Indian Journal of Radio & Space Physics Vol. 34, December 2005, pp. 387-398

Sprites and elves-associated sferics produced by quasi-electrostatic · thundercloud fields-A review

T Datta 1, A Bhattacharya2

, R Bhattacharya3, R Das, S De & A B Bhattacharya

Department of Physics, Kalyani University, Kalyani 741 235, West Bengal, India 1Department of Physics, Serampore College, Serampore 712 201 (WB), India

2Panighata U.D.M High School, Nadia 741 181 (WB), India

' Department of Environmental science, Kalyani University, Kalyani 741 235 (WB), India

Received 14 May 2004; revised 13 December 2004; accepted 15 March 2005

Investigations on sprites and elves glow and a comparison of the observational work obtained at various locations by different workers are reported in this review. As the characteristic of these extraordinary phenomena is not yet well understood, an interpretation of the results is attempted from both experimental and theoretical points of view. The paper critically considers the latitudinal variations of lightning characteristics, as lightning flashes have a reported good relationship with the concerned unusual sprites and elves glow. It also discusses temporal variations of electron density, wh ich is highly relevant as the extraordinary glow experienced between the cloud tops and the ionosphere. Subsequently, the sprites and elves associated sferics data in conjunction with e lectric and magnetic fields have been studied and the role o.f quasi-electrostatic thundercloud fields have been critically examined to focus this mysterious mesospheric glow.

Keywords: Sprites, Elves, Thunderclouds, Sferics PACS No.: 92.60.Nv; 92.60.Pw

1 Introduction Sprites and elves are two classes of phenomena

which have been observed above some strong lightning flashes from especially energetic thundercloudsl.2. To understand the physical processes of the phenomena, Cho and Rycroft3

developed both a quasi-electrostatic code and an electromagnetic code to model the temporally and spatially varying electric and magnetic fields in the atmosphere above the cloud and into the ionosphere following a large positive cloud-to-ground lightning di scharge. Scienti sts4

·6 at different parts of the world

have performed similar or related studies developing new instruments to make new measurements and to test theoretical ideas.

Sprites in the mesosphere have been found to be associated with li ghtning ground flashes of positive polarit/. The relationship between positive electrical charge in the troposphere and a physical process in. the mesosphere include sprite production by electrical heating8

, sprite production by electron runawa/, runaway electron beams 10

"11

, high altitude gamma rays 12

, and infrared glow from C02 emission 13.

Further, Jarzembski and Srivastava 14 have examined the interaction of lightning and sprites with a laboratory model. These studies are based on the

assumption that ordinary thunderclouds characterised by positive dipoles are responsible for the physical processes at higher altitudes. The observations by different groups of scientists 15

•21 described that sprites

generally have a vertical brightness structure and a diameter of 5 km or greater. The brightest feature is usually at about 70 km altitude. The present paper provides an overview of current understanding of sprites and elves associated with thunderclouds and gaps in the field.

2 Background of early observations of sprites and elves

Although there were many eyewitness accounts of upper atmospheric optical flashes called sprites and elves from ground observers and pilots, these firidings did not inspire a systematic search for hard evidence of such phenomena. Sprite was first accidentally documented on ground based videotape recordings in 1989. Video observations from the space shuttle acquired from 1989-1991 provided 17 additional examples to confirm the existence of the sprite phenomenon. Successful video observations from a mountain by Lyons 18

, in 1993, and nighttime aircraft video observations by Sentman and Wescott 16 in the same year established the basic science of the sprite

388 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

phenomenon by acquiring and analyzing data based on hundreds of new events.

The 1994 Sprites campaign and the video entitled "Red Sprites and Blue Jets" popularized the name sprite and provided a vocabulary of terms to describe the visual attributes. Prior to these video observations, inyestigators used a variety of vague descriptive words to describe the individual events. Also during the 1994 campaign, Sentman and Wescott19 obtained the first quantitative measurements of jet and provided the name 'blue jets'. A third phenomenon was discovered in video from the STS-41 mission in 1990 in the lower ionosphere directly above an active thunderstorm. It consisted of a large horizontal brightening several hundred kilometers across at the altitude of the airglow layer. In 1995, Lyons and associates22 confirmed the existence of this type of very brief brightening that they named Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources (ELVES). The partial history outlines the unsophisticated activities using space shuttle videotapes and the dissemination of the results by video presentations during the early phases of sprite research.

3 Latitudinal variations of lightning characteristics Latitudinal variations of lightning characteristics

h b . . d b h 23-28 Whil ave een mvestigate y many aut ors . e some authors have found that the mean number of strokes per flash24 and the intervals between strokes26

depend on latitude, others have found no significant latitude dependence23

'25

. Thomson27 reviewed the available data on these parameters and tested the hypothesis and revealed that the data do not show systematic latitude dependence. Orville28 investigated the possibility that the peak-current of the first return stroke of negative lightning flashes depends on latitude. Using a network of magnetic direction

finders from 25°N to 45°N, he found that the peak­current increased by almost a factor of two. Orville28

suggested that the peak-current might vary as a function of latitude due to the increasing volume of cumulonimbus clouds and the longer lightning channels at lower latitudes.

