1

Introduction

EXPLORING GEOSPACE USING HIGH-FREQUENCY HEATING: NOVEL TECHNIQUES

Traditionally, experimental geospace research, including probing of the atmosphere, the ionosphere, and the magnetosphere (AIM), has been conducted passively (i.e., no controlled input/response capability) using instruments located on the ground or on satellites. From this research has come the recognition that most of the individual processes that underlie the upper atmosphere and ionosphere’s structure and dynamics are coupled and difficult to study in isolation, especially when it is not possible to uniquely identify the driver of a particular process.

Research conducted by the ionospheric modifications (IM) community—a community that uses high-frequency (HF) transmitters to inject energy in the ionosphere and measure its effects using ground-and space-based diagnostics—is focused on understanding the interaction of radio waves with the ionospheric plasma, the local consequences of heating in the ionosphere, and studies of nonlinear plasma physics processes. Some workshop participants noted that while the IM and the AIM community share many scientific goals and require similar ground and space instrumentation, their interactions have heretofore been sporadic.

The guiding principle of the recently completed decadal survey in solar and space physics (NRC, 2013) was that transformational scientific progress is required in order to study the Sun, Earth, and the heliosphere as a coupled system. It was noted at the workshop that the natural processes by which geomagnetic events affect this complex system are very similar to the kind of experiments that can be performed with ionospheric modification, such as those done at the High Frequency Active Auroral Research Program (HAARP; see Box 1.1). Such modifications can be used to perform controlled experiments to better understand natural processes in what is otherwise a very complex system, a point emphasized by numerous participants. Heating can also probe regions of parameter space that are unattainable with other techniques. In addition to perturbing the natural system, several participants noted that active experiments can address universal processes in plasma physics, essentially creating what was described as a cosmic plasma laboratory without walls.

Studies of the nonlinear interaction of HF radio waves with the ionosphere using recently developed powerful and agile ionospheric heaters, such as the EISCAT (European Incoherent Scatter Scientific Association) heater and more recently the completed HAARP heater (Appendix E), have resulted in the development of novel techniques that some participants described as transformational in their implications for understanding the physics of ionosphere-thermosphere-magnetosphere (ITM) regions and their coupling. Among the areas of research highlighted by some participants are the following:

•    Mesosphere-thermosphere diagnostics;

•    Artificial plasma layers (APLs) at altitudes between the F peak and 150 km and associated optical emissions;



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1 Introduction EXPLORING GEOSPACE USING HIGH-FREQUENCY HEATING: NOVEL TECHNIQUES Traditionally, experimental geospace research, including probing of the atmosphere, the ionosphere, and the magnetosphere (AIM), has been conducted passively (i.e., no controlled input/response capability) using instruments located on the ground or on satellites. From this research has come the recognition that most of the individual processes that underlie the upper atmosphere and ionosphere’s structure and dynamics are coupled and difficult to study in isolation, especially when it is not possible to uniquely identify the driver of a particular process. Research conducted by the ionospheric modifications (IM) community—a community that uses high-frequency (HF) transmitters to inject energy in the ionosphere and measure its effects using ground- and space-based diagnostics—is focused on understanding the interaction of radio waves with the ionospheric plasma, the local consequences of heating in the ionosphere, and studies of nonlinear plasma physics processes. Some workshop participants noted that while the IM and the AIM community share many scientific goals and require similar ground and space instrumentation, their interactions have heretofore been sporadic. The guiding principle of the recently completed decadal survey in solar and space physics (NRC, 2013) was that transformational scientific progress is required in order to study the Sun, Earth, and the heliosphere as a coupled system. It was noted at the workshop that the natural processes by which geomagnetic events affect this complex system are very similar to the kind of experiments that can be performed with ionospheric modification, such as those done at the High Frequency Active Auroral Research Program (HAARP; see Box 1.1). Such modifications can be used to perform controlled experiments to better understand natural processes in what is otherwise a very complex system, a point emphasized by numerous participants. Heating can also probe regions of parameter space that are unattainable with other techniques. In addition to perturbing the natural system, several participants noted that active experiments can address universal processes in plasma physics, essentially creating what was described as a cosmic plasma laboratory without walls. Studies of the nonlinear interaction of HF radio waves with the ionosphere using recently developed powerful and agile ionospheric heaters, such as the EISCAT (European Incoherent Scatter Scientific Association) heater and more recently the completed HAARP heater (Appendix E), have resulted in the development of novel techniques that some participants described as transformational in their implications for understanding the physics of ionosphere-thermosphere-magnetosphere (ITM) regions and their coupling. Among the areas of research highlighted by some participants are the following: • Mesosphere-thermosphere diagnostics; • Artificial plasma layers (APLs) at altitudes between the F peak and 150 km and associated optical emissions; 12

