Electron temperature enhancement effects on plasma irregularities associated with charged dust in the Earth's mesosphere

Abstract

Recently, experimental observations have shown that Polar Mesospheric Summer Echoes PMSE may be modulated by radio wave heating the irregularity source region with a ground-based ionospheric heating facilities. It is clear from these past investigations that the temporal behavior of PMSE during ionospheric heating shows promise as a diagnostic for the associated dust layer. To investigate the temporal behavior of plasma irregularities thought to produce PMSE, this work describes a new model that incorporates both finite diffusion time effects as well as dust charging. The hybrid model utilizes fluid ions described by continuity and momentum equations, electrons whose behavior is determined from quasi-neutrality, and charged dust described by the standard Particle-In-Cell PIC method. The model has been used to investigate the temporal behavior of charged dust associated electron irregularities during electron temperature enhancement associated with radio wave heating. The model predicts that the temporal behavior of the irregularities depends on the ratio of the electron-ion ambipolar diffusion time to the dust particle charging time τ d /τ c . The results indicate that typically for τ d /τ c << 1, an enhancement in electron irregularity amplitude occurs for a period after turn-off of the radio wave heating. The work also predicts that for τ d /τ c >> 1, an enhancement in electron irregularity amplitude occurs for a time period after the turn-on of the radio wave heating. Due to the dependence of τ d on irregularity scale-size, these results have important implications for observations of PMSE modification at different radar frequencies. Both continuous and discrete charging model were embedded into this computational model, the results were compared and analyzed.It is evident that significant diagnostic information may be available about the dust layer from the temporal behavior of the electron irregularities during the heating process which modifies the background electron temperature. Particularly interesting and important periods of the temporal behavior are during the turn-on and turn-off of the radio wave heating. Although a number of past theoretical and experimental investigations have considered both these on and off period, this dissertation considers further possibilities for diagnostic information available as well as the underlying physical processes. Approximate analytical models are developed and compared with a more accurate full computational model as a reference. Then, from the temporal behavior of the electron irregularities during the turn-on and turn-off of the radio wave heating, the analytical models are used to obtain possible diagnostic information for various charged dust and background plasma quantities.Finally, two experiment campaigns have been performed at HAARP, Gakona, Alaska. Preliminary observation results look promising for the existence of PMSE turn-on overshoot. However, more careful experiments need to be done before firm conclusions can be drawn. The new designed Echotek dig...

On initial enhancement of mesospheric dust associated plasma irregularities subsequent to radiowave heating

“…It can also be seen in Fig. 1 that during the turn-on of the radiowave for longer irregularity wavelengths (lower radar frequencies), an enhancement in irregularity amplitude is predicted (Scales, 2004;Chen and Scales, 2005;Scales and Chen, 2007). This behavior has not been reported in the current literature, however, the prediction of Fig.…”

“…The dust is taken as immobile and evolution for times much less than the dust diffusion time is considered although the model is not restricted to such time-scales. The plasma model has been described in more detail in previous work (Scales, 2004;Chen and Scales, 2005), therefore only a brief description is provided here.…”

“…It should be noted that x depends on the ratio of the electron temperature during and before the radiowave heating is turned on (Chen and Scales, 2005) denoted by T e /T e0 . It is assumed in the above model that the electron irregularities have achieved steady state prior to the radiowave heating being turned off.…”

“…Figure 1 shows the temporal evolution of electron irregularities during radiowave heating predicted by the model. Parameters are typical of the mesosphere at 85 km as described in Chen and Scales (2005) with n e ∼10 10 m −3 and T e ≈150 • K. The ion mass is taken to be approximately fifty atomic mass units due to the mixture of heavy water cluster ions and NO + at the altitude of interest and ν in ∼10 5 s −1 (Lie- Svenson et al, 2003). The dust density is 5% of the background plasma density and the dust radius is of order 10 nm to allow an equilibrium dust charge of one electron, Z eq d0 =1, before heating.…”

“…These works neglected the finite diffusion time of the electrons by incorporating a Boltzman model and therefore could not accurately model the temporal evolution immediately after the turn-on of the radiowave heating for a range of observing radar frequencies and dust parameters. A new model was introduced by Chen and Scales (2005) to incorporate finite ambipolar diffusion time effects and address these limitations. The results provided some additional insight into the effects of observing radar frequency, and equivalently irregularity wavelength, on the temporal evolution of the turnoff overshoot.…”