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Venus lightning: Comparison with terrestrial lightning
Planetary and Space Science 59 (2011) 965–973
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Venus lightning: Comparison with terrestrial lightning C.T. Russell a,n, R.J. Strangeway a, J.T.M. Daniels a, T.L. Zhang b, H.Y. Wei a a b
University of California, Los Angeles, CA 90095-1567, USA Space Research Institute, Austrian Academy of Science, Graz, Austria
a r t i c l e in fo
abstract
Article history: Received 2 October 2009 Received in revised form 11 February 2010 Accepted 17 February 2010 Available online 26 February 2010
Terrestrial lightning is generated by the separation of electric charge residing on water–ice particles in clouds, a few kilometers above the electrically conducting surface of the Earth. It is detected optically, electromagnetically, and aurally. The majority of discharges occur within or between clouds with about one third discharging to the surface of the Earth. Upward-propagating lightning also occurs with effects extending into the ionosphere. On Venus, the clouds are close to 50 km above the surface of the planet, where the temperatures and pressures are near those of Earth’s surface. In contrast the atmospheric pressure near the surface of Venus is nearly 100 times that of Earth. Thus, while intra- and inter-cloud lightning is expected to occur in a manner similar to that on Earth, we do not expect discharges from the clouds to the surface to occur. Upward-going lightning may be more frequent at Venus because the ionosphere is closer to the clouds. As at Earth, Venus lightning has been detected optically and electromagnetically from a variety of platforms. We find that some of the observed properties of lightning are different at the two planets. Many of the differences in the electromagnetic waves detected by spacecraft can be attributed to effects during ionospheric propagation to the spacecraft. We review the differences in the ionospheres of Earth and Venus and how they affect observations. We use both the Pioneer Venus electric antenna observations as well as the Venus Express magnetic measurements. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Venus Lightning Venus Express
1. Introduction Venus and Earth are often called twin planets because of their similar size, mass, interior structure, and chemical composition. However, they are not identical twins, but rather fraternal twins, because unlike the Earth, Venus has low water content, lacks plate tectonics, possesses a dense atmosphere, and lacks an intrinsic magnetic field. Both planets have clouds and a dynamic atmosphere near the 1-bar level, but the terrestrial clouds are composed of water while the Venus clouds consist of sulfuric acid. As a result of these differences, the sulfur cycle is perhaps as important on Venus as the carbon cycle is at Earth. Nevertheless, the differences do not mean that familiar terrestrial processes do not also occur regularly on Venus. For example, in the past Venus has had extensive volcanism, which may be continuing today without the presence of a wet crust. Its atmospheric composition has evolved as has the Earth’s and there is lightning despite the absence of water clouds. It is important to study such processes under different conditions because we learn how these processes operate much more deeply when seen in different settings.
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0032-0633/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2010.02.010
Comparative planetology can be a powerful tool for the planetary scientist. The atmospheres of Earth and Venus are markedly different. Earth’s surface has a temperature of about 300 K and a pressure of 1 bar, while, as shown in Fig. 1, Venus has a surface temperature of over 700 K and a pressure of close to 100 bar. The composition of the Earth’s atmosphere is predominantly N2, and of the Venus atmosphere, CO2. The clouds of Earth can be generally found from 1 to 10 km above the surface, while on Venus, they occur from 47 to 65 km. Terrestrial clouds contain water particles while Venus clouds contain sulfuric acid particles. However, at the altitudes of the clouds, the temperatures, and pressures are rather similar at the two planets. While we have not been able to photograph Venus clouds with the same cadence and spatial resolution as we have photographed terrestrial clouds, we know from global cloud photography that they circle the planet in 4 days at low latitudes and form a powerful vortex like a hurricane’s eye over the polar regions (Limaye et al., 2009). On Earth, almost two thirds of the discharges occur either within a cloud or between clouds with nearly one third being from the cloud to the surface. On Venus the high altitude clouds and the thickness of the atmosphere are expected to prevent such cloud-to-ground discharges. In the years since the Pioneer Venus mission ended in1992, the understanding of terrestrial lightning
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Fig. 1. Temperature and pressure profile of the Venus atmosphere.
Fig. 2. Artist conception of a lightning discharge in the Venus atmosphere near an aerobot. Courtesy of NASA.
has been revolutionized especially in areas that may be relevant to the Venus situation. Numerous upward-going discharges such as red sprites and blue jets have been observed, and ionospheric effects such as ELVES discovered (Fukunishi et al., 1996). These advances help us understand some of the phenomenology of the Pioneer Venus observations. On Earth, we usually think of the hazards of lightning rather than the benefits. As illustrated in Fig. 2, the electric discharge produces a long narrow channel in which the atmosphere is
heated and the pressure increased. The high pressure produced in this hot channel leads to acoustic signals that we call thunder. The high temperature and pressure also promote chemical reactions that would not take place under conditions of local thermodynamic equilibrium. In the Earth’s atmosphere, nitric oxide is formed this way, which benefits certain plants. The electric current that heats the atmosphere also prefers to flow through material with high electrical conductivity such as lightning rods, wires, trees, and golfers, causing considerable damage to electrical equipment and loss of life each year. Even though a hundred lightning discharges occur every second on average worldwide, this amounts to only about 7/km2/year on average. The rate is strongly dependent on weather patterns, so lightning occurs much more often in some areas than in others and at certain times of the day and the year than at others. Nitric oxide is also produced in the atmosphere of Venus (Krasnopolsky, 1983; 2006). This production may not benefit plant life on Venus but it does suggest that the rates of lightning discharge are similar on the two planets. It is also not dangerous to our technical systems as we do not operate systems in the atmosphere of Venus at present. The only balloons that have been flown in Venus’ atmosphere lasted 4 days in total (Sagdeev et al., 1986a), and we do not believe that it was lightning that silenced them. However, when we do develop long-lived balloons for Venus atmospheric exploration we should consider shielding essential equipment. Lightning has been detected on Venus with almost every technique used on Earth. It has been seen optically from Venera 9 (Krasnopolsky, 1980) and from a terrestrial telescope (Hansell et al., 1995). It has been seen with an electric antenna at ELF and VLF frequencies (Taylor et al., 1979; Scarf et al., 1980; Russell, 1991). It has been seen at ELF frequencies with a fluxgate magnetometer (Russell et al., 2007). It has been detected with radio waves that passed through the ionosphere into the solar wind (Gurnett et al., 1991), with ELF electric signals within the ionosphere (Russell, 1991), and with electric and magnetic antennas within the atmosphere (Strangeway et al., 1993, Ksanfomaliti, 1983). Sometimes ELF waves are not seen. This can be due to many factors, including propagation effects, such as absorption, evanescence, and reflection, as well as the statistical nature of its occurrence both in space and time. It was not detected by the Pioneer Venus star sensor (Borucki et al., 1981), possibly due to the short time the star sensor was exposed over the lightning region. It was not detected by the Vega balloon photometer in the Venus clouds (Sagdeev et al., 1986b) possibly due to the short path length of photons within clouds. While Cassini did see radio emissions when it flew by Venus (Gurnett et al., 2001), these were not interpreted as lightning as they were different in intensity and cadence than on the subsequent Earth flyby. We certainly do not expect lightning at the two planets to produce exactly the same signals because of their different possible discharge paths and because of the different plasma and magnetic field environments in which the signals propagate. In the next sections, we discuss the differences in the plasma and magnetic envelopes of Earth and Venus, how they affect the signals that were detected by the Pioneer Venus Orbiter, and what signals are being detected today by Venus Express.
