Research

Auroral Kilometric Radiation I: Source locations determined by Cluster VLBI Array Observations

We used the Cluster Wideband Data instrument to determine the locations of auroral kilometric radiation (AKR) using very long baseline interferometry. The technique involves cross-correlating individual AKR bursts from all six Cluster baselines using time and frequency filtered waveforms. The resulting differential delay peaks are used to determine source locations with an uncertainty as small as 500 km in a plane perpendicular to the source-spacecraft line of sight or about 200 km when the burst position is projected onto the auroral zone along the magnetic field lines passing through the source. The uncertainty along the line of sight is much larger, but this is mitigated by assuming that the emission arises from a height corresponding to the electron gyrofrequency. We report the locations of over 1700 individual AKR bursts during six observing epochs between 10 July 2002 and 22 January 2003 when the Cluster constellation was high above the Southern or Northern Hemisphere. In general we find that the AKR burst locations lie along magnetic field lines which map onto the nighttime auroral zone as expected from previous AKR studies. For the three observing epochs viewing the Northern Hemisphere, there is a strong tendency for AKR burst locations to be centered within the auroral oval and in the evening sector. The Southern Hemisphere burst locations favor magnetic local midnight to early morning and have somewhat higher invariant magnetic latitudes. The distribution of AKR auroral footprint locations at each epoch had a overall spatial scale between 1000 and 2000 km, much larger than the positional uncertainty of an individual AKR burst location magnetic footprint, but a small fraction of the auroral oval. This indicates that on a timescale of 1-3 hours conditions for suitable generation of AKR emission are found on only a fraction of all magnetic field lines crossing through the auroral oval. For two of the six epochs, there was a significant drift in the mean location of AKR activity over a period of 1-2 hours. The drift was predominantly in latitude at one epoch and in longitude at the other, with average drift speed V ~ 80-90 m s-1 at the AKR emission location.

Publications

Mutel, R., et al. 2003, Locations of auroral kilometric radiation bursts inferred from multispacecraft wideband Cluster VLBI observations. 1: Description of technique and initial results, Journal of Geophysical Research, Volume 108, Issue A11, pp. SMP 8-1, CiteID 1398, DOI 10.1029/2003JA010011

Mutel, R.; Gurnett, D.; Christopher, I. 2004, Spatial and temporal properties of AKR burst emission derived from Cluster WBD VLBI studies, Annales Geophysicae, vol. 22, Issue 7, pp.2625-2632

Auroral Kilometric Radiation II:  The Cause of Frequency Fine Structure 

We describe the statistical properties of narrowband drifting auroral kilometric radiation ('striated'' AKR) based on observations from the Cluster wideband receiver during 2002-2005. We show that the observed characteristics, including frequency drift rate and direction, narrow bandwidth, observed intensity, and beaming angular sizes are all consistent with triggering by upward traveling ion solitary structures (``ion holes''). We calculate the expected perturbation of a horseshoe electron distribution function by an ion hole by integrating the resonance condition for a cyclotron maser instability (CMI) using the perturbed velocity distribution. We find that the CMI growth rate can be strongly enhanced as the horseshoe velocity distribution contracts inside the passing ion hole, resulting in a power gain increase greater than 100 dB. The gain curve is sharply peaked just above the R-mode cutoff frequency, with an effective bandwidth <=50 Hz, consistent with the observed bandwidth of striated AKR emission. Ion holes are observed in situ in the acceleration region moving upward with spatial scales and speeds consistent with the observed bandwidth and slopes of SAKR bursts. Hence we suggest that SAKR bursts are a remote sensor of ion holes and can be used to determine the frequency of occurrence, locations in the acceleration region, and lifetimes of these structures.
   
Publications

Mutel, R. L.; Menietti, J. D.; Christopher, I. W.; Gurnett, D. A.; Cook, J. M. 2006, Striated auroral kilometric radiation emission: A remote tracer of ion solitary structures, Journal of Geophysical Research, Volume 111, Issue A10, CiteID A10203

Menietti, J. D.; Mutel, R. L.; Santolik, O.; Scudder, J. D.; Christopher, I. W.; Cook, J. M. 2006, Striated drifting auroral kilometric radiation bursts: Possible stimulation by upward traveling EMIC waves, Journal of Geophysical Research, Volume 111, Issue A4, CiteID A04214

Auroral Kilometric Radiation III:  Growth Rates, the Shell Distribution, Perturbation by Solitary Waves 

