Spectral observations of Polar Mesospheric Summer Echoes (PMSE)

Abstract

We present spectral measurements of PMSE under the influence of HF heating cycles. The data has been obtained with EISCAT VHF radar, during the EISCAT campaign 2004 in Tromsø. Examples where the PMSE overshoot appears when the electrons are heated with a specific cycle are shown. A detailed analysis of PMSE spectral width during the heating cycles concluded that there is no evidence of a possible correlation between them. In our measurements we found several frequency jumps. By analysing the abruptness one observed (with a discontinuity of the vertical velocity around 56 m/s in less than 10 seconds), we resolve for the first time how this “jump” takes place. During the transition from one Doppler velocity to the opposite, the gradual death of one Doppler velocity “layer” is observed while a new one appears increasing in amplitude until the frequency jump finishes, where the first Doppler has completely disappear and the new one has a value equivalent to the one before with opposite sign. Different models which may explain the source of the frequency jumps were tested, finding that wave steepening could explain some of the cases reported. Not many clear examples of possible signatures of gravity waves were found. In the analysis of the best candidates, we obtained values of vertical phase velocities around /s and vertical wave numbers around 15 m/s and vertical wave numbers around 6x10e-4(1/m)which are reasonable values for atmospherical gravity waves so, we think that PMSE could be modulated by gravity waves during the EISCAT campaign.