Broadband Extremely Low Frequency Research Articles

On O+ Ion Heating by BBELF Waves at Low Altitude: Test Particle Simulations

AbstractWe investigate mechanisms of wave particle heating of ionospheric O+ ions resulting from broadband extremely low frequency (BBELF) waves using numerical test particle simulations that take into account ion‐neutral collisions, in order to explain observations from the Enhanced Polar Outflow Probe (e‐POP) satellite at low altitudes (∼400 km) (Shen et al., 2018, https://doi.org/10.1002/2017JA024955). We argue that in order to reproduce ion temperatures observed at e‐POP altitudes, the most effective ion heating mechanism is through cyclotron acceleration by short‐scale electrostatic ion cyclotron (EIC) waves with perpendicular wavelengths λ⊥ ≤ 200 m. The interplay between finite perpendicular wavelengths, wave amplitudes, and ion‐neutral collision frequencies collectively determine the ionospheric ion heating limit, which begins to decrease sharply with decreasing altitude below approximately 500 km, where the ratio becomes larger than 10−3, νc and fci denoting the O+‐O collision frequency and ion cyclotron frequency. We derive, both numerically and analytically, the ion gyroradius limit from heating by an EIC wave at half the cyclotron frequency. The limit is 0.28λ⊥. The ion gyroradius limit from an EIC wave can be surpassed either through adding waves with different λ⊥ or through stochastic “breakout,” meaning ions diffuse in energy beyond the gyroradius limit due to stochastic heating from large‐amplitude waves. Our two‐dimensional simulations indicate that small‐scale (<1 km) Alfvén waves cannot account for the observed ion heating through trapping or stochastic heating.

Low‐Altitude Ion Heating, Downflowing Ions, and BBELF Waves in the Return Current Region

AbstractHeavy (O+) ion energization and field‐aligned motion in and near the ionosphere are still not well understood. Based on observations from the CAScade, Smallsat and IOnospheric Polar Explorer (CASSIOPE) Enhanced Polar Outflow Probe at altitudes between 325 km and 730 km over 1 year, we present a statistical study (24 events) of ion heating and its relation to field‐aligned ion bulk flow velocity, low‐frequency waves, and field‐aligned currents. The ion temperature and field‐aligned bulk flow velocity are derived from 2‐D ion velocity distribution functions measured by the suprathermal electron imager (SEI) instrument. Consistent ion heating and flow velocity characteristics are observed from both the SEI and the rapid‐scanning ion mass spectrometer instruments. We find that transverse O+ ion heating in the ionosphere can be intense (up to 4.5 eV), confined to very narrow regions (∼2 km across B), is more likely to occur in the downward current region and is associated with broadband extremely low frequency (BBELF) waves. These waves are interpreted as linearly polarized perpendicular to the magnetic field. The amount of ion heating cannot be explained by frictional heating, and the correlation of ion heating with BBELF waves suggests that significant wave‐ion heating is occurring and even dominating at altitudes as low as 350 km, a boundary that is lower than previously reported. Surprisingly, the majority of these heating events (17 out 24) are associated with core ion downflows rather than upflows. This may be explained by a downward pointing electric field in the low‐altitude return current region.

Laboratory generation of broadband ELF waves by inhomogeneous plasma flow

AbstractTransversely accelerated ions and the associated heating of the high‐latitude ionosphere have been attributed to broadband extremely low frequency (BBELF) turbulence. Controlled laboratory tests of the hypotheses on the formation mechanism of BBELF waves have involved only a few examples, e.g., current‐driven and shear‐driven instabilities. In this work, electrostatic fluctuations in the ion cyclotron frequency range have been excited by inhomogeneous energy‐density‐driven instability (IEDDI). This was achieved using the interpenetrating plasma method with a much larger electric field scale size LE comparable to the ion gyroradius ρi, which was challenging earlier because of plasma conditions. The peak frequency of the IEDDI spectrum falls as low as ω≈0.3ωci, where ωci is ion cyclotron frequency. This is an interesting result because the previous attempts could not produce such low‐frequency IEDDI, although it was known theoretically to be possible. The observations made by FAST, Freja, and Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites might be explainable in terms of the reported experimental results.

