Phase Of Solar Cycle Research Articles

The Statistical Analysis of Characteristics of Coronal Mass Ejection During the Descending Phase of Solar Cycle 23 and 24

The characteristics of CMEs we studied are angular width, linear speed, and acceleration for all categories of CMEs such as narrow (W ≤20°), intermediate (20°< W<200°), wide (W ≥ 200°) and linear speed <500 km/s during the descending phase of solar cycle 23 and 24 and compared them. We have found that there are 1951 narrow CMEs during solar cycle 23 that is 1.9 times greater than in solar cycle 24 (1047). On the other side, the number of intermediate CMEs during solar cycle 24 (1571) is 1.14 times more than solar cycle 23 (1162). We observed no noticeable difference between the number of wide CMEs of solar cycle 23 (29) and 24 (36). The angular width of CMEs during the descending phase of solar cycle 23 and solar cycle 24, predominately distributed around 100-600. The fascinating result is that the angular width distributions for the descending phase of solar cycles are approximately identical. On comparing the results of linear speed of both solar cycle, we can say that, (i) 93.7% (1729) and 87.7% (908) of narrow CMEs, (ii) 97% (1328) and 94% (1479) of intermediate CMEs and (iii) 44% (13) and 42% (15) of wide CMEs have speed of <500 km s-1, respectively. Mostly the fractions of narrow and intermediate CMEs decline sharply at the speeds greater than 500 km s-1. The maximum speed observed during the 23rd cycle is 1994 km/s (wide CME) and the 24th cycle is 3163 km/s (wide CME) respectively. It was noticed that the speed of the 24th solar cycle CME is higher than the 23rd solar cycle CME. The major fraction of CMEs has acceleration in the range of -20 to 20 km s-2, all types of CMEs. The narrow and intermediate CMEs mostly show acceleration while wide CMEs show deceleration.

VHF scintillations and plasma drifts observed in southern Taiwan during the declining phase of solar cycle 24

VHF scintillations and plasma drifts observed in southern Taiwan during the declining phase of solar cycle 24

Responses of the African equatorial ionization anomaly (EIA) to some selected intense geomagnetic storms during the maximum phase of solar cycle 24

This study investigates the morphology of the GPS TEC responses in the African Equatorial Ionization Anomaly (EIA) region to intense geomagnetic storms during the ascending and maximum phases of solar cycle 24 (2012–2014). Specifically, eight intense geomagnetic storms with Dst ≤ −100 nT were considered in this investigation using TEC data obtained from 13 GNSS receivers in the East African region within 36–42°E geographic longitude; 29°N–10°S geographic latitude; ± 20°N magnetic latitude. The storm-time behavior of TEC shows clear positive and negative phases relative to the non-storm (median) behavior, with amplitudes being dependent on the time of sudden commencement of the storm and location. When a storm starts in the morning period, total electron content increases for all stations while a decrease in total electron content is manifested for a storm that had its sudden commencement in the afternoon period. The TEC and the EIA crest during the main phase of the storm is significantly impacted by the geomagnetic storm, which experiences an increase in the intensity of TEC while the location and spread of the crest usually manifest a poleward expansion.

