Whistler mode waves from lightning on Venus: Magnetic control of ionospheric access



[1] The fluxgate magnetometer on Venus Express samples the magnetic field near periapsis at 128 Hz. Bursts of plane-polarized magnetic waves in the vicinity of 100 Hz are observed propagating at small angles to the magnetic field. The magnetic field is generally horizontal in the region around periapsis, located at high northern latitudes. When the magnetic field remains within 15° of horizontal during the 2-min periapsis pass, no such waves are observed; but when there are brief periods during which the local magnetic field dips into the atmosphere by more than 15°, the bursts begin to appear. Such radial excursions of the magnetic field occur 25% of the time in the region around periapsis. The bursts are seen only on passes with these excursions. We interpret this magnetic control in terms of the coupling between the electromagnetic wave from lightning discharges refracted vertically by the increasing electron density and the nearly horizontal ionospheric magnetic field along which the energy is guided to the spacecraft. The inferred rate of electric discharges in the Venus atmosphere is about 20% of that seen in the Earth's atmosphere.

1. Introduction

[2] While the Venus atmosphere is quite dry, containing very little water, Venus is also quite cloudy, with hydrated sulfuric acid droplets shrouding the planet some 50–60 km above the surface. At low altitudes, the winds are slow, but in the clouds, the horizontal velocities are observed to range to over 100 ms−1. These “4-d” winds produce much wind shear and probably are accompanied by significant vertical transport as well. Thus, the clouds may well be electrified as terrestrial clouds are. The observations reported herein are over the polar vortex that is possibly associated with rapid downwelling [Piccioni et al., 2007].

[3] Many researchers have reported optical and electromagnetic signals that could be produced by discharges in or from these clouds. Krasnopolsky [1983] reported flashes seen in the Venera-9 visible spectrometer; Hansell et al. [1995] reported detections with an Earth-based telescope. The landing probes of Venera 11, 12, 13, and 14 detected electromagnetic pulses as they descended through the atmosphere and as they sat on the surface [Ksanfomaliti, 1983]. They have been most extensively studied from orbit on Pioneer Venus (PVO) using an electric antenna [Taylor et al., 1979; Scarf et al., 1980]. Finally, at radio frequencies that are able to propagate through the ionosphere with minimal interaction with the plasma, Galileo recorded radio waves similar to terrestrial radio frequency emissions from lightning [Gurnett et al., 1991]. However, as Gurnett et al. [2001] later reported, these high-frequency radio waves are weaker in strength and less frequent in occurrence than terrestrial lightning.

[4] The waves detected in the magnetized Venus ionosphere by PVO were of two distinct types [Russell, 1991]. One class appeared only when the magnetic field had a significant radial component and behaved like an electromagnetic wave would. This wave diminished only slightly in estimated electromagnetic energy flux with increasing altitude. The second class was apparently electrostatic and attenuated rapidly with altitude. It was seen in all four narrowband frequency channels: 100 Hz, 730 Hz, 5.6 kHz, and 30 kHz. We interpret these electrostatic waves as being associated with cloud-to-ionosphere strikes, probably “local.” The electromagnetic waves were most probably due to intracloud discharges, possibly at some distance.

[5] The occurrence of lightning on Venus remained controversial after the Venera and PVO missions ceased because some searches were unsuccessful. A search for scattered light in the PVO star sensor came up empty [Borucki et al., 1991], albeit the total time the star sensor was active and able to see flashes was only a few minutes in total, with very little coverage over the region defined as active by the PVO electric field instrument. The photometer on the Venus balloons also did not observe flashes [Sagdeev et al., 1986], but the balloons were in the clouds and were not looking down on the clouds from above. Thus any light from flashes could be scattered before reaching the detectors on the balloons.

[6] The waves observed by Pioneer Venus were quite strong. In a vacuum, the energy flux of electromagnetic waves is evenly divided between the electric and magnetic components. In the ionosphere where the index of refraction is high, the wave slows down and the energy is mainly carried by the magnetic component. On Pioneer Venus, the plasma density, the magnetic field, and the electric component of the wave were measured and we could calculate the electromagnetic energy of the waves identified as propagating in the whistler mode [Russell et al., 1989]. Furthermore, we could check this estimate with data obtained in the atmosphere beneath the ionosphere on the last few orbits when Pioneer Venus began to enter the atmosphere because of gravitational perturbations and eventually because of drag on the spacecraft [Strangeway et al., 1993]. These calculations indicated that the Pioneer Venus electric waves near 100 Hz could be detected by a state-of-the-art fluxgate magnetometer in the Venus ionosphere [Russell et al., 2006]. The Pioneer Venus magnetometer did not have the bandwidth to detect these waves. When it was decided to include a magnetometer on Venus Express, the decision was made to include a 128-Hz sampling mode that would allow magnetic waves near 100 Hz to be sampled [Zhang et al., 2006]. These data were obtained initially for only 2 min around periapsis. In late December 2006, this interval was extended. As expected, waves with the expected amplitude and temporal structure were observed at periapsis [Russell et al., 2007]. In this paper we report on the properties of the waves seen in these 2-min sampling intervals.

