The DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) spacecraft detects short bursts of lightning-induced electron precipitation (LEP) simultaneously with newly injected upgoing whistlers. The LEP occurs within < 1 s of the causative lightning discharge. First in situ observations of the size and location of the region affected by the LEP precipitation are presented on the basis of a statistical study made over Europe using the DEMETER energetic particle detector, wave electric field experiment, and networks of lightning detection (Météorage, the UK Met Office Arrival Time Difference network (ATDnet), and the World Wide Lightning Location Network (WWLLN)). The LEP is shown to occur significantly north of the initial lightning and extends over some 1000 km on each side of the longitude of the lightning. In agreement with models of electron interaction with obliquely propagating lightning-generated whistlers, the distance from the LEP to the lightning decreases as lightning proceed to higher latitudes.
Cyclotron-resonant loss of trapped electrons via scattering by lightning whistler waves was first indirectly evidenced as whistler-associated perturbations of subionospheric very low-frequency (VLF) signals (Helliwell et al., 1973) and then directly detected (Voss et al., 1984, 1998) as lightning-induced electron precipitation bursts associated with ducted whistlers.
In a subsequent study, Inan et al. (2007) show that large regions of enhanced background precipitation are produced and maintained by high rates of lightning within a localized thunderstorm.
Analysis of the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) spacecraft particle data showed that energetic
electron precipitation exhibits a seasonal dependence consistent with
lightning-induced electron precipitation. Over the United States, energetic
electron fluxes in the slot region (between
According to the model by Abel and Thorne (1998), electron scattering near
the peak of the inner zone,
Geographical distribution of the electron integral flux between 90
and 200 keV at 700 km at pitch angles close to 90
Despite wave propagation models showing the expected location of the
lightning-induced electron precipitation (LEP) events relative to the
location of the lightning (Lauben et al., 1999, 2001), no direct statistical
study to experimentally determine the exact relative location of the
two phenomena made it possible. The DEMETER spacecraft and ground-based lightning networks are
used to infer such a distribution. The region from
Several LEPs corresponding to an intense storm activity over the
Mediterranean region on 6 October 2007. Panel (
Locations of lightning (red crosses) and the associated satellite coordinates corresponding to the detection of LEPs (grey dots).
DEMETER was in a 700 km altitude, polar and circular sun-synchronous orbit (Parrot, 2006). The DEMETER data used for this paper come from two different instruments: (1) the Instrument for Detection of Particles (IDP) (Sauvaud et al., 2006), measuring energetic electron fluxes with a 1 and 4 s time resolution for burst and survey mode, respectively, and in the energy range of 72.9 keV to 2.35 MeV in 256 steps with a 17.8 keV energy resolution and (2) the Electric Field Instrument (ICE), measuring electromagnetic fields up to 20 kHz in burst mode (Berthelier et al., 2006). The lightning position, timing, and peak current, when available, are provided by the following ground-based lightning detection networks: the World Wide Lightning Location Network (WWLLN; USA), South Africa Weather Service (SAWS), European Cooperation for Lightning Detection (EUCLID)–Météorage (EU), and Arrival Time Difference network (ATDnet; UK). ATDnet data were available for 2004 to 2011, whereas EUCLID–Météorage data were available for 2007 to 2008. WWLLN data have an average resolution over Europe; this led us to generally prefer the local ATDnet database for reported cases. However, ATDnet data do not provide intensity and polarity lightning features; therefore, we determined causative lightning peak currents from the EUCLID–Météorage database for cases reported for 2007 to 2008.
DEMETER data were used to search for LEP bursts over Europe for the entire
mission period, i.e., from 2004 to 2010. Such an example of short bursts of
electron fluxes up to 200 keV, detected on 9 October 2007, is displayed in
Fig. 2a in the IDP data (first and third panel from the top), with strong whistlers in ICE
data (second panel from the top) associated with strong lightning peak intensity (fourth panel from the top). Figure 2b shows the simultaneous position of lightning and the
satellite. High-frequency lightning thunderstorms are present during that day, with
148 lightning strokes between 20:48:00 and 20:49:30 UT. Two active cells
trigger electron precipitation over Germany, with one located offshore of
Corsica, the other in southern Italy. This precipitation of electrons
presents flux enhancements over a wide energy range in IDP spectra and a clear
peak in integral fluxes (90.7 to 250.9 keV), while ICE spectra shows intense
0
In order for the DEMETER data to be selected as a correlated LEP and
lightning event, the following set of strict criteria had to be fulfilled
simultaneously:
DEMETER had to be located between 35 and 55 The electron fluxes observed onboard DEMETER have to display short bursts
in the energy range between 90 and 200 keV. An integral electron flux between
90 and 250 keV (where cyclotron resonance between electrons and Whistlers in
the 5 kHz range is expected), which is greater than 50–60 e On board DEMETER, the time occurrence of the electron burst (minimum
recorded duration of 1 s in burst mode) has to follow the time
occurrence of whistlers within 0.6 s (see Peter and Inan, 2004). The time occurrence of the whistlers observed on board DEMETER must
coincide with lightning detected on ground (parent lightning) within 0.2 s,
given the fact that lightning propagates at the speed of light. Note that
the time resolution for the wave measurements is 0.2 s. The latitude of the parent lightning must not differ by more than
15
Note that a 0.4–0.6 s time delay represents the expected duration
between the lightning-generated whistler and the interaction with radiation
belt electrons, which precipitate into the ionosphere (Peter and Inan, 2004). LEP events,
in which several distant and simultaneous thunderstorms and the causative
lightning position could not be clearly determined, have been excluded.