ON THE CHARACTER OF AN ARTIFICIAL SATELLITE DRAG UNDER VARIOUS STATES OF SOLAR AND GEOMAGNETIC ACTIVITY

Purpose: The artificial satellites drag in the atmosphere remains an urgent problem to date. In this work, the artificial satellites data are used in order to study the atmosphere state under various manifestations of solar and geomagnetic activity. The selected satelites were moving uncontrollable being good indicators of the upper atmosphere state. The B-star (drag term) drag coefficient is used in this work. This term is used in the SGP and SDP models to take into account the resistance of the atmosphere to the satelite orbital motion. The data of the drag of two artificial satellites, one moving in elliptical and the other in circular orbits at midlatitudes (orbital plane angles of 58°-60°) were considered. These data include the end of the 23rd solar activity cycle, as well as the growth, the maximum and the decay phases of the 24th solar cycle (years 2005–2017). Seven periods of anomalous drag of the satellites were analyzed. They are: 4 monthly periods (two in 2005 and two in 2011) and 3 yearly periods (within 19.07.2014 to 22.08.2015), five-year long (2005–2010) and six-year long (2011–2017) periods. Design/methodology/approach: The periodogram analysis was made. This allowed to reveal the periodic processes in changes in the state of the atmosphere of different duration. The correlation coefficients of the B-star drag term with the indices of solar and geomagnetic activity were calculated. The analysis of extreme drag of the satellites in the periods of the increased solar and geomagnetic activity (intervals of observation lasting a month) was made. Findings: Using the solar and geomagnetic data we found that some month-long part of the anomalous drag periods were followed by flares on the Sun and the arrival of the coronal mass ejections into the near-Earth space. At time intervals of yearlong observations the highest values (0.5-0.7) were obtained for the coefficients of the B-star parameter correlation with the solar activity indices – solar radiation at the wavelength of 10.7 cm, F10.7, and Lyman alpha radiation, Lα. At monthly time intervals, the largest values of the correlation coefficients were obtained for the B-stars with the electron fluxes with energies above 0.6 and 2 MeV, E, (0.3-0.5), the Lyman alpha radiation, Lα, (0.58–0.73 for a сircular orbit satellite), and the solar constant, TSI, (0.3–0.6), as well as the geomagnetic storms intensity index, Dst , (0.66–0.69). Periodogram calculations show the presence of a whole spectrum of periods in the deceleration of a circular orbit satellite and a dedicated period for an elliptical orbit satellite. Conclusions: The B-star drag term dependences on the indices of solar and geomagnetic activity during some periods of their intensification for the 23–24 cycles of solar activity are considered. The periodogram analysis made together with the analysis of the conditions and parameters of space weather allows to see the general and more detailed picture of the solar and geomagnetic activity influence on the change in the motion of the satellite in the atmosphere. The B-star drag term helps to consider only the atmosphere influence on the artificial satellite movement in the near-Earth space. Key words: artificial satellite, atmosphere, artificial satellite drag, solar activity, geomagnetic activity, space weather

Long- and Short-term Variability of Galactic Cosmic-Ray Radial Intensity Gradients between 1 and 9.5 au: Observations by Cassini, BESS, BESS-Polar, PAMELA, and AMS-02

Abstract Spatial gradients of galactic cosmic-ray (GCR) fluxes are important for studying charged particle transport in the heliosphere. Little is known so far about how these gradients evolve with time. Here we present how the radial gradient (G r ) evolves between 2006 January and 2017 September using Cassini measurements of >300 MeV protons, which we combine with proton spectra obtained by advanced observatories at Earth (BESS, BESS-Polar, PAMELA, and AMS-02). All gradient calculations were performed for a nearly constant heliocentric distance of Cassini from Earth’s orbit and near the ecliptic, thus revealing only how G r changes with time. The variability patterns of G r are well established as they rely on a single data set for ∼9.5 au and accurately cross-calibrated GCR spectra for Earth at 1 au. We show that over solar-cycle timescales, G r is regulated by both the polarity of the solar magnetic field and the solar-cycle phase. During the negative-polarity phase (A < 0, 2006–2014), gradients are stronger and more stable with an average of G r = 3.5 ± 0.3%/au and with evidence of a minimum around the 2009 solar minimum. The gradient peaks at ∼4%/au around the solar maximum and subsequently experiences a gradual drop to ∼2%/au in the A > 0 phase (2014–2017). Regular G r enhancements over yearly or biennial timescales are also observed, in phase with quasi-biennial oscillations in GCR intensity. While all aforementioned results are based on Cassini measurements that are integral in energy, they are most representative for GCR protons in the low gigaelectronvolt range.

