HH0111 what causes lightning
What causes lightning?
Hsiu-Hau Lin
National Tsing Hua University
1876: Franklin and his kite
Franklin demonstrated that lightning is just the same
electric phenomenon — as observed on the ground.
But, what causes the lightning? Hum, interesting…
1992: Gurevich and his theory
called runaway electrons.
Note that at its minimum, the
friction force still does not vanish.
The finite value of Fmin is determined
by the energy lost by the moving electron as it ionizes molecules along its
path. In the absence of an electric
field, a 1-MeV electron traversing
Earth’s atmosphere would lose all its
energy to ionization within a few meters. The electron becomes a runaway because of the electric field, and
even then only where E > Ec.
The phenomenon of RB was predicted in 1992 by one of us (Gurevich), together with Gennady Milikh
and Robert Roussel-Dupre.9 The
basic physical process is the generation of new fast electrons from the
runaway-particle ionization of neutral molecules. Although the majority
of newborn free electrons have low
energies, some will have rather high
energy, e > ec. Those will also be accelerated by the field, become runaway electrons, and may in turn generate more free electrons with e > ec.
As a result, an exponentially growing
runaway avalanche can occur.
Along with the new runaways, a
very large number of slow electrons
are generated, which ultimately
leads to the electrical breakdown of
matter—RB. The full relativistic theory of RB was developed10 by groups
at the Lebedev Physical Institute,
Los Alamos National Laboratory,
Stanford University, and the Sarov
Institute of Physics and Engineering. A full review of the
theory is given in the final paper of reference 10.
Recall that in air at atmospheric pressure, the threshold field for conventional breakdown is about 2 MV/m. By
contrast, the critical field Ec in the same conditions is only
Cosmic rays causes
the runaway breakdown
and trigger the lightning
in the storm clouds.
E > Ec
Figure 1. The combined discharge arising from
runaway breakdown triggered by a cosmic-ray
extensive atmospheric shower (EAS) is shown
schematically during a thunderstorm at the
Tien-Shang Mountain Scientific Station in
Kazakhstan with its gamma-ray detectors and
Y-shaped radio antennas. The discharge occurs
where the cloud’s electric field exceeds a
critical value Ec and produces radio bursts as
well as gamma and other emissions.
with molecules in the atmosphere generates an extensive
atmospheric shower (EAS) that consists of a large number
of different elementary particles and fragments of nuclei.13
For RB, the secondary electrons—arising from the mutual
transformations of electrons, positrons, and gamma rays
Physics Today (May, 2005) 37-43
–500
–250
0
250
Time (microseconds)
500
750
C
curring background
rate, such as
from radioic particle
signal
begins
2
active washout160
(6) or"s
variations
in the
cosmic
before
the
return
0
ray rate from meteorological effects (7).
stroke,
and
the
sawtooth
Based on background measurements made by
–2
that
occurred
the instrument,pattern
the odds that the
event seen
in
–4
Fig. 3 is an accidental
coincidence
with aindiduring the
rise phase
naturally occurring
event, distinct
such
–6
catesbackground
that several
as an atmospheric cosmic ray, are calculated
–8
energetic
particle events
to be much less
than 1 in 3750.
–10
occurredsignal
before
the reThe large saturating
with a com–750
–500
plicated rise turn
beforestroke.
the return
(B) stroke,
Current
shown in Fig.associated
3, is typical ofwith
the 21
enerthe
lightgetic radiation events observed on 20 and
stroke
mea25 July 2002. ning
Of thesereturn
events, only
two had
at other
the hand,
launch
tower. The current provides the
peaks below 1sured
V. On the
for the
10 events observed
13 Septemberelectronics.
2002,
data on
acquisition
(C) Vertical electric fiel
the majority electric
were similar
the event
fieldtobefore
zero time is a result of a downw
shown in Fig. 2, with peaks below 1 V. The
after
zero is the result of an upward-pro
smaller signalThe
sizes field
seen on
13 September
channel.
may be explained
by the distance between
the instrument and the launcher; in September, the instrument was twice as far from
Fig.
