From ElectricStorms

Jump to: navigation, search

International Space Science Institute (ISSI) Teams

The International Space Science Institute (ISSI) is an Institute of Advanced Studies where scientists from all over the world meet in a multi- and interdisciplinary setting to reach out for new scientific horizons. The main function is to contribute to the achievement of a deeper understanding of the results from different space missions, ground based observations and laboratory experiments, and adding value of those results through multidisciplinary research in the framework of International Teams, Workshops, Working Groups, Forums or as individual Visiting Scientists. The program of ISSI covers a widespread spectrum of disciplines from the physics of the solar system and planetary sciences to astrophysics and cosmology, and from Earth sciences to astrobiology.

In preparation of the TLE-related european space missions ASIM and TARANIS, and in order to establish links between various scientific communities interested by the projects, two international teams have been organized at ISSI:

1/ WFM (Team leader: F. Lefeuvre)

The aim of the study is to define how wave field measurements on board magnetospheric satellites may be used to trace Wave Particle Interactions (WPI) involved in magnetosphere / ionosphere / atmosphere couplings. The emphasis is put on processes taking place in the plasmasphere and in the medium and low latitude ionosphere (L<7). The context is the preparation of the CNES micro satellite project TARANIS devoted to the identification of several types of magnetosphere / ionosphere / atmosphere couplings. Its phase A started in January 2005 and has to be completed by June 2006.

At the origin of the study is the need to establish the feasibility of observing runaway electrons associated with Transient Luminous Events (TLEs) generated below 100 km altitude by other means than direct measurement of the energetic particles. In particular remote detection through wave field measurements may be more feasible and provide a more unique signature within the ionospheric plasma environment than other remote diagnostics. As similar questions may be raised for other ionospheric and magnetospheric processes, the study has been extended to the use of wave field measurements to trace accelerated and precipitated particles, or more generally to trace wave particle Interactions (WPI), taking place within the equatorial regions as well as at low altitudes. It is mainly focused on medium and low latitudes. Higher latitudes have been studied in the past and involved other phenomena. In the context of a satellite mission one concentrates on the low frequencies (f < 30 MHz) which are submitted to ionospheric filtering when monitored from the ground. One his in the situation of a space radioastronomer who is looking for long wave length radio waves and pulses. The main difference is that one may use electromagnetic and locally produced electrostatic waves.

An example of the studies one would like to undertake is the examination of resonance interactions as the source of a direct coupling between the magnetosphere and the atmosphere. Several types of resonance interactions take place in the equatorial region of the plasmasphere. They contribute to the magnetosphere / atmosphere coupling via the precipitation of energetic electrons from the radiation belts. As shown by several authors (Callis et al., 1996, 1998; Siskind et al., 2000; Callis, L.B., 2001; Callis et al., 2001, 2002), energetic electrons precipitated from the outer radiation belt (L > 5) are at the origin of NOx formed at high altitudes then transported from the upper atmosphere to the stratosphere during polar winter. After the observation by the SAOZ balloon borne instrument of unexplained enhancements in the concentration of NOx above the South Atlantic Anomaly, on the Eastward side (Huret et al., 2002), i.e. on the side where intense fluxes of precipitated energetic electrons are observed, one may wonder if middle and low latitude energetic electrons could also contribute to the formation of NOx, and so to the coupling between the ionised and neutral parts of the atmosphere. However, in the absence of an efficient transport mechanism, this means that Nitrogen oxides must be produced at low (~ 20 km) altitude, i.e; by relativistic electrons. Now, according to the important fluxes of 10 – 20 MeV electrons observed by the SAMPEX satellite over long time periods (several months or years) at L = 1.2 – 2.5 (Li and Temerin, 2001) this may be possible. One important point in the project is not only to study the possible use of wave field measurements to trace the gyro resonance interactions taking place at the equator but also to study if detectable emissions may be associated with downward directed electron beams.

