Forward Acceleration of Ionospheric Electrons by NAU 40.75-kHz Whistler Waves Over Arecibo

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 11, NOVEMBER 2013 3159

Forward Acceleration of Ionospheric Electrons byNAU 40.75-kHz Whistler Waves Over Arecibo

Lisa A. Rooker, Student Member, IEEE, Min-Chang Lee, Senior Member, IEEE, Rezy Pradipta, Member, IEEE,Laura M. Ross, Student Member, IEEE, Bozhi See, Michael P. Sulzer, Craig Tepley, Senior Member, IEEE,

Sixto A. Gonzalez, and Nestor Aponte

Abstract— We discuss a new mechanism to explain the forwardacceleration of ionospheric electrons by whistler waves. Wesuggest this mechanism to be the key process responsible forour reported enhanced plasma lines detected by the Arecibo430 MHz radar. These plasma lines are characterized by thefrequency-downshifted spectra with a bandwidth of ∼12 kHz.The backscatter radar operation indicates that electrons wereaccelerated upward along the Earth’s magnetic field by theup-going 40.75 kHz whistler waves, which were launched fromthe NAU transmitter. They covered a broad range of altitudes(∼300 km) and lasted for a period of a few minutes. Thismechanism leads to the energization of electrons of ∼13 eVinferred from Arecibo experiments.

Index Terms— Backscatter radar, forward acceleration ofionospheric electrons, four wave interactions, frequency-upshiftedand downshifted plasma lines, whistler wave-ionospheric plasmainteractions.

I. INTRODUCTION

IN THE past 10 years or so, we have been conductingArecibo experiments in Puerto Rico for the controlled

study of whistler wave interactions with ionospheric plasmas[1] and inner radiation belts at L = 1.35 [2]. The main

Manuscript received August 18, 2013; revised September 13, 2013; acceptedSeptember 15, 2013. Date of publication October 17, 2013; date of currentversion November 6, 2013. This work was supported by AFOSR underGrant FA9550-09-1-0391. The Lincoln Laboratory portion of this paperwas supported by the Department of the Air Force under Air Force Con-tract FA8721-05-C-0002. Opinions, interpretations, recommendations, andconclusions are those of the author and not necessarily endorsed by theUnited States Government. A portion of this paper was presented by thefirst author L. A. Rooker for the Student Paper Contest at the 39th IEEEInternational Conference on Plasma Science, Edinburgh, Scotland, 9-12 July2012. L. A. Rooker’s summer research and travel was supported by LutchenFellowship of College of Engineering, Boston University.

L. A. Rooker and B. See were with the Department of Electrical andComputer Engineering, Boston University, Boston, MA 02215 USA (e-mail:[email protected]; [email protected]).

M. C. Lee is with the Department of Electrical and Computer Engineering,Boston University, Boston, MA 02215 USA, and also with the SpacePropulsion Laboratory, Massachusetts Institute of Technology, Cambridge,MA 02139 USA (e-mail: [email protected]; [email protected]).

R. Pradipta is with the Space Propulsion Laboratory, Massachusetts Instituteof Technology, Cambridge, MA 02139 USA, and also with the Institutefor Scientific Research, Boston College, Newton, MA 02467 USA (e-mail:[email protected]).

L. M. Ross is with the Department of Electrical and Computer Engineering,Boston University, Boston, MA 02215 USA, and also with the M.I.T. LincolnLaboratory, Lexington, MA 02421 USA (e-mail: [email protected]).

M. P. Sulzer, C. Tepley, S. A. Gonzalez, and N. Aponte are with theArecibo Observatory, Center for Geospace Studies, SRI International, Arecibo,PR 00612 USA (e-mail: [email protected]; [email protected]; [email protected];[email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2013.2284158

source for the whistler waves is a Naval transmitter, code-named NAU, which is located nearby at Aguada, PuertoRico, emitting radio waves at a power and frequency of100 kW and 40.75 kHz, respectively. The experimental setupis delineated in Fig. 1. While the 40.75 kHz wave propagatesprimarily along Path 1 in the duct formed by the Earth’scrust and the bottom side of the ionosphere, a small portionof the wave energy can leak into the ionosphere propagat-ing along the Earth magnetic field, denoted by Path 2, inthe form of whistler waves. We estimated that ∼7.5% ofNAU transmitted power could be coupled into the ionospherevia refraction and mode conversion. Large-scale ionosphericplasma density irregularities would facilitate the entering ofNAU-launched whistler waves from the neutral atmosphereinto the ionosphere. These ionospheric density irregularitiescan occur naturally during spread F events or be inducedby HF heater waves. Large-scale ionospheric irregularitiestypically refer to those with scale lengths in the range ofhundreds of meters to kilometers.

