Title : An optical flash on Venus detected by the AKATSUKI spacecraft


 Detection of lightning discharges on Venus has been attempted using both radio waves and optical methods for over 40 years. For optical observations, claims of lightning detection were controversial due to the lack of time resolution of optical emissions that is needed to separate lightning from artificial or natural noise. Here we show the first high-time-resolution light curve of a transient optical phenomenon observed by the Lightning and Airglow Camera (LAC), a dedicated instrument on the Venus orbiter Akatsuki. The observed transient was 10 times brighter than a typical terrestrial lightning flash and had a duration of a few hundred milliseconds, whereas that of typical Earth lightning is only a millisecond. These characteristics are not typical, but are well within the variability of Earth lightning. An origin as a bolide flare cannot be excluded, but considering the expected occurrence frequency of meteoroids at Venus, is improbable. The low flash rate and long duration determined by the Akatsuki observation are not inconsistent with non-detection of lightning radio waves by the Cassini spacecraft.


Detection of lightning discharges on Venus has been attempted using both radio waves
Evidence of lightning on Venus has been sought for more than 40 years using both radio waves and optical methodss [1][2][3][4][5][6][7][8][9][10] . However, lack of consensus on its occurrence is due to an absence of combined observations, limited observation periods, and poor performance of instruments which are not optimized for lightning detection. Most of the optical records of lightning lack high temporal resolution, making it difficult to discriminate lightning signals from natural or instrumental artifacts. The Lightning and Airglow Camera (LAC) onboard Venus orbiter, Akatsuki, is the first optical sensor designed specifically for lightning flash measurement on planets other than the Earth 11 . The unique performance of the LAC compared to other equipment used in the previous exploration of Venus is the combination of a high-speed sampling at 20 kHz with a spatial discrimination using a 4 x 8 pixel Avalanche Photo Diode (APD) detector array and high sensitivity using a high-voltage (HV) bias of up to 300 V. Data are captured for an individual 2° x 2° pixel for each event by triggered recording. These features let us distinguish a natural optical lightning flash from other transient signals, such as electrical noise and cosmic rays (as on comparable instruments flown on Earth satellites, e.g. FORTE 12 , GLM or GLIMS). The field-ofview (FOV) of LAC is 8 x 16 degrees, covering a typical footprint >1000km across. We selected a narrow band filter for the OI 777 nm line to detect lightning flashes on Venus, which is expected to be the most prominent emission in the CO2-dominant atmosphere based on laboratory Though the duration of the present event is much longer than that of a typical terrestrial lightning flashes observed from Earth orbit. However, it is within the range of variation of those measured by GLIMS onboard the International Space Station 16 , showing most of the cloud flashes have FWHM of few ms. Also, though the intensity and total photon numbers are somewhat larger than typical for the Earth, they are similarly well inside the observed range. The total optical energy is 1/9 of the lowest flash observed at Venus with a ground telescope 6 , namely, 1x10 8 [J].
The occurrence frequency indicated by our survey, one per ~100 M km 2 hour, is equivalent to that of the ground telescope observation 6 . Figure 2c indicates the location of the present event on the cloud map composited from images taken at 365 nm by Ultra Violet Imager (UVI) onboard Akatsuki in previous 4 days. The flash seems to happen on a dark streak at latitude of about 30 degrees extending in zonal direction, which compose a main part of large scale "Y" structure.
A bolide is an alternative explanation for the flash, which would have an estimated equivalent observed magnitude at Venus of -17.0. The probability of observing such a rare bolide from Venus orbit is estimated to be 0.05-4 percent for the 4 year observation by LAC based on the time-integrated photon numbers. If we consider the fact that the duration of the present event, ~200 ms, is much shorter than that of typical bright terrestrial bolides lasting a few seconds, this scenario seems even less probable. According to Figure 2 the location of the optical event is not near any prominent topographic or volcanic feature, so there seems no direct connection between the present event and volcanic activity. We consider the signal unlikely to be an instrumental artifact as we have detected no similar waveforms for 4 years and have confirmed normal LAC operation 7 times after the detection of the present event, recording LAC output signals by forced triggering in the same high voltage condition.
Optical and radio/magnetic transients detected at Venus, were proposed to be due to lightning 10 . Most optical surveys (e.g. Pioneer Venus and Venus Express 9 ) have indicated no flashes. Of note is that the first claimed optical detection by spacecraft, that of Venera-9 (2) , reported multiple flashes over a 70 s period with a duration of 250 ms and optical energy of ~3x10 7 J, individually not too different from the flash we report here. Those detections were in the first minutes of operation of the (non-imaging) Venera spectrometer instrument, and it is suggested that tumbling debris shed from the spacecraft might have been responsible for some near-field reflection of sunlight 10 . Scattered light of this sort can likely be excluded for this Akatsuki observation, since only a single pixel of the LAC was triggered, and that only once. We did observe scattered light early in the Akatsuki mission when an observation was made near the illuminated limb of Venus, and this had a near-constant intensity, quite different from the brief transient we observed in this paper.
