Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

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Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements
  ~) Pergamon Journal of Atmospheric and Terrestrial Physics, Vol. 57, No. 6, pp. 645~51, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021-9169/95 $9.50 + 0.00 0021-9169(94)00104-9 Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements Harri Laakso,*# Thomas L. Aggson,* F. A. Herrero,* Robert F. Pfaff* and William B. Hansont *Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. tCentel; for Space Sciences, University of Texas at Dallas, Richardson, TX 75083, U.S.A. Received in inal form 29 July 1994 ; accepted 1 August 1994) Abstract--DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubble, 0 are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47~)1.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15-30 m s-' exists. Simultaneously, the F-region plasma was moving downward at a speed of 30-50 m s- L These events appear in the local time sector of 23.00- 23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25-50 m s t are possible at times near midnight in the equatorial F-region. l. INTRODUCTION The vertical neutral wind in the equatorial thermo- sphere has not been studied as thoroughly as the hori- zontal neutral wind. Most ground-based measure- ments of the horizontal wind near the equator have been made using F~.bry-Perot Interferometers with the assumption that the vertical velocity was negligible and could therefore be used as reference for the Doppler shifts measured along the horizontal direc- tion (Biondi et al., 1988 ; Burnside et al., 1981 ; Fried- man and Herrero, 1982). Vertical winds have been inferred from azimuthal scans of the horizontal wind by Burnside et al. (1981). These authors obtained vertical wind estimates ranging from about 5 to 10 m s -I at low latitudes (18°N) when the horizontal wind gradient was small. Recently, Herrero and Meri- wether (1994) have examined the consequences of the strong meridional intensity gradients (MIGs). They found that vertical neutral winds of 25-50 m s- 1 are plausible during MIGs near the equator which is considered more in the discussion. In the auroral ti~ermosphere, vertical winds of roughly + 100 m s -1 are frequently observed (see for example, Smith, 1980). Such winds are believed to occur in response to intense Joule heating during aur- t Now at Department of Geophysics, Finnish Meteoro- logical Institute, Helsinki, Finland. oral storm activity. The small horizontal scales (100- 200 km) of these events make the vertical winds com- parable with the horizontal winds (Herrero et al., 1984). This paper presents an analysis of ion density depletions at low altitudes, 330-340 km, in the local time sector of 22-23 LT near the magnetic and geo- graphic equator on 25 October 1988. We use the elec- tric field and plasma density data to reveal the ambient vertical neutral wind near the equator where its mag- nitude is normally assumed to be less than 5 m s -l. It turns out that the zonal electric field observations are very sensitive to the vertical neutral wind speed. In order to explain the measured density and electric field variations, background vertical wind velocities of about 15-30 m s- ~ downward are needed. Vertical neutral winds of smaller values lead to electric fields which are not consistent with the measurements. These results are discussed in the context of previous results. 2. OBSERVATIONS The San Marco D satellite was launched on 25 March 1988, into a low-altitude (275 x 610 km) orbit near the geographic equator. The satellite carried two orthogonal pairs of 20-m electric field antennas (40 m tip-to-tip) deployed normal to the spin vector (spin period ~ 10 s). The spin axis was perpendicular to the 645  646 II:: -2 ;> -4- 6~ 5- 4. ,,? 3- Z- i05:. 4- 3 01:5 H. Laakso et al. San Marco D ...... ii iiii ......... .4)0 01:51:20 22:58 23:03 339 337 2.5 2.5 316.8 318 5.3 4.7 October 25, 1988 ....... iii ii i ii iiji iii iiii ......... ili ii i .... , , ,.t,l,_,_, ,_, .,., , , I r J ~ , I , , , , ,iiiiiii iiiiiiiiiiii i ili ii ' "'i~: .: ~" :' ::'|i .... ~ ' ' "":"l""':'"';"~",'" ' , I .... , ' ";";': 01:51:40 23:08 335 2.6 319.3 4.1 , , , , I , , J I i I I 1 ~iil i ~i.:il il Iiiii ii .......... 