Spread F- An old Equatorial Aeronomy problem finally resolved?
R. F. Woodman
Radio Observatorio Jicamarca, Instituto Geofísico del Perú, Lima
One of the eldest scientific topics in Equatorial Aeronomy is related to Spread-F. It includes all our efforts to understand the physical mechanisms responsible for the existence of ionospheric F Region irregularities, the spread of the traces in a night-time equatorial ionogram ---which originated its name--- and all other manifestations of the same.
The phenomenon that coined its name as Spread-F, was discovered shortly after the installation of a ionosonde at the Huancayo Observatory, nearly 75 years ago. Now we know that these irregularities are also responsible for the scintillation of satellite signals and for the very strong echoes received by radars at VHF frequencies. The irregularities are observed in situ by satellite and rockets as deep fluctuations in electron density. It is our intention in this paper to review historically the advances we have made on this topic. But, considering its long history and the hundred of papers that have been written about it, we have to be very selective and concentrate only on those we (I) consider that have been landmarks in the progress we have made to understand them.
The progress was very slow at the beginning. Thirty five yeas after its discovery, although quite a bit had been learned about its geographical and temporal behaviour, Jicamarca radar observations showed that none of the theories that had been put forward could explain all of the observations. Gravity, vertical winds and electric fields had been proposed as the source of energy for unstable mechanisms that, given a density gradient, would develop electron density fluctuations (irregularities). The problem was that irregularities were also observed by Jicamarca when-and-where these conditions where stabilizing instead. This apparent paradox was resolved a few years later, again by Jicamarca observations. This time the radar could see the morphology of the irregularities in the context of the background ionosphere thanks to a newly developed imaging technique. Four different types of irregularities were identified: topside, bottom-side, bottom-type and valley-type. Irregularities were observed at the top side of the ionosphere, and although this was an altitude that was gravitationally stable, the images show that they were physically connected with the lower, gravitationally (Raleigh-Taylor) unstable region. It was clear that the bottom unstable region had been convected by buoyancy to the stable region, carrying with it the conditions (low densities) that made it unstable in the first place. Numerical simulations proved that Raleigh-Taylor instabilities would indeed propagate, in a non-linear regime, to the stable top side of the ionosphere. Later, the role of gravity, electric fields and winds was unified into a single theory: the destabilizing force produced by gravity was complemented (or reduced) by forces produced by neutral friction induced by an electric field or a neutral wind (Generalized Raleigh-Taylor instability).
A problem remained, the growth time of the Raleigh-Taylor or the Generalized instability was much too long to explain the observed relatively short time available for its growth. Gravity waves were postulated as a means of producing sufficiently large fluctuations (seeding) to start with, so that the instability would not require a long time to produce a sufficiently large amplification factor. Much effort has been made in the last few decades to prove, without much success, that this is the case. A very recent theory claims that the very large counter streaming velocities, of the order of 200 m/s, between the ionized and the neutral gas, existent at the bottom of the F region during the evening hours, makes the steep gradient in electron density at this altitude (interchange) unstable. The growth time of this instability is very fast. In an hour grows 18 e-folds. When conditions are right, these resultant larger fluctuations produce the necessary seeding for the Rayleigh-Taylor instability to take over, producing the bottom-side type of Spread F or even the plumes that characterize the top side irregularities. If the conditions are not right, the irregularities remain in the unstable, but narrow, steep gradient region, and produce the (almost) ever present bottom-type traces seen by the radar. With this theory, the role and variation of the neutral winds from day to day may help us to understand the day to day variability of the phenomenon, and help us improve its predictability. The latter is much needed for practical applications.
An important experimental advance has been the development of within-the-beam interferometric imaging technique. It allows us to discriminate spatial from temporal variations and see smaller scale irregularities than it was possible with the single-beam time-scanning technique.
Little theoretical effort has been made to explain the valley-type irregularities, but the same mechanism postulated for the seeding above could work on the steep gradients present in this region, particularly when the large pre-reversal electric fields are included. Both of these latter irregularity types are not capable of producing satellite scintillations or spread in the ionogram traces.
One problem would still remain. How are the 3 m and 0.35 m irregularities responsible for the radar echoes produced? This is important since we use them as a tracer to tell us what is happening at the larger scales. Unified interchange instability numerical models using fluid equations show growth at meter scales. But, how valid are the fluid equations used at these length scales? We need non-homogeneous kinetic descriptions to validate the results. This work is still to be done.