High Solar Flux Conditions Research Articles

A Quiet‐Day Ionospheric Electrodynamics Empirical Model in the Low and Middle Latitudes

AbstractIn this paper, we develop a climatology model of the ionospheric electric field and potential at low and middle latitudes on quiet time, utilizing plasma drift observed from the ROCSAT‐1 satellite. The meridional/vertical and zonal E × B drifts are transformed to the electric fields using modified Apex coordinates under the real geomagnetic field model. The electric field data are adjusted to different solar levels through bilinear fitting, and organized into seasonal, longitudinal, latitudinal bins. The fully normalized associated Legendre functions are chosen as the fitting basis modes. Our model can effectively describe the latitudinal dependence of the electrodynamics parameters cover equatorward of 35° magnetic latitude. The fitted drifts hold the fundamental characteristics of the results observed prior to fitting. The model potential can well shows the typical morphological characteristics that are in great agreement with prior research findings. Furthermore, the potential variations exhibit distinct seasonal, solar flux, and UT dependence under moderate and high solar flux conditions. Within the framework of this model, the potential, electric field, and drift can be mutually converted while maintaining self‐consistency.

Three-dimensional shape optimization of fins for application in compact supercritical CO2 solar receivers

The compact solar receiver is a promising option for S-CO2 solar receivers, designed to operate safely and efficiently under high-temperature, high-pressure, and high solar flux conditions. Adding fins with outstanding thermal-hydraulic performance in the mini-channel contributes to maximizing heat transfer performance and minimizing flow resistance for compact solar receivers. However, most of the research on high-performance fins is limited to two-dimensional optimization, ignoring the effect of shape changes in the height direction of the fins. The present study employs a combination of the finite volume method and the adjoint method to optimize single-row cylindrical fin shapes in three-dimensional for compact solar receivers, and the strengthening mechanism of high efficiency and low resistance is further revealed in the view of entropy generation. The results indicate: (1) three-dimensional fins exhibit significant variation in cross-section along the height direction, and each fin is unique; (2) three-dimensional fins exhibit more excellent thermal-hydraulic performance than cylindrical fins, showing an increase of 13 % and 20 % in the performance evaluation criterion (PEC) in case 1 (qw = 160 kW·m−2, vin = 0.487 m·s−1) and in case 2 (qw = 584 kW·m−2, vin = 1.778 m·s−1), respectively; (3) the entropy generation due to heat transfer is reduced by 4.9% and 3.9%; as well as the entropy generation due to pressure drop is decreased by 8.5 % and 16.5 %, in case 1 and case 2, respectively; (4) robustness results show that three-dimensional fins can maintain higher thermal-hydraulic performance than the initial cylindrical fin even under wide working conditions.

Analyzing the impact of various factors on leaf surface temperature based on a new tree-scale canopy energy balance model

Trees are one of the effective ways to regulate the microclimate, while the environmental parameters influence their transpiration rate. These complex processes manifest at the macro scale through leaf surface temperature (LST). Therefore, the key to studying the influence of trees on the microclimate is to calculate the LST. In this paper, we propose a new tree-scale canopy energy balance (CEB) model related to tree canopy height based on the big-leaf model to calculate the LST and analyze the influence of various factors on both LST and each sub-term of CEB. The results indicate that air temperature and solar radiation have a greater effect on LST than relative humidity on it. When the total solar radiation flux remains constant, air parameters primarily affect the latent heat flux of trees through the vapor pressure deficit between leaves and the air. The transpiration rate of trees is influenced not only by air parameters, but also by stomatal resistance. Solar radiation can directly determine the magnitude of the net radiation flux in the CEB, while its influence on latent heat flux is insignificant. Under high solar radiation flux conditions, an increase in wind speed can mitigate the rise of LST.

