Most recent heater-based studies of magnetospheric processes and ionosphere-thermosphere-magnetosphere (ITM) system couplings use the “virtual antenna” concept, which allows generation and injection of ULF/ELF/VLF (ultralow-frequency/extremely low-frequency/very-low-frequency) waves into the various ionospheric and magnetospheric resonators and waveguides and into the radiation belts. Presentations by Dennis Papadopoulos, Herbert Carlson, Meers Oppenheim, Paul Bernhardt, and others at the workshop highlighted recent and ongoing investigations to show how active ionospheric heating cause-and-effect experiments help resolve critical geospace problems and test predictive space weather forecasting models.
Experiments have shown that the interaction of ELF/VLF waves with energetic electrons in the magnetosphere can enhance or suppress the original waves, generate waves with different or new frequencies, and enhance precipitation of energetic particles trapped in the radiation belts. One unexpected example was the observation of coherent electromagnetic emissions known as chorus emissions. Typically a series of strongly nonlinear rising tones, chorus emissions are generated near the magnetic equator by the interaction of energetic electrons trapped in the radiation belts with self-generated or external ELF/VLF signals. It was stated at the workshop that the underlying physics is both critical to understanding the radiation belts and a serious challenge to the textbook understanding of nonlinear plasma physics.
According to one participant, early studies of the interaction of ground-generated VLF signals with radiation-belt electrons gave a major boost to understanding of triggered emissions (Helliwell, 1988) but were hampered by the limited frequency range (3-6 kHz) of the ground VLF transmitter. Utilizing the High Frequency Active Auroral Research Program (HAARP) as a virtual antenna allowed injection of VLF waves covering a very broad frequency range (0.5-5 kHz) and generated what was described as very interesting preliminary results (Golkowski et al., 2008, 2010). However, the limited number of experiments and the lack of satellite diagnostics did not allow a comprehensive resolution of the issues.
Figure 3.1(a) shows the experimental configuration used in the work by Golkowski et al. (2010). Emissions triggered by ELF/VLF waves injected by HAARP are measured either on the ocean at the conjugate point (one-hop signals) or back at Chistochina, Alaska, 37 km northeast of HAARP (two-hop signals). Figure 3.1(b) shows a spectrogram of injected signals and two-hop echoes received in Chistochina 7 seconds (the round-trip transit time) after the initial pulse. Single-frequency signals between 1 and 3 kHz and frequency sweeps from 0.5 to 3.5 kHz were injected. Echoes were produced only for frequencies between 2.0 and 2.8 kHz, indicative of resonance with anisotropic electrons with energy of 5 to 10 keV.
FIGURE 3.1 (a) Schematic of a triggered-wave experiment. Extremely low-frequency/very-low-frequency waves injected into the radiation belts are amplified by interacting with energetic electrons. (b) Two-hop triggered emissions received in Chistochina, Alaska, after reflection at the conjugate point in the South Pacific, connected by red arrows to the signal injected at HAARP. SOURCE: (a) T. Tether, et al., “Future Directions for HAARP,” Committee Report, University of Maryland, College Park, Md., May 2001. Available at http://spp.astro.umd.edu/SpaceWebProj/Tether_Panel.ppt.Courtesy of K. Papadopoulos, University of Maryland. (b) M. Golkowski, U.S. Inan, M.B. Cohen, and A.R. Gibby, Amplitude and phase of non-linear magnetospheric wave growth excited by the HAARP HF heater, Journal of Geophysical Research 115:A00F04, 2010. Available at http://onlinelibrary.wiley.com/doi/10.1029/2009JA014610/abstract. Courtesy of the Journal of Geophysical Research/John Wiley and Sons.
Guides, Resonators and Magnetosphere-Ionosphere Coupling
In the magnetosphere and ionosphere, ULF/ELF waves are guided by various structures. In addition to the well-known Earth-ionosphere waveguide and the associated Schumann resonances, there can be ionospheric ducts that guide magnetosonic (MS) waves, ionospheric Alfvén resonators (IAR) operating between the D/E region of the ionosphere and the inner plasmasphere at a few thousand kilometers, and magnetospheric resonators for Alfvén and compressional waves. These structures control the coupling of disturbances generated in the high-latitude magnetosphere to the middle- and low-latitude ionosphere. These structures control the propagation and coupling of disturbances generated in the high-latitude magnetosphere to the middle- and low-latitude ionosphere.
FIGURE 3.2 Example of energy flow starting as shear Alfvén wave, filtered through the ionospheric Alfvén resonator (IAR) and propagating laterally as magnetosonic in the duct. The IAR resonant spectrum on the ground is shown in the inset. SOURCE: Courtesy of K. Papadopoulos, University of Maryland.
