Equatorial F-regionvertical plasma drifts during solar maxima

Transcription

1 Utah State University From the SelectedWorks of Bela G. Fejer September 1, 1989 Equatorial F-regionvertical plasma drifts during solar maxima Bela G. Fejer, Utah State University E. R. de Paula I. S. Batista E. Bonelli R. F. Woodman Available at:

2 ß JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. A9, PAGES 12,49-12,54, SEPTEMBER 1, 1989 EQUATORIAL F REGION VERTICAL PLASMA DRIFFS DURING SOLAR MAXIMA B. G. Fejer, 1 E. R. de Paula, 1,2 I. S. Batista, 2 E. Bonelli, 3 and R. F. Woodman 4 Abstract. Incoherent scatter radar measurements at Jicamarca are used to study the effects of large solar fluxes and magnetic activity on the F region vertical plasma drifts. The average drifts from the two last solar maxima are almost identical except in the late afternoon-early evening sector where their variations with solar flux and magnetic activity are strongly season dependent. The averag evening winter (May- August) drifts appear to remain almost constant after a certain solar flux level is reached but increase with magnetic activity. The equinoctial evening drifts increase systematically with solar-flux but decrease with magnetic activity. Very large prereversal enhancement velocities, up to about 8 m/s, were often observe during the equinoctial periods when the solar flux was very high. Comparison of incoherent scatter radar drifts with vertical velocities inferred from ionosonde observations indicate that the latter technique substantially underestimates the plasma drifts during periods of large solar fluxes except during winter. Introduction Incoherent scatter radar measurements at the Jicamarca Radio Observatory (11ø57'S, 76ø52'W, magnetic dip 2øN) have provided detailed information on equatorial ionospheric electrodynamics and plasma instabilities [e.g., Farley et al., 197; Woodman, 197, 1972; Fejer and Kelley, 198; Fejer, 1981, 1986]. Most of the F region drift studies have concentrated on the vertical component which plays an important role on the height/latitudinal distribution of ionization at low latitudes and constitutes an important input parameter for equatorial, low latitude, and global ionospheric models [e.g., Sojka and Schunk, 1985; Anderson et al., 1987]. Equatorial F region vertical ionization velocities have also been estimated from ionograms by measuring d(h'f)/dt, where h'f is the virtual height of the bottomside of the F layer [Abdu et al., 1981; Batista et al., 1986]. Although this technique is thought to be reliable only between about 17 and 2 LT, it can provide information on the longitudinal variation of the vertical plasma drifts (east-west electric fields) in the evening sector. The ionosonde measurements have also been quite useful for studies of equatorial spread F [e.g., Lyon et al., 196; Chandra and Rastogi, 1972; Abdu et al., 1981]. Jicamarca observationshow that the vertical plasma drifts depend strongly on solar activity, particularly in the noon to midnight sector [Fejer et al., 1979]. For example, the prereversal enhancement of the upward drifts has considerably higher amplitudes during solar maximum than during the Center for Atmospheric and Space Sciences, Utah State University, Logan. 2Instituto de Pesquisas Espaciais, S o Jos6 dos Campos, S o Paulo, Brazil. 3Departamento de Ffsica, Universidade Federal do Rio Grande do Norte, Natal, Brazil 4Instituto Geoffsico del Peru, Lima. Copyright 1989 by the American Geophysical Union. Paper number 89JA /89/89JA minimum. In fact it is often absent over Jicamarca during the solar minimum winter months (especially May and June). Increased solar activity also enhances the equatorial F region eastward drifts over the entire dawn to dusk period [Fejer et al., 1985]. The main purpose of this paper is to compare the equatorial vertical plasma drifts during the solar maxima of and during both magnetically quiet and disturbed conditions. We show that the effects of both solar flux and magnetic activity on the afternoon and evening drifts are different during the equinoces and solstices. In particular, our observations indicate that the equinoctial prereversal velocity enhancements during the last solar maximum were appreciably higher than during the period. We also compare incoherent scatter drift measurements in the evening sector with the corresponding vertical drifts inferred from the time variation of h'f to show that the ionosonde technique substantially underestimates the drifts during periods of large upward motions. Data and Discussion The technique used for F region plasma drift measurements at Jicamarca was described in detail by Woodman [197, 1972]. The drifts are measured from -6 km where they usually do not change much with altitude. The values to be presented here represent averages between about 3 and km where the signal to noise ratio is highest. The integration time is about 5 min. and the accuracy of the measurements is usually 1-2 rn/s. In the F region over Jicamarca an