Lorenz Progress in Earth and Planetary Science (2018) 5:34
https://doi.org/10.1186/s40645-018-0181-x

Progress in Earth andPlanetary Science

REVIEW

Open Access

Lightning detection on Venus: a criticalreview
Ralph D. Lorenz

Abstract
Claimed detections and nondetections of lightning and related electromagnetic emissions on Venus are qualitatively
contradictory. Here, motivated by the commencement of observations by the Akatsuki spacecraft and by studies of
future missions, we critically review spacecraft and ground-based observations of the past 40 years, in an attempt toreconcile the discordant reports with a minimal number of assumptions. These include invoking alternativeinterpretations of individual reports, guided by sensitivity thresholds, controls, and other objective benchmarks ofobservation integrity. The most compelling evidence is in fact the first, the very low frequency (VLF) radio emissions
recorded beneath the clouds by all four of the Veneras 11–13 landers, and those data are re-examined closely, finding
power-law amplitude characteristics and substantial differences between the different profiles. It is concluded thatsome kind of frequent electrical activity is supported by the preponderance of observations, but optical emissions are
not consistent with terrestrial levels of activity. Venus’ activity may, like Earth’s, have strong temporal and/or spatial
variability, which coupled with the relatively short accumulated observation time for optical measurements, can lead toqualitative discrepancies between observation reports. We note a number of previously unconsidered observations andoutline some considerations for future observations.
Keywords: Venus, Lightning, Electromagnetic emission, Observation

Introduction
As our sister planet Venus attracts particular interest
among solar system objects, it is further important torecognize that many Earth-sized planets now being detected around other stars are as likely to be “Venus-like”
as they are “Earth-like.” Lightning is an important
process for several reasons (e.g. Rakov and Uman, 2003).
First, it is a striking phenomenon in its own right. Second, it may serve as a diagnostic of other processes, suchas volcanism or convection. Third, lightning may be important in atmospheric evolution and prebiotic evolution, notably in fixing nitrogen which is an essential
element in living things but is generally chemically inaccessible. Finally, lightning may be a hazard for vehicles
exploring a planetary atmosphere.
For these reasons, some effort has been devoted to detecting possible lightning on Venus, as indicated by the literature on the topic (Fig. 1)—major stimuli to the topic
have been the Venera and Pioneer Venus results around
Correspondence: Ralph.Lorenz@jhuapl.edu
Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road,
Laurel, MD 20723, USA

1980 and the Venus Express results since 2007. Several
previous surveys have reviewed planetary lightning generally (Desch et al. 2003; Yair 2012; Aplin 2013; Aplin andFischer 2017). Reviews of Venus lightning specifically include Williams et al. 1983, Russell 1991, and Hunten
1995, and the two notably comprehensive reviews in the
Arizona Press “Venus” books now two decades old—
Ksanfomality et al. 1983, Grebowsky et al. 1997. A
comparatively recent Venus-specific review is given in
Takahashi et al. (2008), who outlines the potential of new
optical measurements on the Japanese Akatsuki Venus
Climate Orbiter (Nakamura et al. 2007), which recently
began operations at Venus after an extended journey(Nakamura et al. 2014). It may be noted that lightning has
been repeatedly observed at Jupiter and Saturn and is notin dispute, and firm nondetections are established at
Titan. At Venus, however, there are many superficiallyconflicting positive and negative reports. This review
attempts to assess these reports critically, to develop
scenarios that seem most probable in so far as they are
consistent with the preponderance of observations.

© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Lorenz Progress in Earth and Planetary Science (2018) 5:34

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Fig. 1 The growing corpus of scientific literature on the topic (Smithsonian-ADS refereed paper search 9/28/2017). Search keywords“Venus” and “Lightning” were both required in the title or the abstract; the abstract search turns up more general papers as well as
Venus-lightning-specific articles. The curves have a plateau in the 1995–2007 period as the Pioneer Venus interpretation debate fizzles
out; the pace of papers increases a little after Venus Express data begin to emerge

Broadly speaking, evidence for or against lightning on
Venus may be classified in four general categories, which
are shown schematically in Fig. 2, against the profiles ofelectron density in the ionosphere and the temperatureof the lower atmosphere. The main cloud layer lies between about 50 and 75 km altitude.
A key point in this review is that it is futile to reconcile observations of claimed flash rates (or nondetection)
without considering the robustness of the measurement(e.g., the quantification of false detection rate), the significance of the measurement (the area and duration of
the observations), and the sensitivity (the magnitude oflightning discharge that would be detectable). Measurement sensitivity must also take into account the transfer
of information from where the electrical discharge

occurs to the location of the measurement system: thispropagation may be inefficient or even impossible (e.g.,
blue light from the lower atmosphere is absorbed before
reaching space), or may be highly variable as in the caseof whistler-mode plasma waves. This information propagation is summarized in Fig. 3.
In this connection, it is interesting to consider that thedetection footprints of the various techniques may havea counterintuitive trend—that higher-altitude surveys
may sample a smaller area. Specifically, the extremely
low frequency (ELF) (~ 100 Hz) whistler-mode electromagnetic detections from orbit sample only an area connected to the spacecraft by appropriate ionospheric andmagnetic field conditions and so may sense a range of only
a couple of hundred kilometers from the subspacecraftLorenz Progress in Earth and Planetary Science (2018) 5:34

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Fig. 2 Structure of the Venus atmosphere showing the ionosphere peaking between 100 and 150 km altitude (with daytime densities
exceeding 105 cm−3) and the temperature profile with the main cloud layer around 60 km altitude. Possible sites of electro-optical activity
are noted—eolian and volcanic discharges near the surface, convective lightning in the cloud layer, and possible transient luminous
events like sprites in the mesosphere. The corresponding altitudes of positive indications of lightning are noted at left

point. However, very low frequency (VLF) (10–80 kHz)measurements by descent probes at low altitude may
detect discharges at ranges of thousands of kilometers,
due to ducted propagation by super-refraction in thelower atmosphere (Croft 1983) and/or by “skybounce”—
reflection from the ionosphere (e.g., Simões et al. 2008)—
see also discussion in Ksanfomality et al. (1983). Similar
VLF detectors are now used routinely by networks on
Earth to map and track lightning activity over planetaryscale distances (see, e.g., Wood and Inan 2002 or theWorld-Wide Lightning Location Network WWLLN at
https://webflash.ess.washington.edu).
A key challenge is in the interpretation of electrical ormagnetic signatures claimed to be whistler-mode emissions from lightning discharges. For the Pioneer Venus
electric field data at least, several alternative explanations were offered, and much of the 1980s literature iscluttered with this debate. In part, a difficulty is thatsuch observations somewhat transcend scientific disciplinary boundaries—atmospheric scientists are generally
unfamiliar with plasma physics, and it is difficult for
nonpartisan observers to gauge the robustness of different interpretations. What is clear, however, as we discusslater, is that such electromagnetic signatures are observed fairly consistently, and that lightning discharges
are one possible origin.
A further point centers on the definition of “lightning.” It
is only in the last couple of decades (long after the Venus
lightning debate began in earnest with Pioneer Venus) thata fuller range of optical phenomena associated with atmospheric electrical discharges has been recognized on Earth,namely sprites, elves, blue jets, and other emissions. Given
the different environment on Venus, it seems highly likelythat not only are lightning bolts (if they occur in the conventional sense) possibly different in character, but theseother types of luminous events may be present and have

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Fig. 3 A schematic of lightning sources and detection mechanisms, noting that detections rely on signal propagation mechanisms that may not
be effective for all combinations

similar or different features compared with the Earth. For
the purpose of this review, we consider “lightning” to be
any optical emission associated with an electrical discharge,
thus including ionosphere/mesosphere glows such as
sprites as well as “conventional” lightning flashes.
It is interesting to lay out the development of Venus
lightning studies in parallel (see Table 1) with progressin studying lightning at Earth. In fact, many significant
terrestrial developments have been made since the firstobservations of possible lightning on Venus—notably
the development of terrestrial networks of radio systems
able to provide real-time and essentially global surveillance of lightning activity. Indeed, not only have thesedeveloped from national-scale systems such as the US
National Lightning Detection Network NLDN to theglobal WWLLN, but even an amateur network exists(“Blitzortung.org”) using simple VLF detectors linked toservers that locate strikes by time of arrival (see Fig. 4).
More pertinents, perhaps, from an analogy standpoint
are the observations of terrestrial lightning from space.
Vanguard III, only the 12th satellite to reach the orbit andonly 2 years after Sputnik 1, carried a proton precession
magnetometer, in which some VLF whistler-mode signals
were detected (Cain et al. 1961). Orbital measurements ofwhistlers were more robustly observed and directly attributed to “fractional hop” whistler waves launched fromnear the sites of lightning discharges, by the Canadian satellite Alouette (Barrington and Belrose 1963), and by thesmall US satellites Injuns 3 and 5 (see, e.g., Gurnett andO’Brien 1964). A global map of lightning is derived fromthe detection of VLF sferics measured in 1972 by the
British satellite Ariel 4 (Bullough et al. 1975).
Optical detections of lightning were made by photometers on the Orbiting Solar Observatory (OSO-B or OSO-2,
Vorpahl et al. 1970) and then mapped in more detail by
OSO-5 (Sparrow and Ney 1971). These photometers weresaturated by moonlight (and sunlight) and therefore wererestricted to observations during the new moon. The fieldof view of these instruments was of the order of a few
square degrees (a few × 104km2), and in ~ 200 h ofobservation spread over 15 months, about 7000 lightning
strokes were detected in about 1000 storms, revealingLorenz Progress in Earth and Planetary Science (2018) 5:34

