Radio Emissions from Double RHESSI TGFs
| Nugget | |
|---|---|
| Number: | 294 |
| 1st Author: | Andrey Mezentsev |
| 2nd Author: | |
| Published: | 6 March 2017 |
| Next Nugget: | TBD |
| Previous Nugget: | Pierre Kaufmann |
Introduction
The Earth's atmosphere is a rich source of high-energy radiations, including hard X-rays and gamma-rays, which RHESSI has been monitoring since launch. RHESSI's view is of course from space, looking down at the Earth. We introduced these surprisingly interesting terrestrial observations in earlier Nuggets. Here we describe interesting new features of RHESSI observations of transient gamma-ray flashes and show how their feedback helps to understand the RHESSI data, specifically its clock data. The TGFs, associated with lightning, have extraordinary precision. Reference [1] provides detailed information.
RHESSI detects gamma rays emerging from the atmosphere; these are the byproduct of the bremsstrahlung of relativistic runaway electrons produced deep in the thunderstorm cloud system. Here the electric fields can be high enough to produce the runaway, and the potential difference large enough (millions of volts) to produce gamma rays.
Nowadays it is commonly accepted in the TGF community that the role of a particle accelerator for the runaway electrons producing a TGF is played by an upward propagating intracloud lightning negative "stepped leader". Such a leader, being well developed, concentrates a huge potential difference (proportional to the leader length) and an intense electric field in front of its tip. This area in front of a leader tip serves as a particle accelerator for the TGF; such a scheme matches the fact that quite often TGFs are reported to occur simultaneously with the lightning strokes detected by lightning detection networks.
The associated phenomena of course include tremendous radio emissions over a very broad band. The radio data led to an alternative model for the emissions (Ref. [2]). According to this model the radio emission that is detected by the lightning detection network simultaneously with the TGF could be radiated by the TGF itself, and not necessarily to the lightning leader. This circumstance is very important, because it might help to distinguish between different mechanisms responsible for TGF generation. Accordingly we are interested in the most accurate timing information available.
Both RHESSI and the World Wide Lightning Location Network (WWLLN) have very precise timing capabilities. How do they compare? By studying the simultaneous data over many years, we can hope to analyze any discrepancies and obtain the best possible calibration of these timebases. Note that this is not trivial: in one microsecond, light travels only 300 m, a scale small compared to a thundercloud and very, very small compared with the distance between it and RHESSI.
RHESSI's clock
The corpus of data obtained since RHESSI's launch fifteen years ago, matched with WWLLN detections, has given us a set of double TGFs associated with radio signatures. which These exhibit a very intriguing asymmetric behavior: there is a clear tendency for a radio signature to be simultaneous with the last peak in a multi-peak TGF. Is this a clue that we can exploit for scientific purposes?
