Radar studies of anomalousvelocity reversals in the equatorial ionosphere

Transcription

1 Utah State University From the SelectedWorks of Bela G. Fejer January 1, 1976 Radar studies of anomalousvelocity reversals in the equatorial ionosphere Bela G. Fejer, Utah State University D. T. Farley B. B. Balsley R. F. Woodman Available at:

2 VOL. 81, NO. 25 JOURNAL OF GEOPHYSICAL RESEARCH SEPTEMBER 1, 1976 Radar Studies of Anomalous Velocity Reversals in the Equatorial Ionosphere B. G. FEJER AND D. T. FARLEY School of Electrical Engineering, Cornell University, Ithaca, New York B. B. BALSLEY Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado R. F. WOODMAN Max Planck Institut fiir Aeronomie, Katlenburg-Lindau, Germany Radar observations made at Jicamarca show that the equatorial electrojet current and the E and F region electric fields can reverse from their normal direction during the day or night and during magnetically quiet or disturbed conditions. The nighttime reversals can only be detected by such radar measurements. The observationsupport most of the current hypotheses concerning the electrojet plasma instabilities. The rapid reversalsometimeseen during disturbed conditions indicate that high-latitude currents and electric fields associated with substorm activity strongly perturb the dynamo current system at all latitudes. INTRODUCTION It has been known for many years, primarily through day- nighttime value near the dip equator has been discussed in detail by Gouin and Mayaud [1967] and Hutton and Oyinloye time magnetometer measurements, thath equatorial elec- [1970]. The magnitude of the negative excursions below the trojet current and the associated electric field occasionally corresponding nighttime level can be as large as 907. The reverse from their normal direction. These reversals are most depressions of the H component occur simultaneously with common during magnetically quiet conditions and are pre- changes in the direction of the Z (vertical) component and sumably due to perturbations in the low- and mid-latitude hence imply a reversal of thelectrojet current. This phenometidal winds which drive th equatorial currents. Reversals can non, described by Gouin and Mayaud as a 'counterelectrojet,' alsoccur during disturbed conditions, however, and in this occurs most frequently during sunspot minimum years and at case the time scale can be as short as a few minutes. These times of low magnetic activity. In Africa it is most often observed around 0700, 1200, and 1500 LT [Gouin and Mayaud, rapid reversals must represent coupling between the auroral 1967; Hutton and Oyinloye, 1970]; in India it is most common and low-latitude current systems, since clearly the tidal winds near 1600 LT [Rastogi, 1973b]. cannot reverse suddenly, but the conductivity patterns and Hutton and Oyinloye [1970] noted the absence of sporadic E electric fields at high latitudes can. echoes (Es_q) on equatorial ionograms when the electrojet In this paper we first briefly review earlier work and then current reversed during the day from its normal direction. This proceed to discuss a variety of radar evidence from Jicamarca phenomenon has been studied recently in detail by Rastogi showing the field reversals. First of all, the radar data show [1972a, b, 1973a], who used magnetograms and ionograms conclusively that the 'reversals' really are reversals of the elecfrom various equatorial stations as well as a few radar electron trojet current and the associated electric field; they cannot drift measurements from Jicamarca. No Es_q echoes were possibly be explained as being due to a superposition of magobserved during daytime periods of very small electrojet drifts netic fields generated by the electrojet and the ring currents in or downward F region drifts (the daytime F region drift is the magnetosphere. Secondly, the radar data show that electric normally upward). For magnetically quiet conditions the disfield reversals in both the E and F regions occur at night as well appearance of the Es_q echoes is associated with the drop of H as during the day, a resulthat is perhaps not surprising but below the corresponding nighttime value. These phenomena that cannot be demonstrated with magnetometer data alone, are restricted to electrojet latitudes and are believed to be since the E region electron densities and hence currents are related to the lunar tide [Rastogi, 1974]. very small at night. Finally, we briefly discuss the relevance of During magnetically disturbed periods it is more difficulto some of our observations to electrojet plasma instability theoestablish a magnetic baseline because the nighttime H values ries, spread