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Kasper, Justin

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Kasper

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Kasper, Justin
Author's Publications: search.filters.isAuthorOfPublication.9707aa2c-269e-4514-a910-89be7afdf217

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Now showing 1 - 6 of 6
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    The solar magnetic activity band interaction and instabilities that shape quasi-periodic variability
    (Nature Pub. Group, 2015) McIntosh, Scott W.; Leamon, Robert J.; Krista, Larisza D.; Title, Alan M.; Hudson, Hugh S.; Riley, Pete; Harder, Jerald W.; Kopp, Greg; Snow, Martin; Woods, Thomas N.; Kasper, Justin; Stevens, Michael L.; Ulrich, Roger K.
    Solar magnetism displays a host of variational timescales of which the enigmatic 11-year sunspot cycle is most prominent. Recent work has demonstrated that the sunspot cycle can be explained in terms of the intra- and extra-hemispheric interaction between the overlapping activity bands of the 22-year magnetic polarity cycle. Those activity bands appear to be driven by the rotation of the Sun's deep interior. Here we deduce that activity band interaction can qualitatively explain the ‘Gnevyshev Gap'—a well-established feature of flare and sunspot occurrence. Strong quasi-annual variability in the number of flares, coronal mass ejections, the radiative and particulate environment of the heliosphere is also observed. We infer that this secondary variability is driven by surges of magnetism from the activity bands. Understanding the formation, interaction and instability of these activity bands will considerably improve forecast capability in space weather and solar activity over a range of timescales.
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    First Season MWA EoR Power Spectrum Results at Redshift 7
    (American Astronomical Society, 2016) Beardsley, A.; Hazelton, B.; Sullivan, I.; Carroll, P.; Barry, N.; Rahimi, M.; Pindor, B.; Trott, C.; Line, J.; Jacobs, Daniel; Morales, M.; Pober, J.; Bernardi, G.; Bowman, Judd; Busch, M.; Briggs, F.; Cappallo, R.; Corey, B.; de Oliveira-Costa, A.; Dillon, Joshua; Emrich, D.; Ewall-Wice, A.; Feng, L.; Gaensler, B.; Goeke, R.; Greenhill, L.; Hewitt, J.; Hurley-Walker, N.; Johnston-Hollitt, M.; Kaplan, D.; Kasper, Justin; Kim, Hanna; Kratzenberg, E.; Lenc, E.; Loeb, Abraham; Lonsdale, C.; Lynch, M.; McKinley, B.; McWhirter, S.; Mitchell, D.; Morgan, E.; Neben, A.; Thyagarajan, Nithyanandan; Oberoi, D.; Offringa, A.; Ord, S.; Paul, S.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, A.; Roshi, A.; Shankar, N.; Sethi, Shiv; Srivani, K.; Subrahmanyan, R.; Tegmark, M.; Tingay, S.; al, et
    The Murchison Widefield Array (MWA) has collected hundreds of hours of Epoch of Reionization (EoR) data and now faces the challenge of overcoming foreground and systematic contamination to reduce the data to a cosmological measurement. We introduce several novel analysis techniques such as cable reflection calibration, hyper-resolution gridding kernels, diffuse foreground model subtraction, and quality control methods. Each change to the analysis pipeline is tested against a two dimensional power spectrum figure of merit to demonstrate improvement. We incorporate the new techniques into a deep integration of 32 hours of MWA data. This data set is used to place a systematic-limited upper limit on the cosmological power spectrum of Δ2≤2.7×104 mK2 at k=0.27 h~Mpc−1 and z=7.1, consistent with other published limits, and a modest improvement (factor of 1.4) over previous MWA results. From this deep analysis we have identified a list of improvements to be made to our EoR data analysis strategies. These improvements will be implemented in the future and detailed in upcoming publications.
