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Wednesday, November 19, 2014

Peer reviewed study finds sun's magnetic field significantly affects number of lightning strikes in UK-Environmental Research Letters-BBC

"The study has been published in the journal Environmental Research Letters."

11/18/14, "Sun's magnetic field boosts lightning strikes across the UK," BBC, Matt McGrath

"The number of lightning strikes across the UK has been significantly affected by solar activity, according to new research.

Scientists say the Sun's magnetic field is bending the Earth's own field, increasing our exposure to cosmic rays.

These rays are believed to increase the number of thunderclouds and trigger lightning bolts in some locations.

Over five years, the UK experienced 50% more strikes when the Earth's magnetic field was affected by the Sun.

Fields of influence
 
The manner is which lightning bolts are triggered has long puzzled scientists as the air is known to be a good insulator of electricity. 

Something else needs to come into play to conduct the electrical charges built up in thunder clouds down to the ground.

Since the 1990s, researchers have speculated that the magnetic activity of the Sun could be linked to lightning on Earth.

Current theories hold that high energy particles called galactic cosmic rays provide the necessary link that lets the current flow into a lightning bolt. 

This latest work suggests that the orientation of the Sun's magnetic field is playing a significant role in the number of strikes. 

The researchers believe the field is like a bar magnet, so as our star spins around sometimes the field points towards the Earth and sometimes away. 

"What we found was there is significantly more lightning in the UK when the field is pointing towards the Sun than when its pointing away which was surprising," said Dr Matt Owens from the University of Reading, the lead author on the study.

"What we think is happening is that the Sun's magnetic field is pulling or pushing on the Earth's field and that's letting energetic charged particles down into the atmosphere at different locations and the idea is that these actually trigger lightning."

"For lightning, you need a thin conducting channel like a wire, and galactic cosmic rays can provide this thin column of ionisation in the atmosphere."

The researchers found that over a five year period between 2001 and 2006, the UK experienced a 50% increase in thunder and lightning rates when the solar magnetic field was pointed away from the Earth. 

In Summer, the rates were higher, with an almost doubling of lightning strikes in July compared to when the magnetic field was pointing in the other direction.

Because of good quality records, the scientists confined their work to the UK. They believe that the same effect is playing out over the globe but with different results, so while lightning might increase over Britain, it may have decreased over Canada or Siberia.

Lightning forecast 
 
While the researchers admit that the mechanics of how cosmic rays might trigger lightning is still a theory, they believe that their discovery of an association with the movements of the Sun's magnetic fields, could lead to better predictions of thunder and lightning events. 

As the nature of the Sun's magnetic fields are well known, meteorologists could incorporate this information into weather forecasts. 

"It has real implications," said Dr Owens.

"If you can get a weather forecast good to a week ahead then yes I think we could say something about lightning rates a week ahead as well."

Recent research has also focussed on how climate change is likely to increase the amount of lightning strikes around the world. Dr Owens believes that the mechanism that his study identifies still holds, regardless of the temperature. 

"If you heat up the atmosphere you've got more convection, more water vapour, you get more thunderclouds," he said. 

"If you've got more thunderclouds, you get more lighting but I still think the triggering of that lightning could be dependent upon the Sun and its magnetic field."

The researchers now want to extend their work to look at longer historical records to see if the correlation still stands. 

According to Dr Owens, some scientists are keen to take a more "Benjamin Franklin" approach to prove the theory.

"The fundamental lightning triggering experiments are difficult to do in the lab and difficult to measure in situ," he said. 

"Some of my colleagues have been trying to launch charge sensors on balloons, through thunder clouds - it makes our health and safety officers sweat!" 

The study has been published in the journal Environmental Research Letters."

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11/18/14, "Modulation of UK lightning by heliospheric magnetic field polarity," iopscience.iop.org, ERL  

"Abstract

Observational studies have reported solar magnetic modulation of terrestrial lightning on a range of time scales, from days to decades. The proposed mechanism is two-step: lightning rates vary with galactic cosmic ray (GCR) flux incident on Earth, either via changes in atmospheric conductivity and/or direct triggering of lightning. GCR flux is, in turn, primarily controlled by the heliospheric magnetic field (HMF) intensity. Consequently, global changes in lightning rates are expected. This study instead considers HMF polarity, which doesnʼt greatly affect total GCR flux. Opposing HMF polarities are, however, associated with a 40–60% difference in observed UK lightning and thunder rates. As HMF polarity skews the terrestrial magnetosphere from its nominal position, this perturbs local ionospheric potential at high latitudes and local exposure to energetic charged particles from the magnetosphere. We speculate as to the mechanism(s) by which this may, in turn, redistribute the global location and/or intensity of thunderstorm activity."

