17:15 〜 18:45
[PEM10-P09] Estimation of the FLR frequency and the magnetospheric density from the SuperDARN VLOS data using the Discrete-Time FT
Some of the fluctuations in the solar wind, including those causing sudden impulses (SI), propagate into the magnetosphere and excite eigen-oscillations of the magnetic field lines and the frozen-in plasma via the field-line resonance (FLR). It is known that the gradient methods effectively identify FLR in the observed data. The identified FLR frequency enables us to estimate the plasma mass density along the magnetic field line because, in a simplified expression, 'heavier' field line oscillates more slowly.
We have been applying the gradient methods to the VLOS (Velocity along the Line of Sight) data of the SuperDARN radars. The radars emit azimuthally-collimated beams of radio waves in the HF range, and some of them are backscattered by the ionosphere, while some others are backscattered by the ground and sea surface. The Doppler shift of backscattered signals enables calculation of VLOS. Ionosphere-backscattered signals yield VLOS of the horizontally-moving ionospheric plasma (at mid- to low latitudes, VLOS also has a vertical component because the ambient magnetic field is tilted), while ground/sea-backscattered signals yield VLOS corresponding to the vertical motion of the ionospheric plasma because the length of the ray path of a beam can only be changed by the vertical motion of the ionosphere.
For a 30-min-interval event after an SI, we applied the gradient methods to VLOS data obtained from different beams and range gates, and successfully identified the FLR in both the ionosphere-backscattered signals and sea surface-backscattered signals. Applying the FFT to the FLR signals yielded significantly smaller density for the sea-backscattered signals. This could come from a fairly large frequency spacing of the FFT results due to the fairly small duration (30 min) of the event.
To obtain higher resolution and thus precision of the frequency, we first used the zero-padding method. As a result, the above-stated density difference became smaller. However, the zero-padding method decreases the amplitude of the FFT result, meaning larger statistical error.
We are now testing the Discrete-Time Fourier Transform (FT) method. This method uses the provided data only, meaning no decrease in the amplitude. Another advantage of this method is that it can calculate the FT at any frequency (below the Nyquist frequency). In addition, this method accepts data gaps, unlike the FFT. The test results are so far promising. We will present more details at the meeting.
We have been applying the gradient methods to the VLOS (Velocity along the Line of Sight) data of the SuperDARN radars. The radars emit azimuthally-collimated beams of radio waves in the HF range, and some of them are backscattered by the ionosphere, while some others are backscattered by the ground and sea surface. The Doppler shift of backscattered signals enables calculation of VLOS. Ionosphere-backscattered signals yield VLOS of the horizontally-moving ionospheric plasma (at mid- to low latitudes, VLOS also has a vertical component because the ambient magnetic field is tilted), while ground/sea-backscattered signals yield VLOS corresponding to the vertical motion of the ionospheric plasma because the length of the ray path of a beam can only be changed by the vertical motion of the ionosphere.
For a 30-min-interval event after an SI, we applied the gradient methods to VLOS data obtained from different beams and range gates, and successfully identified the FLR in both the ionosphere-backscattered signals and sea surface-backscattered signals. Applying the FFT to the FLR signals yielded significantly smaller density for the sea-backscattered signals. This could come from a fairly large frequency spacing of the FFT results due to the fairly small duration (30 min) of the event.
To obtain higher resolution and thus precision of the frequency, we first used the zero-padding method. As a result, the above-stated density difference became smaller. However, the zero-padding method decreases the amplitude of the FFT result, meaning larger statistical error.
We are now testing the Discrete-Time Fourier Transform (FT) method. This method uses the provided data only, meaning no decrease in the amplitude. Another advantage of this method is that it can calculate the FT at any frequency (below the Nyquist frequency). In addition, this method accepts data gaps, unlike the FFT. The test results are so far promising. We will present more details at the meeting.