Presented in Table 1 are the comparative results29-35

of the median peak-current /median values by assuming a log-normal distribution for the first strokes and subsequent strokes of negative downward lightning flashes obtained in Brazie5

, compared to similar results obtained by other authors in Itall9

, South Africa30 and Switzerland31

•32

. In the case of first strokes only, data above 20 kA were considered, as there are some indications that below this value, the first stroke peak-current distribution tends to have a different behaviour33

• The comparison obtained at different latitudes in Table 1 is in good agreement with the previous results obtained by Petersen and Rutledge34 and Pinto et al. 35 using lightning location systems, in that the negative first stroke peak-current tends to increase from 45°N to 29°S, and then remains approximately constant below 29°S.

In this context, it should be pointed out that we have reviewed and concluded here the latitudinal variations of lightning characteristics on the basis of available limited observations. For a better understanding, we definitely need more observed data over wide locations.

4 Temporal variation of electron density There is a marked increase of electron density ne

near 70 km altitude. The temporal evolution on the bottom of the ionosphere36 is shown in Fig. 1. The figure reveals that both vertically above the thundercloud (left side) and at a radial distance of 20 km (right side), there is a marked, rapid (- J.l.S time scale) increase in the electron density at 70 km

Table 1-A comparative study of the direct measurements of the Median Peak-Current of negative downward lightning flashes

Investigators Reported Location Latitude Height above Tower height, /median ' first /median '

in year sea level, m m stroke, kA subsequent stroke, kA

Berger32 1967 Lugano 46°N 915 70 30 ±2 (101) 12 (135) (Switzerland)

Garbagnati & 1982 Varese 45°N 993 48 33±2(42) 18 (33) Lo Piparo29 (Italy)

Geldenhys 1989 Durban 29°S 80 60 42 (29) 9 (?) et al.30 (South Africa)

Pinto Jr. 1997 BeloHorizonte 20°S 1430 60 41± 2 (27) 16 (51) et al. 35 (Brazil)

DATI A et al. : SPRITES, ELVES, SFERICS & THUNDERCLOUDS 389

altitude (dashed line) and 80 km (solid line). This feature diminishes a little in a fraction of a ms, but thereafter remains almost constant, for many ms. The enhanced electron density near 70 km altitude may

t last for a time- [<XerrectiveX (ne)maximumr'= 3xl03 s -1 h, where <lerrective is the effective recombination coefficient for electrons and (ne)maximum is the maximum electron density. When a complex chemistry consideration is done it may increase <lerrective and hence reduce the lifetime of the pimple, considerably. During this time, it behaves like significant perturbation to VLF radio signals propagating in the Earth-ionosphere waveguide3

6-38

.

5 Observations and Results In this section, we have attempted to focus some

important properties of sprites and elves as reported by various workers at different parts of the globe and thereby we have made a comparison of the properties of sprites related thundercloud with ordinary · thundercloud.

"" I

E .;

c:

"" I

E .;

c:

2 X 109

1 X 109

~

0

6 X 108

4x 108

2 X 108

-r= 0, z =80 km - r= 0, z =80km

• f'-----·-------------I I I • I • • ' ,...._;_ I I • I

' ' I ,

I r"-

Q=200C POSITIVECG

"t = 25 j.lS

-r=20, z = 80 km - r = 20, z = 80 km

r\ I \

f ' I ', I ' .. .... .... ----1 # I I I

0=200C POSITIVECG

t = 25 j.lS

0=-~J~--~'~~~~~~~ 0 X 10° 5 X 10-4 1 X 10-3 1.5 X 10-3 2 X 10-

3

t, s

Fig. !-Temporal vanauon of electron density36 at varying attitudes (Z), both above the center of the thundercloud (r = 0) and at a radial distance of 20 km

Photographs of some typical unusual flashes as reported by pilots and others39

•40

, both in stratosphere and mesosphere, much above the tropospheric thunderclouds are shown in Fig. 2. To the eye, they resemble material ejected from a very high explosive source. The flashes last only a few thousandths of a second and extend up to about 90 km height, i.e. at the bottom of the ionosphere. To capture the position and altitude of the flashes, special low-light-level cameras were used in aircraft41 and were recorded on colour video.

High altitude blood red flashes and bursts called sprites were photographed as shown in Fig. 3. A large number of ghost-like intense optical flashes was reported during electrical storms in the mesosphere and stratosphere. The satellite launched by the scientists to test nuclear blasts detected radio flashes which were, in general, 10,000 times more intense than the radio signals normally generated by lightning.

It is by now well reported that the phenomena of Sprites and EL YES usually occurred41 in the stratiform regions of mesoscale convective systems. The ring-like elves in most of the cases is centered on the vertical channel to ground, whereas the sprite lies above horizontally extensive, so-called spider lightning stratiform cloud. . The red emission is prevalent in the upper body' of the sprite and blue

Fig. 2-Photograph of some typical unusual fl ashes39

390 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

emission in the lower tendrils. Elves are shaped quite differently from sprites and were identified as brief brightening of the airglow layer in space shuttle imagery. The photograph shown in Fig. 4 reveals doughnut shape characteristic of elves42

. The vertical return-stroke channel of lightning gives rise to an electric field that exibits a null directly over the channel and a maximum field that is azimuthally symmetric about the channel axis and shaped like a doughnut. The most elemental, and likely smallest, sprites are single vertical columns named C-sprites43

('columniform' sprites). The C-sprites exhibit upward branching towards the ionosphere and are named carrots. Large carrot shaped C-sprites with upper end branching44 are reported to extend to a maximum. vertical extent exceeding 60 km, three times more than the largest thunderstorms.