Exposure estimates based on broadband ELF magnetic field measurements versus the ICNIRP multiple frequency rule

The evaluation of exposure to extremely low-frequency (ELF) magnetic fields using broadband measurement techniques gives satisfactory results when the field has essentially a single frequency. Nevertheless, magnetic fields are in most cases distorted by harmonic components. This work analyses the harmonic components of the ELF magnetic field in an outdoor urban context and compares the evaluation of the exposure based on broadband measurements with that based on spectral analysis. The multiple frequency rule of the International Commission on Non-ionizing Radiation Protection (ICNIRP) regulatory guidelines was applied. With the 1998 ICNIRP guideline, harmonics dominated the exposure with a 55% contribution. With the 2010 ICNIRP guideline, however, the primary frequency dominated the exposure with a 78% contribution. Values of the exposure based on spectral analysis were significantly higher than those based on broadband measurements. Hence, it is clearly necessary to determine the harmonic components of the ELF magnetic field to assess exposure in urban contexts.

Multi‐payload measurement of transverse velocity shears in the topside ionosphere

Using a multi‐payload sounding rocket mission, we present the first direct measurement of velocity shear in the topside auroral ionosphere. In regions of large, ∼200 mV/m, transient electric fields we directly measure differences in the plasma drift velocity. From these differences, shear frequencies reaching ±6Hz are measured. These directly measured shears are compared with the shear inferred from single payload measurements. It is shown this traditional measurement of shear overestimates the shear frequency by a factor of two for this event, highlighting the importance of the temporal component of near‐DC electric field structures. Coincident with these strong fields and shears are enhanced emissions of broadband, extremely low frequency (BB‐ELF) plasma waves, and a narrowband wave emission near the H+‐O+bi‐ion resonant frequency.

On the statistical relation between ion upflow and naturally enhanced ion-acoustic lines observed with the EISCAT Svalbard radar

[1] We have investigated characteristics of ion upflow and naturally enhanced ion-acoustic lines (NEIALs) based on the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) data continuously obtained between March 2007 and February 2008. For the ion upflow study we have used approximately 78,000 field-aligned profiles obtained with the ESR. For the NEIAL study we have identified approximately 1500 NEIALs in the ESR data at altitudes between 100 and 500 km. The occurrence frequency of ion upflow shows two peaks, at about 0800 and 1300 magnetic local time (MLT), while only one strong peak is seen around 0900 MLT for NEIALs. The upward ion flux also has only one peak around 1100–1300 MLT. The occurrence frequency of ion upflow varies strongly over season. It is higher in winter than in summer, whereas NEIALs are more frequent in summer than in winter. NEIALs frequently occur under high geomagnetic activity and also high solar activity conditions. Approximately 10% of NEIALs in the F region ionosphere were accompanied by NEIALs in the E region (occurred at altitudes below 200 km). About half of the E region enhanced echoes did not have an F region counterpart. Upshifted NEIALs dominate in the E region whereas downshifted NEIALs are usually stronger above an altitude of 300 km. The high occurrence frequency of NEIALs in the prenoon region (0800–1000 MLT) might be associated with acceleration of thermal ions to suprathermal ones. At the same MLT and geomagnetic latitude suprathermal ions and broadband extremely low frequency (BBELF) wave activity have been observed, according to previous studies.

On the correlation between Broad-Band ELF wave power and ion fluxes in the cusp

Abstract. We present a study of Broad-Band Extremely Low Frequency (BB-ELF) electric fields in the mid-altitude (4–6 Earth radii) cusp during periods of southward interplanetary magnetic field, using data from the Cluster spacecraft. Magnetospheric boundary layers are identified and classified according to particle precipitation characteristics. We find that the BB-ELF is contained within the cusp ion precipitation region, and its onset is closely co-located with the equatorward edge of the cusp ion dispersion signature. Previous studies have shown a positive correlation between BB-ELF and downward ion number flux. In this study, we compare the correlation coefficients of BB-ELF wave power versus the ion number and energy fluxes for upward, downward and total field-aligned fluxes. There is a greater degree of correlation between the total field-aligned flux and wave power than between the downward flux and wave power, which indicates that the BB-ELF wave generation is independent of ion beam direction. Our results support the idea of a local ion – BB-ELF wave interaction.