A more difficult problem is the possible detection of emissions associated with the atmosphere / ionosphere coupling indicated by the observation of TLEs. Energetic runaway electrons above thunderstorms, driven by the intense QE field following a positive cloud-to-ground (+CG) discharge, has been put forth (Roussel Dupré et al., 1994, 1996) as a fundamental new plasma acceleration process (Gurevich et al., 1992; Bell et al., 1995). Models have been developed by Roussel Dupré et al. (1998), Yukhimuk et aL; (1999), Lehtinen et al. (1999). No direct measurements of energetic runaway electron beams have yet been made, and future direct observations are unlikely due to the localized (~20 km) lateral size (Lehtinen et al., 1997) and highly transient (~1ms duration) nature of the beams (Lehtinen et al., 2001). However, possible measurable effects of the beams in the conjugate hemisphere have been investigated (Lehtinen et al., 2001). An important point in the project is the identification of the emissions that could allow tracing of the involved processes. ELF radiation originating from currents flowing within the body of sprites have been suggested (Cummer et al., 1998; Pasko et al., 1998). Considering simultaneous effect of runaway breakdown and extensive atmospheric showers, Gurevich et al. (2002) predicted the generation of wide band bipolar pulses of radio emission in the frequency range 1-10 MHZ. But tests must be made from existing models or even from available satellite data. Till now, the only relevant measurement is the ground-based detection of radio-pulses associated with cosmic ray showers (Ravel et al., 2004). It was performed at a few tens of MHz, but it may exist at lower frequencies.

2/ CARNES (Team leader: C. Hanuise)

The aim of the study is to define simultaneous measurements to be performed at ground, on balloon, and in space in order to trace energy transfers between the atmosphere and the space environment. Two studies will be run in parallel:

• mid- and low-latitude NOX production and transport in relation with thunderstorm activity
• atmospheric dynamical and chemical model

Mid-latitude NOx production

The largest source of production for NOx in atmosphere, is known to be lightning discharges associated with thunderstorms (Kraus et al., 1996; Levy et al., 1999).The production mechanism is generally based on heating. In the middle atmosphere, energetic particle precipitation is an important source of NOx, primarily in the polar regions. This atmospheric NOx plays a crucial role in lower and middle atmosphere chemistry. In the stratosphere, the NOx catalytic cycle is a key ozone-destroying mechanism. As regions with high NOx concentration influence O3 locally or globally, the knowledge of global concentration is important for global climate studies.

Price et al., (1999) evaluated the global NOx production from lightning physics assuming that the main NOx sources are cloud to ground discharges in the troposphere. Difficulties in such estimations arise from the lack of knowledge of the role of intracloud lightning in theses processes, of the lightning energy which is radiated at local and global scales, and of the coupling between the atmospheric layers. Recent measurements during triggered lightning showed that relatively slow discharge processes such as continuing currents in both cloud and cloud to ground flashes and other steady currents can produce NOx (Rahman et al., 2007). NOx production could then be larger than expected and the production altitude could be located at higher altitude, especially at equatorial and tropical latitudes where the tropopause altitude is higher than 15 km.

In addition, under conditions still to be defined, lightning discharges are involved in the triggering of: (i) TLEs (Transient Luminous Events) such as sprites, halos and elves, (ii) TGFs (Terrestrial Gamma ray Flashes), (iii) electron precipitation, the latter effect being called LEPS (Lightning induced Electron Precipitations). The properties of lightning discharges that preferentially leads to these observable phenomena are not understood and may well be important in the overall NOx production. In addition, the TLE and TGF phenomena provide windows of measurability of the dynamics of the otherwise inaccessible mesosphere and ionosphere, which are likely important in determining the production and fate of NOx.

