“WHERE IS THE TURBOPAUSE?” ROCKET CAMPAIGN: OVERVIEW AND FIRST RESULTS

Gerald A. Lehmacher(1), Miguel F. Larsen(1), Sven Bilén(2), Charles L. Croskey(2) John D. Mitchell(2), Franz-Josef Lübken(3), Markus Rapp(3), Richard Collins(4)

(1)Clemson University, Clemson, SC 29634-0978, USA, Email: glehmac@clemson.edu (2)The Pennsylvania State University, University Park, PA 16802-1401, USA, Email sbilen@psu.edu

(3)Leibniz-Institute for Atmospheric Physics, 18225 Kühlungsborn, Germany, Email: luebken@iap-kborn.de (4)University of Alaska, Geophysical Institute, Fairbanks, AK 99775-7320, Email: rlc@gi.alaska.edu

ABSTRACT

The rocket experiment “Where is the Turbopause?” consisted of a salvo of four rockets launched from Poker Flat, Alaska in the night of February 17-18, 2009. The objective was to study enhanced diffusion in the region near and above 100 km, triggered by neutral dynamics, and often observed by chemical release experiments. Besides deploying eight trimethyl aluminium (TMA) trails, from upleg and downleg from each payload, we measured electron and neutral density and fluctuations to detect and quantify turbulent layers by another well-established way. The rocket launches were triggered by small-scale wave activity in the sodium layer and span a period of 2 ½ hours in the same atmospheric volume to study the temporal development of turbulent layers. We present an overview of the experiment, the atmospheric and geophysical conditions, flight performance, and preliminary data. 1. INTRODUCTION

On February 18, 2007, a salvo of four sounding rockets was launched from Poker Flat Research Range, Alaska (65.13 N, 147.49 W) to investigate small-scale neutral dynamics and enhanced diffusion in the upper mesosphere and lower thermosphere, with particular interest in the region above 100 km. The NASA missions 41.077, 41.078, 41.079, and 41.076 were launched in quiet post-magnetic midnight conditions at 08:59, 09:29, 09:59, and 10:49 UT, respectively. All four payloads released chemoluminescent trimethyl aluminum (TMA) trails on upleg and downleg, the second and third payloads carried in addition a deployable instrumented payload with electron nosetip probe forward and a CONE ionization gauge as neutral and electron sensor in the aft. 2. ATMOSPHERIC AND GEOPHYSICAL

CONDITIONS

The launches took place in late winter during a brief period of clear skies over interior Alaska. An arctic cold front, that had brought a few inches of snow, was receding, while a new winter storm was approaching Alaska from the southwest. In the sequence of GOES-11 infrared satellite images one could see fast wave fronts being launched from the approaching low

pressure system. The winter jet stream [1] was sharply diverted to the south by a compact anticyclone in the Northern Pacific (Fig. 1).

The stratosphere was disturbed since a major wave 2 event occurred around January 22, 2009. On February 18, the disturbance had descended to the tropopause level, while the upper stratosphere and lower mesosphere had responded with strong cooling [2] and the stratospheric polar vortex at 10 mbar was established over the Eastern Hemisphere. A smaller stratospheric low was present over the Aleutians leaving central Alaska in a region favorable for upward propagating gravity waves. The cold middle atmosphere is well recognizable in the nightly average temperature profile measured by the NICT Poker Flat Rayleigh lidar (Fig. 2) [3]. There was significant upward wave propagation during that night as seen in temperature variations between 40 and 70 km (not shown); the largest observed period was 3-4 hours and a vertical wavelength of 10 km. Based on the dispersion relation for gravity waves, the horizontal wavelength of such waves was probably several 100 km. Around 70 km, the atmosphere was very stable, followed by a small (10 K) mesospheric inversion layer. The synoptic structure may have been a

Figure 1. Northern hemisphere jet stream winds at 300 mbar about 9 hours before the rocket launches.

___________________________________________________________________________________ Proc. ‘19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7–11 June 2009 (ESA SP-671, September 2009)

mesospheric surf zone associated with planetary wave breaking above 70 km and [4].

Wave perturbations are also evident throughout the sodium layer as observed by lidar from 80 to 105 km [5]. The dominant modulation is established by the semidiurnal tide, however, the layer became more disturbed past midnight, especially at the topside, observations which triggered the salvo of four rockets (Fig. 3).

