Large amplitude transmitter associated and lightning associated whistler waves in the Earth s inner plasmasphere at L < 2,

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010ja016288, 2011 Large amplitude transmitter associated and lightning associated whistler waves in the Earth s inner plasmasphere at L <2 A. Breneman, 1 C. Cattell, 1 J. Wygant, 1 K. Kersten, 1 L. B. Wilson III, 1 S. Schreiner, 1 P. J. Kellogg, 1 and K. Goetz 1 Received 16 November 2010; revised 11 February 2011; accepted 10 March 2011; published 16 June [1] We report observations of very large amplitude whistler mode waves in the Earth s nightside inner radiation belt enabled by the STEREO Time Domain Sampler. Amplitudes range from mv/m (zero peak), 2 to 3 orders of magnitude larger than previously observed in this region. Measurements from the peak electric field detector (TDSMax) indicate that these large amplitude waves are prevalent throughout the plasmasphere. A detailed examination of high time resolution electric field waveforms is undertaken on a subset of these whistlers at L < 2, associated with pump waves from lightning flashes and the naval transmitter NPM in Hawaii, that become unstable after propagation through the ionosphere and grow to large amplitudes. Many of the waveforms undergo periodic polarization reversals near the lower hybrid and NPM naval transmitter frequencies. The reversals may be related to finite plasma temperature and gradients in density induced by ion cyclotron heating of the plasma at 200 Hz, the modulation frequency of the continuous mode NPM naval transmitter signal. Test particle simulations using the amplitudes and durations of the waves observed herein suggest that they can interact strongly with high energy (>100 kev) electrons on a time scale of <1 s and thus may be an important previously unaccounted for source of energization or pitch angle scattering in the inner radiation belt. Citation: Breneman, A., C. Cattell, J. Wygant, K. Kersten, L. B. Wilson III, S. Schreiner, P. J. Kellogg, and K. Goetz (2011), Large amplitude transmitter associated and lightning associated whistler waves in the Earth s inner plasmasphere at L < 2, J. Geophys. Res., 116,, doi: /2010ja Introduction [2] The Time Domain Sampler (burst memory) instrument on the STEREO spacecraft, which selectively captures high time resolution and large amplitude electric fields, has enabled the discovery of waves and electric field structures that have gone undetected or have been masked by time averaged spectral measurements on previous spacecraft missions. Analysis of STEREO burst memory data by Cattell et al. [2008] led to the discovery of very large amplitude (>200 mv/m) whistler waves in the outer radiation belt. Wave particle simulations suggested that these waves were sufficiently large in amplitude that they are able to strongly interact with highenergy electrons via nonlinear energization and pitch angle scattering on a time scale of a single wave electron encounter ( 0.1 s). This possibility is supported by the conjunction between these waves and relativistic electron microbursts as observed on the SAMPEX satellite [Kersten et al., 2011]. 1 Department of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA. Copyright 2011 by the American Geophysical Union /11/2010JA The subsecond time scale is many orders of magnitude quicker than time scales for quasi linear diffusion appropriate to previously known whistler populations which had amplitudes of up to 30 mv/m [Santolík et al., 2003] but are more typically observed with amplitudes of <1 mv/m. [3] In addition to being efficient accelerators of magnetospheric electrons, Kellogg et al. [2010] provided strong evidence that very large amplitude whistlers can trap significant populations of electrons in their electrostatic component. These electrons can be accelerated to high energies as the waves grow in amplitude and may be transported to new plasma regions along with kilovolt plasma potentials brought on by the sudden increase in electron density. [4] The Cattell et al. [2008] study was followed by observations of very large amplitude whistlers on THEMIS in the dawnside radiation belts from L = [Cully et al., 2008], Wind Time Domain Sampler from L =4toL =20 at the magnetosheath (P. J. Kellogg et al., Large amplitude whistlers in the magnetosphere observed with wind waves, submitted to J. Geophys. Res., 2011) and STEREO Time Domain Sampler at stream interaction regions in the solar wind [Breneman et al., 2010]. [5] In all of these studies the whistler waves observed were at least an order of magnitude larger in amplitude than those previously thought to exist. Taken as a whole, they 1of10