In our 1997 HF heating experiments, we demonstrated thatthe Arecibo HF heater can create artificial ionospheric ductsfor controlled conjugate whistler wave propagation betweenArecibo, Puerto Rico, and Trelew, Argentina [3]. TheseO-mode (X-mode) HF heater wave-induced ionospheric ductsappeared in the form of parallel-plate waveguides within(orthogonal to) the meridional plane [4], [5]. However, withoutan HF heater at Arecibo in the past decade, we have beenrelying on naturally occurring, ionospheric irregularities forexperiments to investigate NAU-launched whistler wave inter-actions with space plasmas.

The Arecibo 430 MHz radar has provided powerful diagno-sis of ionospheric plasma effects induced by NAU transmissiondirectly in the ionosphere [1] or by downward streaming390 keV electrons knocked down by NAU signals fromthe radiation belts [2]. VLF whistler waves can interactwith ionospheric plasmas to generate large-scale electrostaticperturbations via thermal instabilities [6], [7]. However, weinvestigate short-scale electrostatic modes (i.e., lower hybridwaves) excited by NAU 40.75 kHz whistler waves, which canaccelerate electrons and ions along and across the geomagneticfield, respectively [8], [1]. These electron and ion accelerationprocesses detected by Arecibo radar lead to significantlyenhanced plasma lines and weak ion line enhancement, respec-tively. Hence, we focus on the study of electron acceleration(leading to significant plasma line enhancement) in spite ofthe fact that NAU launched whistler waves can also accelerate

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3160 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 11, NOVEMBER 2013

Fig. 1. Illustration of experiment setup at Arecibo Observatory, Puerto Rico for controlled study of NAU transmitted 40.75 kHz waves to interact withIonospheric plasmas and inner radiation belts.

Fig. 2. Detection of enhanced plasma lines generated by downward streaming electrons in backscatter radar operation mode, satisfying Bragg scatteringcondition, i.e., kES = 2 kradar. The dip angle (θ) at Arecibo is about 45°, as shown in Fig. 1. The phase energy (Eϕ) of accelerated electrons can beinferred [10].

ions (leading to weak ion line enhancement) via the excitationof lower hybrid waves [1], [8] or by other mechanisms [9].The radar detection of accelerated electrons, viz., the measure-ment of enhanced plasma lines is illustrated in Fig. 2.

From the backscatter radar operation, if plasma lines arisefrom radar-detected beam modes associated with downwardstreaming electrons, then the preferentially detected plasmawaves are expected to yield frequency-upshifted plasma lines

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Fig. 3. Illustration of the four-wave interaction process for the VLFwhistler-mode wave (ωo, ko) to parametrically excite Stokes (ω−

�h , k−) andanti-Stokes (ω+

�h, k+) lower hybrid waves together with the zero-frequency,(ωs , kS) field-aligned, density irregularities. The excited lower hybrid waveshave a single frequency equal to the VLF whistler wave frequency, but aspectrum of wavelengths.

due to the Doppler effect. However, as shown in Section II,enhanced plasma lines with frequency-downshifted spectrawere exclusively measured in our summer 2008 experimentsand also detected in later experiments. The characteristicfeatures of these frequency-downshifted plasma lines arehighlighted in Section III. A new mechanism different fromLabno et al.’s [1] to understand this intriguing phenomenonand to explain how NAU-launched whistler waves can accel-erate ionospheric electrons upward along the Earth’s magneticfield is presented in Section IV. Discussions are given andconclusions are finally drawn in Section V.