The Poker Flat MF and meteor radar [3] revealed primarily the pattern of the semidiurnal tide, but which appeared more disturbed than during previous nights. The night of 17-18 February 2009 was geomagnetically very quiet (Kp ~1-2), so that MF partial reflections were only received between 80 and 90 km, and only after magnetic midnight. The auroral activity was monitored by several all-sky cameras in Alaska and the 4-channel meridional scanning photometer at Poker Flat. Discrete aurora was visible in the North during the launch period

between 10 and 12 UT. Riometer data are available from Ft. Yukon and Poker Flat. One of the all-sky cameras was equipped to observe the green oxygen line at 557.7 nm and alternating the OH layer through a broadband filter from 715 nm and to ~1000 nm. The emission height of the green line airglow layer is around 95 km and for the OH bands around 87 km. At the beginning of the launches, waves with horizontal wavelengths of ~75 km and Doppler shifted periods of ~20 minutes were traveling from north to south; the direction rotated gradually throughout the night towards east; however, later there were even two wave patterns moving in opposite directions. These waves may have traveled through the ducting regions set up by the larger scale waves and tides.

The Poker Flat Incoherent Scatter Radar (PFISR) [6] supported the sounding rocket mission operating in interleaved D, E, and F region modes and 5 beams. Electron densities for the E and F region were very low, nevertheless the radar observations revealed weak, tilted sporadic E layers around 100 km during the launch period and intermittent ionization patches above (Fig. 4), more so in the northward pointing beams. The absolute densities will help us calibrate the in situ electron measurements. 3. ROCKET MEASUREMENTS

As mentioned above, two payloads each were of identical design. Fig. 5 shows drawings of both; 41.076 and 41.077 carried a 24-inch TMA payload for upleg and downleg releases between ~80 and 120 km altitude; 41.078 and 41.079 had in addition a instrumented payload, that was forward ejected by a spring separation

Figure 2. Nightly average temperature profile from the Rayleigh lidar at Poker Flat.

Figure 4. Poker Flat Incoherent Scatter Radar electron densities (preliminary) from 15 minute integrations for the vertical beam.

Figure 3. Sodium densities (preliminary) at the topside of the sodium layer. The first rocket was launched at 00:59 LST (08:59 UT).

system, thereby exposing the CONE instrument [7]. All four rockets were propelled by Terrier Mk 12-Improved Orion motors. The TMA sections stayed attached to the Orion stages. The nosetip probe was biased with +5 VDC to collect negative charges (in ram during upleg) [8]. The CONE outer electrode was also biased positively with +6 VDC to collect negative charges (in ram on downleg). The CONE filament of the inner ionization gauge was turned on T+105 seconds, when the instrumented payload was well separated from the TMA section and the atmospheric pressure was low enough for a safe turn on. The CONE instrument was given additional 70 seconds until apogee to degas filament and electrodes.

Fig. 6 provides a sketch of the four trajectories. The launch site is marked at the origin. 41.077 was launched at 09:59 UT, then 41.078 at 10:29 UT, then 41.079 at 10:59 UT and last 41.076 at 11:49 UT. Three payloads followed the nominal azimuth of 2 degrees; 41.079 took a more westerly trajectory. The nominal launch elevation was not greater than 77 degrees due to ground safety requirements. The payloads reached apogees of 127.2, 120.6, 125.5, and 128.5 km, respectively, consistently below the nominal value of 138 km. The rectangle marks 100 km and the

trajectories are plotted with a thicker line above that level. TMA trails were released on all four flights each on upleg and downleg. Each trail will provide a wind profile and information on small-scale diffusion [9,10]. Camera sites were located at Poker Flat, Fort Yukon (66.56 N, 145.27 W), Coldfoot (67.25 N, 150.15 W), and Toolik Lake (68.63 N, 149.60 W). Fig. 7 shows an the upleg trail of the first rocket 41.077 as observed from Fort Yukon 5 minutes after launch and Fig. 8 the downleg trail from the same payload 10 minutes after launch. All trails show a superposition of the tidal wind spiral and smaller modulations, presumably from inertia gravity waves. The transition from a smooth trail due molecular diffusion to a structured trail due to instability and turbulent diffusion can easily be seen in Fig. 7. Based on the timing of the release and trajectory knowledge, this transition is between 100 and 110 km; the exact heights and wind profiles will be determined by triangulation of images from the different sites. At lower altitudes smaller scale billows indicate more dynamically active regions.

