J. D. Mitchell

Electrical structure of PMSE and NLC regions during the DROPPS Program

The electrical structure of NLC/PMSE regions was investigated by different rocket-borne in situ probe techniques as part of the DROPPS program. Gerdien condenser measurements of very small mobility values suggest concentrations of positively charged aerosols/dust comparable to the density of more mobile positive ions at PMSE/NLC altitudes. Relative electron density values and associated large- and small-scale vertical structure measured by DC Langmuir probes revealed very deep (by a factor of 50) biteouts in PMSE/NLC regions. These biteouts were seen during strong and weak NLC conditions when PMSEs were either present or absent.

DROPPS: A study of the polar summer mesosphere with rocket, radar and lidar

DROPPS (The Distribution and Role of Particles in the Polar Summer Mesosphere) was a highly coordinated international study conducted in July, 1999 from the Norwegian rocket range (Andoya, Norway). Two sequences of rockets were launched. Each included one NASA DROPPS payload, containing instruments to measure the electrodynamic and optical properties of dust/aerosol layers, accompanied by European payloads (MIDAS, Mini-MIDAS, and/or Mini-DUSTY) to study the same structures in a complementary manner. Meteorological rockets provided winds and temperature. ALOMAR lidars and radars (located adjacent to the launch site) monitored the mesosphere for noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSEs), respectively. EISCAT radars provided PMSE and related information at a remote site (Tromso, Norway). Sequence 1 (5–6 July) was launched into a strong PMSE with a weak NLC present; sequence 2 (14 July) occurred during a strong NLC with no PMSE evident. Here we describe program details along with preliminary results.

Evidence for charged aerosols and associated meter‐scale structure in identified PMSE/NLC regions

Evidence for the existence of negatively charged aerosols/dust in PMSE/NLC regions has been obtained by the unique combination of rocket probes flown during the DROPPS Program. Simultaneous current measurements of charged aerosols, ions, and electrons were accomplished by using a configuration of blunt probes, Gerdien condenser, and DC Langmuir probe. The two blunt probes, with different fixed-bias voltages to discriminate the collection of mobile charge carriers, consistently demonstrated the presence of impacting negatively charged aerosols. Their occurrence coincided with an electron bite-out, thus confirming the associated loss process as aerosol attachment.

Intense turbulence observed above a mesospheric temperature inversion at equatorial latitude

[1] Results from a sounding rocket experiment launched on September 19, 2004 from Kwajalein Atoll, Marshall Islands are reported. A large modulation of the temperature profile in the upper mesosphere was observed with a local maximum at 92 km, 40 K warmer than 2 km below. The temperature gradient between 92 and 102 km was near-adiabatic, suggesting strong mixing. Turbulence was observed in the lower part of the mixed layer, as evidenced by neutral and plasma density fluctuations on both the upleg and downleg portions of the flight. The plasma density gradient was less steep in the mixed region. The turbulent energy dissipation rate was found to be 170 mW/kg. The thermal structure can be described as an upper mesospheric inversion layer, possibly caused by enhanced wave breaking or turbulent heat transport.

Large electric potential perturbations in PMSE during DROPPS

Comprehensive vector electric field detectors were flown during the DROPPS rocket experiment to study electrodynamic processes near the mesopause. This paper will discuss the first DROPPS rocket flight, which penetrated a strong polar mesosphere summer echo (PMSE) event that also included a weak noctilucent cloud (NLC) layer. During this flight, strong potential perturbations were observed which at first appeared to be caused by large geophysical electric fields. However, as shown here, the potential perturbations resulted from the rocket wake, and were not caused by an environmental electric field. This result strongly differs from other previous in-situ experiments, which have reported large electric fields in PMSE regions.

Equatorial dynamics observed by rocket, radar, and satellite during the CADRE/MALTED campaign: 1. Programmatics and small‐scale fluctuations

