Solar wind modulation of lightning

雷丁大学（University of Reading）空间与大气电学（太空与大气电学）小组最近的研究表明，高速太阳风到达地球后，整个欧洲的照明速度明显发生了变化。此外，太阳风的磁极已被证明发挥了作用。虽然确切的机制尚不清楚，但迄今为止的证据表明，它可能是通过日球层磁场和/或太阳风中的高能粒子对银河宇宙射线（GCRs）的调制产生的。在到达地球后，这些高能粒子进一步受到地球磁场的影响，这使得除了最高能粒子外，其他所有高能粒子都无法在地面被探测到。低层大气中的高能粒子改变了空气的导电性，这反过来又被认为使闪电在雷雨云中以低得多的电场强度发生。闪电是全球电路的一部分，雷暴就像电池一样，将电荷从地球表面转移到较低的电离层（地球上层大气的带电部分，从海拔60公里左右开始）。由于电离层是导电的，这种电荷从当地的雷暴区传播到全球，然后在全球天气晴朗的地区以小的垂直电流的形式泄漏回地球。因此，电离层和地球表面就像一个球形电容器的两块板，雷暴给电离层充电，使电离层相对于表面的电势达到250千伏左右，而大气的弱导电性为这种电荷泄漏回表面提供了一条途径。项目主管先前的研究表明，不仅仅是电离层下部受到闪电的影响，在地面闪电活动后，大约100公里处的电气化层的浓度也会增强。半球间电离层密度的异常也暗示了闪电是一个原因。关于太阳风对地球上层和下层大气的影响，显然还有很多东西需要了解。由于高层大气和电离层受太阳活动和太阳风变化的支配，任何连接高层和低层大气的机制都提供了一个渠道，使太阳活动能够影响低层大气。在他们迄今为止的工作中，研究人员已经证明，高速太阳风流的通过可以使观测到的闪电率增加多达30%。由于这种太阳风会随着太阳27天的公转周期而旋转，所以它们到达地球是可以预测的。这些信息可能有助于预测雷暴的严重程度。在确定了这个新的研究领域后，在这个项目中，学生将在创新的跨学科研究中研究来自日球层、电离层和低层大气的丰富现有数据，以确定所涉及的机制，确定影响的全球范围，并为改进闪电预报的潜力提供见解。

Recent research by the Space & Atmospheric Electricity (SPATE) group at the University of Reading has demonstrated a clear modulation of lighting rates across Europe by the arrival of high-speed solar wind streams at Earth. Furthermore, the magnetic polarity of the solar wind has been shown to play a role. While the exact mechanism remains as yet unknown, evidence to date suggests that it could be through the modulation of galactic cosmic rays (GCRs) by the heliospheric magnetic field and/or by energetic particles within the solar wind. On arrival at Earth, these energetic particles are further affected by the Earth's magnetic field, which prevents all but the most energetic from being detected at the ground. Energetic particles in the lower atmosphere modify the electrical conductivity of the air, which in turn is thought to enable lightning to occur at much lower electric field strengths within thunderclouds.Lightning forms part of the Global Electric Circuit, with thunderstorms acting like a battery, transferring charge from the Earth's surface to the lower ionosphere (the electrified part of the Earth's upper atmosphere starting at altitudes around 60 km). Since the ionosphere is electrically conducting, this charge is spread globally from the local thunderstorm regions, where it then leaks back to earth as a small vertical current in fair weather regions around the globe. Thus the ionosphere and the Earth's surface act as two plates of a spherical capacitor with thunderstorms charging the ionosphere to a potential of around 250 kV with respect to the surface and the weak conductivity of the atmosphere providing a pathway for this charge to leak back to the surface.Prior research by the project supervisor has demonstrated that it is not just the lower ionosphere that is enhanced by lightning, with the concentration of electrified layers at around 100km being enhanced following lightning activity on the ground. Anomalies in the ionosphere density between hemispheres also hints at lightning as a cause.There is clearly much to be learned about the influence of the solar wind on the Earth's upper and lower atmospheres. Since the upper atmosphere and ionosphere are dominated by variations in solar activity and solar wind, any mechanisms which link the upper and lower atmospheres provides a conduit by which solar activity can influence the lower atmosphere. In their work to date, researchers within the SPATE group have demonstrated that the passage of a high-speed solar wind stream can increase the observed lightning rate by as much as 30%. Since such solar wind streams rotate with the 27 day rotation of the Sun, their arrival at Earth is very predictable. Such information could potentially help in forecasting the severity of lightning storms.Having identified this new area for research, in this project, the student will study the wealth of existing data from the heliosphere, ionosphere and lower atmosphere in innovative cross-disciplinary studies in order to identify the mechanisms involved, determine the global extent of the effects and provide insight into the potential for improving lightning forecasts.