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BOX 1.1 Ionospheric Heaters Mimic Natural Processes Solar radiation interacting with Earth’s atmosphere creates the ionosphere, the shell of ionized plasma enveloping Earth, the discovery of which led to long-range wireless communication. Participants at the workshop, including Jade Morton, John Foster, and Anthea Coster, described research to achieve a fuller understanding of the ionosphere that is motivated by the increasing modern reliance on satellite resources, such as the Global Positioning System (GPS), and concerns about space weather. Plasma instabilities within the global ionosphere, especially during major magnetic storms, disrupt reliable access to satellite communications, GPS navigation, and other civilian, commercial, and defense national satellite resources. The physics of the production of Earth’s bulk ionosphere is straightforward: Solar radiation energy breaks electrons loose from neutral particles in the upper atmosphere, and conservation of momentum makes the low-mass electrons separate at vastly higher velocity than the ions left behind, thereby carrying almost all the initial kinetic energy. Recently completed high-power, high-frequency (HF) research facilities like the High Frequency Active Auroral Research Program (HAARP) can deliver radio-frequency energy densities comparable to that from the Sun, and through wave-particle interactions, found universally in plasmas throughout space (local, interplanetary, intergalactic), can break electrons loose from neutral species at a rate comparable to that by the Sun. From this point on, the physics (whether ionization by solar photon or radio- frequency radiation) is the same as for natural energy flows. Hence, there is great interest in applications for space science, as described by many workshop participants and summarized here. At the workshop, Herbert Carlson noted that a great qualitative advantage of energy deposition from ground-based research facilities is that for the first time it is now possible to conduct controlled experiments, versus simply watching and waiting for the Sun to perturb space and then attempting to learn from studying its response. Carlson and other participants who are active users of heaters emphasized the value of being able for the first time to carry out controlled input-response experiments. In addition, many participants noted that these new capabilities enable exploration of the basic underlying nature of a wide range of properties of the neutral and ionized matter forming our environment. As summarized by Carlson, active experiments provide scientists with a qualitatively new “laboratory in the sky,” to excite input-response discoveries that span over 12 orders of magnitude—a million million-fold—of scale size. He further noted that this capability enables wide-ranging studies, such as the flow of energy in matter, cascading in excited states of atoms and molecules; ionizing matter to probe aspects of the very nature of particle, collective- gas, and wave-particle energy exchange; and exploring fundamental plasma processes, which bear directly on attempts to realize controlled plasma fusion. All of the above is also highly relevant to understanding the geospace response to solar storms and the development of space weather near Earth. In his presentation to the workshop, Carlson noted the decade of HAARP construction coincided with the development of a research community that has learned how to control and use energy deposition in neutral and plasma species. As Carlson put it, both the facility and the community now stand ready to explore and discover new and fundamental physical phenomena, some not yet imagined, of fundamental importance to both civilian/industrial and defense users. He added that additional diagnostic capabilities, especially those that will be provided by the transfer to Gakona of the Poker Flat Incoherent Scatter Radar, make this even more likely. • Generation and injection into the Earth-ionosphere waveguide (EIW) and into the magnetosphere electromagnetic (EM) waves at ULF/ELF/VLF frequencies (ultralow frequency/extremely 13