Sunspot Activity and Coronal Holes during the Declining Phase of Cycle 24

The dynamics of coronal holes (CHs) during the declining phase of solar cycle 24 is studied based on observational data from the Atmospheric Imaging Assembly (AIA), the 19.3 nm channel, of the Solar Dynamic Observatory (SDO). Some features of changes in the areas of polar and mid- and low-latitude CHs in the period April 1, 2014–December 31, 2019, are revealed by a comparison of the variations in the daily total CH area with the index of the sunspot number. The division of all CHs observed in the considered period into polar and nonpolar shows that polar-CH areas dominate and increase while nonpolar-CH areas constantly decrease toward the cycle minimum. Polar-CH areas account for ~80% of the total area of all CHs considered over the studied period. The total polar-CH area increases toward cycle minimum, which is consistent with the general concept of polar CHs as the main source of the Sun’s dipole magnetic field. However, the second subset, which contains selected isolated CHs, shows an interesting behavior in the declining phase of solar activity that was not previously observed. Their areas slowly decrease from the maximum to minimum of the solar cycle. This trend is qualitatively consistent with a decrease in sunspot activity toward the minimum, which suggests a physical relationship between the two phenomena.

Spatial and Temporal Properties of the Solar Cycle from RATAN-600 Microwave Observations

The study involves the obtainment of regular tracers of the solar cycle in the radio range by activity manifestations in the chromosphere and corona. Our research method consists of the systematic processing of the RATAN-600 radio data at frequencies 3–18 GHz obtained during more than one cycle of solar activity and their comparison with other databases of time series characterizing the magnetic activity of the Sun at the photospheric level. In the first phase of this work, we identified an array of approximately 600 active regions that evenly covers all phases of solar cycle 24 in approximately 9 years, for which we reliably identified the local radio sources based on RATAN-600 data, as well as data from other databases, such as spacecraft of the Space Weather Prediction Center of the National Oceanic and Atmospheric Administration and the Solar Dynamics Observatory. The sample is statistically homogeneous and evenly distributed over solar latitudes in both hemispheres. The statistical relationships between the sunspot areas and the parameters of intensity and polarization of local radio sources are obtained. It is shown how the spectral flux density of local radio sources in the chromosphere and corona at different frequencies increases with the increase in the sunspot area (and, consequently, the magnetic flux). The estimates of exponents of this dependence are obtained.

Fof2 prediction with IRI-2016 at Dakar Station during quiet activity over solar cycles 21 and 22

This study deals with comparison between Dakar station ionospheric’s F2 layer critical frequency (foF2) data and the two subroutines (CCIR and URSI) of IRI-2016 model predictions. Comparisons are made for very quiet geomagnetic activity during the four phases of the solar cycles 21 and 22 (minimum, increasing, maximum and decreasing). Model's predictions are suitable with observed data by day than by night. The accuracy is better during increasing phase and poor during the other solar cycle phases. During minimum, maximum and decreasing solar phases, both model subroutines do not express the signature of E×B and on other hand they express an intense counter-electrojet instead of a medium intensity electrojet. Key words: Solar cycle, geomagnetic quiet activity, ionosphere electrodynamic phenomena, IRI-2016 model prediction.

FoF2 Seasonal Asymmetry Diurnal Variation Study during Very Quiet Geomagnetic Activity at Dakar Station