The instrumented payloads provide good electron profiles on upleg from the nosetip probe in ram. A

Figure 6. Radar trajectories for 41.077 (blue), 41.078 (yellow), 41.079 (red), and 41.076 (green).

Figure 8. Downleg TMA trail for 41.077 observed from Fort Yukon 10 minutes after launch.

Figure 7. Upleg TMA trail for 41.077 observed from Fort Yukon 5 minutes after launch.

Figure 5. Payload layout for missions 41.076 and 41.077 (top) and 41.078 and 41.079 (bottom).

comparison of collected negative charge is shown in Fig. 9 for both flights.

The profiles are similar as can be expected from two flights just 30 minutes and a few kilometers apart. The D region consists of layers with strong density gradients alternating with layers of nearly constant electron density. Near 90 km is a local maximum, possibly connected with the atomic oxygen layer. Special features in both profiles are two sporadic ionization layers just above 90 km and around 100 km, which appear at slightly different altitudes. They may correspond to the weak, tilted layers that PFISR observed during the same time (Fig. 4). The probe data observe narrow zones of very low electron densities just below the sporadic layers, resembling electron biteouts, which may be an indication of small charged particles that the probe cannot sample due to the supersonic flow around the small nosetip. The E region structure above 100 km is remarkably different for the two flights, perhaps a manifestation of changing auroral precipitation. NASA provided the body angle and angle-of-attack for the instrumented payloads based on analysis from magnetometer and infrared horizon sensor. These data reveal that the 41.078 encountered the atmosphere at about 40 degrees angle on upleg, while the angle for 41.079 was only 20 degrees. This could help explain the difference in collected current below 80 km, when the plasma is collisional and geometric effects play a major role. On downleg, both payloads crossed the atmosphere almost perpendicular to the velocity vector; 41.078 experienced also a stronger coning modulation of the angle-of-attack due to differences in the average body angle. This had consequences in the sampling of charge and neutral density by the probes (not shown). The structures observed by the electron probes appear

smeared out in comparison to the upleg profiles, since the probes experienced a mix of ram and wake flow. This flow may also explain the loss of signal below 90 km for the nosetip probe and 80 km for the CONE electron probe, since the payload may have charged more negatively and cancelled the positive bias voltage of the probes. Nevertheless, downleg still profiles still display similar features of a very structured E region above 90 km. More analysis is needed to interpret the downleg plasma density profiles.

The CONE neutral measurements were also affected by the shallow body angle and differences in coning modulation. The small body angle leads to larger angle-of-attack sensitivity for the almost spherical probe [11].

Frequency [Hz]

Altit

ude

[km

]

41.078

0 20 40 60 80 100 120 140 160 180 200

70

80

90

100

110

120

130

Figure 11. Spectrogram of 41.078 upleg nosetip probe density fluctuations for frequencies up to 200 Hz. The blue lines are typical inner scales observed in the upper mesosphere converted to frequencies assuming a rocket speed of 1000 m/s.

Figure 10. Preliminary temperature profiles from the CONE ionization gauges measuring neutral density on downleg. The green line is the MSIS-90E temperature profile.

10!10 10!9 10!8 10!7 10!6

70

80

90

100

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120

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Current 0A2

Altit

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0km

2

41.078 up41.079 up

Figure 9. Profiles of collected negative current on the nosetip probes on upleg.

The effects of the payload attitude on the CONE neutral data were reduced by application of both a comb filter to reduce the roll rate modulation and Butterworth low-pass filtering to reduce the coning motion modulation. Preliminary temperature profiles were then calculated by integration of the corrected densities and are shown in Fig. 10. Both temperature profiles show two local temperature minima above 90 and 100 km, with regions of very stable and nearly unstable temperature gradients. Data below 85 km contain still larger uncertainties. OH spectra will be evaluated for temperatures and will provide an additional checkpoint near 87 km. The objective of the experiment was the measurement of turbulence based on spectral analysis of density fluctuations and the comparison with simultaneous observations of diffusion of the tracer TMA. In Fig. 11, we present a spectrogram of electron density fluctuations from the nosetip probe on flight 41.078. The data have filtered to remove spin modulations at 4.4 Hz and higher harmonics. Wide layers of fluctuations are evident around 70, 80 and above 100 km. The blue lines indicate the inner scale corresponding to typical energy dissipation rates of 10 mW/kg (lower curve) and 100 mW/kg (upper curve) observed in the winter mesosphere. Fluctuations below 100 km seem compatible with earlier observations [12], and energy dissipation rates from spectral fitting can be compared with the fast diffusion of the TMA trails at the same altitude [10]. Enhanced neutral fluctuation layers were also found below 100 km (not shown), in particular between 70 and 80 km, where the electron densities (Fig. 9) show no significant background gradient. Electron density fluctuations above 100 km are of unusually small scale, which may be a sign of plasma instability. Analysis of the TMA billow structure and careful spectral analysis of the neutral and electron density structure is required to answer the question which kind of fluid motion is responsible for the enhanced diffusion observed above 100 km. 4. SUMMARY AND OUTLOOK

