Blue Jets Observations & Blue Jets Observations & ModelingModeling

Gennady Milikh, University of Maryland, Gennady Milikh, University of Maryland, College Park, MD, USACollege Park, MD, USA

Presented at the workshop on streamers, sprites, leaders, Presented at the workshop on streamers, sprites, leaders, lightning: from micro- to macroscales October 2007, Leidenlightning: from micro- to macroscales October 2007, Leiden

Discovery of Blue Jets Discovery of Blue Jets

Blue Jets or narrowly collimated beams of Blue Jets or narrowly collimated beams of blue light propagating upwards from the top blue light propagating upwards from the top of thunderstorms were discovered during of thunderstorms were discovered during the Sprites94 aircraft campaign by the the Sprites94 aircraft campaign by the University of Alaska group.University of Alaska group.

In their first paper Wescott, Sentman, In their first paper Wescott, Sentman, Osborne, Hampton, and Heavner [GRL, Osborne, Hampton, and Heavner [GRL, 1995] reported their findings:1995] reported their findings:

Blue Jets Discovery

•Beams of blue light that propagate upward from the tops of thunderclouds at >=18 km.

• Narrowly collimated with an apparent fan apparent fan outout 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., Wescott , Sentman, et al., 19951995

Sprite 94 CampaignSprite 94 Campaign

BJ

The color of Jets

•Main spectral bands are 1P of N2 (478-2531 nm), 2P of N2 (268-546268-546 nm), and 1N of N2

+ (286-587 286-587 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, The red-line emission is strongly quenched below 50 km, thusthus Red/Blue ratio <<1<<1

More jet observationsMore 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.

•At 27 km it broadens to ~2 km, and is ~3 km at 35 km.

•Eight smaller streamers with 50-100 m diameter detected.

•Lifetime of the event ~0.1 s.

•Was not associated with any particular CG lightning.

•The total optical brightness reached 6.7 MR (0.5 MJ of optical energy).

0.4km

2km

3km

50-100m

Blue Starters Blue Starters ((vertically challenged jetsvertically challenged jets))

The starter extending upward to ~25 km [Wescott, Sentman, Heavner et al., GRL, 1996]

Blue Starters Wescott et al., 1996; 2001

•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.

Gigantic Jets

•AA.Speed: 50 km/s 1--5, 160 km/s (5-6), 270 km/s 6--7

Discovered by Pasko et al. [2002]

15 Sep 2001, 0315 UT

•AA.Speed: >1900 km/s 7--8.1 >2200 km/s 8.1--8.2

•Two <17-ms steps: (1) Left trunk 2 branches up to 70 km (2) Right trunk tree +”sprite”.

•Wavelengths 350-890 nm

•33-ms frames show two-trunk tree with filamentary branches.

•Fast growth of the left trunk within 33 ms.

•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 negativenegative breakdown ( –CI).

•No apparent association with CGs.

22 July 2002, 1431 UT

GM

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Sat

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More Gigantic Jets

•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.

Su et al., 2003

•Red circleRed circle Thunderstorm convective core with the top at 16 km at 1431 UT.

•White linesWhite 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

•Wavelengths 400-1000 nm

Event 1

trailing jet

leading jet

Event 4

leading jet

•subsequent VVLF –CI breakdown with the charge moment change 1.7-2 kC·km (tree J1&5) and 1 kC·km (carrot J2&4). Noo CG strokes associated with GJ were detected in the thunderstorm.

NB. 1-s uncertainty in the recorder clock

Summarizing characteristics of Jets/Starters1. 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), ~70-90 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.

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 jetsLeader-streamer structure of jets 11. . Apparently the leader tip is the source for most streamers Apparently the leader tip is the source for most streamers

which form the upper part of a jet. Such leader is presented at which form the upper part of a jet. Such leader is presented at blue jet photos as a long “trunk” from which branches grow. 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 2. The necessity of the leader’s existence in a jet is caused by

two reasons:two reasons:

2.1. At the altitude of about 18 km cold plasma decays in 10 2.1. At the altitude of about 18 km cold plasma decays in 10 s. s. Such source cannot supply jet streamers with the current during Such source cannot supply jet streamers with the current during its lifetime of 0.3 s.its lifetime of 0.3 s.

2.2. In the absence of a leader, unrealistically high charges from 2.2. In the absence of a leader, unrealistically high charges from the thundercloud are required to sustain streamer’s field. the thundercloud are required to sustain streamer’s field.

A Laboratory LeaderA 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]Jet model by Raizer et al. [2007] A bi-leader forms in thundercloud. The A bi-leader forms in thundercloud. The

positive leader moves upward forming the positive leader moves upward forming the trunk of the observed “tree” while its trunk of the observed “tree” while its streamer zone forms the branches.streamer zone forms the branches.

ES required to sustain streamer ES required to sustain streamer growth ~ N. Thus long streamersgrowth ~ N. Thus long streamers grow preferentially upward, grow preferentially upward, producing a narrow cone. producing a narrow cone.

Due to the transfer of Due to the transfer of thundercloud potential by the thundercloud potential by the leader, the Jet streamers can be leader, the Jet streamers can be sustained by a moderate cloud sustained by a moderate cloud charge. charge.

Numerical model of streamersNumerical model of streamers[Raizer et al., 2006, 2007][Raizer et al., 2006, 2007]

The model describes:The model describes:

The motion of the streamer tip. The motion of the streamer tip. The potential of the streamer tip versus its radius, The potential of the streamer tip versus its radius,

electron density, and current. electron density, and current. Electrical processes in the streamer channelElectrical processes in the streamer channel

including attachment and recombination. including attachment and recombination.

Output of the modelOutput of the model

Proven that the similarity law E/N=const Proven that the similarity law E/N=const holds in the atmosphere at h>18 km.holds in the atmosphere at h>18 km.

Streamer propagation in the exponential Streamer propagation in the exponential atmosphere was described.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 self-consistent 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].

ReferencesKasemir, H.W. (1960), J. Geophys. Res., 65, 1873-1878. Niemeyer, L., L. Ullrich and N. Wiegart (1989), IEEE Trans. Electr. Insul., 24, 309-324.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.Wescott, E.M., D. Sentman, D. Osborne, D. Hampton, and M. Heavner (1995), Geophys. Res. Lett., 22, 1209-1212.Wescott, E.M., D.D. Sentman, et all., (1998), J. Atmos. & Solar-Terr Phys., 60, 713-724. Wescott, E.M., D.D. Sentman, et all., (2001), J. Geophys. Res., 106, 21,549-21,554.