Figure 3.2 shows a natural process that couples the magnetosphere to the middle- and low-latitude ionosphere (Lysak, 1999, 2004; Lysak and Song, 2001). Shear Alfvén (SA) waves in the Pc1 frequency range (0.1-5 Hz) generated by ion cyclotron instabilities excite resonant frequencies in the IAR. Mode conversion from SA to MS allows lateral propagation of the energy though the ionospheric duct towards middle and low latitudes. Developing predictive models of such magnetosphere-ionosphere (MI) coupling processes requires quantitative characterization of the three elements involved: the IAR, the ionospheric duct, and the SA/MS coupling. Dennis Papadopoulos informed workshop participants of three examples of recent work at HAARP and EISCAT (European Incoherent Scatter Scientific Association) that show their potential for illuminating these issues:
• Figure 3.3 shows an experiment in which the EISCAT heater injected ULF waves at 3 Hz, and their effect was measured aboard the Fast Auroral Snapshot Explorer spacecraft at altitude 2,550 km (Robinson et al., 2000; Wright et al., 2003). In addition to measuring 3-Hz oscillatory fields in a narrow region that mapped down the magnetic field line to the heated volume, the satellite detected a downward flux of electrons with identical signature to the ELF waves.
• Another goal is determining the Q of the IAR. Figure 3.4 shows a recent experiment in which, guided by the presence of naturally excited frequencies at frequencies 0.25 and 0.5 Hz, HAARP excited the 4th harmonic at 1 Hz (Papadopoulos et al., 2011). Notice the narrowness of the spectral line excited by HAARP compared to the naturally excited lines.
• In a third example, Eliasson et al. (2012) used HAARP to explore the transmission through the ionospheric duct. For the first time, MS waves generated by F-region modulation at HAARP were injected in the duct and their signatures were measured at remote sites Washington State (1,300 miles away), Hawaii (2,900 miles), and Guam (4,800 miles).
FIGURE 3.3 Schematic of artificial 3-Hz injection in the ionospheric Alfvén resonator (IAR) whose top boundary is at an altitude of about 3,300 km. The wave acquires a significant electric field component parallel to the geomagnetic field above the satellite. NOTE: EISCAT, European Incoherent Scatter Scientific Association; FAST, Fast Auroral Snapshot Explorer; ULF, ultralow frequency. SOURCE: T.R. Robinson, R. Strangeway, D.M. Wright, J.A. Davies, R.B. Horne, T.K. Yeoman, A.J. Stocker, M. Lester, M.T. Rietveld, I.R. Mann, C.W. Carlson, and J.P. McFadden, FAST observations of ULF waves injected into the magnetosphere by means of modulated RF heating of the auroral electrojet, Geophysical Research Letters 27:3165-3168, 2000. Available at http://onlinelibrary.wiley.com/doi/10.1029/2000GL011882/abstract. Courtesy of Geophysical Research Letters./John Wiley and Sons.
Several participants saw emerging opportunities for discovery research in magnetospheric physics using ionospheric modifications given the following anticipated developments:
1. The availability of heaters at various latitudes to probe different regions of the magnetosphere, corresponding to different L shells (Figure 3.5);
2. The availability of collocated ISRs; and
3. The launch of a number of new satellites anticipated over the next few years.
FIGURE 3.4 Natural excitation of the ionospheric Alfvén resonator at 0.25 and 0.5 Hz and artificial excitation by HAARP at 1 Hz. SOURCE: Courtesy of K. Papadopoulos, University of Maryland.
FIGURE 3.5 Ionospheric heaters at various latitudes probe different regions of the magnetosphere corresponding to different L shells: Arecibo (L ≈ 1.4), Sura (L ≈ 2.6), HAARP (L ≈ 4.9), and EISCAT (L ≈ 5.9). NOTE: EISCAT, European Incoherent Scatter Scientific Association; HAARP, High Frequency Active Auroral Research Program. SOURCE: Courtesy of K. Papadopoulos, University of Maryland.
Satellite measurements of the effects generated by ionospheric heating and their propagation toward the magnetosphere and the radiation belts were said to be very important in understanding the physics of the interactions (the example of the occasional HAARP over-flights by the French DEMETER microsatellite were cited). Planned satellite studies of the radiation belts over the next few years will provide numerous opportunities for measuring HAARP-induced phenomena. Key missions include the NASA Van Allen Probes, Canada’s e-POP, successfully launched in September 2013, Japan’s ERG, to be launched in 2015, and the Air Force DSX, to be launched in 2015. As discussed below, the Russian Space Agency Resonance mission, to be launched in 2014, could be particularly informative.
Solar Wind-Magnetosphere-Ionosphere Coupling; Saturation of the Polar Cap Potential
The cross-polar-cap (or transpolar) potential (CPCP), the difference between the maximum and minimum of the electrostatic potential in the high-latitude ionosphere, has been observed to play a key role in the solar wind-magnetosphere-ionosphere coupling. Because electric fields are mapped onto the ionosphere along the magnetic field lines from the magnetopause and magnetotail, CPCP is an important indicator of the chain of events coupling the solar wind to the ionosphere.