PFISR nightside observations of naturally enhanced ion acoustic lines, and their relation to boundary auroral features

Abstract. We present results from a coordinated camera and radar study of the auroral ionosphere conducted during March of 2006 from Poker Flat, Alaska. The campaign was conducted to coincide with engineering tests of the first quarter installation of the Poker Flat Incoherent Scatter Radar (PFISR). On 31 March 2006, a moderately intense auroral arc, (~10 kR at 557.7 nm), was located in the local magnetic zenith at Poker Flat. During this event the radar observed 7 distinct periods of abnormally large backscattered power from the F-region. These were only observed in the field-aligned radar beam, and radar spectra from these seven times show naturally enhanced ion-acoustic lines (NEIALs), the first observed with PFISR. These times corresponded to (a) when the polar cap boundary of the auroral oval passed through the magnetic zenith, and (b) when small-scale filamentary dark structures were visible in the magnetic zenith. The presence of both (a) and (b) was necessary for their occurrence. Soft electron precipitation occurs near the magnetic zenith during these same times. The electron density in the vicinity where NEIALs have been observed by previous studies is roughly between 5 and 30×1010 m−3. Broad-band extremely low frequency (BBELF) wave activity is observed in situ by satellites and sounding rockets to occur with similar morphology, during active auroral conditions, associated with the poleward edge of the aurora and soft electron precipitation. The observations presented here suggest further investigation of the idea that NEIALs and BBELF wave activity are differently-observed aspects of the same wave phenomenon. If a connection between NEIALs and BBELF can be established with more data, this could provide a link between in situ measurements of downward current regions (DCRs) and dynamic aurora, and ground-based observations of dark auroral structures and NEIALs. Identification of in situ processes, namely wave activity, in ground-based signatures could have many implications. One specific example of interest is identifying and following the temporal and spatial evolution of regions of potential ion outflow over large spatial and temporal scales using ground-based optical observations.

Coordinated EISCAT Svalbard radar and Reimei satellite observations of ion upflows and suprathermal ions

The relationship between bulk ion upflows and suprathermal ions was investigated using data simultaneously obtained from the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and the Reimei satellite. Simultaneous observations were conducted in November 2005 and August 2006, and 14 conjunction data sets have been obtained at approximately 630 km in the dayside ionosphere. Suprathermal ions with energies of a few eV were present in the dayside cusp region, and the ion velocity distribution changed from an isotropic Maxwellian near the cusp region to tail heating at energies above a few eV in the cusp region. The velocity distribution of the suprathermal ions has a peak perpendicular or oblique to the geomagnetic field, and the temperature of the suprathermal ions was 0.9–1.4 eV. An increase in the phase space density (PSD) of the suprathermal ions, measured with the Reimei, was correlated with bulk ion upflow observed at the same altitude using EISCAT, and with the energy flux of precipitating electrons with energies of 50–500 eV. The PSD also has a good correlation with the electron temperature, which was increased by precipitation, but not with the ion temperature (0.1–0.3 eV) at the same altitude measured with EISCAT. These results suggest that plasma waves such as broadband extremely low frequency (BBELF) wavefields associated with precipitation are connected to the bulk ion upflows in the cusp and effectively cause the heating of suprathermal ions. The heating of suprathermal ions disagrees with anisotropic heating due to O+−O resonant charge exchange.

Electrostatic wave generation and transverse ion acceleration by Alfvénic wave components of broadband extremely low frequency turbulence

We present results here from 2.5‐dimensional particle‐in‐cell simulations showing that the electrostatic (ES) components of broadband extremely low frequency (BBELF) waves could possibly be generated by cross‐field plasma instabilities driven by the relative drifts between the heavy and light ion species in the electromagnetic (EM) Alfvénic component of the BBELF waves in a multi‐ion plasma. The ES components consist of ion cyclotron as well as lower hybrid modes. We also demonstrate that the ES wave generation is directly involved in the transverse acceleration of ions as commonly measured with the BBELF wave events. The heating is affected by ion cyclotron resonance in the cyclotron modes and Landau resonance in the lower hybrid waves. In the simulation, we drive the plasma by the transverse electric field, Ey, of the EM waves; the frequency of Ey, ωd, is varied from a frequency below the heavy ion cyclotron frequency, Ωh, to below the light ion cyclotron frequency, Ωi. We have also performed simulations for Ey having a continuous spectrum given by a power law, namely, ∣Ey∣ ∼ ωd−α, where the exponent α = , 1, and 2 in three different simulations. The driving electric field generates polarization and E × B drifts of the ions and electrons. When the interspecies relative drifts are sufficiently large, they drive electrostatic waves, which cause perpendicular heating of both light and heavy ions. The transverse ion heating found here is discussed in relation to observations from Cluster, Fast Auroral Snapshot, and Freja.

