Hapgood MA

Lucek E

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1INTERPLANETARY SCINTILLATION AND SPACE WEATHER MONITORINGMike Hapgood1 and Elizabeth Lucek 21Space Science Department, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX, UK,Fax: +44 1235 55 5848, Email: M.Hapgood@rl.ac.uk2Space & Atmospheric Physics Group, The Blackett Laboratory, Imperial College, Prince Consort Road, London,SW7 2BZ, UK, Fax: +44 171 594 7772, E-mail: E.Lucek@ic.ac.ukABSTRACTInterplanetary scintillation (IPS) is a technique thatallows us to remotely sense the plasma number densityin the inner heliosphere. Thus, using IPS, we can trackdensity structures propagating out from the Sun andshould be able to predict their time of arrival at theEarth. Since these structures carry the enhanced fluxesof momentum and energy that drive geomagneticactivity, IPS measurements are potentially an importantspace weather activity that can help to predict the onsetof geomagnetic activity. However, in practice, IPS hasproved disappointing as a means of predictinggeomagnetic activity. In this paper, we briefly reviewresults obtained from the Cambridge IPS array during1990-93 with a focus on those results that areparticularly relevant to space weather. We also discusssome of the limitations that have made IPS sodisappointing and suggest ways in which theselimitations might be overcome in a future IPS system.INTRODUCTIONThe amplitude of a radio signal passing through theinner heliosphere is modulated by the motion of solar-wind plasma irregularities across the line of sight. Thismodulation, which may be observed with a suitableradio telescope, is termed interplanetary scintillation.The scintillation amplitude, DS, is proportional to theamplitude of the fluctuations in plasma density, butmany workers (e.g. Ref. 2) have suggested that theamplitude of these fluctuations (and hence DS) isproportional to the absolute plasma density, N. Thusmeasurements of DS can be used to monitor N.In this paper we focus on the IPS data that wererecorded during 1990-93 by the 81.5 MHz IPS array atCambridge, UK (Ref. 1). This system made dailymeasurements of DS for a large number of natural radiosources outside the solar system. Each source wasmeasured as it crossed the local meridian plane. In thismanner a daily skymap of scintillation could be built upsimilar to that shown in Figure 1. To detect temporalchanges in DS (and, by implication, changes in theplasma density) the actual skymap shows gI = DSI/<DSI>where DSI is the measured scintillation amplitude forsource I and <DSI> is the expected amplitude for thatsource (based on a long-term average of themeasurements).Figure 1. IPS scintillation levels displayed on a grid ofdeclination and solar hour angle. Black indicates highscintillation levels, light grey indicates low scintillationlevels and white pixels contain no suitable sources. TheIPS event appears as a coherent patch of highscintillation to the left of the Sun (indicated on the zerosolar hour angle line).In the rest of the paper we first discuss the relationshipbetween IPS and geomagnetic activity (as revealed byanalysis of the Cambridge IPS data) - and hence theability of such IPS observations to help in predictingthat activity (which is an important aspect of spaceweather forecasting). We then discuss a number offactors that constrain the use of existing IPS data inspace weather forecasting. We suggest ways in whichthese constraints might be overcome. These suggestionscomplement those already published by Hewish andDuffett-Smith (Ref. 5) - see their section 3.2Note that, given the limited space available, our aimthroughout this paper has been to outline the key issuesconcerning the use of IPS in space weather activities.Wherever possible, we have not discussed technicaldetail but, instead, have given references to publishedpapers where these details are discussed.IPS AND GEOMAGNETIC ACTIVITYFigure 1 shows a good case where the observation of acoherent patch of enhanced scintillation was followedby enhanced magnetic activity (Ref. 9). This is typicalof a number of good cases that can be found in the1990-93 data and in previous IPS studies. For example,Hewish and Duffett-Smith (Ref. 5), using data from a14-month period near the peak of Solar Cycle 21,reported that all 16 storms (peak Ap ³ 40) during thatperiod were preceded (1 to 7 days) by observations ofenhanced scintillation. These results, together with theexample shown in Figure 1, are suggestive of arelationship between enhanced