1 i J I [ I ~ I I [ i I $ t ~ iiiiiiiii ii'iiiii Ii.i'" ii ii 01:52:00 UT 23:13 LT 333 AIt (kin) 2.6 Gl~t (*) 320.5 GLong (*) 3.4 MLat (e) Fig. 1. Density depletion regions crossed by San Marco D on 25 October 1988, 0151.00~152.10 UT. The upper panel is the y-component of the d.c. electric field Ey) and the bottom panel is the ion density (N,). Density depletion regions shaded in gray are associated with enhanced zonal electric fields, resulting in large vertical plasma flows. orbit plane and aligned roughly with the north-south direction (i.e. approximately along the magnetic field direction). The inner 14.5 m and outer 0.5 m of each antenna are insulated, leaving 5 m of exposed Cu-Be (woven) wire ; details of the d.c. electric field instru- ment are provided by Aggson et al. (1992). In this paper, we use the following frame of ref- erence : the x axis is in the spin plane pointing radially out from the Earth, the z axis is parallel to the spin axis pointing to the north, and the y axis completes a right-handed coordinate system, i.e. the y axis is in the spin plane pointing azimuthally to the east. The d.c. electric field was sampled every 128ms. In addition to electric field data, we present the ion num- ber density measured with an axially symmetric ring collector which is a part of the ion velocity instrument on the San Marco payload (Aggson et al., 1992). Figure 1 presents data from a series of plasma depletions observed on a single night on 25 October 1988. The spacecraft lies in the altitude range of 330- 340 km near the magnetic equator. Magnetic field lines at these altitudes map below 400 km at the mag- netic equator. The local time sector of the spacecraft is 23.00-23.15. The ion density drops, shown in the lower panel, are one order of magnitude and simul- taneously the zonal electric field, Ey, is enhanced. However, the electric field is oriented strongly west- ward, indicating that the plasma flows downward. Such flows can occur occasionally after ~21 LT where the ambient zonal electric field is oriented west- ward (for more details, see Laakso et al., 1994). 3. ELECTRIC FIELDS IN DENSITY DEPLETION REGIONS The equation of momentum of charged particles in the F-region includes three driving forces that may lead to a plasma instability : the ambient electric field, neutral wind, and gravitational term. Thus, the equa- tion of momentum is written dvj qj = ~ (E+vj × B)- vj.(vj- U)+ g, O)  Thermospheric vertical winds over the equator 647 where the subscript j denotes the plasma species (sub- script e and i indicate electrons and ions, respectively), m is the species mass, q is the species charge, v is the species velocity, E is the ambient electric field, B is the ambient magnetiic field, vj~ is the species collision frequency with neutral atoms, U is the neutral wind velocity, and g is the: gravitational acceleration. The inertial term on the left side of expression (1) can usually be set to zero. Imposing the condition of quasi-neutrality, n ---- n~ = n, the perpendicular com- ponent of the current density to the magnetic field, j~ = en ~j±-~±), where e is the electron charge, can be written as (Laakso et al., 1994) [g × B vi. [-E g U × B]~ j± = en ~-. + ~. L~ + ~,. +--~--~, (2) San Marco D - October 7,5, 19~, 0151:Z~0151:37 tit ............................. ;;,i,',;"" t 2 4 6 8 10 ~/nt Fig. 2. Zonal electric field in a density depletion region is plotted against the ion density ratio no/n~, where subscripts 0 and 1 refer to the ion density outside and inside the density depletion region, respectively. A linear least squares fit to the data using equation (3) provides the vertical neutral wind speed and the ambient zonal electric field, yielding he vertical ion drift speed. where t~i = eB/mi is the ion gyrofrequency. The assumption of quasi-neutrality also provides current conservation, i.e. V-j = 0. Let us assume that B = Bet and g = -g~. In a planar geometry, we can assume that the electric current does not depend on the x and z coordinates (Anderson and Haerendel, 1979) which yields that jy is constant across the bubble wall. Then, the electric field in a density depletion region is written (Laakso et al., 1994) where the subscript 0 refers to values outside of the bubble (Ey0, no) and the subscript 1 refers to values inside the bubble (Eyl, nO. Parameter Ux is the vertical neutral wind speed at the magnetic equator. Expression (3) can be used to calculate the vertical neutral wind speed if the ion density and the zonal electric field are known outside and inside a density depletion region. Equation (3) assumes local values of the parameters rather than flux tube integrated