Daytime Ionosphere Research Articles

Comparison between GPS radio occultation electron densities and in situ satellite observations

Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Global Positioning System (GPS) radio occultation (RO) electron densities are compared with collocated in situ observations from the CHAllenging Minisatellite Payload (CHAMP) and Communications/Navigation Outage Forecasting System (C/NOFS) satellites. The comparison is restricted to observations occurring within 2∘ latitude and longitude and 15 min local time. The in situ observations occur at altitudes of ∼300–800 km, and the results of the present study represent the first global comparison of COSMIC electron densities at altitudes ranging from near the F region peak to the topside ionosphere. The correlation coefficient between the COSMIC and in situ observations is greater than 0.90, indicating an overall good agreement between GPS RO electron densities and CHAMP and C/NOFS satellite observations. Furthermore, when averaged over all latitudes and local times, we find a near-zero mean bias and root-mean-square difference of typically less than ±30% between the COSMIC electron densities and collocated in situ observations. The overall good agreement demonstrates that the COSMIC GPS RO observations provide an accurate measure of electron density in the topside ionosphere. The results also reveal a systematic structure to the error in the equatorial and low-latitude daytime ionosphere. This structure is related to the equatorial ionization anomaly and is consistent with the error introduced by the Abel inversion spherical symmetry assumption used to retrieve the COSMIC electron density profiles. The present study thus provides direct observational evidence of the Abel inversion error on GPS RO electron densities.

Electric field penetration into the ionosphere in the presence of anomalous radon emanation

The scientists came to consensus that electric field driven mechanism is more probable to explain ionospheric anomalies before earthquakes than the acoustic-driven mechanism (Pulients and Davidenko, 2014), and it is essential to understand how a vertical electric field from the ground penetrates into the ionosphere. Anomalous radon emanation in the epicentral area is believed to change the atmospheric electrical conditions. Considering the effect of radon emanation on atmospheric conductivity, the electric potential equation is established and solved numerically in the presence of a vertical electric field of 1kV/m on the ground. The results show that radon emanation can strengthen atmospheric conductivity, as a consequence, the resulting electric field is increased by about 60% in the daytime ionosphere. However, the resulting electric field in the ionosphere is very weak (only about 0.3μV/m), which implies that the penetration of vertical electric field of 1kV/m in the seismic area is unlikely to produce daytime ionospheric anomalies before earthquakes.

The calculation of ionospheric absorption with modern computers

New outcomes are proposed for ionospheric absorption starting from the Appleton–Hartree formula, in its complete form. The range of applicability is discussed for the approximate formulae, which are usually employed in the calculation of non-deviative absorption coefficient. These results were achieved by performing a more refined approximation that is valid under quasi-longitudinal (QL) propagation conditions. The more refined QL approximation and the usually employed non-deviative absorption are compared with that derived from a complete formulation. Their expressions, nothing complicated, can usefully be implemented in a software program running on modern computers. Moreover, the importance of considering Booker’s rule is highlighted. A radio link of ground range D=1000km was also simulated using ray tracing for a sample daytime ionosphere. Finally, some estimations of the integrated absorption for the radio link considered are provided for different frequencies.

Ionospheric response to magnetic activity at low and mid-latitude stations

The F2-layer response to the moderate storm of 5–7 April 2010 was investigated using data from two equatorial stations (Ilorin: lat. 8.5°N, 4.5°E; Kwajalein: lat. 9°N, long. 167.2°E) and mid-latitude (San Vito: lat. 40.6°N, long. 17.8°E; Pruhonice: lat. 50°N, long. 14.6°E). Before storm commencement, enhancement, and depletion of NmF2 values were observed in the equatorial and mid-latitude stations, respectively, indicating the latitudinal dependence of the pre-storm event. All the stations with the exception of Kwajalein show positive phase in NmF2 response at the storm onset stage. Positive phase in NmF2 continues over Ilorin and appears on the daytime ionosphere of Kwajalein on 6 April, whereas negative phase suppressed the positive feature in Pruhonice and San Vito until the recovery condition. The differences in the response of F2-layer to the storm for the two equatorial stations were attributed to their longitudinal differences. On the average, both the AE and D st indices revealed poor correlation relationship. More studies are required to ascertain this finding.

Ionospheric redistribution during geomagnetic storms.