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low frequency/very low frequency) by modulated HF heating of the ionosphere, also known as “virtual antennae;” • Generation of artificial ionospheric turbulence (AIT) that increases the level of scintillation and affects the propagation of trans-ionospheric electromagnetic signals, such as the Global Positioning System (GPS), HF communications, and space radar; and • Generation of strong plasma outflows and associated field aligned ducts that focus HF and guide VLF signals. Mesosphere-Ionosphere Diagnostics In presentations and discussions at the workshop, a number of ionospheric modification techniques were cited as providing diagnostic information about the mesosphere/ionosphere. The following list includes techniques noted by one or more participants. • The study of artificial periodic irregularities (APIs) were said to be a powerful technique for determining electron and neutral densities and temperature. APIs are formed in the lower ionosphere by a standing wave due to the interference of incoming and reflected HF radio waves. Probing waves scattered by those irregularities carry information about ambient electron density and vertical velocity. In addition, the neutral density and temperature can be deduced by measuring the temporal relaxation of the irregularities (and finding the rate of ambipolar diffusion). • Polar mesospheric summer echoes (PMSEs) were said to be an indicator of polar mesospheric clouds (PMCs). PMCs have been detected at an increasing rate at lower and lower latitudes and may be linked to global climate change (Thomas et al., 2010). PMCs are composed of dust grains (aerosols) of submicron size at the altitude 80 to 90 km. These dust particles are charged by the attachment of free ionospheric plasma particles and photoemission of electrons by solar radiation, which enables interactions with HF waves. One workshop participant noted that recent observations of PMSE modulations created using the EISCAT heating facility could help to deduce mean grain size, dust density, and photoemission rates from PMSE temporal variations. • Some participants discussed using HF-induced optical emissions to measure neutral densities at altitudes between 200 and 350 km (F-region). The decay rate of an excited oxygen optical emission at 630 nm, O(1D), is determined by the number density and composition of the neutral atmosphere. Thus, by detecting the optical decay rate at different altitudes, the altitude profile of the neutral density can be deduced, which is of special interest in determining the drag force in the ionosphere. • The analysis of optical emissions from quasi-steady-state clouds excited by high-power radio waves were another diagnostic technique discussed at the workshop. The shape of the cloud is determined by neutral diffusion and thermospheric winds. After the HF waves are turned off, the glowing cloud persists long enough that it will expand by diffusion in the neutral atmosphere and move along the direction of the neutral wind. Observations of the cloud shape and the motion immediately after HF power is turned off yield the horizontal components of neutral wind and diffusive flux (Bernhardt et al., 2000). In addition, use of a Fabry-Perot interferometer to measure O(1D) emissions from the cloud can determine the line-of-sight neutral winds at the altitude of the artificial airglow cloud. Artificial Plasma Layers As was explained at the workshop, at low powers, electromagnetic waves propagate in the ionosphere without producing any observable change in the plasma environment. High-power radio waves, on the other hand, reportedly can drive nonlinear processes that yield electrostatic waves that interact with the ambient plasma to accelerate electrons to well above their thermal energies. The energetic electrons collide with neutrals to yield excited species that subsequently radiate as optical 14

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emissions. If the electron energy is high enough, the high-speed electrons can ionize the background neutral gases to yield localized regions of artificial ionization. The recently demonstrated (Pedersen et al., 2010) capability of HAARP’s 3.6-MW transmitter to produce significant artificial plasma in the upper atmosphere was said to open “the door to a new regime in ionospheric radio wave propagation where transmitter-produced plasmas dominate over the natural ionospheric plasma” (p. 1). Eventually, some participants speculated, it may be possible to employ this technique as an active component of communications, radar, and other systems. Imaging of artificial airglow from these “layers” allows their existence, location, and dynamics to be precisely identified; depending on their location, they can also affect satellite communications and navigation. Ionospheric Generation of ULF/ELF/VLF Waves At ULF/ELF/VLF frequencies, traditional dipole antennas are extremely inefficient and require very long wires. However, as noted repeatedly at the workshop, it is possible to generate these frequencies using a virtual ionospheric antenna through ionospheric modification techniques. There are two techniques for generating such waves. The first, known as current modulation, requires the presence of an electrojet current above the HF heater, and thus its use is restricted to high-latitude transmitters (currently, there are currently no HF heating facilities under the equatorial electrojet), and its availability and strength are controlled by the strength of the electrojet. Experiments to date have demonstrated generation of frequencies up to 20 kHz, with the upper frequency limited by the electron temperature relaxation rate at the generation altitude. The generated waves are then injected as EM waves into the EIW and as whistler and shear Alfvén waves (SAW) into the radiation belts (Rietveld et al., 1984, 1989; Barr, 1998; Papadopoulos et al., 1990, 2005). Modulated heating of the D/E-region electrons modulates the plasma conductivity generating a virtual antenna at altitudes between 70 and 85 km. A recently developed alternative technique mentioned at the workshop—ionospheric current drive (ICD; Papadopoulos et al., 2011a,b) does not require the presence of electrojet. As a result, it can be employed at non-electrojet regions with heaters, such as those located at Arecibo and Sura, and it is available to HAARP and EISCAT during conditions when the electrojet is weak or absent. ICD relies on the generation of a diamagnetic current during F-region heating that disappears when the heater is off. Generation of ULF/ELF/VLF waves through HF techniques was discussed at the workshop in connection with applications such as underwater communications and wave injection into the radiation belts. HAARP IS AT A CROSSROADS The HAARP program was initiated in 1990 by congressional action. Congress followed the recommendation of several scientific panels that there was an urgent need for a U.S.-based “world- leading” experimental facility with a heater based on modern electronic phased-array beam steering with wide transmitter frequency coverage and power exceeding theoretical thresholds for triggering strongly nonlinear processes, which was supported by an extensive complement of diagnostic instruments. The facility was completed and achieved its full design power in 2007 and has produced, according to several participants, a number of scientific firsts and breakthrough physics, in addition to being instrumental in training future workers in radio science and space-related disciplines. A dedicated site technical staff (currently at Marsh Creek Ltd.) created powerful software and hardware techniques to operate and maintain this antenna, which participants were told is now at the peak of its performance. 1 According to several workshop participants, the scientists and engineers running experiments at HAARP have been handicapped by the lack of a key diagnostic instrument, namely an incoherent scatter 1 Indeed, there was discussion at the workshop about the vulnerability of HAARP maintenance owing to the consolidation of expertise to a few specific individuals working as private contractors. 15

