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Ruan H.,Hefei University of Technology | Lei J.,Hefei University of Technology | Dou X.,Hefei University of Technology | Zhang S.-R.,Massachusetts Institute of Technology | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

Nighttime ion temperature in the F region is generally thought to be close to neutral temperature at a similar altitude. However, few studies have been conducted that explore the temporal variation between ion and neutral temperatures at night in the F region. In this paper, the similarities and differences of the nighttime ion and neutral temperatures in the F region over Millstone Hill (42.5°N, 71.5°W) were examined using the incoherent scatter radar and Fabry-Pérot interferometer observations during the periods of 1-11 August 2011 and 12-23 January 2012, which represented summer and winter seasons, respectively. Our results showed that in winter, both neutral and ion temperatures exhibited the postmidnight enhancements, which occurred during 3-4 LT and had amplitudes of about 30-90 K. However, the situation was different during the August observation period when the postmidnight enhancement was only observed in the neutral temperatures so that the variation of the nighttime ion and neutral temperatures was almost out of phase. The causes for the postmidnight enhancements of neutral temperature in either winter or summer remain debatable. Our preliminary analysis indicates that the extension of midnight temperature maximum to the latitude of Millstone Hill is the preferred mechanism for the observed postmidnight enhancements of neutral temperature in winter and summer. Key Points Neutral temperature exhibits postmidnight enhancement in winter and summerVariation of nighttime ion temperature agrees with neutral temperature in winterIon temperature shows a contrasting behaviour during the summer nighttime ©2013. American Geophysical Union. All Rights Reserved. Source

Zhang S.-R.,Massachusetts Institute of Technology | Erickson P.J.,Massachusetts Institute of Technology | Foster J.C.,Massachusetts Institute of Technology | Holt J.M.,Massachusetts Institute of Technology | And 7 more authors.
Geophysical Research Letters | Year: 2015

We report a significant poleward surge in thermospheric winds at subauroral and midlatitudes following the 17-18 March 2015 great geomagnetic storm. This premidnight surge is preceded by strong westward winds. These disturbances were observed over three sites with geodetic latitudes 35-42°N in the American sector by Fabry-Perot interferometers at 630nm wavelength. Prior to the wind disturbances, subauroral polarization streams (SAPS) were measured by the Millstone Hill incoherent scatter radar between 20 and 02UT. We identify the observed neutral wind variations as driven by SAPS, through a scenario where strong ion flows cause a westward neutral wind, subsequently establishing a poleward wind surge due to the poleward Coriolis force on that westward wind. These regional disturbances appear to have prevented the well-known storm time equatorward wind surge from propagating into low latitudes, with the consequence that the classic disturbance dynamo mechanism failed to occur. Key Points Premidnight poleward wind surge at 100m/s, preceded by 300m/s westward winds No equatorward wind surge developed throughout the night Disturbance wind driven by strong westward ion flow near dusk and Coriolis force. © 2015. American Geophysical Union. All Rights Reserved. Source

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