Fig. 3-Photograph of some typical blood red sprites40

Fig . 4-Photograph of EL YES with doughnut shape42

Very large sprites with diffuse tops and lower tendrils45 extending down to altitudes of 30-40 km have dubbed angles , jellyfish [Fig. 5(a)], while large collections of C-sprites resemble fireworks46

[Fig. 5(b)]. C-sprites· are caused by small charge moments and from extremely low frequency (ELF) measurement it is revealed that within the body of the sprites, the electric current is of kiloampere order. These currents are consistent with the cm1sequences of dielectric breakdown in air and qualitatively resemble lightning discharge in the form of fireworks in the atmosphere. A subset of the sprites with tendrils is another category reported by Lyons47

, which are most energetic and often one of the largest. The

' .

. 0

., c:\~Qc~ . .. 8.''' . ;.. I . ,~

<'• -~--

Fig. 5-Photograph of (a) very large sprites looking like jellyfish45

, (b) large collections of C-sprites46

DA IT A et al.: SPRITES, EL YES, SFERICS & THUNDERCLOUDS 391

Fig. 6-Photograph of dancing sprites42

tendrils are extending down to altitudes of about 30 km with vertical extension above 60 km.

Sprites are rather exceptional electromagnetic events. The flashes sometimes have an unusual appearance of dazzling arrays42

, which seem to dance for milliseconds, sufficiently above the cloud top (Fig. 6). A diversity of sprites is of garden variet/7

,

whose luminous structure occurs in most cases at more than five storm heights above the ground. If these structures are electrical discharges caused by lightning, why do they occur so far away from the lightning is a question that automatically comes . Very large sprites with downwardly branched tendrils42 are reported sometimes (Fig. 7) . Like botanical trees , lightning and sprites are both double-ended structures that extend bidirectionally, with one positive and one negative ends. The positive end of the sprite develops first, to be followed by negative extension. A marked distinction between lightning and sprites is that lightning often initiates in the strongest field and extends into weaker fields , whereas sprites initiate in the very weak field above the thunderstorm and then extend downward into the stronger field. The lifetimes of lightning and sprites are mainly determined by the speed at which a virgin channel extends. In the lower atmosphere, virgin lightning channels extend 10 km in 100 ms and propagate at a typical speed of 100 km s-1

, whereas sprites extend 30 km vertically and propagate at nearly one-tenth the speed of light in one ms. Sprites last longer (tens to hundreds ms) but at greatly reduced brightness .

Figure 8(a) shows the reported electric and magnetic fields of a typical sprite-associated sferics along with the electric field power spectra48

. Below

Fi g. 7- Photograph of very large sprite with downward-branched tendrils42

1 kHz sprite related sferic is clearly noticed in the power spectral density variation. The electric and magnetic field time domain data and the electric fi eld power spectrum in case of elve-related sferic was also examined by Marshall et al. 48 [Fig. 8(b)]. The inversion of the magnetic field data with respect to the electric field data is due to the arbitrary orientation of the magnetic field sensor. All of the recorded sferics associated with high altitude optical events exhibit an average downward defles;tion, indicating positive CG lightning. The observational setup consisted of a vertical electric field antenna and a horizontal (north-

392 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

~ c.:i -' w u:: ~ (.) w -'

(a) 24 July 1995 0651 :38 hrs UTC

w -0.5 ~-~~-~::---~:----::':::------,J 0 20 40 60 80 100

TIME. ms

"' o. 5 r------,,...--------~---. :g ::> .D

"' c.:i -' w u:: :i CJ

0~--~·----------~--~ t

~ - o. 5 ....._ _ __. __ --:-::----;';:---;:'::---~ 0 20 40 60 80 100

TIME, ms

ELECT. FIELD- POWER SPECTRAL DENSITY

~ -10~ ;. - . . . :--J_ . ~ -20~· :_ _ - :-- -.. 1 ·. 1 ~ -30 . . 'l l . ... j .. . . . . ·'

fu -40 ... • .. ;, : I .

ffi -50 ; : . ~ 0 1000 2000 3000 4000 500 0 a..

FREQUENCY, Hz

E > c.:i ..J w u:: t5 w ..J w

(b) :.t4 July 1995 0657:53 hrs UTC

0.5'------~----~----~ 15 20 25 30

TIME. ms

,g o.s ,-----------------. ::>

t' "' c.:i -' w u:: :i

~ -0.5 ':-----7:------':-----l_

15 20 25 30

TIME. ms

ELECT. FIELD- POWER SPECTRAL DENSITY

FREQUENCY, Hz

Fig. 8-Measured electric and magnetic fields and electric field power spectrum of (a) typical sprite related sferic, (b) sferics48

associated with an observed elves

south) magnetic field coil. The electric field observation system consisted of a 2-meter teflon insulated tripod antenna, a high input impedence voltage follower, and a low pass preamplifier. The capacitance of the antenna was about 100 pF. The follower amplifier had an input impedence of about 1010 ohm. This combination produced a low frequency cutoff of about 5 Hz.

The magnetic field system, on the other hand, consisted of a 620 H coil on a 1.5 m long ferrite core, a differential input preamplifier, a 60 Hz notch filter and a post preamplifier stage. The coil core was oriented in a north-south direction for providing sensitivity to primarily east-west propagating zero­mode waves. Parasitic capacitance in the sensor coil and the effects of the protective aluminium enclosure limited the bandwidth of the magnetic field measurement to below 1 kHz. Another interesting characteristic of the sferic-waveforms is the recovery to the post-stroke baseline. However, many of the waveforms do not return quickly to the pre-stroke, but

instead end at a higher level and then relax back to the pre-stroke level over several milliseconds. Observations further describe a distinctive form of sprites associated with positive CG flashes carrying currents of about 23 kA to about 100 kA in mesoscale thunderstorms49

.

