Blue Jets Observations &
Modeling
Gennady Milikh, University of Maryland,
College Park, MD, USA
Presented at the workshop on streamers, sprites, leaders,
lightning: from micro- to macroscales October 2007, Leiden
Discovery of Blue Jets
 Blue Jets or narrowly collimated beams of
blue light propagating upwards from the top
of thunderstorms were discovered during
the Sprites94 aircraft campaign by the
University of Alaska group.
 In their first paper Wescott, Sentman,
Osborne, Hampton, and Heavner [GRL,
1995] reported their findings:
Blue Jets
Discovery
BJ
•Beams of blue light
that propagate upward
from the tops of
thunderclouds at >=18
km.
• Narrowly collimated
with an apparent fan
out near the terminal
altitude (40-50 km).
•Velocity ~80-115 km/s.
•Intensity ~0.5 MR.
•Brightness decays
simultaneously along
the jet after 0.2- 0.3 s.
Wescott , Sentman, et al.,
1995
Sprite 94 Campaign
The color of Jets
•Main spectral bands are 1P of N2 (478-2531 nm), 2P of N2
(268-546 nm), and 1N of N2+ (286-587 nm).
•Volume emission rate is due to the electron excitation of
the air molecules and collisional quenching.
•The red-line emission is strongly quenched below 50 km,
thus Red/Blue ratio <<1
More jet observations
[Reunion island 03/97, from Wescott et all., 2001]
Blue Jet structure
[Wescott,et al., JGR, 2001]
•At the base of the jet the diameter
~400m.
•The diameter does not vary till ~22 km.
3km
•At 27 km it broadens to ~2 km, and is
~3 km at 35 km.
2km
•Eight smaller streamers with 50-100 m
diameter detected.
•Lifetime of the event ~0.1 s.
•Was not associated with any particular
CG lightning.
50-100m
•The total optical brightness reached 6.7
MR (0.5 MJ of optical energy).
0.4km
Blue Starters
(vertically challenged jets)
The starter extending upward to ~25 km
[Wescott, Sentman, Heavner et al., GRL, 1996]
Blue Starters
•Distinguished from Jets by
much lower terminal altitudes
~20-25 km.
•Apparent speed 27 to 150 km/s.
•Ionization ~3% (427.8 nm).
•Arise out of the anvil during a
quiet interval  no coincidence
with simultaneous CG flashes of
either polarity. Occur in the
same area as –CG flashes.
• Associated with hail and
updrafts (on a few occasions).
•Abrupt decrease in the
cumulative distribution of -CG
flashes for 3 s after the event.
Wescott et al., 1996; 2001
Gigantic Jets
15
Sep 2001, 0315 UT
•Wavelengths
350-890
Discovered by Pasko et al. [2002]
nm
•33-ms frames show two-trunk
tree with filamentary
branches.
•Fast growth of the left trunk
within 33 ms.
•Two <17-ms steps: (1) Left
trunk 2 branches up to 70 km
(2) Right trunk tree +”sprite”.
•A.Speed: 50 km/s 1--5, 160 km/s (5-6), 270 km/s 6--7
•Above the transition altitude of
~40 km resemble sprites.
•Termination at 70km  Edge of
the ionospheric conductivity?
•VLF (‘sferics’) polarity during
re-brightening 18&25 upward
negative breakdown ( –CI).
•No apparent association with
CGs.
•A.Speed: >1900 km/s 7--8.1
>2200 km/s 8.1--8.2
Event 1
More Gigantic
leading jet Jets
Event 4
Su et al., 2003
leading jet
trailing jet
•Red circle Thunderstorm
convective core with the top
at 16 km at 1431 UT.
•White lines: Range of the
line-of-sight to the GJ centre.
•GJ events: 14:0918, 14:1159,
14:1515, 14:2001, and 14:2054
UT
NB. 1-s uncertainty in the
•Wavelengths
nm
recorder 400-1000
clock
GMS 5 Sat. Infrared image
22 July 2002, 1431 UT
•Stages: Leading J, fully developed J (tree & carrot), and trailing J.
•Leading Jet: Emerging point 221-182-244 km (the top of the convective
core), duration 34 ms, speed 10001-12004 km/s.
•Fully developed Jet: Lifetime 171 - 1674 ms, a hybrid of BJ and sprite.
•Trailing Jet: Duration 2331 - 3672 ms, speed 261 - 1204 km/s, terminal
altitude 601 - 684 km.
•subsequent VLF  –CI breakdown with the charge moment change
1.7-2 kC·km (tree J1&5) and 1 kC·km (carrot J2&4). No CG strokes
associated with GJ were detected in the thunderstorm.
Summarizing characteristics of Jets/Starters
1. Emanate from the tops of the electrical core of thunderstorms
as faint blue cones of light that propagate upwards at speeds
of ~100 km/sec .
2. Resemble a toll tree with a thin trunk and the branches on the
top.
3. Termination altitude is ~50 km (jets), ~30 km (starters), ~7090 km (gigantic jets).
4. Are not associated with cloud-to-ground lightning discharges.
5. Occur much less frequently than sprites, although sampling
bias may play a role in this assessment since observations are
more difficult.
Continuation:
6. Brightness of jets exceeds 1 MR.
7. The rate of -CG flashes drops during 3 s after the event.
Why the gigantic jets appear in thunderstorms occur over the
ocean, not in that occur over the land?