2. Different plasma environments of Earth and Venus Earth has a relatively strong intrinsic magnetic field, whose pressure is sufficient to stand off the solar wind over 10 Earth radii from the center of the Earth. The configuration of this magnetic field is shown by the curved dashed line in Fig. 3, that of
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a dipole magnetic field. The field lines do not deviate from this near-dipole configuration until they are far above the surface of the Earth. Dipole field lines extend radially above the north and south magnetic poles and are horizontal near the equator. When lightning-generated electromagnetic waves enter the ionosphere below the electron gyrofrequency, they propagate in what is known as the whistler mode. The electron gyrofrequency is about 1,400,000 Hz in the Earth’s ionosphere and about 560 Hz at Venus. Lightning discharges produce broad band electromagnetic
Fig. 3. Non-ducted entry and propagation of whistler-mode signals in the Earth’s magnetosphere. The curved dashed line represents the magnetic field line (Russell et al., 1972). On Earth, entry into the magnetosphere is promoted by the largely vertical direction of the magnetic field. Thus many lightning-generated waves can reach the spacecraft ‘‘S’’.
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waves with power from tens to millions of Hertz. Since there is a stop band above the electron gyrofrequency, this means that on Earth we expect to see lightning-generated signals to a much higher frequency than on Venus. A second factor that is important is the direction of propagation of the electromagnetic wave as it attempts to enter the ionosphere. Because the signals slow rapidly from the speed of light to a value about a thousand times less, the leading edge of the electromagnetic wave flattens, and the wavefront propagates vertically. This happens equally at Earth and Venus, but with different results because of their differing magnetic configurations. As shown in Fig. 3, as long as the lightning source on the Earth is not near the equator, the energy that is propagating radially enters the ionosphere from the atmosphere and is guided along the nearly vertical magnetic field. This guidance is loose and depends on frequency. The energy is not strongly absorbed by the cold ionospheric and plasmaspheric plasmas, and the signals can bounce back and forth in the magnetosphere several times, before reaching the satellite, S, in Fig. 3. The high frequencies arrive first at the spacecraft because of the nature of the ‘‘dispersion relation’’ of whistler-mode waves, which gives these waves their characteristic descending tone. The path of the waves at Venus is much shorter because the waves are found in the ionosphere, not an extended magnetosphere. Hence at Venus there is no significant ‘‘dispersion.’’ The markedly different magnetic geometry of Venus is illustrated in Fig. 4. The magnetic field is induced and as a result is mainly horizontal. The physics of this induction is that the highly conducting ionosphere produces an electric current that opposes any changing magnetic field. The field of the solar wind averages to zero over a long enough time. To the extent that the ionosphere has finite resistivity, this current will decay with time, allowing the magnetic field to enter the ionosphere and also
Fig. 4. Geometry of Venus’ induced magnetosphere. Field lines draped around the ionosphere slowly sink through the dayside ionosphere and penetrate the atmospheric region. These field lines are mainly horizontal over much of the planet and prevent the lightning-generated whistler waves from reaching satellites throughout much of the Venus ionosphere. On the nightside of Venus, these field lines have a strong vertical component, particularly conducive for guiding electromagnetic waves at frequencies below the local electron gyrofrequency through the ionosphere to the spacecraft. Pioneer Venus was able to probe this near equatorial region and observed copious whistler-mode waves.
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to change direction when the magnetic field direction in the solar wind changes. This generally does not allow easy entry into the ionosphere, except in the center of the wake, where the draped field becomes vertical and points tailward on the right (dark) side of the planet. As a result, the number of whistler-mode waves in the ionosphere due to lightning discharges on Venus might well be much smaller than seen on Earth, even if the discharge rates in the two atmospheres were the same. Fig. 5 illustrates the flattening of the electromagnetic wavefront as it enters the ionosphere. Refraction of the signals by the increasing index of refraction associated with the increasing electron density causes the signal to propagate vertically. At Earth, where the magnetic field can be nearly radial, this generally facilitates entry into the ionosphere. Only if the wave is above the electron gyrofrequency does it cease to propagate. On Venus, where over much of the planet the magnetic field is horizontal, this vertical refraction mitigates against entry into the ionosphere. Whistler mode waves do not propagate across magnetic field lines at frequencies above 43 times the proton gyrofrequency.
Fig. 5. Schematic illustration of the flattening of a wavefront of the electromagnetic wave generated by the lightning signals as it penetrates the increasing density of the lower ionosphere, where the phase velocity decreases. As a result, the wave normals are vertical, almost perpendicular to the horizontal magnetic field seen over much of the ionospheric portion of the Venus Express orbit.
The phase velocity drops to zero for what we call perpendicular propagation, as shown in Fig. 6 for typical conditions in the Venus ionosphere. Thus, we would expect a cone of non-propagation for magnetic fields nearly horizontal in the ionosphere. Once they do couple their energy to the magnetic field, the energy is strongly guided by the magnetic field as is seen in the right-hand panel of Fig. 6, in which the group velocity is restricted to a narrow range of angles along the field. Signals observed at the same frequency in the two ionospheres behave differently, even though the electron densities may be the same. This is because of the importance of the magnetic field in controlling the motions of the electrons and ions, and hence the properties of the electromagnetic wave carried by the gyrating and oscillating charged particles. Fig. 7 shows the electric and magnetic fields measured in the terrestrial ionosphere by the Injun 5 plasma wave instrument at frequencies similar to those at which Pioneer Venus and Venus Express observe lightning signals. What we are seeing is a signal generated over a broad band (from at least 90 to 264 Hz here) and moving upward into the Earth’s ionosphere to the location of Injun 5. Initially the signals are righthanded, but as they propagate upward, signals that were close to the helium gyrofrequency are now closer to the proton gyrofrequency and are converted at what is called the crossover frequency to left-handed polarization. When these now lefthanded waves propagate further along the field line, they encounter the proton gyrofrequency (264 Hz here corresponding to 17,310 nT) and are absorbed by the protons. As they approach this resonant frequency, the waves slow down and the energy moves from the electric component to the magnetic component. This strong dispersion near the proton gyrofrequency earns these waves the name proton cyclotron whistlers, and creates individual bursts that are about 4 s long. Such dispersion is not observed on Venus at the same frequencies because there those waves are well above the proton gyrofrequency and do not resonate with proton motion. Further, the altitude variation of the magnetic field is dissimilar to that on Earth. It is instructive at this point to compare wave amplitudes at Earth and Venus. Surprisingly, there has been very little work done on surveying the wave amplitudes in the terrestrial ionosphere, so we cannot quote average or typical values, but the waves seen in Fig. 7 have a root mean square (rms) amplitude of about 10 pT. The Venus Express rms magnetic amplitudes at similar frequencies are about 300 pT and sometimes much greater. Since the power of electromagnetic waves depends
Fig. 6. Phase speed and group velocity in polar diagrams for Venus ionospheric conditions. The magnetic field is vertical in the diagram. The phase velocity of the whistlermode wave does not propagate across the field and the group velocity is closely parallel to the magnetic field under these conditions.
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Fig. 7. Injun 5 electric and magnetic dynamic spectra of upward traveling EM waves in the terrestrial ionosphere (Mosier and Gurnett, 1969).
inversely on the velocity of the wave for a constant Poynting flux, these waves, that are traveling quite slowly near the end of their lives at 264 Hz, are amplified above their earlier strengths.
3. Pioneer Venus observations Pioneer Venus and Venus Express observations are very complementary. At first glance, their orbits are similar—a 24-h period with periapsis low in the ionosphere. However, the periapsis of Pioneer Venus was close to the equator, while the Venus Express periapsis is close to the north pole. This results in the two sets of low-altitude lightning data being taken in two different magnetic configurations. Moreover, the Pioneer Venus signals were received by an electric field sensor that became noisy in sunlight so that all useful lightning data were taken in eclipse. Fig. 8 shows a sample of Pioneer Venus electric field data with three different types of signals, each quite easily recognizable (Russell and Scarf, 1990). The middle section labeled ‘‘b’’ occurs when the electric field antenna rotates into the wake of the spacecraft moving through the ionosphere and the dipole antenna temporarily finds itself in two different plasma environments. Signals in ‘‘c’’ are whistler-mode signals propagating along the magnetic field below the electron-cyclotron frequency. The channels of the instrument above the electron-cyclotron frequency (narrowband channels at 730 Hz, 5.4 kHz, and 30 kHz) show no signal detection at all. During this period, the magnetic field was 241 to the radial direction, measured from the center of Venus, providing a propagation path from the upper atmosphere to the spacecraft. The magnetic field strength was 24 nT, allowing 100 Hz propagation, but not 730 Hz. In interval ‘‘a’’ bursts are seen at all frequencies, even those well above the electron gyrofrequencies. Here, the magnetic field is nearly horizontal, only 31 from horizontal at the time of the strongest burst, and the electron gyrofrequency is similar to that during the ‘‘c’’ signals. The signals appear to be the equivalent of the terrestrial ELVES, where the electric field of the upward lightning discharge affects a large area above the stroke but does not create electromagnetic waves.