Sprites are characterized by long vertical columns, showing virtually no variation in brightness along their length. The length of the vertical column developed is about 10 km, with a dian1eter of less than 1 km. This three-dimensional triangulation is defined as a 'Columniform' sprite or C-sprite. On 19 June 1995, a large group of columniform sprites was observed from Mt Evans, Colorado50

. Analysis showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. Some of the C-sprites reveal fain t diffuse 'hair' or tendrils extending above and below the column. Hampton et a/.51 also measured the spectra of many sprites from Mt Evans including some of C-sprites.

DATI A etal. : SPRITES, ELVES, SFERICS & THUNDERCLOUDS 393

From a careful analysis of all the observational reports we have made a comparison between the sprites related thundercloud and ordinary thundercloud. The comparison provides some interesting information as presented in Table 2.

6 Theoretical implications The most notable feature of the results we have

reviewed is that the red sprites and elves are usually accompanied by unipolar ' slow tail' wavelets of the order of one millisecond in duration and of a polarity indicating 'positive' lightning. Their power spectrum peaks in the range of several hundred Hz, putting the observations into the 'far field ' range at several thousand km. They apparently propagate in the expected TEM (transverse electromagnetic) mode and when extrapolated to the source the total ELF (extremly low frequency) electromagnetic energy approaches a mega Joule.

A mechanism for the generation of slow tails52 was suggested by Waie3

. According to him, the dispersion of an impulsive source alone is insufficient for forming the observed slow tail waveforms; rather relatively long and uniform lightning current pulses are required for the purpose. In his model he assumed that the pulse width was determined by the source current waveform and subsequent dispersion in the Earth-ionosphere waveguide. An alternate source mechanism is based on more recent calculations of Hale and Baginski54

, who produced a ' millisecond' slow tail from a nearly impulsive source. Hale55 gave an explanation for this in terms of the transition between an 'initial' quasi-static field and a 'final' solution involving the ionosphere. The time of thi s transition is controlled by the round-trip propagation delay to the ionosphere. The energy of the pulses can be sufficiently large, up to several percent of the total electrical energy associated with the stroke. The mechanism can explain relatively uniform unipolar

millisecond ELF waveforms observed to accompany the CGs associated with red sprites and elves, and thus serve as an important diagnostic of their occurrence.

However, the explanation for quasi-static field solution does not completely coincide with the duration of the optical phenomena, which are usually longer in sprites (ms) and shorter in the case of elves (hundreds of ms). We can, however speculate the mechanisms that control the duration of the optical phenomena. It is likely to consider that the elves56

occur when the transfer of substantial amounts of charge occurs in hundreds of microseconds or Jess, giving rise to large fields due to the acceleration and velocity of the charge. Another possibility is the acceleration of electrons by the quasi-static electric field to relativistic velocities, which ultimately 'run­away' to the ionosphere, with a transit time of less than a ms. The observation of a 'doughnut' shape in elves would indicate the dominance of electromagnetic radiation, due to the on-axis null in the radiation pattern.

A schematic diagram showing a connection among sprites, positive CG strokes and Q-bursts (an electromagnetic events) is exhibited7

,43 in Fig. 9. The

positive CG strokes are assumed to be the electrostatic source for the sprites and electromagnetic source for the Q-burst. Radiation upon ground attachment of the CG stroke is represented by dB/dt in the figure, while the change in the total dipole moment !1M from the associated event excites Q­bursts at ELE in the earth ionosphere cavity. A more difficult problem appears in explaining why the observed optical sprites are substantially delayed sometimes, from the associated slow tail electromagnetic signal. One possibility is that they are powered by continuing currents, which are delayed by the observed time period. There are further possibilities involving the post-stroke 'dendric'

Table 2-A comparison of the properties of sprites related thundercloud and ordinary thundercloud

Sprites rel ated thundercloud

Mesospheric phenomenon; strong lightning flashes are reported from specially energetic thundercloud.

Mesoscale Convective System (MCS ) is associated with the occurrence of sprites, MCS areas are larger than 104 km2

.

Observations reveal that dominant charge within the sprites associated cloud layer is positive and at the bottom of the cloud.

Sprites generally have a vertical brightness structure at about 70 km height and a diameter of 5 km or greater. These high altitude optical events are associated wi th positive CG fl ashes carrying current of about 100 kA in mesoscale thunderstorm.

Ordinary thundercloud

Tropospheric phenomenon; lightning usually associated at the core of the thundercloud.

Ordinary thundercloud area is one llundred times smaller than MCS area.

These observed structure is contrasted with the behaviour in ordinary active thunderclouds.

These dominant features are absolutely absent in usual tropospheric thunderstorms.

