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It is well established that lightning-generated whistlers play an important role in the physics of the inner magnetosphere and impact the radiation belt dynamics via wave-particle interactions that lead to particle precipitation. Whistlers contribute to the generation of slot region and plasmaspheric hiss. Recently, we identified a new kind of whistler, specularly reflected (SR) whistler, in which the lightning energy injected at low latitudes first undergoes a specular reflection in the conjugate hemisphere and then propagates to the magnetosphere (Sonwalkar and Reddy, Science Advances, 2024, in Press). The existence of SR whistlers in the magnetosphere contradicts previous understanding that lightning energy injected at low latitudes cannot escape the ionosphere (Thorne and Horne, JGR, Vol. 99, A9, 1994; Bortnik et al., JGR, Vol. 108, A5, 2003). A survey of data from Van Allen Probes shows that SR whistler is a common phenomenon. To assess the impact of SR whistlers on the radiation belt physics, we calculated the lightning energy reaching the magnetosphere in the form of SR whistlers relative to that reaching as magnetospherically reflected (MR) and ducted whistlers. Using ray tracing simulations and taking into account various propagation losses and the global distribution of lightning flashes, we performed a comparative study of SR, MR, and ducted whistlers. Our research shows: (1) SR whistlers occupy the same region of the magnetosphere as the MR whistlers and have similar wave normal angles and intensity. (2) SR and MR whistlers carry most of the lightning energy reaching the magnetosphere. (3) Ducted whistlers represent ~1-4% of the lightning energy reaching the magnetosphere. (4) When SR whistlers are considered, the global lightning energy contribution to the magnetosphere doubles, implying that the previous estimates of the impact of lightning energy on radiation belts and its role in the physics of the inner magnetosphere may need substantial revisions. 

The lightning-generated whistlers play an important role in the physics of the radiation belts. Magnetospherically reflected (MR) and recently identified specularly reflected (SR) whistlers are the principal contributors to the lightning energy reaching the plasmasphere (Sonwalkar and Reddy, Science Advances, 2024 in press; Sonwalkar and Reddy, AGU 2024). Using plasma wave data from Van Allen Probes and lightning data from the World Wide Lightning Location Network (WWLLN), we performed a correlative study of 22 cases of SR whistlers accompanied by MR whistlers observed in the plasmasphere and the associated causative lightning flashes observed on the ground. Our results can be summarized as follows: (1) The whistlers were observed for 2 < L < 2.5 and 10°S < λm <15°N. (2) Causative lightning flash locations were between 500 and 5000 km of the satellite geomagnetic footprint. (3) Ray tracing analysis in a typical magnetosphere showed that in most cases, the causative lightning was located within 1500 km of ionospheric lightning energy injection points that generated SR and MR whistlers, though lightning energy injected into the ionosphere as far as 3000-4000 km from the lightning location led to detectable SR and MR whistlers. (4) Most of the lightning flashes were located at 10° < λm < 30°, consistent with the observed latitudinal distribution of lightning that shows the majority of lightning flashes occur at low latitudes (<30°) (Orville and Spencer, Monthly Weather Review, 1979). (5) The typical lightning flash energy that generated SR and MR whistlers ranged between 250 J and 6000 J. A whistler propagation model that takes into account lightning location and intensity and various propagation losses could explain the observed intensities of SR and MR whistlers. Our results imply that combining ground-based observations of global lightning activity with the whistler propagation model should provide the levels of lightning-generated whistler mode waves in the plasmasphere, leading to a powerful new space weather technique to monitor lightning-generated plasmaspheric whistler mode waves from the ground. 