We calculate growth rates and corresponding gains for RX and LO mode radiation associated with the  cyclotron maser instability for parameterized horseshoe electron velocity distributions. The velocity distribution function was modeled to closely fit the electron distribution functions observed in the auroral cavity. We systematically varied the model parameters as well as the propagation direction to study the dependence of growth rates on model parameters. The growth rate depends strongly on loss cone opening angle, which must be less than 90 deg for significant CMI growth. The growth rate is sharply peaked for perpendicular radiation (kpar = 0), with a full-width at half-maximum 1.7 deg, in good agreement with observed k-vector orientations and numerical simulations. The fractional bandwidth varied between 0.0001 and 0.01, depending most strongly on propagation direction. This range encompasses nearly all observed fractional AKR burst bandwidths. We find excellent agreement between the computed RX mode emergent intensities and observed AKR intensities assuming convective growth length  Lc ~ 20-40 km and group speed 0.15c.  The only computed LO mode growth rates compatible observed LO mode radiation levels occurred for number densities more than 100 times the average energetic electron densities measured in auroral cavities. This implies that LO mode radiation is not produced directly by the CMI mechanism but more likely results from mode conversion of RX mode radiation. We find that perturbation of the model velocity distribution by large ion solitary waves (ion holes) can enhance the growth rate by a factor of 2-4. This will result in a gain enhancement more than 40 dB depending on the convective growth length within the structure.   Similar enhancements may be caused by strong EMIC waves.
Publications  

Mutel, R. L.; Peterson, W. M.; Jaeger, T. R.; Scudder, J. D. 2007, Dependence of cyclotron maser instability growth rates on electron velocity distributions and perturbation by solitary waves, Journal of Geophysical Research, Volume 112, Issue A7, CiteID A07211

Auroral Kilometric Radiation IV:  Determination of Angular Beaming Pattern

Simultaneous observations of AKR emission using the four-spacecraft Cluster array were used to make the first direct measurements of the angular beaming patterns of individual bursts. By comparing the spacecraft locations and AKR burst locations, the angular beaming pattern was found to be narrowly confined to a plane containing the magnetic field vector at the source and tangent to a circle of constant latitude. Most rays paths are confined within 15 deg of this tangent plane, consistent with numerical simulations of AKR k-vector orientation at maximum growth rate. The emission is also strongly directed upward in the tangent plane, which we interpret as refraction of the rays as they leave the auroral cavity. The narrow beaming pattern implies that an observer located above the polar cap can detect AKR emission only from a small fraction of the auroral oval at a given location. This has important consequences for interpreting AKR visibility at a given location. It also helps re-interpret previously published Cluster VLBI studies of AKR source locations, which are now seen to be only a subset of all possible source locations.
Publications

Mutel, R.L., Christopher, I. W., and Pickett, J. S., 2008, Cluster Multi-spacecraft determination of AKR angular beaming, submitted to Geophysics Research Letters, Jan 2008

Precession in BL Lac Radio Jet?


Stirling et al. [2003] recently reported the discovery of a 2.3 yr periodic variation in the structural position angle of the parsec-scale radio core in the blazar BL Lac. We searched for independent confirmation of this periodic behavior using 43 GHz images of the radio core during 10 epochs overlapping those of Stirling et al. Our maps are consistent with several periodicities, including one near the period reported by Stirling et al. By comparing our position angle measurements with those of Stirling et al., we find strong, consistent evidence for position angle variations of the inner core during the observed epochs. However, the claim of periodic variation is not convincing, especially when the most recent epochs (2000.60-2003.78) are included. A definitive resolution will require continued monitoring of the core structure over several periods.

Publications

Mutel, R. L. and Denn ,G. R. 2005, Is the Radio Core of BL Lacertae Precessing?, Astrophysical Journal, 623, 79-84.

Magnetic Field Structures in the Radio Cores of Four Blazars: Rotation Measures and Detection of a Jet Boundary Layer

We have monitored the morphology and magnetic field structure of the parsec-scale radio jets of four blazars (1751+096, 1823+568, 2136+006, 2202+422)  at nine epochs from 1998.7 - 2002.5 using the VLBA. The jet components all have low rotation measure (RM) and nearly perpendicular magnetic field structure at all epochs, consistent with  transverse  shocks and little foreground thermal gas. In contrast, the core has high variable RM, up to 8,200 rad-m-2 at some epochs, but the true RM may be underestimated by blending from emerging jet components. We also detected weak peripheral emission surrounding the main jet at some epochs with nearly parallel magnetic field and high fractional polarization. We suggest this may be a boundary layer caused by interaction with the ambient medium.

