Multipoint measurements of magnetospheric substorms: onset timing and tail reconnection rates

地球拥有的磁场在形状上近似为偶极子——非常类似于一个简单的条形磁铁产生的磁场。磁力线从地球的一个磁极发出，并向大气层外延伸数千公里进入太空，然后返回到另一个半球的磁极。这些磁场线穿过的空间区域不是真空，而是充满了等离子体——一种由带电粒子组成的导电气体。这些粒子大多来自地球大气层，是由紫外线照射电离高海拔大气中的气体而产生的。太阳也有很强的磁场。随着核过程在太阳内部产生能量，太阳大气层的外层向外扩展，穿过太阳系（形成太阳风），并携带着太阳磁场的残余（行星际磁场）。当太阳风和行星际磁场到达地球时，它们与地球磁场相撞，并在地球周围改变方向。地球磁场在太阳风中形成的空洞被称为磁层。在磁层内部，等离子体和磁场主要来自地球。在磁层之外，它们起源于太阳。在地球白天一侧的行星际磁场和地球磁场的交界处，磁场线有时方向相反。当这种情况发生时，磁力线可以合并或“重新连接”越过边界。换句话说，从地球表面开始和结束于相对半球的封闭磁力线可以打开，这样一端固定在地球上，另一端向外延伸到太阳风中。由于太阳风不断地远离太阳，新打开的磁力线被拖拽和拉伸，远离地球。因此，由于在昼侧边界的磁重联过程，地球的偶极磁场在地球的夜侧被拉伸，形成一条指向太阳的长磁尾。如果地球的磁场不断被剥离并被拖进尾巴，那么最终地球向阳的一面将没有磁场。然而，尾部的一个过程周期性地减少尾部开放磁场的数量，并将封闭磁场返回到白天——这个过程就是磁重联。通过重新连接两条开放的磁力线，就产生了一个封闭的磁场（就像把一块橡皮筋的两个松散的末端打在一起一样）。然而，由此产生的闭合场是高度拉伸的，就像拉伸的橡皮筋一样，它收缩回地球，将一些磁层等离子体弹射回地球。尾部重接过程不稳定。一般来说，磁场会在尾部积聚，直到达到某个临界点。不知何故，重联被触发，被拉伸的磁场从尾巴上移除并返回到地球。尾场形成的时期被称为亚暴生长阶段，而与重连和开放场线闭合相关的尾能量爆炸释放被称为亚暴扩展阶段。然而，引发膨胀阶段的过程（即触发弹射器的机制）仍不清楚——这是日地物理学中最大的不确定性之一。这次调查将使用位于尾部的航天器上的仪器测量和从地球上进行的观测，以确定磁层亚暴的触发机制。

The Earth possesses a magnetic field which is approximately dipolar in shape - very similar to the magnetic field produced a simple bar magnet. Magnetic field lines emerge from the planet at one magnetic pole and extend out of the atmosphere and many thousands of kilometres into space, before returning to the magnetic pole in the opposite hemisphere. Rather than being a vacuum, the region of space that these field lines pass through is filled with plasma - an electrically conducting gas made up charged particles. Most of these particles originate in the Earth's atmosphere having been produced by ultraviolet sunlight which ionises gases in the high altitude atmosphere. The Sun also possesses a strong magnetic field. As nuclear processes generate energy in the solar interior, the outer layer of the solar atmosphere expands outwards through the solar system (forming the solar wind), and carries with it remnants of the Sun's magnetic field (the interplanetary magnetic field). When the solar wind and interplanetary magnetic field arrive at the Earth, they collide with the Earth's magnetic field and are diverted around the planet. The cavity carved out of the solar wind by the Earth's magnetic field is called the magnetosphere. Inside the magnetosphere the plasma and magnetic field originate mainly from the Earth. Outside of the magnetosphere, they originate from the Sun. At the boundary between the interplanetary and terrestrial magnetic fields on the dayside of the Earth, the field lines sometimes orient themselves in opposite directions. When this happens, the field lines can merge or 'reconnect' across the boundary. In other words, closed magnetic field lines that start and finish at the Earth's surface in opposite hemispheres can be opened so that one end stays fixed to the Earth while the other extends outwards into the solar wind. Since the solar wind is constantly streaming away from the Sun, the newly-opened magnetic field line is dragged and stretched away from the Earth. Therefore, because of the process of magnetic reconnection at the dayside boundary, the Earth's dipolar magnetic field is stretched out on the planet's nightside to form a long magnetic tail that points away from the Sun. If the Earth's magnetic field was continuously being peeled away and dragged into the tail, eventually there would be no field left on the dayside of the planet. However, a process in the tail periodically acts to reduce the amount open magnetic field in the tail and return closed field to the dayside - this process is magnetic reconnection. By reconnecting two open magnetic field lines a closed magnetic field is produced (like typing together the two loose ends of a piece of elastic). However, the resulting closed field is highly stretched and, just like a stretched elastic band, it contracts back towards the Earth, catapulting some of the magnetospheric plasma Earthward. The reconnection process in the tail is not steady. Generally magnetic field builds up in the tail until some critical point is reached. Somehow, reconnection is triggered and stretched magnetic field is removed from the tail and returned to the Earth. The period when tail field is building is known as the substorm growth phase, while the explosive release of energy in the tail associated with reconnection and the closure of open field lines is known as the substorm expansion phase. However, the processes that cause the triggering of the expansion phase (i.e. that mechanisms that trigger the catapult) remain unclear - it is one of the biggest uncertainties in solar-terrestrial physics. This investigation will use measurements from instruments on spacecraft located in the tail and observations made from the Earth in order to determine the triggering mechanism of magnetospheric substorms.