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Sondrestrom, Greenland

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Operation of a wideband digitizing receiver captured one intense auroral roar emission and several weak emissions during a campaign in March, 2000. The intense emission showed a striking new feature: pulsation of the radio emission with period 7-11 Hz. The pulsations come and go over a period of minutes, with the pulsation frequency slowly varying. The auroral roar signal appears to be modulated only at the lower end of its frequency range, over about a 5 kHz range. We put forth that this phenomenon may be associated with flickering aurora. See Hughes and LaBelle, 2001 for details.

Operation of the medium frequency interferometer shows that most auroral roar events at Sondrestrom (73 degrees magnetic latitude) come from the south at elevations of 40-50 degrees, consistent with their generation in the aurora but detection poleward of it. The direction of arrival (DOA) of auroral roar signals is variable within a single event; single events appear to be comprised of components originating from different directions, and highly time variable. The statistical data also show evidence that a significant fraction of auroral roar emissions observed at ground level in Sondrestrom originate in the topside of the F-region. See Hughes et al. 2001 for details.

A study of auroral roar observations coincident with measurements of ionospheric density structure from a scanning incoherent scatter radar shows that the ionosphere is usually disturbed when auroral roar occurs; in only one case out of 13 does the ionosphere appear laminar. The data show that during times of auroral roar emission, the "double resonance" condition (f_uh=2f_ce) usually holds somewhere in the F-region ionosphere for the observed frequency of the auroral roar emissions; cases where this is not true can be explained by the limited spatial sampling of the scanning radar.
See Shepherd et al. 1998 for details.

The Medium Frequency Interferometer allowed this result to be improved significantly. In five examples of auroral roar emissions measured coincident with scanning radar operations, all five cases show that the "double resonance" condition is met in the F-region in the part of the sky from which the emissions are coming. Furthermore, ray tracing in the measured ionospheric structure shows that LO-mode signals originating at the "double resonance" condition can reach the MFI location on the ground. In some of these cases, the source appears to be at high altitude in the topside of the F-region. See Hughes and LaBelle, 2001 for details

Instrument | Top

Dartmouth Programmable Frequency Receiver (PFR)

This receiving system consists of a loop antenna of approximately 10 square meters. The antenna response is a dipole, with the null in the horizontal plane oriented such as to eliminate the largest source of local interference. A low-noise preamplifier at the antenna has frequency response 100 kHz to above 5 MHz, and transmits these signals through a 50-ohm coaxial cable to the observatory as little as a few hundred feet or as much as a mile away, depending on the station.

The PFR is a superheterodyne receiver tunable to 0-5 MHz using IF frequency of 10.7 MHz and crystal filter with bandwidth 7.5 kHz. The local oscillator is controlled directly by a PC running DOS. In the standard mode, frequency is stepped from 30 kHz to 5 MHz in 10-kHz steps, repeating the 498-frequency sequence each 2 seconds. Other programs are used on occasion, including faster frequency switching. In the standard mode, data are collected 20-24 hrs/day, archived on disk in the PC at the station, backed up onto CD-ROM monthly by a local operator, and mailed to Dartmouth (except at Arviat and Taloyoak, where data are backed up annually onto tape by visiting personnel from SED Inc.)

For more information, see: Weatherwax, A.T., Ground-based observations of auroral radio emissions, Ph.D. thesis, Dartmouth College, Hanover, N.H., 1994.

Selected Publications | Top

73. Shepherd, S.G., J. LaBelle, R. Doe, M. McCready, and A.T. Weatherwax, Ionospheric structure associated with auroral roar emissions, J. Geophys. Res., 103, 29253, 1998.

83. Hughes, J.M., and J. LaBelle, First observations of flickering auroral roar, Geophys. Res. Lett., 28, 123, 2001.

84. Hughes, J.M., J. LaBelle, and J. Watermann, Statistical and case studies of 2f_ce auroral roar observed with a medium-frequency interferometer, J. Geophys. Res., 106, 21147, 2001.

85. Hughes, J.M., and J. LaBelle, Joint incoherent scatter radar/medium frequency interferometer observations of auroral roar, J. Geophys. Res., 106, 21157, 2001.

90. LaBelle, J., and R.A. Treumann, Auroral Radio Emissions, 1. Hisses, Roars, and Bursts, to appear in Space Sci. Rev., 2002.

Note: Numbers refer to Full Publication List

Also Note: Most abstracts are freely available through NASA's Astrophysics Data System Bibliographic Services (ADS). Some are available locally.  Full texts are only available to users within institutions that subscribe to the corresponding web-based journal.