394 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

IONOSPHERE

85 km

MESOSPHERE

45 km

FAST .c.£ 2

STRATOSPHERE

15 km .. .. ~ TRAILIN G STR AT IFORM DEEP ANVIL-

TROPOSPHERE CO~ 6'-'{ ~~~

Fig. 9- Schematic diagram showing the proposed connections - among sprites, positive CG strokes and Q-bursts43

spreading of charge deposited by lightning 10 and also the expected lower electric field for 'runaway breakdown'. A third possibility would appear to be that the requisite energy is stored in the ionosphere above the sprite for the delay period. Hale55 has suooested that 'clouds' of free electrons may be bb

accelerated up magnetic field lines into the ionosphere by the quasi-static electric field related to positive lightning. With the decay of the field a portion of the electrons, propelled by mutual repulsion, may be accelerated back down into the sprite region and thus initiating the optical emissions by impact with the

2? atmosphere -.

7 Models for sprite production Following some lightning flashes from highly

energetic thunderclouds, sprites are observed in the lower ionosphere. With a view to describe the physical processes leading to these and associated phenomena, several models have been proposed by different workers. The models developed have advantages and shortcomings in all the cases for explaining the features.

The sprite-associated field can be calculated by employing the positive return stroke model of Coorai7

. The basic idea of this model is as follows. The total charge brought to ground by the return stroke is initially stored on the corona sheath of leader channel. When return strokes move upwards, the corona sheath collapses by draining its charge into the hiohly conductino return stroke core. In this model b b

each point of the leader channel is treated as a cunent

source, causing an injection of cunent into the return stroke. The variation of the cunent as a function of time is evaluated by repeating measurement until a match 'is obtained between the models and measured sprite field. Jarzembzki and Srivastava 14 pointed out how the interaction takes place between the lightning and sprites with a laboratory model. These studies are based on the assumption that ordinary thundercloud characterized by positive dipoles are responsible for the physical process at the higher altitudes.

Numerous studies7'16

'21

.47 imply that sprites are not

found over ordinary thunderclouds but rather over Mesoscale Convective System (MCS). The electrical structure of MCS has been evaluted with balloon soundings of electric field. The vertical electric charge distribution is observed on the basis of one­dimensional interpretation of Poisson's equation, which seems to be more accurate for laterally homogeneous stratiform regions. Perhaps the most significant evidence against the role for upper positive charge in MCS stratiform region is the work of Marshall48

, who measured the electric field above the stratiform region during the positive ground flashes. It was observed that electric field behaviour over electrified cloud is non-unique and is just opposite to that found over ordinary thunderclouds. Also the dominant charge within the sprites associated cloud was positive and at the bottom of the cloud.

Theoretical models8'58 for sprites show a strong

dependence of intensity on positive charge magnitude and more importantly on the altitude of the positive charge. Pasko et al. 8 found a thousand-fold enhancement of optical intensiti8 at 75 km altitude for a doubling of lightning charge from 5 km to 10 km. Likewise, Bell et al. 9, using a simple electrostatic model ,8 but taking runaway electrons as a sprite excitation source, concluded that a minimum of +250 C from at least 10 km altitude is required to reproduce the optical intensity. The comparative studies of these theoretical models with the ideas discussed here would seem to require still larger tl'ansfer of charge. Based on the evidences of different models discussed here, we believe that the pictures for charge transfer by sprite-producing ground flashes found in

7 59 . h .. Boccippio et al. and Lyons (w1th t e pos1t1ve charge reservoir in the lower part of ~he Cloud), may be a more accurate representation.

8 Discussion The dynamic and electrostatic f(eld ·distributions

within . a thunderstorm anvil are very . complex in

DATTA et al.: SPRITES, EL YES, SFERICS & THUNDERCLOUDS 395

nature. Byrne et al. 60 reported the results of two balloon flights carrying corona probes for measuring the electric fields. The balloons were allowed to pass through the anvil upstream of the precipitation core and measured extensive regions of both net positive and negative charge. MCS are characterised by extensive regions of stratiform precipitation with a total area, which is usually an order of magnitude greater than an ordinary thunderstorm61

-63

• In fact, MCS areas are larger than 104 km2

, an area that is one hundred times greater than an ordinary thundercloud. This MCS is associated with the occurrence of sprites61

. The stratiform region of the MCS is further characterized by a radar bright band just beneath the ooc isotherm, by vertical air motions64

·65 of the order

of tens of em s - 1• All these features clearly represent

substantial departures from the meteorological behaviours of ordinary thunderstorms. Lyons61 and later Laing and Fritsch66 suggested that MCS's are the product of more isolated thunderstorm convection earlier in the day and the tendency for sprites to occur later in the diurnal cycle follows from this meteorological evolution.

Probable explanations for the physical origin of the lower posi tive charge reservoir have been established by Marshall and Lin67

, who gave evidence of the predominance of the lower positive charge in the decaying stratiform phase of smaller thunderstorms. Observation by Marshall et a/.48 and also reports by other workers68

.70 revealed that dominant charge

within the sprites associated cloud layer was positive and at the bottom of the cloud. These observed structures are also contrasted with the behaviour in ordinary active thunderclouds.

It is believed in many models that red sprites and elves are triggered by strong CG discharges with large return stroke currents and continuing currents and large rise times of currents 7'

12'69

.73

. A large charge transfer within a few milliseconds of a flash can first break down the atmosphere at z > 60 km, causing sprites. If, however, the rate of large charge transfer is very low, so that the effective time scale of the source is greater than the local relaxation time at z > 60 km, the breakdown field at those altitudes will not be reached and sprites will not occur.