Table 1 Timeline of lightning studies. Venus dates correspondto relevant publications, rather than data acquisition, generally
only a year or so earlier
Venus
1959–1964 Mariner 2 encounter with
Venus; hot greenhouseatmosphere identified
1967

First in situ
measurements—Venera 4

1970–1

Optical flashes mapped by
OSO-2 and OSO-5 satellites

1972

Venera 8 reaches surface

1975

Veneras 9 and 10 landers +Orbiters. V-9 orbiter records
possible flashes

1979

VLF Discharges measured by
Venera 11, 12 landers

1979–80

Pioneer Venus Orbiter
detects electric ELF whistlers

1983

Veneras 13 and 14 confirm
VLF discharges

1986

VEGA balloons float in Venus
atmosphere

1989

1989

Earth
Whistlers first observed bysatellites—Vanguard III,
Alouette, Injuns 3 and 5

RF lightning emissions
mapped by UK satellite Ariel
4

NLDN provides continuous
US continental-scale
lightning mapping
Magellan spacecraft
launched

Sprites first photographed

1994

Terrestrial gamma-ray flashes(TGFs) discovered

1997

FORTE satellite: both opticaland RF detections TRMM
satellite studies storms andlightning

2004

DEMETER satellite. WorldWide Lightning Network(WWLN) established2007

Venus Express detects
magnetic ELF Whistlers

2008
2010
2016

C/NOFS satellite
Akatsuki launched
GOES-16 with Geostationary
Lightning Mapper

graphically the enhancement of lightning activity over land
relative to the oceans at the same latitude. Surveillance byflash meters on US VELA satellites (installed to detect the
optical emission from nuclear detonations) revealed asmall population of particularly intense lightning
discharges (Turman 1977) and later dedicated lightning
sensors on the Defense Meteorological Support Program(DMSP) satellites gave additional information on the
optical power distribution of lightning flashes overall(Turman 1978; see also Christensen et al. 1979).
Modern satellite sensors dedicated to lightning mapping (e.g., the Lightning Imaging Sensor (LIS) on the

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Tropical Rainfall Mapping Mission (TRMM), the Optical Transient Detector (OTD) (see, e.g., Finke 2009),
and more recently, the global lightning imager on
GOES-16 have yielded systematic observations of activity, enabling correlation with geographical or convective parameters. There have also been satellite missions
dedicated to the study of lightning-related phenomena,
notably the Fast On-orbit Rapid recording of Transient
Events (FORTE) and Communications/Navigation
Outage Forecasting System (C/NOFS), both of which
carried optical flash detectors (photodiodes) as well as
antennas to measure electromagnetic signatures. Theseobservations (e.g., Fig. 5), and those from a related instrument (Global Lightning and Sprite Measurements
on Japanese Experiment Modules JEM-GLIMS on theInternational Space Station, e.g., Sato et al. 2015), havegiven us simultaneous views of different phenomena associated with discharges, phenomena which have only
been studied separately at Venus. There is also a
powerful synergy between the ground-based networks
and spacecraft sensors since they can perform intercalibrations to assess detection thresholds and efficiencies.

Venus observation review
In this section, we review pertinent details of the various
Venus observations, arranged chronologically. The key
parameters are summarized in Table 2.
Veneras 11–14 lander electromagnetic observations

Observations by the Veneras 11 and 12 probes, which descended to Venus’ surface in December 1978, were the firststrong report of lightning on Venus (Ksanfomality et al.
1979). Note that although the Venera 9 orbiter observations(see next section) were acquired earlier, their interpretation
as lightning was not developed (Krasnopolsky 1979; submitted November 10, 1979) until after Ksanfomality’s Venera
11/12 paper was published in the May/June 1979 issue of(originally submitted in February 1979).
The Veneras 11 and 12 probes carried an instrument
suite called “Groza” (thunderstorm—a clear indication of
the purpose of the system) which comprised a magnetic
field antenna (a loop) to detect sferics and a microphone
to detect thunder. Signals in two wavebands (at 10 and
36 kHz) were recorded. Veneras 13 and 14 carried an
improved instrument (Groza-2) which also included aseismometer, Fig. 6.
The acoustic element of the instrument was notuseful for detecting thunder during descent, in that,aeroacoustic noise of the air rushing past provided
too strong a background for sensitive detection. On
the surface, some lander-generated noise was identified, and the background signal level was in fact used
to estimate the wind speed on Veneras 11 and 12,
but no thunder was detected.

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Fig. 4 Map of lightning activity in a 2 h period, detected more or less globally (coverage in South America and Africa by this amateur network is
somewhat sparse of stations, which need only a small VLF receiver and an internet connection). A single recent strike is detected (lines) by stations
from Japan to Australia and New Zealand, allowing its location in the Philippines—some 8000 km from New Zealand—to be reconstructed (circle)
within a second or so. It is also apparent from this plot that there may often exist regions thousands of kilometers across where no lighting is present
at a given time. Creative Commons license—by contributors to the Blitzortung network

The electromagnetic field sensor detected substantial
activity, which showed a different vertical profile foreach lander (suggesting that the signals were not artifacts). Usually, these have been displayed together withscales adjusted; it is instructive here to show them on acommon scale Fig. 7.
Ksanfomality et al. (1979) and Ksanfomality (1980) claimthat some of the detected bursts showed a periodicity commensurate with the slow rotation of the Venera 11 lander
during the latter part of the descent, consistent with spin
modulation due to the directionality of the antenna as it
swept past a distant (i.e., of small extent in azimuth) source.
However, this suggestion does not seem quite consistent
with the measurements of the probe rotation from angularvelocity sensors (~ 7°/s—see Fig. 4 of Karyagin et al. 1980);
the combination of this rate (50 s period) and the bi-lobed
sensitivity pattern of the antenna should lead to a 25 s
period modulation of a distant source, not the 40–80 s

modulation observed. Furthermore, inspection of the radiation pattern of the antenna (Fig. 10 of Ksanfomality et al.
1983) suggests that the sensitivity varies with azimuth by afactor of 2–3, yet the observed change in signal amplitude
appears rather larger. Thus, an alternative interpretation
advanced in the present review is that in fact the pseudoperiodic variations are bursts of the original emission(implying only one to four sources were being observed,
each generating one or a few bursts), rather than spin
modulation of a single continuous source.
On Venera 12, one electromagnetic burst was recorded after landing. The burst (see Fig. 10 later in thisarticle) lasted the entire observation window of 8 s, thesewindows being repeated at 3–4 min intervals. This
sparse sampling only constrains the burst to have a duration of between 8 and 360 s. No bursts were recordedon the surface by other landers (a total observation
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Fig. 5 A single (multiple) lightning flash recorded by the C/NOFS satellite on Earth in multiple ways that have only separately been seen on Venus. Threebrief optical flashes (top) are recorded, the first near-synchronous with a stroke detection by the ground-based VLF World-Wide Lightning Network(WWLN), shown in the red line. Electric field antennas on the satellite then showed a waveform (middle) that can be decomposed in the spectrogram(bottom) into whistlers in the VLF range (kHz to 10 s of kHz), similar to the VLF noise recorded by the Venera landers, and ELF waves of a few hundred
Hertz (which may be whistler-mode) in the ionospheric plasma, similar to those recorded by Pioneer Venus and Venus Express. Figure courtesy
Bob Holzworth

The near-continuous activity generally observed by theprobes during most of the descent (the Venera 11 instanceabove notwithstanding) implies that if the activity is typically in 5–70 s bursts, the observation (instrument plus signal propagation characteristics) was sensitive enough todetect bursts from long enough distances that a sufficientnumber of sources was detectable at any moment to prevent gaps being observed between bursts. If we ignore fora moment that the probes were descending, we can consider each record as a time series. For Venera 11 (see, e.g.,
Fig. 2. of Ksanfomality et al. 1983), the activity was moderate (~ 20 uVm−1 Hz−0.5) between 0530 and 0545 h (60 to
30 km) and strong (~ 100 uVm−1 Hz−0.5) from then until
0605 (15 km) then moderate to the surface. For Venera 12(ibid, Fig. 3) some sporadic activity (with 1–2 minvariations) was seen for the first 15 mins of descent, thenessentially zero for the next 25 min, then moderate
activity for the last 20 min of descent.
Considering the first part of Venera 11, if a single source
causes ~ 5 bursts of ~ 30 s duration (as indicated in thehigh-resolution record 0606–0611 previously interpreted
as spin modulation—Ksanfomality et al. 1983; Fig. 8), thenthe continuous record seen 0546–0552 (ibid, Fig. 5) implies two or more sources within range, such that the
bursts overlap.
Note that in addition to the averaged field strength(recorded with a 0.25 s time constant, and in some casesin the papers cited above plotted after averaging over 20 s
periods), the radio system was also equipped with a pulse
counter. This indicated (Ksanfomality et al. 1979) pulse
rates (threshold not specified) from 1or 2 per second to
10 or 20 for part of Venera 11’s descent, and the text indicates even higher levels were seen. Pulse counter data isalso presented in Ksanfomality et al. (1982) for Venera 14,
with a threshold of some 200 μV/m/Hz0.5—at this quite
high threshold, the rates were 0 to 2 pulses per 10 s above
about 25 km, with some periods of 5–7 pulses per 10 s
below that level.
The possibility of some artifact (e.g., noise generated byaerodynamic flutter during descent) does not appear tohave been discussed in detail in the literature. However, aslow systematic (smooth) altitude dependence would be
expected, as the vehicles’ terminal velocity slowed as theyreached the denser lower atmosphere—such a dependencePioneer Venus
Orbiter

