ISSN 00167932, Geomagnetism and Aeronomy, 2015, Vol. 55, No. 5, pp. 643–649. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.F. Yakovets, G.I. Gordienko, Yu.G. Litvinov, 2015, published in Geomagnetizm i Aeronomiya, 2015, Vol. 55, No. 5, pp. 659–665.
Midlatitude Summer Nighttime Anomaly in NmF2 According to the Data of Vertical Sounding in AlmaAta A. F. Yakovets, G. I. Gordienko, and Yu. G. Litvinov Institute of Ionosphere, Almaty, Kazakhstan email:
[email protected] Received March 16, 2015
Abstract—The morphology of the midlatitude summer nighttime anomaly in the diurnal behavior of the electron concentration maximum in the F2 layer in various seasons and at various solar activity levels was studied with the use of vertical sounding data in Almaty in 1999, 2008, 2011, and 2012. It was shown that the anomaly is manifested most distinctly in the summer months (June–August) and disappears in the equinox months (March, September). The maximum value of the electron concentration falls on the solar zenith angles, which correspond to the cessation of the arrival of ionizing radiation at the heights of the F2layer maximum. The anomaly is clearly manifested in the solar activity minimum and barely seen in the maximum. It was shown that the summer anomaly parameters at the boundary of the northeastern Asia (Almaty, 76°55′ E) insignificantly change as compared to the parameters of its center (Japan, 130° E). Mechanisms of anomaly formation, as well as of its diurnal and seasonal behavior, are discussed. DOI: 10.1134/S0016793215050199
1. INTRODUCTION It is widely known that diurnal variations in the electron concentration in the maximum of the midlat itude ionospheric F2 layer (NmF2) in summer deviate from the behavior typical for the Chapman layer gov erned by the solar radiation flux. For the first time, this effect was detected over the Antarctic according to data from groundbased vertical ionospheric sounding (Penndorf, 1965). This effect was called “the Weddell Sea Anomaly” (WSA). The WSA is characterized by the fact that NmF2 exceeds NmF2 observed around the local noon in the local summer during the night time hours. A similar effect (an evening increase in NmF2 in the summer months) was detected also in the North ern Hemisphere (Rishbeth, 1968). At first, this effect was in no way related to the WSA, because certain dif ferences exist between them. The WSA the electron concentration peak occurs at night, close to the mid night, whereas the peak in the Northern Hemisphere falls in the hours near the dusk. Moreover, the North ern Hemisphere anomaly is more weakly pronounced than the WSA. The modeling performed by Rishbeth (1968) made it possible to interpret this effect as a result of the later sunset in summer and the shift of the F2 layer to higher altitudes by equatorward thermo spheric wind at the time when ionospheric photoion ization still occurs. Current satellite technologies, including the radio occultation method (Burns et al., 2008), plasma den sity measurements in situ on board the CHAMP satel lite (Liu et al., 2010), topside ionospheric sounding
(Deminov and Karpachev, 1988), and measurements of the total electron content by the TOPEX/Poseidon system (Horvath and Essex, 2003) made it possible to show that the anomaly of the diurnal NmF2 behavior has a global character. The anomalies in the diurnal behavior in both the Northern and Southern hemi spheres obtained the joint name of midlatitude sum mer nighttime anomaly (MSNA) (Lin et al., 2010). Modeling of the MSNA was performed with cur rent atmospheric–ionospheric models (Knyazeva et al., 2010; Karpachev et al., 2010; Thampi et al., 2011). Analysis of the measurement results and modeling presented the consideration of various mechanisms of MSNA formation, including a vertical shift of the layer due to the thermospheric meridional and zonal wind in the existing configuration of the geomagnetic field, ionospheric plasma drift in the crossed electric and magnetic fields, and variations in the plasma fluxes from the plasmasphere to the ionosphere. Although each of these mechanisms contribute something to MSNA formation, the dominant role in the longitudi nal distribution of anomaly regions is played by the dependence of the velocity of the plasma vertical drift caused by the meridional wind on the geomagnetic field inclination. The maximum amplitudes of the effect are observed at longitudes corresponding to the maximum distance between the geomagnetic and geo graphic equator. In the Southern Hemisphere, these longitudes are located near 30° W (with the maximum amplitude in the WSA region), whereas in the North ern Hemisphere these longitudes are located near 135° E (the northeastern Asia anomaly with a maximum
