Decrease In foF2 Research Articles

Study of Ionospheric Variability During Super Substorms

This paper study variability of three ionospheric parameters foF2, h′F and hmF2 to investigate the middle latitude ionospheric effect at Boulder, Colorado, USA (40°N, l105.0° W) during super substorms (SSSs) of 24 August 2005, and 7 September 2017 and 8 September 2017 respectively. Continuous wavelet transform (cwt) implemented to identify the low and high frequency and longer and shorter duration present in the signal. The result shows decrease in foF2 during SSSs of 24 August 2005 and 8 September 2017 and increase in foF2 during 7 September 2017. The highest fluctuation in h′F is noticed during SSS of 24 August 2005. The cwt shows that the coupling between solar wind and magnetosphere occurs between ~ 16 to 32 minutes for SSS of 24 August 2005 and between 27.9 to 64 minutes during super substorm of 7 and 8 September 2017 for all the ionospheric parameters respectively. This study leads to understand the impact of SSSs on communication signals due to energy injected in ionosphere during the coupling mechanism between magnetosphere-ionosphere.

Simultaneous ionosonde investigations of the ionospheric F2 layer critical frequency and peak height at both ends of the geomagnetic tube

Based on the results of simultaneous ionosonde observations during low solar and weak magnetic activities, a coupling was found between diurnal and quasi-periodic variations in ionospheric parameters over magnetically conjugated regions, where the Ukrainian Antarctic Station (UAS) and Millstone Hill Observatory are located. A significant impact of the summer hemisphere on the nighttime variations of the F2 layer critical frequency foF2 in the magnetically conjugated region in the winter hemisphere was found. The most characteristic manifestation of this impact is the control of foF2 variations over the UAS not by the local sunset (sunrise), but by the sunset (sunrise) over Millstone Hill. It was found that the sunset over Millstone Hill leads to an increase in foF2 over the UAS, while the sunrise leads to a decrease in foF2 with a subsequent sharp increase. Both phenomena are associated with changes in the photoelectron flux from the northern hemisphere, corresponding changes in the electron temperature in the ionosphere above the UAS and the effect of these changes on the compression or rarefaction of the ionospheric plasma and changes in the plasmaspheric fluxes of H + ions. It was shown that the transition from nighttime to daytime conditions over both observation points was characterized by a significant decrease in the F2 layer peak height, and the difference in the values of this ionospheric parameter over Millstone Hill and UAS at night is due to seasonal differences in the thermospheric circulation and the difference in the behavior of the ionospheric parameters in the Northern and Southern hemispheres. Manifestations of atmospheric gravity waves, caused by the passage of local sunrise terminators, as traveling ionospheric disturbances with periods of about 90 and 75 – 120 mins over Millstone Hill and UAS, respectively, were found. These waves were most likely generated in the region located between the ionospheric F1 and F2 layers, where the sharp gradients in the electron and ion densities occur during changes in the intensity of solar radiation. It is confirmed that wave disturbances in atmospheric and ionospheric parameters can be transferred between magnetically conjugated regions by slow magnetohydrodynamic waves generated both at the heights of the ionospheric dynamo region due to the modulation of atmospheric and ionospheric parameters by atmospheric waves and the occurrence of external currents, and at the top of the plasmaspheric tube, where sharp plasma compression and heating or rarefaction and cooling occur during the passage of the solar terminator. Keywords: the ionosphere, F2 region, ionosonde measurements, geomagnetic field tube, magnetoconjugate region coupling, atmospheric gravity waves, traveling ionospheric disturbances, generation of slow magnetohydrodynamic waves

FoF2 Long-Term Trend at a Station Located near the Crest of the Equatorial Ionization Anomaly