Lightning-generated whistlers profoundly affect the energetic particle population in Earth’s radiation belts, influencing space weather and endangering astronauts and satellites. We report the discovery of specularly reflected (SR) whistler in which the lightning energy injected into the ionosphere at low latitudes reaches the magnetosphere after undergoing a specular reflection in the conjugate ionosphere, contradicting previous claims that lightning energy injected at low latitudes cannot escape the ionosphere. SR whistlers provide a low-latitude channel to transport lightning energy to the magnetosphere. We calculate the relative contributions of SR, magnetospherically reflected, subprotonospheric, and ducted whistlers to the lightning energy reaching the magnetosphere. When SR whistlers are considered, the global lightning energy contribution to the magnetosphere doubles, implying that the previous estimates of the impact of lightning energy on radiation belts may need substantial revisions. Whistler dispersion and intensity analyses quantitatively confirm our results and suggest new remote-sensing methods of the magnetosphere, ionosphere, Earth-ionosphere waveguide, and lightning flashes. 

The lightning energy from global thunderstorm activity plays an important role in the physics and dynamics of the inner magnetosphere. Lightning-generated whistlers, including magnetospherically reflected (MR) and recently discovered specularly reflected (SR) whistlers, have been observed on Van Allen Probes (Sonwalkar and Reddy, AGU Fall Meeting, 2021). MR whistlers propagate in nonducted mode and undergo reflections within the magnetosphere. SR whistlers propagate in nonducted mode, undergoing their first reflection (specular) at the Earth-ionosphere boundary (~90 km) in the conjugate hemisphere and subsequently undergo magnetospheric reflections similar to an MR whistler. We inspected 31 days of RBSP-A and RBSP-B data during October-November 2012 period when 6-s continuous-burst data (~3400 6-s spectra on RBSP-A and ~6300 6-s spectra on RBSP-B) were available. Whistlers, either MR or SR, were detected on ~1430 spectra in the L-shell range ~1.1 to 5.0 at all near-equatorial geomagnetic latitudes covered by the Van Allen Probes. Whistlers were observed more frequently at nighttime (~1170, 82%) relative to daytime (~260, 18%), consistent with greater D-region losses at daytime. SR whistlers accompanied by MR whistlers were predominantly observed in the L-shell range of ~2 – 3, magnetic latitude λm of ~17° S to 15° N and MLT 0 – 3.6, and in the altitude range of ~ 6000-12,000 km. The MR and SR whistlers occurrence increased from L~2.0 to L~2.5 and then declined with increasing L-shell, a pattern consistent with reported observations of MR whistlers (Edgar, Ph.D. thesis, 1972) for which whistler occurrence increased with L from 2.0 to 2.4 and then declined with increasing L-shell. SR and MR whistlers were detected during geomagnetically quiet to moderately disturbed conditions. The maximum Kp index, 24 hours before detecting whistlers, ranged between 0.33-6.0. Our results indicate that SR whistlers are a common phenomenon, and their impact on the inner radiation belt should be similar to that of MR whistlers. 

The lightning energy from global thunderstorm activity plays an important role in the physics and dynamics of the inner magnetosphere. Lightning-generated whistlers, including magnetospherically reflected (MR) and recently discovered specularly reflected (SR) whistlers, have been observed on Van Allen Probes (Sonwalkar and Reddy, AGU Fall Meeting, 2021). MR whistlers propagate in nonducted mode and undergo reflections within the magnetosphere. SR whistlers propagate in nonducted mode, undergoing their first reflection (specular) at the Earth-ionosphere boundary (~90 km) in the conjugate hemisphere and subsequently undergo magnetospheric reflections similar to an MR whistler. We inspected 31 days of RBSP-A and RBSP-B data during October-November 2012 period when 6-s continuous-burst data (~3400 6-s spectra on RBSP-A and ~6300 6-s spectra on RBSP-B) were available. Whistlers, either MR or SR, were detected on ~1430 spectra in the L-shell range ~1.1 to 5.0 at all near-equatorial geomagnetic latitudes covered by the Van Allen Probes. Whistlers were observed more frequently at nighttime (~1170, 82%) relative to daytime (~260, 18%), consistent with greater D-region losses at daytime. SR whistlers accompanied by MR whistlers were predominantly observed in the L-shell range of ~2 – 3, magnetic latitude λm of ~17° S to 15° N and MLT 0 – 3.6, and in the altitude range of ~ 6000-12,000 km. The MR and SR whistlers occurrence increased from L~2.0 to L~2.5 and then declined with increasing L-shell, a pattern consistent with reported observations of MR whistlers (Edgar, Ph.D. thesis, 1972) for which whistler occurrence increased with L from 2.0 to 2.4 and then declined with increasing L-shell. SR and MR whistlers were detected during geomagnetically quiet to moderately disturbed conditions. The maximum Kp index, 24 hours before detecting whistlers, ranged between 0.33-6.0. Our results indicate that SR whistlers are a common phenomenon, and their impact on the inner radiation belt should be similar to that of MR whistlers. 