Fig. 8. Plasma wave measurements on the Pioneer Venus Orbiter in eclipse in four narrowband filters (Russell and Scarf, 1990).
The signals seen in interval ‘‘a’’ are only a low-altitude phenomenon. Fig. 9 shows a statistical analysis of such waves versus altitude (Ho et al., 1992). The 30 kHz waves are seen no higher than 180 km. The 5.4 kHz and 730 Hz waves fall off rapidly in occurrence, but can be seen to 300 km on occasions. In contrast, the 100 Hz waves are seen when the magnetic field is nearly radial, so that the vertically propagating waves can move as whistler-mode waves and do not noticeably decrease in occurrence with altitude. This dependence is further evidence that the atmosphere is the source of these waves, and shows that the waves on Venus are similar to our postulated terrestrial counterparts.
4. Venus Express observations Venus Express includes a fluxgate magnetometer sampling the magnetic field at a rate of up to 128 Hz. Venus Express did not
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Fig. 9. Altitude variation of the rate of occurrence of the different impulsive signals seen above the night ionosphere by the Pioneer Venus plasma wave instrument. The dashed and dotted lines are non-propagating signals that may be due to discharges from the clouds to the ionosphere. The solid line shows the occurrence rate of the propagating VLF waves that are inside the cone of allowed propagation around the magnetic field (Ho et al., 1992).
include a magnetic cleanliness program. Instead spacecraft generated signals are detected by using a gradiometer sensor mounted on the body of the spacecraft. This sensor allows the spacecraft sources to be separated from the natural signals detected at the outboard sensor 1 m from the spacecraft deck (Zhang et al., 2006). The use of the dual sensors allowed the correction of the data at data rates of 1 Hz and lower. The 128 Hz data were not corrected but rather analyzed in spacecraft coordinates after filtering. The analysis techniques used to detect the whistler-mode waves involve filtering the 128 Hz data to a passband of frequencies whose range contains very few spurious spacecraft signals. The technique is described in detail by Russell et al. (2008a). One of the key results of the Venus Express measurement was simply the detection of the magnetic component of the waves whose electric component was detected by Pioneer Venus (Russell et al., 2007). Despite the unfavorable magnetic geometry of the Venus ionospheric field, strong electromagnetic bursts were seen with energies comparable to those expected from Pioneer Venus measurements (Russell et al., 2006). These waves were nearly circularly polarized and propagated nearly parallel to the magnetic field (Russell et al., 2007). The entry of energy into the ionosphere was facilitated by time variations of the interplanetary magnetic field. This leads ultimately to twisted magnetic fields in the ionosphere, some of which deviate strongly enough from the horizontal to allow propagation into the ionosphere from below. Fig. 10 shows the maximum peak-to-peak amplitude on a pass through the lower ionosphere versus the maximum field inclination. A deviation of 171 is enough to allow the energy to penetrate into the ionosphere (Russell et al., 2008b). Whistler-mode waves can reach very significant amplitudes of over 1 nT peak-to-peak and the duration of an interval of activity can last several seconds, as illustrated in Fig. 11. We can learn more about the signal using the Fourier coefficients on these axes to compute the in-phase and quadrature power, thereby determining the direction of propagation, the ellipticity of the wave, and the percent polarization (Means, 1972). To do this, we add the band-passed 128 Hz signal to the 4-s cleaned data in spacecraft coordinates and analyze the resulting signal relative to the magnetic field. This allows us to determine the handedness of the waves and the direction of propagation relative to the magnetic field. Fig. 12 shows a power spectrum of the signals from 0620:11 to 0620:19 on April 15, 2007, shown in Fig. 11.
Fig. 10. Peak-to-peak amplitudes of burst maxima on each pass versus maximum inclination of the magnetic field to the horizontal.
Fig. 11. Band-passed filtered 128 Hz magnetic data showing a strong burst accompanied by weaker wave activity on April 15, 2007.
The signal was bandpass-filtered in the frequency range 42–60 Hz using a sharp filter with an attenuation of 100 db outside the passband. The compressional signal shows the power in the field strength, i.e. in the component along the field, and the transverse signal contains the remaining power. The signal is strongly transverse to the field. From the cross-correlation we can calculate the signal amplitudes as well as the direction of propagation, the percent polarization and the ellipticity in sub-bands of the spectrum. These quantities are shown for four bands in Table 1. The locations of these bands are shown in Fig. 12. The lowest frequency range is the strongest here. The signals propagate close to the magnetic field and are very coherent with a percent polarization of 95%. This large percent polarization indicates that the signal is well above noise or equivalently that the signal is produced by a single dominant source. We would expect this of a
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strong lightning discharge. The time series in Fig. 11 shows that one burst dominates over others. The ellipticity is positive and near unity. The positive sign means that the signal is right-handed and we would expect a nearly circular ellipticity of unity for a nearly parallel propagating whistler-mode signal. We see that at high frequencies the percent polarizations drop and the ellipticities drop while the direction of propagation to the field remains fairly constant. The drop in percent polarization is probably due to the weakening of the signal relative to the noise sources. The ellipticity change may be due to the approach
Fig. 12. Power spectrum of waves seen in Fig. 11. Transverse and compressional powers are shown separately. Bands analyzed in Table 1 are shown by labels a–d.
Table 1 Spectral properties of whistler-mode signals on April 15, 2007. Frequency range (Hz) 42.2–49.0 49.1–54.0 54.2–58.7 58.7–60.1
Amplitude (pT)
yRB (deg)
Pol. (%)
Ellipt.
115 67 58 18
14 20 5 17
95 85 69 68
0.82 0.75 0.71 0.60
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of the signals to the electron gyrofrequency. We note that the signal is right-hand polarized across the frequency band. This is consistent with the signals being whistler-mode waves. However, the signals are not strongly filtered above the 64 Hz Nyquist frequency. If the signals were in the band 64–128 Hz, they would be seen as left-handed waves due to aliasing. If the signal were from the next higher band 128–192 Hz, they would again appear right-handed, but it would be less likely that they fell into a single band. In short, the persistence of the right-handed polarization implies that the wave frequency is lower than 64 Hz and has not been altered by aliasing. Fig. 13 shows 2 s of whistler-mode wave activity on July 1, 2006. This event has been previously discussed by Russell et al. (2008b). Here we have added the band-passed signal to the 4-s average field in spacecraft coordinates. The varying ratios between axes during this interval indicate that there are different sources for these bursts. Table 2 shows the wave properties in four frequency bands. Here, the lowest frequency band is the weakest with weak polarization that is very elliptical, not circular. Here the lowest frequency band is closest to the noise. The second band propagates more closely to the field, within 91, is highly polarized, and well above the noise, and its ellipticity is closer to unity and therefore much more circular. The third band propagates nearly along the magnetic field, but its low percent polarization and nearly linear ellipticity indicates it is not far above the noise or that there are several sources contributing to this signal. The fourth band is significantly different from the others. It is left-hand polarized. Since left-handed waves do not propagate at this frequency, the wave frequency must be above the Nyquist frequency of 64 Hz. Importantly this signal seems not to be weak relative to the noise. It has a percent polarization of 71% and propagates nearly along the magnetic field. The drop in absolute power seen in Fig. 14 may be due to the instrument’s ‘‘anti-aliasing filter’’. In short, the combination of the two events in Figs. 12 and 14 suggests that the signals detected on Venus Express are often in the nominal passband of the instrument, but can be at times strong above its Nyquist frequency of 64 Hz. As mentioned above, Krasnopolsky’s (1983; 2006) observations of nitric oxide in the Venus atmosphere are consistent with the rate of lightning being similar at Venus and Earth. Pioneer Venus observations also suggested this. Nevertheless, it is very difficult to make this comparison using the Venus Express or the Pioneer Venus data because of the access problem. We see only a fraction of the events in the ionosphere. The ionosphere is a good
Fig. 13. Two-second interval of whistler-mode waves at periapsis on July 1, 2006 in spacecraft coordinates. Four-second magnetic field has been added to bandpass-filtered data.