2 Figure 1. STEREO orbit during the SM coordinate perigee passes on 6 November (black), 17 November (blue), 29 November (purple), and 12 December (red). The STEREO A (STEREO B) orbit is represented by the solid (dashed) line. The arrows indicate the direction of the spacecraft velocity. The STEREO burst memory capture locations are indicated by the asterisks. show that very large amplitude whistlers are an ubiquitous phenomenon in a wide variety of plasma regions. [6] Because they interact so strongly with trapped electrons, very large amplitude whistlers may play a very important role in magnetospheric dynamics. In this paper we report the first observations of very large amplitude, oblique whistlers in the Earth s inner radiation belt (L < 2). 2. Instrumentation and Observation Overview [7] The STEREO spacecraft measure electric fields from the S/WAVES instrument suite [Bougeret et al., 2008] with three orthogonal 6 m cylindrical antennas [Bale et al., 2008]. High time resolution, large amplitude electric field waves and pulses are recorded by the burst memory which samples for short duration (<0.5 s) at up to 250 khz. The burst memory buffer ranks waveform captures according to a quality measure and amplitude. Telemetry constraints dictate that only a limited subset of these waves are transmitted to Earth. A survey mode called TDSMax records the maximum electric field every minute, thus providing a rough indication of the overall amount of large amplitude wave activity in a region and indicating where large amplitude waves existed but were not telemetered by the burst memory. S/WAVES also has a low and high frequency receiver (LFR HFR) which provides a spectral survey of electric field from 2.5 khz to 16 MHz. The upper hybrid lines from these spectrograms are used to determine when the spacecraft are within the high density plasmasphere. Low frequency magnetic field measurements, sampled at 8 Hz, are made with the fluxgate magnetometer on the IMPACT suite [Luhmann et al., 2008]. Because the plasma instrument was not yet turned on, no electron pitch angle or energy data are available for the times of the burst memory captures presented in this paper so we were unable to analyze electron distributions. [8] The two STEREO spacecraft each made four perigee passes through the Earth s radiation belts in 2006 before entering their prime mission orbits around the Sun. These passes are shown in Figure 1 along with the locations of the burst memory captures. All of the orbits have perigee at or inside of L 2 in the nightside equatorial region. [9] During the 6 and 17 November perigee passes, the burst memory on STEREO A (STEREO B) captured 78 (33) very large amplitude waveforms in the lower frequency range of the whistler mode as defined by f ci < f f ce,wheref ci ( f ce ) is the ion (electron) cyclotron frequency, as summarized in Table 1. When rotated to a maximum variance coordinate system the largest amplitude waves exceed 100 mv/m on these passes. A few waveform captures on STEREO A on 17 November had waves with amplitudes of 300 mv/m and show clear signs of electron trapping as described by Kellogg et al. [2010]. These waveforms were detected at what is likely to be the edge of a plasmaspheric plume. No burst memory captures in the plasmasphere were telemetered for 29 November and 12 December because largeramplitude waves occurred at larger L shells so waveforms inside of the plasmasphere were not transmitted due to telemetry constraints. Table 1. Burst Memory Waveform Statistics for STEREO Perigee Passes in Plasmasphere a Date Times in PS Total VLF Waves Max Amp Avg Amp Median Amp 6 November (STEREO A) November (STEREO B) November (STEREO A) November (STEREO B) a Amplitudes (zero peak) of maximum variance component in mv/m. 2of10

3 [10] In the next part of this paper we discuss in detail burst memory captures from the 6 November 2006 perigee pass of each spacecraft. The large amplitude whistlers on this day may be associated with subionospheric NPM naval transmitter signals and lightning strikes which act as pump waves that grow to large amplitudes while propagating to the spacecraft. We conclude with TDSMax observations for all perigee passes which show that large amplitude whistlers are a common feature in the Earth s plasmasphere. 3. Wave Observations [11] The first type of very large amplitude whistler wave observed on 6 November 2006 is associated with signals from the NPM naval transmitter in Lualualei, Hawaii, used by the U.S. Navy for submarine communication. The transmitter, located at 20.4 N, E, emits 424 kw of power at 21.4 khz in a continuous broadband (d f > 200 Hz) or pulsed narrowband (d f 10 Hz) mode. On 6 November 2006 the naval transmitter NPM was in pulsed narrowband mode with a 2 s on, 3 s off keyed transmission from UT and again from UT, otherwise it operated in continuous broadband mode. Eight transmitter associated whistlers were observed by STEREO A on 6 November 2006 from 0906: :03 when the spacecraft was near perigee at L = 1.16, lat = 14.5, MLT SM 0100 and 750 km altitude. At this time, STEREO A s magnetic footprint was almost directly over the transmitter which was in broadbandcontinuous mode operation. No transmitter associated whistlers were seen on STEREO B which passed through perigee 1.5 h earlier on a nearly identical orbit, but when the naval transmitter NPM was operating in the pulsed narrowband mode. The second type of whistler observed by the burst memory on 6 November 2006 