Solar wind governing the response of Venusian atmosphere and ionosphere

The ionosphere of Venus is primarily formed by photoionization of a gaseous blanket around Venus. The impact ionization by energetic solar charged particles also plays an important role in the variability of Venusian ionospheric ion, electron density and their temperature profiles. The microscopic variations in the solar wind velocity, particle flux and orientations of frozen-in interplanetary magnetic field determine the solar wind interaction with the Venusian ionosphere. The ion and electron density profiles obtained by Pioneer Venus Orbiter and Pioneer Venus Entry Probes have been analysed in the light of simultaneous solar wind velocity and particle flux. Marked changes in height profiles of ion, electron densities and their temperatures have been found to correlate with the simultaneous changes in the solar wind velocity and particle flux. It is shown that the solar wind plays a more important role in controlling the physical properties and behavior of daytime as well as nighttime ionosphere of Venus, whereas the solar xuv sustains the primary ionization process.     

Effects of large-scale magnetic fields in the Venus ionosphere

Theoretical models of the ionosphere of Venus have been constructed in the past without due consideration of the fact that the ionosphere is sometimes magnetized. This paper examines some differences between the magnetized and unmagnetized dayside Venus ionosphere using the Pioneer Venus Orbiter Langmuir probe and magnetometer data. Particular attention is given to the evaluation of the altitude profiles of the thermal electron heating and comparison of the magnitude of the magnetic force(¯vׯB) ׯB with other forces in the ionosphere. Several examples illustrate how heating profiles are different in the magnetized ionosphere with effective heating below ～200 km altitude reduced by orders of magnitude compared to the field-free ionosphere. The force associated with the magnetic field is comparable to other forces in the magnetized ionosphere. The measured plasma density, electron temperature and magnetic field thus suggest that large-scale magnetic fields should be included in future ionosphere models.     

Empirical model of the composition of the Venus ionosphere: Repeatable characteristics and key features not modeled

In-situ measurements of positive ion composition of the ionosphere of Venus are combined in an empirical model which is a key element for the Venus International Reference Atmosphere (VIRA) model. The ion data are obtained from the Pioneer Venus Orbiter Ion Mass Spectrometer (OIMS) which obtained daily measurements beginning in December 1978 and extending to July 1980 when the uncontrolled rise of satellite periapsis height precluded further measurements in the main body of the ionosphere. For this period, measurements of 12 ion species are sorted into altitude and local time bins with altitude extending from 150 to 1000 km. The model results exhibit the appreciable nightside ionosphere found at Venus, the dominance of atomic oxygen ions in the dayside upper ionosphere and the increase in prominence of atomic oxygen and deuterium ions on the nightside. Short term variations, such as the abrupt changes observed in the ionopause, cannot be represented in the model.     

Temporal and spatial variations observed in the ionospheric composition of Venus: Implications for empirical modelling

In situ measurements of the thermal ion composition of the ionosphere of Venus have been obtained for a period of two Venus years from the Bennett rf ion mass spectrometer on the Pioneer Venus Orbiter. Ion measurements within an altitude interval of 160 to 300 kilometers, corresponding to an overall latitude interval of about ?4° to 34°N, are assembled from the interval December 1978 to March 1980. This time interval corresponds to two revolutions of Venus about the Sun, designated as two “diurnal cycles”. The distributions of several ion species in this data base have been sorted to identify temporal and spatial variations, and to determine the feasibility of an analytical representation of the experimental results. The first results from the sorting of several prominent ions including O⁺, O₂⁺, and H⁺ and several minor ions including CO₂⁺, C⁺, and H₂⁺ reveal significant diurnal variations, with superimposed modulation associated with solar activity and solar wind variations. The diurnal variation consists of strong day to night contrast in the ion concentrations, with differences of one to two orders of magnitude, depending upon ion mass and altitude. The concentrations of O₂⁺, O⁺, CO₂⁺ and C⁺ peak throughout the dayside decreasing sharply at the terminators to nightside levels, lower by one to two orders of magnitude relative to the dayside. The diurnal variations of the light ions H⁺ and H₂⁺ peak during the night, exhibiting asymmetric nightside bulges favoring the pre-dawn sector, near 0400 solar hour angle. Superimposed upon the diurnal distributions are modulation signatures which correlate well with modulation in the F_10.7 index, indicating a strong influence of solar variability on the ion production and distribution. The influence of solar wind perturbations upon the ion distributions are also indicated, by a significant increase in the scatter of the observations with increasing altitude as higher altitudes, approaching 300 kilometers, are sampled. Together, these temporal and spatial variations make the task of modelling the ionosphere of Venus both very interesting and challenging.     