SIERRA observations of Alfvénic processes in the topside auroral ionosphere

The NASA SIERRA rocket mission was a multiple‐payload, auroral plasma physics experiment that flew into a 100 nT auroral substorm at altitudes up to 735 km over the Poker Flat Research Range in Alaska on 14 January 2002. The flight environment was composed of an equatorward region of inverted‐V electron precipitation followed by a poleward region of mostly field‐aligned suprathermal electron bursts. In the inverted‐V region, the measured δE/δB implied a stationary field‐aligned current configuration. In the poleward region, the average electric field was much larger, 50 mV/m compared with 20 mV/m, included time‐varying components that exceeded 100 mV/m with periods of 0.5–5 s, and δE/δB was Alfvénic. Our multiple‐payload observations show that the perpendicular scale size of most of these waves is on the order of a kilometer or less. At smaller scales, broadband, extremely low‐frequency (BBELF) fluctuations associated with observed Alfvén waves had spectral ratios (δE/δB) appropriate for inertial Alfvén waves larger and smaller than the electron inertial length (λe = c/ωpe), but the phase angle differences between δE and δB indicate mostly propagating Alfvén waves for spatial scales much larger than λe and a transition to spatial perturbations, embedded in the plasma, for scales near λe. The data are consistent with the model of kilometer‐scale oblique Alfvénic arcs that are unstable to the current shear‐driven instabilities discussed in previous theoretical investigations, lending support to the conclusion that the observed BBELF fluctuations are likely driven by the wave‐associated field‐aligned current shear.

The relationship between field‐aligned currents and low‐frequency electromagnetic fluctuations

Current sheets having a transverse scale near the electron inertial length c/ωp have been shown to be unstable using a reduced MHD description with electron inertia [Seyler and Wu, 2001]. This robust instability arises from the advection of the parallel current which lowers the magnetic energy resulting in vortex formation. The instability is localized where the parallel current shear and B⟂ are near a maximum. Observations from Freja, FAST, and CLUSTER show fluctuation enhancement in regions where current shear maximizes. The simulation spectral ratios δE/δB and δE/δn are found to be consistent with satellite data. The agreement between observations and simulations support the simulation results as the generation mechanism of BBELF (broad‐band extremely low‐frequency) fluctuations.

Modeling transverse heating and outflow of ionospheric ions from the dayside cusp/cleft. 1 A parametric study

Abstract. The transport patterns of non-thermal H + and O + field-aligned flows from the dayside cusp/cleft, associated with transverse heating by means of wave-particle interactions and in combination with the poleward motion due to the magnetospheric convection are investigated. This has been accomplished by developing a steady-state, two-dimensional, trajectory-based code. The ion heating is modelled by means of a Monte Carlo technique, via the process of ion cyclotron resonance (ICR), with the electromagnetic left-hand circular polarized component of a broad-band, extremely low-frequency (BBELF) turbulence. The altitude dependence of ICR heating from 1000 km to 3 Earth radii (RE) is modelled by a power law spectrum, with an index a, and a parameter w0 that is proportional to the spectral density at a referenced gyrofrequency. Because of the finite latitudinal extent of the cusp/cleft, the incorporation of the horizontal convection drift leads to a maximum residence time tD of the ions when being energized. A large set of simulations has been computed so as to study the transport patterns of the H + and O + bulk parameters as a function of tD , a, and w0. Residence time effects are significant in O + density patterns while negligible for H +. When comparing the results with analytical one-dimensional theories (Chang et al., 1986; Crew et al., 1990), we find that mean ion energies and pitch angles at the poleward edge of the heating region are slightly influenced by tD and may be used as a probe of ICR parameters ( a, w0). Conversely, poleward of the heating region, upward velocity and mean energy dispersive patterns depend mainly on tD (e.g. the magnitude of the convection drift) with latitudinal profiles varying versus tD . In short, the main conclusion of the paper is that any triplet (tD , a, w0) leads to a unique transport-pattern feature of ion flows associated with a cusp/cleft ionospheric source. In a companion paper, by using high-altitude (1.5–3 RE) ion observations as constraints, the results from the parametric study are used to determine the altitude dependence of transverse ion heating during a significant number of passes of the Interball-2 satellite.Key words. Magnetospheric physics (auroral phenomena) – Space plasma physics (numerical simultation studies; wave-particle interactions)

Modeling transverse heating and outflow of ionospheric ions from the dayside cusp/cleft. 2 Applications

