The high latitude D-region and mesosphere revealed by the EISCAT incoherent scatter radars during solar proton events
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Spcc Rcs. Vol. 18. No. 3, pp. (318~(3)92. 1996 copright Gel 1995 COSPAR
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THE HIGH LATITUDE D-REGION AND MESOSPHERE REVEALED BY THE EISCAT INCOHERENT SCATTER RADARS DURING SOLAR PROTON EVENTS
P. N. Collis
EISCAT Scientific Association, Box 812, S-981 28 Kiruna. Sweden
Energetic protons precipitating into the polar atmosphere during solar proton events cause increased ionisation rates at mesospheric heights, producing a target for incoherent scatter radars at altitudes not normally accessible by that technique. Such measurements fall into two general classes; they am obtained either fortuitously, when the event erupts during an ongoing radar operation, in which case the experiment mode may not be optimal for low altitude studies, or a suitablydesigned experiment is initiated on notification of the event. The main parameters derived from these observations are the electron concentration, spectral width of the backscattered signal (from which aeronomic information may be derived) and Doppler shift (from which neutral winds may be derived). We present a review of the results obtained ftom the EISCAT incoherent scatter radars during solar proton events. Particular emphasis is given to data obtained during the most recent solar cycle maximum when strong events were observed in the years 1989-1991. The review concentrates on the height range 50 to 90 km, and covers topics including the height- and time-variation of election concentmtion and negative ions, wave-like behaviour of the neutral winds, comparisons of measured spectral widths with model calculations, and effective recombination coefficients.
Solar proton events (SPE) erupt with little prior notice but can last for many days. Their probability of occurrence is strongly related to the 1 l-year solar cycle, with very few events being observed near solar minhnum. Incoherent scatter radar (ISR) observations of the associated ionospheric effects ate fortuitous if the radar happened to be operating during the onset of the event. More often, operations have been instigated on notification of such conditions.
The ability to measure the D-region effects of SPEs with ISRs depends on the strength of the event and the sensitivity and capabilities of the radar. Results concerning effective recombination coefficients in the D-region were obtained by combining electron density measurements with the Chatanika radar and proton flux observations by satellite during the extremely strong SPE of August 1972 which occurred in solar cycle 20 /I/. A similar study /2/ was carried out using EISCAT UHF data from February 1984, during the decliing phase of solar cycle 2 1.
The EISCAT system first became operational in 1981, after the maximum of solar cycle 21. However, it is only during the most recent solar cycle 22 maximum (1989-1991) that strong SPEs have been experienced when the EISCAT system has been capable of providing measurements of incoherent scatter spectra from mesospheric heights. Furthermore, the EISCAT VHF radar, which is more appropriate for such observations by virtue of its lower operating frequency, only became available in 1987. To the best of the authors knowledge, alI the major results in this field in recent years have stemmed from the EISCAT facility. As the next solar mtimum, with its anticipated dearth of events, is now approaching, it is timely to review the advances gained from the EISCAT observations during the past few years.
(3)84 P. N. CoIlis
EISCAT AND ITS APPLICATION TO SOLAR PROTON EVENTS
EISCAT, the European Incoherent SCATter radar facility, is located in the aumral zone in northern Scandinavia /3,4/. It is thus well situated to observe the effects of the proton precipitation in the northern polar cap and aurora1 zone during SPEs. The available data fall naturally into two categories: power profile observations of D-region electron density with a height resolution of -3 km during regular ionospheric experiments, and mesospheric spectral measurements with -1 km height resolution from a dedicated experiment mode which was operated specifically for SPEs. No details of these experiment schemes arc given hem, but brief descriptions can be found in /5/.
The spectral observations have more potential than the power profile measurements for scientific studies since they anyway yield backscattered power, from which the electmn density may be derived, but they additionally provide aeronomic information /6/. The Doppler shift of the spectrum is a measure of the line- of-sight ion drift, which is generally assumed to be a tracer of the neutral wind at these heights due to the large ion-neutral collision frequency. For antema directions away from the vertical the Doppler shift is governed almost entirely by the horizontal component of the wind, as this is normally much greater than the vertical component. The spectral width is a function of several parameters, most notably neutral temperature and density, positive ion mass and density of negative ions. One area of promise in this field is the comparison of the observed spectra with those predicted from ion-chemical models /7,8/. Independent information on one or more of these parameters from other ground-based or rocket-borne instruments is especia.lly valuable in interpreting the radar data. In most cases, however, the models used are relatively simple and only gross effects, such as the changes in negative ion density through twilight, can be interpreted easily.