The lightning energy from global thunderstorm activity plays an important role in the physics and dynamics of the inner magnetosphere. Lightning-generated whistlers, including magnetospherically reflected (MR) and recently discovered specularly reflected (SR) whistlers, have been observed on Van Allen Probes (Sonwalkar and Reddy, AGU Fall Meeting, 2021). MR whistlers propagate in nonducted mode and undergo reflections within the magnetosphere. SR whistlers propagate in nonducted mode, undergoing their first reflection (specular) at the Earth-ionosphere boundary (~90 km) in the conjugate hemisphere and subsequently undergo magnetospheric reflections similar to an MR whistler. We inspected 31 days of RBSP-A and RBSP-B data during October-November 2012 period when 6-s continuous-burst data (~3400 6-s spectra on RBSP-A and ~6300 6-s spectra on RBSP-B) were available. Whistlers, either MR or SR, were detected on ~1430 spectra in the L-shell range ~1.1 to 5.0 at all near-equatorial geomagnetic latitudes covered by the Van Allen Probes. Whistlers were observed more frequently at nighttime (~1170, 82%) relative to daytime (~260, 18%), consistent with greater D-region losses at daytime. SR whistlers accompanied by MR whistlers were predominantly observed in the L-shell range of ~2 – 3, magnetic latitude λm of ~17° S to 15° N and MLT 0 – 3.6, and in the altitude range of ~ 6000-12,000 km. The MR and SR whistlers occurrence increased from L~2.0 to L~2.5 and then declined with increasing L-shell, a pattern consistent with reported observations of MR whistlers (Edgar, Ph.D. thesis, 1972) for which whistler occurrence increased with L from 2.0 to 2.4 and then declined with increasing L-shell. SR and MR whistlers were detected during geomagnetically quiet to moderately disturbed conditions. The maximum Kp index, 24 hours before detecting whistlers, ranged between 0.33-6.0. Our results indicate that SR whistlers are a common phenomenon, and their impact on the inner radiation belt should be similar to that of MR whistlers. 

Nu whistlers are a special case of magnetospherically reflected (MR) whistlers in which, on a spectrogram, 2- and 2+ whistlers are joined at their lower cutoff frequency, fmin. An MR whistler is labeled 2- if the lightning energy has crossed the equator twice before reaching the satellite, and it is labeled 2+ if the lightning energy after crossing the equator twice has undergone a magnetospheric reflection before reaching the satellite (Smith and Angerami, JGR, 73, 1-20, 1968). A magnetospheric reflection occurs when the local lower hybrid frequency is close to the wave frequency (Kimura, Radio Science, 269, 1966). Nu whistlers are, therefore, observed when the satellite is at a location where flh ~ fmin, so both 2- and 2+ MR whistlers have almost identical propagation paths, hence the identical time delay. In addition to conventional Nu whistlers described above, some of the OGO-1 observations contain a second set of Nu whistler-type traces, which, according to previous authors (Smith and Angerami, JGR, 73, 1-20, 1968; Edgar, Ph.D. thesis, 1972), resulted from multipath propagation, possibly due to large scale irregularities in the magnetosphere. Recent whistler observations from the Van Allen probe have led to the discovery of a new kind of whistler - the specularly reflected (SR) whistler (Sonwalkar and Reddy, AGU Fall Meeting 2021, SM43C-02). An SR whistler propagates in nonducted mode, undergoing its first reflection (specular) at the Earth-ionosphere boundary (~90 km) in the conjugate hemisphere and subsequently undergoing magnetospheric reflections similar to that of an MR whistler. Using ray tracing analysis, we show that the second set of Nu whistlers are Nu specularly reflected whistlers. Our research implies that there are two types of Nu whistlers: Nu MR whistler and Nu SR whistler. Nu whistlers potentially provide a new method to determine the ion composition at the satellite location. 

