MESOSPHERIC AND LOWER THERMOSPHERIC WINDSAT MIDDLE EUROPE AND NORTHERN SCANDINAVIA

DURING THE LEONID 1999 METEOR STORM

WERNER SINGER AND PETER HOFFMANN

Leibniz-Institut für Atmosphärenphysik, Schloss-Str. 6,18225 Kühlungsborn, GermanyE-mail: singer@iap-kborn.de

NICHOLAS J. MITCHELLDep. of Physics, University of Wales, Aberystwyth,

Ceredigion SY23 3BZ, Wales, UK

and

CHRISTOPH JACOBIInstitut für Meteorologie, Universität Leipzig, 04103 Leipzig, Germany

(Received 5 July 2000; Accepted 30 August 2000)

Abstract. We report observations of winds in the mesosphere and lowerthermosphere during the Leonid meteor storm of November 17/18, 1999. Theobservations were obtained at five radar sites in Middle Europe and NorthernScandinavia using meteor radars in Germany and Northern Sweden, MediumFrequency (MF) radars in Germany and Northern Norway and Low Frequency(LF) wind measurements in Germany. We present hourly means of zonal andmeridional winds covering the altitude range 82 km to 106 km. At mid-latitudes(52°- 54°N) we observe strong eastward and southward directed winds during thestorm phase of the Leonid shower in the early morning hours of November 18whereas eastward and northward directed winds are dominating at high latitudes(67°- 69°N). Strong semidiurnal and weaker diurnal tidal oscillations are observedin the wind field at both latitudes at altitudes above 90 km.

Key Words: Leonids 1999, Lower thermosphere, mesosphere, meteors, winds

1. Introduction

The Leonid meteor storm is caused by meteoroid streams associatedwith the comet Temple-Tuttle. Meteoroid streams with high dust fluxrates generate meteor storms with spectacular displays of numerouspersistent trails. The knowledge of the background wind field at

Earth, Moon and Planets82–83: 565–574, 2000.c©2000Kluwer Academic Publishers. Printed in the Netherlands.

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mesospheric/lower thermospheric altitudes can contribute to improvethe understanding of trail motions and to separate between intrinsicmotion of the trail and motion of the heated gas. Ground-based windmeasurements by MF and meteor radars allow a continuous andreliable monitoring of the upper atmospheric wind field in the altituderange from 70 to about 100 km and can provide appropriate data.After presenting the radar systems, we discuss our observationalresults obtained during the main phase of the 1999 Leonid meteorstorm.

2. Observation sites and radar experiments

Radar observations of the mesospheric and lower thermospheric windfield were carried out during the 1999 Leonid meteor storm inGermany at Juliusruh (meteor and MF radar) and Collm (LF windprofiler) as well as in Northern Sweden at Kiruna (meteor radar) andin Northern Norway at Andenes (MF radar). The observations atMiddle Europe are about 1600 km apart from those at NorthernScandinavia.

We will discuss in more detail the meteor radar experiments asthese systems provide besides the winds also height-dependentmeteor flux rates. The meteor radar of the Leibniz-Institute ofAtmospheric Physics is located at Juliusruh on the island Rügen,Germany (54°38'N, 13°24'E). It is a commercially produced all-skyinterferometric meteor radar ("SkiYMet") operating at a frequency of32.55 MHz and a peak envelope power of 12 kW. The radar uses asingle crossed-element antenna on transmission and an interferometeron reception consisting of five crossed-element antennas (Hocking etal., 2000). The antenna system provides a near uniform angulardistribution of detections. A range accuracy of 2 km and angularaccuracy of 1 to 2 deg in meteor location were possible. The accuracyof the estimated mean winds is generally in the order of 5 to 7 m/s(Hocking and Thayaparan, 1997). In the Leonid experiment theobjective was to locate as many meteors as possible as well as todetermine entrance velocities. A two-point coherent integration wasused in order to optimise the time resolution for entrance speedestimation, resulting in a sample resolution of 0.94 ms at a pulserepetition frequency of 2144 Hz. The radar was configured for a cut-off altitude of 120 km during the Leonid shower and was operated inthis mode from November 15, 21:00 UT until November 20, 14:00UT. Before and after that time the radar was operated in theoptimised mode for wind measurements up to 110 km altitude. The

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meteor data have been analysed for the mean winds using 1-hourbins of data at each height of observation. The data are also binnedaccording to height with an approximate vertical resolution of 3 km(Hocking and Thayaparan, 1997).

The University of Wales Aberystwyth meteor radar is located atEsrange (67°53'N, 21°06'E) near Kiruna in Northern Sweden. Thisradar is a SkiYMet system too. It operates, however, with a peakpower of 6 kW. The radar uses crossed-element antennas to ensure anear uniform azimuthal sensitivity to meteor echoes. During the timeof the Leonid shower the radar was operating in a mode optimised formeteor drift measurements of mesospheric winds and so it did notreliably record meteors at heights greater than 110 km. The same dataanalysis has been applied as for the Juliusruh radar.