2. Instrument and Data Processing

[7] Because Venus Express was a low-cost mission and employed an existing bus, there was no magnetic cleanliness program. Instead, two magnetometer triads were installed: one on the top deck and the second on the end of a 1-m boom. The two magnetometers were aligned and used in a gradiometer mode to identify spacecraft-associated waves so that they could be removed. The magnetometer has been described in detail by Zhang et al. [2006], and the cleaning process has been described by Zhang et al. [2008]. The Alfvenic character of interplanetary fluctuations has been used to establish the magnetometer's DC level to better than 1 nT [Leinweber et al., 2008].

[8] In addition to the DC spacecraft fields, there is AC interference, in particular from the reaction wheels. These waves are found in the passband in which we expect to see the waves due to lightning. They also can vary in frequency from pass to pass, and at times, during a pass. Examples of these waves have been published by Russell et al. [2008]. A very simple test of whether a wave is in the ambient plasma or due to spacecraft noise is simply to subtract the measurements of the two magnetometers. Natural waves have the same amplitude at the two locations and disappear in the difference wave. The reaction wheel noise is stronger in the sensor on the spacecraft deck and does not disappear in subtraction [Russell et al., 2008]. As we demonstrate below in section 3, the bursts we discuss in this report pass this test.

[9] In this study we use the outbound sensor and pass the time series through a bandpass filter from 42 to 60 Hz. This range of frequencies is chosen to avoid strong signals from the reaction wheels [Russell et al., 2008]. The magnetometer has an antialiasing filter, but this filter does allow strong signals above the Nyquist frequency of 64 Hz to enter the telemetry stream. These signals will appear to be below 64 Hz, but their polarization will be reversed. When reaction wheel noise appears in this passband, we simply eliminate this pass from consideration. We examine herein observations obtained in 2006, after the initial commissioning of the instruments in orbit.

3. Observations

[10] In the Pioneer Venus observations, whistler mode waves could not be detected in the day-lit hemisphere because of solar-induced noise in the plasma wave electric antenna. One of the advantages of the Venus Express magnetic measurement is that the waves can be observed in sunlight as well as darkness. Figure 1 shows a filtered waveform from the outer fluxgate sensor on 1 July 2006, at 308-km altitude and 0642 LT. The wave packets have been rotated into their principal axis system where the minimum variance direction is along the third axis, and the maximum variance direction is along the first. Clearly, the waves are confined to two directions and have similar amplitudes in these two directions. Figure 2 focuses on 3 s of this record revealing substructure within the burst. Overall, the wave packets last about 1/4 to 1/2 s, but clearly substructure can occur on 100-ms scales. A hodogram of these waves is shown in Figure 3, over 400 ms of data. The waves are confined to a plane as is expected of whistler mode signals. The waves' phase velocity (k vector) along the minimum variance direction is propagating at only 9° to the background magnetic field. As mentioned above, the magnetometer samples continuously in a gradiometer mode with simultaneous samples at the end of the boom and on the spacecraft upper deck. Ambient waves will be the same amplitude at the two magnetometers so that subtracting the two time series will result in a time series of only noise. In Figure 4, we demonstrate this subtraction for the event shown in the hodogram of Figure 3. The top trace is the output of the y component outboard sensor. The middle trace is the y component inboard sensor. The bottom trace is the difference. In the difference trace, the burst of waves vanishes into the noise level, indicating it is an ambient signal.

Figure 1.

Waveform of 8-ms samples of the magnetic field at periapsis on 1 July 2006, bandpass filtered from 42 to 60 Hz and rotated into principal axis coordinates.

Figure 2.

Three seconds of the record shown in Figure 1 showing substructure in the burst.

Figure 3.

Hodogram of 400 ms of the burst shown in Figure 2 in the principal axis system.

Figure 4.

By component of the 128-Hz magnetic field in spacecraft coordinates measured on the outboard and inboard sensors and the difference between the two readings. Naturally occurring, nonspacecraft sources would be identical at the two locations and cancel in subtraction. The burst's absence indicates that the burst in Figure 3 is not due to spacecraft sources.

