U.-P. Hoppe

First artificially induced modulation of PMSE using the EISCAT Heating Facility

A coordinated experiment involving ionospheric heating and VHF observations of polar mesosphere summer echoes (PMSE) has recently been conducted at the EISCAT facility near Tromso, Norway. We have demonstrated for the first time that ionospheric heating can influence VHF radar returns associated with PMSE. Artificially elevating the electron temperatures within the PMSE layer has been shown to reduce the echo power. Based on this and other results from the experiment, it is suggested that the observed reduction in PMSE power is related to an enhancement of the electron diffusivity through the heating.

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.

First in‐situ observations of neutral and plasma density fluctuations within a PMSE layer

The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, CUPRI and EISCAT observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.

Evidence of substantial ozone depletion in winter 1995/96 over northern Norway

In winter 1995/96, the stratospheric ozone layer over most of the European Arctic was exposed to extremely low temperatures and strong PSCs. At the same time, total ozone monitoring instruments at Tromso and Andoya, Northern Norway, observed values up to 50% below the long-term average values of the Dobson instrument at Tromso. The deviation increased systematically with time of the year, reaching a maximum in early March. Measurements with the ozone DIAL system at the ALOMAR facility at Andoya showed significant reduction of ozone mixing ratios in the 430 to 580 K potential temperature region. A maximum reduction of almost 50% occurred at about 500 K between early January and early March. Comparison with 3D model calculations revealed large reductions with respect to passive tracer ozone profiles in the 400 to 600 K region, reaching a maximum of about 60% by the end of March at the 475 K level. The 3D chemical model showed ozone depletion in the same altitude region as the measurements but seemed to underestimate the depletion systematically. The comparison also indicated ozone depletion of up to 30% in the 475 to 675 K region in extra-vortex air which remained close to the vortex edge during a period in mid-March.

Multiple-frequency studies of the high-latitude summer mesosphere : implications for scattering processes

Abstract The characteristics of polar mesosphere summer echoes (PMSE) are studied at 53.5 and 224 MHz. Observations at 2.78 MHz, simultaneous with the ones at the other two frequencies, were carefully compared for indications of PMSE, but no obvious relation was found. Relationships between relative scattering cross-section, spectral width and vertical velocity are studied for the 224 MHz radar, and observations at 53.5 MHz are compared with those at 224 MHz. Results of aspect sensitivity measurements at 53.5 MHz are presented. The implications of these characteristics for several possible scattering mechanisms are discussed. We rule out incoherent scatter and chemically induced fluctuations from the evidence that we have. In view of the extremely low temperatures near the high-latitude mesopause in summer, we discuss several scenarios involving heavy cluster ions and charged aerosol particles.

A study of the vertical motion field near the high-latitude summer mesopause during MAC/SINE

Abstract We present the results of high-resolution observations of the vertical velocity field obtained with the EISCAT and SOUSY VHF radars near the high-latitude summer mesopause during the MAC/SINE campaign in northern Norway in 1987. The data reveal an energetic motion field with maximum amplitudes of ~ 10 m/s and characteristic periods of ~5–30 min. Motions exhibit a high degree of vertical coherence and a quasi-periodic structure, with typical durations of 5–10 cycles. Estimates of the mean vertical velocity are downward at lower levels and are near zero or positive at greater heights. The mean vertical velocity variance is found to be ~5 m2/s2, consistent with other high-latitude measurements. Frequency spectra computed for each radar are found to exhibit considerable variability, while vertical wavenumber spectra are seen to be somewhat variable in amplitude and to have slopes approaching −3 at lower wavenumbers. These results are suggestive of an energetic spectrum of gravity wave motions near the mesopause that has a large vertical flux of wave energy, that may have observed wave frequencies differing significantly from intrinsic frequencies due to Doppler shifting by large horizontal winds, and that is consistent with the separability of the frequency and wavenumber dependence of the motion spectrum and with gravity wave saturation at sufficiently small vertical scales.

Mean state densities, temperatures and winds during the MAC/SINE and MAC/EPSILON campaigns

Abstract During 1987 two major field campaigns were conducted, mainly in northern Norway (in summer and late autumn), in which a total of 41 (26+15) in-situ temperature profiles were obtained by different techniques such as passive falling spheres, ionizalion gauges and mass spectrometers. Simultaneously, ground-based measurements of OH-temperatures and sodium lidar temperatures were performed for approximately 85 h and 104 h, respectively. In addition, a total of 67 (37 + 30) wind profiles were measured by in-situ techniques. Several radar systems measured winds almost continuously before, during and after the campaigns. The mean temperature profile for the summer campaign showed major deviations from a recently published reference atmosphere (CIRA 1986), whereas the differences between observations and model are smaller in autumn. In general, both the summer and autumn mean wind profiles agreed with CIRA 1986. Minor differences were attributed to tidal biases of the observations and ageostrophic components.

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.

A comparison of PMSE and other ground-based observations during the NLC-91 campaign

Abstract During the period July–August 1991, observations were made of Polar Mesospheric Summer Echoes (PMSE) at 46.9 MHz and 224 MHz by the CUPRI and EISCAT radars, respectively, at two sites in northern Scandinavia. Those observations are compared here with observations of noctilucent clouds, energetic particle precipitation and magnetic disturbances. The appearance and morphology of PMSE are found to be closely correlated at the two frequencies and the two sites, 200 km apart. No correlation is found between PMSE and noctilucent clouds or magnetic disturbance. No correlation is found between energetic particle precipitation and the appearance of PMSE at 46.9 MHz for the whole time period. At 224 MHz, there is no evidence for a correlation before the beginning of August and only one event suggesting a possible correlation after the beginning of August. A minimum in occurrence frequency for PMSE is found between 16 and 21 UT (17–22 LST) which may be related to an expected minimum in background wind strength in that time interval.

Studies of Polar Mesosphere Summer Echoes by VHF radar and rocket probes

Abstract At radar frequencies in the range 50 MHz to 250 MHz, at times even to over 1 GHz, strong enhancements of scattering cross section occur between ≅ 80km and ≅ 95km altitude in summer at high latitudes. These echoes, termed “ Polar Mesosphere Summer Echoes” (PMSE), have attracted considerable experimental effort. Observations of this phenomenon are reviewed in the context of atmospheric dynamics and of scattering processes. Recent rocket and radar measurements indicate that a partial reflection from a multitude of ion layers and constructive interference causes at least some of the PMSE. It is discussed which further observations are necessary and some possible practical consequences of PMSE are pointed out.