upward plasma drift velocity of rn/s corresponds to an eastward electric field of about 1 mv/m. Comparison of average velocities during two solar maxima. Figure 1 shows the averages for all vertical drift data measured during and for equinox (March, April, September, and October) and southern hemisphere winter (May-August). There were only a few measurements during the summer months (November-February). Therefore, we will discuss these data later and in less detail. The average 1.7 cm fluxes for the days of observations during the and equinoxes were 148 and 216, and the corresponding sunspot numbers were 95 and 161, respectively. The winter data for these two periods correspond to average fluxes of 15 and 183, and sunspot numbers of 111 and 117. The average velocities for closely resemble the values given by Fejer et al. [1979] where slightly different seasonal periods were considered. The 1971 data which were used in the previous study to improve the statistics, were not included in the present study to allow for an easier distinction between solar flux and magnetic activity effects. We can see that the increased solar activity leads mainly to larger upward drifts in the afternoon and early evening sectors during equinox. In this case, the peak of the equinoctial prereversal enhancement drift changed by about 18 m/s between the two solar maxima. Smaller changes between the curves are not significant, since there is considerable variability on the data, as we will show later. Moreover, the various periods were characterized by different levels of magnetic activity. Figure 1 shows that in contrasto the equinoctial data, the average winter patterns for and are essentially identical. In particular, there was no change in the amplitude of the evening prereversal enhancement drifts for this season. This result is partly due to 12,49

3 12,5 Fejer et al.: Brief Report m/$ 6 4o 2o -2o 2o -2o -4o ] i [ WINTER O 4 8 Fig. 1. Average Jicamarca F region vertical drifts (positive upward) for equinox (March, April, September and October) and winter (May-August) during the and solar maxima. ] I I I ] i i i I I 1 I I I I I I I I I I [ I rn/s JICAMARCA VERTICAL DRIFTS KP(2 + - i - EQUINOX j ( - - o r.œ, - WINTER -213 ' & the relatively small difference between the average flux and sunspot number between these two periods for winter. We have also averaged the three winter days with the highest solar indices (average 1.7 cm flux and sunspot number of 6 and 15, respectively) during In this case the average drift profile closely resembled the winter data shown in Figure 1 except for an increase of the maximum evening prereversal velocity enhancement from 19.5 rn/s to 22. m/s (for the winter data this maximum was 17.5 m/s). These results show that during solar maxima the increase of the maximum prereversal enhancement is indeed considerably larger during equinox than during local winter. Fejer et al. [1979] showed that for all seasons, the afternoon and evening upward drifts were substantially higher during than during the solar minimum. The equatorial F region eastward drifts in the late afternoon and evening sectors also increase systematically with solar activity from the solar minimum to the maximum [Fejer et al., 1985]. The quiet time equatorial cross field plasma drifts are driven by the E and F region dynamos [Rishbeth, 1971; Heelis et al, 1974; Richmond et al., 1976; Farley et al., 1986]. These variations of the vertical and zonal evening drifts are attributed to the increase of the F region dynamo electric fields with solar activity. Therefore, Figure 1 indicates that the winter F region east-west dynamo electric fields (which drive the vertical evening drifts) do not change much between high levels of solar activity. We have also computed the averages. There were very few measurements during As expected, the winter averages for this period are essentially identical to those shown in Figure 1. The equinoctial curve also agrees very well with the plot except for a decrease of the prereversal velocity peak from 51 m/s to 45 m/s. To improve our statistics for magnetically quiet and disturbed conditions, from now on we will use the data for the last solar maximum. Day-to-day variations. Figures 2a and 2b show the scatter plots of the and drift data for the periods with Kp < 2 +. The thick lines denote the averages of these quiet time curves. The smaller amount of data in the dusk-midnight sector for equinox and summer is due in part to the frequent occurrence of equatorial spread F. Woodman [197] has shown scatter plots of the data including observations during both quiet and disturbed -. I [ I I [ I! I! I!, I I I!, I I I O 4 8 Fig. 2a.. Scatter plots of the quiet time F region vertical plasma drifts for The thicker curves denote the quiet time averages. m/s - I i I i i i i i i i i i i i I I I i i i i 1 i JICAMARCA VERTICAL DRIFTS KP < o o -,v -.,.-o - -!! I I!!! I I I I [ I I I I! I I I I!! O 4 8 Fig. 2b. Same as Figure 2a but for the solar maximum.

