Whistler-induced Electron Precipitation Research Articles

Lightning-driven inner radiation belt energy deposition into the atmosphere: implications for ionisation-levels and neutral chemistry

Abstract. Lightning-generated whistlers lead to coupling between the troposphere, the Van Allen radiation belts and the lower-ionosphere through Whistler-induced electron precipitation (WEP). Lightning produced whistlers interact with cyclotron resonant radiation belt electrons, leading to pitch-angle scattering into the bounce loss cone and precipitation into the atmosphere. Here we consider the relative significance of WEP to the lower ionosphere and atmosphere by contrasting WEP produced ionisation rate changes with those from Galactic Cosmic Radiation (GCR) and solar photoionisation. During the day, WEP is never a significant source of ionisation in the lower ionosphere for any location or altitude. At nighttime, GCR is more significant than WEP at altitudes <68 km for all locations, above which WEP starts to dominate in North America and Central Europe. Between 75 and 80 km altitude WEP becomes more significant than GCR for the majority of spatial locations at which WEP deposits energy. The size of the regions in which WEP is the most important nighttime ionisation source peaks at ~80 km, depending on the relative contributions of WEP and nighttime solar Lyman-α. We also used the Sodankylä Ion Chemistry (SIC) model to consider the atmospheric consequences of WEP, focusing on a case-study period. Previous studies have also shown that energetic particle precipitation can lead to large-scale changes in the chemical makeup of the neutral atmosphere by enhancing minor chemical species that play a key role in the ozone balance of the middle atmosphere. However, SIC modelling indicates that the neutral atmospheric changes driven by WEP are insignificant due to the short timescale of the WEP bursts. Overall we find that WEP is a significant energy input into some parts of the lower ionosphere, depending on the latitude/longitude and altitude, but does not play a significant role in the neutral chemistry of the mesosphere.

Temporal signatures of radiation belt electron precipitation induced by lightning‐generated MR whistler waves: 2. Global signatures

We use the methodology presented in a companion paper to calculate the temporal and spatial precipitation signatures of energetic radiation‐belt electrons due to pitch‐angle scattering by magnetospherically reflecting (MR) whistler waves generated by lightning discharges at geomagnetic source latitudes of λs = 25°, 35°, 45°, and 55°. We show precipitated energy and number fluxes, as well as average precipitated energy as a function of time and L‐shell for all four λs cases, and then extrapolate the results of the λs = 35° case in longitude to produce a time‐sequence of geographic ‘hot spots’ which are affected by the MR whistler induced electron precipitation. We then discuss the total precipitation in both hemispheres due to the various λs cases. Our major findings are that the precipitation region moves to higher L‐shells as a function of time, on short (0.1 sec, at the start of the event) and long (10 sec) timescales, corresponding to the first hop of the wave, and the MR portion of the whistler wave, respectively. There is also structure within the long‐timescale precipitation on the order of ∼1 sec, reflecting the periodic MR of the underlying whistler wave. As λs increases, an additional precipitated flux signature which is more incoherent and discontinuous, begins to form at higher L‐shells and later times, due to MR whistler wave reflections from the plasmapause. At lower L‐shells, a pronounced maximum occurs in the number flux of ∼1 keV electrons at L ∼ 2–3 due to the Landau resonance. The geographic hot spot affected by the precipitation can split into two separate regions per hemisphere, and occur simultaneously in both hemispheres so that up to four distinct precipitation hot spots can occur on the Earth at any instant, driven by a single lightning discharge. We also discuss potential sources of error, and comparison to related modeling efforts, and to observations using a either satellite‐borne instruments or ground‐based techniques.

Investigating radiation belt losses though numerical modelling of precipitating fluxes

Abstract. It has been suggested that whistler-induced electron precipitation (WEP) may be the most significant inner radiation belt loss process for some electron energy ranges. One area of uncertainty lies in identifying a typical estimate of the precipitating fluxes from the examples given in the literature to date. Here we aim to solve this difficulty through modelling satellite and ground-based observations of onset and decay of the precipitation and its effects in the ionosphere by examining WEP-produced Trimpi perturbations in subionospheric VLF transmissions. In this study we find that typical Trimpi are well described by the effects of WEP spectra derived from the AE-5 inner radiation belt model for typical precipitating energy fluxes. This confirms the validity of the radiation belt lifetimes determined in previous studies using these flux parameters. We find that the large variation in observed Trimpi perturbation size occurring over time scales of minutes to hours is primarily due to differing precipitation flux levels rather than changing WEP spectra. Finally, we show that high-time resolution measurements during the onset of Trimpi perturbations should provide a useful signature for discriminating WEP Trimpi from non-WEP Trimpi, due to the pulsed nature of the WEP arrival.