The optical detection in ground-based telescopic data 6 had flashes that must have been less than ~50 ms in duration, since these flashes did not persist from one frame to the next in an imaging sequence obtained at 18.8 frames per second. Those flashes (of which 7 were observed in ~4 hours of observation over 5 nights) had higher optical energies (10 8 -2x10 9 J) than that reported here.
It may be noted that the VLF (radio) signatures identified by Ksanfomality in Venera 13 & 14 data as lightning sferics were persistent, with many pulses per minute for a period of tens of minutes. Similarly the Venera-9 detections spanned 70 s (2) . If the phenomena responsible for these effects were also responsible for the flash we detected, it is puzzling that we observed only a single event. On the other hand, such low occurrence rate is not inconsistent with non-detection of radio waves caused by lightning by the Cassini spacecraft.
We conclude that the present event is most likely a lightning flash in Venus' atmosphere, though the possibility of an unusual bolide is not zero. It represents the first reliable optical observation from orbit, and indicates a low flash rate in the clouds. This low flash rate may imply the result of inefficient charge separation processes, due to the low mass loading and particle collision rates in the clouds 17, 18 and the lack of frequent deep convective motions, which are necessary conditions for generating electric fields that can surpass the local gas breakdown value for the CO2 -dominated atmosphere of the planet. If the event was a bolide, it is nonetheless remarkable since no such event has previously been detected.

Author Contributions:
Yukihiro Takahashi [Estimation of bolide frequency] The observed peak value of 0.66 x 10 9 [ph/sec/6 digit] is equivalent to 4.5 x 10 7 [J/sec/6 digit] at the light source. Since emission energy at 777 nm with bandwidth of 9 nm is 1/140 of the total blackbody energy at 6000 K, the total optical energy will be 6.3 x 10 9 [J/sec/6 digit]. We confirmed that even if we take a bolide spectrum reported by a previous study, which is different from blackbody, the ratio contributing to 777 nm with bandwidth of 9 nm is almost the same as for blackbody radiation. diameter, and we find a good fit with these data in the 3-10-m size range assuming an average geometric albedo of 0.25. We are unable to match the crater flux curve over the 3-10-m size range with our data for a typical main-belt mean albedo of 0.11 (near the average for C and S types in the main belt), further supporting the notion that small near-Earth asteroids have higher mean albedos than large main-belt asteroids 16 . We note, however, that the true geometric albedo distribution for the near-Earth asteroid population is uncertain.
The only telescopic flux measurement to overlap our population directly is that based on a debiasing of Spacewatch data 17 . In particular, the Spacewatch estimate (albeit with large error) of the flux of 4-and 10-m diameter bodies striking the Earth is in agreement with our satellite energy estimates within error limits.
We also find that extrapolation of our power law is in agreement with the flux of smaller bodies 18 for energies between 10 25 kton and 10 23 kton (Fig. 4), these data being based on observations with ground-based cameras of fireballs having energies of ,10 23 kton. However, the influx measurements derived from infrasonic/acoustic measurements are slightly higher on average than the satellite of 1-10-m bodies striking the Earth 21 .
Using the best fit of these satellite data and extrapolating the power law to higher energies, we find that the Earth is struck by an object with the energy of Tunguska (assumed to be 10 Mton) every 1,000 2200 þ800 years (with an allowed range from 400 to 1,800 years on the basis of our most extreme assumptions for luminous efficiency). We estimate that the Earth is on average struck annually by an object of energy ,5 kton (with a possible range of 2-10 kton), and struck each month by an object with 0.3 kton of energy. Every ten years, an object of energy ,50 kton strikes Earth. A Figure 4 The flux of small near-Earth objects colliding with the Earth. Data are shown over a range of 14 magnitudes of energy. In addition to data shown in Fig. 3, this plot also shows the Earth collision hazard in the 1,000 Mton and larger energy range, based on modelling the albedo distribution of near-Earth asteroids 29 . At smaller sizes, the powerlaw number distribution derived from a decade-long survey of ground-based observations of fireballs 18  probability with Venus will be 1.1-1.8 times larger than that with the Earth (their Table 1. See below). Then the probability of 2.25 % with the Earth will be in the rage of 2.48 -4.05 % with Venus, while 0.045 % will be 0.050 -0.081 %.
Considering the discussions above, the possibility of bolide detection by LAC in the 4 year observation period will be 0.05 -4 %, which correspond to one event detection in 100 -8,000 years.