In August 1994, the Mesospheric and Lower Thermospheric Equatorial Dynamics (MALTED) Program was conducted from the Alcântara rocket site in northeastern Brazil as part of the International Guara Rocket Campaign to study equatorial dynamics, irregularities, and instabilities in the ionosphere. This site was selected because of its proximity to the geographic (2.3°S) and magnetic (∼0.5°S) equators. MALTED was concerned with planetary wave modulation of the diurnal tidal amplitude, which exhibits considerable amplitude variability at equatorial and subtropical latitudes. Our goals were to study this global modulation of the tidal motions where tidal influences on the thermal structure are maximum, to study the interaction of these tidal structures with gravity waves and turbulence at mesopause altitudes, and to gain a better understanding of dynamic influences and variability on the equatorial middle atmosphere. Four (two daytime and two nighttime) identical Nike-Orion payloads designed to investigate small-scale turbulence and irregularities were coordinated with 20 meteorological falling-sphere rockets designed to measure temperature and wind fields during a 10-day period. These in situ measurements were coordinated with observations of global-scale mesospheric motions that were provided by various ground based radars and the Upper Atmosphere Research Satellite (UARS) through the Coupling and Dynamics of Regions Equatorial (CADRE) campaign. The ground-based observatories included the Jicamarca radar observatory near Lima, Peru, and medium frequency (MF) radars in Hawaii, Christmas Island, and Adelaide. Since all four Nike-Orion flights penetrated and overflew the electrojet with apogees near 125 km, these flights provided additional information about the electrodynamics and irregularities in the equatorial ionospheric E region and may provide information on wave coupling between the mesosphere and the electrojet. Simultaneous with these flights, the CUPRI 50-MHz radar (Cornell University) provided local sounding of the electrojet region. A description of the campaign logistics and the measurements performed with the Nike-Orion instrumentation and their implications for turbulence due to gravity waves and tidal instability in the mesosphere and lower thermosphere (MLT) are presented here. From a study of electron density fluctuations measured by rocket probes, we have found evidence for equatorial mesospheric neutral-atmospheric turbulence between 85 and 90 km. Furthermore, falling-sphere data imply that gravity wave breaking was a source for this turbulence. Mean motions and the various planetary, tidal, and gravity wave structures and their coherence and variability are the subjects of a companion paper.

Rocket probe observations of electric field irregularities in the polar summer mesosphere

Electric field wave measurements gathered on a sounding rocket flown in the presence of polar mesospheric summer echoes reveal a distinct layer of irregularities between 83 - 86 km with broadband amplitudes of > 10 mV/m rms. The waves are characterized by bursty, spiky waveforms with lower frequencies (∼ 10 Hz) dominant in the upper portion of the layer near 85 km and broader band emissions, extending to higher frequencies (∼ 1000 Hz) dominant in the lower portion of the layer near 83.5 km. The lower altitudes correspond to a region of weak optical emissions associated with a noctilucent cloud. The waves appear in and around regions where charged/neutral aerosols (1 - 10 nm) and large electron density depletions were observed. The irregularities likely result from a variety of processes including space charge inhomogeneities, mixed neutral and plasma motions, and complex effects associated with charged aerosols of varying sizes.

Middle atmosphere electrical structure during MAC/EPSILON

Abstract Extensive use of rocket-launched probes during the MAC/EPSILON campaign at the Andoya Rocket Range, Norway, has enabled the characterization of the regions electrical environment for all four flight series. The first rocket salvo was conducted during daylight (15 October 1987) and the subsequent three occurred at night (21 and 28 October and 12 November 1987), all of them during geomagnetically disturbed conditions. Measurements of polar electrical conductivity, ion mobility and number density are presented, and their associated structure is investigated for local auroral ionization effects. This is believed to be the first time that Gerdien condenser mobility measurements have indicated a heavy-ion presence (positively charged aerosols) in the auroral mesopause region.

Electron density fluctuations in the equatorial mesosphere: Neutral Turbulence or plasma instabilities?

Electron density fluctuations were measured with rocket borne nose-tip probes launched on August 19, 1994 at 12:08 LT and August 24, 1994 at 10:39 LT from Alcântara, Brazil (2.3°S, 44.4°W) during the MALTED/Guara campaign. The spectral analysis of the fluctuations shows evidence for neutral turbulence in the region of 85 to 90 km for both flights. Falling spheres launched 44 and 13 minutes prior to the main payloads indicate superadiabatic lapse rates in this height region. Also for the two flights, the electron density fluctuations between 90 and 110 km are dominated by the equatorial electrojet which has been observed simultaneously by the 50-MHz CUPRI VHF radar. The in situ data show significant plasma irregularities associated with the electrojet at altitudes well below those that return detectable radar echoes, and where they can interfere with the fluctuations attributed to neutral turbulence.

Studies of high latitude mesospheric turbulence by radar and rocket 2: measurements of small scale turbulence

Abstract Measurements of mesospheric small scale turbulence and associated larger scale wave structures were obtained during equinox by rocket probes flown in coordination with the nearby Poker Flat MST Radar. One rocket series (29 March 1985) included a parachute-borne, electrostatic probe for measuring relative fluctuations in positive ion density, while the other salvo (1 April 1985) included a fixed bias nose tip probe to measure electron current and associated small scale density irregularities. The measured small scale plasma density variations are indicative of neutral air motions. A − 5 3 value for the spectral index, characteristic of turbulence in the inertial subrange, was generally observed for plasma density fluctuations in the region of strong MST radar returns. A comparison of the variance calculated from the power density spectra of the nose tip probes current with the MST radars S/N ratio demonstrated consistent measurements of electron density fluctuations. Also, both measurements indicated the presence of saturating waves. The occurrence of 1–3 km wavelike perturbations, superposed on a larger wave (~7 km) in the wind velocity field, was identified by the MST radar. Consistency between MST radar, meteorological and probe measurements of the wave structures is observed in the region where turbulence was detected. The 1–3 km waves are believed to be more important in the transport of energy and momentum and also in the production of turbulence in the lower mesosphere.