Assessment of IRI-2016 hmF2 model options with digisonde, COSMIC and ISR observations for low and high solar flux conditions

The attempt of this work is to evaluate the three options (BSE-1979, AMTB-2013, SHU-2015) embedded in IRI-2016 for the F2 layer peak height (hmF2) estimation under low and high solar flux levels. We compare the model hmF2 with digisonde observations from Qaanaaq, Milllstone Hill, Ramey, Jicamarca and Port Stanley as well as Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric radio occultation data over the five stations spanning from northern hemisphere to southern hemisphere. In the case of Millstone Hill, we also use incoherent scatter radar (ISR) measurements. For the comparison with digisonde data, SHU-2015 is the most accurate option for reproducing hmF2 at low and middle latitude stations (Milllstone Hill, Ramey, Jicamarca) with the smallest Root-Mean-Square-Error and the highest correlation with observations. At high latitude (Qaanaaq and Port Stanley), BSE-1979 becomes the best option. This is true for both low and high solar flux conditions. Similar results are obtained for the comparison between ISR and IRI-2016 hmF2 model options over Millstone Hill, where SHU-2015 option has the smallest deviations from the observation data in both low and high solar activity conditions. As for the comparison with COSMIC data, SHU-2015 edges out the other two options under low solar activity level with better performance at three stations. There is no optimal option under high solar activity level, which is to our surprise, cause SHU-2015 was developed based on a large amount of radio occultation data. At Jicamarca, all options show dramatic deviations from COSMIC observations, showing even negative correlations. We propose that the performance of the three options do not vary much with solar condition, while they differ a lot with location, for example, latitude.

Climatology of plasmaspheric total electron content obtained from Jason 1 satellite

AbstractWe used more than 40 million total electron content (TEC) measurements obtained from the GPS TurboRogue Space Receiver receiver on board the Jason 1 satellite in order to investigate the global morphology of the plasmaspheric TEC (pTEC) including the variations with local time, latitude, longitude, season, solar cycle, and geomagnetic activity. The pTEC corresponds to the total electron content between Jason 1 (1336 km) and GPS (20,200 km) satellite altitudes. The pTEC data were collected during the 7 year period from January 2002 to December 2008. It was found that pTEC increases by about 10–30% from low to high solar flux conditions with the largest variations occurring at low latitudes for equinox. During low solar flux condition, pTEC is largely independent of geomagnetic activity. However, it slightly decreases with increasing geomagnetic activity at low latitudes during high solar flux. The seasonal variations such as the annual and semiannual anomalies in the ionosphere also exist in the low‐latitude plasmasphere. In particular, the American sector (around 300°E) shows strong annual asymmetry in the plasmaspheric density, being larger in December than in June solstice.

Comparative aeronomy: Molecular ionospheres at Earth and Mars

AbstractThe ionospheres in our solar system vary not only in their electron densities but also in the dominance of atomic versus molecular ions at their altitudes of peak plasma density. With the exception of Earth's F layer composed of atomic oxygen ions and electrons, all other planets have their peak ionospheric layers composed of molecular ions and electrons embedded in a dense neutral atmosphere. At Mars, both of its ionospheric layers have molecular ions, with the M1 layer at a lower altitude than the more robust M2 layer above it. The terrestrial ionosphere has a prominent region of molecular ions (the E layer) below the dominant F layer. In this paper, we explore the production and loss of molecular ion layers observed under the same solar irradiance conditions at Mars and Earth. We compare observations of M1 and M2 electron densities with terrestrial ionosonde data for the peak densities of the E and F layers during low, moderate, and high solar flux conditions. The subsolar peak densities of molecular ion layers have high correlations at each planet, as well as between planets, even though they are produced by separate portions of the solar spectrum. We use photochemical‐equilibrium theory for layers produced by soft X‐rays (M1 and E) versus the M2 layer produced by extreme ultraviolet (EUV) to identify the key parameters that cause similarities and differences. The yield of our comparative study points to the roles of secondary ionization and temperature‐dependent plasma recombination rates as areas most in need of further study at each planet.

