Proton cyclotron waves at Mars and Venus

Transcription

1 Advances in Space Research 38 (26) Proton cyclotron waves at Mars and Venus C.T. Russell a, *, S.S. Mayerberger a, X. Blanco-Cano b a Institute of Geophysics and Planetary Physics, University of California, 63 Charles Young Drive East, 3845 Slichter Hall, Los Angeles, CA , USA b Universidad Nacional Autonoma de Mexico, Coyoacan, Mexico Received 22 September 24; received in revised form 7 February 25; accepted 27 February 25 Abstract Proton cyclotron frequency waves are present upstream of the Mars bow shock and have been interpreted as due to the ionization of exospheric hydrogen and the subsequent acceleration of the proton by the solar wind electric field. These waves are left handed polarized on the spacecraft frame and propagate at a small angle to the magnetic field, presumably growing from the free energy of newly accelerated protons. A second consequence of this acceleration is the loss of the planetõs hydrogen exosphere. Proton cyclotron waves have not previously been reported for Venus. Herein, we report the occurrence of such waves in the Venus magnetosheath. However, we do not find clear examples of such waves in the solar wind upstream of the Venus shock. At Mars, the waves are found up to at least 12 planetary radii to the side of the planet. A process that could produce such an extended pattern of newly created ionization and resultant waves is a multi-step one; ionization near the planets, acceleration of ions by the solar wind electric field, reneutralization of ions allowing them to travel across field lines, and a second ionization far from the planet producing the waves. Ó 25 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Mars; Venus; Ion cyclotron waves 1. Introduction Waves at the proton cyclotron frequency were first detected in a planetary exosphere when the Phobos mission arrived at Mars (Russell et al., 199). These waves had small amplitudes (.15 nt), were left-hand elliptically polarized in the spacecraft frame, and propagated at a small angle to the magnetic field. Russell et al. (199) concluded that the waves were growing from the free energy provided by ring beam protons created when the newly ionized protons were accelerated by the solar wind electric field. This conclusion was supported by the observations by PhobosÕ ASPERA instrument of the picked-up protons produced in the extended exosphere around Mars (Barabash et al., 1991). Waves * Corresponding author. Tel.: ; fax: address: nina@igpp.ucla.edu (C.T. Russell). are also seen extensively in the Mars Global Surveyor (MGS) data with similar properties (Brain et al., 22; Mazelle et al., 24). Waves at the proton cyclotron frequency are important because their power is often expected to be proportional to the rate of newly born ions (Huddleston et al., 199, 1992, 1998) and hence atmospheric loss. However, in the alternate model of Sauer et al. (21) and Dubinin et al. (24) this simple proportionality does not hold. These waves were not originally reported to occur at Venus. While much data exists from the Pioneer Venus magnetometer in the vicinity of Venus (Russell et al., 198), the data analysis tools available at that time were at an early stage of development (Russell, 1983) and it is possible that such waves were overlooked. It is the purpose of this paper to report on a reanalysis of the Pioneer Venus data using the same analysis methods used to discover the Mars waves and compare their properties at the two planets /$3 Ó 25 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:1.116/j.asr

2 746 C.T. Russell et al. / Advances in Space Research 38 (26) Mars proton cyclotron waves Magnetic Field [nt] B x B y B z B MGS Universal Time December 9, 1997 Fig. 1. Time series of magnetic field measurements obtained with Mars Global Surveyor during period in which proton cyclotron waves are present in solar wind in Mars Solar Orbital coordinates, equivalent to the terrestrial GSE system. Spectral Density [nt 2 /Hz] Compressional Frequency [Hz] MGS December 9, 1997 Transverse 1 Fig. 2. spectrum of time series shown in Fig. 1 illustrating peak in power spectral density at local proton gyrofrequency. The Phobos spacecraft was specifically instrumented to study the hydrogen exosphere. While Mars Global Surveyor was not so instrumented, it can detect indirectly, the newly ionized hydrogen from the exosphere through the generation of an electromagnetic wave that can be detected by the MGS magnetometer (Brain et al., 22). The strength of that wave is a measure of the loss rate of the exospheric hydrogen (Russell et al., 199). Using the MGS data, Brain et al. (22) found left-hand polarized waves (..1 Hz) near the local proton gyrofrequency propagating at small to moderate angles to the magnetic field and with amplitude decreasing with distance from Mars. To compare the MGS data with our earlier results and to compare with our survey of Venus data below, we examined MGS data taken during the aerobreaking phase, using the Banal program (Russell, 1983) that had been used on the Phobos data earlier. This program uses both the Means (1972) and Rankin and Kurtz (197) methods to determine the characteristics of the waves. To be chosen as a proton cyclotron event, the waves had to be left-handed with a polarization greater than 5%, propagating within 35 of the magnetic field and within 4% of the expected proton gyrofrequency. Furthermore, the direction of propagation returned by the two techniques had to agree within 15. At Mars, the expected proton cyclotron frequency is about.6 Hz for the 4 nt typical ambient magnetic field value. For a more limited sample of data, the waves were examined with a separate spectral analysis program that performs dynamic spectral analysis of both the real and imaginary part of the spectral matrix. First, we show an example of a proton cyclotron wave at Mars to illustrate the nature of the waves. Fig. 1 shows a time series of the magnetic field seen in the solar wind. Fig. 2 shows a power spectrum of these waves. The arrow shows that the peak power is close to the local proton cyclotron frequency. Wave analysis shows that the waves are left-hand polarized, transverse with an ellipticity of.94, a percent polarization of 86%, and an amplitude of.23 nt. We note that because of our selection criteria this event is not necessarily typical of those analyzed by Brain et al. (22) or Mazelle et al. (24). We can gain a perspective of an entire pass with the use of dynamic spectra as shown in Fig. 3. The top panel shows a dynamic spectrum of the transverse wave power that should be compared with the compressional power in the panel below. The white line shows the proton cyclotron frequency calculated from the strength of the magnetic field. It passes through several regions of enhanced power that mark the occurrence of proton cyclotron waves including those illustrated in Figs. 1 and 2. The compressional power in these waves, is much reduced. The next panel down shows that the proton cyclotron waves are left-hand circularly polarized and the bottom panel shows that the waves propagate at a small angle to the magnetic field. A coherency mask has been used to block out power, ellipticity and direction of propagation for waves with coherency less than.5. Ω P