This paper aims to study the foF2 seasonal asymmetry diurnal variation at Dakar station from 1976 to 1995. We show that equinoctial asymmetry is less pronounced and somewhere is absent throughout 21 and 22 solar cycles. The absence of equinoctial asymmetry may be due to Russell-McPherron mechanism and the vertical drift E × B . The solstice anomaly or annual anomaly is always observed throughout both 21 and 22 solar cycles as measured at Dakar ionosonde. The maximum negative value of σfoF2, fairly equal to -65%, is observed during the decreasing phase at solstice time; this value appeared usually at 0200 LT except during the maximum phase during which it is observed at 2300 LT. The maximum positive value, fairly equal to +94%, is observed at 0600 LT during solar minimum at solstice time. This annual asymmetry may be due to neutral composition asymmetric variation and solar radiation annual asymmetry with the solstice time. The semiannual asymmetry is also observed during all solar cycle phases. The maximum positive value (+73%) is observed at 2300 LT during solar maximum, and its maximum negative (-12%) is observed during the increasing phase. We established, as the case of annual asymmetry, that this asymmetry could not be explained by the asymmetry in vertical velocity E × B phenomenon but by the axial mechanism, the “thermospheric spoon” mechanism, and the seasonally varying eddy mixing phenomenon.

Solar cycle modulation of the connection between boreal winter ENSO and following summer South Asia high

In this study, the solar cycle modulation of the connection between boreal winter El Niño-South Oscillation (ENSO) and following summer South Asia High (SAH) are revealed. For El Niño accompanied by low solar cycle (EL & LS) or La Niña accompanied by low solar cycle (LA & LS), the boreal winter ENSO prominently affects the following summer's SAH variation. The SAH is enhanced (weakened) and expanded (narrowed) in the meridional and zonal directions in the EL & LS (LA & LS) phase. The composite difference in the SAH between El Niño and La Niña events is obviously stronger in the low solar cycle phase. In low solar cycle phase, El Niño signal and charging process are stronger, which lead to stronger warming sea surface temperature (SST) appear in tropical Indian Ocean and Maritime Continent extending to northwestern Pacific, resulting in larger SAH variations via warming tropospheric temperature. However, for El Niño with a high solar cycle (EL & HS) or La Niña with a high solar cycle (LA & HS), the variations in the SAH and the composite difference of the SAH between El Niño and La Niña events are quite weak in the high solar cycle phase. The charging process over tropical Indian Ocean induced by El Niño is weaker in the high solar cycle phase, which leads to weaker warming SST appear over tropical Indian Ocean, resulting in smaller SAH variations.Further investigation based on longer historic data also ensures that the 11-year solar cycle modulates the ENSO-SAH connection, with a more robust connection in the low solar cycle phase. The 11-year solar cycle should be considered when investigating the connection between the preceding winter ENSO and following summer SAH variations.

Source-region characteristics of anemone active regions in the ascending phase of solar cycle 24

Context. Active regions in close proximity to coronal holes, also known as anemone regions, are the best candidates for studying the interaction between closed and open magnetic field topologies at the Sun. Statistical investigation of their source-region characteristics can provide vital clues regarding their possible association with energetic events, relevant from space weather perspectives. Aims. The main goal of our study is to understand the distinct properties of flaring and non-flaring anemone active regions and their host coronal holes, by examining spatial and magnetic field distributions during the rise phase of the solar cycle, in the years 2011–2014. Methods. Anemone regions were identified from the minimum-distance threshold, estimated using the data available in the online catalogs for on-disk active regions and coronal holes. Along with the source-region area and magnetic field characteristics, associated filament and flare cases were also located. Regions with and without flare events were further selected for a detailed statistical examination to understand the major properties of the energetic events, both eruptive and confined, at the anemone-type active regions. Results. Identified anemone regions showed weak asymmetry in their spatial distribution over the solar disk, with yearly average independent from mean sunspot number trend, during the rise phase of solar cycle 24. With the progression in solar cycle, the area and minimum-distance parameters indicated a decreasing trend in their magnitudes, while the magnetic field characteristics indicated an increase in their estimated magnitudes. More than half of the regions in our database had an association with a filament structure, and nearly a third were linked with a magnetic reconnection (flare) event. Anemone regions with and without flares had clear distinctions in their source-region characteristics evident from the distribution of their properties and density analysis. The key differences included larger area and magnetic field magnitudes for flaring anemone regions, along with smaller distances between the centers of the active region and its host coronal hole.