Medium‐scale gravity wave activity in the thermosphere inferred from GOCE data

AbstractThis study is focused on the effect of solar flux conditions on the dynamics of gravity waves (GWs) in the thermosphere. Air density and crosswind in situ estimates from the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) accelerometers are analyzed for the whole mission duration. The analysis is performed in the Fourier spectral domain averaging spectral results over periods of 2 months close to solstices. A new GW marker (called ) is introduced here to characterize GWs activity under low, medium, and high solar flux conditions, showing a clear solar damping effect on GW activity. Most GW signal is found in a spectral range above 8 mHz in GOCE data, meaning a maximum horizontal wavelength of around 1000 km. The level of GW activity at GOCE altitude is strongly decreasing with increasing solar flux. Furthermore, a shift in the dominant frequency with solar flux conditions has been noted, leading to larger horizontal wavelengths (from 200 to 500 km) during high solar flux conditions. The correlation between air density variability and GW marker allows to identify most of the large‐amplitude perturbations below 67° latitudes as due to GWs. The influence of correlated error sources, between air density and crosswinds, is discussed. Consistency of the spectral domain results is verified in the time domain with a global mapping of high‐frequency air density perturbations along the GOCE orbit. This analysis shows a clear dependence with geomagnetic latitude with strong perturbations at magnetic poles and an extension to lower latitudes favored by low solar activity conditions. These results are consistent with previous Challenging Minisatellite Payload (CHAMP) data analysis and with general circulation models.

Extended ionospheric amplitude scintillation model for GPS receivers

Ionospheric scintillation is a phenomenon that occurs after sunset, especially in the low-latitude region, affecting radio signals that propagate through the ionosphere. Depending on geophysical conditions, ionospheric scintillation may cause availability and precision problems to Global Navigation Satellite System users. The present work is concerned with the development of an extended model for describing the effects of the amplitude ionospheric scintillation on GPS receivers. Using the α-μ probabilistic model, introduced by previous authors in different contexts, the variance of GPS receiver tracking loop error may be estimated more realistically. The proposed model is developed with basis on the α-μ parameters and also considering correlation between amplitude and phase scintillation. Its results are interpreted to explain how a receiver may experience different error values under the influence of ionospheric conditions leading to a fixed scintillation level S4. The model is applied to a large experimental data set obtained at São José dos Campos, Brazil, near the peak of the equatorial anomaly during high solar flux conditions, between December 2001 and January 2002. The results from the proposed model show that depending on the α-μ pair, moderate scintillation (0.5 ≤ S4 ≤ 0.7) may be an issue for the receiver performance. When S4 > 0.7, the results indicate that the effects of scintillation are serious, leading to a reduction in the receiver availability for providing positioning solutions in approximately 50% of the cases.

On the second order statistics for GPS ionospheric scintillation modeling

AbstractEquatorial ionospheric scintillation is a phenomenon that occurs frequently, typically during nighttime, affecting radio signals that propagate through the ionosphere. Depending on the temporal and spatial distribution, ionospheric scintillation can represent a problem in the availability and precision for the Global Navigation Satellite System's users. This work is concerned with the statistical evaluation of the amplitude ionospheric scintillation fading events, namely, level crossing rate (LCR) and average fading duration (AFD). Using α‐μ model, the LCR and AFD are validated against experimental data obtained in São José dos Campos (23.1°S; 45.8°W; dip latitude 17.3°S), Brazil, a station located near the southern crest of the ionospheric equatorial ionization anomaly. The amplitude scintillation data were collected between December 2001 and January 2002, a period of high solar flux conditions. The obtained results with the proposed model fitted quite well with the experimental data and performed better when compared to the widely used Nakagami‐m model. Additionally, this work discusses the estimation of α and μ parameters, and the best fading coefficients found in this analysis are related to scintillation severity. Finally, for theoretical situations in which no set of experimental data are available, this work also presents parameterized equations to describe these fading statistics properly.