Critical frequency foF2 long-term trends at Dakar station (14.4°N, 342.74°E) located near the crest of the equatorial ionization anomaly EIA, are analysed taking into account geomagnetic activity, increasing greenhouse gases concentration and Earth’s magnetic field secular variation. After filtering solar activity effect using F10.7 as a solar activity proxy, we determined the relative residual trends slopes α values for three different levels of geomagnetic activity. For example, at 1200 LT, the value of α goes from &#450.27%/year for very magnetically quiet days to -0.19%/year for magnetically quiet days and to -0.13%/year for all days. It appears from the slopes α obtained, that they increase with the level of geomagnetic activity and their negative values are qualitatively consistent with the expected decreasing trend due to the increase in greenhouse gases concentration but are greater than 0.003%/year which would result from a 20% increase in CO2 emissions which actually took place during the analysis period. Regarding Earth’s magnetic field magnitude, B secular variation and the dip equator secular movement, Dakar station is located near the crest of the equatorial ionization anomaly, Earth’s magnetic field magnitude, B decreases there and the trough approaches the position of Dakar during the period of analysis. These two phenomena induce a decrease in foF2 which is in agreement with the decreasing trend observed at this station.

Ionospheric Response to the St. Patrick's Day Space Weather Events in March 2012, 2013, and 2015 at Southern Low and Middle Latitudes

AbstractThe changes in critical frequency of the F2 layer (foF2) and foF2 deviation (ΔfoF2) have been determined for three geomagnetic storms in March of the years 2012, 2013, and 2015 at low‐latitude stations, Darwin (geomag. lat. 21.96°S) and Townsville (28.95°S), and midlatitude stations, Brisbane (36.73°S), Canberra (45.65°S), and Hobart (54.17°S). The moderate storm during 15–16 March 2012 (Dst = −87 nT) showed a decrease in foF2 at midlatitude and no effect at low‐latitude stations. For the intense storm of 17–18 March 2013 (Dst = −132 nT) and the super storm of 17–18 March 2015 (Dst = −222 nT), some middle‐ to low‐latitude stations showed a short‐duration increase in foF2, but all stations showed a long‐duration decrease in foF2 during the recovery phases with ΔfoF2% varying from 26% (Darwin) to 36.6% at Hobart for the March 2013 storm and above 40% for the March 2015 storm at all of the stations. Short‐duration (~2–4 hr) increase in foF2 seems to be associated with the prompt penetrating electric fields. Long‐duration (>6 hr) decrease in foF2 is mainly accounted to the decrease in thermospheric O/N2 density ratio because of storm‐induced high‐latitude circulation of gas with depleted O/N2 density ratio to lower latitudes and partly due to disturbance dynamo electric fields. A comparison of ionosonde given foF2 for equinoctial storms (March 2013 and 2015) with similar strength Southern Hemisphere winter storms (July 2012 and June 2015) has been made with the IRI‐2016 model foF2 for Darwin, Brisbane, and Canberra stations.

Changes in the middle and upper atmosphere parameters during the January 2013 sudden stratospheric warming

We present the results of complex obser-vations of various parameters of the middle and upper atmosphere over Siberia in December 2012 – January 2013, during a major sudden stratospheric warming (SSW) event. We analyze variations in ozone concentration from microwave measurements, in stratosphere and lower mesosphere temperatures from lidar and satellite measurements, in the F2-layer critical frequency (foF2), in the total electron content (TEC), as well as in the ratio of concentrations of atomic oxygen to molecular nitrogen (O/N2) in the thermosphere. To interpret the observed disturbances in the upper atmosphere, the experimental measurements are compared with the results of model calculations obtained with the Global Self-consistent Model of Thermosphere—Ionosphere—Protonosphere (GSM TIP). The response of the upper atmosphere to the SSW event is shown to be a decrease in foF2 and TEC during the evolution of the warming event and a prolonged increase in O/N2, foF2, and TEC after the SSW maximum. For the first time, we observe the relation between the increase in stratospheric ozone, thermospheric O/N2, and ionospheric electron density for a fairly long time (up to 20 days) after the SSW maximum at midlatitudes.

Изменения параметров средней и верхней атмосферы во время внезапного стратосферного потепления в январе 2013 г.