4 Fejer et al.' Brief Report 12,51 conditions. As we will see later, this leads to larger variability on the patterns. The main results in Figures 2a and 2b are as follows: 1. The largest values of the evening prereversal enhancement drifts occur during equinox. These drifts increase with solar activity. 2. The relatively small average value of the peak upward velocity enhancement during local summer is largely due to the occurrence of the maxima over a period of about 2 hours from November through February. That is, the maximum prereversal enhancements for each of the summer months is much larger than the 4 month average. This can be seen also in Figure 2 of Fejer et al. [1979] for the period. Although there were only a few days of observations during the summer months, these data suggest also an increase of the maximum upward velocities with solar activity. 3. The variability of the quiet time drifts decreases with increasing solar activity. This is particularly evident in the winter data. Fejer et al. [ 1979] showed a large increase in the variability of the winter drifts from solar maximum to solar minimum which is consistent with our results. Since the average drifts for were not available until now, a number of equatorial ionospheric modeling studies for this period have used the averages published by Fejer et al. [ 1979]. The much larger equinoctial prereversal enhancement velocities observed in our data suggesthat some of the low latitude modeling results for the last solar maximum could be affected appreciably. Variation with magnetic activity. Vertical drifts measured during strongly disturbed periods often show large fluctuations relative to their quiet time values [Gonzales et al., 1979; Fejer, 1986]. Figures 3a and 3b show all the data for Kp > 3 and also the corresponding quiet time seasonal averages. The summer data are not shown since rots i i i i i i i i i i i i i i i i i i i i i i EQUINOX JICAMARCA VERTICAL DRIFTS KP> p / KP Fig. 3a. Scatter plots of the F region vertical plasma drifts during magnetically active periods. The quiet time averages are also shown. m/s JICAMARCA VERTICAL DRIFTS KP>3 - t i i i i i i i i i i i i i i i i i i i i i i 6j EQUINOX. *--'* KP'('2+ ' - o -4O - I i i I i i i i i i i i i i i i i i i i I i i,,,,, O Fig. 3b. Same as Figure 3a, but for the period. there are drift measurements from only 2 disturbe days. The average Kp for equinoctial drifts, shown in Figure 3, was 4.. The average Kp corresponding to the data was only about 3.5. The main results in Figures 3a and 3b are as follows: 1. The largest sudden drift perturbations are usually downward during the day and upward at night. However, perturbations with the same sign as the quiet time drifts are also observed near dawn and dusk. 2. In general, the evening prereversal enhancement drifts increase with magnetic activity in the winter and decrease during equinox. These results are particularly clear in the winter data and in the equinoctial data where all the disturbed velocity patterns near dusk are above and below the quiet time averages, respectively. The summer data also suggest an increase in the drifts near dusk, but the smaller number of data during this season does not give us as much confidence in this result. The average drift velocities for Kp < 2 + and Kp > 3 for the equinox and for the winter are shown in Figure 4. This figure shows clearly the different behavior of the winter and equinoctial drifts with magnetic activity near the dusk sector. Note also that except near dawn, the nighttime downward drifts decrease with magnetic activity. The increase of the evening upward plasma drifts with magnetic activity during winter is even more pronounced in the Jicamarca solar minimum winter data [Fejer, 1986] although there are also clear exceptions. Several studies have used ionosonde data to determine the relationship between h'f, magnetic activity and the occurrence of equatorial spread F at different longitudes [Lyon et al., 196; Chandra and Rastogi, 1972]. In these studies, however, the quiet and disturbe days were chosen so that the greater part of the night and of the previous day were quiet, or disturbed at all longitudes, which is not the best criterium for studies at a single station. These studies have shown that both h'f and spread F dependence on magnetic activity vary appreciably with longitude. Therefore, the Jicamarca results presented here are not necessarily typical of other longitudinal sectors. Recently, Batista [1986] used ionosonde observations from Fortaleza, Brazil (3øS, 38øW; magnetic dip 2øS) and Huancayo, Peru (12øS, 75øW; magnetic dip.6øn) between October 1978 and September 1979 to study the effect