[1]The abundance of plasma in the daytime ionosphere is often seen to grow greatly during geomagnetic storms. Recent reports suggest that the magnitude of the plasma density enhancement depends on the UT of storm onset. This possibility is investigated over a 7year period using global maps of ionospheric total electron content (TEC) produced at the Jet Propulsion Laboratory. The analysis confirms that the American sector exhibits, on average, larger storm time enhancement in ionospheric plasma content, up to 50% in the afternoon middle-latitude region and 30% in the vicinity of the high-latitude auroral cusp, with largest effect in the Southern Hemisphere. We investigate whether this effect is related to the magnitude of the causative magnetic storms. Using the same advanced Dst index employed to sort the TEC maps into quiet and active (Dst<−100 nT) sets, we find variation in storm strength that corresponds closely to the TEC variation but follows it by 3–6h. For this and other reasons detailed in this report, we conclude that the UT-dependent peak in storm time TEC is likely not related to the magnitude of external storm time forcing but more likely attributable to phenomena such as the low magnetic field in the South American region. The large Dst variation suggests a possible system-level effect of the observed variation in ionospheric storm response on the measured strength of the terrestrial ring current, possibly connected through UT-dependent modulation of ion outflow.

Oscillations detected near the lower boundary of the Venus ionosphere from the radio occultation measurements of the Venera-15 and Venera-16 satellites

The possibility of detecting wave processes in the atmosphere-ionosphere system of the planet from perturbations of radio signals in radio occultation experiments is studied. A method of interpretation of the radio-raying data that does not employ integral transforms and is well-adapted to problems of detecting refractive index oscillations observed during sensing of the stratified gas envelope of the planet is presented. Periodic oscillations of the radio signal power that are linearly related with the rate of variation in the frequency of the signal used for probing of the atmosphere and ionosphere are found, which evidences wave perturbations of the medium near the lower boundary of the daytime ionosphere of Venus.

A theory of ionospheric response to upward‐propagating tides: Electrodynamic effects and tidal mixing effects

The atmospheric tide at ionospheric heights is composed of those locally generated and those propagated from below. The role of the latter in producing the variability of the daytime ionosphere is examined using the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. The impact of upward‐propagating tides is evaluated by running simulations with and without tidal forcing at the lower boundary (approximately 96 km), which imitates the effect of tides from below. When migrating diurnal and semidiurnal tides at the lower boundary is switched on, the intensity of E region currents and the upward velocity of the equatorial F region vertical plasma drift rapidly increase. The low‐latitude ionospheric total electron content (TEC) first increases, then gradually decreases to below the initial level. The initial increase in the low‐latitude TEC is caused by an enhanced equatorial plasma fountain while the subsequent decrease is due to changes in the neutral composition, which are characterized by a global‐scale reduction in the mass mixing ratio of atomic oxygen O1. The results of further numerical experiments indicate that the mean meridional circulation induced by dissipating tides in the lower thermosphere is mainly responsible for the O1 reduction; it acts like an additional turbulent eddy and produces a “mixing effect” that enhances net downward transport and loss of O1. It is stressed that both electrodynamic effects and mixing effects of upward‐propagating tides can be important in producing the variability of ionospheric plasma density. Since the two mechanisms act in different ways on different time scales, the response of the actual ionosphere to highly variable upward‐propagating tides is expected to be complex.

Variability in ionospheric total electron content at Mars

The Mars Express (MEX) mission includes a multi-purpose radio instrument called the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When used in its ionospheric-penetrating subsurface sounder (SS) radar mode, a by-product of the MARSIS observations is the ray-path-integral of electron densities, called the total electron content (TEC). We have used the initial TEC database of approximately 1.2 million TEC values spanning the period June 2005 to September 2007 to study the basic characteristics of TEC morphology and variability. We find quantitative agreement between the TEC values measured and those computed from model simulations of global diurnal behavior. With the basic photo-chemistry of the martian ionosphere a well understood process, it is the departures from average conditions that need specification and modeling. Here we use MARSIS TEC to do this quantitatively. We explore the specification of variability using different ways to define it: standard deviations from sample averages versus departures from control curves.For global studies, we computed the standard deviation (σ in %) of mean values of TEC (in TECU of 1015e−/m2) sorted by latitude, longitude, solar zenith angle (SZA), local time, season, and locations with/without strong crustal magnetic fields (50nT at 150km). For daytime conditions (SZA<75°), the global average 〈TEC〉 is ~6 TECU with σ=~20%, while for nighttime (SZA>105°) 〈TEC〉 is ~0.3 TECU with σ=~75%. Daytime variability is enhanced in the latitude region 0–30°S, a pattern that needs validation by later observations before its source can be identified. Nighttime variability is noticeably larger in regions of strong crustal magnetic fields (B)—an effect noted by previous authors.For regional studies, high resolution latitude patterns of variability in the southern hemisphere – within the longitude sector 150–210° of strong crustal-B values – were computed as percentage changes with respect to zonally-averaged patterns outside the region of interest. We present evidence for the first time of B-fields affecting the variability of the daytime ionosphere by small amounts (~±5%). Under nighttime conditions, the B-field associated variability is ~±20%. The results also reveal an anti-correlation between daytime and nighttime variability ordered by the inclination angle (I) of the B-fields. TEC variability is greater as I approaches vertical at night, but higher during the day (by smaller amounts) where I approaches horizontal patterns.