Galileo flyby

Galileo flyby

1.54 m ground- CCD camera
based telescope

Cassini flybys

Venus Express

Venus Express

Huestis and Slanger
1993

Gurnett et al. 1991

Belton et al. 1991

Hansell et al. 1995

Gurnett et al. 2001

Russell et al. 2007a,
2007b. Hart et al. 2013

Moinelo et al. 2016

VIRTIS near-IR
spectrometer

Magnetometer

Radio and plasma
wave

CCD Camera

Radio

UV spectrometer

BrightspectraBursts at
64 Hz

Electric
field

Brightspots onimage

Brightspots onimage

Electricfield

Spikes in
opticalintensity

OpticalpulsesE-field
bursts at
100 HzNo

Indirectly

Yes

Yes

Yes

Yes

No
(spectralscanner)

No

(Yes)

Yes

Nondetection~ 700 s of activity
per 225 days
(10 min/day)Firm
nondetection

7 flashes

Nondetection

9 impulses

1 event

Not stated~ 0.1–1 nT

7E7–1E9 J

4 × 1E9 J

10–16
V2 m-2 Hz-1

> 70 kR

2E8 J

Plasma noise,spacecraft noise
N/A

Observations
before/after
encounter–

Plasma, spacecraft
noise
N/A

–

N/A
Variationwith altitude

Earthlightning

Cosmic rays

Meteor trail waspreferred
explanation–

Dark frames

Cosmic rays

Plasma noise.
Spacecraft noise

Statistics ofoff-diskpulses> 20 μV/μ/Hz0.5 –As above–

20–40 μV/
μ/Hz0.5

One burst (V12)150 pulses in 8 s
4240 bursts

Spacecraft noise:
but time (altitude)
dependence argues
against–Tens ofμV/μ/Hz0.5

Near-continuous
noise (few to~ 40 pulses per
second)Debris fromspacecraft?

Dark sky

1 burst of ~ 2
1E8 W 3E7 J/
flashes/s over 70 s flash

Other explanations
for data

Controls

Amplitude or
Sensitivity

Detections(varies)~ 200 km

~ Disk

~ 70% of disk

Part of disk

Disk

< 1000 km2~1 million
km2

Unknown

1 million km2(?)

Few million
km2 (?)
Depends on
propagation

1500 km2~ 50,000 s total

3.75 h53 min~ 600 orbits,
10 min each~ 80 s257 h

4 s/c × 8 s every
3–6 min for ~ 1 h

4 spacecraft, ~1 h each descent

2 spacecraft
× 25,000 s (night)

Area observed Duration

In many cases, many papers examine the same data (especially Pioneer Venus and Venus Express, see main text)—here lists only the papers from which the table entries have been derivedaPioneer Venus
Orbiter

Borucki et al. 1991

Star scannerElectric field
detector

Pioneer Venus
Orbiter

Scarf et al. 1980, 1987

10 and
36 kHzimpulses

No(spectralscanner)

Signature Instrument
designedforlightning?

Visible spectrometer Spikes in
opticalintensity
Groza (V11, 12)Groza-2 (V13, 14)magnetic andacoustic sensors

Veneras 9 and
10 orbiter

Krasnopolsky 1979,
1983a,bInstrument

Ksanfomality et al. 1979, Veneras 11 andKsanfomality et al. 1982, 12 lander.
Ksanfomality et al. 1983 Veneras 13 and14 lander

Platform

StudyaTable 2 Key characteristics of Venus lightning observations

Lorenz Progress in Earth and Planetary Science (2018) 5:34
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Fig. 6 The Venera 13 lander (left) with major systems noted (figure courtesy W. Huntress)—the GROZA-2 label points to the location of the loop antenna
mounted externally just above the shock-absorbing landing ring. The disk structure near the top stabilized the probe during descent after the parachutewas released. The GROZA-1 instrument is shown at the right (figure courtesy L. Ksanfomality) showing the loop antenna and mounting strut, together witha microphone for detecting thunder—note the heavy-duty insulation on the cable to protect it from the hot corrosive Venus atmosphere

was not observed. Further, it is seen (e.g., Ksanfomality
et al. 1983, Fig. 10) that the signals do not cease abruptly
at impact but rather fall off within the last few hundred
meters of descent, more consistent perhaps with terrain
obscuration of signal propagation. (The possibility ofnear-surface atmospheric composition changes causing
radio absorption cannot be completely excluded, however). Finally, specifically to test whether charging of thevehicle itself could be responsible for the observed signals,
a discharge current meter was installed on the Groza-2
experiments on Veneras 13 and 14 (see Ksanfomality et al.
1982)—those results do not appear to indicate any strong
local contribution to the observed discharges but may beconsistent with the presence of charged dust in the Venus
lower atmosphere (Lorenz 2018).
It is useful to put the Groza experiments in context
with a similar investigation, the Lightning and Radio
Emission Detector (LRD) on the Galileo Probe (e.g.,
Lanzerotti et al. 1992; Rinnert et al. 1989, 1998). It wasnoted that the noise background of this instrument was
found to be higher in flight (due to the operation of anearby radiometer instrument with an electric motor)
than in laboratory/field tests.
It may also be noted that the LRD instrument detected
between 10 and several hundred RF pulses per second
throughout the descent (over 150,000 pulses total), whereas
no optical lightning detections were claimed (the few hundred counted optical pulses being attributed to spinmodulated sunlight early in descent, or to cosmic rays—see

Rinnert et al. 1998). This underscores the comparative
long-range ability of electromagnetic vs optical sensing(and makes the optical nondetection of the VEGA balloon
at Venus perhaps unsurprising). The LRD used a ferrite coilantenna (i.e., sensing the magnetic component of the signal,
like the Groza instruments) and counted pulses in narrow
bands at 3, 15, and 90 kHz, with the lowest frequencies seeing highest activity; sensed pulse amplitudes reached a few
tens of nanotesla. The LRD detected appreciable azimuthal
anisotropies (i.e., preferred source directions).
Terrestrial tests of the LRD showed that intensity fell
off with increasing frequency approximately as f−2; sucha strong fall-off was also seen at Jupiter (Rinnert et al.
1985). The Groza instrument at Venus found a spectral
index varying between 0.5 and 2, typically 1. This fall-off
seems consistent with an interpretation of the signals as
due to electrical discharges. Note that the sometimes
challenging units of electromagnetic observations have
perhaps limited external scrutiny into these results—for
example, even though a magnetic antenna was used,(Ksanfomality et al. 1979, Ksanfomality et al. 1983 hereafter referred to as K83) expresses the Groza signals aselectric field intensity—e.g., the 10 kHz signals on
Venera 12 (K83 Fig. 3) reaching a value of ~ 15 μV/m/
Hz0.5 (recorded over a bandwidth of 1.6 kHz) or a peakfield of 0.6 mV/m. For classic electromagnetic
propagation in free space, the electric field E relates tothe magnetic field B as E = cB, with c being the speed oflight—note that this relation does not hold for theLorenz Progress in Earth and Planetary Science (2018) 5:34

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Fig. 7 The electromagnetic field intensity (5 kHz bandwidth centered on 10 kHz) from Veneras 11–13 (Ksanfomality et al. 1982,1983) averaged
over 20 s intervals, plotted on a common scale which shows the dramatically different character of the Venera 11 profile with the 30–15 km
altitude range (about 20% of the descent duration) having several times higher intensity than the rest. Previous presentations have tended torescale each profile, de-emphasizing this difference

whistler-mode propagation in a plasma), and so, thiscorresponds to 0.002 nT, or 5 × 10−5 nT/Hz0.5. The
Galileo sensors at Jupiter, by contrast, recorded a power
spectral density of up to 10−3 nT2/Hz or ~ 1 × 10−6 nT/
Hz0.5 at a similar frequency (15 kHz), a significantly
lower level than Venus. On the other hand, a handful of
waveforms were recorded by Galileo, the largest having
a peak amplitude of a couple of tens of nanotesla(Lanzerotti et al. 1996; Rinnert et al. 1998).
In many respects, the Groza records are among the richest datasets pertaining to possible Venus lightning. Their

presentation in various graphical forms in Ksanfomality
et al. 1979 and 1983 is useful, but the examination withnew statistical methods would benefit from access to original digital data, which is unfortunately not publicly accessible (sadly, also true of the Galileo LRD experiment).
Note that Ksanfomality’s papers (1982, 1983) describe
anomalies encountered by the Pioneer Venus probes ascircumstantial support for electrical activity. While atthe time, the possibility of some kind of discharge had
been considered one possible explanation for theseanomalies (the failure of external temperature sensors

Lorenz Progress in Earth and Planetary Science (2018) 5:34

Fig. 8 Amplitude distribution of pulse heights (inset) recorded at ahigh data rate during part of the Venera 12 descent (see, e.g.,
Ksanfomality et al. 1983). About 30% of the time, the amplitude wasat the noise level of ~ 5 uV/m/Hz0.5—spikes of ~ 100 occur less than
1% of the time. The data (analyzed here by digitizing the plotted
figure) do not permit robust discrimination of a power law from abroken power law or an exponential; the single power law with an
exponent of − 1.25 is shown as an example, although there is somehint of a steeper slope at the high end of the curve (right). The
distribution arises either from an intrinsic amplitude distribution, or
because discharges occur at a variety of ranges, or both. The
distribution is also broadly consistent with the aggregate of results
shown in Fig. 7

on all four probes, at the same altitude of ~ 12 km) theseanomalies have since been attributed to a failure of electrical insulation in the Venus environment at this altitude (e.g., Seiff 1995; Harland and Lorenz 2006) and soshould not be considered as indicating electrical activity(although they do not exclude it either).
Venera 9 and 10 orbiter spectrometer

These observations, initially reported in Krasnopolsky(1979), are discussed at rather more length in Krasnopolsky (1983a, 1983b, 2006). As this UV-visible-NearIRspectrometer scanned over 10 s from 300 to 800 nm, observing a 160 × 9 km patch of the Venus nightside, the recorded light level saw several irregular jumps—seven suchscans had these jumps. Interpreting this as a time seriesrather than a spectrum, there was a 70 s series of flashes,
about two per second, each lasting about 250 ms, each ofabout 3 × 107 J of optical energy. Although claimed to be“similar to the duration of a flash on Earth,” thesedurations (much longer than the instrument responsetime of 5 ms) are puzzling. The observation was at 9° S
around 7 pm local solar time; the “thunderstorm region”
is described as being of 50,000 km2 area, presumably
derived by mapping the instrument footprint migration
during the 70 s.