The Solar Corona during the Total Eclipse on 1806 June 16: Graphical Evidence of the Coronal Structure during the Dalton Minimum

Abstract Visible coronal structure, in particular the spatial evolution of coronal streamers, provides indirect information about solar magnetic activity and the underlying solar dynamo. Their apparent absence of structure observed during the total eclipses throughout the Maunder minimum has been interpreted as evidence of a significant change in the solar magnetic field from that during modern solar cycles. Eclipse observations available from the more recent Dalton minimum may be able to provide further information, with sunspot activity being between the levels seen during recent solar cycles and in the Maunder minimum. Here, we show and examine two graphical records of the total solar eclipse on 1806 June 16, during the Dalton minimum. These records show significant rays and streamers around an inner ring. The ring is estimated to be ≈0.44 R ⊙ in width and the streamers in excess of 11.88 R ⊙ in length. In combination with records of spicules or prominences, these eclipse records visually contrast the Dalton minimum with the Maunder minimum in terms of their coronal structure and support the existing discussions based on the sunspot observations. These eclipse records are broadly consistent with the solar cycle phase in the modeled open solar flux and the reconstructed slow solar wind at most latitudes.

Physical Conditions in Coronal Holes

The physical conditions in the chromosphere of polar and equatorial coronal holes in the decay phase of the 24-th solar activity cycle are studied. Models of the chromosphere are constructed based on the intensities of the HeI 10830 A and Hα lines and using a non-LTE program. The calculations showed that the temperature in equatorial coronal holes is similar to that of polar coronal holes. The temperature in coronal holes is lower than that of the unperturbed chromosphere and increases depending on height in the chromosphere. The difference between the atmospheres of coronal holes and the quiet Sun begins at the level of the upper photosphere- lower chromosphere.

Analysis of Large Deflections of Prominence–CME Events during the Rising Phase of Solar Cycle 24

Motivated by the need to improve the ability to forecast whether a certain coronal mass ejection (CME) is to impact Earth, and by the insufficiency of statistical studies that analyze the whole erupting system with the focus on the governing conditions under CME deflections, we performed a careful analysis of 13 events along a one-year time interval showing large deflections from their source region. We used telescopes imaging the solar corona at different heights and wavelengths on board the Project for Onboard Autonomy 2 (PROBA2), Solar Dynamics Observatory (SDO), Solar TErrestrial RElations Observatory (STEREO), Solar and Heliospheric Observatory (SOHO) spacecraft and from National Solar Observatory (NSO). By taking advantage of the quadrature position of these spacecraft from October 2010 until September 2011, we inspected the 3D trajectory of CMEs and their associated prominences with respect to their solar sources by means of a tie-pointing tool and a forward model. Considering the coronal magnetic fields as computed from a potential field source surface model, we investigate the roles of magnetic energy distribution and kinematic features in the non-radial propagation of both structures. The magnetic environment present during the eruption is found to be crucial in determining the trajectory of CMEs, in agreement with previous reports.

Simulation of the Interplanetary Bz Using a Data-driven Heliospheric Solar Wind Model

Abstract Aimed to be ready for the transition from research to operation, we have developed a solar wind model by coupling a data-driven empirical coronal model with a magnetohydrodynamics heliospheric model. We performed a data-driven simulation of the solar wind for a two-year period during the declining and minimum phases of solar cycle 23. Comparisons with OMNI and Ulysses spacecraft data show that the model can reproduce the large-scale variations of the solar wind plasma parameters. The evolution of geocentric solar magnetospheric (GSM) B x and B z components are also reasonably duplicated by the model in terms of polarity and strength. Apparent signatures of the Russell–McPherron (R-M) effect are found from both observed data and simulated results, indicating that during the investigated interval the R-M effect is the dominant mechanism that controls the large-scale evolution of the north–south component of the interplanetary magnetic field in the GSM frame. The results demonstrate that the established model can provide valuable space weather information about the solar wind.