The properties of the low altitude magnetic belt in the Venus ionosphere

When the solar wind dynamic pressure is high, the Venus ionosphere usually contains a belt of steady magnetic field at the very lowest altitudes to which Pioneer Venus probes. The current layer that flows on the high altitude side of this low altitude belt is centered at an altitude which ranges from 170 to 190 km with a most probable altitude of 182 km. This altitude is independent of solar zenith angle and hence the current system is flowing horizontally rather than vertically as proposed by Cloutier and co-workers. The lower edge of the magnetic belt was probed only on the lowest altitude passes of Pioneer Venus. This boundary is even more stable in location. The belt has decayed to 90% of its maximum strength usually by 162 km and to 50% of its maximum strength by 155 km. We interpret these data to indicate that the observed magnetic structure of the Venus ionosphere is a product of temporal evolution rather than of spacecraft motion through a spatially varying static structure.     

Electron densities and temperatures in the Venus ionosphere: Effects of solar EUV,solar wind pressure and magnetic field

The Venus ionosphere is influenced by variations in both solar EUV flux and solar wind conditions. On the dayside the location of the topside of the ionosphere, the ionopause, is controlled by solar wind dynamic pressure. Within the dayside ionosphere, however, electron density is affected mainly by solar EUV variations, and is relatively unaffected by solar wind variations and associated magnetic fields induced within the ionosphere. The existence of a substantial nightside ionosphere of Venus is thought to be due to the rapid nightward transport of dayside ionospheric plasma across the terminator. Typical solar wind conditions do not strongly affect this transport and consequently have little direct influence on nightside ionospheric conditions, except on occasions of extremely high solar wind dynamic pressure. However, both nightside electron density and temperature are affected by the presence of magnetic field, as in the case of ionospheric holes.     

Latitudinal influences on the quiet daytime D-region

Four versions of a steady-state quiet D-region model are presented. They differ from each other as a result of latitudinal differences in total neutral particle concentrations, nitric oxide concentrations and cosmic ray ionization rates. The total ion concentration profiles of all four versions have minima near 70 km which range from about 10⁸ m^?3 at high latitudes to 3.5 × 10⁷ m^?3 at equatorial latitudes for a solar zenith angle of 60°. Neutral density differences among the four cases result in important vertical shifts for the respective D-region profiles relative to one another. A “C-layer” is evident for the high and mild-latitude models at large solar zenith angles. The altitude where the negative ion/electron concentrations ratio is unity varies from about 63 to 67 km. The computed results are compared briefly with the extensive data base in the literature.     

Magnetic flux ropes in the Venus ionosphere: In situ observations of force-free structures?

Force-free magnetic structures with cylindrical geometry appear under a variety of conditions in nature. Filamentary helical magnetic structures are observed to be associated with prominences and flares in the solar atmosphere, and can arise in superconductors and laboratory plasmas. Another example of cylindrical quasi-force-free configurations appears to exist in the Venus ionosphere. Magnetic flux ropes with diameters of ～20 – 30 km have been observed by the Pioneer Venus Orbiter to be a nearly ubiquitous feature of the dayside Venus ionosphere. Models of flux ropes suggest that many of these structures tend to be quasi-force-free, i.e., $\text{J}$×$\text{B}$～0, while others are correlated with pressure variations in the ambient thermal plasma, $\text{J}$×$\text{B}$=-?(nkT).     