was the lightning associated whistler. These were dispersive in nature, with a falling frequency characteristic of lightning whistlers propagating through the plasmasphere. Thirteen lightning associated whistlers were observed on STEREO B from 0746: :34 followed by eighteen on STEREO A from 0911: :17. Both types of whistlers had amplitudes that ranged from mv/m zero peak, much larger than previously observed in the inner radiation belt. For comparison, typical NPM naval transmitter signals were observed on DEMETER at 700 km in the same footprint 30 min prior to STEREO A perigee and had amplitudes on the order of 0.1 mv/m. [12] Figure 2 shows a typical lightning associated whistler from each spacecraft along with various quantities calculated from the waveforms. The waveforms (Figure 2a) are rotated to a hybrid magnetic field aligned/minimum variance coordinate system where the ^z direction is along the magnetic field, and the maximum variance electric field lies in the ^x ^z plane. The STEREO B lightning associated whistler (Figure 2, left) is elliptically polarized with a small electric field component in the magnetic field direction. The power spectrogram (Figure 2b) shows that the signal is dispersive with a falling tone, typical for lightning whistlers. Figure 2c plots the wave normal angle at each frequency (angle between the wave vector and magnetic field direction) for the respective waves, calculated using the linear cold plasma dispersion relation (with appropriate ion species) by measuring the electric field polarization ratio perpendicular to the background magnetic field (ie x /E y =(n 2 S)/D, where n is the index of refraction and S and D are the usual cold plasma sum and difference parameters. See equation 42 in chapter 1 of Stix [1992]) for each frequency time box. The STEREO B lightning associated whistler has moderate to oblique wave normal angles ranging from This wave is right hand polarized with respect to the magnetic field at all frequencies, as is shown in the chirality plot (Figure 2d). A rigorous determination of the wave propagation direction cannot be made without simultaneous wave magnetic and electric field measurements, or a propagation analysis from multiple antennae, neither of which is available on STEREO. However, lightning flash data from the World Wide Lightning Location Network (WWLLN) [Rodger et al., 2006] indicate that the north magnetic footprint of both spacecraft passed through a large lightning storm during the time of observations for both STEREO A and STEREO B. [13] Figure 3 shows a burst memory transmitter associated waveform capture in the hybrid magnetic field aligned/ minimum variance coordinate system, along with a power spectral density (Figure 3a), wave normal angle spectrogram (Figure 3c), chirality spectrogram (Figure 3d) and waveform modulus (Figure 3e). The power spectral density plot indicates that peak power is near the NPM naval transmitter frequency 21.4 khz, with the small variation likely being due to Doppler shift. The waveforms are elliptically polarized with a small electric field component along the magnetic field direction, similar to the STEREO B lightning associated whistlers. They have a distinct modulation with individual packets most often separated by 5 ms, equal to the pulse separation of the transmitter which is modulated at 200 Hz while in continuous mode. This is easily seen in the individual waveforms (Figure 3b) and the waveform modulus (Figure 3e). [14] Figures 2 and 3 indicate that the lightning associated and transmitter associated whistlers have comparable amplitudes of 60 mv/m at the position of the spacecraft (>750 km altitude). Rocket observations indicate that lightning whistler amplitudes of mv/m are common at altitudes of km [Holzworth et al., 1999], while signals from VLF transmitters similar to NPM naval transmitter at these altitudes are <1 mv/m [Kelley et al., 1985]. These amplitudes are smaller than those observed at STEREO and are generally peaked at frequencies less than 1 khz for the lightning whistlers, suggesting that the whistlers observed on STEREO are not attenuated versions of whistlers propagating from lower altitudes. However, we note that the ionospheric conductivity profile can have local order of magnitude or more fluctuations above thunderstorms [Holzworth et al., 1985], and we cannot rule out this possibility. Regardless of the absolute attenuation, the relative attenuation between lightning and transmitter VLF waves is expected to be similar because they both propagate in the whistler mode. The fact that the lightningassociated and transmitter associated whistlers are observed by STEREO to have similar amplitudes suggests that the waves observed in this study grow to large amplitudes within the ionosphere or inner radiation belt. [15] We discuss three mechanisms that could explain the large whistler amplitudes. The first is the linear conversion of electromagnetic whistler energy to quasi electrostatic whistler energy caused by the scattering of whistler waves off of 3of10