We present the results of complex obser-vations of various parameters of the middle and upper atmosphere over Siberia in December 2012 – January 2013, during a major sudden stratospheric warming (SSW) event. We analyze variations in ozone concentration from microwave measurements, in stratosphere and lower mesosphere temperatures from lidar and satellite measurements, in the F2-layer critical frequency (foF2), in the total electron content (TEC), as well as in the ratio of concentrations of atomic oxygen to molecular nitrogen (O/N2) in the thermosphere. To interpret the observed disturbances in the upper atmosphere, the experimental measurements are compared with the results of model calculations obtained with the Global Self-consistent Model of Thermosphere—Ionosphere—Protonosphere (GSM TIP). The response of the upper atmosphere to the SSW event is shown to be a decrease in foF2 and TEC during the evolution of the warming event and a prolonged increase in O/N2, foF2, and TEC after the SSW maximum. For the first time, we observe the relation between the increase in stratospheric ozone, thermospheric O/N2, and ionospheric electron density for a fairly long time (up to 20 days) after the SSW maximum at midlatitudes.

Ionospheric parameters as the precursors of disturbed geomagnetic conditions

Geomagnetic storms and substorms are the principal elements of the disturbed Space Weather conditions. The aim of the study was to reveal the ionospheric precursors that can be used to forecast geomagnetic disturbance beginning. To study the ionospheric processes before, during and after magnetic storms and substorms data from Sodankylä Geophysical Observatory was used (geomagnetic coordinates: 64.1oN, 119.2oE). In earlier works the Main Effect (ME) was revealed for substorms. It consists of the following steps: (a) the increase of critical frequency foF2 from its quiet median before and during the substorm growth phase, four-five hours before To moment that is the moment of the expansion phase onset, (b) the foF2 decrease to the level lower than its median just after To and until Te that is the moment of the end of the expansion phase, (c) the issue “a” repeated during the recovery phase (d) two bell-shape spikes in the cutoff frequency values foEs: first spike occurs three hours before To, second spike – during the expansion phase within the interval between To and Te. In the present work it is shown that ME manifestations can be used as precursors of magnetic substorms at high-latitudes (geomagnetic latitudes 50oN–65oN). In particular, the foF2 growth some hours before To can be used asa precursor of substorm development. The first foEs bell-shaped spike also can be used for short-term forecasting, two–three hours in advance of a substorm. Furthermore, the storms between 2008 and 2012 were studied. It was revealed that the similar ME also takes place in the case of magnetic storms but within the different time scale. More specifically, the first ME maximum in foF2 values occurs one-two days before the storm beginning and can be used as its precursor. In addition, the foEs spike takes place approximately ten hours before a storm and also can be used for the prediction of the storm beginning.

A severe negative response of the ionosphere to the intense geomagnetic storm on March 17, 2015 observed at mid- and low-latitude stations in the China zone

In this paper, the critical frequency of F2 layer of the ionosphere (foF2) and the total electron content (TEC) recorded at mid- and low-latitude observation sites near 120°E in the China zone were used to investigate the response to a severe geomagnetic storm on March 17, 2015 (the minimum Dst −223 nT at 23 UT). The results showed that the strong geomagnetic storm caused a massive effect on the ionosphere. The characteristics of foF2 and TEC did not show obvious perturbation during the main phase. Severe depletion of foF2 and TEC was observed at all stations during the storm recovery period. The maximum absolute discrepancy in TEC compared with the past 27-day average value was 78 TECU, and the minimum percentage deviations reached −71% at Fuzhou (26.1°N,119.3°E). The minimum percentage deviations of decrease in foF2 reached −65% at Sanya (18.1°N,109.3°E) and Mohe (53.5°N,122.3°E). This was an infrequent negative effect that foF2 and TEC sustained throughout the day with extremely low level on March 18. The O/N2 rate showed a distinct reduction on March 18 in the China zone, which may be mainly responsible for the severe depletion of foF2 and TEC. The spread-F seemed to be developed at first but was then suppressed to some extent during the main phase. During the recovery phase, the spread-F was suppressed at Sanya, while it developed at Wuhan and Mohe. The disturbance electric fields and thermospheric circulation may contribute to this phenomenon.