Dependence of Venus ionopause altitude and ionospheric magnetic field on solar wind dynamic pressure

The shape of the dayside Venus ionopause, and its dependence on solar wind parameters, is examined using Pioneer Venus Orbiter field and particle data. The ionopause is defined here as the altitude of pressure equality between magnetosheath magnetic pressure and ionospheric thermal pressure; its typical altitudes range from ～300 km near the subsolar point to ～900 km near the terminator. A strong correlation between ionopause altitude and magnetosheath magnetic pressure is demonstrated; correlation between magnetic pressure and the normally incident component of solar wind dynamic pressure is also evident. The data support the hypothesis of control of the ionopause altitude by solar wind dynamic pressure, manifested in the sheath as magnetic pressure. The presence of large scale magnetic fields in the ionosphere is observed primarily when dynamic pressure is high and the ionopause is low.     

Corotating interplanetary streams and associated ionospheric disturbances at Venus and Earth

During the summer of 1979, solar coronal structure was such that a sequence of recurrent regions produced a corresponding sequence of corotating solar wind streams, with pronounced downstream signatures. One of these stream events passed Earth on July 3, and was observed later at Venus late on July 11th, with similar characteristics. Corresponding in-situ measurements at Earth from the Atmospheric Explorer-E satellite and at Venus from the Pioneer Venus Orbiter are examined for evidence of comparable perturbations of the planetary ionospheres. The passage of the stream shock front is found to be associated with pronounced fluctuations in n(0⁺) which appear as pronounced local depletion of ion concentrations in both ionospheres. The ionosphere disturbances appear to be closely associated with large variations in the solar wind momentum flux. The implied local ionospheric depletions observed at each planet are interpreted to be the consequence of plasma redistribution, rather than actual depletions of plasma.     

Partially transmitted lightning signals recorded by pioneer Venus orbiter

Impulsive electric fields appearing on all four frequency channels of the Pioneer Venus electric field detector in the night ionosphere of Venus are characteristic of lightning generated signals. Based on our knowledge of the electron density and magnetic field in the Venus ionosphere, we suggest that lightning waves could be partially transmitted upwards into the ionosphere. The leakage of these lightning waves into the ionosphere on encountering electron density holes may be treated as reradiation into the ionosphere from the hole. Since this radiation pattern is frequency dependent, we should not expect to see all frequency components for every lightning stroke observed.     

Global comparison of the model results of GSM TIP with IRI for summer conditions

This paper presents the results of the numerical calculations thermosphere/ionosphere parameters which were executed with using of the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP)and comparison of these results with empirically-based model IRI-2001. Model GSM TIP was developed in West Department of IZMIRAN and solves self-consistently the time-dependent, 3-D coupled equations of the momentum, energy and continuity for neutral particles (O₂, N₂, O), ions (O⁺, H⁺), molecular ions (M⁺) and electrons and largescale eletric field of the dynamo and magnetospheric origin in the range of height from 80 km to 15 Earth’s radii. The empirically derived IRI model describes the E and F regions of the ionosphere in terms of location, time, solar activity and season. Its output provides a global specification not only of N_e but also on the ion and electron temperatures and the ion composition. These two models represent a unique set of capabilities that reflect major differences in along with a substantial approaches of the first-principles model and global database model for the mapping ionosphere parameters. We focus on global distribution of the N_e, T_i, T_e and TEC for the one moment UT and fixed altitudes: 110 km, h_mF2, 300 km and 1000 km. The calculations were executed with using of GSM TIP and IRI models for August 1999, moderate solar activity and quiet geomagnetic conditions. Results present as the global differences between the IRI and GSM TIP models predictions. The discrepancies between model results are discussed.     

