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 Beams of blue light that propagate upward from the tops of thunderclouds at >=18 km. apparent fan out Narrowly collimated with an apparent fan out near the terminal altitude (40-50 km). Velocity ~ km/s. Intensity ~0.5 MR. Brightness decays simultaneously along the jet after s. Wescott, Sentman, et al., 1995 Sprite 94 Campaign BJ

The color of Jets Main spectral bands are 1P of N 2 ( nm), 2P of N 2 ( nm), and 1N of N 2 + ( 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 <<1The red-line emission is strongly quenched below 50 km, thus Red/Blue ratio <<1

More jet observations [ 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. At 27 km it broadens to ~2 km, and is ~3 km at 35 km. Eight smaller streamers with 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 m

Blue Starters ( vertically 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 >2200 km/s Two <17-ms steps: (1) Left trunk  2 branches up to 70 km (2) Right trunk  tree +”sprite”. Wavelengths 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? negativeVLF (‘sferics’) polarity during re-brightening 18&25  upward negative breakdown ( –CI). No apparent association with CGs.

22 July 2002, 1431 UT GMS 5 Sat. Infrared image More Gigantic Jets Stages: Leading J, fully developed J (tree & carrot), and trailing J. Leading Jet: Emerging point km (the top of the convective core), duration 34 ms, speed km/s. Fully developed Jet: Lifetime ms, a hybrid of BJ and sprite. Trailing Jet: Duration ms, speed km/s, terminal altitude 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 nm Event 1 trailing jet leading jet Event 4 leading jet V osubsequent VLF  –CI breakdown with the charge moment change kC·km (tree J 1&5 ) and 1 kC·km (carrot J 2&4 ). N o CG strokes associated with GJ were detected in the thunderstorm. NB. 1-s uncertainty in the recorder clock

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), ~ 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 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 /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 E s /N=const, E s 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 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 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].

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