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radar (ISR). Further, these participants noted that such an instrument is now available, built from the next- generation ISR technology incorporated in AMISR. AMISR’s novel modular configuration allows it be relocated with relative ease, thus enabling the study of upper atmospheric activity around the globe. In addition, remote operation and electronic beam steering allow researchers to operate and position the radar beam instantaneously. 2 AMISR radars have so far been constructed in two locations. The first, in Poker Flat, Alaska, known as PFISR, has been completed and is already being used for scientific investigations. The second, the Resolute Bay ISR (RISR) in Nunavut, Canada, has two antennas pointed in complementary directions, the north-directed radar currently operating and the south-directed radar complete but not yet fully operational. Richard Behnke and Bob Robinson from the National Science Foundation informed the workshop that they would like to move PFISR to the HAARP facility for 1 year (or possibly more) of coordinated experimental campaigns. With HAARP complete as of 2007, and with the potential of the addition of an ISR and other diagnostic instruments, HAARP users at the workshop spoke enthusiastically about new experiments and the potential for new scientific discoveries. At the same time, there was considerable discussion at the workshop regarding the future of the facility, given the Air Force’s current plans to wind-down its support and plan for the facility’s decommissioning. HAARP’S UNIQUE CAPABILITIES HAARP is located at 62.39° north latitude, 145.15° west longitude, which translates to 63.09° north magnetic latitude and 92.44° west magnetic longitude. At this magnetic latitude, the facility can observe regions rich with geophysical phenomena. Under nominal geomagnetic conditions, the region is between the mid-latitudes and auroral zone and is magnetically conjugate to a region of the magnetosphere close to the Van Allen radiation belts. As geomagnetic conditions vary, so does the conjugate position. Under moderately active conditions, HAARP can be in the auroral zone, or even in the polar cap at higher levels of activity. In addition, it is reasonably isolated from populated areas, which results in very dark skies. Hence, the facility has great potential as a location for an observatory (Figure 1.1). The HAARP ionospheric research instrument (IRI) is physically capable of transmitting any frequency between 2.8 and 10 MHz with an instantaneous bandwidth of at least 200 kHz. This frequency range is the broadest of any heating facility, going both lower and higher than all others. The low end of the band is just below twice the electron gyro frequency in the ionosphere over HAARP. It is the only heating facility that is able to transmit below this important value. The ability to transmit as high as 10 MHz ensures that the facility can probe into the F-region even under high plasma density conditions. Such a broad range allows operation throughout a complete solar cycle. Because each of the HAARP transmitters can generate from 10 W to 10 kW, the total transmitted power can range from 3,600 W to 3.6 MW, while maintaining a consistent antenna pattern. As already noted, the HAARP antenna consists of an array of 180 crossed dipoles, which are arranged in a 12 by 15 rectangular grid and are phased to provide steering. At the low end of the frequency band (2.8 MHz), the array main lobe beam width is about 15°, while at the upper end of the band (10 MHz), it is about 5°. Below about 5.8 MHz, the antenna beam can be steered anywhere in the sky without the possibility of grating lobes. The beam can be repositioned within about 15 μs, which means that the beam can be swept in a nearly continuous motion across an area, or it can be stepped from place to place almost instantly. The transmitter has great flexibility in modulations, including AM, FM, phase, pulse, and any signal that can be represented in a “.wav” file. 2 Advanced Modular Incoherent Scatter Radar, AMISR Overview, available at http://amisr.com/amisr/about/ amisr-overview/, accessed September 4, 2013. 16