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Table 2 Spectral properties of whistler-mode signals on July 1, 2006. Frequency range (Hz) 41.9–45.0 45.5–50.8 51.4–55.8 56.3–59.8
Amplitude (pT)
yRB (deg)
Pol. (%)
30 77 68 53
451 91 71 91
38 85 45 71
Ellipt. 0.33 0.77 0.33 0.38
Fig. 15. Percent access time as a function of local time of periapsis for the first Venus year of operation. Access time is the fractional time the magnetic field tilts at an angle greater than 171 from the horizontal. The burst rate in the lower panel is not simply controlled by this parameter. From noon to midnight, the rate dropped to zero, possibly because of the weakness of the magnetic field at the satellite altitude so that the signal could not propagate from the atmosphere to the spacecraft.
Fig. 14. Power spectrum of waves seen in Fig. 13. Transverse and compressional powers are shown separately. Bands analyzed in Table 2 are shown by labels a–d.
shield. We can illustrate this with Fig. 15, in which we calculate based on the study shown in Fig. 10 that signals can propagate from the atmosphere to the spacecraft based solely on the direction of the field. The percent access time is nearly symmetric around noon, but the observed burst rate is not. Thus, there must be some other factor affecting the access to the spacecraft. One possible factor is the strength of the magnetic field. A weak magnetic field could prevent the signal from reaching the spacecraft.
5. Discussion and conclusions While the observational evidence for electric discharges in the Venus atmosphere is overwhelming, the theoretical community seems still divided on whether the properties of the upper atmosphere of Venus as we understand them permit such discharges. Two recent papers exemplify this. In a study of the possibility of sprites in the atmospheres of planets other than Earth, Yair et al. (2009) predict the appearance of sprites at Venus. In contrast, Michael et al. (2009) examine the expected abundance of aerosols and the expected electrical conductivity of the upper atmosphere and predict that there is too little of the former and too much of the latter to support lightning. In our opinion, we do have the conditions on Venus that lead to strong potential differences in the clouds as on Earth. Both H2O and H2SO4 are polar molecules. They both become solid at similar temperatures and the nephelometers on the Pioneer Venus atmospheric probes saw solid particles of varying sizes, thought to promote
differential charging in terrestrial clouds (Ragent and Blamont, 1979; Knollenberg and Hunten, 1979). The evidence that lightning is taking place is not based on a few observations, nor on a few properties of the observed waves. Rather the observations behave precisely as one would expect if there were lightning discharges in the atmosphere below the spacecraft. Whistler mode signals with the correct polarization were observed on Pioneer Venus only when the magnetic field dipped into the atmosphere, allowing the upward propagating ways to reach the spacecraft. When the field was horizontal the wave amplitude decreased rapidly with altitude. In this situation the waves appeared at frequencies where no electromagnetic wave could propagate. Not only the frequencies and the polarization were correct but also the wave amplitude. Pioneer Venus did make observations in the atmosphere at the bottom of the ionosphere as it entered the atmosphere at the end of the mission. The electromagnetic energy flux in the atmosphere was the same as in the ionosphere when the field was vertically out of the atmosphere. Moreover as they should, vertical fields led to nearly constant electromagnetic flux with altitude. When Venus Express arrived with a completely different detection technique the same wave power was seen. Observationally there is complete closure. There are no conflicting observations. Observations that should agree, do agree, but clearly we need to learn more about the electrical conductivity and aerosols of the cloud layer. We look forward to the day when we can deploy more balloon-based measurements in the Venus atmosphere. From the properties we have measured we have no reason to believe that the lightning produced at Venus is produced by a process fundamentally different than on Earth. The signals produced by lightning in the atmosphere of Earth and Venus appear to be quite similar at ELF frequencies. Nevertheless, the differing magnetic configurations facilitate entry of the signals into the terrestrial ionosphere while they tend to prevent the entry into the Venus ionosphere. The differing strengths of the magnetic fields also affect the appearance of the signals, producing dispersion in the case of the terrestrial signals (near
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the proton and helium gyrofrequencies) and not producing dispersion at higher frequencies at Venus. Nevertheless, the evidence for pervasive lightning at Venus is strong. Pioneer Venus detected the whistler-mode radiation expected from lightning in the equatorial regions as well as signals that could correspond to terrestrial ELVES. Venus Express detects similar whistler-mode signals at high latitudes near the north pole. The amplitudes of the signals at Venus are strong, stronger that the signals seen by Injun 5 in the terrestrial ionosphere. Nitric oxide observations and reasonable assumptions about the access of signals to the spacecraft suggest that the rate of lightning strokes is comparable to that on Earth. In short, the generation of lightning is one more way in which Venus and Earth are fraternal twins.
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Limaye, S.S., Kossin, J.P., Rozoff, C., Piccioni, G., Titov, D.V., Markiewicz, W.J., 2009. Vortex circulation on Venus: dynamical similarities with terrestrial hurricanes. Geophys. Res. Lett. 36, L04204. Means, J.D., 1972. Use of three dimensional covariance matrix in analyzing the polarization properties of plane waves. J. Geophys. Res. 77, 5551–5559. Michael, M., Tripathi, S.N., Borucki, W.J., Whitten, R.C., 2009. Highly charged cloud particles in the atmosphere of Venus. J. Geophys. Res. 114, E4. Mosier, S.R., Gurnett, D.A., 1969. VLF Measurements of the Poynting flux along the geomagnetic field with the Injun 5 satellite. J. Geophys. Res. 74, 24. Ragent, B., Blamont, J., 1979. Preliminary results of the Pioneer Venus nephelometer experiment. Science 203, 790–792. Russell, C.T., 1991. Venus lightning. Space Sci. Rev. 55, 317–356. Russell, C.T., Scarf, F.L., 1990. Evidence for lightning on Venus. Adv. Space Res. 10, 125–136. Russell, C.T., McPherron, R.L., Coleman Jr., P.J., 1972. Fluctuating magnetic fields in the magnetosphere, 1, ELF and VLF fluctuations. Space Sci. Rev. 12 (6), 810. Russell, C.T., Strangeway, R.J., Zhang, T.L., 2006. Lightning detection on the Venus Express Mission. Planet. Space Sci. 54, 1344–1351. Russell, C.T., Zhang, T.L., Delva, M., Magnes, W., Strangeway, R.J., Wei, H.Y., 2007. Lightning on Venus inferred from whistler-mode wave in the ionosphere. Nature 450, 661–662. Russell, C.T., Zhang, T.L., Strangeway, R.J., Wei, H.Y., Delva, M., Magnes, W., 2008a. Electromagnetic waves observed by Venus express at periapsis: detection and analysis techniques. Adv. Space Res. 41, 113–117. Russell, C.T., Zhang, T.L., Wei, H.Y., 2008b. Whistler mode waves from lightning on Venus: magnetic control of ionospheric access. J. Geophys. Res. 113, E00B05. Sagdeev, R.Z., Linkin, V.M., Blamont, J.E., Preston, R.A., 1986a. The Vega Venus balloon experiment. Science 231 (4744), 1407–1408. Sagdeev, R.Z., Linkin, V.M., Kerzhanovich, V.V., Lipatov, A.N., Shurupov, A.A., Blamont, J.E., Crisp, D., Ingersoll, A.P., Elson, L.S., Preston, R.A., Hildebrand, C.E., Ragent, B., Seiff, A., Young, R.E., Petit, G., Boloh, L., Alexandrov, Y.N., Armand, N.A., Bakitko, R.V., Selivanov, A.S., 1986b. Overview of VEGA Venus balloon insitu meteorological measurements. Science 231, 1411–1414. Scarf, F.L., Taylor, W.W.L., Russell, C.T., Brace, L.H., 1980. Lightning on Venus: orbiter detection of whistler signals. J. Geophys. Res. 85, 8158–8166. Strangeway, R.J., Russell, C.T., Ho, C.M., Brace, L.H., 1993. Plasma waves observed at low altitudes in the tenuous Venus nightside ionosphere. Geophys. Res. Lett. 20, 2767–2770. Taylor, W.W.L., Scarf, F.L., Russell, C.T., Brace, L.H., 1979. Evidence for lightning on Venus. Nature 279, 614–616. Yair, Y., Takahashi, Y., Yaniv, R., Ebert, U., Goto, Y., 2009. A study of the possibility of sprites in the atmospheres of other planets. J. Geophys. Res. 114, E9. Zhang, T.L., Baumjohann, W., Delva, M., Auster, H.-U., Balogh, A., Russell, C.T., Barabash, S., Balikhin, M., Berghofer, G., Biernat, H.K., Lammer, H., Lichtenegger, H., Magnes, W., Nakamura, R., Penz, T., Schwingenschuh, K., Voros, Z., Zambelli, W., Fornacon, K.-H., Glassmeier, K.-H., Richter, I., Carr, C., Kudela, K., Shi, J.K., Zhao, H., Motschmann, U., Lebreton, J.-P., 2006. Magnetic field investigation of the Venus plasma environment: expected new results from Venus Express. Planet. Space Sci. 54, 1336–1343.
