Thermospheric winds over Abuja during solar minimum period

Journal Pre-proofs Thermospheric winds over Abuja during Solar Minimum Period W.T. Sivla, O. Ogunjobi, F. Tesema PII: DOI: Reference:

S0273-1177(19)30851-8 https://doi.org/10.1016/j.asr.2019.11.042 JASR 14567

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Advances in Space Research

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20 June 2019 19 November 2019 29 November 2019

Please cite this article as: Sivla, W.T., Ogunjobi, O., Tesema, F., Thermospheric winds over Abuja during Solar Minimum Period, Advances in Space Research (2019), doi: https://doi.org/10.1016/j.asr.2019.11.042

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Thermospheric winds over Abuja during Solar Minimum Period Sivla W.T.1,*, Ogunjobi O2. and Tesema F3,4. 1Department

of Physics and Astronomy, University of Nigeria, Nsukka, Enugu State, Nigeria

2Department 3Washera

of Physics, Federal University Lokoja, Lokoja, Kogi State, Nigeria

Geospace and Radar Science Laboratory, Bahir Dar University, Bahir Dar, Ethiopia.

4Department

of Arctic Geophysics, University Centre in Svalbard, Longyearbyen, Norway E-mail: [email protected]

Abstract Monthly variations of averaged nighttime thermospheric winds have been investigated over Abuja, Nigeria (Geographic: 9.060N, 7.50E; Geomagnetic: 1.600S). The reports are based on Fabry-Perot interferometer measurements of Doppler shifts and Doppler broadening of the 630.0 nm spectral emission. The results were obtained during a period of weak solar activity with the solar flux (F10.7) typically below 70 solar flux units.Inspection of the average monthly thermospheric winds from October 2017 to December 2017 found December meridional winds to be more equatorward than the October and November winds. Zonal winds are eastward with pre-midnight maximum speeds going above 100 m/s. Compared to Jicamarca ( zonal winds in the Peruvian sector for the same month of October) the magnitude of maximum Abuja zonal wind speed is weaker. We compare the observed diurnal variation with the recently updated Horizontal wind model (HWM 14). Most of the observational features of thermospheric wind diurnal variation are captured in the model variation. The HWM14 generally showed good agreement with the Abuja Ocober and November zonal wind observations but overestimates the December meridional winds. Expected longer period analysis of the results from Abuja will stimulate a better understanding of wind climatology over the west African sector. Keywords: Zonal and Meridional winds; Low solar activity; Geomagnetic equator; Pressure gradient force

1

1. Introduction Thermospheric neutral winds can alter the ionospheric compositionand regulate ionospheric phenomena such as equatorial spread F, scintillation, and high latitude convection (Emmert et al., 2006). They furnish the energy needed for global scale dynamo (Sridharan, 1998). Many electrodynamic and plasma physics processes such as sporadic E, Sq dynamo and the equatorial electrojet are driven by the atmospheric neutral winds (Larsen and Fesen, 2009). The greatest influence of thermospheric winds occurs at middle and low latitudes (Fisher et al., 2015), due to the geometry of the earth’s magnetic field. Here we have the zonal component of the wind perpendicular to the field and the meridional component aligned with the field. In the equatorial thermosphere the principal driving force of upper thermospheric winds is the pressure gradient force. According to Rishbeth(1972b) the zonal component thermospheric winds in the equatorial thermosphere is given by 𝑑𝑈𝑥

∂𝑝

∂2𝑈𝑥

𝜌 𝑑𝑡 = ― ∂𝑥 -𝑛𝑖𝑚𝑖𝜗𝑖𝑛(𝑈𝑥 ― 𝑉𝑖𝑥) + 𝜇 ∂𝑧2 .

(1)

