Transient dynamics of secondary radiation from an HF pumped magnetized space plasma

• In order to systematically analyze the transient wave and radiation processes that are excited when a high-frequency (HF) radio wave is injected into a magnetized space plasma, we have measured the secondary radiation, or stimulated electromagnetic emission (SEE), from the ionosphere, preconditioned such that geomagnetic field-aligned plasma irregularities are already present. The transient dynamics experiments were made using a duty cycle of the HF radio wave of 200 ms (180 ms on and 20 ms off) and 100 ms (80 ms on and 20 ms off) for various frequencies near the fifth harmonic of the local ionospheric electron cyclotron frequency. Within the first 10 ms after the radio pulse turn-on, frequency downshifted structures of the SEE exhibit an overshoot with a maximum at 3 ms < t < 8 ms, whereas the upshifted spectral components do not exhibit this feature. The relative magnitude of the overshoot is strongly dependent on the frequency offset of the pump from the harmonic of the electron cyclotron frequency. A transient blue-shifted frequency component is identified. This component is upshifted from the pump by 14 kHz < Δ f< 55 kHz and exists only within the first 10 ms after the radio pulse turn-on. On a longer time scale we analyze the amplitude modulation, or "ringing," of the reflected radio wave, (also known as "quasi-periodic oscillations" or "spikes"). The ringing has a frequency of the order 15-20 Hz and we show that this phenomenon is also present in the SEE sidebands and is synchronized with the ringing of the reflected HF wave itself.

Ämnesord

NATURVETENSKAP  -- Fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences (hsv//eng)

Nyckelord

Quasi periodic variation

Amplitude modulation

Frequency shift

Blueshift

exhibits

harmonics

Electron cyclotron frequency

cycles

experimental studies

geomagnetic field

ionosphere

electromagnetic emission

high frequency

Radio wave

Plasma

radiation

dynamics

transient phenomena

Physics

Fysik

Publikations- och innehållstyp

ref (ämneskategori)

art (ämneskategori)