GEOPHYSICAL RF.•EARCH LETTERS, VOL. 19, NO. 3,PAGES 245-247, FEBRUARY 7, 1992 K-SPECTRA OF LOWFREQUENCY FLUCTUATIONS IN LABORATORY PLASMA SIMULATING CONDITIONS OFEQUATORIAL SPREAD-F G. Prasad, D. Bors and Y.C. Saxena Institute for Plasma Research _A.b•r.a..:.cA. Low frequency electrostaticfluctuations (•o < 12i) travelling in ExB direction areobserved in a laboratory plasma simulating conditions similar to night time equatorial E-region. The density andpotential fluc- tuations in theregion of theplasma'where effective g parallel to density gradient Vn has similar spectral char- acteristics to that of theresults of our earlier experiment ofbottom side of equatorial spread F. The k-spectrum of density in this region of plasmaexhibits two marked slopes with indices n= -2.5-3,0.3and-4.64- 0.3 whereas thepotential spectrum exhibits one slopewith index- 5.2:5 0.2. The fluctuations aregenerated in theregion whereVn is antiparallel to g and appear in the other re- gion dueE xB motion. Results are in agreement with in-situ measurements in F-region,theoretical predictions and numerical simulation of Rayleigh-Taylor and drift wave instabilities. Introduction The electrostatic fluctuations associated with the equatorial spread F (ESF) arise dueto the special ge- ometry of the bottom side of the F - region plasma, viz., the density gradient (Vn) antiparallel to thegravity (g) and perpendicular to the magnetic field,together with the fact that plasma is weakly ionized with magnetized electrons andions (r,,:,• << fl,,; •'h• << lll). The night time equatorial F - region plasma density profile of Belle etal. (1986) indicate two distinct regions; region 1 corresponding to bottom side of thedensity peak, having geometry described asabove and region 2 above thepeak density witit Vn parallel tog. Both, the Rayleigh-Taylor (R-T) instability and drift wave instability are known to play important role in determining thecharacteristics of the fluctuations (Ossakow 1981; LaBelle et al. !986; Ke- skinen et fl. 1984; Chaturvedi andKaw1976; Sudan and Keskinen 1984). While linear theories predict the excitation of these tinctnations on the bottom side the F-region, the in-situ measurements indicate presence of these fluctuations onboth sides of theF-region peak (Kelley et31. 1981 & references therein). The numerical simulation ofnon-linear evolution oftheR-T instability by Scannapieco etal. (1976) have shown that the fluctu- ations are generated in the bottom side ofF-region peak which then propagate beyond F-region peak by ExB polarisation motion. Copyright 1992 by the American Geophysical Union. Paper number 92GL00046 0094-8534/92/92GL4){X)46503.00 We have earlier reported (Prasad et fl. 1991) an experiment to simulate cha.rcteristics of the bottom side of equatorial F-region andshown that the k-spectra for density and potential fluctuationshave similar indices to those observed in in-situ observations and predicted by analytical and numerical results. In a modification of this experiment we have simulatedconditions of both the region I and region 2 of the F-region plasma and studied the density and potential fluctuations in these regions. The spectralcharacteristics of thesefluctuations are presented and compared with in-situ observations, analytical predictions and numerical simulations. Experimental set-upand Data analysis The experiment has been performed in a toroidal plasma device BETAand the experimental setup i• described in brief by Prasad et fl. (!991). The details of experi- mental parameters, diagnostics and data acquisition are similar to that mentioned in that paper. The plasma is produced, in a toroidal vessel placed in toroidal magnetic field, by striking a discharge between cathode (incandes- cent tungsten filament) andthe vessel wall (anode), in Argon gas a,t a.pressure of 10 -4 torr. A magnetic field of 1 kG is applied. The system is evacuated to 10 -ø torr before filling tl•e gas. The toroidal plasma has a major radius 45 cm and a minor radius 15 cm. The position of the filament is adjusted to tailor the profiles. The results reported here are obtained with the filament lo- cated at major radius of 45 cm. The data is acquired at different radial and azimuthal locations using 8 chan- nel CAMAC based data acquisitionsystem. The data is analysed using standard FFT technique (Smithet fl. 1974) to obtain frequency, wave number, coherence and power spectrum S(w) of fluctuations.The k-spectrum of these fluctuations is obtained in both region 1 and re- gion 2 by using a. fixed pairof probes using the technique outlined by Beallet fl. (1982). The fixedpair of probes in region 1 of the plasma with angleof seperation 30 ø between thetnlocated at a radial distance (r) of 8 cm resolveswave number from-0.75 to 0.75 cm-• . Simi- larly the fixedprobe pair in region 2 of the plasma placed at the same radial but different azimuthal location with ang!uar separation of 60" resloves wave numbers from -0.375 to 0.375 cm"•. Results and discussion The plasma has a peak density of 8x 10 TM cm -s and ambientelectricfield of 3.0 xo!t/cm at 1 kG toroidal magnetic field value. The effectof g is simulated by 245