"1. Introduction

The electrification of thunderclouds is generally accepted
to result from strong updrafts in the mixed-phase region (e.g., [4]), where the presence of both a downward flux of graupel and and upward flux of ice crystals can produce charge transfer and separation by collisions [24]. Indeed, parameterization of these processes [8] in numerical weather prediction models has led to successful forecasts of lightning flash rates [30]. A number of observational studies, however, suggest lightning flash rates may also be modulated to some degree by external factors such as solar magnetic activity [25], though the mechanism(s) and even sign of response are still under debate. On solar-cycle timescales, both positive [27] and negative [21] correlations have been found between sunspot number and thunderstorm activity. On shorter time scales, lightning over the USA has been found to decrease following impulsive reductions in galactic cosmic ray (GCR) flux by strong heliospheric magnetic field (HMF) enhancements [6]. Conversely, smaller, recurrent HMF structures appear to increase UK lightning and thunder rates, possibly via lower energy solar particles compensating for the GCR decrease [26]. These thunderstorm trends have all been linked with changes in energetic charged particle flux incident on Earth and hence changes in global atmospheric conductivity [9] and/or global lightning trigger rates [22].

The HMF is formed by the solar wind dragging coronal magnetic loops anti-sunward while both magnetic foot points remain rooted on the rotating solar surface [19]. Thus the large-scale HMF forms an Archimedean spiral, making an angle of approximately 45° to the Earth–Sun line in near-Earth space. If the overall polarity of the HMF is pointing toward (T ) the Sun, near-Earth HMF will have magnetic field components and in geocentric solar ecliptic coordinates (where X points towards the Sun, Y points away from Earthʼs orbital motion and Z is normal to the ecliptic plane), whereas away polarity (A) will result in and . At any one instant, there are typically 2 or 4 large-scale T or A 'sectors' in Earthʼs orbital plane, meaning the Earth is embedded in a given sector for an average of 7–14 days (e.g., [18]). This study investigates the effect of HMF polarity on lightning occurrence."

"2. Data

There are a number of methods for detecting thunderstorm activity, but long-term studies at mid-to-high geomagnetic latitudes are best performed by radio networks, which detect radio sferics from lightning strokes. In this study, lightning stroke rates, RL, were obtained from the UK Met Officeʼs arrival time difference (ATD) network of radio receivers in Western Europe [15]. It detects the VLF component of broad-band emission ('sferics') from lightning and uses the relative timing to determine location. The ATD system is primarily sensitive to cloud-to-ground (CG) lightning over Europe, but can detect lightning worldwide with reduced sensitivity. In order to ensure uniformity of the lightning measurements and enable more direct comparison with the UK-based thunder-day data described below, lightning data are limited to events within a radius of 500 km of central England. Ongoing development of lightning detection systems makes long-term thunderstorm activity studies problematic. However, between September 2000 and May 2005 the ATD system was not subject to any modifications affecting its sensitivity. After this period the radio network was expanded and increased in sensitivity to form ATDnet, which detects a larger number of smaller sferics. During May 2005 to 2007, this increase was only moderate, with the annual mean of RL increasing by around 50%. We include these data in the study by normalizing RL after May 2005 by 0.64, though accept that this results in the inclusion of smaller lightning events than in the initial period. (We note that our results are largely unchanged whether these later data are included or not.) Post May 2007, further developments to ATDnet meant the sensitivity increased by around 400%. Without any means to discriminate between CG lightning and smaller, inter-cloud lightning, these data are dominated by qualitatively different lightning events from pre-2007. In order to not preferentially emphasize any one season, we only consider whole years of data, 2001 through 2006 inclusive. Data are converted to daily mean lightning stroke rates to remove any diurnal variations and to enable direct comparison with RTH data.

Audible thunder records from UK Met Office manned observing sites can serve as an independent, if low fidelity, validation of radio observations. This observation is subject to false positives (such as vehicle noise or explosions) and is of a lower time resolution compared with the ATD lightning data, but crucially its detection efficiency is not subject to the ionospheric effects which could potentially affect RL. We compute a 'thunder day rate', RTH, defined as the fraction of UK manned stations which reported thunder on a given day. Any stations which did not report a single instance of thunder in a given year were assumed to not be logging such information and excluded from the study. Note that RTH will be affected by the mean altitude, and hence audibility, of thunderstorms over the UK and thus may be sensitive to somewhat different forms/intensities of thunderstorm activity than RL."...







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