Dennis Papadopoulos sees the ionospheric control of the solar wind-magnetosphere-ionosphere system behavior as a critical outstanding issue in magnetospheric physics. In his view, high-frequency (HF) ionospheric heating experiments at high latitudes, combined with incoherent and coherent radar measurements of the ionospheric dynamics and with multi-satellite observations of the global magnetosphere configuration during both quiet and extreme solar-wind driving conditions, provide unique opportunities to illuminate this process.
Dynamics of the Radiation Belts
Dennis Papadopoulos led a discussion at the workshop that included consideration of active experiments and the dynamics of Earth’s radiation belts. The radiation belts form a natural resonator for many types of waves; as mentioned in Chapter 1, Alfvén and whistler waves can be ducted by gradients of plasma and magnetic field. They oscillate many times along a magnetic field line, being reflected, for example, by the conjugate ionosphere. On the other hand, experiments have shown that these waves interact efficiently with energetic particles (protons and electrons, respectively) via cyclotron resonance, and thus play a critical role in the dynamics of Earth’s radiation belts.
Papadopoulos stated that radiation belt studies led to the concept of a magnetospheric cyclotron maser in which energetic charged particles serve as the active gain medium, and the electromagnetic cavity is formed by magnetic flux tubes filled with background (cold) plasma and their ionospheric footprints. Further, he noted that an inhomogeneous distribution of cold plasma plays a twofold role: first, it ensures ducting for the Alfvén and whistler-mode waves along the magnetic field, and second, it makes cyclotron resonant interactions between particles and waves possible, most importantly near the equatorial plane where the magnetic field inhomogeneity is smallest.
Despite the importance and intensive study of magnetospheric-maser interactions, Papadopoulos believes key problems remain unresolved. These include the origin and effects of discrete emissions with fine spectral structure, spatio-temporal dynamics of radiation belts, the role of magnetosphere-ionosphere interactions in the dynamics of magnetospheric cyclotron masers, and similarities and differences between various particular maser systems.
Papadopoulos and other participants discussed how the combination of ionospheric heaters, ISRs, and space-based measurements could provide unique opportunities to resolve these issues. In particular, the scientific potential of a combination consisting of the HAARP HF transmitter, an ISR, and the Russian Resonance satellite mission was cited. The Resonance mission comprises two fully instrumented pairs of micro-satellites in magneto-synchronous orbit (an orbit that rotates with Earth’s magnetic field).
FIGURE 3.6 (a) Magneto-synchronous orbit; the satellite velocity perpendicular to the magnetic field line is equal to the flux tube velocity, allowing the spacecrafts to cover the flux tube over times between 30-45 min. (b) Resonance mission strategy. SOURCE: A. Petrukovich and the Resonance team, “RESONANCE: Project for Studies of Wave-Particle Interactions in the Inner Magnetosphere,” Report of the Resonance Team, HAARP/ RESONANCE Workshop, University of Maryland, College Park, Md., November 8-9, 2011. Available at http://spp.astro.umd.edu/SpaceWebProj/Haarp_Resonance/ap_res2011.pdf. Courtesy of K. Papadopoulos, University of Maryland.
In this orbit, the actual magnetic field line above the HAARP facility can be monitored for periods of 30 minutes or more (Figure 3.6).1 In addition to providing long integration times, Papadopoulos asserted that this unique magneto-synchronous configuration provides an opportunity to perform “revolutionary” active experiments in which satellite-based instruments provide information about the natural processes
1 For further discussion of Resonance and HAARP coordinated observations, see Zelenyi et al. (2004).
FIGURE 3.7 Schematic of bi-static solar radar for coronal mass ejection (CME) detection. The transmitted signal at a given frequency is shown along with the expected spectral echoes. A CME will produce a more distinct Doppler shift. SOURCE: P. Rodriguez, E. Kennedy, and P. Kossey, “High Frequency Radar Astronomy With HAARP,” 2003 IEEE Radar Conference, 2003, available at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA514972.
occurring in the tube, changes that occur as a result of HF interaction, and how these changes vary in response to changes in the amplitude and phase of the influence. Finally, in addition to ULF/ELF/VLF wave injection, Papadopoulos noted that HF heating allows for controlled modification of the reflection coefficient at F-region heights.