Here 𝑈𝑥 is the equatorial zonal wind speed, 𝜌 is the density of the atmosphere, 𝑝 is the neutral air pressure, 𝑛𝑖 and 𝑚𝑖 are the number density and mass of the ionized species respectively, 𝜗𝑖𝑛 is the ion-neutral collision frequency, 𝑉𝑖𝑥 is the zonal ion drift, 𝜇 is the viscosity coefficient and 𝑧 the altitude. The ionized particles act as breakers on zonal wind flow in the upper atmosphere but can enhance wind speeds when accelerating in the same direction. The ion drag force significantly regulates the thermospheric neutral wind (Liu et al., 2006).The spatial heating of the earth’s upper thermosphere gives a general pattern of neutral motion observed at all geomagnetic quiet times. In addition to the normal diurnal pattern, the zonal wind also varies with changes in the seasons, geomagnetic activity and the solar cycle. Propagating tides from the lower layers of the atmosphere contribute considerably to thermospheric wind behaviour during periods of low solar activity. F region neutral winds can be measured using Fabry-Perot interferometers, Incoherent scatter radar and data derived accelerometers on polar orbiting satellites. Several papers described thermospheric wind behavior in the low latitudes(e.g. Biondi and Meriwether, 1985; Burnside et al.,1989; Biondi et al., 1990, 1991, 1999; Hari and Murthy, 1995; Meriwether and Biondi, 1995; Vila et al. 1998, Martinis et al., 2001, Meriwether et al.,1986, 2008, 2011; Tesema et al., 2017). Using h’F data at two equatorial points Hari and Murthy (1995) showed, the nighttime equatorial thermospheric meridional wind flow to be mainly equatorward with a reversal to the poleward direction around midnight hours during equinoxes. Equatorial and low latitude thermospheric winds at quiet time variations with the season and solar flux from 1980 to 1990 were determined by Biondi et al.(1999), using data from the Fabry–Perot Interferometers at Arequipa, Peru and Arecibo, Puerto Rico. 2

Thermospheric wind studies using wind data from accelerometer readings on polar orbiting satellites in the equatorial latitudes have also been reported (e.g. King-Hele and Walker, 1983; Wu et al., 1994; Liu et al., 2006; Häusler et al., 2007; Häusler and Lühr 2009).Wu et al. (1994) examined a large dataset of Dynamic Explorer satellite (DE 2) measurements of equatorial latitudes (˂±9°) thermospheric horizontal wind component in the range from 200 to 400 km for quiet (Kp˂3) and active (Kp˃3) geomagnetic conditions. The dayside hourly averaged neutral winds were found to be insensitive to variations in geomagnetic activity. From their detail investigation on the equatorial zonal wind during the declining phase of solar cycle 23 using Challenging minisatellite payload (CHAMP) measurements, Liu et al. (2006) observed solar flux to have a

significant effect on daytime and nighttime winds. Wind observations over Africa have been hampered due to the scarcity of Fabry-Perot Interferometers. The first ground observations of thermospheric winds over Africa using Fabry-Perot interferometer data were reported by Vila et al. (1998). Results from the short-lived campaign over Korhogo (Ivory Coast, 9.250 N, 3550 E, Dip latitude -2.50) showed a complex behaviour by nighttime F2 layer winds during the period of regular measurements from 1994-1995. There have been reports of new observations from the FPI’s in Oukaimeden observatory (31.20 N, 80 W, magnetic latitude 22.70 N) in the Atlas Mountains over Morocco (Kaab et al., 2017) and Bahir Dar University in Ethiopia (11.60 N, 37.40 E, magnetic latitude 3.70 N) (Tesema et al., 2017). The first results by Tesema et al. (2017) of six monthson nighttime monthly averaged thermospheric winds and temperatures for the equatorial east African sector near the geomagnetic equator revealed significant night-tonight variation in the zonal and meridional winds, temperatures and relative intensities.With support from the Nigeria Space Research and development Agency, a Fabry Perot Interferometer has been installed in Abuja, Nigeria (Wu, 2016). In this study, we present results of the low latitude thermospheric winds from observations by the recently installed ground-based FPI over Abuja, Nigeria (Geographic: 9.060 N, 7.50 E; Geomagnetic: 1.600 S). The short observation period of three months from October 2017 to December 2017 falls within the low solar activity period of solar cycle 24. Our results are compared with predictions from the new version of the horizontal wind model (HWM 14). The results are then compared with the same period from the FPI in Jicarmarca in a different longitudinal sector which is also close to the geomagnetic equator.