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amplitude at the longitudes of Japan and Okhotsk Sea (Lin et al., 2010)). Almaty (76°55′ E) is located at the western bound ary of the longitudinal sector of the northeastern Asia anomaly (Knyazeva et al., 2010), so studying the char acteristics of the latter at this longitude presents sub stantial interest. Our goal was to analyze the MSNA parameters over Almaty, including their diurnal behavior, seasonal variations, and dependence on solar activity. The data were obtained from vertical ionospheric sounding. 2. MEASUREMENT METHOD AND RESULTS Observations of the ionosphere are conducted at the Institute of Ionosphere with a digital ionosonde conjugated to a computer that is used for the collec tion, storage, and processing of ionograms in the digi tal form. The information is read from ionograms by a semiautomatic method. The ionospheric sounding was conducted in a 15minute regime. Initial process ing of the ionograms includes reading the critical fre quency of the F2 layer (foF2). The ionosonde provides an accuracy of the foF2 reading of ~0.05 MHz. The critical frequency of the layer (in MHz) is related to the electron concentration in the layer maximum (NmF2) as expressed in the number of electrons per cubic centimeter by the formula: NmF2 = 1.24 × 104 (foF2)2. In order to analyze the diurnal NmF2 behavior in particular months, we used its median values, bear ing in mind a substantial daytoday scatter in NmF2 and the fact that, unlike the average value, the median reacts weakly to a separate, strong deviation of the electron concentration. To analyze the MSNA evolution as a function of the season, we drew the diurnal behavior of the elec tron concentration for March, April, May, June–July, August, and September 2011 (Fig. 1). The points show separate NmF2 measurements. The solid curve shows the median, the behavior of which makes it possible to exclude substantial daytoday scatter of the concen tration caused by various disturbances due to varia tions in the solar ionizing radiation, fluctuations in the density and composition of the thermosphere and by sources of wave processes of various natures. The vast majority of data of the analyzed electron concentra tion measurements was obtained under low magnetic activity (Dst > –50 nT). Measurements during which moderate and high magnetic activity (Dst ≤ –50 nT) was observed were rejected from the analysis. The monthly mean values of the solar radio emission flux at a wave of 10.7 cm for each month and the solar zenith angles (χ) calculated for the middle of the analyzed month corresponding to the peaks of the evening max imum in NmF2 are shown on the margin of the figure. The anomaly presents the effect of a positive deviation in NmF2 in the evening hours (~2000 LT in summer)
from the smooth diurnal behavior. Now we consider the anomaly manifestation beginning from the spring equinox month and ending by the fall equinox month. In March (Fig. 1a), the diurnal behavior of the median shows no pronounced evening peak. It appears sharply in April (Fig. 1b). In May (Fig. 1c), the evening maxi mum in NmF2 is approximately the same as in April, whereas in June–July the positive deviation in NmF2 in the evening hours increases (Fig. 1d). A strong pos itive deviation in NmF2 remains in August (Fig. 1e). In September the evening maximum differs little from the diurnal fluctuations in the median caused by statis tical scatter. The time corresponding to the evening maximum (shown in the figures by vertical arrows) took the following values: 1845, 1915, 2015, 1930, and 1800 LT in April, May, June–July, August, and Sep tember, respectively. The sunset times and the solar zenith angle change rates were taken from astronomi cal tables, and