scintillation andgeomagnetic activity. However, much further work isrequired to determine a substantive relationship suchthat we can use IPS as a predictor of geomagneticactivity. In particular two issues need attention:1. Are there observations of enhanced scintillationthat are not followed by enhanced geomagneticactivity? That is, IPS events that would yield falsealerts of geomagnetic storms. We shall show belowthat such events are common.2. Is there a statistically significant correlationbetween enhanced scintillation and geomagneticactivity?These issues have been considered in a number ofpapers. For example, Lucek and Clark (Ref. 7) manuallyexamined a series of 1235 daily skymaps taken by theCambridge array. They used each skymap, together withmaps recorded on previous days, to predict geomagneticactivity on that day or the two following days. Theseforecasts were considered successful if either (a) asudden storm commencement or a sudden impulse, or(b) geomagnetic activity with Ap ³ 30, were observedduring the period covered by the prediction. Even withthese broad criteria for success they found that therewas a very high rate of false alarms, which occurredabout twice as frequently as real events. Given this highlevel of false alarms, they concluded that IPS datacannot be used alone to produce reliable forecasts ofgeomagnetic activity. Lucek and Clark also noted thatthe temporal and spatial resolution of the data wasinsufficient to allow estimation of the speed anddirection of features seen in skymaps. This may accountfor a substantial number of the false alarms.Another approach to these issues is to find a morequantitative method of analysing sky maps. Severalworkers have devised IPS indices that are sensitive tosystematic changes in skymaps. One such index is theI35 index devised by Harrison et al. (Ref. 4), which iscalculated for selected portions of a skymap, e.g. allsources in a given elongation range from the Sun. TheI35 index ranges from -1 to +1; positive values indicate asystematic enhancement of scintillation while negativevalues indicate a corresponding reduction. Harrison et alused this index to show that the correlation between IPSand the geomagnetic activity index Ap takes the form ofa triangular distribution (see example in Figure 2below). For any value of I35, Ap can take any valuebetween zero and some maximum, where that maximumincreases with increasing I35.Figure 2. Scatter plot of Ap against I35 measured oneday ahead.Hapgood and Harrison (Ref. 3) investigated thistriangular distribution in detail. They showed that (a) I35is well-correlated with the plasma number densitymeasured in the near-Earth solar wind - with the peakcorrelation occurring when the density is measured onthe same day as I35, (b) that the correlation between Apand I35, as shown in Figure 2, is best when Ap ismeasured on the day after I35 was measured, and (c) thescatter of Ap values within the triangular distributionshows a trend such that higher Ap values are associatedwith a more southward interplanetary magnetic field.They concluded that the triangular distribution was theresult of two factors:1. The momentum and energy flux in the solar wind,which sets the maximum possible level ofgeomagnetic activity. This is largely determined bythe density of solar wind, which is correlated withI35. The upper envelope of the triangulardistribution reflects this correlation.2. The coupling between the solar wind and themagnetosphere. This is determined by factors, suchas the orientation of the interplanetary magneticfield (IMF), which are not correlated with I35. Thescatter within the triangular distribution reflects thislack of correlation.3They concluded that IPS can contribute to predictions ofgeomagnetic activity by providing an estimate of themaximum potential activity. However, otherindependent sources of information are required todetermine how much of this potential is realised.A broader study of the correlation between IPS indicesand geomagnetic activity has been undertaken by Lucekand Clark (Ref. 8). They started with the I35 index, asdefined by Harrison et al (Ref. 4), and developedseveral modified versions to try to increase thecorrelation between Ap and the IPS index. They foundthat some of these modified indices gave small butstatistically significant improvements in the correlation.They emphasised the importance of comparing Ap withthe IPS index measured one or more days earlier - inorder to provide a basis for predicting Ap. However, inthis case the maximum