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Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
Venus lightning: Comparison with terrestrial lightning C.T. Russell a,n, R.J. Strangeway a, J.T.M. Daniels a, T.L. Zhang b, H.Y. Wei a a b
University of California, Los Angeles, CA 90095-1567, USA Space Research Institute, Austrian Academy of Science, Graz, Austria
a r t i c l e in fo
abstract
Article history: Received 2 October 2009 Received in revised form 11 February 2010 Accepted 17 February 2010 Available online 26 February 2010
Terrestrial lightning is generated by the separation of electric charge residing on water–ice particles in clouds, a few kilometers above the electrically conducting surface of the Earth. It is detected optically, electromagnetically, and aurally. The majority of discharges occur within or between clouds with about one third discharging to the surface of the Earth. Upward-propagating lightning also occurs with effects extending into the ionosphere. On Venus, the clouds are close to 50 km above the surface of the planet, where the temperatures and pressures are near those of Earth’s surface. In contrast the atmospheric pressure near the surface of Venus is nearly 100 times that of Earth. Thus, while intra- and inter-cloud lightning is expected to occur in a manner similar to that on Earth, we do not expect discharges from the clouds to the surface to occur. Upward-going lightning may be more frequent at Venus because the ionosphere is closer to the clouds. As at Earth, Venus lightning has been detected optically and electromagnetically from a variety of platforms. We find that some of the observed properties of lightning are different at the two planets. Many of the differences in the electromagnetic waves detected by spacecraft can be attributed to effects during ionospheric propagation to the spacecraft. We review the differences in the ionospheres of Earth and Venus and how they affect observations. We use both the Pioneer Venus electric antenna observations as well as the Venus Express magnetic measurements. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Venus Lightning Venus Express
1. Introduction Venus and Earth are often called twin planets because of their similar size, mass, interior structure, and chemical composition. However, they are not identical twins, but rather fraternal twins, because unlike the Earth, Venus has low water content, lacks plate tectonics, possesses a dense atmosphere, and lacks an intrinsic magnetic field. Both planets have clouds and a dynamic atmosphere near the 1-bar level, but the terrestrial clouds are composed of water while the Venus clouds consist of sulfuric acid. As a result of these differences, the sulfur cycle is perhaps as important on Venus as the carbon cycle is at Earth. Nevertheless, the differences do not mean that familiar terrestrial processes do not also occur regularly on Venus. For example, in the past Venus has had extensive volcanism, which may be continuing today without the presence of a wet crust. Its atmospheric composition has evolved as has the Earth’s and there is lightning despite the absence of water clouds. It is important to study such processes under different conditions because we learn how these processes operate much more deeply when seen in different settings.
n
Corresponding author. Tel.: + 1 310 825 3188; fax: + 1 310 206 8042. E-mail address: [email protected] (C.T. Russell).
0032-0633/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2010.02.010
Comparative planetology can be a powerful tool for the planetary scientist. The atmospheres of Earth and Venus are markedly different. Earth’s surface has a temperature of about 300 K and a pressure of 1 bar, while, as shown in Fig. 1, Venus has a surface temperature of over 700 K and a pressure of close to 100 bar. The composition of the Earth’s atmosphere is predominantly N2, and of the Venus atmosphere, CO2. The clouds of Earth can be generally found from 1 to 10 km above the surface, while on Venus, they occur from 47 to 65 km. Terrestrial clouds contain water particles while Venus clouds contain sulfuric acid particles. However, at the altitudes of the clouds, the temperatures, and pressures are rather similar at the two planets. While we have not been able to photograph Venus clouds with the same cadence and spatial resolution as we have photographed terrestrial clouds, we know from global cloud photography that they circle the planet in 4 days at low latitudes and form a powerful vortex like a hurricane’s eye over the polar regions (Limaye et al., 2009). On Earth, almost two thirds of the discharges occur either within a cloud or between clouds with nearly one third being from the cloud to the surface. On Venus the high altitude clouds and the thickness of the atmosphere are expected to prevent such cloud-to-ground discharges. In the years since the Pioneer Venus mission ended in1992, the understanding of terrestrial lightning
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Fig. 1. Temperature and pressure profile of the Venus atmosphere.
Fig. 2. Artist conception of a lightning discharge in the Venus atmosphere near an aerobot. Courtesy of NASA.