II. ENHANCED PLASMA LINES IN IONOSPHERIC F REGION

As reported in our earlier work [1], NAU launched40.75 kHz whistler waves are intense enough to excite lowerhybrid waves and field-aligned, zero-frequency, plasma densitystriations via a four-wave interaction process in ionosphericF region. These excited lower hybrid waves and densityirregularities are meter-scale electrostatic modes. This processcan be understood in terms of a diagram shown in Fig. 3. Therequired wave frequency and wave vector matching conditionsare

ωo = ω+�h − ωs , ωo = ω−

�h + ω∗s

ko = k+ − kS, ko = k− + kS (1)

where (ωo, ko) stands for the VLF whistler wave, (ω−�h , k−)

for the Stokes lower hybrid waves, (ω+�h, k+) for the anti-

Stokes lower hybrid waves, and (ωs , kS) for the low-frequency,

field-aligned, density irregularities. Note that the wave vector(kS) of the zero-frequency mode has two components, oneacross (kS⊥) and the other along (kS||) the geomagnetic field;|kS⊥ | � kS|||. Labno et al. [1] assume that k+ ≈ kS andk− ≈ −kS in the wave vector matching conditions (1). Inother words, the wavelengths (λ+ = 2π /k+, λ− = 2π /k−) ofexcited lower hybrid waves are approximately equal to thescale lengths (λS = 2π /kS) of excited meter-scale densityirregularities. Note that the wavelength (λo = 2π /ko) ofNAU launched 40.75 kHz is several hundred meters. BecausekS|| � ko of the 40.75 kHz whistler waves, Labno et al.’sassumption that k+ ≈ kS and k− ≈ −kS in (1) is justified.Hence, as depicted in Fig. 3, the excited lower hybrid wavesin either scenario/case can have both upward and downwardwave vectors for Stokes or anti-Stokes components. Thus,it is equally likely that electrons can be accelerated for-ward and backward according to the mechanism proposed byLabno et al. [1]. This mechanism explains the detection ofenhanced plasma lines in the F region with both frequency-upshifted and frequency-downshifted features well.

However, enhanced plasma lines with frequency-downshifted spectra were exclusively measured in oursummer 2008 experiments at Arecibo with the radartransmitted vertically via the stationary linefeed. Plasmaline measurements were recorded using a coded-long pulsetechnique to detect whistler wave-plasma interactions inthe altitude range 90–645 km. Additionally, ionogramswere recorded for every 5 min to monitor the backgroundplasma conditions. A VLF receiver was deployed to trackwhether the NAU transmitter was on or off. NAU-generated40.75 kHz waves were detected continuously during theentire experiment. Displayed in Fig. 4 is a series of enhancedplasma line data recorded on August 4/5, 2008.

III. CHARACTERISTIC FEATURES OF ENHANCED

PLASMA LINES

These frequency-upshifted plasma lines have outstandingfeatures distinctively different from those observed in ourDecember 2004 experiments [1], as highlighted below.

1) In our earlier experiments, the Arecibo radar couldonly detect upshifted plasma lines corresponding toplasma modes generated by down-going electrons [1].Now, although the radar has the capability to detectboth upshifted and downshifted plasma lines, we onlymeasured “downshifted” plasma lines in our Summer2008 experiments.

2) These downshifted plasma lines have a rather narrowfrequency bandwidth (∼12 kHz) around 4.5 MHz com-pared to the 1.5 MHz bandwidth of the plasma linescentered at 4 MHz reported in [1]. Also, these down-shifted plasma lines have significantly weaker intensi-ties.

3) They covered a much broader range of altitudes(∼300 km) than that of F-region plasma lines (∼120 km)seen in December 2004 experiments [1].

4) They lasted for a much longer period of time (> a fewminutes) than those (>10 s) reported in [1].

3162 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 11, NOVEMBER 2013

Fig. 4. Compilation of 3-D (altitude-frequency-intensity) plots to show plasma line enhancement features. From top left to right, we have data recorded at23:00 (LT), 23:31 (LT), and 00:00 (LT) on August 4/5, 2008, respectively. From bottom left to right, we have data recorded at 00:30 (LT), 01:00 (LT), and01:31 (LT) on August 5, 2008, respectively.