Neutral upper atmospheres of Venus and Mars

Numerous measurements of the neutral upper atmosphere above 100 km have been made from spacecraft over Venus and over Mars. The Venus exospheric temperatures are unexpectedly low (less than 300°K near noon and less than 130°K near midnight). These very low temperatures may be partially caused by collisional excitation of CO₂ vibrational states by atomic oxygen and partially by eddy cooling. The Venus atmosphere is unexpectedly insensitive to solar EUV variability. On the other hand, the Martian dayside exospheric temperature varies from 150°K to 400°K over the 11-year solar cycle, where CO₂ 15-μm cooling may be less effective because of lower atomic oxygen mixing ratios. On Venus, temperature increases with altitude on the dayside (thermosphere), but decreases with altitude from 100 to 150 km on the nightside (cryosphere). However, dayside Martian temperatures near solar minimum for maximum planet-sun distance and low solar activity are essentially isothermal from 40 km to 200 km. During high solar activity, the thermospheric temperatures of Mars sharply increase. The Venus neutral upper atmosphere contains CO₂, O, CO, C, N₂, N, He, H, D and hot nonthermal H, O, C, and N, while the dayside Mars neutral upper atmosphere contains CO₂, O, O₂, CO, C, N₂, He, H, and Ar. There is evidence on Venus for inhibited day-to-night transport as well as superrotation of the upper atmosphere. Both atmospheres have substantial wave activity. Various theoretical models used to interpret the planetary atmospheric data are discussed.     

Model calculations of the dayside ionosphere of Venus

Model calculations of the dayside ionosphere of Venus are presented. The coupled continuity and momentum equations were solved for O₂⁺, O⁺, CO₂⁺, C⁺, N⁺, He⁺, and H⁺ density distributions, which are compared with measurements from the Pioneer Venus ion mass spectrometer. The agreement between the model results and the measurements is good for some species, such as O⁺, and rather poor for others, such as N⁺, indicating that our understanding of the dayside ion composition of Venus is incomplete. The coupled heat conduction equations for ions and electrons were solved and the calculated temperatures compared with Pioneer Venus measurements. It is shown that fluctuations in the magnetic field have a significant effect on the energy balance of the ionosphere.     

Spatial and temporal variations of the ion velocity measured in the venus ionosphere

The average velocity of the Venus ionosphere is nightward and approximately symmetric about the sun-Venus axis. We report here on temporal and spatial deviations from this average flow and their effects on the ionosphere. Temporal variability of the ion flux affects the main ionization source on the nightside. The influence of the solar wind is seen in the correlation between nightside ion density and ionopause height. Spatial asymmetries include a low-altitude superrotation (v-dawn < v-dusk) component related to superrotation of the neutral atmosphere, and a high-altitude prograde (v-dawn > v-dusk) component that is shown to be the result of asymmetric pressure gradients on the dayside.     

Temporal extrapolation of TEC using WNN during 2007–2018 and comparison against GIM,IRI2016 and Kriging

In this paper, a new method of temporal extrapolation of the ionosphere total electron content (TEC) is proposed. Using 3-layer wavelet neural networks (WNNs) and particle swarm optimization (PSO) training algorithm, TEC time series are modeled. The TEC temporal variations for next times are extrapolated with the help of training model. To evaluate the proposed model, observations of Tehran GNSS station (35.69°N, 51.33°E) from 2007 to 2018 are used. The efficiency of the proposed model has been evaluated in both low and high solar activity periods. All observations of the 2015 and 2018 have been removed from the training step to test the proposed model. On the other hand, observations of these 2 years are not used in network training. According to the F10.7, the 2015 has high solar activity and the 2018 has quiet conditions. The results of the proposed model are compared with the global ionosphere maps (GIMs) as a traditional ionosphere model, international reference ionosphere 2016 (IRI2016), Kriging and artificial neural network (ANN) models. The root mean square error (RMSE), bias, dVTEC = |VTEC_GPS ? VTEC_Model| and correlation coefficient are used to assess the accuracy of the proposed method. Also, for more accurate evaluation, a single-frequency precise point positioning (PPP) approach is used. According to the results of 2015, the maximum values of the RMSE for the WNN, ANN, Kriging, GIM and IRI2016 models are 5.49, 6.02, 6.34, 6.19 and 13.60 TECU, respectively. Also, the maximum values of the RMSE at 2018 for the WNN, ANN, Kriging, GIM and IRI2016 models are 2.47, 2.49, 2.50, 4.36 and 6.01 TECU, respectively. Comparing the results of the bias and correlation coefficient shows the higher accuracy of the proposed model in quiet and severe solar activity periods. The PPP analysis with the WNN model also shows an improvement of 1 to 12 mm in coordinate components. The results of the analyzes of this paper show that the WNN is a reliable, accurate and fast model for predicting the behavior of the ionosphere in different solar conditions.     

