scholarly journals Evidence for thermospheric gravity waves in the southern polar cap from ground-based vertical velocity and photometric observations

2001 ◽  
Vol 19 (5) ◽  
pp. 533-543 ◽  
Author(s):  
J. L. Innis ◽  
P. A. Greet ◽  
P. L. Dyson
Keyword(s):  
Gravity Waves ◽  
Vertical Velocity ◽  
Auroral Oval ◽  
Local Times ◽  
Polar Cap ◽  
Photometric Observations ◽  
Fabry Perot ◽  
Waves And Tides ◽  
Thermospheric Dynamics ◽  
Vertical Winds

Abstract. Zenith-directed Fabry-Perot Spectrometer (FPS) and 3-Field Photometer (3FP) observations of the λ630 nm emission (~240 km altitude) were obtained at Davis station, Antarctica, during the austral winter of 1999. Eleven nights of suitable data were searched for significant periodicities common to vertical winds from the FPS and photo-metric variations from the 3FP. Three wave-like events were found, each of around one or more hours in duration, with periods around 15 minutes, vertical velocity amplitudes near 60 ms–1 , horizontal phase velocities around 300 ms–1 , and horizontal wavelengths from 240 to 400 km. These characteristics appear consistent with polar cap gravity waves seen by other workers, and we conclude this is a likely interpretation of our data. Assuming a source height near 125 km altitude, we determine the approximate source location by calculating back along the wave trajectory using the gravity wave property relating angle of ascent and frequency. The wave sources appear to be in the vicinity of the poleward border of the auroral oval, at magnetic local times up to 5 hours before local magnetic midnight.Key words. Meteorology and atmospheric dynamics (thermospheric dynamics; waves and tides)

scholarly journals Thermospheric vertical winds in the auroral oval/polar cap region

2002 ◽  
Vol 20 (12) ◽  
pp. 1987-2001 ◽  
Author(s):  
P. A. Greet ◽  
J. L. Innis ◽  
P. L. Dyson
Keyword(s):  
Auroral Oval ◽  
Atmospheric Dynamics ◽  
Polar Cap ◽  
Frequency Distributions ◽  
Meteorology And Atmospheric Dynamics ◽  
Fabry Perot ◽  
Thermospheric Winds ◽  
Thermospheric Dynamics ◽  
Vertical Winds ◽  
Range Of Variation

Abstract. Thermospheric mean vertical winds from high-resolution Fabry-Perot Spectrometer observations of the l630 nm emission (from ~ 240 km altitude), over a four year interval 1997–2000, from Mawson (67.6° S, 62.9° E, Inv 70.5° S) and Davis (68.6° S, 78.0° E, Inv 74.6° S) are presented. Combining the four years of data shows Mawson mean hourly vertical winds vary between -10 ms-1 and +4 ms-1, while Davis mean hourly vertical winds vary between - 0 ms-1 and +10 ms-1. Mean hourly vertical winds from Mawson show little change with Kp, while at Davis the range of variation increases with increasing geomagnetic activity. Histograms of frequency distributions of such winds, and their variations with Kp and l630 nm emission intensity, are presented and discussed. Variations in mean hourly thermospheric winds and l630 nm emission intensities show at least three significant associations between mean vertical winds and the auroral oval. Mean vertical winds within the auroral oval are smaller than those outside the oval, particularly those in the polar cap. A downward wind associated with entry of the observing region into the auroral oval can be seen in both Mawson and Davis hourly mean vertical winds. Large vertical winds are seen poleward of the auroral oval/polar cap boundary, most significantly upward winds occur within ± 2 hr of magnetic midnight. Under moderately quiet geomagnetic conditions Davis passes through the auroral oval into the polar cap in the evening, but at higher Kp it passes into the polar cap earlier and larger, and more sustained mean vertical winds are observed.Key words. Meteorology and atmospheric dynamics (thermospheric dynamics)

scholarly journals High time resolution measurements of the thermosphere from Fabry-Perot Interferometer measurements of atomic oxygen

2007 ◽  
Vol 25 (6) ◽  
pp. 1269-1278 ◽  
Author(s):  
E. A. K. Ford ◽  
A. L. Aruliah ◽  
E. M. Griffin ◽  
I. McWhirter
Keyword(s):  
Gravity Waves ◽  
Time Resolution ◽  
Lower Thermosphere ◽  
Line Emission ◽  
Auroral Oval ◽  
High Time ◽  
High Time Resolution ◽  
Short Period ◽  
Fabry Perot ◽  
Thermospheric Winds