5) They occurred during the continuously descending ofF-region layer in the process of strong developing spreadF [see attached range-time-altitude (RTI) plot in Fig. 5].Contrastingly, F-region plasma line enhancement wasdetected on December 20/21 and December 26, 2004[1] when the F-region layer was quite stable.

6) We have imager (showing 6300 A and more intense 5577A airglows) and Fabry–Perot data, indicate eastward anddownward plasma drifting in our 2008 experiments.

IV. NEW SOURCE MECHANISM

Based on the above-noticed features from radio and opticaldiagnostics, we propose the following source mechanism, asdepicted in Fig. 6. The distinctive difference between this newmechanism and that of Labno et al. includes the followingaspects.

A. Wave Vector Matching Conditions

In this new mechanism

k+|| = ko, k+⊥ = kS, k−|| = ko, k−⊥ = −kS (2)

In [1]

k+ = kS, k− = −kS . (3)

B. Scale Lengths of Excited Lower Hybrid Waves andZero-Frequency Modes

In this proposed mechanism, 10-m-scale (e.g., 15 m) elec-trostatic wave/modes are excited under the wave vector-matching condition given by (2).

In [1], 10-m-scale (e.g., 1.5 m) wave modes are generatedunder the wave vector matching condition given by (3).

C. Preferred Directions to Accelerate Ionospheric Electrons

In this new physical process, electrons are preferen-tially accelerated forwardly with the up-propagating, NAU-launched, whistler waves, as depicted in Fig. 6. Contrastingly,electrons are equally likely to be accelerated in upward anddownward directions (i.e., forwardly and backwardly) by theLabno et al. mechanism, as illustrated in Fig. 3.

D. Energies of Accelerated Electrons

Based on the frequency spectra of measured plasma lines,the proposed mechanism accelerates ionospheric electrons inthe phase energy range ∼13.0 ± 1.2 × 10−5 eV, while themechanism of Labno et al. energizes electrons in the range7.2–15 eV.

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Fig. 5. RTI plot of backscattered radar echoes showing continuous descending of ionospheric layers during F-region plasma line measurements, which resultin data gaps, shown as white stripes on the RTI plot.

Fig. 6. Depiction of four-wave interaction process leading to acceleration of electrons upward along the magnetic field line due to NAU launched whistlerwaves.

In summary, the proposed mechanism causes forwardacceleration of electrons by NAU-launched 40.75 kHzwhistler waves. The parallel wave vector of excited10-meter scale lower hybrid waves is determined by thewhistler wave vector. This mechanism can explain the upwardstreaming electrons that can only lead to the radar detection ofdownshifted plasma lines with narrow frequency bandwidth.This process occurs in a much broader range of altitudes(several hundred kilometers). Thus, NAU whistler waves will

deposit significant energy in F region before they propagateinto radiation belts.

E. Detection With HF-VHF Radar

Arecibo 430 MHz radar cannot detect the NAU wave-excited field-aligned density striations and lower hybrid modesdirectly. However, these ionospheric plasma modes can bedetected by a HF-VHF radar, if we deploy such a radar at

3164 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 11, NOVEMBER 2013

Guadeloupe, a French island about 250 km to the south ofPuerto Rico. We then will be able to tilt the radar beam at aright angle to the geomagnetic field in the ionospheric regionabove Arecibo, to detect these field-aligned plasma modesdirectly. For example, the 15 and 1.5 m field-aligned modescould be detected by a HF radar operating at 10 MHz, and aVHF radar at 100 MHz, respectively.

V. CONCLUSION

We propose that upward propagating, NAU-launched,40.75 kHz, whistler waves can accelerate ionospheric electronsforwardly along the background magnetic field via a newsource mechanism. This process can cause the downshiftedplasma line enhancement detected by the Arecibo 430 MHzradar. This new mechanism has a striking feature distinguish-ing it from that of Labno et al., i.e., the excited Stokesand anti-Stokes lower hybrid waves have their parallel wavevectors equal to the wave vector of the up-going whistler wave.Contrastingly, in the mechanism discussed in [1], the wavevectors of whistler wave-excited lower hybrid waves are sameas those of concomitantly excited zero-frequency, field-alignedmodes.