parameters ; the latter must be used at hitch altitudes of 500 km and above (Anderson and Haerendel, 1979). In our cases, we do not expect that including the flux-tube integrated variables would produce any major deviation from the present result, because the spacecraft altitude is relatively low, in the altitude range of 330-340 km. If the integration is extended down to an altitude of 200 km, the integration path would then extend to about + 8 ° off the magnetic equator. At that altitude, the plasma density becomes small, the ion-neutral col- lision frequency becomes very high, and the neutral wind speed becomes small. Outside the magnetic equator, Ux is affected by meridional winds which are much larger than vertical winds. However, their effect is small in this case because the integration path extends only to +8 ° Thus, we may conclude that the flux-tube integrated equations are not expected to affect the analysis presented in this paper in any significant way. The ion-neutral collision frequency for O ÷ in O can be written vt, = C 6.7 10 -16 n O), where n O) is the atomic oxygen number density and C is an empirical correction factor. According to Burnside et al. (1987) and Sipler et al. (1991), C~ 1.9+0.4. Using the atomic oxygen number density model of U.S. Stan- dard Atmosphere 1976, the ion-neutral collision fre- quency can be approximately written as [ x x0q vl, = v~°. exp - H (x) J' x >t 150 kin, (4) where v~n = 23-1-5 s-1 is the ion-neutral collision fre- quency at an altitude ofxo = 150 kin. The scale height, H x), is taken from U.S. Standard Atmosphere 1976. 4. VERTICAL NEUTRAL WIND SPEED IN THE N1GHTSIDE F-REGION Figure 2 displays the measured electric fields vs. the measured values of the density ratio, no/n~, in the second bubble (centered at ~0151.30 UT) of Fig. 1. In regions where the density depletions are less than 25%, that is no/n~ < 1.3, the data points are ignored, because the method turns out to be inaccurate. The reason is that Ey~ in (3) becomes independent of Ux when no/n1 ---, 1. A linear least squares fit of function Ey~ = K~ no~nO + Ko to the data points provides two constants. Constant Ko gives the vertical neutral wind speed Ux when the gravitational acceleration, the magnetic  648 H. Laakso et al. .... r ...... i ...... i ...... J • • i -- Vaal~tm~tlwlndqtmd Ithrt~D I--v~,~-~ I Io~,~ .,,, ,-~ .,--..,, o,I 0 ..................................................................................... .6e 22:30 ~:45 "23:0O ..... ~3115" Local Time Fig. 3. Vertical neutral wind plotted against the local time. The method explained in Fig. 2 is applied to each bubble to estimate the vertical neutral wind. Error bars show the standard deviation of the data points in each depletion. A dashed line shows an averaged vertical E x B drift speed. Figure 4 compares the Ey measurements and theor- etical expression (3) for the bubbles of Fig. 1. The measured zonal electric field, Ev, is indicated by a solid line (this is the same line as the solid line in the upper panel of Fig. 1). Two derived zonal electric fields are presented : electric fields with U~ m 0 m s- ~ (no ver- tical neutral wind) indicated by a dashed line and electric fields with optimized vertical neutral wind speeds (given by Fig. 3) shown by a dotted line. Assuming no vertical neutral wind yields much too large zonal electric fields in density depletion regions. Using the optimized vertical neutral wind speeds, the predicted zonal E-field and the measured zonal E-field are in good agreement. field, and the ion-neutral collision frequency are known. The sum K~ + K0 is equal to Eyo which is also measured and which gives the vertical ion drift speed. During this event, the vertical neutral wind speed is about -26 m s-~ where the negative sign indicates the downward speed. Thus, the neutral atmosphere is moving downward with the declining F-region. The ambient zonal electric field is about 1.2 mV m -~ which corresponds to a vertical ion drift of about -44 m s -t. If, in addition, vertical neutral wind speed measure- ments were available (unfortunately, the instrument did not work on San Marco D), the ion-neutral col- lision frequency could be alternatively determined with this method. Figure 3 shows calculated vertical neutral wind speeds plotted against local time for the electron den- sity depletion regions crossed by San Marco D on 25 October 1988, 01.50-01.52 UT. The neutral wind speeds are mostly between - 15 and - 30 m s- ~ with an averaged