correlation coefficient betweenAp and the IPS index was about 0.45. This left 80% ofthe variance in Ap unexplained by the correlation. Thusthey concluded that the measured correlation was wellbelow the value that would allow IPS data to act as areliable predictor for geomagnetic activity.LIMITATIONS OF THE IPS TECHNIQUEOur experience of using the IPS data has allowed us toidentify a number of factors that can limit the use ofthese data to predict geomagnetic activity:Density structures that miss the Earth. The IPStechnique is best suited to detecting density structures atsome elongation from the direction of the Sun. (Theelongation depends on the frequency of the telescopeused to collect IPS data. For the Cambridge array it iselongations of 30° to 110°.) Thus IPS detects structuresthat are propagating at an angle to the Earth-Sun line.However, these structures are thought to have angularextents of order 45° as seen from the Sun. Thus many,but not all, structures detected by IPS will intercept theEarth. However, we should expect that a significantfraction of the density structures detected by IPS willmiss the Earth. Thus this factor leads to a class of IPSevents that will not be associated with enhancedgeomagnetic activity. However, it is possible thatfurther analysis of skymaps may help us to estimatewhich density structures will intercept the Earth. This isan important topic for future IPS studies. In particular, itwould be interesting to compare IPS observations ofdensity structures with spacecraft observations ofcoronal mass ejections (CMEs) - such as those made bythe LASCO instrument on SOHO. Unfortunately, as yet,there is little or no overlap between IPS datasets andspacecraft observations of CMEs.IMF orientation. It is very well known that this factor,in particular the IMF north-south component Bz,controls the flow of energy and momentum from thesolar wind into the magnetosphere. Thus only densitystructures associated with southward IMF will beeffective in generating geomagnetic activity. Since IPSdetects density structures but has no correlation with Bz(Ref. 3), we may expect that a significant fraction of IPSevents will not be associated with enhancedgeomagnetic activity. In any prediction scheme basedon IPS data, the IMF orientation is an additionalindependent factor that must be taken into account.Ionospheric scintillation. The motion of plasmairregularities in the Earth's ionosphere is also a source ofscintillation in natural radio signals. This ionosphericscintillation is an additional signal that can confuse ourinterpretation of IPS results. Unfortunately, ionosphericscintillation can be triggered by enhanced geomagneticactivity - and therefore is likely to occur in conjunctionwith the IPS events that could help to predict enhancedgeomagnetic activity. Several papers have reportedapparent enhancements of IPS that may actually beionospheric scintillation. For example, Lucek and Clark(Ref. 8) found that the highest correlation betweenscintillation levels and Ap occurred between IPS data inthe 90° to 110° elongation band and the Ap valuerecorded the same day (rather than the IPS index beingmeasured one day before Ap). They attributed thismarked increase to the effects of ionosphericscintillation. Lucek and Rodger (Ref. 6) made a moredetailed study of the effect of ionospheric scintillation.They used data from the ionosonde at Slough, 90 kmsouth-west of Cambridge, to identify days when therewere plasma irregularities in the ionosphere. Byexcluding such days they were able to obtain asignificant increase in the correlation between Ap andIPS indices measured one day before Ap. Theyconcluded that contamination by ionosphericscintillation is a major problem in the interpretation ofIPS data. However, there could be methods to monitorand reduce the impact of ionospheric scintillation onIPS. Their development is an important issue for thefuture development of IPS as a practical tool for spaceweather activities.Seasonal effects. As noted above, IPS detects densitystructures when they lie at an angle to the Earth-Sunline, i.e. within a cone of solid angle centred on the Sun.However, only a fraction of that solid angle is viewablefrom a particular site on a particular day. This fractionexhibits marked seasonal variations. This is illustratedin the table below that shows the solid angle, between30° and 90° from the Sun, which, in principle, could beviewed from Cambridge (52° N) in different seasons.We also show this as a fraction of the total solid