of foF2 and hmF2. To evaluate the ability of the proposed method to reproduce rapid local changes that are common under disturbed conditions, quality metrics are calculated for two test stations whose measurements were not assimilated in IRI UP, Fairford (51.7°N, 1.5°W) and San Vito (40.6°N, 17.8°E), for IRI, IRI UP, and IRTAM models. The proposed method turns out to be very effective under highly disturbed conditions, with significant improvements of the foF2 representation and noticeable improvements of the hmF2 one. Important improvements have been verified also for quiet and moderately disturbed conditions. A visual analysis of foF2 and hmF2 maps highlights the ability of the IRI UP method to catch small-scale changes occurring under disturbed conditions which are not seen by IRI.     

Letter to the editor

Using a method suggested by the authors earlier, the long-term trends of the F2-layer critical frequency, foF2 are derived for a set of ionospheric stations with a wide latitudinal and longitudinal coverage. All the trends are found to be negative. A pronounced dependence on geomagnetic latitude is found, the trend magnitude increasing with the latter. No globe scale longitudinal effect in trends is detected. For the majority of the stations there is also a pronounced seasonal effect, the trend magnitude being higher in summer than in winter.     

Mid-latitude ionospheric perturbation associated with the Spacelab-2 plasma depletion experiment at Millstone Hill

Elevation scans across geomagnetic mid latitudes by the incoherent scatter radar at Millstone Hill captured the ionospheric response to the firing of the Space Shuttle Challenger OMS thrusters near the peak of the F layer on July 30, 1985. Details of the excitation of airglow and the formation of an ionospheric hole during this event have been reported in an earlier paper by Mendillo et al.. The depletion (factor 2) near the 320 km Shuttle orbital altitude persisted for 35 min and then recovered to near normal levels, while at 265 km the density was reduced by a factor of 6; this significant reduction in the bottomside F-region density persisted for more than 3 hours. Total electron content in the vicinity of the hole was reduced by more than a factor of 2, and an oscillation of the F-region densities with 40-min period ensued and persisted for several hours. Plasma vertical Doppler velocity varied quasi-periodically with a 80-min period, while magnetic field variations observed on the field line through the Shuttle-burn position exhibited a similar 80-min periodicity. An interval of magnetic field variations at hydromagnetic frequencies (95 s period) accompanied the ionospheric perturbations on this field line. Radar observations revealed a downward phase progression of the 40-min period density enhancements of -1.12° km–1, corresponding to a 320-km vertical wavelength. An auroral-latitude geomagnetic disturbance began near the time of the Spacelab-2 experiment and was associated with the imposition of a strong southward IMF Bz across the magnetosphere. This created an additional complication in the interpretation of the active ionospheric experiment. It cannot be determined uniquely whether the ionospheric oscillations, which followed the Spacelab-2 experiment, were related to the active experiment or were the result of a propagating ionospheric disturbance (TID) launched by the enhanced auroral activity. The most reasonable conclusion is that the ionospheric oscillations were a result of the coincident geomagnetic disturbance. The pronounced depletion of the bottomside ionosphere, however, accentuated the oscillatory behavior during the interval following the Shuttle OMS burn.     

Non-trough foF2 enhancements at near-equatorial dip latitudes

Fine resolution series from three equatorial ionosondes of the IEEY network in West Africa have revealed small-scale daytime peak F2 structures, superposed on the slowly varying minimum or –trough distribution in the ±5° magnetic latitude zone. We report this new morphology, concentrating on foF2 enhancements of two types: near-equatorial crests (which travel either northwards or southwards) and magnetic field-aligned domes, whose onsets last only tens of minutes. Both types are observed to start at mid-morning or early afternoon hours. We relate their occurrence with the available variations of Vz = E × B upward drift which feeds the equatorial plasma fountain. We suggest the foF2 enhancements to be triggered by brief slow-downs of the Vz velocity near F2 peak altitude in our West African sector. Their short latitude extent differentiates them from the larger-scale tropical crest system. Further analysis of these features should lead to weather-like models of the low latitude ionosphere variations, where unstable local coupling between processes seems to be the trigger.     

Solar-cycle variation of the daily foF2 and M(3000)F2

Daily values of the ionospheric characteristics foF2 and M(3000)F2 for a given hour and month are correlated with the corresponding daily values of sunspot number using measured data collected at seven European locations. The significance of applying different-order polynomials is considered and the times are confirmed when the higher-order terms are important. Mean correlation coefficients for combined data sets over all hours, months and stations are determined, together with the standard errors of estimates. Comparisons are made with corresponding figures for monthly median values derived from the same data sets.     