Abstract. In this paper, we consider major ion energization mechanisms in the dayside cusp/cleft region. This includes transverse ion heating by ion cyclotron resonance (ICR), ion energization through structures of field-aligned electric potential drops, and transverse heating by lower hybrid (LH) waves. First, we present and discuss three typical cusp/cleft crossings associated with one of the first two mechanisms mentioned above. Then, we develop a procedure for finding the altitude dependence of ICR heating for any data set in the high-altitude cusp/cleft under the absence of field-aligned potential drops. This has been accomplished using a large set of numerical simulations from a two-dimensional, steady-state, Monte Carlo, trajectory-based code, as discussed in detail in the first companion paper (Bouhram et al., 2003). The procedure is applied and tested successfully for the first two events, by using patterns of ion moments along the satellite track as constraints. Then, we present a statistical study that uses 25 cusp/cleft crossings associated with steady IMF conditions, where ICR heating is expected to occur alone. It is pointed out that the ICR heating increases gradually versus geocentric distance as s 3.3 ± 1.8 . The inferred values of the wave power and the spectral index associated with the component responsible for ICR heating are lower than those characterizing the broad-band, extremely low-frequency (BBELF) turbulence usually observed in the cusp/cleft. This strengthens the idea that more than one wave-mode is contained in the BBELF turbulence, and only a small fraction of the observed turbulence is responsible for ICR heating. Then, we study the occurrence versus magnetic local time (MLT) of field-aligned potential drops. According to previous statistical studies, such structures are not common in the cusp and tend to be associated with the cleft region. We also discuss the effects of LH heating in the cusp on the observed ion distributions. However, this mechanism turns out to be of less importance than ICR heating.Key words. Magnetospheric physics (Auroral phenomena) – Space plasma physics (charged particle motion and acceleration; wave-particle interactions)

Instability of inertial Alfvén waves in transverse sheared flow

We study the stability of dispersive Alfvénic structures with transverse scales near the electron inertial length c/ωpe. As demonstrated by Seyler and Wu [2001], inertial Alfvén waves can be driven unstable by the parallel current sheets associated with the Alfvénic structures. In this paper we extend that study by including the effect of the transverse sheared flow due to the E × B drift. We show that the sheared flow decreases the growth rate of the instability, but the instability still exists even in the presence of the shear. In addition, we show that the sheared flow itself can drive the plasma unstable. Owing to a resonance between the sheared flow velocity and the phase velocity of the Alfvén wave, energy can be transferred from the sheared flow to the wave, driving the plasma unstable. We present an extensive analytical study of the linear stability. The result of this paper shows how inertial Alfvén waves can be generated in the auroral region and may explain the origin of broadband extremely low frequency (BBELF) waves associated with Alfvénic structures.

Electromagnetic waves and bursty electron acceleration: implications from Freja

Abstract. Dispersive Alfvén wave activity is identified in four dayside auroral oval events measured by the Freja satellite. The events are characterized by ion injection, bursty electron precipitation below about 1 keV, transverse ion heating and broadband extremely low frequency (ELF) emissions below the lower hybrid cutoff frequency (a few kHz). Large-scale density depletions/cavities, as determined by the Langmuir probe measurements, and strong electrostatic emissions are often observed simultaneously. A correlation study has been carried out between the E and B field fluctuations below 64 Hz and 10 Hz, respectively, (the DC instruments upper threshold) and the characteristics of the precipitating electrons. This study revealed that the energisation of electrons is indeed related to the broadband ELF emissions and that the electrostatic component plays a predominant role during very active magnetospheric conditions. Furthermore, the effect of the ELF electromagnetic emissions on the larger scale field-aligned current systems has been investigated, and it is found that such an effect cannot be detected. Instead, the Alfvénic activity creates a local region of field-aligned currents. It is suggested that dispersive Alfvén waves set up these local field-aligned current regions and, in turn, trigger more electrostatic emissions during certain conditions. In these regions, ions are transversely heated, and large-scale density depletions/cavities may be created during especially active periods.Key words. Ionosphere (particle acceleraton; wave-particle interactions) Magnetospheric physics (auroral phenomena)

Ion outflow and associated perpendicular heating in the cusp observed by Interball Auroral Probe and Fast Auroral Snapshot

