Dynamics of the Polar Cap Ionosphere
Part 1. Applications of Digisonde animations in ionospheric research

A UMLCAR (University of Massachusetts Lowell, Centre for Atmospheric Research) Digital Ionosonde (DPS-4) has operated at the Australian Antarctic polar cap station Casey (-80.8o geomagnetic latitude) since early 1993, primarily to study the dynamics of the southern polar cap ionosphere.  To gain a better understanding of the ionospheric motions involved and to identify dynamical features, animated data displays have been developed using ionospheric drift and ionogram data combined together with other geophysical data sets.  These displays generally show ionospheric skymaps (echo locations), drift velocities and Doppler ionograms (vertical profiles), produced with a time resolution of up to a frame every 3 minutes.  Static displays of other geophysical data set (eg. Total Electron Content (TEC) from Global Positioning System (GPS) satellites or Interplanetary Magnetic Field (IMF) measurements) can be incorporated into the animations.  Ionospheric changes can then be viewed frame-by-frame using commercial animation software.

The displays provide a simple means of displaying large amounts of data and identifying, for example, drifting polar patches, or the response of ionospheric drift to changes in the IMF.  Since the ionosonde takes measurements throughout the bottomside ionosphere, different height regions (e.g.. E and F) can be examined independently or compared directly.  Doppler and directional information featured in the animations provide a powerful technique for studying the evolution of ionospheric phenomena such as polar patches.

Some of the applications of the ‘ionomovie’ animations in ionospheric research will be demonstrated.

1. INTRODUCTION

The Digisonde Portable Sounder (DPS-4) (Reinisch et al., 1995), like most modern digital ionosondes, is able to provide high temporal and spatial resolution measurements of the ionosphere.  These measurements contain a wealth of information including Doppler shifts, angles of arrival and group ranges from significant numbers of ionospheric echo sources simultaneously.  The data sets thus generated can be of considerable size, requiring complex and time consuming computer analysis.  Initial display techniques essentially produced ‘one-dimensional’ drift velocity or azimuth versus time plots (as in Buchau et al. 1988). These combined measurements over all heights and frequencies producing average values at a given time. Recently, significant advances have been made in the development of new display methods of digisonde data for ionospheric research (Parkinson et al., 1999a), making good use of colour as a means of identifying specific features in the plots, and adding a range/height dimension.  However, these types of visualisation packages are still limited to generating static displays whose complexity and content can make it difficult to study smaller scale or transient features present in the data.

This paper describes a method to provide an alternative view of digisonde data by using animation techniques to better demonstrate the dynamic nature of the ionosphere and ionospheric features. By incorporating drift data and ionograms, and displaying multiple ranges, the new displays can give a virtual three-dimensional view of the ionosphere, and how it changes with time.

The use of animations in ionospheric research is not new, with early ionosondes recording ionogram images on film that could then be projected. Computers have increased the flexibility of this technique, but still display a series of ionograms only, an approach that provides a limited view of the information available (giving effectively only a simple vertical profile).  The animation methods described here (tentatively called ‘ionomovies’) add a horizontal dimension to the animation by incorporating ‘skymap’ displays (showing spatial distribution and Doppler shift of ionospheric echo sources in a horizontal view), generated by the DPS-4 Drift Data Analysis (DDA) software (Scali et al., 1995). The DDA software locates echo sources by applying a Doppler sorted interferometry (DSI) technique to measurements taken during a ‘drift mode’ sounding program of the DPS–4.  Echo sources may be sorted according to vertical group range (disregarding refraction effects), allowing skymaps to be generated for specific height ranges (eg. E- and F-regions). Corresponding velocity vectors deduced from the skymaps may also be included on the displays.

The DPS-4 has several ionogram modes with the most commonly used being beamforming (BEM) ionograms storing coarse angle of arrival and single level Doppler information.  Concurrent ionograms and drift files may be included in the same ‘frame’ of the ionomovies. With the current configuration of the DPS-4, a cycle of one ionogram followed by a drift measurement can be completed every three minutes. Higher repetition rates may be available with future upgrades.

To increase the usefulness of the ionomovies in investigating ionospheric features and their evolution, they were designed to allow simple comparison with other geophysical parameters. By making the animation displays flexible enough to allow the inclusion of other data sets (e.g.. total electron content, satellite scintillation or interplanetary magnetic field), the interrelationships between these parameters and dynamic features in the ionospheric environment can be investigated.

Although applicable to any research of dynamic or transient events in the ionosphere, this study demonstrates the use of ionomovies in the identification and/or characterisation of sporadic-E events or Polar patches.

2. DPS OPERATION AND DATA ANALYSIS

The standard operating mode of the Casey DPS-4 involves running an ionogram (usually beamforming) program every ten minutes, followed by a number of drift programs covering a range of frequencies and heights. For best temporal resolution of features using the ionomovies, an operating mode was devised that completed an ionogram and a basic drift program every three minutes. It is expected that, with future upgrades of the Casey DPS, this time may be reduced to a two or even one minute cycle.

BEM ionograms are recorded by using the four receive antennas to form seven ‘beams’ into which the strongest ionospheric echoes (at each selected frequency and range) are sorted. One beam contains vertical echoes (which are further separated into ordinary and extraordinary modes) out to a zenith of 20 deg, and the six remaining beams include off vertical reflections in 60 degree azimuthal segments. The ‘beam number’ is stored in the data file along with amplitude and coarse Doppler (ie. positive or negative) information.