4 Figure 2. Lightning whistlers observed on 6 November 2006 on (left) STEREO B at 0746: and (right) STEREO A at 0917: (a) The waveforms are rotated to a hybrid magnetic field aligned/ minimum variance coordinate system where the ^z direction is along the magnetic field and the maximum variance electric field lies in the ^x ^z plane. The components are Ez (orange), Ex (blue), and Ey (green). (b, c, d) Frequency time spectrograms of relative power, wave normal angle, and chirality. Chirality is defined as red for right hand polarization and black for left hand polarization with respect to the magnetic field. The dotted lines indicate the NPM naval transmitter frequency of 21.4 khz and the estimated lower hybrid frequency. small scale ( m) field aligned density striations [Bell and Ngo, 1990]. These quasi electrostatic waves are highly oblique with wave normal angles at or near the resonance cone and have a large longitudinal electric field component with little magnetic field wave component. This scenario does not appear to be responsible for the large wave amplitudes presented here. The STEREO B lightning associated whistlers are only moderately oblique and do not have large longitudinal electric field components and thus they do not show the telltale signs of interaction with small scale field aligned density striations. [16] The second possibility is similar to the first but involves a magnetic field gradient rather than a density gradient. Gushchin et al. [2008] have shown with a controlled laboratory plasma experiment that a local magnetic field enhancement can act as a lens, focusing whistler waves and increasing their amplitudes. Magnetic field perturbations of this sort in the magnetosphere can come from MHD oscillations or diamagnetic currents formed at nearby plasma or field gradients. No magnetic field oscillations are observed in the 8 Hz fluxgate magnetometer data. Higher frequency fluctuations may exist but cannot be detected with STEREO instrumentation. [17] The final possibility is that the subionospheric transmitter associated and lightning associated whistler signals act as pump waves that grow via wave/particle cyclotron or Landau interaction with electrons or ions within the ionosphere or inner radiation belt. Many past studies have shown that pump whistler waves can become unstable via interaction with electrons in the inner radiation belt [e.g., Helliwell, 1988]. This may explain the periodicity in the transmitter associated whistler burst captures which, with 4 of 10

5 Figure 3. Naval transmitter associated whistler observed on STEREO A on 6 November 2006 at 0906: in the same coordinate system as defined in Figure 2. (a) Power spectral density; (b) waveform; (c) spectrogram of wave normal angles; (d) chirality; (e) wave modulus. The vertical lines highlight the 5 ms modulation inherent in the transmitter signal. the exception of the first capture, are separated by s, on the order of the bounce period of high energy electrons in the inner radiation belt [Schulz and Lanzerotti, 1974]. [18] The NPM naval transmitter signals are not observed near perigee on STEREO B when the NPM naval transmitter operated in pulsed narrowband mode suggesting that the continuous broadband signal is needed to excite the instability and growth of the waves. Free energy may come from a temperature anisotropy or electron beam. [19] No particle data were available during the 6 November 2006 STEREO perigee passes, and we cannot demonstrate the existence of such populations for these events. 4. Mode Conversions [20] The STEREO A NPM naval transmitter associated and lightning associated whistlers have an interesting property not usually observed with magnetospheric whistlers. Figure 3c shows that the wave normal angle of the naval transmitter associated whistler alternates in periodic fashion from moderate to highly oblique. Every period is associated with a reversal in wave polarization, defined as the sense of electric field rotation with respect to the magnetic field direction, as indicated by the chirality in Figure 3d. The reversals represent mode conversions between right and left handed waves. They are also observed in the frequency domain of the STEREO A lightning associated whistlers (Figure 2d) which become left hand polarized at roughly f LHR < f < f NPM. These polarization reversals are more directly seen in the hodogram plots of Figure 4 which shows segments of the total burst memory capture at select times for the transmitter associated whistler in Figure 3. The wave starts as right hand elliptically polarized (Figure 4, left), with a sense of rotation indicated by the arrow. The wave then becomes linearly polarized (Figure 4, middle), predominantly perpendicular to the magnetic field. By the end of Figure 4, right, the wave has gone back to elliptical polarization but now rotates about the magnetic field in the lefthand sense. As indicated in Figure 3c this pattern repeats a few times over the length of the burst memory wave 5of10