3 C.T. Russell et al. / Advances in Space Research 38 (26) Fig. 3. Dynamic spectra of the waves seen on the outbound MGS pass on December 9, 1997 from 212 to 24 UT. From top to bottom are shown transverse power, compressional power, ellipticity and direction of propagation relative to the magnetic field. The white line shows the local proton gyrofrequency. A mask has been applied to the spectrum covering regions with a coherency less than.5.

4 748 C.T. Russell et al. / Advances in Space Research 38 (26) A search for waves at venus at the proton cyclotron frequency Despite MarsÕ and VenusÕ similar interaction with the solar wind and theoretical calculations predicting that Venus has hydrogen and oxygen exospheres similar to that of Mars (Nagy et al., 199); such waves have never been reported at Venus. Waves were studied at lower frequencies in the solar wind and the Venus magnetosheath (e.g., Luhmann et al., 1983), but not at the local proton gyro frequency. In an attempt to determine whether the absence of these waves is a sampling artifact, an oversight or whether there is a basic difference in the two exospheres, over 25 h of high time resolution data from the Pioneer magnetometer were examined. As for Mars, a search for left-handed waves with polarization greater than 5%, propagation within 35 of the magnetic field and within 4% of the expected proton gyrofrequency was undertaken with the help of the Banal program (Russell, 1983). At Venus, the expected proton cyclotron frequency is on average about 2.5 times that at Mars,.5 Hz, for the usual 1 nt ambient magnetic field value. Figs. 4 and 5 show time series and power spectra in the magnetosheath of waves satisfying our criteria. However, we did not find such waves further from Venus in our survey. In Fig. 6, we repeat the dynamic spectral analysis shown in Fig. 3 for Mars, but now we analyze the pass from which Figs. 4 and 5 were obtained. The top two panels show dynamic spectra of the transverse and compressional parts of the waves. The bottom two panels show the ellipticity and the direction of propagation. Again, like at Mars the proton cyclotron waves at the time of the event in Figs. 4 and 5 are mainly transverse and left-handed circularly polarized propagating nearly along the field. A coherency Magnetic Field in Spacecraft Coordinates [nt] bx b y b z B : : : : :54 Universal Time May 3, 1979 Fig. 4. Time series of magnetic field measurements in Venus Solar Orbital coordinates obtained with Pioneer Venus in magnetosheath during period in which proton cyclotron waves are present. Spectral Density [nt 2 /Hz] Frequency [Hz] mask has been used to block out power, ellipticity and direction of propagation for waves with coherency less than.5. The event shown in Figs. 4 and 5 is propagating at an angle of 4 to the magnetic field with a percent polarization of 8%, an ellipticity of a.94, is lefthanded and has an amplitude of 2 nt. No comparable waves are detected in the solar wind unlike Mars. 4. Discussion and conclusions Compressional Transverse The appearance of waves at the local proton cyclotron frequency is the signature of the ion pickup process. Ions may be created by photoionization, impact ionization or charge exchange between the hydrogen exosphere and the solar wind. However, at Mars many of these waves at the proton cyclotron frequency occur far beyond, where one would expect them to occur for simple hydrogen exospheres (Brain et al., 22). When an ion is created it drifts in the direction of the solar wind flow. Thus, it is unexpected that the waves appear far to the side of Mars. At Io in the jovian magnetosphere, a similar extended wave source region occurs. To explain this, we proposed a charge-exchange mechanism that created a spray of fast-neutrals (Russell et al., 21; Wang et al., 21). This mechanism is illustrated in Fig. 7, but adapted for the solar wind interaction with Venus and Mars. This mechanism will produce wave power proportional to the solar wind electric field as measured in the planetary reference frame. The beam of fast neutrals moves to the side of the planet and downstream of it, in a sheet perpendicular to the magnetic field. P PVO May 3, 1979 Fig. 5. spectrum of part of the interval shown in Fig. 4 illustrating peak in power spectral density just above local proton gyrofrequency.