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• CLUSTER • Van Allen Probes Geomagnetic Field • GRACE • DMSP Artificial Plasma Ducts • ePOP • RESONANCE F ULF/ELF/VLF Propagating Modified Region to Conjugate Point Ionospheric Current Drive Striations E Artificial Ionized Layer Optical emissions Ionospheric Regions D Modified Conductivity ELF/VLF Waves • Ionosonde • HF/VHF Radars • Optical Photometer ULF/ELF/VLF Scintillation • Magnetometers Receivers Receiver MUIR HAARP FIGURE 1.1 HAARP has a privileged location that allows for conditions that cover critical geospace phenomena. It is within the auroral zone at storm times, mapped in the plasmasphere during quiet times, and in the subauroral region during substorms. SOURCE: Top: E. Mishin, Air Force Research Laboratory, “Using HAARP to Better Understand Space Weather,” presentation to the Committee on the Role of High-Power, High-Frequency-Band Transmitters in Advancing Ionospheric/Thermospheric Research: A Workshop, May 2013. Courtesy of Evgeny Mishin. Bottom: Courtesy of K. Papadopoulos, University of Maryland. In addition to the IRI, the HAARP site has a number of diagnostic instruments and facilities to support additional instruments. Some of the diagnostics are owned by the HAARP program (some with an associated principal investigator [PI]), while others are owned by PI institutions and supported by the HAARP program. These diagnostic instruments include magnetometers, riometers, an ionsonde, ultrahigh-frequency (UHF) and very-high-frequency (VHF) radars, optics, GPS scintillation receivers, ELF and VLF receivers, a seismometer, meteorological monitors, an HF receive antenna, and a spectrum monitor. 17

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TABLE 1.1 High-Frequency Ionospheric Heating Name Platteville Arecibo Sura EISCAT HAARP SPEAR Platteville, Arecibo, Vasil’sursk, Tromsø, Gakona, Svalbard, Location Colorado Puerto Rico Russia Norway Alaska Norway First opened 1970 1970 1979 1980 1995 2003 New facility Temporary Closed in Status operating Operating Operating closing June Operating 1975 early 2014 2013 Years of 1970-1975 1970-present 1979-present 1980-present 1995-present 2003-present operation Geographic 40.2 N 18.3 N 59.1 N 69.6 N 62.4 N 78.2 N coordinates 104.7 W 66.8 W 46.1 E 19.2 E 145.2 W 16.0 E Geomagnetic 48.2 N 28.1 N 53.2 N 66.1 N 63.5 N 74.8 N latitude Geomagnetic dip angle 66.9 44.8 73.6 77.6 75.8 82.1 (degrees) Frequency 5.1 and 3.9-5.5 2.7-10.0 4.5-9.0 2.8-10.0 4.45-5.82 (MHz) 8.175 5.4-8.0 Radiated power 1.4 0.9 0.75 1.2 3.6 0.11 (MW) Antenna gain 22-25 19 22 and 26 23-26 up to 40 22 (dB) 28-31 Effective 180-340 radiated power 100 95 and 240 150-280 up to 3600 17 630-1260 (MW) SOURCE: Adapted from B. Isham, “Overview of HF experiments at EISCAT Tromsø,” 13th Radio Frequency (RF) Ionospheric Interactions Workshop, Santa Fe, New Mexico, April 22-25, 2007. Courtesy of B. Isham, Interamerican University, Bayamón, Puerto Rico. HAARP COMPARED TO OTHER HEATERS Table 1.1 and Figure 1.2 display some of the technical characteristics of HAARP compared with other HF facilities currently operating, or soon to be operational, and with the original HF facility in Platteville, Colorado. HAARP has a frequency band equaled only by the original transmitter in Plateville and a transmitted power density 36 times that of Platteville and three times that of the high-power array at EISCAT. This capability, unique to the HAARP heater, allows for the generation of artificial ionization layers. Not apparent, however, in Table 1.1 and Figure 1.2 are HAARP’s extremely fast and flexible beam-forming and beam-pointing capabilities, which enable many unique experiments discussed elsewhere in the text and the many instruments available at HAARP, with the notable exception being the absence of an ISR such as the powerful and capable ISRs located at Arecibo, EISCAT, and SPEAR. Table 1.1 and Figure 1.2 make clear that HAARP represents the culmination of achievement in HF ionospheric heaters. It has already been noted that as the HF power delivered to the ionosphere increases, the response of the geophysical environment changes discontinuously, 3 thus indicating the 3 See Joint Services Program Plans and Activities, Air Force Geophysics Laboratory, and Navy Office of Naval Research (1990). 18

