SOLAR-TERRESTRIAL ATMOSPHERIC INTERACTIONS: A FRESH, SPARKY LOOK
We live in a strongly climate-oriented era, where each one of us is called to choose between sides; are we among the "believers" or the "infidels" of the "Global Warming?". Two decades ago, a part of the scientific community started engaging in intense discussions around the "unusual" temperature trends which, for the northern hemisphere, had been going undoubtedly uphill. The first serious counter arguments regarding the validity of these findings targeted the inherited errors that all observational tools possess. New observations and improved algorithms started to appear fairly quickly, a fact that partially appeased debates and concerns. Skeptics of the global warming theory try to find other kinds of evidence to second-guess the mainstream CO2 increase and steer away from anthropocentric related theories. And suddenly, along came a cloud...
Life on Earth as we know it is a direct result of cloud existence. In simple words, the clouds are for the Earth what igloos are for the Eskimos; a thermal insulator which continuously works as an anti-freeze mechanism. The clouds' regulatory power is simply vast; they control the amount of Sun radiation reaching the surface, the atmospheric water cycle and even our every day habits and mood. For most people, clouds are simply interpreted; they appear in the horizon when rain is coming, but their physics can be as complicated as a Moon landing attempt.
As many groundbreaking ideas in the history of science, clouds had their own fair share; it has been suggested that such physical mechanisms exist and could potentially control cloud properties by what happens millions of kilometers away, even outside of our own Galaxy (Greek word "γάλα" for "milk", hence the Milky Way).
The Galactic Cosmic Rays (GCR) are highly charged particles (made mostly of hydrogen ions) which originate from a variety of interstellar processes (e.g. star explosions, mini, super novas etc.). GCR traveling through the universe at speeds much lower than that of the light, reach our planet, its atmosphere and even its surface. Overall, the intensity of the GCR reaching the Earth's atmosphere is largely controlled by the Sun. During periods of increased solar activity (e.g. violent events such as solar flares) the amount of GCR reaching the Earth is usually suppressed. Presently, around 80 observatories around the world monitor GCR activity in real time.
Along with some other gases (e.g. radon, the only radioactive element in gaseous from) and because of their charge-carrying property, GCR are considered as the "official ionizing agent" of our atmosphere. Limited observations as well as laboratory experiments came forward to support that there could an abridging between the GCR bearing electrical charge and cloud microphysical properties such as particle and ice content. Is then the GCR to blame (hence the Sun) for the climatological trends observed during the past few decades? Are extra-terrestrial processes able to control cloud formation therefore global climate? A barrage of reports supporting planetary cloud formation-GCR interactions, studies which completely discredited them and many other lying undecided somewhere in between, briefly completes the picture of maybe the most intense scientific duel in modern atmospheric physics.
Is it then possible that one side is completely wrong while the other is completely foul-less? Are we about to pass a "guilty", or for that matter, an "innocent" verdict on the only ionization source of our atmosphere? Could there be something else that we are we missing?
From the era when Δίας (Zeus) punished the sinners with his lightning bolts, to the ingenious Benjamin Franklin, lightning has always been one of the most mesmerizing, mysterious and at the same time violent phenomena in the Earth's repertoire. In its overwhelming majority, lightning relates to severe weather, destructive hail and hazardous conditions for both humans and animals. Most significantly though, lightning relates to the electrical charge and ice residing inside a cloud.
At the same time, lightning may well be the only meteorological parameter that can be observed on a global scale at every given second. Technology available since the mid 80's, allows world wide monitoring of lightning activity with relatively low cost infrastructure. Pioneering efforts by NASA lead to the first operational lightning observational platforms from space since the early 90's (Optical Transient Detector-OTD and Lightning Imaging Sensor-LIS). Among the ground-based observational networks, the U.S. National Lightning Detection Network (NLDN) could be easily characterized as a substantial technological advancement, with more than 100 ground-based monitoring stations and data availability extending back to beginning of the previous decade.
Could lightning provide some link between the GCR and the atmosphere? It certainly fits the profile of a potential candidate since it is dictated by some of the processes that also control clouds; Unlike cloud-related postulations, only a small number of studies have directly addressed and tested the "lightning" hypothesis. The first attempts to such claims mainly focused on the theoretical expectation that lightning activity must have a reaction during intense solar events and consequent sudden GCR reductions (Forbush events - FE). Letherbridge (1986), used empirical lightning observations to temporally place the time window (between 3-5 days) within which lightning activity is in fact enhanced during a relative GCR increase (Ground Level Enhancements). It was only recently that the "lightning" hypothesis was tested, based on 16 years worth of daily NLDN data. Chronis (2009), in his study documented the Letherbridge "mirror hypothesis", in that the U.S. continental lightning activity was suppressed 4-5 day following FE. These findings were also corroborated in monthly time-scales as the study highlighted positive trends between monthly lightning activity over the continental U.S. and incoming GCR.
Are these findings capable of introducing new concepts to the hypothesized GCR-atmospheric interactions? What if indeed we have a few sparks less during a solar flare? How would this affect the climate? It is a valid question although one must bare in mind several important facts; decreased flux of incoming GCR may be isolated events but they come in bunches. For example during a period of solar maxima (e.g. 2002-2003) we have numerous GCR reductions of various magnitudes. Most importantly, lightning is seldom considered as a cause rather as a result. If there is indeed a established GCR-lightning modulation then what causes this? In other words, considering everything else being "equal", what would be their substantial difference between an electrically active and an "insulated" cloud? Answers to questions such as what happens to the distribution to the suspended water droplets or ice crystals after the electrical field has been discharged or what is the true mechanism that initiates the electrical spark are needed in order to have clear picture of the lightning world.
Most likely, the answer is lying far away from the thunder following the lightning strike, residing deep in the cloud's mysterious microphysical world. Future synergistic observational capabilities like the Geostationary Lightning Mapper combined with existing world-wide lightning detection networks (Zeus-http://www.zeus.iag.usp.br/, World Wide Lightning Location Network, http://webflash.ess.washington.edu/) may hold the key to challenging questions regarding atmospheric electricity, cloud microphysics and how these may be affected by such extra-terrestrial influences. In the end, it seems that our primal fear of lightning may turn into enigma-induced respect and surprise, even during the era of the genome project and space exploration.
Image courtesy: www.Solarmonitor.org
One of the largest solar flares ever recorded (30 Oct, 2003)
Note: All references in this document can be found at: http://www.docstoc.com/docs/21735326/Investigating-Possible-Links-between-Incoming-Cosmic-Ray-Fluxes-and-Lightning-Activity-over-the-US
Dr. Themis G. Chronis
Institute of Inland Waters
Hellenic Center for Marine Research