We have presented first results from a recent rocket investigation of dynamics and small-scale structure in the mesosphere and lower thermosphere. The launch conditions were monitored by a large array of supporting instrumentation, including radars, lidars, and passive optical instruments. The launch conditions were ideal for our goal: to study enhanced diffusion above 100 km and compare turbulent parameters to be derived from diffusion of the TMS trail versus the observation of density fluctuations. In the future detailed analysis, we hope to characterize the tidal and wave field from the jet stream at 10 km up to the E region at 120 km to understand the multiple regions of

energy and momentum deposition and mean-flow accelerations during that night. In addition, we plan to study the properties of the narrow sporadic E layers and associated biteout structure, as well as plasma irregularities in the E region. 5. ACKNOWLEDGMENTS

This project was funded by NASA grant NNX07AJ99G to Clemson University and associated NASA grants to Penn State University and University of Alaska Fairbanks. The missions were professionally executed by NSROC and NASA teams from GSFC Wallops Flight Facility and the staff at Poker Flat Research Range. We are grateful for the support from Mike Nicolls and Craig Heinselman for PFISR, Don Hampton Brian Lawson, and others for instrument support at PFRR and remote sites. Seji Kawamura from NICT supported the MF and meteor radar. The TMA camera sites were operated by Lamar Durham, Brian Turpin, Lucas Hurd, and Patti Larsen from Clemson University. The CONE instruments were manufactured and prepared by Hans-Jürgen Heckl and Boris Strelnikov at IAP. Shelton Simmons helped with the vacuum calibrations at Clemson. The authors wish to thank also all other individuals that were part of the successful missions and are not mentioned here. 6. REFERENCES

1. http://squall.sfsu.edu/crws/jetstream.html 2. http://www.cpc.ncep.noaa.gov/products/stratosphere/ strat-trop/ 3. http://salmon.nict.go.jp/systemsum/instrument_e.html 4. Sassi, F., R. R. Garcia, B. A. Boville, & H. Liu, On temperature inversions and the mesospheric surf zone, J. Geophys. Res., 107(D19), 4380, 2002. 5. Collins, R.L., Hallinan, T.J., Smith, R.W., & Hernandez, G., Lidar observations of a large high-altitude sporadic Na layer during active aurora. Geophys. Res. Lett., 23 (24), 3655–3658, 1996. 6. http://isr.sri.com/iono/amisr/ 7. Giebeler, J., Lübken, F.-J. & Nägele, M., CONE – a new sensor for in situ observations of neutral and plasma density fluctuations, ESA SP-355, 311-318, 1993. 8. Blood, S. P., J. D. Mitchell, C. L. Croskey, T. D. Raymund, E. V. Thrane, T. A. Blix, U.-P. Hoppe. D. C. Fritts, & F. J. Schmidlin, Studies of high-latitude mesospheric turbulence by radar and rocket. 2: measurements of small-scale turbulence, J. Atmos. Terr. Phys., 50, 963-976, 1988. 9. Larsen, M. F., Winds and shears in the mesosphere and lower thermosphere: Results from four decades of chemical release wind measurements, J. Geophys. Res., 107 (A8), 1215, 2002. 10. Bishop, R. L., M. F. Larsen, J. H. Hecht, A. Z. Liu, & C. S. Gardner, TOMEX: Mesospheric and lower

thermospheric diffusivities and instability layers, J. Geophys. Res., 109, D02S03, 2004. 11. Rapp, M., Gumbel, J., & Lübken, F.-J., Absolute density measurements in the middle atmosphere, Ann. Geophys., 19, 571-580, 2001. 12. Lübken, F.-J., W. Hillert, G. Lehmacher, & U. von Zahn, Experiments revealing small impact of turbulence on the energy budget of the mesosphere and lower thermosphere, J. Geophys. Res., 98, 20,369-20,384, 1993.