By using this methodology, we found 60 clear DEMETER observations of
electron precipitation events related to lightning over Europe (Fig. 3).
Considering the large number of LEPs at times characterized via
subionospheric VLF signal perturbations (e.g., 80 and 81 LEPs
for two 4 h periods; Peter and Inan, 2004), the number of events presented here
is relatively small. This is mostly due to the strict rules used during the
selection process and the limitation of the IDP, which cannot
detect energies lower than 90 keV. All LEP events are located between 40 and
60
Seasonal distribution (black) and seasonal normalized distribution (grey) of the detected lightning–LEP events. The seasonal distribution has been normalized by the seasonal total number of lightning flashes occurring over the domain and scaled according to the winter season.
Spatial distribution of the latitudinal and longitudinal lightning–satellite distance with latitudinal and longitudinal histograms.
Panel (
Lightning–satellite latitudinal and longitudinal distances relative to the
parent lightning are shown in Fig. 5. Spatial distributions account for a
longitudinal distribution that is slightly biased to the east from the lightning
position, whereas the latitudinal distribution is directly north of the
lightning position. Note that a 100 keV electron only drifts at a rate of
1
LEP over Europe associated with an active storm in the geographical conjugate area on 6 April 2008. From top to bottom: integral electron energy flux from 90 to 250 keV; wave-frequency–time spectrogram between 0 and 20 kHz (the arrows indicate four intense whistlers); energy–time electron spectrogram from 100 to 200 keV; lightning occurrences from ATDnet; lightning occurrences from SAWS. The map shows the trajectory of the spacecraft (grey) when LEP are detected.
High-resolution ICE data VLF spectrogram up to 10 kHz at
the time of the three simultaneous lightning strokes and the associated
whistler (1) in Fig. 7. The vertical and horizontal dashed lines
correspond to the time and the frequency difference, respectively, used to
compute the dispersion parameter
Figure 6a gives the lightning latitude as a function of the
lightning–satellite distance. Note that lightning–satellite distances are
greater at lower than at higher latitudes. The solid line gives the best fit,
while 1
Furthermore, from time to time, bursts of energetic electrons are observed on
board DEMETER without corresponding lightning activity over Europe. For
example, on 6 April 2008, an enhanced precipitation flux period was observed
over France on board DEMETER with no lightning activity detected by the
ATDnet network (Fig. 7). No 0
Let us first check that the characteristics of the selected whistlers are
compliant with ducted propagation along the field line. To achieve this goal
we take into account the general form of Eckersley's law:
Complementary measurements of energetic electrons and VLF waves performed
simultaneously on board DEMETER allowed us to unambiguously determine the
occurrence of electron short bursts associated with whistlers. More rarely,
we were able to relate these events to lightning whose position is given by
ground-based networks. In about 60 cases, the identification of the exact
position of the lightning, not only that of the storm cell, was possible
based on relative timings. The analysis of the database clearly shows that
the electron precipitation in the energy range of 90 to 200 keV is located north of the parent lightning in a wide region
with an average difference in latitude reaching up to 7.7
A statistical study of lightning distribution associated with spontaneous electron precipitation has been carried out. A total of 60 instantaneous clear lightning-induced electron precipitation have been detected by DEMETER during passes over Europe. The study shows that lightning leads to electron precipitation over a very large area located north of lightning up to 1500 km. This area spreads over 1000 km on both sides of the causative lightning. A clear correlation between the lightning latitude and the lightning–satellite latitudinal distance has been highlighted and is in good agreement with whistler-wave propagation models and their interactions with radiation belt electrons (Lauben et al., 2001). Furthermore, ducted-whistler-generated electron precipitation detected by DEMETER over France and associated with lightning activities in the conjugate hemisphere has been found. These results appear to confirm previous indirect studies using the ionospheric perturbations measured by VLF transmitters and receivers and caused by high-energy electron precipitation associated with lightning.
We thank the World Wide Lightning Location Network, the UK Met Office, Météorage, and the South Africa Weather Services for providing the lightning data. The topical editor, S. Milan, thanks two anonymous referees for help in evaluating this paper.