Page 11 of 25

One point to note is that in fact, this event was seen
during the very first night observation of the instrument,
perhaps when instrument anomalies would be more likely
to occur. A further consideration is that while no localized
Venus emission interpreted as lightning was recorded by
the instrument on Venera 10, the light was unexpectedly
measured off-disk and was interpreted as a dust trail froma comet Krasnopolsky and Krysko, 1979. The presence ofa dust trail (something no mission to Venus before orsince has indicated) cannot be ruled out entirely but is atleast superficially improbable. Instead, it is possible thatboth this observation and/or the Venera 9 “lightning”
burst may have been caused by light reflected from somematerial (e.g., thermal blanket) released from the spacecraft: a tumbling reflector could give quasiperiodic flashes
with durations consistent with the 0.25 s durations observed. The star tracker on Magellan was spoofed by particles shed from its that spacecraft’s thermal blankets whensolar wind conditions led to surface charging (e.g.,
Harland and Lorenz 2005). It is also reported that the
Venera 9 instrument temperatures increased from 20 to
60 °C, during its 2-month mission, a factor which degraded the sensitivity of the spectrometer instrument(Krasnopolsky 1983a).
When considered qualitatively, “Venera 9 saw lightning,but some aspects of the observation are puzzling.” A
quantitative perspective, however, is much more illuminating. An important quantity recently explained(Krasnopolsky, personal communication, June 2017) tothe author of the present paper is the duration ofsubsequent nightside observations (for Veneras 9 and 10
combined) where no lightning features were observed,
50,000 s. Thus, one “burst” of 70 s of emission was seen in
about 14 h of observation of an area ~ 1500km2, theactivity being present ~ 0.1% of the time.
Krasnopolsky claims that the spikes are enveloped
somewhat by the optical sensitivity of the instrument (i.e.,
fewer/weaker flashes at the ends of the passband, whereas
electrical noise or cosmic rays might be expected to beuniform with time/wavelength), supporting an external
optical origin of the signal. However, this is also fully
consistent with the sunlight reflections-from-debris hypothesis suggested above. If the signal was really due tolightning, it imposes an altitude constraint for the opticalemission (as noted by Krasnopolsky 2006) in that the blue
end of the spectrum at 20 km or below (i.e., volcanic
lightning) would be attenuated severely by the atmosphere, and thus, whatever light was observed by Venera 9
had to come from above the deep atmosphere. However,since lightning emission has a highly nonuniform line
emission spectrum (e.g., Borucki et al. 1985, 1996), thesomewhat uniform peak amplitudes observed by Venera 9
seems perhaps more consistent with periodically reflectedsunlight.

Lorenz Progress in Earth and Planetary Science (2018) 5:34

Pioneer Venus electric field detector

The Pioneer Venus Orbiter was placed in December
1978 into an elliptical polar orbit around Venus with a
24 h period. It carried an electric field instrument whichmeasured the power of electric field variations in four
narrow wavebands. Taylor et al. (1979) and Scarf andRussell, 1983 reported that signals seen over nightside
periapses, most prominently in the 100 Hz band, could
be interpreted as whistler-mode emissions fromlightning. These signals were “impulsive” in the sense ofbeing short compared with the 0.5 s sampling interval
and the 0.7 s decay time of the instrument. Because thespacecraft environment was noisy in sunlight, thesesignals could only be reliably detected at night, andthey were detected only at low altitude (e.g., Scarf
et al. 1980), which given the parameters of the orbit,
meant low latitudes.
A variety of plasma phenomena can occur in space,and alternative explanations were offered for these signals and an extensive debate in the literature ensued.
We do not recapitulate the blow-by-blow debate here

Page 12 of 25

but refer the reader to comprehensive reviews by Russell(1991) and Grebowsky et al. (1997) Fig. 9.
For the present discussion, we may note that while allobservations (electromagnetic or optical) may suffer
noise or artifacts, there are features of the signals thatare claimed to be consistent with a lightning origin, notably their altitude dependence.
Scarf et al. (1987) reported identifying 4240 bursts in thefirst 2124 orbits, at altitudes between 150 and 2900 km, indicated by them as a total observation period of 257 h.
Then, whatever phenomenon a “burst” actually corresponds
to—perhaps passage over a thunderstorm—there wereabout 15 per hour. Note that the “new” burst definition
used by Scarf et al. (1987), counting short events distinctly,
corresponds to a rate five times higher than that used previously (assessing whether a signal existed in a 30 s window
or not.) A more elaborate analysis was made by Ho et al.
(1991) who attempted to deconvolve the response time of
the instrument—in effect counting any 0.5 s window withmeasureable “extra” energy as an event—see Fig. 10. This
yielded the rather high rate of ~ 0.14 events per second.

Fig. 9 NASA artists impression of the Pioneer Venus orbiter. The Orbiter Electric Field Detector (OEFD) measures the potential between two wire
mesh sphere electrodes (arrowed)

Lorenz Progress in Earth and Planetary Science (2018) 5:34

Page 13 of 25

Fig. 10 Several different Venus lightning detections plotted with arbitrary vertical scales, but on the same abscissa, showing the characteristic
variability on second and sub-second timescales. It is evident too that the rate of events (bursts/pulses/flashes/strokes) is strongly dependent on
the time resolution adopted

Although some claims of geographical association ofwhistler detections with elevated terrain (presumed to
be volcanic) were made (e.g., Scarf et al. 1987), samplingbiases may not have been fully taken into account, andthis correlation is now considered weak at best—one
refutation is by Taylor Jr and Cloutier (1986).1

Considerably more robust, however, is the finding that
activity appeared to be concentrated in the duskmidnight sector, with much less in the midnight-dawn
sector. Note that whereas on Earth, most convection occurs in the afternoon when solar heating is strong, on
Venus convection may instead be driven by cloud-top

Lorenz Progress in Earth and Planetary Science (2018) 5:34

cooling (e.g., Imamura et al. 2014) at night. This latter
scenario would probably cause peak convective lightning
activity in the dusk-midnight sector, as observed.
Pioneer Venus optical

Borucki et al. (1981, 1991) examined data from thestar scanner on the Pioneer Venus Orbiter—an instrument and data-mining analysis much like that of
Sparrow and Ney (1971) at Earth. Borucki et al.’s
nondetection of events (or rather a detection ratethat did not exceed the false alarm background due
to energetic particles) in fact derived from only
450 s of effective search time (Borucki et al. 1981;
another 88 s, but covering a wider area, was addedin further analysis, Borucki et al. 1991). The shortobservation period results from the fact that the observations relied on lightning emissions not being
observed directly but rather scattered into the detector as stray light—even the “ashen light” from theVenus nightside saturated the detector when observed directly. Despite this short observing time,
the area-time product of a few × 107 km2s wasenough to derive a useful upper limit on the flash
rate of 1–4 × 10−7 flashes/km2/s.

Page 14 of 25

lightning origin (even though Borucki et al. (1996) conclude that discharges in the Venus atmosphere would
produce some UV light, it would be scattered orabsorbed before reaching an orbiting detector if
launched from the clouds.) The possibility, however, of it
being due to upper atmosphere luminosity from electrical discharge (i.e., sprites, elves, etc.—phenomena
which only became well-known in the 1990s, after thisobservation and analysis were completed) might bear reexamination. Indeed, Pérez-Invernón et al. (2016)
present models of posssible mesospheric optical signatures of lightning (i.e., transient luminous events (TLEs,
such as sprites and elves) but curiously do not cite the
Huestis and Slanger work. They note that in addition to
the green and red oxygen lines, nitrogen emission would
occur in the UV and near-IR. As an example, a lightning
discharge with a total released energy of 2 × 1010 J
would launch 2 × 1020 green photons and 3.7 × 1019
photons 120–280 nm, much of which would be in the
PV UVS passband.
UV observations were made (lasting a few tens ofminutes around periapsis) over the first 2 years; only
this single anomaly was reported. Thus, this single“burst” of ~ 72 s was the only one in ~ 1 million seconds of observation.