Abstract. Recent advances in the performance of CCD detectors have enabled a high time resolution study of the high latitude upper thermosphere with Fabry-Perot Interferometers (FPIs) to be performed. 10-s integration times were used during a campaign in April 2004 on an FPI located in northern Sweden in the auroral oval. The FPI is used to study the thermosphere by measuring the oxygen red line emission at 630.0 nm, which emits at an altitude of approximately 240 km. Previous time resolutions have been 4 min at best, due to the cycle of look directions normally observed. By using 10 s rather than 40 s integration times, and by limiting the number of full cycles in a night, high resolution measurements down to 15 s were achievable. This has allowed the maximum variability of the thermospheric winds and temperatures, and 630.0 nm emission intensities, at approximately 240 km, to be determined as a few minutes. This is a significantly greater variability than the often assumed value of 1 h or more. A Lomb-Scargle analysis of this data has shown evidence of gravity wave activity with waves with short periods. Gravity waves are an important feature of mesosphere-lower thermosphere (MLT) dynamics, observed using many techniques and providing an important mechanism for energy transfer between atmospheric regions. At high latitudes gravity waves may be generated in-situ by localised auroral activity. Short period waves were detected in all four clear nights when this experiment was performed, in 630.0 nm intensities and thermospheric winds and temperatures. Waves with many periodicities were observed, from periods of several hours, down to 14 min. These waves were seen in all parameters over several nights, implying that this variability is a typical property of the thermosphere.

scholarly journals Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar

1998 ◽  
Vol 16 (10) ◽  
pp. 1367-1379 ◽  
Author(s):  
N. J. Mitchell ◽  
V. St. C. Howells
Keyword(s):  
Gravity Waves ◽  
Vertical Velocity ◽  
Middle Atmosphere ◽  
Lower Thermosphere ◽  
Motion Field ◽  
Vhf Radar ◽  
Height Range ◽  
The Mean ◽  
Vertical Winds ◽  
Mean Variance

Abstract. The EISCAT VHF radar (69.4°N, 19.1°E) has been used to record vertical winds at mesopause heights on a total of 31 days between June 1990 and January 1993. The data reveal a motion field dominated by quasi-monochromatic gravity waves with representative apparent periods of ~30–40 min, amplitudes of up to ~2.5 m s–1 and large vertical wavelength. In some instances waves appear to be ducted. Vertical profiles of the vertical-velocity variance display a variety of forms, with little indication of systematic wave growth with height. Daily mean variance profiles evaluated for consecutive days of recording show that the general shape of the variance profiles persists over several days. The mean variance evaluated over a 10 km height range has values from 1.2 m2s–2 to 6.5 m2s–2 and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km has a slope (spectral index) of –1.36 ± 0.2, consistent with observations at lower heights but disagreeing with the predictions of a number of saturation theories advanced to explain gravity-wave spectra. The spectral slopes evaluated for individual days have a range of values, and steeper slopes are observed in summer than in winter. The spectra also appear to be generally steeper on days with lower mean vertical-velocity variance.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

scholarly journals Plasma convection jets near the poleward boundary of the nightside auroral oval and their relation to Pedersen conductivity gradients

2010 ◽  
Vol 28 (4) ◽  
pp. 969-976 ◽  
Author(s):  
H. Wang ◽  
H. Lühr ◽  
A. J. Ridley
Keyword(s):  
Auroral Oval ◽  
Flow Speed ◽  
Local Times ◽  
Closed Field ◽  
Plasma Convection ◽  
Polar Cap ◽  
Ionospheric Conductivity ◽  
Ion Flow ◽  
One Year ◽  
Conductivity Gradient

Abstract. In this work, we have shown that the ionospheric azimuthal plasma velocity jets near the open-closed field line boundary on the nightside can be associated with the peak in the ionospheric conductivity gradient. Both model and DMSP observations have been utilized to conduct this investigation. The model tests show that when the gradient of conductivity in the poleward boundary becomes sharper, convection peaks appear around the poleward edge of the aurora. The model results have been confirmed by DMSP observations. Hundreds of large ion flow events are identified from one year DMSP observations, with flow speed larger than 500 m/s that occurred poleward of the aurora. Among them, 280 (74%) events are found to be associated with conductivity gradient peaks. Most of the convection jets occur in winter when conductivity gradients are expected to be large. The convection jets tend to occur at later local times (21:00–22:00 MLT) at 70°–72° MLat. These events are preceded by increasing of the merging electric field suggesting that they occur after the expansion of the polar cap. Both observation and model results show that the conductivity gradient at the polar cap boundary are one of the important elements in establishing the convection jets.