4. inStill
the launcher as
it was
July. photograph of the second
All the energetic
radiation events,
regardlessflash on 20 July
triggered
lightning
of size, have signals
exponential
decays was for 6 s and
2002.withThe
exposure
that are completely consistent with the detector
contains
strokes
response. Furthermore,
unlikeall
gamma
rays fromfor this flash (the
narrow
channels). The widradioactive sources
and and
unliketwisting
cosmic ray
events, all the er
energetic
radiation
eventsis obcentral
column
from the burning of
served in association
with lightning
havethat
com- trailed from the
the
copper
wire
plicated rising portions, indicating that multiple
rocket;
energetic particles
(x-rays, that
gamma burning
rays, or en- was due to the
initial
stagethecurrent
ergetic electrons)
are producing
signals. Thethat precedes the
attenuation of x-rays
in the
0.32-cm
aluminum
leader
and
return
stroke sequences. The
window on the top of the instrument increases
structure at the bottom of the picture is
rapidly below about 30 keV, with 30-keV x-rays
thetimes
toptheir
of the
launch
attenuated to 0.42
original
number tower. The return
Electric field (kV/
–10
–750
2003: Dwyer and his rockets
. The current provides the external trigger (threshold $ –7.1 kA) for the
s. (C) Vertical electric field measured 260 m from the launch tower. The
me is a result of a downward-propagating negatively charged dart leader.
e result of an upward-propagating return stroke discharging the leader
of the second
on 20 July
for 6 s and
his flash (the
nels). The widthe burning of
led from the
s due to the
precedes the
equences. The
the picture is
er. The return
d attached to
Science 299 (2003) 694-697
31 JANUARY 2003 VOL 299 SCIENCE www.sciencemag.org
strokes terminate on a rod attached to
the launch tubes.
lightning
http://gpb.pbslearningmedia.org/resource/oer09.sci.phys.maf.newlightng/a-new-theory-of-lightning/
Do cosmic rays cause light
2008: someone objects!
ASK THE EXPERTS
Joseph R. Dwyer, a professor of physics and space sciences at the
Florida Institute of Technology, has wondered the same thing:
■■■■
www.SciAm.com/asktheexperts
Do cosmic rays cause lightning?
main
ductiv
a few
mend
a larg
away
— B. Whiteside, Woodridge, Ill.
Joseph R. Dwyer, a professor of physics and space sciences at the
Florida Institute of Technology, has wondered the same thing:
main stumbling block arises because lightning must form a conductive channel to propagate. This channel, extremely hot and just
a few centimeters in width, acts like a metal wire, allowing treAlthough some researchers have proposed that cosmic rays
mendous electric currents to flow through. It remains unclear how
instigate
lightning, others,
a large, diffuse discharge produced
by cosmic-ray-induced
run- including me, have voiced doubts
away breakdown would result
in such
a narrow,
hotAt
channel.
about
this
theory.
present, the debate remains unsettled.
Joseph R. Dwyer, a professor of physics and space sciences at the Florida
Decades of measurements inside thunderstorms have failed to
Institute of Technology, has wondered the same
thing:
find electric
fields large enough to spontaneously spark lightning.
A mechanism
proposed in 1992 by physicist Alex V. Gurevich of
How do three
tiny bones
the Lebedev
amplify sound
into Physical Institute in Moscow and his collaborators
suggests
that the movement of large showers of energetic particles
— P. Madsen, Brooklyn, N.Y.
the
inner
ear?
researchers have proposed that cosmic rays
Although some
produced by high-energy cosmic rays — which originate from exDouglas E. Vetter, assistantploding
professor stars
of neuroscience
the Tufts lightning’s massive energy discharge.
— mightattrigger
instigate lightning, others,
including
me,
have
voiced doubts
University Sackler School of Graduate Biomedical Sciences, replies:
For Gurevich’s mechanism to work, many charged particles must
pass through the storm at once. Because cosmic-ray showers
about this theory. At present, the debate
remains unsettled.
alone do not produce enough such particles, Gurevich postulated
Although some researchers have proposed that cosmic rays
instigate lightning, others, including me, have voiced doubts
about this theory. At present, the debate remains unsettled.
Decades of measurements inside thunderstorms have failed to
find electric fields large enough to spontaneously spark lightning.
A mechanism proposed in 1992 by physicist Alex V. Gurevich of
the Lebedev Physical Institute in Moscow and his collaborators
suggests that the movement of large showers of energetic particles
produced by high-energy cosmic rays — which originate from exploding stars— might trigger lightning’s massive energy discharge.
The hammer, anvil and stirrup bones of the middle ear— also
known as the malleus, incus and stapes, respectively, and as ossicles, collectively— are arranged in a lever system. Their leveraging capabilities, combined
withathe
concentration of vibration
that
thunderstorm
gives the cosmic-ray shower a boost by inenergies from the larger eardrum to the much smaller stirrup, efthe
number of energetic particles through a process
ficiently transmit the forcescreasing
that allow us
to hear.
runaway
breakdown.
The middle-ear ossicles called
lie between
the eardrum
and the co-
Doug
Unive
Th
know
sicles
aging
energ
ficien
Th
chlea
to the
syste
Witho
would
face m
summary
Our understanding about nature is dynamical — it
changes from time to time.
Scientists’ answers to the same question may vary
with time — we do learn new stuffs.
The most reliable source for valuable information is
your own brain — be nice to it :)
And…we still do not fully understand what in the
world causes the lightning!