F observations, and E region current models. exhibit large variations, due primarily to ring currents gener- MAGNEToMETER AND IONOGRAM STUDIES ated by interactions between the geomagnetic field and the solar wind. In the equatorial zone most of these magneto- The occurrence of daytime depressions of the H (horizontal) spheric effects can be removed by using the Dst(H), or storm component of the geomagnetic field below the corresponding time variation of H, to correct the nighttime H level. The expression Dst(H) is the average of the variation of H relative Now at Applied Physics and Information Science Department, to its quiet day value around a circle of constant latitude. University of California, La Jolla, California Rastogi [1972b] has shown that no daytime Es_q is observed in Copyright 1976 by the American Geophysical Union. disturbed periods when H is below the nighttime value cor- 4621

3 4622 FEJER ET AL..' ANOMALOUS EQUATORIAL DRIFT REVERSALS rected with the Dst(H) values. Another way of removing mag- with the radar directed perpendicular to the earth's magnetic netospheric effects in equatorial magnetometerecords is to field. The electric field can easily be derived from the drifts. subtracthe magnetic field variations observed at an off-equa- Power profiles of the electrojet echoes are also useful in detertorial station from the H variations observed in the electrojet mining exactly when the electrojet current reverses, since then zone. the driving source of the electrojet instabilities is removed and' the echoes disappear. These profiles are obtained by using the JICAMARCA RADAR DRIFT MEASUREMENTS 'modified range-time-intensity (RTI)' technique discussed by Fejer et al. [1975]. The radar Doppler techniques used to measure both eastwest E region and vertical F region drifts have been described in various papers [e.g., Balsley and Woodman, 1969; Woodman and Hagfors, 1969; Balsley, 1973]. The E region electron drifts are derived from the east-west velocities of electron density irregularities embedded in the equatorial electrojet, while the F region drifts are determined by an incoherent scatter technique JICAMARCA 25 FEBRUARY 1971 EXPERIMENTAL RESULTS AND INTERPRETATION Daytime ret)ersals. Figure 1 shows a few examples of altitude profiles of backscattered echo power (modified RTI) obtained with the vertically directed Jicamarca antenna together with related magnetic field data for February 25, 1971, -'1102, -:1106 II ' 1123 ' 'l. " '.. ß, ' ' -,., Li.I 1136 '/ '12OI J '1210 I... I I.I!! I, I L... I I I I I I! 0 õ0 RELATIVE ECHO POWER (db)... I 1...! i I i... I... I... I... i... I000 II LOCAL TIME Fig. 1. Samples of profiles power scattered vertically from the equatorial electrojet at 50 M Hz together with related magnetic field data. The fluctuations shown in the lower magnetic field curve should be due primarily to electrojet effects. The arrows indicate the times of the power profileshown, and the horizontal line is the estimated electrojet baseline. The dashed portions of magnetic field curves correspond to times when no radar data were available.

4 . FEJER ET AL.' ANOMALOUS EQUATORIAL DRIFT REVERSALS 4623 which was a magnetically disturbed day. The Kp indices for the are, on the average, about 153' above the baseline shown, a period shown were 6-, 5-, and 6. The presence or absence of feature which suggests thathe true baseline should be 153' radar echoes is seen in the power profiles and is also shown higher or that perhaps there is a small threshold belowhich superimposed on the magnetic field curves. The arrows in- irregularities are not generated; (2) large negativexcursions dicate the times of the power profiles. Except for the gaps of the horizontal component of the magnetic field are obshown in the figure the time resolution of the vertical power served, particularly at about 1100 and 1400 LT, and during measurements was 30 s (only a few are shown). The overall these excursions no echoes are observed; (3) no sudden change altitude resolution in these profiles was about 3 km. The in the height of the echoing region, such as that described by variations of the H component due to the electrojet were Fejer et al. [1975] and discussed further below, is seen, aldetermined by subtracting the horizontal component of the though echoes appeared and disappeared several times maglaetic fl.,eld at Huancayo (12øS, 75øW, magnetic dip 0.5øN) throughout the period of observation. A similar example of and Paramaribo (6øN, 55øW, magnetic dip 31øN). The mag- the disappearance of the VHF bac,kscattered echoes during a netic' field variation AH (= AHHuancayo -- Z ['/Paramarlbo) is magnetically perturbed