Relation of the <Emphasis Type="Italic">F</Emphasis> region to stratospheric parameters: Possible relation to the atmospheric dynamics

Time variations in the correlation coefficient r(h, fo) between the h(100) stratospheric parameter and the foF2 ionospheric parameter for more than two solar activity cycles (1979–2004) are considered. It is obtained that, for daytime values of the correlation coefficient r(h, fo) when all available years are considered, the absolute value of r(h, fo) decreases in time; i.e., the correlation weakening from the 1980s to the 1990s. This weakening is interpreted as a weakening of the eastward winds in the MLT region of the atmosphere, where presumably a filtration of internal waves providing interrelation of various atmospheric layers occurs.     

Ionospheric measurements of relative coronal brightness during the total solar eclipses of 11 August, 1999 and 9 July, 1945

Swept-frequency (1/10 MHz) ionosonde measurements were made at Helston, Cornwall (50°06N, 5°18W) during the total solar eclipse on August 11, 1999. Soundings were made every three minutes. We present a method for estimating the percentage of the ionising solar radiation which remains unobscured at any time during the eclipse by comparing the variation of the ionospheric E-layer with the behaviour of the layer during a control day. Application to the ionosonde date for 11 August, 1999, shows that the flux of solar ionising radiation fell to a minimum of 25±2% of the value before and after the eclipse. For comparison, the same technique was also applied to measurements made during the total solar eclipse of 9 July, 1945, at Sörmjöle (63°68N, 20°20E) and yielded a corresponding minimum of 16 ± 2%. Therefore the method can detect variations in the fraction of solar emissions that originate from the unobscured corona and chromosphere. We discuss the differences between these two eclipses in terms of the nature of the eclipse, short-term fluctuations, the sunspot cycle and the recently-discovered long-term change in the coronal magnetic field.     

Automatic scaling of critical frequency <Emphasis Type="Italic">fo</Emphasis>F2 from ionograms recorded at São José dos Campos,Brazil: a comparison between Autoscala and UDIDA tools

This paper considers a dataset of ionograms recorded by the CADI ionosonde installed at São José dos Campos (SJC; 23.2°S, 45.9°W, magnetic latitude 14.1°S), Brazil, to compare two autoscaling systems: Autoscala, developed by the Istituto Nazionale di Geofisica e Vulcanologia, and the UDIDA-scaling, developed by the Universidade do Vale do Paraíba. The analysis, focused on the critical frequency of the F2 layer, foF2, shows that the two systems work differently. The UDIDA-scaling gives always a value of foF2 as output, regardless of the presence of the ionogram trace and its definition, while Autoscala tends to reject ionograms for which the digital information is considered insufficient. As a consequence, the UDIDA-scaling can autoscale more foF2 values than Autoscala, but Autoscala can discard a larger number of ionograms for which the trace is unidentifiable. Discussions are made on the accuracy of the foF2 values given as output, as well as on the main shortcomings characterizing the two systems.     

Revealing the classes of ionospheric disturbances on the basis of multiyear data on the critical frequency of the <Emphasis Type="Italic">F</Emphasis>2 layer

The bases of the classification method of ionospheric disturbances caused by solar-geomagnetic activity on the basis of the critical frequency of the F2 layer are developed. Data for the total solar activity cycle from 1975 to 1986 were used for studying variations in the critical frequency of the ionospheric F2 layer. The critical frequency was measured at the Moscow ionospheric observatory (55°45′N, 37°37′E) at an interval of 1 h. The gaps in the critical frequency values were filled in by the cubic interpolation method. The solar activity level was estimated using the F10.7 index. The geomagnetic disturbance was determined using the Kp · 10, Dst, and AE indices. According to the developed classification, an index of ionospheric activity is introduced. An analysis of the obtained values of the index for years of solar activity minimum and maximum shows that an increase in the absolute values of the index as a rule occurs at an increase in global geomagnetic and/or auroral disturbances. This fact indicates the sufficient information content of the developed index for characterizing ionospheric activity in any season. Moreover, using the sign of the index, one can form an opinion regarding an increase or decrease in the concentration of the ionospheric F2 layer, because the values of the considered index correspond to real oscillations in the critical frequency of the midlatitude ionosphere.     

Morphological aspects of a new type of counter electrojet event

The study describes the time and space morphologies of a rather new type of counter electrojet event on the basis of data from the excellent chain of magnetic and ionospheric observatories along the Indo-Russian longitude sector. Abnormally large westward currents are observed during almost the whole of the daytime hours on a series of days. These events do not form any vortices in the current system and do not apparently seem to be associated with tidal effects or any solar magnetosphere events or geomagnetic disturbances. The existence of a westward electric field over the equatorial ionosphere has been confirmed by the absence of an equatorial type of sporadic E in the ionograms at Thumba precisely during the periods when H at Trivandrum minus H at Alibag is negative. The equatorial F region anomaly was also absent on the counter electrojet day. Such counter electrojet events during the northern winter months of low solar activity years are suggested to be the result of the modified wind system in the ionosphere associated with stratospheric warming events.     