9 Conclusions In conclusion, the great maJOrity of sprites and

elves are initiated basically by ground flashes of positive polarity. All of the reported high altitude optical events were accompanied by relatively

uniform and primarily unipolar 'millisecond' slow tails, in a direction indicating positive lightning. These are probably due to wavelets launched by the deposition of large amounts of negative charge in the atmosphere in times of a millisecond or less. The fine structure due to higher frequency components is quite variable, in a manner not completely understood and the low sampling rates of the observations available so far do not allow for precise analysis. This may be better understood with wider bandwidth measurements at shorter distances, to reduce the effects of propagation on the observed waveforms .

The millisecond 'slow tails' seem to be associated with sprites and/or elves, and hence may provide a useful diagnosis for their occurrence. They may also provide part of the excitation for the Schumann resonance spectrum in the Earth-ionosphere cavity, which is also excited by the 'impulses' of return strokes and longer duration continuing currents. Schumann resonance perturbations have been observed to accompany high altitude optical observations7

• Additional theoretical analysis and computer modelling is required to further substantiate 'quasi-static' explanation for the ELF portion of sferics.

Although relating to the electromagnetic environment, the millisecond time-scale of slow tails does not correspond to the duration of elves (which are shorter) or sprites (which are both longer and delayed in time) and hence require more complex mechanisms. It appears into the high altitude electromagnetic environment on time scales of several milliseconds following large CGs. While probably not solely sufficient to explain sprites and elves, slow tails are a critical component in determining the coupling mechanisms of the energy sources to the high altitude optical phenomena. On the theoretical side, several models have been proposed. Every model has its own advantages and shortcomings in explaining the documented features. At this stage, more observations are needed.

In the case of elves, X-rays and perhaps, other electromagnetic spectral measurements could be used to distinguish whether purely electromagnetic energy coupling or high-energy electrons are involved.

Though optical images are likely to be the principal experimental form of sprite detection, more specific information can be investigated by developing a new model. An electrostatic model can be developed, based on the idea that electrostatic breakdown favours rapid charge transfer. These models must include a

396 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

VLF measurement of associated ionospheric heating effect and continuous wave radar probes of sprites to determine electron densities. The associated field strength is to be measured with VLF band receiver with the required modification to accommodate a large dynamic range of field intensities.

We think that ultraviolet and infrared spectral studies would give better information about the problem. X-ray and other electromagnetic spectral measurement could be used to determine the coupling mechanisms of the energy sources to the high altitude optical phenomena, whether purely electromagnetic or high energy electrons are involved for its production.

10 Summary Summarising what has been discussed, the

following important properties related to sprites and elves can be listed:

(i) Short-lived luminous shapes (ii) Electrical discharge phenomena associated with

large thunderstorm called mesoscale convective systems

(iii) These migratory storms contain regions of active convention adjacent to regions of weaker stratiform convection

(iv) Ground flashes with negative polarity predominate in the active convection regions, whereas less frequent b~t more energetic flashes with positive polarity predominate m the stratiform regions

(v) The great majority of sprites and elves are initiated by ground flashes of positive polarity

Acknowledgements The authors are grateful to Dr. E R Williams,

Massachusetts Institute of Technology, Cambridge, USA for various helps during the preparation of this paper. The authors are thankful to the two reviewers for their valuable comments and suggestions. R Das and S De like to express thanks to the UGC for financial supports to them.

References I Sentman D D & Wescott E M, Red sprites and Blue Jets:

Thunderstorm-excited optical emission in the stratosphere, mesosphere and ionosphere, Phys Plasmas (USA),2 (1995) 2514.

2 !nan U S, Rapid Cateral expansion of optical Juminesity in lightning induced ionospheric flashes referred to as 'elves', Geophys Res Lett (USA), 24 (1997) 583.

... l i!

3 Rycroft M J & Cho M, Modelling electric and magnetic fields due to thunderclouds and lightning from cloud-tops to the ionosphere, J Atmos Sol-Terr Phys (UK). 60 (1998) 889.

4 Glukhov V S & !nan U S, Particle simulation of the time dependent interaction with the ionosphere of rapidly varying lightning EMP, Geophys Res Lett (USA), 23 ( 1996) 2193.

5 Nickolaenko A P & Hayakawa M, Heating of the lower ionosphere electron by electromagnetic radiation of lightning discharges, Geophys Res Lett (USA), 22 (1995) 3015.

6 Rowland H , Lightning driven EMP in the upper atmosphere, Geophys Res Lett (USA), 22 (1995) 361.

7 Boccippio D J, Williams E R, Heckman S J, Lyons W A, Baker I T & Boldi R, Sprites, ELF transients and positive ground strokes, Science (UK), 269 (1995) 1088.

8 Pasko V P, !nan U S, Taranenko T N & Bell T F, Heating, ionization and upward discharges in the mesosphere due to intense quasi-electrostatic thundercloud fields, Geophys Res Lett (USA), 22 (1995) 365.

9 Bell T F, Pasko V P & !nan U S, Running electrons as a source of Red Sprites in the mesosphere, Geophys Res Lett (USA) , 22 (1995) 2127.

10 Roussell-Dupre R A & Gurevich A V, on Runaway break down and upward propagating discharges, J Geophys Res (USA). 101 (1996) 2297.

11 Lehtinen N G, A two dimensional model of runaway electron beams driven by quasi-electrostatic thundercloud fields , Geophys Res Lett (USA), 24 (1997) 2639.

12 Taranenko Y N, !nan U S & Bell T F, Interaction with the lower ionosphere of electromagnetic pulses from lightning, heating, attachment and ionization, Geophys Res Lett (USA) , 20 (1993) 1539.