3

Fig. 1. World map showing locations of the Abuja and Jicamarca FPIs. The broken line through the two sites indicates the geomagnetic equator. The “star” and “asterisk”symbols show the locations of Abuja, Nigeria, and Jicamarca, Peru. 2.Instrumentation The FPI is an optical instrument designed to make spectral measurements with high accuracy (Kaab et al., 2017). The Abuja FPI measures thermospheric winds by recording the wind-induced Doppler shift in the O 630 nm nightglow emission from ~250 km. The FPI consists of a 5.0cm diameter interference filter and a 4.2cm diameter etalon. The etalon has a fixed spacing of 1.5cm. The etalon coating reflectivity of about 80% enhances the transmission of the 630.0nm emission. In the design, a narrow band which is centered at 630.0nm isplaced in the optical path to isolate the redline emission. The image is focused onto an Andor CCD by an objective lens. The four cardinal directions are sampled with an elevation angle of 45 degrees by use of a single mirror rotator. The exposure time for each of these directions is 600 seconds. The instrument also uses a neon lamp to monitor the instrument drift. The wind error is signal intensity dependent and varies widely from a few meters per second to larger values. The cloud sensor which covers a field of view of 800 reports the observing conditions and measures the infrared radiation from the sky to ascertain the value of the sky temperature and this when compared with ambient ground temperature gives the cloud coverage (Makela et al., 2012). The sky is mostly clear for values of approximately -200 C or less while for higher values the skies are cloudy. Table1. Number of nights in each month, corresponding averaged solar flux and geomagnetic index Ap Month

Nights

Ap

Averaged F10.7

4

October

1, 2,4,6,7,9,11,12,13,14,15,16,17,18,19,21

11,5,5,8,3,1,28,26,37,30,25,8,5,4,12,8

68.31

November

1,2,4,9,10,11,13,14,15,16,17,21,23,27

2,8,8,18,18,20,6,11,11,11,14,4,27,8,4

63.62

7,3,28,3,23

62.52

December

1,2,5,15,17

Ap

200 100

Solar flux (F10.7) (s.f.u)

0

20

40

60

80

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120

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40

60

80

100

120

140

200 100 0

20

Days from September 1st 2017 (day1) to January 31st 2018 ( Day 153)

Fig. 2. Solar flux and Ap index variation from September 2017 to January 2018 3.Data, Data Analysis and Results Thermospheric wind data have been collected since September 2017 from an equatorial latitude new site at Abuja, Nigeria (9.060N, 7.50E) using the Fabry-Perot interferometer (FPI). The map in Figure 1 shows the location of the FPI deployed at Abuja in Nigeria, Africa (9.060N, 7.50E) and Jicamarca, the site another FPI, but in a different longitude sector. A summary of the observations considered in this study is presented in Table 1. The Table lists the number of usable observing nights from October 2017 to December 2017, the corresponding averaged solar flux (F10.7) and geomagnetic activity index Ap. Data collection from the FPI was limited by technical hitches and cloudy skies. We do not consider observations taken during cloudy nights in our analysis. The considered time interval from October 2017 to December 2017 is characterized by low solar flux values, generally varying between 60 and 70 s.f.u [1sfu= 10 ―22𝑊 𝑚2𝐻𝑧 ―1]. To derive the horizontal wind components, a series of zenith measurements were utilized to produce the zero wind reference that may be interpolated in time to get a zenith reference for each of the individual horizontal measurements (Meriwether et al., 2011). The average values describing the nightly variation of the measured zonal or meridional winds for a particular month were obtained by taking all the data points for the given period. The data points were then sorted into bins at one hour interval. The points for each bin were averaged to get the wind speeds. The vertical bars attached to each point indicate the standard deviation in the nightnight variations in the data. These bars do not reflect the uncertainties in the wind measurements. 5

The zonal and meridional winds are highly variable as can be inferred from the standard deviation of the averaged diurnal three months variation as depicted in the right panel of figure 3. The standard deviations (𝜎) of the averaged winds for each of the one-hour interval are plotted as vertical lines as shown in Figure 3. The standard deviation (𝜎) is not consistent for the zonal winds as shown in the right panel of Figure 3. Large variability values of 𝜎 = +/ - 200 can be observed in the early evening October zonal winds from 1900 LT to 2200 LT. Variability values as high 𝜎= +/ - 250 m/s occur on the November zonal winds. From very high variability values in the early evening meridional winds at 1900 LT, a decrease in trend is observed to about 2200 LT. Largest variability of values of about 𝜎 = +/ - 300 m/s are observed in the November and December meridional winds as displayed in the right panel of Figure 3. 3.1 Monthly Variations in the Thermospheric winds The monthly averages of the FPI measurements from October 2017 to December 2017 over Abuja illustrate

0 18

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Zonal Wind (m/s)

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Meridional Meridional Wind Wind Meridional of No. No. of Speed(m/s) No. of (m/s) (m/s) Points points Points

thermospheric wind behaviour for the transition from September equinox to December solstice.