Hence, NAU-launched whistler waves can only accelerateionospheric electrons forwardly via the proposed mechanism,while both forward and backward electron acceleration canoccur via Labno et al.’s mechanism under much more strictconditions.

The marked difference between these two mechanisms canalso be understood in terms of the scale lengths of theirgenerated lower hybrid waves and zero-frequency modes.While meter-scale (e.g., 1.5 m) electrostatic plasma modesare produced by Labno et al.’s mechanism, the new mech-anism creates corresponding plasma mode with 10-meterscale (e.g., 15 m). Consequently, the parallel phase velocitiesof excited lower hybrid waves to accelerate electrons are∼6.1 × 105 m/s in the case of Labno et al. and ∼1.4 × 107 m/sin the present case. Compared to the typical thermal velocity(∼1.3 × 105 m/s), it is seen that the meter-scale lower hybridwaves can yield acceleration of more thermal ionosphericelectrons, while the 10-meter scale waves can only acceleratefewer tail electrons. This is consistent with the observationsthat enhanced plasma lines in [1] are much more intense thanthose in the concerned case displayed in Fig. 4.

Finally, we notice that the process discussed in [1] didnot occur in our August 2008 experiments; namely, wedid not measure frequency-upshifted plasma lines. This puz-zlement can be understood from the fact that the back-ground plasma was so turbulent during our 2008 experi-ments. This fact was supported by the RTI plots showingthe continuous descending of F-region layer (see Fig. 5),as well as both the Fabry–Perot and imager data indicat-ing the eastward and downward plasma drifts over Arecibo.Therefore, it is rather difficult for 40.75 kHz whistler wavesto excite meter-scale electrostatic plasma modes, whichrequire much higher thresholds in the four wave interactionprocess.

In conclusion, the proposed mechanism can be based onto reasonably explain the detection of enhanced plasma lines

with only frequency-downshifted spectra in our 2008 Areciboexperiments. In the comparison with our earlier experimentsto record frequency-upshifted plasma lines [1], we can under-stand how distinctively different features of enhanced plasmalines are caused by these two mechanisms. The mechanismdiscussed in [1] has more severe requirements for its occur-rence than the newly proposed process.

ACKNOWLEDGMENT

The Arecibo Observatory is operated by SRI Internationalunder a cooperative agreement with the National ScienceFoundation (AST-1100968), and in alliance with SistemaUniversitario Ana G. Méndez, and the Universities SpaceResearch Association. Dr. S. Basu of Air Force ResearchLab (AFRL) passed away recently. Some of us had beenDr. Bas’s friends and colleagues for over 3 decades, enjoyingand benefiting collaboration with him for research. Dr. Basuwill be always remembered by us as an outstanding scientistand a gentleman.

REFERENCES

[1] A. Labno, R. Pradipta, M. C. Lee, M. P. Sulzer, L. M. Burton,J. A. Cohen, et al., “Whistler-mode wave interactions withionospheric plasmas over Arecibo,” J. Geophys. Res., vol. 112,pp. A03306-1–A03306-5, Mar. 2007.

[2] R. Pradipta, A. Labno, M. C. Lee, W. J. Burke, M. P. Sulzer,J. A. Cohen, et al., “Electron precipitation from the inner radi-ation belt above Arecibo,” Geophys. Res. Lett., vol. 34, no. 8,pp. L08101-1–L08101-5, 2007, doi: 10.1029/2007GL029807.

[3] M. J. Starks, M. C. Lee, and P. Jastrzebski, “Interhemispheric propaga-tion of VLF transmissions in the presence of ionospheric HF heating,”J. Geophys. Res., vol. 106, no. A4, pp. 5579–5591, 2001.

[4] M. C. Lee, R. J. Riddolls, W. J. Burke, M. P. Sulzer, S. P. Kuo,and E. M. C. Klien, “Generation of large sheet-like ionosphericplasma irregularities at Arecibo,” Geophys. Res. Lett., vol. 25, no. 16,pp. 3067–3070, 1998.