As background to the discussion of solar radar, Paul Bernhardt noted that studies of the solar corona with HF radars were performed between 1963 and 1969 using a special radar facility in El Campo, Texas, which operated as both a transmitting and a receiving array at 38.25 MHz (Rodriquez, 2000). The results from these experiments “suggested a radar cross section for the Sun’s corona of approximately the same size as the optical disk, with occasional expansion by an order of magnitude or more” (Rodriquez, 2000, p. 155). More recently, the Sura heater in Russia was used as transmitter and the UTR-2 facility in the Ukraine as receiver in experiments designed to be close equivalents to an operational solar radar (Rodriquez, 1998).
Coronal mass ejections (CMEs) play an important role in geomagnetic disturbances.2 Workshop discussions included the potential for solar radars to detect Earthward-moving CMEs, providing several days of advance warning of possible geomagnetic storms. In addition, it was noted that wave scattering in the solar corona might provide information on coronal densities and irregularities. Bernhardt noted that detection of CMEs from Earth using optical techniques is not possible because the geo-effective CMEs are not visible when looking directly at the Sun’s photosphere. HF radar scatter from CMEs has been attempted with minimal success using HAARP transmitting at HF and the Jicamarca radar at VHF frequencies, respectively. Bernhardt believes the new Arecibo HF facility in Puerto Rico is especially suited to attempt radar measurements of the Sun and Moon because of its relatively low latitude.
Bernhardt further stated that the potential of HF heaters as bi-static radars for CME monitoring could be tested when the Earth-Sun geometry permits bi-static reception (Figure 3.7) by using the HAARP heater at its maximum frequency as the transmitter and the Arecibo dish as a receiving antenna. The HAARP/Arecibo combination has a 10-dB advantage over El Campo and 14-dB advantage over the Sura/UTR-2 combination. He noted that this configuration could be used for routine detection and
velocity of average CMEs, velocities of large CMEs at up to 20 solar radii, and polarimetric measurements of magnetic fields in the solar corona and measurements of coronal turbulence using Doppler spectra.
Eliasson, B., X. Shao, G.M. Milikh, E.V. Mishin, and K. Papadopoulos. 2012. Numerical modeling of artificial ionospheric layers driven by high-power HF-heating. Journal of Geophysical Research 117:A10321, doi:10.1029/2012JA018105.
Golkowski, M., U.S. Inan, A.R. Gibby, and M.B. Cohen. 2008. Magnetospheric amplification and emission triggering by ELF/VLF waves injected by the 3.6 MW HAARP ionospheric heater. Journal of Geophysical Research 113:A10201, doi:10.1029/2008JA013157.
Golkowski, M., U.S. Inan, M.B. Cohen, and A.R. Gibby. 2010. Amplitude and phase of nonlinear magnetospheric wave growth excited by the HAARP HF heater. Journal of Geophysical Research 115:A00F04, doi:10.1029/2009JA014610.
Helliwell, R.A. 1988. VLF wave simulation experiments in the magnetosphere from Siple Station, Antarctica. Reviews of Geophysics 26(3):551-578, doi:10.1029/RG026i003p00551.
Lysak, R.L. 1999. Propagation of Alfvén waves through the ionosphere: Dependence on ionospheric parameters. Journal of Geophysical Research 104:10017-10030.
Lysak, R.L. 2004. Magnetosphere-ionosphere coupling by Alfvén waves at midlatitudes. Journal of Geophysical Research 109:A07201, doi:10.1029/2004JA010454.
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Robinson, T.R., R. Strangeway, D.M. Wright, J.A. Davies, R.B. Horne, T.K. Yeoman, A.J. Stocker, M. Lester, M.T. Rietveld, I.R. Mann, C.W. Carlson, and J.P. McFadden. 2000. FAST observations of ULF waves injected into the magnetosphere by means of modulated RF heating of the auroral electrojet. Geophysical Research Letters 27:3165.
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Rodriquez, P. 2000. Radar studies of the solar corona: A review of experiments using HF wavelengths. Pp. 155-165 in Radio Astronomy at Long Wavelengths (K.S. Balasubramaniam, S.L. Kiel, and R.N. Smartt, eds.). Geophysical Monograph 119. American Geophysical Union.
Wright, D.M., J.A. Davies, T.K. Yeoman, T.R. Robinson, S.R. Cash, E. Kolesnikova, M. Lester, P.J. Chapman, R.J. Strangeway, R.B. Horne, M.T. Rietveld, and C.W. Carlson. 2003. Detection of artificially generated ULF waves by the FAST spacecraft and its application to the “tagging” of narrow flux tubes. Journal of Geophysical Research (Space Physics) 108:1090, doi:10.1029/2002JA009483.
Zelenyi, L.M., et al. 2004. “Russian Space Program: Experiments in solar-terrestrial physics.” In Multi-Wavelength Investigations of Solar Activity (A.V. Stepanov, E.E. Benevolenskaya, and A.G. Kosovichev, eds.). Proceedings IAU Symposium No. 223. 2004 International Astronomical Union, doi:10.1017/S1743921304006921.