6 Figure 4. Each column represents waveform hodograms (mv/m) in selected time snippets for the NPM naval transmitter event starting at 0906: in the same coordinate system as defined in Figure 2. The magnetic field points out of the page in the top row of plots. The waveforms are band passed to only include frequencies between 21 and 23 khz. The sense of rotation about the magnetic field is indicated by the arrows. The transmitter associated whistler is (first column) right hand elliptically polarized, (second column) linearly polarized, and (third column) left hand elliptically polarized. This cycle repeats a few times as indicated in the chirality spectrogram plot in Figure 3d. The field aligned (Ez) component is small throughout the capture. 6of10

7 Figure 5. Three representative wavelet spectrograms (E x component in the coordinate system described in Figure 3) for STEREO A burst memory captures. The horizontal lines are the NPM naval transmitter frequency of 21.4 khz and the estimated lower hybrid and H+ ion cyclotron frequencies, and the vertical lines indicate when the polarization reversals occur. The wavelet spectrograms at (a) 0922: and (b) 0850: show a correlation between power at 200 Hz and f LHR < f < f NPM, while the wavelet spectrogram at (c) 0735: shows that the polarization reversals also occur at 2f NPM. The bell shaped curve is called the cone of influence, delineating where edge effects (or spectral leakage) become important. Below this curve, signals may be contaminated with artificial noise and thus are subject to scrutiny. capture. For the possible range of background magnetic field values and densities encountered by STEREO A for the times considered, the burst wave frequencies always satisfy the conditions f ci f f L, where f ci and f L are the local ion cyclotron and left hand cutoff frequencies [e.g., Stix, 1992, chap. 1, p. 11]. No left hand polarized wave modes exist in a cold plasma for these frequencies, indicating that warm plasma effects are required to explain the propagation of these waves. [21] The polarization reversals are associated with the following conditions: (1) there is wave power at f LHR < f < f NPM,or2f NPM ; (2) there is enhanced electric field wave power at 200 Hz; and (3) the H+ ion cyclotron frequency or lower hybrid frequency is 200 ± 60 Hz. These conditions can be observed in the three wavelet spectrograms in Figure 5. All three spectrograms show significant wave power at 200 Hz, near the H+ ion cyclotron frequency in the first two and near the lower hybrid frequency in the third. In the first wavelet spectrogram this power exists throughout the burst memory capture, but the polarization reversal, which occurs at the location of the vertical line, only happens once the wave frequency passes below the NPM transmitter frequency. In the second wavelet spectrogram the growth of the whistler at f LHR < f < f NPM is associated with the sudden onset of power at 200 Hz. Thus the growth of these whistlers may, at least in some cases, be directly related to wave power at 200 Hz. The third wavelet spectrogram shows that the polarization reversals can occur at multiples of f NPM. This is a whistler wave with an unidentified source near the plasmapause boundary as estimated from the upper hybrid line in the spectral data (not shown). This wave is polarized in the same sense as the STEREO A lightning associated whistlers. [22] In most cases the enhanced power at 200 Hz occurs when the NPM naval transmitter is in continuous mode and is, therefore, likely related to the continuous broadband mode modulation frequency. This suggests the following scenario. The modulation signal cyclotron resonates with local H+ ions or electrons when the cyclotron frequency or lower hybrid frequency is near 200 Hz, resulting in local plasma heating. Whistlers interacting with this heated plasma undergo large changes in wave normal angle and polarization reversals at f LHR and f NPM. Because the highly oblique rays propagate much more slowly than the moderately oblique ones (in a cold plasma), the smoothly decreasing frequency of the whistlers could not be observed if the source of the plasma modification were far from the spacecraft (for example, in the lower ionosphere). Powerful ground based transmitters like NPM have been shown to cause dramatic nightside density and ion and electron temperature fluctuations in the ionosphere [e.g., Parrot et al., 2007]. This influence may extend well into the inner radiation belt, perhaps as far as the plasmapause, and may be most significant when the transmitter operates in continuous broadband mode. The plasma modification may take the form of field aligned density striations, which naturally form out of a heated plasma because of enhanced conductivity along field lines as compared to across them. Whistler waves incident on such density striations can become highly oblique and largely electrostatic. Because of instrumental limitations and lack of data coverage during the 6 November perigee passes we are unable to verify the existence of density striations or a heated plasma component. [23] Indirectly, the wave/plasma interaction due to an inhomogeneous plasma may be hinted at in the amplitude modulation of the naval transmitter associated whistler shown in Figure 3e. The overall wave envelope amplitude has no apparent relation to the obliqueness, but the variation in the modulus becomes large and extends to near zero when the waves become highly oblique. This may suggest that there is 7of10