The lightning energy from global thunderstorm activity plays an important role in the physics and dynamics of the inner magnetosphere. Lightning-generated whistlers, including magnetospherically reflected (MR) and recently discovered specularly reflected (SR) whistlers, have been observed on Van Allen Probes (Sonwalkar and Reddy, AGU Fall Meeting, 2021). MR whistlers propagate in nonducted mode and undergo reflections within the magnetosphere. SR whistlers propagate in nonducted mode, undergoing their first reflection (specular) at the Earth-ionosphere boundary (~90 km) in the conjugate hemisphere and subsequently undergo magnetospheric reflections similar to an MR whistler. We inspected 31 days of RBSP-A and RBSP-B data during October-November 2012 period when 6-s continuous-burst data (~3400 6-s spectra on RBSP-A and ~6300 6-s spectra on RBSP-B) were available. Whistlers, either MR or SR, were detected on ~1430 spectra in the L-shell range ~1.1 to 5.0 at all near-equatorial geomagnetic latitudes covered by the Van Allen Probes. Whistlers were observed more frequently at nighttime (~1170, 82%) relative to daytime (~260, 18%), consistent with greater D-region losses at daytime. SR whistlers accompanied by MR whistlers were predominantly observed in the L-shell range of ~2 – 3, magnetic latitude λm of ~17° S to 15° N and MLT 0 – 3.6, and in the altitude range of ~ 6000-12,000 km. The MR and SR whistlers occurrence increased from L~2.0 to L~2.5 and then declined with increasing L-shell, a pattern consistent with reported observations of MR whistlers (Edgar, Ph.D. thesis, 1972) for which whistler occurrence increased with L from 2.0 to 2.4 and then declined with increasing L-shell. SR and MR whistlers were detected during geomagnetically quiet to moderately disturbed conditions. The maximum Kp index, 24 hours before detecting whistlers, ranged between 0.33-6.0. Our results indicate that SR whistlers are a common phenomenon, and their impact on the inner radiation belt should be similar to that of MR whistlers. 

Nu whistlers are a special case of magnetospherically reflected (MR) whistlers in which, on a spectrogram, 2- and 2+ whistlers are joined at their lower cutoff frequency, fmin. An MR whistler is labeled 2- if the lightning energy has crossed the equator twice before reaching the satellite, and it is labeled 2+ if the lightning energy after crossing the equator twice has undergone a magnetospheric reflection before reaching the satellite (Smith and Angerami, JGR, 73, 1-20, 1968). A magnetospheric reflection occurs when the local lower hybrid frequency is close to the wave frequency (Kimura, Radio Science, 269, 1966). Nu whistlers are, therefore, observed when the satellite is at a location where flh ~ fmin, so both 2- and 2+ MR whistlers have almost identical propagation paths, hence the identical time delay. In addition to conventional Nu whistlers described above, some of the OGO-1 observations contain a second set of Nu whistler-type traces, which, according to previous authors (Smith and Angerami, JGR, 73, 1-20, 1968; Edgar, Ph.D. thesis, 1972), resulted from multipath propagation, possibly due to large scale irregularities in the magnetosphere. Recent whistler observations from the Van Allen probe have led to the discovery of a new kind of whistler - the specularly reflected (SR) whistler (Sonwalkar and Reddy, AGU Fall Meeting 2021, SM43C-02). An SR whistler propagates in nonducted mode, undergoing its first reflection (specular) at the Earth-ionosphere boundary (~90 km) in the conjugate hemisphere and subsequently undergoing magnetospheric reflections similar to that of an MR whistler. Using ray tracing analysis, we show that the second set of Nu whistlers are Nu specularly reflected whistlers. Our research implies that there are two types of Nu whistlers: Nu MR whistler and Nu SR whistler. Nu whistlers potentially provide a new method to determine the ion composition at the satellite location.