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FIGURE 1.2 Effective radiated power versus frequency for current high-frequency heating facilities. The Arecibo facility is under construction and will come on line in 2014. NOTE: 1 GW is equal to 90 dBW. SOURCE: E. Kennedy, Naval Research Laboratory, “Heating facilities update: High-frequency Active Auroral Research Program (HAARP),” 4th Radio Frequency (RF) Ionospheric Interactions Workshop, Santa Fe, New Mexico, April 19-22, 1998. presence of thresholds (see Figure S.3 in the Summary). Scientists do not know what thresholds may lay ahead, but some participants asserted that HAARP offers the best opportunity to discover as well as to explore new fundamental physics and to unearth opportunities for new systems development. In making this claim, it was noted that while HAARP’s “first light” came in 1995, its operation at full power began only in the past few years. In its first experiment at full power, the threshold for ionospheric production was exceeded. It was further asserted that when the new Arecibo facility begins its expected operation in 2014, it will, with HAARP, enable a new surge of exploration of comparative instability processes, utilizing known major latitude dependencies. REFERENCES Barr, R. 1998. The generation of ELF and VLF radio waves in the ionosphere using powerful HF transmitters. Advances in Space Research 21:677-687, doi=10.1016/S0273-1177(97)01003-X. Bernhardt, P.A., M. Wong, J.D. Huba, B.G. Fejer, L.S. Wagner, J.A. Goldstein, C.A. Selcher, V.L. Frolov, and E.N. Sergeev. 2000. Optical remote sensing of the thermosphere with HF pumped artificial airglow. Journal of Geophysical Research 105:10657-10672. Joint Services Program Plans and Activities, Air Force Geophysics Laboratory, and Navy Office of Naval Research. 1990. “HAARP: HF Active Auroral Research Program,” February. Available at http://www.viewzone.com/haarp.exec.html. NRC (National Research Council). 2013. Solar and Space Physics: A Science for a Technological Society. The National Academies Press, Washington, D.C. 19

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Papadopoulos, K., C.L. Chang, P. Vitello, and A. Drobot. 1990. On the efficiency of ionospheric ELF generation. Radio Science 25(6):1311-1320, doi:10.1029/RS025i006p01311. Papadopoulos, K., T. Wallace, G.M. Milikh, W. Peter, and M. McCarrick. 2005. The magnetic response of the ionosphere to pulsed HF heating. Geophysical Research Letters 32:L13101. Papadopoulos, K., N.A. Gumerov, X. Shao, I. Doxas, and C.L. Chang. 2011a. HF-driven currents in the polar ionosphere. Geophysical Research Letters 38:L12103. Papadopoulos, K., C.-L. Chang, J. Labenski, and T. Wallace. 2011b. First demonstration of HF- driven ionospheric currents. Journal of Geophysical Research 38:L20107. Pedersen, T., B. Gustavsson, E. Mishin, E. Kendall, T. Mills, H.C. Carlson, and A.L. Snyder. 2010. Creation of artificial ionospheric layers using high-power HF waves. Geophysical Research Letters 37:L02106, doi:10.1029/2009GL041895. Rietveld, M.T., R. Barr, H. Kopka, E. Nielson, P. Stubbe, and R.L. Dowden. 1984. Ionospheric heater beam scanning: A new technique for ELF studies of the auroral ionosphere. Radio Science 19(4):1069-1077, doi:10.1029/RS019i004p01069. Rietveld, M.T., P. Stubbe, and H. Kopka. 1989. On the frequency dependence of ELF/VLF waves produced by modulated ionospheric heating. Radio Science 24:270-278, doi=10.1029/RS024i003p00270. Thomas, G., D. Marsh, and F.-J. Lübken. 2009. Mesospheric ice clouds as indicators of upper atmosphere climate change: Workshop on Modeling Polar Mesospheric Cloud Trends, Boulder, Colo., December 10-11, 2009. Eos, Transactions AGU 91(20):183-183, doi:10.1029/2010EO200004, 2010. 20