scholarly journals M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

2014 ◽  
Vol 32 (4) ◽  
pp. 333-351 ◽  
Author(s):  
P. E. Sandholt ◽  
C. J. Farrugia ◽  
W. F. Denig
Keyword(s):  
Spatial Scales ◽  
Current System ◽  
Auroral Oval ◽  
Local Times ◽  
Type I ◽  
Polar Cap ◽  
Interplanetary Coronal Mass Ejections ◽  
Expansion Phase ◽  
Earth Magnetic Field ◽  
Substorm Activity

Abstract. We study substorms from two perspectives, i.e., magnetosphere–ionosphere coupling across the auroral oval at dusk and at midnight magnetic local times. By this approach we monitor the activations/expansions of basic elements of the substorm current system (Bostrøm type I centered at midnight and Bostrøm type II maximizing at dawn and dusk) during the evolution of the substorm activity. Emphasis is placed on the R1 and R2 types of field-aligned current (FAC) coupling across the Harang reversal at dusk. We distinguish between two distinct activity levels in the substorm expansion phase, i.e., an initial transient phase and a persistent phase. These activities/phases are discussed in relation to polar cap convection which is continuously monitored by the polar cap north (PCN) index. The substorm activity we selected occurred during a long interval of continuously strong solar wind forcing at the interplanetary coronal mass ejection passage on 18 August 2003. The advantage of our scientific approach lies in the combination of (i) continuous ground observations of the ionospheric signatures within wide latitude ranges across the auroral oval at dusk and midnight by meridian chain magnetometer data, (ii) "snapshot" satellite (DMSP F13) observations of FAC/precipitation/ion drift profiles, and (iii) observations of current disruption/near-Earth magnetic field dipolarizations at geostationary altitude. Under the prevailing fortunate circumstances we are able to discriminate between the roles of the dayside and nightside sources of polar cap convection. For the nightside source we distinguish between the roles of inductive and potential electric fields in the two substages of the substorm expansion phase. According to our estimates the observed dipolarization rate (δ Bz/δt) and the inferred large spatial scales (in radial and azimuthal dimensions) of the dipolarization process in these strong substorm expansions may lead to 50–100 kV enhancements of the cross-polar-cap potential due to inductive electric field coupling.

scholarly journals Mesospheric temperature soundings with the new, daylight-capable IAP RMR lidar

2016 ◽  
Vol 9 (8) ◽  
pp. 3707-3715 ◽  
Author(s):  
Michael Gerding ◽  
Maren Kopp ◽  
Josef Höffner ◽  
Kathrin Baumgarten ◽  
Franz-Josef Lübken
Keyword(s):  
Gravity Waves ◽  
Signal To Noise Ratio ◽  
Correction Method ◽  
Spectral Filtering ◽  
Mesospheric Temperature ◽  
Fabry Perot ◽  
Small Bandwidth ◽  
Waves And Tides ◽  
Temperature Errors ◽  
Thermal Tides

Abstract. Temperature measurements by lidar are an important tool for the understanding of the mean state of the atmosphere as well as the propagation of gravity waves and thermal tides. Though, mesospheric lidar soundings are often limited to nighttime conditions (e.g., solar zenith angle  >  96°) due to the low signal-to-noise ratio during the day. By this, examination of long-period gravity waves and tides is inhibited, as well as soundings in summer at polar latitudes. We developed a new daylight-capable Rayleigh–Mie–Raman (RMR) lidar at our site in Kühlungsborn, Germany (54° N, 12° E), that is in routine operation since 2010 for temperature soundings up to 90 km or  ∼  75 km (night or day) and soundings of noctilucent clouds. Here we describe the setup of the system with special emphasis on the daylight suppression methods like spatial and spectral filtering. The small bandwidth of the Fabry–Pérot etalons for spectral filtering of the received signal induces an altitude-dependent transmission of the detector. As a result, the signal is no longer proportional to the air density and the hydrostatic integration of the profile results in systematic temperature errors of up to 4 K. We demonstrate a correction method and the validity of correction by comparison with data obtained by our co-located, nighttime-only RMR lidar where no etalon is installed. As a further example a time series of temperature profiles between 20 and 80 km is presented for day and night of 9–10 March 2014. Together with the other data of March 2014 these profiles are used to calculate tidal amplitudes. It is found that tidal amplitudes vary between ∼  1 and 5 K depending on altitude.

scholarly journals Average thermospheric wind patterns over the polar regions, as observed by CHAMP