Comparison of GPS-TEC measurements with NeQuick2 and IRI model predictions in the low latitude East African region during varying solar activity period (1998 and 2008–2015)

This paper examines the performances of NeQuick2, the latest available IRI-2016, IRI-2012 and IRI-2007 models in describing the monthly and seasonal mean total electron content (TEC) over the East African region. This is to gain insight into the success of the various model types and versions at characterizing the ionosphere within the equatorial ionization anomaly. TEC derived from five Global Positioning System (GPS) receivers installed at Addis Ababa (ADD, 5.33°N, 111.99°E Geog.), Asab (ASAB, 8.67°N, 116.44°E Geog.), Ambo (ABOO, 5.43°N, 111.05°E Geog.), Nairobi (RCMN, ?4.48°N, 108.46°E Geog.) and Nazret (NAZR, 4.78°N, 112.43°E Geog.), are compared with the corresponding values computed using those models during varying solar activity period (1998 and 2008–2015). We found that different models describe the equatorial and anomaly region ionosphere best depending on solar cycle, season and geomagnetic activity levels. Our results show that IRI-2016 is the best model (compared to others in terms of discrepancy range) in estimating the monthly mean GPS-TEC at NAZR, ADD and RCMN stations except at ADD during 2008 and 2012. It is also found that IRI-2012 is the best model in estimating the monthly mean TEC at ABOO station in 2014. IRI show better agreement with observations during June solstice for all the years studied at ADD except in 2012 where NeQuick2 better performs. At NAZR, NeQuick2 better performs in estimating seasonal mean GPS-TEC during 2011, while IRI models are best during 2008–2009. Both NeQuick2 and IRI models underestimate measured TEC for all the seasons at ADD in 2010 but overestimate at NAZR in 2009 and RCMN in 2008. The periodic variations of experimental and modeled TEC have been compared with solar and geomagnetic indices at ABOO and ASAB in 2014 and results indicate that the F10.7 and sunspot number as indices of solar activity seriously affects the TEC variations with periods of 16–32?days followed by the geomagnetic activity on shorter timescales (roughly periods of less than 16?days). In this case, NeQuick2 derived TEC shows better agreement with a long term period variations of GPS-TEC, while IRI-2016 and IRI-2007 show better agreement with observations during short term periodic variations. This indicates that the dependence of NeQuick2 derived TEC on F10.7 is seasonal. Hence, we suggest that representation of geomagnetic activity indices is required for better performance over the low latitude region.     

SAMI2 model results for the quiet time low latitude ionosphere over India

Efficacy of SAMI2 model for the Indian low latitude region around 75°E longitudes has been tested for different levels of solar flux. With a slight modification of the plasma drift velocity the SAMI2 model has been successful in reproducing quiet time ionospheric low latitude features like Equatorial Ionization Anomaly. We have also showed the formation of electron hole in the topside equatorial ionosphere in the Indian sector. Simulation results show the formation of electron hole in the altitude range 800–2500?km over the magnetic equator. Indian zone results reveal marked differences with regard to the time of occurrence, seasonal appearances and strength of the electron hole vis-a-vis those reported for the American equatorial region.     

Theoretical simulation of electron density and temperature distribution at Indian equatorial and low latitude ionosphere

The electron density and temperature distribution of the equatorial and low latitude ionosphere in the Indian sector has been investigated by simultaneously solving the continuity, momentum and energy balance equations of ion and electron flux along geomagnetic field lines from the Northern to the Southern hemisphere. Model algorithm is presented and results are compared with the electron density and electron temperature measured in situ by Indian SROSS C2 satellite at an altitude of ∼500 km within 31°S–34°N and 75 ± 10°E that covers the Indian sector during a period of low solar activity. Equatorial Ionization Anomaly (EIA) observed in electron density, morning and afternoon enhancements, equatorial trough in electron temperature have been simulated by the model within reasonable limits of accuracy besides reproducing other normal diurnal features of density and temperature.