Publications

Mutel, R., Denn, G., and Dreier, C. 2005, Magnetic Field Structure in the Radio Core of BL Lac: Variable Core Rotation and Detection of a Jet Boundary Layer,ASP Conference Proceedings, Vol. 340. Edited by J. Romney and M. Reid. San Francisco: Astronomical Society of the Pacific

A Radio Cerenkov Search for ultra-high energy (UHE) Neutrinos (Ph.D. thesis of Ted Jaeger)

Determining the origin and characteristics of extragalactic cosmic rays is of fundamental importance in astrophysics.  Direct detection of high energy cosmic rays from distant extragalactic sources is difficult due to interaction with cosmic microwave background photons.  The cross section depends strongly on particle energy: For energies greater than 10^19.5 electron volts, the so-called GZK limit [1], cosmic-ray particles have a strong probability of pair production and pion photo-production for propagation paths much greater than a few million light years. Since most known extragalactic sources are at much greater distances,  direct direction of these cosmic rays is not possible.  However, a predicted consequence of such interactions is the production of ultra high energy (UHE) neutrinos, which travel unimpeded in the intergalactic medium and hence may be detectable.  Any UHE neutrino flux should then be correlated with sources of the highest energy cosmic rays and may provide the only method to evaluate UHE cosmic ray production models.  In addition, some models of active galaxies and gamma-ray burst sources predict copious production of UHE neutrinos directly.  Currently no UHE neutrinos have ever been detected.

We will search for Cerenkov radio bursts from UHE neutrino lunar impacts using the Very Large Array, consisting of 27 large radio telescopes operated by the National Radio Astronomy Observatory in New Mexico.  We will use sub-arrays pointed around the lunar limb. This proposal pertains to Phase A of this effort, which consists of using two sub-arrays of 4 antennas at an observing frequency of 1.4 GHz. The figure at right shows the relevant geometry - two pairs of primary antenna beams  of either side of the lunar limb (central blue disk).

By observing a total of 50 hours (5x10 hour observing sessions centered at lunar transit), we will improve of the flux lower limit of the most sensitive upper limit (GLUE experiment) a factor of 10, and probe the flux-energy region predicted by topological defect models. Phase B, which will be proposed when all 27 EVLA antennas are available, will use 12 sub-arrays at 5 GHz, and will test neutrino production models for active galaxies and gamma-ray bursters, as well as the high-energy tail of the GZK interaction.


Talks

Jaeger, T. and Mutel, R. 2008, URSI Commission J talk, Boulder,CO

Time-Lapse Imaging of Algol's Radio Magnetosphere (Ph. D. Thesis of Bill Peterson)

Determining the spatial structure and physical processes in stellar magnetospheres is a fundamental problem in stellar astrophysics. The research program described in this proposal addresses two key questions that inform the larger picture: (1) What is the structure and time-evolution of extended stellar magnetospheres as mapped by their non-thermal radio emission? (2) What plasma physical magnetospheric processes and environments are responsible for the strong coherent, sometimes periodic radio bursts detected in late-type dwarf magnetospheres?

To address these questions, multi-epoch observations of several radio-luminous, nearby late-type stars will be made using the High Sensitivity Array (HSA), the world’s most powerful radio imaging VLBI array. Radio magnetospheric images made with the HSA will have 3-5 times the brightness sensitivity and twice the angular resolution of any previous radio images of stellar magnetospheres. This will allow the first detailed imaging of magnetospheric structure, including possible large-scale loops analogous to solar CME events. The HSA images will also provide a critical test of the polar model, derived from x-ray and optical studies, in which magnetic flux is concentrated near the poles. Rather than survey a large number of stars, we will target a few relatively nearby active binaries for detailed multi-epoch study, observing over a complete orbital period when possible. We will coordinate with M. Güdel (ETH Zurich) to schedule coeval HSA - x-ray observations on Chandra, XMM-Newton, and other x-ray telescopes.  The joint x-ray radio studies will allow modeling of the overall spatial distribution of thermal and non-thermal plasma and energy release in radio - x-ray flares with unprecedented sensitivity and angular resolution.