In order to illustrate the quality of the data being produced in the ISIS data preservation and analog-to digital transformation project, instructions for viewing ionograms from three different ISIS 2 passes will be presented.
First open a second Web Browser window at the URL
http://cdaweb.gsfc.nasa.gov/cdaweb/sp_phys/
Select ISIS under the option "Select one or more Sources:", ignore "Select one or more Instrument Types:" and click on "Submit" at the bottom of the form.
Fill out the request form for a 10 second interval on June 10, 1978 from 1978/06/10 14:54:49 to 1978/06/10 14:54:59 and select "Plot Data" under " Select an activity:". Ignore "Plotting Options" and scroll down to "Variable parameters (required for Listing and Plotting data only)" and select BOTH of the two options:
[X] Sounder amplitude at 223 virtual heights for fixed & swept
frequency
[X] Interpolated fixed & sweep frequencies
and click "Submit" at the bottom of the form.
Soon, an ionogram panel with a frequency vs. time panel below it appears (scrolling usually required to see the bottom panel). The latter indicates that about 2 s of the fixed-frequency portion of the ionogram (at 1.0 MHz) and about 8 s of the swept-frequency portion of the ionogram (from 0.1 to almost 5.0 MHz) are displayed in the ionogram panel. The actual frequency marks, i.e., the 11 vertical lines, correspond to the frequencies 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2, 3 and 4 MHz. The 1.25 MHz marker is above the last "4" in the time label "14:54:54". A list of the possible fixed frequencies, and other ionogram information, is given in the description at the first address given above.
In this ionogram, the extraordinary (X)-mode ionospheric reflection trace is the thin trace that starts at zero range between the 1.25 and 1.5 MHz markers and extends off the right edge at an apparent range of about 1,000 km. It is the trace that would be used to determine the vertical electron-density Ne profile from the height of the satellite (approximately 1,400 km for the polar circular orbit of ISIS 2) to the height of the Ne maximum (approximately 400 km in the present case). The ordinary (O)-mode trace is similar but it extends to lower frequency where it is lost in intense plasma-resonance signals near 1.25 MHz. It is the trace that would be used to confirm the vertical Ne profile derived from the X-mode trace.
The above mentioned plasma resonance signals near 1.25 MHz correspond to a combination of the 3 fH and fT resonances where fT is the upper hybrid frequency given by fT**2 = fN**2 + fH**2 where fH is the electron gyrofrequency and fN is the electron plasma frequency. This identification is possible from the self-consistent set of resonance and cutoff frequencies observed on this ionogram. Namely, the clear cutoff of the X-mode trace at zero apparent range, the distinct gyrofrequency resonance (at about 0.4 MHz) and the harmonics 2 fH, 3 fH, 4 fH and 5 fH (the last one being the resonance of only about 100 km apparent range slightly above the 4.0 MHz marker), and the resonance at fN between the 1.0 and 1.25 MHz markers. The fN resonance extends to more than 3,000 km apparent range) and has three distinct "spurs" extending to lower frequencies (i.e., to the left).