The production of NOx and O3 by jets was suggested by Mishin (1997), who evaluated the role of fast heated electrons in the discharge process, similar to NOx production during solar proton events. Mechanisms as runaway electric breakdown (Rousel Dupré et al, 1994, Roussel Dupré and Gurevich, 1996, Roussel Dupré et al, 1998, Yukhimuk et al., 1999) involved in sprite formation may also play a role in these processes. Runaway breakdown triggered by cosmic radiation produce TGFs in the Earth atmosphere with very intense HF-VHF bipolar pulses at the top of thunderstorm clouds. Recent studies show also that the acceleration of thermal electrons at the tip of sprite streamers, is sufficient to produce runaway breakdown (Moss et al, 2006) and NO-gamma emissions (Liu and Pasko, 2006)

Pasko et al. (1997) suggested that the neutral density is non-uniform because gravity waves are launched by the updraft associated with the mesospheric current systems associated with thunderstorm activity. Such gravity wave produced by a thunderstorm was observed by Sentman et al (2005). The gravity waves produced at low latitude by thunderstorms and wind over mountains contribute to the forcing of the stratosphere and play a significant role in the global dynamics system in which atmospheric species are transported from low latitude region to polar regions through a large scale motion. The observation by the GOMOS/ENVISAT satellite, in the absence of magnetic storm, of a layer of strongly enhanced NO2 in the north polar mesosphere simultaneously with intense mesospheric warming indicated a strong air descent in the polar region, transporting a large quantity of NO from the upper mesosphere-lower thermosphere to the lower mesosphere illustrating this effect (Hauchecorne et al, 2007).

On another side, precipitated electrons may modify the atmosphere conductivity and may affect the production of lightning discharges, so that there are difficulties in properly indentifying the causative sequence of events and the hierarchies of the different phenomena. Several authors have pointed out associations between precipitated particles and NOx events. However the association is better defined at high latitudes that at mid- and low-latitudes.

At high latitudes, effects of proton precipitation on the production of NOx are known for long. More recently, analyses of ENVISAT GOMOS data have pointed out associations between NOx increases and proton events associated with the 2003 Halloween solar events and geomagnetic storms. Effects of electron precipitations have been studied by several authors (Callis et al., 1996, 1998; Siskind et al., 2000; Callis, L.B., 2001; Callis et al., 2001, 2002). It is now well established that energetic electrons precipitated from the outer radiation belt (L > 5) are at the origin of NOx formed at high altitudes then transported from the upper atmosphere to the stratosphere during polar winter.

At mid- and low- latitudes few effects have been pointed out. After the observation by the SAOZ balloon borne instrument of unexplained enhancements in the concentration of NOx on the eastward side of the South Atlantic Anomaly (Huret et al., 2002), i.e. on the side where intense fluxes of precipitated energetic electrons are observed, one may wonder if middle and low latitude energetic electrons could also contribute to the formation of NOx, and so to the coupling between the ionised and neutral parts of the atmosphere. In the absence of an efficient transport mechanism, this would mean that Nitrogen oxides be produced at low (~ 20 km) altitude by relativistic electrons.

Although this has not been demonstrated so far, it seems possible that fluxes of 10 – 20 MeV electrons observed at L=1.2 - 2.5, over time periods of months or even years, by low altitude satellites like SAMPEX (Li and Temerin, 2001) could trigger NOx events. GOMOS measurements on ENVISAT being performed with timescales compatible with plasma events (of the order of a few hours or more), it should be now possible to test that hypothesis.

Atmospheric model

Numerical models of chemical processes, in a medium where both neutral and ionized species are present, need to be developed in the altitude domains of production of NOx to provide guidelines for the study of the generation and effects of the transient transfers of energy between the atmosphere and the near-Earth space. These models need to be associated to models of atmospheric circulation for including the effects of the dynamics of the stratosphere and mesosphere for a global representation of NOx concentrations. Observations of lightning, TLEs, TGFs, electromagnetic emissions, accelerated particle and other parameters will be needed as inputs for these models. poker francais

In a preliminary step
- The TRANSCAR model of ion chemistry (Blelly et al., 1996) is being extended towards lower altitudes.
- Contributions from Coronal Mass Ejections (CME) to NOx effects have been recently published (Renard et al., 2006).
- A new five constituent model of lower ionospheric / mesospheric chemistry has been developed to explain recoveries from intense daytime / nightime ionization events (Inan et al., 2007; Lehtinen and Inan, 2007).

In the future it is planned to associate the developments of ion chemistry (TRANSCAR) and neutral chemistry (REPROBUS) models, and thus introduce the ion chemistry in the LMDZ code.

Personal tools