The NorFA '97 Summer School

9 June 1997

VLF measurements during EISCAT/Heating campaign June 9,1997



INTRODUCTION
Our task was to measure very low frequency (VLF) waves during the EISCAT/Heating campaign at June 9 1997, 8 - 12 UT. Measurements were made at Liikkuvankangas (67.30N, 26.20E) in Northern Finland. The EISCAT UHF transmitter and one of the receivers are located in Ramfjordmoen (69.6N, 19.2E, L=6.2) near Tromsø. Two other receivers are located in Kiruna and Sodankylä. The heating facility is located near the EISCAT station at Ramfjordmoen.


WHAT ARE THE VLF EMISSIONS ?
VLF emissions are electromagnetic waves that propagate in the magnetospheric plasmas in whistler mode. The frequency range of these VLF waves is between about 0.3 - 30 kHz. The frequency ranges of radio waves of different types are shown in Table 1 .

VLF emissions can be produced by :
  • self-generation of waves in wave-particle interactions in the magnetosphere
  • strokes of lightning on the Earth
  • power line radiation on the Earth

    VLF waves produced by above mentioned processes are very weak. When waves propagate in the magnetosphere they interact with magnetospheric particles. In these so-called wave-particle interactions VLF waves get energy from the particles and are intensified enough to be observed on the ground. The interactions between VLF waves and electrons can, for example, lead to a pitch angle diffusion of the electrons and this can lead to the precipitation of the electrons into the ionosphere.

    One common type of VLF emissions is so-called chorus type emission which consists of discrete rising emissions close to each other at frequency range from a few hundred Hz to few kHz. An example of chorus bursts measured at Kalkkoaivi 1994 in Northern Finland is shown in Figure 1. Emission bursts are seen between 2 and 3 kHz.

    VLF waves can also be generated artificially. One way to do this is the heating of the ionosphere. This can be done by powerful HF radio waves which are amplitude-modulated. Heating produces changes of the Pedersen and Hall conductivities and thus the current density. The current density can be varied periodically with the frequency of the modulating wave. Thus the heated area can operate as an antenna radiating at the modulation frequency. The principle of the ionospheric heating is shown in Figure 2 . One heating facility is located near EISCAT station in Ramfjordmoen. Also the power line radiation (PLR) can generate VLF emissions.

    In Finland VLF measurements have been made every year since 1990. Here you can find the list of previous campaigns. During winter times measurements are mainly done during EISCAT and auroral campaings.


    INSTRUMENTATION
    The instrumentation for simultaneous measurements of VLF and ULF waves and optical aurora is shown in Figure 3.

    Ground based instrumentation for VLF wave measurements includes loop antennas, pre- and main amplifiers and the system to record the observed VLF waves. The examples presented here were made by two antennas; one antenna measuring EW component and the other NS component of the VLF wave.

    Antennas consist of 81 turns of copper wire (cross-section 0.75 mm^2). These turns are inside aluminium body which size is 1.25 m * 1.25 m. In antennas there are also fastened preamplifiers (amplify 40 dB). Signals from the antennas are first frequency filtered, here we have used the frequency range 50 Hz - 9.2 kHz. Filtered signals are then amplified by preamplifiers (amplify 30 dB) and main amplifiers (42 dB). After all VLF signals are recorded by normal VHS HiFi video tape recorder to the audio tracks of the video tape (dynamic range 90 dB).

    At winter times the auroral activity is measured by low-light-level TV camera and wide angle photometer. The signal from the camera is recorded to the same video tape as VLF signals. Before recording time signal from the accurate timer is multiplexed to the video signal. By this way we get the timing of our measurements.

    Magnetic variations are measured by two induction coil magnetometers, in NS and EW directions. The measured frequency range is 0.01-9 Hz. The magnetometer signal is only printed, more detailed and accurate magnetometer data is measured at Pittiovaara (67.42N, 26.39E) station.

    Heating facility in Ramfjordmoen consists of three antenna arrays and 12 transmitter amplifiers. The maximum power of each transmitter is 100 kW. So the total power of transmitter system is 1.2 MW. The radio wave frequencies can be modulated by frequency synthesizer. All 12 frequency synthesizer are connected to one common frequency source. This system is of cource computer controlled.

    Antenna array 1 consists of 144 crossed dipole antennas. The size of the array 1 is about 400*400 meters. The frequency range is 5.5-8 MHz and the beam widths are 7 degrees.

    Arrays 2 and 3 consists of 36 crossed dipole antennas and they cover the frequency area 3.85-5.65 MHz and 5.5-8.8 MHz. The sizes of the arrays are 270*270 meters (array 2) and 200*200 meters (array 3). The beam widths are 14.5 degrees. During this campaing the modulation period was 20 s and the frequencies were 985 Hz, 3015 Hz, 4515 Hz, 925 Hz, 1375 Hz, 2375 Hz, 985 Hz, 3015 Hz, 4515 Hz.


    CALIBRATION
    The calibration was made by using a signal generator and a coil with 15 turns and a diameter of 1 m. When the current in the coil is known, the magnetic field density can be calculated (Mannninen J., Lic. Thesis, 1995). The calibration frequency was 1 kHz, in which the signal generator used has nearly maximal signal amplitude. Other frequensies were not used, because the response of the VLF amplifier used has a constant value in the whole frequency range used.


    DATA ANALYSIS
    VLF data has been digitized from video tapes by using 2-channel 16-bit AD converter. The sampling rate of the conversion is 40 kHz. The digitized data can then be analyzed by LabView-based programs that are developed in Sodankyla Geophysical Observatory for the analysis of VLF and ULF waves. Analysis method is normal complex FFT to the data. From the Fourier transformation coefficients it is possible to determine the following parameters:
  • signal power
  • ellipticity
  • orientation of the polarization ellipse
  • sense of rotation

    The power of the VLF signal can be devided as:
  • total power
  • right-handed polarized power
  • left-handed polarized power
  • linearly polarized power

    As results we get the total and linear power, left-handed and right-handed power, polarization angle and ellipticity.


    PRELIMINARY RESULTS
    In the experiment we recieved the heater signal at the following intervals: 08:10-08:10, 09:10-09:19, 10:11-10:19, and 11:10-11:19. By comparing to magnetogram obtained in Sodankyla these seem to correlate with increasing northward magnetic field component. Similar correlation is found also by A. Oikarinen et al., JATP, in press, 1997. During the time period 08:10-10:10 enhancements of the electron dencity around the height of 88 km was observed by EISCAT. The correlation with the recieved heating singal during the periods 08:10-08:10 and 09:10-09:19 is not clearly pronounced and thus requires further analysis. For a further analysis of VLF powers we choose a time period 09:10-09:11: Here we show data for total, right-handed, and left-hand powers. The total power consists of the heating signal at modulation frequency 3015 Hz starting from 09:10:48 UT, power-line harmonic radiation as constant frequences below about 1300 Hz, and spikes due to lightning in broad frequency ranges. The heating signal is right-hand polarized as the left-hand power is absent. This is also seen in the ellipticity power diagram that indicates that the wave is 100% right-handed.

    Here you can find one example of discrete emissions from our measurements.

    AUTHORS
    Reijo Manninen, University of Oulu
    Anja Strømme, University of Tromsø
    Petri Toivanen, Finnish Meteorological Institute
    Antti Anttila, University of Turku
    Anita Kullen, KTH Stockholm
    Tom Grydeland, University of Tromsø
    Andris Vaivads, University of Umea
    Jyrki Manninen, Sodankulä Geophysical Observatory