Trends in the critical frequency foF2 after 2009

The problem of how the deep minimum of solar activity of 2008–2009 influenced trends in the critical frequency of the F2 layer and whether the negative trends in foF2 are conserved in the following five years is considered in the paper. The initial data series used in previous publications by the authors are completed by data to 2014 (2015). Methods of searching for trends that were described in the past several times by the authors were applied. It was found that, in the vast majority of cases, the monotonous change in the curves used to derive the foF2 trend was distorted in years close to the anomalously low minimum of solar activity in 2008–2009. That decreased the obtained trend in the critical frequency as compared with the results based on the data to 2009. However, subsequent years again lead to a tendency toward monotonous decrease in foF2. The causes of the anomalies indicated above lie apparently in the inadequacy of the solar activity index F10.7 for the description of changes in the aeronomical parameters determining the foF2 behavior in the period of a very deep minimum of activity that is widely described in some publications.

Midlatitude ionospheric responses to the 2013 SSW under high solar activity

AbstractIonospheric responses to sudden stratospheric warming (SSW) are not well understood, particularly in the midlatitudes and under high solar conditions. During the 2013 SSW, ionospheric disturbances were observed in eight locations on the meridional chain from 30.5°N to 42.8°N in northern China. The midlatitude ionosphere responded strongly to the SSW despite being under high solar flux. The F2 layer maximum electric density increased by more than 80%, and the peak height was elevated more than 60 km. Well‐set and organized semidiurnal variations were recorded in early and middle January during the SSW in eight observation locations. The expected foF2 decrease in the afternoon hours was not clearly discernible; however, nighttime enhancements occurred frequently. The time‐period spectra of the average foF2 and zonal winds and meridional winds at altitudes of 86–95 km presented quasi‐16 day planetary wave‐like oscillations during the warming event. The coupling between the atmosphere and ionosphere may be strengthened by the quasi‐16 day waves. The amplified diurnal, semidiurnal, and terdiurnal tides in foF2 were also recorded during the warming, in good agreement with earlier observations. Importantly, the variations in the semidiurnal tides included a 16 day periodic component, indicating that the modulated semidiurnal tides may transmit these 16 day planetary wave‐like oscillations to the F region through wind dynamo. Although the PW‐tide interaction theory is not novel, it is of significance in the midlatitude ionospheric response to SSW.

Ionospheric effects of solar flares and their associated particle ejections in March 2012

Flares of March 4–9, 2012 were accompanied by an intensification of solar electromagnetic and corpuscular radiations and five coronal mass ejections. Bursts of X-rays and increased solar cosmic ray fluxes caused an increase in ionospheric absorption manifesting itself in data from vertical sounding stations as enhancements of the lowest frequency of reflections up to 4–6MHz at the daytime and as the disappearance of reflections in the ionograms of high latitude stations. Interplanetary coronal mass ejections (ICME) generated March 7–8 moderate and March 8–11 intense magnetic storms accompanied by ionospheric disturbances. At the peaks of both magnetic storms there were abrupt afternoon–evening decreases in the ionospheric F2-layer critical frequency (foF2). During the March 7–8 storm, the foF2 decrease concurred with the reversal of the interplanetary magnetic field azimuthal component (IMF By) which initiated restructuring of magnetospheric convection; during the March 8–11 storm, with the abrupt weakening of the interplanetary magnetic field southward component (IMF Bz) which triggered a substorm.