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    Low frequency observations of linearly polarized structures in the interstellar medium near the south Galactic pole
    (American Astronomical Society, 2016) Lenc, E.; Gaensler, Bryan; Sun, X; Sadler, Evan; Willis, A. G.; Barry, Nicholas; Beardsley, A. P.; Bell, Marjorie; Bernardi, G.; Bowman, Jason; Briggs, Florence; Callingham, J. R.; Cappallo, R. J.; Carroll, P.; Corey, B. E.; Oliveira-Costa, A. de; Deshpande, A. A.; Dillon, J. S.; Dwarkanath, K. S.; Emrich, D.; Ewall-Wice, A.; Feng, L.; For, B.-Q.; Goeke, R.; Greenhill, Lincoln; Hancock, P.; Hazelton, B. J.; Hewitt, Justina; Hindson, L.; Hurley-Walker, N.; Johnston-Hollitt, M.; Jacobs, Daniel; Kapińska, A. D.; Kaplan, Daniel; Kasper, Justin; Kim, Andrew Hyung-Do; Kratzenberg, E.; Line, J.; Loeb, Abraham; Lonsdale, C. J.; Lynch, M. J.; McKinley, B.; McWhirter, Sarah; Mitchell, Daniel; Morales, M. F.; Morgan, E.; Morgan, James; Murphy, Teresa; Neben, A. R.; Oberoi, D.; Offringa, A. R.; Ord, Stephen; Paul, S.; Pindor, B.; Pober, J. C.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, Adrianne; Roshi, A.
    We present deep polarimetric observations at 154 MHz with the Murchison Widefield Array (MWA), covering 625 deg^2 centered on RA=0 h, Dec=-27 deg. The sensitivity available in our deep observations allows an in-band, frequency-dependent analysis of polarized structure for the first time at long wavelengths. Our analysis suggests that the polarized structures are dominated by intrinsic emission but may also have a foreground Faraday screen component. At these wavelengths, the compactness of the MWA baseline distribution provides excellent snapshot sensitivity to large-scale structure. The observations are sensitive to diffuse polarized emission at ~54' resolution with a sensitivity of 5.9 mJy beam^-1 and compact polarized sources at ~2.4' resolution with a sensitivity of 2.3 mJy beam^-1 for a subset (400 deg^2) of this field. The sensitivity allows the effect of ionospheric Faraday rotation to be spatially and temporally measured directly from the diffuse polarized background. Our observations reveal large-scale structures (~1 deg - 8 deg in extent) in linear polarization clearly detectable in ~2 minute snapshots, which would remain undetectable by interferometers with minimum baseline lengths >110 m at 154 MHz. The brightness temperature of these structures is on average 4 K in polarized intensity, peaking at 11 K. Rotation measure synthesis reveals that the structures have Faraday depths ranging from -2 rad m^-2 to 10 rad m^-2 with a large fraction peaking at ~+1 rad m^-2. We estimate a distance of 51+/-20 pc to the polarized emission based on measurements of the in-field pulsar J2330-2005. We detect four extragalactic linearly polarized point sources within the field in our compact source survey. Based on the known polarized source population at 1.4 GHz and non-detections at 154 MHz, we estimate an upper limit on the depolarization ratio of 0.08 from 1.4 GHz to 154 MHz.