has been revolutionized especially in areas that may be relevant to the Venus situation. Numerous upward-going discharges such as red sprites and blue jets have been observed, and ionospheric effects such as ELVES discovered (Fukunishi et al., 1996). These advances help us understand some of the phenomenology of the Pioneer Venus observations. On Earth, we usually think of the hazards of lightning rather than the benefits. As illustrated in Fig. 2, the electric discharge produces a long narrow channel in which the atmosphere is
heated and the pressure increased. The high pressure produced in this hot channel leads to acoustic signals that we call thunder. The high temperature and pressure also promote chemical reactions that would not take place under conditions of local thermodynamic equilibrium. In the Earth’s atmosphere, nitric oxide is formed this way, which benefits certain plants. The electric current that heats the atmosphere also prefers to flow through material with high electrical conductivity such as lightning rods, wires, trees, and golfers, causing considerable damage to electrical equipment and loss of life each year. Even though a hundred lightning discharges occur every second on average worldwide, this amounts to only about 7/km2/year on average. The rate is strongly dependent on weather patterns, so lightning occurs much more often in some areas than in others and at certain times of the day and the year than at others. Nitric oxide is also produced in the atmosphere of Venus (Krasnopolsky, 1983; 2006). This production may not benefit plant life on Venus but it does suggest that the rates of lightning discharge are similar on the two planets. It is also not dangerous to our technical systems as we do not operate systems in the atmosphere of Venus at present. The only balloons that have been flown in Venus’ atmosphere lasted 4 days in total (Sagdeev et al., 1986a), and we do not believe that it was lightning that silenced them. However, when we do develop long-lived balloons for Venus atmospheric exploration we should consider shielding essential equipment. Lightning has been detected on Venus with almost every technique used on Earth. It has been seen optically from Venera 9 (Krasnopolsky, 1980) and from a terrestrial telescope (Hansell et al., 1995). It has been seen with an electric antenna at ELF and VLF frequencies (Taylor et al., 1979; Scarf et al., 1980; Russell, 1991). It has been seen at ELF frequencies with a fluxgate magnetometer (Russell et al., 2007). It has been detected with radio waves that passed through the ionosphere into the solar wind (Gurnett et al., 1991), with ELF electric signals within the ionosphere (Russell, 1991), and with electric and magnetic antennas within the atmosphere (Strangeway et al., 1993, Ksanfomaliti, 1983). Sometimes ELF waves are not seen. This can be due to many factors, including propagation effects, such as absorption, evanescence, and reflection, as well as the statistical nature of its occurrence both in space and time. It was not detected by the Pioneer Venus star sensor (Borucki et al., 1981), possibly due to the short time the star sensor was exposed over the lightning region. It was not detected by the Vega balloon photometer in the Venus clouds (Sagdeev et al., 1986b) possibly due to the short path length of photons within clouds. While Cassini did see radio emissions when it flew by Venus (Gurnett et al., 2001), these were not interpreted as lightning as they were different in intensity and cadence than on the subsequent Earth flyby. We certainly do not expect lightning at the two planets to produce exactly the same signals because of their different possible discharge paths and because of the different plasma and magnetic field environments in which the signals propagate. In the next sections, we discuss the differences in the plasma and magnetic envelopes of Earth and Venus, how they affect the signals that were detected by the Pioneer Venus Orbiter, and what signals are being detected today by Venus Express.
2. Different plasma environments of Earth and Venus Earth has a relatively strong intrinsic magnetic field, whose pressure is sufficient to stand off the solar wind over 10 Earth radii from the center of the Earth. The configuration of this magnetic field is shown by the curved dashed line in Fig. 3, that of
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a dipole magnetic field. The field lines do not deviate from this near-dipole configuration until they are far above the surface of the Earth. Dipole field lines extend radially above the north and south magnetic poles and are horizontal near the equator. When lightning-generated electromagnetic waves enter the ionosphere below the electron gyrofrequency, they propagate in what is known as the whistler mode. The electron gyrofrequency is about 1,400,000 Hz in the Earth’s ionosphere and about 560 Hz at Venus. Lightning discharges produce broad band electromagnetic
Fig. 3. Non-ducted entry and propagation of whistler-mode signals in the Earth’s magnetosphere. The curved dashed line represents the magnetic field line (Russell et al., 1972). On Earth, entry into the magnetosphere is promoted by the largely vertical direction of the magnetic field. Thus many lightning-generated waves can reach the spacecraft ‘‘S’’.
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waves with power from tens to millions of Hertz. Since there is a stop band above the electron gyrofrequency, this means that on Earth we expect to see lightning-generated signals to a much higher frequency than on Venus. A second factor that is important is the direction of propagation of the electromagnetic wave as it attempts to enter the ionosphere. Because the signals slow rapidly from the speed of light to a value about a thousand times less, the leading edge of the electromagnetic wave flattens, and the wavefront propagates vertically. This happens equally at Earth and Venus, but with different results because of their differing magnetic configurations. As shown in Fig. 3, as long as the lightning source on the Earth is not near the equator, the energy that is propagating radially enters the ionosphere from the atmosphere and is guided along the nearly vertical magnetic field. This guidance is loose and depends on frequency. The energy is not strongly absorbed by the cold ionospheric and plasmaspheric plasmas, and the signals can bounce back and forth in the magnetosphere several times, before reaching the satellite, S, in Fig. 3. The high frequencies arrive first at the spacecraft because of the nature of the ‘‘dispersion relation’’ of whistler-mode waves, which gives these waves their characteristic descending tone. The path of the waves at Venus is much shorter because the waves are found in the ionosphere, not an extended magnetosphere. Hence at Venus there is no significant ‘‘dispersion.’’ The markedly different magnetic geometry of Venus is illustrated in Fig. 4. The magnetic field is induced and as a result is mainly horizontal. The physics of this induction is that the highly conducting ionosphere produces an electric current that opposes any changing magnetic field. The field of the solar wind averages to zero over a long enough time. To the extent that the ionosphere has finite resistivity, this current will decay with time, allowing the magnetic field to enter the ionosphere and also
Fig. 4. Geometry of Venus’ induced magnetosphere. Field lines draped around the ionosphere slowly sink through the dayside ionosphere and penetrate the atmospheric region. These field lines are mainly horizontal over much of the planet and prevent the lightning-generated whistler waves from reaching satellites throughout much of the Venus ionosphere. On the nightside of Venus, these field lines have a strong vertical component, particularly conducive for guiding electromagnetic waves at frequencies below the local electron gyrofrequency through the ionosphere to the spacecraft. Pioneer Venus was able to probe this near equatorial region and observed copious whistler-mode waves.
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to change direction when the magnetic field direction in the solar wind changes. This generally does not allow easy entry into the ionosphere, except in the center of the wake, where the draped field becomes vertical and points tailward on the right (dark) side of the planet. As a result, the number of whistler-mode waves in the ionosphere due to lightning discharges on Venus might well be much smaller than seen on Earth, even if the discharge rates in the two atmospheres were the same. Fig. 5 illustrates the flattening of the electromagnetic wavefront as it enters the ionosphere. Refraction of the signals by the increasing index of refraction associated with the increasing electron density causes the signal to propagate vertically. At Earth, where the magnetic field can be nearly radial, this generally facilitates entry into the ionosphere. Only if the wave is above the electron gyrofrequency does it cease to propagate. On Venus, where over much of the planet the magnetic field is horizontal, this vertical refraction mitigates against entry into the ionosphere. Whistler mode waves do not propagate across magnetic field lines at frequencies above 43 times the proton gyrofrequency.
Fig. 5. Schematic illustration of the flattening of a wavefront of the electromagnetic wave generated by the lightning signals as it penetrates the increasing density of the lower ionosphere, where the phase velocity decreases. As a result, the wave normals are vertical, almost perpendicular to the horizontal magnetic field seen over much of the ionospheric portion of the Venus Express orbit.
The phase velocity drops to zero for what we call perpendicular propagation, as shown in Fig. 6 for typical conditions in the Venus ionosphere. Thus, we would expect a cone of non-propagation for magnetic fields nearly horizontal in the ionosphere. Once they do couple their energy to the magnetic field, the energy is strongly guided by the magnetic field as is seen in the right-hand panel of Fig. 6, in which the group velocity is restricted to a narrow range of angles along the field. Signals observed at the same frequency in the two ionospheres behave differently, even though the electron densities may be the same. This is because of the importance of the magnetic field in controlling the motions of the electrons and ions, and hence the properties of the electromagnetic wave carried by the gyrating and oscillating charged particles. Fig. 7 shows the electric and magnetic fields measured in the terrestrial ionosphere by the Injun 5 plasma wave instrument at frequencies similar to those at which Pioneer Venus and Venus Express observe lightning signals. What we are seeing is a signal generated over a broad band (from at least 90 to 264 Hz here) and moving upward into the Earth’s ionosphere to the location of Injun 5. Initially the signals are righthanded, but as they propagate upward, signals that were close to the helium gyrofrequency are now closer to the proton gyrofrequency and are converted at what is called the crossover frequency to left-handed polarization. When these now lefthanded waves propagate further along the field line, they encounter the proton gyrofrequency (264 Hz here corresponding to 17,310 nT) and are absorbed by the protons. As they approach this resonant frequency, the waves slow down and the energy moves from the electric component to the magnetic component. This strong dispersion near the proton gyrofrequency earns these waves the name proton cyclotron whistlers, and creates individual bursts that are about 4 s long. Such dispersion is not observed on Venus at the same frequencies because there those waves are well above the proton gyrofrequency and do not resonate with proton motion. Further, the altitude variation of the magnetic field is dissimilar to that on Earth. It is instructive at this point to compare wave amplitudes at Earth and Venus. Surprisingly, there has been very little work done on surveying the wave amplitudes in the terrestrial ionosphere, so we cannot quote average or typical values, but the waves seen in Fig. 7 have a root mean square (rms) amplitude of about 10 pT. The Venus Express rms magnetic amplitudes at similar frequencies are about 300 pT and sometimes much greater. Since the power of electromagnetic waves depends
Fig. 6. Phase speed and group velocity in polar diagrams for Venus ionospheric conditions. The magnetic field is vertical in the diagram. The phase velocity of the whistlermode wave does not propagate across the field and the group velocity is closely parallel to the magnetic field under these conditions.
