The central nightside venus ionosphere: dependence of ion concentration and plasma holes location on solar wind dynamic pressure

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Adv. Space Res. Vol. 27, No. 11, pp. 1863-1868, 2001 © 2001 COSPAR. Published by Elsevier Science l,td. All rights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 PII: S0273-1177(01)00320-9

THE CENTRAL NIGHTSIDE VENUS IONOSPHERE: DEPENDENCE OF ION CONCENTRATION AND PLASMA HOLES LOCATION ON SOLAR WIND DYNAMIC PRESSURE K.K. Mahajan 1 and K.-I. Oyama 2

National Physical Laboratory, New Delhi - 110012, India 2. Institute of Space & Astronautical Science, Yoshinodai, Sagamihara, 229, Japan ,

ABSTRACT The aeronomy experiments on the Pioneer Venus Orbiter (PVO) have established three characteristic features of the nightside ionosphere of Venus: (1) disappearing ionospheres, (2) large spatial/temporal variability of ion densities and (3) plasma holes/troughs. As the nightside ionosphere is essentially maintained by the transterminator flow of O+ ions from the dayside, this flow should then be the major parameter controlling these features. Disappearing ionospheres, for example, have indeed been found to occur during episodes of high solar wind dynamic pressure (Psw), when the height of the terminator ionopause is greatly reduced and thus the transterminator flow severely diminished. In this paper, we study the other two features, viz the temporal/spatial ion density variability and the location of plasma holes/troughs by analyzing O÷ density profiles measured by the ion mass spectrometer experiment on the PVO. We find that most of the variability in the central nightside ionosphere is related to Psw, which is seen to control the O÷ peak density. Further, we observe that the altitude of the plasma holes/troughs is also dependant on Psw. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION Early radio occultation measurements had shown that a substantial ionosphere exists on the nightside of Venus. In situ measurements of the ion composition by the ion mass spectrometer (OIMS) and the retarding potential analyzer (ORPA) on the Pioneer Venus Orbiter (PVO) showed that the dominant ion at altitudes above about 160km in the nightside is O+ (Taylor et al., 1980a, Miller et al., 1980). Since O+ ions are not expected to survive during the long Venus night, the presence of a large quantity of these ions in the nightside has been successfully explained by the transterminator O+ flow from the dayside to the nightside (e.g., Knudsen et al., 1980). Among the many interesting features seen in the nightside ionosphere, the most prominent are: (1) the large temporal and spatial variability (e.g., Taylor et al., 1979, 1995), (2) the ionospheric holes/troughs (e.g., Brace et al., 1980, 1982a, Taylor et al., 1980a) and (3) the disappearing ionospheres (e.g., Cravens et al., 1982). Since the transterminator flow from the dayside is the major source of the nightside ionosphere, the concentration and distribution of ions in the nightside ionosphere will then be determined by the ion fluxes at the terminator. These fluxes depend upon the height of the terminator ionopause, which is known to be controlled by the solar wind dynamic pressure, Psw (e.g., Brace et al., 1980). Accordingly, it is expected that most of the features of the nightside ionosphere should be related to Psw. The disappearing ionospheres, for example, have indeed been found to occur during episodes of high Psw. Cravens et al. (1982) proposed that on such occasions, the altitude of the terminator ionopause falls sharply, resulting in a severe reduction in the transterminator flow of the ionospheric plasma from the dayside to the nightside. It seems to us that, like the disappearing ionospheres, the other two important features of the nightside ionosphere (viz the large temporal/spatial varability and the plasma holes/troughs), may also be related to Psw.

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In this paper we examine this possibility. Since O÷ flow from the dayside is the major source of the nightside ionosphere, we essentially analyse the O ÷ profiles measured by the OIMS (Taylor et al., 1980b). DATA BASE The observations used in our analysis are from the PVO-UADS (United Abstract Data System) data base, which contains 12 s-averaged data from all aeronomy experiments for the hour around the PVO periapsis. The PVO primary mission, during which the data used here were obtained, has been described by Colin (1980). The PVO was in an elliptical orbit and its periapsis was maintained near 160 km for three nightside seasons, between the end of 1978 and 1980. To avoid the complexities of near terminator ionosphere, we have limited our analysis generally to the central nightside--i.e., 21:00 to 03:00 local solar time. The orbit numbers for these nightside passes fall in the range 48 to 103, 273 to 328 and 499 to 553 and pertain to a period of high solar activity. We have mainly used data from the OIMS, the Bennett ion mass spectrometer. The design and operation of this instrument have been described by Taylor et al. (1980b). Another important parameter used in our analysis is the solar wind dynamic pressure, Psw, which is based upon the Plasma Analyser, OPA (Intriligator et al., 1980) measurements on the PVO. The PVO-UADS data is available from the World Data Center at NASA-GSFC.

