Substorm is a basic mode of coupling among the solar wind, magnetosphere, and ionosphere, and is an important space weather event driven by the solar wind. Substorms explosively release energy, accelerating and heating particles with a power of up to 1 billion kilowatts, roughly equivalent to the total power generation capacity of a country. Understanding the solar wind-magnetosphere-ionosphere coupling related to substorms is one of the main scientific objectives of the joint China-Europe SMILE satellite mission. The causal chain of substorms starts with the southward magnetic field of the solar wind, undergoes magnetic reconnection at the magnetopause, produces strong electric currents and magnetic field disturbances in the ionosphere, and is characterized by the AE geomagnetic index, often accompanied by auroral eruptions. The study of substorms has always been controversial, with the difficulty of directly verifying the intermediate causal chain across scales and layers forming opposing "inside-out" and "outside-in" phenomenological models.

In the face of the challenges posed by the Asia-Pacific power outage, the research team led by Academician Chi Wang has proposed an innovative solution that involves the precise measurement of response times between the "starting point - key nodes - endpoint" as a limiting factor. By reverse inferring the causal chain between these key points, a new phenomenological theoretical model of the Asia-Pacific power outage has been developed.

The causal chain in the traditional model of substorms relies on the Dungey cycle image, where magnetic field lines convect to the tail and accumulate, with a response time of approximately one hour. However, through satellite-ground joint observations, researchers such as Academician Chi Wang and Dr. Lei Dai have discovered a type of substorm where the response time for the generation of substorm currents from magnetospheric reconnection at the top of the magnetosphere is within 10-20 minutes. This type of substorm originates from the continuously reversing north-south magnetic field carried by Alfven waves in the solar wind (IMF Bz). The oscillating north-south magnetic field is compressed and amplified by the Earth's bow shock as it reaches the magnetopause, triggering intermittent non-steady-state magnetic reconnection. The enhanced magnetic reconnection at the magnetopause is accompanied by a rapid rise in the AU and AE geomagnetic indices, ultimately resulting in the production of substorm currents and substorm bursts within 10-20 minutes. This type of substorm occurs frequently in fast solar wind and CIR-triggered magnetic storms.

Based on the observed response time and causal chain, a new phenomenological model for substorms was proposed by researchers including Academician Chi Wang and Dr. Lei Dai. The model is shown in the figure below. Magnetic reconnection at the magnetopause causes enhanced convection, and the enhanced convection electric field is mapped along the magnetic field lines to the high-latitude ionospheric cusp region. In this mapping, the potential difference caused by the convection electric field is conserved because the convection electric field is the electrostatic field perpendicular to the magnetic field lines, which are equipotential lines. The convection electric field in the high-latitude ionospheric cusp region penetrates into the auroral zone, driving the two-cell convection pattern of the ionosphere and inducing the east-west convection electrojet in the auroral zone, as reflected by the increase in the AU index. The convection electric field in the auroral zone causes the Pedersen current and the 1st and 2nd field-aligned currents. After the two-cell convection is enhanced, the nighttime convection electric field is mapped to the transition region between the magnetospheric dipole field and the magnetotail via the 2nd field-aligned current, driving the convection in the magnetosphere to produce latitudinal flow deviation and reduce the tail current, resulting in magnetic dipolarization and the formation of the substorm current wedge effect and ionospheric substorm current. In the entire causal chain, it takes about 10-20 minutes to rebuild the two-cell convection pattern [Kennel 1996], while the electric field and potential difference are mapped along the magnetic field lines very rapidly, in a matter of minutes. This type of directly driven substorm is independent of the causal chain of the Dungey cycle, and its causal chain goes directly from the ionosphere to the magnetospheric transition region.

From the perspective of the new substorm model, the contradiction between the traditional "inside-out" and "outside-in" models stems from the fact that substorms themselves have two types, each with its own independent causal chain. One type of substorm relies on the Dungey cycle and responds to the accumulation of magnetic field lines in the tail, and its causal chain is more likely to be "outside-in" in the tail. The other type of substorm is directly driven by the ionosphere, and its causal chain is more likely to be "inside-out" in the tail.

 

The new substorm phenomenological model is directly related to the two major scientific questions proposed by the SMILE mission (the mode of interaction between the solar wind and the magnetosphere, and the full-process cycle of substorms), and is expected to be studied and tested in detail in the SMILE satellite mission. The research results were published in The Astrophysical Journal.