Pioneer Venus ultraviolet spectrometer

An optical anomaly was noted (Huestis and Slanger1993) in the analysis of data from the Pioneer Venus
Orbiter ultraviolet spectrometer, whose primary purpose was to investigate the nightglow between 150
and 360 nm wavelength. In one observation (orbit 75),
three successive scans were observed to have a strong
emission (at wavelengths matching the N-O band system). These short-wavelength scans had an interval of
24 s, as the spacecraft spun with a 12 s period andshort-wavelength (155–258 nm) and long-wavelength(258–360 nm) scans were interleaved. No anomalous
brightness in the two intervening long-wavelengthscans was observed. One could interpret this as a
72 s long event with only shortwave emission in narrow bands, or perhaps just coincidence that the 3–5
major flashes (discarding those with only one or two
photons—see their Fig. 9) over a 72 s period just happened to occur during the shortwave scans. It should
be noted that the “strong emission,” corresponding to
70 kR (kiloRayleighs—where one R = 1010 photons/m2/s),
was indicated by only about 60 photons in each scan,compared with a typical 30 photons due to the
airglow.
Huestis and Slanger (1993) offered a meteor trail(900 km long!) as a possible explanation, considering thismore plausible than a lightning or auroral origin. Theirinterpretation seems improbable but cannot be excluded.
The observation seems incompatible with a direct

Pioneer Venus gamma ray detector

The Pioneer Venus Orbiter in fact carried an instrument
whose purpose was not to study Venus at all; the Orbiter
Gamma-Ray Burst Detector (OGBD) was flown to help
measure astrophysical gamma-ray bursts, by using timeof-arrival differences between it and other spacecraft inthe solar system to triangulate the source direction. This
instrument operated for some 14 years until 1993
when Pioneer Venus’ orbit decayed. It was only a year
afterwards that terrestrial gamma-ray flashes (TGFs)
associated with lightning on Earth were discovered
Fishman et al. 1994.
Lorenz and Lawrence (2015) reviewed the OGBD dataset
archived on the National Space Science Data Center(NSSDC) to assess whether there was evidence of an enhancement of gamma-ray flux near Venus, but unfortunately, those data were too coarsely binned in time topermit any conclusion. Should high-time-resolution records
be discovered (in contrast to astrophysical bursts, TGFs are
only milliseconds long), the question might bear reexamination. The authors also modeled the propagation of
gamma rays through the Venus atmosphere, to assess thealtitude above which gamma rays would need to be released
to be usefully detectable. They found that gamma rays fromsources at 65–75 km altitude (depending on energy) are attenuated by a factor of 100. Radiation and particle transport
studies were performed by Bagheri and Dwyer (2016), withsimilar results. Should gamma-ray instrumentation be flown

Lorenz Progress in Earth and Planetary Science (2018) 5:34

on a future Venus orbiter (e.g., to measure sulphur in theclouds), a high-time-resolution capability would be desirable
to detect possible lightning flashes.
VEGA balloon

The two VEGA balloons deposited into Venus’ atmosphere in 1985 floated for ~ 48 h at an altitude of 50–
55 km. Each gondola was equipped with a photodiodelight gauge, including an electronic high-pass filter todetect flashes. No flash detections were reported:
Sagdeev et al. (1986) notes that the intermediatebrightness flash counter on VEGA 2 did increment once,
indicating a possible flash, but the measurement is suspicious because it was made near the terminator (wherevarying cloud-top altitudes could cause strong changes
in ambient illumination) and that the lower-level threshold counter should also have incremented but did not.
The VEGA data have recently been restored and madeavailable for the NASA Planetary Data System (PDS)—
Lorenz et al. (2018); it may be noted that the VEGA-2
lander pressure-temperature profile therein is the only
high-quality in situ atmospheric dataset that reaches all
the way to the surface of Venus.
Although the observation period was relatively long, it
may be noted that the observation was mostly in themidnight-dawn sector of the Venus day (on Earth, theperiod least likely to see lightning). Furthermore, the detection area would have been relatively small, only a few
tens of kilometers across at most, i.e., area of ~ 1000 km2.
The platform is pseudoLagrangian, being advected withthe air mass. Thus, if one adopts the proposition thatsome air masses have lightning and some do not (e.g., due
to moisture content), then the range of locations sampled
is very small (in contrast, e.g., to an orbiter survey ofsimilar duration). Thus, the nondetection is a rather weak
constraint on lightning on Venus.
Galileo radio

Gurnett et al. (1991) reported the detection of nine electric field impulses over a period of 53 min during theGalileo spacecraft’s Venus flyby in 1991 at about 4–5 RV(Venus radii), i.e., a range of about 20,000 km. Although
at the time, they reported that “lightning is the most
likely source,” the lead on this investigation has informed us (Gurnett, personal communication, 23 May
2017) that he no longer believes lightning to be responsible, noting that impulsive signals could arise from (1)spacecraft electronics, (2) plasma waves, (3) dust impacts, (4) thermal stresses that cause discontinuous
mechanical motions that are coupled to the antennas via
microphonic effects, and (5) other unknown effects. Although control observations were carried out during two
1 h intervals (when no impulses were seen), these were
well before the Venus flyby and well after the Venus

Page 15 of 25

flyby. Notably, the high-voltage component of a plasma
instrument (PLS) was active during the close approach
period but was off during the control intervals. Thus, it
is possible that the operation of the PLS high voltage
unit close to Venus was responsible for the signals interpreted at the time as lightning. It may also be noted that
if the impulses really were due to lightning, the observed
rate is much lower than would be observed for terrestrial lightning at a comparable radial distance.
Galileo optical

Belton et al. (1991) examined nightside images of Venus
acquired by the Galileo spacecraft during its flyby en routeto Jupiter. Ten images were devoted to this lightning
search; however, since exposure times were less than asecond, the nondetection is not a strong constraint.
Mt. Bigelow ground-based optical

Hansell et al. (1995) installed the CCD detector at the
153 cm telescope located on Mt. Bigelow, Arizona (morepopularly known locally as the “61-inch.”), and searched
for light flashes on the nightside of Venus. Their study
carefully employed coronagraphic optics, using two
masks designed in accordance with the specific geometryfor each individual night of viewing. An occulting mask
was used in the imaging plane, and a Lyot mask wasused to block diffracted light by the edges and support
structure of the secondary mirror. The CCD detector
was operated at 18.8 frames/s for ∼ 30 pixel images ofVenus. The observations were made at mainly 777.4 nm(i.e., in the near-infrared—an atomic oxygen line expected from laboratory simulations of lightning discharge in a carbon dioxide atmosphere by Borucki et al.
1996); a few observations were made at 656.3 nm as acontrol. For eight nights in 1993, the total viewing timewas 3 h at 777.4 nm and 45 min at 656.3 nm. The dusk
side of Venus was facing Earth. Seven events met thestringent criteria (including the requirement that an
event must be seen on more than one pixel) that the experimenters used for isolating lightning flashes, as shownin Fig. 1. Six events were detected in the 777.4 nm line.
The seventh occurred while observations were being madeat 656.3 nm. The 777.4 nm flashes occur at a rate of
2.7 × 10−12 flashes/km2/s and imply Venus lightning
flashes with optical energies from 7 × 107 to 2 × 109 J.
Although the statistics are rather poor, Hansell et al.
(1995) did note that three of the flashes occurred
within 10 min of each other, while some nights had no
flashes at all, suggesting a possibly clustered behavior.
However, the locations of flashes were distant fromeach other, suggesting this may have been a sporadic
coincidence. The detection of a flash at 656.3 nm (thehydrogen-alpha line, not initially expected to yieldlightning detections) was interpreted as being due toLorenz Progress in Earth and Planetary Science (2018) 5:34

light from a carbon emission line at 658 nm identified
in laboratory experiments simulating lightning discharge in various planetary atmospheres (Borucki et al.
1985; Borucki et al. 1996).
A remark on the flash duration is in order.
Krasnopolsky’s (1979) detections—see earlier—were
about 250 ms long, not typical of terrestrial flashes (ashad been claimed). The Hansell et al. (1995) data analysis procedure entailed subtracting a running average
image (of the two frames before and the two frames
after) from each image. Thus, if a flash were of 250 ms
duration, it would have spanned all five frames, and thebias (average) image would contain just as much energy
as the frame under study. In other words, the analysis
procedure excluded long pulses of light. Note also thatthe analysis procedure excluded single-pixel events. The
only events recorded, then, had an apparent span ofmany hundreds of kilometers, although the sources may
have been rather smaller but blurred by the Earth’s atmosphere (observations of Venus are necessarily madewith Venus low in the sky and thus at large air masses,
susceptible to seeing conditions). Note that some otherphenomena do have durations of 250 ms (e.g., blue jets),and meteor fireballs or trails can have durations of
the order of a second or longer (e.g., McAuliffe andChristou 2006).
Note that Hansell and colleagues repeated the experiment the following year (Hansell, personal communication, 2010), although since this was a null result and theconcluding part of work for a thesis (Hansell, Universityof Arizona, 1996), no journal publication was made. This
experiment used a dichroic splitter so that both the
778 nm and hydrogen-alpha line would be observed simultaneously. The same processing applied to the earlier
imaging campaign failed to yield any detections at either
wavelength, although it should be noted that the new observation was made on the sunrise side of Venus’ dark
disk (i.e., local solar times between about midnight and6 a.m., where the electromagnetic results from Pioneer
Venus seem to show a pronounced decrease in activity.)
Cassini radio

The Cassini flybys (Gurnett et al. 2001) provided a muchbetter search for lightning at Venus than was available fromthe Galileo flyby. The reasons are as follows. First, the flyby
altitudes for Cassini, 284 and 598 km, were much closer
than for Galileo, which would make the lightning signals
stronger by a factor of 1600 to 10,000 in power. Second, because Cassini had longer antennas, its sensitivity was better
than for Galileo, by at least a factor of 10. Third, the integration time constant of the Cassini receiver (1 ms for someof the observations) is much better suited to detect the very
short impulses from lightning than the Galileo receiver,which had an integration time constant of about 100 ms.