Long‐Term Variations of >16‐MeV Proton Fluxes: Measurements From NOAA POES and EUMETSAT MetOp Satellites

AbstractBased on the recalibrated data of >16‐MeV proton omnidirectional integral fluxes obtained from NOAA POES and EUMETSAT MetOp satellites during the period from 1978 to 2014, solar cycle phase lags, the variations of proton radiation belt, and the relationships between trapped proton fluxes and F10.7 flux, cosmic ray, or sunspot number are investigated in detail. It is found that (a) the solar cycle phase lags between energetic proton fluxes and sunspot number are within 2.5 years, dependent on solar cycle, Lm, B/B0, and proton energy; (b) at the magnetic equator, the ratios of the maximal to minimal values of 4‐month smoothed monthly >16‐MeV proton fluxes during the period from 1978 to 2014 increase and then decrease with increasing Lm, showing the maximal value ~11.24 at Lm = 1.15; and (c) at a magnetic field line with Lm ≤ 1.15, the ratios of the maximal values in 1987 to the minimal values in 2002 for yearly >16‐MeV proton fluxes decrease with increasing B/B0, while at a magnetic field line with Lm ≥ 1.16, they increase and then decrease with increasing B/B0, and the maximal ratio for all magnetic field lines can reach above 20. An exponential equation with an offset is feasible to describe the relationship between the logarithm of >16‐MeV proton fluxes and F10.7 flux. It can be readily used to improve the NOAAPRO model.

Characterization of African Equatorial Ionization Anomaly During the Maximum Phase of Solar Cycle 24

AbstractThis study characterizes the African equatorial ionization anomaly (EIA) during the maximum phase of Solar Cycle 24 (2012–2015). Total electron content (TEC) data were obtained from a chain of 13 African Global Navigation Satellite System (GNSS) receivers (within 36–42°E geographic longitude; 29°N to 10°S geographic latitude; ±20°N magnetic latitude) to study quiet time variations of TEC and thereafter construct the EIA profiles. The correlations of TEC for pairs of conjugate stations within the African EIA were determined. TEC station‐to‐trough ratio (TEC‐STR) was used as an index to determine the strength of the EIA crests. Furthermore, the variability of the EIA crests on a 3‐hourly basis over each month was investigated. Overall, equinoxes recorded the highest values of TEC, with the widest latitudinal location of the EIA crests, and June solstice recorded the least, with the characteristic equatorward collapse of the EIA crests. TEC and the location of the crests of the EIA also increased with solar activity. TEC showed good correlations for conjugate pairs of stations, especially for stations closer to the trough. The African EIA morphology showed asymmetry in the locations of the Northern Hemisphere (NH) and Southern Hemisphere (SH) crests. The formation of EIA starts around 0600–0800 LT and decays around 2100–2300 LT. Generally, SH crest forms first before NH crest and also decays first; we attributed this to the effects of the meridional neutral winds. Comparing TEC at African EIA crests with other longitudinal sectors, the Asian sector recorded the highest TEC, while the American sector recorded the least.

Clustering of Fast Coronal Mass Ejections during Solar Cycles 23 and 24 and the Implications for CME–CME Interactions

Abstract We study the clustering properties of fast coronal mass ejections (CMEs) that occurred during solar cycles 23 and 24. We apply two methods: the Max Spectrum method can detect the predominant clusters, and the declustering threshold time method provides details on the typical clustering properties and timescales. Our analysis shows that during the different phases of solar cycles 23 and 24, CMEs with speeds ≥1000 km s−1 preferentially occur as isolated events and in clusters with, on average, two members. However, clusters with more members appear, particularly during the maximum phases of the solar cycles. Over the total period and in the maximum phases of solar cycles 23 and 24, about 50% are isolated events, 18% (12%) occur in clusters with two (three) members, and another 20% in larger clusters ≥4, whereas in a solar minimum, fast CMEs tend to occur more frequently as isolated events (62%). During different solar cycle phases, the typical declustering timescales of fast CMEs are τ c = 28–32 hr, irrespective of the very different occurrence frequencies of CMEs during a solar minimum and maximum. These findings suggest that τ c for extreme events may reflect the characteristic energy build-up time for large flare and CME-prolific active regions. Statistically associating the clustering properties of fast CMEs with the disturbance storm time index at Earth suggests that fast CMEs occurring in clusters tend to produce larger geomagnetic storms than isolated fast CMEs. This may be related to CME–CME interaction producing a more complex and stronger interaction with Earth’s magnetosphere.

