On the height variation of the equatorial F-region vertical plasmadrifts

Transcription

1 Utah State University From the SelectedWorks of Bela G. Fejer May 1, 1987 On the height variation of the equatorial F-region vertical plasmadrifts J. E. Pingree Bela G. Fejer, Utah State University Available at:

2 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. A5, PAGES , MAY 1, 1987 On the Height Variation of the Equatorial F Region Vertical Plasma Drifts J. E. PINGREE AND B. G. FEJER School of Electrical Engineering, Cornell University, Ithaca, New York We have used improved incoherent scatter radar measurements at the Jicamarca Radio Observatory to study the height variation of the F region vertical plasma drift velocity (driven by the zonal electric field) during moderately quiet conditions. Preliminary results indicate a nearly linear change of the vertical drift velocity with altitude between 200 and 700 km, but with considerable day-to-day variations in the value of the slope. On the average, the velocity gradients are positive in the late night and morning periods and negative during the afternoon and evening hours. Simultaneous vertical and zonal drift measurements confirm that the measured height variation of the vertical drift is consistent with the existence of a curl free electric field in the low latitude ionosphere. The time dependence of the Jicamarca vertical drifts extrapolated to higher altitudes closely resembles the diurnal variation of the drift component due to the zonal electric field observed at F region heights over Arecibo. INTRODUCTION The equatorial ionosphere has been extensively studied with incoherent scatter radar measurements at the Jicamarca Radio Observatory (12øS, 76.9øW, magnetic dip 2øN). These observations have shown that the F region vertical plasma drifts (driven by east-west electric fields) have large day-to-day, seasonal and solar cycle variations, and can be strongly affected by magnetic activity [e.g., Woodman, 1970; Gonzales et al., 1979; Fejer et al., 1979; Fejer, 1981], whereas the zonal plasma drifts (driven by vertical electric fields) are considerably less variable [Woodman, 1972; Fejer et al. 1981, 1985]. The Jicamarca drift data have also been used in theoretical and numerical. models of the low latitude ionosphere [e.g., Anderson, 1981; Richmond et al., 1980]. The early studies suggested that near the F-region peak and above (usually between about 300 and 500 km), where the incoherent scatter drift measurements are most accurate, the plasma drifts were essentially height independent except during periods of rapid drift variations and near sunset [Woodman, 1970; McClure and Petersen, 1972]. As a result, almost all studies since then have used drift data averaged in altitude, typically between 300 and 400 km, since this improves the statistical accuracy of the measurements. Recently, improved observations indicated that both the east-west and vertical plasma drifts are height dependent. Radar and rocket measurements near sunset showed the occurrence of a shear in the zonal drift velocity with westward drifts in the valley between the E and F regions and eastward drifts at higher altitudes [Valenzuela et al., 1980; Kudeki et al., 1981]. Near the F region peak and above, however, incoherent scatter radar observations indicated that for time scales of about 30 min and longer, the zonal plasma drifts did not change much with altitude [Fejer et al., 1985]. Simultaneous rocket observations from Chamical, Argentina, and radar observations from Jicamarca showed that the evening reversal of the F region vertical drifts occurs earlier at the higher altitudes [Valenzuela et al., 1980]. More recently, Murphy and Heelis [1986], using averaged Jicamarca vertical and zonal drifts, suggested that the altitude gradients of the equatorial plasma drifts are usu- ally very small, but that neglecting them altogether would be inconsistent with a curl-free low-latitude electric field. We present here the first detailed study of the altitudinal variation of F-region vertical plasma drifts. We use improved Jicamarca drift measurements and data analysis to measure the height gradient of the vertical drift as a function of time. These gradients are usually small but are still easily measurable, and are consistent with a curl-free electric field. MEASUREMENT TECHNIQUE The experimental procedure used for F region plasma drift measurements at Jicamarca was described by Woodman and Haglots [1969] and Woodman [!970, 1972]. The incoherent scatter antenna is split into two beams that are both perpendicular to the Earth's magnetic field, one to the east and the other to the west of vertical with a net split of about 7 ø. The two line-of-sight drift velocities are measured typically between 200 and 700 km with an altitude resolution of km and with an integration time of about 5 min. These components are then combined to give the vertical and east-west drifts. The estimated error during daytime is typically about 1-2 m/s for the vertical drifts and m/s for the east-west. The nighttime errors are larger, particularly in the post midnight period during solar minimum when the signal to noise ratio is very small due to reduced electron densities. The results to be presented here were obtained by integrating the data for only 1 min initially. This permits the removal of data contaminated by echoes from satellites and