5 12,52 Fejer et al.' Brief Report 6O JICAMARCA VERTICAL DRIFTS - KP.(2 + KP>3 EQUINOX o - 2O - WINTER ! I!!! I I I! I I I I I I I I I I I I I Fig. 4. Average plasma drifts for quiet and disturbed periods during equinox and winter. Notice the different behavior with magnetic activity for the two seasons. northward Bz changes which lead to a sudden decrease in the polar cap potential drop. The two largest upward velocity perturbations on the winter nighttime data are examples of events associated with sudden northward B z changes. The disturbance dynamo electric fields, driven primarily by Joule heating at high latitudes, increase linearly with the high-latitude energy input. Their effects on the low and equatorial electric fields were modeled by Blanc and Richmond [198]. Disturbance dynamo electric field (drift) perturbations are usually observed at equator about hours after the onset of geomagnetic storms [Fejer et al., 1983]. These perturbations decrease both the upward drifts during the day and the downward drifts at night. The observed increase of the upward drift velocities in the late afternoon-early evening period for the winter months could be explained by the penetration of magnetospheric electric fields associated with increases in the polar cap potential drop. We have no explanation for the larger efficiency in the penetration of electric fields in the evening sector during winter, but this could perhaps be associated with asymmetry in the conductivity distribution between the two hemispheres during solstice. Numerical modeling studies of convection electric field effects at low latitude have used conductivity distributions consistent with equinoctial periods only. The decrease of the equinoctial drifts near the time of the prereversal enhancement is also difficult to explain, but is consistent with perturbations resulting from disturbance dynamo electric fields. It is clear that additional experimental of magnetic activity on vertical plasma drifts in the afternoonmidnight sector and also on the occurrence of equatorial spread F. The drifts were inferred from the time variation of h'f. Figure 5 shows the results for Huancayo for quiet and disturbed periods. These measurementshow increased h'f and upward drift velocities near dusk for local winter (May- July) and summer (November-February) during magnetically disturbed conditions, in agreement with our observations. The winter observations from Fortaleza, Brazil, also show higher h'f and upward velocities during strongly disturbed evening periods. No magnetic field dependence on the vertical drifts near dusk was detected for equinox on either the Huancayo or on the Fortaleza ionosonde data. We will show later that, except for winter months, this technique underestimates the prereversal enhancement drifts during solar maxima. Therefore, the decrease of the equinoctial upward drifts near dusk seen in the Jicamarca data might not be detectable from the ionosonde observations. Our observations indicate an increase of the prereversal enhancement of the upward drifts with magnetic activity during winter and a decrease for equinox. The quiet time equatorial cross-field plasma drifts are driven by E and F region dynamos. At magnetically disturbed times, magnetospheric electric fields and disturbance dynamo effects also become important. The first of these perturbation processes is associated with changes in the polar cap potential drop which can cause fast electric field changes extending down to the equatorial region [Gonzales et al., 1979; Fejer, 1986]. Several global convection models have been developed to explain the penetration of high-latitudelectric fields and currents to middle and low latitudes [e.g., Senior and Blanc, 1984; Spiro et al., 1988]. The models predicthat an increase (decrease) in the polar cap potential drop results in upward (downward) equatorial velocity perturbations during the day and downward (upward) at night. The high-latitud electric fields should penetrate most efficiently into the equatorial ionosphere near dusk and in the postmidnight-early morning hours. The most frequent and largest equatorial drift perturbations are downward during the day and upward at night [Gonzales et al., 1979; Fejer, 1986; $omayajulu et al., 1987]. These perturbations are associated with sudden UANCAYO Kp<2 L EQUINOX Kp>4+ SUM R - h,f. WlNTER LL I ( I-- co?. 125 > o I I I I I I TM[ Fi[. 5. Height of h'f and vc ical plasma vd ities obt ned from the Huancayo ionosondc data du n[ ma[nctic ] quiet d sturb pc ods (adapt from Batista []986]).

