Streamer waves — large-scale oscillations of the coronal plasma sheet — were thought to require fast coronal mass ejections (>500 km/s) as drivers. Shen, Tiwari, and colleagues observe two recurrent streamer waves driven by consecutive slow CMEs (<500 km/s) accompanied by only modest flare activity. The energy threshold for excitation is lower than assumed.
But the deeper finding concerns the wave periods. The two waves, driven by CMEs of different energies, had dramatically different amplitudes (0.41 vs. 0.77 solar radii) and wavelengths (4.02 vs. 6.17 solar radii). Yet their periods — 2.66 and 2.53 hours — and the rates at which those periods changed were nearly identical.
This means the streamer is not a passive medium responding proportionally to its driver. It is a resonant structure that selects its own oscillation frequency from its eigenmodes, using the CME merely as a trigger. The driver energy determines the amplitude and wavelength — how strongly the structure rings — but the temporal structure is set by the streamer itself. Different drivers produce the same period because the period is an intrinsic property of the plasma slab.
Three-dimensional stereoscopic reconstruction confirms significant deceleration of both waves, and the second wave shows an increasing amplitude with heliocentric distance — defying the expectation of dissipative decay. The energy source for this amplification must be the streamer's own stored energy, further supporting the eigenmode interpretation.
Coronal streamers are eigenmode resonators, not passive waveguides. The CME does not imprint its energy signature on the wave's temporal structure but merely rings the streamer at its own natural frequency, which is why different drivers produce the same period.