other man made objects (an increasing problem over the last few years) without a significant loss in time coverage. These interference echoes appear in only one integration period and were eliminated during the post processing when a number of integration periods are combined. Furthermore, the statistical errors in the east-west drift measurements, which have increased considerably in the last few years, have been reduced recently by a factor of about three. The antenna positions were changed slightly (R. F. Woodman, private communication, 1985) to correct for the change of the earth's magnetic field inclination over the last decade. The measurements re- quire the radar to be pointed perpendicular to the magnetic field [Woodman and Haglots, 1969; Woodman, 1972]. Copyright 1987 by the American Geophysical Union. Paper number 6A /87/006A RESULTS AND DISCUSSION The height variation of the F region drifts was studied using data from eight days during March 5-6 and 11-12, and April 4763

3 4764 PINGREE AND FEJER' BRIEF REPORT JICARMARCA APRIL 2-3, OO 08:07 09:08 I0:00 11:02 11:54 13:12 14: : 7 LT 600 5OO :19 18: '23 21' '22!!!. i._,1 _. 00:23 01:25 5OO i S,p-,, 0 25 UPWARD VELOCITY (m/s) Fig. 1. Height variation of the vertical plasma drifts over Jicamarca during April 2-3, The integration time of each profile was 30 min. 1-4, Figure 1 shows the height variation of the vertical drifts during April 2-3, In this case the integration time was about 30 min, and only every other profile is shown. The velocity increases with altitude from 0900 LT to 1300 LT, with the largest gradient at around 1000 LT. In the afternoon and early evening periods the gradient is negative with an earlier reversal at the higher altitudes. After 1900 LT the velocities were negative (downward), except at the lowest altitudes where the measurements were contaminated by spread F echoes. The velocity gradients are negative up to about 2200 LT. The data during the early morning period are not shown because the corresponding error bars were very large except over a small range of altitudes around the F region electron density peak. Other examples of large negative gradients in (m/s) -4O -I00 8O 4O 2OO I00-2OO JICAMARCA F- REGION UPWARD VELOCITY 5-6 Morch March 1986 _ / I-4 Aprd 1986 F-REGION EASTWARD VELOCITY O Fig. 2. Plot of the Jicamarca vertical and zonal plasma drifts during March-April 1986 measured with an integration time of 15 min. The smooth curves indicate the spline fits to the data. the vertical velocity near sunset were presented by McClure and Peterson [1972]. We have used a weighted linear least squares fit to each vertical drift profile to determine the average velocity and the rate of change of the velocity with height. The same procedure was used for the east-west drifts, but in this case the data were not good enough to estimate the slope accurately, so that only the weighted averages of the zonal drift profiles were computed. The standard procedure of averaging the data over a height range of about 100 km where the signal to noise ratio is the highest, gives essentially the same mean velocities as the least squares average. Figure 2 shows a composite plot of the vertical and east-west drift data obtained with an integration time of 15 min during the 8 days in March and April The thicker curves indicate the average velocities obtained by fitting cubic splines to the data. The curve produced by the spline fit is a series of cubic polynomials which join together so that the curve and its first and second derivatives are con- tinuous at the end points. The coefficients of the polynomials are adjusted to minimize the mean squared error and to satisfy the above continuity conditions exactly. In this case, the data from all eight days were grouped into half hour local time intervals, and the corresponding weighted averages were used to determine the spline coefficients. Thus, the total effective integration time is 30 minutes. The daytime vertical drift data show large day-to-day variations even during relatively quiet conditions, while the zonal drifts show more random departures from the average curve in agreement with previous studies [e.g., Fejer et al., 1979, 1981]. Some of the data in the sunset-midnight period were contaminated by spread F echoes. The large random fluctuations in the postmidnight period are a result of the reduced electron densities which decrease the signal to noise ratio. Figure 3 shows the time variation of the vertical velocity altitude gradient obtained by combining all the results from March and April The thick curve is a spline fit made using the procedure described above, whereas the light curves are the results of running averages of five adjacent points. Smoothing the data over 75 rain intervals reduces the statistical fluctuations. The average gradient is negative in the after-