the zenith angles χ corresponding to the evening maximum were calculated. All of the zenith angles correspond to the position of the Sun below the mathematical horizon, when the ionizing radiation almost ceased traveling to the F2layer heights. The dependence of the anomaly parameters on solar activity can be seen in Fig. 2, in which the sound ing data in the period of the summer solstice for low (2008) (Fig. 2a), moderate (2012) (Fig. 2b), and high (1999) (Fig. 2c) solar activity are shown. It follows from the graphs that the minimum manifestation of the summer anomaly corresponds to the maximum solar activity. We then compared the results obtained by us in 2008 to the measurements conducted in Japan in July 2008 at the center of the northeastern Asia anomaly zone (Thampi et al., 2011). These measurements were con ducted by radio occultation, which makes it possible to obtain the electron concentration profile beginning from the bottom layer and up to heights substantially exceeding the height of the layer maximum. Thampi et al. (2011) showed that the local time corresponding to the night maximum peak depends on geographic latitude: the time shifts towards midnight at the increase in latitude. For the geographic latitude of 45° N, which almost coincides with the latitude of Almaty, this time was found to equal 2030 LT, which coincides with the time obtained by us. The deviations in NmF2 in evening hours from the smooth diurnal behavior were also similar. This shows that the anomaly param eters at the zone boundary (Almaty, 76°55′ E) vary insignificantly relative to its center (130° E). Note that we conducted measurements of the concentration in the F2layer maximum, so the comparison of param eters for points distanced in longitude was performed for the anomaly observed in NmF2. At the same time, Thampi et al. (2011) showed that, if one follows the changes in the electron concen tration at fixed heights, one can see that the manifes tation of the anomaly is intensified with an increase in
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(a)
NmF2, cm–3
1.4E+006 March 1–31, 2011 F10.7 = 116 1.2E+006 1.0E+006 8.0E+005 6.0E+005 4.0E+005 2.0E+005 0.0E+005 0
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χ = 93°
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Fig. 1. Diurnal behavior of the electron concentration in the F2layer maximum in (a) March 2011, (b) April 2011, and (c) May 2011.
height: at particular heights, NmF2 in the evening hours could exceed the daytime values by a factor of two. The cause of this effect becomes clear when con GEOMAGNETISM AND AERONOMY
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sidering the problem of the mechanism of the summer anomaly formation. The electron concentration in the F2 layer is governed by the processes of production, 2015
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1.3E+006
June–July, 2011 F10.7 = 95
NmF2, cm–3
1.1E+006
χ = 97°
9.0E+005 7.0E+005 5.0E+005 3.0E+005 1.0E+005 0
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1.1E+006 August 1–19, 2011 F10.7 = 102 9.0E+005
χ = 97°
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September 1–30, 2011 1.3E+006 F10.7 = 135 1.1E+006 9.0E+005 7.0E+005 5.0E+005 3.0E+005 1.0E+005 0
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Fig. 1 (continuation). Diurnal behavior of the electron concentration in the F2layer maximum in (d) June–July 2011, (e) August 2011, and (f) September 2011.
loss, and transport. The production process (photo ionization) takes place in the daytime hours. At night the plasma production is weak. The rate of plasma
chemical losses depends strongly on height, because it is determined by the density of the neutral atmo spheric component participating in collisions with the
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(a)
7.0E+005
NmF2, cm–3
June–July, 2008 6.0E+005 F10.7 = 65.8
χ = 99°
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June–July, 1999 F10.7 = 168
1.8E+006 1.6E+006 1.4E+006 1.2E+006 1.0E+006 8.0E+005 6.0E+005 4.0E+005 2.0E+005 0.0E+005
χ = 95°
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Fig. 2. Diurnal behavior of the electron concentration in the F2layer maximum in June–July of (a) 2008, (b) 2012, and (c) 1999.