Magnetic and Velocity Field Topology in Active Regions of Descending Phase of Solar Cycle 23

We analyse the topology of photospheric magnetic fields and sub-photospheric flows of several active regions (ARs) that are observed during the peak to descending phase of the solar cycle 23. Our analysis shows clear evidence of hemispheric preferences in all the topological parameters such as the magnetic, current and kinetic helicities, and the 'curl-divergence'. We found that 68\%(67\%) ARs in the northern (southern) hemisphere with negative (positive) magnetic helicities. Same hemispheric preference sign is found for the current helicities in 68\%(68\%) ARs. The hemispheric preferences are found to exist statistically for all the time except in few ARs observed during the peak and the end phases of the solar cycle. This means that magnetic fields are dominantly left(right)-helical in scales smaller than individual ARs of northern(southern) hemisphere. We found that magnetic and current helicities parameters show equator-ward propagation similar to the sunspot cycle. The kinetic helicity showed similar hemispheric trend to that of magnetic and current helicity parameters. There are 65\%(56\%) ARs with negative (positive) kinetic helicity as well as divergence-curl, at the depth of 2.4\,Mm, in the northern (southern) hemisphere. The hemispheric distribution of the kinetic helicity becomes more evident at larger depths, e.g., 69\%(67\%) at the depth of 12.6\,Mm. Similar hemispheric trend of kinetic helicity to that of the current helicity supports the mean field dynamo model. We also found that the hemispheric distribution of all the parameters increases with the field strength of ARs. The topology of photospheric magnetic fields and near surface sub-photospheric flow fields did not show good association but the correlation between them enhances with depths which could be indicating more aligned flows at deeper layers of ARs.

Statistical Characteristics and Correlation of Low‐Latitude F Region Bottom‐Type Irregularity Layers and Plasma Plumes Over Sanya

AbstractIonospheric F region bottom‐type irregularity layer has been frequently detected at equatorial latitude and regarded as a possible precursor of equatorial backscatter plumes producing scintillations. In this study, the VHF radar observations of ionospheric F region bottom‐type irregularity layers and plasma plumes over the low‐latitude station Sanya (18.4°N, 109.6°E, dip lat. 12.8°N) during the equinoctial months (March, April, September, and October) of 2011–2018 are analyzed. The results show an equinoctial asymmetry in the occurrence of F region backscatter plumes, wherein the occurrence could be greater in spring or autumn months under different solar activities and background ionospheric conditions (F layer height and density). On the other hand, the bottom‐type irregularity layer occurred almost exclusively in the descending phase of solar cycle (2015–2018). No clear correlation is found between the occurrences of bottom‐type irregularity layer and plasma plume at low latitude. The F layer virtual height (h′F) observations from the Sanya ionosonde show that the average h′F is higher during the months when the plume occurrence is higher, suggesting that the asymmetric plasma vertical drift could play a role in the equinoctial asymmetry of plume occurrence. Larger vertical drift (and higher h′F) could cause higher occurrence of equatorial plasma plumes which extend to low latitudes along magnetic field lines. In contrast, during the days when bottom‐type irregularity layers occur, the h′F is relatively lower. We surmise that only when the background F region is situated at low altitudes (with h′F < ~250 km), the low‐latitude bottom‐type irregularity layer could be generated.