Analysis of post-sunset F-region vertical plasma drifts during Counter Electrojet days using multi frequency HF Doppler Radar

In this paper, through a case study, an attempt has been made to bring out the relationship between post noon E-region electric field and post sunset F-region vertical plasma drift on quiet time Counter Electrojet (CEJ) days. Study carried out using the data from a multi frequency HF Doppler Radar and Digital Ionosonde located over Trivandrum (8.5° N; 77° E; 0.5° N dip lat.) a geomagnetic dip equatorial station in India during quite time CEJ days of the years 2004 and 2006, revealed some interesting aspects of the E region electrodynamics and post sunset F region electrodynamics. It has been observed that, in contrast to the normal electrojet (EEJ) days, the Pre-Reversal Enhancement (PRE) is either weakened or inhibited on CEJ days and the field reversal takes place much earlier than that on a normal day. It is suggested that even after the effects of the field reversal ceases to show up in the ground magnetic data, the reversed field may persist and shows up as a decrease in the PRE experienced by the F-region. In other words, the study indicates that the EEJ associated electrodynamics have a significant role in controlling the PRE.     

A plan for compilation of experimental data on drifts for IRI

Methods for constructing a dynamical part of the IRI model are described.     

Comparisons of foF2 with IRI model and equatorial vertical drifts

Measurements of the critical frequency, foF2 recorded over Ibadan: 7.4°N, 3.9°E (geographic), 6°S (dip angle) have been compared with the International Reference Ionosphere (IRI-2007) model for solar maximum geomagnetically quiet conditions, with a view to determining what modifications might bring about better predictions for the model. Our results reveal that the present version of IRI essentially reproduces diurnal trends and the general features of the experimental observations for all seasons, except for nighttime June solstice periods, which the model seriously overestimated. The model errors ranging from 50% to 125% over the four seasons considered in this study. It is also indicated that the percentage relative deviations between the observed and the modeled values vary approximately from −11% to 12% (March), −34% to 11% (June), −16% to 12% (September), and −10% to 13% (December). An unexpected feature of foF2 is obvious and remarkable reduction in values during nighttime June solstice periods compared to that in other seasons. Relationship between equatorial vertical drift and foF2 is also investigated. However, cross correlation analysis reveals strong anti-correlation between vertical drift and critical frequency during the daytime hours, but exceptionally opposite is the case for the nighttime sector. The discrepancies which are noted, particularly during June solstice season are attributed to processes most likely within the thermosphere and from meteorological influences during quiet magnetic conditions.     

Characteristics of the equatorial F-region vertical plasma drift observed during post-sunset and pre-sunrise hours

Post-sunset and pre-sunrise vertical plasma drifts at the equatorial F-region have been investigated using the HF Doppler radar and ionosonde observations. Observed vertical plasma drift features during the sunrise are found to complement that observed during the evening. The post-sunset vertical plasma drift is characterized by an upward enhancement, a pre-reversal enhancement and a reversal in the drift direction. Similarly, the pre-sunrise plasma drift is characterized by a sudden downward excursion followed by an upward turning. The wavelet analysis of the plasma drift shows the presence of fluctuations in the period range 4–32 min and the short period fluctuations are attributed to the atmospheric gravity waves.     

Very low frequency interferometry for the Chang’E-2 project

Very low frequency interferometry among two astronomical experiments has been proposed and accepted for further study for the second phase of China’s lunar exploration programme (the Chang’E Programme), which is envisaged to operate a lander and a rover on the surface of the moon. This experiment is an interferometer experiment in the very low frequency (VLF, f < 15 MHz) regime of radio frequencies with at least degree-level angular resolution. The goals include observing solar storm activities, Coronal Mass Ejections, Auroral Kilometric Radiation, and planetary radiation in the solar system, studying the origin of Cosmic Rays, spectral properties of pulsars, surveying ionized hydrogen in the Galaxy, and exploring coherent radio emissions.     

Solar cycle effects upon the relationship of Ne and Te in the F-region

Employing Atmsophere Explorer-C measurements made in 1974, just prior to solar minimum, Brace and Theis /1/ demonstrated that a remarkably consistent inverse relationship existed between the electron density N_e and temperature T_e in the F-region. In this paper we use later data from AE-C, taken when solar activity was rising (1975–1978), and Dynamics Explorer-2 data taken at solar maximum (1981), to examine how the temperature and density relationship changes with solar activity. We find that the solar maximum T_e is a factor of two larger than the solar minimum T_e for the same values of N_e. T_e does not necessarily increase with solar activity, however, because N_e increases enough to approximately cancel the effect of higher solar extreme ultraviolet heating. We find that the effect of solar activity can be accounted for by a simple function of the F_10.7 cm index that multiplies the solar minimum equation of Brace and Theis /1/.     