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    A high reliability survey of discrete Epoch of Reionization foreground sources in the MWA EoR0 field
    (Oxford University Press (OUP), 2016) Carroll, P. A.; Line, J.; Morales, M. F.; Barry, N.; Beardsley, A. P.; Hazelton, B. J.; Jacobs, D. C.; Pober, J. C.; Sullivan, I. S.; Webster, R. L.; Bernardi, G.; Bowman, J. D.; Briggs, F.; Cappallo, R. J.; Corey, B. E.; de Oliveira-Costa, A.; Dillon, J. S.; Emrich, D.; Ewall-Wice, A.; Feng, L.; Gaensler, Bryan; Goeke, R.; Greenhill, Lincoln; Hewitt, J. N.; Hurley-Walker, N.; Johnston-Hollitt, M.; Kaplan, D. L.; Kasper, Justin; Kim, HS.; Kratzenberg, E.; Lenc, E.; Loeb, Abraham; Lonsdale, C. J.; Lynch, M. J.; McKinley, B.; McWhirter, S. R.; Mitchell, D. A.; Morgan, E.; Neben, A. R.; Oberoi, D.; Offringa, A. R.; Ord, Stephen; Paul, S.; Pindor, B.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, A. E. E.; Roshi, A.; Shankar, N. Udaya; Sethi, S. K.; Srivani, K. S.; Subrahmanyan, R.; Tegmark, M.; Thyagarajan, Nithyanandan; Tingay, S. J.; Trott, C. M.; Waterson, M.; Wayth, R. B.; Whitney, A. R.; Williams, A.; Williams, C. L.; Wu, C.; Wyithe, J. S. B.
    Detection of the Epoch of Reionization HI signal requires a precise understanding of the intervening galaxies and AGN, both for instrumental calibration and foreground removal. We present a catalogue of 7394 extragalactic sources at 182 MHz detected in the RA=0 field of the Murchison Widefield Array Epoch of Reionization observation programme. Motivated by unprecedented requirements for precision and reliability we develop new methods for source finding and selection. We apply machine learning methods to self-consistently classify the relative reliability of 9490 source candidates. A subset of 7466 are selected based on reliability class and signal-to-noise ratio criteria. These are statistically cross-matched to four other radio surveys using both position and flux density information. We find 7369 sources to have confident matches, including 90 partially resolved sources that split into a total of 192 sub-components. An additional 25 unmatched sources are included as new radio detections. The catalogue sources have a median spectral index of -0.85. Spectral flattening is seen toward lower frequencies with a median of -0.71 predicted at 182 MHz. The astrometric error is 7 arcsec. compared to a 2.3 arcmin. beam FWHM. The resulting catalogue covers approximately 1400 sq. deg. and is complete to approximately 80 mJy within half beam power. This provides the most reliable discrete source sky model available to date in the MWA EoR0 field for precision foreground subtraction.
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    First limits on the 21 cm power spectrum during the Epoch of X-ray heating
    (Oxford University Press (OUP), 2016) Ewall-Wice, A.; Dillon, Joshua S.; Hewitt, J. N.; Loeb, Abraham; Mesinger, A.; Neben, A. R.; Offringa, A. R.; Tegmark, M.; Barry, N.; Beardsley, A. P.; Bernardi, G.; Bowman, Judd D.; Briggs, F.; Cappallo, R. J.; Carroll, P.; Corey, B. E.; de Oliveira-Costa, A.; Emrich, D.; Feng, L.; Gaensler, Bryan; Goeke, R.; Greenhill, Lincoln; Hazelton, B. J.; Hurley-Walker, N.; Johnston-Hollitt, M.; Jacobs, Daniel C.; Kaplan, D. L.; Kasper, Justin; Kim, HS; Kratzenberg, E.; Lenc, E.; Line, J.; Lonsdale, C. J.; Lynch, M. J.; McKinley, B.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Thyagarajan, Nithyanandan; Oberoi, D.; Ord, Stephen; Paul, S.; Pindor, B.; Pober, J. C.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, A. E. E.; Roshi, A.; Shankar, N. Udaya; Sethi, Shiv K.; Srivani, K. S.; Subrahmanyan, R.; Sullivan, I. S.; Tingay, S. J.; Trott, C. M.; Waterson, M.; Wayth, R. B.; Webster, R. L.; Whitney, A. R.; Williams, A.; Williams, C. L.; Wu, C.; Wyithe, J. S. B.