2007 ◽  
Vol 25 (5) ◽  
pp. 1093-1101 ◽  
Author(s):  
H. Lühr ◽  
S. Rentz ◽  
P. Ritter ◽  
H. Liu ◽  
K. Häusler
Keyword(s):  
Auroral Oval ◽  
Magnetic Activity ◽  
Local Times ◽  
Polar Regions ◽  
Hemispheric Differences ◽  
Polar Cap ◽  
Seasonal Differences ◽  
Thermospheric Wind ◽  
Wind Distribution ◽  
Wind Patterns

Abstract. Measurements of the CHAMP accelerometer are utilized to investigate the average thermospheric wind distribution in the polar regions at altitudes around 400 km. This study puts special emphasis on the seasonal differences in the wind patterns. For this purpose 131 days centered on the June solstice of 2003 are considered. Within that period CHAMP's orbit is precessing once through all local times. The cross-track wind estimates of all 2030 passes are used to construct mean wind vectors for 918 equal-area cells. These bin averages are presented in corrected geomagnetic coordinates. Both hemispheres are considered simultaneously providing summer and winter responses for the same prevailing geophysical conditions. The period under study is characterized by high magnetic activity (Kp=4−) but moderate solar flux level (F10.7=124). Our analysis reveals clear wind features in the summer (Northern) Hemisphere. Over the polar cap there is a fast day-to-night flow with mean speeds surpassing 600 m/s in the dawn sector. At auroral latitudes we find strong westward zonal winds on the dawn side. On the dusk side, however, an anti-cyclonic vortex is forming. The dawn/dusk asymmetry is attributed to the combined action of Coriolis and centrifugal forces. Along the auroral oval the sunward streaming plasma causes a stagnation of the day-to-night wind. This effect is particularly clear on the dusk side. On the dawn side it is evident only from midnight to 06:00 MLT. The winter (Southern) Hemisphere reveals similar wind features, but they are less well ordered. The mean day-to-night wind over the polar cap is weaker by about 35%. Otherwise, the seasonal differences are mainly confined to the dayside (06:00–18:00 MLT). In addition, the larger offset between geographic and geomagnetic pole in the south also causes hemispheric differences of the thermospheric wind distribution.

scholarly journals Statistical analysis of thermospheric gravity waves from Fabry-Perot Interferometer measurements of atomic oxygen

2008 ◽  
Vol 26 (1) ◽  
pp. 29-45 ◽  
Author(s):  
E. A. K. Ford ◽  
A. L. Aruliah ◽  
E. M. Griffin ◽  
I. McWhirter
Keyword(s):  
Gravity Waves ◽  
Time Resolution ◽  
Atomic Oxygen ◽  
Source Region ◽  
Auroral Oval ◽  
Wave Activity ◽  
Similar Proportion ◽  
Short Period ◽  
Fabry Perot ◽  
The Waves

Abstract. Data from the Fabry-Perot Interferometers at KEOPS (Sweden), Sodankylä (Finland), and Svalbard (Norway), have been analysed for gravity wave activity on all the clear nights from 2000 to 2006. A total of 249 nights were available from KEOPS, 133 from Sodankylä and 185 from the Svalbard FPI. A Lomb-Scargle analysis was performed on each of these nights to identify the periods of any wave activity during the night. Comparisons between many nights of data allow the general characteristics of the waves that are present in the high latitude upper thermosphere to be determined. Comparisons were made between the different parameters: the atomic oxygen intensities, the thermospheric winds and temperatures, and for each parameter the distribution of frequencies of the waves was determined. No dependence on the number of waves on geomagnetic activity levels, or position in the solar cycle, was found. All the FPIs have had different detectors at various times, producing different time resolutions of the data, so comparisons between the different years, and between data from different sites, showed how the time resolution determines which waves are observed. In addition to the cutoff due to the Nyquist frequency, poor resolution observations significantly reduce the number of short-period waves (<1 h period) that may be detected with confidence. The length of the dataset, which is usually determined by the length of the night, was the main factor influencing the number of long period waves (>5 h) detected. Comparisons between the number of gravity waves detected at KEOPS and Sodankylä over all the seasons showed a similar proportion of waves to the number of nights used for both sites, as expected since the two sites are at similar latitudes and therefore locations with respect to the auroral oval, confirming this as a likely source region. Svalbard showed fewer waves with short periods than KEOPS data for a season when both had the same time resolution data. This gives a clear indication of the direction of flow of the gravity waves, and corroborates that the source is the auroral oval. This is because the energy is dissipated through heating in each cycle of a wave, therefore, over a given distance, short period waves lose more energy than long and dissipate before they reach their target.

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