If we take into account that the event shows short duration (few 100s ms) compared to typical bolide (few seconds), the detection probability of the event we observed will be even much smaller than 0.05 -4 %.
The US government satellite surveillance of optical flashes has yielded a record  , 2001). Thus, our adopted values of v 0 , are actually somewhat larger than those of the disruptions that created the observed asteroid families, but are appropriate for the putative breakup of a parent body as large as 1000-1500 km in diameter, which, as we show later, would be necessary to cause the LHB in this scenario.
For each of the seven initial disruption locations, we placed about 3000 test particles (20,756 particles in total), and numerically integrated their orbital evolution for up to 100 million years. When a test particle goes within the physical radius of the Sun or that of planets, we consider the particle to have collided with that body, and remove it from the computation. Also, when the heliocentric distance of a test particle gets larger than 100 AU, the particle's integration is stopped. The ''survivors'' are particles that remain within 100 AU heliocentric distance without colliding with the Sun or any planet for 100 Myr. For the numerical integration we used the regularized mixed-variable symplectic method (SWIFT_RMVS3 by Levison and Duncan (1994)).
We should explain here the reasons why we have selected initial conditions that are very close to the m 6 resonance.
We do not necessarily think that these are realistic initial conditions of actual asteroid disruptions in the main belt. Large asteroids are not likely to remain close to a strong resonance, such as the m 6 , for hundreds of million years (which is the time interval between the planet formation era and the LHB). Thus, large collisional disruptions, if they occur, would be expected to be centered away from strong resonances. However, the fragments that eventually collide with the inner planets are delivered into planet crossing orbits by entering strongly unstable resonance zones. In this manuscript our focus is on obtaining the collision probability of asteroid fragments that are delivered to the inner solar system via the m 6 . We use as many test particles as computationally feasible in order to obtain the basic orbital statistics data for relevant research. In this sense, we think our choice of initial conditions of test particles close/in the m 6 resonance is justified and useful for our purpose.

Asteroid flux on terrestrial planets
The lower part of Table 1 summarizes the collision probability of test particles on the planets and on the Sun, for each of the simulations. Except for the cases (4) and (5), approximately 70% of the particles collided with the Sun. About 10-15% of the particles were removed by going further away than 100 AU. In our numerical model, the typical dynamical behavior of particles coming from the m 6 that eventually hit the terrestrial planets and the Sun as well as those particles that are ejected from the system due to scattering by Jupiter, is qualitatively the same as has been demonstrated by previous numerical studies as Gladman et al. (1997) or Morbidelli and Gladman (1998). Because our numerical integrations are focused Table 1 The number of test particles Ntp, osculating orbital elements (a, e, I, x, X, l) of each disruption center, ejection velocity v0, and the collision probability of asteroids that hit the Sun and planets in our numerical integrations The fraction of particles that went beyond 100 AU and that of the particles that have survived over 100 million years are also shown. No collisions with Neptune or Uranus were observed in our simulations. [Assessment of shape of bolide lightcurves] The light curve we observed is positively-skewed (i.e. the long tail is towards the right, after the peak). A nonfragmenting bolide, on the other hand, has an energy deposition profile (and thus, lightcurve) that is negatively skewed, with its peak towards the end of the event. This is because a bolide entering an atmosphere from above encounters air density that increases exponentially with depth. A survey of lightcurves obtained by the GLM (Jenniskens et al., 2018) suggests that the majority of bolide lightcurves are negatively skewed.
The initial growth of intensity should be exponential with a timescale that is the same for all bolides at a given planet, namely ~H/Vsin(gamma), where H is the atmospheric density scale height (~10 km), V the entry velocity (11-30 km/s typical) and gamma the entry angle (most probably 45 degrees, but sin(gamma) cannot be more than 1.0 in any case). So a bolide's onset is almost always going to have a ramp-up with a timescale of the order of 10/(20*0.7) ~ 0.7 seconds.
Only an improbably fast would ramp up in the ~30 ms we observed.
However, it is possible that we are not observing the full lightcurve of an integral body, but rather only the peak flashes of a disintegrating bolide which is overall small enough that the initial entry with a ~1s ramp-up is invisibly faint. Since entirely arbitrary breakup histories can be posited due to the stochastic nature of fracture, any lightcurve can be theoretically generated. Many of the lightcurves in Jenniskens et al. (2018) have multiple peaks due to breakup events, and the individual peaks may be positively skewed (e.g. figure below). If the onset of the lightcurve is due to a breakup event, then its ramp rate is not constrained by speed/scale height considerations above. The sharp peak in the event shown has a width of <100 ms.
Thus, the shape of the lightcurve we observed at Venus does not exclude a bolide origin, but requires an even larger and thus less probable bolide.