Testing the importance of precipitation loss mechanisms in the inner radiation belt

Manmade control of the radiation belts for the protection of space‐based infrastructure has been suggested on the basis of theoretical calculations. In this paper we put forward an experimental test of the relative importance of either whistler‐induced electron precipitation (WEP) or manmade VLF transmitters as the most significant inner radiation belt loss mechanisms for 2 < L < 2.4. The experimental identification of seasonality in inner radiation belt electrons lifetimes would provide strong evidence for the relative significance of WEP‐ or manmade transmitter‐driven losses depending on the relative phasing found.

Significance of lightning‐generated whistlers to inner radiation belt electron lifetimes

The behavior of high‐energy electrons trapped in the Earth's Van Allen radiation belts has been extensively studied, through both experimental and theoretical techniques. While the evidence for whistler induced electron precipitation (WEP) from the radiation belts is overwhelming, and the mechanisms behind WEP are well understood, the overall significance of WEP on radiation belt loss rates has not been clear. In this paper we investigate the L‐shell variation and significance of WEP‐driven loss of Van Allen belt electrons by combining in situ measurements of electron precipitation, local WEP rates determined from Trimpi perturbations, and global lightning distributions. Our modeling suggests that long‐term WEP driven losses are more significant than all other inner radiation belt loss processes for electron kinetic energies in the range ∼50–150 keV in the L‐shell range L = 2–2.4. These calculated lifetimes are comparable to the observed decay rates of artificially injected high‐energy electrons. The upper energy limit of the WEP significance range increases with decreasing L to ∼225 keV at L = 2. For electron energies above this range manmade VLF transmitters and plasmaspheric hiss should dominate over all other loss processes. However, as our lifetimes are based on rather conservative parameter estimates, these conclusions should represent the lower bounds for the energy ranges over which WEP losses are significant. For lower L‐shells the coupling of lightning activity to the production of WEP events rapidly decreases, such that by L ∼ 1.7 WEP will be unimportant in the overall loss processes.

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

Wave‐particle interactions driven by whistler mode waves in the inner Van Allan belt is an important loss process for the energetic electrons found in this region. In this paper we seek to investigate the significance of Whistler‐Induced Electron Precipitation (WEP) to inner belt electron lifetimes, by combining experimental satellite results, ionospheric remote observations, and global lightning distributions. We find that long‐term WEP driven losses at L = 2.23 are more significant than all other inner radiation belt loss processes for electron kinetic energies in the range 40–350 keV. This suggests a faster depletion rate and thus lower lifetimes than previously calculated. Thunderstorm generated whistlers are an important factor for determining the lifetimes of medium energy electrons in the inner belt.

D region reflection height modification by whistler‐induced electron precipitation

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−3 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−1. 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.

Decay of whistler‐induced electron precipitation and cloud‐ionosphere electrical discharge Trimpis: Observations and analysis

There are two distinctly different causes of phase and amplitude perturbations of subionospheric VLF transmissions (termed “Trimpis”): (1) ionization enhancement in the ionospheric D region due to whistler‐induced electron precipitation (WEP) and (2) modifications to the ionosphere induced directly by lightning. The latter appear to be subdivided into ionization anomalies produced by cloud‐ionosphere electrical discharge (CID), or “red sprites,” and heating anomalies which may not involve ionization at all. Here we consider only Trimpis (WEP and CID) produced by ionization and find that the magnitude of the Trimpi perturbation (which includes both the amplitude and phase perturbations) of both Trimpi types decays logarithmically rather than exponentially with time. While this has been previously shown for CID Trimpis, the decay of WEP Trimpis was previously thought to be exponential.