Spatial and Temporal Variability The PVO measurements have established that the Venus nightside ionospheric density varies orders of magnitude within the same orbit path or from one orbit path to adjacent paths (e.g., Taylor et al., 1980a; Brace et al., 1980). This is not a surprising result in view of the large variability of the nightside ionopause altitude. The dependence of the nightside ion density between 170 to 200 km on the ionopause altitude has indeed been demonstrated by Miller and Knudsen (1987) from the ORPA measurements. In addition to variability caused by changes in the ionopause altitude, we find that the density in the O ÷ peak also responds to changes in Psw. Figure 1 shows a plot of O ÷ peak density versus Psw for all the central nightside outbound orbits during the first three seasons. An inverse relationship is seen (inbound orbits showed a similar relationship). The lowest values of density are for orbits with disappearing ionospheres when O ÷ ions were completely absent during a particular orbit. We assigned a value of 5 ions/cm3 for such orbits, which is close to the limiting sensitivity of OIMS (Taylor et al., 1980a). The inverse relationship between Psw and peak O ÷ density seen by us and between Psw and ionopause altitude seen by Miller and Knudsen (1987) clearly shows that the major cause of the large density variations in the nightside ionosphere is the solar wind dynamic pressure. Brace et al. (1990) arrived at similar conclusions by analysing OETP data on a long term basis, though their studies were based upon Ne values averaged over rather large altitude ranges, namely (1) below 170 km, (2) 170 to 300 km, and (3) 300 to 600 km. 106 o'J I

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Location of Plasma Holes The nightside O ÷ profiles invariably exhibit a region of plasma depletion above the periapsis altitude, which can be ascribed to solar wind interaction. In general, when Psw is low, one sees a "full-up" ionosphere with the depletion occurring at some upper boundary called the ionopause (see, e.g., Taylor et al, 1980a, 1995). During episodes of high Psw, the O ÷ depletion is observed to occur at and above the periapsis altitude, thus resulting in disappearing ionospheres (Cravens et al., 1982). However, when Psw is moderate, this depletion generally occurs at altitudes varying between a few tens to a few hundreds of kilometers above the periapsis. This depletion region is well below the conventional ionopause and can be identified as a sharp density gradient in the O ÷ profiles. Taylor et al. (1980a) gave the name "trough" ( in analogy with the Earth's ionosphere), while Brace et al. (1982a) called it "hole". Figure 2 shows a few O ÷ profiles demonstrating the three states of the nightside ionosphere, (1) Disappearing ionospheres, Orbit 48, (2) Plasma troughs/holes, Orbits 56 and 67, and (3) Full-up ionosphere, Orbit 53 (see also Taylor et al., 1995). 8~

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Fig. 2 Sample O ÷ profiles demonstrating the various states of nightside ionosphere. The 100 ions/cm3 line is shown for each orbit. It seems to us that the altitude of the O+ depletion region could provide an important result regarding solar wind interaction and location of plasma troughs/holes. We have, therefore, scaled the depletion altitudes for all the central nightside O+ profiles during the first three Venus seasons. For uniformity, these altitudes have been read for O+ density of 100/cm3. In some cases, when O÷ did not fall to 100/cm3 in the steep gradient, we extrapolated the gradient to obtain 100/cm3 altitude. For disappearing ionospheres, periapsis altitude (nominally 160 km) was taken as the depletion altitude (see also Luhmann, 1992). Figure 3 shows a plot o f O + depletion altitudes versus Psw for the outbound orbits. It can be noted that the depletion altitude falls sharply at low and moderate values of Psw. However, it nearly levels off to 160 km (the periapsis altitude) for Psw above about 5 nPa. An important feature observed is that the depletion altitude is generally low, between 160 and 500 km for a large number of orbits. It rises to 1000 km and more for a few orbits only. Data from outbound orbits showed similar results.