Study of total electron content variations over equatorial and low latitude ionosphere during extreme solar minimum

In recent years with the advancement in satellite based navigational applications, study of Total Electron Content (TEC) has gained significant importance. It is well known that due to dynamical behaviour of equatorial and low latitude ionosphere, the levels of ionization is relatively high herein. The sustained decrease in solar extreme ultraviolet radiations during the current minimum is greater than any in recent history. This gives us the opportunity to study the observations of global positioning system total electron content (GPS-TEC) dual frequency signals from the GPS satellites continuously recorded at Trivandrum (an equatorial station) and Delhi (a low latitude station) during the extremely low solar activity period from January 2007 to June 2009. This study illustrates the diurnal, seasonal and annual variations of TEC during the extended solar minimum period. This study also investigates the behaviour of daytime ionosphere around spring and autumn equinoxes at low solar activity period. The results clearly reveal the presence of equinoctial asymmetry which is more pronounced at equatorial station Trivandrum. The diurnal variation of TEC shows a short-lived day minimum which occurs between 0500 to 0600 LT at both the stations. Delhi TEC values show its steep increase and reach at its peak value between 1200 and 1400 LT, while at the equator the peak is broad and occurs around 1600 LT. Further, the daily maximum TEC ranges from about 5 to 40 TEC units at Trivandrum and about 10 to 40 TEC units at Delhi, which correspond to range delay variations of about 1 to 8 m at the GPS L1 frequency of 1.575 GHz. The Maximum values of TEC were observed during spring equinox rather than autumn equinox, showing presence of semi annual variation at both the locations. The minimum values of TEC were observed during the summer solstice at Trivandrum indicating the presence of winter anomaly at equatorial region while Delhi TEC values were minimum during winter solstice showing absence of winter anomaly. Also the TEC values at both the locations have been decreasing since 2007 onwards exhibit good positive correlation with solar activity.

DaytimeDregion parameters from long-path VLF phase and amplitude

[1] Observed phases and amplitudes of VLF radio signals propagating on very long paths are used to validate electron density parameters for the lowest edge of the (D region of the) Earth's ionosphere at low latitudes and midlatitudes near solar minimum. The phases, relative to GPS 1 s pulses, and the amplitudes were measured near the transmitters (∼100–150 km away), where the direct ground wave is dominant, and also at distances of ∼8–14 Mm away, over mainly all-sea paths. Four paths were used: NWC (19.8 kHz, North West Cape, Australia) to Seattle (∼14 Mm) and Hawaii (∼10 Mm), NPM (21.4 kHz, Hawaii) and NLK (24.8 kHz, Seattle) to Dunedin, New Zealand (∼8 Mm and ∼12 Mm). The characteristics of the bottom edge of the daytime ionosphere on these long paths were found to confirm and contextualize recently measured short-path values of Wait's traditional height and sharpness parameters, H′ and β, respectively, after adjusting appropriately for the (small) variations of H′ and β along the paths that are due to (1) changing solar zenith angles, (2) increasing cosmic ray fluxes with latitude, and (3) latitudinal and seasonal changes in neutral atmospheric densities from the (NASA) Mass Spectrometer Incoherent Scatter- (MSIS-) E-90 neutral atmosphere model. The sensitivity of this long-path (and hence near-global) phase and amplitude technique is ∼ ± 0.3 km for H′ and ∼ ± 0.01 km−1 for β, thus creating the possibility of treating the height (H′ ∼70 km) as a fiduciary mark (for a specified neutral density) in the Earth's atmosphere for monitoring integrated long-term (climate) changes below ∼70 km altitude.

Analysis of daytime ionosphere behavior between 2004 and 2008 in Antarctica

In this paper we present 5-year study of the lower ionosphere behavior obtained from VLF sounding done at Comandante Ferraz Brazilian Antarctic Station. The results suggested a strong influence of ‘meteorological processes’ especially during wintertime, when VLF measurements showed that the D-region was strongly affected by planetary waves in all years. This effect was superposed to the well known control by solar radiation. The 16-day wave was the most important PW modulating the lower ionosphere behavior. Since planetary waves are very predominantly of tropospheric origin, this result is an evidence of the vertical couplings in the atmosphere–ionosphere system.

