Mid-latitude rocket measurements op plasma irregularities associated with topside density depletion

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Short paper

MID-LATITUDE ROCKET MEASUREMENTS OF PLASXA IRREGULARITIES ASSOCIATED WITH TOPSIDE DENSITY DEPLETION

G.L. Gdalevich+ , V.F. Gubsky+ , L. Natorf++ , V.D. Ozerov+ , A.W. Wernik++ + Space Research Institute, Academy of Sciences

of the USSR, Moscow, USSR

++ Space Research Centre, Polish Academy of Sciences, Warsaw, Poland (Received

in

final

form

16 August

1984)

Abstract: A plasma irregularities spectrum analyzer and a Langmuir probe @xperiment on a rocket experiment conducted at mid-latitudes during quiet nighttime winter conditions revealed the existence of an isolated plasma density depletion between 700 and 1100 km. A substantial enhancement of intensity of the irregularities coinciding with the depletion was observed over a broad band of irregularity sizes ranging from tens of meters to several kilometers. The power spectral index was equal 0.8420.17 as compared to 1.7120.56 for the irregularities outside the plasma depletion.

1.

Introduction Small-scale F-region plasma density irregularities at mid-latitudes have

been studied much less is because

the

weakness

intensively

than

those

at low and high latitudes. This

and infrequent occurrence of mid-latitude

ities make measurements difficult. Early in-situ measurements

irregular-

indicated that

the amplitude of mid-latitude irregularities is usually less than 1% and that almost all the power is contained in scale-sizes smaller than 1 km (Dyson et al.9 1974). The mid-latitude irregularities associated with subtrough density gradients have been studied by Basu(l978). The IXS amplitude of irregularities with sizes from several hundreds of meters to several

kilometers usually

exceeds lo%, but most of the contribution to the nns value cornea from large irregularities which surpass smaller onea in amplitude. Basu suggested that the gradient drift instability is responsible for the generation of the irreg-

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analysis of a large amount of Esro 4 in-situ data ularities. A statistical indicates that during the nighttime irregularities larger than 1.5 km and with amplitudes larger than 5% fill a zone which extends to less than 40' latitude (Clark and Raitt, 1976). In this short paper we present preliminary results of plasma irregularity

measurements made on a Vertical-10 rocket launched from Kapustin Jar, iJSSR (q = 48.5'N, h=

45.7'E) at 2137 LT on December 21, 1981. The day was geomag-

netically moderately quiet(CKp

= 12+) and the rocket reached apogee at 1511

km. The payload included several neutral and plasma diagnostic instruments but only

the

irregularity spectrum analyzer and Langmuir probe data will be discussed here.

2. Experimental setup and results A three electrode spherical ion trap (Belashin et al., 1982) was used as the ion concentration probe. The amplified ion current was measured and, after filtering, its fluctuations were analysed with the spectrum analyzer. In the analyzer the input current fluctuations are converted into voltage fluctuations. They are then processed to achieve the trend reduction, normalized to 10 seconds voltage average and band limited from 1 to 14 Hz

and from 14 to

220 Hz. Two signals are amplified and sampled every 32 ms and 2 ms in the lower and upper frequency bands, respectively, A total number of 128 samples is used for further analysis. Fourier sin and cos transforms at 16 frequencies in both bands are computed and telemetred to the ground station where the power spectrum estimates are calculated. Spectra are measured every 5 s. A detailed description of the spectrum analyzer will appear elsewhere. The electron concentration was measured with the cylindrical

Langmuir

probe. The spectrum analyzer gave over 200 spectra in the altitude range

200 -

1500 km. Six consecutive spectra were averaged in order to reduce the error of the spectrum estimate. Figure 1 shows the spectral intensity profile at two frequencies: 1.58 and 25.39 Hz. The intensities were additionally averaged over four frequencies for further reduction of the error. The effective bandwidth is 5.86 and 93.75 Hz at low and high frequency, respectively. If the wave phase and plasma drift velocities are much less than the rocket velocity, which is assumed, then at the lower frequency the vertical irregularity size changes from 2.66 km at 300 km to 0.83 km at 1400 km. At the higher frequency the corresponding sizes are 0.17 and 0.05 km. A readily noticed feature is the enhanced spectral intensity in the height range 700 - 1200 km during ascent with a maximum at 950 km. The enhancement is particularly large at the higher frequency, The smaller effect at the lower frequency indicates that the spectrum slope is smaller. Indeed, if a spectrum of the form f-" is assumed, then in the height range 775 - 1177 km the average power spectral index n is 0.84 20.17, as compared to 1.7120.56 outside this region. During descent, the spec-

LOG ELECTRON

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POWER (Hz-‘1

for ascent (dots) 1. Spectral intensity profiles and descent (crosses) at 1.59 Hz (right curves) and 25.39 Hz (left curves) and the electron density profiles (thin line) . The vertical dashed lines show the spectrum analyzer noise level,

Pig,

trdl intensity is small and exhibits only minor variations. The power spectral index is ?.&0,2. IIIFigure 1 the ascent and descent electron density profiles are shown as well. A distinct depletion is seen between 700 and $100 km during ascent. The electron density is deficient by up to 40% with respect to that for the descent. The depletion region is characterized by the presence of large scale irregularities. The horizontal separation between profiles is about 17 km in the E - W direction at an altitude 1000 km. Thus, the eastward horizontal gradient is much larger than the vertical one and the gradient reaches maximum around 950 km. Below about 350 km the ascent and descent

profiles differ due to a larger

horizontal separation and to electron density gradients. The disturbance caused

by the rocket exhaust cannot be excluded as a factor contributing to

the divergence of the profiles at these heights. 3.

Conclusions Spectrum analyzer and Langmuir probe data show that enhanced irregularity

intensity coincides with an isolated plasma density depletion. Although the

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presented here refer to two frequencies Only, the spectral intensity has increased over the entire range of the measured frequencies indicating that irregularitieswith sizes ranging from several tens of meters to several kilometers coexisted. We have also found that the spectrum of plasma fluctuations is substantially flatter inside than it is outside the depletion. Various plasma instabilitiesand non-plasma sources have been postulated to explain the small-scale irregularitiesobserved at mid-latitudestfor a review see Pejer and Kelley, 1980). The gradient drift instabilities are attractive candidates for generation of irregularities observed in our experiment. Linear instability can not, however, account for simultaneous presence of irregularitiesdiffering in the scale sizes by more than two orders of magnitude. Thus, the non-linear gradient drift instability or multi-step instability mechanisms must be implemented. An important problem to be solved by the theory is a small power spectral index inside the density depletion,

data

Acknowledgement The irregularityspectrum analyzer was designed and constructed by Yr. 2. Krysinski at the Space Research Centre of the Polish Academy of Sciences, References Basu S, Belashin A.P., Gdalevich G.L., Zhdanov V.I. and Ozerov V.D. Clark D.H. and Raitt W.J. Dyson P.L., "i&lure J,P, and Hanson W.B. Fejer B-G, and Kelley ?I,C.

1976 1974

J.Geophys.Res. 9, 152 Cosmical Inatrunents, p.89, Wauka, Moskva(in Russian) Planet.Space Sci. 4, 873 J.Geophys.Res. 79, 1497

1980

Rev.Geophys.Space Phys, 2,

7975 f982

401