Validation of theα-μModel of the Power Spectral Density of GPS Ionospheric Amplitude Scintillation

Theα-μmodel has become widely used in statistical analyses of radio channels, due to the flexibility provided by its two degrees of freedom. Among several applications, it has been used in the characterization of low-latitude amplitude scintillation, which frequently occurs during the nighttime of particular seasons of high solar flux years, affecting radio signals that propagate through the ionosphere. Depending on temporal and spatial distributions, ionospheric scintillation may cause availability and precision problems to users of global navigation satellite systems. The present work initially stresses the importance of the flexibility provided byα-μmodel in comparison with the limitations of a single-parameter distribution for the representation of first-order statistics of amplitude scintillation. Next, it focuses on the statistical evaluation of the power spectral density of ionospheric amplitude scintillation. The formulation based on theα-μmodel is developed and validated using experimental data obtained in São José dos Campos (23.1°S; 45.8°W; dip latitude 17.3°S), Brazil, located near the southern crest of the ionospheric equatorial ionization anomaly. These data were collected between December 2001 and January 2002, a period of high solar flux conditions. The results show that the proposed model fits power spectral densities estimated from field data quite well.

On the distribution of GPS signal amplitudes during low-latitude ionospheric scintillation

Ionospheric scintillations are fluctuations in the phase and/or amplitude of trans-ionospheric radio signals caused by electron density irregularities in the ionosphere that affect the performance of Global Navigation Satellite Systems receivers. We used an entire month of high-rate (50 Hz) measurements of the GPS L1 (1.575 GHz) signal amplitude to investigate the statistics of L-Band signals during ionospheric scintillation events. The scintillation measurements used in this study were made by a GPS-based scintillation monitor installed in Sao Jose dos Campos, Brazil, near the equatorial anomaly peak. The observations were made over 32 days during high solar flux conditions when typical values of F10.7 were above 150 A— 10Â?22 W/m2/Hz. This data set allowed us to test the Nakagami-m and Rice probability density functions (PDFs) in the description of the distribution of L-Band scintillating signals with better statistical confidence than previously possible. In addition, we parameterized and tested the ability of the Â?---μ distribution, which is a more general and yet simple and flexible fading model to describe the distribution of signal amplitudes during scintillation events. The results show a slight advantage of the Nakagami-m PDF over the Rice distribution. We also show that the Â?---μ PDF outperforms the Nakagami-m and Rice PDFs in the statistical characterization of amplitude scintillation. The reason for such a performance is the fact that the Â?---μ model was specially tailored to the ionospheric scintillation events, resulting in a better fit with experimental data, specifically in the region of small amplitudes, which is particularly interesting for scintillation studies.

Analysis of the Characteristics of Low-Latitude GPS Amplitude Scintillation Measured During Solar Maximum Conditions and Implications for Receiver Performance

Ionospheric scintillations are fluctuations in the phase and/or amplitude of trans-ionospheric radio signals caused by electron density irregularities in the ionosphere. A better understanding of the scintillation pattern is important to make a better assessment of GPS receiver performance, for instance. Additionally, scintillation can be used as a tool for remote sensing of ionospheric irregularities. Therefore, the study of ionospheric scintillation has both scientific as well as technological implications. In the past few years, there has been a significant advance in the methods for analysis of scintillation and in our understanding of the impact of scintillation on GPS receiver performance. In this work, we revisit some of the existing methods of analysis of scintillation, propose an alternative approach, and apply these techniques in a comprehensive study of the characteristics of amplitude scintillation. This comprehensive study made use of 32 days of high-rate (50 Hz) measurements made by a GPS-based scintillation monitor located in Sao Jose dos Campos, Brazil (23.2°S, 45.9°W, −17.5° dip latitude) near the Equatorial Anomaly during high solar flux conditions. The variability of the decorrelation time (τ0) of scintillation patterns is presented as a function of scintillation severity index (S4). We found that the values of τ0 tend to decrease with the increase of S4, confirming the results of previous studies. In addition, we found that, at least for the measurements made during this campaign, averaged values of τ0 (for fixed S4 index values) did not vary much as a function of local time. Our results also indicate a significant impact of τ0 in the GPS carrier loop performance for S4 ≥ 0.7. An alternative way to compute the probability of cycle slip that takes into account the fading duration time is also presented. The results of this approach show a 38% probability of cycle slips during strong scintillation scenarios (S4 close to 1 and τ0 near 0.2 s). Finally, we present results of an analysis of the individual amplitude fades observed in our set of measurements. This analysis suggests that users operating GPS receivers with C/N0 thresholds around 30 dB-Hz and above can be affected significantly by low-latitude scintillation.