period occurred on January 19, 1973, but is not shown here. presumed to be due to variations in the electrojet current alone, since Paramaribo is ore'side the equatorial zone. The The fact that no echoes are observed during daytime when baseline shown was obtained by sub.tracting the nighttime H H is below the baseline agrees with predictions of the gradient values of a magnetically quiet day. Values of H above the drift instability theory, provided that the electron density baseline correspond to eastward currents, and negative values gradient is positive throughout the electrojet region. At night correspond to westward currents. This procedure has been or even late in the afternoon, however, there can be both used by Cohen and Bowles [1963] and Farley and Balsley positive and negativelectron density gradients in the elec- [1973]. trojet region, in which cas echoeshould be observed before The following are the most important features observed in and after the anomalous reversal but with the echoing regions Figure 1' (1) the values of the electrojet magnetic field at the at different heights. Figure 2 showsome power profiles taken times of a,p{3earance andisappearance of the vertical echoes (a) " I IO : I00- - cll ring such a reversal on January 16, 1973, together with 16,JANUARY NO ECHO _ iiiiii] uj cjo- - :i::. I... I,..!... I...! t_,,,.i... o 2o o RELATIVE ECHO POWER (db) o I I..] 2o (b) ioo 050O... NO DATA... ECHO ECHOES ß.1.,! I t... i... I....!... I... I :..I...! I000 i LOCAL TI ME Fig. 2. Samples of scattered power profiles and magnetic field data from Huancayo for a magnetically quiet day. The magnetic baseline corresponds to the nighttime H value.

5 4624 FEJER ET AL.: ANOMALOUS EQUATORIAL DRIFT REVERSALS corresponding magnetic data from Huancayo. No simultaneous drift measurements are available. This day was magnetically quiet (all Kp indices were smaller than 3), and so nighttime magnetic level corrections were not necessary. After 1824 LT, echoes were continuously observed until Our interpretation of the January 16 data is as follows: the echo shown at 1806 LT, which is similar to the echoes obtained during the preceding hour, corresponds to the anomalously reversed electrojet; at 1811 the current is in the process of reversing again, and the echo disappears; after 1821 the current resumes flowing in the normal daytime direction, and the echo reappears in a new locatioh; finally, at 2047 the echo dropout (not shown here) marks the usual nighttime reversal, which cannot be detected on the magnetogram because by this time the electron densities and hence currents have dropped to very low values. The change of echo location associated with the normal nighttime reversal has been discussed by Fejer et al. [1975]. It has been shown that F region vertical drifts are usually well correlated with the electrojet drift velocity [Balsley, 1969; Balsley and Woodman, 1969]. For a typical daytime situation the electrojet electron drift velocity is westward, and the vertical F region drift is upward; therefore a downward daytime F region drift should correspond to an eastward E region elec- tron drift and hence to a disappearance of the electrojet echo. Such counterelectrojet effects were observed between about 1200 and 1700 LT on February 1-2, These days were fairly quiet magnetically, with Kp indices of 2-4. Figure 3 shows the total power backscattered from the electrojet region, the F region vertical drift velocity, and the magnetic field variations at Huancayo on February 1. Corrections for Dst(H) were not important and were omitted. The F region drifts were obtained by using the technique developed by Woodman and Hag. fors [1969], and the scattered power from the electrojet was measured at an elevation angle of 45øW. We can see that as H becomes smaller than the nighttime value, the vertical drift velocity reverses, and the echo power drops to the back- m/s db 80 I FEBRUARY 1973 ELECTROJET BACKSCATTERED POWER I I I I I I I I I I I I I I I F REGION VERTICAL DRIFT,A,H HUANCAYO ioo 7' ' 0700 I, I I )0 '' I 5100 ' ' 19 tool LOCAL TI ME Fig. 3. The total power scattered from the electrojet at a radar elevation angle of 45øW, the F region vertical drift velocity, and the magnetic field at Huancayo. The dashed line in the top figure indicates the approximate background noise level. "',., 5øø F DEC ,-,w.,- v o9 - i-o - J _ m 50 Vi= o OO I i I i I I I I I I I I I d LOCAL TIME Fig. 4. An illustration of an anomalous nighttime electrojet reversal [after Balsley and Woodman, 1971]. The Huancayo Kp indices are shown above the time axis. ground noise level. The daytime F region vertical drift velocity is downward