Modelling ionospheric vertical drifts over Africa low latitudes using Empirical Orthogonal functions and comparison with climatological model

For the first time, empirical model of daytime vertical $\mathbf{E}\times \mathbf{B}$ drift based on Empirical Orthogonal functions (EOF) decomposition technique is presented. Day-to-day variability of $\mathbf{E}\times \mathbf{B}$ drift inferred from horizontal (H) geomagnetic field data around dip latitude for the period of 2008–2013 is used to both develop and validate the model. Results show that the EOF technique is promising with modelled values and data giving correlation coefficient values of at least 0.90 for geomagnetic conditions of both $K_p?3$ and $K_p>3$ within 2008–2013. Independent model validation shows that in situ $\mathbf{E}\times \mathbf{B}$ values from ion velocity meter (IVM) instrument on-board C/NOFS satellite are closer to model $\mathbf{E}\times \mathbf{B}$ estimates than the climatological Scherliess-Fejer (SF) model incorporated within the International Reference Ionosphere (IRI).     

Modulation of cosmic rays: Perpendicular diffusion and drifts in a heliosphere with a solar wind termination shock

In this study the roles of polar perpendicular diffusion and drifts are illustrated in a model containing a heliosheath and diffusive shock acceleration as applied to the solar wind termination shock. Of particular interest is the relation of polar perpendicular diffusion to particle drifts and how the effectiveness of the termination shock acceleration of galactic and anomalous protons is influenced by this relation. We found that drifts have a more prominent effect than the polar enhancement of perpendicular diffusion so that its omission from termination shock models would produce unrealistically large shock acceleration and consequently also larger modulation effects throughout the heliosphere. The computed spectra at a heliolatitude of 35° are almost similar for the two polarity epochs indicating that the two Voyager spacecraft might not observe differences between the two cycles in future.     

Low latitude ionospheric scintillation and zonal irregularity drifts observed with GPS-SCINDA system and closely spaced VHF receivers in Kenya

In this study we have used VHF and GPS-SCINDA receivers located at Nairobi (36.8°E, 1.3°S, dip −24.1°) in Kenya, to investigate the ionospheric scintillation and zonal drift irregularities of a few hundred meter-scale irregularities associated with equatorial plasma density bubbles for the period 2011. From simultaneous observations of amplitude scintillation at VHF and L-band frequencies, it is evident that the scintillation activity is higher during the post sunset hours of the equinoctial months than at the solstice. While it is noted that there is practically no signatures of the L-band scintillation in solstice months (June, July, December, January) and after midnight, VHF scintillation does occur in the solstice months and show post midnight activity through all the seasons. VHF scintillation is characterized by long duration of activity and slow fading that lasts till early morning hours (05:00 LT). Equinoctial asymmetry in scintillation occurs with higher occurrence in March–April than in September–October. The occurrence of post midnight VHF scintillation in this region is unusual and suggests some mechanisms for the formation of scintillation structure that might not be clearly understood. Zonal drift velocities of irregularities were measured using cross-correlation analysis with time series of the VHF scintillation structure from two closely spaced antennas. Statistical analyses of the distribution of zonal drift velocities after sunset hours indicate that the range of the velocities is 30–160 m/s. This is the first analysis of the zonal plasma drift velocity over this region. Based on these results we suggest that the east–west component of the plasma drift velocity may be related to the evolution of plasma bubble irregularities caused by the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocities and scintillation could be attributed to the asymmetry of neutral winds in the thermosphere that drives the eastward electric fields.     

Comparison of mesopause region meteor radar winds,medium frequency radar winds and low frequency drifts over Germany

During 2004 and 2005 measurements of mesospheric/lower thermospheric (80–100 km) winds have been carried out in Germany using three different ground-based systems, namely a meteor radar (36.2 MHz) at the Collm Observatory (51.3°N, 13°E), a MF radar (3.18 MHz) at Juliusruh (54.6°N, 13.4°E) and the LF D1 measurements using a transmitter (177 kHz) at Zehlendorf near Berlin and receivers at Collm with the reflection point at 52.1°N, 13.2°E. This provides the possibility of comparing the results of different radar systems in nearly the same measuring volume. Meteor radar winds are generally stronger than the winds observed by MF and especially by LF radars. This difference is small near 80 km but increases with height. The difference between meteor radar and medium frequency radar winds is larger during winter than during summer, which might indicate an indirect influence of gravity waves on spaced antenna measurements.     