Superrotation of the ionosphere and quiet time zonal ion drifts at low and middle latitudes observed by Republic of China Satellite-1 (ROCSAT-1)

[1] Local-time variations of zonal drifts at different latitudes, longitudes, and seasons are investigated using ion drift measurements from the Republic of China Satellite-1 (ROCSAT-1). We select a quiet time period defined when Dst > −100 nT and Kp ≤ 3. The ion drifts in the magnetic zonal direction are latitude (apex height) dependent and differ for different longitude sectors and seasons. Superrotation of the ionosphere is observed in the zonal drift measurements, particularly at the lower latitudes. The ionospheric superrotation generally decreases with increasing latitude and is attributed mainly to the F region dynamo where the features of the neutral wind and conductance affect the characteristics of the superrotation at different latitudes, longitudes, and seasons. We suggest that the latitudinal variation of the superrotation is mainly produced by the decrease in latitude of the neutral winds in the F region. A decrease of conductance with increasing latitude is also present, but evidence shows that neutral winds decrease faster with latitude than the conductance for high solar flux conditions. The longitudinal and seasonal variations of the superrotation are more influenced by changes in the F region conductance that is modified by vertical E × B ion drift motions and neutral wind-induced ion drift motions along magnetic field lines.

Plasmasphere effects for GPS TEC measurements in North America

Plasmasphere effects on total electron content (TEC) measurements conducted using Global Positioning System (GPS) receivers are typically neglected for the North American region, because of the relatively high magnetic latitudes there, but model and measurement cases for this region are presented here to demonstrate the magnitude of the effects for GPS TEC measurements away from vertical and for the associated equivalent vertical TEC values. For high solar flux conditions, the effects of high, distant electron content can range up to 25 TEC units for equivalent vertical TEC, or up to 65 TEC units for slant TEC determinations derived from vertical TEC measurements. The effects of the plasmasphere for TEC calibrations conducted in North America include systematic overestimation of the equivalent vertical TEC and excessive latitudinal gradients, especially at night. These may not be readily evident in the calibration results, and some alternatives for addressing these circumstances are considered.

Climatology of postsunset equatorial spread F over Jicamarca

We use radar observations from 1996 to 2006 to study the climatology of postsunset equatorial 3‐m spread F irregularities over Jicamarca during all seasons. We show that the spread F onset times do not change with solar flux and that their onset heights, which occur near the altitude of the evening F region velocity vortex, increase linearly from about 260 to 400 km from solar minimum to solar maximum. Higher onset heights generally lead to stronger radar echoes. During the equinox, spread F onset occurs near vertical drift evening reversal times, while during the December solstice, they occur near the drift reversal times close to solar minimum and near the time of the prereversal velocity peak for high solar flux conditions. On average, radar plume onset occurs earlier with increasing solar flux in all seasons. Plume onset and peak altitudes increase with solar activity, and the peak heights are generally highest during the equinox. The F region upward drift velocities that precede spread F onset increase from solar minimum to solar maximum and are approximately proportional to the maximum prereversal drift peak velocities.

A quiet time empirical model of equatorial vertical plasma drift in the Peruvian sector based on 150 km echoes

Equatorial vertical plasma drift is an important consequence of the E and F region dynamos. Understanding the climatology of vertical drift can provide significant insight into many ionospheric phenomena. In this study we present the first empirical model of vertical plasma drifts at 150 km altitude, observed by the Jicamarca Unattended Long‐term studies of the Ionosphere and Atmosphere (JULIA) coherent scatter radar located in Peru. The model, called JULIA Vertical Drift Model (JVDM), describes the local time, seasonal, and solar flux behavior of the equatorial vertical drifts in the Peruvian sector. The model is valid from 0800 through 1600 local time, which is typically when JULIA makes vertical drift measurements. During very high solar flux conditions, however, the model is unreliable before 1000 local time owing to a lack of JULIA data. The model includes a climatology of the 150 km equatorial vertical drifts as well as an estimate of the day‐to‐day variability, which can be significant. The daytime drifts typically peak between 1000 and 1200 LT and have amplitudes of 25–30 m/s ± 10 m/s. The model has been validated against the global empirical model of Scherliess and Fejer, with a total rms difference of under 4 m/s for 1000 to 1600 LT. This model will allow researchers to study daily variations in the equatorial electric field by subtracting the climatological mean. Model coefficients and software are available online at http://geomag.org/models and http://www.earthref.org.