Spatial and time variations in the <Emphasis Type="Italic">foF</Emphasis>2 (night)/<Emphasis Type="Italic">foF</Emphasis>2 (day) ratio: Specification of some effects

A series of important aspects of the time and spatial variations in the critical frequency ratio fo(night)/fo(day) considered by one of the authors [Danilov, 2007, 2008] is specified. The list of the Eastern-Hemisphere stations, for which an analysis of the above indicated ratio was performed, is completed. The available stations of the Western Hemisphere are considered. It is shown that the character of the variations in the fo(night)/fo(day) ratio is independent of limitations imposed on the Ap index of magnetic activity for the analyzed days. The consideration of the long-term variations in geomagnetic activity using the annual mean value of the Ap index does not influence the principal conclusions of the study, although in some cases changes statistical characteristics of the obtained behavior of the fo(night)/fo(day) ratio after 1980. A comparison of the results, based on the interpretation of the trends of the fo(night)/fo(day) value with the changes in the stratosphere-ionosphere coupling from the 1980s to the 1990s obtained by the authors earlier, confirms the assumption that there occurs a systematic change in the zonal wind in the upper atmosphere.     

Semiannual and annual variations in the height of the ionospheric F2-peak

Ionosonde data from sixteen stations are used to study the semiannual and annual variations in the height of the ionospheric F2-peak, hmF2. The semiannual variation, which peaks shortly after equinox, has an amplitude of about 8 km at an average level of solar activity (10.7 cm flux = 140 units), both at noon and midnight. The annual variation has an amplitude of about 11 km at northern midlatitudes, peaking in early summer; and is larger at southern stations, where it peaks in late summer. Both annual and semiannual amplitudes increase with increasing solar activity by day, but not at night. The semiannual variation in hmF2 is unrelated to the semiannual variation of the peak electron density NmF2, and is not reproduced by the CTIP and TIME-GCM computational models of the quiet-day thermosphere and ionosphere. The semiannual variation in hmF2 is approximately isobaric, in that its amplitude corresponds quite well to the semiannual variation in the height of fixed pressure-levels in the thermosphere, as represented by the MSIS empirical model. The annual variation is not isobaric. The annual mean of hmF2 increases with solar 10.7 cm flux, both by night and by day, on average by about 0.45 km/flux unit, rather smaller than the corresponding increase of height of constant pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle variations of thermospheric winds. Although geomagnetic activity, which affects thermospheric density and temperature and therefore hmF2 also, is greatest at the equinoxes, this seems to account for less than half the semiannual variation of hmF2. The rest may be due to a semiannual variation of tidal and wave energy transmitted to the thermosphere from lower levels in the atmosphere.     

Studie über Höhenschwankungen der F2-Schicht: II

Zusammenfassung Die scheinbaren Minimalhöhen der F2-Schicht (h F2) zeigen eine charakteristische Abhängigkeit von der jeweiligen Sonnentätigkeit mit einer Doppelperiode innerhalb eines Sonnenfleckenzyklus. Nur für die peruanische Station Huancayo ergeben die statistischen Berechnungen eine einfache Periode mit grossen Höhen für die untere Grenze der F2-Schicht zur Zeit geringer Sonnentätigkeit und mit einer tiefliegenden unteren Begrenzung zum Sonnenfleckenmaximum.

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The virtual heights of the ionospheric F2-layer depends on the solar activity characteristically with a double-period for one sunspots-cycle. Only at the Peruvian station Huancayo appears a single period with great heights at small solar activity and with deep-seated boundary of F2-layer at strong solar activity. *** DIRECT SUPPORT *** A2919023 00002

     

Simultaneous ionospheric and magnetospheric observations of azimuthally propagating transient features during substorms

During the 6^th August 1995, the CUTLASS Finland HF radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s. Data from such scans, during the substorm expansion phase, revealed pulses of equatorward flow exceeding 600 m s^–1 with a duration of 5 min and a repetition period of 8 min. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. These transient features, which propagate eastwards away from local midnight, have been interpreted as ionospheric current vortices associated with fieldaligned current pairs. The present study reveals that these ionospheric convection features appear to have an accompanying signature in the magnetosphere, comprising a dawnward perturbation and dipolarisation of the magnetic field and dawnward plasma flow, measured in the geomagnetic tail by the Geotail spacecraft, located at L = 10 and some four hours to the east, in the postmidnight sector. These signatures are suggested to be the consequence of the observation of the same field aligned currents in the magnetosphere. Their possible relationship with bursty Earthward plasma flow and magnetotail reconnection is discussed.     