4 PINGREE AND FEJER' BRIEF REPORT O5 E JICAMARCA MARCH-APRIL 1986 [ [ [ O Fig. 3. Daily variations of the height gradients of the vertical plasma drifts. The smooth average curve was obtained with an integration time of 30 min. noon and early evening periods, and positive at other times. There are large fluctuations in the derivative as a function of time, particularly when the drifts are highly variable. Valenzuela et al. [1980] pointed out that the vertical plasma drifts decrease with altitude in the evening period. Prakash and Muralikrishna [1981] used east-west drift measurements from irregularities in the equatorial electrojet and average F region vertical drift measurements from Jicamarca to show that the ratio between the zonal electric field in the E and F region increases from morning to afternoon hours and from premidnight to postmidnight hours. These results are consistent with the daily variation shown in Figure 3. Murphy and Heelis [1986] used the Jicamarca average zonal and vertical plasma drifts together with the assumption of an irrotational electric field to estimate the altitudinal gradient of the equatorial plasma drifts. They have shown that the velocity scale height, V:/(OV:/Oz), is minimum in the late afternoon-early evening period. Our observations indicate that the decrease of the velocity gradient length is a consequence of both the small drift velocities and the large negative gradients during that period. The local azimuthal and vertical electric field components can be obtained directly from the drift data, since E---v x B, where v is the plasma drift velocity and B is the geomagnetic field. An upward (eastward) drift velocity of 40 m/s corresponds to an eastward (downward)' electric field of about 1 m¾/m. Near the magnetic equator, for a static dipole magnetic field with components only in the meridional plane, the curl-free electric field condition, i.e., curl E -- curl (-v x B) -- 0, can be written 1 Ova, 2 v Ov r c3 b r rr --+ =0 (1) In addition, we have also assumed that the vertical and zonal plasma drifts are symmetric about the magnetic equator (i.e., Off?O--} 0). This will be true in general for a dipole magnetic field with equipotential field lines. A detailed discussion of the consequences of a curl-free low latitude ionospheric electric field for both dipole and nondipole magnetic fields was presented by Murphy and Heelis [1986]. For the purpose of this work the dipole approximation is compatible with the accuracy of our measurements. We can verify the curl-free condition by evaluating the terms in (1) using our vertical and zonal drift measurements. Near the magnetic equator r - R q- z, where R is the radius of the earth. We assume that the time and longitudinal variations of the zonal drifts are interchangeable. Therefore, the time variation of the zonal drift velocity can be used to determine the first term on the left-hand side of (1). The time derivatives of the east-west drift velocity were computed directly from the spline fitted polynomial curve shown in Figure 2. The daily variation of the first two terms in (1) were combined to give the solid curve in Figure 4. The dashed curve represents the average gradient of the vertical drift velocity with height (see Figure 3). The curl free condition is met when the sum of the two curves is zero. Figure 4 clearly indicates that in spite of the large day-to-day variations in drift velocities and in the velocity gradient, the equatorial plasma drift measurements are consistent with the occurrence of an irrotational electric field at least during moderately quiet conditions. The curl-free condition would be satisfied even better if one of the curves were shifted by approximately 1 hour. Murphy and Heelis [1986] also pointed out that if the vertical velocity is proportional to the radial distance squared, the last two terms in (1) cancel. Hence, the curl-free condition can be satisfied by a vertical drift velocity with two components: one which cancels the first term in (1), and another proportional to r 2. Our observations allow us to make an estimate of the slope of the profile, but are not accurate enough to show any change in the slope with altitude. In order to see such a slight curvature of the profile drift measurements over a much larger range of altitudes are needed. Our measurements can also be used to estimate the vertical drifts at altitudes higher than we can presently measure. This procedure is illustrated in Figure 5, where the lowest curve is the same as in Figure 2a, and the two middle curves (corresponding to magnetic latitudes of about 15 ø and 20 ø at 350 km, respectively) were estimated assuming that the linear height dependence continues at higher altitudes. In order to minimize any possible bias, each profile was individually extrapolated, and then the results from the different days were averaged. The top curve in Figure 5 shows the average quiet time meridional velocity (driven by the zonal electric field) measured at the Arecibo Observatory during the solar minimum period [Ganquly et al., 1987]. The magnetic latitude of Arecibo is about 31øN. Note that the negative velocity gradient with height between 1300 and 2100 LT (see Figure 3) results in a much earlier drift reversal at high altitudes. This also restricts the prereversal enhancement of the upward drifts to the lower altitudes (latitudes). The diurnal variation of the Jicamarca drift extrapolated to 1200 km is in surprisingly good agreement with the Arecibo data particularly during daytime when our measurements are most accurate. The field line which Arecibo observes at F region heights actually crosses the magnetic equator at approximately 2800 km. The detailed comparison of the Arecibo and Jicamarca data is beyond the scope of the present work ; v z JICAMARCA MARCH -APRIL [ 12 ] 16 ' ' 2]0 ] 010 ] 0 ]4 08 Fig. 4. Time plot of the curl E components calculated from the Jicamarca F region drifts.