The MF radars are operated by the Leibniz-Institute of AtmosphericPhysics at Juliusruh on 3.18 MHz and at Andenes, Norway (69°18’N,16°01’E) on 1.98 MHz (Kremp et al., 1999; Singer et al., 1997).Both systems apply the spaced antenna method and horizontal windsare obtained on the basis of the full correlation analysis in an altituderange between 70 km and 92-94 km. Hourly mean height profiles ofhorizontal wind vectors are estimated from samples of 2-5 minutestime resolution and a vertical resolution of 2 km at Juliusruh and 4km at Andenes with an accuracy comparable with the meteor winds.The observations at Juliusruh are restricted to daytime and twilightconditions due to the high night-time noise level at Central Europe.

The LF wind profiler at Collm (52°N, 15°E) performs totalreflection radio wind measurements at oblique incidence using theionospherically reflected sky wave on 177 kHz, 225 kHz and 270kHz (Schminder et al., 1997). The height estimation is done on the177 kHz measuring path. The system applies also the spaced antennamethod. The high radio wave absorption on daytime restricts themeasurements to night-time and twilight conditions. As result of thediurnal course of the reflection height the winds at different altitudes(85-105 km) are measured at different times.

3. Upper mesospheric winds during Leonid 1999 meteor storm

The meteor observations of the Leonid 1999 storm by the meteorradars indicate that the storm maximum occurs in the period 01:00and 04:00 UT of November 18, 1999 as shown in Figure 1 from theobservations of Juliusruh at altitudes between 75 and 120 km. Thepresented hourly meteor rates contain sporadic, shower, and stormmeteors, not only Leonids. The meteor storm activity peaks at

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Juliusruh more precise on 02:08 UT as shown in a detailed study ofthe fine structure of the Leonid 1999 storm at three sites (Singer etal., 2000).

Figure 1. Meteor rates [hour-1] during the Leonid meteor storm 1999observed by meteor radar at Juliusruh, Germany.

Figure 2. Hourly meteor rates during the maximum of the Leonid meteorstorm on November 18, 1999 obtained at Juliusruh (full lines) and Kiruna(dashed lines).

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by the different modes of radar operation and the differences intransmitted power.

In the following we concentrate our study of upper atmosphericwinds to the period between 17 and 18 November as well as to thepeak of the Leonid storm in the morning hours of November 18,1999. The term zonal wind stands for winds directed towards east(positive sign) or towards west (negative sign), the term meridionalwind stands for winds directed towards north (positive sign) ortowards south (negative sign).

TABLE I

SKiYMETJuliusruh54.3°N

MF radarJuliusruh54.3°N

LF profilerCollm52°N

SkiYMETKiruna67.9°N

MF radarAndenes69.3°N

85 kmU0 7.5 2.5 -10.4 -2.3V0 -3.5 -0.1 -6.6 -2.1

U12 / P12 2.9 / (6.3) 1.9 / (8.7) 5.3 / 5.7 9.3 / 8.2U24 / P24 1.8 / (17.7) 2.8 / (8.6) 3.5 / 7.391 km

U0 12.8 6.3 2.8 -5.5V0 -4.9 1.1 -1.5 -0.3

U12 / P12 12.2 / 11.7 1.4 / (11.2) 24.3 / 10.4 13.0 / 9.4U24 / P24 16.0 / 8.0 12.3 / 6.7 9.4 / 11.094 km

U0 15.5 17.5 5.9V0 -4.7 -1.6 -4.4

U12 / P12 30.5 / 10.4 4.4 / 8.8 21.0 / 9.7U24 / P24 14.2 / 6.6 8.6 / 7.8

Explanations (mean winds on Nov. 17-19, 1999):U0[m/s] = prevailing zonal wind, positive towards east;V0[m/s] = prevailing meridional wind, positive towards north;U12 /U24[m/s] = amplitude of the semidiurnal/diurnal tidal wind (zonalcomponent);P12 / P24[UT] = phase of the semidiurnal/diurnal tidal wind (zonalcomponent), defined as time of the occurrence of the eastward windmaximum, values in brackets are uncertain due to low tidal amplitudes.

3.1. UPPER MESOSPHERIC WINDS

The variability of the upper mesospheric wind field in height andtime has been studied for a 48 hour period on 17/18 Novembercentred to the storm maximum. Hourly mean winds are presented

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for the altitudes 85 km, 91 km, and 94 km where both meteor radarsprovide a good data coverage. In addition, the prevailing winds andthe mean diurnal and semidiurnal tidal wind components areestimated for a 3-day period from 17 to 19 November andsummarized in Table I. For clearness, the tidal winds are restricted tothe zonal components, the meridional components not shown hereare of comparable magnitude. The tidal variability is noticeableabove 90 km altitude.