speed of about -25 m s-~. At these alti- tudes, the term gB/vi, is smaller than UxB and Ey0 (see expression (3)), and thus possible errors in vin do not have much effect on the results. For reference, Fig. 3 also shows the vertical plasma speed in the F-region, which is indicated by a dashed line. This speed is 40- 45 m s- ~, which means that plasmas move downward approximately twice as fast as neutral particles. The error bars in Fig. 3 show the standard deviation of the calculated neutral wind speeds in each bubble. A large standard deviation is an indication that the ion density variations and the zonal electric field vari- ations are not well correlated. Plausible reasons for the lack of correlation are large-scale turbulence, a spin modulation in the measurements, and our simple approach (planar geometry and local electro- dynamics). 4.1. Finite bubble yeometry Equation (3) contains two simplifications: infinite sheet model (vs. finite sheet) and local electro- dynamics (vs. flux tube integrated variables, see Anderson and Haerendel, 1979). These selected events occurred at such low altitudes that the latter assump- tion can be justified (see Laakso et al., 1994). However, the former one needs some consideration. We assumed in (3) that the bubble is made of two infinite sheets in the meridional plane. Although bubbles are elongated in the vertical direction, in reality, of course, they are not infinite. Assuming that the cross section of a bubble is an ellipsoid at the magnetic equator, equation (3) may be written where F is the geometrical factor. For a bubble with an elliptical cross section, F is written (Ossakow and Chaturvedi, 1978 ; Hanson and Bamgboye, 1984) x-1 F(X)-b , x>l, (6) -x+l a and variables a and b are the major and minor axis of an ellipsoid, respectively. The case of two infinite sheets is obtained by selecting a = oo. There is good reason to believe that the bubble walls are semi-infinite in the x direction. A finite elliptical cross-section is expected to yield some d.c. electric field signatures outside the bubble, as suggested by Fig. 5. These signatures should be oriented opposite to the electric field in the bubble. Such electric field signatures have not been observed during these events which is an indication that the bubbles are elongated along the x direction with b << a. We have studied the magnitude of vertical neutral  'E E Thermospheric vertical winds over the equator 2 4 6 8 San Marco D October 25, 1988 |i | i i i i s i Sllg|l lllil;;lll||l |lllll II|l|lltlrlllll|l~lllllll|llll | "V"v I~ I i I I I, II I l I j I I t 'q ' , ',",, t i I iI • • model with neutral wind model with no neutral wind 01:51:00 01:51:20 01:51:40 01:52:00 UT 22:58 23:03 23:08 23:13 LT 339 337 335 333 Alt 0an) 2.5 2.5 2.6 2.6 GLat (*) 316.8 318 319.3 320.5 GLong (0) 5.3 4.7 4.1 3.4 MLat (*) Fig. 4. Zonal electric fields for density depletion regions of Fig. 1. The measured field is indicated by a solid line and is the same as the line in the upper panel of Fig. 1. The calculated field without a vertical neutral wind is indicated by a dashed line, and the calculated field with the optimized vertical neutral wind is shown by a dotted line. 649 ...•.."""--.+... Earth E~ Spacecraft orbit "'-.. ..... ..," ...,.-""+'...,..+ "...• ..... .•.'• Time Fig. 5. Sketched eqttipotentials inside and outside a bubble. A bubble of finite size yields electric field signatures of opposite sign outside the bubble. Such signals become weaker in more elongated bubbles. The bottom curve shows an expected zonal electric field on the given spacecraft orbit. winds for finite elliptical bubble geometries. Figure 6 plots the calculated vertical neutral wind against the geometrical ratio b/a for the four downdrafting events of Fig 1. The two infinite sheet model is obtained by selecting b/a = 0, and a circular cross-section is obtained with b/a = 1. Figure 6 shows that in the .. , ...... + .... , .... i .... , .... i .... , .... , ......... 20 -- 0151:2"/-0151:3"/ _. --'" g 0 ................. ;..~.'-2 ° GO 0.2 0.4 0.6 0.S i .0 bit Fig. 6. Vertical neutral wind speed was determined as a function of the geometrical ratio, b/a, for four equatorial babbles. If the geometrical ratio is 0.3-0.7, the ambient ver- tical neutral wind is almost zero. These values, however, are not plausible since they would produce relatively strong electric field signals outside the bubble, which are not observed.
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