8.6 Sprites and other upper atmospheric discharges

Upper atmospheric discharges are electrical phenomena occurring above thunderstorms. These events, including sprites, blue jets, elves, and gigantic jets, transfer energy between atmospheric layers and impact global electrical circuits. Understanding them provides insights into atmospheric physics and potential climate interactions.

Sprites, the most common type, appear as reddish-orange tendrils at 50-90 km altitudes. They last milliseconds and often occur in clusters. Blue jets, elves, and gigantic jets have distinct characteristics, altitudes, and durations. These phenomena reveal complex interactions between Earth's atmosphere and electrical processes.

Types of upper atmospheric discharges

• Upper atmospheric discharges encompass various electrical phenomena occurring above thunderstorms in Earth's atmosphere
• These discharges play crucial roles in energy transfer between different atmospheric layers and impact global electrical circuits
• Understanding these phenomena provides insights into atmospheric physics, chemistry, and potential climate interactions

Sprites vs blue jets

Top images from around the web for Sprites vs blue jets

• 

  ESD - A dynamical systems characterization of atmospheric jet regimes View original

  Is this image relevant?

• 

  Sprite (lightning) - Wikipedia View original

  Is this image relevant?

• 

  Sprites | Green Comet View original

  Is this image relevant?

• 

  ESD - A dynamical systems characterization of atmospheric jet regimes View original

  Is this image relevant?

• 

  Sprite (lightning) - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Sprites vs blue jets

• 

  ESD - A dynamical systems characterization of atmospheric jet regimes View original

  Is this image relevant?

• 

  Sprite (lightning) - Wikipedia View original

  Is this image relevant?

• 

  Sprites | Green Comet View original

  Is this image relevant?

• 

  ESD - A dynamical systems characterization of atmospheric jet regimes View original

  Is this image relevant?

• 

  Sprite (lightning) - Wikipedia View original

  Is this image relevant?

1 of 3

• Sprites occur at higher altitudes (50-90 km) characterized by reddish-orange tendrils extending upward
• Blue jets propagate from cloud tops to about 40 km altitude with a distinctive blue color
• Sprites typically last for milliseconds while blue jets can persist for several hundred milliseconds
• Sprites often appear in clusters while blue jets tend to be more isolated events

Elves and halos

• Elves (Emission of Light and Very Low Frequency perturbations due to Electromagnetic Pulse Sources) manifest as rapidly expanding rings at altitudes around 100 km
• Halos appear as diffuse reddish glows at altitudes of 70-80 km, often preceding or accompanying sprites
• Elves last for less than a millisecond while halos can persist for several milliseconds
• Both phenomena result from electromagnetic pulses generated by lightning strokes

Gigantic jets

• Gigantic jets bridge the gap between tropospheric thunderstorms and the ionosphere, reaching altitudes of 70-90 km
• They exhibit a tree-like structure with a blue trunk transitioning to red at higher altitudes
• Gigantic jets transfer large amounts of charge from the troposphere to the ionosphere
• These events are relatively rare compared to other upper atmospheric discharges

Physical characteristics of sprites

Morphology and structure

• Sprites typically consist of a bright head region followed by tendrils extending both upward and downward
• The overall shape can vary from columnar to carrot-like structures
• Sprite clusters can cover horizontal areas of up to 50 km in diameter
• Fine-scale filamentary structures within sprites have diameters of 10-100 meters

Color and luminosity

• Dominant red color results from nitrogen molecular emissions in the first positive band
• Blue hues occasionally observed in the lower portions due to ionized molecular nitrogen
• Sprite brightness can range from 10 to 100 kR (kilorayleigh)
• Luminosity varies with altitude, peaking around 65-75 km

Temporal evolution

• Sprite initiation occurs within a few milliseconds after the parent lightning stroke
• The bright head region develops first, followed by the downward-propagating tendrils
• Total sprite duration typically ranges from 10 to 100 milliseconds
• Secondary sprite events, called afterglows, can occur in the same region within seconds of the initial sprite

Formation mechanisms

Quasi-electrostatic field theory

• Explains sprite formation through the buildup and relaxation of electric fields above thunderstorms
• Charge moment changes in the underlying lightning create transient electric fields at mesospheric altitudes
• When the field exceeds the local breakdown threshold, it initiates electron avalanches and streamer formation
• This theory accounts for the observed time delay between lightning and sprite initiation

Electromagnetic pulse theory

• Attributes sprite triggering to the electromagnetic pulse (EMP) generated by lightning return strokes
• The EMP propagates upward and creates a region of enhanced electric field in the lower ionosphere
• This enhanced field can initiate electron avalanches and subsequent sprite formation
• Explains the rapid formation of elves and the upper portions of sprites

Runaway electron breakdown

• Involves the acceleration of high-energy electrons in the presence of strong electric fields
• These runaway electrons collide with atmospheric molecules, producing secondary electrons and initiating avalanches
• The process can occur at lower electric field strengths compared to conventional breakdown
• May contribute to the initial stages of sprite formation and the production of X-rays associated with sprites

Sprite triggering conditions

Relationship to lightning

• Sprites typically associated with positive cloud-to-ground (+CG) lightning strokes
• Charge moment change of the parent lightning crucial for sprite initiation (typically >300 C km)
• Time delay between lightning and sprite varies from a few to tens of milliseconds
• Multiple sprites can be triggered by a single lightning stroke

Meteorological factors

• Sprites predominantly occur above mature mesoscale convective systems (MCS)
• Stratiform precipitation regions of MCS provide favorable conditions for sprite production
• Cloud top temperatures and heights correlate with sprite occurrence probability
• Convective available potential energy (CAPE) influences the likelihood of sprite-producing storms