13 Pichard R H, Infrared glow above thunderstorms, Geophys Res Lett(USAj, 24 (1997) 2635.

14 Jarzembski M A & Srivastava V, Low pressure experimental simulation of electrical discharges above and inside a cloud, J Atmos Terr Phys (UK), 59 (1997) 271.

15 Franz R C, Nemzek R J & Winckler J R, Television image of a large upward electrical discharge above a thunderstorm, Scince (USA), 249 (1990) 48.

16 Sentman D D & Wescott EM, Video observation of upper atmospheric optical flashes recorded from an aircraft, Geophys Res Lett (USA), 20 (1993) 2857.

17 Sentman D D, Wescott E M, Osborne Hampton D L & Heavner M J, Preliminary results from the sprites 94 aircraft Campaign. l. red Sprites, Geophys Res Lett (USA), 22 (1995) 1205.

18 Lyons W A, VLF heating of the lower ionosphere, Geophys Res Lett (USA), 17 (1993) 729.

19 Sentman D D & Wescott E M, Red sprites & Blue jets, (University of Alaska video production), 9 July (1994).

20 Boeck W L, Jr Vauchan 0 H, Blakeslee R J, Vonnegut B, Brook M & McKune J, Observation of lightning in the Stratosphere, J Geophys Res (USA), 100 (1995) 1465.

21 Winckler J R, Lyons W A, Nelson T E & J Nemzek R, New high resolution ground based studies of sprites, J Geophys Res (USA), 101 (1996) 6997.

22 Fukunishi H, Takahashi Y, Kubota M, Sakanoi K, Inan lJ S & Lyons W A, Elves: Lightning-induced transient Juminious events in the iower ionosphere, Geophys Res Lett (USA), 23 (1995) 2150.

23 Bruce C E R & Golde R H, The lightning discharge, J lnst Elect Engg (UK), 88 (1941) 487 .

DATI A et al.: SPRITES, ELVES, SFERICS & THUNDERCLOUDS 397

24 Pierce E T, Latitudinal variation of lightning parameters, J Appl Meteorol (USA), 9 (1970) 194.

25 Harris D J & Salman Y E, The measurement of lightning characteristics in Northern Nigeria, 1 Atmos Terr Phys (UK), 34 (1972) 775.

26 Takeuti T, Nakano M & Nagatani M, Lightning discharges in Guan and Philippine Islands, 1 Meteorol Soc Japan, 53 (1975) 360.

27 Thomson E M, The dependence of lightning return stroke characteristics on latitude, 1 Geophys Res (USA), 85 (1980) 1050.

28 Orville R E, Peak current variations of lightning return strokes as a function of latitude, Nature (UK), 343 ( 1990) 149.

29 Garbagnati E & Lo-Piparo G B, Parameter von blitzstromen, Elektrotech Z E1Z A (Germany), I 03 ( 1982) 61 .

30 Geldenhys H J, Eriksson A J & Bourn G W, Fifteen years of data of lightning current measurements on 60m mast, Trans South Afr Inst Elect Eng (South Africa), 80 (1989) 130.

31 Berger K A, Novel observations on lightning discharges: results of research on Monte san Satratore, 1 Franklin Inst (UK), 283 (1967) 478.

32 Berger K A, Anderson R B & Kroninger H, Parameters of lightning flashes, Electra (UK), 80 (1975) 23.

33 Anderson R B & A J Eriksson, Lightning parameters of engineering application, Electra (France), 69 (1980) 65.

34 Petersen W A & Rutledge S A, some characteristics of cloud to ground lightning in tropical northern Australia, 1 Geophys Res (USA), 97 (1992) 11553.

35 Jr Pinto 0, Gin R B B, Pinto I RCA & Jr Mendes 0, Cloud to ground lightning flash characteristics in the Southern Brazil during the 1992-93 summer season, 1 Geophys Res (USA), 101 (1996) 29627.

36 Inan U S, VLF Signatures of the ionosphere disturbances associated with sprites, Geophys Res Lett (USA), 22 (1995) 3461 .

37 Rycroft M J, A review of whistlers and energetic-electron precipitations, International Union of Radio Science, (Oxford University Press, UK), (1993) 631.

38 Strangeways H J, Lightning, Trimpis and Sprites: In Review of Radio Science, 1993-1995, International Union of Radio Science, (Oxford University Press, UK), (1996) 741.

39 G I Q (Geophysical Institute Quaterly), Spectacular flashes recorded above thunderstorms 12, No. 3 ( Summer 1994 ) (Private Communication).

40 Bhattacharya A B, Chatterjee M K, Mali P, Monda! N C & Sarkar S K, Sprites, their relationship to intence quasi­electrostatic thundercloud field and the physical mechanisms for their production, Indian 1 Phys, 74B(l) (2000) 81.

41 Sentman D D, Wescott E M, Video observations of upper atmospheric optical flashes recorded from an aircraft, Geophys Res Lett (USA), 20 (1993) 2857.

42 Huang E, Wiliams E R, Baldi R, Heckman S, Lyons W A, Taylor M & Nelson T, Monitoring of Global Lightning Activities using ELF Radio Waves, 1 Geophys Res (USA), 104 (1999) 16943.

43 Wescott E M, Sentman D D & Osborne D L, Video observations of upper atmosphere discharges from an aircraft, EOS Trans.AGU, 74 (1993) 155.