RESULTS FROM ELECTRON DENSITY MEASUREMENTS
Long-term measurements have shown that it is extremely rare for the EISCAT radars to observe ionisation caused by energetic electron precipitation below about 70 km during normal auroral disturbances. Thus SPEs offer the only possibility of using the ISR technique to probe altitudes in the middle and lower mesosphere.
x 1030 -1032 UT "x x 0 1144-1146 UT
x x xx
log Ne (m-3)
Fig. 1. Two profiles of electron density from different antenna elevation angles near local noon during the EISCAT UHF operation on 20 Ott 1989, from /5/.
500 9.6 10.0 104 108
log Ne (rnT3)
Fig. 2. Night-time profiles of electron density from the EISCAT UHF operations during the evenings of 30 Nov and 1 Dee 1989, from /S/.
High-Latitude D-Region During SOIU Proton EWTB (?)SS
Figure 1 shows two examples of electron density profiles taken by the UHF radar near local noon on 20 Ott 1989 when a particularly strong SPE of _ 24opaDl
pfu was in progress. The unit pfu is proton flux unit, expressed as the integral proton flux cm s- si exceeding some threshold energy, in this case 10 MeV. The two profiles am separated by 74 minutes, but am virtually identical, indicating the constancy of these events. The profiles are even slightly separated in the horizontal direction. as the antenna elevation was lowered for the later one to obtain better definition of the lower boundary of the ionosphere near 50 km at this time.
The large electron densities observed below about 75 km during SPEs in daytime decrease significantly at night, even though the proton precipitation may continue, due to electron attachment onto neutrals to form negative ions. Figure 2 displays two night-time profiles of electron density from 30 Nov and 1 Dee 1989. The proton event was more intense on 1 Dee than on 30 Nov. as revealed by the larger electron densities on that day. The night-time densities on 1 Dee above 80 km were the same as the daytime ones in Figure 1, but with a steep gradient below this height where negative ions formed in the absence of solar illumination.
The quantity h, the ratio of negative ion number density to electron number density, is a widely-used parameter in D-region aeronomy. A very simple determination of 5 may be obtained by comparing Ne profiles during sunlit and dark conditions when the proton flux is uniform, and assuming that the number of electrons lost is equal to the number of negative ions formed. However, it is known that them already exist some negative ions during daytime, and since they cannot be measured with this method a mom refined approach is to assume a model h profile during the daytime N, measurement. Figure 3 shows results from both these approaches, as well as the assumed daytime h pmfrle. It should be noted that mom accurate determinations of h can be gained from analyses of spectral measurements, as shown in mom detail later.
Fig. 3. Broken lines: profiles of differences between daytime and
\ . nighttime electron density, +,._ normal&i to the nighttime values, for the periods before z and after sunset on 23 Ott (+) y and before and after sunrise on 2 70- 25 Ott (o), 1989. Full circles $ (Day): assumed daytime profile z of k (from Ulwick, 1973). Full lines (Night): deduced nighttime profiles of h for 23 (+) and 25 (0) Ott, from /5/.
60' 10-l IO0 IO' 102
The solar control of negative ion densities in the lower D-region during SPEs was suspected already from early riometer observations in the late 1950s, and evidence of asymmetric behaviour of radio absorption at sunset and sunrise was also observed then, eg. P/. Figure 4 illustrates the variation of electron density at selected D-region heights over a 2-day period. Day/night differences am obvious only below about 75 km; higher altitudes am dominated by irmgular changes due to electron precipitation.
The progressive decrease of electmn density at sunset, starting first at the lower altitudes, is shown in detail in Figure 5. Careful analysis of these data indicates that the time of the commencement of the N depletion at a given he