Ionospheric conditions

• Lower ionospheric electron density affects sprite initiation and morphology
• Sporadic E layers can inhibit sprite formation by screening out the quasi-electrostatic field
• Variations in the D-region of the ionosphere impact sprite characteristics and occurrence rates
• Solar activity and geomagnetic conditions modulate ionospheric properties relevant to sprite formation

Observation techniques

Ground-based optical methods

• Low-light video cameras with image intensifiers capture sprite emissions
• Photometers provide high-temporal resolution measurements of sprite luminosity
• Spectroscopic observations reveal the molecular species involved in sprite emissions
• Triangulation techniques using multiple camera sites determine sprite altitudes and spatial extents

Satellite observations

• Space-based imagers on satellites (ISUAL, GLIMS) provide global coverage of sprite occurrences
• Limb observations allow for vertical profiling of sprite structures
• Satellite measurements complement ground-based observations by covering oceanic and remote regions
• Detection of sprite-induced perturbations in the ionosphere using radio occultation techniques

Radio frequency detection

• VLF (Very Low Frequency) radio receivers detect electromagnetic signatures associated with sprites
• ELF (Extremely Low Frequency) observations provide information on the charge moment changes of sprite-producing lightning
• Broadband HF (High Frequency) measurements capture the impulsive radio emissions from sprite streamers
• Lightning mapping arrays help correlate sprite occurrences with specific lightning events

Global distribution of sprites

Geographical patterns

• Sprites occur most frequently over continental regions with high lightning activity
• Hotspots include the Great Plains of North America, Central Africa, and South America
• Oceanic sprite occurrences less common but observed over marine storm systems
• Latitudinal distribution shows a preference for mid-latitude and tropical regions

Seasonal variations

• Sprite activity follows the seasonal migration of thunderstorm activity
• Northern Hemisphere experiences peak sprite occurrence during summer months (June-August)
• Southern Hemisphere sprite maximum occurs during austral summer (December-February)
• Some regions (Central Africa) show less pronounced seasonal variations due to year-round thunderstorm activity

Diurnal cycle

• Sprite occurrence generally peaks during nighttime hours
• Afternoon to evening transition period shows increased sprite activity in many regions
• Diurnal patterns vary with geographical location and local thunderstorm climatology
• Observations biased towards nighttime due to easier optical detection in darkness

Effects on atmospheric chemistry

Nitrogen oxide production

• Sprites generate significant amounts of nitric oxide (NO) and nitrogen dioxide (NO2) in the mesosphere
• NOx production rates estimated at 10^27 to 10^28 molecules per sprite event
• These nitrogen oxides can persist for hours to days in the upper atmosphere
• Potential impact on ozone chemistry and energy balance in the mesosphere and lower thermosphere

Ozone depletion potential

• Sprite-produced NOx participates in catalytic ozone destruction cycles
• Local ozone depletion of up to 15% possible in active sprite regions
• Long-term effects on global ozone budget still under investigation
• Interaction between sprite-induced chemistry and solar-driven processes affects ozone dynamics

Ionization of upper atmosphere

• Sprites create localized regions of enhanced ionization in the mesosphere and lower ionosphere
• Electron densities in sprite streamers can reach 10^6 to 10^8 cm^-3
• Ionization effects can persist for several minutes after the sprite event
• Potential impact on radio wave propagation and atmospheric electrical properties

Sprites in planetary atmospheres

Jovian sprites

• Theoretical predictions suggest sprite occurrence in Jupiter's upper atmosphere
• Differences in atmospheric composition and pressure profiles affect potential Jovian sprite characteristics
• Lightning observations by spacecraft (Voyager, Galileo, Juno) provide context for possible sprite activity
• Challenges in direct observation due to Jupiter's bright dayside and limitations of current instruments

Venusian electrical discharges

• Ongoing debate about the existence of lightning on Venus
• If present, Venusian lightning could potentially trigger sprite-like phenomena
• Differences in atmospheric chemistry (CO2-dominated) would result in unique emission spectra
• Future missions with enhanced detection capabilities needed to confirm Venusian upper atmospheric discharges

Potential for exoplanetary sprites

• Theoretical models suggest the possibility of sprite-like phenomena on exoplanets with Earth-like atmospheres
• Variations in planetary magnetic fields, atmospheric composition, and pressure profiles would influence sprite characteristics
• Detection of exoplanetary sprites could provide insights into atmospheric properties and electrical activity
• Challenges in observation due to the small scale and transient nature of sprite events

Research challenges and future directions

Improved detection methods

• Development of more sensitive optical instruments for ground-based and space-based sprite detection
• Implementation of multi-spectral imaging techniques to better characterize sprite emissions
• Advancement in radio detection methods to capture sprite-associated electromagnetic signatures
• Integration of machine learning algorithms for automated sprite identification and classification

Modeling and simulation

• Refinement of 3D sprite initiation and propagation models incorporating detailed plasma physics
• Coupling of sprite models with global atmospheric chemistry and climate models
• Improved simulations of sprite-induced chemical reactions and their long-term atmospheric effects
• Development of planetary sprite models for various solar system bodies and exoplanets

Climate change impacts

• Investigation of potential changes in sprite occurrence due to shifting global thunderstorm patterns
• Analysis of sprite-induced chemical perturbations in a warming atmosphere
• Exploration of possible feedback mechanisms between sprite activity and climate variables
• Long-term monitoring of sprite characteristics and distributions as indicators of upper atmospheric changes