Page 16 of 25

Furthermore, the ability of the Cassini instrument to detect
lightning was explicitly demonstrated during the CassiniEarth flyby in August 1999, during which over 1000
impulses consistent with lightning were detected at radial
distances inside 14 RE. That no impulses significantly above
the receiver noise level were detected during either of the
Cassini-Venus flybys (total observing time of several hours
while viewing essentially the entire planet) allows us tomake the very strong statement that “lightning similar toterrestrial lightning did not exist at Venus during the two
Cassini flybys.” Crudely, one might interpret the null result
as indicating a flash rate lower than about one-thousandthof Earth’s. Of course, this does not rule out the possibility
that episodes of terrestrial-like lightning could occur at
Venus during other times, such as occurs for the very episodic lightning at Saturn (intense lightning activity was observed in association with a large storm on that planet,around the equinox season in 2009, but relatively little activity has been detected by the 2004–2017 Cassini mission
either optically or by radio before or since this one storm—see, e.g., Fischer et al. 2011).
Venus Express magnetometer

The most persuasive recent evidence for lightning are
the transient AC magnetic field pulses observed by
Venus Express (Russell et al. 2008a). Like other electromagnetic evidence, these suffer from the ambiguity of having alternative explanations (spacecraft artifacts, plasma
noise, etc.), but the recurrent nature of the signals (having
been observed hundreds of times) and the fact that theirpolarization and detection appear consistent withwhistler-mode ELF emissions lend some support to theclaim that they have a lightning-like origin.
Specifically, they have been observed close to the periapsis of Venus Express (~ 250 km altitude; due to Venus
Express’s orbit, this happens nearly over the north pole),
when magnetic field lines are inclined in such a way as tolink the spacecraft with Venus’ atmosphere. The signals
are detected at 64 Hz (the sample rate of the magnetometer is limited to 128 Hz) and have a typical amplitude of
0.1 nT, sometimes reaching 1.5 nT (Daniels et al. 2012).
Hart et al. (2013) describe the bursts as lasting between 6 s and up to a minute. Note, however, that the
spacecraft is moving at almost 10 km/s, so in 1 min, thevehicle traverses about 600 km. If the range sensed by
the whistler-mode observation is only a couple of hundred kilometers, then it may be that the upper bound on
burst duration is truncated not by the intrinsic durationof bursts of activity but by the duration of the visibility
window of a given source.
Hart et al. (2013) report that bursts occupied about
1% of the time the spacecraft spent at about 250 km
altitude, the occurrence falling off to higher altitudes(~ 0.3% at 390 km), but also towards lower altitudes.

Lorenz Progress in Earth and Planetary Science (2018) 5:34

Russell et al. (2008b) note that in the first Venus year ofoperation, there were 12,223 s of useful data, during which
61 bursts were detected, i.e., one per 200 s. They assumed
that the magnetic field permitted detection only 25% of
the time. They also make the reasonable (but unsupported) assumption that the detection samples a region
200 km in radius (roughly the spacecraft altitude above
the ionosphere) or 0.027% of the surface and extrapolate
to a rate of 18/s globally.
Some care is needed in that different definitions of theevent or “burst” have been used. Daniels et al. (2012)
show magnetometer waveforms and hodograms and describe the “components” as lasting around 100 ms (since
the data are generated from heavily bandpass-filteredsignals between 42 and 60 Hz, 100 ms corresponds toonly a few “wiggles” of the signal) whereas Russell et al.
(2007a) describe them as 0.2–0.5 s. They show the peakrate of bursts defined that way as ~ 0.0045 per second at
200–225 km altitude, whereas the “reduced” burst rate,
defined as the number of 5 s intervals containing such asignal as 0.0014 per second—thus on average, when a
5 s burst occurs, it contains ~ 3 smaller pulses.
These rates, based on several years of operation, are adisconcerting order of magnitude less than the initial report of Russell et al. (2007a). That study used 37 orbits(days) of data selected when the background noise dueto the spacecraft reaction wheels was particularly low,such that bursts with a peak-to-peak amplitude of 0.2 nT
can be picked out. Reader attention is needed to note thatthe Daniels et al. (2011) counts of 0.2 nT are for 0.2 nT
half amplitude—i.e., events double in amplitude comparedwith Russell et al. (2007a). Lightning events, like manyother phenomena in nature, have highly skewed populations, and any attempt to compare counts without taking
the detection threshold into account is hopeless (see, e.g.,
Lorenz 2009 for a similar problem with dust devils).
Daniels et al. (2011) break their counts up into several
amplitude bins—> 0.4 nT events make up only 10% of thetotal of > 0.2 nT events, so one might similarly extrapolatethat > 0.1 nT events are ten times more abundant, consistent with the Russell et al. (2007a) numbers. Russell et al.
(2011) describe bursts as about 4 s long and having a
rate of 0.05/s at peak. Thus, even considering observations from a single instrument, the event rate mayvary by a factor of ~ 40 (0.0014 to 0.05/s) dependingon the threshold and time window considered! The samecaution applies to optical detections—see Fig. 11 andassociated discussion later.
Venus Express near-infrared

Moinelo et al. (2016) examined nighttime data from thevisible (280–1100 nm) channel of the VIRTIS imaging
spectrometer on Venus Express. Their search resulted inthousands of transient signal detections, but these could

Page 17 of 25

all be explained by cosmic rays impinging on the detector, and analysis showed that the events were randomly distributed along the spectral dimension, rather
than being associated with some expected line emission
from Venus’ atmosphere. The total observing time was~ 50,000 s (Moinelo, personal communication, 2017).
Ground-based optical (II)García Munõz et al. (2011) utilized narrowband fast imaging of the Venus disk from an array of ground-based
telescopes. The search targeted the oxygen emission line
at 777 nm which should be a prominent line in lightning(as on Earth—most terrestrial spaceborne sensors use afilter to isolate this wavelength to improve the signal tonoise). Two sites (Calar Alto and Observatorio delTeide) and three instruments (AstraLux, FastCam, andWide FastCam) were used for observations during
November 2010 to January 2011. The analysis of thenight-side imaging of Venus showed no signal of opticalemissions. It is likely that many other optical surveys
with negative results also exist since nondetections tend
not to be reported (e.g., an anonymous reviewer of thispaper suggested that the late D. Sentman at Universityof Alaska made such observations but no formal publication resulted).

Implications and discussion
Synthesis of observations

In Tables 3 and 4, we review the evidence outlinedin the previous section. While indeed the reports
are not mutually consistent with a simple picture ofEarth-like lightning, some conclusions can be
drawn, most notably that some sort of atmosphericelectricity effect is present but that it yields neither
optical flashes nor VLF radio emissions observablefrom space in anything like the frequency that occurs for Earth.
In principle, it might be possible to aggregate the observational constraints and apply them to models oflightning distribution (spatial, temporal, and amplitude)
to test various hypotheses. For example, if lightning is
exclusively due to volcanoes, sources will be geographically fixed and will have no local time dependence(although various observation efficiencies, e.g., the ionosphere, may have such dependence). However, the dataat hand do not seem adequate to justify such an exercise—the Venera lander datasets are too brief, the Pioneer Venus results already dismissed as equivocal, while
the Venus Express magnetometer results are confined tonear the north pole and are likely conditioned more bypropagation conditions than by local time or geographic
effects on their sources.
Observations similar to those of Hansell et al. (1995)
would be useful to repeat, even with telescopes ofLorenz Progress in Earth and Planetary Science (2018) 5:34

Page 18 of 25

Fig. 11 Optical flashes recorded by one of the two photodiode detectors on the C/NOFS satellite (downloaded from https://cdaweb.sci.gsfc.nasa.gov/
index.html/). The top panel shows data for the lowest (most sensitive) threshold brightness—flashes are only detectable in the gray eclipse periods.
The counts are the number of samples (~ 4000) per 0.5 s integration period that exceed a flux threshold, here 0.23 mW/m2—count rates are as high asa couple of hundreds. The bottom panel concatenates several eclipse periods together into an observation comparable in length with Hansell et al.’s(1995) telescopic Venus survey which detected seven flashes in about 4 h. With a 100 times higher threshold than the top panel, only a handful ofcounts are now obtained, and periods of 2 h or more exist with no counts at all (note that the instantaneous field of view of this sensor isonly ~ 1000 × 2000 km)

Lorenz Progress in Earth and Planetary Science (2018) 5:34

Page 19 of 25

Table 3 Summary of characteristics of Venus electrical activity
Property

Estimate andconfidence

SupportComment

Rate of bursts

15/h~ 1/min~ 18/hPV E-field bursts [30 s] (Scarf et al. 1987)
Venera 11 bursts 13–17 km
Venus Express bursts (see text)

Rate of pulses (during burst) ~ 0.14/s~ 0.1–0.7/s
2–20/sPV E-field pulses (Ho et al. 1990)
Venera 14 descent pulses (> 200 μV/m/Hz0.5)
Venera 11 descent pulses (unknown threshold)

Duration of burst events

Well-determined:
5–100 s• Venera 12 surface burst > 8 s, less than
3–6 min• VEx magnetometer bursts 6 s to > 60 s
• Venera 9 burst 70 s
• PV E-field bursts

Overlaps between bursts on Veneras 11–14
descent do not constrain burst duration

Flash duration

Unclear• Venera 9 claims ~ 250 ms (long for terrestrial
lightning) but instrument response?
• Hansell ground-based detections < 100 ms(but 5-frame running mean bias subtraction
reduces sensitivities to flashes > 50 ms, anddiscards transients > 250 ms altogether)• Veneras 11–14 pulses 10–60/s, means up
to ~ 8–50 ms long

Venus Express, Pioneer Venus ELF signatures
longer than “flash” due to propagation
delay, so no constraint (see Fig. 5 bottom
panel)

Geographic distribution

Global?

• PV OEFD detections mostly mid-latitude(claimed association with highlands specificallywas not substantiated by data, but not
excluded either• VEx MAG detections only made near northpole (low altitude required for observation)• Venera lander detections made at low
latitude, but long detection range gives
little localization

Source region size

< few hundred
kilometers• Spin modulation of Venera 11 source• PV, VEx cover ~ 600 km in 60 s
• Hansell flashes cover 3–15 pixels(2.2 arcsec pixels each ~ 600 km across)

Local time distribution

Strongest observed inevening (18–24 h). But
few daysideobservations• Strongly indicated by distribution of PV
OEFD and VEx MAG bursts• Statistics of optical detections too poor
to provide useful constraints

Evening cooling of cloud-tops drives
convection?