A study of evolution/suppression parameters of equatorial postsunset plasma instability

Abstract. Evening equatorial pre-reversal vertical ion E×B

Solar and geomagnetic trends of equatorial evening and nighttime F region vertical ion drifts

F region vertical ion drifts were inferred from the evening and nighttime ionosonde data for two magnetic equatorial stations in West Africa: Ouagadougou (geographic: 12°N, 1.5°W; 5.9°N dip) and Ibadan (geographic: 7.9°N, 3.9°E; 6°S dip). We examine and discuss the short‐term patterns of behavior of ionospheric variability over Ouagadougou for 1986–1987 years of low solar activity (F10.7 = 80) and 1988–1989 years of high solar activity (F10.7 = 180) for quiet time, while that of Ibadan is for undisturbed (Kp ≤ 3.0) and disturbed (Kp > 3.0) geomagnetic conditions during the 1958 International Geophysical Year (IGY) period, corresponding to high solar flux conditions (F10.7 = 208). Our results indicate that the evening and nighttime ion drift exhibits strong variations with the phase of the solar cycle but only small variations with geomagnetic activity. The characteristic values of evening prereversal velocity enhancements (PRE) vary between about 2–14 m/s and 12–22 m/s and 17–42 m/s and 18–40 m/s for low and high solar flux, unperturbed and perturbed conditions, in that order. The solar minimum evening reversal times are strongly season dependent, while the morning reversal times are season independent except during December solstice, which occurs earliest. During solar maximum, reversal times near dawn and dusk are essentially season independent except during June solstice season, which occurs late. The average occurrence time (1900 LT) of PRE is strongly independent of solar and magnetic variations apart from June solstice of high solar activity periods.

The F-Region Equatorial Ionospheric Electrodynamics Drifts

The ionospheric plasma drift is one of the most essential parameters for understanding the dynamics of ionospheric F-region. F-region electromagnetic drifts are calculated for three seasonal conditions from ionosonde observations acquired during quiet period of a typical year of high and low solar activity at Ibadan (7.4oN, 3.9oE, dip 6oS), Nigeria. The vertical plasma drifts derived from h' (f) ionosonde data are compared with vertical drifts obtained by incoherent scatter radar and AE-E satellite measurements during nighttime periods under similar solar and geomagnetic conditions. We find comparable variability among the ionosonde drifts at Ibadan, Jicamarca VHF radar drifts, and AE-E satellite drifts during high solar flux and geomagnetic quiet conditions at equinox and solstices periods. The equinoctial average evening upward drifts enhancements by the three methods are roughly similar and occur at the same local time (19 LT) for all the seasons. Additionally, the evening reversal time from upward daytime to downward nighttime does not vary much except during the winter months; and occurs earliest in summer and equinox, but least during winter period. Also the data indicate asymmetry of evening reversal times about the dip-equator between the Peruvian, Indian, and the African equatorial regions. Our observations are in conformity with some results obtained at other equatorial ionospheric stations Journal of the Nigerian Association of Mathematical Physics Vol. 10 2006: pp. 27-34

Quiet time equatorial F region vertical plasma drift model derived from ROCSAT‐1 observations

We have used five years of measurements on board the ROCSAT‐1 satellite to develop a detailed quiet time global empirical model for equatorial F region vertical plasma drifts. This model describes the local time, seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions. The model results are in excellent agreement with measurements from the Jicamarca radar and also from other ground‐based and in situ probes. We show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier, especially during December and June solstice. The late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons, the morning and afternoon December solstice drifts have significantly different longitudinal dependence, and the daytime upward drifts have strong wave number‐four signatures during equinox and June solstice. The largest evening upward drifts occur during equinox and December solstice near the American sector. The longitudinal variations of the evening prereversal velocity peaks during December and June solstice are anti‐correlated, which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere.