September Equinox Research Articles

Prolonged Quasi‐6‐Day Wave Activities in the Northern Hemisphere MLT Region Due To Antarctic Stratospheric Minor Warming Around September Equinox of 2013 and 2014

AbstractThe geopotential height observations from the Aura Microwave Limb Sounder show that the quasi‐6‐day wave (Q6DW) events with westward zonal wavenumber 1 (W1) in the Northern Hemisphere (NH) mesosphere and lower thermosphere (MLT) during September 2013 and 2014 had a prolonged lifetime of ∼45–50 days and reached their maximum amplitudes after the September equinox, while the climatological Q6DW‐W1 is completely dissipated in the background atmosphere before the equinoxes. The Eliassen‐Palm flux diagnostic results indicate that Q6DW‐W1 during September 2013 and 2014 obtained an additional source at ∼60°‐80°S and ∼40–50 km as the September equinox approached, which is related to the baroclinic/barotropic instability in this region. Further investigation on the background atmosphere reveals that several stratospheric minor warming (SMW) events occurred in the Antarctic region during September 2013 and 2014 due to the enhancement of wavenumber 1 activities, accompanied by the increase in the stratospheric temperature, changes in the shape of the polar vortex and the reversal in the zonal mean circulation. The strong planetary wave breaking during the September 2013 and 2014 Antarctic SMW events significantly weakened the strength of the polar night jet (PNJ) and made its peak height descend below ∼35 km, which generated the baroclinic/barotropic instability at the new upper boundary of the PNJ (∼60°‐80°S and ∼40–50 km) by an anomalous double‐jet configuration in the background winds. This unusual instability provided additional wave source and energy for the trans‐equatorial propagation of Q6DW‐W1, which finally led to prolonged wave activities in the NH MLT region.

The Signature of Sporadic E of an Equatorial Ionosphere of the Low Latitude Region

Sporadic E is usually referred to as the large unpredictable formation of regions of very high electron density in the E region as one of the anomalies that is present in the equatorial region. This work studies the variability of sporadic E in the region with respect to the propagation of low frequency communication and impact on the equatorial region during the occurrence of this anomaly. Hourly data for the year 2010, a year of low solar activity obtained in-situ from a Digisonde Portable Sounder (DPS-4) from the African sector, Ilorin, Nigeria (8.5oN, 4.5oE, -2.96 dip) and the Southern American sector online at Jicamarca, Peru (12 oS, 76.8 oW, 0.74 dip) and Fortaleza, Brazil (3 oS, 38 oW, -7.03dip) were used for this study. The result obtained shows that sporadic E is usually observed at the region during the daytime and early evening (0600- 1700 local time) and is more prevalent during solstice months. Two types of sporadic E were observed in the region: blanket and transparent sporadic E. 12% of the transparent sporadic E was observed during the March equinox while 9% of blanket sporadic E was observed during this period; September equinox shows 15% of transparent and 7% blanket sporadic E respectively, while June solstice has 21% transparent and 7% blanket sporadic E. During the December solstices, 14% transparent and 7% blanket sporadic E was observed. During the March equinox, the blanket sporadic E was observed around 0200 hour local time, and that of the transparent sporadic E at about 1000 hour local time. Observation during the September equinox shows that it occurs around 1100 hour and 0200 hour respectively. It was observed that there is no correlation between sporadic E and geomagnetic storm. Sporadic E has been observed to be one of the anomalies that are responsible for the scintillation and scattering of the lower frequency signal in the said region when present.

Variability of the equatorial ionization anomaly over the South American sector: Effects of electric field and effective meridional wind

Vertical Total Electron Content (VTEC) Maps over the South American Continent were utilized to investigate the temporal and longitudinal climatology of Equatorial Ionization Anomaly (EIA) using more than 350 Global Navigation Satellite Systems (GNSS) receivers. At a temporal resolution of 10 min, the EIA motions, morphologies, and evolutions were mapped using VTEC keogram along magnetic meridians lines. Between 2014 and 2019, characteristics of the EIA were studied at two different South American magnetic meridians (i.e., 3.36° E and 7.58° E) separated by ∼555 km at an altitude of 300 km. The aim of this study is to examine the EIA’s variability, monthly variations and occurrences at evenly spaced longitudinal sectors. The effects of effective meridional winds component and E × B drift velocity on the daytime asymmetry of EIA anomalies were studied using a physics-based numerical model, Sheffield University Plasmasphere-Ionosphere model at Instituto Nacional de Pesquisas Espaciais (SUPIM-INPE). We found that the EIA parameters such as strength, shape, intensity, and latitudinal positions are affected by the eastward electric field and effective meridional wind. The monthly variations in the EIA over two magnetic meridian sectors demonstrate a semiannual variation. The EIA crests were more symmetric in equinox than in solstice seasons. The asymmetries of the EIA observed during the December solstice are more intense than during the June solstice, whereas September equinox is less symmetric than March equinox seasons. Moreover, this study indicates that the vertical drift and the meridional neutral wind plays a very significant role in the development of the EIA asymmetry by transporting the plasma up the field lines. There was a notable contraction of the EIA southern hemispheric (SH) crests from the December solstice to the June solstice. Meanwhile, the EIA crest positions in the northern hemisphere (NH) expand from the December solstice to the June solstice. According to our observations, the March equinox season had the most EIA occurrences, which were then followed by the September equinox, the December and June solstices. The intensities of the EIA crests also considerably decreased with solar descending phases. Through modeling, this work provides the scientific community with new insights into the evolution/development of EIA and their latitudinal asymmetry, as well as the role of E × B drift and thermospheric neutral wind in assessing the statistical analysis of EIA variability using the largest VTEC database over the South American sector.

Analysis of total electron content over the African low-latitude region during the maximum phase of solar cycle 24 (2012–2014)

This study examined the ionospheric variation over the African low-latitude region using Total Electron Content (TEC) and its wavelet power spectrum (WPS). TEC data were obtained from five Global Positioning System (GPS) stations located in the trough and crests of the equatorial ionization anomaly (EIA) in both hemispheres during quiet periods of the maximum phase of solar cycle 24 (2012–2014). Our results revealed a striking equinoctial asymmetry (EA) with stronger TEC and higher solar flux in September equinox than March equinox of 2012–2013, with the reverse being the case in 2014. As such, the difference in solar flux between both equinoxes might have played a dominant role in driving the EA. Furthermore, we found a hemispheric asymmetry in TEC with higher value towards the crest in the northern hemisphere (southern hemisphere) in September equinox (March equinox) of 2012–2013. In 2014, nevertheless, TEC was remarkably higher towards the crest located in the southern hemisphere in September equinox only. The chaotic time series methods used to examine the TEC variations showed positive Lyapunov exponents (LE) at all stations. This indicated the presence of chaoticity in the African EIA. Nevertheless, stations located towards the southern crest (Eldoret and Dodoma) had the maximum LE values while those at the magnetic equator (Adis Ababa) and northern crest (Nama and Al Wajh) had the minimum values. Consequently, the changes in the internal dynamics of the equatorial ionosphere have impacts on its chaotic behavior.

Infestation patterns of a major wood boring pest, Psiloptera fastuosa (Buprestidae: Coleoptera) in Tasar, Terminalia arjuna (Myrtales: Combretaceae) plantation

Tasar silk is a significant cash crop in the tropics. In Tasar culture, arjuna (Terminalia arjuna) (Combretaceae) are often planted on farms because the Tasar worm largely consumes their leaves, Antheraea paphia (Lepidoptera: Saturniidae). The wood-boring Psiloptera fastuosa (Coleoptera: Buprestidae) causes severe threats to Tasar plantations during their mating season (September equinox to December solstice). After hatching, the coleopteran grubs puncture and penetrate the arjuna stem, mature into pupae inside, pass into adults, and exit through emergence holes, causing severe damage to younger plants. Though such infestations are highly fatal, no precautions have been proposed to protect the arjuna plantations. Therefore, sustainable Tasar plantation management is highly challenging because of limited bio-ecological information and the infestation pattern of the pest. This study investigates the origins and implications of P. fastuosa infestations in T. arjuna plantations. Our study focuses on seasonal changes in P. fastuosa assaults on its primary host and infestation frequency in an Indian agroecosystem. Second, we examined whether host plant age and size affected the P. fastuosa invasion. The occurrence and distribution of emergence holes for adult beetles on the arjuna bark and the intensity of wood galleries on the stem aid in assessing the beetle infestation. It was observed that 4 to 8-year-old arjuna plants wilted more frequently than younger (< 4 years) and older (> 8 years) plants. Since most infestations occur on the main trunks up to the middle of the plant's height during autumn, insecticidal treatments can be performed around the middle of the main trunks of 4–8-year-old arjuna plants during fall. The findings of this study will benefit the management and propagation of arjuna plants for tropical Tasar silk production.

Long-term equatorial ionosphere variations over Ethiopia

This paper focuses on the investigation of the long-term behavior of the equatorial ionosphere in Ethiopia. To explore the extended ionospheric variations, we analyze Total Electron Content (TEC) fluctuations in conjunction with solar activity indices, including F10.7 radio flux, EUV radiation, and Sunspot Number (SSN). The dataset used encompasses GPS-derived TEC records spanning 11 consecutive years from 2007 to 2017, corresponding to the period of solar cycle 24. Our analysis shows that the highest TEC levels generally occur during the post-noon hours, while the lowest values are observed around post-midnight or early morning. Notably, exceptions to this pattern are found in 2011 and 2013 when the highest TEC levels are observed during the December solstice. Otherwise, the seasonal TEC variation exhibits a semi-annual pattern, with peak TEC values occurring during the equinoxes and the lowest values during the solstices. Further examination demonstrates that in both solar activity phases, TEC magnitude is higher during the March Equinox compared to the September Equinox, and slightly higher during the December Solstice compared to the June Solstice. This reveals an asymmetry in the seasonal variations, encompassing the equinoxes, solstices, and the ascending and descending phases of the solar cycle. Our study confirms that long-term fluctuations in ionospheric TEC closely correspond to variations in solar activity indices. Correlation analysis indicates a strong link between all solar activity indices and TEC. Notably, while a negative correlation is observed in 2007, the correlation in 2011 is remarkably stronger. Additionally, we find that the long-term TEC variation exhibits stronger correlations with EUV and F10.7 radio flux than with SSN, with correlation coefficients of 0.86, 0.85, and 0.78, respectively. Lastly, our study extends previous research by identifying the long-term trends of vertical TEC (vTEC) in relation to various driving factors such as F10.7, SSN, and EUV.

Characterize the Long-term Ionospheric Response to the Changes in Solar Activity at Low-Latitude Stations of the East African Sector

In this research, we examine the long-term ionospheric response to solar conditions from 2009-2019 using GPS-TEC observations from 8 stations in East Africa at low/equatorial latitudes. To determine the effects of solar activity on ionospheric vertical-Total-Electron-Content (vTEC), solar proxies (EUV, F10.7, and SSN) were used. We have applied statistical analysis and used quadratic fits with solar indices to find the types of trends, forecast vTEC, and describe the daily, monthly, and seasonal variations of vTEC. We stated the fluctuations and dependency of GPS-vTEC on solar activity. From seasonal analysis, the greater values of GPS-vTEC were observed in the equinox’s season from 2009-2016, followed by the December and June solstices, respectively. The highest values of GPS-vTEC were recorded in the years 2014 (70 and 67 TECU), 2013 (57 and 65 TECU), 2012 (56 and 60 TECU) at the March and September equinoxes, and the 2015 March equinox (68 TECU), and the lowest value of GPS-vTEC was recorded (2-3 TECU) for all seasons from 03 to 07 EAT. The changes in vTEC have confirmed worthy agreement with the changes in solar parameters, and EUV flux has a stronger association with vTEC than the F10.7 and SSN by 2% and 7%, respectively. Daily and monthly variations of vTEC showed positive associations with the solar parameters (EUV, F10.7, and SSN) from 2009-2019. We got both linear and nonlinear trends; however, when we approximated (EUV)2,(F10.7)2, and (SSN)2 coefficients, linear trends in vTEC were dominant. This analysis confirms that a quadratic polynomial can well capture the long-term solar activity dependency of vTEC in a statistical sense; based on the anticipated and observed vTEC values, the modeled outcome is suitable using EUV, F10.7, and SSN indices. But the modeled result is better using EUV and F10.7 than SSN in the region. In the predictions of vTEC, the maximum deviations and number of errors were observed during solar maxima years for EUV, F10.7, and SSN compared with the deviations and number of errors in solar minima years. Therefore, F10.7 is a better proxy of EUV flux in solar cycle 24 using monthly values, and the EUV flux is a better controller of GPS-vTEC changes compared to F10.7 and SSN, even if F10.7 is a proxy of EUV based on daily values.

Response of the ionosphere to equatorial electrojet at latitudes near the northern EIA crest in the East African longitudinal sector

Equatorial Electrojet (EEJ) and Equatorial Ionization Anomaly (EIA) are two distinct ionospheric phenomena which are caused by the horizontal configuration of the geomagnetic field at the equator. On the day side of the earth, these activities are controlled by a common zonal electric field, and their interactions must be researched for a number of scientific reasons. In this study, we used multiple Global Navigation Satellite System (GNSS) stations at Al Wajh, Sola village, Nama, and Sheba in 2012 to investigate the correlations between the instantaneous and integrated equatorial electrojet (EEJ) and the variations of vertical total electron content (vTEC) at the northern crest of EIA in the East Africa and Middle East longitudinal sector. The equinoctial and solstitial months have the highest and lowest values of the monthly mean EEJ and vTEC, respectively, and as a result, both parameters show a semi-annual variability. The monthly mean vTEC also demonstrates equinoctial and solstitial asymmetries, resulting in vTEC values that are higher at the September equinox and the December solstice than at the March equinox and the June solstice, respectively. These asymmetries have been also observed in the composition of [O]/[N2] ratio. On days with strong counter electrojet (CEJ), EEJ peaks are smaller and occur later than on days with weak CEJ. Additionally, when CEJ is high, EIA is suppressed and vTEC exhibits low values at all crest points. In the early hours (10:00–12:00 Lt), Nama and Sheb showed higher correlation coefficient values with significant levels than Alwj and Sola. According to the correlation analysis, the anomaly crest develops at the lower latitude stations between 10:00 and 12:00 Lt and then moves to the higher latitude stations later. Generally, the correlations are higher and more significant in the equinoxes than the solstices. The correlations between integrated EEJ (IEEJ) and vTEC were also found to be strong and significant at the lower latitude stations throughout all seasons. These findings suggest that both the instantaneous and integrated EEJ can be used as a proxy for the eastward zonal equatorial electric field and must be considered in regional and global models of total electron content of equatorial and low latitude ionosphere.

Performance evaluation for vertical TEC predictions over the East Africa and South America: IRI-2016 and IRI-2020 versions

In this paper, we evaluate and report the International Reference Ionosphere (IRI) model's vertical Total Electron Content (TEC) regional profile. Diurnal, monthly, seasonal, and storm-time characteristics of IRI estimates over the equatorial region's ionosphere are validated. We compared the vertical TEC derived from IRI-2020 and its predecessor, IRI-2016, with the GPS-TEC measurements. Results show that IRI (both versions) agrees with observed TEC during solar cycle maximum periods. Exceptionally, over Turkwel station, IRI-2020 produced double peak profiles on the March equinox, June solstice, and September equinox and overestimated with larger discrepancies. Over the other three stations, both IRI versions reproduced the seasonal averaged observed TEC with only slight time lags and discrepancies. During geomagnetically perturbed times, IRI-2020 better indicated positively enhanced GPS-TEC than IRI-2016. However, during the March 17, 2015 storm, IRI-2020 underestimated the storm-enhanced TEC over Bahir Dar station at all phases of the storm. IRI-2016 shows good performances for negative storms where lower TEC measurements are recorded.

Response of the F2-peak electron density and height to plasma depletion bays in the equatorial nighttime ionosphere observed by FORMOSAT-7/COSMIC-2

FORMOSAT-7/COSMIC-2 (F7/C2) satellite cluster, launched in June 2019, performs radio occultation experiments observing the vertical electron density profiles in the low-latitude ionosphere. The F7/C2 F2-peak electron density (NmF2) and height (hmF2) are used to identify and examine the behavior of the plasma depletion bay (PDB) structures in the equatorial nighttime ionosphere. The NmF2 reveals three distinct PDBs within 60°W–15°E, 30−90°E, and 110−160°E during June solstice, while a single PDB appears within 150−60°W during December solstice. All the four PDBs’ features simultaneously appear during March and September equinoxes. Asymmetries in the hmF2 over the PDB regions suggests that the electron density increases (decreases) are related to the NmF2 ascending (descending) in the summer (winter) hemispheres. Agreements between F7/C2 observations and HWM93 results show that the asymmetry of vertical motions driven by the neutral wind is the main causal mechanism for the longitudinal variation of PDBs.

Mesoscale Structure and Properties of the Terrestrial Magnetotail Plasma Sheet From the Magnetospheric Multiscale Mission

AbstractUsing Magnetospheric Multiscale mission (MMS) orbits in the Earth's magnetotail from 2017 to 2020, plasma conditions and the 3D spatial structure of inner‐magnetotail plasma environments (with a focus on the plasma sheet (PS)) are studied with different approaches. Threshold conditions for distinguishing the PS, PS boundary layers, and lobes are derived from the statistical properties of background plasma parameters. Our results support previous studies that employed similar methods using Cluster data. However, stronger currents are observed in both the lobes and PS, likely due to the smaller spacecraft separation (≲70 km) that can resolve thin electron‐scale currents. Threshold conditions are used together with magnetic field and electric field measurements to image the spatial structure of the PS. Results are in good agreement with a global neutral sheet model based on solar wind conditions and magnetospheric configurations. Furthermore, the Earth's dipole tilts toward the Sun around June solstice, which warps the magnetotail as much as ∼2–4 RE in Z geocentric solar magnetospheric. This warping effect is relaxed toward September equinox. Consequently, as MMS travels through the magnetotail from dawn to dusk during this period, there is an apparent dawn‐dusk asymmetry in plasma conditions between June and September. Kink‐like flapping waves and interplanetary magnetic field twisting are other mesoscale processes attributed with a few RE of flaring near the flanks. These findings reveal important insights into the mesoscale structure and dynamics of the magnetotail.

Diurnal and seasonal variations of the occurrence of ionospheric irregularities over Nigeria from GNSS data

We investigate the occurrence of ionospheric irregularities in Nigeria during 2014, using the Global Navigation Satellite System (GNSS) total electron content (TEC) rate of change index (ROTI). We categorized days with |Dst| &amp;lt; 30 nT as quiet days and days with |Dst| ≥ 50 nT as disturbed days, during both the daytime and nighttime periods. Our results reveal significant latitudinal differences in both quiet and disturbed conditions. The quiet-time observations indicate that irregularities were consistent across all the stations. The occurrence was strong and consistent at CLBR station in Calabar, Nigeria, which is located further away from the geomagnetic equator (and closer to the southern anomaly crest) than the other receivers. TEC variability is greatly influenced by the geographical location of the Earth’s magnetic field, while its spatial patterns of variability are modulated by the Earth’s diurnal rotation. Near the September equinox, the occurrence of irregularities was inhibited during disturbed periods, but an opposite pattern was observed during the March equinox across all stations. Two peaks of occurrence were observed: one in March and the other in September respectively. The strength of irregularities was generally greater in March equinox compared to September equinox, and they were more pronounced during pre-midnight hours. This pattern could be attributed to the suppressing role of the dynamo electric field, requiring further investigation.

Nighttime wavenumber-four and plasma depletion bays observed by FORMOSAT-5/AIP, ICON/IVM, and COSMIC-2/RO data

We examine nighttime longitudinal structures of ion density and ion velocity in wavenumber-four (WN4) and plasma depletion bays (PDBs) observed by FORMOSAT-5 (F5) and ICON, as well as electron density by FORMOSAT-7/COSMIC-2 (F7C2) in 2020. Three PDBs significantly occur in June and December solstices, while WN4 prominently appear in March and September equinox. F5/AIP (advanced ionospheric probe) ion velocities tend to be westward (eastward) at the PDB (WN4) longitudes, which shows that the ion density maximum and minimum are coincident with the eastward ion velocity maximum and minimum, respectively. ICON/IVM (ion drift meter) field-aligned velocities further reveal that the summer to winter trans-equatorial flow is essential to form PDBs, which further results in upward and downward ion drifts in the upstream and downstream areas (summer and winter hemisphere), respectively. Furthermore, F7C2 radio occultation electron density profiles reveal that three PDBs could prominently appear in June and December solstice in the upper ionosphere. F5/AIP and ICON/IVM ion velocities show that the zonal plasma drift is essential to form nighttime WN4 and PDBs, and the summer to winter trans-equatorial flow is important to PDBs.

Equinoctial Asymmetry in the Upper Ionosphere: Comparison of Satellite Observations and Models

AbstractThe terrestrial ionosphere displays significant equinoctial asymmetry despite the upper atmosphere receiving similar levels of solar ionization energy at a given location and local time in March and September during similar solar activity conditions. This intriguing feature is not well understood or modeled, particularly in the upper ionosphere, and causes of the asymmetry are not fully established and quantified. Yet, their study is important to provide better insights into the atmosphere‐ionosphere coupling processes. Analysis of Langmuir probe data from ESA's Swarm B satellite at ∼525 km altitude reveals that the daytime electron density is larger for all latitudes during the March equinox than during the September equinox, while the electron temperature shows an inverted asymmetry except at low latitudes. Simultaneously obtained neutral density data from Swarm GPS accelerations indicate that the thermosphere is denser during the March equinox. The asymmetry seen by Swarm electron density observations is also present in electron densities obtained using GPS radio occultation measurements from the COSMIC satellites. Simulations were performed using physics‐based ionosphere models (SAMI3, WACCM‐X, and TIE‐GCM) to determine their ability to produce the observed asymmetry, understand the generation mechanism(s), and establish the relative role of physical drivers. Results produced by TIE‐GCM are closest to the observations when seasonally varying eddy diffusivity is considered in the model. The asymmetry produced by other models is noticeably weaker. Modeling of the asymmetry by SAMI3 driven with the TIE‐GCM neutral atmosphere shows that both neutral density and winds play a critical role, but the density has a greater effect.

Reducing the Ionospheric Contamination Effects on the Column O/N2 Ratio and Its Application to the Identification of Non‐Migrating Tides

AbstractPrior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (O/N2) in the context of non‐migrating tides. The retrieval of thermospheric O/N2 from far ultra‐violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N2. Consequently, efforts to characterize the tidal variability in O/N2 have been limited by ionospheric contamination from O+ + e radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval of O/N2 from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modified O/N2 datasets to delineate the response of thermospheric composition to non‐migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra‐Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON‐FUV data set, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction.

Universal Time variations in the magnetosphere

We study the dependencies of Earth’s magnetosphere on Universal Time, UT. These are introduced because Earth’s magnetic axis is not aligned with the rotational axis and complicated because it is eccentric, which makes the offset of the magnetic and rotation poles considerably greater in the Southern hemisphere and the longitudinal separation of the magnetic poles less than 180°: hence consequent UT variations in the two hemispheres are not in equal in amplitude nor in exact antiphase and do not cancel, as they would for a geocentric dipole. We use long series of a variety of geomagnetic data to demonstrate the inductive effect of motions of the polar caps in a “geocentric-solar” frame, which is phase-locked to the Russell-McPherron (R-M) effect on solar-wind magnetosphere coupling. This makes the response of the magnetosphere-ionosphere system different for the two polarities of the Y-component of the Interplanetary Magnetic Field in the GSEQ reference frame, explaining the difference in response to the March and September equinox peaks in solar wind forcing. The sunward/antisunward pole-motion effect is detected directly in satellite transpolar voltage data and is shown to have a greater effect on the geomagnetic data than the full dipole tilt effect which generates the equinoctial pattern, the potential origins of which are discussed in terms of the dipole tilt effect on ionospheric conductivities and the stability of the near-Earth tail. Persistent UT variations in Region-1 and Region-2 field-aligned currents and in partial ring current indices are presented: their explanation is an important challenge for numerical modelling of the magnetosphere-ionosphere-thermosphere system which we need to quantify the relative contributions of the various mechanisms and to give understanding of the effect of arrival time on the response of the system to large, geoeffective disturbances in interplanetary space.Plain language summary: The effect on terrestrial space weather of Earth’s magnetic axis not being aligned with the rotational axis is investigated. It is complex because not only do these two axes not align in direction (the “dipole tilt”), the magnetic axis does not pass through the centre of the Earth, which sets a requirement for an “eccentric” model of the field and not the commonly-used “geocentric” one. For many years, it has been known that the dipole tilt gives a peak in geomagnetic activity at the equinoxes (the semi-annual variation) through the “Russell-McPherron” (R-M) effect. However, although the variation with Universal Time is consistent with the R-M effect for the September equinox, it is not for the March equinox. We here solve this long-standing puzzle by investigating the effects of the motions of the two poles in a frame fixed with respect to both the Earth and the Sun for an eccentric dipole model. But solving one puzzle generates many others. We present observations of the Universal Time variations that these mechanisms combine to generate, which set an important challenge to the numerical modelling of the near-Earth space environment.

Correlation analysis of normalized ROTI and S4 as observed during different geomagnetic conditions of the 2013 September equinox over the stations within EIA African sector

It has been a concern of space scientists and engineers for many decades to have a well-defined proxy to characterize the fluctuations experienced by radio-based signals propagating through the ionosphere. Scintillation indices were deduced using the signal intensity to show the level of fluctuations in phase and amplitude of radio signal passing through the ionosphere. Subsequently, more than two decades now, the rate of change of Total Electron Content (TEC) index (ROTI) was as well used to characterize the level of fluctuations associated to ionospheric irregularities on radio signals. This paper examines the level of relationship between the rate of change of TEC index (ROTI) and the ionospheric amplitude scintillation (S4) within the belt of equatorial ionization anomaly of West African sector. The study was carried out using the Scintillation Network Decision Aid (SCINDA) Global Positioning System (GPS) data obtained from ground-based stations located at Dakar (Senegal) and Cape Verde. The comparative analysis of ROTI and S4 was done with normalized ROTI. The results obtained have shown that the magnitude of normalized ROTI and S4 corresponds very well, higher in Cape Verde compared to Dakar. It was observed that the correlation coefficient could be ∼0.6 when all the satellites are used. However, the relationship increases with individual satellite in which the coefficient of correlation could be up to ∼0.97 and R2 ∼ 0.94. It can be concluded that the relationship between the S4 and ROTI depends on the location and the level of ionospheric disturbance condition as observed in this work.

Equinoctial asymmetry during solar minima at low to mid latitude

Equinoctial asymmetry (EA) of F2-layer critical frequency (foF2) during minimum years of solar cycle 22–23 (1996) and 23–24 (2008) has been studied for low and mid latitude stations ranges from 20°N - 55° N. Year 2008 provided an opportunity to study this phenomenon in extreme low EUV conditions. Pronounced/reasonable increase in EA strength at low/mid latitudes from 1996 to 2008 indicates role of EUV in EA. In addition to an increase in strength of EA from 1996 to 2008, variation in its strength and sign (positive if Mar Equinox foF2means > Sep Equinox foF2means and vice versa) with latitude and longitude are also observed during both years. Similar to previous studies, EA is observed in daytime during both years. However at low latitude stations, two peaks of foF2 are also observed at night: the first peak in summer and the second peak in the beginning of the September equinox which shows superposition of semi-annual and annual components. Hourly analysis showed asymmetry (Sep Equinox foF2means > Mar Equinox foF2means) from sunset to mid-night at low mid to mid latitude. It appears that the Mid latitude Summer Nighttime Anomaly (MSNA) is extending to September equinox with decreasing magnitudes from Summer to Sep equinox. IRI-16 showed EA during daytime only with over- and under- estimation as compared to observed data. Serious discrepancy between observed and IRI-2016 model values especially during deep minimum suggests modification of model for extreme low EUV condition as well as incorporation of parameters related to global wind pattern in the model.

