Introduction / Context:
Geomagnetic storms evolve through recognizable phases associated with solar wind/IMF conditions. Understanding these phases helps explain disruptions to satellites, power grids, and navigation systems—an interdisciplinary topic spanning geophysics and space weather operations.
Given Data / Assumptions:
- Phases often identified: sudden commencement (SSC), initial phase, main phase, and recovery phase.
- “Unstable” here refers to enhanced geomagnetic activity and strong ring-current development.
- We choose the phase with maximum sustained disturbance.
Concept / Approach:
After an SSC or initial compression, the main phase is characterized by southward interplanetary magnetic field coupling and enhanced magnetospheric convection, leading to ring-current intensification (Dst index decreases markedly). This sustained energy input produces the most severe, widespread disturbances—auroral expansions, magnetic fluctuations, and potential technological impacts. Thus, the magnetosphere is most “unstable” during the main phase, with recovery following thereafter.
Step-by-Step Solution:
Recognize SSC/initial phases as brief onset/compression events.Identify main phase as the interval of peak energy input and strongest Dst depression.Select “main phase.”
Verification / Alternative check:
Storm indices (Dst, Kp) typically peak in disturbance during the main phase; satellite observations show strongest ring-current development then, corroborating this choice.
Why Other Options Are Wrong:
initial phase / SSC: Mark onset or sudden compression, usually short-lived compared to the sustained disturbances of the main phase.None of the above: Not applicable because “main phase” matches standard storm morphology.
Common Pitfalls:
Equating the dramatic spike of an SSC with overall severity; the SSC is a sharp kick, but the main phase drives the lasting instability and technological risks.
Final Answer:
main phase
Discussion & Comments