Latitude and solar-cycle patterns in the response of the ionosphere F2-layer to geomagnetic activity

Adv. Space Res. Vol. 20, No. 9, pp. 1689-1692, 1997 Q1997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Greet Britain 0273-1177/97S17.00+0.00 PII: SO273-1177(97)00573-5

Pergamon

LATITUDE AND SOLAR-CYCLE PATTERNS IN THE RESPONSE OF THE IONOSPHERE F2-LAYER TO GEOMAGNETIC ACTIVITY H. Rishbeth and P. R. Field Department of Physics and Astronomy, University of Southampton, Southampton SO17 IBJ, U.K.

ABSTRACT The M-layer response to geomagnetic activity is studied using the daily mean and local-time variations of the (disturbed/quiet) ratio of the peak electron density NmF2. Data for 53 stations are binned by local time, season, and geomagnetic activity. The mean level has a well-defined variation with latitude, attributed to neutral composition changes caused by a global-scale storm circulation. There are systematic differences in the local-time average of the (disturbed/quiet) ratio of NmF2 between three solar cycles, which may indicate long-term changes in thermospheric composition that possibly arise from different levels of solar activity in the three cycles. Comparison with storm simulations using a coupled thermosphere-ionosphere-plasmasphere model reinforce the conclusion that the principal FZ-layer storm effects are due to composition changes. 01997 COSPAR.Publishedby Elsevier Science Ltd. INTRODUCTION Studies of how magnetic storms affect the F-layer (Appleton and Piggott, 1952, Martyn, 1953, Matsushita, 1959) led to a statistical picture of a typical F-layer storm with (i) an initial phase with enhanced peak electron density NrnF2 (or critical frequencyfiF2) lasting a few hours after the start of the storm, which may coincide with a geomagnetic sudden storm commencement (SSC); (ii) a main phase lasting a day or so, in which jWrrF2is usually depressed from its quiet-day value (negative storm) but sometimes enhanced (positive storm); and (iii) a recovery phase. Positive storms are more prevalent in winter at mid-latitudes and at all seasons in low latitudes. Although individual storms vary greatly in their behaviour, this idealized picture is widely used. Seaton (1956) suggested that negative storm effects are caused by changes in the atomic/molecular concentration ratio of the neutral air. These changes, which alter the balance between electron production and loss, are now attributed to a global circulation in the thermosphere. This storm circulation, which is superimposed on the quiet-day solar-driven circulation, is driven by high-latitude energy inputs during the active phase of a magnetic storm (Duncan, 1969, and many others). This idea was eventually verified by theoretical modelling (Burns et al., 1989, Fuller-Rowe11 et al., 1994). Other processes such as electrodynamic drifts and neutral air winds contribute to the complexity of FZlayer storms; see the review by Priilss (1995). This paper summarizes the results of an analysis of F2-layer ionosonde data (Field and Rishbeth, 1997) and computer modelling using a global coupled thermospheric model (Field et al., 1997). Both works support the ‘composition change’ theory of F2-layer storm effects, which may be discussed in terms of the steady-state equation for the peak electron density hW2 Pn(NinF2) = 1.3 Pn[O] - 0.7 Pn[Nj + C

(1)

in which C is proportional to the solar ionizing flux (Rishbeth, 1986). Field and Rishbeth (1997) showed that the storm(*)-to-quiet ratio of NmF2 closely approximates the corresponding ratio of 0 and N2 gas concentrations (NmF2*INmF2) = [O/NJ*/[O/NJ

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AC/DC DATA ANALYSIS The so-called AC/DC analysis, introduced by Wrenn et al. (1987) and Rodger et al. (1989), separates the changes of i’VmF2 into a mean (DC) component, averaged over local time (LT), and a local time (AC) component. These components bear some relation to the stormtime variations (Dst) and disturbance daily variations (SD) used by Martyn (1953) and Matsushita (1959), except that stormtime ST (the time elapsed from the SSC) is not taken into account in the analysis, so evolutionary behaviour is not studied. The iv’mF2 data are taken from the period 1957-1990, which includes three sunspot minima and maxima. Most of the 53 stations (listed by Field and Rishbeth, 1997) have >20 years of data, though it was necessary to use some southern stations with shorter runs. For each station, Pn(NmF2*lNmF2) is computed for each LT (00-23) and for each 2-month period (Jan-Feb, Mar-Apr, . . . ), taking Ap C 7 and Ap > 30 to represent quiet and disturbed conditions respectively. All such values of NmF2 are used, no attempt being made to separate data from the initial, main or recovery phases of a storm (because, although NmF2 behaves differently during the initial phase, this phase represents only a small percentage of the data). The values of Pn(NmF2*lNmF2) are smoothed with a 7-hour centred moving window to reduce transient features, the period of 7 hours being chosen because it is less than the typical response time of the thermosphere to magnetic activity (Wrenn et al., 1987). From the smoothed curve are computed the mean DC component R , the AC amplitude fi , and the local time t^ at which IVmF2 attains its maximum value, as in the equation Pn(NmF2*lNmF2)