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Fig. 3. Meridional and zonal wind variation from October to December 2017 for a time bin of 1 hour. The upper panels show the number of data points in each local time bin for each monthly variation. The vertical bars show the range of variation within each averaging bin. Positive values are northward for the meridional winds and eastward for the zonal winds.

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200 OCT NOV DEC

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Fig. 4. Comparison of meridional and zonal winds shown in Figure 3 above.

Figure 3 displays the averaged results of the meridional and zonal winds from October to December, 2017 with the standard deviations and the corresponding number of measurements in each local time bin. The standard deviations display the variability of the winds for each local time bin. High variability is observed in the early evening winds, especially at 1900 LT for the October and December winds. High variability is also displayed by November and December morning winds at 0500 LT. The meridional winds are highly variable, with the largest variablilty generally occurring at 1900 LT for the three months. For the zonal winds, variability is significant in the November and December winds. High variability is exhibited by the zonal winds in the early evening hours and more pronounced from 0100 to 0500 LT. October zonal winds display the least variability. 200

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HWM14 FPI

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Fig. 5. Comparison of the FPI wind results with the HWM results. 7

Figure 4 illustrates the comparison in variation for the three months. The December meridional winds are observed to be more equatorward than the October and Novemberwinds from 0200 LT to 0300 LT. With an equatorward speed of up to 80 m/s, the October winds switch polewards from midnight with a maximum speed of 40m/s observed at 0400 LT. The November winds are polewards for most of the night. The zonal winds are generally eastwards for most of the night with maximum speeds going above 100 m/s as displayed in figure 4 (right panel). From an early westward speed of about 10 m/s the December zonal winds maintain eastward speeds above 60 m/s with a maximum going above 140 m/s recorded at 0230 LT. After attaining a maximum speed of about 110m/s from an earlier speed of about 90 m/s the November speeds show a steady gradual decrease to speeds between 30 m/s and 50 m/s from 0300 LT to 0500 LT. The October wind speeds are mostly maintained above 60 m/s though with lower magnitudes than the December winds. Maximum December wind speeds of 110 m/s occur at 0200 LT.

4. Discussion The above results represent nighttime thermospheric meridional and zonal winds over Abuja. The three-month results from October 2017 to December 2017 represent the first measurements ground-based FPI measurements of equatorial thermospheric winds over Abuja. The measurements coincide with solar minimum activity period characterized by low solar flux values, generally below 70 s.f.u. At periods of low solar activity, the strength of the tidal wave forcing from below may likely overshadow the role of EUV forcing from tidal wave harmonics through normal EUV absorption (Meriwether et al., 2011). Thus our results are important as they may be important in revealing the contributions of tidal wave forcing to the features observed in our diurnal wind variation. The pressure gradient force which drives the equatorial zonal winds increases in the eastward direction towards sunset, together with the reduced ion drag account for the relatively large zonal winds noticed in the October and November before midnight. This large early evening wind which has been observed is surprisingly absent in our December winds. Our December zonal winds flow with speeds below 50 m/s from 1900-2200 LT as depicted in Figure 4, right panel. Under solar minimum conditions, the influence of ion drag force is less pronounced with viscous drag serving as the dominant contributor to the force balance of the thermosphereionosphere system (Hsu et al., 2016). The ion drag force which results from collisions between the neutrals and F-region plasma partly balances the eastward pressure gradient force (Fisher et al.,2015). The accelerated ascent of the ionosphere by the evening prereversal leap of eastward electric fields minimises the influence of ion drag force (Anderson and Roble, 1974; Maruyama and Watanabe, 2003). 8