6 Fejer et al.: Brief Report 12,53 and numerical studies are needed to understand the changes in the prereversal enhancement drifts with magnetic activity. Comparison between incoherent scatteradar and ionosonde drifts. Ionosonde observations have been used for several years to estimate the vertical motion of the equatorial ionosphere by measuring the time rate of change of h'f [Abdu et al., 1981; Batista et al., 1986]. This technique is expected to work best when the height of the F layer is above about 3 km [Bittencourt and Abdu, 1981] which corresponds usually to the period around the prereversal enhancement of the vertical drifts. The seasonal and longitudinal variations of vertical drifts inferred from ionosonde measurements from Fortaleza, Brazil and Huancayo, Peru were discussed by Batista et al. [1986]. Simultaneous incoherent scatter drift measurements at Jicamarca and ionosonde observations from Huancayo during showed that the two techniques were in good agreement during evening hours for winter months. However, the maximum prereversal enhancement drift velocities obtained from the ionosonde data were systematically smaller than the corresponding Jicamarca drifts during both equinox and summer. Batism et al. [ 1986] have presented the monthly averages of the ionosonde vertical drifts for both Huancayo and Fortaleza for October 1978 to September The average solar flux during this time period is comparable to that of our data. The average drifts shown in Figures 1, 4, and 5, clearly show that although the winter Jicamarc and ionosonde drifts are in good agreement, the equinoctial and summer prereversal enhancement drifts are largely underestimated by the ionosonde technique. We have also compared the Jicamarca and ionosonde drifts for October 1978 and observed the same disagreement. The ratio of about 2 between evening drifts measured at Jicamarca for and those inferred from ionosonde measurements is much larger than obtained by Batista et al. [ 1986] for a similar comparison using the data. This suggests that for equinox and probably also for summer the accuracy of the ionosonde inferred drifts decreases with increasing solar flux. In the future, we intend to determine the correction curves for the drifts inferred from h'f using simultaneous data from Huancayo and Jicamarca. Once properly corrected, the ionosonde drift should provide important information on the longitudinal dependence of the equatorial vertical drifts. Conclusions We have seen that the main effect of the increased solar flux between the and the solar maxima was the occurrence of larger afternoon and evening equinocial drifts. Very large prereversal enhancement velocities were observed during the equinoctial periods of when solar activity was substantially higher than in the previous solar maximum. On the other hand, the winter average drifts for and were essentially identical. The afternoon and evening winter drifts increase with magnetic activity probably as a result of the penetration of magnetospheric electric fields into the equatorial ionosphere. For equinox, the vertical drifts near dusk seem to decrease with magnetic activity during solar maxima. Vertical plasma drifts calculated from the time variation of h'f also suggest an increase of the upward plasma drifts with magnetic activity for the winter months. The accuracy of this technique decreases with increasing solar flux except for winter months. Acknowledgments. We thank the staff of the Jicamarca Radio Observatory for the measurements and D. Thompson for help with the computations Utah State University. This work was supported by the National Science Foundation through grant ATM , and by the Air Force contract F C-19. E.R. de Paula was supported by the Fundo Nacional de Desenvolvimento Cientffico e Tecno16gico under contract FINEP 537/CT and by the Conselho Nacional de Desenvolvimento Cienfffico e Tecno16gico-CNPq of Brazil. The Jicamarca Observatory is operated by the Instituto Geoffsico del Peru with support from the National Science Foundation. The Editor thanks D. N. Anderson and C. Mazaudier for their assistance in evaluating this paper. References Abdu, M. A., J. A. Bittencourt, and I. S. Batista, Magnetic declination control of the equatorial F region dynamo electric field development and spread F, J. Geophys. Res., 86, 11,443, Anderson, D. N., M. Mendillo, and B. Hermiter, A semiimperical low-latitude ionospheric model, Radio Sci., 22, 292, Batista, I. S., Dinamo da regiao F equatorial: Assimentrias sazonais e longitudinais no setor Americano, Ph.D. thesis, Rep. 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7 12,54 Fejer et al.: Brief Report production mechanism of electric currents and fields in the ionosphere, J. Geophys. Res., 81, 547, Rishbeth, H., The F region dynamo, Planet. Space Sci., 19, 263, Senior, C., and M. Blanc, On the control of magnetospheric convection by spatial distribution of ionospheric conductivities, J. Geophys. Res., 86, 261, Sojka, J., and R. W. Schunk, A theoretical study of the global F region for June solstice, solar maximum, and low magnetic activity, J. Geophys. Res., 9, 5285, Somayajulu, V. V., C. V. Derasia, C. A. Reddy, and K. S. Viswanathan, Penetration of magnetosphericonvective electric field to the equatorial ionosphere during the substorm of March 22, 1979, Geophys. Res. Lett., 14, 876, Spiro, R. W., R. A. Wolf, and B. G. Fejer, Penetration of high-latitude electric field effects to low latitudes during Sundial 1984, Ann. Geophys., 6, 39, Woodman, R. F., Vertical velocities and east-west electric fields at the magnetic equator, J. Geophys. Res., 75, 6249, 197. Woodman, R. F., East-west ionospheric drifts at the magnetic equator, Space Res., 12, 969, I.S. Batista and E.R. de Paula, Instituto de Pesquisas Espaciais (INPE), 121 S. Jos6 dos Campos, Sgo Paulo, Brazil. E. Bonelli, Departamento de Ffsica, Universidade federal do Rio Grande do None, Natal, Brazil. B.G. Fejer, Center for Atmospheric and Space Sciences, Utah State University, Logan, UT R.F. Woodman, Instituto Geoffsico del Peru, Lima, Peru. (Received August 29, 1988; revised May 2, 1989; accepted May 5, 1989.)