Eficiencia espectral

In attempting to investigate ion-neutral coupling it is inevitable that combined measurements from instruments determining ionospheric and neutral parameters be used. W hile it is possible for incoherent scatter radars to estimate the neutral wind, these measurements are indirect and depend on the value used for 0 + diffusion velocity which in turn is dependent on the value of the 0 + - 0 collision frequency. As this is currently subject to considerable speculation the derived neutral winds must be regarded as less accurate than direct measurements from FPIs. The same problems apply to derivations of the neutral wind from ionosonde data. Experimenters have sought to devise combinations of instruments to allow comparison o f the measurements from these instruments to establish a better understanding o f the limits of our current theoretical framework for ion-neutral coupling.

2.7.1 Ionosonde - FPI Comparisons

The first reported comparison o f FPI and ionosonde data using servo theory principles was for the southern hemisphere mid-latitude by Yagi and Dyson (1985). They found reasonable overall agreement between observed changes in the height of the nighttime F-layer and the changes calculated from servo theory using neutral winds from Fabry-Perot measurements.

Full diurnal calculations of the horizontal wind from ionospheric electron

density profiles were first given by Miller et al. (1986, 1987) using values o f hmF2

obtained from the true height analysis of ionograms at Arecibo (3 0 °N geomagnetic), vhere the results were compared with meridional winds from Fabry-Perot interferometer and incoherent scatter radar data, revealing good agreement.

Gurubaran and Sridharan (1993) used the servo equations to estimate the balance height o f the F-region ionization maximum by inputting neutral wind and

temperature data from an FPI and comparing these with independently measured hmF2

by means o f ground-based ionosonde, at Mount Abu/Ahmedabad, India, a low latitude site. The comparison (e.g. Figure 2.7) reveals fairly good agreement, reproducing most of the observed features and providing direct experimental evidence for the two legions to behave as a closely coupled system. They note that the influence o f electric fields, though they appear to be less significant at this latitude, could still be inferred from the data.

F e b .n .1 9 9 l Mt.Abu/Ahm«dobo<J A p - l/> 2 0 0- 100 2 - 1 0 0 20 0- - 3 0 0 C s l H T t O l f d • • Meosured 4 0 0 - D 3 0 0 - 200 19 21 23 01 03 05 Tim e (1ST) H rs.

F ig u re 2.7 F P I - Io n o so n d e co m p ariso n s at low latitu d e (after G u ru b a ra n and S rid h a ra n , 1993)

Thayer et al. (1995) combined the use of an FPI at Thule Air Base, Greenland with a digital ionosonde located at Qanaq, Greenland to obtain a continuous record of F-region neutral winds, electron density profiles and F region ion drifts at high latitude. This combination of ground-based observations allowed the investigation of ion/neutral coupling at a temporal resolution of about 15 min. They investigated the observed response of the neutral wind to convection changes in the ion drift inside the

polar cap for southward and northward IMF 6 % conditions, and concluded that the ion

drag term alone could not describe the observed response of the neutral wind during northward IMF.

Recently Dyson et al. (1997) have reported results at southern mid-latitude from comparison of FPI neutral winds with those derived from Digisonde

measurements of hmF2. They find that in implementing the servo equations the best

fits between the data sets are found by using a Burnside factor of 2.0, as opposed to the recommended factor o f 1.7 . They suggest that lower values o f the Burnside factor could only be tolerated by inferring large electric fields or large discrepancies in the atomic oxygen densities derived from the MSIS-86 model.

Previously no direct comparisons of FPI neutral winds to neutral winds derived from ionosonde data have been performed at high latitudes. This is probably due to the difficulty in taking possibly large electric fields into account. An experimental arrangement to perform this comparison at Kiruna/Tromso is described in the following chapter.

2.7.2 EISCAT INDI Experiment

A further example o f the experimental possibilities for investigating ion-neutral coupling is the EISCAT Ion Neutral Dynamics Investigation (INDI) experiment. The EISCAT radar system will be described in detail in Chapter 3. This experiment resulted from early attempts to compare FPI and EISCAT ISR data being complicated by the lack of coordination between the two instruments (Rees et al., 1984). For the INDI experiment an FPI is used at Kiruna. Early versions o f the INDI experiment attempted to scan the radar through 360® every 15 min, observing the same volumes as the FPI. This was not possible as the radar had to return through 3 6 0 ° after each scan, which took too long.

As a result, it was decided that the radar need scan only through the meridian, which allows the determination of the meridional component o f the neutral wind, the

main object of the comparative study. A modified version o f the EISCAT CP-3

experiment was used (Rishbeth and Williams, 1985), with only the seven central positions o f the original scan from the north of Tromso to above Kiruna. In this way, the scan time was reduced to 15 min from a standard CP-3 scan time o f 30 min, each of the seven measurements taking approximately 2 min. Figure 2.8 is a plan o f the INDI experiment geometry. This shows the relation between the FPI and EISCAT scattering volumes.

The results of the experiment have been used to estimate the 0 + - 0 collision parameter by adjusting the value used in the derivation of winds from the radar until the best fit was achieved. It has been pointed out that much care is necessary in taking into account the variability of the data used and some re-evaluation o f the methods used has been undertaken in similar experiments as a consequence (Davis et al., 1995; Buonsanto et al., 1997c). The values o f the Burnside factor derived using this

technique tend to be around 1.2 , significantly lower than the recommended value o f 1.7 (Salah, 1993) and in keeping with the recent trend to downward revision o f the Burnside factor.

Qeornagnetio

North

Kiruna

C alibration

Zenith

Figure 2.8 Plan view o f experimental geometry (after Davis et al., 1995)