Chang’E-1 precision orbit determination and lunar gravity field solution

In this paper we present results assessing the role of Very Long Baseline Interferometry (VLBI) tracking data through precision orbit determination (POD) during the check-out phase for Chang’E-1, and the lunar gravity field solution CEGM-01 based on the orbital tracking data acquired during the nominal phase of the mission. The POD of Chang’E-1 is performed using S-band two-way Range and Range Rate (R&RR) data, together with VLBI delay and delay rate observations. The role of the VLBI data in the POD of Chang’E-1 is analyzed, and the resulting orbital accuracies are estimated for different solution strategies. The final orbital accuracies proved that the VLBI tracking data can improve the Chang’E-1 POD significantly. Consequently, CEGM-01 based on six-month tracking data during Chang’E-1 nominal mission phase is presented, and the accuracy of the model is assessed by means of the gravity field power spectrum, admittance and coherence between gravity and topography, lunar surface gravity anomaly and POD for both Chang’E-1 and Lunar Prospector (LP). Our analysis indicates that CEGM-01 has significant improvements over a prior model (i.e. GLGM-2), and shows the potential of Chang’E-1 tracking data in high resolution lunar gravity field model solution by combining with SELENE and LP tracking data.     

Radio and optical investigations of storm time evolution of post-midnight equatorial plasma bubbles (EPBs) and their drifts over Indian sector

We investigated the spatio-temporal evolution of disturbed time post mid-night Equatorial Plasma Bubbles (EPBs) using Canadian Advanced Digital Ionosonde (CADI) located at dip equatorial station, Tirunelveli (8.73°N, 77.7°E, 0.23°N Dip. Lat.), an all-sky imager (ASI) observations at low latitude station Panhala (16.48°N, 74.6°E, 11.1°N Dip. Lat.) and Gadanki Ionospheric Radar Interferometer (GIRI) at Gadanki (13.5°N, 79.2°E; 6.5°N Dip. Lat.) which is situated at few degrees towards east and south of Panhala on 02–03 February 2017 night. During this night, IMF Bz showed its periodic variation starting from 16:00 UT to 23:00 UT accompanied by decrease in SYM-H to as low as ?35 nT indicating the onset of weak magnetic storm. The analyzed results suggested that cause of post-midnight EPBs could be due to manifestation of fluctuating eastward/westward electric field due to combined under-shielding/over-shielding Electric Fields and disturbance dynamo electric fields that led to rise and fall of the F-layer over dip equator. Interestingly, the EPBs over Panhala showed eastward motion initially that quickly reversed to westward later. Along with westward motion they also started growing until 21:30 UT. However, most of these EPBs disappeared with time except the one that started descending/shrinking towards southern side (i.e. towards equator). The rising and shrinking of EPBs is found to be fairly correlated with the equatorial vertical drifts. The westward drift of EPBs at Panhala and its anti-correlation with vertical drifts has been confirmed from CADI zonal/vertical drifts. Accordingly, the study also investigated the role of storm induced vertical Hall electric field as a possible cause for westward drifts and its anti-correlation with vertical drifts. However, GIRI observations do not show any significant westward drift on this night at Gadanki suggesting that there is a longitudinal gradient in the zonal drift of these EPBs. In addition to longitudinal drift reversal, the latitudinal gradient in zonal drifts also has been noticed. The present work highlights the role of storm induced disturbances in the generation and evolution of post-midnight EPBs which is believed to be triggered by weak magnetic disturbances in the deep low solar minimum.     

Effects observed in the equatorial and low latitude ionospheric F-region in the Brazilian sector during low solar activity geomagnetic storms and comparison with the COSMIC measurements

The main objective of the present investigation has been to compare the ionospheric parameters (NmF2 and hmF2) observed by two ground-based ionospheric sounders (one at PALMAS- located near the magnetic equator and the other at Sao Jose dos Campos-located in the low-latitude region) in the Brazilian sector with that by the satellite FORMOSAT-3/COSMIC radio occultation (RO) measurements during two geomagnetic storms which occurred in December 2006 and July 2009. It should be pointed out that in spite of increasing the latitude (to 10°) and longitude (to 20°) around the stations; we had very few common observations. It has been observed that both the peak electron density (NmF2) and peak height (hmF2) observed by two different techniques (space-borne COSMIC and ground-based ionosondes) during both the geomagnetic storm events compares fairly well (with high correlation coefficients) at the two stations in the Brazilian sector. It should be pointed out that due to equatorial spread F (ESF) in the first storm (December 2006) and no-reflections from the ionosphere during nighttime in the second storm (July 2009), we had virtually daytime data from the two ionosondes.     