during the whole period when H is below the nighttime value, just as one would expect if the east-west electric fields in the E and F regions are closely coupled and both reverse at approximately the same time. Nighttime reversals. By using only magnetometer data, anomalous daytime reversals during both quiet and disturbed conditions can be detected, but nighttime reversals cannot. Radar data, however, show clearly that anomalous reversals occur also at night. Figure 4 [from Balsley and Woodman, 1971] shows a comparison between the electrojet drift velocity at Jicamarca and the magnetic field strength at Huancayo on December 20-21, The Kp values for Huancayo are shown at the bottom. For drift velocities smaller than 4-50 m/s the accuracy is no better than 4-20 m/s. Positive drifts correspond to eastward currents (westward electron drifts) and negative drifts to westward currents, the typical daytime and nighttime situations, respectively. We can see that after the very disturbed period before about 1300 LT the electrojet drift velocity became very small until about 2200, after which echoes characteristic of the daytime were observed until about midnight. Normal nighttime drifts were detected only after 0300 LT. Recall that reversed daytime drifts cannot be ob- served by electrojet radar experiments, since the irregularities are not generated. The current could have actually reversed slightly after 1300, although the magnetometer data suggest that any such reversal was probably very weak. Another example of an anomalous nighttime reversal of the electrojet drift is shown in Figure 5 [from Balsley, 1969] together with the variation of the height of the F region electron density maximum, from whose slope we can infer the approximate vertical drift velocity in the F region. Both the E region drift and the inferred F region drift are reversed from their normal sense for a period of over 3 hours starting shortly after midnight, implying a similar reversal of the east-west electric field. The Huancayo Kp indices for the period shown were 3, 2, l, 3, and 4. Figure 6 shows a series of F region vertical drift measurements at Jicamarca. The Huancayo Kp indices are given below each curve. The top curve shows the typical quiet day behavior of the F region vertical drift, i.e., upward during the day and downward at night. The other three curves correspond to magnetically disturbedays, and all show anomalous nighttime reversals. Although it is unrelated to the main subject of this paper, we note here that these disturbed periods with electric field reversals and upward drifts were associated with the appearance of spread F, particularly in the postmidnight

6 _ FLIER ET AL.: ANOMALOUS EQUATORIAL DRIFT REVERSALS 4625 period. This positive correlation between disturbed conditions and the appearance of postmidnight spread F is typical of sunspot minimum (but not maximum) conditions [Singleton, 1968; Farley et al., 1970; Chandra and Rastogi, 1972; Bowman, 1975] and is contrary to the negative correlation which is often quoted. DISCUSSION AND CONCLUSIONS m/s 6( Aug I Sp.F o - 2o -40 zo -60 l,3, I,2, I,I, Au. I9 2 I We have seen from the radar data that the equatorial electric field in the E and F regions can reverse during the day or night and during magnetically quiet or disturbed conditions. The results presented have implications both for the theory of the electrojet instabilities, where they confirm and extend previous conclusions, and for models of the dynamo electric field and current system. Instability theory. The gradient drift instability [e.g., Rogister and D'/lngelo, 1970; Fejer et al., 1975] is excited in the electrojet when the electron density increases upward and the electron drift is toward the west (usual daytime conditions) or both are reversed. During daytime current reversals, then, this instability should not be excited, except perhaps near sunrise or sunset, when the profile may become irregular with vertical gradients of both signs. Farley and Balsley [1973] and Sudan et al. [1973] suggested that the obliquely and vertically moving small-scale irregularities seen by the radar are generated by large-scale type 2 (gradient drift) irregularities via a two-dimensional coupling mechanism. When the primary gradient drift instability is not excited, this mechanism obviously will i, 3, I,2, I,, I, 5, I, 5, I,5, i,5, i- I3-I S p. 19T2 _ v -... I I I 5 I 5 I 5 I 5 I? 1-31 Oct.