5 C.T. Russell et al. / Advances in Space Research 38 (26) Fig. 6. Dynamic spectrum of the waves seen on the Pioneer Venus pass through periapsis on May 3, 1979 that includes the intervals shown in Figs. 4 and 5. Comments in caption of Fig. 3 apply. In summary, we find waves at the proton cyclotron frequency with similar appearances in the magnetosheaths of both Mars and Venus. Thus, hydrogen is being lost to the solar wind at both planets but at Venus the waves are restricted to the magnetosheath, i.e., closer to the planet. In contrast at Mars, the region of occurrence extends well beyond the distance expected for a hydrogen exosphere. A way to accomplish this is

6 75 C.T. Russell et al. / Advances in Space Research 38 (26) V Near Wake Pickup Ions Distant Torus Pickup Ions B E hν Fast Neutral hν Mars, Venus Slow Neutral hν Solar Wind Flow Fast Neutral Atmosphere Neutrals Ionization Neutralization Neutralization Ionization of Neutral Slow Neutral Fig. 7. Possible mechanism for producing an extended exosphere at Venus and Mars. Close to the planet but in the flowing solar plasma protons are created from the neutral hydrogen exosphere by photo-or impact-ionization. These protons are accelerated in the solar wind electric field and then subsequently neutralized by charge exchange. The fast neutral, so produced, crosses the magnetized solar wind to great distances until it is ionized forming a proton ring beam that is unstable to proton cyclotron waves. through the production of fast neutrals. This production allows the pick-up process to extend far to the sides of the planet and allows more of the incoming solar wind to be involved. A test of this hypothesis is an examination of the role of the magnetic field in the amplitude and occurrence of these waves. The ion acceleration is proportional to the cross product of the magnetic field and the solar wind velocity. The fast neutral transport should be perpendicular to the magnetic field. We plan to undertake such a test in a future study. Acknowledgment This work was supported by the National Aeronautics and Space Administration under research grant NNG4GJ14G. References Barabash, S., Dubinin, E., Pissarenko, N., et al. Picked-up protons near mars: phobos observations. Geophys. Res. Lett. 18, , Brain, D.A., Bagenal, F., Acuna, M.H., et al. Observations of lowfrequency electromagnetic plasma waves upstream from the Martian shock. J. Geophys. Res. 17 (A6), 22. Dubinin, E., Sauer, K., McKenzie, J.F. Non-linear stationary waves and solitons in ion-beam-plasma configuration. J. Geophys. Res. 19, A228, 24. Huddleston, D.E., Johnstone, A.D. Relationship between wave energy and free energy from pickup ions in the Comet Halley environment. J. Geophys. Res. 97, 12,217 12,23, Huddleston, D.E., Johnstone, A.D., Coates, A.J. Determination of Comet Halley gas emission characteristics from mass loading of the solar wind. J. Geophys. Res. 95, 21 3, 199. Huddleston, D.E., Strangeway, R.J., Warnecke, J., et al. Ion cyclotron waves in the Io torus: wave dispersion, free energy analysis, and SO þ 2 source rate estimates. J. Geophys. Res. 13, 19,887 19,889, Luhmann, J.G., Tatrallyay, M., Russell, C.T., Winterhalter, D. Magnetic field fluctuations in the Venus magnetosheath. Geophys. Res. Lett. 1, , Mazelle, C., Winterhalter, D., Sauer, K., et al. Bow shock and upstream phenomena at Mars. Space Sci. Rev. 111, , 24. Means, J.D. Use of three dimensional covariance matrix in analyzing the polarization properties of plane waves. J. Geophys. Res. 77 (28), , Nagy, A.F., Kim, J., Cravens, T.E. Hot hydrogen and oxygen in the upper atmospheres of Venus and Mars. Ann. Geophys. 8, , 199. Rankin, D., Kurtz, R. Statistical study of micropulsation polarizations. J. Geophys. Res. 75, , 197. Russell, C.T. Interactive analysis of magnetic field data. Adv. Space Res. 2 (7), , Russell, C.T., Snare, R.C., Means, J.D., Elphic, R.C. Pioneer Venus Orbiter fluxgate magnetometer. IEEE Trans. Geosci. Remote Sens. GE-18 (1), 32 36, 198.

7 C.T. Russell et al. / Advances in Space Research 38 (26) Russell, C.T., Luhmann, J.G., Schwingenschuh, K., et al. Upstream waves at Mars: Phobos observations. Geophys. Res. Lett. 17, 897 9, 199. Russell, C.T., Wang, Y.L., Blanco-Cano, X., Strangeway, R.J. The Io mass-loading disk: constraints provided by ion cylotron wave observations. J. Geophys. Res. 16, 26,233 26,242, 21. Sauer, K., Dubinin, E., McKenzie, J.F. New type of soliton bi-ion plasmas and possible implications. Geophys. Res. Lett. 28, , 21. Wang, Y.L., Russell, C.T., Raeder, J. The Io mass-loading disk: model calculations. J. Geophys. Res. 16, 26,243 26,26, 21.