The spatial properties of ionospheric ion outflows associated with perpendicular heating processes in the cusp are studied using a conjunction study from two satellites and ground radar systems. Low‐energy outflowing ions are measured in a wide longitudinal range, between 13,000 and 19,000 km in altitude, over the dayside polar cap by the Hyperboloid experiment aboard the Interball Auroral Probe (AP). These observations are related to conjugate convection field measurements by the Saskatoon‐Kapuskasing pair of the Super Dual Auroral Radar Network (SuperDARN). Data analysis suggests that outflowing ions originate from a wide magnetic local time range associated with both the dayside cusp region and the dayside cleft region. A direct cusp crossing by the Fast Auroral Snapshot (FAST) satellite at 2000‐km altitude shows a correlation between transverse ion heating in a thin latitudinal region (∼1.3°) and the presence of broadband extremely low frequency (BBELF) turbulence, in addition to more intense electrostatic waves near and just above the lower hybrid (LH) frequency. This event is unusual because on the basis of the statistical survey of Freja and FAST data, most of the ion‐heating events in the midaltitude auroral zone are correlated with enhanced emissions in the BBELF range. Furthermore, the electron population has energies that are too small to drive LH waves unstable. This event offers the opportunity to analyze the contribution of cusp magnetospheric ion injections to the heating of the ambient H+ and O+ ions. Kinetic instability calculations demonstrate that LH waves are destabilized by the ring distributions, which result from injections of high‐energy magnetosheath ions. Calculations show that a preheating mechanism by BBELF turbulence near the ion gyrofrequencies is also required so that LH heating is able to occur. The altitude dependence of the LH perpendicular heating is then analyzed by modeling the transport of the injected magnetospheric ions along magnetic field lines. It is shown that LH heating acts as an additional process from ∼2000 up to 10,000 km in altitude. In addition, trajectory calculations show that the low‐energy outflowing H+ and O+ ions observed along Interball AP orbit in the polar cap are heated inside the cusp at altitudes extending up to 15,000 km. These results are then assembled to construct a possible heating scenario inside the cusp for this data set. The contribution of the different energization mechanisms to the ion heating as a function of altitude is then discussed.

Instability at the electron inertial scale

Current sheets having a transverse scale near the electron inertial length c/ωp are shown to be unstable. Two general classes of modes are found to occur, the single and double tearing modes and a current shear driven current‐convective interchange mode. The tearing modes are global modes that relax the equilibrium to a lower magnetic energy state. The current‐convective interchange mode is localized near points at which k · B0 = 0. The interchange mode has features in common with the collisionless tearing modes but is a local rather than global instability. An extensive analysis of the linear stability is presented, and the results are confirmed from linear initial value and eigenvalue calculations. Nonlinear three‐dimensional initial value simulations are performed to determine the effects of the mode on the equilibrium. The δE/δB spectrum of the field fluctuations is computed and compared to satellite observations. The results support the hypothesis that short‐scale broadband extremely low‐frequency (BBELF) waves associated with Alfvénic structures are possibly the result of current decay through the electromagnetic current‐convective instability.

On the mass dependence of transverse ion acceleration by broad-band extremely low frequency waves

Recent data from the Fast Auroral Snapshot Explorer (FAST) and other satellites indicate that broad-band extremely low frequency (BBELF) waves account for most of the transverse ion acceleration in the aurora. These waves tend to accelerate ions of different masses to approximately the same energy. We have shown previously that a downward parallel electric field is necessary in order to explain this result in terms of a cyclotron resonant acceleration process. In this paper we use triangular electric field distributions to investigate how strong and how extensive a field is necessary to equalize the energization of ions of different mass.

Ionospheric E region remote sensing with ELF radio atmospherics

Radio remote sensing of the ionospheric E region can be difficult, particularly in the nighttime E region valley which lies between two regions of higher electron density. We show that extremely low frequency (ELF) electromagnetic waves launched from below penetrate this region because of their low attenuation and can be reflected from both the D region and the top of the E region valley. This double reflection, also caused by sporadic E layers, produces a measurable effect on the subionospheric propagation of ELF waves, which we demonstrate with numerical Earth‐ionosphere waveguide simulations. This sensitivity opens the possibility of remotely sensing the E region and detecting sporadic E layers with broadband ELF propagation measurements using lightning discharges as the source. From nighttime observations of ELF lightning radiation over the same propagation path on different days, we extract E region electron density profiles that show significant variability not only from day to day but over the course of a single night. This technique is inherently a path‐integrated measurement, enabling tomographic large‐scale ionospheric remote sensing with multiple sources and receivers.