    We present first results from radio observations with the Murchison Widefield Array seeking to constrain the power spectrum of 21 cm brightness temperature fluctuations between the redshifts of 11.6 and 17.9 (113 and 75 MHz). Three hours of observations were conducted over two nights with significantly different levels of ionospheric activity. We use these data to assess the impact of systematic errors at low frequency, including the ionosphere and radio-frequency interference, on a power spectrum measurement. We find that after the 1-3 hours of integration presented here, our measurements at the Murchison Radio Observatory are not limited by RFI, even within the FM band, and that the ionosphere does not appear to affect the level of power in the modes that we expect to be sensitive to cosmology. Power spectrum detections, inconsistent with noise, due to fine spectral structure imprinted on the foregrounds by reflections in the signal-chain, occupy the spatial Fourier modes where we would otherwise be most sensitive to the cosmological signal. We are able to reduce this contamination using calibration solutions derived from autocorrelations so that we achieve an sensitivity of 104 mK on comoving scales k≲0.5hMpc−1. This represents the first upper limits on the 21 cm power spectrum fluctuations at redshifts 12≲z≲18 but is still limited by calibration systematics. While calibration improvements may allow us to further remove this contamination, our results emphasize that future experiments should consider carefully the existence of and their ability to calibrate out any spectral structure within the EoR window.
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    The Importance of Wide-Field Foreground Removal for 21 Cm Cosmology: A Demonstration With Early MWA Epoch of Reionization Observations
    (American Astronomical Society, 2016) Pober, J. C.; Hazelton, B. J.; Beardsley, A. P.; Barry, N. A.; Martinot, Z. E.; Sullivan, I. S.; Morales, M. F.; Bell, M. E.; Bhat, N. D. R.; Bowman, J. D.; Briggs, F.; Cappallo, R. J.; Carroll, P.; Corey, B. E.; de Oliveira-Costa, A.; Deshpande, A. A.; Dillon, Joshua. S.; Emrich, D.; Ewall-Wice, A. M.; Feng, L.; Goeke, R.; Hewitt, J. N.; Hindson, L.; Hurley-Walker, N.; Jacobs, D. C.; Johnston-Hollitt, M.; Kaplan, D. L.; Kim, Han-Seek; Kittiwisit, P.; Kratzenberg, E.; Kudryavtseva, N.; Lenc, E.; Line, J.; Loeb, Abraham; Lonsdale, C. J.; Lynch, M. J.; McKinley, B.; McWhirter, S. R.; Mitchell, D. A.; Morgan, E.; Neben, A. R.; Oberoi, D.; Offringa, A. R.; Ord, Stephen; Paul, Sourabh; Pindor, B.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, A. E. E.; Roshi, A.; Sethi, Shiv K.; Shankar, N. Udaya; Srivani, K. S.; Subrahmanyan, R.; Tegmark, M.; Thyagarajan, Nithyanandan; Tingay, S. J.; Trott, C. M.; Waterson, M.; Wayth, R. B.; Webster, R. L.; Whitney, A. R.; Williams, A.; Williams, C. L.; Wyithe, J. S. B.; Bernardi, Gianni; Greenhill, Lincoln; Kasper, Justin
    In this paper we present observations, simulations, and analysis demonstrating the direct connection between the location of foreground emission on the sky and its location in cosmological power spectra from interferometric redshifted 21 cm experiments. We begin with a heuristic formalism for understanding the mapping of sky coordinates into the cylindrically averaged power spectra measurements used by 21 cm experiments, with a focus on the effects of the instrument beam response and the associated sidelobes. We then demonstrate this mapping by analyzing power spectra with both simulated and observed data from the Murchison Widefield Array. We find that removing a foreground model which includes sources in both the main field-of-view and the first sidelobes reduces the contamination in high k_parallel modes by several percent relative to a model which only includes sources in the main field-of-view, with the completeness of the foreground model setting the principal limitation on the amount of power removed. While small, a percent-level amount of foreground power is in itself more than enough to prevent recovery of any EoR signal from these modes. This result demonstrates that foreground subtraction for redshifted 21 cm experiments is truly a wide-field problem, and algorithms and simulations must extend beyond the main instrument field-of-view to potentially recover the full 21 cm power spectrum.