The investigations carried out on equatorial latitude upper thermospheric winds using monthly analysis (Meriwether et al., 2011; Tesema et al., 2017,) have revealed nighttime patterns similar to those in our observations. The seemingly unreliable meridional observations at 1900 LT are likely due to atmospheric scattering contamination. Our West African sector zonal wind speeds revealed maximum speeds generally below 100 m/s which is generally in agreement with the investigations of Tesema et al.(2017) in the east African sector FPI which is also close to the magnetic equator. With more coverage studies will be carried out to fully understand the interhemispheric flow from the summer to winter hemisphere. 4.1. Comparison to a Horizontal wind Model Comparisons of our measured thermospheric wind monthly averages to the Horizontal wind Model (HWM14) are shown in Figure 5. The latest version of the climatological wind model, HWM14 was updated using FPI measurements from Brazil and PARI (Fisher et al., 2015). In general, zonal wind speeds in our study compare quite well with the HWM results for the three months of measurements. The HWM14 agrees perfectly well with the zonal winds for the months of October and November. The early evening FPI zonal winds lag and are observed to lead the model winds from 1000 LT for the month of December. Some significant disagreements are quite visible for some time intervals in the meridional wind variation; 18:00-21:00 LT for the October winds, 19:00-23:00 LT for November winds. The FPI winds generally lag HWM winds for the December meridional winds. The horizontal wind model (HWM) which is an empirical model integrating various data of thermospheric neutral wind observations (Liu et al., 2006) will likely reveal more representative observations of upper thermospheric winds than our measurements spanning over a short interval of three months. Since no ground-based neutral wind measurements over Africa were used in the construction of HWM14 the FPI data from the installed FPIs in Morocco (Fisher et al., 2015), Ethiopia (Tesema et al.,2017), and now from Abuja will contribute immensely to the validation of the latest HWM version. 4.2 Comparison with South American Sector October Results Compared with October zonal winds from Jicamarca, Peru at the same time some differences appear in the premidnight maximum speeds. Jicamarca in a different longitude sector as shown in Figure 1 revealed maximum eastward speeds going above 150 m/s from 1900 LT to about 2015 LT while in our observations maximum speeds of about 100m/s occur from 2000 to 2200 LT. Zonal thermospheric winds of up to 150 m/s have been reported in investigations from the Peruvian sector (e.g Martinis et al., 2001, Meriwether et al., 2012). A steady sharp decrease is noticeable in the Jicamarca zonal winds from 2000 to 2200 LT as can be seen in Figure 6. This rate in decrease is gradual from 2200 LT to the early morning hours.

9

The meridional winds show some resemblance with our observed winds from midnight to 0300 LT. There is a great disparity in the pre-midnight meridional winds as Abuja winds are observed to be polewards. Tesema, (2017) suggested the uplift of the F-region plasma by a strong pre-reversal enhancement may be responsible for the evening zonal wind enhancements observed over Peru in earlier studies. 200 Zonal Wind Meridional Wind

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Wind Speed (m/s)

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Fig. 6. October 2017 averaged zonal and meridional wind variations over Jicamarca, Peru. 5. Conclusions We have presented low solar activity period nighttime observations of thermospheric neutral winds using the recently installed Fabry-Perot Interferometer in Abuja, Nigeria. Our three months observations revealed general variations generally similar to previous investigations in other equatorial longitude sectors around the September equinox and December solstice. From our investigation we draw the following conclusions: 1. The Abuja December meridional winds are observed to generally lead October and November winds equatorwards from 2000 -0400 LT. Unexpectedly the December early evening zonal wind is strongly westward but lead the October and November winds eastwards from 2230-0500 LT. 2.As expected premidnight maximum eastward speeds are observed in the October and November Abuja FPI winds. The November and December averaged monthly winds are not in agreement with November and December 2015 East African sector winds where flow is polewards.

3. The latest version of HWM14 correctly captures the October and November zonal winds but generally overestimates the November and December averaged meridional winds.

10

4. The analysis of October diurnal variations from the Jicamarca, Peru, which is in a different longitude sector also close to the geomagnetic equator as our Abuja site show similarities but with enhanced pre-midnight maximum zonal speeds and pre-midnight poleward winds. Upcoming detail studies with monthly average temperatures, over a longer period of time, will surely provide a comprehensive study of the climatology of nighttime equatorial thermospheric winds over Abuja augment studies already carried out on wind behaviour over Africa.

Acknowledgments. We are deeply grateful to Dr. Qian Wu who provided the wind data used in this study. Horizontal wind model (HWM14) is avialable at http://github.com/timduly4/pyglow/. Solar flux and geogmagnetic indexes are from the websites “http://omniweb.gsfc.nasa.gov/” and “ http://swdcwww.kugi.kyoto-u.ac.jp/” respectively. The Jicamarca wind data is available in the Madrigal database. We thank the two reviewers for their constructive suggestions regarding the draft of the paper.

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