These "spurs" have been attributed to ion motions because they occur at delay times (apparent range/(c/2)) approximately equal to multiples of the proton gyro period. They have been known for some time, but this example from the ISIS 2 digital data base has revealed for the first time how sensitive they are to the antenna orientation. Indeed, it was this antenna spin dependence that identified the present example which was apparent in a scan of the entire pass. This pass was recorded when the sounder was operating in a mode where the sounder transmitter is always on but the ionograms alternate between one at fixed frequency and one normal ionogram (combined fixed and swept frequency). This mode of operation, called the G mode, was not used often but is useful for some investigations. To illustrate the benefit of the G mode for the study of ionospheric sounder-stimulated resonances, click on "back" and then enter a time interval that includes part of the above ionogram and also part of the previous fixed-frequency ionogram, i.e., select the 15 s interval from 14:54:40 to 14:54:55. Click "submit" under "Plotting Options" (or the "submit" at the bottom of the form). (One could also select the entire pass from 14:43 to 14: 55 but more time is required for such a request.) The resulting display reveals 11 s of fixed-frequency operation at 1.0 MHz followed by 4 s of swept-frequency operation (0.1 to almost 1.5 MHz). The fixed-frequency operation clearly shows the first proton spur on the fN resonance for about a 2 s time interval centered on 14:54:45 at approximately 700 km apparent range. Note that only the first spur appears in the fixed-frequency (1.0 MHz) display and that in the swept-frequency display only the first spur extends low enough in frequency, i.e., to the left, to cross the 1.0 MHz marker (explaining why it is the only one detected in the fixed-frequency presentation). Thus these spurs are greatly enhanced for certain orientations of the sounder antenna (presumably parallel to the magnetic-field direction). (The antenna spin-modulation dependence of the short-duration resonance feature at 1.0 MHz appears to be opposite to that of the spur since it has a bite out centered on the appearance of the spur.)
The plasma resonances stimulated by the Alouette and ISIS topside sounders have been studied for many years by a large number of scientists. Most of the features have been explained, in terms of the echoing of electrostatic waves stimulated by the sounder, but many questions remain. For example, questions concerning the ion modulation effects on electron plasma wave phenomena (illustrated in the above example) and questions concerning the origin of dominant diffuse resonances that have been attributed to plasma emissions stimulated by the sounder. A prime example of the latter is the long-duration diffuse resonance between fH and 2 fH seen in the swept-frequency portion of the above ionogram near 15:54:53 (the center one of the three long-duration resonances on the left of the swept-frequency portion of the ionogram). The knowledge gained from investigating such resonant phenomena in the terrestrial ionosphere have been applied to measure Ne and the magnetic field strength in the terrestrial magnetosphere and in Jupiter's Io plasma torus (see, e.g., Benson and Osherovich [J. Geophys. Res., 97, 19,413, 1992] and Osherovich et al. [J. Geophys. Res., 98, 18,751, 1993] and references therein).
After a few minutes (a long time is required when an entire pass is requested - 12 minutes of data in this case) a display of eight 90 s panels of ionogram data will be displayed with a plot of the frequency variation with time displayed in a ninth panel at the bottom. From this bottom panel, it can be seen that the swept-frequency range extends to 10 MHz (rather than 20) on these ionograms and that the short (3.3 s) fixed-frequency portion at the beginning of each ionogram was selected as 0.12 MHz. Each of these ionograms has a total duration (fixed plus swept frequency) of about 14 s. It can also be seen that the automatic frequency interpolation was not able to be performed from about 00:24:30 to 00:25:00 and after about 00:31:20.
Next, select "back" and enter the more restricted time interval from 00:30:56 to 00:31:06 in order to expand the swept-frequency portion of a single ionogram with the sounder transmitter on. The resulting display has two panels: an expanded portion of an ionogram in the top panel with a plot of the frequency in the lower panel (scrolling may be required to see the bottom panel). The ionogram reveals two intense ionospheric reflections (traces with increasing apparent range with increasing time, i.e., increasing frequency) and two intense plasma resonances (vertical traces that resemble stalactites). The plasma resonance on the left (which extends downward to well over 1500 km apparent range) is at the ambient plasma frequency fN and the one on the right (which extends downward to approximately 1500 km apparent range) is at the upper hybrid frequency fT. The intense ionospheric reflection on the left, which terminates at fN, is due to propagation in the slow branch of the X mode (called the Z-mode); the one on the right is due to propagation in the fast X-mode (which corresponds to the free-space X mode at frequencies well above fX - the wave cutoff frequency at zero apparent range). The weaker trace near 1000 km apparent range, which starts slightly beyond the upper hybrid resonance and merges into the X-mode trace, is due to propagation in the O-mode.
Please contact Bob Benson at u2rfb@lepvax.gsfc.nasa.gov if you would like more information about these examples and explanations.
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