8 a rapid oscillatory exchange of energy between the wave and the plasma when the wave is in the process of mode converting. The extent to which these waves interact with the plasma is unknown, but such large amplitude waves, in addition to their strong interaction with high energy electrons, may provide strong heating of the ambient plasma. 5. Anomalous Wave Polarization [24] Figures 3 and 2 show that the polarization differs significantly between the three different whistlers. The transmitter associated and STEREO B lightning associated whistlers are polarized primarily perpendicular to the magnetic field, while the STEREO A whistler has a field aligned component comparable to the perpendicular component. In a purely cold plasma the field aligned component has an amplitude given by E z ¼ E x n 2 cos k sin k = n 2 sin 2 k P ð1þ [e.g., Verkhoglyadova et al., 2010], where E x is the maximum variance perpendicular component of the electric field, k is the wave normal angle, n is the refractive index and P is the cold plasma parameter from Stix [1992]. For a 10 khz wave in the inner plasmasphere with E x = 50 mv/m and with the observed frequencies and wavenormal angles, we expect a field aligned component of E z <1mV/m.ThisiswhatisobservedintheSTEREOB lightning associated whistlers and the STEREO A naval transmitter associated whistlers, but not on the STEREO A lightning associated whistlers and the whistler with an unknown source in Figure 5c, both of which have anomalously large field aligned components of E z 40 mv/m. [25] The polarization of the 200 Hz signal (normally the same as the naval transmitter associated whistler polarization) develops the same large parallel component as the STEREO A lightning associated whistlers as it grows to large amplitudes. This further suggests that the frequencies f LHR < f < f NPM and 200 Hz may sometimes grow together as in Figure 5b. The anomalous field aligned component may be the result of warm plasma effects [Kellogg et al., 2010] or may indicate the presence of an acoustic like wave with polarization along the magnetic field direction. 6. TDSMax observations [26] Burst memory measurements presented above indicate that large amplitude whistlers were observed in the Earth s plasmasphere on 6 and 17 November. Due to telemetry constraints, no burst memory waveform captures from the plasmasphere were transmitted on 29 November or 12 December. Figure 6 shows TDSMax for all 4 days when the spacecraft were within five Earth radii. Large amplitude electric field signals are seen throughout the plasmasphere on all perigee passes, particularly on 6 and 17 November 2006 where amplitudes (antenna coordinates) exceed 150 mv/m on both spacecraft. Smaller, but still large amplitude waves are observed on 29 November and 12 December Therefore all eight perigee passes show that populations of largeamplitude whistler waves exist in the Earth s plasmasphere. Although geomagnetic activity is very low for all of the perigee passes, it is not expected to have much influence on the inner radiation belts except during very large solar storms (Reeves et al. [2003]). 7. Discussion [27] Utilizing data from the burst memory on STEREO we have presented observations of inner radiation belt whistlers 2 to 3 orders of magnitude larger than previously observed. The measurements of wave amplitudes provided by STE- REO TDSMax indicate that very large amplitude whistlers are a common feature inside the plasmasphere. The large amplitudes may result from wave/particle cyclotron or Landau interaction within the ionosphere or plasmasphere between electrons and small amplitude pump whistlers originating from the NPM naval transmitter (and other ground transmitters) and lightning strikes. [28] Cattell et al. [2008] have shown that very large amplitude, oblique whistlers can cause significant pitch angle scattering and electron energization of high energy test particle electrons on a time scale of 0.1 s. These simulations, based on those of Roth et al. [1999], were run for this paper with conditions representative of the inner plasmasphere with the following results: resonant particles were seen to decrease their pitch angle by up to 20 and increase their energy by 25 kev in the same short time scale. Thus these whistler waves are able to pitch angle scatter and energize electrons on a time scale orders of magnitude shorter than that of quasi linear diffusion, the time averaged manner in which small amplitude whistler waves interact with electrons. In addition, these oblique whistler waves with a perpendicular wavelength smaller than or equal to the ion gyroradius can have a transverse resonance (w k? v? ) with ions. A highly oblique 5 khz whistler with a perpendicular wavelength of 400 m will resonate