Discussion A major surprise which came out of the PVO mission was the existence of large holes in the Venus nightside ionosphere. The density in these holes was found to be lower than that in the surrounding ionosphere

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K. K. Mahajanand K.-I.Oyama

by one or two orders of magnitude. The initial announcements came from the OEPT measurements (Brace et al., 1980, 1982a), and were supported by the OMAG experimenters (Luhmann et al., 1981) who discovered the existence of strong quasi-vertical magnetic fields within the holes (Bx > By or Bz), while the magnetic field in the surrounding ionosphere was found to be normally weak and more nearly horizontal. Luhmann and Russell (1992) in a detailed analysis of holes from OMAG measurements concluded that the magnetic fields in the holes were larger and more organized than in the surrounding ionosphere. Other characteristics reported about the holes were the presence of two electron populations (with one cold component and one hot component) and a different ion composition with H ÷ as the major ion, while O ÷ is the major ion outside (Grebowsky et al., 1983). Various theories which explain these holes include (1) the emergence of magnetic flux which enters the dayside ionosphere and is convected nightward by the bulk ion flow (Brace et al., 1982a), (2) parallel electric fields that can be generated in the overlying plasma sheet (Grebowsky and Curtis, 1981), (3) nightward ion convection (Grebowsky et al., 1983), and (4) upward ion flow (Hartle and Grebowsky, 1990). Marubashi et al. (1985) interpreted the troughs/holes as resulting from PVO crossings of the ionopause. So another possible hypothesis to explain these holes would be to invoke some plasma clouds above the O ÷ depletion regions. Although such clouds have been observed above the dayside ionopause by Brace et al. (1982b), one then needs to explain the large vertical sizes needed for such clouds. Our analysis of OIMS central nightside data during the first three seasons of the PVO mission shows that there were 75 holes with O ÷ density decreasing by about a factor of 10 (in some cases by larger factors) in each of these holes. In addition to these, there were several other holes with density decreasing by lesser factors. But all these holes occurred just above the O ÷ depletion region. Some orbits showed holes both in the inbound and outbound passes, some in inbound or outbound, while others did not show any holes. Most of the holes were seen when the depletion altitude was below 500 km. Holes were rarely seen when the depletion altitude was above about 1000 km and never seen when it was above 1500 km. Keeping these altitude limits in mind and by looking at Figure 3, one can infer that a hole will generally be seen when Psw is moderate (2.5 to 4.5 n Pa) and a disappearing ionosphere will be seen when Psw is high, in agreement with conclusions of Luhmann and Russell (1992). !

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Fig. 3 A plot of ion depletion altitude in the central nightside ionosphere versus solar wind dynamic pressure, Psw for outbound orbits. We find that the plasma above the holes is rather disturbed. Figure 4 shows some sample O ÷ profiles to demonstrate this. All profiles exhibit prominent holes, but the distribution of O ÷ density above the holes (i.e., above the depletion altitude) is not quite the same. Orbits 509 and 533 show a rather sharp decrease of O ÷ with altitude and the scale height is less than 200 km. On the other hand, orbits 529 and 502 show little decrease of

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O + density with altitude, thus indicating scale height of several thousand kilometers. Further, orbits 70 and 523 show extremely disturbed profiles above the holes. It is difficult to obtain any scale height from such profiles. The simultaneously measured electron temperatures and model ion temperatures (Miller et al., 1980) give scale heights of 500 to 600 km for the orbits shown in Figure 4. Although H + was reported to be the dominant ion in the holes for some orbits (Grebowsky et al., 1983), an examination by us of all the orbits containing holes indicates that this is not a general feature. Further, the large electron temperature seen in the holes (Brace et al., 1980, 1982a) is indeed expected when electron density is that low (Mahajan et al., 1994). .... 7

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Fig. 4 Sample O+ profiles showing large holes. The plasma above the holes for most of the orbits is disturbed. CONCLUSIONS From the above analysis we infer that the solar wind dynamic pressure is the major parameter which controls the behaviour of the nightside ionosphere. It controls the ion concentration and the height of the plasma holes/troughs. We also note that the plasma above the holes is a disturbed plasma. We believe that the above features of the central nightside ionosphere will be helpful in further studies of Venus holes/troughs. Acknowledgements

