A coherent radiation mechanism for type IV solar radio bursts

Abstract

A new coherent radiation mechanism, involving nonlinear interaction of whistler solitons with upperhybrid waves, excited by energetic electrons of energies of 10 keV-100 keV, is proposed for type IV solar bursts of both moving (type IV M) and stationary (type IV S) types. We show that the type IV M bursts occur when the interaction of whistler solitons and upperhybrid waves takes place in the coronal transients whereas the type IV S bursts originate provided this interaction takes place in stationary loops where density has been increased. The emitted radiation is right-hand circularly polarized with ~100~ polarization. Increase of brightness temperature, Tb, at lower frequencies and also its decrease, at all frequencies, with the passage of time is predicted for type IV M bursts; this agrees fully with the observations. Furthermore, the decrease of Tb, with time for stationary type IV component, is easily explained if the source which supplies energetic electron to the loop, becomes weaker with time.

An Eruptive Hot-Channel Structure Observed at Metric Wavelength as a Moving Type-Iv Solar Radio Burst

“…Earlier studies have used this mechanism to explain continuum radio bursts such as the t-IV burst (Winglee & Dulk 1986;Lakhina & Buti 1985). Yet, further studies are required to check whether this mechanism can properly account for the radio characteristics reported here.…”

“…Another coherent emission mechanism that is associated with trapped electrons is the electron maser cyclotron emission, developed when electrons are lost from the trap to form a positive gradient of the velocity distribution function along the perpendicular direction in the velocity phase space (Wu et al 1979). Earlier studies have used this mechanism to explain continuum radio bursts such as the t-IV burst (Winglee & Dulk 1986;Lakhina & Buti 1985). Yet, further studies are required to check whether this mechanism can properly account for the radio characteristics reported here.…”

“…The t-IV bursts are broadband continuum emission, further classified as the moving t-IV (t-IVm) and static t-IV bursts (Weiss 1963;Melrose, 1980), generally believed to be excited by energetic electrons trapped within certain magnetic structures, such as magnetic arches, loops, or plasmoid structures (Smerd & Dulk 1971;Vlahos et al 1982;Stewart 1985). Different emitting mechanisms have been proposed, including the synchrotron and gyro-synchrotron emission (Boischot 1957;Bastian & Gray 1997;Bastian et al 2001), the plasma emission (Duncan 1981;Stewart 1982;Ramesh et al 2013), and the electron cyclotron maser emission (Kujipers 1975;Lakhina & Buti 1985;Winglee & Dulk 1986).…”