charged component. The rate of plasma transporta tion is governed by the velocity of the meridional wind. Depending on the meridional wind direction, plasma is transported to higher altitudes (the wind is equator GEOMAGNETISM AND AERONOMY
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ward), where the recombination rate is small, or to lower altitudes (poleward wind), where the losses are high. The ionosphere as a whole shifts upwards at night and downwards in the daytime by the equator 2015
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ward and poleward neutral winds, respectively, so hmF2 is higher at night than in the daytime. The latter leads to the fact that heights located in the daytime higher than hmF2 at night are closer to the layer maxi mum. This combination of physical and geometric effects explains why the diurnal anomaly is better pronounced in the behavior of Ne at fixed heights as compared to the concentration behavior in the layer maximum. Two factors determining the anomaly formation in the summer months are considered in the literature. First (Thampi et al., 2011), the meridional wind changes its direction from polarward to equatorqard much earlier in summer at the middle latitudes than in other seasons (around 14.30 LT), when the photoion izing radiation flux is still high. That is why photoion ization, in combination with the rise of the ionosphere to the heights where the recombination rate is low, leads to the formation of an evening increase in NmF2. Second, in addition to the factor of an early change in the thermospheric wind direction, seasonal variations in the meridional wind velocity are manifested in the fact that the equatorward meridional wind has the maximum and minimum velocities in the summer and winter hemispheres, respectively, at night (Kawamura et al., 2000). In the equinox periods, the velocity of the equatorqard neutral wind has intermediate values. Now we consider possible causes of the obtained good correspondence between the anomaly parame ters over Japan and Almaty. Knyazeva et al. (2010) pre sented calculations of the global distribution of the electron concentration using the Upper Atmosphere Model (UAM); the calculations (Namgaladze et al., 1991) included the thermosphere, ionosphere, and plasmasphere and took into account the displacement of the geomagnetic and geographic poles of the Earth. The model calculations have shown that the anomaly in the Northern and Southern Hemispheres is formed at longitudes at which the geomagnetic equator is shifted maximally relative to the geographical one into the summer hemisphere. An important role is played in the formation of the longitudinal variation in MSNA by the longitudinal dependence of the vertical component Vzi of the velocity of ion transport by the meridional neutral wind (Vxn), which is approximately equal to Vzi ≅ Vxn cosIsinI, where I is the inclination of the magnetic field. The longitudinal dependence of the product cosI sinI as calculated for the Southern Hemisphere showed that, at the same Vxn, the vertical component can differ almost by a factor of two between the longitudes with maximum and minimum values of cosIsinI. Such a substantial change in cosIsinI is caused by a substantial change in the mag netic inclination I with longitude in the Southern Hemisphere. At the same time, in the Northern Hemisphere, the longitudinal variations in I are sub stantially lower (Jee et al., 2009), so the variations in cosIsinI are not as strong. That can apparently explain the results of our comparison, which has