F-region ionospheric parameters observed in the equatorial and low latitude regions during medium solar activity in the Brazilian sector and comparison with the IRI-2007 model results

The ionospheric sounding observations using the Canadian Advanced Digital Ionosondes (CADIs) operational at Palmas (PAL; 10.2°S, 48.2°W; dip latitude 6.6°S; a near-equatorial station), and São José dos Campos (SJC, 23.2°S, 45.9°W; dip latitude 17.6°S; a low-latitude station located under the southern crest of the equatorial ionospheric anomaly), Brazil, are analyzed during the different seasons viz., winter (June and July 2003), spring (September and October 2003), summer (December 2003 and January 2004), and fall (March and April 2004). The period used has medium solar activity (sunspot number between 77.4 and 39.3). The seasonal mean variations (using only geomagnetically quiet days) of the ionospheric parameters foF2 (critical frequency of the F-region), hpF2 (virtual height at 0.834 foF2; considered to be close to hmF2 (peak height of the F-region)), and h’F (minimum virtual height of the F-region) are calculated and compared between PAL and SJC. The prominent differences between PAL and SJC are as follows: h’F variations show strong post-sunset enhancement at PAL during the seasons of spring, summer, and fall; hpF2 variations show pre-sunrise uplifting of the F-layer at both stations during all the seasons and the hpF2 values during the daytime are lower at SJC compared with PAL during all the seasons; the foF2 variations show mid-day bite-out at PAL during all the seasons and SJC shows strong equatorial ionospheric anomaly during summer and fall seasons. Also, the seasonal variations of the ionospheric parameters foF2 and hpF2 (with ±1 standard deviation) observed at PAL and SJC are compared with the IRI-2007 model results of foF2 and hmF2. In addition, variations of the foF2 and hpF2 observed at SJC are compared with the IRI-2001 model results of foF2 and hmF2. It should be pointed out that the ionospheric parameter hpF2 is much easier to obtain using computer program developed at UNIVAP compared with hmF2 (using POLAN program). During the daytime due to underlying ionization hpF2 estimated is higher (approximately 50 km) than the true peak height hmF2. During the nighttime hpF2 is fairly close to hmF2. The comparison between the foF2 variations observed at PAL and SJC with the IRI-2007 model results shows a fairly good agreement during all the seasons. However, the comparison between the hpF2 variations observed at PAL and SJC with the hmF2 variations with the IRI-2007 model results shows: (1) a fairly good agreement during the nighttime in all the seasons; (2) the model results do not show the pre-sunrise uplifting of the F-layer at PAL and SJC in any season; (3) the model results do not show the post-sunset uplifting of the F-layer at PAL; (4) considering that, in general, hpF2 is higher than hmF2 during the daytime by about 50 km, the model results are in good agreement at PAL and SJC during all the seasons except summer at SJC, when large discrepancies in the observed hpF2 and modeled hmF2 are observed. Also, it has been observed that, in general, hmF2 values for SJC calculated using IRI-2001 are higher than IRI-2007 during the daytime in winter, summer, and fall. However, hmF2 values for SJC calculated using IRI-2001, are lower than IRI-2007 during the nighttime in spring.     

IUE observations of globular clusters and blue horizontal branch stars

This paper presents a review of ultraviolet photometry and spectroscopy of globular clusters and blue horizontal branch stars. Current observations with IUE are placed within the framework of earlier photometric studies carried out by OAO and ANS. In order to provide basic information on the nature of the horizontal branch star, whose light dominates in the ultraviolet, observations of field horizontal branch stars as well as individual stars in globular clusters are discussed. Finally the IUE observations of globular clusters in the Magellanic Clouds are examined, since they provide a sequence spanning the age range from 7 × 10⁶ years to 10¹⁰ years.     

Variation of F-region critical frequency (foF2) over equatorial and low-latitude region of the Indian zone during 19th and 20th solar cycle

The effect of solar cycle and seasons on the daytime and nighttime F-layer ionization has been investigated over the equatorial and low-latitude region during 19th (1954–1964) and 20th (1965–1976) solar cycle. The F-layer critical frequency (foF2) data observed from the three Indian Ionosonde stations has been used for the present study. The dependence of foF2 on solar cycle has been examined by performing regression analysis between the foF2 values and R₁₂ (twelve month running average sunspot number). The result shows that the magnitude of the cycle, seasons and the location of station has considerable effects on foF2. There is a significant nonlinear relationship between the foF2 values and R₁₂ during 19th solar cycle as compared to 20th solar cycle. Further, the nighttime saturation effect is prominently seen during the 19th solar cycle and summer season. It is also observed that the most profound saturation effect appears at the equatorial ionization anomaly crest region. Seasonally, it is seen that all the stations exhibits semiannual anomaly. The phenomenon of winter anomaly decays as we move higher along the latitude and is prominently seen during the intense solar activity.     

Models of the high latitude stratosphere and mesosphere

Methods of calculation of interhemispheric and longitudinal differences are discussed. It is noted that in models one should take into account the solar activity influence. Network rocket sounding accuracy, devices and technique compatibility are analysed.