with 20 kev ions. Because they interact so strongly with high energy electrons and ions the whistlers may have a major influence on inner radiation belt dynamics. The region in which they are observed (L = ) has traditionally been thought to be dominated by coulomb collisions at L < 1.2 and slow pitch angle diffusion of high energy electrons due to cyclotron interaction with VLF waves from 1.2 < L <2.4.Abel and Thorne [1998] showed that precipitation rates of high energy electrons, calculated from VLF wave flux model estimates, compared favorably to observed electron precipitation rates. However, Starks et al. [2008] found an average of 20 db less VLF wave power in the inner radiation belt than predicted by ionospheric absorption models and assumed by Abel and Thorne [1998]. More sophisticated absorption models have only increased the discrepancy between required and observed VLF wave fluxes [e.g., Tao et al., 2010]. Much of this missing wave energy appears to be lost inside of the ionosphere after interaction with small scale field aligned density irregularities of scale size m [e.g., Bell et al., 2008]. [29] The very large amplitude whistlers presented here may represent an additional component of the missing VLF wave energy. They have gone unnoticed in the past because no previous spacecraft sampling this region had a highresolution burst mode capable of detecting such waves. The large amplitude whistlers may also provide an additional 8of10

9 Figure 6. Maximum electric field values (antenna coordinates) in mv/m each minute (peak peak) from TDSMax for all eight STEREO perigee passes when the spacecraft are at less than five Earth radii. source of high energy electron scattering at L < 1.2 where coulomb collisions tend to dominate. [30] The polarization plots presented in Figures 2 and 3 show that the STEREO A lightning associated and transmitterassociated whistlers undergo periodic mode conversions between left hand and right hand polarized waves. This may result from wave interaction with a heated plasma associated with continuous broadband mode signal of the NPM naval transmitter. Perhaps the most well known method of mode conversion is linear mode conversion at crossover frequencies which exist between adjacent ion cyclotron frequencies [e.g., Gurnett and Bhattacharjee, 2005, section ]. This type of mode conversion is responsible for the creation of ion cyclotron whistlers observed in association with lightning whistlers. This possibility is ruled out because the largest crossover frequency at the Earth is <1 khz. [31] Nonlinear mode conversion has also been associated with certain parametric instabilities. Trakhtengerts and Rycroft [1997], for example, have detailed the decay of whistler waves on field aligned density striations into lower hybrid and lower frequency ion acoustic waves in the magnetosphere, somewhat analogous with observations presented here. It is unclear, however, what process would produce left hand polarized waves in the whistler frequency range. The detailed 9of10

10 wave observations from the upcoming Radiation Belt Storm Probes (RBSP) mission may resolve this issue. 8. Conclusions [32] Very large amplitude, oblique whistler mode waves have been observed on STEREO in the Earth s plasmasphere. Their large amplitude and oblique nature may be due to instability resulting from the interaction between local electrons and the VLF pump waves provided by the lightning and transmitters. The whistlers observed on STEREO A have undergone periodic mode conversions, possibly as a result of interaction with plasma heated at the H+ ion cyclotron or lower hybrid frequencies. This heating appears to be related to the growth of the 200 Hz modulation signal from the NPM naval transmitter while operating in continuousbroadband mode. Wave particle simulations indicate that these whistlers interact strongly with both the ambient plasma,aswellaswithhigh energy electrons and possibly ions. Therefore they should be accounted for in models that fully describe wave/particle dynamics in the inner plasmasphere. [33] Acknowledgments. We thank J. Albert and J. Kozyra for discussion of relevant topics, T. Bell, K. Graf, and M. Spasojevic for information on the NPM naval transmitter, and A. Davis for providing high resolution STEREO orbital data. Lightning location data were supplied by the World Wide Lightning Location Network ( a collaboration among over 40 universities and institutions. This research was supported by NASA grants NNX07AF23G and NAS and was carried out inpart using computing resources at the University of Minnesota Supercomputing Institute. [34] Masaki Fujimoto thanks the reviewers for their assistance in evaluating this manuscript. References Abel, B., and R. M. Thorne (1998), Electron scattering loss in Earth s inner magnetosphere: 1. Dominant physical processes, J. Geophys. Res., 103, , doi: /97ja Bale, S. D., et al. (2008), The electric antennas for the STEREO/WAVES experiment, Space Sci. Rev., 136, Bell, T. F., and H. D. Ngo (1990), Electrostatic lower hybrid waves excited by electromagnetic