angle inthat cone (5.4 steradians). These figures assume that IPScan be measured down to a minimum elevation of 5°above the local horizon.4Season Solid angle - steradians FractionSummer 4.7 86%Equinox 4.0 74%Winter 3.6 66%These figures show that, even for an optimum system,there are significant variations in the solid angle (andhence the volume of the heliosphere) sampled each day.As one would expect, this sampling is worse in winter.At a technical level the solution to this samplingproblem would be straightforward. We would simplyrequire an additional IPS array in the southernhemisphere, so that the seasonal variations at the twosites will cancel out. Of course, the implementation ofthis solution poses significant financial and operationalproblems.Non-optimum sampling The Cambridge IPS array wasoperated as part of a radio astronomy observatory andthus, quite reasonably, was optimised for astronomicalapplications. In particular, the array sampled radiosources only over a limited range of declination (-8° to+ 75°) and only when those declinations were south ofthe celestial pole. This further reduced the solid anglethat was viewed as shown in the table below (using thesame format as the table above).Season Solid angle - steradians FractionSummer 3.3 61%Equinox 2.9 53%Winter 2.6 48%In winter the Cambridge IPS array sampled less thanhalf of the solid angle in which we may expect to IPSevents. Thus there is a considerable possibility thatmany IPS events were not observed by the array.SUMMARYIf a future IPS system were built specifically for spaceweather applications, it would be important to optimiseits design for that application. For example:1. Use of several arrays giving: improved coverage ofinner heliosphere, reduced seasonal variations inthat coverage, better time resolution, cross-comparison to identify sources of interference andimprove statistics.2. Sampling over a greater range of declination toimprove coverage. However, sampling at lowerelevations may bring the risk of increasedinterference from man-made sources. This might beless of an issue if future IPS observations weremade at remote locations with low levels of man-made radio noise.3. Observations on several different meridians by eacharray giving better time resolution.4. Observations over a range of frequencies giving:observations over a wider range of elongations,overlap between elongation ranges at differentfrequencies to help identify interference, use ofhigher frequencies less susceptible to ionosphericinterference.ACKNOWLEDGEMENTSThe 1990-93 operation of the Cambridge IPS arrayfollowed a major refurbishment supported jointly byMullard Radio Astronomy Observatory (MRAO),Rutherford Appleton Laboratory, British AntarcticSurvey and the Space Environment Laboratory. Theefforts of these four groups are gratefullyacknowledged. We also thank the MRAO for theoperation of the array and the provision of the data.REFERENCES1. Duffett-Smith. P.J., A. Purvis and A. Hewish, "Astatistical study of faint radio sources at 81.5 MHz",Mon. Not. R. Astr. Soc. 190, 891-901, 1980.2. Erskine, F.T., W.M. Cronyn, S.D. Shawhan, E.C.Roelof and B.L. Gotwols, "Interplanetary scintillation atlarge elongation angles: response to solar wind densitystructure", J. Geophys. Res. 83, 4153-4164, 1978.3. Hapgood, M. and R. Harrison, "On the potential ofinterplanetary scintillation for predicting geomagneticactivity", Geophys. Res. Lett. 21, 637-640, 1994.4. Harrison, R.A., M.A. Hapgood, V. Moore, and E.A.Lucek,, "An interplanetary scintillation index", Ann.Geophysicae 10, 519-526, 1992.5. Hewish, A. and P.J. Duffett-Smith, "A new method offorecasting geomagnetic activity and proton showers",Planet. Space Sci. 35, 487-491, 1987.6. Lucek, E.A. and A.S. Rodger, "The use of spread Fand sporadic E observations in interplanetaryscintillation forecasts of geomagnetic activity", Ann.Geophysicae 13, 1237-1239, 1995.7. Lucek, E.A. and T.D.G. Clark, "The use ofinterplanetary scintillation maps in the prediction ofgeomagnetic activity", Ann. Geophysicae 14, 131-138,1996.8. Lucek, E.A. T.D.G. Clark and V. Moore, "The use ofvarious interplanetary scintillation indices withingeomagnetic forecasts", Ann. Geophysicae 14, 139-148,1996.9. Moore, V., E.A. Lucek, D.G. Sime and R.A.Harrison, "The study of the solar wind usinginterplanetary scintillation observations from theMRAO, Cambridge, array", J. Br. Int. Soc. 47, 164-168,1994.

Interplanetary Scintillation and Space Weather Monitoring

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