5 4766 PINGREE AND FEJER: BRIEF REPORT ARECIBO z : -2 ß 40 J JICAMARCA H=lPOOkm (estimated) : 20. / JICAMARCA H=800 km ' 20! / (erx stimated) o a'o o'o i i i i i i i i i Fig. 5. Average Jicamarca vertical drifts measured at 350 km, estimated drifts at 800 and 1200 kin, and average northward/upward Arecibo drift perpendicular to the earth's magnetic field during quietime solar minimum conditions. However, it is clear that the Jicamarca observations can reproduce the general features of the Arecibo data. Note that the time of reversal of the high altitude (latitude) daytime drift velocity from upward to (southward) downward is determined by the time when the velocity gradient changes from positive to negative (see Figure 3). This result is in excellent agreement with the computations by Richmond et al. [1980] (see their Figure 7), which show that the estimated reversal time does not change for magnetic latitudes higher than about ø, approximately the Arecibo latitude. For a more detailed comparison, one must compute the zonal electric field above Jicamarca and map it down along the field line to F region heights above Arecibo, taking into account changes in magnetic field intensity with latitude and altitude. Simultaneous drift observations from Jicamarca and Arecibo indicate that these measurements are also consistent on a day-to-day basis, as will be discussed in a future work. Murphy and Heelis [1986] estimated th, height variation of the equatorial vertical drift velocity for height independent zonal drifts and also for zonal drifts proportional to the radial distance r. The data in Figure 5 are in better agreement with their height independent zonal drift case, as would be expected since, at least during solar maximum, the Jicamarca zonal drifts do not change much with altitude from the F region peak to about km (i.e., over the height range of the Jicamarca drift measurements) [Fejer et al., 1985]. We have also examined the height variation of the vertical drifts using a few days of data from other seasons and also during solar maximum. Our preliminary results indicate that during these periods the average height variation follows the pattern described above. This is consistent with the small dependence of the zonal plasma drifts on season and solar cycle [Fejer et al., 1981, 1985]. CONCLUSIONS Recent improvements in the Jicamarca F region plasma drift measurements and data analysis now allow for the accurate calculation of the derivative of the vertical drift velocity with height. We have shown that the time and height vari- ations of the equatorial plasma drifts are consistent with the assumption that the ionospheric electric field is irrotational. Extrapolation of measured drifts to high altitudes produces a diurnal variation with a form similar to that observed at Arecibo Observatory. The decrease of the equatorial vertical drift with height in the afternoon and early evening sector is probably related to the earlier reversal of the vertical drift velocity at higher altitudes (latitudes) and to the occurrence of appreciable prereversal enhancements only near the magnetic equa- tor. Acknowledgments. We thank R. Woodman and the staff of the Jicamarca Radio Observatory for help with the measurements, W. Swartz for help with the analysis program, and D. Farley for helpful discussions. The Jicamarca Radio Observatory is operated by the Instituto Geofisico del Peru, with support from the National Science Foundation. This work was supported by the Division of Atmospheric Sciences of the National Science Foundation through grants ATM and ATM The Editor thanks R. G. Robie and another referee for their assistance in evaluating this paper. REFERENCES Anderson, D. N., Modelling the ambient low latitude F-region ionosphere, d. Atmos. Terr, Phys., 43, 753, Fejer, B. 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Hagfors, Methods for the measurement of vertical ionospheric motions near the magnetic equator by incoherent scattering, J. Geophys. Res., 74, 1205, Woodman, R. F., Vertical drift velocities and east-west electric fields at the magnetic equator, J. Geophys. Res., 75, 6249, Woodman, R. F., East-west ionospheric drifts at the magnetic equator, Space Res., 12, 969, B. G. Fejer and J. E. Pingree, School of Electrical Engineering, Cornell University, Phillips Hall, Ithaca, NY (Received November 6, 1986; revised February 5, 1987; accepted February 6, 1987.)