DIAS effective sunspot number as an indicator of the ionospheric activity level over Europe

DIAS (European Digital Upper Atmosphere Server) effective sunspot number — R12eff was recently introduced as a proxy of the ionospheric conditions over Europe for regional ionospheric mapping purposes. Although a pre-processing step for the real-time update of the Simplified Ionospheric Regional Model (SIRM) to real-time conditions, R12eff is available in real time by DIAS system (http://dias.space.noa.gr) for independent operational use. In this paper we discuss the efficiency of R12eff to specify ionospheric conditions over Europe. For this purpose, the diurnal R12eff’s reference pattern was determined on monthly basis and for different solar cycle phases. The deviation of the real-time R12eff estimates from the reference values, ΔR12eff was found to be highly correlated with the foF2 storm-time disturbances, especially during large scale effects indicating that DIAS-R12eff can provide a reliable estimator of the ionospheric activity level over a substantial part of Europe and a powerful tool for ionospheric specification applications.     

Sunrise effects in the lowerD-Region by solar eclipse

Summary In contrast to the well known fact that the ionospheric absorption minimum falls behind the maximum phase of the solar eclipse, in the present work the reverse effect is discovered and analysed, namely that the ionospheric absorption minimum falls before the maximum phase of the eclipse, while the maximum absorption comes behind it. Such anomalies can be observed only on paths of long waves at oblique incidence of the layer (length of the path over 1300 km). The theoretical analysis shows that the established effects are due to the considerable increase of the factor of negative ions, respectively of the effective recombination coefficient in the first phase of the eclipse for heights between 65 and 70 km, i. e. in the lower boundary of theD-Region (CR-Layer). In the second phase of the eclipse falls rapidly because of which at a constant electron production inCR-layer a strong increase of the absorption is observed similar to the known sunrise effects in theD-Region. The electron drift is intensified about the maximum phase of the eclipse.     

Ionospheric total electron content and critical frequencies over Europe at solar minimum

The paper presents results obtained by analyzing high-resolution ionospheric vertical total electron content (vTEC) data set evaluated from a chain of European ground-based Global Positioning System (GPS) stations and its equivalent slab thickness, as well as the F2-layer critical frequency foF2 and propagation factor M(3000)F2 from nearby ionosonde stations over the period 2006–2007. The study covers data within an area between 36°N and 68°N geographic latitude, and 7°W and 21°E geographic longitude during these last two years of minimum solar activity in the 23rd solar cycle. It reveals 15 extraordinary events, all of which exhibited some form of large short-lived vTEC and foF2 enhancements of the duration of small-magnitude solar-terrestrial events. The results clearly show a well-defined vTEC and foF2 storm-like disturbance patterns developed under these conditions. They prove that there are still some open questions related to the large electron density variations during weak disturbances that require additional study for both their relevance to different Global Navigation Satellite Systems (GNSS) applications and their role in the formation and evolution of the daytime ionosphere at middle latitudes.     

The lunar tide in sporadic E

It seems that the wind shear theory is accepted for the explanation of sporadic E at mid and low latitudes. Some examples from Arecibo are displayed to show this. The effect of lunar tides should then modify the wind-shear theory in a manner that yields the observed features of the lunar tide in the critical frequency foEs and the height h′Es of the sporadic E. This is shown to imply that the phase of the lunar tide in h’Es should be the same as the phase of the lunar tide in the eastward wind and that the phase of the lunar tide in foEs is three hours later. Hourly values of foEs, fbEs (the blanketing critical frequency) and h′Es from several observatories are analysed for the lunar semidiurnal tide. It is found that the phase of the tide in foEs is often about 3 hours later than for h′Es in agreement with the theory. Seasonal variations in the tide are also examined with the statistically most significant results (largest amplitudes) usually occurring in summer. After reviewing the many difficulties associated with determining the lunar tide in Es, both experimentally and theoretically, the analysed phase results are compared with what might be expected from Hagan’s global scale wave model. Agreement is only fair (a success rate of 69% among the cases examined) but probably as good as might be expected.     

Hydroxyl (6/2) airglow emission intensity ratios for rotational temperature determination

OH(6/2) Q1/P1 and R₁/P₁ airglow emission intensity ratios, for rotational states up to j = 4.5, are measured to be lower than implied by transition probabilities published by various authors including Mies, Langhoff et al. and Turnbull and Lowe. Experimentally determined relative values of j transitions yield OH(6/2) rotational temperatures 2 K lower than Langhoff et al., 7 K lower than Mies and 13 K lower than Turnbull and Lowe.     

Long-term trends in the ratio of the daytime and nighttime values of <Emphasis Type="Italic">foF</Emphasis>2

The consideration of the relation between the daytime and nighttime values of the critical frequency F2, foF2 of the ionospheric F2 layer, started in the previous publication of the authors, is continued. The main regularities in variations in the correlation coefficient R(foF2) characterizing this relation are confirmed using larger statistical material (more ionospheric stations and longer observational series). Long-term trends in the R(foF2) value are found: at all stations the negative value of R(foF2) increases with time after 1980.