Table 4 Summary of hypotheses regarding Venus lightning
Scenario

SupportComment/challenge

Conclusion

There is no electrical activity on
Venus• Sensitive Cassini searchyielded null result• Various optical searches
have yielded null
results• A priori theoreticaldifficulty of charge
separation• Requires alternative mechanisms (e.g., local plasma noise) for all
orbital electromagnetic evidence (Pioneer OEFD, VEx
magnetometer)• Also requires alternative mechanism for atmosphere/surface
electromagnetic evidence (Veneras 11–14)• Implies Venera 9 optical detection spurious (not improbable per
the present paper)• Implies ground-based optical detection spurious

Unlikely

Sporadic electrical discharges occur
with characteristics different fromEarth

• Consistent withelectromagnetic
signatures• Occasional opticaldetections• If optical signals are present at all, perhaps they are transient
luminous events (TLE’s, i.e., sprites or similar)• Cassini radio nondetection was “unlucky”—ionosphericconditions?

Not excluded

Frequent lightning similar to Earth• Venera 9 original
interpretation• Sensitive Cassini search yielded null result, but detected ample
Unlikely
activity at Earth with similar observation• Orbital electromagnetic signatures imply lower rates• Various optical searches have yielded null results or low flash rates• A priori theoretical difficulty of charge separation

Lorenz Progress in Earth and Planetary Science (2018) 5:34

somewhat smaller aperture (which are quite abundant).
Modern digital video cameras allow high frame rates suitable for detecting transients, and the computer storage
and processing hardware and software—demanding in the
mid-1990s for this application—are easy to acquire. Notably, software to detect bright transients in planetary video
sequences has been developed and made publicly available(e.g., http://pvol2.ehu.eus/psws/jovian_impacts/) and has
been successful in detecting impact flashes on Jupiter andthe moon (e.g., Hueso et al. 2013).
Chemical evidence for lightning

On Earth, lightning was the principal mechanism of nitrogen fixation prior to the evolution of nitrogen-fixing bacteria and the Haber process. It has been long noted (e.g.,
Krasnopolsky 1983; Bar-Nun 1980) that the abundance ofNO or other nitrogen-bearing species could be profoundly
influenced by lightning, and the observed abundance has
been taken as at least circumstantial evidence forlightning. Krasnopolsky (2006) suggested an abundance of
5.5 ± 1.5 ppb below 60 km by ground-based telescopic
observations and argued that this was consistent with a
global flash rate is ∼ 90 flashes/s for a flash energy ∼ 109 J(of course, the chemistry only constrains the product of
flash rate and energy—larger but rarer flashes would yield
the same production.). The production of NO by shockchemistry associated with meteors has not been evaluatedbut could conceivably reduce the apparent need forlightning to yield the observed NO abundance.
Lightning and volcanos

In many respects, the study of present-day volcanism on
Venus has parallels with the study of lightning. While
the past modification of the Venus surface by volcanism
is indisputable, the detection of the present-day activity
is not. Such detection would of course be a scientificallyappealing discovery, and a number of claims have beenmade. However, the indirect evidence (of changes in thesulphur dioxide abundance in the atmosphere) has alternative explanations. The claimed direct evidence of
microwave (Bondarenko et al. 2010) and near-infrared(e.g., Shalygin et al. 2012) emission is subject in bothcases to high false-positive susceptibility due to thebackground subtraction or modeling required to isolate
a volcanic signature, and the claims are not widely considered adequate evidence to assert discovery.
In fact, the investigations of lightning on Venus and ofpresent-day volcanism have not only scientific and sociological similarities but also intersections. It is often pointed
out that lightning discharges occur in the ash plumes of
volcanos on Earth, and in fact, the lightning networks suchas WWLN are now used to remotely monitor volcanic
activity, as in the case of the 2015 eruption of the Chilean
volcano Calbuco (Van Eaton et al. 2016).

Page 20 of 25

Some geographical associations of electric field signatures in Pioneer Venus data with regions on Venus weretaken as circumstantial support of a volcanic lightning
interpretation. However, the logic was in fact somewhat
circular, and the associations were not statistically robust(e.g., Taylor Jr and Cloutier (1986). That said, such an
association is not excluded either (e.g., Russell 1991).
Observations on future missions able to locate thesources of electrical discharges without strong latitude,
local solar time, or ionospheric propagation biases would
be useful in assessing the possible association oflightning with highlands or volcanos specifically. Note,however, that a surface-fixed source of “lightning,” evenassociated with a surface feature interpreted to be volcanic, is not a guarantee that active volcanism is occurring. A correlation could emerge in another way, forexample, in that atmospheric motions that manifest near
the Venus cloud tops are now known to occur in asurface-fixed reference frame, due to gravity waves excited by topography (e.g., Fukuhara et al. 2017).
Triboelectric discharges on Venus surface

It is striking that on only two occasions when smallscale surface changes could have been observed on
Venus, namely in the roughly 1 h intervals between the
successive image panoramas on each of the Veneras 13
and 14 landers, sediment transport was observed on oneof these (Lorenz 2016). In contrast, typically one might(depending on location) need to wait for days or months
to observe surface movement on Earth; similarly, only afew surface changes were observed on Mars by the
Viking landers over several years.
It has been recognized that triboelectric charging may be
an important effect on Titan (Lorenz 2014; Mendez-Harper
et al. 2017) as well as Mars, in environments where the lackof surface electrical conductivity via liquid water prevents
rapid charge leakage. In the dry high-pressure near-surface
atmosphere of Venus, it will be even harder for the charge
to leak away from triboelectrically charged sediments.
It seems plausible that triboelectric charging associatedwith eolian sediment transport might occur on Venus; if it
does, the proximity of Venus wind speeds to the transport
threshold and the surface transport observation on Venera
14 may imply that it is widespread and frequent. It may
not be ubiquitous, however. An obvious prerequisite is theavailability of sand, dust, or gravel to be transported, andsuch material may be strongly supply-limited in many
places (e.g., Weitz et al. 1994); although ash streaks from anumber of volcanos can be recognized, the dominantsource of sediment may be ejecta from the ~ 1000 impact
craters on Venus. Topographic obstacles may play animportant role in funneling winds at a regional and local
scale. These caveats aside, while only a couple of sites with
Magellan-resolvable sand dunes are known (e.g., Lorenz

Lorenz Progress in Earth and Planetary Science (2018) 5:34

and Zimbelman 2014), wind streaks are found over all latitudes and longitudes.
Although we have little direct information on Venus’
near-surface winds, they are unlikely to be uniform—
some places will be windier than others. Notably, slope
winds due to diurnal heating will be strongest on
large, steep slopes at low latitudes, and an evaluation(Dobrovolskis 1993) suggested two daily peaks in speed
will occur, around 0600 and 1800 h local solar time (upslope and downslope respectively). Since sediment transport will be favored downhill, this predicted maximum inparticle motion, and thus, triboelectric charging will occur
in the evening, consistent with the apparent increase inobserved electrical activity then compared with earlymorning. It should be underscored that the high pressure
at Venus’ surface is a challenge to the prospect of discharges; however, in that, the breakdown field at these
conditions is very high. In this respect, volcanic ash clouds
may be more likely to yield discharges, since the charged
dust in this case is lofted to a higher altitude where thebreakdown field is lower.

Conclusions and recommendations for future
surveysThis review reaches much the same conclusion as many
before it, namely that more observational data are required to make a robust assessment about atmosphericelectrical activity on Venus. To date, the most compelling indications of some kind of persistent atmosphericelectrical activity at Venus appear to be the VLF detections by Veneras 11–14. The ELF emissions observed by
Pioneer Venus, and perhaps more robustly by Venus
Express, appear fully consistent with such electrical activity although they do have potential other explanations.
As summarized in Table 3, rates of activity seem broadly
coherent among the quite different datasets.
Of the optical observations, the ground-based observation by Hansell et al. (1995) appears the most significant
in terms of robustness and area and time observed, although the very small number of flashes is challenging
to interpret quantitatively. The Venera 9 observation has
a number of aspects that make the interpretation difficult and suggest a possible artifact. Circumstantially, thenondetection, or at least lack of reports of detectedflashes, in a number of other datasets (Pioneer Venus
star tracker, VEGA, Venus Express) indicates that flashes
are not a prominently observable feature of Venus, although in some cases (Pioneer Venus, VEGA), the area
and/or duration of the observation is small.
It is clear that a larger area-time product, desirablywith a low optical energy detection threshold, is required
to assert strong upper limits on, or to obtain secure statistics of, what is likely to be a strongly spatiotemporally variable phenomenon. The present state ofPage 21 of 25