Intermission
Models of Blue Jets
 The earlier models suggested that BJ’s are either
gigantic positive streamers [Pasko et al., 1996] or
negative streamers [Sukhorukov et al., 1996],
such model require enormous charge of a few
hundred C, and unable to explain the low
propagation velocity.
 A beam of runaway electrons [Russel-Dupre and
Gurevich, 1996] has the same problem.
 Recently Petrov and Petrova [1999] and Pasko
and George [2002] assumed that Jets are similar
to the streamer zone of a leader.
Leader-streamer structure of jets
 1. Apparently the leader tip is the source for most streamers
which form the upper part of a jet. Such leader is presented at
blue jet photos as a long “trunk” from which branches grow.
 2. The necessity of the leader’s existence in a jet is caused by
two reasons:
 2.1. At the altitude of about 18 km cold plasma decays in 10 s.
Such source cannot supply jet streamers with the current during
its lifetime of 0.3 s.
 2.2. In the absence of a leader, unrealistically high charges from
the thundercloud are required to sustain streamer’s field.
A Laboratory Leader
•In a leader channel the gas is heated
above 5,000K, thus maintaining its
conductivity as in an arc channel.
•The leader tip continuously emits a fan
of streamers at the rate of 109 1/s,
which forms the streamer zone, and the
current heats up the leader channel.
Space charge of the stopped streamers
covers the leader channel which
prevents its expansion and cooling.
The key problem is how a self-consistent E-field in the
streamer zone is formed.
Jets as a fractal tree [Pasko and
George, JGR, 2002]
•Jets are similar to the streamer zone of a
leader
• Starting from the point base the positive
streamers are branching, as described by
the Niemeyer’s algorithm [1989]
• The E-field is generated by the branches
and the cloud charge
• The scaling law is applied Es/N=const, Es is
from the laboratory experiments
• The model simulates the propagation of
branching streamer channel.
• It shows transitions from starters to jets
when the cloud charge increases
• It resembles blue jets in terms of their
altitude and conical structure.
• The model does not have the electron sink
due to recombination and attachment
• The charge is collected by hail, which is a
slow process. Similar problem of insufficient
current supply in conventional lightning was
resolved using concept of bi-leader [Kasemir,
1960].
Recently Tong et al., [2005] used a similar
model but for negative streamers and get
jets at 300 C.
Jet model by Raizer et al. [2007]
 A bi-leader forms in thundercloud.
The positive leader moves upward
forming the trunk of the observed
“tree” while its streamer zone forms
the branches.
ES required to sustain streamer
growth ~ N. Thus long streamers
grow preferentially upward,
producing a narrow cone.

 Due to the transfer of
thundercloud potential by the
leader, the Jet streamers can be
sustained by a moderate cloud
charge.
Numerical model of streamers
[Raizer et al., 2006, 2007]
The model describes:
 The motion of the streamer tip.
 The potential of the streamer tip versus its radius,
electron density, and current.
 Electrical processes in the streamer channel
including attachment and recombination.
Output of the model
 Proven that the similarity law E/N=const
holds in the atmosphere at h>18 km.
 Streamer propagation in the exponential
atmosphere was described.
• Despite a progress in understanding of the
physical mechanisms leading to Blue Jet
formation and propagation some outstanding
problems remain unresolved such as how a selfconsistent E-field in the streamer zone is formed.
• Further
progress depends on the
development of leader / streamer models
based on the laboratory experiments.
Atmospheric effects due to Blue Jets
• Blue jets can produce perturbations of the ozone layer [Mishin, 1997].
• Can effect the atmospheric conductivity [Sukhorukov & Stubbe, 1998].
• Gigantic jets could produce a persistent ionization which recovers over
minutes. Such recovery signatures may be observable in subionospheric
VLF data [Lehtinen & Inan, 2007].
References
Kasemir, H.W. (1960), J. Geophys. Res., 65, 1873-1878.
Niemeyer, L., L. Ullrich and N. Wiegart (1989), IEEE Trans. Electr. Insul., 24, 309324.
Pasko, V.P., M.A. Stanley, J.D. Mathews, U.S. Inan, and T.G. Woods (2002),
Nature, 416, 152-154.
Pasko, V.P. and J.J. George (2002), J. Geophys. Res. 107(A12), 1458,
doi:10.1029/2002JA009473.
Pasko, V.P., U.S. Inan and T.F. Bell (1996), Geophys. Res. Lett., 23, 301-304.
Petrov, N.I., and G.N. Petrova (1999), Tech. Phys., 44, 472-475.
Raizer, Y.P., G.M. Milikh, M.N. Shneider and S.V. Novakovski (1998), J. Phys. D.
Appl. Phys. 31, 3255-3264.
Raizer, Y.P., G.M. Milikh, and M.N. Shneider (2007), J. Atmos. & Solar-Terr Phys.,
69, 925-938.
Roussel-Dupre, R. and A.V. Gurevich (1996), J. Geophys. Res., 101, 2297-2311.
Su, H.T., R.R. Hsu, A.B. Chen, et al. (2003), Nature, 423.
Sukhorukov, A.I., E.V. Mishin, P. Stubbe, and M.J. Rycroft (1996), Geophys. Res.
Lett., 23, 1625-1628.
Tong, L., K. Nanbu, and H. Fukunishi (2005), Earth Planets Space, 57, 613-617.
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