Daytime ionosphericDregion sharpness derived from VLF radio atmospherics

[1] We described and applied a technique to measure the local midlatitude daytime ionospheric D region electron density profile sharpness from the Earth-ionosphere waveguide mode interference pattern in the spectra of radio atmospherics (or sferics for short), which are the high-power, broadband, very low frequency (VLF, 3–30 kHz) signals launched by lightning discharges. VLF propagation simulations are used to show that the upper VLF frequency spectral minima of sferics on several hundred kilometers long propagation paths depend critically on the effective D region sharpness while depending only weakly on the effective D region height. This enables the straightforward extraction of the sharpness parameter from measured VLF spectra, which generally exhibit well-defined minima at upper VLF frequencies. By applying this technique, we calculated the profile sharpness during morning, noontime, and afternoon periods in 3 different days using sferics from ∼660–800 km away. The measured sharpness showed a weak dependence on the solar zenith angle, with values between 0.35 and 0.45 km−1 for angles from 20° to 75°. This is different from the previous narrowband measurement since the sharpness derived from narrowband VLF signals highly depends on the solar zenith angle. To better understand this discrepancy, we also used simulations to calculate the equivalent exponential profiles for International Reference Ionosphere (IRI) profiles and the empirical FIRI model profiles. The equivalent exponential profiles can best duplicate the sferic spectral characteristics for IRI and FIRI models. We find that both the magnitudes and solar zenith angle variations of the sharpness for our broadband measurements, previous narrowband measurements, and both models are completely different. This suggests the daytime ionosphere, particularly at larger solar zenith angles, may not be well described by a simple two-parameter exponential model.

Lower ionosphere model for noon quiet conditions and conditions of sudden ionospheric disturbances according to the data on VLF propagation

The parameters of a model of the lower daytime ionosphere for quiet conditions and conditions of SID within the 25–75 km height interval are estimated according to experimental SPA data (at frequencies of 11.9 and 13.6 kHz), the strength of electromagnetic fields (15–25 kHz), and the phase velocity (10.2 kHz) obtained at various long paths within the 40° S–40° N latitude band for a period of high solar activity (the Wolf number is 100). At heights of 45–75 km, the created model provides the electron concentration profile and altitude dependence of the parameter of the loss coefficient type. At heights of 25–45 km, the model gives (in the cold plasma approximation) an equivalent description of the dependence on height of the ion concentration. On the basis of six samplings of VLF data formed over two-month intervals, seasonal variations of the model parameters are estimated. Joint consideration of the data for quiet and disturbed conditions and also insertion of the effective ion “layer” with a concentration maximum at a height of 35 km is a peculiar features of the model creation method.

Sounding of the ionosphere disturbed by the “Sura” heating facility radiation using signals of the GPS satellites

We present the results of experimental studies of the properties of the plasma-density disturbances created during heating of the ionospheric F2 region by high-power HF radio waves radiated by the “Sura” heating facility (Radiophysical Research Institute, Nizhny Novgorod). These experiments are specific in that they were performed in a sunlit (daytime) ionosphere when the generation of ionospheric turbulence has specific features and the turbulence intensity level is low enough. The plasma-density disturbances induced by high-power HF radio emission were sounded by signals of the GPS satellites, the line of sight to which crossed different parts of the disturbed ionosphere region. Threshold powers of the excitation of artificial plasma-density variations as well as spatial, temporal, spectral, and energy characteristics of the generated disturbances are determined.

Asymmetric response of the topside ionosphere to large-scale IGW generated during the November 30, 1979, substorm

We used bottomside ground observations and topside sounding data from the Intercosmos-19 satellite to study a Travelling Ionospheric Disturbance (TID) that occurred in response to Large-Scale Internal Gravity Wave (LSIGW) propagation during a substorm on November 30, 1979. We built a global scheme for the wavelike ionospheric variations during this medium substorm (AE max ∼800 nT). The area where the TID was observed looks like a wedge since it covers the nighttime hours at subauroral latitudes but contracts to a ∼02 h local sector at low latitudes. The ionospheric response is strongly asymmetric because the wedge area and the TID amplitude are larger in the winter hemisphere than in the summer hemisphere. Clear evidence was obtained indicating that the more powerful TID from the Northern (winter) hemisphere propagated across the equator into the low latitude Southern (summer) hemisphere. Intercosmos-19 observations show that the disturbance covers the entire thickness of the topside ionosphere, from h mF2 up to at least the 1000 km satellite altitude at post-midnight local times. F-layer lifting reached ∼200 km, N e increases in the topside ionosphere by up to a factor of ∼1.9 and variations in N mF2 of both signs were observed. Assumptions are made concerning the reason for the IGW effect at high altitudes in the topside ionosphere. The relationship between TID parameters and source characteristics determined from a global network of magnetometers are studied. The role of the dayside cusp in the generation of the TID in the daytime ionosphere is discussed. The magnetospheric electric field effects are distinguished from IGW effects.

