Now，scientists have data from satellites and ground—based observations from which we know that the auroral brilliance is an immense electrical discharge similar to that occurring in a neon sign.
(介词提前的定语从句：from which，which指前面的data)
如今，科学家们根据来自卫星和陆基观察的数据得知，极光是发光原理类似于霓虹灯的巨大的电流释放。
解析：
主句：scientists have data from satellites and ground—based observations
从句1：from which we know
从句2：the auroral brilliance is an immense electrical discharge
从句3：similar to that occurring in a neon sign
从句1修饰主句中的data
从句2作为从句1的宾语从句，表示科学家从data中的得知的内容
从句3修饰从句2整句
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