The effect of month and breed on plasma adrenocorticotropic hormone concentrations in equids

Use of plasma adrenocorticotropic hormone (ACTH) concentrations for the diagnosis of PPID in equids requires awareness of other factors that might influence ACTH concentrations, including breed and time of year. This study was designed to investigate effects of breed on plasma ACTH concentrations and potential interactions between breed and time of year. Ten breeds were selected from a laboratory database containing data on plasma ACTH concentrations. Breeds represented were Arabians, Cobs, Connemara ponies, donkeys, Irish Sports horses, New Forest ponies, Shetland ponies, Thoroughbreds, Warmbloods and Welsh breeds. Multivariable analysis was used to determine the effects of month and breed on plasma ACTH concentrations.A circannual pattern of ACTH concentration was documented, with a nadir in April and a peak in September. Arabian horses and donkeys had significantly higher ACTH concentrations than some other breeds, primarily from May to November, whereas in Shetland ponies and Welsh breeds, relatively higher ACTH concentrations only occurred from July to November. The annual increase in plasma ACTH appeared to commence in late April/early May in response to increasing daylight and decreased again promptly after the late September equinox as daylength fell below 12 h per day. Inter-breed differences in plasma ACTH were substantial and indicated that breed and time of year should be considered when interpreting plasma ACTH concentrations in equids suspected for PPID.

On the high percentage of occurrence of type-B 150-km echoes during the year 2019 and its relationship with mesospheric semi-diurnal tide and stratospheric ozone

• Anomalously high occurrence of 150-km radar echoes over Kototabang during September 2019. • In the years 2017 and 2019, SW2 dominates DW1 due to the larger stratospheric ozone, when compared to other years. • Dominant SW2 along with solar minimum conditions may lead to large echo occurrence through interchange instability. The Kototabang (0.20°S, 100.3°E, 10.36°S dip latitude) Equatorial Atmosphere Radar (EAR) observations of valley region field-aligned irregularities (FAI) reveal maximum percentage of occurrence (PO) of high signal to noise ratio (greater than 3 dB) type-B 150-km echoes during boreal summer (June-August) and winter (December-January) of solar moderate to minimum years 2016–2019. The PO is anomalously high in the solar minimum year 2019 and particularly during the September equinox when a major austral sudden stratospheric warming (SSW) event has occurred. Its possible relation with mesospheric tides and stratospheric ozone is investigated. The space–time spectral analysis of upper mesospheric temperature information obtained from the spaceborne radiometer instrument Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) reveals that migrating semi-diurnal tide (SW2) is dominant during June-August, when the PO of the 150-km echoes maximizes, whereas migrating diurnal tide (DW1) is dominant during equinox months. It also reveals quasi-biennial variability of the DW1 tide probably due to similar variability of the stratospheric ozone. It is suggested that the DW1 tides generated due to solar radiation absorption by stratospheric ozone and tropospheric water vapour can have phases opposite to each other, leading to the suppression of DW1 tide during September 2017 and 2019 resulting in the relative dominance of SW2 tide over DW1 tide. The meridional wind shear associated with the relatively dominant SW2 tide results in an interchange instability developed on the gradient of daytime descending ion layer along with solar minimum conditions leads to plasma irregularities responsible for these echoes. The large eastward propagating diurnal tide with zonal wavenumber 3 (DE3) observed during June-September 2019 is unlikely to cause the instability, due to its weak meridional component.