[5] J. A. Cohen, R. Pradipta, L. M. Burton, A. Labno, M. C. Lee,B. J. Watkins, et al., “Generation of ionospheric ducts by theHAARP HF heater,” Phys. Scripta, vol. T142, pp. 014040-1–014040-7,Dec. 2010, doi: 10.1088/0031-8949/2010/T142/014040.

[6] M. C. Lee and S. P. Kuo, “Excitation of magnetostatic fluctuations byfilamentation of whistlers,” J. Geophys. Res., vol. 89, pp. 2289–2294,Apr. 1984.

[7] L. Stenflo, P. K. Shukla, and M. Y. Yu, “The theories for excitation ofelectrostatic fluctuations by thermal modulation of whistlers,” J. Geo-phys. Res., vol. 91, no. A10, pp. 11369–11371, 1986.

[8] M. C. Lee and S. P. Kuo, “Production of lower hybrid waves andfield-aligned plasma density striations by whistlers,” J. Geophys. Res.,vol. 89, pp. 10873–10880, Dec. 1984.

[9] P. K. Shukla, L. Stenflo, R. Birgham, and R. O. Dendy, “Ponderomotiveforce acceleration of ions in the auroral region,” J. Geophys. Res., SpacePhys., vol. 101, pp. 27449–27451, Dec. 1996.

[10] H. C. Carlson, V. B. Wickwar, and G. P. Mantas, “Observations offluxes of suprathermal electrons accelerated by HF excited instabili-ties,” J. Atmos. Terrestrial Phys., vol. 44, no. 12, pp. 1089–1100, 1982.

Lisa A. Rooker (S’12) received the B.S. degreein electrical engineering from Boston University,Boston, MA, USA.

She conducted active experiments at Arecibo,Puerto Rico, and Gakona, AK, USA. as well astheoretical and numerical analysis of whistler wavegeneration and interactions with space plasmas. Herresearch work was presented at the 39th IEEEICOPS in 2012 and the IEEE Region I StudentConference for the Student Paper Competition in2013.

Ms. Rooker is a Student Member of the IEEE Nuclear and Plasma ScienceSociety.

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Min-Chang Lee (SM’92) received the Ph.D. degreein electrophysics and applied plasma physics fromthe University of California San Diego, La Jolla, CA,USA.

He has developed an integrated educational andresearch program with the Massachusetts Instituteof Technology (MIT), Cambridge, MA, USA, andBoston University (BU), Boston, MA, USA, inelectromagnetic wave interactions with plasmas forspace and energy applications, including theoreticalstudy, field experiments, and laboratory simulation

experiments. He is currently a Professor of electrical and computer engineer-ing at BU. He is affiliated with MIT.

Dr. Lee is the IEEE Boston Chapter Chair of the Nuclear and Plasma ScienceSociety.

Rezy Pradipta (M’12) received the B.S., M.S., andPh.D. degrees in physics, nuclear science and engi-neering, and applied math from the MassachusettsInstitute of Technology, Cambridge, MA, USA.

He is currently a Post-Doctoral Researcher withthe Institute for Scientific Research, Boston Col-lege, Boston, MA, USA. He has been conduct-ing experiments with Prof. M.-C. Lee’s Group atArecibo, Puerto Rico, and Gakona, AK, USA. aimedat investigating HF heater wave and whistler waveinteractions with space plasmas. He carried out HF

ionospheric heating experiments to simulate gravity wave generation causedby natural or artificial anomalous heat sources.

Dr. Pradipta is a member of the IEEE Nuclear and Plasma Science Society.

Laura M. Ross (S’11) received the B.S. degree inphysics from the Massachusetts Institute of Tech-nology (MIT), Cambridge, MA, USA, and the M.S.degree in electrical engineering from Boston Uni-versity, Boston, MA, USA.

She has conducted experiments at Arecibo, PuertoRico and Gakona, AK, with a focus on parametricinstabilities in ionospheric plasma. She is currentlywith the MIT Lincoln Laboratory.

Ms. Ross is a Student Member of the IEEENuclear and Plasma Science Society.

Bozhi See received the B.S. degree in electricalengineering from Boston University, Boston, MA,USA.

He conducted experiments at Arecibo Observatoryon radar detection of electron acceleration causedby very low frequency whistler waves for his seniorhonor thesis research.