[11] An event observed on 23 December 2006, at 284-km altitude, 88° latitude, 1042 LT, and 88.5° solar zenith angle is shown in Figure 5. This event is propagating at only 6° to the local magnetic field. Figure 6 shows a hodogram of 250 ms of data at the maximum of the event. Figure 7 repeats our gradiometer difference test for the burst in the hodogram in Figure 6. Again the burst disappears in the difference trace indicating its source is not on the spacecraft.

Figure 5.

Bandpass-filtered measurements in principal axis system for 400 ms at periapsis on 23 December 2006. Comments of Figure 1 apply.

Figure 6.

Hodogram of 250 ms of the burst shown in Figure 5.

Figure 7.

By component of the 128-Hz field in spacecraft coordinates measured on the outboard and inboard sensors and the difference between the two readings. The absence of the burst on the bottom trace indicates that the burst in Figure 6 is not due to spacecraft sources.

[12] Figure 8 shows projections of the background magnetic field along the orbit in the plane containing the solar direction and the orbit plane normal. The 8-s averaged magnetic field is shown every minute. The event shown in Figure 7 occurs at the second vector before 0750 UT where the field is dipping most strongly into the atmosphere. This suggests that we should examine the control of the occurrence of these waves by the magnetic field orientation. We demonstrate this control statistically in section 4.

Figure 8.

Venus Express orbit together with projections of 8-s samples of the magnetic field once per minute in the x-z plane in Venus Solar Orbital coordinates. Magnetic field is seen to be dipping into atmosphere around time of burst of noise.

4. Magnetic Field Control of Burst Occurrence

[13] To illustrate the nature of the magnetic field control of the appearance of the whistler mode waves, we display the entire 2-min passes for three consecutive orbits together with the angle between the ambient magnetic field and the local horizontal direction for the same interval plotted with 1-s resolution. These orbits have very similar altitudes and local times but may differ in the steadiness and orientation of the magnetic field.

[14] Figure 9 shows the measurements on 8, 9, and 10 June 2006. On 10 June, the ambient magnetic field is quiet and close to horizontal for the entire 2-min pass. No wave bursts are seen in the right-hand panel. In contrast on 9 June, the ambient magnetic field is time varying moving up to 30° away from horizontal. Strong wave packets accompany these variations at the beginning of the interval. These wave packets are analyzed in detail by Russell et al. [2007]. June 8 has similar field variations to those of 9 June, but no wave events are seen. This observation indicates that the magnetic orientation is not the only controlling factor. In fact, the most probable explanation of the lack of waves on this pass is that there are no atmospheric discharges below the satellite on this day.

Figure 9.

Three consecutive periapsis passes on 8, 9, and 10 June. The left-hand panels show the angle of the field to the radial direction with 90° being horizontal as calculated from 1-s samples of the field. The right-hand panels show bandpass-filtered 8-ms magnetic field measurements covering the frequency range from 40 to 62 Hz. The component shown is from the y axis sensor in spacecraft coordinates. This is the sensor with the lowest noise level.

[15] Another 3-day interval, 30 June, 1 and 2 July, is shown in Figure 10 in the same format as Figure 9. In this interval, there is a burst on 1 July, but no activity on 30 June or 2 July. The most inclined fields occur on 30 June, but they are not accompanied by whistler mode bursts. The 1 July ambient fields do deviate from the horizontal by about 20° and are sufficient to allow the entry of waves into the ionosphere as in the previous example, but clearly 30 June shows that on some days, there just are not waves present near the dipping field lines. July 2 has an ambient magnetic field that is quite horizontal and says little about the possible presence of lightning near the satellite at this time. We note that more reaction wheel noise is present here than on other passes, but we feel we would have seen a typical amplitude burst if present.

Figure 10.

Three consecutive passes on 30 June, 1 July, and 2 July 2006. Comments of Figure 9 apply.

[16] Figure 11 shows three consecutive days in December 2006, when the spacecraft was near 251 km, 0141 LT, and 96.8° solar zenith angle. December 22 has a horizontal ambient magnetic field and no wave events. December 23 has a field inclined nearly 40° to the horizontal and has a strong wave event, near 1 nT peak-to-peak. December 24 deviates nearly 20° to the horizontal, and no wave bursts are seen. Again we have a pattern of inclined field being associated with occasional wave events and horizontal fields being associated with quiet intervals.

Figure 11.

Three consecutive passes on 22, 23, and 24 December 2006. Comments of Figure 9 apply.

[17] Figure 12 summarizes our survey of the 2006 Venus Express data. Plotted is the peak-to-peak amplitude of the strongest bursts on each pass versus the maximum deviation of the ambient field from the horizontal. We see that when the field deviation was less than 15° from the horizontal direction everywhere on the pass, no whistler mode wave bursts were detected. When the deviation was greater than 17°, the wave events could occur. We attribute this magnetic control to the effect of the ionosphere on the electromagnetic waves propagating from the clouds below and the properties of the propagation of whistler mode waves at the conditions in the Venus ionosphere as discussed in section 5.