- 01 Nov , 6, I, 7, I, LOCAL TI ME Fig. 6. A series of F region vertical drift records obtained both quiet (top curve) and disturbed magnetic conditions at Jicamarca. The Huanca o Kp indices and the presence of spread F echoes are also shown. not operate, and hence we would not expect to receive radar echoes. Our results presented here and data reported by Ras- shorter-wavelength waves detectable by radar. Apparently, togi [1972a, 1973b] support this picture. In spite of the very this process does not occur, however, since no reversed type 1 high sensitivity of the Jicamarca radar, no echoes are received echoes have ever been observed at Jicamarca near midday at during the midday reversals. moderate or large elevation angles. At low elevation angles the During a very strong daytime reversal, on the other hand, small-scale type 1 irregularities could perhaps be generated one might expect to receive some type 1 ('two stream') echoes. directly by a suf ciently strong reversed current flow. No such The so-called two-stream driving term of the instability does echoes have been seen, but no extended systematic search for not depend on the electron density gradient but only requires them has ever been made. We conclude tentatively that either that the drift velocity exceed a threshold value that is some- the drift velocity during a daytime reversal never exceeds the what greater than the ion acoustic velocity. If large-amplitude two-stream threshold or else the two-dimensional coupling type 1 waves of long wavelength traveling horizontally were mechanism is ineffective; long-wavelength waves either are not excited, these could in turn, via a similar two-dimensional excited or do not grow to suf cient amplitude. coupling, generate the vertically and obliquely traveling At night or near sunrise or sunset, negative as well as positive vertical electron density gradients are generally present in JICAMARCA the electrojet region, and so radar echoes can be obtained for 5-6 JAN either direction of current flow. The observed shift in height of the echoing region upon reversal is also consistent with the gradient drift theory, which has been discussed previously [Fejer et al., 1975]. Other examples of 50-MHz radar measurements during counterelectrojet conditions have been reported by Carter et al. [1976]. These measurements were taken in central Africa during magnetically disturbed conditions at and near the time of the 1973 solar eclipse. In a few instances early or late in the day at Fort Lamy, Chad, currents were apparently observed to flow simultaneously in both directions within the scattering volume; the radar echoes appeared to contain two components with Doppler shifts of opposite sign. Such echoes have never been seen at Jicamarca, perhaps because Jicamarca (dip angle LOCAL TIM E of 2 ø) is considerably closer to the center of the electrojet than Fort Lamy (4.5 ø dip). Fig. 5. Another example of an anomalous nighttime electrojet Dynamo theory. The reversals during quiet magnetic conreversal. The vertical drift velocity of the F region, indicated roughly ditions are presumably due to alterations in the normal tidal by the slope of the hmax curve, reversed at approximately the same time as the electrojet, indicating a high correlation between the east-west wind patterns and are believed to be related to the lunar tide electric fields in the two regions [after Balsley, 1969]. [Rastogi, 1974]. The rapid reversals during disturbed condi-

7 4626 FEJER ET AL.: ANOMALOUS EQUATORIAL DRIFT REVERSALS tions must be related to magnetospheric and high-latitude Balsley, B. B., Electric fields in the equatorial ionosphere: A review of phenomena. Onwumechili et al. [1973] have Shown that there is techniques and measurements, J. Atmos. Terr. Phys., 35, , sometimes a good correlation between magnetic fluctuations Balsley, B. B., and R. F. Woodman, On the control of the F-region observed at the equator at Huancayo and at Eskadelmuir, drift velocity by the E-region electric field: Experimental evidence, J. Scotland, in the auroral zone. Carter et al. [1976] noted a Atmos. Terr. Phys., 31, , similar correlation during disturbed periods between the mag- Balsley, B. B., and R. F. Woodman, Ionospheric drift velocity measnetic and drift velocity fluctuations in Chad and the magnetic urements at Jicamarca, Peru (July 1967-March 1970), Upper Atmos. fluctuations at Dixon Island in the auroral zone. In both cases Geophys. Rep. 17, World Data Center A, Boulder, Colo., Bowman, G. G., Ionospheric spread-f at Huancayo, sunspot activity the fluctuations were essentially simultaneous in universal and geomagnetic activity, Planet. Space Sci., 23, , time, and the stations were separated by about 5 hours in local Carter, D. A., B. B. Balsley, and W. L. Ecklund, VHF Doppleradar time (75 ø in longitude). Since thes events have rapid onsets observations of the African equatorial