Long term changes in the ionosphere over Indian low latitudes: Impact of greenhouse gases

Increased concentration of greenhouse gases due to anthropogenic activities warm the troposphere and have a cooling effect in the middle and upper atmosphere. Ionospheric densities and heights are affected due to cooling. Carbon dioxide is one of the most dominant gases for the cause of long term ionospheric trends along with other radiatively active greenhouse gases. Regular ionospheric soundings are made over Ahmedabad (23.1°N, 72.7°E), since 1953. Long term changes in the ionosphere as a consequence of the cooling of the mesosphere and thermosphere due to the increased concentration of greenhouse gases have been studied. Ionospheric observations over Ahmedabad, a low latitude station in the anomaly crest region, for the years 1955–2003 are examined to study the long term changes in the critical frequencies of the various ionospheric layers and the height of the maximum ionization as characterized by hPF2. A decrease in foF2 (1.9MHz for midday, 1.4MHz for midnight) and hPF2 (18km for midday, 17km for midnight) during about five decades are noted. An increase is noted in foF1 (0.4MHz). The foF2 data are also examined over an equatorial station Kodaikanal (10.2°N, 77.5°E), situated near the magnetic equator for the years 1960–1995 and a decrease of 0.5MHz for midday and 0.7MHz for midnight are noted in ~35 years.

Long‐term trend of foF2 at a West African equatorial station linked to greenhouse gas increase and dip equator secular displacement

Long‐term variability of foF2 at Ouagadougou station (12.4°N, 358.5°E; dip, 1.45°), a West African equatorial station, is studied, analyzing an annual mean data series at 12 LT for the period 1966–1998. After filtering the solar activity effect using Rz as a solar activity proxy, a downward trend of −0.015 MHz/yr is obtained, that is a ~5% decrease during the period of 33 years here considered, equivalent to a ~0.2%/yr. The downward trend is qualitatively consistent with a decreasing trend expected from increasing greenhouse gas concentration but greater than the ~0.003%/yr that would result from the 20% increase in CO2 that actually took place during the period of analysis. The foF2 decreasing trend is not in agreement with the trend expected from the secular variation of the dip angle at the location of Ouagadougou, but here an additional mechanism is considered, that is the secular movement of the dip equator toward Ouagadougou. This implies an approaching of the trough of the equatorial ionization anomaly and thus an foF2 decrease at Ouagadougou that is qualitatively in agreement with the observed trend.

Variations in foF2 at the end of the 1990s and the beginning of the 2000s according to the SPIDR system data

Variations in the critical frequency of the F2 layer at 22 ionospheric stations within the period 1990–2010 according to the SPIDR system data are considered. A confirmation of the negative trends in foF2 for seven stations considered by one of the authors earlier on the basis of median data is obtained. It is found that both negative (a decrease in foF2 with time) and positive (growth in foF2 with time) trends of the critical frequency are observed. During the later part of the considered period (after 1997), negative trends dominate. This fact manifests itself, apparently, in an increase in the role of the decrease in the thermospheric neutral temperature in the formation of foF2 trends.

Relationship between variations in the characteristics of the high-latitude ionosphere and magnetic PC indices

It is shown in a joint analysis of ionospheric vertical sounding data at the arctic Heiss Island and antarctic Vostok stations and the geomagnetic PC index, which characterizes the geoefficient component of the interplanetary magnetic field, that, during a disturbed geomagnetic period when PC > 2 in years of solar activity (SA) maxima in the winter season, positive phases of ionospheric disturbances are predominantly observed. In the nighttime hours, an increase in the critical foF2 frequencies by a factor of 2–3 can occur. In a disturbed geomagnetic period at the PC > 1.5 level in the summer season, negative phases of ionospheric disturbances are mainly observed. In years of maximum and moderate SA, the decrease in foF2, as compared to their median values, happens at night (∼30%). In years of low SA, the decrease value is much lower. At a substantial decrease in the PC index level, in the region of the geomagnetic pole at the Vostok station, in some cases, a substantial increase in the electron density level in the F region occurs with a delay of 0.5 h. At the same time, a significant correlation (r = −0.57) is observed between variations in the PC index and foF2.