Logarithmic decay and Doppler shift of plasma associated with sprites

VLF phase and amplitude perturbations in ‘early/fast Trimpis’ have been observed simultaneously on two or more transmitters at two or more receiver sites and simultaneously with sprites observed optically in Colorado. In all cases the early/fast Trimpis could apparently be resolved into two components: the RORD (rapid onset, rapid decay) component coincident with the sprite and a ‘classic Trimpi’ component having the slower onset and decay of Trimpis generally thought to be caused by whistler-induced electron precipitation from the radiation belts. The phase and amplitude perturbations can decay at very different rates, and even change in sign from positive to negative perturbation (‘overshoot’) during decay. This implies a spatial separation between the rapid decay plasma responsible for the RORD and the slow decay plasma responsible for the classic Trimpi component of up to a few tens of km. This is much less than the expected separation of sprite plasma and electron precipitation plasma induced by the same cloud–ground lightning. The discovery of very strong early/fast Trimpis in the tropics (Darwin, Australia), where electron precipitation is unlikely, which showed similar effects like overshoot but much more clearly, caused an examination of recent Colorado data of high resolution. The Colorado Trimpis, which are clearly associated with sprites, show the same features as the Darwin Trimpis where optically detected sprites were not available. Both can be explained in terms of sprite plasma decaying from the bottom up without recourse to electron precipitation.

Are VLF rapid onset, rapid decay perturbations produced by scattering off sprite plasma?

Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler‐induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.

Rapid onset, rapid decay (RORD), phase and amplitude perturbations of VLF subionospheric transmissions

Rapid onset (few ms), rapid decay (~ls) perturbations or RORDs occur frequently on the west-to-east signal from NWC to Dunedin, more often than not with classic Trimpis. They do not appear on an NWC mimic signal directly injected into the antenna and so cannot be broadband bursts. There is no delay between the initiating sferic and RORD start, implying that they are produced not by whistler-induced electron precipitation but directly by lightning. Observations on a multi element array show that classic Trimpis and RORDs initiated by the same sferic usually come from measurably different directions, so the lightning-induced ionisation enhancements (LIEs) which cause them must be laterally displaced. They may also be vertically displaced to explain the differing decay rates (30s versus 1 s). We conclude that RORDs are VLF echoes from vertical columns of ionisation at around 40km altitude and having vertical dimensions of some tens of km and horizontal dimensions of 1–2km, since such a column would scatter sufficient signal to fit observed amplitudes. Cloud-to-ionosphere (CID) lightning discharges (also called “cloud-to-space” and “cloud-to-stratosphere” discharges) of these visible dimensions have been observed on mountain observatories and on board the Space Shuttle.

Seasonal, latitudinal and diurnal distributions of whistler‐induced electron precipitation events

The seasonal, latitudinal, and diurnal distributions of whistler‐induced electron precipitation events, detected as subionospheric signal perturbations (Trimpi events), have been studied by means of data sets acquired in 1982‐1983 at Palmer and Siple stations, Antarctica. The data sets, substantially larger than any previously examined, confirm previous indications of a broad (∼ 4 hour) maximum in hourly event occurrence rates. The peak was centered ∼ 1‐2 hours after local midnight at Palmer for the roughly north‐south Argentina Omega 12.9‐kHz path, but was shifted several hours later for the 23.4‐kHz NPM path, which has a westerly arrival bearing at Palmer. The previously reported seasonal variation, with peaks at the equinoxes, was confirmed; activity persisted in the austral winter, particularly following a magnetic storm, but in general occurred on fewer days and during shorter daily periods. Comparisons of occurrence data at Siple (L ≃ 4.3) and Palmer (L ≃ 2.4) for particular signal sources showed the number of days of activity to be larger at Palmer by a factor of ∼ 2 or more. Comparisons of five months of simultaneous Palmer and Siple receptions of 21.4‐kHz NSS signals on north‐south paths suggest that most of the events observed at Siple occurred as the result of ionospheric perturbations relatively close to the L shell of Palmer, that is, in the range L = 2 – 3. This result is consistent with the expected variation with latitude of the energy of electrons precipitated due to equatorial gyroresonance at typical whistler frequencies.