=

R

+ R .fit - i)

(3)

The functionfir - t^) is empirical, with zero mean value and a maximum at f = i , which is not always welldefined. It varies in shape from one season or station to another. The DC variations vary quite systematically with latitude and season, much as expected from the old storm studies. In low latitudes and winter midlatitudes, the DC component is generally positive, +O.l to +0.2 (i.e. NmF2* exceeds NmF2 by lo-20%); in summer midlatitudes it is negative by a similar amount; and at equinox, its latitude variation is flat and slightly positive between magnetic latitudes i-30”, but negative at higher latitudes. Above 50-60” latitude, the DC component is influenced by the migration of the main ionospheric trough. The AC amplitude is typically 0.2 below latitudes f40”, but larger at higher latitudes. The AC component generally peaks at 15-20 LT at midlatitudes, but at 00-06 LT at low magnetic latitudes. No obvious systematic variations with longitude are seen in these components. Field and Rishbeth (1997) find that the DC results agree with what would be expected from the composition change theory of FZlayer storms. Moreover, the DC values of On(NmF2*/NmF2) agree well with DC values of Pn([OINj*/[O/N,]) taken from the MSISE-90 model (Hedin, 1991) and with the modelling results from the CTIP (below). The interpretation of the AC component is more complicated; it probably depends on winds and the accompanying horizontal advection, and at low latitudes on electrodynamic drifts. Appreciable differences exist between the mean DC levels of Pn(NmF2*/NmF2) for the three 11-year periods 1957-67, 1968-78 and 1979-89. They might be related to different distributions of F10.7 in the periods (there being no significant differences in the distributions of Ap); the IGY/IQSY period 1957-67 has the largest proportion of values of F10.7 > 200, and the 1967-78 period the smallest. This may imply some long-term conditioning of the thermosphere, which causes the response to a given level of Ap to depend on past history, possibly related to long-term changes in the thermospheric circulation and hence in neutral gas composition. STORM MODELLING WITH THE GLOBAL CTIP MODEL The coupled thermosphere-ionosphere-plasmasphere model CTIP (Millward et al., 1996) was used to simulate storm changes at equinox and at solstice. The grid size is 2” meridionally, 18” zonally, 1 scale height vertically, with 1 min time step. After the computation has settled down for 10 days with quiet conditions (Kp 2), the simulated storm is started at ‘zero stormtime’ (0 ST), assumed to occur at 12 UT, with imposed high latitude energy inputs (particle precipitation and electric fields) for a fairly severe Kp 5 storm, for 12 hours (O-12 ST), followed by a 60-hour recovery period at Kp 2 (12-72 ST), all at moderate solar activity (F10.7 = 100).

Latitude and Solar Cycle Fields

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+lO 0 i-10 %

+lO

change

0 -10 -25:-40

+lO 0 -10 +lO 0 -10 +lO-

t

l

,

20

0

,

40

0

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60

I

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I

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Stormtime ST (hours) Fig. 1. Percentage changes in NmF2 (full curves) and [O/N,] ratio (dashed curves), computed from the CTIP coupled model for stormtime O-72 hours, at nine zones of magnetic latitude as shown. Left: December solstice, geographic longitude 0” (European/African sector). Right: June solstice, geographic longitude 126’ (Asian/Australian sector). Black dots show local midnight. 1 tests the agreement of the CTIP simulations with the composition change theory. It compares the computed variations of (MnF2*/MuF2) (full curves) with those of neutral gas concentration ratio [O/NJ (dashed curves) throughout the storm, in nine latitude zones. There are big fluctuations early in the storm, when the neutral air winds and electromagnetic drifts are changing rapidly, but subsequently the NmF2 and composition curves generally follow each other quite closely. The negative storm effects at summer midlatitudes are clearly visible, with weaker positive storm effects in winter. In the longitude 0” sector (left), high southern magnetic Figure