44 Wescott E M, Sentman D D, Heavner M J, Hampton D L, Lyons W A & Jr Vaughan 0 H, Observations of 'Columniform' sprites, 1 Atmos Sol-Terr Phys (UK), 60 (1998) 733.

45 Sentman D D, Wescott EM, Hampton D L, Heavner M J & Osborne D L, Preliminary results from the sprites 94 aircraft Campaign: 1. Red Sprites. Geophys Res Lett (USA), 22 (1995) 1205.

46 Wescott E M, Sentman D D, Osborne D, Hampton D & Heavner M, Overview of the sprites 94 aircraft Campaign and results, EOS Trans AGU, 75 (1994) 108.

47 Lyons W A, Characteristics of lurrunious structures in the stratosphere above thunderstorms as imaged by low light video, Geophys Res Lett (USA), 21 (1994) 875.

48 Marshall T C, Stolzenburg M & Rust W D, Electric field measurements above mesoscale convective systems, 1 Geophys Res (USA), 101 (1996) 6979.

49 Wescott E M, Sentman D D, J Heavner M, Hampton D L, Lyons W A & Nelson T, Observations of 'Columniform' sprites, 1 Atmos Sol-Terr Phys (UK) , 60 (1998) 733.

50 Lyons W A & Nelson T E, The colorado SPRITES' 95 Campaign: initial results, EOS Trans AGU, 76 (1995) 113.

51 Hampton D L, Heavner M J, Wescott EM & Sentman D D, Optical spectral characteristics of sprites, Geophys Res Lett (USA), 23 (1996) 89.

52 Hepburn F, Atmospheric waveforms with very low­frequency components below I Kc/s known as slow tails, J Atmos Terr Phys (UK), 10 (1957) 266.

53 Wait J R, On the theory of the slowtail portion of atmospheric waveforms, 1 Geophys Res (USA), 65 (1960) 1939.

54 Hale L C & Baginski M E, Current to the ionosphere following a lightning stroke, Nature (UK), 329 (1987) 814.

55 Hale L C, The coupling of ELF/ ULF energy from lightning and MeV particles to the middle atmosphere, ionosphere, and global circuits, 1 Geophys Res (USA), 99 (1994) 21089.

56 De S, Datta T, Das R & Bhattacharya A B, The Probable Role of Lightning Plasma Over Transient Luminous Events in the Lower Ionosphere, XIII National Space Science Symposium 17-20 Feb, 2004, M.G.University, Kottayam, Kerala, (India), p 83.

57 Cooray V, A model for positive return strokes, Inst.Phys.Conf.Ser.No., ( 1995) 143.

58 Pasko V P, loan U S & Bell T F, Sprites as luminous columns of ionozation produced by electrostatic thundercloud fields, Geophys Res Lett (USA), 23 (1996) 649.

59 Lyons W A, Sprite observations above the U.S. high plains in relation to their parent thunderstorm systems, 1 Geophys Res (USA), 101 (1996) 29641.

60 Byrne C J, Few A A & Stewart M F, Electric field measurements within a severe thunderstorm anvil, 1 Geophys Res (USA), 94 (1989) 6297.

61 William E R & Heckman S J, The local diurnal variation of cloud electrification and the global diurnal variation of negative charge on the Earth, 1 Geophys Res, (USA), 98 (1993) 5221.

62 Bhattacharya A B, Chatterjee M K, Mali P, Monda! N C & Sarkar S K, Sprites, their relationship to intense quasi­electrostatic thundercloud field and the physical mechani sms for their production, Indian 1 Phys, 74B (2000) 113.

63 Bhattacharya R, Datta T & Bhattacharya A B, Observations of sprites and ELVES glow over unusual thunderclouds, Sci & Culture (India) •. 69 (2003) 437.

64 Zipser E J, Mesoscale and Convective-Scale downdrafts as distinct components of squall line structure, Man Weather Rev (USA) , 105 (1977) 1568.

398 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

65 Williams E R, The role of ice on the electrification and dynamics of thunderstorms, Conf. on Cloud Physics, American Meteorological Society (USA), (1990) 282.

66 Laing A & Fritsch 1 M, The global distribution of mesoscale convective complexes, Quart J Roy Met Soc (UK),1997.

67 Marshall T C & Lin B, Electricity in dying thunderstorms, J Geophys Res (USA) , 97 (1992) 9913.

68 Williams E R, Zhang R & Boccippio D J, Microphysical growth state of ice particles and large scale electrical structure of clouds, J Geophys Res (USA), 99 (1994) 10787.

69 Rutledge S A & Petersen W A, Vertical radar reflectivity structure and cloud-to ground lightning in the stratiform region of MCS: further evidence of in situ charging in the stratiform region, Mon Weather Rev (USA), 122 (1994) 1760.

70 Stephard T R, Rust E D & Marshall T C, Electric fields and charge near oc in stratiform clouds, Mon Weather Rev (USA) , 124 (1996) 920.

71 Milikh G M, Papadopoulos K & Chang C L, On the physics of high altitude lightning, Geophys Res Lett (USA). 22( 1995) 85.

72 Rowland H L, Femsler R F, Huba J D & Bernhardt P A, Lightning driven EMP in the upper atmosphere, Geophys Res Lett (USA), 22 (1995) 316.

73 Sukhorukov A I, Rudenchick E A & Stubbe P, Simulation of the strong lightning pulse penetration into the lower ionosphere, Geophys Res Lett (USA), 23 (1996) 2911.