Ionosphere around equinoxes during low solar activity

The seasonal behaviors of the ionosphere have been investigated for several decades, but the differences of the ionosphere between the March and September equinoxes are still an open question. In this analysis we utilize the data of ionospheric electron density (Ne) profiles from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission radio occultation measurements, total electron density (TEC) from TOPEX and Jason‐1, and TEC from Global Positioning System (GPS) receivers as well as global ionosonde measurements of the F2 layer peak electron density (NmF2) to investigate the behaviors of the daytime ionosphere around equinoxes during low solar activity (LSA). The analysis reveals that during LSA the equinoctial asymmetry in ionospheric plasma density is mainly a low‐latitude phenomenon. The differences of equinoctial TEC and NmF2 have considerable amplitudes at low latitudes in both hemispheres and less significant at higher latitudes. With increasing altitude, the asymmetry in COSMIC Ne becomes weaker in the Southern Hemisphere, and the northern pronounced asymmetry regions move toward the magnetic equator. The ionospheric equinoctial asymmetry may be considered as a manifestation of the annual variation, whose annual phase significantly shifts away from the solstices. The F layer peak height (hmF2) extracted from COSMIC Ne profiles also shows an equinoctial asymmetry at low latitudes, indicating the existence of equinoctial differences in low‐latitude neutral winds, specifically in the Northern Hemisphere. It reveals that, besides the important effect of the neutral wind, other processes should play roles in the forming of the observed equinoctial asymmetry in the ionosphere.

Amplitude and frequency variations of coherent radio signals during probing of the daytime Venusian ionosphere

A new procedure for analyzing the data obtained during radio probing of the Venusian ionosphere is proposed. The experimental results of the two-frequency probing with the use of the “Venera 15” and “Venera 16” orbiters that provide evidence that the daytime Venusian ionosphere has a layered structure are presented. It is shown that there exists a lower part of the daytime ionosphere at altitudes of 80 to 120 km.

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.

An Emulation Research on the Radio Occultation Exploration of Martian Ionosphere

The technical system of the Sino-Russian joint satellite-to-satellite Mars ionosphere occultation is analyzed and introduced. The analogue computation of the observed values of the radio waves of the ionosphere occultation event is carried out by adopting the three-dimensional ray tracking method and the electron density profile inversion is conducted by means of the simulated occultation observational data, with the result showing that the emulation algorithm is reliable. By taking advantage of the emulation method the case computation and analysis of the inversion errors caused by the observational error of the occultation radio wave phase and the satellite orbital error are respectively carried out, and it is obtained from the result that the effect of the phase measuring error of the 5% circle on the result of the daytime ionosphere occultation exploration may be neglected, while the absolute error of the night electron density measurement is less than 4 × 108 m −3, and the main effect of the satellite orbital error on the occultation leads to the lifting or falling of the ionospheric height. The result shows that the technical system of the Sino-Russian joint Mars ionosphere occultation exploration is advanced. It can be expected that the high accuracy electron density profile is obtained and the technical system can be applied to the exploration of the lunar ionospheric environment.

Cross modulation of whistler mode and HF waves above the HAARP ionospheric heater

[1] The HF facility of the High Frequency Active Auroral Research Program (HAARP) is employed to generate ELF/VLF radiation by modulation of overhead auroral electrojet currents for magnetospheric wave injection and propagation in the Earth-ionosphere waveguide. HAARP induced ducted magnetospherically amplified whistler mode signals are observed to return to the vicinity of the HF facility and experience cross modulation with simultaneous HF transmissions. The cross modulation effect results from modification of the attenuation properties of the ionospheric medium by HAARP HF waves in the daytime ionosphere. The cross modulation concept is subsequently investigated as a new method for ELF/VLF wave generation with the HAARP heating facility using two co-located HF beams. The new generation method is observed to generate radiation from 630 Hz–37 kHz. Observed amplitudes are an order of magnitude lower than fundamental frequencies generated using conventional AM, but of the same order as AM second harmonics.