Figure 12.

Peak-to-peak amplitudes of burst maxima on each pass versus maximum inclination of the magnetic field to the horizontal.

5. Discussion

[18] The evidence for the necessity of inclined ionospheric magnetic fields for the access of whistler mode waves from the atmosphere into the ionosphere is clear from our statistical survey. However, in order to understand why this access control occurs, we need to examine the expected characteristics of the electromagnetic waves that enter the ionosphere from below. Figure 13 shows a cartoon of the wavefronts produced by an intracloud discharge in Venus' atmosphere. The waves at first propagate with spherical wavefronts at the speed of light. When they encounter the ionosphere, the part of the wavefront that enters the ionosphere slows down considerably to a very small fraction of the velocity of light depending on the electron density and the strength of the magnetic field. The flat wavefront corresponds to vertical propagation of the wave. The magnetic field in the ionosphere, as we have seen, tends to be horizontal so long as the interplanetary magnetic field is steady. Inclined fields can be produced by time variations of the magnetic fields that in turn can lead to interconnection of different field directions within the ionosphere.

Figure 13.

Cartoon of electric discharge in the Venus clouds and the resultant wavefronts of electromagnetic waves. When the wavefronts enter the ionosphere where the phase velocity decreases substantially, the wavefront flattens and the wavenormals become vertical.

[19] Figure 14 shows the phase and group velocities of a whistler mode wave at 100 Hz propagating at various angles to the magnetic field in a plasma with a density of 5000 electrons cm−3 and a magnetic field strength of 20 nT. These velocities are calculated from the full dispersion relation for cold plasma waves [Stix, 1962]. On the left is the phase velocity. This wave does not travel perpendicular to the magnetic field. In fact, there is a cone of nonpropagation of the order of 10°, an angle similar to the angle of inclination needed for the waves to reach Venus Express. On the right is the group velocity in a similar display. Once the energy couples to the magnetic field, it is strongly guided by the magnetic field. Thus, our observations are consistent with what one would expect for the waves in the ionosphere that would be produced as a result of atmospheric electrical discharges.

Figure 14.

Phase and group velocities of whistler mode waves at 100 Hz propagating at various angles to the magnetic field in a plasma with a density of 5000 cm−3 and a magnetic field strength of 20 nT.

[20] We can use the first year's statistics of the magnetic field orientation and the number of discrete whistler wave bursts during the 2-min sampling intervals to make a rough comparison with terrestrial rates. If we use the conservative assumption that access to the ionosphere occurs only when the field is inclined 15° or more to the horizontal, then the statistics of cone angle occurrence indicates that access is allowed only 25% of the time. In the first Venus year of operation up to 16 December 2006, we received 12,223 s of 128-Hz data that could be analyzed. During this interval, 61 individual bursts of noise were detected, giving a rate of 0.005 per second. Normalizing this rate for access time, i.e., the duration of the fields inclined more than 15° to the horizontal, we get a rate of 0.02 per second below the clouds. If the spacecraft can see a circle of 200-km radius in the clouds below it, a distance equal to its height above them, it is seeing only 0.027% of the surface. If we use this ratio to normalize our observations, we get a global rate of 18 strokes s−1 or 20% the terrestrial rate. We do not know if our region of study near Venus' polar vortex is representative of the entire planet or if our area of seeing is correct, but these numbers do tell us that lightning is a significant and important process in the Venus atmosphere. Finally, we note that our measurements were gathered over a full Venus year covering all local times. While we would not necessarily expect the results of our observations to agree with the rate seen on a brief Cassini swing-by, observing radio waves and not the whistler mode waves studied here, we do agree qualitatively with the result of Gurnett et al. [2001] that the rate at Venus may be lower than on Earth.

6. Summary

[21] In summary, Venus Express observes bursts of whistler mode noise in the Venus ionosphere. These waves are almost circularly plane-polarized and propagate at a small angle to the magnetic field. They were seen only when the local ionospheric magnetic field was inclined 17° or more to the horizontal. Such a cone of evanescence would be expected for electromagnetic radiation produced in the atmosphere and refracted vertically as it enters the ionosphere. The observations are consistent with lightning being prevalent in the Venus atmosphere and creating the whistler mode waves seen in the ionosphere, both those observed with the electric antenna on Pioneer Venus at low latitudes and those detected with the Venus Express magnetometer at high latitudes.


[22] This work was supported by the National Aeronautics and Space Administration under research grant NNG06GC62G.