Drift mode measurements record up to 128 complex samples (depending on desired Doppler frequency resolution) from each receive antenna for each frequency and range bin. These undergo a Fourier transform to obtain Doppler and phase spectra and are subsequently stored in a data file. With further processing using the DDA package (Scali et al., 1995a), echo sources are obtained from DSI (Scali et al., 1995b) whereby echoes are sorted according to Doppler shift, and directions of arrival are derived from phase measurements over the four antennae of the receive array. This results in a SKY data file containing echo coordinates, ranges and Doppler shifts from which Skymaps are produced. Further processing with DDA on these SKY files yields ‘drift velocities’ deduced by a least-squares fit to at least three echoes.

This procedure for determining drift velocities depends on a number of assumptions. It assumes minimal refraction effects. It also assumes that the ionosphere is moving with a uniform, bulk flow, and that the echo sources detected are imbedded in that bulk flow. The validity and limitations of digisonde drift measurements is a subject of some debate. Comparative studies at high latitudes have shown that digital ionosondes basically measure the ion motion in the F-region (Grant et al., 1995; Smith et al., 1998). However, Monselesan and Morris (1999), discuss possible discrepancies in drift measurements introduced by the presence of plasma waves or irregularities, primarily in the E-region, but also of importance at F-region heights. It is noted that the current study does not attempt to address these possible limitations but simply to show a new method of visualising digisonde data from which other conclusions can then be drawn.

The DPS data were converted to animated displays via a number of steps using software packages developed for the purpose. Firstly the DPS binary files (ionograms and SKY files) are converted to a text format to enable easier sorting and display. At this point, approximate group heights of echoes from the SKY files are calculated using group range and elevation values contained in the data files. Using data from a SKY file, a plot is created displaying Skymaps (horizontal locations) of ionospheric echoes sorted into two virtual height ranges (on separate axes) and colour coded according to Doppler shift. The corresponding ionogram (recorded immediately before the drift file) is filtered by amplitude and colour coded according to beam direction and polarisation and displayed in a separate panel on the plot. Any other data that may be relevant to the feature being studied (e.g. IMF or TEC) are then displayed in a further separate panel on the same plot. The resulting image is saved as a GIF (Graphical Interchange Format) image file and merged with all subsequent images using a commercial animation package to form the final display.

Figure 1: Example of one frame of an ionomovie, 9 April, 1998

Figure 1 shows an example of one such image from an animation sequence. The left top panel in the figure shows echoes in a horizontal plane in the virtual height range 90 to 200 km, approximately E-region. The right top panel shows the range 200 to 600 km (F-region). Echoes are shown as either cyan (positive Doppler) or red (negative Doppler). The outer circle of the plots represents a zenith angle of 45 degrees. The approximate solar azimuth is shown as a star symbol on the outer circle. Superimposed on each Skymap plot is the calculated drift velocity vector for that height range (the outer circle represents a velocity of 1500 m/s).
The lower left panel shows the corresponding ionogram, while the lower right panel, in this case, shows the three orthogonal interplanetary magnetic field values (in GSM (Geocentric Solar-Magnetospheric) coordinates). All directions shown are in magnetic compass coordinates (approximately -92 degrees deviation from geographic directions).

3. APPLICATIONS AND EXAMPLES

The main idea behind the development of the ionomovies, apart from simplifying the display and analysis of large volumes of data, was to assist in a study on polar patches. Polar patches are regions of increased plasma density, up to 1000 km in horizontal extent, which convect across the polar cap in an antisunward direction. The ionosonde signatures of a patch are significant increases in critical frequencies on ionograms or U-shaped traces on constant frequency, range versus time plots (James and MacDougall, 1997; MacDougall et al., 1996). It was expected that the ionomovie technique could also be used to identify and further investigate the formation and motion of polar patches. Although results from this study are still very preliminary a number of patches have been identified in ionomovie animations.

Figure 2: Passage of a small Polar patch through the field-of-view of the DPS-4 at Casey, 16 February, 1996.

Figure 2 shows an example of a small patch convecting through the field-of-view of the DPS. The figure shows just the Skymap image sequence from an ionomovie, clearly showing the passage of a patch to the North-east (compass) of the station. The Doppler shift is seen to change sign as the patch passes the point of closest approach. This patch was also identified as a U-shaped trace in the data of Parkinson et al., 1999b. Larger patches tend to be less easily identified than this example and appear to take the form of ‘fronts’ (the leading or trailing edges of the patch) passing across the field-of-view of the instrument.

Other possible applications of the ionomovie technique could include a detailed investigation of drift velocity shear in the ionosphere (by the choice of appropriate virtual height ranges) and the possible association with the formation of sporadic-E layers (Parkinson et al., 1997; Parkinson et al., 1998). Examples have been seen when the azimuths of the drift vectors in the approximate E- and F-region Skymaps have shown up to 180 degrees difference.

Figure 3: Skymap image showing apparent drift shear from 7 April, 1999.

Figure 3 shows

• a Skymap where the echo and Doppler distribution in the 200-600 km range (F-region) indicates what appear to be two
• separate drift directions at right angles. Without the use of the ionomovie animation such an occurrence would simply be
• averaged by the DDA software to give a single drift velocity. While the actual interpretation of this event is beyond the
• scope of the current study, it does further demonstrate the advantages of the ionomovie display technique.

A new dynamic display technique of Digisonde DPS-4 data has been developed in the form of ionomovie animations. This has great potential for studying the formation of dynamics of small or transient features and motions in the ionosphere. In this paper, an example of how this technique can help in the identification and study of polar patches has been presented. It is anticipated that, with the flexibility allowed in this technique, it will prove useful in many other areas of ionospheric study including sporadic-E and ionospheric drift-shear, convection pattern changes with IMF, and ionospheric irregularities and instabilities associated with the high latitude ionosphere.

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