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Fig. 7. Injun 5 electric and magnetic dynamic spectra of upward traveling EM waves in the terrestrial ionosphere (Mosier and Gurnett, 1969).
inversely on the velocity of the wave for a constant Poynting flux, these waves, that are traveling quite slowly near the end of their lives at 264 Hz, are amplified above their earlier strengths.
3. Pioneer Venus observations Pioneer Venus and Venus Express observations are very complementary. At first glance, their orbits are similar—a 24-h period with periapsis low in the ionosphere. However, the periapsis of Pioneer Venus was close to the equator, while the Venus Express periapsis is close to the north pole. This results in the two sets of low-altitude lightning data being taken in two different magnetic configurations. Moreover, the Pioneer Venus signals were received by an electric field sensor that became noisy in sunlight so that all useful lightning data were taken in eclipse. Fig. 8 shows a sample of Pioneer Venus electric field data with three different types of signals, each quite easily recognizable (Russell and Scarf, 1990). The middle section labeled ‘‘b’’ occurs when the electric field antenna rotates into the wake of the spacecraft moving through the ionosphere and the dipole antenna temporarily finds itself in two different plasma environments. Signals in ‘‘c’’ are whistler-mode signals propagating along the magnetic field below the electron-cyclotron frequency. The channels of the instrument above the electron-cyclotron frequency (narrowband channels at 730 Hz, 5.4 kHz, and 30 kHz) show no signal detection at all. During this period, the magnetic field was 241 to the radial direction, measured from the center of Venus, providing a propagation path from the upper atmosphere to the spacecraft. The magnetic field strength was 24 nT, allowing 100 Hz propagation, but not 730 Hz. In interval ‘‘a’’ bursts are seen at all frequencies, even those well above the electron gyrofrequencies. Here, the magnetic field is nearly horizontal, only 31 from horizontal at the time of the strongest burst, and the electron gyrofrequency is similar to that during the ‘‘c’’ signals. The signals appear to be the equivalent of the terrestrial ELVES, where the electric field of the upward lightning discharge affects a large area above the stroke but does not create electromagnetic waves.
Fig. 8. Plasma wave measurements on the Pioneer Venus Orbiter in eclipse in four narrowband filters (Russell and Scarf, 1990).
The signals seen in interval ‘‘a’’ are only a low-altitude phenomenon. Fig. 9 shows a statistical analysis of such waves versus altitude (Ho et al., 1992). The 30 kHz waves are seen no higher than 180 km. The 5.4 kHz and 730 Hz waves fall off rapidly in occurrence, but can be seen to 300 km on occasions. In contrast, the 100 Hz waves are seen when the magnetic field is nearly radial, so that the vertically propagating waves can move as whistler-mode waves and do not noticeably decrease in occurrence with altitude. This dependence is further evidence that the atmosphere is the source of these waves, and shows that the waves on Venus are similar to our postulated terrestrial counterparts.
4. Venus Express observations Venus Express includes a fluxgate magnetometer sampling the magnetic field at a rate of up to 128 Hz. Venus Express did not
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Fig. 9. Altitude variation of the rate of occurrence of the different impulsive signals seen above the night ionosphere by the Pioneer Venus plasma wave instrument. The dashed and dotted lines are non-propagating signals that may be due to discharges from the clouds to the ionosphere. The solid line shows the occurrence rate of the propagating VLF waves that are inside the cone of allowed propagation around the magnetic field (Ho et al., 1992).
include a magnetic cleanliness program. Instead spacecraft generated signals are detected by using a gradiometer sensor mounted on the body of the spacecraft. This sensor allows the spacecraft sources to be separated from the natural signals detected at the outboard sensor 1 m from the spacecraft deck (Zhang et al., 2006). The use of the dual sensors allowed the correction of the data at data rates of 1 Hz and lower. The 128 Hz data were not corrected but rather analyzed in spacecraft coordinates after filtering. The analysis techniques used to detect the whistler-mode waves involve filtering the 128 Hz data to a passband of frequencies whose range contains very few spurious spacecraft signals. The technique is described in detail by Russell et al. (2008a). One of the key results of the Venus Express measurement was simply the detection of the magnetic component of the waves whose electric component was detected by Pioneer Venus (Russell et al., 2007). Despite the unfavorable magnetic geometry of the Venus ionospheric field, strong electromagnetic bursts were seen with energies comparable to those expected from Pioneer Venus measurements (Russell et al., 2006). These waves were nearly circularly polarized and propagated nearly parallel to the magnetic field (Russell et al., 2007). The entry of energy into the ionosphere was facilitated by time variations of the interplanetary magnetic field. This leads ultimately to twisted magnetic fields in the ionosphere, some of which deviate strongly enough from the horizontal to allow propagation into the ionosphere from below. Fig. 10 shows the maximum peak-to-peak amplitude on a pass through the lower ionosphere versus the maximum field inclination. A deviation of 171 is enough to allow the energy to penetrate into the ionosphere (Russell et al., 2008b). Whistler-mode waves can reach very significant amplitudes of over 1 nT peak-to-peak and the duration of an interval of activity can last several seconds, as illustrated in Fig. 11. We can learn more about the signal using the Fourier coefficients on these axes to compute the in-phase and quadrature power, thereby determining the direction of propagation, the ellipticity of the wave, and the percent polarization (Means, 1972). To do this, we add the band-passed 128 Hz signal to the 4-s cleaned data in spacecraft coordinates and analyze the resulting signal relative to the magnetic field. This allows us to determine the handedness of the waves and the direction of propagation relative to the magnetic field. Fig. 12 shows a power spectrum of the signals from 0620:11 to 0620:19 on April 15, 2007, shown in Fig. 11.
Fig. 10. Peak-to-peak amplitudes of burst maxima on each pass versus maximum inclination of the magnetic field to the horizontal.
Fig. 11. Band-passed filtered 128 Hz magnetic data showing a strong burst accompanied by weaker wave activity on April 15, 2007.
The signal was bandpass-filtered in the frequency range 42–60 Hz using a sharp filter with an attenuation of 100 db outside the passband. The compressional signal shows the power in the field strength, i.e. in the component along the field, and the transverse signal contains the remaining power. The signal is strongly transverse to the field. From the cross-correlation we can calculate the signal amplitudes as well as the direction of propagation, the percent polarization and the ellipticity in sub-bands of the spectrum. These quantities are shown for four bands in Table 1. The locations of these bands are shown in Fig. 12. The lowest frequency range is the strongest here. The signals propagate close to the magnetic field and are very coherent with a percent polarization of 95%. This large percent polarization indicates that the signal is well above noise or equivalently that the signal is produced by a single dominant source. We would expect this of a
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strong lightning discharge. The time series in Fig. 11 shows that one burst dominates over others. The ellipticity is positive and near unity. The positive sign means that the signal is right-handed and we would expect a nearly circular ellipticity of unity for a nearly parallel propagating whistler-mode signal. We see that at high frequencies the percent polarizations drop and the ellipticities drop while the direction of propagation to the field remains fairly constant. The drop in percent polarization is probably due to the weakening of the signal relative to the noise sources. The ellipticity change may be due to the approach
Fig. 12. Power spectrum of waves seen in Fig. 11. Transverse and compressional powers are shown separately. Bands analyzed in Table 1 are shown by labels a–d.