shown an insignificant decrease in the anomaly ampli tude upon a substantial change in longitude. It follows from Fig. 2 that the anomaly is better pronounced in the solar activity minimum. It is explained by the fact that the velocity of the meridi onal thermospheric wind is lower under high solar activity than under low activity due to an increase in the friction force between the neutral component and ions (Knyazeva et al., 2010). 3. CONCLUSIONS The morphology of the midlatitude summer night time anomaly over Almaty in various seasons and at various solar activity levels was studied on the basis of the ionospheric vertical sounding over seven months of 2011 and the summer months of 1999, 2008, and 2012. It was shown that the anomaly is not seen in the equi nox months in the diurnal behavior of the electron concentration maximum in the F2 layer. The maxi mum effect of the anomaly is seen in the summer months (July–August). The maximum value of the electron concentration in the evening peak corre sponds to the solar zenith angles, when the ionizing radiation almost ceases to reach the heights of the F2 layer maximum. The anomaly is distinctly manifested in the solar activity minimum but scarcely seen in the solar maximum. It was shown that the parameters of the summer anomaly at the boundary of the northeast ern Asia zone (AlmaAta, 76°55′ E) change insignifi cantly as compared to the parameters at its center (Japan, 130° E). The mechanisms of the formation of the anomaly and of its diurnal and seasonal behavior are discussed. ACKNOWLEDGMENTS The authors thank the reviewer for valuable com ments. The work was carried out within the project “To Study the Structure and Dynamics of the Cosmic Ray Flux, Geomagnetic Field, Ionosphere, and Atmosphere to Diagnose and Forecast the State of Near Space” of the Republic Budget Program 076. REFERENCES Burns, A.G., Zeng, Z., Wang, W., Lei, J., Solomon, S.C., Richmond, A.D., Killeen, T.L., and Kuo, Y.H., Behavior of the F2 peak ionosphere over the South Pacific at dusk during quiet summer conditions from cosmic data, J. Geophys. Res., 2008, vol. 113, no. A12305. doi:10.1029/2008JA013308 Deminov, M.G. and Karpachev, A.T., Longitudinal effect in the nighttime midlatitude ionosphere according to the “Interkosmos19” satellite data, Geomagn. Aeron., 1988, vol. 28, no. 1, pp. 76–80. Horvath, I. and Essex, E.A., The Weddell Sea anomaly observed with the Topex satellite data, J. Atmos. Sol.– Terr. Phys., 2003, vol. 65, no. 6, pp. 693–706. doi:10.1016/S13646826(03)00083X
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MIDLATITUDE SUMMER NIGHTTIME ANOMALY Jee, G., Burns, A.G., Kirn, Y.H., and Wang, W., Seasonal and solar activity variations of the Weddell Sea Anom aly observed in the TOPEX total electron content mea surements, J. Geophys. Res., 2009, vol. 114, no. A04307. doi:10.1029/2008JA013801 Karpachev, A.T., Gasilov, N.A., and Karpachev, O.A., Causes of the longitudinal variations in NmF2 at middle and subauroral latitudes in summer nighttime condi tions, Geomagn. Aeron., (Engl. Transl.), 2010, vol. 50, no. 4, pp. 482–488. Kawamura, S., Otsuka, Y., Zhang, S.R., and Fukao, S., A climatology of middle and upper atmosphere radar observations of thermospheric winds, J. Geophys. Res., 2000, vol. 105, no. A6, pp. 12,777–12,788. doi:10.1029/ 2000JA900013 Knyazeva, M.A., Zubova, Yu.V., and Namgaladze, A.A., Numerical modeling of the Weddell Sea anomaly in the behavior of the ionospheric F2 region, Vestn. MGTU, 2010, vol. 13, no. 4/2, pp. 1068–1077. Lin, C.H., Liu, C.H., Liu, J.Y., Chen, C.H., Burns, A.G., and Wang, W., Midlatitude summer nighttime anomaly of the ionospheric electron density observed by
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FORMOSTAT3/COSMIC, J. Geophys. Res., 2010, vol. 115, no. A03308. doi:10.1029/2009JA014084 Liu, H., Thampi, S.V., and Yamamoto, M., Phase reversal of the diurnal cycle in the midlatitude ionosphere, J. Geophys. Res., 2010, vol. 115, no. A01305. doi:10.1029/ 2009JA014689 Namgaladze, A.A., Korenkov, Yu.N., Klimenko, V.V., Kar pov, I.V., Surotkin, V.A., and Naumova, N.M., Numer ical modelling of the thermosphere–ionosphere–prot onosphere system, J. Atmos. Sol.–Terr. Phys., 1991, vol. 53, nos. 11/12, p. 1113–1124. Penndorf, R., The average ionospheric conditions over the Antarctic, Geomagn. Aeron. Antarct. Res. Ser., 1965, vol. 4, no. 1, pp. 1–45. Rishbeth, H., The effect of winds on the ionospheric F2 peak, J. Atmos. Terr. Phys., 1968, vol. 30, no. 1, pp. 63–71. Thampi, S.V., Balan, N., Lin, C., Liu, H., and Yama moto, M., Midlatitude Summer Nighttime Anomaly (MSNA) – observations and model simulations, Ann. Geophys., 2011, vol. 29, no. 1, pp. 157–165. doi:10.5194/ angeo291572011
Translated by A. Danilov
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