whistler mode waves scattering from planar magneticfield aligned plasma density irregularities, J. Geophys. Res., 95, , doi: /ja095ia01p Bell, T. F., U. S. Inan, D. Piddyachiy, P. Kulkarni, and M. Parrot (2008), Effects of plasma density irregularities on the pitch angle scattering of radiation belt electrons by signals from ground based VLF transmitters, Geophys. Res. Lett., 35, L19103, doi: /2008gl Bougeret, J. L., et al. (2008), S/WAVES: The radio and plasma wave investigation on the STEREO mission, Space Sci. Rev., 136, , doi: /s Breneman, A., C. Cattell, S. Schreiner, K. Kersten, L. B. Wilson, P. Kellogg, K. Goetz, and L. K. Jian (2010), Observations of large amplitude, narrowband whistlers at stream interaction regions, J. Geophys. Res., 115, A08104, doi: /2009ja Cattell, C., et al. (2008), Discovery of very large amplitude whistler mode waves in Earth s radiation belts, Geophys. Res. Lett., 35, L01105, doi: /2007gl Cully, C. M., J. W. Bonnell, and R. E. Ergun (2008), THEMIS observations of long lived regions of large amplitude whistler waves in the inner magnetosphere, Geophys. Res. Lett., 35, L17S16, doi: / 2008GL Gurnett, D. A., and A. Bhattacharjee (2005), Introduction to Plasma Physics, 462 pp., Cambridge Univ. Press, Cambridge, U. K., ISBN: Gushchin, M. E., T. M. Zaboronkova, V. A. Koldanov, S. V. Korobkov, A. V. Kostrov, C. Krafft, and A. V. Strikovsky (2008), Whistler waves in plasmas with magnetic field irregularities: Experiment and theory, Phys. Plasmas, 15(2), , doi: / Helliwell, R. A. (1988), VLF wave stimulation experiments in the magnetosphere from Siple Station, Antarctica, Rev. Geophys., 26, , doi: /rg026i003p Holzworth, R. H., M. C. Kelley, C. L. Siefring, L. C. Hale, and J. D. Mitchell (1985), Electrical measurements in the atmosphere and the Ionosphere over an active thunderstorm: 2. Direct current electric fields and conductivity, J. Geophys. Res., 90, , doi: /ja090ia10p Holzworth, R. H., R. M. Winglee, B. H. Barnum, Y. Li, and M. C. Kelley (1999), Lightning whistler waves in the high latitude magnetosphere, J. Geophys. Res., 104, 17,369 17,378, doi: /1999ja Kelley, M. C., C. L. Siefring, R. F. Pfaff, P. M. Kintner, and M. Larsen (1985), Electrical measurements in the atmosphere and the ionosphere over an active thunderstorm: 1. Campaign overview and initial ionospheric results, J. Geophys. Res., 90, , doi: / JA090iA10p Kellogg, P. J., C. A. Cattell, K. Goetz, S. J. Monson, and L. B. Wilson (2010), Electron trapping and charge transport by large amplitude whistlers, Geophys. Res. Lett., 37, L20106, doi: /2010gl Kersten, K., et al. (2011), Observation of relativistic electron microbursts in conjunction with intense radiation belt whistler mode waves, Geophys. Res. Lett., 38, L08107, doi: /2011gl Luhmann, J. G., et al. (2008), STEREO IMPACT investigation goals, measurements, and data products overview, Space Sci. Rev., 136, Parrot, M., J. A. Sauvaud, J. J. Berthelier, and J. P. Lebreton (2007), First in situ observations of strong ionospheric perturbations generated by a powerful VLF ground based transmitter, Geophys. Res. Lett., 34, L11111, doi: /2007gl Reeves, G. D., K. L. McAdams, R. H. W. Friedel, and T. P. O Brien (2003), Acceleration and loss of relativistic electrons during geomagnetic storms, Geophys. Res. Lett., 30(10), 1529, doi: /2002gl Rodger, C. J., S. Werner, J. B. Brundell, E. H. Lay, N. R. Thomson, R. H. Holzworth, and R. L. Dowden (2006), Detection efficiency of the VLF World Wide Lightning Location Network (WWLLN): Initial case study, Ann. Geophys., 24, Roth, I., M. Temerin, and M. K. Hudson (1999), Resonant enhancement of relativistic electron fluxes during geomagnetically active periods, Ann. Geophys., 17, Santolík, O., D. A. Gurnett, J. S. Pickett, M. Parrot, and N. Cornilleau Wehrlin (2003), Spatio temporal structure of storm time chorus, J. Geophys. Res., 108(A7), 1278, doi: /2002ja Schulz, M., and L. J. Lanzerotti (1974), Physics and Chemistry in Space, vol. 7, Particle Diffusion in the Radiation Belts, 215 pp., Springer, New York. Starks, M. J., R. A. Quinn, G. P. Ginet, J. M. Albert, G. S. Sales, B. W. Reinisch, and P. Song (2008), Illumination of the plasmasphere by terrestrial very low frequency transmitters: Model validation, J. Geophys. Res., 113, A09320, doi: /2008ja Stix, T. (1992), Waves in Plasma, Springer, New York. Tao, X., J. Bortnik, and M. Friedrich (2010), Variance of transionospheric VLF wave power absorption, J. Geophys. Res., 115, A07303, doi: /2009ja Trakhtengerts, V. Y., and M. J. Rycroft (1997), A new parametric reflection mechanism for ducted whistlers and an explanation of precursors, J. Atmos. Sol. Terr. Phys., 59, , doi: /s (97) Verkhoglyadova, O. P., B. T. Tsurutani, and G. S. Lakhina (2010), Properties of obliquely propagating chorus, J. Geophys. Res., 115, A00F19, doi: /2009ja A. Breneman, C. Cattell, K. Goetz, P. J. Kellogg, K. Kersten, S. Schreiner, L. B. Wilson III, and J. Wygant, Department of Physics and Astronomy, University of Minnesota, Tate Lab Room 148, 116 Church St. S.E., Minneapolis, MN 55455, USA. (awbrenem@hotmail.com) 10 of 10