Hourly means of zonal and meridional winds obtained at mid-latitudes by the meteor radar and MF radar at Juliusruh as well as bythe LF profiler at Collm are presented in Figure 3. Wind speeds up to45 m/s are observed during the storm phase. MF (*) and LF (•) windswere included if their altitudes were within ±1 km of meteor windaltitude. The dashed lines represents the tidal fits to the hourly meanvalues. The tidal fits in general reproduce well the measured diurnalvariability except for the meridional component at 91 km during thestorm phase.

Figure 4 depicts the hourly mean winds obtained with the Kirunameteor radar and the Andenes MF radar for the same period withzonal winds in the left panel and meridional winds in the right panel.The MF radar data (+) were selected using the same criteria as atmid-latitudes. Also at high latitudes, the observed diurnal variabilityis generally well reproduced by the tidal fit. The zonal winds vary inthe same way at middle and high latitudes during the storm phasewhereas the meridional winds show an opposite variation.

The hourly mean winds from the different observations are ingeneral agreement. Some of the details show differences, but thezonal winds agreement between meteor and MF radar observations at85 km is good.

3.2. MESOSPHERIC/LOWER THERMOSPHERIC WINDS DURING THESTORM MAXIMUM

Both meteor radars at Juliusruh and Kiruna have detected meteorechoes above 100 km during the activity maximum of the Leonid1999 storm as shown Figure 2. The obtained meteor rates allow theestimation of height profiles up to 106 km altitude. Hourly meanprofiles of the zonal and meridional winds from both sites arepresented in Figures 5 and 6. Zonal winds agree in shape andmagnitude between 90 and 100 km.

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Figure 3. Hourly mean zonal winds (left panel) and meridional winds (rightpanel) after meteor radar (full lines), MF radar (*) and LF drift observations(•) at Middle Europe on 17/18 November 1999. The hatched bar indicatesthe Leonid storm phase. The dashed lines represent tidal fits to the meteorwinds (for details see text).

Figure 4. Hourly mean zonal winds (left panel) and meridional winds (rightpanel) after meteor radar (full lines) and MF radar observations (+) atNorthern Scandinavia on 17/18 November 1999. The hatched bar indicatesthe Leonid storm phase. The dashed lines represent tidal fits to the meteorwinds (for details see text).

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Figure 5. Hourly mean height profiles of zonal wind during the maximumof the Leonid meteor storm on November 18, 1999 obtained by meteorradars at Juliusruh (full lines) and Kiruna (dashed lines). Winds above 100km (thin lines) should be treated cautiously due to possible electric fieldinfluences.

Figure 6. Same as Figure 5 but meridional wind.

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The meridional winds are different below about 95 km with strongnorthward directed winds up to 70 m/s at 85 km altitude at highlatitudes. The winds above 100 km are given for information onlyand should be treated cautiously. At these altitudes the trail motionmay be strongly influenced by electric fields and we can not say howreal the data are.

4. Summary

Observations of winds in the upper mesosphere/lower thermospherehave been carried out at five radar sites in Middle Europe andNorthern Scandinavia during the Leonid meteor storm of November17/18 , 1999 using meteor radars, MF radars, and a LF wind profiler.48-hour records of hourly mean winds collected by meteor radars at54.3°N and 67.9°N are presented for 85, 91 , and 94 km altitude andare supplemented by MF radar observations as well as LF windprofiler data from the same geographical region.

The observations from the different experiments are in generalagreement. Some of the details show differences, but the agreementbetween the zonal winds observed by the meteor and MF radars at 85km is good. Similar results of meteor and MF wind comparisonsbased on a 3-years data set were reported by Hocking andThayaparan (1997).

We observe eastward directed zonal winds of about 40 m/s above90 km in mid- and high latitudes at the beginning of the activitymaximum on November 18, 00 UT. The winds are decreasing andchanging to westerlies between 03 and 04 UT. The meridional windsare of opposite direction at mid- and high latitudes with southwarddirected winds of 40 m/s at mid-latitudes and a reversal of the winddirection is again observed between 03 and 06 UT.

At both latitudes tidal wind oscillations are evident above 90 kmwith a strong semidiurnal tidal wind amplitude between 10 and 30m/s and a diurnal tidal wind amplitude between 10 and 15 m/s.

Acknowledgements

The authors thank J. Weiss, D. Keuer and R. Latteck from theInstitut für Atmosphärenphysik for operating the Skiymet radar atJuliusruh and the MF radars at Juliusruh and Andenes as well as D.Kürschner from the Institut für Geologie und Geophysik der

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Universität Leipzig for performing LF drift observations. W.S. andP.H. express their gratitude to W.K. Hocking for most helpfulsupport in software development for the SKiYMET radar. This studywas partially supported by the Deutsche Forschungsgemeinschaft bycontract BR 2023/1-1. Editorial handling: Mark Fonda.

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