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latitudes occur at even higher geographic latitudes; the same applies to high northern latitudes in the longitude 126” sector (right), but in this sector the high southern magnetic latitudes correspond to more moderate geographic latitudes, which must be taken into account in detailed study of the CTIP results. Some other conclusions drawn from the CTIP simulations are as follows: *

* * * * * *

Latitude variations of DC and AC amplitudes broadly follow those given by the AC/DC data analysis; AC phase shows some consistency in UT as well as in LT; (Storm - quiet) differences of height hmF2 become small after stormtime 20 ST approximately, apart from some LT-dependent perturbations; (Storm - quiet) differences of neutral gas temperatures peak at 12-15 ST at latitudes below f55” (earlier at high latitudes) and recover to normal after about 60 ST; Meridional wind speeds fluctuate considerably during the first day of the storm, returning almost to normal after 24 ST; The results at low latitudes require further study, with more detailed modelling of electromagnetic drifts; The variations in sub-aurora1 latitudes are affected by migration of the main ionospheric trough.

CONCLUSIONS The AC/DC analysis, originally used to describe the clear-cut storm effects of the South Atlantic/Antarctic sector, has been applied to worldwide data to yield results which broadly tit the predictions of the storm circulation theory. The great importance of neutral composition changes in F-layer storms is confirmed by the results of global modelling using a coupled thermosphere-ionosphere-plasmasphere model. It is therefore concluded that neutral composition changes, produced by the global thermospheric storm circulation driven by high latitude energy inputs, are responsible for the main features of F-layer storm behaviour at midlatitudes. ACKNOWLEDGMENTS This work was supported by the UK Particle Physics and Astronomy Research Council, and used the CRAYYMP system at the Rutherford Appleton Laboratory. We thank M. Mendillo for helpful discussions. We are grateful to the ionospheric observatories and to WDC-A for Solar Terrestrial Physics for their work in collecting the critical frequency data, taken from the NGDC Combined WDC Ionospheric Digital Database CD-ROM, issued July 1994. The CTIP is a collaborative project between Space Physics Laboratory, University College London, Dept. of Statistics and Mathematics, Sheffield University, and NOAA Space Environment Laboratory, Boulder, Colorado. We thank the ionospheric observatories whose data made this work possible. REFERENCES Appleton, E.V. and W.R.Piggott, J. Atmos. Terr. Phys., 2, 236 (1952). Bums, A.G., T.L.Killeen, G.Crowley, B.A.Emery, and R.G.Roble, J. Geophys. Res., 94, 16961 (1989). Duncan, R.A., J. Atmos. Terr. Phys., 31, 59 (1969). Field, P.R. and H.Rishbeth, J. Atmos. Solar-Terr. Phys., 59, 163 (1997). Field, P.R., H.Rishbeth, R.J.Moffett, D.W.Idenden, T.J.Fuller-Rowe11 et al., submitted to J. Atmos. Solar-Terr. Phys., (1997).

Fuller-Rowell, T.J., M .V .Codrescu, R.J.Moffett, and SQuegan, J. Geophys. Res., 99, 3893 (1994). Fuller-Rowell, T.J., M.V.Codrescu, H.Rishbeth, R.J.Moffett, and S.Quegan, J. Geophys. Res., 101, 2343 (1996). Hedin, A.E., J. Geophys. Res., 96, 1159 (1991). Martyn, D.F., Proc. Roy. Sot. Lord, A218, 1 (1953). Matsushita, S., J. Geophys. Res., 64, 305 (1959). Millward, G.H., R.J.Moffett, SQuegan, and T.J.Fuller-Rowell, in STEPHandbook, edited by R.W.Schunk, pp.239-280, Utah State University (1996). Prolss, G.W., Handbook of Atmospheric Electrodynamics, Vol. 2, pp. 195-248, CRC Press, Boca Raton (1995). Rishbeth, H., J. Atmos. Terr. Phys., 48, 511 (1986). Rodger, A.S., G.L.Wremr, and H.Rishbeth, J. Atmos. Terr. Phys., 51, 851 (1989). Seaton, M.J., J. Atmos. Terr. Phys., 8, 122 (1956). Wrenn, G.L., A.S.Rodger, and H.Rishbeth, J. Atmos. Terr. Phys., 49, 901 (1987).