the art of optical detections at Venus is somewhat similar to the handful of orbits with a high detection threshold (totaling 4 h), as shown in the lower panel of Fig. 11
for terrestrial data from the C/NOFS satellite—the interpretation of only a few flashes is challenging as any geographical or temporal dependence is not statisticallyrobust. It may be hoped that the Lightning and Airglow
Camera (LAC) on Akatsuki may be operated for sometime, yielding statistically useful numbers of detections.
The rate of optical detections for an instrument at Earthviewing from about 1000 km altitude, and thus viewing
about 1 million km2 (e.g., FORTE, Kirkland et al. 2001)
is of the order of 100–200 per hour. The eight initial
LAC observations reported by Takahashi et al. (2018)
cover a few million km2 for ~ 20 min, and thus, manydetections would have been expected if the optical
flashes comparable with terrestrial ones occurred at aterrestrial rate, given a total survey of 10~ 20 million
km2-h. On the other hand, perhaps detectable Venus
flashes may be as rare as terrestrial “superbolts” (~ 109 J
optical energy). Forty such flashes were seen (Turman
1977) in 2 months of long-distance observations, i.e.,
full-disk ~ 130 million km2, from the Vela satellites.
Thus, the rate of these events is ~ 2 × 10−4 per million
km2-h, and so, an instrument with observing
opportunities like LAC (~ 20 min in each 10-day orbit)
is unlikely to achieve a detection, even if in operation fora decade. The subtlety of planet-averaged lightning rates
not representing the highly clustered (i.e., “conditional”)
distribution of flashes (e.g., Fig. 4) should be noted—it is
not enough to stare with a very sensitive detector at only
one place if that place does not see a storm. Thus, there
remains high value in obtaining further LAC observations. The question of irregular spatio-temporal sampling of an irregular spatio-temporal phenomenon suchas lightning deserves further quantitative exploration, e.
g., by Monte-Carlo simulations that can include, e.g.,geographical, time of day, or other dependencies, but
this is beyond the scope of the present review.
It may be noted that a sufficiently long optical survey may
yield optical transient detections even if lightning never occurs on Venus; satellite optical observations (such as Vela,
designed to detect nuclear detonations) detect flashes oflarge meteoroids (bolides) entering the Earth’s atmosphere(Brown et al. 2002). The reported events with optical energies of the order of 4 × 109 J occur at a rate of ~ 100/year at
Earth, or about 2.5 × 10−5 per million km2-h, with lessenergetic events proportionately more often. Thus a 106 J
optical event might occur at ~ 0.1 per million km2-h, suchthat at least a few such events can be expected in someyears of LAC observation. Note, however, that the durationof bolide entries is long compared with a lightning flash; inthe Venus atmosphere, meteor “flares” are expected to last acouple of seconds (e.g., McAuliffe and Christou 2006), soLorenz Progress in Earth and Planetary Science (2018) 5:34

the triggering logic of an instrument like the LAC, or the
exposure time of a camera, cannot be optimal for searching
for lightning and for bolides simultaneously.
The superficially bland optical appearance of Venus andits interpretation as a uniform, unchanging atmosphere
has given way to a much more variable perspective revealed in the near-infrared (for example by the Akatsuki
2-micron camera—see Fig. 12); thus, it would not be surprising if the vertical motions required for charge separation were as localized as they are on Earth. Not only
might the diurnal distribution of lightning production beconcentrated (e.g., dusk-midnight) but there may be ageographical preference (e.g., at volcanoes or downwind oflarge mountainous provinces that may produce uplift ingravity waves). There may also be a meteorological influence on detection efficiency—e.g., lightning emissions
may (as on Earth) be more easily detected at the edges oflarge convective storms, rather than at their center where
the obscuring clouds may be thicker.
Above the ionosphere, surveys of whistler-mode signals
from orbit have already been comparatively extensive, yielding results that remain broadly consistent from 1 year to
the next, and between the Pioneer Venus and Venus Express datasets. While further observations, e.g., from a low
circular orbit, would have some value in refining geographical and local time distributions, they are unlikely by themselves to be any more persuasive to the broader community,
or informative on the generation mechanism. A combination of an ELF electrical/magnetic detection with anothertype (most obviously an optical sensor, but possibly gamma

Fig. 12 The deeper view into Venus’ clouds afforded by near-infraredwindows (this example from the Akatsuki 2-micron camera) shows
structures on a range of length scales—if lightning is associated withcloud convection as on Earth, it might be expected that lightning will
be similarly patchy (see also Fig. 4)

Page 22 of 25

ray, or even VHF radio) would be much more compelling(as performed on FORTE and C/NOFS at Earth—see, e.g.,
Fig. 5) and Suszcynsky et al. 2000. As noted by Russell(2011), a low circular polar orbit is desirable for radar
mapping, and a modest lightning instrument would be avaluable but relatively inexpensive augmentation to a radar
mapping mission. We may note that such combined opticaland RF surveys of lighting (e.g., Light et al. 2001) at Earthhave been performed on a rather small satellite platform(e.g., FORTE, 210 kg, flown in 1997).
Both the Pioneer Venus electric and the Venus Express
magnetic whistler-mode observations have been madeover ~ 10 years for each mission near periapsis (~tens ofminutes during each 24 h orbit, or a duty cycle of ~ 2%).
Thus, the total observing time is ~ 0.2 years. A radar mapper in a low circular orbit allowing continuous observation
would require at least one Venus day (~ 0.7 years) andprobably several to sweep through the full range of longitudes and relay the data. Thus, even allowing for lightningobservations only at night, piggybacking a lightning payload on a mapping mission will yield a factor of several improvements in observation duration for electromagnetic
observations, with better geographical diversity, comparedwith prior surveys. An optical monitor on a low orbiter,with a ~ 25% duty cycle for night-time only operation,
would yield a vastly longer observation time than has beenpossible to date (typically a few hours).
Below the ionosphere, a long-lived (months?) aerial
platform like a balloon would be useful in having amuch longer observation period than a surface probe,
which will be limited to a few hours. The Venera results
suggest that a simple instrument like Groza or the
Galileo LRD (an instrument which even with the 1980s
technology had a mass of 2.5 kg, a power demand of
3 W and an average data rate of only ~ 1 bit per second—Lanzerotti et al. 1992) would yield rich VLF datafrom an easily accommodated coil or loop antenna. A
modern instrument, operating on a platform near the
cloud tops (where temperatures and pressures are benign) could be implemented with a few hundred gramsof hardware—it is basically an AM radio, and even only
a week of operation would yield a dataset 100 times larger than a probe descent like Venera. Optical detection
on an airborne platform would likely have a much
shorter range (tens of kilometers) due to atmosphericabsorption and scattering but demands so little mass or
power that a flash detector should probably be included
anyway. A measure of the DC electric field would also
be useful to assess fair weather electricity (e.g., the character of the global electric circuit) and possible charging
mechanisms; the vertical tether on a balloon is an obvious platform on which to make a simple measurement.
The incremental value of further measurements on
short-lived landers or probes is not obvious, unlessLorenz Progress in Earth and Planetary Science (2018) 5:34

direction-finding capability (easily implemented, as insome terrestrial sensors, simply by having two coils andmeasuring the intensity ratio between them) is included.
The crude direction-finding implemented by the single coilon the Venera probes with modest directivity coupled withsomewhat uncertain spin rate information (Ksanfomality,
personal communication, 2017) could not disentangle spin
modulation of a finite-extent source from temporal variability in it, nor reliably isolate multiple source regions. A
two-axis sensor could resolve these ambiguities, making
even a single hour-long probe descent worthwhile, although a long-lived aerial platform would of course bepreferable. Desirably, technology developments may eventually permit long-duration operation, and/or multiple
platforms can be sent such that useful constraints on
temporal or spatial variability can be determined.
Although further spacecraft measurements at Venus
are sorely needed, as described above, it is likely that
useful insights from ground-based observations can still
be made. More sensitive optical surveys can be performed
with high-speed cameras that are now readily available,
and automated searches with sophisticated statistical tests
have been developed (e.g., a lightning search at Jupiter by
Luque et al. 2015). Additionally, lightning can be detectedby radio methods (e.g., Zarka et al. 2008; Konovalenko
et al. 2013) reported observations of Saturn lightning
using the large UTR-2 radiotelescope at 20–25 MHz(robust detections were made with fluxes of 100–700
Jansky). A positive simultaneous detection at Venus byboth optical and radio means would be powerful evidenceof lightning or related activity.

Endnote1
Note that this Taylor (Harry H., of Goddard Space
Flight Center) is not the same Taylor (William W., of
TRW, Inc) who authored the 1979 “discovery” paper.
Abbreviations
C/NOFS: Communications/Navigation Outage Forecasting System (satellite);
ELF: Extremely low frequency, a few Hz to a few hundred Hz; FORTE: Fast
On-orbit recording of Transient Events (satellite); kR: kiloRayleigh (unit of
optical flux); LRD: Lightning and Radio Detector (on Galileo probe);
OEFD: (Pioneer Venus) Orbiter Electric Field Detector; OGBD: (Pioneer Venus)
Orbiter Gamma-Ray Detector; TGF: Terrestrial gamma-ray flash; TLE: Transient
luminous event; VEx: Venus Express; VLF: Very low frequency, typically 3–
100 kHz
Acknowledgements
The author thanks Seth Hansell, Vladimir Krasnopolsky, Leonid Ksanfomality,
and Don Gurnett for a candid discussion of their observations. Theiracknowledgement here for responding to questions should not be taken as
their endorsement of any contentions in this paper. Bob Holzworth is
thanked for making C/NOFS data available and providing Fig. 4. Twoanonymous referees provided constructive comments.
Funding
RL acknowledges the support of NASA Venus Climate Orbiter Participating
Scientist grant NNX16AC78G.

Page 23 of 25

Author’s contributions
RL conceived and designed the study and wrote the paper. The author readand approved the final manuscript.
Author’s information
Ralph Lorenz is a planetary scientist on the Principal Professional Staff of theJohns Hopkins University Applied Physics Laboratory (APL). He is a member of
the Cassini RADAR team, a Co-I on the Huygens Surface Science Package, aCollaborator on the InSight Seismometer team, and a NASA Participating
Scientist on the JAXA Akatsuki mission. He is involved in formulating mission
and instrument concepts at Titan, Europa, Venus, and Mars. He holds five NASAGroup Achievement awards. He is the author of seven books, including “DuneWorlds,” “Lifting Titan’s Veil,” “Spinning Flight,” and “Space Systems Failures.”
Prior to joining APL in 2006, Lorenz was at the Lunar and Planetary Lab,
University of Arizona. He has a B.Eng. in Aerospace Systems Engineering fromthe University of Southampton in the UK and a Ph.D. in Physics in 1994 fromthe University of Kent at Canterbury and worked for the European Space
Agency on the design of the Huygens probe, for which he performed thelightning hazard assessment.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Received: 16 October 2017 Accepted: 4 April 2018

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