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Investigation of precursory phenomena in the ionosphere, atmosphere and groundwater before large earthquakes of M > 6.5

Signature of the 29 March 2006 eclipse on the ionosphere over an equatorial station

This study documents some results of the effect of the 29 March 2006 eclipse on the ionosphere over Ilorin, Nigeria (longitude 4.57°E, latitude 8.53°N, dip 4.1°S), an equatorial station in the West African region. The maximum obscuration of the eclipse at this station was 99 percent and it occurred before midday. True height electron density profile analysis below the F2 peak was employed in the study. The effect on the E and F1 layers was a drastic decrease in electron density, with maximum decrease percentages of 60 and 68 for the E and F1 layers, respectively. A decrease in foF2 began at about 1 hour 20 min after those in the lower layers had started. Variation of electron density with height showed that the decrease in the electron density occurred through out the E and F1 heights at about the same time while that of the F2 region began at lower heights and extended progressively toward the peak of electron density height of the layer. The recovery in the E and F1 layers has already reached an advanced stage before the effect of the eclipse got to the maximum in the F2 region. A major departure of hmF2 from the normal variation was observed and discussed.

Spatial and time variations in critical frequencies of the ionospheric F region above the zone of equatorial earthquake preparation

Spatial and time variations in the critical frequencies foF2 before the strong earthquake of August 15, 1963, with the magnitude M = 7.75 are analyzed. The epicenter of the earthquake was located in the vicinity of the magnetic equator in the American longitudinal sector. The data of the topside ionosonde on board Alouette-1 and of the series of ground-based ionosondes has been used for this purpose. The ground-based ionosondes made it possible to detect an insignificant anomalous decrease in foF2 within the zone of earthquake preparation a few days prior to the earthquake. This result confirms the conclusion drawn earlier on the basis of the satellite data. The modification of the ionosphere at the F-region level is more evident in the satellite than in the ground-based data. It is also noted that the character of the time variations in foF2 a day before the earthquake is similar to the so called “quiet time” Q-disturbances in the ionosphere, when the electron concentration at the F region maximum differs from the median values by more than 20% under undisturbed geophysical conditions.

F2-Region Response to Geomagnetic Storms at the Equatorial Anomaly During 1989–2001

The storm-time effects in the F2-region during the period of 1989–2001, at Ahmedabad (23.2°N, 72.4°E), a station at the ionization anomaly crest in the Indian region, are studied. In general, the negative storm effects (decrease in foF2) are more pronounced than the positive storm effects. Seasonally, the occurrence of negative storm effects is more in the summer and equinox as compared to that in the winter. Diurnal variation of storm effects shows that negative storm effects occur more often in the post-midnight period and less in the daytime. The negative storm effects associated with strong storms (Dst < -100 nT) and that occur in the post-midnight are mostly accompanied by a substantial increase in h'F. The complicated diurnal and seasonal variation of storm effects is explained in terms of (a) changes in E × B drifts (Fountain effect) associated with the zonal ionsopheric electric field perturbations due to prompt penetration of the magnetospheic electric fields to the low latitudes and the electric fields generated by the disturbance dynamo and (b) the downwelling of the thermospheric circulation due to storm-induced circulation. A possibility of changes in the neutral gas composition due to storm-induced circulation that may bring the gas with depleted [O]/[N2] ratio up to the latitude of Ahmedabad during strong storm events, is also expressed to explain the post-midnight negative storm effects accompanied by a substantial increase in h'F.

The time-weighted magnetic indices ap(τ), PC(τ), AE(τ) and their correlation to the southern high latitude ionosphere

In this investigation different data transformations of the ionospheric characteristic foF2 from high latitude stations in the Southern Hemisphere are considered and correlated with time weighted accumulation of the geomagnetic indices ap, AE and PC. The analysis, performed during periods of severe magnetic activity, shows a decrease in foF2 as a consequence of geomagnetic perturbations and a characteristic response recovery time t ranged between 15–20 hours. The best correlation coefficients are obtained by comparing the time weighted accumulation of the auroral index AE and a data transformation based on the ratio NmF2(t)/NmF2M where NmF2(t) is the hourly maximum electron density at the F2 peak layer and NmF2M is its monthly median value.