We are thankful to WDC at NASA/GSFC, Greenbelt for the supply of PVO-UADS data and are grateful to the PIs of the PV mission for making this data available to the international scientific community. Our sincere thanks to Y. Kakinami for his help in the initial stages of work and to R K Choudhary for his help in preparing the camera ready copy. KKM was a Visiting Professor at ISAS when this work was initiated. He is thankful to CSIR for the award of the Emeritus Scientist Grant to undertake this work. References

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Cravens, T.E., L.H. Brace, H.A. Taylor, C.T. Russell, W.C. Knudsen, K.L. Miller, A. Barnes, J.D. Mihalov, F.L. Scarf, S. Quenon and A.F. Nagy, Disappearing ionospheres on the nightside of Venus, Icarus, 51, 271-282, 1982. Grebowsky, J.M., and S.A. Curtis, Venus nightside ionospheric holes in the nightside ionosphere, Geophys. Res. Lett., 8_, 1273-1276, 1981. Grebowsky, J.M., H.G. Mayr, S.A. Curtis and H.A. Taylor, Jr., Venus nighttime horizontal plasma flow, magnetic cogestion and ionospheric hole production, J. Geophys. Res., 88, 3005-3012, 1983. Hartle, R.E., and J.M. Grebowsky, Upward ion flow in ionospheric holes on Venus, J. Geophy. Res., 95, 31-37, 1990. Intriligator, D.S., J.H. Wolfe and J.D. Mihalov, The Pioneer Venus Orbiter Plasma Analyzer experiment, IEEE Trans. Geosci. Remote Sensing, GE-18, 39-43, 1980. Knudsen, W.C., K. Spenner, K.L. Miller and V. Novak, Transport of ionospheric O+ ions across the Venus terminator and implications, J. Geophys. Res., 85, 7803-7810, 1980. Luhmann, J.G., Pervasive large-scale magnetic fields in the Venus nightside ionosphere and their implications, J. Geophys. Res., 97, 6103-6121, 1992. Luhmann, J.G., and D.S. Russell, Magnetic fields in Venus nightside ionospheric holes: Collected Pioneer Venus Orbiter magnetometer observations, J. Geophys. Res., 97, 10, 267-10, 282, 1992. Luhmann, J.G., R.C. Elphic, C.T. Russell, J.A. Slavin and J.D. Mihalov, Observations of large scale steady magnetic fields in the nightside Venus ionosphere and near wake, Geophys. Res. Lett., 8, 517-520, 1981. Mahajan, K.K., S. Ghosh, R. Paul, and W.R. Hoegy, Varability of dayside electron temperature at Venus, Geophys. Res. Lett., 21, 77-80, 1994. Marubashi, K., J.M. Grebowsky, H.A. Taylor Jr., J.G. Luhmann, C.T. Russell and A. Barness, J. Geophys. Res., 90, 1385-1398, 1985. Miller, K.L., and W.C. Knudsen, Spatial and temporal variations of the ion velocity measured in the Venus ionosphere, Adv. Space Res., 7, 12, 107-110, 1987. Miller, K.L., W.C. Knudsen, K. Spenner, R.C. Whitten and V. Novak, Solar zenith angle dependence of ionospheric ion and electron temperatures and density on Venus, J. Geophys. Res., 86, 7559-7764, 1980. Taylor, H.A., Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, P.A. Cloutier, R.E. Daniell, Jr., and T.M. Donahue, Ionosphere of Venus: First observations of day-night variations of the ion composition, Science, ~ 9699, 1979. Taylor, H.A., Jr., H.C. Brinton, S.J. Bauer, R.E. Hartle, P.A. Cloutier and R.E. Daniell, Jr., Global observations of the composition and dynamics of the ionosphere of Venus: Implications for the solar wind interaction, d. Geophys. Res., 85, 7765-7777, 1980a. Taylor, H.A., Jr., H.C. Brinton, G.R. Cordier, B.H. Blackwell and T.C.G. Wagner, Bennett ion mass spectrometer on the Pioneer Venus bus and orbiter, IEEE Trans. Geosci., Remote Sensing, GE-18, 44-49, 1980b. Taylor, H.A., Jr., L. Kramer, P.A. Cloutier and S.S. Walker, Signatures of solarwind interaction with the nightside ionosphere of Venus, Earth, Moon and Planets, ~ 173-199, 1995.