Michael P. Sulzer received the Ph.D. degree inelectrical engineering from Pennsylvania State Uni-versity, University Park, PA, USA.

He has been a Staff Scientist with Arecibo Obser-vatory, Arecibo, Puerto Rico, since 1981. He is aSenior Research Associate and the Head of radiosciences with Arecibo Observatory, where he hasbeen conducting radar projects related to ionosphericphysics and HF ionospheric heating. He has beenactively organizing HF heating sessions for nationaland international conference, and serving as a men-

tor/advisor for REU students at Arecibo annually as well as journal andproposal reviewer.

Dr. Sulzer is an active member and participant in NSF CEDAR, AGU, andURSI.

Craig Tepley (S’74–M’80–SM’91) received theB.S.E.E. degree from Cleveland State University,Cleveland, OH, USA, in 1975, and M.S. and Ph.D.degrees in electrical engineering from Case WesternReserve University, Cleveland.

He was a Visiting Graduate Researcher withArecibo Observatory, Arecibo, Puerto Rico, wherehe combined incoherent scatter radar observationsof the total ionization content of lower ionosphericsporadic layers with twilight optical spectroscopicmeasurements of various metallic species in an effort

to understand the composition and intricate dynamics of these layers. His Post-Graduate Research with the University of Michigan, Ann Arbor, MI, USA,in 1980, helped to establish remote and automated optical observatories forairglow and auroral studies at Calgary, Alberta, at Arequipa, Peru, and toa lesser extent at Stara Zagora, Bulgaria. At those three sites, as well asat Arecibo, spectrophotometric techniques were combined with Fabry–Perotinterferometry to investigate the temperatures, winds, and basic physiochem-ical structure of the upper and lower thermosphere. He joined the staff atArecibo in 1983, and has been involved in many aspects of experimental workthroughout the earth’s atmosphere and ionosphere, including the developmentof Doppler Rayleigh and resonance fluorescence lidars, which combined withother existing radar and optical instrumentation, broaden the observationalcapabilities of the Arecibo Observatory for upper atmospheric researchthroughout the middle and lower atmosphere.

Dr. Tepley is a member of AGU and formally of OSA.

Sixto A. Gonzalez is the Current Director for Spaceand Atmospheric Sciences with Arecibo Observa-tory, part of the Center for Geospace Studies withSRI International. He has 20 years of professionalexperience as a Scientist and Research Managerand a demonstrated talent for thinking outside thebox and finding innovative solutions. He was anAdvisor and Research Supervisor of more than 35individuals, including high school, undergraduate,graduate, and post-doctoral students. His currentresearch interests include the studies of the earth’s

upper atmosphere using incoherent scatter radars, satellites, optical instru-ments together with physics-based numerical models, and improving accuracyand precision of the data obtained with the incoherent scatter radars. He hasoffered over 100 lectures at national and international meetings includinginvited or distinguished seminars at Boston University, Boston, MA, USA,University of Washington, Seattle, WA, USA, and Penn State University,University Park, PA, USA. He has served on multiple organizing committeesand convened numerous workshops, national, and international meetings. Hehas served on numerous proposal and research program evaluation panelsfor NASA and NSF. He has reviewed more than 50 manuscripts primarilyfor JGR, GRL, and JASTP. He served on NSF CEDAR Steering Committee,including one term as a Chair.

Nestor Aponte received the Ph.D. degree in electri-cal engineering from Cornell University, Ithaca, NY,USA.

He was with Arecibo Observatory, Arecibo, PuertoRico, in 1998, and he has been involved in radarprojects for ionospheric research. From 2002 to2003, he was an Assistant Professor of electricalengineering with Cedarville University, Cedarville,OH, USA. His current research interests includeradar studies of the equatorial F-region energy bal-ance, simultaneous measurements of ionospheric

densities, temperatures, and drift velocities at Jicamarca, Peru, advectionof the equatorial anomaly over Arecibo by small-storm related disturbancedynamo electric fields, instantaneous electric field measurements, and derivedneutral winds at Arecibo, and molecular ion composition measurements in theF1-region at Arecibo.