electrojet, J. Geophys. Res., and occur nearly simultaneously in regions separated by large 81, , distances, they cannot be due to winds or gravity wave per- cycle, Ann. Geophys., 28, , Chandra, H., and R. G. Rastogi, Equatorial spread F over a solar turbations. Nor can they be due exclusively to magnetospheric Cohen, R., and K. L. Bowles, The association of plane-wave electronring currents, since the radar data give direct evidence that the density irregularities with the equatorial electrojet, J. Geophys. Res., 68, , E region currents at the equator are involved. The variations in Farley, D. T., and B. B. Balsley, Instabilities in the equatorial elec- E region conductivity and electric fields at high latitudes due to trojet, J. Geophys. Res., 78, , precipitation events and other substorm activity apparently Farley, D. T., B. B. Balsley, R. F. Woodman, and J.P. McClure, alter drastically the current flow and the electric field pattern Equatorial spread F: Implications of VHF radar observations, J. over the entire earth. Some fairly recent discussion of this Geophys. Res., 75, , Fejer, B. G., D. T. Farley, B. B. Balsley, and R. F. Woodman, Vertical problem is given by Matsushita and Balsley [1972]. It appears structure of the VHF backscattering region in the equatorial electhat the details of this coupling between high and low latitudes trojet and the gradient drift instability, J. Geophys. Res., 80, are as yet not well understood , In summary, then, our results indicate the following. Gouin, P., and P. N. Mayaud, A propos de l'existence possible d'un 1. Typically, there are no Es_ echoes during anomalous contre-electrojet aux latitudes magn6tiques 6quatoriales, Ann. Geophys., 23, 41-47, daytime electrojet reversal periods because (1) the electron Hutton, R., and J. O. Oyinloye, The counter-electrojet in Nigeria, density gradient is in the wrong direction to generate type 2 Ann. Geophys., 26, , irregularities and (2) the electron drift is too small to generate Matsushita, S., and B. B. Balsley, A question of DP-2, Planet. Space observable type 1 irregularities. Sci., 20, , Onwumechili, A., K. Kawasaki, and $.-I. Akasofu, Relationships 2. There are Es_ echoes during anomalous nighttime elecbetween the equatorial electrojet and polar magnetic variations, trojet reversal periods that arise because the irregular night- Planet. Space Sci., 21, 1-16, time density profile has both positive and negative vertical Rastogi, R. G., Equatorial sporadic E and plasma instabilities, Nagradients. The Es_ echoes change altitudes, a feature consist- ture Phys. Sci., 237, 73-75, 1972a. ent with the fact that reversed drifts require reversed gradients Rastogi, R. G., Sudden disappearance of Es_q and the reversal of the equatorial electric fields, Ann. Geophys., 28, , 1972b. for type 2 irregularity generation. Rastogi, R. G., Equatorial sporadic-e and the electric field, J. Atmos. 3. While slow anomalous reversals during quiet magnetic Terr. Phys., 35, , 1973a. conditions are attributed to tidal wind variations, rapid revers- Rastogi, R. G., Counter electrojet currents in e Indian zone, Planet. als during disturbed magnetic conditions are attributed to Space Sci., 21, , 1973b. Rastogi, R. G., Lunar effects in the counter-electrojet near the maghigh-latitude/equatorial coupling effects. netic equator, J. Atmos. Terr. Phys., 36, , A high degree of correlation between east-west electric Rogister, A., and N. D'Angelo, Type 2 irregularities in the equatorial fields in the E and F regions is demonstrated, at least for quiet electrojet, J. Geophys. Res., 75, , conditions. Singleton, D. G., The morphology of spread F occurrence over half a Acknowledgments. This work was supported in part by National Science Foundation grant DES and by grant from the National Oceanic and Atmospheric Administration. The Jicamarca Observatory is operated by the Geophysical Institute of Peru, Ministry of Education, and is supported by the government of Peru, the National Science Foundation, and the National Aeronautics and Space Administration. The Editor thanks M. K. Hudson and R. G. Rastogi for their assistance in evaluating this paper. sunspot cycle, J. Geophys. Res., 73, , Sudan, R. N., J. Akinrimisi, and D. T. Farley, Generation of smallscale irregularities in the equatorial electrojet, J. Geophys. Res., 78, , Woodman, R. F., and T. Hagfors, Methods for the measurement of vertical ionospheric motions near the magnetic equator by incoherent scattering, J. Geophys. Res., 74, , REFERENCES Balsley, B. B., Nighttime electric fields and vertical ionospheric drifts near the magnetic equator, J. Geophys. Res., 74, , (Received April 19, 1976; accepted June 18, 1976.)