D region reflection height modification by whistler‐induced electron precipitation

Abstract

[1] We examine the electron density profile expected in the lower ionosphere due to a 0.2-s whistler-induced electron precipitation (WEP) burst with experimentally determined properties. The ionization rate in the lower ionosphere due to a single such WEP event has a height variation with a rather broad maximum, leading to additional electron densities of $5 electrons cm À3 stretching over altitudes of $75-92 km. For ambient nighttime conditions a single WEP burst with these parameters will lead to a significant electron density changes only for altitudes below $85 km. We go on to consider the cumulative response of the nighttime D region to a sustained series of WEP bursts observed through Trimpi perturbation activity on one night in the Antarctic. For altitudes >70 km, significant long-term changes in electron densities due to WEP bursts can occur. The additional WEP-produced ionization leads to increases in the high-altitude electron densities, until a new equilibrium level is reached. Peak changes in electron density are $16 times ambient at 85 km and $7 times ambient at 90 km, occurring in the $15-min period during which the WEP rate is at its peak ($4.5 per min). The simulation suggests that electron density levels ''settle'' into an new quasi-equilibrium state during the $3-hour period where the ionization at 85-km altitude is 10-12 times ambient due to WEP bursts occur at $3 min À1 . The ionization changes produced by WEP bursts lead to lower reflection heights for VLF and LF radio waves (in the Earth-ionosphere waveguide). While significant short-term changes in reflection heights are likely, realistic long-term changes in WEP occurrence rates do not appear likely to be able to explain the reported $2 km decrease in LF reflection heights observed during the last 35 years.

Investigating radiation belt losses though numerical modelling of precipitating fluxes

“…2, and integrating over the energy range 1-1500 keV. The WEP burst ionisation rate q(E) is found by an application of the expressions in Rees (1989), as has been previously described by Rodger et al (2002).…”

“…Pasko and Inan, 1994;Rodger et al, 2002). For example, it has been reported that the observed Trimpi recovery signatures may be used to determine the energy content of WEP bursts (Pasko and Inan, 1994).…”

Inner radiation belt electron lifetimes due to whistler‐induced electron precipitation (WEP) driven losses

“…It is more instructive to consider the WEP driven electron losses shown as a fraction of the number of electrons at a given energy in a magnetic flux tube having unit area perpendicular to the geomagnetic field at the top of the atmosphere. Following the approach described in Voss et al [1998] the percentage loss from the overall flux tube population at a given energy has been calculated for the 100 km altitude level at L = 2.23 for electrons with energy 45–1500 keV (after Rodger et al [2002]), and is shown in Figure 1b. Clearly, a single WEP burst causes an extremely small loss from the overall particle population in a given unit area flux tube.…”

“…We will instead make use of observed Trimpi, as it has been suggested that WEP may also be produced by unducted whistlers [ Lauben et al , 1999], which are unlikely to observed on the ground in the conjugate hemisphere. A study of Trimpi perturbations observed at Faraday, Antarctica (65.3°S, 64.3°W, L = 2.5) reported that at the typical times of highest Trimpi perturbation rates (05:30–06:30 UT) the most likely rate was 1/min observed over 115 days from 1993 and 1994 [ Rodger et al , 2002]. It has previously been found that Trimpi perturbations observed in the Antarctic Peninsula are strongly associated with high‐current cloud to ground lightning occurring around 34°N, 76°W [ Clilverd et al , 2002].…”

“…However, Trimpi observations through much of the day are often masked by ionospheric effects and noise levels, and thus lower rates are observed. Following the approach of Rodger et al [2002] we argue that because of the large size and ionization profile of observed WEP patches [ Clilverd et al , 2002], the vast majority of observed Trimpi perturbations will be due to approximately equal WEP fluxes occurring across all of this L‐shell range. Under these assumptions, the WEP rate experienced by energetic electrons at L = 2.23 for Antarctic Peninsula longitudes is taken to have the constant value throughout the day of 1/min.…”