Table 1 Spectral properties of whistler-mode signals on April 15, 2007. Frequency range (Hz) 42.2–49.0 49.1–54.0 54.2–58.7 58.7–60.1
Amplitude (pT)
yRB (deg)
Pol. (%)
Ellipt.
115 67 58 18
14 20 5 17
95 85 69 68
0.82 0.75 0.71 0.60
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of the signals to the electron gyrofrequency. We note that the signal is right-hand polarized across the frequency band. This is consistent with the signals being whistler-mode waves. However, the signals are not strongly filtered above the 64 Hz Nyquist frequency. If the signals were in the band 64–128 Hz, they would be seen as left-handed waves due to aliasing. If the signal were from the next higher band 128–192 Hz, they would again appear right-handed, but it would be less likely that they fell into a single band. In short, the persistence of the right-handed polarization implies that the wave frequency is lower than 64 Hz and has not been altered by aliasing. Fig. 13 shows 2 s of whistler-mode wave activity on July 1, 2006. This event has been previously discussed by Russell et al. (2008b). Here we have added the band-passed signal to the 4-s average field in spacecraft coordinates. The varying ratios between axes during this interval indicate that there are different sources for these bursts. Table 2 shows the wave properties in four frequency bands. Here, the lowest frequency band is the weakest with weak polarization that is very elliptical, not circular. Here the lowest frequency band is closest to the noise. The second band propagates more closely to the field, within 91, is highly polarized, and well above the noise, and its ellipticity is closer to unity and therefore much more circular. The third band propagates nearly along the magnetic field, but its low percent polarization and nearly linear ellipticity indicates it is not far above the noise or that there are several sources contributing to this signal. The fourth band is significantly different from the others. It is left-hand polarized. Since left-handed waves do not propagate at this frequency, the wave frequency must be above the Nyquist frequency of 64 Hz. Importantly this signal seems not to be weak relative to the noise. It has a percent polarization of 71% and propagates nearly along the magnetic field. The drop in absolute power seen in Fig. 14 may be due to the instrument’s ‘‘anti-aliasing filter’’. In short, the combination of the two events in Figs. 12 and 14 suggests that the signals detected on Venus Express are often in the nominal passband of the instrument, but can be at times strong above its Nyquist frequency of 64 Hz. As mentioned above, Krasnopolsky’s (1983; 2006) observations of nitric oxide in the Venus atmosphere are consistent with the rate of lightning being similar at Venus and Earth. Pioneer Venus observations also suggested this. Nevertheless, it is very difficult to make this comparison using the Venus Express or the Pioneer Venus data because of the access problem. We see only a fraction of the events in the ionosphere. The ionosphere is a good
Fig. 13. Two-second interval of whistler-mode waves at periapsis on July 1, 2006 in spacecraft coordinates. Four-second magnetic field has been added to bandpass-filtered data.
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Table 2 Spectral properties of whistler-mode signals on July 1, 2006. Frequency range (Hz) 41.9–45.0 45.5–50.8 51.4–55.8 56.3–59.8
Amplitude (pT)
yRB (deg)
Pol. (%)
30 77 68 53
451 91 71 91
38 85 45 71
Ellipt. 0.33 0.77 0.33 0.38
Fig. 15. Percent access time as a function of local time of periapsis for the first Venus year of operation. Access time is the fractional time the magnetic field tilts at an angle greater than 171 from the horizontal. The burst rate in the lower panel is not simply controlled by this parameter. From noon to midnight, the rate dropped to zero, possibly because of the weakness of the magnetic field at the satellite altitude so that the signal could not propagate from the atmosphere to the spacecraft.
Fig. 14. Power spectrum of waves seen in Fig. 13. Transverse and compressional powers are shown separately. Bands analyzed in Table 2 are shown by labels a–d.
shield. We can illustrate this with Fig. 15, in which we calculate based on the study shown in Fig. 10 that signals can propagate from the atmosphere to the spacecraft based solely on the direction of the field. The percent access time is nearly symmetric around noon, but the observed burst rate is not. Thus, there must be some other factor affecting the access to the spacecraft. One possible factor is the strength of the magnetic field. A weak magnetic field could prevent the signal from reaching the spacecraft.
5. Discussion and conclusions While the observational evidence for electric discharges in the Venus atmosphere is overwhelming, the theoretical community seems still divided on whether the properties of the upper atmosphere of Venus as we understand them permit such discharges. Two recent papers exemplify this. In a study of the possibility of sprites in the atmospheres of planets other than Earth, Yair et al. (2009) predict the appearance of sprites at Venus. In contrast, Michael et al. (2009) examine the expected abundance of aerosols and the expected electrical conductivity of the upper atmosphere and predict that there is too little of the former and too much of the latter to support lightning. In our opinion, we do have the conditions on Venus that lead to strong potential differences in the clouds as on Earth. Both H2O and H2SO4 are polar molecules. They both become solid at similar temperatures and the nephelometers on the Pioneer Venus atmospheric probes saw solid particles of varying sizes, thought to promote
differential charging in terrestrial clouds (Ragent and Blamont, 1979; Knollenberg and Hunten, 1979). The evidence that lightning is taking place is not based on a few observations, nor on a few properties of the observed waves. Rather the observations behave precisely as one would expect if there were lightning discharges in the atmosphere below the spacecraft. Whistler mode signals with the correct polarization were observed on Pioneer Venus only when the magnetic field dipped into the atmosphere, allowing the upward propagating ways to reach the spacecraft. When the field was horizontal the wave amplitude decreased rapidly with altitude. In this situation the waves appeared at frequencies where no electromagnetic wave could propagate. Not only the frequencies and the polarization were correct but also the wave amplitude. Pioneer Venus did make observations in the atmosphere at the bottom of the ionosphere as it entered the atmosphere at the end of the mission. The electromagnetic energy flux in the atmosphere was the same as in the ionosphere when the field was vertically out of the atmosphere. Moreover as they should, vertical fields led to nearly constant electromagnetic flux with altitude. When Venus Express arrived with a completely different detection technique the same wave power was seen. Observationally there is complete closure. There are no conflicting observations. Observations that should agree, do agree, but clearly we need to learn more about the electrical conductivity and aerosols of the cloud layer. We look forward to the day when we can deploy more balloon-based measurements in the Venus atmosphere. From the properties we have measured we have no reason to believe that the lightning produced at Venus is produced by a process fundamentally different than on Earth. The signals produced by lightning in the atmosphere of Earth and Venus appear to be quite similar at ELF frequencies. Nevertheless, the differing magnetic configurations facilitate entry of the signals into the terrestrial ionosphere while they tend to prevent the entry into the Venus ionosphere. The differing strengths of the magnetic fields also affect the appearance of the signals, producing dispersion in the case of the terrestrial signals (near
C.T. Russell et al. / Planetary and Space Science 59 (2011) 965–973
the proton and helium gyrofrequencies) and not producing dispersion at higher frequencies at Venus. Nevertheless, the evidence for pervasive lightning at Venus is strong. Pioneer Venus detected the whistler-mode radiation expected from lightning in the equatorial regions as well as signals that could correspond to terrestrial ELVES. Venus Express detects similar whistler-mode signals at high latitudes near the north pole. The amplitudes of the signals at Venus are strong, stronger that the signals seen by Injun 5 in the terrestrial ionosphere. Nitric oxide observations and reasonable assumptions about the access of signals to the spacecraft suggest that the rate of lightning strokes is comparable to that on Earth. In short, the generation of lightning is one more way in which Venus and Earth are fraternal twins.
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